Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

Abstract

We report X-ray reflectivity measurements of polystyrene thin films supported on Si substrates at various heating and cooling rates. At a heating rate of 0.05 1C min À1 , the width of the glass transition w does not show any noticeable thickness dependence. However, at faster (0.5 1C min À1 ) and slower (0.01 1C min À1 ) heating rates, w shows remarkable thickness dependence: it broadens at a faster rate and narrows at a slower rate with decreasing film thickness. Our results suggest that the heterogeneous character of the dynamics can be transformed into a homogeneous one in a glassy film with thickness comparable to a critical thickness that depends on the rate of temperature variation. Polymer Journal (2011) 43, 390-397; doi:10.1038/pj.2010.145; published online 19 January 2011Keywords: confinement effects; glass transition; polymer thin films; transition breadth; transition broadening; X-ray reflection INTRODUCTIONThe molecular dynamics in liquid, especially close to and below the glass transition temperature T g , are very heterogeneous. 1 From a theoretical viewpoint in which the mean structural relaxation in glassy systems occurs through rearrangements of correlated particles, the spatial particle region surrounded by other subsystems composed of particles with differing mobilities is called a cooperatively rearranging region. 2,3 Because of the dynamic heterogeneity, temporal relaxations depend on the subsystems, and the glass transition generally occurs within a certain temperature range called the width of glass transition w. Polymers confined in a particular size of cooperatively rearranging region can be good candidates for investigating the spatial size of dynamic heterogeneities. A striking result in confined systems is that T g of thin films often exhibits a remarkable thickness dependence. 4,5 This result strongly suggests that interfaces in a confined system have an important role. Many studies have revealed the properties of interfacial regions and have provided extensive evidence that the molecular mobility is much higher in the free surface region than in the bulk region at the same temperature; 6-9 dynamics near the free surface are very fast. On the other hand, the interaction between a polymer and a solid substrate can suppress molecular relaxation in an interfacial region, leading to peculiar slower dynamics in this region. [9][10][11][12][13] Measurements of the relaxation rate using a 20-nm-thick fluorescently labeled layer that has been incorporated into polymer films have revealed that the rate of dynamics depends on the distance from both the free surface and the interface. These dynamics vary continuously, exhibiting bulk-like characteristics toward the interior layer sandwiched between the surface and the interfacial layers. 9

Fabrication and Characterization of Polystyrene Surface with Atomic-Scale Surface Roughness

“…The in-plane heterogeneous periodicity of surface undulation gives diffuse scattering normal to the q z direction in reciprocal space; therefore, the broad peak observed in the q z scan profiles might originate from the diffuse scattering component superimposed on the specular reflection along the q z direction. Another possibility of the emergence of the broad peak is the presence of a periodic layered structure normal to the PS surface; however, this is not probable because the periodic layered structure normal to the sample surface would exhibit a periodic interference fringe pattern in the XR profiles, as reported in previous studies [1,4,7,20,23,24]. We observed only one peak at q z = 0.04Å −1 in the XR profiles.…”

Depth-dependent inhomogeneous characteristics in supported glassy polystyrene films revealed by ultra-low X-ray reflectivity measurements

“…This result is again consistent with the a glass value reported for 70-nm-thick films of molecular weight 2.34 Â 10 5 g mol À1 measured at heating rates of 0.5 and 0.01 1C min À1 . 32 However, in the case of the thinner films (B12 and 6 nm), the a glass value was found to increase with decreases in the heating rate. Figure 2 illustrates the temperature dependence of the T g of PS films obtained at various heating rates.…”

“…The details of the analysis have been previously described. 32,33 RESULTS Figure 1 illustrates the temperature dependence of the normalized thicknesses that were obtained at heating rates of 0.14 1C Figure 1 are the fitted curves obtained by assuming the 'tanh' profile approximation: 4…”

Glass Transition of Ultrathin Sugar Films Probed by X-Ray Reflectivity

“…This temperature-dependent thickness behavior is probably attributable to the glass transition upon cooling. The glassy-to-viscous state transition of T g is typically determined as the intersection of the two straight lines showing different linear expansivities [18]. In this way, we determined the T g of trehalose nano-film as 95°C.…”

“…The importance of these findings in the pharmaceutical field cannot be overstated. Specular X-ray reflectivity (XRR) is a unique and powerful analytical method (Figure 2) that is frequently applied to condensed soft-matter films, including glass transition studies of ultrathin polymer films [16][17][18][19][20], but which has not been applied in amorphous sugar studies. XRR can evaluate the layered structure of a material, such as the film thickness, electron density, surface roughness, and interfacial width [21,22].…”