Improvement of barrier properties of poly(ethylene terephthalate)/organoclay nanocomposites.

摘要

The presence of nanoclays in poly(ethylene terephthalate) (PET) as an impermeable phase and for increasing the tortuous path of permeates can result in outstanding property improvements in terms of decreasing gas permeability. PET nanocomposites can be prepared by in-situ or melt-mixing methods. The latter approach is more economical, practical and environmental friendly due to the absence of monomers and/or organic solvents. The microstructure of polymer nanocomposites substantially plays an important role in the determining macroscopic properties of final products. To achieve significant performance enhancements, good dispersion of the nanoclay in the matrix and thermodynamic compatibility between the nanoclay and the polymer are required.;In this work, poly(ethylene terephthalate) (PET) nanocomposites were prepared by water-assisted melt compounding in a twin-screw extruder. In order to facilitate the diffusion of PET molecules into clay galleries, steam was fed into the extruder. Subsequently, the molecular weight (M W) reduction of the PET matrix, due to hydrolysis by water, was compensated by solid-state polymerization (SSP). Effects of feeding rate, compatibility between PET and nanoclay (Cloisite Na+, Cloisite 30B and Nanomer I.28E) and SSP on the microstructure of the nanocomposites were studied. The results revealed more delamination of organoclay platelets in PET-C30B nanocomposites processed at low feeding rate compared to those processed at high feeding rate. The presence of water resulted in a larger number of single and double layers of C30B nanoparticles as well as an increased aspect ratio of clay particles in PET nanocomposites. The effect of water on the microstructure of PET nanocomposites was strongly dependent on the nanoclay modifier. Processing with water had negative effects on the PET-I28E nanocomposites, because of its lower compatibility. Studies on the structure of PET after SSP by rheological measurements, Nuclear magnetic resonance ( 1H NMR and 13C NMR) spectra showed the linear molecular structure of PET.;The effect of organoclay concentration on the rheological, thermal, mechanical and barrier properties of the PET nanocomposites prepared by different methods was studied in the second part of this work. XRD, SEM and TEM analyses displayed intercalated/exfoliated morphology in all PET/C30B nanocomposites, with a higher degree of intercalation and delamination for the water-assisted process. Enhanced mechanical and barrier properties were obtained in PET-C30B nanocomposites compared to the neat PET. The nanocomposites exhibited higher tensile modulus and lower oxygen permeability after SSP. Elongation at break was significantly higher for SSP nanocomposites than for nanocomposites processed by conventional melt mixing. Elongation at break for conventional PET nanocomposites containing 2 wt% C30B was 6%, but for nanocomposites after SSP, the elongation at break was around 145%. Compared to the neat PET, improvements of 45% in tensile modulus and 42% in barrier behavior were found for nanocomposites containing 6 wt% C30B. The pseudo-inclusion model was used to predict the tensile modulus of PET nanocomposites and to determine the effect of intercalation microstructure on the model predictions. Moreover, the Nielsen and Bharadwaj models were employed for the prediction of permeability in the presence of organoclay.;Studies of solid-state polymerization of PET and PET nanocomposites with different concentrations of Cloisite 30B and particle size were carried out at different reaction times in the third part of this work. Moreover, the effect of particle size on the rate of SSP was also investigated. Viscometry, titration, rheological and dynamic scanning calorimetry (DSC) measurements were used to analyze the samples from SSP. The weight-averaged molecular weight (MW) of PET was shown to increase significantly following SSP. The Maron-Pierce model was utilized to evaluate the molecular weight of PET in the nanocomposites before and after SSP. It was also found that the extent and the rate of the SSP reaction in nanocomposites were lower than those for the neat PETs, due to the barrier effect of clay platelets. Titration results showed that by the presence of C30B the concentration of carboxyl groups was increased in PET nanocomposites while by SSP, these end groups were significantly reduced. Increases of half-time of crystallinity and reductions of both crystallization temperature and percentage of crystallinity with molecular weight of PET were found from DSC results.;In the last phase of this research work, PET nanocomposites containing an unmodified nanoclay (Cloisite Na+) were prepared by the slurry method. A slurry of Cloisite Na+ and water was pumped into the twin-screw extruder to be melt-mixed with PET. To improve the nanoclay dispersion carboxymethyl cellulose (CMC) and poly(ethylyene glycol)(PEG) were added to the slurry. Mechanical and barrier properties showed more improvements for the nanocomposites containing PEG. The color of the nanocomposites prepared by this method was better than those obtained by conventional melt-mixing of PET and Cloisite 30B. (Abstract shortened by UMI.).