Elastomers strain-induced

Nishikawa E, Finkelmann H. 1999. Smectic A liquid single crystal elastomers strain induced break down of smectic layers. Macromol Chem Phys 200 312 322. 

The crystallization of PDMS in the undeformed state has long been of interest, including unfilled elastomers - and block copolymers reinforced with silica. Of greater interest, however, is PDMS, generally in the filled state, in elongation - - where strain-induced crystallization provides considerable reinforcement of the elastomer. Strain-induced crystallization is of practical as well as fundamental importance. - Theoretical models have been developed to characterize this type of crystallization. - ... 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

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. 

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

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. 

P. Xu and J.E. Mark, Strain-induced crystallization in elongated polyisobutylene elastomers, Polym. Gels Netiv., 3(3) 255-266,1995. 

The curve shown by a soft and strong material like natural rubber is shown next. The small initial slope and modulus show the material to be soft. At higher elongations, however, strain-induced crystallization occurs and this reinforces the elastomer. As a result its ultimate strength is large and it is therefore quite strong. In other words, one has to be strong to pull it apart. 

An example of a relevant optical property is the birefringence of a deformed polymer network.256 This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity.92,296-302 Infrared dichroism has also been particularly helpful in this regard.82,303 In the case of the crystallizable polysiloxane elastomers, this orientation is of critical importance with regard to strain-induced crystallization, and the tremendous reinforcement it provides.82... 

Raman and IR spectroscopic studies dealing with the qualitative and/or quantitative determination of rubber compounding ingredients, i.e., the elastomer itself [22, 26-31], fillers [32, 33], vulcanisation chemicals and other additives [34-37], are not included here. The same applies to studies dealing with the crosslinking of elastomers by means of chemicals other than sulfur or peroxide [38-41], self-crosslinking of elastomers blends [42-44], crystallisation (strain-induced) [45-48] and oxidation/ageing [49-53]. 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

Cross-linked elastomers (the other main class of polymers studied by ESR) can be pre-strained above Tg to any desired extent (up to fracture) and their orientation stabilised by cooling below Tg before testing. At high pre-strains, strain-induced crystallization may occur providing a morphology essentially similar to that of synthetic fibres Cross-linked polymers may therefore be used to explore in a systematic manner, the role of strain and orientation in rmlecular fracture. 

The Gough Joule effect, shown as an increase in modulus with an increase in temperature and the retraction of stressed rubber on heating An ability of some elastomers to undergo strain-induced crystallization The susceptibility of un.saturated rubbers to ozone attack and subsequent cracking in the stretched state... 

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