Factors Affecting the Crystalline Melting Point

Further information on the effect of polymer structure on melting points has been obtained by considering the heats and entropies of fusion. The relationship between free energy change AF with change in heat content A// and entropy change A5 at constant temperature is given by the equation 

In thermodynamic language it is said that a reaction will occur if there is a decrease in the free energy, i.e. AF is negative. Since at the melting point melting and crystallising processes are balanced AF is zero and the expression may be written 

Where is the melting point Af/n, the heat of fusion AS , the entropy of fusion 

The entropy term is a measure of the degree of freedom of the molecules and thus a measure of its flexibility. Measurement of the heats and entropies of fusion has provided interesting information on the relative importance of various factors 

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The values of the heat of fusion,aHf and the melting point, Tm for the copolymers shown in Pig. 4. and Fig. 5. are very typical for ethylene copolymers in this composition range. Their values in SPC was always higher than those in CPC. That is to say, the crystallinity of the copolymer obtained by SPC was always larger than those obtained by CPC for the same comonomer content in the copolymer. It is expected that the crystallinity be smaller for higher comonomer content in the copolymer. But the crystallinity will be affected by various factors, such as the... 

The screw design and elements used to melt the formulation depends on the formulation particle size (pellets versus powder), the resin softening temperature or melting point (amorphous versus crystalline), and whether the polymer viscosity is heat or shear sensitive. Other factors to be considered include liquid addition, lubricity of the formulation, and other additives present that may affect the screw s ability to feed and forward convey. 

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