Gel preparation

Poly(vinyl-alcohol) (PVA) gels were made by crosslinking with glutaraldehyde at pH = 1.0 in aqueous solutions.⁹³ For the experiments a fully hydrolyzed and fractionated PVA sample was used (M_w,PVA = 110 kDa). Crosslinks were introduced at 4% (w/w) polymer concentrations, the molar ratio of monomer units to the molecules of crosslinker was 200. Gels were cast between two glass microscope slides with 2 mm thick spacers. After gelation the samples were equilibrated with 100 mM NaCl solution to remove HCl and uncrosslinked polymer (sol fraction). Then the gels were dried in an oven at 95 °C, reswollen in 100 mM NaCl solution for 24 hours, and exposed to two (PVA1), four (PVA2) six (PVA3) and eight (PVA4) cycles of freezing for 12-14 hours at − 20 °C and thawing the samples for 10 hours at 25 °C.

The preparation of polyacrylic acid (PAA) gels has been described previously.^71,72 Poly(acrylic acid) gels were synthesized by free-radical copolymerization of partially neutralized acrylic acid and N,N′-methylenebis(acrylamide) in aqueous solution. The monomer concentration was 30% (w/w). The concentration of the crosslinker was 0.3 (PAA1), 0.6 (PAA2), 0.9 (PAA3) and 1.2 % (PAA4). In the initial mixture 35% of the monomers were neutralized by sodium hydroxide. After the components were mixed, dissolved oxygen that would inhibit the polymerization reaction was eliminated by bubbling nitrogen through the solution. The polymerization reaction was initiated by ammonium persulfate (0.5 g/L). After gelation the acrylic acid units were first neutralized with NaOH solution and then dried at 90 °C for 48 hours. The dry polymer was ground into a powder having an average particle size less than 10 microns.

PVA/PAA composite gels were prepared by crosslinking PVA in solution containing PAA particles. In the initial solution the concentrations of both polymers were 4%. First the HCl was added to the solution (pH ≈ 1) to suppress the swelling of PAA. Then the crosslinker glutaraldehyde was added to the suspension. The molar ratio of the monomer units to the crosslinker was the same as in the pure PVA gels. The composite gels were treated by the freeze thaw process similarly to the PVA gels. The sample code is defined by the composition and the code of the corresponding PVA and PAA gel. For example, PVA1/PAA1 is a composite gel made by crosslinking a PVA solution containing PAA particles (crosslinker concentration: 0.3 %) and applying 2 freeze-thaw cycles.