MECHANICAL, MICROSTRUCTURAL AND SOLUBILITY PROPERTIES OF PECTIN POLY(VINYL ALCOHOL) BLENDS/

Citrus pectin was blended and cast into films with poly(vinyl alcohol) (PVOH). PVOH and pectin were miscible in all proportions. Dynamic mec hanical analysis revealed that pectin controls exhibited no thermal tr ansitions, whereas PVOH controls exhibited a glass transition temperat ure (T-g) over a broad temperature range commencing at about 0 degrees C and ending about 50 degrees C. A mixture of 49% pectin, 21% PVOH an d 30% glycerol exhibited lower storage moduli and mon flexibility than comparable mixtures of either pectin/PVOH or pectin/glycerol. Scannin g electron microscopy and phase contrast optical microscopy indicated that the mixture was biphasic and a compatible composite either of PVO H in pectin or pectin in PVOH depending on which macromolecule was in excess. Elongation to break measurements revealed that pectin/PVOH fil ms underwent a brittle to ductile transition with increasing PVOH comp osition. The addition of glycerol to pectin/PVOH films increased ducti lity significantly when films were relatively brittle. Initial moduli (IM) as a function of composition gave complex curves which exhibited either one or two local maxima depending on such factors as degree of hydrolysis and molar mass of the PVOH in addition to the moisture cont ent of the film. Solubility studies in water revealed that, at 30 and 50 degrees C, only films with 30% PVOH or less were soluble. At 70 deg rees C, all compositions were soluble but films containing pectin diss olved more rapidly than those without. The solution kinetics of pectin /PVOH films with 30% or less PVOH were approximated with zero-order ki netics and activation energies were about 3-5 kcal mol(-1). In general , addition of PVOH to pectin films resulted in films with more PVOH-li ke properties and addition of pectin to PVOH films resulted in films w ith mon pectin-like properties. (C) 1998 Elsevier Science Ltd. All rig hts reserved.