Local movements in amorphous polymers around the glass transition temperature studied by steady state anisotropy

BEATRIZ ARAOZ; PEDRO F. ARAMENDIA

Sao paulo-Cubatao_ Brasil

Encuentro; IX Encontro latino-americano de fotoquimica e fotobiología; 2008

CEPEMA-USP

The glass transition temperature (Tg) is one of the fundamental properties of an amorphous polymeric matrix. Around Tg a great increase of mobility at the atomic scale takes place and the correlation length of these movements rises quickly. Several studies were focused on the dynamics of glass transition but its nature has not been solved yet. Below Tg, the system fluctuates at a very low speed and the different environments are equaled in several hundreds of seconds, consequently the system is heterogeneous at the micro scale and a particular dynamic process displays a broad distribution of rates. Above Tg, the system fluctuates faster and the behaviour is fluid-like. In this way, the dynamic behaviour becomes homogeneous and tends to show a single characteristic time (mono-exponential kinetic regimen). Thus, the origin of the glass transition is accepted to be a dynamical event rather than a phase transitions. When domain size becomes comparable to the characteristic distance of the fluctuations, the Tg is influenced by nanoconfinement effects; because of this, we propose to monitor the variation of Tg with polymer thickness by fluorescence anisotropy of embedded probes. Thickness control and preparation is easily performed by spin-coating. In this work, we study local movements in thin films of polyalkyl methacrylates (butyl, PBMA; ethyl, PEMA and propyl, PPMA) doped with Rhodamine 6G, around Tg by steady state anisotropy. Our experiments show a temperature dependence of anisotropy in films of 200 nm, 100 nm and 50 nm thickness, the change in the anisotropy is remarkable well over the Tg.