Crystallization behavior of poly lactic acid (PLA) in addition of Cardo materials having fluorene moiety was investigated. From DSC results, it was found that PLA added Cardo materials had two melting peaks: the peak at a lower temperature was corresponding to the melting temperature of δ－form crystal and the higher one is that of α－form crystal structure of PLA. From Temperature-controlled XRD results, the δ－form crystal structure of PLA can also be detected at high temperature regions as well as the α－form crystal structure in PLA matrix when Cardo materials were added into PLA. Furthermore, the isothermal crystallization processes of PLA with Cardo materials were estimated by means of rheorogical measurement and optical microscopic observation. These results indicated as follows; (a) the activation energy on crystallization were 43.7 kJ/mol for neat PLA, 68.1 kJ/mol for PLA with BCF (5 wt%), and 60.1 kJ/mol for PLA with BPF (5 wt%), and (b) the crystal growth rate of the PLA with Cardo materials was getting much slower than that of neat one.
High-contrast X-ray computerized tomography (CT) observation reveals that the cavitation is commonly found in the glass fiber (GF) / polymer composites prepared by injection molding. Generally, the cavities in materials often lead to the poor mechanical properties, so that the reduction of the cavities is a serious issue in the mass-production of the plastic products. In this study, the cavitation is found to be localized in the middle of the test piece by X-ray CT observations, which is strongly related to the skin / core layer structure commonly known in the field of the injection molding. This fact strongly suggest that the cavitation is caused by the volume shrinkage during the cooling process after stopping the plastic flow, and that the phenomenon is coupled with the sink-mark. The analysis of the volume of cavities and sink-mark suggest not-simple cooling process.
X-ray total reflection by the surface of fibers produces strong streaks in the direction perpendicular to the fiber axis. In order to analyze higher-order crystalline structures in the fiber, the streaks overwhelm valuable scattering from the structures we intend to analyze. Shish-kebab structure is one of the most important higher-order crystalline structures because it plays a key role for imparting high strength to general fibers. It is known that the streaks due to the X-ray total reflection from the fiber surface can be eliminated by immersing fibers into a liquid of which electron density was matched to that of the PET fibers because the contrast between two neighboring objects is proportional to the square of the electron density difference between the two objects. For the sake of the precise structural analyses, we have conducted two-dimensional small-angle X-ray scattering (2d-SAXS) measurements at room temperature for poly(ethylene terephthalate) (PET) fibers prepared with high-speed melt spinning being immersed in non-solvent of which electron density was matched to that of the PET fibers. For this purpose, a bundle of the fibers was set in a quartz capillary of which axis is parallel to the fibers (the edge-view image). As a consequent, it was found that the spacing between the neighboring shish structures and that between the neighboring kebab structures were both increased with an increase in the spinning velocity. We also present a succeeded example of a sophisticated method for end-view 2d-SAXS measurements for the PET fibers dipped in the electron-density matching non-solvent, where the incident X-ray beam was exposed to the fibers from the direction parallel to the fiber axis. The result of this measurement was consistent with that of the measurements for the fibers in the quartz capillary.
Phase separated structure induced by radical copolymerization of poly(dimethyl siloxane)-α,ω-diacrylate (PDMS-DA) and N,N-dimethyl acrylamide (DMAA) was investigated by small angle X-ray scattering (SAXS). The copolymerized gel of PDMS-co-PDMAA had bicontinuous phase separated domains (hydrophilic and hydrophobic) with periodicity in nano meter scale. In present work, the structure of the gels in three different conditions (1: dry state, 2: swollen in water, 3: swollen in water/methanol mixed solvent) was analyzed by SAXS. The hydrophilic region was confirmed to be swollen selectively by water and methanol. The electron density of hydrophilic region varied according to the mixture ratio and adsorption amount of the solvents, although the phase separated structure also changed by swelling. The experimental scattering intensity was compared with theoretical one calculated by multiplying square of the electron density difference and volume fraction of each domain. In dry state and in swollen state with water, the scattering intensity was interpreted by two phases. In contrast, the methanol content dependence of the SAXS intensity of the gels in does not agree with the model composing the two regions but fits quite well with three-region model in water/methanol mixed solvent. The solvent poor region near the interface between the phase separated domains was disclosed when the electron density of hydrophilic domains was close to that of hydrophobic (PDMS) domains.
