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By D East, G C Margerison

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As would be expected from this account, the percentage of the polymer present in crystalline regions depends to some extent on its thermal history; for example, whether it was cooled slowly or rapidly from the liquid state. In that some of the physical properties of the solid such as its density and melting point are dependent on its crystallinity, t this fact gives rise to a certain indeterminacy in these quantities for a given polymer. Some polymers show a much greater tendency to solidify with high degrees of crystallinity than do others.

We start with the general statement that every physico-chemical system at equilibrium has a definite value of an energy parameter G (the Gibbs Free Energy) in just the same way as it has a definite volume V. The actual value taken by G (like V) depends on the temperature and pressure of the system as well as on the number of moles present. The importance of G is simply this : if we mix two systems with G 28 AN INTRODUCTION TO POLYMER CHEMISTRY values Gx and G2 in a thermostat to keep the temperature constant and under constant pressure, then only those changes occur, be they physical or chemical, which result in the value of G for the mixture being less than G^-fG^· If a change does occur, the new equilibrium position corresponds to the lowest value of G accessible to the particular system.

The situation becomes considerably more complicated, however, when the temperature is lowered and this translational INTRODUCTION FIG. 11. freedom is lost. With familiar low molecular weight liquids, this loss of translational freedom occurs at a definite temperature known as the freezing point—the random structure of the liquid being replaced by the ordered arrangement of the crystalline solid. For the liquid polymer, the analogous process in which all the chains disentangle themselves and line up to form an ordered array is obviously highly unlikely—in other words perfectly crystalline polymers cannot be obtained by simply cooling the melt.

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