Crystallization and melting behaviour of segmented thermotropic polymers: 3. Analysis of two polyesters based on 4,4′-(alkanedioyldioxy)dibenzoyl dichlorides and hydroquinones

Abstract

A differential scanning calorimeter has been used to investigate the crystallization and melting behaviour of two segmented nematic polyesters based on 4,4′-(alkanedioyldioxy)dibenzoyl dichlorides and hydroquinones. Isothermal crystallizations from the nematic melt have been carried out on both samples. The results are discussed in terms of variation of the successive melting profiles as a function of various parameters, such as the crystallization temperature and time and, in more detail, the conditions of the preliminary treatment in the nematic state. The complex melting behaviour evidenced by the polyesters can be justified on the basis of a new scheme, introduced in our previous paper, which accounts for the slow evolution of the nematic melt towards its thermodynamic equilibrium. Above the crystal-nematic transition a certain registry of neighbouring chains persists in the melt, which becomes poorer and poorer upon increasing temperature and time and eventually disappears when equilibrium is attained. These results raise some fundamental questions on the ability of thermotropic polymers to crystallize from the nematic melt and underline that a careful adjustment of the process parameters must be achieved to obtain reproducible results and to control the final properties of the material.