Contents
Cover
Half Title page
Title page
Copyright page
Preface
Contributors
Chapter 1: High Performance Polymers: An Overview
1.1 Introduction
1.2 Poly(ether amide) and Poly(ether amide-imide)
1.3 Poly(arylene ether)
1.4 Benzoxazine Polymers
1.5 Poly(ether ether ketone) (PEEK)
1.6 Polytriazole
1.7 Hyperbranched Conjugated Polymers
1.8 Alternating Copolymers
1.9 References
Chapter 2: Synthesis and Properties of Polyoxadiazoles
2.1 Introduction
2.2 Synthesis of Polyoxadiazoles in Poly(phosphoric acid)
2.3 Thermal and Mechanical Properties of Polyoxadiazoles
2.4 Application Fields
2.5 References
Chapter 3: Conjugated Polymers Based on Benzo[1,2-b:4,5-b’]dithiophene for Organic Electronics
3.1 Introduction
3.2 General Synthetic Methods for BDT Monomers and Polymers
3.3 Application of BDT-Based Polymers in OFET and PSC
3.4 Outlook
3.5 References
Chapter 4: Polysulfone-Based Ionomers
4.1 Introduction
4.2 Polysulfone Backbone and Selection of the Ionic Function
4.3 Ionomer Synthesis and Characterization
4.4 Conclusion
4.5 References
Chapter 5: High-Performance Processable Aromatic Polyamides
5.1 Introduction
5.2 Monomers
5.3 Polymerization
5.4 Major Problem with Aromatic Polyamides
5.5 Approaches to Processable Polyamides
5.6 Processable Linear Aromatic Polyamides
5.7 Processable Hyperbranched Aromatic Polyamides
5.8 Properties
5.9 Applications
5.10 Conclusion
5.11 References
Chapter 6: Phosphorus-Containing Polysulfones
6.1 Introduction
6.2 Synthesis of Phosphorus Containing Polysulfones
6.3 Properties of Phosphorus-Containing Polysulfones (P-PSF)
6.4 High Performance Applications of Phosphorus-Containing Polysulfones
6.5 References
Chapter 7: Synthesis and Characterization of Novel Polyimides
7.1 Introduction
7.2 Synthesis of Polyimides
7.3 Properties of Aromatic Polyimides
7.4 Conclusions
7.5 References
Chapter 8: The Effects of Structures on Properties of New Polytriazole Resins
8.1 Introduction
8.2 The Preparation of Polytriazole Resins
8.3 Reactivity of Crosslinkable Polytriazole Resins
8.4 Glass Transition Temperatures of Polytriazole Resins
8.5 Mechanical Properties of Polytriazole Resins
8.6 Dielectric Properties of Polytriazole Resins
8.7 Thermal Stabilities of Polytriazole Resins
8.8 Conclusions
8.9 Acknowledgement
8.10 References
Chapter 9: High Performance Fibers
9.1 PIPD or “M5” Rigid Rod
9.2 “Zylon” PBO Rigid Rod Polymer Fibers
9.3 Aromatic Polyamide-Rigid Rod “Kevlar” Poly(p-Phenylene Terephthalamide) Fibers
9.4 Spectra, Dyneema UHMWPE Flexible Polymer Chain
9.5 Carbon Fibers
9.6 Advances in Improving Performance of Conventional Fibers
9.7 Conclusions
9.8 Acknowledgments
9.9 References
Chapter 10: Synthesis and Characterization of Poly (aryl ether ketone) Copolymers
10.1 Introduction
10.2 General Synthetic Methods of PAEK Copolymers
10.3 Synthesis and Characterization of Structural Poly (aryl ether ketone) Copolymers
10.4 Synthesis and Characterization of Liquid Crystalline Poly (aryl ether ketone) Copolymers
10.5 Synthesis and Characterization of Poly (aryl ether ketone) Copolymers with Pendent Group
10.6 Synthesis and Characterization of poly (aryl ether ketone) copolymers with Containing 2,7-Naphthalene Moieties
10.7 References
Chapter 11: Liquid Crystalline Thermoset Epoxy Resins
11.1 Liquid Crystals
11.2 Liquid Crystalline Thermosets Based on Epoxy Resins
11.3 Synthesis and Physical Properties of LCERs
11.4 References
Index
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List of Contributors
Mehdi Afshari is currently Research and Development Scientist at Fiberweb Inc., and Adjunct Assistant Professor at North Carolina State University. He obtained his PhD in Polymer and Fiber Science from Amirkabir University of Technology in 2002. In 2006, he joined North Carolina State University as a Research Associate and then became a Research Assistant Professor. From 2002–2006 he was an Assistant Professor in Textile Engineering Department at Yazd University, Iran.
