Die Kristallisation von Mehrkomponentensystemen aus hochpolymeren Stoffen

The differences in the crystallization behaviour between a single component system and a multicomponent system are discussed. Examples for multicomponent systems are homopolymers with a broad distribution in molecular weight, mixtures of different homopolymers, swollen polymers, block copolymers, and statistical copolymers. A distribution in molecular weights manifests itself mainly in extended chain crystallization experiments in that way that a fractionation with respect to the chain length takes place. It causes also a broadening of the melting range. The presence of a second noncrystallizable homopolymer which is miscible with the crystallizable homopolymer leads to a reduction of the melting point and a change in the glass transition temperature. The crystallization remains spherulitic. The noncrystallizable component is expelled from the crystals. If the diffusion rate of this component is large, it is also expelled from the spherulites, otherwise it is incorporated into the spherulites. When the noncrystallizable component is expelled from the spherulites, the growth rate of the spherulites decreases during growth. The temperature range in which crystallization takes place is limited by the melting point of the crystallizable component and by the glass transition temperature of the two-component system. If the crystallizable component is not dissolved completely in the noncrystallizable component, this part which is not dissolved crystallizes much more rapidly than the part which is dissolved. Below the glass transition temperature only the part which is not dissolved crystallizes. This gives a possibility to determine the solubility above the melting point. By swelling, the glass transition temperature and therewith the crystallization temperatures are decreased. When, during swelling of an amorphous sample, the glass transition temperature is decreased below the temperature where swelling is performed one observes a front of spherulites penetrating into the sample simultaneously with the swelling agent. On the other hand, when the glass transition temperature remains above the swelling temperature, one can crystallize the sample after swelling is completed by raising the temperature. As in a pure polymer, one then observes the growth of spherulites from statistically distributed centers; the growth rate of the spherulites increases however with increasing time. Block copolymers of a crystallizable component and a non-crystallizable component sometimes are not able to crystallize. This is the case, if the chains of the noncrystallizable component have a cross section which is larger than that of the chains of the crystallizable component and if, in addition, the latter chains are not so long that they can fold several times in order to compensate the difference in the cross sections. When crystallization takes place, spherulites are formed only if the amount of the crystallizable component exceeds a well defined limit. Otherwise only a diffuse birefringence is developed. In this case a much larger supercooling is necessary to crystallize the sample than in the case of spherulitic crystallization. From long period measurements one can conclude how many times each noncrystallizable chain is folded. From the melting and swelling behaviour one learns whether the noncrystallizable chains form loops or tie molecules. With statistical copolymers consisting of crystallizable units and noncrystallizable units the melting point, the rate of crystallization, the degree of crystallization at the end of the process, and the melting point decrease with increasing amount of noncrystallizable units. The noncrystallizable units are incorporated partly also into the crystals.