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Nano-hybrid self-crosslinked PDMA/Silica hydrogels

We have developed a simple synthetic strategy to obtain highly extensible and yet mostly elastic nano-composite ‘self-crosslinked’ hydrogels. We discovered that the free radical polymerization of N,N-dimethylacrylamide in water can lead, above a certain concentration, to gels without any added difunctional cross-linker. These so called “self-crosslinked” hydrogels were prepared and their weak mechanical properties were improved by introducing silica nanoparticles.

The ‘self-crosslinked’ gels reported here are made from relatively high amounts of silica nano-particles (Silica/Polymer weight ratio from 0.5 to 5, i.e. volume fractions from 0.03 to 0.21).

The improvement in properties relies on the synergy between a lightly swollen cross-linked polymer matrix and strongly interacting silica nanoparticles creating a hydrogel with combination of physical and chemical cross-links. Several important conclusions can be drawn from our results.

The addition of a small amount of silica particles led to a dramatic change in macroscopic mechanical properties without any loss in transparency.

From mechanical tests performed with gels in the preparation state, experiments showed that small strain modulus is controlled by the product of the density of entanglements by the weight ratio of silica nanoparticles, a remarkable result suggesting that each silica particle contributes an additional density of elastic chains which is proportional to the existing entanglement density of the polymer in water.

The strong adsorption of the PDMA polymer on each particle led up to a factor of 20 in elastic modulus increase while retaining an extensibility of 1000%, a remarkable result. Instantaneous recovery in strain is seen to be quite high for loading/unloading cycles at 0.5 strain, inferior to 0.08 strain for a silica volume fraction of 0.10.

However, after higher strain levels beyond about 0.05 volume fraction of silica, the gel starts to show a significant level of hysteresis and a non negligible residual deformation.

Tensile, hysteresis and relaxation experiments all point to the following physical picture :
at a few percent of Silica volume fraction, the viscoelastic poorly cross-linked matrix become physically cross-linked by the particles and behaves as an elastic gel with very high extensibility and a modulus of the order of 2-3 kPa. When additional silica is added the resulting interactions between silica particles become comparatively more important, resulting in a modulus increasing up to 20-40 kPa at 20-25 vol% Silica but the gel demonstrates also an increasingly significant strain rate dependence, hysteresis and residual deformation.

In summary our results demonstrate that hybrid hydrogels, with mechanical properties similar to the clay-filled hydrogels, can be obtained with silica particles provided that the polymer strongly adsorbs on the surface of the particles. These results are very promising and further optimization of the mechanical properties can most likely be obtained by tuning the ratio of chemical vs. physical cross-links as well as the level of particle/matrix interaction.

Linn Carlsson, Séverine Rose, Dominique Hourdet, Alba Marcellan publié dans Soft Matter