Fundamentals of Conjugated Polymer Blends, Copolymers and Composites: Synthesis, Properties, and Applications

Since their discovery in 1977, the evolution of conducting polymers has revolutionized modern science and technology. These polymers enjoy a special status in the area of materials science yet they are not as popular among young readers or common people when compared to other materials like metals, paper, plastics, rubber, textiles, ceramics and composites like concrete. Most importantly, much of the available literature in the form of papers, specific review articles and books is targeted either at advanced readers (scientists/technologists/engineers/senior academicians) or for those who are already familiar with the topic (doctoral/postdoctoral scholars). For a beginner or even school/college students, such compilations are bit difficult to access/digest. In fact, they need proper introduction to the topic of conducting polymers including their discovery, preparation, properties, applications and societal impact, using suitable examples and already known principles/knowledge/phenomenon.   Further, active participation of readers in terms of “question & answers”, “fill-in-the-blanks”, “numerical” along with suitable answer key is necessary to maintain the interest and to initiate the “thought process”. The readers also need to know about the drawbacks and any hazards of such materials. Therefore, I believe that a comprehensive source on the science/technology of conducting polymers which maintains a link between grass root fundamentals and state-of-the-art R&D is still missing from the open literature. 

• Language: English
• Publisher: Wiley
• Release date: Apr 30, 2015
• ISBN: 9781119137108

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Preface

The conjugated polymers (CPs) are considered as path-breaking discovery, a Serendipity indeed!!!, that has not only revolutionized the area of material science but also changed the face of nanotechnology. It is apt to highlight and worth mentioning here that though their discovery has been rewarded with Year 2000’s Chemistry Nobel Prize to the discoverers Prof. Heeger, Prof. Shirakawa, and Late Prof. MacDiarmid, their wealth of prevalent applications that were actually based on strategic combination of CPs with a variety of organic/inorganic materials (bulk- or nano-size) in the form of blends (BLNs), conjugated copolymers (CCPs), composites (CMPs) [bulk or nanocomposites (NCs)], or hybrids (HYBs) have played a pivotal role in demonstrating and advancing their techno-commercial utility. Interestingly, the above area is the best example of power of interdisciplinary research, cross-country collaborations, and industrial partnership, for successful implementation of knowledge and ideas into life-changing products. Therefore, I strongly feel that an interdisciplinary research-oriented dedicated book covering the elementary concepts and recent advancements in the area is necessary to expose current challenges, highlight future perspectives, and stimulate thought process for evolution of novel materials and technologies. This edited book is the first of its own kind that provides a single-source solution to specifically address the fundamentals and applications of CP-based mixed systems, i.e., BLNs, CCPs, and CMPs with special focus on interdisciplinary and application-oriented research and comprehensive literature account. Accordingly, the book is organized into 14 chapters that are subdivided into four sections viz. Multiphase Systems: Synthesis, Properties, and Applications (Chapters 1–4); Energy Harvesting and Storage Materials (Chapters 5–8); Advanced Materials for Environmental Applications (Chapters 9–11); and Sensing and Responsive Materials (Chapters 12–14).

The first section highlights the fundamental aspects of mixed system constituted by strategic combination of doped and undoped CPs with a variety of organic/inorganic materials (bulk- or nano-size) to form BLNs, CCPs, and CMPs/NCs/HYBs. These advanced materials have been demonstrated for techno-commercial utility in diverse areas including electronics and optoelectronics; energy harvesting and storage, environmental pollution, and corrosion control; biology and biomedicals; sensing and responsive materials, etc. Chapter 1 deals with their definitions, types, importance, synthetic routes, and practical applications including the state of the art in the area. The special emphasis has been given to the combination of CPs with a variety of materials like carbonaceous fillers [e.g., fullerene derivatives like PCBM, carbon nanotubes (CNTs), nanofibers (CNFs), graphene analogs, carbon dots (C-Dots), graphene quantum dots (GQDs)]; inorganic nanofillers (e.g., silica or various clays); metal/inorganic nanoparticles (NPs) (e.g., Au/Ag NPs, dielectric/magnetic/redox-active particles like ZnO, V2O5, MnO2, Co3O4, Cu2O, Y2O3, Nb2O5, RuO2, WO3, TiO2, BaTiO3, γ-Fe2O3, BaTiO3, Fe3O4, etc.); semiconductors [quantum confined nanomaterials (QCNs) and nanocrystals (NCRs) like quantum dots (QDs), nanorods (NRs), nanowires (NWs), nanotubes (NTs), tripods, tetrapods, multipods, etc.] to form advanced BLNs, CCPs, and CMPs/NCs/HYBs. Chapter 2 provides a comprehensive account of the transition metal oxides-filled polyaniline (PANI) matrix-based NCs, including their synthesis, properties, and applications. Chapter 3 focuses on the organic/inorganic NCs/HYBs based on strategic combination of various CPs with QCNs (like inorganic semiconducting NPs/NCRs and carbonaceous analogues, e.g., GQDs or C-Dots). The specific advantages of such systems, various synthesis strategies, and their optoelectronic properties have also been discussed. The applicability of these NCs has been demonstrated and discussed for optoelectronic applications [like solar cells (both photovoltaic and dye-sensitized solar cells) and light-emitting diodes (LEDs)], via suitable examples from open literature. Finally, Chapter 4 highlights the graphene-based CP NCs and gives brief account of their optoelectronic and biological applications.

The second section discusses the energy harvesting and storage possibilities with above systems with special reference to organic photovoltaics (OPVs), thermoelectrics (TEs), Li-ion batteries (LIBs), and supercapacitors (SCs). In particular, Chapter 5 highlights the promise of CPs-based BLNs, CCPs, and CMPs/NCs/HYBs as an active material for solar cells and reviews the performance of related OPV devices. The inherent advantages of these systems (in terms of panchromatic solar absorption, phase segregation driven bulk-heterojunction morphology, efficient charge separation and improved transport, leading to efficient PV action) and various material combinations have also been discussed with suitable examples. Chapter 6 outlines the specific advantages of doped CPs (ICPs)-based TE materials (e.g., low density, solution processability, low thermal conductivity, tunable conductivity, and Seeback coefficient) over conventional inorganic TE materials. Particularly, the ICP/inorganic NCs (containing inorganic NPs, CNTs, graphene, metals, and their compounds), their TE properties, and actual TE performances have been comprehensively discussed via suitable examples. Chapter 7 sheds light on ICP/inorganic NCs [based on combination of ICPs like PANI, polypyrrole (PPy), or polythiophene (PTh) with inorganics like LiFePO4, MnO2, V2O5, Si, SnO2, Fe2O3, etc.] as high-performance cathodic and anodic materials for LIBs. It has been systematically highlighted that ICPs-based matrices act as soft cushion for inorganic fillers to accommodate for drastic volume changes and to mitigate mechanical stress, thereby preventing the electrode failure and improving cyclability. In addition, they improve the conductivity of the system and facilitate Li+ insertion/extraction rates, resulting in enhancement of rate capacity of LIBs. Finally, Chapter 8 talks about SCs, which are most promising energy storage alternative for portable electronics and HYB electric vehicles (HEVs). In particular, the PPy/inorganic NCs for electrochemical SCs are reviewed, with emphasis on required structural, chemical, and physical requirements, potential materials, important device parameters and SC performance, well supported by related literature examples.

The third section outlines the environmental applications of above systems including their utility in electromagnetic interference (EMI) shielding, water purification, and corrosion protection of metals. In particular, Chapter 9 covers overview of fundamentals of EMI shielding, theoretical aspects of shielding, and experimental techniques for measurement of shielding effectiveness, followed by detailed discussion on potential EMI shielding materials with main emphasis on ICP-based BLNs and CMPs. Chapter 10 explains corrosion phenomenon, possible solutions, and characterization techniques, in context of ICPs/CP-based NCs, which limit the corrosion by providing the anodic protection, ennobling the surface, and by exerting barrier effect toward a variety of corrodants. The superior anticorrosion performances of organic/inorganic filler-loaded NCs have been highlighted via appropriate examples and data. Chapter 11 emphasizes the pollutant adsorption capabilities of ICPs/CPs-based CMPs/NCs/HYBs and demonstrates their potential for wastewater treatment. Proper attention has been given to adsorption phenomenon, governing equations, and important parameters, with parallel comparison of purification capabilities of PANI-, PPy-, and PTh-based systems.

