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Foaming of Transient Polymer Hydrogels
journal contribution
posted on 2018-02-13, 13:18 authored by Rémi Deleurence, Tamar Saison, François Lequeux, Cécile MonteuxFoams made with polymer hydrogels
can be used in a variety of applications,
such as scaffolds for biomedical applications or decontamination processes.
However, from a practical point of view, it is difficult to introduce
bubbles into viscous or viscoelastic fluids and to produce large volumes
of hydrogel foams. In the present article, we investigate the foaming
process of poly(vinyl alcohol) (PVA)/borax transient hydrogels, where
PVA chains reversibly bind to borax molecules. In a previous article,
we showed that foams obtained with PVA/borax mixtures are highly stable
because of both high interfacial and bulk viscosities and can be used
to quickly absorb liquids, which make them suitable for detergency
or decontamination processes. To produce these foams, we use a two-step
foaming process which consists in first shearing a PVA solution to
obtain a PVA foam and second adding borax to the PVA foam under continuous
shearing. The obtained PVA/borax foams are stable for weeks. In this
study, we observe a shear-induced collapse of the foams for formulations
containing a low borax/PVA ratio, whereas they remain stable under
shear for high PVA/borax ratios. Using scaling arguments, we find
that the shear-induced collapse of the foams and bubbles is obtained
below a critical ratio, NE/NB = 15, of the number of entanglements per chain, NE, and the number of borax per chain, NB. Rheology measurements show that the samples
present a shear-thickening behavior that increases with the borax
concentration. We suggest that during the foaming process when the
shearing rate is of the order of 100 s–1, the viscosity
of these samples diverges, leading to a viscous to fragile transition.
To mimic the fast stretching of the PVA/borax thin films during the
foaming process, we study the stretching of individual PVA/borax catenoid-shaped
thin films at high stretching rates. We observe that the films containing
low PVA/borax ratios do not minimize their surface area unlike what
is theoretically expected for standard surfactant films. Moreover,
the films tend to be unstable and fracture because the PVA/borax network
does not have time to rearrange and relax stresses for high stretching
rates.