Crystallization-induced stress

V.V. Martynov and V.V. Kokorin, The crystal stmcture of thermally- and stress-induced martensites in... 

It should be noted that the fraction of ECC in samples obtained by other methods described in Sect. 2 is approximately as small as that of the framework in the orientation-ally crystallized samples. These methods differ in details but depend on the mechanical treatment of the crystallizing system and are therefore given the common name stress-induced crystallization . Although the structure of the samples obtained by these methods has some features in common with that of orientationally crystallized samples, the thermodynamics and kinetics of orientational crystallization are fundamentally different from the mechanism of stress-induced crystallization. 

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

Both stress-induced crystallization and orientational crystallization can be used for the preparation of polymer materials with mechanical property values (e.g. tenacities and elastic moduli) much higher than those for polymer films and fibers obtained by conventional processing. We believe that the advantage of orientational crystallization over more complex methods consists in the possibility of obtaining samples of elastic moduli and tenacities in a one-step continuous process. 

Very frequent are the cases of stress-induced crystallizations. A typical case is that of slightly vulcanized natural rubber (1,4-m-polyisoprene) which, under tension producing a sufficient chain orientation, is able to crystallize, while it reverts to its original amorphous phase by relaxation [75],... 

An applied stress lowers the symmetry of the crystal and can make defects with different orientations inequivalent. A review of stress techniques has been written by Davies (1988). The degeneracy of the ground state and also of the spectroscopic transition energies can be lifted. In this section we suppose that the defects cannot reorient and consider only the splitting of the transition energies. The stress-induced reorientation of defects is discussed in the next section. 

The stress-induced crystallization influences the ability to sustain the stress of deformation occurring in the spin line. Optical micrographs of fluffs allow the assumption of different deformation or stress behavior of core and skin layers of spun fibers, as shown in Figure 13.1. The reason for this kind of yam break can... 

The critical state of stress-induced crystallization at high spinning speeds is governed by the viscoelasticity of the polymer in combination with its crystallization behavior. Any kind of coarse particle obviously disturbs the structure and affects the resistance against deformation. The development of stress is controlled by the rheological properties of the polymer. Shimizu et al. [4] found that increasing the molecular weight drastically promotes the crystallinity under stress conditions. 

Sometimes crystallographers consider that measuring a crystal at very low temperature is a kind of panacea, able to solve all defects of the sample, all kinds of experimental errors, and enhance the response indefinitely. Young students might be disappointed to leam that these miracles do not take place. A bad crystal sample remains as such even at 10 K, and sometimes it becomes even worse because the cooling process and the residual stress induced by a temperature gradient may produce further damage to the sample. Many other kinds of experimental problems and sources of error (for example absorption, extinctions, disorder, etc.) are not attenuated by the low temperature. 

Meanwhile, development of coordination catalyst was proceeding full scale. The polyisoprene prepared using this coordination catalyst (TiClj, AIR ) proved to be more suitable in physical properties than the one made by lithium metal or organolithium compounds in hydrocarbon media. The Ziegler polyisoprene, as it was called, has greater stereoregularity and stress-induced crystallization properties than polyisoprene made by the alkyl lithium catalyst. How-... 

Perhaps, the most dramatic example of the effects of drawing on the spectra of a polymer comes from the stress-induced crystal transition of poly(butylene terephtha-late) 292,29 298 299). The phase transition of PTMT has been investigated above and below the glass transition temperature by means of dynamic infrared measurements... 

Certain polymers can be crystallized by mechanical stress. Namely, the stress induced elongation decreases the entropy of the chains. For this reason, an additional decrease in entropy, which is required for crystallization is comparatively small. Strain induced crystallization phenomena are of great practical importance since the elastomeric properties can be tailored in some way. In particular, PIB readily undergoes strain induced crystallization already close to room temperature. 

Unlike the thermomechanical inversion of heat, the inversion of internal energy is possible only for chains with d In 0/dT 0. It is also evident from Eq. (48) that for d In 1 and vice versa. For values a = (6 — 10) x 10"4 K 1 and d In polymeric networks, XtJ = 1.3 — 2.2 for extension and X,j > 0.5 for compression. It must be emphasized that this thermomechanical inversion of internal energy is not connected with the stress-induced crystallization and arises from the different signs of inter- and intrachain contributions to the internal energy. The extreme of the internal energy occurs at the deformation... 

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