Distributions of glass-transition temperature and thermal expansivity in multilayered polystyrene thin films studied by neutron reflectivity

Abstract

We performed neutron reflectivity measurements on multilayered polymer thin films consisting of alternatively stacked deuterated polystyrene (d-PS) and hydrogenated polystyrene (h-PS) layers ∼200 Å thick as a function of temperature covering the glass-transition temperature T(g), and we found a wide distribution of T(g) as well as a distribution of the thermal expansivity α within the thin films, implying the dynamic heterogeneity of the thin films along the depth direction. The reported anomalous film thickness dependences of T(g) and α were reasonably understood in terms of the distributions, showing that the surface mobile layer and the bottom hard interfacial layer are, respectively, responsible for the depressions of T(g) and α with decreasing film thickness. The molecular mobility in each layer is also discussed in relation to the distribution of T(g), based on the results on mutual diffusion at the layer interface.

Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

“…Recently Kanaya et al made further LEM measurements on PS samples in the form of thin multilayer dPS/hPS films, measured using the longitudinal field (LF) configuration [13]. These results confirmed the presence of the reduction of T g near the surface and provided good consistency with neutron reflectivity measurements made on the same samples [14].…”

Formation of hydrated layers in PMMA thin films in aqueous solution

“…Although most applications are in dry environments, analysis of PMMA films in aqueous solutions is of relevance because of their potential as coatings for various types of medical implants [5][6][7] and general use in other applications where protective surface coatings in aqueous solution are required. Studying PMMA films in situ in aqueous solutions is challenging, but neutron reflectometry has been previously used in various studies which characterise thin films of hydrated polymers prepared via spin-coating, in particular with studies of glass transition temperature (T G ) [11,12] and solvent retention [13,14]. Formation of blisters in PMMA [8] films in aqueous solution have been observed with AFM and optical microscopy, where it was suggested that deformation depends on film thickness and Young's modulus of PMMA.…”

Modulus, Confinement, and Temperature Effects on Surface Capillary Wave Dynamics in Bilayer Polymer Films Near the Glass Transition

“…With some exceptions [1], studies of confined polymers have revealed large deviations in properties, e.g., glass transition temperature (T g ), compared with bulk behavior [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Supported films of linear polymers without attractive polymer-substrate interaction report a reduced T g relative to bulk T g (T g;bulk ) upon confinement [2][3][4][5][7][8][9][10][11][12][13][14].…”

“…Supported films of linear polymers without attractive polymer-substrate interaction report a reduced T g relative to bulk T g (T g;bulk ) upon confinement [2][3][4][5][7][8][9][10][11][12][13][14]. For example, the seminal 1994 study by Keddie et al [2] revealed that T g À T g;bulk ¼ $ À 24 C for a 17-nm-thick polystyrene (PS) film on silica.…”

“…The enhanced surface mobility propagates into the film several tens of nanometers [3,4], and in the absence of attractive polymersubstrate interactions, the propagation of the free-surface effect causes a reduction of average film T g with decreasing thickness [3,4]. Studies have probed the film surface and interfacial regions, and the local T g s differ from the average T g across the film [3][4][5]. For example, in bulk PS films in which a 14-nm-thick free-surface layer has been labeled with a fluorescent dye, T g À T g;bulk ¼ $ À 32 C in the surface layer [3,4].…”