Susanta Banerjee is currently an Associate Professor in the Materials Science Centre, Indian Institute of Technology, Kharagpur, India. His research interests includes high performance polymers, membrane based separations and polymeric materials in general. He has published more than 100 research papers in international journals and supervised more than 25 students for their master and PhD degrees.
Peng Chen is working at Ningbo Institute of Materials Technology and Engineering in China and earned his PhD in Polymer Engineering. He was Research Associate at North Carolina State University from 2006–2008.
Lei Du is an honorable Professor at East China University of Science & Technology.
Yanpeng E is pursuing his PhD in the School of Materials Science and Engineering at ECUST. His current research interests include click chemistry, adhesives, and heat-resistant polymers and composites.
Dominique de Figueiredo received his PhD in 2002 from the Universidade Federal do Rio de Janeiro, Brazil and in 2002 she moved to Germany where she worked at GKSS on her postdoctoral activities. In 2006 she was awarded with the direction of a Helmholtz-University Young Investigators Group on the development of functionalized polyoxadiazole nanocomposites in cooperation with the Technical University of Hamburg-Harburg. After moving to the chemical industry, she gained her six sigma green belt in 2009. She is currently working at Lehmann & Voss & Co. in Hamburg with the development of LUVOCOM® high performance engineering thermoplastic compounds.
Yanhong Hu is engaged in the research of polymer design and synthesis, especially for the resins and their carbon fibers reinforced composites with high properties, such as interface modification of fiber reinforced resin matrix composites. She is also familiar with the analyses and characterization of polymers and chemicals. She has published articles in more than 30 journals in recent years as well as authoring 4 patents.
Farong Huang is a member of the Chinese Society of Composites and Director of Key Laboratory for Specially Functional Polymeric Materials and Related Technology of the Ministry of Education at ECUST. His research interests focus on the design, synthesis and chemical modification of specialty polymers, the surface, interfaces and manufacture techniques of advanced polymeric composites, and the functional polymeric materials. He is an author or coauthor of more than 200 article as well as 4 books. He has contributed to more than 30 Chinese patents.
Cristina Iojoiu graduated as a Chemical Engineer and obtained her PhD in 2001 from the Montpelier 2 University, France and “Gh. Asachi” University, Iasi, Romania. She is a CNRS scientist at LEPMI, France and her research focuses on the development of electrolytes for electrochemical devices based on polymers and ionic liquids (PEMFC, lithium batteries and photovoltaic cells). She has published more than 40 peer-reviewed papers, 3 book chapters and is co-inventor of 8 patents.
P. Kannan obtained his PhD degree from Anna University, Chennai in 1988 and worked as a Research Associate at the Indian Institute of Science, Bangalore during 1988–1991. He joined the Department of Chemistry, Anna University in 1991 as a lecturer and has been a Professor since 2008. He worked as a Post-Doctoral Fellow at Washington University of St. Louis, during 1999–2000. Fourteen students have earned their PhD under his supervision. He has published 90 research papers in national and international journals.
His research interests are in the domains of fire retardant, photo-crosslinkable, liquid crystalline polymers, LC thermosets, polymer-metal complexes, optical data storage, and photo and electrically switchable polymers.
Richard Kotek is Associate Professor in the College of Textiles at North Carolina State University since 1999. He graduated from the Man-Made Fibers Institute at Lodz Polytechnic, Poland. Following completion of his PhD he worked at the Man-Made Fibers Institute and then as Research Associate at Duke University, and finally joined the R&D department at BASF.
Yujing Li is pursuing her PhD in the School of Materials Science and Engineering at ECUST. Her current research interests include click chemistry, novel rigid polytriazole resins and their composites.
Samarendra Maji is working as a postdoctoral researcher in the Department of Chemistry at the Philipps-Universität Marburg. He obtained his PhD from the Indian Institute of Technology, Kharagpur, India. His research interests includes high performance polymers, biodegradable and biocompatible polymers, structure property-relationship studies of polymers.
