Abstract
The effect of plasticizer species and the degree of hydrolysis (DH) on the free volume properties of poly(vinyl alcohol) (PVA) were studied using positron annihilation lifetime spectroscopy. Both glycerol and propylene glycol caused an increase in the free volume cavity radius, although exhibited distinct plasticization behavior, with glycerol capable of occupying existing free volume cavities in the PVA to some extent. The influence of water, normally present in PVA film under atmospheric conditions, was also isolated. Water added significantly to the measured free volume cavity radius in both plasticized and pure PVA matrices. Differences in plasticization behavior can be attributed to the functionality of each plasticizing additive and its hydrogen bonding capability. The increase in cavity radii upon plasticizer loading shows a qualitative link between the free volume of voids and the corresponding reduction in Tg and crystallinity. Cavity radius decreases with increasing DH, due to PVA network tightening in the absence of acetate groups. This corresponds well with the higher Tg observed in the resin with the higher DH. DH was also shown to impact the plasticization of PVA with glycerol, indicating that the larger cavities—created by the weaker hydrogen bonding acetate groups—are capable of accommodating glycerol molecules with negligible effect on the cavity dimensions.
Highlights
Poly(vinyl alcohol) (PVA) is a water-soluble, synthetic, semi-crystalline polymer with excellent film forming capability, good mechanical properties, and optical transparency [1]
The characteristics of the PVA resin are dependent on its degree of polymerization (DP), as well as its degree of hydrolysis (DH), which defines the fraction of hydroxyl groups present on the backbone
There is a very modest decrease in the I3 parameter with propylene glycol concentration. This effect has been previously observed in plasticized films [13,33], and in some cases has been attributed to a hole-filling mechanism [10], the increase in cavity radius with plasticizer concentration suggests that a likely reason for the decrease in I3 could be the result of quenching of o-Ps by the plasticizer or an increased number of sites where the positronium precursors can be trapped, rather than a decrease in void concentration
Summary
Poly(vinyl alcohol) (PVA) is a water-soluble, synthetic, semi-crystalline polymer with excellent film forming capability, good mechanical properties, and optical transparency [1]. Its application in food packaging makes use of the excellent barrier properties of PVA to oxygen and carbon dioxide [2], but PVA is valued for its solubility, nontoxicity, and biodegradability [3,4], which contribute to its low overall environmental impact. These properties, alongside its resistance to organic solvents, have led to its increased use in the laundry industry as a film for packaging unit-dose detergents. PVA is prepared from the hydrolysis of poly(vinyl acetate) (PVAc).
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