■ Mechanical response of surface-attached hydrogel thin films: poroelasticity and phase transition

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Description of the PhD project

Hydrogels are hydrophilic polymer networks swollen in water. Hydrogel coatings are suitable candidates for widespread applications in biomedical or surface engineering and optics due to the combination of biocompatibility, chemical versatility and mechanical stability. Surface-attached hydrogel films are advantageously multifunctional and multiscale while chemically well-controlled, as opposed to existing polymer coatings. The latter are either coated layers with poorly controlled chemistry or nanometer-thin polymer brushes and layer-by-layer assemblies. Surface-attached hydrogel films are chemically stable polymer layers owing to covalent cross-linking and surface-grafting. They are multiscale materials with thickness ranging from a few nanometers to several micrometers. The nature of the polymer can be adjusted for targeted properties (responsive polymers, rubber/glassy polymers). The network architecture allows the films to absorb and expel very large amount of water yielding unprecedented high amplitude volume change (four-fold or more) for responsive coatings.
However, applicability of hydrogel coatings relies on a better understanding of the timescales involved in their mechanical response under the action of external stimuli: temperature, pH… or mechanical stress. Studies using bulk gels demonstrated that these typical timescales arise from the water transport within the stressed polymer network. Denoted as poroelasticity, the coupling between network elasticity and water flow is expected to be especially relevant to thin hydrogel films due to confinement, which enhances stresses and the associated drainage. Preliminary works in the lab showed that confinement within contacts can induce some overlooked phase transition phenomena (LCST, glass transition...) resulting in drastic changes in mechanical properties. Similarly, the description of the effects of such phase transition on frictional response is extremely challenging.
We aim at better understanding the interplay between poroelasticity and phase transitions within thin hydrogel coatings and their effects on the mechanical and frictional response of hydrogel coatings. 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. The permeation and elastic properties of the polymer network will be varied with the crosslinks density. The chemical nature of polymers will be finely tuned to probe glass transition (using glassy/rubber polymers) or microphase separation (using polymers with LCST/UCST properties) as water content varies.
- Original lateral contact methods will be used to probe the change in films mechanical properties during poroelastic drainage and specifically across phase transitions. We will also develop original rotational friction experiments to separate the contributions of poroelasticity and interfacial interactions to friction.

Keywords

Hydrogel, film, surface-grafting, CLAG chemistry, click chemistry, contact methods, friction, phase transitions, coupled transport phenomena

Research unit

UMR7615 Soft Matter Sciences and Engineering

Description of the research Unit/subunit

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.

Name of the supervisor
Yvette Tran (yvette.tran (arobase) espci.fr)

Name of the co-supervisor
Emilie Verneuil (emilie.verneuil (arobase) espci.fr) and Antoine Chateauminois (antoine.chateauminois (arobase) espci.fr)

3i Aspects of the proposal

The mechanical and lubricating properties of thin hydrogels films are relevant to many applications in the biomedical field (catheters, contact lenses, bioadhesives, biosensors), in surface engineering and microfluidics (microvalves or microcages for lab-on-a-chips). Existing start-ups, such as Microfactory, are currently developing hydrogel actuators technology integrated in microfluidic chips. Other potential development also involves the production of lab-on-a chips based on hydrogel actuators for biotechnological applications. Thin hydrogels films can also be used as anti-mist and anti-frost coatings for glass windows or plastic visors. The design of such coatings strongly relies on a better understanding of the relationships between the physical-chemistry of hydrogel networks and their mechanical and tribological properties (friction, scratch resistance).
The project involves the complementary expertise of the partners in widely different areas ranging from chemistry to physics and mechanics of soft matter. It relies on (i) the synthesis and physico-chemical characterization of thin (micrometric) hydrogel films grafted onto glass or silicon substrates via a CLAG chemistry route developed in the lab, (ii) the experimental investigation of their mechanical and frictional properties in relation to stress-induced water transport and phase transition phenomena within the hydrogel networks (iii) the theoretical description of mechanical and frictional properties within the framework of poroelastic models taking into account the network properties and the occurrence of transitions (glass transition, LCST/UCST phase separation…) during the course of swelling/collapse.
This project involves the participation of an international partner, Professor Chung Yuen Hui from the Department of Mechanical and Aerospace Engineering at Cornell University (USA). C.-Y. Hui is an internationally renowned expert in the field of mechanics of soft matter. Within the framework of this project, he will bring his expertise in the modeling of poroelastic contact phenomena within hydrogels. We especially intend to develop a theoretical description of the interplay between poroelasticity and phase transition in hydrogel films and its implications on gel friction. Since several years, C.-Y. Hui uses to spend about one month on a yearly basis at ESPCI which will provide several opportunities for an in-depth collaborative work on the topic of the project. It is also planned that both the PhD student and the (co)supervisors will visit Cornell University during the course of the project. Moreover, C.-Y. Hui is the co-author with the other projects’ partners of a recently submitted publication on friction of poroelastic contacts.

Expected Profile of the candidate

For this project we will be seeking for a candidate with a strong background in soft matter science and in physical chemistry. The candidate should be able to handle both the aspects related to the synthesis of hydrogel thin films (using the Cross-Linking And Grafting CLAG route through thiol-ene click chemistry we have already developed and validated in our lab) and the experimental characterization of their contact and frictional properties using existing custom-built devices. Although the project involves some theoretical modeling, the emphasis will be put on the experimental skills of the candidate and his/her abilities to tackle both physical-chemical and physical-mechanical characterizations.

Important dates

Call for applications : from July 16th to September 17th 2018
Eligibility check results : Late September
3i Committee evaluation results : Late October
Interviews from the shortlisted candidates with the Selection Committee : Mid-December (week of December 10th)
Final results : Late December





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