Solid-state 13C NMR and synchrotron SAXS/WAXS studies of uniaxially-oriented polyethylene

We report solid-state ¹³C NMR and synchrotron wide-and small-angle X-ray scattering experiments (WAXS, SAXS) on metallocene linear low density polyethylene films (e.g., Exceed™ 1018 mLLDPE; nominally 1 MI, 0.918 density ethylene-hexene metallocene copolymer) as a function of uniaxial draw ratio, λ. Combined, these experiments provide an unambiguous, quantitative molecular view of the orientation of both the crystalline and amorphous phases in the samples as a function of draw. Together with previously reported differential scanning calorimetry (DSC), gas transport measurements, transmission electron microscopy (TEM), optical birefringence, small angle X-ray scattering (SAXS) as well as other characterization techniques, this study of the state of orientation in both phases provides insight concerning the development of unusually high barrier properties of the most oriented samples (λ=10). In this work, static (non-spinning) solid-state NMR measurements indicate that in the drawn Exceed^TM films both the crystalline and amorphous regions are highly oriented. In particular, chemical shift data show the amorphous phase is comprised increasingly of so-called “taut tie chains” (or tie chains under any state of tautness) in the mLLDPE with increasing draw ratio – the resonance lines associated with the amorphous phase shift to where the crystalline peaks are observed. In the sample with highest total draw (λ=10), virtually all of the chains in the non-crystalline region have responded and aligned in the machine (draw) direction. Both monoclinic and orthorhombic crystalline peaks are observed in high-resolution, solid-state magic-angle spinning (MAS) NMR measurements of the oriented PE films. The orientation is comparable to that obtained for ultra-high molecular weight HDPE fibers described as “ultra-oriented” in the literature. Furthermore, the presence of a monoclinic peak in cold-drawn samples suggests that there is an appreciable internal stress associated with the LLDPE. The results are confirmed and independently quantified by Herman's Orientation Function values derived from the WAXS measurements. The degree of orientation approaches theoretically perfect alignment of chains along the draw direction. We deduce from this observation that a high fraction of the non-crystalline chains are either tie chains that directly connect adjacent lamellae or are interlocking loops from adjacent lamellae. In either case, the chains are load-bearing and are consistent with the idea of “taut tie chains”. We note that transmission electron micrographs recorded for the ultra-oriented Exceed showed the lamellae are often appreciably thinner and shorter than they are for cast or blown Exceed 1018. Combined with higher crystallinity, the thinner lamellae statistically favor more tie chains. Finally, the remarkably large decrease in permeability of the λ=10 film is primarily attributed to the high degree of orientation (and loss of entropy) of the amorphous phase.     

Solid-state variable-temperature H MAS NMR studies on deuterated polyethylene

Solid-state varible-temperature/magic angle spinning(VT/MAS) ¹H NMR measurements were carried out on deuterated polyethylene. From these experimental results it was found that the ¹H chemical shift induced by conformational and morphological changes of the polyethylene sample is within the linewidth of ≈ 0.5 ppm. Furthermore, from MAS/dipolar decoupling experiments it was found that the resonance frequency of the proton varies linearly with the inverse square of the deuterium decoupling power. This experimental finding is discussed theoretically.     

聚苯乙烯/纳米铝复合材料的流变特性及纺丝研究

为有效制得Z箍缩氘代聚苯乙烯/纳米铝(DPS/AlNPs)导电丝阵材料,采用PS中掺入AlNPs制备PS/AlNPs复合材料纤维进行模拟研究。研究了温度及剪切速率等因素对PS/AlNPs复合材料流变性能的影响、复合材料熔体的结构变化及流动状态与可纺性能的关系,以及PS/AlNPs纤维的形貌、热稳定性能和力学性能。结果表明：PS/AlNPs熔体属于典型剪切变稀型非牛顿流体,熔体的表观粘度与温度呈现负相关,240~260 ℃时复合材料的非牛顿指数介于0.462~0.546,结构黏度系数介于1.8~2.1,黏流活化能介于77.2~104.6 kJ·mol^-1,具有良好的可纺性。PS/AlNPs纤维表面光滑,对AlNPs粒子包覆良好且对其抗氧化非常有利,其中当AlNPs质量分数为1%时纤维的断裂伸长率突出、掺量为5%时其断裂强度较高。     

聚乙烯/银纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚乙烯/银纳米颗粒复合物的结构、极化率和红外光谱、热力学性质、力学特性进行计算, 分析其随模拟温度和银颗粒尺寸的变化规律. 模拟结果表明: 聚乙烯/银纳米颗粒复合物为各向同性的无定形结构, 温度升高可提高银纳米颗粒的分散均匀性; 银纳米颗粒表面多个原子层呈现无定形状态, 并在银颗粒和聚乙烯基体的界面形成电极化层, 界面区域随颗粒尺寸和温度的增加分别减小和增加; 与聚乙烯体系相比, 聚乙烯/银纳米颗粒复合物的极化率高很多, 且随温度的升高和银颗粒尺寸的减小而增大; 银颗粒尺寸直接影响界面电偶极矩的强度和振动频率, 红外光谱峰强度和峰位随颗粒尺寸发生变化; 聚乙烯/银纳米颗粒复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数, 热容随颗粒尺寸的变化较小, 但随温度的升高而明显减小, 具有显著的温度效应; 热压力系数随温度的变化较小, 但随颗粒尺寸的增加而减小, 具有明显的尺度效应, 温度稳定性更好; 聚乙烯/银纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚乙烯体系更高的杨氏模量和泊松比, 并且都随温度的升高和银颗粒尺寸的增大而减小, 加入银纳米颗粒可有效改善聚乙烯的力学性质. 关键词： 分子动力学模拟 聚合物纳米复合物 纳米颗粒     