Atsushi Morikawa studied organic materials science at the Tokyo Institute of Technology. In 1992 he obtained his doctor’s degree on siloxane dendrimers and polyimide-silica hybrid materials by sol-gel method. After a two-year spell as a research engineer at Asahi Chemical Company, he transferred to the Faculty of Engineering at Ibaraki University to continue his research on poly(ether ketone) dendrimers and polyimides.
Oana Petreus is a senior researcher at “Petru Poni” Institute of Macromolecular Chemistry from Iasi, Romania. She has a PhD in chemistry (1979) and has published 3 books and more than 120 articles in international journals. She is the author of 17 patents. She leads and is a coworker to many national and international scientific programs and is a specialist in synthesis of organic and macromolecular compounds with phosphorus, halogens, nitrogen and sulfur used as flame retardants.
Tudor Petreus, MD and PhD (cell and molecular biology), is an Assistant Professor and researcher at “Gr.T.Popa” University of Medicine and Pharmacy Iasi, Romania and vice president of Iasi Branch of the Romanian Society for Biomaterials. He has published 3 books (as co-author) and more than 50 journal articles. He is leading a research team on matrix metalloproteinase’s inhibitors. His other main research areas are extracellular matrix proteins investigation and the biomaterials-cell interface investigation by a proteomic approach.
Racki Sood obtained in 2009 a Masters of Science diploma in the field of Functionalized & Advanced Materials - Erasmus Mundus Joint program (Grenoble-INP, France & University of Liege, Belgium). She is now working on her PhD in the field of Polymer Electrolytes for Fuel Cells collaborating with three laboratories in France: LEPMI-St. Matin D’Heres; SPrAM-CEA-Grenoble; LMPB-Claude Bernard 1- Lyon University.
P. Sudhakara obtained his PhD from Anna University, Chennai in 2010. He was awarded Senior Research Fellowship by Council of Scientific & Industrial Research (CSIR), New Delhi, India in 2009. He joined as a Post-doctoral Researcher in the Department of Mechanical Engineering, Changwon National University, Changwon, South Korea in 2011. He has published 8 papers in national and international journals in the domain of photocross-linkable, liquid crystalline polymers, fire retardant LC thermosets, and natural fiber composites.
Liqiang Wan obtained his PhD in ECUST. He is now an Associate Professor at Key Laboratory for Specially Functional Polymeric Materials and Related Technology of the Ministry of Education at ECUST. His major interests are in the high properties resins and their fiber reinforced composites. He has published more than 20 papers, submitted more than 10 Chinese patent applications of which 4 have been approved.
Guibin Wang received his BS degree in 1988 from Jilin Institute of Engineering and Technology. He went to work at Jilin University where he obtained his PhD in 2000 in polymer chemistry and physics. In 2001 he became a full Professor. Wang’s research interests are in synthesis, modification and processing of high performance polymer.
Wei You obtained his BS from University of Science and Technology of China in 1999. He obtained his PhD from the University of Chicago in 2004 and completed his postdoctoral training at Stanford University in 2006. He later joined the University of North Carolina at Chapel Hill as an Assistant Professor in Chemistry. His research interests include organic solar cells, molecular electronics and spintronics.
Huaxing Zhou is currently a PhD candidate at the University of North Carolina at Chapel Hill. He obtained his BS degree in polymer chemistry from University of Science and Technology of China in 2007. His current research interests focus on rational design of conjugated polymers for organic solar cells.
Chapter 1
High Performance Polymers: An Overview
V. Mittal
The Petroleum Institute, Chemical Engineering Department, Abu Dhabi, UAE
Abstract
During the last years, several new families of high performance polymers and engineering plastics have been reported which find enhanced application potential in the more challenging areas like aerospace, defense, energy, electronics, automotives etc. as compared to the commodity or conventional polymers. Such polymers provide improved set of properties like higher service-temperatures at extreme conditions and good mechanical strength, dimensional stability, thermal degradation resistance, environmental stability, gas barrier, solvent resistance, electrical properties etc. even at elevated temperatures.
