(3) Surface and Interface Structure of Softmatter and Dynamics

Glass Transition in Polymer Thin Films
We investigated glass transition temperature Tg and thermal expansivity in polymer thin films using neutron and X-ray reflectivity measurements.  In addition we studied dynamics of polymer thin films using inelastic and quasielastic neutron scattering
Neutron and X-ray Reflectivity Measurements
Glass transition temperature Tg decreases with film thickness
Thermal expansivity in glassy state decreases with film thickness

These findings gave us a chance to start quasielastic neutron scattering
Phys. Rev. E69, 022801 (2004)
Phys. Rev. E69, 06183 (2004)
Dynamics of Polymer Thin Films by Inelastic Neutron Scattering
In order to study the origins of the decreases in glass transition temperature and thermal expansivity with film thickness we performed inelastic neutron scattering, and found that mean square displacement <u2> and the density of phonon states G(w) decrease with film thickness.  These decreases shjow the potential hardening in thin films, and have been assigned to a hard layer at the interface between the polymer and substrate.
Dynamic scattering law
S(Q,w) of polymer thin film.
Decrease in MSD <u2>
Decrease in DPS G(w)
Phys. Rev. Lett., 95. 56102 (2005))
Phys. Rev. E74, 021801 (2006)
Cause of Potential Hardening is an interface hard layer beteeen polymer and substrate.
Phase Separation and Dewetting of Polymer Blend Thin Films
Phase separation and dewetting of polystyrene (PS) and Poly(vinyl methyl ether ) (PVME) blend thin films were studied as a function of film thickness by means of light scattering (LS), AFM, optical microscope, and neutron reflectivity.
Time evolution of LS profiles
for various film thickness.
Film thickness dependence
of characteristic wavelength

In a range of thickness above 1ƒÊm, phase separation occurs preferentially, and below 100 nm dewetting occurs preferentially.  In between (100 nm - 1ƒÊm) phase separation and dewetting coexist and no characteristic length is observed in LS profiles.
Concentration Fluctuations before Dewetting Revealed
by Neutron Reflectivity
Dewetting process of PS/PVME blend thin film looks like spinodal decomposition type one, however very long incubation time was observed.  We have studied the concentration fluctuations in thin films before dewetting by neutron reflectivity, and found that they did.   Another interesting finding is a very thin layer formation of PVME after dewetting.  This must be due to the very high afinity of PVME to substrate.
Time evolution of NR profiles as a function
of annealing time.