Chain conformation of polyethylene in the solid state as studied by 13C cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy

High-resolution ¹³C nuclear magnetic resonance (n.m.r.) spectra of melt-quenched polyethylene and polyethylene single crystals are measured by the cross-polarization/magic angle spinning technique. Melt-quenched polyethylene and polyethylene single crystals have four small upfield peaks, a shoulder on the main peak and three small peaks, respectively. Based on the ¹³C n.m.r. resonance lines of cyclic paraffin C₆₄H₁₂₈ reported previously, it is concluded that the main peak and the three upfield peaks arise from the trans zigzag structure region and the folded structure region, respectively. From these peak intensities, it is estimated that the stem length of polyethylene single crystals is approximately 125 Å. Taking into account an error in the estimation of the small peak intensities, the calculated stem length of 125 Å is consistent with the crystal thickness (120–150 Å) observed directly by electron microscopy. It can be concluded, therefore, that polyethylene single crystals mainly contain sharply folded structure. Melt-quenched polyethylene may contain sharply folded structure to some extent in addition to loose loops.     

I.r. study of solution-grown crystals of polyethylene: correlation with the model from neutron scattering

The crystal stem arrangement in solution-grown polyethylene crystals has been investigated by the mixed-crystal infra-red technique. Parallel neutron scattering measurements are reported separately. The behaviour of the CD₂ bending vibration is shown to be consistent with a model having 75% adjacent re-entry, 50% dilution of a molecule along the fold plane (110 direction) and a degree of superfolding equivalent to an average molecular weight ($\text{M}\text{?}{}_{\mathrm{w}}$) of 21 000 for each sheet. This is in agreement with results from neutron scattering, which have previously been interpreted using this model. Computer calculations of stem positions based on these parameters are used to calculate the distributions of doublet splittings for several molecular weights. The main features of low-temperature FTi.r. spectra as a function of molecular weight are reproduced in this way.     

MD Simulation Study of the Influence of Branch Content on Crystallization of Branched Polyethylene Chains with Uniform Branch Distribution

Linear low-density polyethylene (LLDPE) chains with different levels of branch content (BC), ranging from 10 to 80 branches/1000 C, distributed uniformly along the chain were simulated in vacuum at a temperature of 350 K. The influence of BC on the relaxation and crystallization of LLDPE chains was studied. The collapse of the branched chains was found to occur via a local followed by a global collapse mechanism with branches acting as nucleation points for the collapse of the molecule leading to a faster collapse of chains with higher BC. The trans population was found to be dominant at all branch contents; however, it decreases with increasing BC. Increasing BC was found to decrease order and to strongly influence chain conformation. Chain conformation undergoes a transition from lamellar to a more random coil-like structure near a critical BC of 50 branches/1000 C. Branches were observed to be excluded from the lamella and to self assemble at high BC. This work also provides insight into the conformation adopted during the coil-globule transition experienced by a single chain in an infinitely dilute solution much below the temperature.     

Neutron scattering studies on solution-grown crystals of polyethylene: a statistical preference for adjacent re-entry

Neutron scattering studies of solution-grown polyethylene crystals demonstrate a strong statistical preference for adjacent re-entry, with 75% of stems occupying adjacent lattice sites. The number of sheets of stems for one molecule increases with molecular weight, with an average molecular weight per sheet of 21 000. These are the main features of computer models which give the best agreement with scattering data obtained over an angular range ($\text{q</1}\text{A}\text{?}{}^{\mathrm{?1}}$). Scattering in this q range is directly sensitive to typical stem separations. Any preference for alternate re-entry would result in a peak at $\text{q=0.7}\text{A}\text{?}{}^{\mathrm{?1}}$, which was not observed. The results are discussed in terms of possible crystallization mechanisms.     

