Poly(L-lactic acid)s with molecular weights ranging from 500 to 43,000 were synthesized by the direct polycondensation of L-lactic acid. Both initial and total hydrolytic degradation rates in a buffer solution increased with decreasing Mw. Poly(L-lactic acid) with the lowest Mw showed a significantly high degradation rate. This may be due to its Tg lower than the temperature of degradation experiment at which the water molecules easily diffuse into the matrix. Although the hydrolytic degradation can be theoretically described by the first-order reaction, the weight loss of the low Mw poly(L-lactic acid) in the buffer solution deviated from the behavior predicted. The rate of the drug elution from the film showed a maximum at the first day, irrespective of the Mw of poly(L-lactic acid) suggesting that the drug dispersed near the film surface elutes most rapidly. The drug elution rate from the film with the highest Mw was not significantly different from those from low Mw films. This may be due to the similar degree of crystal to those of the low Mw film. These results indicate that the drug elution behavior is affected not only by the molecular weight but also the degree of crystal of the matrix poly(L-lactic acid).
In order to improve the brittleness of Poly(lactic acid) (PLA), the effect of blending several kinds of polyamide 12 (PA12) with PLA through Reactive Processing (RP) method was investigated. During RP, a maleic anhydride-grafted PLA (MAPLA) was synthesized as a pre-compatibilizer before blending PA12. From the result of static tensile test of PLA/PA12 with MAPLA, the elongation at break was increased and impact strength was increased as well. From the SEM observation results of PLA/PA12 with/without MAPLA, the diameter of PA12 phase in PLA with MAPLA was getting smaller than that of PA12 phase in PLA without MAPLA, which was good agreement with the mechanical properties of PLA/PA12 with/without MAPLA.
We investigated the aggregation structure and surface properties of ethylene-maleimide copolymer thin films as a function of fraction of fluoride side group by means of small angle X-ray scattering (SAXS), grazing incidence SAXS (GISAXS), DSC, and contact angle measurements. The maleimide copolymers were synthesized through the imidization of poly(ethylene-alt-maleic anhydride) with stearylamine and fluoroalkyl amine. The thin films were obtained by spin-coating on silicon wafers and annealed at appropriate temperatures. The copolymer having fluoroalkyl side chains (SFχ) form ordered structure when fluorine side chain content is more than 40%. The copolymer SFχ attached with stearyl and fluoroalkyl side chains formed the layered structure no less than 40 mol% of fluoroalkyl side group. In the thin film, the layer was found to be aligned parallel to the substrate as revealed by GISAXS and AFM. The surface free energy of the SFχ was revealed to decrease steeply around 40mol% of fluoroalkyl side group. The surface segregation of the fluorine groups linked to the ordered structure formation.
Poly(vinyl acetate)/poly(methyl glycidyl ether) (PVAc/PMGE) blends were found to be miscible in wide ranges of temperature and composition. We made dielectric relaxation measurements for this miscible blend at several compositions to examine the degree of dynamic heterogeneity. Two α-relaxation peaks were observed in both temperature and frequency dependencies of dielectric loss curves. These two relaxation modes were attributed to the segmental motion of each component. From the temperature dependence of each segmental relaxation time, we determined two effective glass transition temperatures (TgeffPVAc and TgeffPMGE) as functions of the blend composition. Existence of the intrinsic Tgeff of the components, which were different more than 20K, indicates that the PVAc/PMGE blend is ‘dynamically heterogeneous’. We found that the self-concentration model could semiquantitatively explain the composition dependence of each Tgeff.
We succeeded in the simulation of microphase separation in the semi-dilute solution of an ultra-high-molecular-weight block copolymer using a neutral solvent and a differentiating non-solvent. The solutions of polystyrene-block-poly(tert-butyl methacrylate) (PS-b-PtBuMA, the weight average molecular weight Mw is 8.2 × 105g/mol, the volume fraction of polystyrene fPS is 0.43) in a mixture of tetrahydrofuran (THF)/water as a good and neutral solvent and a differentiating non-solvent, respectively, were used. Microphase separation was induced by the addition of water into a disordered block copolymer/THF solution. The phase diagram where microphase separation was induced was indicated by means of spectroscopy using UV-vis spectrometer. The role of water in the semi-dulite solution was revealed by the spatial distribution of the solvents simulated by the self-consistent field theory (SCFT) approach. By increase of water content, the volume fraction of PS phase decreased in order to avoid an increase of enthalpy because of a contact between PS and water. THF and water collectively move into the PtBuMA phase and the ratios of water to THF in both phases are almost identical to each other. This means that the solvent mixture of THF and water behaves like strongly selective solvent as a whole.