The fourth and final section is based on the redox property, electroactivity, and polarizability of CPs/ICPs-based systems. More specifically, it deals with the advanced sensing and responsive materials covering applications in the areas of chemical sensors, biosensors, and electrorheological (ER) fluids. Chapter 12 provides current trends in chemical sensor research particularly, where CPs/ICPs-based NCs are exploited as detecting elements. In particular, the device architecture, active layer forming methods, sensing mechanism, important performance indices (sensitivity, selectivity, response/recovery time, performance stability), sensing performance in the presence of various analytes, and specific advantages have been highlighted and thoroughly discussed via suitable examples. Chapter 13 overviews recent works dealing with synthesis and characterization of CP/CP NCs and highlights their applications related to biosensors including catalytic biosensors and bio-affinity biosensors. The last contribution, i.e., Chapter 14, focuses on the recent R&D and state of the art of the PANI NCs-based suspensions and ER fluids based on the same. Especially, the advantages of using anisotropic nanostructured PANI as ER dispersal phase have been highlighted, and performance of related NCs has been compared.

The above chapters cover all perspectives of these multiphase systems, including their relevance, synthetic routes, and practical applications that have been complemented by comprehensive literature account covering past and present research, key developments, and state of the art in the area. Extra care has been taken to simplify the matter and to facilitate the understanding via the specifically designed and selected graphics, i.e., illustrations, schematics, and figures; tabulated information, case studies, and many more literature examples. Due attention has also been given at the end of specific chapters to address the current challenges and future aspects and to stimulate the grey matter and ignite though process. I feel that a broader picture will evolve after reading all the chapters; that will eventually help to improve the existing understanding of the subject and stimulate further innovation and interdisciplinary research. I hope this compilation will also act as a smooth entry point for new researchers to become acquainted with the field, and facilitate advanced readers to enrich their existing understanding. The key challenges and opportunities have also been thrown open for related academicians, material scientists, nanotechnologists, and industry personals to dig deeper into the subject and come out with cheaper, viable, and efficient versions of the existing materials and technologies.

I cannot end here without expressing my regards and gratitude to all those who have made any direct or indirect contributions toward this book. I am greatly indebted to renowned authors from diverse background, who kindly agreed to extend their expertise for the respective chapters. Their valuable contributions provided an excellent overview of these multiphase systems in terms of academic importance, technological relevance, and potential applications. I am extremely grateful to Mr. Martin Scrivener for giving me this opportunity and for his constant encouragement, tremendous faith, and valuable suggestions. I would also like to thank the entire Wiley-Scrivener editorial team who helped me to overcome numerous difficulties and extended strong technical support at every stage of publication. I am extremely thankful to all my peers, colleagues, associates, students, and friends, who extended all the possible help at every stage of the book and made it become reality eventually. Thanks are also due to CSIR and NPL, India, for providing the financial support, necessary research facilities, and conducive research environment. In particular, I am extremely grateful to Prof. Vikaram Kumar, Ex. Director, CSIR-NPL, India, for pushing me into the breadth of field via his visionary initiative called Polymer Electronics Journal Club (PEJC), an interdisciplinary research discussion forum started by him at NPL in 2004. This work is a reflection of the things I learnt at the PEJC via heated discussions over a range of interdisciplinary research topics.

My special regards and thanks to my parents, sisters, and wife who stood by me in all shades of life and shown immense faith in my abilities. I am specially thankful to my pet Harry who always gave me a superstar treatment, greeted me every day with loving enthusiasm, and proved a lucky mascot for me. Last but not least, I am extremely thankful to my critics and all those who pushed me beyond my limits, which actually and eventually helped me to improve at every stage. Finally, I would like to thank the God who has shown me the path.

Parveen Saini

New Delhi, India

January-2015

Part 1

MULTIPHASE SYSTEMS: SYNTHESIS, PROPERTIES AND APPLICATIONS

Chapter 1

Conjugated Polymer-based Blends, Copolymers, and Composites: Synthesis, Properties, and Applications

Parveen Saini*

Conjugated Polymers and Graphene Technology Laboratory, Polymeric and Soft Materials Section, Materials Physics and Engineering Division, CSIR – National Physical Laboratory, New Delhi, INDIA

*Corresponding author: [e-mail]

Abstract

The conjugated polymers (CPs) are considered as path-breaking discovery that has revolutionized the area of material science and nanotechnology. It is apt to highlight here that though their discovery has been rewarded with year 2000’s Chemistry Nobel Prize to the discoverers Prof. Heeger, Prof. Shirakawa, and Late Prof. MacDiarmid, their wealth of prevalent applications, which were actually based on strategic combination of CPs with a variety of organic/inorganic materials (bulk or nano size) in the form of blends (BLNs), conjugated copolymers (CCPs), composites (CMPs) [bulk or nanocomposites (NCs)], or hybrids (HYBs), has played a pivotal role in demonstrating and advancing their techno-commercial utility. This chapter sheds light on fundamental aspects of CPs-based BLNs, CCPs, and CMPs/NCs/HYBs. In particular, their definitions, types, importance, synthesis techniques, and practical applications have been discussed with the help of suitable examples and the state of the art in the area. Finally, current challenges and future scope were highlighted to conclude the chapter.

Keywords: Conjugated polymers (CPs), intrinsically conducting polymers (ICPs), polyaniline (PANI), polypyrrole (PPY), polythiophene (PTh), carbon nanotubes (CNTs), graphene, blends, copolymers, composites, nanocomposites, hybrids, interpenetrating network (IPN), solution processing, melt blending, electrical conductivity, thermoelectric, optoelectronics, organic photovoltaics (OPVs), solar cells, organic light-emitting diodes (OLEDs), memory devices, field-effect transistors (FETs), energy storage, supercapacitor, Li-ion battery, gas sensors; biosensors, corrosion protection, water purification, electromagnetic interference (EMI), antistatic; electrostatic dissipative (ESD), electrorheological fluids

1.1 Introduction

Polymers have emerged as one of the most promising materials of last century. Today we cannot imagine living without them as they have become an integral part of our daily life with wide applicability in diverse areas like medicine, automobiles sector, hospitality industry, and high-tech areas such as space and defense [1–9]. Interestingly, polymers in general sense are known for their excellent electrical insulation properties so that terms conductivity and polymer are often considered as mutually exclusive [4,7,10–16]. However, the accidental discovery (indeed a serendipity!!!) of organic conjugated polymers (CPs) [17,18,19] has provided the direct evidence that with special structure features and under special circumstances, even polymers can transport current. The identification of doping has further extended the concept and demonstrated that the suitable perturbations of electronic structures can cause regulation of conductivity (just like in case of conventional inorganic semiconductors), to display broad range of values covering insulating, semiconducting or metallic regions [18,20,21]. The enormous significance of this path-breaking and revolutionary discovery was acknowledged by awarding the Nobel Prize in Chemistry for year 2000 to the discoverers; Prof. Hideki Shirakawa, Late Prof. Alan G. MacDiarmid, and Prof. Alan J. Heeger [22–24]. The common peculiar structural feature of CPs is the presence of highly delocalized π-electron system consisting of alternate single and double bonds, i.e., π-conjugation in the polymer backbone that can be readily oxidized/reduced (or doped/undoped) to yield an insulating, semiconducting, or metallic material.

Although the conductivity was first reported in polypyrrole (PPY)-based systems [25–27], the discovery went largely unnoticed and remained less understood, until the reporting of metallic conductivity (>10⁵ S/cm) in case of organic polymer based on of p-doped crystalline polyacetylene (PA) films that were actually exposed to I2 vapors (electron acceptor). Later, PA doped with n-type dopants (electron donors) was also demonstrated to display similar conductivity values [18]. This revolutionary work gave impetus to a surge of activities directed toward the exploration, synthesis, and application development of organic conducting macromolecules. In the due course of time, several other CPs such as PPY, polythiophene (PTh), poly(p-phenylene) (PPP), poly(p-phenylene vinylene) (PPV), polyaniline (PANI), and their analogues were also evolved [15–17,28–36]. The chemical structures of constitutional units of undoped forms of some very common CPs are shown in Figure 1.1.

