Polyisobutylene crystal

In dispersed systems the nature of the filler also plays a controlling role in the way the crystallization proceeds. Examples are reported in [105], whose authors have used X-ray analysis to estimate the degree of crystallinity of polyisobutylene filled with different concentrations of a number of filler materials, after 100 cycles of 50% stretching. Polyisobutylene crystallizes as a result of such treatment. The results are given in Table 1. 

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

When X = Y, as in polyethylene, poly-(tetrafluoroethylene), polyisobutylene, and poly -(vinylidene chloride), the polymers are highly crystalline products with sharply definable melting points (except for polyisobutylene, which crystallizes readily on stretching but with difficulty on cooling). Oriented specimens of high strength may be obtained, exactly as in the crystalline condensation polymers. 

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

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

Ethylene-propylene rubber (EPR or EPDM) is, basically, a copolymer of ethylene and propylene. Because of the random arrangement of the monomers in the chain, crystallization does not occur, and the material behaves as a rubber. Just as with polyisobutylene, vulcanization with sulphur is impossible (the chain is saturated). Also here, a small amount of another monomer is incorporated, which enables the vulcanization and thus the use as a technical elastomer. EPR has a high resistance against ageing and chemical attack, and is, compared with other specialty rubbers, relatively cheap. 

Polyisobutylene forms crystalline domains when the material is stretched. Studies of the kinetics of this stress induced crystallization were the motive for the first... 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.6. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (IH) and so, unlike polyisobutylene, is incapable of crystallizing upon being stretched. Isotactic polypropylene, however, having all the methyl groups on one side, crystallizes easily. 

In addition to the thermodynamic requirements, kinetic factors relating to the flexibility and mobility of a chain in the melt must also be considraed. Thus, polyisobutylene -f CH2C(CH3)2 n might be expected to crystallize because the chain is symmetrical, but it will only do so if maintained at an optimum temperature for several months. This is presumably a result of the flexibility of the chain, which allows extensive convolution, thereby impediug stabilization of the required long-range alignment. 

During the last several years, much effort has been spent on developing new materials, based on iPP/elastomers blends. This interest is related to the fact that addition of the rubber phase improves the impact strength of the iPP, The present paper reports on a study of the isothermal crystallization and melting behaviour of thin films of isotactic polypropylene blended with an ethylene--propylene diene terpolymer and three samples of polyisobutylene with different molecular mass. 

The polymer polyisobutylene (—C(CH3)2—C—) presents a case where G(cl) = 0. This polymer is entirely amorphous (except on stretching, when it crystallizes) and the linear increase of /My (D) with dose has been established over a wide range of initial molecular weights. From these results it has been concluded that the scissions take place at random positions. ... 

Polyisobutylene Fluorinated polyamide Fluorinated polyamide Polymer liquid crystals(s) Polymethylemethacrylate Polyoxymethylene... 

Orientations in elongated mbbers are sometimes regular to the extent that there is local crystallization of individual chain segments (e.g., in natural rubber). X-ray diffraction patterns of such samples are very similar to those obtained from stretched fibers. The following synthetic polymers are of technical relevance as mbbers poly(acrylic ester)s, polybutadienes, polyisoprenes, polychloroprenes, butadiene/styrene copolymers, styrene/butadiene/styrene tri-block-copolymers (also hydrogenated), butadiene/acrylonitrile copolymers (also hydrogenated), ethylene/propylene co- and terpolymers (with non-conjugated dienes (e.g., ethylidene norbomene)), ethylene/vinyl acetate copolymers, ethyl-ene/methacrylic acid copolymers (ionomers), polyisobutylene (and copolymers with isoprene), chlorinated polyethylenes, chlorosulfonated polyethylenes, polyurethanes, silicones, poly(fluoro alkylene)s, poly(alkylene sulfide)s. 

Studies of uniaxial extension on noncrystallizable elastomer, poly(phenyl methyl siloxane) showed results which are consistent and comparable with those obtained for PDMS, suggesting that the crystallization is not important for this type of reinforcement [20]. Other examples for reinforcement effects achieved with the addition of silica fillers include polyisobutylene [24], poly(ethyl acrylate) [3], poly (tetra methylene oxide) [29,30], and some high-temperature polymers such as aromatic polyamides [14,33,34], polyi-mides [15,38,39], polybenzoxazoles [16,17], and polyben-zobisthiazoles [16,17]. Results indicated that the modulus increases with increase in silica content while the tensile... 

The good barrier properties of VDC copolymers are a consequence of crystallinity and low free volume in the amorphous phase. The sjunmetric nature of the VDC unit in the polymer leads to nested packing that is adequate for crystallization and that leaves very little dead volume in the amorphous phase. Both polyisobutylene and PVDC have unusually low permeability to water compared to their monosubstituted counterparts, polypropylene and PVC (86). The values listed in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

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