Chunye Xu

Electrical properties of polyethylene and carbon black particle blends prepared by gelation/crystallization from solution

Abstract Composite materials based on low molecular weight polyethylene (LMWPE), ultra-high molecular weight polyethylene (UHMWPE) and carbon black (CB) particles were prepared by gelation/crystallization from solution. The positive temperature coefficient (PTC) intensity for the 90/10 (LMWPE/UHMWPE) composition exceeded five orders of magnitude for the specimens heat-treated at a suitable temperature, which was almost equal to that observed with LMWPE–CB blends prepared by a kneading method. In comparison with LMWPE–CB blends, much promoted reproducibility of PTC effect and inhibition of the negative temperature coefficient (NTC) effect were achieved.

Relaxation mechanism in several kinds of polyethylene estimated by dynamic mechanical measurements, positron annihilation, X-ray and 13C solid-state NMR

Abstract Relaxation processes of several kinds of polyethylene films and fibers with different molecular orientational degrees and crystallinities were extensively investigated by the dynamic mechanical relaxation, positron annihilation and 13 C nuclear magnetic relaxation ( 13 C NMR). From complex dynamic tensile modulus, the activation energies of α 1 and α 2 relaxations were determined to be 97–118 and 141–176 kJ/mol, respectively. The activation energy of β relaxation was 114–115 kJ/mol. These values were similar to those of α 1 relaxation reported already. For γ relaxation mechanisms, there existed two mechanisms, γ 1 and γ 2 , the activation energies being 9–11 and 23–25 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that non-crystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From 13 C NMR measurements of 13 C longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value is close to the activation energy (23–25 kJ/mol) of the γ 2 relaxation estimated by the dynamic mechanical measurement. The result by 13 C NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by 13 C NMR indicated that the γ 2 relaxation mainly is due to the motion of the C–C central bond of a short segment (e.g. three to four CH 2 ) within interfacial-like amorphous phase. The γ and β relaxation peaks by the dynamic mechanical measurements corresponded to the first and second lifetime transition of ortho -positronium indicating, in turn, a drastic change in free volume by local mode relaxation.

Study of carbon films from PAN/VGCF composites by gelation/crystallization from solution

A carbon film with a cross-sectional area much larger than that of a commercial carbon fiber (>6000 times) and a thickness of about 0.3 mm was obtained using a new method. In this method, composite materials of polyacrylonitrile (PAN) and vapor-grown carbon fiber (VGCF) prepared by gelation/crystallization from dilute solutions were used as starting materials The gelation/crystallization method was adopted to ensure high orientation of PAN chains. The composite materials were heat-treated at 200–300°C in an oxidizing atmosphere for thermal stabilization and then heat-treated to 1500°C in argon gas to promote carbonization. The tensile modulus and electric conductivity for the carbon materials with cross-sectional areas of about 0.6 mm2 (thickness 0.3 mm and width 2 mm) reached 18 GPa and 10 Ω−1 cm−1, respectively. The mechanical and electrical properties of the final carbonized materials were sensitive to the PAN/VGCF composition and the draw ratio. These phenomena were analyzed using Fourier transform IR and X-ray diffraction.

Deformation mechanism of polyethylene spherulites estimated by crystal orientation distribution function and small angle light scattering under Hv polarization condition

The deformation mechanism of polyethylene spherulites was studied by using melt films of high density polyethylene with a 78% crystallinity in terms of crystallite orientation distribution estimated by X-ray diffraction technique and small angle light scattering under Hv polarization condition. In doing so, a model relating crystal orientation to the deformation of polyethylene spherulite was proposed. The distribution function of crystallites within a spherulite was assumed to be a function of crystal lamella. The distribution functions of reciprocal lattice vectors of given crystal planes were derived from two functions of the crystallites and lamellae with several parameters. The best values of each parameter were determined by the simplex method associated with direct research method to obtain the object function on the basis of trial and error. As a result, it was found that at an initial elongation ratio of 1.5, the most important factor in fitting the model to experimental results, was the preferred orientation of lamellar axis but most of the crystallites within the lamella were maintained without taking characteristic orientation. The Hv scattering from a deformed three-dimensional spherulite was formulated on the basis of the proposed model. The scattered intensity distribution in an undeformed state showed the typical four-leaf clover pattern and the lobes extending in the horizontal direction indicated the deformation of spherical to ellipsoid structure with a draw ratio of 1.5. The calculated patterns were in good agreement with those observed.

Young's Modulus Estimated from Orientation Distribution Functions of Crystal and Amorphous Phases for Simultaneous Biaxially Stretching Ultra-High Molecular Weight Polyethylene Films Prepared by Gelation/Crystallization from Solutions

Youngs modulus of simultaneous biaxially stretched films was much lower than that of uniaxial films with the same draw ratio. To address this problem, a theoretical approach was carried out by using a three-dimensional model. In this model system, the oriented crystalline layers are surrounded by an anisotropic amorphous phase and the strains of the two phases at the boundary are identical. Youngs modulus was calculated by using the generalized orientation factors of crystallites and amorphous chain segments calculated from the orientation function of crystallites and that of amorphous chain segments. The orientation function of crystallites was determined from the orientation functions of the reciprocal lattice vectors of the crystal planes. The theoretical values of Youngs modulus were calculated using the values of elastic compliance proposed by Zehnder. The values were in fairly good agreement with the experimental values of specimens with different draw ratios. Judging from the induced equations, the calculated results provided that the Youngs modulus of simultaneous biaxially stretching film is attributed to the contribution from the compliance associated with shear modulus and consequently the Youngs modulus automatically takes low values.

Morphology of poly(vinyl alcohol) gels assuring the greatest significant drawability

Atactic poly(vinyl alcohol) (at-PVA) and syndiotactic poly(vinyl alcohol) (st. PVA) prepared by gelation/crystallization using dimethyl sulphoxide/water mixtures were drawn in a hot oven at 160 °C under nitrogen. The degrees of polymerization of at- and st-PVA were 2000 and 1980, respectively. The drawability of at- and st-PVA films was affected by the composition of the solvent mixture as well as by quenching temperature. The drawability of at- and st-PVA films prepared by using the solvent mixture containing 60% of dimethyl sulphoxide and 40% of water became more pronounced as the temperature of gelation/crystallization decreased and the draw ratio reached maximum value at −80 °C. Namely, the greatest significant drawability was the same condition for at- and st-PVA films in spite of the different stereo-regularity. Even in this common best condition for significant drawability, however, the morphological properties of swollen gels and of the resultant dry gel films are different each other, dependent upon the tacticity. For at-PVA, small-angle light scattering under Hv polarization condition could not be observed in the swollen gels and in the dry films when the solutions were quenched at temperatures <−10 °C. In contrast, for st-PVA, the X-type scattering pattern from swollen gels became clearer as the temperature decreased but the pattern became indistinct under drying process at ambient condition. On the other hand, the fibrillar textures within the at- and st-PVA dry films became finer and the orresponding crystallinity became lower as the temperature of gelation/crystallization decreased. Thus it turned out that the morphological properties of the swollen gels and of the dried films play an important role to assure the greatest significant drawability.