Elastomer crystallizing, strain-induced

Synthesis of Elastomers with Strain-Induced Crystallization... 

One major factor that improves the ultimate load-carrying capacity of elastomers is strain-induced crystallization where the crystallites act as additional cross-links and induce strength by aligning the strong bonds along the direction of deformation. Elastomers that cannot undergo strain-induced... 

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 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... 

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... 

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

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

Although elastomers are usually amorphous, strain-induced crystallization occurs in rubbers such as cA-l,4-polybutadiene, butyl rubber, and NR. Crystallization under stress, discovered 200 years ago [239], increases the modulus and most failure properties of rubber and is essential to performance in many... 

Methods to study crystallization of deformed elastomers include x-ray diffraction [207,208,263-265], optical birefringence [266,267], infrared or Raman spectroscopy, electron microscopy [268], dilatometry [269, 270], NMR [271], and mechanical measurements [193,262,272]. Strain-induced crystallization is manifested in the latter by both greater hysteresis (Fig. 23) and a longer time for stress decay (Fig. 24). However, the shape of the stress-strain curve during extension does not obviously reveal the onset of crystallization [207,208,262]. 

Natural mbber crystallizes on elongation—a phenomenon called strain-induced crystallization—what enhances mechanical properties. However, a filler in the form of carbon black is typically added to natural rubber to additionally modify the mechanical properties. Elastomers which cannot undergo strain-induced crystallization contain even more fillers. Carbon black is used in such cases also, but silicone rubbers are filled with silica. 

Kundler, L Finkelmann, H., Strain-Induced Director Reorientation in Nematic Liquid Single Crystal Elastomers. Makromol. Rapid Commun. 1995,16, 679-686. 

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