Rigid-rod structure

Several groups have studied introduction of phenylene ethynylene units into PPV backbones. The first material of this type, copolymer 163, was reported by Bunz and coworkers [193] (Chart 2.36). The material displayed blue luminescence in solution (A ax = 460 nm), but due to the polymer s rigid-rod structure, very strong aggregation in the solid state gave rise to... 

Polymerization of terephthalic acid with 4,6-diamino-l,3-benzenediol via oxazole formation (Eq. 2-219) proceeds with a sharp and continuous decrease in reaction rate with increasing polymer molecular weight [Cotts and Berry, 1981]. Reaction becomes progressively more diffusion-controlled with increasing molecular size due to the increasing rigid-rod structure of the growing polymer. 

The CTE of polyimides decreases with the introduction of a rigid-rod structure into polyimide molecules, and is also controlled by changing the composition of high- and low-thermal-expansion polyimides. Figure 15.7 shows... 

In many cases the disorder introduced into rigid-rod structures in order to depress the crystalline melting point to a temperature below the degradation temperature is such that the polymer can crystallise only to a small extent or not at all. In such cases, the polymer exhibits a frozen nematic structure at temperatures below the glass transition, with a typical micaceous sheen. 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

The absolute moleeular weights of these materials have been established by GPG by means of the universal calibration technique that involves the acquisition of intrinsic viscosity data. As with the aforementioned platinum polyyne polymers 166 (M = Pt(P"Bu3)2 x = Z), the intrinsic viscosities were found to be independent of the nature of the solvent and a Mark-Houwink a-value of 1.5 was determined, which is indicative of a rodlike structure. Interestingly, it was found that in this case, GPG, using PS standards, underestimates the true molecular weight of the polymers. This was attributed to the high molar mass of a repeat unit in the Ni polymer 207 compared to PS, which overcompensates for the effect of the rigid-rod structure that would be expected to lead to the opposite situation. Absolute values of of up to ca. 1 x 10 were determined for the polymer samples. 

PROPERTIES OF SPECIAL INTEREST Chiral-helical rigid-rod structure and yields liquid crystals in solution. Potentially useful as models for the understanding of the structure and properties of biological molecules. Unreachve toward hydrogenation at ambient temperature and pressure and resistant toward acid hydrolysis. One of the few soluble polyisocyanides of high molecular weight. ... 

Polymers with Pt-Pt bonds as part of the polymer main chain have also been reported (Eq. 6.7) [35, 36]. These yellow materials (e. g. 6.10) are particularly interesting as they possess conjugated rigid-rod structures, but are essentially insoluble or are only sparingly soluble in organic solvents studies to date have been comparatively limited in scope. Analogues of 6.10 with diphosphine spacers, as well as neutral analogues with acetylide spacers, have also been prepared and were also found to be insoluble in common solvents [36]. 

Compared with cured samples of conventional bisphenol A epoxy (Epon 828), the liquid crystal epoxy with the same curing condition exhibits better thermal and electric properties as shown in Table 10.7 [91]. The results are expected because of the highly ordered rigid-rod structure of the liquid crystal epoxy, and a shorter gel time than that of bisphenol A epoxy was observed. 

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