For a lightly cross-linked poly (methyl methacrylate) (PMMA) with a gel fraction of 0.65, experimental studies are made on swelling, dynamic viscoelasticity, and uniaxial and biaxial extension at constant strain rates. The values of average molecular weight between cross-links, Mc, (or between a cross-link and a trapped entanglement) are determined by four methods; swelling, the equilibrium modulus, the Mooney-Rivlin plot and the Young's modulus. These values are in the same order but somewhat smaller values are obtained from the latter two methods. The density of weakly attached or untrapped entanglement strands is evaluated from the plateau modulus of (G' −Geq), where G' and Geq are the storage and equilibrium moduli. The entanglement molecular weight, Me, evaluated by this method is certainly smaller than Mc, and is slightly larger than Me of uncross-linked atactic PMMA melt. In uniaxial extension, the stress growth coefficient shows stronger strain-hardening than that of uncross-linked PMMA melts with very high molecular weight component. On the other hand, in biaxial extension, the stress growth coefficient exhibits weak strain-softening followed by small upturn.
Cross-linked poly(methyl methacrylate)s with various degrees of cross-linking were prepared, and their swelling, dynamic viscoelasticity and uniaxial extension behavior was investigated. For each sample, the values of molecular weight Mc between cross-links (and trapped entanglements) evaluated from the equilibrium modulus Geq and the Mooney-Rivlin constants (C1+C2) are comparable, while Mc evaluated from the gel fraction is somewhat higher than those values. It is suggested from the data of excess storage modulus (G'−Geq) and the Mooney-Rivlin constant C2 that untrapped entanglement density decreases slightly while trapped entanglement density increases slightly with increasing degree of cross-linking. The strain-hardening tendency at constant strain rates becomes weaker with increasing cross-linking degree, and the strain-hardening does not appear when Mc approaches the entanglement molecular weight Me. It is suggested for cross-linked polymers with rather high cross-linking density that chain breakage will occur before chain strands between cross-links stretch enough to result in the strain-hardening.
In this study, we mainly investigated the flow behavior of nematic and smectic liquid crystals. Mechanical rubbing, chemical modification and photo-alignment methods were applied to make up different kinds of surface orientation on the substrates. After these various treatments, the flow behavior of liquid crystals in slit cell having parallel, perpendicular or homeotropic orientation on substrates was investigated. It was observed that the value of apparent viscosity η becomes higher in the order of η para, η perpen and η homeo in the nematic phase. Furthermore, on the mechanical parallel and perpendicular rubbing layer, the effect of electric field on the flow behavior of liquid crystals was examined. We found that the apparent viscosity becomes higher with an increase of the value of electric field.
Two dimensional jets of a visocoelastic fluid have been studied using aqueous surfactant solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). The solution shows strong viscoelasticity even at low concentration condition 0.01[mol/l] and 0.07[mol/l] for CTAB and NaSal, respectively. Shapes of jets issuing from slits are observed, and the birefringence of sample solution is measured along the flow direction from the inside to the outside of the slit. Elastic stresses of jets are evaluated from their shapes, and the degree of orientation of thread-like micelles is calculated from the value of birefringence. The elastic stress of the slit jets agrees with that of capillary jets, and the value of birefringence increases and is followed by decreasing inside of the slit near the exit. The increase and decrease in the value of birefringence is related to a change in the orientation of micelles caused by the contraction and expansion of the flow near the exit. The increase in the value of birefringence is also observed with die swelling.
Effects of cell destruction on compression moduli of four closed-cell polyethylene were studied by the dynamic viscoelastic measurements in compression mode and the static compression test. In the dynamic viscoelastic measurements, dynamic modulus after cell destruction was smaller than that before cell destruction, and in foams after cell destruction the relations with the dynamic modulus and the initial shrinkage for foams of small elastic modulus was different from those for foams of large elastic modulus. In the static compression test, stress-strain curves of virgin foams gave a bend in the small strain region as observed for expanded polystyrene foams, whereas after cell destruction the stress monotonically increased with strain for foams having low elastic modulus. On the other hand for foams of high elastic modulus, stress-strain curves gave a bend in the small strain region as observed for virgin foams after cell destruction. It is considered that the ratio of the resistance against the pressure inside the cells to the total resistance varies with the elastic modulus of foams.
For evaluation of thermal endurance in foamed plastics, temperature and time characteristics of compression dynamic modulus of four expanded polystyrene foams were investigated by the dynamic viscoelastic measurements in compression mode. Log t which dynamic modulus 10 or 20% fall as compared with the start time of measurement, has been found to relate linearly with the reciprocal of the absolute temperature from the time characteristics of dynamic modulus in foams before destruction of cell. Thermal endurance temperature that is a measure to evaluate thermal endurance is not suitable for being given by the temperature characteristics of the dynamic modulus, and is suitable for being given by the temperature for setting up durable time from the relations between log t and the reciprocal of the absolute temperature.