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121.
Rheological investigation of the influence of molecular structure on natural and accelerated UV degradation of linear low density polyethylene 总被引:1,自引:0,他引:1
Ibnelwaleed A. Hussein 《Polymer Degradation and Stability》2007,92(11):2026-2032
Metallocene and Ziegler-Natta (ZN) linear low density polyethylenes (LLDPEs) of different branch types and contents as well as linear high density polyethylene (HDPE) were exposed to natural and accelerated weather conditions. The degree of UV degradation of exposed samples was measured by rheological techniques and results were compared with unexposed polymers. Dynamic shear measurements were performed in an ARES rheometer in the linear viscoelastic range. The degree of enhancement or reduction in viscosity and elasticity was used as a measure of the degree of cross-linking or chain scission, respectively. The degradation results of LLDPE suggest that both cross-linking and chain scission are taking place. Chain scission dominated the degradation at high levels of short chain branching (SCB) and long exposure times. The degradation mechanism of m-LLDPE and ZN-LLDPE is similar; however, m-LLDPE showed a higher degradation rate than ZN-LLDPE of similar Mw and average SCB. ZN-LLDPE was found to be more stable than a similar m-LLDPE. Comonomer type had little influence on degradation. Dynamic shear rheology was very useful in revealing the influence of different molecular parameters and it exposed the degradation mechanism. 相似文献
122.
Proteins with molecular mass (M(r)) <20 kDa are often poorly separated in 2-D sodium dodecyl sulfate polyacrylamide gel electrophoresis. In addition, low-M(r) proteins may not be readily identified using peptide mass fingerprinting (PMF) owing to the small number of peptides generated in tryptic digestion. In this work, we used a 2-D liquid separation method based on chromatofocusing and non-porous silica reversed-phase high-performance liquid chromatography to purify proteins for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric (MALDI-TOFMS) analysis and protein identification. Several proteins were identified using the PMF method where the result was supported using an accurate M(r) value obtained from electrospray ionization TOFMS. However, many proteins were not identified owing to an insufficient number of peptides observed in the MALDI-TOF experiments. The small number of peptides detected in MALDI-TOFMS can result from internal fragmentation, the few arginines in its sequence and incomplete tryptic digestion. MALDI-QTOFMS/MS can be used to identify many of these proteins. The accurate experimental M(r) and pI confirm identification and aid in identifying post-translational modifications such as truncations and acetylations. In some cases, high-quality MS/MS data obtained from the MALDI-QTOF spectrometer overcome preferential cleavages and result in protein identification. 相似文献
123.
Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70‐eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that
- Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions.
- Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real‐world applications.
- Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library.
- Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the “Extractor” exhibits relatively abundant molecular ions compared with the “Inert” ion source, while the “High efficiency source” (HES) provides mass spectra with depleted molecular ions compared with the “Inert” ion source or NIST library mass spectra.
124.
Well established routes for obtaining stiff and strong polyethylene (PE) involve solid state drawing either of solution crystallized gel films or melt crystallized spherulitic PE. The aim of this work is to show the potential of melt deformation as an alternative route for obtaining highly oriented products. Our previous work on the melt deformation route showed that oriented PE fibers could be directly extruded under appropriately controlled conditions [8,9]. Here, we show that PE films (or filaments) can also be melt drawn in the temperature window 130–160 °C, thus yielding oriented products. The advantage of melt drawing over direct melt extrusion is that it allows a wider operational latitude and thus does not require such carefully controlled conditions.The morphology produced by melt deformation is different from solid state deformation and consists of extended chain fibrils with platelet overgrowths. The relative amount of fibrils and platelets depends on operating parameters. The temperature window of PE melt drawing is identified with the regime where some flow induced crystallization takes place. The conditions for melt drawability are of wider generality for crystallizable flexible chain polymers. They are: (i) adequate strain rate to overcome entropie resistance to chain extension, (ii) but not high enough to activate the elastic response of the transient networks in the entangled system, (iii) sufficient strain to fully extend the chain, (iv) appropriate temperature for flow-induced crystallization and strain hardening, and (v) cooling to freeze the oriented structure.Ultra high molecular weight PEs were not the most suitable for melt drawing due to their high recoverable elongation in the melt (melt elasticity) in addition to added limitations imposed by their nascent grain systeme. Our work suggests that an optimum molecular weight for melt drawing is¯M
w(400–900)×103 with further possibilities for improvement through multimodal distributions. 相似文献
125.