Keywords: Poly(ether amide) and poly(ether amide-imide), poly(arylene ether), benzoxazine polymers, poly(ether ether ketone) (PEEK), polytriazole, hyperbranched conjugated polymers, alternating copolymers
1.1 Introduction
During the last years, several new families of high performance polymers and engineering plastics have been reported which find enhanced application potential in the more challenging application areas like aerospace, defense, energy, electronics, automotives etc. as compared to the commodity or conventional polymers. Such polymers provide improved set of properties like higher service-temperatures at extreme conditions and good mechanical strength, dimensional stability, thermal degradation resistance, environmental stability, gas barrier, solvent resistance, electrical properties etc. even at elevated temperatures. For example, aromatic polyesters and polybenzamide have decomposition temperatures around 480–500°C, whereas polybenzimidazole, polypyrrole and poly(p-phenylene) decompose around 650°C. Various other categories of high performance polymers include poly(phenylene ether), polysulfones, poly(aryl ether ketone), poly(oxadiazole), poly(imide), poly(ether amide), poly(ether amide imide), poly(naphthalene), liquid crystalline polymers and poly(amide imide) etc [1]. The raw materials involved in the synthesis of poly(phenylene ether) are described in Figure 1.1, whereas Figure 1.2 shows the chemical structures of the monomers used for the synthesis of poly(oxadiazole) polymers [1]. Poly(aryl ether ketone)s have aromatic groups in the main chain and both the ether group and the keto group are in the backbone. Liquid crystal polymers partly maintain the crystal structure is in the liquid phase above the melting point and exhibit a long range orientational order. Molecular structure/processability/property relationships of many of high performance polymers and engineering plastics have been reported in the literature along with their applications, a brief overview of a few of which is provided in the following sections.
1.2 Poly(ether amide) and Poly(ether amide-imide)
Vora [2] reported the synthesis and properties of high-performance thermoplastic fluoro-poly(ether amide)s (6F-PEA), fluoro-poly(ether amide-imide)s (6F-PEAI), and their co-polymers. The synthesis was based on the 6F-polyimide chemistry using using the novel state-of-the-art 2-(3,4’-carboxy anhydrophenyl-2(4-carboxyphenyl) hexafluoropropane (6F-TMA) and 2,2’-bis(4-carboxyphenyl) hexafluoropropane (6F-DAc) monomers. Various co-polymers like fluoro-copoly(ether amide-(ether imide))s (6F-co(PEA-PEI)), fluoro-copoly(ether amide-(ether amide-imide))s (6F-co(PEA-PEAI)) and fluorocopoly(ether amide-imide-(ether imide))s (6F-co(PEAI-PEI)) were also synthesized. The authors synthesized the films of the polymers and studied their their solution properties, solubility, morphology, thermal and thermo-oxidative stability, and moisture absorption. The polymers had high viscosity and high degree of polymerization with narrow polydispersity between 1.7 and 2.9. Figure 1.3 shows the synthesis schemes for these polymers and copolymers. It was ascertained by XRD spectroscopy that the polymers were amorphous in nature as no peaks were observed in the diifractograms measured in the range of 10 to 35° 2θ. The polymers were observed to be soluble in almost all organic solvents and the films prepared by thermal curing at elevated temperature were either partially soluble or insoluble in such solvents indicating increased solvent resistance. The polymers were observed to possess moderate to high glass transition temperatures (Tg). The TGA analysis indicated that the 5% weight losses for the polymers in air were in the range of 480–515°C. The weight loss in nitrogen was observed on an average about 15–25°C higher than in air. The polymers also had excellent thermal resistance in isothermal heating at temperature 300°C for 300 h. Most of the polymers had low moisture uptake at 100% relative humidity at 50°C over 100 h. The amorphous nature of the polymers led to their easy processability into films, sheets, molded articles, etc. Dielectric constant values of all the synthesized fluorinated polymers was observed in the range of 2.85 to 3.1 measured at 1 kHz at 25°C. These values were also lower than the values reported for the commercially available non-fluorinated polymers. The authors also commented on the enhancement of polymer properties by the addition of inorganic montmorillonite clays as filler.
Xie et al. [3] also reported the synthesis of polyimides with low moisture absorption and high hygrothermal stability. Four different aromatic dianhydrides, viz. 4,4’-oxydiphthalic anhydride (ODPA), 3,3’,4,4’-benzophenone tetracarboxylic dianhydride, 4,4’-(hexafluoroisopropylidene)diphthalic anhydride, and pyromellitic dianhydride were used during the synthesis and the resulting polyimides are shown respectively in Figure 1.4. The authors observed better solubility over a wider range of solvents in the case of chemically imidized films when compared to the films prepared by thermal methods owing to more compact structure due to stronger aggregation of the polyimide molecules during thermal imidization. The authors also suggested that the cemical method leads to incomplete imidization which was the cause of lower stability of the films generated by this method.
