Description of the PhD project
Hydrogels are hydrophilic polymer networks swollen in water that are advantageously biocompatible, chemically versatile and soft. As such they are considered as good candidates for applications in biomedicine (eg tissue engineering), surface engineering (eg biocompatible coatings on surgical implants) and optics (eg adaptative or anti-mist coatings).
The potential applicability of hydrogel coatings relies on a control of their mechanical and swelling properties: either in a contact or during swelling, hydrogel films experience large stresses due to confinement, which control their mechanical sustainability. Understanding their mechanical response under external stimuli (chemical or mechanical stress) requires a better description of the timescales and stresses involved. In bulk gels these typical times were shown to arise from water transport within the stressed polymer network.
By working on model surface-attached hydrogel thin film, we recently demonstrated our ability to control and describe some of their mechanical properties in relation to both network elasticity and solvent flow. Our purpose is now to develop a fundamental understanding of the response of thin hydrogel films to friction and swelling. Several aspects of both practical and fundamental interest will be addressed:
- Within sliding contacts, the contribution to friction of water permeation in the gel and of surface interactions must be sorted out. Both transient and steady friction will be studied. To this end, the gel architecture and surface chemistry will be varied.
- For some polymers, phase transitions are expected as water content vanishes. We aim at understanding the interplay between water transfers and phase transitions within hydrogel films and their effects on the contact response.
- Due to confinement, the gel/substrate interface is submitted to large tensile or shear stresses and may fail. We will aim at describing the stresses transferred to this interface in relation to the grafting density of the polymer, so as to control and predict the failure of the coatings during swelling or in contacts.
To this end, a multi-disciplinary approach will be developed: Surface-attached hydrogel films (1-10 µm) will be synthesized using a well-controlled CLAG (Cross-Linking And Grafting) chemistry route. Their permeation and elastic properties will be varied with the crosslinks density. The chemical nature of polymers will be changed to probe glass transition or microphase separation effects. Original rheology methods developed in the lab will be used to probe the films mechanical properties. We will develop rotational friction experiments in addition to classical linear ones so as to separate the contributions of poroelasticity and interfacial interactions to friction. Finally, the grafting of the gel to the substrate will be varied by tuning the surface density of anchoring sites and its effect on the delamination threshold will be explored.
Description of the research Unit/subunit
UMR7615 Soft Matter Sciences and Engineering
Soft Matter Science deals with systems where macroscopic properties are highly related to microscopic structure. The relevance of this field pertains to the understanding and the controlled design of matter at sub-micrometer scales. Within this context, SIMM leans on its various competences to clarify links between complex objects (of characteristic sizes typically between ten microns and ten nanometers) and their macroscopic properties. SIMM works on the elaboration of basic concepts in this domain - mainly focused on mechanical behaviors of complex systems. It tackles problems which originate in industrial questions, at the crossroads of the traditional disciplines of physical-chemistry and chemical engineering.
The laboratory develops its activity in sciences (physics and chemistry) and engineering of soft matter, by using the truly multidisciplinary competences of its staff which include the design of systems and original techniques, and the mastery of measurements of the properties (especially of mechanical ones). Our purpose is to develop new concepts, taking our inspiration from the numerous original applied situations in this field and tackling the challenge they offer.