(Keynote) Dimensional Change of Oriented Polymers upon Heating

Abstract

Molecular orientation can be incorporated to the polymeric materials through either stretching in a solid state or elongation in a molten state. When a polymer has molecular orientation in a glassy state, shrinkage may proceed when temperature is increased above its glass transition temperature. Degree of shrinkage may vary depending on the temperature reached and the degree of molecular orientation. In the case of crystalline polymers, however, shrinkage may be suppressed because of the occurrence of crystallization prior to the shrinkage. It should be noted that the crystallization rate can be accelerated significantly with the increase of molecular orientation. Accordingly, there can be a critical degree of molecular orientation; below which orientation relaxation proceeds upon heating, whereas above which spontaneous orientation enhancement caused by crystallization proceeds upon heating. In some cases, even the spontaneous elongation of materials takes place along with the progress of crystallization. On the other hand, it is well known that the dynamics of polymeric materials is characterized by the wide range of relaxation time. This means that the state of molecular orientation can be manipulated through the combination of deformation/flow history and temperature history. Dimensional change of oriented polymeric materials upon heating is mainly governed by the oriented structure of relatively long relaxation time such as network structure of molecular entanglement. Based on this concept, we could prepare fibers which showed no birefringence but exhibited significant shrinkage with the increase of temperature. This is possible because birefringence merely reflects the degree of orientation of relatively short segments, i.e. short relaxation time, in the polymer chain. Concept of manipulating the oriented structure of wide range of relaxation period leads to the idea of controlling the state of molecular entanglement. Based on this concept, we speculated that the polymeric materials of high strength and high toughness can be prepared if the material has rather homogeneous state of molecular entanglement, i.e. narrow distribution of molecular weight between adjacent entanglement points. As-spun poly(ethylene terephthalate) (PET) fibers with rather homogeneous state of molecular entanglement was prepared through the control of melt spinning process. Applying the drawing and annealing processes to such as-spun fibers, high-strength PET fibers were prepared. To verify the applicability of the concept of homogeneous state of molecular entanglement, PET fibers prepared under various processing conditions were analyzed using the laser Raman spectroscopy. It was found that the narrow distribution of internal stress field and limited widening of such distribution upon applying the tensile stress to the fibers are the key factors for realizing the high-strength and high-toughness.