Network chains crystallization

If of the G network chains crystallize, leaving G-G amorphous chains, there are G+Gc amorphous elastic elements (amorphous chains plus amorphous sub-chains) that comprise the seml-crystalllne network. It Is here assumed that a chain enters a crystallite only once or not at all. By and large, multiple entry Into crystalline regions Is probably rare except In very lightly crossllnked networks. Let each elastic element (chain or subchain) behave In a Gaussian fashion so that we may write for the elastic free energy F of a specified network... 

An example of a relevant optical property is the birefringence of a deformed polymer network [246]. This strain-induced birefringence can be used to characterize segmental orientation, both Gaussian and non-Gaussian elasticity, and to obtain new insights into the network chain orientation (see Chapter 8) necessary for strain-induced crystallization [4,16,85,247,248]. 

Most probable positions of the chains are determined by the use of a characteristic vector r. This vector is representative of an average network chain of N links (the average links per chain). It deforms affinely whereas the actual network chains might not, and its value depends only upon network deformation. Crystallization leaves r essentially unaltered since the miniscule volume contraction brought about by crystallization can be ignored. But real network chains are severely displaced by crystallization. These displacements, however, must be compatible with the immutability of r. So in a sense, the characteristic vector r limits the configurational variations of the chains to those consistent with a fixed network shape and size at a given deformation. 

Such curves, illustrated in Figure 7, show no deviations from linearity which could be attributed to strain-induced crystallization. Similarly, birefringence-temperature measurements also carried out at a > au show no deviations from linearity that could be attributed to crystallization, or to other inter-molecular orderings of the network chains. Typical results of this type are shown in Figure 8 (16),... 

Figure 4. Nonuniform (nonaffine) straining due to network inhomogeneties, for example, of short network chains, causing local crystallization and relaxation (38).

Note The aging of a gel can involve polymerization, crystallization, aggregation, syneresis, phase changes, formation of branch points and junction points as well as scission and chemical changes to constitutional units of network chains. 

Low-strain oscillatory measurements show that PDM-PMAS copolymers with longer side-chains (C e and Cig) form a network structure at temperatures below the side-chain Intermolecular side-chain crystallization may be responsible for this behavior. 

That crystallization increases the elastic stress has already been demonstrated in Figure 6-8, in which the Mooney-Rivlin plot shows a rise at high extension ratios. However, it should be remembered that part of this increase is due to finite extensibility of network chains. In Figure 6-13 we show the stress-strain curves of natural rubber at two temperatures. At 0 °C there is considerable strain-induced crystallization, and we observe a dramatic rise in the elastic stress above X = 3.0. Wide-angle X-ray measurements show the appearance of crystallinity above this strain. At 60 °C there is little or no crystallization, and the stress-strain curve shows a much smaller upturn at high strains. The latter is presumably due only to the finite extensibility of the polymer chains in the network. 

A pseudo-affine model predicts the variation of 2 with the deformation of a semi-crystalline polymer. It assumes that the distribution of crystal c axes is the same as the distribution of network chain end-to-end vectors r, in a rubber that has undergone the same macroscopic strain. Figure 3.12 showed the affine deformation of an r vector with that of a rubber block. 

In a stretched rubber, the molecules elongate, and the r vectors move towards the tensile axis. Fience the variation of Pi with extension ratio will differ from the pseudo-affine model. For moderate strains the increase of Pi with extension ratio is linear, but at high extensions the approximation used in Eq. (3.12), that both q and q are large, breaks down. Treloar (1975) described models which consider the limited number of links in the network chains. Figure 3.33 shows that the orientation function abruptly approaches 1 as the extension ratio of the rubber exceeds v. Although the model is successful for rubbers, it fails for the amorphous phase in polypropylene (Fig. 3.32), presumably because the crystals deform and reduce the strain in the amorphous phase. 

Holland, V. F. and Lindenmeyer, P. H. (1965) Direct observation of dislocation networks in folded-chain crystals of polyethylene, J. Appl. Phys., 36, 3049-3056. 

By comparing the results of Mandelkem et al. [335-339] with those of Narh et al. [332], the conclusion can be reached that the crosslinking sites in the gel network consist of fringed micelles of relatively small, extended chain crystals. They melt at higher temperatures than the chain folded crystals that are also present in the gels and probably also contribute to the crosslink sites. 

Special discussion is required for gels where molecular network chains are crasslinked due to the local crystallization of fragments of polymer chains (jellies type Ib after Pap-kov). 

Sun, C.-C. Mark, J. E., The Effect of Network Chain Length Distribution, Specifically Bimodality, on Strain-Induced Crystallization. J. Polym. Sci., Polym. Phys. Ed. 1987,25, 2073-2083. 

Biodegradable shape-memory polymer networks with single POSS moieties located in the center of the network chains would promote POSS crystallization even within a constraining network structure. Successful synthesis of POSS initiated poly(e-caprolactone) (PCL) telechelic diols, utilizing a POSS diol as initiator, was reported by Lee et al. [116]. The POSS-PCL diols were terminated with acrylate groups and photocured in the presence of a tetrathiol crosslinker. Scheme 1 shows the chemical reaction for the synthesis of POSS-PCL network. 

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