Semicrystalline 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). 

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 properties of block copolymers, on the other hand, cannot be calculated without additional information concerning the block sizes, and whether or not the different blocks aggregate into domains. The results of calculations using the methods developed in this book can be inserted as input parameters into models for the thermoelastic and transport properties of multiphase polymeric systems such as blends and block copolymers of immiscible polymers, semicrystalline polymers, and polymers containing various types of fillers. A review of the morphologies and properties of multiphase materials, and of some composite models which we have found to be useful in such applications, will be postponed to Chapter 19 and Chapter 20, where the most likely future directions for research on such materials will also be pointed out. 

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. 

For many years, molecular orientation and crystallinity have been observed to improve the barrier properties of polymers (1-3). In extreme cases, drawing of semicrystalline polymers has been shown to reduce permeability by as much as two orders of magnitude. A crude understanding of the dependence of the transport parameters on penetrant size and chain packing can be... 

More detailed aspects of transport in heterogeneous media have been given in the excellent reviews of the subject by Barrer (55) and Petropolus (51) The models described by these authors and others include the effects of size, shape, and anisotropy of the crystalline phase on the tortuosity. Models of highly ordered anisotropic media have been demonstrated to have tortuosities in the range of 30, which reflects the rather dramatic role that orientation can have on the barrier properties of semicrystalline polymers. 

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. 

Information on how orientation during melt crystallization affects the transport properties of polymers is sparse however, increases in the permeability have been attributed to the "shish kebab" morphology (ill). Most of the work involving barrier properties of oriented semicrystalline polymers has dealt with materials drawn at temperatures well below the melting point. The transport properties of cold-drawn polyethylene (34f 42-46), polypropylene (42,42), poly(ethylene terephthalate) (12,42-4 9), and nylon 66 (22) among others have been reported. 

The effect of biaxial strain on the transport properties of semicrystalline polymers has been demonstrated for several systems (42) However, particular attention has been paid to poly(ethylene terephthalate)(47-49) because of its wide use in blow molding applications. 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

Oriontation-Induced Effects. Orientation and combined heat and orientation processing affect the transport properties of glassy polymers. Especially when crystallites are present, the effects can become surprisingly large. As noted for rubbery semicrystalline materials, the obvious improvements in barrier properties associated with organization of lamellar crystalline domains with their platelets perpendicular to the direction of penetrant flow can produce significant... 

The relationship between the gas-transport properties and composition of semicrystalline binary blends of cellulose (CELL) and PVA has been assessed by following the kinetics of CO2 sorption [95]. The blends are thermodynamically miscible with Xai = —0.985, consistent with the presence of favorable interactions due to hydrogen bonding between the two different polymers. As sorption takes place only in the amorphous regions, the absolute level of CO2 equilibrium sorption is relatively low with the highest value for pure CELL The sorption curves for the blends lie intermediate to those for the pure components. Accordingly, both the diffusion coefficient and permeabUity were increased in line with the CELL content, with little or no pressure dependency. 

The mass transport mechaiusm of gases permeating in a nanocomposite is similar to that in a semicrystalline polymer. The nanocomposite is considered to consist of a permeable phase where non-permeable nanoplatelets are dispersed. There are mainly three factors that influence the permeabiUty of nanocomposites the volume fraction of the nanoparticles, their relative orientation to the diffusion direction and their aspect ratio. The gas transport behavior of two different nanoclay-reinforced EVA membranes has been analyzed using oxygen and nitrogen gases and the results were compared with neat EVA. EVA nanostructured polymer blends exhibit excellent barrier properties. 

Crystallinity of polymers is one of the factors determining their gas transport properties. The concept of gas transport through amorphous regions of the semicrystalline polymers with crystalline fractions being essentially impermeable dominates in the contemporary literature [68]. Universality of this postulate introduced by van Amerongen more than 50 years ago [69] was later questioned [70, 71]. 

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