Chain Extension Of Polyethylene Terephthalate And Membrane Preparation Via Thermally Induced Phase Separation

Poly(ethylene terephthalate) (PET) has been widely used in packaging fields due to its transparency, chemical resistance, and barrier properties. However, during melt processing the molecular weight of PET may be easily reduced when subjected to thermal oxidation and hydrolytic degradation. Both degradations lead to reduction in molecular weight and an overall decrease in melt viscosity and melt strength, which limited its processing and application.In the research work of this group, bisphenol-A dicyanate (BADCy) was tried as an extender of PET and compared with conventional ones like diglycidyl ether of bisphenol-A (DGEBA) and methylenediphenyl diisocyanate (MDI). It was found that BADCy being more effective and worthy further studying. FTIR was employed to confirm the reaction between the PET and BADCy during the chain extension. The extending effect was characterized with torque, intrinsic viscosity, and the carboxyl content. As a result, a longer time of chain extension did not result in higher molecular weight, because of the negative effect of reaction heat. The more the amount of BADCy introduced the higher and the earlier the torque maximum exhibited. The enhanced stirring rate enhanced a higher rate of the chain extension reactions, which led to higher temperatures. However, the high temperature accelerated the chain cleavage. Measurement of MFR, storage modulus and dynamic viscosity showed that BADCy indeed extended the molecular weight of PET and reduced mobility of the extended PET. DSC analysis represented that less perfect crystallization of the extended PET for the difficulty in crystallization, lower melting temperatures.It was noticed that BADCy could undergo a cyclotrimerization forming a tri-mer. Such a tri-mer possesses three reactive sites and 3-fold larger molecular weight than the BADCy monomer. It was natural to consider that the tri-mer might be a potential chain extender for PET and the idea was tried in this work.The content of tri-mers was determined using FTIR. The average molecular weights (Mn and Mw) and polydispersity of BADCy tri-mers were determined by GPC for investigating the details of cyclotrimerization. The melt torque, intrinsic viscosity, carboxyl content, and gel content of modified PET were characterized. With increasing the cyclotrimerization reaction temperature of BADCy monomer, the conversion to tri-mers was increased. With a fraction of BADCy tri-mer in the chain extender, the modified PET exhibited higher melt torque, intrinsic viscosity, melt modulus, and melt viscosity than those modified BADCy monomer alone. BADCy tri-mer had a positive effect on the chain extension of PET. However, the fraction of tri-mers in the monomer/tri-mer mixture should be limited to a certain value, otherwise the chain extension would be weakened.In this Paper, the PET membrane was obtained by thermally induced Phase separation (TIPS). The solvent of dimethyl sulfoxide (DMSO), diphenylmethanone and N-methyl pyrrolidone (NMP) were selected as the diluents of PET in TIPS process. SEM was employed to study the structure and morphology of membranes. It was found that the membrane morphology can be controlled with different diluents for the interactions between polymer and diluents. The membrane of cellular structure and lacy structure were formed by liquid-liquid phase separation in PET/NMP and PET/DMSO system with weak interactions, respectively, while the particulate structure was formed by solid-liquid phase separation in PET/di-phenylmethanone system with strong interaction. The pore or particulate size decreased with the increasing of PET concentration and molecular weight. As the cooling temperature increased, the pore or particulate size increased.