温度梯度对聚乙烯耐慢速开裂性能的影响

通过调控温度梯度得到含取向球晶的聚乙烯(PE)试样,研究取向球晶结构对PE耐慢速开裂性能的影响。结果表明:在温度梯度作用下,PE球晶呈椭圆形,片晶沿温度梯度方向取向生长。当温度梯度下制备的试样受到轴向应力作用,取向晶片垂直于受力方向,易发生晶片层间破坏,导致其耐慢速裂纹开裂性能降低。随温度梯度增加,取向球晶增多,耐慢速开裂性能下降更明显。     

Structural characterization and mass transfer properties of polyurethane block copolymer: influence of mixed soft segment block and crystal melting temperature

An attempt has been made to investigate the influence of mixed soft segment on structure and mass transfer properties of segmented polyurethane (SPU). For this purpose polyurethane block copolymer containing soft segment such as polycaprolactone glycol (number‐average molecular weight 3000, PCL 3000), PCL 3000–polypropylene glycol (number‐average molecular weight 3000, PPG 3000), PCL 3000–polytetramethylene glycol (number‐average molecular weight 2900, PTMG 2900), PPG 3000–PTMG 2900, were synthesized using a two‐step or three‐step synthesis process. All the SPUs were modified with the hydrophilic segment, i.e. diol‐terminated poly(ethylene oxide) (number‐average molecular weight 3400, PEG 3400). Fourier‐transform infrared, wide‐angle X‐ray diffraction, differential scanning calorimetry, and dynamic mechanical thermal analysis were used to characterize the polyurethanes. The mass transfer properties were measured by equilibrium sorption and water vapor permeability measurements. Mixed blocks loosen the inter‐chain interaction due to phase mixing which decreases the crystallization of the soft segment in the resulting SPU. The crystallinity of mixed polyol block SPU increases when both polyols are crystallizable in the pure state. Highest loss tan δ value was observed for the sample containing PTMG 2900–PPG 3000 mixed soft segment due to their flexible and phase mixed structure which increases the chain mobility; this sample performed best among all four SPUs in equilibrium water sorption as well as water vapor permeability owing to their loose and nearly amorphous structure. Soft segment crystal melting further enhances the water vapor permeability significantly, which would make the membrane suitable for breathable textiles, packaging and medical applications. Copyright © 2006 Society of Chemical Industry     

添加剂对氨基酸晶体生长的影响

氨基酸大多生长为针状或片状晶习,存在堆密度低、流动性差等问题,严重影响产品后加工处理过程。因此,实现氨基酸晶习的定向调控具有重要意义。添加剂对氨基酸晶习调控直接有效,广泛应用于工业生产中。本文主要从抑制和促进晶体生长两个角度,综述了添加剂对氨基酸晶体生长的作用机理。添加剂对晶体生长的抑制机理主要有两点：一是添加剂分子吸附到晶面上,阻碍溶质分子的扩散和聚集;二是添加剂分子嵌入晶格并占据生长位点。而添加剂促进晶体生长的机理为：添加剂加快了溶质分子在晶体表面的聚集速度、使晶体表面粗糙化和降低了溶剂层脱除能垒。最后,针对添加剂对氨基酸晶体生长影响的研究,从晶体工程的角度提出了通过分子模拟设计的添加剂定向控制氨基酸晶体生长,从而调控晶习的展望。     

Photografting of methacrylic acid on polyethylene film: Effect of mixed solvents consisting of water and organic solvent

Photografting of methacrylic acid (MAA) on linear low‐density polyethylene film (thickness = 30 μm) was investigated at 60°C in mixed solvents consisting of water and organic solvent with xanthone as the photoiniator. The organic solvents examined were acetone, methanol, tetrahydrofuran, and dioxane. A maximum percentage of grafting was observed at a certain concentration of organic solvent in the mixed solvent, depending on the nature of the organic solvent. Distribution of the grafted chains in the resultant MAA‐grafted film, which was examined by scanning electron microscopy and attenuated total‐reflection IR spectroscopy, was largely influenced by the use of the mixed solvent. The grafted samples swelled in the alkaline region and shrank in acidic medium, showing a pH‐responsive character. The extent of the pH‐responsive function was closely related to the location of grafted chains, which could be controlled by proper choice of the composition of organic solvent in the mixed solvent. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2454–2461, 2002     

The ablative behavior of high linear chains and its role in the thermal stability of polyethylene: a combined P‐TREF−TGA study

