| Popis: |
Heat-induced gelation was studied for aqueous solutions of methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC). It has been demonstrated that the evolution in time of the elastic shear modulus at different cellulose concentration can be superimposed on a universal gelation master curve, independent of the cure time. This would indicate self similarity of the network at different scales, irrespective of concentration (fractal similarity). An attempt was made to elucidate the gel network structure and the validity of the scaling law. In the case of MC, the elasticity of the gel (at 95°C) followed a power law with concentration: G&'C1.9. In contrast, no single scaling laws could be found in case of HPMC. The calorimetric traces are distinctly asymmetric, which suggests an aggregation process. Model fitting to the aggregation model of the apparent excess heat capacity-temperature function enabled evaluation of the van't Hoff enthalpy. The calculated thermodynamic parameters suggest that the aggregation process involves molecular clusters. The number of clusters and indeed the number of molecular chains comprising a cluster are not dependent upon the concentration but upon the substitution type (MC>HPMC). Heat-induced self association of a new regioselective substituted 3-0-ethyl-propyl cellulose ethers in water have also been studied by means of differential scanning calorimetry and oscillatory shear rheology. The measured degrees of substitution were close to 1 but the ratio of ethyl to propyl groups was varied. The aggregation process is intimately coupled with phase separation as shown by the appearance of clouding in the same temperature range. The correlation between rheology and thermal analysis clearly demonstrates that aggregation leads to formation of a gel network. It was found that as the ethyl groups are replaced by propyl groups the enthalpies of the thermal transitions increases strongly whereas the cloud point temperature decreases. In addition the increase in the amount of propyl groups increases the gel strength. Reversibility was observed on cooling with a marked hysteresis for samples containing high levels of propyl groups. Hysteresis on cooling was explained in term of additional consolidation of the structure occurring at temperature much higher than the aggregation temperature, possibly involving backbone-backbone interactions. Indeed, when the samples are heated to temperatures greater than 30°C higher than the onset of aggregation promotes irreversible structure formation, which persists as a hard sediment upon cooling. |
