Barrier properties, polymer transport

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

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). 

ID CNTs inserted in the 2D clay platelet network is believed to be responsible for this, as this unique nanostructure provides larger tortuosity and obstacle for the heat transport. The large improvement in thermal stability of chitosan may arise from following two reasons (1) good heat barrier properties of CNTs and clay for polymer matrix during formation of chars and (2) formation of carbonaceous layer. 

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... 

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. 

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. 

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. 

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