Changes in Morphology with Crystallization Temperature

It is of course well-known that the morphology of a crystal depends in an clearly visible way on the crystallisation temperature, or more precisely the degree of undercooling. Primarily, this is caused by the temperature dependant rate at which molecules get attached and integrated into a crystalline structure. The slower this rate, the simpler are the crystalline structures in terms of morphology. However, we want to emphasize that even crystals of rather complex morphology like dendrites may stUl be single crystals where all molecules are perfectly ordered with respect to each other. 

We investigated crystallization of polymers by following the process in real time under a microscope. In a first approach, we have chosen films of thickness significantly larger than the thickness of one lamella. In Fig. 11.1, a typical result, reminiscent of spherulitic growth, is shown. 

We note that such a transition from a circular to a quadratic envelope of the crystals has also been reproduced by computer simulations [13,14]. There, this transition is due to the reduction of the growth front nucleation probability. Wliile the disk-like pattern consists of multiple crystals, the square-shaped pattern represents a single crystal. We thus assume that, for the given film thickness, we observed a transition from a poly crystalline structure to a single crystal within the temperature interval from 45 to 50°C. 

Wlien increasing the temperature to Tc = 55° C (Fig. 11.3), growth proceeded similar to Fig. 11.2. This can be seen from the dominance of the diagonals. However, the hierarchy of side-branches is less visible because there are fewer but wider side-branches and the gaps between them have been almost completely filled. This was possible as, at such slow growth, transport was not the limiting step. In addition, as more perfect crystals could be formed, the probability for the nucleation of screw dislocations was reduced. It should be 

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