Yeong-Tarng Shieh

Interaction of supercritical carbon dioxide with polymers. I: Crystalline polymers

Supercritical fluid (SCF) technology involving carbon dioxide is recently receiving wide attention due to its vast potential application in various fields such as cleaning, extraction, synthesis, etc., in addition to its environmental benefits. To fully exploit the use of SCFs in new technologies, it is important to understand how SCFs interact with materials. To this end, we have undertaken a systematic study involving a wide pressure and temperature range to investigate the interaction of supercritical carbon dioxide (SC CO2) with nine different crystalline polymers, namely, substituted and unsubstituted polyethylene (four varieties), polypropylene, nylon 66, poly(ethylene terephthalate), poly(oxymethylene), and poly(vinylidine fluoride). Critical factors such as changes in appearance and weight, temperature, pressure and time of the supercritical fluid treatment, and dimension of samples have been observed. The influence of SC CO2 on the thermal properties of treated polymers has been investigated through TGA analysis. Further, changes in the mechanical properties such as yield strength, ultimate elongation, and modulus of elasticity of the investigated crystalline polymers were also observed. A discussion has been included to show the possible implications of the observed changes in realizing various applications.

Interaction of supercritical carbon dioxide with polymers. II. Amorphous polymers

In continuation of our goal to implement supercritical fluid (SCF) technology for various applications such as precision cleaning, foaming, and impregnation of materials, a systematic study has been performed involving amorphous polymers. Eleven different polymers of amorphous nature have been subjected to supercritical carbon dioxide (SC CO2) treatment under a wide pressure and temperature range (1000–3000 psi and 25–70°C, respectively). The influence and impact of such treatment on the appearance, weight change, and thermal and mechanical properties were followed systematically. In addition, the effect of treatment conditions and dimension of the samples on weight changes were also monitored. It has been found that amorphous polymers can absorb carbon dioxide to a greater extent than crystalline polymers and, in turn, the phenomenon of plasticization was also very high. In addition to morphology, the polarity of the polymer is also crucial in determining the solubility in carbon dioxide. Comparison was also made with the behavior of crystalline polymers.

Crystallization behavior, crystal transformation, and morphology of polypropylene/polybutene-1 blends

Abstract The effects of composition on crystallization behavior, crystal transformation, and morphology of crystalline/crystalline PP/PB-1 blends are complicated. The crystallization rate of PP decreases with increasing PB-1 content in the cooling or isothermal crystallization process. Avrami constants of the PP crystallization are, however, insignificantly dependent on the PB-1 content. The crystallization rate and spherulite size of PB-1 are both enhanced by PP in PB-1-rich blends. As prepared by the solution–precipitation method, pure PB-1 exhibits crystal forms I′ (or III) and II, whereas the PB-1 in the blends exhibits only crystal form I′; pure PP and PP in the blends both exhibit only crystal form α. Crystal transformation from form I′ to form II for PB-1 in the blends can occur upon dynamic heating and cooling, but the transformation is retarded with increasing PP content. The crystal form II of PB-1 can be transformed to form I upon aging at room temperature, and this transformation can be facilitated by the PP in PB-1-rich blends. The similar helix conformation, of crystal form α of PP and of crystal forms I′ and I of PB-1, is found to be a driving force in preventing crystal form I′ of PB-1 in PP-rich blends from transforming to form II upon dynamic heating and cooling, and in facilitating crystal transformation from form II to form I for PB-1 in PB-1-rich blends upon aging at room temperature.

Role of Si-H and Si-H2 in the photoluminescence of porous Si

FTIR spectra reported by Brandt et al. [Solid State Commun. 81, 307 (1992)], Tischler et al. [Appl. Phys. Lett. 60, 639 (1992)], and Tsai et al. [Appl. Phys. Lett. 59, 2814 (1991), and 60, 1700 (1992)] have shown the presence of Si‐H and Si‐H2 lines in porous Si which Canham [Appl. Phys. Lett. 57, 1046 (1990)], Tischler and Tsai have considered essential to the observation of visible photoluminescence (PL). Hydrogen has been ascribed as a surface passivating agent which eliminates nonradiative transitions thus enabling PL to occur. Photo‐oxidation has been described as replacing some of the surface passivants with oxygen, generating nonradiative transitions which quench the PL. We have markedly reduced the Si‐H and Si‐H2 content of anodized samples by treatment with CCl4 vapor. Comparison of the photoluminescent emission and excitation spectra for CCl4 treated and untreated samples show very similar results. Comparison of the FTIR spectra of photo‐oxidized samples which exhibit no PL and as‐anodized sampl...

Silane grafting Reactions of low-density polyethylene

Reactions of vinyl trimethoxysilane grafting onto low-density polyethylene (LDPE) were investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermal gravimetric analysis. The silane grafting reactions were induced by a fixed amount of dicumyl peroxide at 0.2 part of reagent per hundred parts with respect to LDPE. Fourier transform infrared data demonstrated that the extent of the silane grafting reaction was increased as the amount of silane used, the reaction time, or the reaction temperature was increased. The apparent activation energy of the silane grafting reaction was 9.7 kJ mol−1. Differential scanning calorimetry was used to follow the silane grafting reactions in situ at a heating rate of 20°C per minute. The silane grafting reaction was exothermic starting at about 150°C and ending at about 230°C, indicating a completion of the reaction in 4 min. The grafting reaction heat has linear relations to the amount of silane used. The grafting reaction heat of about 1 J/g of sample was generated during reaction per part of reagent per hundred parts of silane used. The reaction heat of silane grafting onto LDPE per mol of silane used was 14.5 kJ mol−1 silane, and the reaction heat of peroxide that reacted with LDPE was −12 kJ mol−1 peroxide.

