Glassy polymers copolymers

Table 10 contains some selected permeabiUty data including diffusion and solubiUty coefficients for flavors in polymers used in food packaging. Generally, vinyUdene chloride copolymers and glassy polymers such as polyamides and EVOH are good barriers to flavor and aroma permeation whereas the polyolefins are poor barriers. Comparison to Table 5 shows that the large molecule diffusion coefficients are 1000 or more times lower than the small molecule coefficients. The solubiUty coefficients are as much as one million times higher. Equation 7 shows how to estimate the time to reach steady-state permeation t if the diffusion coefficient and thickness of a film are known. 

Kambour, R. P. and Gruner, C. L., Effects of polar group incorporation on crazing of glassy polymers styrene-acrylonitrile copolymer and a bisphenol polycarbonate, J. Polym. Sci., Polym. Phys. Ed., 16, 703 (1978). 

Since glassy polymers tend to become plasticized by C02, it is important to determine the magnitude to which CO2 effects plasticization. This is achieved in monitoring nonlinear effects of gas permeability of mixtures containing CO2. CO2 shows a higher sorption than expected. This is attributed to the presence of acrylonitrile moieties in the ABS copolymer. 

Microhardness (MH), has been shown to be a convenient additional technique to detect accurately the ferro to paraelectric phase changes in these copolymers. The increase of MH as a function of VF2 polar sequences observed at room temperature is correlated with the contraction of the p-all-trans unit cell On the other hand, the fast exponential decrease of MH with increasing temperature, observed above Tc, is similar to that obtained for glassy polymers above Tg and suggests the existence of a liquid crystalline state in the high temperature paraelectric phase. This phase is characterized by a disordered sequence of conformational isomers (tg-, tg+, tt) as discussed for Condis crystals [109]. 

Pioneering contact mode AFM studies by Meiners et ai. (1995) show that chemical sensitivity at the surface of thin polymer films can be achieved by measuring the microscopic friction and stiffness for glassy block copolymers. This provides invaluable information to complement topography on the nature of the block at the surface. 

Folkes, M. J, and Keller, A. (1973). The morphology of regular block copolymers. In The physics of glassy polymers, (ed. R. N. Haward). Applied Science, London. Fredrickson, G. H. and Bates, F. S. (1996). Annual Reviews of Materials Science, 26, 501. Goodman, I. (ed.) (1982). Developments in block copolymers, Vol. 1. Applied Science, London. 

The usual requirement is a rubbery core and an outer shell of a glassy polymer. The rubbery core is generally based on polybutadiene or butyla-crylate or copolymers, the choice affecting strongly the ductile-brittle transition temperature. 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

Alentiev, A. Yu., Loza, K. A., and Yampol skii, Y. P. (2000). Development of methods for prediction of gas permeation parameters of glassy polymers Polyimides as altenating copolymers, J. Membrane Sci. 167, 91. 

Block copolymers of styrene and butadiene are used to make thermoplastic rubbers, or impact-resistant glassy polymers, depending on the length, arrangement and relative proportions of the blocks. Polyurethanes are an example of what are called segmented block copolymers and these we have mentioned in Chapter 3. 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

Stereo-transmission electron microscopy of craze tips has shown that the meniscus instability is the operative craze tip advance mechanism in a wide variety of glassy polymers Figure 4 shows a craze tip in a thin film of a styrene-acrylonitrile copolymer (PSAN). The void fingers are clearly visible. No isolated voids can be... 

Finally, there are complex fluids that are intermediate between solid and liquid in more than one of the ways listed above. Liquid crystalline polymers (LCPs) are both viscoelastic and liquid crystalline. Ordered block copolymers are viscoelastic and anisotropic. Glassy polymers possess long viscoelastic time scales both because they are glassy and because they are polymeric. Filled polymer melts possess the properties of both polymer melts and suspensions. 

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