Tetsuya Ogita

Molecular-weight dependence of morphology and mechanical properties of ultrahigh-molecular-weight polyethylene gel films

Abstract Films of three kinds of ultrahigh-molecular-weight polyethylenes (UHMWPE) with molecular weights of 6 × 106, 3 × 106 and 1 × 106 were produced by gelation/crystallization from dilute solutions. The dried gel films were stretched up to their maximum draw ratios. The molecular-weight dependences of morphology and mechanical properties of the drawn films have been examined using wide-angle X-ray diffraction, small-angle X-ray scattering, birefringence, crystallinity and stress relaxation modulus. It turned out that the long period, molecular orientation and crystallinity were hardly affected by molecular weight, while the stress relaxation modulus was strongly affected. The master curves indicated that the decrease in the stress relaxation modulus was more pronounced as the molecular weight of the test specimens decreased. This was thought to be due to a drastic increase of the slippage of molecular chains, leading to the disappearance of tie molecules and/or entanglement meshes under the external applied stress.

Relationship between drawability of poly(vinyl alcohol) films prepared from semi-dilute solutions and phase separation of the solutions studied in terms of stereo-regularity and degree of polymerization

Abstract The phase separation of poly(vinyl alcohol) (PVA) in dimethyl sulfoxide/water mixtures was investigated by using light scattering techniques in terms of the degree of polymerization and stereo-regularity. Atactic (at-) and syndiotactic (st-) PVA, the degrees of polymerization ( P ) being 17 900 and 1980, respectively, were used as test specimens. The analysis was mainly performed in comparison with previous results obtained for at-PVA ( P = 2000) whose degree of polymerization is similar to that ( P = 1980) of st-PVA, but lower than that ( P = 17 900) of at-PVA used in the present work. The phase separation occurred rapidly when the content of dimethyl sulfoxide in the mixed solvent was 50–70vol%. This tendency was found to be significant for st-PVA with high stereo-regularity, but to be almost independent of the degree of polymerization of at-PVA. The logarithm of scattered intensity from at- and st-PVA solutions increased linearly with time in the initial stage of the phase separation. If this phenomenon could be analysed within the framework of the linear theory of spinodal decomposition proposed by Cahn ( J. Chem. Phys. , 1965, 42 , 93) the phase diagram of st-PVA was classified into four regions: (I) a homogeneous sol region, (II) a sol region under spinodal temperature, indicating only spinodal decomposition, (III) a gel region under the spinodal curve, indicating simultaneous advance of both gelation and spinodal decomposition, and (IV) a gel region above the spinodal line, independent of liquid—liquid separation. On the other hand, for at-PVA, region II could not be observed and region IV existed only in a very narrow temperature range, which is similar to the result observed for at-PVA ( P = 2000) solutions in previous work. The drawability of the dried gel film was affected by the composition of the solvent mixture and by quenching temperature of the solutions, but it was hardly affected by the stereo-regularity. This interesting phenomenon is discussed in terms of morphology of gels and films, as studied by crystallinity, X-ray diffraction, birefringence and small angle light scattering under H v polarization condition. As the results show, it turned out that the greatest significant drawability is related to a continuous tissue with honeycomb-like structure associated with rapid phase separation of PVA solutions due to thermodynamic instability.

A light scattering study of orientation of liquid‐crystalline rodlike textures of poly(γ‐benzyl‐L‐glutamate) in an electric field by saturation electric birefringence method

The orientation behavior of liquid crystalline solutions of poly (‐γ‐benzyl‐L‐glutamate) (PBLG) in chloroform under an electric field was investigated with respect to characteristic electro‐optical properties of macromolecules using small angle light scattering and birefringence techniques. The Hv light scattering pattern showed a circular type in the absence of an applied field. This indicates large orientation disorder of optical axes with respect to the molecular cluster axis. When an electric field was applied, the scattering pattern became X type, characterizing the scattering from rodlike textures being a sort of molecular cluster and the scattering lobes were extended to the horizontal direction. This means the preferential orientation of the molecular cluster in the direction of an electric field. In order to analyze the variation of the scattering pattern with increasing electric field, we have carried out theoretical calculations of the basis of a model. In this model system, the orientation dis...

Morphology and mechanical properties of ultrahigh-molecular-weight polyethylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×106) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Youngs modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.

Crsslinking effect of ultrahigh molecular weight polyethylene-low molecular weight polyethylene blend films produced by gelation/crystallization from solutions

Polyethylene-polyethylene blend films were prepared by gelation/crystallization from semidilute solution by using ultrahigh molecular-weight (mw) polyethylene (UHMWPE) (mw=6×106) and low molecular weight polyethylene (LMWPE) (mw=4×104). The UHMWPE/LMWPE compositions chosen were 50/50, 67/33, and 91/9. Elongation was carried out in a hot oven at 115–130°C. The drawn films were exposed to an electron beam under nitrogen flow. Radiation doses chosen were 10, 20, 40, and 100 Mrad. crosslinking caused a significant effect in improving high temperature resistance for the blend film with draw ratio of 20 in the case of irradiation doses less than 20 Mrad. The elongation beyond 20 times and high doses beyond 20 Mrad hampered the crosslinking effect and the specimens were easily torn manually. This is thought to be due to the fact that the excess irradiation dose causes main chain scission apart from crosslinking.

