Polyethene, crystallites

Why should polypropene melt so much higher than polyethene (175° vs. 110°) The answer lies in the differences between the way the polymers crystallize. Polyethene crystallites have extended zig-zag chains that have very low barriers to rotation about the C-C bonds. Because of interferences between the methyl groups, polypropene does not crystallize in extended... 

Figure 29-3 Representation of attractive interactions between the hydrogens in a crystallite of polyethene. This drawing is incomplete in that it does not show the interactions of the depicted chains with the other chains in front and behind.

In terms of polymer matrices for composite materials, there will be a compromise between solvent and water resistance. Thus non-polar resins are likely to be less resistant to hydrocarbon solvents, which have low polarity, but more resistant to moisture absorption. Polar resins behave in the opposite way. Strongly polar solvents, such as dimethyl sulphoxide or similar, can interact with polar structures in the resin and are difficult to resist. Crystalline thermoplastic polymers are often better for such applications. For example, polyethene will only dissolve in hydrocarbon solvents (of similar solubility parameter) at temperatures above the crystalline melting point. Polar semi-crystalline polymers such as the polyamides or nylons can be dissolved in highly polar solvents, such as cresol, because of a stronger interaction than that between molecules within the crystallites. High performance thermoplastic polymers such as polyether ether ketone (PEEK) have been promoted for their resistance to organic solvents (see Table 3.5) [12], The chemical resistance of unsaturated polyester and vinyl ester and urethane resins is indicated in Table 3.6 [15]. 

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