Shipeng Zhu

Hydrophobic Modification of Poly(Ethylene Terephthalate) with Epoxy-Modified Polysiloxane by Reactive Extrusion

Hydrophobic poly(ethylene terephthalate) (PET) was prepared by reactive blending of PET with double epoxy groups modified polysiloxane (diepsi) or multiepoxy groups modified polysiloxane (multiepsi). The structure of the modified PET was characterized by 1H NMR and intrinsic viscosity. The grafted ratio of diepsi and multiepsi was 1.3 and 0.03 wt%, respectively. With the introduction of diepsi and multiepsi to PET, the water contact angle of modified PET increased from 73° to a maximum of 106°, showing the good hydrophobicity of the modified PET. The samples of modified PET were washed with different solvents and their water contact angles did not significantly decrease, indicating good durability of the hydrophobicity. Moreover, the films of modified PET prepared by solution casting (phenol and 1, 1, 2, 2-tetrachloroethane mixed solution) showed excellent hydrophobicity with the contact angle 151°. Scanning electron microscopy images showed the surface of the modified PET films to be rougher than those of unmodified PET because the introduction of polysiloxane to PET resulted in phase separation during the solvent evaporation process.

Effects of Polymerization and Crosslinking Technologies on the Crystallization Behaviors and Gel Network of Crosslinked Polyethylene

In the present study, we investigated the influences of polymerization and crosslinking technologies on the branched-chain structures, crystallization behaviors, and gel networks of two types of low-density polyethylenes, LDPE1 and LDPE2, polymerized under different polymerization conditions in commercial large-capacity production lines. High temperature 13C NMR analysis suggests that the molar contents of both long-chain branches and short-chain branches of LDPE2 were markedly higher than those of LDPE1 due to a higher polymerization temperature for LDPE2. The high content of chain branches of LDPE2 leads to its relatively higher storage modulus, entanglement density, and gel content, as indicated by dynamic mechanical analysis. In addition, when the crosslinking temperature reached 180°C, the LDPE2 crosslinked with 2 wt% dicumylperoxide (XLPE2-D2) displayed a relatively wider processing time window and a faster crosslinking rate than XLPE1-D2. Successive self-nucleation and annealing fractionation indicated that increasing the concentration of dicumylperoxide can improve the crosslinking degree, but resulted in lower crystallinity and thinner lamellar crystals.

Effect of moisture on the measured tensile strength of polyacrylonitrile carbon fibers

The aim of this work is to investigate the effect of moisture on the measured tensile strength of polyacrylonitrile (PAN) carbon fibers immersed in epoxy matrix under various moisture conditions. Tensile tests are carried out for the specimens, and a sharp decrease in measured tensile strength of the carbon fibers/epoxy system A immersed under high-moisture conditions is observed. Scanning electron microscope (SEM) is used to observe the effect of moisture on the surface structures of specimens. The results show that the decrease in the measured tensile strength is attributed to the microvoids of epoxy matrix formed in the preparation of the specimens. And the formation of microvoids in carbon fiber/epoxy matrix depends on viscosity change and curing condition of epoxy system. During the early stage of epoxy curing, both the solvent and absorbed water in the epoxy system escape from the immersed carbon fibers and form channels in the epoxy matrix. Once the viscosity of the epoxy system increased rapidly at a low curing temperature, the channels cannot be filled quickly by the surrounding resin and form microvoids finally. In order to verify these results, epoxy system B with a high curing temperature is chosen to prepare the specimens for tensile tests, and SEM images demonstrate that there are no microvoids that can be observed on the surfaces of specimens, even at such a high-moisture condition. And the measured tensile strengths remain unaffected in both low and high-moisture conditions. These results indicate that epoxy system with a high curing temperature is suitable for preparing the specimens for tensile tests regardless of moisture condition in the surrounding air.