Amorphous polymer chains

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

When the crystallinity of polyethylenes is increased, the gas permeability through the film decreases. The factors involved are the tortuosity of the gas path through the amorphous phase, and the effect of the crystals in restricting the mobility of the amorphous polymer chains (chain immobilisation factor). The logarithm of the permeability of nitrogen, argon and carbon dioxide decreased almost linearly with increased crystallinity of PE, with the ratio of the gas values remaining almost constant for a particular PE. 

An amorphous bulk polymer contains chains that are arranged in less than a well-ordered, crystalline manner. Physically, amorphous polymers exhibit a Tg but not a T, and do not give a clear x-ray diffraction pattern. Amorphous polymer chains have been likened to spaghetti strands in a pot of spaghetti, but the true extent of disorder that results in an amorphous polymer is still not fully understood. 

PVA is used in the treatment of textiles and paper. PVA also acts as the starting material for the synthesis of a number of poly(vinyl acetals) with the general structure as given in Equation 6.66. The acetal rings on these random amorphous polymer chains restrict flexibility and increase the heat deflection temperature to a value higher than that of PVAc. 

In Chapter 6 we talked about the ability of solvent molecules to interact with and surround amorphous polymer chains, leading to the formation of polymer solutions. A closely related phenomenon utilizes a low-molar mass compound to penetrate a polymer and reduce the forces of attraction between chains. Such a compound is called a plasticizer. It must be compatible with the polymer and is almost always nonvolatile. Solvent molecules actually plasticize a polymer sample before forming a solution. However most solvents are not good permanent plasticizers because they diffuse to the surface and evaporate. 

The stractme of an amorphous polymer chain comprises various structural levels as it has been cleai-ly presented by Jancar [4] (i) constitution (atomic/molecular structm-e of monomer units), (ii) configuration (spatial arrangement of primary bonds independent of bond rotation), and (iii) conformation (spatial arrangement, which can be altered by bond rotation). The majority of amorphous polymer chains used as matrices contain the carbon backbone (ethyleiie/prop-ylene rubber, EPR poly(vinyl chloride), PVC polystyrene, PS poly(methyl methacrylate), PMMA poly(vinyl acetate), PVAc etc.). The approximate characteristics of the vinyl-type backbone are as follows C-C bond length b = 0.15 nm, C-C bond angle 9= 109°, C-C bond energy = 350 kj/mol. 

Stress-induced reactions of polymers are the object of intensive research, with the relative literature having been recently reviewed by Porter and Casale " this field is commonly called mechanochemistry. Most of the available data are on the stress reactions of amorphous polymer chains, having mainly been obtained on both dilute and concentrated solutions. In the case of dilute solution, theories are based on the rotation and elongation of chains by hydrodynamic forces on the other hand imposition of shear on entangled chains (concentrated solutions) places tension on the temporary network resulting from interchain entanglements. 

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