SAXS characterization of thermoresponsive soft matter-based systems

AZZARONI, O.; PICCO, A.; BRUVERA, I.J.; CEOLIN, M.

Campinas

Encuentro; Reuniao Anual de Usuários do LNLS. XX RAU; 2010

SAXS characterization of thermoresponsive soft matter-based systems Azzaroni, O.1, Picco,A.1, Bruvera, I.J.2, and Ceolin M.1 1 Instituto de Investigaciones Fisicoquimicas Teoricas y Aplic - La Plata BA Argentina 2 Universidad Nacional de La Plata - La Plata Bs.As Argentina Soft matter materials are attractive because of their great versatility as building blocks in multi-functional systems as well as the possibility of the the tuning in their mechanical, structural and chemical properties. Among them, stimuli (pH, temperature, pressure, external fields, etc.) responsive molecules and supramolecules have attracted wide interest because of the possibility of tight control of their properties and possible specfic uses as mechanical actuators at nano(meso)metric scale. In this report we present Small Angle X-ray Scattering experiments on the thermally induced structural changes observed in hyperbranched polyethileneimine(core)- palmitic (shell) (HPEI-C16) colloidal suspensions and hyperbranched polynipam (hbPNIPAM). The experiments were performed using 0.1488 nm wavelength and 410 mm sample-to-detector distance. HPEI-C16 room temperature SAXS displays a clear Bragg peak indicating the existence of long range ordered structure with an interplanar distance of 5.65 nm. From the line width of the Bragg peak a crystallite size of 113 nm (at room tem-erature) was deduced. The thermal evolution of the line area indicates that the ordered phase vanishes aroun 30 C giving rise to a SAXS diagram typical for a colloidal suspension. Also the thermal evolution of the peak position indicates that the interplanar distance slightly decreaces from 5.65 nm at 22 C down to 5.60 nm just before line vanishing. The reduction of the interplanar distance coincides with a reduction of the crystallite size (reflected in the line width) as expected from the reduction of the interplanar distance. Taken together, the SAXS results for HPEI-C16 indicates the occurence of an ordered low temperature phase in close resemblance to those observed in liquid crystals. The thermoresponsive polymer hbPNIPAM was also characterized using SAXS. A clear transition from a globular low temperature structure (as indicated for a typical bell-shaped Kratky plot) to an extended and aggregation prone structure was determined to occur around 30 C. Both, the transitions observed in HPEI-C16 and hbPNIPAM were verified to be fully reversible. Acknowledgements: This work was partially supported by CONICET, ANPCyT and Centro Interdiciplinario de Nanociencia y Nanotecnologia-ANPCyT (Argentine), Max Planck Society (Germany) and Laboratorio Nacional de Luz Sincrotron (Brazil).