The structure−property evolution of commercial poly(ethylene terephthalate) (PET) fibers obtained from the different drawing and heat-setting stages in industrial processing was systematically investigated. Upon combination of thermal analysis (DSC and DMA) with crystallization and orientation (WAXD and SAXS), the variation of crystallization and microstructures mainly containing lamellar and microfibrillar crystals following the processes were discussed in connection with properties. Results indicate the significant tenacity increase of fiber in the drawing process is mainly attributed to the orientation development of the interlamellar amorphous region, the interfibrillar extension of amorphous molecular chains, and its entanglement with the lamellae. Accordingly, a decline of shrinkage can be seen as a fact of the coiling of amorphous molecular chains, the formation of rigid amorphous fraction, and the increase of crystallinity. Thus, a new four-phase model has been proposed to clarify the structure−property relationships of the commercial PET industrial fibers.

Three kinds of modified poly(ethylene terephthalate) (PET) were prepared by solution blending combined with melt post-polycondensation, using 4,4′-thiodiphenol (TDP), 4,4′-oxydiphenol (ODP) and hydroquinone (HQ) as the bisphenols, respectively. The effects of TDP, ODP and HQ on melt post-polycondensation process and crystallization kinetics, melting behaviors, crystallinity and thermal stability of PET/bisphenols complexes were investigated in detail. Excellent chain growth of PET could be achieved by addition of 1 wt% bisphenols, but intrinsic viscosity of modified PET decreased with further bisphenols content. Intermolecular hydrogen bonding between carbonyl groups of PET and hydroxyl groups of bisphenols were verified by Fourier transform infrared spectroscopy. Compare to pure PET, both the crystallization rate and melting temperatures of PET/bisphenols complexes were reduced obviously, suggesting an impeded crystallization and reduced lamellar thickness. Moreover, the structural difference between TDP, ODP and HQ played an important role on crystallization kinetics. It was proposed that the crystallization rate of TDP modified PET was reduced significantly due to the larger amount of rigid benzene ring and larger polarity than that of PET with ODP or HQ. X-ray diffraction results showed that the crystalline structure of PET did not change from the incorporation of bisphenols, but crystallinity of PET decreased with increasing bisphenols content. Thermal stability of modified PET declined slightly, which was hardly affected by the molecular structure of bisphenols.

Time‐resolved rheometry and size exclusion chromatography system [advanced polymer chromatography (APC)‐multi‐angle laser light scattering (MALLS)‐refractive index detector (RID)] were employed to investigate post‐polycondensation process of nonstirred high‐viscosity poly(ethylene terephthalate) (PET) of molten film forming by high‐flow inert gas sweeping at atmospheric pressure. In view of chemical reaction kinetics together with mass transfer to adjust reactive parameters, including moisture content of the sample, temperature, concentration of the active groups, residence time, and mass transfer factor, this work aims to distinguish the different reactions in reprocessing of high‐viscosity PET. Results indicated that hydrolysis is observed initially before thermal degradation and polycondensation reactions on remelting process. Promotion of thermal degradation is stronger than polycondensation with the increase in temperature. Fiber‐grade PET exhibits polycondensation as the dominant reaction at 275 °C in a reasonable reaction time, which leads to the increase of average molecular weight and the phenomenon of double distribution proved by APC‐MALLS‐RID system. However, the average molecular weight of industrial yarn‐grade PET is very difficult to improve when the thermal degradation reaction is not effectively inhibited. Importantly, the experimental study of the interfacial mass‐transfer area and mass‐transfer intensification demonstrated that mass transfer is the rate‐determining factor for the overall post‐polycondensation of high‐viscosity melt. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47484.

Melt polycondensation has recently been reported to prepare high‐viscosity poly(ethylene terephthalate) (PET), the reaction efficiency is greatly improved in over 10‐folds compared with conventional solid state polycondensation (SSP). Melt postpolycondensation of common PET chips was conducted in specified film thickness to obtain industrial PET. Based on the investigation of reaction conditions, film reaction kinetics were determined in the principle of end groups analysis. It was positively regulated that the intrinsic viscosity of PET could be achieved in condition of high vacuum, thin melt film and proper temperature, degradation reaction would be increased at exorbitant temperature. An apparent reaction kinetic model was proposed and was verified by experiments. Results indicated the activation energy of melt postpolycondensation of PET was 88.22 kJ/mol and the reaction rate constant was significant higher than that of solid state polycondensation.