Reorganization of Structure to Alter the Properties of PET

Abstract

This research focuses on the study of the unique behavior and properties exhibited by as-received Polyethylene Terephthalate (PET) subsequently processed by a simple precipitation method, and mainly involves the comparison of the properties of as-received and precipitated PET's at the microscopic and macroscopic levels. As-received PET is dissolved in Trifluoro Acetic acid (TFA) at 50C and then precipitated by adding the solution drop wise to acetone stirred at a very high rate. The ar-PET is by nature a slowly crystallizing polymer. Bulk PET has been observed to reorganize both morphologically and conformationally by the precipitation method used.

Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR), and X-ray studies of precipitated (ppt) PET show structures and morphologies that are different from those of the samples obtained by ordinary solution and melt processing techniques. Unlike as-received PET, the ppt PET is apparently repeatedly and rapidly crystallizable from the melt, with non-crystalline portions of the sample that do not evidence a glass transition nor a crystallization peak during the heating DSC scan. DSC results suggest that the precipitation method has transformed PET into a repeatedly, rapidly crystallizable polymer that is found to achieve very high levels of crystallinity even when crystallized from the melt at a very high cooling rate. The fact that the precipitated PET has high levels of crystallinity is also supported by FTIR analysis, which shows a higher amount of trans conformer. The reorganization induced in the ppt PET is different from ordinary solvent induced crystallization.

Shrinkage of films made from both PET's is observed with an increase in temperature. Heating the as-received film, prepared by melt pressing and rapid quenching in water, results in the abrupt shrinking of the material at around the glass transition temperature of the polymer, whereas the ppt PET film continues to contract at a controlled rate with rising temperature. This suggests that the amorphous regions in the ppt PET film are organized differently than that of the as-received material. Density measurements also support the fact that the non-crystalline regions in ppt-PET are different from the amorphous portions of ar-PET. Atomic Force Microscopy suggests a molecular-level difference between the precipitated and the as-received PET's even in their melts. Melting of the ppt PET has not erased the structural organization. Preliminary observations of the stress-strain behavior of their films indicate that as-received PET exhibits tough plastic behavior characteristic of glassy amorphous polymers, whereas ppt PET shows more rigid behavior similar to semi-crystalline polymers like isotactic polypropylene. Characterization of macroscopic properties, such as rheology using the Minimelter, indicates that the melt viscosity of ppt PET is less than that of the as-received sample, which will make processing easier. Blending of the as-received and the precipitated PET's has also been done.

Overall it is observed that ppt-PET is different from ar-PET in terms of both its microscopic (organization and conformation) and macroscopic behaviors.