Linear Dynamic Viscoelasticity of Dual Cross-Link Poly(Vinyl Alcohol) Hydrogel with Determined Borate Ion Concentration

We investigated the linear dynamic viscoelasticity of dual cross-link (DC) poly(vinyl alcohol) (PVA) (DC-PVA) hydrogels with permanent and transient cross-links. The concentrations of incorporated borate ions to form transient cross-links in the DC-PVA hydrogels (CBIN) were determined by the azomethine-H method. The dynamic viscoelasticity of the DC-PVA hydrogel cannot be described by a simple sum of the dynamic viscoelasticity of the PVA gel with the same permanent cross-link concentration and the PVA aqueous solution with the same borate ion concentration (CB = CBIN) as in the DC-PVA gel. The DC-PVA hydrogel exhibited a considerably higher relaxation strength, indicating that the introduction of permanent cross-links into temporary networks increases the number of viscoelastic chains with finite relaxation times. In contrast, the relaxation frequency (ωc) (given by the frequency at the maximum of loss modulus) for the DC-PVA hydrogel was slightly lower but comparable to that for a dilute PVA solution with the same CB. This signifies that the relaxation dynamics of the DC-PVA hydrogels is essentially governed by the lifetime of an interchain transient cross-link (di-diol complex of boron). The effect of permanent cross-linking on the relaxation dynamics was observed in the finite broadening of the relaxation-time distribution in the long time region.

RETROGRADATION PROPERTIES OF HEAT MOISTURE TREATED (HMT) SAGO AND ARENGA STARCHES

This study aimed to investigate the retrogradation rate of heat moisture treated sago and arenga starches using different approaches, including a thermal approach using DSC (differential scanning calorimetry), a rheological approach using dynamic viscoelasticity as rheological and syneresis level. The autoclaving procedures prepared the HMT starches at 20% moisture content and warmed to 120°C for 60 min and 90 min for sago and arenga starches, respectively. The Avrami equation was used to express starch retrogradation kinetics based on gelatinization enthalpy (ΔH). The Avrami exponent (n) of HMT and native starches were close to 1.0 (0.77 – 1.20) indicates rapid nuclei growth of the crystal. HMT has a significant influence on the retrogradation of sago starch, both from the values of n and k of the Avrami equation. On the other hand, it does not have a significant effect on arenga starch. Based on the thermal approach (DSC), HMT significantly affects sago starch’s retrogradation rate, but there was no effect on arenga starch. The influence of HMT on the retrogradation rate of arenga starch was observed on rheology and syneresis approaches, although it was not as high as sago starch.

The Dynamic Viscoelasticity of Dental Soft Polymer Material Containing Citrate Ester-Based Plasticizers

The aim of this study was to investigate the dynamic viscoelasticity of dental soft polymer material containing citrate ester-based plasticizers. Three kinds of citrate ester-based plasticizer (Citroflex® C-2: TEC, Citroflex® A-2: ATEC, and Citroflex® A-4: ATBC), with the combination of 5 wt% ethyl alcohol, were used as the liquid phase. The dynamic viscoelastic properties of nine ethyl methacrylate polymers: (A, B, C, D, E, F, G, H, and I) were immersed in 37 °C distilled water for 0, 1, 3, 7, 14 and 30 days, respectively. The dynamic viscoelastic properties were measured at 37 °C with an automatic dynamic mechanical analyzer. The shear storage modulus (G′), shear loss modulus (G″), and loss tangent (tan δ) were determined at 1 Hz. These parameters were statistically analyzed by two-way and one-way ANOVA and Tukey’s multiple comparison test at a predetermined significance level of 0.05. A significant difference was found among the materials in terms of the dynamic viscoelasticity. The materials containing citrate ester-based plasticizer ATBC showed the most stable dynamic viscoelasticity. Considering the limitations of this study, the results suggest that the inclusion of citrate ester-based plasticizer can improve the durability of dental soft polymer materials.