Thickness Anomalies in Supported Polystyrene Films with Thicknesses Comparable to the Radius of Gyration

Abstract

We report on X-ray reflectivity of ultrathin polystyrene films with thickness h, which is comparable to the radius of gyration R g . Tiny jumps in thickness, accompanying an increase in surface roughness and decrease in electron density, are reproductively observed at around 80 C and 92 C only with increasing temperature. For films thinner than R g , the jump at 92 C disappears. However, films of 2R g thick, the jump at 80 C is hardly observed with a faint trace in slope at this temperature. For much thicker films, neither jump appears in heating experiments. The thickness dependent complex behavior observed in non-equilibrium condition is considered to be some indication of surface and confined effects, because the characteristic film thicknesses coincide with the order of R g .KEY WORDS: Ultrathin Polystyrene Film / Glass Transition / Thickness Jump / X-Ray Reflectivity / Physical properties of systems are strongly affected by symmetry, dimensionality, and the number of constituent particles. Part of such dependences comprises an interesting field of physics, widely known as confined effects. Numerous experimental and theoretical studies have reported many intriguing confined effects on the glass transition of polymeric materials, and therefore glass-forming polymers are recognized to be an excellent example of such systems. As reproducible experimental results are easily obtained, polystyrene (PS) thin films with atomically flat surfaces have been used as standard glass-forming polymer films. For substrate-supported PS films, a large reduction in the glass transition temperature (T g ) dependent on the film thickness is observed for films less than 40 nm thick.1,2 More interestingly, the so-called free-standing PS films showed T g reduction much larger than those of supported films.3,4 Since the free-standing PS films are free from the interaction between the solid substrates, the strong reduction in T g is considered to reflect enhanced mobility inherent in the surface region exposed to air. The positron annihilation technique was used to estimate the free volume in the surface region, and indicated that the effective T g of PS was T g (bulk)-25 C and T g (bulk)-43 C for the surface region to a depth of 5 nm and 2 nm, respectively.5,6 Scanning viscoelasticity microscopy 7 showed enhanced mobility on the PS surface, and subsequent atomic force microscopy measurements confirmed a soft surface region 3-4 nm thick with low T g . 8 Regarding thermodynamical considerations, concentration of the ends of molecules in the surface region was pointed out, 9 suggesting a decrease in polymer density on the surface, subsequently confirmed by simulations. 10,11 Since the reduced density naturally leads to an increase in the free-volume and enhanced mobility of chain segments, several experimental studies on the surface glass transition of PS successfully employed a two-layer 1 or three-layer 5 model where the surface layer is characterized by constant thickness and peculiar physical properties (e.g., density, and viscosity...

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.…”

“…After nonlinear least-squares refinement, the electron density and surface roughness of the PS samples were converged to 0.33 e/Å 3 and 11.7Å, respectively. The electron density of the PS sample is almost comparable to that of the PS thin films at room temperature [20] and the surface roughness is sufficiently small to evaluate the surface structure by XR measurements. Thus, the XR analysis revealed that the PS surface fabricated by the aforementioned method exhibits atomic-scale flatness.…”

Confinement enhances dispersion in nanoparticle–polymer blend films

“…On the other hand, very little is known or understood about the nature of dispersion of nanoparticles in thin polymer films [5][6][7]9,10,12 . This is a bit surprising since, a vast amount of research has been devoted to understanding of structure and properties of thin polymer films, especially those related to the perturbation of chain conformation [17][18][19][20][21] and glass transition due to confinement [5][6][7][8]11,[22][23][24][25] . Although there is significant progress in understanding the chain conformation with confinement, some conflicting reports still exist [18][19][20]26,27 .…”

“…This is a bit surprising since, a vast amount of research has been devoted to understanding of structure and properties of thin polymer films, especially those related to the perturbation of chain conformation [17][18][19][20][21] and glass transition due to confinement [5][6][7][8]11,[22][23][24][25] . Although there is significant progress in understanding the chain conformation with confinement, some conflicting reports still exist [18][19][20]26,27 . Similarly, in the case of glass transition of thin polymer films, contradictory reports on finite size effects on T g in polymer thin films have emerged [5][6][7][8][22][23][24] .…”

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

“…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…”