Lower Critical Solution Temperature Phase Transition of Poly(PEGMA) Hydrogel Thin Films
Résumé
Surface-attached hydrogel films with well-controlled chemistry are a new approach of polymer
thin layers and an actual alternative to polymer brushes and layer-by-layer assemblies. The
advantage is that the thickness of hydrogel films can widely range from a few nanometers to
several micrometers. Hydrogel films can also remarkably respond to stimuli such as temperature:
(i) the thickness change is of great amplitude, four-fold and more, which could not be reached with
the geometry of polymer brushes or layer-by-layer assemblies, (ii) the time response is very short
(less than one second) and (iii) the swelling-to-collapse transition is narrow (a small temperature
change of a few degrees may be enough). Poly(NIPAM) is the most temperature-responsive
polymer investigated with the LCST around 32°C. However, it is relevant to have available
polymers responding to various transition temperatures with the advantage of keeping the same
chemistry. Poly[(oligo(ethylene glycol) methacrylate] (PEGMA) meets these specifications since
its transition temperature can be finely tuned with the number of oligo ethylene glycol units while
it attractively combines biocompatibility with PEG side chains. Here, we report the synthesis and
the temperature-responsive properties of poly(PEGMA) hydrogel thin films. We used a simple,
versatile and well-controlled approach through thiol-ene click reaction, so-called CLAG (CrossLinking And Grafting), to synthesize surface-attached poly(PEGMA) hydrogel films with various
thickness. We show that the transition temperature of poly(PEGMA) hydrogel films ranges from
15°C to 60°C if the number of PEG units is from 2 to 5. This transition temperature can also be
finely adjusted for hydrogel films containing copolymers or mixing homopolymers of PEGMA
with suitable ratio. Moreover, the LCST properties, swelling-to-collapse amplitude and transition
temperature, is not sensitive to salt. In particular, there is no effect on the LCST properties of
surface-attached poly(PEGMA) hydrogel films in phosphate saline buffer, which is promising for
applications in biology such as injectable hydrogels, drug delivery systems, hydrogel-based
microfluidic valves and flow switches for biotechnologies.