The shear tests for amorphous polymers, (polyethylene terephthalate) (PET) and polycarbonate(PC) are performed under various conditions of strain rates and temperatures below the glass transition. Diffuse slip line(DSL) yield mode in which amorphous polymers deform plastically without a macroscopic shear band and necking has been observed under the conditions of relatively higher temperature and lower strain rate. The dependence of yield mode of PET and PC on the temperature and the strain rate is shown on a common yield mode map drawn with temperature difference from their glass transition temperatures, which suggests strongly that mobility of segment chains under the mechanical stress should be responsible for the generation and growth of diffuse slip lines. The growth rate of the diffuse slip line is given by the rate process theory, and the activation energy and the activation volume are estimated to be 240 kJ/mol and 3.1 nm3 for PET, and 110 kJ/mol and 5.3 nm3 for PC.
In our previous study, diffuse slip line (DSL) yield mode without an inhomogeneous deformation, local shear band and necking, has been found by in-situ observations of simple shear deformation for glassy polyethylene terephthalate (PET) under the conditions of low strain rates and high temperatures. Growth process of DSL can be formulated as a thermally activated process, that is, Eyring's flow model. The growth rate and density of DSL increase with increasing stress and strain, respectively. Plastic deformation proceeds by molecular process which activated segments of polymer chains exchange their positions with free volume under applied shear stress. In the present study, several parameters necessary for an empirical simulation of stress-strain curves, that is, activation parameters, variation rate of DSL length with stress, and variation rate of DSL density with strain have been determined experimentally. Using these parameters, stress-strain curves are culculated by an analogical analysis to Johnston-Gilman’s mathematical method for the yield of metals.
High-Polymer and rubber are important materials. Evaluation method of stress-strain behavior of them is demanded in order to use them with reliability. But it is difficult especially in unloading process and in fatigue. And the mechanical properties of high-polymer and rubber are deteriorated more easily than that of metal. The constitutive equation that can consider the deterioration and that can evaluate deformation in unloading process is demanded. Nitrile Butadiene Rubber (NBR) is hard rubber used as O-ring and gasket, etc. In this study, the deterioration tests of NBR in water at 60C for 0 month, 3 months, 7 months, 9 months and 12 months were carried out. Compression tests of NBR in water at 25C were carried out with deteriorated material. VBO model, one of constitutive equation, is applied to evaluate the deformation of them. The mechanical properties used in VBO model were taken from the constant-strain-rate tests and relaxation tests. And the influence of deterioration to mechanical properties used in VBO model was obtained. The stress-strain curve simulated with VBO model well corresponded to that of test in loading process, but didn’t in unloading process. Therefore we improved VBO model with the strain acceleration term. We showed the probability that the improved model could evaluate the stress-strain behavior in unloading process.
Polyvinylphenol (PVPh) and Poly-2-vinylpyridine (P2VP) are known to form polymer complex through the hydrogen bonding interaction between the nitrogen of P2VP and hydroxyl group of PVPh. We conducted differential scanning calorimetry for this particular blend in the wide range of blend composition and examined the effect of H-bonding interaction on the glass transition behavior.
Damping behavior is investigated for five types of high density polyethylene (HDPE) samples with broader molecular weight distributions. The melt of moderately entangled HDPE shows a weaker damping than the Doi-Edwards (DE) prediction, maybe due to a broader molecular weight distribution. On the other hand, the highly entangled HDPE melts with broader molecular weight distributions show stronger damping than the DE prediction, as in the case of highly entangled systems with narrow molecular weight distributions. The strong damping in the former case appears different in origin from the latter.
The single relaxation time of glassy epoxy networks having different crosslink densities was evaluated during uniaxial compression processes by using a simple nonlinear single relaxation model to study the effect of crosslinked molecular structures on the transient nonlinear viscoelastic behavior of glassy epoxy networks. 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 during the compression by fitting the model to the experimental stress-strain curves obtained at various strain rates and at temperatures T 18℃ lower than the glass transition temperature Tg of the material being compressed. With increasing strain εn, τSS obtained for each epoxy network steeply decreased in the pre-yield region of strain and then reached to low steady values appearing in strain ranges larger than the yield strain, reproducing previously reported typical behavior observed for glassy polymers under large deformation. When compressed at an identical strain rate, τSS-εn relations for all epoxy networks almost coincided each other despite the difference in the crosslink density. This result indicates that, if the deformation is performed at the same condition i.e., strain rate and T-Tg, strain-induced variation of relaxation time in the epoxy glasses is almost not influenced by crosslinked molecular structures.