Abstract This article focuses on the rubberlike elasticity of elastomers, with particular emphasis on rubberlike materials that exhibit high deformability and recoverability. It begins with a discussion of the variety of practical ways to form and characterize a rubber-elastic network, including random chemical crosslinking, highly specific chemical end-linking, polymerizations with multi-functional monomers, physical aggregation, and crosslinking in solution and in the deformed state. It then considers the effects of network structure on elastomeric properties, along with the results of elasticity experiments regarding the mechanical properties of elastomeric materials. It also examines the evolution of theories of rubber elasticity describes the specific properties of swollen polymer gels where the possibility of solvent exchange leads to some dramatic transformations in the system. Finally, it evaluates new emerging classes of rubber-elastic materials, such as liquid crystalline elastomers, where the internal microstructure added to the random network leads to some unique mechanical properties.

Because of the great importance of the surface properties of the polysiloxanes, this topic is treated separately in this chapter. Hydrophobic polysiloxanes having simple aliphatic or aromatic side groups have surfaces that show essentially no attraction to water. In fact, polysiloxanes can serve as water repellants. This property is very useful for applications such as protective coatings on historical monuments and for controlling the surfaces of other polymers, sensors, and quantum dots. Hydrophobic surfaces can be readily regenerated if the surface becomes damaged. Regeneration occurs by rearrangements of the polysiloxane chains so that the hydrophobic methyl groups are once again covering the surface. The flexibility of the siloxane chain backbone facilitates this process. It is also possible to prepare hydrophobic films using methyl-modified siloxane melting gels. Glass surfaces or wool fibers can be coated with polydimethylsiloxane (PDMS) to make them more hydrophobic. In some cases, it is necessary to modify a polysiloxane surface to make it hydrophilic or hydrophobic. Hydrophobization is one aspect of the general topic of modifying and managing the properties of polymer surfaces. An important example involves soft contact lenses that contain PDMS, which is often used because of its very high permeability to oxygen, which is required for metabolic processes within the eye. Such lenses do not feel comfortable however because they do not float properly on the aqueous tears that coat the eye. There are a number of ways to modify the surfaces. There is even a way to make “unreactive” silicones react with inorganic surfaces. In some applications it is useful to have hydrophilicity in the bulk of the polymer instead of just at the surface. One way of doing this is by simultaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains and hydrophobic PDMS chains. Another way is to make a PDMS network with a trifunctional organosilane R’Si(OR) end linker that contains a hydrophilic R’ side chain, such as a polyoxide. Treating only the surfaces is another possibility, for example, by adding hydrophilic brushes by vapor deposition/hydrolysis cycles. Such hydrophilic polysiloxanes can also serve as surfactants.

Random copolymers are prepared by the copolymerization of a mixture of cyclic oligomers. Although the resulting polymer can be quite blocky (figure 8.1), taking the reaction to equilibrium can give a polymer that is essentially random in its chemical sequencing. One reason for preparing copolymers is to introduce functional species, such as hydrogen or vinyl side groups, along the chain backbone to facilitate cross linking. Another reason is the introduction of sufficient chain irregularity to make the polymer inherently noncrystallizable. Specific examples of comonomers include imides, perylenediimide, urethane-ureas, epoxies, other siloxanes, amides, styrene, divinylbenzene, acrylics, silsesquioxanes, polythiophenes, and poly(lactic acid). One novel combination is the preparation of polysiloxanebased episulfide resins. An unusual application is the use of monomethylitaconate- grafted polymethylsiloxane to induce crystal growth of CaCO3. Polysiloxanes containing thermally curable brenzoxazine moieties in the main chain are also in the category. These and other copolymers have been extensively characterized by nuclear magnetic resonance (NMR) spectroscopy. The sequential coupling of functionally terminated chains of different chemical structure can be used to make block copolymers, including those in which one or more of the blocks is a polysiloxane. If the blocks are relatively long, separation into a two-phase system invariably occurs. Frequently, one block will be in a continuous phase and the other will be dispersed in domains having an average size the order of a few hundred angstroms. Such materials can have unique mechanical properties not available from homopolymer species. Sometimes similar properties can be obtained by the simple blending of two or more polymers. Examples of blocks used with polydimethylsiloxane (PDMS) include imides, epoxies, butadienes, ε-caprolactones, amides having trichlorogermyl pendant groups, urethanes, ureas, poly(ethylene glycols), polystyrene, vinyl acetates, acrylates or methacrylates, 2-vinylpyridine, and even other polysiloxanes. Some results have also been reported for polyesters, polyethers, hydroxyethers of bisphenol A, bisphenol A arylene ether sulfones, vinylpyridinebenzoxazines, methyloxazolines, terpyridines, polysulfones, γ-benzyl-Lglutamate, and carboranes. Two other examples are foamed polypropylene and melamine resins. Even ABA, ABC triblock copolymers, and ABCBA pentablock copolymers involving PDMS have been reported.

