A reappraisal of the ‘thermoreversible’ gelation of aqueous poly(vinyl alcohol) solutions through freezing–thawing cycles
Introduction
Aqueous solutions of atactic poly(vinyl alcohol) (PVA) do not produce physical gels when merely cooled at low temperature unlike what is commonly observed with atactic poly(vinyl chloride) [1], [2], [3]. Gelation of PVA solutions can, however, be achieved by submitting them to a series of freezing–thawing cycles [4], [5], [6]. The elastic modulus is seen to increase and a fibrillar network can be seen.
The freezing–thawing process can be accounted for from a thermodynamic viewpoint by realizing that water crystallization results in creating interstitial domains of high polymer concentrations. If some of the polymer chains (the chains of highest syndiotacticity for instance) belonging to these highly concentrated domains stand in a situation of appropriate undercooling, then they are liable to crystallize leading eventually to the gelation of the whole system. Yet, it is not clear why the number of freezing–thawing cycles should play such a major role. Indeed, there is no thermodynamic argument to state that polymer concentration would be further increased in the highly concentrated domains or to suppose that chains of lower syndiotacticity should eventually crystallize. In this paper, we report on a self-exclusion chromatography (SEC) analysis of the polymer chains recovered from the dissolution of gel samples obtained from various numbers of freezing–thawing cycles as well as on a study by means of microdifferential calorimetry of the resulting gels. The outcomes of the present study have led us to a reappraisal of the phenomenon in terms of the existence of a permanently cross-linked gel (chemical gel) rather than in terms of a thermoreversible gel.
Section snippets
Materials
The PVA used in this study is a commercial product (ERKOL 28/99) with a degree of hydrolysis of 99.8%. The weight-average molecular weight as determined by GPC is 107,000 g/mol with a polydispersity of 1.77. The tacticity as measured by 1H NMR spectroscopy is syndio=0.2, hetero=0.5 and iso=0.3.
Sample preparation
PVA solutions (polymer concentrations of 5, 10, 15 and 20 wt%) were prepared in Pyrex tubes by mixing the appropriate amount of polymer and water (milli-Q grade) at 90 °C under conditions of vigorous
Results and discussion
A SEC study had already been made by Lozinsky et al. [7] but after only one freezing–thawing cycles. These authors had then detected no change with respect to the starting polymer. Here, again a comparison has been made between the PVA starting material and the polymer that has been gelled through several freezing–thawing cycles. Typical SEC spectra observed for all samples after more than two cycles are shown in Fig. 1. Both the variation of the concentration Ci, and the determination of CiMi
Concluding remarks
The experiments presented here suggest to reconsider the origin of the gelation phenomenon in poly(vinyl alcohol) submitted to freezing–thawing cycles. The results hint at some chemical cross-linking possibly arising from the creation of free radicals due to the shearing produce while water crystals grow within the polymer solution. Investigations should focus on this point to throw some light on this most unusual gelation mechanism.
Acknowledgements
R. Hernandez is indebted to the Spanish government for a grant in aid for a stay at Institut Charles Sadron.
References (14)
- et al.
Eur Polym J
(1989) - et al.
J Control Release
(1991) - et al.
Compar Biochem Phys Part C: Pharm, Tox Endocrinol
(1995) - et al.
Cryobiology
(1999) - et al.
Colloid Polym Sci
(1976) Thermoreversible gelation of polymers and biopolymers
(1992)Russian Chem Rev
(1998)