Crystalline forces

Many high molecular weight synthetic polymers, such as polyethylene and polypropylene, have a large percentage of their molecules in the crystalline state. Prior to dissolution, these polymers must usually be heated almost to their melting points to break up the crystalline forces. Orthodichlorobenzene (ODCB) is a typical mobile phase for these polymers at 150°C. The accuracy and stability of the Zorbax PSM columns under such harsh conditions make them ideal for these analyses (Fig. 3.8). 

The barrier to internal rotation in biphenyl is smaller than crystalline forces, thus the considerable nonplanarity of the molecule disappears in the solid state. G. Bastiansen, Acta Chem. Scand., 1952, 6. 205 C. P. Brock, K. L. Haller, J. Phys. Chem., 1984, 88, 3570 G. P. Charbonneau, Y. Delugeard, Acta Crystallogr. Sect. B, 1976, 32, 1420. 

The significance of crystallinity and the relative accessibility or the rate of hydrolysis is overshadowed hy the fact that starch, which is readily hydrolyzed, provides a good yield of levoglucosan and, furthermore, the crystalline forces are weakened at the elevated temperatures. In addition to these factors, trace amounts of inorganic impurities or contamination could, as discussed later (see Section IV,2 p. 467), account for some of the observed differences. ... 

In metals, a special type of crystalline force comes into play, characterized by a largely nondirec-tional nature. Fixed bond angles do not play a major role in metals, and the stablest crystal structures for most elementary metals are those with the densest packing. Important exceptions are the Group 1 metals... 

It is obvious that the effects of retardation are due to the action of the crystalline forces... 

In highly crosslinked polymers and highly crystalline polymers the chains are held together by primary bonds and by crystalline forces. In both cases, forces are too strong to permit plasticizer to penetrate into the polymer. 

Xu Z H, Ducker W and Israelachvili J N 1996 Forces between crystalline alumina (sapphire) surfaces in aqueous sodium dodecyl sulfate surfactant solutions Langmuir 12 2263-70... 

Ultra-high vacuum (UHV) surface science methods allow preparation and characterization of perfectly clean, well ordered surfaces of single crystalline materials. By preparing pairs of such surfaces it is possible to fonn interfaces under highly controlled conditions. Furthennore, thin films of adsorbed species can be produced and characterized using a wide variety of methods. Surface science methods have been coupled with UHV measurements of macroscopic friction forces. Such measurements have demonstrated that adsorbate film thicknesses of a few monolayers are sufficient to lubricate metal surfaces [12, 181. 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Polymers can be crystalline, but may not be easy to crystallize. Computational studies can be used to predict whether a polymer is likely to crystallize readily. One reason polymers fail to crystallize is that there may be many conformers with similar energies and thus little thermodynamic driving force toward an ordered conformation. Calculations of possible conformations of a short oligomer can be used to determine the difference in energy between the most stable conformer and other low-energy conformers. 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Polymer Composition. The piopeities of foamed plastics aie influenced both by the foam stmctuie and, to a gieatei extent, by the piopeities of the parent polymer. The polymer phase description must include the additives present in that phase as well. The condition or state of the polymer phase (orientation, crystallinity, previous thermal history), as well as its chemical composition, determines the properties of that phase. The polymer state and cell geometry are intimately related because they are determined by common forces exerted during the expansion and stabilization of the foam. 

---