Cooperative phenomena in semi-dilute solutions of poly(oxyethylene)

Preparation of Hydroxypropylcellulose Membranes using Slow Ionotropic Reactions Configured in an Experimental Design

This study applied a Face-Centered Central Composite Design to prepare various hydroxypropylcellulose (HPC) membranes using slow ionotropic reactions. After the assessment of various salting-out agents, sodium carbonate was selected as the ideal electrolyte capable of inducing sufficient membrane growth. The factors studied included the concentration of hydroxypropylcellulose [HPC] (1-2%w/v), concentration of salt [Salt] (10-20%w/v) and the salting-out reaction time (SORT) (96-144 hours). The experimental and predicted responses in the design included the measurement of the Brinell Hardness Number (BHN), gel thickness, moisture absorption and membrane erosion. Surface morphological examination revealed that the membranes varied from highly porous to closely packed networks. Statistically, the experimental and predicated response values showed no significant differences (p 0.05) based on a one way ANOVA. A significant decrease in BHN was observed as the [HPC] was increased. This was attributed to an increase in the elasticity of the membrane. The increase in moisture absorption was accompanied by an increase in gel thickness and subsequently an increase in membrane erosion around the peripheral areas of the gel structure. In the case of the [Salt], all response values were reduced above 15%w/v, except in the case of the BHN. In the case of SORT, there was a substantial increase in the responses above 120 hours, except once again for BHN. Above 120 hours the matrix became loose due to extensive infiltration and crystallization of salt ions. The interactions plots indicated that the changes among the different factor levels were found to be significant (p 0.05). The average p-values for the changes between levels for each response were: BHN - p 0.035, moisture absorption - p 0.029, gel thickness - p 0.011, and erosion - p 0.042. This technology may be applicable in the development of membrane scaffolds for interconnecting tissues via their role in cell seeding, adhesion and regeneration.

On the theory of aggregation and micellization: PEO–PVP copolymer in water

We develop a theoretical approach to micellization of the PEO-PVP block-copolymer in water. This copolymer is a weak polyelectrolyte due to protonation of VP blocks. The theory accounts for non-linear ion screening, and predicts strong position dependence of both ion concentration and the effective Debye length. We consider both the case when the local Debye length is small compared to the core radius and the case when it is large. We found that the effective (local) pH is not uniform even inside one micellar core, hence non-uniform protonation of the core with higher charge density near the surface. In many cases the core charge is concentrated in a relatively thin surface layer. Considering statistical weights of non-equilibrium micelles and their continuous evolution we show that kinetics of both formation and dissociation of typical block-copolymer or surfactant micelles can be extremely slow. Thus micelle formation at the genuine (equilibrium) critical micelle concentration (c.m.c.) is totally suppressed (involves astronomical time scales) if the micelles are big enough. An 'apparent' critical micelle concentration (c.m.c.*) is introduced to account for this effect. The apparent c.m.c.* could be much higher than the genuine equilibrium c.m.c., i.e. a significant hysteresis is inherent in these systems. We also determine the ranges of meta-stability of micelles depending on the experimental time-scales.