8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P8.4Thu 811:10-14:00Glass transition in thin polymer filmsLaura R. Arriaga, 1 Francisco Monroy, 2 and Dominique Langevin 31 Laboratoire de Physique des Solides, Université Paris Sud XI, UMR 8502, bat 510,91405, Orsay, France2 Universidad Complutense de Madrid, Madrid, Spain3 Laboratoire de Physique des Solides, Orsay, FranceThe shift of the glass transition temperature T g in thin polymer films is presently the objectof much interest. Although literature results on films (5-50 nm thick) are quite scattered, ithas been recently postulated that T g should change only in extremely thin films (below 2 nm)[A. V. Lyulin et al. , J. Non Cryst. Sol. , 2011]. We present an experimental study of ultrathinfilms of various polymers deposited on a free water surface: poly-butylmethacrylate (PTBMA),poly-hydroxystyrene (P4HS) and poly-methylmethacrylate (PMMA), which have rather rigidchains, poly-butylacrylate (PTBA) and poly-vinylacetate (PVAc), with more flexible chains.Particularly interesting are the differences in thermal behavior between both groups of polymers[1]. While PTBA and PVAc films show weak temperature dependence, PTBMA, P4HS andPMMA films undergo a solid-liquid transition at a temperature well below the T g of the bulkpolymer. This thermal softening is accompanied by a conformational change from a collapsedstate at low T to a more extended conformation at high T, as revealed by the surface pressureisotherms. At low T, the shear rheology of the films resembles that of soft glassy materials. Thebehavior of the shear moduli beyond the fluid-solid transition is similar to that observed recentlywith dispersions of soft colloidal spheres [D. A. Sessoms et al. , Phil. Trans. R. Soc. A, 2009]:upon cooling, the system undergoes first a glass transition, followed by a jamming transition,which could be responsible for inhibiting a large viscosity increase. Another scenario couldinvolve the disappearance of entanglements upon heating, which would account better for theobserved lack of reversibility in the ultrathin layers studied. In this case the glassy behaviorwould be due to the freezing of segmental motion as in pure 3D polymers. Both scenarii will bediscussed in the presentation.[1] L. R. Arriaga, F. Monroy and D. Langevin, Soft Matter (2011, DOI: 10. 1039/C1SM05338H)4