Gelation is the cross-linking process that leads to the network structures required for rubberlike elasticity. In some cases, gelation can be reversible. There have been numerous studies involving theory and simulations exploring gelation and the mechanical properties of the resulting networks. Cross linking with free radicals is still quite common. Radiation has often been used to carry out the cross linking, as have new techniques known as “click” chemistry. Hydrosilylation is also popular. Networks have even been designed with movable cross links. Finally, reactive groups can be placed at the chain ends or within the chains themselves. Related studies have involved polydimethylsiloxane (PDMS)-based organogelators, web-to-pillar transitions of gels, and silica aerogels. There has also been interest in polysiloxanes in interpenetrating hydrogels with high oxygen permeabilities and viscoelastic magnetic gels. Organic-inorganic hybrids with relatively low melting temperatures also exist, some of which can be made to be self-healing. Gels are also formed in swelling experiments, which are useful for equilibrium experiments to characterize network structures. One of the recent topics in this area involves stimuli-responsive gels, under the descriptive title of “self-walking gels” “wormlike motion of gels,” and “peristaltic motion of gels.” The earliest studies of networks formed in solution were undertaken to investigate some subtle aspects of the elastic free energy expression— whether or not an additional term in the logarithm of the volume was required. Other studies focused on the properties of networks in general. As can be gathered from chapter 4, it is difficult to obtain information on the topology of a network. Some studies have therefore taken an indirect approach. Networks were prepared in a way as to simplify their topologies, and their properties were measured and interpreted in terms of reduced degrees of network-chain entanglement. The two techniques employed involved separating the chains prior to cross linking by either dissolution or stretching. After cross linking, the solvent is removed or the stretching force is relaxed, and the network is studied (unswollen) with regard to its stress-strain properties, typically in elongation.

The polysiloxane of greatest commercial importance and scientific interest is poly(dimethylsiloxane) (PDMS), [Si(CH3)2 –O –]x, a member of the symmetrical dialkyl polysiloxanes, with repeat unit [SiR2 –O –]x. This polymer is discussed extensively in the following chapters, particularly in chapter 5. Other members of this series are poly(diethylsiloxane) [Si(C2H5) –O–]x, and poly(di-n-propylsiloxane) [SiC3H7)2–O–]x. An example of an aryl member of the symmetrically substituted series is poly(diphenylsiloxane), with repeat unit [Si(C6H5)2–O–]x. This polymer is unusual because of its very high melting point and the mesophase it exhibits. The closely related polymer, poly(phenyl/tolylsiloxane), has also been prepared and studied. The unsymmetrically substituted polysiloxanes have the repeat unit [SiRR’O–]x, and are exemplified by poly(methylphenylsiloxane) [Si(CH3) (C6H5) –O–]xand poly(methylhydrosiloxane) [Si(CH3)(H) –O–]x. In some cases, one of the side chains has been unusually long, for example C6H13, C16H33, and C18H37, including a branched side chain—CH(CH3– (CH2)m–CH3. Another example has methoxy-substituted aromatic fragments as one of the two side chains in the repeat unit. Such chains have stereochemical variability in analogy with the vinyl polymers such as polypropylene [CH(CH3) –CH2–]xand vinylidene polymers such as poly(methyl methacrylate) [C(CH3)(C = OOCH3) –CH2–]xOne can also introduce optically active groups as side chains, the simplest example being the secondary butyl group—CH(CH3)(C2H5). Another example involves redox-active dendritic wedges containing ferrocenyl and carbonylchromium moieties. Other substituents have included phenylethenyl groups, cyclic siloxane groups, and Cr-bound carbazole chromophores. In a reversal of roles, some polymers were prepared to have PDMS side chains on a poly(phenylacetylene) main chain. Siloxane-terminated solubilizing side chains are used to improve the properties of thin-film transistors. Silalkylene polymers have methylene groups replacing the oxygen atoms in the backbone. Poly(dimethylsilmethylene) is an example, [Si(CH3)2–CH2]x. A variation on this theme is to include aryl groups, for example, in poly(dimethyldiphenylsilylenemethylene) [Si(CH3)2CH2Si(C6H5)2]x. Other aryl substituents, specifically tolyl groups, have also been included as side chains. It is also possible to insert a silphenylene group [Si(CH3)2–C6H4–] into the backbone of the polysiloxane repeat unit to give [Si(CH3)2–C6H4– Si(CH3)2O–], in which the phenylene can be para or ortho or meta. A specific example is poly(tetramethyl-p-silphenylene-siloxane).

