Hydrogen Bonding between Molecular Chains

In a similar manner to a helices, extended structures can be held together by hydrogen bonds with the hydrogen bonds running perpendicular to the chain axis. Silk is an example of a protein that is found in the p structure. The amino acid composition of silk is rich in glycine (44.5%), alanine (29.3%), and serine (12.1%) amino acids with small hydrocarbon side chains that form sets of antiparallel hydrogen bonds between molecular chains. Models of polypeptide chains with sequences of poly(gly-ala) and poly(ala-gly-ala-gly-ser-gly) show that the most probable structure contains all the gly residues on one side of the chain and all the ala residues on the other side of the chain, and therefore by packing the chains in... 

The molecular structure of cellulose, unlike that of starch, allows for strong hydrogen bonding between polymer chains. This results in the formation of strong water-resistant fibers such as those found in cotton, which is 98% cellulose. Cotton actually has a tensile strength greater than that of steel. The major industrial source of cellulose is wood ( 50% cellulose). 

The molecular arrangement within the crystal units cells of nylon is governed by the need to maximize hydrogen bonding between adjacent chains. Hydrogen bonding within crystallites is facilitated by the fact that nylon chains adopt planar zig-zag conformations with dipoles perpendicular to the chain axis to thin the plane of the molecule. Examples of nylon crystallite structures are shown in Figs. 23.8 and 23.9 for nylon 6 and nylon 66, respectively. In the... 

The molecular interactions (hydrogen bonding between starch chains) after cooling of the gelatinized starch paste have been called retrogradation (Hoover, 2001). During retrogradation. 

Some unexpectedly complex liquid solid interactions have been detected and studied by ultrasonic impedance measurements (ultrasonic impedometry). Small amounts of water and alcohols have pronounced effects on the physical state of hydrophilic polymers specifically, the high frequency shear modulus and crystallinity index of a poly (vinyl alcohol) film increases with water content to a maximum before normal solution phenomena occur. These effects are attributed to the increased molecular order owing to water hydrogen bonded between polymer chains. The unusual effects of moisture on a novel poly(vinyl chloride)/plasticizer system and on hydrophilic polymers other than poly (vinyl alcohol) are also described. 

The basic requirement for cellulose dissolution is that the solvent is capable of interacting with the hydroxyl groups of the AGU, so as to eliminate, at least partially, the strong inter-molecular hydrogen-bonding between the polymer chains. There are two basic schemes for cellulose dissolution (i) Where it results from physical interactions between cellulose and the solvent (ii) where it is achieved via a chemical reaction, leading to covalent bond formation derivatizing solvents . Both routes are addressed in details below. 

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