Rajagopalan et al. [4] reported the synthesis of sulphonated polyetherimide and subsequent synthesis of ionic polymer metal composites (IPMC) by depositing platinum on both sides of the polymer membrane by electroless plating process for use in actuators. The TGA and NMR analysis confirmed the successful incorporation of sulfonic groups in the polymer backbone. The content of sulfur in the polymer membrane was measured to be 4.68% by EDX analysis and the degree of sulfonation could also be controlled. SEM micrographs of the composite membrane also confirmed the uniform formation of small platinum particles on the surface of polymer membrane as shown in Figure 1.5. The thickness of platinum coating was observed to be 15–18 μm. The surface of the uncoated membrane was very smooth whereas platinum deposition led to the formation of rough surface morphology. The ionic polymer–metal composite actuator showed good harmonic and step responses similar to an electro-active polymer.
Guhathakurta et al. [5] characterized the polyelectrolytes based on sulfonated PEI and triazole. Bisphenol A based polyetherimide was sulfonated using trimethylsilylchlorosulfonate (TMSCS) as sulfonating agent. Polyelectrlayes were prepared by solution blending of sulfonated PEI and triazole in the presence of dimethylacet-amide. The amount of sulfonated PEI and traizole was altered, the PEI had also different degrees of sulfonation. The effect of degree of sulfonation in the sulfonated PEI and triazole concentration in the blend on size, shape and crystal morphology of triazole crystals in sulfonated polyetherimide were examined. It was observed that at a constant triazole weight percent, increased sulfonation level caused enhanced nucleation density, reduction of crystallite size and their uniform distribution throughout the polymer matrix as shown in Figure 1.6. The crystal domains were also elevated at lower sulfonation level and embedded at higher level of sulfonation.
1.3 Poly(arylene ether)
Dhara et al. [6] reviewed the synthesis and properties of poly(arylene ether) polymers. The role of trifluoromethyl groups on the polymerization process and polymer properties was also demonstrated. Kim et al. [7] also reported the synthesis of hyperbranched pol(arylene ether) polymer. The selective and sequential displacement of the fluorine group and the nitro group of 5-fluoro-2-nitrobenzotriflu-oride was used as a basis for the synthesis of the hyperbranched poly(arylene ether) containing pendent trifluoromethyl groups as shown in Figure 1.7.
1.4 Benzoxazine Polymers
Liu et al. [8] reported improved processability of main-chain benzoxazine polymers by synthesizing novel benzoxazine main-chain oligomers which are low in viscosity. Bisphenol-F based benzoxazine monomers were obtained from the reaction of bisphenol-F isomers, para-formaldehyde and aniline in toluene. Main chain benzoxazine oligomers were obtained by the reaction involving bisphenol-F isomers, aniline, 4,4’-diamino-diphenylmethane and para-formaldehyde. The polymer films from both monomers and oligomers were obtained by casting followed by thermal curing. For the thermoset polymerized from benzoxazine monomers, the glass transition temperature (Tg) was determined to be 154°C. On the other hand, the crosslinked polybenzoxazine derived from benzoxazine oligomers had a glass transition temperature of 213°C. The increase in glass transition temperature for the cross-linked polymers from oligomers was due to the presence of the difunctional amine linkage. The polybenzoxazines derived from the oligomers also showed an increase in 5% weight loss temperature as compared to polybenzoxazines derived from the monomers owing to reduced evaporation rate around 300°C. Figure 1.8 shows the synthesis strategy of bisphenol-F isomer-based benzoxazine monomer and oligomers.