Many of the properties of the polysiloxanes have been tabulated in handbooks of polymer science and engineering. Recent work has included the stretching of polydimethylsiloxane (PDMS) chains, in some cases to their rupture points. The nature of the bonding in siloxane molecules has been of long-standing interest. Force fields for calculations of PDMS properties have been revised over the years and are now at an advanced state of development. Some of the simplest approaches employ the methods of molecular mechanics. Most of the experimental results have been obtained on solutions of polysiloxanes in thermodynamically good solvents. The first member of this series, poly(dimethylsiloxane) (PDMS), [–Si(CH3)2O–]x, has been studied extensively with regard to its configuration-dependent properties. PDMS (figure 2.1) is very similar in structure to the polyphosphate chain in that the successive bond angles are not equal. The Si–O bond length in polysiloxanes is 1.64 Å, and bond angles at the Si and O atoms are 110 and 143°, respectively. This inequality of bond angles causes the all-trans form of the molecule (with rotational angles ϕ = 0°) to form a closed structure after approximately eleven repeat units. The torsional barrier for rotations about the skeletal bonds is very low, which accounts for the high dynamic flexibility and low glasstransition temperature of the PDMS chain. Not surprisingly, low temperature properties are superb. Trans states are of lower energy than gauche states (ϕ = ±120°) in the PDMS chain. This conformational preference may arise from favorable van der Waals interactions between pairs of CH3 groups separated by four bonds in trans states. This factor is apparently more important than favorable coulombic interactions between oppositely charged Si and O atoms separated by three bonds, which are larger in gauche states because of the reduced distance. Comparisons between experimental and theoretical values of various configuration-dependent properties, however, yield a value for this energy difference that is significantly larger than that obtained from the semi-empirical calculations of interactions between nonbonded atoms.

Elemental silicon on which the entire technology is based is typically obtained by reduction of the mineral silica with carbon at high temperatures: . . . SiO2 + 2C → Si 2CO (2.1) . . . The silicon is then converted directly to tetrachlorosilane by the reaction . . . Si + 2Cl2 → SiCl4 (2.2) . . Tetrachlorosilane can be used to form an organosilane by the Grignard Reaction . . . SiCl4 + 2 RMgX → R2SiCl2 + 2 MgClX (2.3). . . This relatively complicatreaction has been replaced by the so-called Direct Process or Rochow Process, which starts from elemental silicon as is illustrated by the reaction . . . Si + 2 RCl → R2SiCl2 (2.4) . . . This process also yields RSiCl3 and R3SiCl, which­­ can be removed by distillation. Compounds of formula R2SiCl2 are extremely important as intermediates to a variety of substances having both organic and inorganic character. Hydrolysis gives dihydroxy structures, which can condense to give the basic [–SiR2O–] repeat unit. The nature of the product obtained depends greatly on the reaction conditions. Basic catalysts and higher temperatures favor higher molecular weight linear polymers. Acidic catalysts tend to produce cyclic small molecules or low molecular weight polymers. The hydrolysis approach to polysiloxane synthesis has been largely replaced by ring-opening polymerization of organosilicon cyclic trimers and tetramers, with ionic initiation. These cyclic monomers are produced by the hydrolysis of dimethyldichlorosilane. Under the right conditions, at least 50 wt % of the products are cyclic oligomers. The desired cyclic species are separated from the mixture for use in ring-opening polymerizations such as those described in the following section. In addition, “click” chemistry has been developed for new synthesis techniques in general, and polymerizations in particular. These approaches have been used to prepare polysiloxane elastomers and polydimethylsiloxane (PDMS) copolymers that can function as thermoplastic elastomers. New synthetic strategies for structured silicones, based on B(C6F5)3 have also been developed. Another new approach involves enzymes, such as the lipase enzymatically catalyzed synthesis of silicone aromatic polyesters and silicone aromatic polyamides.

