Water transport properties in poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) biopolymers

Water sorption and diffusion have been investigated in poly(3-hydroxybutyra te) (PHB) and three poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers [P(HB-KV)] by means of a Cahn electromicrobalance. Permeability of these s amples have been determined using a gravimetric permeation cell. Two experi mental setups were used for the gravimetric sorption measurements, under dy namic and static conditions, respectively. The differences observed in the results obtained using these techniques are discussed. The sorption measure ments have evidenced the tendency of water molecules to form aggregates or clusters in the polymer. In addition, the static sorption method revealed t he potential of PHB and P(HB-HV) to undergo molecular relaxations, eventual ly leading to a partial desorption of the previously sorbed water after an induction period. The clustering effect was adequately described by the pol ycondensation model. On the other hand, the interpretation of the diffusivi ty in terms of mobility coefficients has revealed a competition between a p lasticization effect and clustering. As a whole, water transport properties in PHB and its copolymers can be considered to be very close in magnitude to those of common thermoplastics such as PVC and PET. (C) 1999 John Wiley & Sons, Inc.