Intracatenary Forces

The macroconformation of chainlike macro molecules in crystalline polymers is principally determined by two factors, which are inter- and intracatenary forces. Calculation of potential barriers of isolated molecules, that is, in a vacuum, is based exclusively on intracatenarily effective forces (see also Section 4.1.2). Microconformations calculated in this way correspond to an internal energy minimum. According to the equivalence principle, all structural units should adopt geometrically equivalent positions in relation to the crystallographic axes, whereby a monomeric unit, for example, may serve as a structural unit. Thus the regular sequence of microconformations should lead to a regular macroconformation. 

The question is now whether intercatenary effects can lead to changes in the microconformation caused by intracatenary forces, and, if so, to what extent. Of course, intercatenary forces quite definitely affect the mutual packing of main chains, and so, give rise to density differences. But the 

The chains try to pack as densely as possible. The minimum interchain distances produced thereby are determined by the van der Waals radii. So, with the aid of the equivalence principle, knowledge of these radii allows the macroconformation in the crystalline state to be estimated even though knowledge of the nature and effect of individually active forces may be absent. 

The distance between H atoms on adjacent carbon atoms in the T conformation of polyethylene can be calculated as 0.25 nm from the bond length of 0.154 nm and valence angle of 109.6. This distance is greater than the sum of the van der Waals radii of 0.24 nm for the two hydrogen atoms. Consequently, crystalline poly(ethylene) occurs in the T conformation. 

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The conformation of molecules is determined in the gaseous state completely, and in the crystalline state almost completely by intracatenary forces. Solvents interact with solutes, and, thus, can alter their conformations. The effect of this interaction, however, is weak when the solute is surrounded by like molecules. Thus, butane has practically the same conformation energy in the gaseous state, 3.35 kJ/mol, as in the liquid state, 3.22 kJ/mol. 

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