Polysiloxanes are the most studied inorganic and semi-inorganic polymers because of their many medical and commercial uses. The Si-O backbone endows polysiloxanes with intriguing properties: the strength of the Si-O bond imparts considerable thermal stability, and the nature of the bonding imparts low surface free energy. Prostheses, artificial organs, objects for facial reconstruction, vitreous substitutes in the eyes, and tubing take advantage of the stability and pliability of polysiloxanes. Artificial skin, contact lenses, and drug delivery systems utilize their high permeability. Such biomedical applications have led to biocompatibility studies on the interactions of polysiloxanes with proteins, and there has been interest in modifying these materials to improve their suitability for general biomedical application. Polysiloxanes examines novel aspects of polysiloxane science and engineering, including properties, work in progress, and important unsolved problems. The volume, with ten comprehensive chapters, examines the history, preparation and analysis, synthesis, characterization, and applications of these polymeric materials.

A relatively new area that involves silicon-containing materials is the synthesis of “ultrastructure” materials (i.e., materials in which structure can be controlled at the level of 100 Å). An example is the “sol-gel” hydrolysis of alkoxysilanes (organosilicates) to give silica, SiO2. The reaction is complicated, involving polymerization and branching, but the overall reaction may be written . . . Si(OR4 + 2H2O → SiO2 + 4ROH (9.1) . . . where the Si(OR)4 organometallic species is typically tetraethoxysilane such as tetraethylorthosilicate (TEOS, with R being C2H5). In this application, the precursor compound is hydrolyzed and then condensed to yield branched polymers. Eventually a continuous swollen gel is formed. The gel is dried at moderately low temperatures to remove volatile species, and then it is fired into a porous ceramic object that can then be densified and machined into a final ceramic part. The production of ceramics by this novel route triggered interest in the ceramics community because of advantages over the conventional powder-processing approach to ceramics. Advantages include (i) the higher purity of the starting materials, (ii) the relatively low temperatures required, (iii) the possibility of controlling the ultrastructure to reduce the microscopic flaws that lead to failure, (iv) the ease with which ceramic coatings can be formed, and (v) the ease with which ceramic alloys can be prepared (e.g., by hydrolyzing solutions of both silicates and titanates). The sol-gel approach has been used to form ceramic-like phases in a variety of polymers. Poly(dimethylsiloxane) (PDMS) is the most popular. PDMS is relatively weak and stands to benefit most from reinforcement. PDMS is easily absorbs the precursor materials generally used in the solgel process. Nearly monodisperse silica microparticles can be obtained using siloxane elastomer mixtures. In some cases, the PDMS has been part of a copolymer, with ureas, imides, amideimides, and dianilines. In other approaches, the particle surface is modified, for example, with a polysiloxane. Siloxane/silica nanocomposites have also been used as “culture-stone-protective materials.” Sol-gel hydrolysis and condensation can be carried out within a polymeric matrix to generate particles of the ceramic material, typically with an average diameter of a few hundred angstroms.

The general approach used in choosing a polymer suitable for a particular application is: . . . Polymerization ↔ Structure ↔ Properties ↔ Application . . . For example, if one wants a polymer for fire-resistant fabrics, then a polymer with good high-temperature properties is required, which implies aromatic structures, which suggest condensation polymerizations. More relevant here, however, would be that a polymer remains elastomeric at low temperatures. This requirement evokes a polymer with high flexibility (low glass transition temperature), which indicates use of the polymerization techniques used with the polysiloxanes. An example of a relevant optical property is the birefringence of a deformed polymer network. This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity. Infrared dichroism has also been helpful in this regard. In the case of the crystallizable polysiloxane elastomers, orientation is of critical importance with regard to strain-induced crystallization and the tremendous reinforcement it provides. Segmental orientation has also been characterized by fluorescence polarization, deuterium nuclear magnetic resonance (NMR), and polarized infrared spectroscopy. Infrared spectroscopy has been used to characterize the structures of silica-filled polydimethylsiloxane (PDMS). Other optical and spectroscopic techniques are also important, including positron annihilation lifetime spectroscopy, spectroscopic ellipsometry, confocal Raman spectroscopy, and photoluminescence spectroscopy. Surface-enhanced Raman spectroscopy has been made tunable using gold nanorods and strain control on elastomeric PDMS substrates. A great deal of information is now being obtained on filler dispersion and other aspects of elastomer structure and morphology through the use of scanning probe microscopy, which consists of several approaches. One approach is that of scanning tunneling microscopy (STM), in which an extremely sharp metal tip on a cantilever is passed along the surface while measuring the electric current flowing through quantum mechanical tunneling. Monitoring the current then permits maintaining the probe at a fixed height above the surface. Display of probe height as a function of surface coordinates then gives the desired topographic map. One limitation of this approach is the requirement that the sample be electrically conductive. Atomic force microscopy (AFM), on the other hand, does not require a conducting Surface.