The effect of carbonyl group on sorption of CO2 in glassy polymers

Abstract The interaction of CO2 and polymers was investigated by examining the sorption isotherms and the desorption time variation of characteristic IR absorption bands. From the time variation of the bending mode (ν2) of CO2 at near 660 cm−1 and the antisymmetric stretching mode (ν3) of CO2 at near 2340 cm−1, the CO2 within the polymer film containing carbonyl groups (e.g. poly(methyl methacrylate) (PMMA)) was much more difficult to outgas than the polymer film containing phenyl rings (e.g. polystyrene (PS)). From the red shift of the stretching mode of carbonyl groups at near 1740 cm−1 for the PMMA film upon CO2 treatments, the bonding force of the carbonyl group–CO2 interaction is much weaker than that of the conventional hydrogen bonding and Lewis acid–base interaction. The CO2 was surprisingly found to be present within PMMA for about 6 months after CO2 treatments as demonstrated by the shifting of loss maxima of dynamic mechanical analyses (DMA) toward low temperatures.

Silane grafting reactions of LDPE, HDPE, and LLDPE

Vinyl trimethoxysilane and vinyl triethoxysilane grafting reactions, induced by dicumyl peroxide, of LDPE, HDPE, and LLDPE were investigated. The apparent activation energy of vinyl trimethoxysilane grafting reactions was positive when the reactions were induced by 0.2 phr of the peroxide. The apparent activation energies were negative when 0.05, 0.1, 0.15, and 0.25 phr of peroxide were used. The extents of vinyl trimethoxysilane grafting reactions of polyethylenes were in the order of LLDPE > LDPE > HDPE, although the extents of peroxide cross-linking were in the order of LDPE > LLDPE > HDPE. As compared with vinyl trimethoxysilane, vinyl triethoxysilane produced a relatively high extent of grafting reactions of LDPE but showed a relatively low rate of water cross-linking reactions of the silane-grafted LDPE. The investigation of effects of the amount of peroxide on the vinyl trimethoxysilane grafting reaction heats demonstrated that the extent of silane grafting reactions increased proportionally as peroxide was increased until a certain amount (the value was dependent on the amount of silane used). Beyond this amount of peroxide, the silane grafting did not increase, whereas the peroxide cross-linking appeared to increase significantly with increasing amounts of peroxide.

Thermal properties of silane‐grafted water‐crosslinked polyethylene

Vinyl trimethoxysilane grafting reactions of low-density polyethylene (LDPE) were performed in an extruder, followed by crosslinking with boiled water. The thermal properties of both silane-grafted and silane-grafted water-crosslinked LDPE were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC data showed that the silane grafts on the LDPE molecules were thermal stable in the absence of moisture under 130°C under which the silane-grafted LDPE could be processed or recycled. The silane grafts on the LDPE molecules reduced the melting point of LDPE and gave rise to an endotherm shoulder at about 85°C. TGA data showed that the decomposition temperature of the silane-grafted LDPE was much higher than that of LDPE. It was demonstrated that the cause of the increase in the decomposition temperature was not due to the silane grafts but due to the peroxide-induced crosslinking reactions during the silane grafting reactions performed in an extruder. Silane-grafted water-crosslinked LDPE displayed multiple melting behavior resulting from phase separation during crosslinking of LDPE with water. The phase separation gave rise to two melting points, including one at about 94°C, and the other at about 107°C.

Thermal degradation of MDI-based segmented polyurethanes

Thermal degradations of 4,4′-diphenylmethane diisocyanate-based thermoplastic polyurethane elastomers were conducted and investigated as functions of heating conditions by using thermogravimetric analysis, ultraviolet-visible (UV-vis) spectroscopy, gel permeation chromatography (GPC), and Fourier transform infrared (FTIR) spectroscopy. The extent of degradation increased with increasing temperatures and times. The degradation was accompanied by crosslinking and was more significant under air than under nitrogen, indicating that a free-radical mechanism was involved. The degradation mainly was due to unstable hard segments and gave a red shift in the UV-vis spectra. The degradation, leading to considerable discoloration, was demonstrated by UV-vis spectroscopy, starting from 240 °C in air for 10 min. Heated in nitrogen for the same period of time, the samples did not show considerable discoloration until 280 °C. The UV-vis data suggested that the degradation occurred through cleavages of NH bonds and CH bonds on the hard segments. Chain scission of polymer main chains, as demonstrated by GPC data, occurred at a temperature as low as 200 °C in nitrogen, although cleavage of NH bonds was not detectable by UV-vis and FTIR spectroscopy at these conditions. FTIR spectroscopy also provided evidence of cleavage of NH bonds and depolymerization of urethane linkages. Irganox 1010 was found to be an efficient antioxidant.

Reactive compatibilization of PP/PBT blends by a mixture of PP-g-MA and epoxy resin

In this study, dual compatibilizers composed of the commercially available maleic anhydride-grafted polypropylene (PP–MA) and a multifunctional epoxy resin were demonstrated to effectively compatibilize the immiscible and incompatible blends of PP and poly(butylene terephthalate) (PBT). The PP–MA with a low MA content is totally miscible with PP to make the PP phase quasi-functionalized, so that the multifunctional epoxy has the chance to react with PBT and PP–MA simultaneously to form PP–MA-co-epoxy-co-PBT copolymers at the interface. These desired copolymers are able to anchor along the interface and serve as efficient compatibilizers. The compatibilized blends, depending on the quantity of dual compatibilizers employed, exhibit higher viscosity, finer phase domain, and improved mechanical properties. Epoxy does not show compatibilization effects for the PP/PBT blends without the presence of PP–MA in the blends.