Crystalline structures, hydrogen bonding

In Section 8.9 some effects of crystallinity and hydrogen bonding on Tg were considered. The effect of molecular weight was discussed in Section 8.7, and the effect of copolymerization was discussed in Section 8.8. This section discusses the effect of chemical structure in homopolymers. 

The lack of a center of symmetry in the monomer repeat imit structure of PA6 imparts a directionality to the PA6 chains. Consequently, PA6 can form the a-crystalline, planar hydrogen bonded sheet structure only between antiparallel chains. Parallel chains, on the other hand, form a 7-crystalline, pleated sheet structure in which the chains assume a twisted helix conformation to permit lull H-bonding. Due to these symmetry and entropic restrictions, PA6 is somewhat less crystalline and less ordered, usually containing a mixture of a and 7-crystalline forms in the injection molded parts. As a consequence of these crystallinity and morphology differences, PA6 is intrinsically more ductile than PA66, since it is easier to deform a less ordered polymer phase. 

Fig. 5-3. Schematic representation of the pore structure of (A) dextran and (B) agarose. The effect of the agarose crystalline microstructure, hydrogen bonding between the individual polymer chains, on the pore geometry and size is clearly evident.

There is a fair amount of work reported with films at the mercury-air interface. Rice and co-workers [107] used grazing incidence x-ray diffraction to determine that a crystalline stearic acid monolayer induces order in the Hg substrate. Quinone derivatives spread at the mercury-n-hexane interface form crystalline structures governed primarily by hydrogen bonding interactions [108]. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

Water [579] is present in the structure of true crystalline hydrates [580] either as ligands co-ordinated with the cation (e.g. [Cu(OH2)4]2+ in CuS04 5 H20) or accommodated outside this co-ordination sphere within voids left in anion packing, further stabilized by hydrogen bonding (e.g. the remaining water molecule in CuS04 5 H20). 

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