Chitoshi Nakafuku

Effects of Molecular Weight on the Melting and Crystallization of Poly( L -lactic acid) in a Mixture with Poly(ethylene oxide)

The melting and crystallization behavior of poly(L-lactic acid) (PLLA) of different molecular weight (MW=2000, 66000, and 152000) in the binary mixture with poly(ethylene oxide) (PEO) of the different molecular weight (MW=3000, 100000, and 5000000) was studied by DSC, wide angle X-ray diffraction measurement and optical microscopy. Melting and crystallization process of PLLA depends on molecular weights of PLLA and PEO. Melting temperature (Tm) of the low molecular weight PLLA decreased drastically below the weight fraction of PLLA 0.2. Tm of high molecular weight PLLA decreased with decreasing PLLA in the mixture with low molecular weight PEO, while drastic decrease did not occur in the mixture with high molecular weight PEO. Temperature change of the morphological features of PLLA in the binary mixture depends on the molecular weight of PLLA and weight fraction of PEO.

High pressure d.t.a. study on the melting and crystallization of isotactic polypropylene

The melting and crystallization behaviour of isotactic polypropylene (iPP) in the α (monoclinic) and γ (triclinic) crystalline modifications has been studied by differential thermal analysis (d.t.a.) under high pressure. The d.t.a. melting thermograms of the sample prepared by slow cooling from the melt at atmospheric pressure (α form) and high pressure (γ form) show single endothermic peaks up to about 4600 bar in isobaric measurements. Two isolated melting point curves, corresponding to the melting of the α and γ modifications, were obtained and the melting points were fitted to a quadratic equation as a function of the pressure (bar). The slope of the curve at atmospheric pressure has been determined accurately for each crystalline form. Two curves were observed for the pressure dependence of the crystallization peak temperature in the d.t.a. curve. The origin of the separated crystallization curves is discussed in relation to the degradation of the iPP melt-crystallized under high pressure.

Crystal Structure of High Pressure Phase of Polytetrafluoroethylene

The crystal structure of high pressure phase of polytetrafluoroethylene (PTFE) has been determined by X-ray methods. A high pressure X-ray diffraction apparatus which enables to get the fiber pattern of crystalline sample under purely hydrostatic high pressure at high temperature has been constructed. A small specimen of PTFE fiber was held in the bore of a beryllium window for X-ray. The fiber pattern of high pressure phase of this polymer has been recorded on a cylindrical film at 5500km/cm2, 78°C. It is found that in this phase the molecules have a planar zigzag arrangement different from that of a helix at atmospheric pressure. The unit cell is orthorhombic, with a=8.73 A, b=5.69 A, c=2.62 A. The result of the X-ray diffraction studies agrees with that of the other physical properties of PTFE in high pressure phase.

Effect of pressure on the phase diagram of binary mixtures of n-alkanes

Abstract The effect of pressure on the phase diagram of n-triacontane ( C 30 ) n-docosane (C22) binary mixture and n-triacontane ( C 30 ) n-hexacosane (C26) binary mixture was studied up to 500 MPa by high-pressure differential thermal analysis. The C 30 C 22 mixture at 0.1 MPa has a eutectic-type phase diagram, with the hexagonal phase appearing just below the melting temperature on the C30 side. The eutectic temperature, Te, is 45.1°C and the eutectic composition lies between 15 and 20 wt% C30. The hexagonal phase and the low-temperature transition disappear above 100 MPa. The change of the phase diagram with pressure was interpreted by the different pressure dependence of the melting temperature, Tm, of C30 and C22 and that of Te. Pressure dependence of Te was 0.247 K MPa−1, which is smaller than that of C22 (0.279 K MPa−1) and C30 (0.276 K MPa−1). The C 30 C 26 mixture has a solid-solution-type phase diagram, showing an almost linear melting curve from the Tm of C30 to the Tm of C22, and the lower critical transition temperature on the hexagonal transition curve at 0.1 MPa. At elevated pressure, the melting curve changes to become downwardly convex and the transition curve moves close to the melting curve.

Unit cell variations of polyethylene crystal with temperature and pressure

Abstract The unit cell dimensions for the drawn and the extended chain crystal samples of polyethylene (PE) have been measured at temperatures between 20° and 130°C under hydrostatic high pressures up to 4000 kg/cm2, by the use of high pressure and high temperature X-ray diffraction apparatus. A clear difference was found in the variation of a, b and c dimensions of orthorhombic unit cell in PE with temperature and pressure. The temperature changes of linear compressibility for each axis direction of the unit cell and for cell volume were determined on the bases of the variation of cell dimensions. For the a-axis direction, a drastic increase of the compressibility was observed above ∼90°C but for the b- and c-axis directions, it was constant for all the temperature region of the measurement. The values of Gruneisen constant, γ, were evaluated at various temperatures from the compressibility data for both samples. The value of γ of the ECC sample was nearly constant below 90°C and gradually increased above 90°C; in the drawn sample, however, a rather steep increase was observed above 90°C.