锂离子电池正极材料LiMn2O4的低热固相合成与性能表征 总被引:6,自引:0,他引:6
锂离子电池具有比能量高、环境污染小等优点,广泛应用于手提电话、便携式电脑、摄像机等设备中。其正极材料的研究是锂离子电池的研究重点。层状结构的LiCoO2、LiNiO2和尖晶石结构的LiMn2O4是仅有的三种能在3.5V以上电位可嵌入Li的正极材料[1~3]。目前市售的锂离子电池主要采用LiCoO2作正极材料,但由于Co资源缺乏和价格相对昂贵,而锰资源丰富,价格低廉且无毒,对环境友好,因此世界各国都在大力进行以LiMn2O4为正极材料的锂离子电池的实用化研究。LiMn2O4传统的制备方法是高温固相反应合成法[4~7],但由于Mn的变价多,与Li形成贫Li或… 相似文献
126.
127.
128.
极低密度聚乙烯与其它聚乙烯的共混 总被引:3,自引:0,他引:3
从结构角度,用DSC,WAXD,SAXS研究了聚乙烯(PE)家族中极低密度聚乙烯(VLDPE)与其它PE的互容性.HDPE/VLDPE是共晶互容的,以其大量无规部分“溶解”了HDPE的结晶缺陷部分,提高了HDPE的Tc,Tm,Xc,结晶峰半高宽变窄,晶胞参数随组成而有最低值.VLDPE与LLDPE结构极为相似,DSC及WAXD证明其共混物是共晶相容体系.LDPE/VLDPE的结晶度符合按组成的计算值,但晶胞参数a,b以及晶粒尺寸增大,DSC上有分别相应于两组份的两个Tm;VLDPE的Tc,Tm峰高之和高于按组份的计算值,LDPE的Tm,Tc则低于计算值.认为是正如LLDPE/LDPE,LDPE向充满整个体积的VLDPE中不断填入,以VLDPE为晶核而结晶,形成相分离的不相容体系. 相似文献
129.
Crystalline [Y(OH2)3(NCMe)(benzo-15-crown-5)][ClO4]3·benzo-15-crown-5-CH3CN can be obtained by slowly cooling a reaction mixture of Y(ClO4)3·n H2O with benzo-15-crown-5 in a solution of acetonitrile and methanol (3 : 1) from 60°C to room temperature. The crystal structure of this complex has been determined at –150 and 20°C. The complex is triclinic,P
. At –150°C the cell parameters area = 11.986(4),b = 12.071(7),c = 16.364(5) Å, = 93.56(3), = 98.68(3), = 109.68(4)°, vol = 2187 Å3, andD
calc = 1.61 g cm–3 forZ = 2 formula units. 3633 independently observed [F
o 5(F
o)] reflections were used in the final least-squares refinement leading to an agreement index ofR = 0.048. The Y(III) ion coordination geometry approximates a tricapped trigonal prism with three water molecules and three benzo-15-crown-5 oxygen atoms forming the prism, with the two remaining benzo-15-crown-5 oxygen atoms and the acetonitrile molecule completing the coordination as capping atoms. The three water molecules hydrogen bond a second crown ether molecule and two of the perchlorate anions. The two acetonitrile molecules have contacts with perchlorate oxygen atoms close enough for some weak interaction. One perchlorate is ordered, one is partially disordered as is the coordinated solvent molecule, and the third anion is totally disordered. The two unique crown ether molecules have distinctively different conformations.For Part 20, see reference [1]. 相似文献
130.
Jeong Hyun Yeum Byung Chul Ji Chul Joo Lee Ji Youl Lee Sang‐Seob Lee Sam Soo Kim Joon Ho Kim Won Seok Lyoo 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1103-1111
To prepare water‐soluble, syndiotacticity‐rich poly(vinyl alcohol) (PVA) microfibrils for various industrial applications, we synthesized syndiotacticity‐rich, low molecular weight PVA by the solution polymerization of vinyl pivalate (VPi) in tetrahydrofuran (THF) at low temperatures with 2,2′‐azobis(2,4‐dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of poly(vinyl pivalate) (PVPi). Effects of the initiator and monomer concentrations and the polymerization temperature were investigated in terms of the polymerization behaviors and molecular structures of PVPi and the corresponding syndiotacticity‐rich PVA. The polymerization rate of VPi in THF was proportional to the 0.91 power of the ADMVN concentration, indicating the heterogeneous nature of THF polymerization. The low‐temperature solution polymerization of VPi in THF with ADMVN proved to be successful in obtaining water‐soluble PVA with a number‐average degree of polymerization (Pn) of 300–900, a syndiotactic dyad content of 60–63%, and an ultimate conversion of VPi into PVPi of over 75%. Despite the low molecular weight of PVA with Pn = 800, water‐soluble PVA microfibrillar fibers were prepared because of the high level of syndiotacticity. In contrast, for PVA with Pn = 330, shapeless and globular morphologies were observed, indicating that molecular weight has an important role in the in situ fibrillation of syndiotacticity‐rich PVA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1103–1111, 2002 相似文献