Hybrid graphene oxide (GO) with metal ions have become very promising for many applications including catalyst, thermal conductive paper, magnetic devices, energy storage and electronics. In this paper, GO was mixed with trivalent rare earth ions, LaCl 3 to prepare GO/ LaCl 3 hybrid composites. The GO/ LaCl 3 hybrid composites were tunable by controlling the ratio of LaCl 3 to GO. The interaction between GO and LaCl 3 includes the coordination of La 3+ with carboxyl groups at the edges of GO, the coordination of La 3+ with epoxides and hydroxyls, and the electrostatic force between La 3+ and aromatic bonds of GO. The La 3+ preferred the coordination with the oxygen-containing groups of GO sheets first. However, the electrostatic absorption of La 3+ was the main factor that controls the deposit of GO/ LaCl 3 composites. The GO/ LaCl 3 hybrid structures were also investigated.

Octadecylamine-modified graphene-oxide (OMGO) polybutadiene nanocomposites with different OMGO loadings were prepared by solution mixing. The dispersion of OMGO in chloroform is greatly improved compared to GO. Toughness and elongation of PBD–OMGO nanocomposites increase by 332% and 191% respectively compared with pure PBD. However, Young's modulus of PBD–OMGO nanocomposite decreases by 10% at 2-wt% loading. Graphene sheet crumpling accounts for the increased toughness, the absence of modulus reinforcement and the absence of a Payne effect for PBD–OMGO. The oxidation susceptibility of PBD is greatly reduced after the addition of OMGO, which is particularly desirable in the tire industry.

Graphite oxide (GO) was modified using 3-aminopropyltriethoxysilane (APTES) to investigate the impact of dispersion and interfacial bonding on the mechanical properties of reinforced silicone elastomer, poly (dimethylsiloxane), PDMS. Although a 71% enhancement of Young's modulus was observed (comparable to thermoplastics) at a loading of 3.0 wt% APTES-modified GO (A-GO), the observed enhancement factor is just 8% of that expected for randomly dispersed perfect graphene sheets. We attribute this less-than-ideal enhancement to crumping and break-up of the graphene sheets caused by ultrasonic dispersion. In the absence of TEOS crosslinker, the A-GO PDMS melt does not solidify so modulus enhancement is not due to crosslinking through the APTES-modified filler particles. Modulus enhancement due to rubber-like elasticity of the filler itself, however, may be active. Characterization indicates that amino groups are chemically attached to the surface of GO in A-GO. The improved dispersibility enhances the toughness, but only at low loadings. In fact, at 3-wt% loading the toughness is less than that of pure PDMS due to loss of elongation caused by filler aggregation.

Graphite oxide (GO) was modified using 3-aminopropyltriethoxysilane (APTES) to investigate the impact of dispersion and interfacial bonding on the mechanical properties of reinforced silicone elastomer, poly (dimethylsiloxane), PDMS. Although a 71% enhancement of Young's modulus was observed (comparable to thermoplastics) at a loading of 3.0 wt% APTES-modified GO (A-GO), the observed enhancement factor is just 8% of that expected for randomly dispersed perfect graphene sheets. We attribute this less-than-ideal enhancement to crumping and break-up of the graphene sheets caused by ultrasonic dispersion. In the absence of TEOS crosslinker, the A-GO PDMS melt does not solidify so modulus enhancement is not due to crosslinking through the APTES-modified filler particles. Modulus enhancement due to rubber-like elasticity of the filler itself, however, may be active. Characterization indicates that amino groups are chemically attached to the surface of GO in A-GO. The improved dispersibility enhances the toughness, but only at low loadings. In fact, at 3-wt% loading the toughness is less than that of pure PDMS due to loss of elongation caused by filler aggregation.