Figure 1.1 Chemical structures of some undoped CPs.

These undoped polymers display weak conductivity that often lies in insulating or semiconducting range (10–10 to 10–5 S/cm). Among them, semiconducting CPs are of special interest due to distinguished optical and electronic properties and prevalent applications in the area of optoelectronics. As already briefed for PA, the controlled doping of other CPs (undoped forms) can also transform them into corresponding doped materials; which on the basis of nature and concentration of dopant, display either semiconducting or metallic conductivity (10–6 to 10⁵ S/cm). It is extremely important to note that the electronic conductivity of CPs is called intrinsic, as it is caused by the presence of particular molecular structure, rather than contributed by externally added conducting inclusions [e.g., metals, carbonaceous fillers like graphite, graphene, or carbon nanotubes (CNTs)]; which are often being parts of conventional polymer-based conducting blends or composites. Therefore, these doped π-conjugated macromolecules are also known as intrinsically conducting polymers (ICPs). In particular, the display of metal like electrical and optical properties by the highly doped forms of CPs (i.e., ICPs) also entitled them to be called synthetic metals [17,22–24,37]. Consequently, it is important to distinguish between highly doped metallic CPs (better known as ICPs) and their relatively less conducting (insulating or semiconducting) counterparts, i.e., undoped (or base forms), which in general simply termed as CPs. The same terminology has been practiced in the present chapter for specifically referring the undoped (semiconducting) and highly doped (conducting or metallic) forms as CPs and ICPs, respectively.

It is worth mentioning that, once the techno-commercial potential of the CPs was realized, much of the focus of industrial, academic, and scientific research got directed toward exploration and establishment of their real world applications. In this context, though many eminent individuals (including discoverers) played key roles toward idea generation and application demonstration; the foundation of modern research in the area was laid by Prof. Sir Richard H. Friend’s (better known as Father of Organic Electronics) at Cavendish Laboratories in early 1990. His ground-breaking research propelled novel applications (like organic LEDs, transistors, and lasers), which inspired many similar fascinating ideas and highlighted the actual worth of discovery of CPs. The same was indeed!!! also recognized later by Nobel Committee in terms of announcement of Chemistry Nobel Prize for the year 2000 to the original discoverers of these Macromolecules. This Colossal recognition for teamwork from one side and parallelly mounting pressure on other side for further innovations in the area, have sparked many interdisciplinary research initiatives, cutting across boundaries of many fields as well as geographic domains. The subsequent advances and vertical expansion of the field of CPs/ICPs have led to a variety of materials and technologies with great potential for various techno-commercial applications [15,16] (Figure 1.2), e.g., batteries [38–45], organic light-emitting diodes (OLEDs) [46–55] and display devices [56,57], solid-state lighting [58–61], organic field-effect transistors (OFETs) [62–68], organic solar cells (OSCs)/photovoltaics (OPVs)[69–78], dye-sensitized solar cells (DSSCs) [79–84], nonlinear optics [68,85–95], lasers [47,96–105], thermoelectric power generation (TEG) [106–116], electrorheological fluids [117–119], memory devices [120–127], electromagnetic interference (EMI) shielding [11,14,128–137], electrostatic charge dissipation (ESD) [138–144], electrochromic (EC) devices [145–150] and smart windows/goggles/mirrors [38,150–153], supercapacitors [154–161], actuators and artificial muscles [162–169], corrosion inhibitors and anticorrosion coatings [170–178], and sensors (including electronic nose and tongue) for detection of a variety of analytes (including gas sensors, chemical, biosensor, strain, humidity, light, and pH) [179–190]. In parallel, highly focused application oriented R&D in the area necessitated the development of their improved version or analogues having superior properties, lower cost, better stability (mechanical, thermal, electronic, and environmental) and facile processing, so as to boost the overall performance in intended applications.

Figure 1.2 Some potential applications of organic CPs.

In further quest for development of more efficient materials, clue had been provided by ongoing mixed (interdisciplinary) research. Intelligently!!! the immediate inspiration was drawn from mixed systems (i.e., blends, alloys or composites) based on conventional polymers, metals, and ceramics. Soon it was realized that the already established wide applicability of CPs/ICPs can be further expended by formation of multiscale/multiphase systems, e.g., a wide variety of electronically, electrochemically, and/or optoelectronically active blends (BLNs), conjugated copolymers (CCPs) and composites (CMPs) [both bulk or nanocomposites (NCs)] or hybrids (HYBs) [11,14–16,52,109,113,120,128,131,132,191–205]. The next section of the chapter covers the fundamental aspects of CP-based BLNs, CCPs, and NCs/HYBs. In particular, their definitions (including etymology), types, properties, synthetic routes, and practical applications have been discussed with the help of suitable examples from the open literature.

1.2 CPs/ICPs-Based Blends

A blend is merely a physical mixture (no chemical bonding) of two or more phases with different chemical compositions as opposed to the copolymer where the constituent phases are actually joined together by chemical bonds [7,206]. In more refined scientific sense, a polymer blend represent a class of materials [analogous to metal (stiff material)-based alloys], in which at least two soft phases (one of them should be polymers) are blended/mixed together to create a new material, with different physico-mechanical properties [5,7–9,206]. Often the blends can be specifically designed to combine attractive features of both components, and sometimes the microstructure of the blend has new properties that were missing in the either components [3,8,9,193,206–209]. CP/ICPs blends refer to special class of blends [52,75,107,113,128,129,140,143,191,193,200–202,204,205,210–219] involving combination of undoped/doped CPs (soft material) with another soft material (e.g., another CP/ICP, conventional insulating polymers like thermoplastics, rubber, or elastomer, suitable functionalized derivative of carbonaceous materials like fullerenes or graphene), to generate a material with distinguished electrical, electrochemical, electronic, optoelectronic, thermal, chemical, or mechanical properties. Sometimes, ICPs have been used as electrically conducting filler (in place of metal- or carbon-based materials) to form the electro-conducting blends, which may be useful for applications like antistatics, electrostatic dissipation, EMI shielding, energy devices, memory devices, corrosion protection, etc. [122,128,132,140,141, 143,155,201,202,214,215,220,221]. However, more often, undoped forms, i.e., CPs (insulating yet electroactive or semiconducting) are combined with other soft phases [insulating polymer or semiconducting material based on another CP, organic quantum confined nanomaterials (QCNs) like fullerene derivatives], to form electrochemically responsive or optoelectronically active blends with wide applicability in the areas like corrosion protection, solar cells (OPVs or DSSCs), OLEDs, display devices, solid-state lighting, OFETs, etc. [52,54,60,65,77,171,193,200,204,213,216,217,220,222–225].

