Rheological and Mechanical Relaxation Behavior of a Thermally Crosslinkable Poly(Ethylene Terephthalate)

Poly(ethylene terephthalate) PET, is a material of great commercial importance, in the form of fibers, bottle resin, and to a lesser extent, as a molding resin. In 1990, the annual world production of PET fibers was about 9 million metric tons and the annual world production of bottle resin was 1.2 million metric tons (1). Owing to the widespread use of PET, investigations of the decomposition routes and methods for reducing it flammability (Limiting Oxygen Index (LOI) ' 18%) have been a subject of significant effort over the past several decades (2). Past approaches for improving flame retardancy have included compounding with inorganic powders, such as antimony or phosphorous compounds, as well as halogen substitution along the polymer chains. Recognizing the strong correlation between the tendency of a polymeric material to crosslink at elevated temperature and resulting “self-extinction,” we have recently focused on the preparation of PET copolymers containing residues which upon exposure to extreme environments would lead to crosslinking and, in turn, decrease melt flow substantially. It is anticipated that such a reduction in melt flow would aid in the formation of a char and cessation of flame propagation (3). This effect has been demonstrated recently by incorporating a terephthalic acid derivative of benzocyclobutene (BCB), XTA, along the backbone of PET, the result being a favorable increase in LOI, from 18% to 33% (3). We note that materials with LOI . 27% exhibit self-extinction under ambient atmospheric conditions. The addition of latent crosslinking groups along the PET backbone is expected to affect not only flame retardancy, but rheological and mechanical properties as well. Previous research by the authors on thermotropic liquid crystalline polymers (LCPs) also containing XTA as a comonomer has shown that the time dependence of the rheological material functions at elevated temperature, as well as solid-state microstructure, are strongly affected by the mole fraction of XTA present in the system (4). In a similar fashion, many material characteristics are likely to be affected by XTA copolymerization in PET. One possibility is that Rheological and Mechanical Relaxation Behavior of a Thermally Crosslinkable Poly(Ethylene Terephthalate)