Influence of early thermal-oxidative ageing on the structure and properties of polyoxymethylene copolymer.

Thermal-oxidative ageing of polyoxymethylene (POM) copolymer in the oven at 100°C for 1, 2, 3, 5, 7, 10, 14 and 21 days and the influence of early thermal-oxidative ageing on POM structure and properties were studied by means of wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry and tensile test. Based on the results, we found that the early thermal-oxidative ageing of POM copolymer can be divided into three regions. The region I is the initial 3 days. In this region, some molecular chains rearranged, resulting in internal stress relaxation, increase of crystallinity degree and grain size due to the perfection of crystal structure; both extended chain crystal (ECC) and folded chain crystal (FCC) increased and ECC grew faster than FCC. The region II is from 3 days to 10 days, and in this region, chain scission took place in amorphous region and led to chemi-crystallization. The region III is after 10 days. In this region, the structure and performance of POM copolymer reached a stable situation at this stage. In this work, the difference between skin and core were also analysed.

Facile Method to Fabricate Highly Thermally Conductive Graphite/PP Composite with Network Structures.

Thermally conductive polymer composites have aroused significant academic and industrial interest for several decades. Herein, we report a novel fabrication method of graphite/polypropylene (PP) composites with high thermal conductivity in which graphite flakes construct a continuous thermally conductive network. The thermal conductivity coefficient of the graphite/PP composites is markedly improved to be 5.4 W/mK at a graphite loading of 21.2 vol %. Such a great improvement of the thermal conductivity is ascribed to the occurrence of orientations of crystalline graphite flakes with large particles around PP resin particles and the formation of a perfect thermally conductive network. The model of Hashin-Shtrikman (HS) is adopted to interpret the outstanding thermally conductive property of the graphite/PP composites. This work provides a guideline for the easy fabrication of thermally conductive composites with network structures.

Molecular characterization of a gene for aldose reductase ( CbXYL1) from Candida boidinii and its expression in Saccharomyces cerevisiae.

Candida boidinii produces significant amounts of xylitol from xylose, and assays of crude homogenates for aldose (xylose) reductase (XYL1p) have been reported to show relatively high activity with NADH as a cofactor even though XYL1p purified from this yeast does not have such activity. A gene coding for XYL1p from C. boidinii (CbXYL1) was isolated by amplifying the central region using primers to conserved domains and by genome walking. CbXYL1 has an open reading frame of 966 bp encoding 321 amino acids. The C. boidinii XYL1p is highly similar to other known yeast aldose reductases and is most closely related to the NAD(P)H-linked XYL1p of Kluyveromyces lactis. Cell homogenates from C. boidinii and recombinant Saccharomyces cerevisiae were tested for XYL1p activity to confirm the previously reported high ratio of NADH:NADPH linked activity. C. boidinii grown under fully aerobic conditions showed an NADH:NADPH activity ratio of 0.76, which was similar to that observed with the XYL1p from Pichia stipitis XYL1, but which is much lower than what was previously reported. Cells grown under low aeration showed an NADH:NADPH activity ratio of 2.13. Recombinant S. cerevisiae expressing CbXYL1 showed only NADH-linked activity in cell homogenates. Southern hybridization did not reveal additional bands. These results imply that a second, unrelated gene for XYL1p is present in C. boidinii.

Transposon mutagenesis to improve the growth of recombinant Saccharomyces cerevisiae on D-xylose.

Saccharomyces cerevisiae L2612 transformed with genes for xylose reductase and xylitol dehydrogenase (XYL1 and XYL2) grows well on glucose but very poorly on d-xylose. When a gene for d-xylulokinase (XYL3 or XKS1) is overexpressed, growth on glucose is unaffected, but growth on xylose is blocked. Spontaneous or chemically induced mutants of this engineered yeast that would grow on xylose could, however, be obtained. We therefore used insertional transposon mutagenesis to identify two loci that can relieve this xylose-specific growth inhibition. One is within the open reading frame (ORF) of PHO13, and the other is approximately 500 bp upstream from the TAL1 ORF. Deletion of PHO13 or overexpression of TAL1 resulted in a phenotype similar to the insertional mutation events. Quantitative PCR showed that deletion of PHO13 increased transcripts for TAL1, indicating that the growth inhibition imposed by the overexpression of XYL3 on xylose can be relieved by an overexpression of transcripts for downstream enzymes. These results may be useful in constructing better xylose-fermenting S. cerevisiae strains.

Optimal growth and ethanol production from xylose by recombinant Saccharomyces cerevisiae require moderate D-xylulokinase activity.

D-Xylulokinase (XK) is essential for the metabolism of D-xylose in yeasts. However, overexpression of genes for XK, such as the Pichia stipitis XYL3 gene and the Saccharomyces cerevisiae XKS gene, can inhibit growth of S. cerevisiae on xylose. We varied the copy number and promoter strength of XYL3 or XKS1 to see how XK activity can affect xylose metabolism in S. cerevisiae. The S. cerevisiae genetic background included single integrated copies of P. stipitis XYL1 and XYL2 driven by the S. cerevisiae TDH1 promoter. Multicopy and single-copy constructs with either XYL3 or XKS1, likewise under control of the TDH1 promoter, or with the native P. stipitis promoter were introduced into the recombinant S. cerevisiae. In vitro enzymatic activity of XK increased with copy number and promoter strength. Overexpression of XYL3 and XKS1 inhibited growth on xylose but did not affect growth on glucose even though XK activities were three times higher in glucose-grown cells. Growth inhibition increased and ethanol yields from xylose decreased with increasing XK activity. Uncontrolled XK expression in recombinant S. cerevisiae is inhibitory in a manner analogous to the substrate-accelerated cell death observed with an S. cerevisiae tps1 mutant during glucose metabolism. To bypass this effect, we transformed cells with a tunable expression vector containing XYL3 under the control of its native promoter into the FPL-YS1020 strain and screened the transformants for growth on, and ethanol production from, xylose. The selected transformant had approximately four copies of XYL3 per haploid genome and had moderate XK activity. It converted xylose into ethanol efficiently.