1.2.1 Classification of CPs/ICPs-Based Blends

Like classical or conventional polymer blends, depending upon interactions (as well as solubility parameters and thermodynamics) mediated compatibility between the phases, CPs- or ICPs-based blends can also be broadly classified into three basic categories:

1.2.1.1 Immiscible Blends

These blends display macroscopic phase-segregated morphology (i.e., they are heterogeneous in nature) due to weak interactions (e.g., van der Waals forces) and limited compatibility (i.e., large solubility mismatch) between the constituent phases [8,9,204,206,209,217,218,226,227]. In such a system, the two constituent phases form separated domains (Figure 1.3a–c) with only slight molecular-level interpenetration between them (Figure 1.3d). Consequently, the distinct properties of individual phases are observed in the blend, e.g., two separate glass transition temperatures (corresponding to each phase) in case of polymer/polymer blends. In such systems, as the concentration of minor phase (say polymer B) is increased, distinct changes in the morphology can be seen, e.g., at low concentration (Figure 1.3a), spherical globs of minor phase (polymer B) are formed that are separated from each other by major phase (polymer A). At higher concentration of polymer B, globs become big enough to coalesce into interconnected mass, such that a bi-continuous yet phase-segregated network of both phases is formed (Figure 1.3b). Upon further increasing the amount of polymer B, the interconnected domains of polymer A gets converted into isolated orbs (Figure 1.3c) now dispersed inside pool of polymer B. Nevertheless, the physical sizes of droplets/domains of minor phases are large (micron/submicron range) compared to corresponding compatible or miscible blends [8,9,193,206,217]. It is opportune to highlight here that immiscible blends is by far the most populous group and has a special relevance particularly in context of organic electronics and opt oelectronics (e.g., OPVs, OLEDs, TFTs, and memory devices) [107,120,193,205,210,213,216–218,220,222–225,228], due to distinguished advantages in terms of exciton (i.e., excited state of coulombically bound electron–hole pair) dissociation, charge recombination, and charge migration/transport [70,75,77,81,193,204,229–232]. The well known bulk heterojunction (BHJ)-type donor–acceptor (D-A) blends involving combination of CP (donor) with another electron-accepting soft phase (e.g., acceptors like another CP, fullerene derivatives, or functionalized analogues of graphene) belong to above category of blends [15,16,62,64,120,196,227,233–238]. The most common example is phase-segregated D-A blend constituted by proportionate mixing of P3HT-based donor (D) and fullerene derivative [[6,6]-phenyl C61 butyric acid methyl ester (PCBM)]-based acceptor (A), such that molecularly inhomogeneous and bi-continuous (yet semi-inter-penetrating-type) D-A networks forms throughout the bulk of the photoactive layer [218,227,231].

Figure 1.3 Schematic representation of morphological difference between immiscible (a-d) and miscible (e-h) blends. The change in the phase distribution with the increase in concentration of polymer B (from left to right) showing phase-separated domains in immiscible blend and interpenetrating domains in miscible blends.

1.2.1.2 Compatible Blends

They are in actual sense an improved version of immiscible polymer blends, wherein a suitable compatibilizer (i.e., molecularly architectured third phase consisting of subunits with specific compatibility with both the immiscible primary phases) is deliberately added (to form the ternary system) so as to improve the compatibility between the otherwise originally immiscible phases (i.e., donor and acceptor components in CP-based blends) [193,206,207,211,217,224,231,239]. As a result, the formed ternary blend exhibits macroscopically uniform physical properties (due to sufficiently strong interactions between the components), superior optoelectronic properties, and better phase stability compared to immiscible blends [207,211,239]. Strategically designed block or graft copolymers with covalently linked suitable donor and acceptor functionalities are often used as compatibilizers in OPVs [192,193,200,217,239–241], e.g., the use of additive compatibilizers in the form of diblock copolymers (DBCs) functionalized with P3HT (D) grafts on one block and fullerene (A) grafts on the other block, have resulted in the generation of macroscopically homogenous blends with good phase stability and stable performance of photovoltaic cells or optoelectronic devices [207,217,231,242].

1.2.1.3 Miscible Blends

They represent a system with a single-phase structure, formed due to molecular-level (homogeneous) mixing of constituents (Figure 1.e–h), that probably mediated by existence of sufficiently strong interactions (ionic or dipolar) and matching solubility parameters [6–8,206]. In such systems, the phases are always mixed at molecular (chain level) so that the minor phase droplet size is extremely small (limited by the chain aggregate size). In miscible blends, there is a molecular-level interpenetration of constituent phases (Figure 1.3h). In general, a miscible blend of two polymers is going to have properties somewhere between those of the two constituent polymers. In such cases, individual features are suppressed and only the features corresponding to formed mixed phases appears in the blend (e.g., single glass transition temperature in case of conventional insulating polymer-based blends). The molecular-level blending is difficult for CPs though emulsion/latex-based systems can provide a bit of success in this direction [243]. Nevertheless, in context of CPs, they are less known due to miscibility limited [244] difficulties in forming a true perfectly compatible blend (or better called nanoblends).

It should be remembered that the use of the term polymer-alloy for a polymer blends is strongly discouraged, as the former term includes multiphase copolymers but excludes incompatible polymer blends.

1.3 CPs/ICPs-Based Copolymers (CCPs)

Copolymer or hetero-polymer is a polymer derived from two (or more) monomeric species [5–7,245], as opposed to a homopolymer where only one monomer is used. Copolymers that are obtained by copolymerization of two monomer species are sometimes termed bi-polymers. Similarly, those obtained from three and four monomers are referred as tri-polymer and quarter-polymers respectively. The CCPs are special case of copolymers that display appropriate morphological features (e.g., molecular-level phase segregation leading to high density of D-A heterojunctions across active layer thickness), optoelectronic properties (e.g., tailored bandgap, feasible positioning of energy levels, desired carrier mobility, controlled exciton migration/dissociation rates) and resistance to performance drift (that may occur due to heat/solvent/filed induced augmentation of phase segregation, leading reduction of number density of D-A interfaces), to suit optoelectronic applications, though they may used for other areas too.

1.3.1 Types of CPs/ICPs-Based Copolymers

As a copolymer consists of at least two types of constituent units, so depending upon the number, arrangement, and distribution of co-monomer units/segments along the copolymer’s backbone; presence and disposition of side chains/grafts; and specific assemblage of chain segments (to form higher structures like star assemblies, dendrons, cross-linked networks), a wide variety of CCPs can be realized (Figure 1.4) that can be broadly classified as [5–7,15,16,246,247].

Figure 1.4 Schematic representation of various type of copolymers viz. (a) statistical, (b) random, (c) alternating, (d) diblock, (e) triblock, and (f) graft copolymer, containing distributed monomer units (e.g., A, B, or C).

1.3.1.1 Statistical Copolymers

In statistical copolymers, portion of a macromolecule comprise of many constitutional units and has at least one feature which is not present in the adjacent portions [246]. Here, the sequential distribution of the monomeric units (Figure 1.4a) obeys known statistical laws; e.g., Markovian statistics of zeroth (Bernoullian), first, second, or a higher order. Kinetically, the elementary processes leading to the formation of a statistical sequence of monomeric units are such that the arrangement of monomeric units may tends toward alternation, clustering of like units, or possess no ordering. In simple binary copolymerization, the nature of this sequence distribution can be indicated by the numerical value of a function either of the reactivity ratios or of the related run number [248,249]. Some examples are statistical block-copolymers (BCPs) of PTh analogues [250,251], e.g., poly(3-heptylselenophene)-stat-poly(3-hexylthiophene) (P3HseT-s-P3HT) and poly(3-hexylthiophene)-stat-(3-thiohexylthiophene) (P3HT-s-P3THT).

Interestingly, their structural and optoelectronic properties as well as parameters of formed devices change in a nonlinear manner as a function of composition, which highlights the distinct properties that can be achieved with conjugated statistical copolymers. For example, statistical copolymer P3HT-s-P3THT (Figure 1.5), with 3-hexylthiophene:3-thiohexylthiophene monomer ratios ranging from 50:50 to 99:1, is head-to-tail (HT) regioregular in both its hexylthiophene–hexylthiophene and hexylthiophene–thiohexylthiophene linkages, which is not observed in poly(3-thioalkylthiophene) homopolymers. The polymer sequence is random, and the ¹H NMR spectra have eight distinct aromatic signals that correspond to the eight possible HT–HT regioisomer triads and differ from the spectra expected for the corresponding block or homopolymer systems. Further, when testing the copolymers (donor) in BHJ PV devices with PC71BM-based acceptor, the copolymers display an 11–18% increase in the open-circuit voltage (Voc) relative to the P3HT:PC71BM device due to the deeper HOMO level of the 3-thiohexylthiophene unit. This increase is independent of copolymer composition over the 50:50 to 85:15 range and is still observed when there is just one 3-thiohexylthiophene unit in the polymer chain. This shows that statistical copolymers containing as low as 1% of a deep HOMO unit can be used to increase the Voc of the device relative to the parent polymer, and also demonstrate their importance in context of organic electronics and optoelectronics. Nevertheless, it is important to point out that such copolymers are often described in the literature as random copolymers, which is an improper technical use of the prefix random and the same practice should be abandoned.

