Qian Xing

Phase morphology, crystallization behavior and mechanical properties of poly(L-lactide) toughened with biodegradable polyurethane: Effect of composition and hard segment ratio

Polyester-based biodegradable polyurethane (PU) with different hard segment ratios was selected to modify the impact toughness of poly(L-lactide) (PLLA). The influence of blending composition and hard segment ratio of PU on the phase morphology, crystallization behavior and mechanical properties of PLLA/PU blends has been investigated systematically. The results showed that the PU particles were uniformly dispersed in PLLA matrix at a scale from submicrons to several microns. The glass transition temperature of PU within these blends decreased compared to that of neat PU, but rose slightly with its content and hard segment ratio. The presence of PU retarded the crystallization ability of PLLA, whereas enhanced its elongation at break and impact resistance effectively. As the PU content reaches up to 30 wt%, the phenomenon of brittle-ductile transition occurred, resulting in a rougher fracture surface with the formation of fibril-like structure. Moreover, under the same concentrations, the elongation at break and impact strength of PLLA blends decreased slightly with the increase of hard segment ratio of PU.

Hydrogen-bonding induced change of crystallization behavior of poly(butylene succinate) in its mixtures with bisphenol A

In the present work, the blend of poly(butylene succinate) (PBS) and bisphenol A (BPA) was prepared by solution mixing, and the intermolecular interactions between the two components were characterized by a combination of nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results showed that intermolecular hydrogen-bonding forms between the carbonyl group of PBS and phenol hydroxyl of BPA. With the increase of BPA content, more hydrogen bonds were formed. The effect of hydrogen bonding on the crystallization behavior of PBS was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results showed that the overall isothermal crystallization kinetics and the spherulite growth rate of PBS decrease with the increase of BPA content, while the PBS spherulite size increases with BPA content.

Effect of solubility of a hydrazide compound on the crystallization behavior of poly(L-lactide)

Tetramethylenedicarboxylic di-(2-hydroxybenzohydrazide) (TMBH) has been proved to be an effective nucleating agent for poly (L-lactide) (PLLA). The correlation between the solubility of TMBH in a PLLA melt during the annealing treatment and its nucleating efficiency has been systematically investigated. The phase diagrams of the binary system consisting of PLLA and TMBH were constructed by differential scanning calorimetry and polarized optical microscopy. Depending on the annealing temperature (Tf) and TMBH concentration (CTMBH), TMBH may remain crystallites, or partially/completely dissolve into a PLLA melt, which greatly affected the crystallization kinetics and crystalline morphology of PLLA. When Tf was relatively lower and/or TMBH concentration was higher, TMBH crystallites remained solid in the PLLA melt after annealing treatment (e.g. Tf@180 °C and CTMBH@1 wt%), which could effectively enhance the crystallization rate and nucleation density of PLLA. The solubility of TMBH in the PLLA melt would be increased gradually with the increase of Tf and/or the decrease of CTMBH, e.g. TMBH would be partially dissolved into the PLLA melt for TMBH-1 at 200 °C. Furthermore, TMBH would be completely dissolved into the PLLA melt as Tf was raised to 230 °C with CTMBH ranging from 0.1 to 1 wt%. In this case, the soluble TMBH molecules would assemble into rod-like structures with lower specific area during the cooling process, which decreased the nucleating efficiency of TMBH and produced rod-like PLLA crystallites.

Correlation between chain microstructure and mechanical properties of two polypropylene/poly (ethylene-co-propylene) in-reactor alloys

This study concentrated on evaluating the structure-property correlation of two polypropylene/poly (ethylene-co-propylene) in-reactor alloys (A and B). Systematic investigation and comparison have been performed on the molecular structure and phase morphology of the polypropylene (PP)-based alloys. Especially, the bivariate distribution of chemical composition and molecular weight for each fraction was obtained simultaneously by adopting cross fractionation chromatography. The results illustrated the following structure difference which determines the ultimate mechanical properties of the PP in-reactor alloys. It has been found that the relative content of components polypropylene homopolymer (HPP) and ethylene-co-propylene random copolymer (EPR) is a little more in sample A than those in sample B, and meanwhile both of them in sample A show higher molecular weight and broader molecular weight distribution. Furthermore, the fraction of HPP in sample A eluted at a higher temperature, meaning that this fraction possesses better stereo-tacticity and can form more perfect crystals with higher crystallinity. These advantages in structure characteristics endow sample A with better rigidity-toughness balance than sample B.

Deformation investigation on iPP/SiO2 composites: Influence of stretching temperature and particle size on morphology evolution and crystalline structure of thin films

In the present work, structure changes during stretching of isotactic polypropylene (iPP) and iPP/silicon dioxide (SiO2) composites have been investigated systematically. The α-form crystal structure of both iPP and iPP/SiO2 composites is destroyed and transforms into the mesophase as the samples are stretched at a low temperature (35°C), while stretching at high temperatures (90°C and 120°C) can restrain the appearance of defects and keep the perfection of crystal structure. FTIR results reveal that the stretching temperatures show no obvious difference of the effect on the orientation of pure iPP, however, the orientation of iPP/SiO2 composites is greatly changed by the tensile temperature. In the case of micron-sized SiO2 particles (average particle diameter d > 1 μm), the orientation of the composites is lower than that of pure iPP at all stretching temperatures. The above results suggest that the stretching temperature and the SiO2 particle size have great influence on the structure variation and orientation behavior of iPP/SiO2 composites.

Photosynthesis, transpiration, and water use efficiency of Leymus dasystachys on the Hunshandake desert

Net photosynthetic rate (PN), transpiration rate (E), and stomatal conductance (gs) declined from upper leaves to the lower ones during dry and rainy seasons, indicating that long-term carbon budget should take into account PN variations for different leaf types. Relatively greater PN in the dry season suggested that this species is more able to maintain higher PN under drought, but the relatively higher E in the dry season might reduce water use efficiency (PN/E) for the species. Significant correlations between PN and gs indicated that gs may be the critical factor for PN variability in the desert region.