Surface Mobility of Amorphous o-Terphenyl: A Strong Inhibitory Effect of Low-Concentration Polystyrene

Abstract

Previous work has shown that a surface wave on amorphous o-terphenyl (OTP) decays by viscous flow at high temperatures and by surface diffusion at low temperatures. We report that the surface mass transport can be efficiently suppressed by low-concentration polymers. Surface-grating decay has been measured for OTP containing 1 wt % polystyrene (PS, Mw = 1-8 kg/mol), which is miscible with OTP. The additive has no significant effect on the decay kinetics in the viscous-flow regime, but a significant effect in the surface-diffusion regime. In the latter case, surface evolution slows down and becomes nonexponential (decelerating over time). The effect increases with falling temperature and the molecular weight of PS. These results are attributed to the very different mobility of PS (slow) and OTP (fast) and their segregation during surface evolution, and relevant for understanding the surface mobility of multicomponent amorphous materials.

Direct Measurement of Lateral Molecular Diffusivity on the Surface of Supersaturated Amorphous Solid Dispersions by Atomic Force Microscopy

“…A similar trend was previously shown when comparing neat OTP samples with 1% (w/w) spiked polystyrene samples and qualitatively explained with a heterogeneous decay, where polystyrene molecules enrich on peaks owing to their slower surface mobility. 25 Our study underlines this observation for 1% (w/w) polymer spiked samples. As seen in Figure 5a, a small deviation is already present for neat IMC with β values lower than unity.…”

“…As described previously, 8,25 the height information for several pictures over the measurement time is subsequently fitted either with a single exponential decay function or with a…”

“…As described previously, , the height information for several pictures over the measurement time is subsequently fitted either with a single exponential decay function or with a stretched exponential (see also Figure below) to obtain the decay constant K . Obtained log K values from either fit did not differ significantly (difference <0.1 s –1 ) and log K values shown in Figure and Figure are from the stretched exponential fit.…”

“…24 To this date, only a single study on a two component system, an amorphous mixture of OTP with 1% (w/w) polystyrene, exists in the literature. 25 All of the studies above report a surface diffusion many magnitudes faster than the respective bulk diffusion.…”

“…This group has since almost exclusively pioneered the work on organic amorphous compounds with further measurements on nifedipine, polyalcohols, griseofulvin, and polystyrene . To this date, only a single study on a two component system, an amorphous mixture of OTP with 1% (w/w) polystyrene, exists in the literature . All of the studies above report a surface diffusion many magnitudes faster than the respective bulk diffusion.…”