Morphology and mechanical properties of ultrahigh-molecular-weight polyethylene films prepared from solutions in mixtures of diphenyl and diphenylether as co-solvent

In an attempt to check solvent dependence of drawability of ultra high-molecular-weight polyethylene films, gels were prepared from diphenyl/diphenylether solution, which is well known as Θ-solvent. The content of diphenyl in the mixed solvent was chosen to be 25 wt%. Rapid gelation occurred when the solution had been cooled down to temperature lower than 147°C. The gel shrunk drastically with the further decrease of temperature and formed a bulk specimen. The maximum draw ratio was 20 and the corresponding Youngs modulus reached 27 GPa. The value of Youngs modulus was higher than the maximum value of melt film stretched up to 30 times, but it was much lower than the value of film which was prepared by gelation from decalin solution and was elongated up to 300. Thus, it turned out that the drastic shrinkage in the process of rapid gelation hampers a suitable level of the entanglement mesh that effectively transmits the drawing force.

A light scattering study of orientation of liquid–crystalline rodlike textures of poly(γ‐benzyl‐L‐glutamate) in an electric field perpendicular to the direction of sample cell surface by means of saturation electric birefringence method. III

The orientation behavior of liquid crystalline solutions of poly(γ‐benzyl‐L‐glutamate) (PBLG) in chloroform was investigated with respect to characteristic electro‐optical properties of macromolecules using polarizing microscope and small angle light scattering techniques, when an electric field was applied parallel to an incident beam. The birefringence phase exhibited an irregular patchwork of predominantly blue, yellow, and pink regions under cross polars without applying the electric field and the corresponding Hv light scattering was circular. When the field was applied, the brightness of the phase decreased with increasing voltage and the corresponding Hv patterns became a typical X‐type characterizing the scattering from rodlike textures. The analysis was made for a model system. In this model system, rods orient randomly in the three‐dimensional space in the absence of the electric field, whereas when the electric field is applied, the rods orient predominantly parallel to the electric field. The ...

Polarized light scattering studies on the orientational behaviour of liquid crystalline rods immersed in shear flow under the action of an electric rectangular pulse

Abstract This paper deals with theoretical treatments for the deformation mechanism of liquid crystal rods by light scattering under Hv polarization condition, when the rods oriented by a shear flow are acted on by a rectangular electric pulse along the direction of the velocity gradient of flow. As in one example, the calculations were carried out to check whether the complicated orientational behaviour of superstructures such as rods and/or spherulites cause significant effect on the profile of Hv light scattering patterns. In the present system, the orientation distribution function of rods was obtained as the solution of the rotational diffusion equation for rotational ellipsoidal particles. In actual calculations for light scattering patterns, the orientational fluctuation with respect to the rod axis is considered to explain the circular type pattern under no external excitation which has been reported in previous experiments. When the orientation functions provide curves showing two peaks by the proper choice of parameters concerning electric field strength and velocity gradient, the corresponding Hv light scattering pattern showed four small dull lobes in the vertical direction indicating the preferential orientation of rods with respect to the shear flow direction in addition to the four sharp large lobes in the horizontal direction indicating the preferential orientation of rods with respect to the electric field direction. This indicates that the Hv light scattering pattern influences the two kinds of orientation of rods.

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.

General analysis of the measurements of the lateral crystal lattice moduli of semicrystalline polymers by x-ray diffraction and the application to nylon 6

A mathematical representation based on a linear elastic theory is proposed by which one may investigate the dependences of molecular orientation and crystallinity on the crystal lattice moduli and linear thermal expansion coefficients in the direction perpendicular to the chain axis as commonly measured by x-ray diffraction. In the theoretical calculation, a previously introduced model was employed in which oriented crystalline phase is surrounded by oriented amorphous phase and the strains of the two phases at the boundary are identical. The mathematical analysis indicated that the lateral crystal lattice moduli and linear thermal coefficients as measured by x-ray diffraction may be different from the intrinsic crystal moduli and linear thermal coefficients of a crystal unit cell, depending on the structure of the polymer solid. The numerical calculation was applied to nylon 6. As a result, it may be confirmed that the lateral crystal lattice moduli measured by x-ray diffraction are sensitive to the morphology of the bulk speciments and close to the intrinsic crystal moduli if the morphology of the test specimen can be represented by a parallel model with respect to the original stretching longitudinal direction.