Amorphous polymers transport properties

Since pneumatic conveying is largely applied to transport granular polymers and on the other hand even smallest amounts of attrition of these solids cannot be tolerated the results presented are focused on these materials. Polymers of four chemically different polymer classes were examined. Polypropylene (PP) and polyethylene (PE) belong to the semicrystalline polymers, which possess both, an amorphous phase and a crystalline phase. The polymethylmethacrylates (PMMA) and polystyrenes (PS) are fully amorphous. Some material properties of the polymers are summarized in Table 1. 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

Mandelkern et al. (1968) have proved that the WLF formulation, which has had an outstanding success in explaining the segmental mobility and flow properties of completely amorphous polymers, is not applicable to the transport process involved in the growth of spherulites in melts of semi-crystalline polymers. Rather, a temperature-independent energy of activation, specific to a given polymer and dependent on its glass temperature, suffices to explain the experimental data now available. Mandelkern s equation reads ... 

The remainder of this chapter will focus on the thermal conductivities of amorphous polymers (or the amorphous phase, in the case of semicrystalline polymers). See Chapter 20 for a discussion of methods for the prediction of the thermal conductivities of heterogeneous materials (such as blends and composites) in the much broader context of the prediction of both the thermoelastic and the transport properties of such materials. 

The effects of molecular order on the gas transport mechanism in polymers are examined. Generally, orientation and crystallization of polymers improves the barrier properties of the material as a result of the increased packing efficiency of the polymer chains. Liquid crystal polymers (LCP) have a unique morphology with a high degree of molecular order. These relatively new materials have been found to exhibit excellent barrier properties. An overview of the solution and diffusion processes of small penetrants in oriented amorphous and semicrystalline polymers is followed by a closer examination of the transport properties of LCP s. 

Presently, the amount of data on transport in uniaxially oriented amorphous polymers is small in comparison with that of semicrystalline materials. The transport properties of oriented natural rubber (22), polystyrene (i3.,ii), polycarbonate (22.), and polyvinyl chloride (22,22) among others have been reported. One of the more complete descriptions of the effects of uniaxial orientation on gas transport properties of an amorphous polymer is that by Wang and Porter (34) for polystyrene. 

The hydrolytic stability and very high thermal endurance of this plastic in conjunction with a good balance of mechanical properties suit it for hot water and food handling equipment, range components, TV applications, alkaline battery cases, and film for hot transparencies. The unmodified product is transparent with a slightly yellow tint. Low flammability and low smoke suite it for aircraft and transportation applications. As with the other amorphous polymers, susceptibility to attack by organic solvents is a deficiency. 

Transport Properties. Sorption and transport properties are highly dependent on the post-vitrification history of glassy polymers (77) hence one would expect parameters such as physical aging, antiplasticization and amorphous orientation to affect transport properties. The reduction in diffusivity and permeability due to aging, orientation, and antiplasticization can be modeled via entropy or fi ee volume arguments (77). In addition, diffusive jumps of penetrant molecules in glassy polymers can be affected by (facilitated by) the segmental mobility that is manifested in sub-Tg relaxations 78),... 

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