Correlation of Diffusional Exponent with Structural Sequence of (N-Vinylpyrrolidone-alt-Acrylic Acid) Hydrogels: A Predictive Hydrogel Design Approach

Correlation of diffusional exponent of water into (N-Vinylpyrrolidone-alt-Acrylic acid) hydrogels with the structural sequence of hydrogel repeating units (i.e. hydrophilic and/or hydrophobic moieties) through a scaling law relationship has been investigated. The diffusional exponent (n) was found to depend linearly on Y/X² , where X equals to molar ratio of monomers in the feed (i.e. ([M₁ ]/[M₂ ])_feed) and Y equals to molar ratio of repeating units in the copolymer (i.e. ([M₁ ]/[M₂ ])_copolymer). Copolymers rich in hydrophilic acrylic acid (i.e. larger Y/X² ) were found to form more coiled polymeric chains as acrylic acid content increased due to H-bonding interactions, and hence results in lower diffusion coefficients and linear lowering of the n-value. On the other hand copolymers rich in partially hydrophobic N-vinylpyrrolidone (i.e. smaller Y/X²) formed more uncoiled polymeric chains as N-vinylpyrrolidone content increased due to larger amounts of positively charged protonated nitrogen atoms at pH 5.5 that tend to repel each other, which eventually lead to higher diffusion coefficients and a linear increase in n-value. These characteristic findings could be significant in sustained drug release studies, where the structural sequence of a hydrogel plays a dominant role in the release kinetics of a drug at certain positions and/or times.