Melting and crystallization of ultra-high-molecular-weight polyethylene in a mixture with tetracontane under high pressure

Abstract The melting and crystallization processes of ultra-high-molecular-weight polyethylene (UHMW PE) in a mixture with tetracontane (TC) were studied at elevated pressures up to 500 MPa by high-pressure differential thermal analysis. At atmospheric pressure, melting-point depression of UHMW PE occurs with increasing TC content, but it does not occur at 500 MPa. The phase transition to the hexagonal phase of UHMW PE is impeded by the addition of TC, and does not occur below a weight fraction of PE equal to 0.4 at 500 MPa. The content of extended chain crystals (ECC) of UHMW PE formed by high-pressure crystallization decreases with TC, and below a weight fraction of PE equal to 0.35, ECC do not form even on crystallization at 500 MPa.

Orientational order and thermodynamic properties of mainchain trimer liquid crystals: a combined use of 2H NMR, PVT and high-pressure differential thermal analysis

Abstract The trimer compounds (4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls) (CBA-T n , n =9, 10) with n indicating the number of carbon atoms in the spacer exhibit a nematic order on cooling from the melt. The orientational characteristics of the central and terminal mesogenic cores were separately investigated in the liquid-crystalline (LC) state for CBA-T10 by the 2 H NMR method. The ratio of the quadrupolar splittings was found to remain nearly constant over the entire range of the nematic state, suggesting that the conformation of the intervening spacer is also stable. The pressure-dependence of the phase boundary curves has been studied for transitions such as crystal↔nematic LC↔isotropic melt by the high-pressure differential thermal analysis. The transition entropies have been obtained according to the Clapeyron relation. A combined use of the PVT data led to an estimate of the constant-volume transition entropies, which is in reasonable agreement with the conformational transition entropies estimated previously by the rotational isomeric state analysis of the spectroscopic data.

Crystallization of poly(ethylene oxide) in a mixture with poly(methyl methacrylate) under high pressure

Abstract The effects of mixing with poly(methyl methacrylate) (PMMA) on the melting and crystallization process of poly(ethylene oxide) (PEO) were studied under high pressures up to 500 MPa by d.t.a. The intensity of the X-ray diffraction peak of PEO in the sample crystallized at 500 MPa was compared with the intensity of the sample crystallized at 0.1 MPa. Melting point ( T m ) depression of PEO observed at 0.1 MPa with decreasing PEO in the mixture was almost constant up to 500 MPa. The depression rate of the crystallization temperature ( T c ) of PEO with PMMA content decreased at elevated pressures up to 500 MPa. The intensity of the exothermic peak of crystallization of PEO in the d.t.a. curve of the sample of medium PEO content increased with pressure. The intensity of the wide-angle X-ray diffraction line of PEO in the sample crystallized at 500 MPa was larger than that of the sample crystallized at 0.1 MPa. Optical microscopic observation of the high pressure crystallized lower PEO content blend showed partial spherulite formation, indicating increased crystallizability at elevated pressure.

High pressure crystallization of poly(ethylene oxide) and poly(methyl methacrylate) mixture

Abstract Dynamic mechanical and thermal properties of the binary mixture of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) crystallized at 500 and 0.1 MPa were studied. The result shows that the separation of the PEO and PMMA molecules occurs at high pressure.

Effect of pressure on the melting and crystallization of pseudo binary mixtures of polyethylene and high melting temperature diluents

Abstract The melting and crystallization behaviour of pseudo binary mixtures of polyethylene (PE)/1,3,5-tribromobenzene and PE/hexamethylbenzene were studied at elevated pressures up to 500 MPa by differential thermal analysis (d.t.a.). The phase diagram of these binary mixtures under high pressure was determined and compared with that of a PE/1,2,4,5-tetrachlorobenzene binary mixture. The pressure dependence of the eutectic temperature T e of the PE/1,3,5-tribromobenzene mixture at 1 atm was 0.33°C/MPa and for PE/hexamethylbenzene 0.31°C/MPa. The eutectic composition does not move at elevated pressure. In both mixtures, the area between the liquidus curve of the diluents and the eutectic line of the hypoeutectic region in the phase diagram increases with pressure but the area between the liquidus curve for PE and the eutectic line in the hypereutectic region decreases with pressure. In the PE/1,3,5-tribromobenzene system, the phase transition from the orthorhombic phase to the hexagonal phase of PE does not occur under high pressure but the transition did occur in the PE/hexamethylbenzene system. The d.t.a. study at 1 atm showed that the formation of an extended chain crystal of PE was largely impeded in the mixture with 1,3,5-tribromobenzene and 1,2,4,5-tetrachlorobenzene.