Orientation polymer transport properties

Such polymer composites (that will not be treated in this chapter) can be used as precursors to the C3 materials where the polymer is converted into a carbon phase with a low content of heteroatoms. A well-developed sp2 structure is desired, with its basic structural units being oriented perpendicular to the fiber axis. The required excellent mechanical and transport properties in the weak direction of the initial fiber can thus be delivered. This material is now called carbon and finds widespread application in energy-related structural material applications such as electric passenger cars, as construction material for airplanes and as the core structure of turbine blades for windmills and compression turbines. 

As mentioned earlier, suspensions of particulate rods or fibers are almost always non-Brownian. Such fiber suspensions are important precursors to composite materials that use fiber inclusions as mechanical reinforcement agents or as modifiers of thermal, electrical, or dielectrical properties. A common example is that of glass-fiber-reinforced composites, in which the matrix is a thermoplastic or a thermosetting polymer (Darlington et al. 1977). Fiber suspensions are also important in the pulp and paper industry. These materials are often molded, cast, or coated in the liquid suspension state, and the flow properties of the suspension are therefore relevant to the final composite properties. Especially important is the distribution of fiber orientations, which controls transport properties in the composite. There have been many experimental and theoretical studies of the flow properties of fibrous suspensions, which have been reviewed by Ganani and Powell (1985) and by Zimsak et al. (1994). 

The remainder of this chapter will focus on the oxygen permeabilities 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 permeabilities of heterogeneous materials (such as blends, composites and oriented semicrystalline polymers) 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. 

We first give a concise review of the effects of orientation and crystallinity on the barrier properties of polymeric materials, paying particular attention to their effects on the solubility and diffusion coefficients. This will provide useful background for considering the transport properties of liquid crystal polymers which, because of their unique properties, may have some role to play in the quest for improved barrier 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. 

L. Hardy, E. Espuche, G. Seytre, and I. Stevenson. Gas transport properties of poly(ethylene-2,6-nc htalene dicarboxylate) films Influence of crystallinity and orientation. J. Appl Polym. Sci., 89(7) 1849-1857, August 2003. 

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),... 

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. 

Many theories have been developed (involving solitons, excitons, polarons and bipolarons) [17] to explain the conductivity phenomenon under the assumption that the chains of conductive polymers are being arranged and at least somewhat oriented in fibrils. But now, it must be explained why our dispersed (and later flocculated) polymer showed principally the same transport properties as the fibrillar conductive polymers, as can be concluded from conductivity versus temperature and thermopower measurements. 

Experimental results on the band dispersion in o-bond polymers are very limited due to difficulty in preparing thin films with oriented chains [20, 31, 32, 62]. Here, we introduce the band dispersion of quasi-one-dimensional polymer polyethylene. Early work on the band structure study was carried out on systems with alkyl chains and was aimed at understanding the electronic structure of polyethylene, in particular, the possible existence of one-dimensional band structure in thin films where molecular chains assemble via weak interchain interactions. There is renewed interest in the band dispersions as they determine carrier transport properties in nanoscale molecular electronics [63]. 

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