Choi et al. [9] also reported the synthesis of functional benzoxazine monomers and polymers containing phenylphosphine oxide. Phosphorus-containing group was introduced into polybenzoxazine via monomer modification. Three phosphorus-containing bisphenol compounds, bis(4-hydroxyphenyl)phenylphosphine oxide (BHPPO), bis(4-hydroxyphenoxyphenyl) phenylphosphine oxide (BPPPO), and bis(4-hydroxyphenoxy)phenylphosphine oxide (BPHPPO) were synthesized as starting materials for the synthesis of benzoxazine monomers. Polymerization was carried out by ring opening polymerization initiated thermally. The presence of phenylphosphine oxide group in the polymer chain led to an improvement in the thermal stability of polybenzoxazines. Figure 1.9a shows the scheme of the synthesis of bis(4-benzyloxy-phenoxy)phenylphosphine oxide (BBHPPO) and bis(4-hydroxyphenoxy)phenylphosphine oxide (BPHPPO). A schematic representation of the synthesis of BPHPPO-based benzoxazine monomers (R1, R2, and R3 represent methyl, phenyl, and 3-ethynylphenyl, respectively) is shown in Figure 1.9b. Thermal degradation patterns were found to be similar for all the BHPPO-based, the BPPPO-based and the BPHPPO based benzoxazine polymers. Methylamine and aniline-based polymers showed a distinct two-stage degradation pattern while the acetylene functionalized polymers showed a one-stage degradation pattern. The extent of char yield was also different in the different polymers. As an example, in BPPPO and BPHPPO, the aniline-based polymers showed a char yield of 51% (thus significant improvement of thermal stability) as compared to the methylamine-based polymers (31% char yield).
1.5 Poly(ether ether ketone) (PEEK)
Sulfonation of polymers is an important chemical modification process utilized for enhancing proton conductivity of proton conductive polymers. Inan et al. [10] reported the sulfonation of PEEK by reaction with concentrated sulfuric acid. The glass transition temperature was reported by the authors to be affected significantly by the degree of sulfonation of the polymer. Figure 1.10 shows the enhancement if the glass transition temperature as a function of degree of sulfonation. At 80% degree of sulfonation, the glass transition temperature increased from 167°C for pure PEEK to 238.5°C upon sulfonation owing to the incorporation of bulkier sulfonyl groups in the polymer chain.
Blends of PEEK with other high performance polymers have also been reported for applications like fuel cells. Inan et al. [10] reported the blend of sulfonated PEEK with poly(vinylidene fluoride) (PVDF). Effect of the type and molecular weight of the fluorinated polymer was investigated for low temperature fuel cell applications. Figure 1.11 shows the SEM micrographs of the blend membranes of sulfonated PEEK with varying amounts of PVDF of different molecular weights. The blend membranes were observed to have homogeneous structure and no phase separation was observed. Phase separated morphology was however observed for PVDF-HFP (Figure 1.11d) in all concentrations suggesting that PVDF-HFP and sulfonated PEEK are thermodynamically immiscible due to their dissimilar structures.
Fu et al. [11] reported the acid-base blend membranes based on 2-amino-benzimidazole (basic polymer) and sulfonated poly(ether ether ketone) (SPEEK) (acidic polymer) for direct methanol fuel cells. A novel polymer, polysulfone-2-amide-benzimidazole (PSf-ABIm), using carboxylated polysulfone and 2-amino-benzimidazole was synthesized for this purpose. The blend membrane of SPEEK/PSf-ABIm showed high performance a s represented by Figure 1.12. The blend membrane with 3 wt% PSf-ABIm was evaluated continuously for 120 h and little or no decline in performance was found after 120 h. On the other hand, the Nafion 112 membrane standard was observed to have a decline in performance due to a much higher amount of methanol crossover.
The authors opined that the membranes based on acidic and basic polymer blends can offer a promising strategy to replace lithium ion batteries in portable electronic devices like laptop computers and cell phones.
1.6 Polytriazole
Boaventura et al. [12] reported the generation of polytriazole based proton conducting membranes. Sulfonated polytriazole membranes were doped with three different agents: 1H-benzimidazole-2-sulfonic acid, benzimidazole and phosphoric acid. Figure 1.13 also shows the storage modulus and tan δ curves for pure polymer membrane and membranes after doping with different amounts of 1H-benzimidazole-2-sulfonic acid (BiSA) and benzimidazole (BI). The glass transition temperature was observed to generally decrease with increasing the doping agent concentration. This was attributed to the plasticization of the chain backbone after doping. The plasticizing effect in the case of doped membranes also led to the reduction in the storage modulus. It was further observed that adding BI and BiSA to polytriazole did not significantly improve the conductivity of the membranes, whereas doping with phosphoric acid led to the generation of membranes with conductivity of 2.10-3 S cm-1 at 120°C and 5% relative humidity.