Three-dimensional real-space images of the carbon fiber (CF)/polymer composites were obtained by a high-contrast X-ray computerized tomography (CT). Since both CF and polymers do not include heavy atoms, CF has not been supposed to be suitable to X-ray CT observation. In this study, we used the X-ray CT apparatus which is designed to enhance the contrast of the materials consisting only of light-weight atoms. Besides the usage of the appropriate apparatus, the experimental conditions were found to be important : cutting the sample into a thin rod, and obtaining sufficient number of projections. In our case, we used 1mm × 1mm × 4mm sample, and 720 projections with 0.25 degree intervals in order to obtain the 3μm voxel resolution in the reconstructed three-dimensional images. Eventually, each CF in polystyrene was clearly visualized in three dimensions.
Douglas fir wood flour was liquefied in the presence of phenol/catalytic amounts of sulfuric acid mixture, after which formalin was added and reacted. Then, the free phenol was distilled off under reduced pressure from the reaction mixture to obtain the liquefied and resinified wood. The liquefied and resinified wood was melt spun, stabilized by three dimensional curing and finally carbonized at 900°C. Carbon fiber made from liquefied wood without reaction with formaldehyde resulted in the formation of macropores in it, which were found by SEM observations. The resinification of the liquefied wood with adequate amounts of formaldehyde prevented the formation of the macropores, which was endorsed by carbonization of synthetic novolak fibers. Measurements of carbonization yield by TGA comes into line with these observations.
We have investigated the flow rate of methanol aqueous solutions through alginate sulfate (AS) electrolyte membranes under a constant pressure gradient. The solvent flow rate for the AS membranes with low cross-linking densities is considerably higher than that of alginic acid membrane when the methanol fraction (φ) is low. The solvent flow rate drastically decreases as φ increases, and no solvent permeation is observed when φ > 0.6. This strong φ dependence of flow rate is not simply explained by the φ dependence of swelling degree (Θ), because the reduction in Θ by increasing φ is considerably small. The confocal microscopy reveals a higher order network structure with a mesh size of ca. 10μm in the water-swollen AS membrane. The network structure results from the phase separation of the hydrophilic and hydrophobic domains. The hydrophilic and hydrophobic domains are mainly composed of the sulfated blocks and non-sulfated blocks, respectively. The mesh size of the network structure becomes considerably smaller as φ increases. The solvent permeability of the AS membranes is mainly controlled by the mesh size of the higher order network structure.
Gold nanoparticle chains are expected to be one of novel photonics materials in the next generation. We previously reported the formation of the gold nanoparticle chains from a colloidal gold solution on a glass substrate coated with oriented thin films of poly(tetrafluoroethylene) (PTFE). According to the proposed formation mechanism, some requisite conditions for the fabrication of the gold nanoparticle chains were extracted. In this study, the requisite conditions were reproduced in a flow cell using a glass slide with the oriented thin layers of PTFE. By applying temperature gradient to the cell and injecting a colloidal gold solution, a continuous flow was induced in the cell. By this method, the gold nanoparticle chains were successfully formed over the wider area compared to the previous reports. The gold nanoparticle chains were mostly located in the regions where the PTFE thin films were peeled off from the glass substrate. It was suspected that the negatively-charged gold nanoparticles could be electrostatically adsorbed on the glass surface owing to the positive contact charging of the glass slide after the peeling of the PTFE films.