Thermophysical characterization of the deformation behavior of polyethylene/kaolin composites

The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.     

Thermal transport enhancement of multi-walled carbon nanotubes/ high-density polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) enhanced high-density polyethylene (HDPE) composites were prepared and their thermophysical properties were measured. The thermal diffusivity of the composites increases with the increase in the amount of MWCNTs. A thermal diffusivity of more than three times that of pure HDPE was obtained for 38 vol. % MWCNTs/HDPE composites. An equation based on an effective medium approach model was used to discuss the thermal diffusivity enhancement of MWCNTs/HDPE composites as a function of the volume fraction of MWCNTs. The results from this analysis can be a predictive guideline for further improvements in the thermal transport properties of MWCNTs/HDPE composites. Moreover, the intrinsic longitudinal thermal conductivity k_z of an individual MWCNT was deduced from the measured results on the MWCNTs/HDPE composites. PACS 67.55.Hc; 61.46.Fg; 66.30.Xj     

Synthesis and characterization of radiation crosslinked nano silver- polyvinyl alcohol/polyethylene oxide composites

Poly (vinyl alcohol)/poly (ethylene oxide) (PEO/PVA) blends were modified by gamma irradiation in the presence of acrylic acid (AAc) monomer. The modified PVA/PEO blends were then complexed with silver nitrate salt and lithium trifluoromethanesulfonate. Transmission electron microscopy was used to determine the distribution as well as the particle size of the silver nanoparticles (NP) formed in the matrix. The UV–vis absorbance spectra of the prepared grafted nanocomposite membranes confirmed the formation of Ag NP based on their surface plasmon band at 438?nm. The electrical properties of the blended electrolyte polymer films were characterized and discussed.     

Physico-chemical characterization of the fibre/matrix interaction in polyethylene fibre/epoxy matrix composites

The interphase in polyethylene fibre/epoxy matrix composites is studied with FT-IR microspectroscopy using a set-up to investigate the matrix as close to the fibre as a few μm or less. It is shown that moisture present on the fibre surface is able to influence the polymerization reaction of the epoxy/anhydride matrix in an irreversible manner. This effect is enhanced for composites from the more hydrophilic polyvinylalcohol fibre. The fibre/matrix interaction in these thermoplastic fibre composites is also studied with DSC through the characterization of the fibre melting. A decreased 'DSC interaction parameter' is found if the composition of the interphase is changed by moisture. For a composite with an epoxy/amine matrix, on the other hand, the DSC interaction parameter is unaffected by moisture from the fibre surface.     

Study on the surface properties of wood/polyethylene composites treated under plasma

Wood/polyethylene (PE) composites are widely used in many fields for its excellent properties, but they are hard to adhere for the surface lacking of polarity. So low-pressure glow discharge of air plasma was used to improve the adhesion properties of wood/PE composites. The composites were treated by plasma under different discharge power. And the changes on the surface properties of the treated and untreated composites were studied by contact angle, Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) analysis. The measurement showed that the contact angle decreased after plasma treatment, and the contact angle decreased gradually with the increasing of discharge power. The FTIR analysis results showed that the polar groups such as hydroxyl, carbonyl and carboxyl were formed on the surface of the composites treated under plasma. SEM and AFM results showed that the roughness of plasma treated samples increased. XPS analysis results indicated that the content of carbon element decreased while the content of oxygen element in the composition of wood/PE composites surface element increased and it reached a balance in a higher power, meanwhile a lot of carboxyl groups were formed. The newly formed polar groups are benefit for the adhesion of composites. The shear bonding strength test showed that the adhesion properties of wood/PE composites improved effectively after plasma treatment.     

Fracture of composites based on polyethylene and elastic particles

The composites based on low-density polyethylene with elastomer filling particles are studied. A fracture mechanism induced by the fracture of filler particles or their separation from the matrix polymer is revealed. The fracture of the composites is caused by the growth of formed rhombic pores. The natural relative elongation in a neck is shown to be an important characteristic of a polymer. If the relative elongation in a neck is lower than the strain of appearance of rhombic pores, they form at the stage of uniform tension after necking, and the composite remains plastic. If the relative elongation in a neck is higher than the strain of formation of rhombic pores, they nucleate during necking, and the material undergoes quasi-brittle fracture. Good adhesion between the matrix polymer and elastic particles hinders the appearance of rhombic pores in a neck and, thus, retains high deformation properties of the composites.     