Figure 1.5 Schematic representation of statistical copolymer (left) and HOMO levels of P3HT-s P3THT copolymers in the solid state determined by cyclic voltammetry (squares) and Voc of the optimized devices (circles) as a function of the amount of 3-hexylthiophene monomer in the polymer backbone. The dashed lines are a guide for the eye, and the arrows indicate the appropriate axis.

Reprinted from [251] with permission from ACS.

1.3.1.2 Random Copolymers

They refer to copolymers that consist of random arrangement (Figure 1.4b) of repeat units (e.g., A and B) along the backbone (or main chain). It should be noted that the random copolymer is a special case of a statistical copolymer in which the probability of finding a given monomeric unit at any given site in the chain, is independent of the nature of the neighboring units at that position. Again, as already cautioned, the term random should not be used to refer to all types of statistical copolymers except in above mentioned special case.

Random copolymers have wide applicability in the diverse areas such as electronics/optoelectronics, anticorrosion coatings, and EMI shielding [15,16,34,197,203,252–264]. For example, a low-bandgap random copolymer PDPP2FT-seg-2T (Figure 1.6) has been reported by combination of PDPP2FT [an alternating copolymer consisting of N-alkylated diketopyrrolopyrrole (DPP) unit flanked by two furan rings (2F) alternating with thiophene (T)] with tail-to-tail coupled unit of two 3-hexylthiophene rings (bithiophene, 2T) in an average of one of approximately five repeat units. The PDPP2FT-seg-2T firms display increased elasticity (due to disturbance of crystallinity) yet good optoelectronic properties, and provided a means to design flexible and mechanically strong donor films suitable for highly efficient flexible OPV devices. Similarly, many soluble CCPs have been demonstrated for providing good corrosion protection and inhibition efficiency, for metallic surfaces under wide range of hostile environments [142,197,261–263]. Likewise, EMI shielding performance of doped poly(aniline-co-alkyl-aniline)-type CCPs has also been reported [264].

Figure 1.6 Summary of the synthetic strategy used to generate segmented copolymers. Two monomers, the dibromide (DPP2F) (a) and the distannane (T) (b), are reacted in the presence of Pd0. Shortly after initialization of the reaction (when macromonomers began to form), additional T and dibrominated bithiophene (2T) (c) were added to the reaction mixture to form the segmented polymer, PDPP2FT-seg-2T. Separately, the homopolymers PT2T and PDPP2FT were also prepared.

Reprinted from [252] with permission from RSC.

1.3.1.3 Alternating Copolymers

They consist of alternating sequences (Figure 1.4c) of monomeric units along the polymeric backbone. It may be considered as a homopolymer derived from an implicit or hypothetical monomer. It has been established that alternating arrangement of suitable donor (D) (electron-donating) and acceptor (A) (electron-accepting) units in linear alternating copolymers, leads to reduction of bandgap (Figure 1.7) and related expansion of absorption range from visible to red and near-infrared (IR) part of the solar spectrum [62,63,85,192,232,265–271].

Figure 1.7 Schematic representation of a donor–acceptor (D–A)-type alternating copolymers (left) and bandgap engineering using such system.

It may be attributed to the fact that, the interaction of the donor–acceptor moieties enhances the double bond character between the repeating units. This stabilizes the low-bandgap quinonoid-like forms within the polymer backbone, leading to reduction of bandgap energy. This has special relevance in context of organic optoelectronics, where the above molecular design strategy allows the formation of low-bandgap (1.2–1.5eV) conjugated copolymers {e.g., poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothia-diazole) (PCDTBT)} having good charge carrier motilities (~10–2 cm²/V.s). These materials can efficiently harvest the incident solar flux as they tends to display broad absorption features and better spectral overlap with the terrestrial solar radiation (note: photon flux maximum of the solar spectrum at the standard air mass 1.5 conditions lies between 1.3 and 2.0 eV). Their good carrier mobility enables improved charge transport and reduced carrier recombination rates. The high mobility (1 cm²/V.s) systems based on strong acceptors [64] have shown definite promise in thin-film transistors.

Realizing the need for further improvements, design guidelines have been proposed for the synthesis of new low-bandgap conjugated copolymers with deeper highest occupied molecular orbital (HOMO) level (to harvest more incident photons), so that both Jsc (short-circuit current density) and Voc (open-circuit voltage) can be maximized [232,272]. This strategy is called weak donor–strong acceptor approach (Figure 1.8), where the weak donor must have a low HOMO energy level and a strong acceptor is required to obtain a narrow bandgap via intramolecular charge transfer transitions. It has also been shown that the lowest unoccupied molecular orbital (LUMO) of D-A CCPs largely resides on the acceptor moiety [273–275]. Therefore, it is envisioned that incorporating a more electron deficient acceptor to lower the LUMO would lead to a smaller bandgap and maintain the low HOMO energy level in the newly designed materials [271]. It is noteworthy that, besides the designing of donor–acceptor (D-A) CCPs-based donor materials, the acceptor–donor (A-D)-type alternating copolymers have also been reported that can act as low-bandgap (<1.5 eV) acceptors [196]. Therefore, designing of above alternating CCPs is not a straight forward exercise and involves judicious selection of suitably designed donor (e.g., selection of substituents to regulate mobility) or acceptor (e.g., selection of substituents to fine tune bandgap) segments, and coherent strategies to chemically link them [64,276]. In addition, most low-bandgap polymers have an inherently common problem of limited environmental stability (thermal/air) [277,278]. However, by judicious selection of D-A segments some improvement can be done. Recently, a high-molecular-weight yet soluble poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzo thiadiazole)] was synthesized by either a Stille or Suzuki coupling reaction [269], and found to display good thermal and air stability with enhanced performance of PCBM-based BHJ PV device.

Figure 1.8 Schematic representation of a donor–acceptor (D–A)-type alternating copolymers (left) and bandgap engineering using such system weak donor-strong acceptor concept and energy levels.

Reprinted from [232] with permission from ACS.

Recently, it has also been demonstrated that the strict control over donor and acceptor segments as well as regulation of physical size of the particles (Figure 1.9), may facilitate realization of CCPs nanoparticles (NPs) with good optoelectronic and photonic pr operties [279]. For example, Suzuki–Miyaura dispersion polymerization enabled the formation of highly monodisperse CCP NPs [e.g., poly(dioctylfluorene-alt-diethylhexylfluorene) (P1), poly(dioctylfluorene-alt-benzothiadiazole) (P2), poly(diethylhexyl-co-benzothiadiazole-co-benzoic acid) (P3), poly(thiophene-alt-benzothiadiazole) (P4)], with precise control over size, emission characteristics, surface functionality, and shape. In particular, their ability to spontaneously self-assemble into photonic crystals, highlighting their potential as novel building blocks for self-assembled photonic materials. They display well-defined emission spectra and emit highly fluorescent with emission color (P1:blue; P2: green; P3: yellow & P4: red) corresponding to actual bandgap. Most importantly, these photonic crystal acts as both a filter and a frequency converter via photoluminescence (PL), which makes it a potential candidate for future photonic applications, e.g., in telecommunications or optical sensing.

Figure 1.9 Scanning electron micrographs of (a) 915 nm monodisperse particles of P1, (b) crystalline sample of 420 nm monodisperse particles of P2, (c) a CP particle P3 covered with aminated silica particles attached via EDC-NHS coupling to the carboxylic acid moiety of P3, and (d) 517 nm monodisperse particles of P4. Scale bars are 2 μm except for (c) where it is 200 nm. (e) Fluorescence spectra of dispersions of P1 (left curve), P2 (middle, solid line curve), P3 (dashed middle, dashed line curve), and P4 (right curve) particles. (f) Photographs of fluorescent dispersions of P1–P4 (left to right) in water.

Reprinted from [279] with permission from NPG.

