Crystal, Crystallization strain-induced

It is well known that by the uncertainty principle, the time-dependent processes give rise to the finite linewidth of an optical transition [170]. This homogeneous linewidth, however, can seldom be observed since crystal strain induces a spread in resonance frequencies, which exceeds in most cases the homogeneous width. The spectral line is then inhomogeneously broadened. 

Oriented In-Plane Texture. In this kind of film the properties (H and in the various in-plane directions (texture and nontexture directions) are different. The texture of the film can be supported by the texture of the substrate and the crystal lattice can be smaller in the texture direction than in the transverse direction. This can be the source for strain-induced magnetic anisotropy (magnetostriction). It is also found that the crystal is aligned in the texture direction (92). 

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]. 

We shall deal in this lecture with recent improvements in the elastomers synthesis, that should be able to cope with the above mentioned requirements, without resorting to important investments for new plants or to cumbersome feedstocks. The improvement of the elastomer synthesis relies upon new catalytic systems that allow a control of elastomer tacticity in order to achieve a strain induced crystallization, and suitable monomer combinations in order to minimize the hysteresis loss of the elastomer in a wide range of temperatures and frequencies. 

Synthesis of Elastomers with Strain-Induced Crystallization... 

The properties of elastomers are much improved by strain-induced crystallization, which occurs only in polymers with high stereoregularity. The polymerization of butadiene using completely soluble catalysts composed of a) rare earth carboxylates, b) Lewis acids and c) aluminum alkyls leads to polymers with up to 99 % cis-1,4 configuration. These polymers show more strain-induced crystallization than the commercial polybutadienes and consequently their processability is much improved. 

Very active catalysts for the preparation of strictly alternating butadiene-propylene copolymers (BPR) consist of V0(0R)2C1/i-Bu Al (R = neopentyl). The CH3 side groups in BPR stiffen the polymer chain and were expected to promote the formation of strain-induced structures. The fact that we could not detect strain-induced crystallization is probably due to an atactic configuration of the propylene units. 

Alternating isoprene-ethylene copolymers (IER) were prepared with the same catalyst. Due to the strictly alternating sequences of diene and olefin units and the absence of chiral carbon atoms IER shows strain-induced crystallization, but at lower temperatures compared to natural rubber. 

X-ray diffraction pictures taken with a flat-film camera show that crosslinked SE-BR samples crystallize on stretching. Sharp reflections are observed at an extension ratio of 4 1 (Figure 4). With samples having different degrees of stereoregularity the order for increasing strain-induced crystallization is the same as the order for the rate of low temperature crystallization. 

The absence of asymmetric carbon atoms in IER could explain why the strain-induced crystallization is more pronounced in this case. Numerous strong reflections of the type shown in Figure 11 were obtained by X-ray diffraction of stretched IER samples at 30 °C. It is therefore evident that a well-formed crystal lattice exists. 

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

Comparison of Strain Induced Crystallization Behavior of NR with High Trans SBR s from Ba-Mg-Al and Ba-Li Catalysts... 

Since the tack and green strength of these rubbers may depend on their ability to undergo strain induced crystallization, this behavior was studied and compared with that shown by NR. Information on the strain induced crystallization behavior of these experimental rubbers was obtained from x-ray diffraction measurements and from rheo-optical studies. 

Figure 18 shows the percent crystallinity obtained by birefringence measurements for NR at various elongations as a function of temperature. The relative shapes of the curves in this Figure show the pronounced temperature and strain dependence on the strain induced crystallization of NR. Of particular importance is the relatively high amounts of crystallinity that develop at room temperature. 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

Two additional properties that may depend on the strain induced crystallization behavior of NR are green strength and building tack. A comparison of the performance of the experimental high trans SBR s with NR was, therefore, carried out. 

A study of the difference in monomer sequence length distribution and hence of the possibility of strain-induced crystallization and other structural parameters,including long-chain branching, of samples prepared with the above-mentioned various catalyst systems is under way and constitutes the subject of a subsequent paper. 

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