Ethylene copolymers contain short chain branches (SCBs) which have great influence on their properties. An ethylene/1‐hexene copolymer distinguished in terms of the number of butyl SCBs was precisely separated based on differences in crystallizability using the preparative temperature‐rising elution fractionation (P‐TREF) method and was then studied via TGA and DSC. A comparison between the results obtained and the literature suggested a short chain branch distribution (SCBD) functionality for a included in the general linear form T_m (°C) = ?a(SCBD) × (SCB) + b. P‐TREF?TGA results showed that the highly linear chains acted as ablative layers which could increase the thermal stability and durability of polyethylene in the absence of any mineral additive. Furthermore, the P‐TREF?TGA data displayed an interesting interrelationship between temperature at maximum rate of degradation (T_max) and the number of butyl SCBs over all the heating rates (10, 25, 50 and 100 °C min^‐1). The role of the number of butyl SCBs in thermal degradation was exhausted by higher heating rates, whereas the ablation capability was enhanced. Kinetic studies demonstrated that the activation energy dropped on increase in butyl branch content within the backbone. © 2015 Society of Chemical Industry     

Hydrogen evolution during the oxidation of formaldehyde on Au:: The influence of single crystal structure and Tl-upd

Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140–160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H₂ is largely suppressed.     

The characterization of carbonaceous species from CO hydrogenation on single crystal Ru(0001) and Ru(11¯20) catalysts with high-resolution electron energy-loss spectroscopy

The CO hydrogenation on single-crystal ruthenium catalysts has been studied utilizing an elevated-pressure micro-reactor and high resolution electron energy loss spectroscopy (HREELS). It is found that carbonaceous deposits identified following CO hydrogenation are essentially identical to those observed in the study of methane decomposition. Three distinct forms of carbonaceous intermediates are identified; these are methylidyne (CH), vinylidene (CCH₂), and graphitic carbonaceous species.     

Hydrogen evolution during the oxidation of formaldehyde on Au:: The influence of single crystal structure and Tl-upd

Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140-160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H₂ is largely suppressed.     

The activation of fibroblast growth factors (FGFs) by glycosaminoglycans: influence of the sulfation pattern on the biological activity of FGF-1

Six synthetic heparin-like oligosaccharides have been used to investigate the effect of the oligosaccharide sulfation pattern on the stimulation of acidic fibroblast growth factor (FGF-1) induced mitogenesis signaling and the biological significance of FGF-1 trans dimerization in the FGF-1 activation process. It has been found that some molecules with a sulfation pattern that does not contain the internal trisaccharide motif, which has been proposed for high affinity for FGF-1, stimulate FGF-1 more efficiently than those with the structure of the regular region of heparin. In contrast to regular region oligosaccharides, in which the sulfate groups are distributed on both sides of their helical three-dimensional structures, the molecules containing this particular sulfation pattern display the sulfate groups only on one side of the helix. These results and the fact that these oligosaccharides do not promote FGF-1 dimerization according to sedimentation-equilibrium analysis, confirm the importance of negative-charge distribution in the activation process and strongly suggest that FGF dimerization is not a general and absolute requirement for biological activity.     

Development and evaluation of an improved soil test for phosphorus: 1. The influence of phosphorus fertilizer solubility and soil properties on the extractability of soil P

