Article pubs.acs.org/Macromolecules
Influence of Backbone Rigidity on Nanoscale Confinement Effects in Model Glass-Forming Polymers Amit Shavit and Robert A. Riggleman* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ABSTRACT: Despite nearly 20 years of active research, the effects of nanoscale confinement on the properties of glass forming polymers remain poorly understood. In particular, the effects of varying polymer chemistry have received comparatively little attention, as many experimental and simulation studies have focused on model polymer systems. In this paper, we use molecular dynamics simulations to investigate the confinement effects of glass-forming polymers with varying backbone rigidity. We find that our model polymers with more rigid backbones experience reduced confinement effects compared to flexible polymers, which is in good qualitative agreement with recent experiments. Furthermore, we find that for a single material, the magnitude of the confinement effect can vary strongly depending on the property investigated to measure the confinement effects. For example, relaxation times studies using the intermediate scattering function can vary dramatically from the bond autocorrelation function. We attribute this finding to enhanced ordering in the vicinity of the free surface in our model polymers with more rigid backbones. Our results indicate that other physical effects beyond glassy dynamics, such as local ordering, may play a role in nanoscale confinement of polymer glasses.
1. INTRODUCTION After more than 15 years of study since the original article by Keddie et al. demonstrating the effects of confinement on the glass transition temperature (Tg) in nanoscopic polymer films1 supported on a silicon substrate, there is not yet a consensus on the origins of the Tg shift, and furthermore, some doubt that a Tg shift occurs at all.2−6 Understanding and controlling the effects of confinement on glass-forming polymers is essential to further development of photolithography and semiconductor manufacturing, as well as several emerging technologies that will depend on the properties of confined glasses, such as sensors, flexible displays, and responsive materials. By far, the most commonly studied experimental system consists of polystyrene (PS) confined to either supported1,7−12 or free-standing thin films,13−15 although recently studies of other polymers such as poly(n-methacrylate) (PnMA),16,17 polycarbonate (PC), and poly(vinyl acetate) (PVAc)15,18 have emerged. From most of the literature in this area, the view that has developed is that the shift in Tg depends critically on the interactions of the polymer with the substrate. For systems with either a weak interaction or repulsion with the underlying substrate, the glass transition temperature is found to decrease upon confinement. This is argued to be a result of an increase in the free volume (decrease in the density) near the relevant surface that promotes mobility. Alternatively, if there is a strong affinity between the polymer and its supporting substrate, then Tg typically increases.19 Near Tg, the relaxation times of a glass-forming system change rapidly with T, so a small shift in Tg corresponds to relaxation times that are expected to change by several orders of magnitude. A common element of most of the experiments © XXXX American Chemical Society
described above is that they measure a Tg based on thermodynamic data and infer that the dynamics are substantially different in the film due to the large changes in Tg. Very recently, the Ediger group measured the rotational dynamics of fluorescent probe molecules distributed throughout free-standing thin polymer films14,20,21 as they are heated through Tg. A range of confinement effects are observed: the dynamics in 28 nm films of poly(α-methylstyrene) are bulk-like at all temperatures, while thin (