Miscibility studies of poly-p-phenylene terephthalamide/polyamide molecular composites

Miscibility characteristics of poly-p-phenylene terephthalamide (PPTA) with nylon 6, nylon 66, nylon 11, and nylon 12 have been examined by means of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Mixtures of PPTA/nylon 6, 66, 11, and 12 and amorphous nylon were prepared by coagulation in water from isotropic sulfuric acid solutions. The WAXD investigation shows the presence of small amounts of disordered crystals in the regenerated pure PPTA and to a lesser amount in the blends of PPTA/amorphous nylon or PPTA/aliphatic crystalline nylons. DSC studies show the melting point depression of nylon crystals. At low PPTA contents, a small melting point elevation occurs in nylon 6, presumably due to nucleation. The Flory-Huggins interaction parameter (χ) was subsequently calculated from the melting point depression of the nylons.     

Precipitation of calcium carbonate by ultrasonic irradiation

Supersaturated solution of calcium carbonate ([Ca²⁺]=1.2 mmol/L, [HCO₃⁻]=3.2 mmol/L, pH=8.8, T=30±0.5 °C), a scale forming component, was irradiated by an ultrasonic homogenizer (24 kHz, 15–250 W/cm²) to study the factors that affect its precipitation rate. The factors of (1) depth of horn immersion, (2) ultrasonic intensity and horn tip size and (3) cavitation, which can affect the precipitation rate were investigated in this study. Ultrasonic irradiation was observed to accelerate the precipitation of calcium carbonate and it was found that there exists an optimum range of horn immersion depth for maximizing the precipitation rate. The experimental data also established that the precipitation rate was proportional to ultrasonic intensity and diameter of horn tip. These findings were correlated to the effects of physical mixing, that arises due to ultrasonic irradiation. However, the effect of cavitation in accelerating the precipitation rate was found to be small. Thus it is forwarded that the physical mixing effect, especially macrostreaming is the main factor that accelerates the precipitation rate of calcium carbonate during ultrasonic treatment. Further, neither the morphology nor the size of the calcium carbonate crystals formed were found to be affected by the ultrasonic irradiation.     

有机化凹凸棒土/尼龙6复合材料的光谱分析研究

采用硅烷偶联剂对凹凸棒土进行表面接枝改性,并通过熔融共混技术在双螺杆挤出机上制备了有机化凹凸棒土/尼龙6复合材料.利用X射线光电子能谱(XPS)、红外光谱(FTIR)、X射线衍射(XRD)和扫描电镜(SEM)对改性结果和复合材料的晶体结构和微观相貌进行表征.结果表明:改性后凹凸棒土表面Si,N,C元素的质量分数提高,结...     

Dielectric and impedance studies of DBSA doped polyaniline/PVC composites

Polyaniline (PANI) was doped with dodecylbenzene sulfonic acid (DBSA) and then mixed with PVC by solution blending method to prepare DBSA doped PANI (PAND)/PVC composites. FTIR spectroscopy indicates the strong dipole–dipole interaction between the individual components of the composites. The ac electrical properties of the synthesized composites were investigated by complex impedance spectroscopy in the frequency range of 0.5–10⁶ Hz at room temperature. Both dielectric loss factor and permittivity increase with the decrease of frequency exhibiting strong interfacial polarization at low frequency. Addition of PAND in PVC reduces the charge trapping centers by increasing the number of conducting channels participating in the relaxation process; hence an increase in conductivity is observed.     

Electrochemically assisted co-deposition of calcium phosphate/collagen coatings on carbon/carbon composites

Calcium phosphate (CaP)/collagen coatings were prepared on the surface of carbon/carbon (C/C) composites by electrochemically assisted co-deposition technique. The effects of collagen concentration in the electrolyte on morphology, structure and composition of the coatings were systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adhesive strength of the coatings was also evaluated by scratch tests and tensile bond tests. It was demonstrated that the coatings of three-dimensional collagen network structure was formed on the C/C composites from the electrolyte containing collagen. The surface of the collagen network was covered by uniform CaP aggregates. The coatings were actually composites of CaP and collagen. Hydroxyapatite (HA) was a favorable composition in the coatings with the increase of the collagen concentration in the electrolyte. The formed collagen network increased the cohesive and adhesive strength of the coatings. The adhesive strength between the coatings and substrates increased as the collagen concentration in the electrolyte increased. The coatings prepared at the collagen concentration of 500 mg/L in the electrolyte were not scraped off until the applied load reached 32.0 ± 2.2 N and the average tensile adhesive strength of the coatings was 4.83 ± 0.71 MPa. After C/C coated with composite coatings (500 mg/L) being immersed in a 10⁻³ M Ca (OH)₂ solution at 30-33 °C for 96 h, nano-structured HA/collagen coatings similar to the natural human bone were obtained on the C/C.     

Fluorine NMR studies of solid SiF4

Fluorine NMR second moments and spin-lattice relaxation times were measured for solid SiF₄. The results indicate a hindered molecular motion and piezoelectricity in solid SiF₄.