1.3.1.4 Block Copolymers

A block copolymer (BCP) is constituted by linear arrangement of blocks (Figure 1.4d and e), a block being defined as a portion of a polymer molecule in which the monomeric units have at least one constitutional (or configurational) feature absent from the adjacent portions [7,246,248]. In a BCP, the distinguished feature is constitutional, i.e., each of the blocks comprises units derived from a characteristic species of monomer. If the alternating sequence of two block is identified the polymer is termed as diblock copolymer (DBC) whereas the copolymer with three distinct alternating block sequences is known as triblock copolymer (TBC) [246,280–284]. Interestingly, just like immiscible blends, BCPs also undergo microscopic phase separation (owing to limited miscibility of phases or blocks) leading to formation of assembled supramolecular structures. However, BCPs are superior compared to immiscible blends (which can be thermodynamically demixed at macroscopic level), in terms of thermodynamic stability of microphase-segregated morphology, due to the inherent advantage that phases are covalently bonded to each other and cannot be demixed (phase segregated) macroscopically (e.g. oil droplets in water) without breaking the bonds [193,217,248,280,281]. It is worth mentioning here that the microphase separation and formation of self-assembled supramolecular structures adopted by the blocks, is governed by the balance between unfavorable enthalpy driven segment–segment interactions, and the entropic elasticity of the polymer chains, such that the competition between the Flory–Huggins interaction and Maier–Saupe interaction plays a crucial role in the self-assembly of BCPs [192,241,280,285] (Figure 1.10a). Consequently, on the basis of block size/nature, several thermodynamic equilibrium morphologies can be obtained (Figure 1.10b) [240,241,286]. For example, in AB- and ABA-type BCPs, blocks of sufficiently different lengths lead to morphology, internally constituted by nanometer-size spheres of one block distributed in a pool of other block. Using less different block lengths, hexagonally packed cylinder-type geometry can be obtained. Similarly, blocks of similar length form layers (often called lamellae in the technical literature). In between cylindrical and lamellar phase is the tri-continuous gyroid phase. Interestingly, the cylindrical or gyroidal nanostructures are ideal morphologies for active layer of OPVs, since they possess co-continuous D-A networks and a large number of BHJs, such that the domain size of phases (especially donor) falls within the exciton diffusion length (i.e., the distance exciton can traverse/diffuse before being captured by a trap) for that particular system (typically 10–20 nm).

Figure 1.10 (a) Typical phase diagram of a coil-coil DBC. f: Volume fraction of one block; χ: Flory-Huggins interaction parameter; N: Degree of polymerization; L: Lamellae; H: Hexagonally packed cylinders; Q230: Double-gyroid phase; Q229: Body-centered spheres; CPS: Closed-packed spheres; DIS: Disordered.

Reprinted from [286] with permission from ACS. (b) Structures of the different phases described in (a) where f is the volume fraction of block. Reprinted from [241] with permission from Elsevier.

The nanoscale structures created from BCPs could potentially be used for creating devices for use in computer memory, nanoscale templating, and nanoscale separations. In particular, the BCPs containing strategic combination of blocks constituted by suitable donor (D) and acceptor (A) units, have shown great potential in the areas like OPVs, OLEDs, memory devices, transistors, sensors, ECs, thermoelectrics, etc. [107,120–122,145,146,192,223,240,280,285,287–312]. For example, DBCs of type donor–acceptor (D–A) [292,299,301] and TBCs of type donor–acceptor–donor (D-A-D) [283,289,313,314], acceptor–donor–acceptor (A-D-A) [315–319], donor–acceptor–acceptor (D-A-A) [79,320–322], or donor–donor–acceptor (D-D-A) [210,323,324], have been demonstrated to improve the performance of OSCs.

1.3.1.5 Graft Copolymers

A graft copolymer is a polymer comprising molecules with one or more species of block connected (Figure 1.4f) to the main chain as side chains having constitutional (or configurational) features that differ from those in the main chain. In a graft copolymer, the distinguishing feature of the side chains is constitutional, i.e., the side chains comprise units derived from at least one species of monomer different from those which supply the units of the main chain [325–333]. For example, post-functionalization of poly(3-hexylthiophene) (P3HT) with suitable end-function group terminated P3HT gives well-defined block and graft copolymers via grafting-to or grafting-from strategies (that will be discussed in following sections) [192,280,297,330,334], with efficient OPV performance. Similarly, P3HT-b-C60 DBCs (Figure 1.11a) can be prepared by grafting of fullerene derivative {i.e., [6,6]-phenyl-C61-butyric acid (PCBA)}, with the BCP of regioregular P3HT [i.e., P3HT-b-P(MMA-r-HEMA)], where MMA and HEMA represent methyl methacrylate and 2-hydroxyethyl methacrylate units, respectively [335].

Figure 1.11 (a) Chemical structure of P3HT-b-C60 DBC containing PCBM (acceptor) units grafted to P3HT (donor)-based backbone. TEM images of the self-assembled structure of P3HT-b-C60 DBC (a) before and (b) after thermal annealing at 150°C for 24 h. Inset shows a higher-magnification image.

Reprinted from [335] with permission from RSC.

It is important to point out that these graft copolymers also exhibit peculiar morphology (Figure 1.11b) imparted by self-assembly and microphase separation (like linear BCPs, immiscible/compatibilized blends, etc.). However, their distinguished advantage over D-A blends is the stability of phase-segregated morphology (Figure 1.11c) even after annealing. Consequently, they don’t undergo annealing triggered macro-phase separation (which often occur in D-A blends with domain size >100 nm) and maintain their micromorphology, with phase separation of domain remain stable (even after annealing) to lie within the limiting size of excitons diffusion length (~10–20 nm). This demonstrates their promise as organic electronic/optoelectronic material and makes them potential material candidate for OPVs, OLEDs, transistors, and other strategically important devices.

1.3.2 Sub-Classification of Linear or Graft BCPs

It is worth mentioning that purely conjugated semiconducting polymer-based blocks having high rigidity (due to stiff aromatic backbone), are regarded as rod-like or stiff whereas the flexible blocks based on purely aliphatic main chains (i.e., soft material) are considered as coil-like or flexible. Nevertheless, on the basis of disposition of the above blocks, their relative rigidity (whether stiff rod-like or soft coil-like), volume fraction and length, as well as the presence/absence of non-conjugated blocks, these linear or graft BCPs can be further sub-classified into three main categories [192,241,281,286,291,292,297,307,309,310,336,337] shown schematically in Figure 1.12.

Figure 1.12 Schematic representation of various types of BCPs viz. (a) coil-coil, (b) rod-rod, and (c) rod-coil copolymers.

1.3.2.1 Coil–Coil-Type BCPs

Here both donor and acceptor units are attached (Figure 1.12a) as side groups (or grafts) to the non-conjugated (soft polymer-based) main chain (backbone). Therefore, the system is made up of BCPs, where blocks are constituted by suitable donor and acceptor grafted blocks of non-conjugated polymer [192,241,282,303,336,338,339]. Typical examples are fully functionalized DBCs like PvDMTPA-b-PPerAcr and PvDMTPD-b-PPerAcr, where the first block, i.e., PvDMTPA and PvDMTPD represent donor blocks made up of substituted triphenylamines (poly(bis(4-methoxyphenyl)-4’-vinylphenylamine) and substituted tetraphenylbenzidines (poly(N,N’-bis(4-methoxyphenyl)-N-phenyl-N’-4-vinylphenyl-(1,1’-biphenyl)-4,4’-diamine) units, respectively. The second block consists of perylene diimide side groups attached to a polyacrylate backbone (PPerAcr) via a flexible spacer. The coil–coil-type copolymers are widely used as electronic and optoelectronic materials, e.g., in OSCs both as compatibilizer for D-A immiscible blends as well as chemically linked D-A active material, for improved phase stability and superior device performance [192,336].