This chapter presents details related to the molecular basis of rubber-like elasticity. Rubber-like materials consist of relatively long polymeric chains having a high degree of flexibility and mobility, which are joined into a network structure. The requirement of flexibility and mobility is associated with the very high deformability. The response of rubbers is almost entirely intramolecular. Externally applied forces transmitted to the long chains through the linkages at their extremities change the conformations of the chains, and each chain acts like a spring in response to the external stress. A network is obtained by linking polymer chains together, and this linkage may be either physical or chemical. Chemical crosslinks may be obtained by randomly joining segments in already formed chains, by random copolymerization, or by end-linking functionally terminated chains. It has been observed that the copolymerization of monomers where at least one type has three or more reactive sites also leads to randomly cross-inked networks.

Molecular dynamics techniques have been used to investigate the pronounced permeability and selectivity of polymers of intrinsic microporosity (PIM). While helium exhibited greater permeability coefficients due to higher diffusivity resulting from its small molecular size, the higher permeability coefficients of carbon dioxide were attributed to the favorable values of the enthalpy of mixing of the gas molecules with the polymeric chains. The simulation results also showed that the fractional free volume and the Connolly surface of the PIMs had greater values resulting from the inability of the rigid and contorted structures of the PIMs to pack space efficiently and thus creating interconnected nanochannels throughout the matrix of the polymers of microporosity. Replacing the heteroatoms of the PIM with carbon atoms resulted in a major decrease in the fractional free volume with a corresponding decline in the diffusion coefficients whereas replacing the site of contortion with a flexible methylene group did not have much effect. Attaching alkyl side groups to the polymeric chains was shown to reduce the free volume as a result of the side chains filling the voids of the micropores. The results were employed in the design of tailor-made membranes for use in the separation of natural gas components.

Elastomers and rubberlike materials form a critical component in diverse applications that range from tyres to biomimetics and are used in chemical, biomedical, mechanical and electrical engineering. This updated and expanded edition provides an elementary introduction to the physical and molecular concepts governing elastic behaviour, with a particular focus on elastomers. The coverage of fundamental principles has been greatly extended and fully revised, with analogies to more familiar systems such as gases, producing an engaging approach to these phenomena. Dedicated chapters on novel uses of elastomers, covering bioelastomers, filled elastomers and liquid crystalline elastomers, illustrate the established and emerging applications at the forefront of physical science. With a list of experiments and demonstrations, problem sets and solutions, this is a self-contained introduction to the topic for graduate students, researchers and industrialists working in the applied fields of physics and chemistry, polymer science and engineering.

Polysiloxanes can be formed into a variety of elastomeric networks that are of fundamental as well as commercial interest. This review, which primarily focuses on the author’s own work, illustrates the use of experiments and simulations to establish structure–property relationships for these materials. Studies of this type have been very useful in obtaining a better molecular understanding of rubberlike elasticity in general.

This review focuses on a new type of para-catenated aromatic polymer being used in the preparation of high-performance films and fibers of exceptional strength, thermal stability, and environmental resistance, including inertness to essentially all common solvents. Polymers of this type include the cis- and trans-poly(p-phenylene benzobisoxazole) (PBO), the cis- and trans-forms of the corresponding poly(pphenylene benzobisthiazole) (PBT), and the structurally similar poly(5,5ʹ-bibenzoxazole-2.2ʹ-diyl-l,3-phenylene) (AAPBO) and poly(2,5-benzoxazole) (ABPBO) and their sulfurcontaining analogues. Because of their rigidity, these polymers become highly oriented in solution and some display liquid crystalline behavior. The purpose of this paper is to summarize the authorsʹ theoretical work on the structures, conformational energies, intermolecular interactions, electronic properties, electrical conductivity, and electrooptical properties of these chains, including, in some cases, the so-called articulated forms and the protonated forms known to exist in strong acids. The emphasis is on how such studies provide a molecular understanding of the unusual properties and processing characteristics of this new class of materials.