1.7 Hyperbranched Conjugated Polymers
Hyperbranched conjugated polymers (HBPs) are specialty high performance polymers which possess advanced structure and properties as compared to conventional linear conjugated polymers. Tang et al. [13] reported the synthesis and photovoltaic properties of three HBPs photosensitizers (H-tpa, H-cya, and H-pca). The chemical structures of these Hyperbranched polymers are shown in Figure 1.14. The polymer had the same conjugated core structure and different functional terminal units. The polymers were synthesized following Wittig-Horner polymerization method. The polymers had broad absorption band in the range of 260–600 nm which was consistent with the hyperbranched structure of conjugation chain length. Two distinct absorption bands were exhibited by all the polymers: one absorption band is in the UV region (271–284 nm) and the other is in the visible region (413–455 nm). The authors reported that the donor-π-acceptor architecture in hyperbranched molecule benefited intramolecular charge transfer and consequently increased the generation of photocurrent. It was observed that the three-dimensional (3D) steric configuration of generated hyperbranched polymers effectively suppressed the aggregation of dyes on TiO2 film, which was beneficial for achieving good photovoltaic functional performance.
Qu et al. [14] reported the synthesis of carbazole-based hyperbranched conjugated polymers. These carbazole-based hyperbranched conjugated polymers which were linked with triphenylamine and benzene moieties were synthesized by Sonogashira coupling polycondensation of N-octadecyl- and N-octyl-3,6-diethynylcar-bazoles with tris(4-iodophenyl)amine and 1,3,5-tribromobenzene. Figure 1.15 shows the synthesis strategy for these polymers. The generated polymers were solvent-soluble polymers and had number-average molecular weights in the range of 3500–21,000. The absorption spectrum of polymers was red-shifted as compared to carbazole which confirmed the extension of conjugation length. The fluorescence quantum yields of the hyperbranched polymers reached 67% in CHCl3, which were larger than those of polyacetylenes carrying carbazole moieties in the side chains. There were differences in the fluorescence quantum yields among the polymers, as the fluorescence quantum yields of poly(1/3) and poly(1/4) which had longer N-alkyl chains were larger than those of poly(2/3) and poly(2/4), respectively. The branched structure of the polymers was observed to be effective to suppress the decay of fluorescence. The polymers were observed to be electronically redox-active.
1.8 Alternating Copolymers
Lim et al. [15] reported the synthesis of a new thienylenevinylene–benzothiadiazole copolymer, poly{1, 2-(E)-bis[2-(5-bromo-3-dodecyl-2-thienyl)-5-thienyl]ethene-2, 1, 3-benzothiadiazole} (PETVTBT) for potential use in solar cells. Thus generated alternating copolymer comprised of electron-rich 1,2-(E)-bis[2-(5-bromo-3-dodecyl-2-thienyl)-5-thienyl]ethane and electron-deficient 2, 1, 3-benzothiadiazole units. Figure 1.16 shows the synthesis scheme of the PETVTBT polymer. The copolymer was thermally stable with a decomposition temperature of 390°C (loss of less than 5% of weight). The differential scanning calorimetry analysis revealed the glass transition temperature of 149°C for the polymer. The UV-vis absorption spectrum of PETVTBT covered a broad absorption range 350–700 nm. The optical band gap of PETVTBT was observed to be 1.57 eV, which lied near to the ideal band gap for a polymer solar cell. The potential of the usefulness of the generated copolymer in polymer solar cell applications was further confirmed by low HOMO level of about 5.1 eV and relatively high hole mobility of 0.025 cm2/Vs.
1.9 References
1. J.K. Fin, High Performance Polymers, New York, William Andrew, 2008.
2. R.H. Vora, Materials Science and Engineering B, Vol. 168, p. 71, 2010.
3. K. Xie, J.G. Liu, H.W. Zhou, S.Y. Zhang, M.H. He, and S.Y. Yang, Polymer, Vol. 42, p. 7267, 2001.
4. M. Rajagopalan, J.-H. Jeon, and I.-K. Oh, Sensors and Actuators B, Vol. 151, p. 198, 2010.
5. S. Guhathakurta, and K. Min, Polymer, Vol. 50, p. 1034, 2009.
6. M.G. Dhara, and S. Banerjee, Progress in Polymer Science, Vol. 35, p. 1022, 2010.
7. Y.J. Kim, M.A. Kakimoto, and S.Y. Kim, Macromolecules, Vol. 39, p. 7190, 2006.
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