1.3.2.2 Rod-Coil-Type BCPs

Here one block is formed by a conjugated system (made up of donor units) as the main chain (Figure 11.2c) and the second one is a coil block with acceptors units attached as pendant side groups [123,192,281,297,309,310,325,340]. It should be kept in mind that rod-like segments tend to form crystalline aggregates, whereas coil-like units tend to assume random/amorphous configuration. Consequently, rod–coil BCPs tend to form locally crystalline-organized structures, including isotropic, nematic, monolayer and bilayer smectic liquid crystal phases, and various morphologies (lamellar, gyroidal, cylindrical, etc.) [297], that are responsible for distinguished electronic and optoelectronic properties. Some examples are poly(alkoxyphenylenevinylene)-block-polyisoprene, poly(3-hexylthiophene)-block-poly(vinylphenyloxadiazole), and poly(3-hexylthiophene)-block-polystyrene [123,341,342]. These copolymers also have wide applicability in OPVs both as active material as well as compatibilizers for donor–acceptor blends. Besides, they are also useful for OLEDs, memory devices, and transistor applications [123,192,193,207,210,239,281, 310,326,343].

1.3.2.3 Rod–Rod-Type BCPs

Here the main chain is made up of two separate alternating blocks (Figure 1.12b) of donor and acceptor units. Typical examples are poly (3-hexylthiophene)-b-[3-(2-ethylhexyl)thiophene], poly[(3-hexylthiophene)-block-(3-(4,4,5,5,6,6,7,7,7-nona-fluoroheptyl) thiophene)], poly(9,9-dioctylfluorene-co-benzothiadiazole) [344–346]. This category of copolymers represents a true fully conjugated system with direct chemical linkage (though sometimes a bride may be used [347–349]) between donor and acceptor blocks. They have been demonstrated for wide applicability in organic electronics and optoelectronics, e.g., active layer of solar cells, memory devices, and OLEDs [122,192,241,290–292,296,297,303].

In addition to above discussed polymer types, rod–coil- or coil–coil-type polymers having one or two sacrificial blocks (made up of non-conjugated polymers), may also be formed for subsequent use to generate ideal phase-segregated morphology [192,240,303]. Besides, higher-order structures like star, brush, and comb copolymers; hyper-branched/dendritic assemblies; gradient copolymers; statistical copolymers; cross-linked systems; and TBCs having different co-monomers subunits [180,192,211,250, 259,300,309,342,350–359] have also been designed. However, they are less frequently used than their other counterparts and their detailed discussion is beyond the scope of chapter.

1.4 CPs/ICPs-Based Composites/Nanocomposites/Hybrids

Composites (CMPs) represent class of materials made from two or more constituent phases (non-gaseous, one of which should be continuous) with significantly different physical or chemical properties, that when combined, produce a characteristically different material compared to the individual components [8,14,206]. The individual constituent phases remain separate and retain their identity in the resultant composite, and may be partially or completely (depending on strength of interactions between phase, scale at which they interact and their actual dimensions) recovered by suitable disintegration technique.

In general, there are two main material constituents (Figure 1.13) in any CMP material: the matrix (or continuous phase) and filler (or dispersed/discontinuous/discrete phase). Besides, a third phase so-called interfacial region is also present which is responsible for communication between the matrix and filler [11,14,206]. Interface possesses unique combination of properties that are not exhibited by filler or matrix alone and is responsible for the macroscopic properties of formed CMPs. A synergism produces material properties unavailable from the individual constituent material; therefore, formation of conventional CMPs is often motivated by the idea of realization of new material which may be stronger, lighter, or less expensive compared to unfilled counterparts (or neat matrices). Consequently, filler for conventional CMPs may be reinforcement, diluents, or a cheap inert material. However, modern CMPs with advanced functionality [8,9,11,14,85,109,112,119,140,157,178,191,199,231,238,360–371] may contain special additives like electrically/thermally conducting fillers (e.g., ICPs, metal particles, carbonaceous materials like carbon black, graphite flakes, CNTs, or graphene), semiconducting materials [e.g., CPs, QCNs, and nanocrystals (NCRs) like quantum dots (QDs), nanorods (NRs), nanowires (NWs), nanotubes (NTs), tripods, tetrapods, and multipods; electrochemically active molecules, e.g., CPs, other redox materials like transition metal oxides (TMOs)] as fillers.

Figure 1.13 Schematic representation of composite material and its various phases viz. continuous phase (or matrix), discontinuous (dispersed/discrete) phase (or filler: fiber-like phase), and inter-phase/interfacial region (glowing outer region of filler’s surface).

It is interesting to point out that, as the physical dimensions of the filled inclusions decreases (either by size reduction or by de-agglomeration as shown in Figure 1.14, which exposes otherwise inaccessible internal surfaces of filler particles) to become less than 100 nm (in at least one dimension), drastic changes in physical and chemical properties occurs due to large increase in surface-to-volume ratio (due to formation of more surfaces shown by the glow around particles in Figures 1.13 and 1.14) and surface atoms dominated (compared to atoms in the bulk) interactions in corresponding nanoscale CMPs (or NCs) [11,14,361,364,365,371].

Figure 1.14 Schematic representation of formation of new surface (represented by yellow outer glow) and increase in surface-to-volume ratio upon size reduction/de-agglomeration of large filler particles/domains (micron size or larger) to form filler NPs.

It is worth mentioning here that in case of bulk CMPs, the concentration of interfacial regions is too low compared to matrix and reinforcements and often gets neglected. However, in case of NCs, where filler extend ultra-large interfacial area per unit volume to host polymer matrix, the concentration of filler’s bulk and interfacial regions become comparable. Consequently, significant changes in the properties of NCs are observed compared to their bulk counterparts such as enhanced strength, better optical properties (e.g., improved clarity), good electrical properties, and/or improved thermal/oxidative stability [11,14,360,361,364,365,371] along with novel and interesting electrochemical, structural, or optoelectronic properties [85,109,119,171,195,240,327,362,363,370,372–375]. Interestingly, very low concentration of nanofiller (NF) is required (compared to bulk material) to endow the host polymeric matrix with intended property, which is also beneficial from the view point of minimal disturbance of the physical and chemical properties of the host matrix in the formed NCs. However, the NFs have inherent agglomeration tendencies and utmost care has to be taken to achieve their de-agglomeration and to affect their uniform dispersion inside the host matrix, so as to eventually realize perfect/ideal NCs (i.e., disordered exfoliated-type systems that will be discussed in following sections). It is important to note that all other mixed phase materials, which do not comply the definition of BLNs, CCPs, or CMPs/NCs in general referred to as HYB materials, though in the absence of the strict guidelines, the terms CMPs/NCs, BLNs, and HYBs (especially CMPs/HYBs) have been loosely used interchangeably or synonymously, which is unhealthy scientific practice and may not be always correct. This also makes the selection and citation of true mixed system (i.e., whether belong to category of BLNs, CMPs/NCs, or HYBs) from the available wide literature an extremely tedious and sort of impossible task. For example, P3HT/PCBM or analogous systems are mentioned as BLN [217,218,225,227,236,239,376,377], CMP [75,231,377–380], NC [381,382], or even BLN-NC!!! [382] by various works. Nevertheless, henceforth, readers and authors are requested and expected to be more responsible and careful, to correctly identify and quote the above systems.