The single relaxation time of epoxy glass, a thermoset having crosslinked molecular structures, was evaluated during tensile yielding process by using a nonlinear single relaxation model. The model consisted of two elastic springs expressing linear viscoelastic behavior and a dashpot with variable viscosity as a single parameter representing strain-induced structural change. We calculated the strain-dependent relaxation time τSS by fitting the model to experimental stress-strain curves observed at various strain rates and temperatures. The relaxation time τSS steeply decreased with increasing strain at the beginning of stretching, and then attained to low steady values at strains slightly larger than the yield strain. The steady values of τSS were almost inversely proportional to the strain rate, and slightly shorter at a higher stretching temperature. These dependences of τSS for epoxy glass on strain, strain rate and temperature were qualitatively identical to those for thermoplastic glassy polymers without crosslinked molecular structures. Strain-dependent relaxation times for thermoplastic glassy polymers are known as the result of change in glassy structures. Thus, it is concluded that the nonlinear viscoelastic behavior of crosslinked epoxy glass arises from strain-induced structural change.
We have developed a sophisticated sample cell with which one can impose horizontal temperature gradient on the sample under an optical microscope. The temperatures of the hotter and the cooler sides of the sample are individually maintained respectively by electric heaters and circulating water of which temperature is controlled accurately in a huge reservoir. Thus, with this cell we can attain the temperature difference (ΔT) up to 30.0°C for the sample which is embedded in the horizontal gap with 1 mm. The accuracy of the temperature control is ± 0.1°C. Furthermore, temperature jump (T-jump) experiments can be conducted by sudden switching of the water circulation paths from two individual reservoirs maintained at different temperatures T1 and T2 (corresponding to the T-jump from T1 to T2). Even by this primitive method, a satisfactorily quick T-jump can be accomplished (for instance, within 90 sec for 30.0°C T-jump from 50.0°C to 20.0°C). Both the linearity of the horizontal temperature gradient (ΔT) across the sample and homogeneity of the temperature normal to ΔT (namely, parallel to hotter and cooler side walls) were rigorously examined by employing a liquid crystal sample of which nematic-isotropic phase boundary was visually observed under the optical microscope with the hotter and the cooler temperatures being set respectively above and below the nematic-isotropic transition temperature. As a result, we found that the nematic-isotropic boundary is firmly parallel to the side walls, ensuring homogeneity of the temperature normal to ΔT. To examine the linearity of ΔT, both of the hotter and the cooler temperatures were step-wisely changed with keeping ΔT constant. At each step the horizontal position of the nematic-isotropic boundary was measured and was plotted as a function of the cooler side temperature. As a result, the plots exhibited good linearity for ΔT below 30.0°C. We show some interesting results as examples of application of our horizontal temperature gradient cell to research works on the non-equilibrium transient phenomena ; the first observation of overshoot crystallization in a mixture of n-alkane (paraffin) and easy determination of the equilibrium melting temperature of polymer.
When a rubber O-ring was used as a sealing material of a pressure vessel for high-pressure hydrogen gas such as 70MPa, which is the hydrogen pressure for fuel cell electric systems, the O-ring was extruded from the pressure vessel due to an increase in volume by gas absorption, and consequently cracks initiated in the previous study. Therefore, it is important to clarify the influence of hydrogen pressure on the increase in volume and mechanical properties in order to design the rubber O-ring for high-pressure hydrogen gas. From this viewpoint, unfilled and carbon black-filled sulfur-vulcanized NBR composites were exposed to hydrogen gas at a maximum pressure of 100MPa ; then, the influence of hydrogen exposure on volume increase and tensile properties of the composites was investigated. The residual hydrogen content which was measured 35 min after decompression increased with an increase in the hydrogen pressures ranging from 0.7 to 100MPa for the unfilled composite (NF) and the filled composite (CB). In contrast, the volume of NF and CB was hardly changed less than 10MPa, and the increase in volume by gas absorption was observed more than 10MPa. The elastic modulus of NF and CB decreased with an increase in volume, and this tendency can be successfully explained in term of the decrease in the crosslink density and elongation by volume increase. Although the tensile strength of NF hardly depended on the volume of specimen, the tensile strength of CB clearly decreased with an increase in volume. This is considered to be because a boundary structure between carbon black and rubber matrix was changed by hydrogen exposure.
We propose a framework that can be used to study the local thermodynamic stability of materials at finite temperatures, by reconstructing free energy surface based on metadynamics, constraint molecular dynamics and local atomic deformation tensor analysis methods. We apply the proposed framework to fcc embedded atom copper models, and estimate the activation energies, volumes, and critical local deformation tensor for a stacking fault nucleation event in copper single crystal.