从聚丙烯流动曲线估算分子量分布

聚丙烯的控制降解技术由于能够带来很好的经济效益，国内外都在进行研究．其中分子量分布是一个重要数据，它对于聚丙烯的注塑、纺丝、薄膜等加工过程都有重要的影响．分子量分布一般用重均分子量Mw和数均分子量Mn之比D=Mw/Mn来表征，称为分子量分布指数．它可以     

The Degree of Disorder in Hardwood Kraft Pulps Studied by Means of LODP

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.     

An analytical solution of a newly proposed peak broadening equation and extension of polydispersities to higher molecular weight averages in size exclusion chromatography (SEC)

Summary Herein is reported an analytical solution to the peak broadening or peak dispersion/flattening equation based on the recently proposed Instrumental Spreading Shape Function and its application to correction for imperfect resolution (inadequate peak separation and/or excessive peak broadening) for higher molecular weight averages. The relationship of these higher MW averages with the familiar Weight Average and number average molecular weights is also discussed. Criteria for perfect resolution are specified and a true molecular weight calibration curve is accordingly defined.     

Thermal analysis study of the effect of molecular weight on constrained amorphous phase in poly(phenylene sulfide)

We report a thermal analysis study of the effect of molecular weight on the amorphous phase structure of poly(phenylene sulfide), PPS, crystallized at temperatures just above the glass transition temperature. Thermal properties of Fortron PPS, having viscosity average molecular weights of 30000 to 91000, were characterized using temperature modulated differential scanning calorimetry (MDSC). We find that while crystallinity varies little with molecular weight, the heat capacity increment at the glass transition decreases as molecular weight decreases. This leads to a smaller liquid-like amorphous phase, and a larger rigid amorphous fraction, in the lower molecular weight PPS. For all molecular weights, constrained fraction decreases as the scan rate decreases.This research is supported by the U. S. Army Research Office through contract DAAH04-96-1-0009. The authors thank Hoechst Celanese for providing different molecular weight Fortron samples and Dr. George Collins for providing sample information. The authors acknowledge the assistance of Elizabeth Oyebode and Leonardo Grimaldi with sample preparation and MDSC work.     

Fluoropolyethers end‐capped by polar functional groups. IV. A novel approach to the evaluation of reactivities of hydroxy‐terminated ethoxylated fluoropolyethers in the tin(IV)‐catalyzed reactions with isophorone diisocyanate

The effect of catalyst dibutyltin dilaurate (DBTDL) on the kinetics of urethane formation reactions of α,ω‐bis(hydroxy)‐terminated fluoropolyethers Fomblin® Z‐DOL TXs (FPEs) of various molecular weights and poly(oxyethylene) glycol PEG‐400 with isophorone diisocyanate (IPDI) in hexafluoroxylene (HFX) and tetrahydrofuran (THF) at 40 °C and NCO:OH = 2:1 have been studied in a broad range of catalyst (0.10–9.00) ×10^?4 M and total reagents (10.0–60.1 wt %) concentrations. The rate of tin‐catalyzed second‐order reactions (with respect to diol and diisocyanate) was found to be proportional to the square root of catalyst concentration [DBTDL]^0.5 both in low polar (HFX) and polar (THF) solvents. Effect of catalyst saturation was revealed for all the reaction systems at higher DBTDL concentrations as well as the appearance of the limiting catalyst concentrations C_lim below which the rates of reaction were close to zero. Based on these findings new effective rate coefficients have been derived k = k_cat/(C ? C) that are independent of the total reagent concentration in the range of 10.0–60.1 wt % ([OH] = 0.10–0.91 equiv/L). This new approach highlights that the rate of the tin‐catalyzed urethane formation reactions of α,ω‐bis(hydroxy)‐terminated fluoropolyethers Z‐DOL TXs with IPDI in HFX at 40 °C and NCO:OH = 2:1 increases significantly with increasing MW of FPE from 776 up to 3405. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5354–5371, 2004     

气相渗透法中的溶质吸附效应

在气相渗透仪上，用四个低分子量有机化合物和五个已知平均分子量的窄分布聚苯乙烯低聚体，对ＶＰＯ方法中的溶质吸附效应作了研究，提出了一种对吸附效应作改正的方法．所得结果表明，在扣除溶质吸附效应后，仪器常数Ｋ无分子量依赖性，提高了测定低分子量聚合物数均分子量的准确性．     

聚对苯乙烯磺酸钠添加剂的分子量对水煤浆浆体性质的影响

选用了变质程度不同的八种煤和三种不同分子量的聚对苯乙烯磺酸钠（PSS）添加剂，详细考查了该添加剂的分子量对水煤浆浆体性质的影响。结果发现，在考查PSS相对分子量的范围内（质均分子量为5.34×104～33.39×104），八种煤的水煤浆成浆性随着分子量的增大而增加，水煤浆成浆性与PSS添加剂的平均分子量的关系可归因于添加剂在煤粒上的吸附，分子量小的PSS在煤粒上的吸附量大于分子量大的PSS；PSS分子量的增加有利于水煤浆的流变性由胀性向假塑性转变；PSS分子量的增加使水煤浆的静态稳定性得到显著的改善。     

The effects of substituent size and molecular weight on the liquid crystalline properties of poly(2-alkyl-1,4-phenylene terephthalate)s

A series of low molecular weight, thermotropic poly(2-alkyl-1,4-phenylene terephthalate)s was prepared by the solution polycondensation reaction of terephthaloyl chloride and alkylhydroquinones containing n-alkyl substituents of increasing size from methyl to dodecyl. Samples of the low molecular weight polymers so obtained were also further polycondensed in the solid state to obtain high molecular weight polymers. The liquid crystalline phase behaviors and textures were determined, and the effects of polymer structure and molecular weight on these properties are discussed. All of the polymers obtained formed thermotropic, nematic mesophases, which were less stable for the lower molecular weight polymers, as expected, than were the mesophases formed by the higher molecular weight polymers. © 1994 John Wiley & Sons, Inc.     

Diffusion-averaged molecular weights of «nonboiling» crude oil fractions

Summary The successful application of the formerly presented concept of diffusion-averaged molecular weight, M_d, to vacuum residues of crude oils, or their fractions, is described. The samples for establishing the calibration curve, logDvs. logM, were crude oil fractions with polydispersities smaller than 1.1. In a surprisingly good approximation the calibration curve is independent of the origin of these fractions if 500<M<10000. The validity of our calibration curve was demonstrated with more than 10 crude oil residues and with some of their hydrogenation residues [2].     

Degradation-fragmentation of marine plastic waste and their environmental implications: A critical review

This review critically evaluates the plastic accumulation challenges and their environmental (primarily) and human (secondarily) impacts. It also emphasizes on their degradation and fragmentation phenomena under marine conditions. In addition, it takes into account the leachability of the various chemical substances (additives) embedded in plastic products to improve their polymeric properties and extend their life. Regardless of their effectiveness in enhancing the polymeric function of plastic products, these additives can potentially contaminate air, soil, food, and water. Several findings have shown that, regardless of their types and sizes, plastics can be degraded and/or fragmented under marine conditions. Therefore, the estimation of fragmentation and degradation rates via a reliable developed model is required to better understand the marine environmental status. The main parameter, which is responsible for initiating the fragmentation of plastics, is sunlight/UV radiation. Yet, UV- radiation alone is not enough to fragment some plastic polymer types under marine conditions, additional factors are needed such as mechanical abrasion. It should be also mentioned that most current studies on plastic degradation and fragmentation centered on the primary stages of degradation. Thus, further studies are needed to better understand these phenomena and to identify their fate and environmental effects.