Although the NCs are as old as nature itself, their importance was realized quite later and its foundations were laid only 25 years back. Therefore, it’s worth to outline a brief account of the same. I must point that Nature is a maestro nanotechnologist!!! that interestingly and incredibly has designed few of the most perfect and widely encountered NCs. The wood, shellac, and bone represent the simplest examples of natural NCs [11,14,364]. Intelligently and deliberately, in an attempt to emulate nature, the inspired man quickly learned from its ambiance and started the production of a variety of bulk composites, e.g., straw-reinforced mud bricks, concrete, and fiber-reinforced plastics (FRPs). In parallel, the efforts were also started and intensified to know, learn, and perfect the art of formation of NCs. Soon, the efforts paid off and the real breakthrough in NC science and technology came in the early 1990s, when Toyota Central Research Laboratories in Japan reported the formation of nano-clay-filled Nylon-6 NC [364,365,371,383,384], for which a very small amount of NF loading resulted in a drastic improvement of thermal and mechanical properties. This has triggered a surge of dedicated efforts to synthesize NCs based on other NF(s)/matrix combinations [11,171,194,198,199,215,240,245,362–364,366–373,375,385–390]. In this context, CPs/ICPs-based NCs is a relatively new entry, that refer to special class of multiphase materials where suitable CPs/ICPs (or their copolymers) are combined with another organic/inorganic phase (e.g., conventional insulating polymer; fillers like graphene/CNT analogues; metallic, semiconducting, dielectric, ferroelectric, magnetic NPs; NCRs and QCNs like QDs, NRs, NWs, NTs, tripods, and tetrapods, multipods; occasionally another CPs/ICPs) to form advanced polymer NCs with fascinating properties (e.g., interesting redox chemistry, electrochemical behavior, electronic, electromagnetic, or optoelectronic properties) and wide spectrum of dependent applications [e.g., in the area of organic electronics (transistors, memory devices) and optoelectronics (OPVs, OLEDs, display devices, solid-state lightings), sensing (gas sensor, biosensor, strain/vibration sensor, electronic nose/tongue), electrochemical actuators, energy storage (e.g., Li-ion battery, supercapacitors), catalysis (normal catalysis, electrocatalysis, or photocatalysis), purification (gas or water), thermoelectrics, corrosion control, EMI shielding, antistatics, electrostatic dissipatives (ESDs), nonlinear optics, biomedicals, and tissue engineering] [11,14,54,85,109,119,126,127,139,142,144,155,157,171,180,183, 194,195,198,199,204,215,231,237,240,362,363,373,375,388,389,391–402]. It is important to note that CP/ICPs analogues can be used either as matrix material or as filled inclusion, though it may also serve the purpose of both matrix as well as filler, e.g., for all polymer electronic/optoelectronic devices.

1.4.1 Categorization of CPs/ICPs-Based NCs

Depending upon state of aggregation of NF in the matrix and assumed thermodynamically equilibrated morphologies, CPs/ICPs-based NCs can be categorized into three main classes:

1.4.1.1 Ordered Intercalated NCs

They retain the domain structure of filler and consist of ordered regions (crystallographically regular) formed by slightly exfoliated filler domains, with polymer chains intercalated between and running through the nanosize sub-domains (e.g., layers in clay or graphitic compounds) of filled inclusion (Figure 1.15b), such that the filler sub-domains and polymer chains are present in alternate stack form. These intercalated domains are dispersed within the remaining disordered/ordered matrix regions formed by the entangled polymer chains. Typical example includes graphite/clay intercalation compounds having CP/ICP chains running through the stacks of clay or graphene sheets. They have wide applicability in the areas such as sensing, corrosion control, EMI shielding, and organic electronics [144,178,393,397,399].

Figure 1.15 Schematic representation of formation of polymer-based CMP systems viz. (a) conventional bulk CMP (tactoid), (b) intercalated NCs, and (c) exfoliated NCs, via physical mixing (dry/solution phase mixing or melt compounding) and in-situ polymerization routes.

1.4.1.2 Disordered Intercalated or Flocculated NCs

They partially retain the ordered intercalated morphology in some regions, with the presence of few partially delaminated (into constituent sub-domains) and intercalated domains randomly dispersed in the matrix. In that way, they assume almost the same structure as that of intercalated NCs, except the presence of floccus formed due to the localized interaction and delamination. Consequently, their properties lies somewhere between ordered intercalated phase and disordered exfoliated system. They may not have special relevance in context of CP/ICP-based systems, though they are always formed in part (as by-product) along with ordered intercalated and disordered exfoliated NCs.

1.4.1.3 Disordered Exfoliated NCs

They consist of completely delaminated/exfoliated filler sub-domains (Figure 1.15c) that are randomly dispersed (crystallographically irregular) within the host polymer matrix. In true sense, they represent a perfect NC having good intimacy and inter-communication between the phases. Most CP/ICP-based exfoliated NCs have wide applicability and mostly formed by in-situ polymerization (ISP) approach, though occasionally solution phase blending (under agitated conditions like stirring, sonication, or high shear exfoliation) or high-temperature/shear melt-compounding routes (e.g., extrusion mixing, batch compounding, exfoliation) may also be employed [14–16,109,119,144,171,183,198,199,362,372,375,389,391,394,397,399, 400,402–410].

Again, it should be noted that, though there is some analogy between traditional phase-segregated bulk CMP (or tactoid; Figure 1.15a) and immiscible BLNs (macroscopic phase segregation), the term BLN is strictly reserved for mixture of two soft phases, whereas, in CMPs at least one of the phase (matrix or filler) has to be constituted by stiff material. Besides, many CPs/ICPs assume rigid/rod-like chain configuration (stiff phase) under some conditions (e.g., temperature, doping level, solvent, and in co-polymeric phases) and flexible coil-like configuration (soft phase) under others. Therefore, depending upon the robustness of filler (stiff or soft/flexible), rigidity of matrix polymer (stiff or soft) and continuity of the phases, utmost care has to be taken to term the formed mixed system as BLNs (two separable, soft phases), NCs (one continuous phase with intercalated/exfoliated and nanosize filler), and HYB. Nevertheless, like BLNs, the thermodynamic stability of the phase-separated morphology of the NCs is dependent on the strength (weak or strong) and nature (physical, chemical, or covalent) of interactions between phases. Often additives like compatibilizers, coupling agents, or auxiliary molecules are used to improve the phase stability. Besides, chemical routes have also been developed to chemically link/graft the filler with/over matrix polymer’s backbone, so as to improve intimacy between the phases and stability of mixed phase [14,231,242,329,362,373,375,391,400,407].

1.5 Interpenetrating/Semi-Interpenetrating Polymer Network (IPN/SIPN)

Polymer comprising two or more polymer networks which are at least partially interlaced on a molecular scale (Figure 1.16) but not covalently bonded to each other and cannot be separated unless chemical bonds are broken [206,411]. A mixture of two or more preformed polymer networks is not an IPN. An IPN may be further described by the process by which it is synthesized e. g. when an IPN is prepared by a process in which the second component network is polymerized following the completion of polymerization of the first component network, the IPN may be referred to as a sequential IPN. In contrast, a process in which both component networks are polymerized concurrently, the IPN may be referred to as a simultaneous IPN. An IPN is distinguished from other multipolymer combinations, such as polymer blends, blocks, and grafts, in two ways: (1) an IPN swells, but does not dissolve in solvents; and (2) creep and flow are suppressed.

Figure 1.16 Schematic representation of IPN of polymers formed by entangled chains of polymer A and polymer B. The isolated networks of individual polymers are also shown to depict and clarify the concept.

The conventional polymer-based IPNs are widely used in polymer science [8,9,385,412–416]; however, in context of CPs/ICPs, there are only few related examples [417,418] and mostly the term IPN is used in loose sense to denote the three-dimensional (3D) co-continuous-type morphology (so-called inter-penetrating network) of donor (e.g., P3HT or any other polymer) and acceptor (e.g., PCBM, graphene analogues, semiconducting nanomaterials) materials [113,231,237,336,378,379,391,419] formed within the device’s active layer. Similar, to the IPNs, corresponding semi-interpenetrating polymer network (SIPN) systems are also identified. They can be defined as: Polymer system comprising one or more polymer network(s) and one or more linear or branched polymer(s) characterized by the penetration on a molecular scale of at least one of the networks by at least some of the linear or branched chains [206,411]. SIPNs are different from IPNs because the constituent linear-chain or branched-chain macromolecule(s) can, in principle, be separated from the constituent polymer network(s) without breaking chemical bonds, and, hence, they are polymer blends. Again, like IPNs, SIPNs may also be classified as sequential and simultaneous SIPN, on the basis of process by which they are synthesized. There is possibility that a linear or branched polymer may be incorporated into a network by means other than polymerization, e.g., by swelling of the network and subsequent diffusion of the linear or branched chain into the network. In conventional polymers, SIPNs are more widely employed compared to IPNs due to relatively facile synthesis and better processing control [8,9,412–415,420,421]. Similarly, they are more popular compared to IPNs in case of CP/ICP-based systems too [204,237,352,379,387,422–425]. Again, there is a lot of confusion in terms of loose usage of IPNs to denote both the actual system as well as acquired morphology.

Nevertheless, it is expected that with the passage of time and further developments, the definitions become clearer and correct usage of terms will eventually become more regular practice.