Linear viscoelastic behavior was investigated for a poly(dimethyl siloxane) (PDMS) gel formed through a bulk double-liquid crosslinking reaction of two types of vinyl-terminated PDMS prepolymers of the molecular weights Mpre ≈ 35×103. Time-temperature superposition worked, and master curves at 20 °C were constructed for the storage and loss moduli, G' and G", in a wide range of angular frequency ω (= 103 −10−5 s−1) by combining the data obtained from dynamic oscillatory tests and creep tests. With decreasing ω to 10−1 s−1, G" decreased in proportion to ω0.6 and G' rapidly decreased to its equilibrium plateau at the modulus Ge = 2800 Pa. On a further decrease of ω well in the plateau regime of G', G" decreased in proportion to ω0.3. Thus, the gel exhibited the fast and slow relaxation processes characterized with these types of power-law behavior of G". The molecular weight between the crosslinks evaluated from the Ge data (as well as the equilibrium swelling ratio in toluene), Mc ≈ 340 ×103, was about ten times larger than Mpre. The crosslinking reaction was made in the bulk state but still gave such a scarce gel network (with Mc ≈ 10 Mpre) possibly because a large amount of sol chains and dangling chains had diluted the trapped entanglements during the reaction. From the analysis of the G' and G" data on the basis of the above Mc value and the intrinsic Rouse relaxation time, the fast relaxation process was assigned as the Rouse-like constraint release (CR) process of individual gel strands. The polydispersity of the strands was found to be essential for the power-law behavior (G"∝ωn with n ≈ 0.6) to be observed in the plateau regime of G'. The slow relaxation process was related to fluctuation of the crosslinking points, which is equivalent to cooperative Rouse-CR motion of many gel strands connected at these points.
Bread dough exhibits unique viscoelastic behavior, and the relationships between the properties of bread dough and the quality of final products have not been clear. The objectives of this study are to predict the baking characteristics using physical properties of bread dough. Several kinds of dough were prepared at three levels of second fermentation time (35, 50 and 65 min at 38 °C), which seriously affects baking characteristics. Breads were made from these kinds of bread dough. Specific gravity and pH of the dough decreased with increasing time of the second fermentation. The bread dough with 50 min of second fermentation time had the lowest value of mechanical loss tangent of 0.87±0.03. Dielectric loss tangent of the dough increased with increasing second fermentation time. The non-liner multiple regression models using the values of physical properties of the dough as independent variables provided a good prediction of the loaf volume (R2= 0.87∼0.94).
Effect of molecular weight on structural evolution in compressively strained commercial poly(methyl methacrylate)(PMMA) was studied using low and high molecular weight samples abbreviated here as L-PMMA and H-PMMA, respectively. The weight-averaged molecular weight of L-PMMA was 8×104, whereas that of H-PMMA was 338×104. The polydispersity indices MW/Mn were 2.8 for L-PMMA and 4.7 for H-PMMA. Structural evolution under various amounts of compressive strain in L-PMMA and H-PMMA was monitored at an arbitrarily selected temperature 20 °C less than their glass transition temperature Tg in a time range of 0 to 105 s by constant-rate mechanical compression test and differential scanning calorimetry(DSC) technique. Well-defined structural evolution was observed in a manner qualitatively quite similar in both PMMA samples as progressive developments of the yield stress in constant rate compression test and the endothermic peak at temperatures less than Tg in DSC measurement. The structural evolution was dependent on time period passed under strain and the amount of strain, yet apparently independent of molecular weight. Such quite similar behaviors of the structural time evolution in L-PMMA and H-PMMA are likely to be attributed to relatively wide molecular weight distribution of each sample tailing off in a lower molecular weight range.
Aqueous solutions of thread-like micelles of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) are known to be classified into three types depending on their rheological properties that change with the concentration ratio of salt to detergent: When the concentration of NaSal, CS, is much lower than that of CTAB, CD, the dynamic modulus, G*, is similar to that of dilute polymer solutions without entanglements (Type I). With increasing salt concentration (CS < CD), G* becomes similar to that of entangled semidilute polymer solutions (Type II). For the case of CS ≥ CD, G* is described with the Maxwell model having a single relaxation time (Type III). In this study, nonlinear viscoelastic behavior of type II solutions was examined. G* for the solution with CD = 1.0×10−1 molL−1 and CS = 3.25 ×10−2 molL−1 was very similar to that of the ordinary entangled polymer solutions. The damping function determined from the stress relaxation experiments was close to that of the entangled polymers and well described with the Doi-Edwards model. The characteristic time for nonlinear rheology, τeq, was determined from the stress overshoot of viscosity growth function. τeq was found to be about thirty times smaller than the characteristic time for nonlinear stress relaxation. These features were in accord with those of the ordinary entangled polymer solutions. However, shear rate dependence of the steady viscosity was not described with the Cox-Merz rule, suggesting that a shear induced structure might have been developed at long times during a steadily flowing state.
To establish the design methodology of skin care products with good application feeling based on physical evidence, the correlation between rheological and sensory properties of skin care products was examined. To clarify the effect of surface properties of skin, fluidity tests were performed using glass plate and artificial leather other than skin. The results of fluidity test showed a strong correlation with the results of sensory evaluation for each substrate. To evaluate the surface free energy of skin care properties and substrates, contact angles were measured. Application feeling was excellent when hydrophilic-oleophilic property of skin care product was similar to that of substrate. It was found that the application feeling of skin care product depends not only on the viscoelasticity of itself, but also on the surface free energy of substrate and skin care product.
Annealing effects on the elastic behavior of sphere-forming polystyrene-block-polyisoprene-block-polystyrene (SIS) and polystyrene-block-polyisoprene-block-poly(2-vinylpyridine) (SIP), annealed in a cone-plate attached to a rheometer at 150 °C with and without additional shear, are examined by dynamic mechanical analysis performed at 70 °C. Plateau modulus GN obtained at 70 °C rapidly increased within the first one hour, while GN very gradually increased at longer times. By comparing the data with and without additional shear, the initial increase of GN is attributed to increase in restoring force of the lattice caused by the relaxation of lattice distortion. The gradual increase at longer times is attributed to reformation of network structure by reentrant of pulled-out end blocks into the spherical domains since it was faster for SIS than SIP; S and P chains are immiscible with each other so that it takes longer time for SIP to reform the network structure. Contribution of pull out of end blocks and network reformation on elastic properties for molded film are not so large.