Polymer cross-linking conformational

When a polymer network goes inside an FLC medium, we may consider the random networks of cross-Unk chains to be stretched between two smectic planes spaced L apart (Fig. 6.5). Gaussian like cross-links with orientationally uncorrelated chains, freely joined by spherical particles and connected by harmonic springs can be considered as shown in Fig. 6.6. r(z) = [x(z),y(z)], describes the path of cross links conformations where z stands for the direction of cross-link alignment without any loops and overhangs. Further, we assume the cross-link... 

A cross-linked polymer has a density of 0.94 g cm" at 25°C and a molecular weight between crosslinks of 28,000. The conformation of one bond in the middle of the molecule changes from trans to gauche, and the molecule opens up by 120°. In w-butane, the trans to gauche transformation requires about 3.3 kJ mol". Estimate a value for AH of stretching based on this model, and use the law of cosines to estimate the magnitude of the opening up that results. 

In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

The temperature of esterification has a significant influence on isomerization rate, which does not proceed above 50% at reaction temperatures below 150°C. In resins produced rapidly by using propylene oxide and mixed phthaUc and maleic anhydrides at 150°C, the polyester polymers, which can be formed almost exclusively in the maleate conformation, show low cross-linking reaction rates with styrene. 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

At the gel point, (3 —l) = l/p, which with the foregoing expression gives Eq. (14), thus establishing equivalence of the two procedures. The primary molecules in a condensation polymer must almost invariably conform to a most probable distribution (see Chap. VIII). The random cross-linking of primary molecules otherwise distributed in size has no counterpart in polyfunctional condensation, therefore. 

The reversible recovery of a deformed elastomer to its original (undeformed) state is due to an entropic driving force. The entropy of polymer chains is minimum in the extended conformation and maximum in the random coil conformation. Cross-linking of an elastomer to form a network structure (IX) is... 

The reactions of intramolecular cross-linking is a rather poorly investigated area in the field of macro-molecular reactions. However, the problems of regularities of such processes are related to such important problems of polymer chemistry as chemical modification of polymers, networks formation, sorption of low molecular reagents by polymers, intramolecular catalysis, conformational transitions and so on. In spite of the great importance of the study of regularities of cross-linking reactions, the experimental and theoretical analysis of such processes is complicated by many difficulties. ... 

The simplified two-network experiment is performed in the following manner A thin strip of the uncross-linked polymer is stretched by about 60% and maintained with constant length throughout the remainder of the experiment. The force is monitored at all times. After a predetermined relaxation period, the temperature is decreased to below the glass transition temperature to quench all overall conformational changes. The sample is cross-linked in the glassy... 

Through control of the amount of cross-linking, nature of the packing material, and specific processing procedures, spheres of widely varying porosity are available. The motion in and out of the stationary phase depends on a number of factors including Brownian motion, chain size, and conformation. The latter two are related to the polymer chain s hydrodynamic volume—the real, excluded volume occupied by the polymer chain. Since smaller chains preferentially permeate the gel particles, the largest chains are eluted first. As noted above, the fractions are separated on the basis of size. 

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