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681.
Mitsuhiro Nakae Hiroki Uehara Tetsuo Kanamoto Toshio Ohama Roger S. Porter 《Journal of Polymer Science.Polymer Physics》1999,37(15):1921-1930
The drawing behavior of the ultra‐high molecular weight polyethylene (UHMW‐PE) melts has been studied by comparing the stress/strain curves for two types of samples as polymerized using conventional Ziegler and newer metallocene catalyst systems. Two UHMW‐PE samples, having the same viscosity average molecular weight of 3.3 × 106, but different molecular weight distribution, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180°C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X‐ray measurements and thermal analyses. The metallocene‐catalyzed sample having narrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that obtained for the Ziegler‐catalyzed sample, ∼ 50. The stress/strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was achieved at 150°C and a strain rate of 5 min−1, independent of sample molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the metallocene‐catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Ziegler sample, 50–55 and 0.90 GPa, respectively, were significantly higher than those for the metallocene sample, 20 and 0.65 GPa, respectively, reflecting the markedly higher drawability for the former than the latter. The stress/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1921–1930, 1999 相似文献
682.
C. J. Porter C. J. Proctor E. A. Larka J. H. Beynon 《Journal of mass spectrometry : JMS》1982,17(7):331-334
Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m2+, formed from m1+ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m1+ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C5H5O]+ ions formed from [C6H5O]+ ions by these two independent methods is different as verified by comparison of the behaviour of [C5H5O]+ ions formed from several other compounds. 相似文献
683.
Porter CA Martin GK Stagner BB Lonsbury-Martin BL 《The Journal of the Acoustical Society of America》2006,120(2):884-900
This study investigated noise-induced changes in suppression growth (SG) of distortion product otoacoustic emissions (DPOAEs). Detailed measurements of SG were obtained in rabbits as a function of f2 frequencies at four primary-tone levels. SG measures were produced by using suppressor tones (STs) presented at two fixed distances from f2. The magnitude of suppression was calculated for each ST level and depicted as contour plots showing the amount of suppression as a function of the f2 frequency. At each f2, SG indices included slope, suppression threshold, and an estimate of the tip-to-tail value. All suppression measures were obtained before and after producing a cochlear dysfunction using a monaural exposure to a 2-h, 110-dB SPL octave-band noise centered at 2 kHz. The noise exposure produced varying amounts of cochlear damage as revealed by changes in DP-grams and auditory brainstem responses. However, average measures of SG slopes, suppression thresholds, and tip-to-tail values failed to mirror the mean DP-gram loss patterns. When suppression-based parameters were correlated with the amount of DPOAE loss, small but significant correlations were observed for some measures. Overall, the findings suggest that measures derived from DPOAE SG are limited in their ability to detect noise-induced cochlear damage. 相似文献
684.
Sivaramakrishnan Muthukrishnan Vivekanand S. Shete Toby T. Sanan Shubham Vyas Shameema Oottikkal Lauren M. Porter Thomas J. Magliery Christopher M. Hadad 《Journal of Physical Organic Chemistry》2012,25(12):1247-1260
We designed, synthesized, and screened a library of analogs of the organophosphate pesticide metabolite paraoxon against a recombinant variant of human serum paraoxonase‐1. Alterations of both the aryloxy leaving group and the retained alkyl chains of paraoxon analogs resulted in substantial changes to binding and hydrolysis, as measured directly by spectrophotometric methods or in competition experiments with paraoxon. Increases or decreases in the steric bulk of the retained groups generally reduced the rate of hydrolysis, while modifications of the leaving group modulated both binding and turnover. Studies on the hydrolysis of phosphoryl azide analogs as well as amino‐modified paraoxon analogs, the former being developed as photoaffinity labels, found enhanced tolerance of structural modifications when compared with O‐alkyl‐substituted molecules. Results from computational modeling predict a predominant active site binding mode for these molecules, which is consistent with several proposed catalytic mechanisms in the literature and from which a molecular‐level explanation of the experimental trends is attempted. Overall, the results of this study suggest that while paraoxonase‐1 is a promiscuous enzyme, there are substantial constraints in the active site pocket, which may relate to both the leaving group and the retained portion of paraoxon analogs. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
685.
A novel molecular model, the latent entanglement, is proposed to explain the relationships between ductilities of polyethylene reactor powders and different conditions of synthesis. According to Hoffman's variable cluster model, irregularities in the crystallite fold surface increase as the synthesis temperature is decreased. The concept of latent entanglement stems from the potentiality of these defects to convert into active entanglements when the involved chain segments are drawn out of the crystals. 相似文献