Dynamics and structure of hydrogels under deformation studied by light scattering – Dynagel

In this multidisciplinary project, we planned to work on main three parts.
(1) Chemistry:
Our strategy is to synthesize model damping hydrogels capable to dissipate localized fracture energy at the crack by introduction of breakable but reformable bonds to the gel. We tested the following model hydrogels developed in our laboratory.
(i) hydrophobically modified hydrogels are chemically crosslinked polyelectrolyte gels having hydrophobic micelle forming groups. this system was abandoned.
(ii) PVA dual cross-link gels having well controlled reversible bonds were synthesized (not planned initially, replacement of the gel above).
(iii) Hybrid gels are composed of chemically crosslinked polymer network and silica nanoparticles. Reversible adsorption of polymer chains on silica nanoparticle surface can dissipate a lot of energy.
(2) mechanical tests:
Mechanical tests of hydrogels by traction were performed. Several experimental difficulties such as prevention of water evaporation from gel and fixation of soft gels to apparatus were to overcome.
(3) optical characterizations:
(i) Characterization of structural and dynamic properties of deformed hydrogels by single light scattering. We planned to set up a CCD camera-based small-angle light scattering microscopy for gels under deformation (uniaxial compression equivalent to equibiaxial elongation). This task of development a mechano-light scattering microscope is challenging.
(ii) Microrheology by multiple light scattering is a new technology which has not been usually used for hydrogels. By measuring the movements of probe particles dispersed in the gels thanks to scattered light, we can determine micrometric mechanical properties of gels at short time scales. Applicability of the technique should be proved.

Remarkable results are following.
(1) Chemistry
We have successfully synthesized two different tough hydrogels for the mechanical tests and optical characterizations. Especially the PVA dual cross-link gels are promising, they are made by very simple method with very low cost chemistry. Compared to previously reported other tough gels, our gel has very simple dynamic properties, can be a good model for various physical characterizations of gels.
(2) mechanical tests:
The mechanical properties of the PVA dual crosslink gels were far beyond our expectation, we investigated them more in detail than initially planned. With our new international partner, modeling was done, it will help characterizations of various tough gels.
(3) optical characterizations:
We have developed prototypes of mechanical test apparatus coupled with light scattering measurements, more improvements are required.
The new technique of microrheology was developed and clarified. Thanks to the setup, short time scale characterizations by light of our tough gel were successfully done, detailed molecular mechanism of the reversible bonds was revealed.
These experimental setups and the knowhow on the technique will be useful for the characterization of other soft matter in future projects.

With our PVA dual crosslink gels, we demonstrated a universal behavior of the mechanical properties of hydorgels having reversible bonds under large deformation. It can be served as a guideline to characterize other similar highly deformable hydrogels. We showed that the lifetime of the reversible bonds is an important characteristic time of the system, in comparison with the time scale of the deformation and crack propagation. Gels with tunable lifetimes should be systematically studied for the next step. We have already started to design and synthesize such a tunable gel and will continue the study for a thesis.
We developed two original experimental setups. They will be used for the characterization of other polymer solutions and gels. We obtained a funding for a thesis to use the microrheological technique. There are several companies in Europe commercializing apparatus for the microrheology. We are in contact with them and new collaborations will probably be opened.

We have published 4 articles in good journals in the field of polymer science (3 Macromolecules, 1 ACS MacroLetters).
Another 4 articles have been submitted or under preparation.
1. T. Narita, K. Mayumi, G. Ducouret, P. Hébraud, Macromolecules 2013, 46, 4174
2. S. Rose, A. Marcellan, D. Hourdet, C. Creton, T. Narita, Macromolecules 2013, 46, 4567
3. S. Rose, A. Marcellan, D. Hourdet, T. Narita, Macromolecules 2013, 46, 5329
4. K. Mayumi, A. Marcellan, G. Ducouret, C. Creton, T. Narita, ACS MacroLett. 2013, 2, 1065
We made 18 presentations in conferences (15 international, 3 French). We won a poster prize.

Hydrogels are soft and deformable and have the ability to change volume by absorbing/desorbing water. Although there are many applications of synthetic polymer hydrogels in life sciences and engineering, these gels are generally mechanically fragile compared with biological gels. Thus applications of hydrogels as deformable materials are limited.
Recently several Japanese groups synthesized new tough (fracture resistant) hydrogels of high deformability and high water content having original network architectures. Yet while the reported toughness results have been spectacular, no explanation which could be generalized, has been proposed.
Generally these gels have unique structures compared with conventional chemically crosslinked hydrogels, but the local dynamics of the polymer network and its role in macroscopic viscoelastic properties has not been investigated. In order to have highly fracture resistant materials, it is important to prevent cracks to progress. What are effects of the structure and dynamics on the structural relaxation occurring at crack tips during gel fracture? What are the length and time scales of this structural relaxation? While scattering techniques have been previously used for the structural analyses in the (un)deformed conditions, the dynamics of the network under large deformation have not been used to understand the fracture process of the gels.
In this context our project focuses on the structure and dynamics of novel hydrogels under large deformation. We will apply two techniques.
(1) Characterization of structural and dynamic properties of deformed hydrogels by single light scattering (mechano-LS microscope).
(2) Microrheology of the gels by multiple light scattering. We will use the multiple dynamic light scattering (diffusing-wave spectroscopy, DWS). Local high frequency viscoelasticity of the gels will be probed.
We will use two model hydrogels: hybrid gels and hydrophobically modified hydrogels developed in the laboratory.
Hybrid gels are composed of chemically crosslinked polymer network and silica nanoparticles dispersed prior to the polymerization. Such gels can be almost one order of magnitude tougher than the unmodified gels. Mechanical experiments suggest that there are interactions between the polymer and silica which serve as reversible physical crosslinks and that their reversible nature is the origin of dissipative properties which make the gel fracture resistant. Hydrophobically modified hydrogels are chemically crosslinked polyelectrolyte gels having hydrophobic micelle forming groups. Although these hydrogels are just moderately tough, they show significant hysteresis in uniaxial compression at large strain.
With the mechano-LS microscope, we can study the structure and dynamics of silica particles/micelles. DWS allows us to characterize the local dynamics and rheological properties of the physical crosslink at high frequency.
Various macroscopic characterizations such as swelling properties and mechanical tests will be performed and compared with the light scattering experiments.
We expect that from these techniques we will be able to identify the difference in dynamic properties of these complex gels relative to a conventional gel under deformation and gain therefore insight in the molecular and structural origin of the enhanced macroscopic mechanical properties.
The strong point of our project is that it is composed of both dynamic studies by optics and mechanical analyses complemented by the optical microrheology. Considering the international situation in this field, our project will be very well positioned, because of its multidisciplinary approach, to bring genuine advances and to develop a general understanding of the mechanisms controlling toughening of soft hydrogels, which should then lead to new and more general guidelines for tough gel design for industrial applications.