Chain-extended polymers

The last two decades of research on high performance rigid-rod polymers and molecular composites have produced a wide variety of material systems for evaluation. The purpose here is not to present a comprehensive review but rather to describe these relatively recent research developments with regard to the synthetic methods and approaches that have been utilized. In doing this some properties of the polymers are discussed as well as some of the problems encountered in conjunction with their synthesis and evaluation. In an effort to limit the scope of the review, we will address only aromatic heterocyclic rigid-rod polymers, extended chain polymers and derived molecular composites. 

Table 12-In Vitro Inhibition of Aureobasidium Pullulans By Fungicides Bound To Polymers, Extended-Chain Acrylate Ester Functions ...

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

PBO andPBZT. PBZ, a family of/ -phenylene-heterocycHc rigid-rod and extended chain polymers includes poly(/)-phenylene-2,6-benzobisthiazole) [69794-31-6] trans-V 27V) and poly(/)-phenylene-2,6-benzobisoxazole) [60871-72-9] (ot-PBO). PBZT and PBO were initially prepared at the Air Force Materials Laboratory at Wright-Patterson Air Force Base, Dayton, Ohio. PBZT was prepared by the reaction of... 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

There is no unanimity in regard to the exact mechanism of ECC formation under high pressure. Wunderlich et al. [11-18] suggested that when a flexible polymer molecule crystallizes from the melt under high pressure, it does not grow in the form of a stable extended chain, rather it deposits as a metastable folded chain. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

The most widely used method for preparing extended-chain crystals involves solid-phase polymerization when the monomer exists as a single crystal. The polymerization of single crystals of the monomer permits the preparation of a polymer material with a maximum orientation a polymeric single crystal composed of fully extended macromolecules. Such polymer crystals are needle-shaped22. ... 

Fig. 5 a, b. Models of the crystallization of flexible-chain polymers with the formation of a folded-chain crystals and b extended-chain crystals b... 

Applying the TABS model to the stress distribution function f(x), the probability of bond scission was calculated as a function of position along the chain, giving a Gaussian-like distribution function with a standard deviation a 6% for a perfectly extended chain. From the parabolic distribution of stress (Eq. 83), it was inferred that fH < fB near the chain extremities, and therefore, the polymer should remain coiled at its ends. When this fact is included into the calculations of f( [/) (Eq. 70), it was found that a is an increasing function of temperature whereas e( increases with chain flexibility [100],... 

From studies in solution and in the sohd state, a number of structures both sheet-like [14] and helical [19, 21, 24—36] have been proposed over the years for these polymers. Poly(/9-alanine) 3 for example crystallizes as extended chains [14]... 

Because of the atoms freedom to rotate about single bonds, a chain of carbon atoms can achieve various positions in space. On one extreme is the zig-zag extended chain and on the other is a coil. Such spatial structures become particularly important in determining properties of very long chained compounds known as polymers (Chapter 5). 

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