Effects of shear stress on the crystallization of linear polyethylene and polybutene‐1
Authors: Haas T. W. , Maxwell B. 
Published: December 2004

The effects of shear stress on the crystallization kinetics and morphology of linear polyethylene and polybutene‐1 were studied with the aid of a specially designed apparatus. With this equipment, it was possible to heat a thin polymer sample between glass slides to a melt temperature, quench the sample to a crystallization temperature, and then deform the sample in shear by applying a constant load to one of the glass slides. During the deformation, the crystallization process was observed and photographed under a polarizing microscope. Also, the displacement of the glass slide was simultaneously recorded which made possible a determination of the shear strain as a function of time.

The results demonstrate that two phenomena may occur in the initially supercooled polymer samples in response to the applied shear stress. In one case, the sample deformed until it fractured, generally exhibiting no evidene of crystallization; in the other, the sample deformed until an inflection point was reached after which the sample became rigid. This latter phenomenon was attributed to crystallization.

At low shear stresses, the inflection point was associated with the growth of spherulites which simply became large enough to bridge the glass slides and prevent further deformation of the sample. This generally occurred prior to the completion of the radial growth of the lamellae.

At high shear stresses, however, no evidence of crystallization was seen in the microscope until the inflection point was reached. At this point, birefringence was observed in the sample. The resulting structure generally could not be resolved in the microscope, thereby indicating very profuse nucleation.

The results obtained clearly demonstrate that the application of a sufficiently high shear stress to an initially supercooled melt has a substantial effect on the rates of crystallization of both polyethylene and polybutene‐1. This was shown most dramatically at temperatures close to the melting point, e.g., both polyethylene at 130°C and polybutene‐1 at 113°C, which require over 104 sec to crystallize under quiescent conditions, crystallized at approximately 0.05 seconds.

The application of a shear stress to a polymer melt is envisaged as resulting in molecular orientation. In accord with the theories of Flory, and Krigbaum and Roe, the associated decrease in entropy of the melt may be considered to increase the supercooling. Under high stresses at which large increases in supercooling result, crystallization occurs more rapidy at the high temperatures and with polymers of lower molecular weight. At low shear stresses, the influence of temperature and molecular weight on the crystallization kinetics is essentially the same as that obseved under quiescent conditions.

Observations through the microscope have shown that the application of a shear stress to a polymer melt leads to large increases in the number of crystalline structures formed and to the formation of oriented morphologies. This latter phenomenon arises due to nucleation lines formed by impurities and spherulites in the deforming melt. The impurities and spherulites apparently cause a disturbance which is thought to result in a local increase in stress of the melt and, hence, a local increase in supercooling. Lamellae then nucleate on these lines and grow out radially.