A free-volume interpretation of the decoupling parameter in bioactive-compound diffusion from a glassy polymer

• The kinetics of vitamin release in condensed biopolymer systems was examined.
• Linearity exists between diffusion coefficient and inverse of fractional free volume.
• Cooperativity between vitamin and biopolymer interactions was established.

Delivery of techno- and biofunctionality in all-natural processed foods is an area of steadily increasing fundamental and technological interest. We prepared a range of condensed biopolymer-vitamin systems and monitored the kinetics of bioactive compound release as a function of environmental temperature. Distinct patterns of structural relaxation were noted for the polymer matrix and microconstituent above the glass transition temperature of the high-solid sample. Then, there was a qualitative agreement between fractional free-volume of polymer and effective diffusion coefficient of vitamin within the glass transition region. These observations encouraged us to develop for the first time a mathematical expression that argues for linearity in the relationship between diffusion coefficient and inverse of fractional free volume yielding the so-called decoupling parameter of polymeric motion and small-molecule diffusion. Quantifying the cooperativity of vitamin and biopolymer interaction allows informed manipulation of bioactivity release, and the generic nature of the fundamental treatise invites validation in systems beyond the current range.

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