Session X46: Invited Session: Deformation and Fracture of Soft Materials

Responsive Gel-Gel Phase Transitions in Artificially Engineered Protein Hydrogels

Artificially engineered protein hydrogels provide an attractive platform for biomedical materials due to their similarity to components of the native extracellular matrix. Engineering responsive transitions between shear-thinning and tough gel phases in these materials could potentially enable gels that are both shear-thinning and tough to be produced as novel injectable biomaterials. To engineer a gel with such transitions, a triblock copolymer with thermoresponsive polymer endblocks and an artificially engineered protein gel midblock is designed. Temperature is used to trigger a transition from a single network protein hydrogel phase to a double network phase with both protein and block copolymer networks present at different length scales. The thermodynamics of network formation and resulting structural changes are established using small-angle scattering, birefringence, and dynamic scanning calorimetry. The formation of the second network is shown to produce a large, nonlinear increase in the elastic modulus as well as enhancements in creep compliance and toughness. Although the gels show yielding behavior in both the single and double network regimes, a qualitative change in the deformation mechanism is observed due to the structural changes.   

Direct probes of molecular interactions at buried interfaces

The molecular interactions at buried interfaces play an important role in adhesion, friction, and wetting. In my talk, I will discuss the use of interface-sensitive sum frequency generation (SFG) spectroscopy to study acid-base interactions at solid-liquid and solid-solid interfaces. The shift of the sapphire hydroxyl peak in contact with several polar and non-polar liquids and polymers can be used to determine the interaction energy. The magnitude of the interaction energies cannot be predicted based on measuring water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role and these effects are not accounted for in the current theoretical models. The importance of these interactions in controlling segregation of polymer blends and friction and adhesion of soft substrates will be discussed.   

Adhesion of hydrogels under water by hydrogen bonding: from molecular interactions to macroscopic adhesion

Hydrogels are an essential part of living organisms and are widely used in biotechnologies, health care and food science. Although swelling properties, cell adhesion on gel surfaces and gel elasticity have attracted much interest, macroscopic adhesion of hydrogels on solid surfaces in aqueous environment is much less well understood. We studied systematically and in aqueous environment, the reversible adhesion by hydrogen bonding of macroscopic model hydrogels of polydimethylacrylamide (PDMA) or of polyacrylamide (PAAm) on solid surfaces functionalized with polyacrylic acid (PAA) polymer brushes. The hydrogels were synthesized by free radical polymerization and the brushes were prepared by grafting polytertbutyl acrylate chains and converting them by pyrolisis into polyacrylic acid. A new adhesion tester based on the flat punch geometry was designed and used to control the contact area, contact time, contact pressure and debonding velocity of the gels from the surface while the samples were fully immersed in water. The adhesion tests were performed at different pH and temperatures and the modulus of the gel and grafting density and molecular weight of the brushes was varied. Macroscopic adhesion results were compared with phase diagrams in dilute solution to detect molecular interactions. While the PDMA/PAA pair behaved very similarly in solution and in macroscopic adhesion tests, the PAAm/PAA pair showed an unexpectedly high adhesion level relatively to its complexation ability in dilute solution. Surprisingly, time dependent experiments showed that the kinetics of H-bond formation and breakup at interfaces was very slow resulting in adhesion energies which were very dependent on contact time up to one hour of contact. At the molecular level, neutron reflectivity showed that the equilibrium brush conformation when in contact with the gels was more extended at pH2 (H-bonds activated) than at pH9 (H-bonds deactivated) and that a certain applied pressure was necessary to bring the brush and the gel in contact. These combined results suggest that the mechanisms of formation and breakup of interactions at surfaces are more complex and cannot be understood from bulk interactions in solution alone.   

Self-healing polymers and mechanochemistry

The forces that accompany macroscopic deformation and material failure are many orders of magnitude larger than the forces between the individual atoms of a molecule. The magnitude of macroscopic forces, in combination with the fact that they are directional, creates an opportunity to direct new, stress-responsive chemistry on demand in polymer materials. This talk will present studies of reactions under large, directional forces, and their applications in new classes of stress-responsive polymers, including a class of self-healing polymers in which mechanical activation of chemical reactions leads to improved structure and properties under conditions that are typically destructive to both.   

Soft active materials---when mechanics meets chemistry

Soft materials, such as elastomers and gels, can mimic a salient feature of life: deformation in response to diverse stimuli. For example, an electric field can cause an elastomer to stretch several times its length. As another example, a change in pH can cause a gel to swell many times its volume. The deformation is large and reversible. These soft active materials are being developed for soft robots, adaptive optics, self-regulated fluidics, and programmable haptic surfaces. This talk describes recent work in my group on the mechanics of soft active materials. We formulate theories to answer commonly asked questions. How do mechanics, chemistry, and electrostatics work together to generate large deformation? What is the maximal energy that can be converted by a material? We also develop experimental methods to characterize nonlinear time-dependent mechanical behavior. Also described in this talk are hydrogels of exceptional toughness and stretchability.