The objective of this work was to develop and evaluate a soil test suitable for estimating the phosphorus status of soils whether they were fertilized with soluble or sparingly soluble P fertilizers or both. Four New Zealand soils of contrasting P sorption capacity and exchangeable Ca content were incubated alone or with monocalcium phosphate (MCP), reactive North Carolina (NC) phosphate rock or unreactive Florida (FRD) rock, at 240 mg P kg^–1 soil, to allow the P sources of different solubilities to react with each soil and provide soil samples containing different amounts of extractable P, Ca and residual phosphate rock. The phosphorus in the incubated soils was fractionated into alkali soluble and acid soluble P fractions using a sequential extraction procedure to assess the extent of phosphate rock dissolution. Eight soil P tests [three moderately alkaline — Olsen (0.5M NaHCO₃) modified Olsen (pretreatment with 1M NaCl) and Colwell; three acid tests — Bray 1, modified Bray 1 and Truog; and two resin tests — bicarbonate anion exchange resin (AER) and combined AER plus sodium cation exchange resin (CER)] were assessed in their ability to extract P from the incubated soils.The 0.5M NaHCO₃ based alkaline tests could not differentiate between the Control and FRD treatments in any soil nor between the Control, NC and FRD treatments in the high P sorption soils. The acid extractants appeared to be affected by the P sorption capacity of the soil probably because of reabsorption of dissolved P in the acid medium. The AER test gave results similar to Olsen. Only the combined AER + CER test extracted P in amounts related to the solubility of the P sources incubated with each soil. Furthermore, when soil samples were spiked with FRD and NC and extracted immediately, the P extracted by the AER + CER test, over and above the control soils, increased with the amount and chemical reactivity of the rocks. There was no extraction of rock P by any of the alkaline extractions.Increases in the amounts of P extracted (P) by each soil test from the fertilized soils, over and above the control soils were compared with the amounts ofP dissolved from the fertilizers during incubation (measured by P fractionation). Soil P sorption capacity had least influence on the amounts of P extracted by the AER + CER and Colwell tests. However, the Colwell test was unable to differentiate between all P sources in all four soils and suffered from the disadvantage of producing coloured extracts. The AER + CER test appeared to have the potential to assess the available P status of soils better than the other tests used because of its ability to extract a representative portion of residual PR (in accordance with the amount and reactivity) and dissolved P, and thus to differentiate between fertilizer treatments in all four soils.     

Simultaneous isomerization of n-heptane and saturation of benzene over Pt/Beta catalysts: The influence of zeolite crystal size on product selectivity and sulfur resistance

In this work we show that decreasing the zeolite crystal size has a clear benefit on product selectivity and sulfur resistance of Pt/Beta catalysts during the simultaneous hydroisomerization of n-heptane and hydrogenation of benzene. The higher isomerization selectivity of the catalyst prepared from a nanocrystalline Beta zeolite can be ascribed to a faster diffusion of the iso-C₇ products through the small crystallites preventing cracking reactions to occur, while the improved sulfur resistance can be related to a better dispersion of Pt owing to its higher surface area and mesoporosity.     

Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

The role of a multifunctional catalyst for de-NO_x process has been investigated. The NO_x storage capacity of H₃PW₁₂O₄₀·6H₂O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M⁰ (zero-valent metal) and perhaps (ii) (metal–H)^δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO_x and to generate cationic catalytic sites (M^x+). These cationic sites seem to be the clue for their important activity toward NO_x reduction. This catalyst presents an outstanding resistance to SO₂ poisoning which can be related to NO and NO₂ absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO_x reduction capacity and NO_x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H₂ mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO_x process to occur and second, the formation of oxygenate active species must be indispensable for NO_x reduction into nitrogen.     

Fabrication of polycrystalline (Y0.7Gd0.3)2O3:Eu ceramics: The influence of initial pressure and sintering temperature on its morphology and photoluminescence activity

Nanocrystalline (Y_0.7Gd_0.3)₂O₃ powder, synthetised via polymer complex solution method, was compacted into 25 pellets applying high pressures (173-867 MPa) for 30 s that were subsequently sintered at different temperatures (800-1400 °C) for 18 h. The morphology and optical characteristics of the starting powder and prepared ceramic samples were monitored and discussed in order to identify the changes induced with the variations of initial compacting pressure, which influence is often neglected, and with sintering temperature. The grain size tends to decrease significantly with increasing pressure, even when elevated temperatures are used for annealing, while low compacting pressure resulted in grain coarsening and, in some cases, even in anomalous morphology of ceramic samples. Luminescence emission in ceramic samples decays faster than in nanopowders, that is in complete agreement with the grain formation and gradual transformation to the bulk material. Judd-Ofelt intensity parameters and branching ratios were calculated taking into account the difference in effective refractive index for nanopowder and ceramic samples.