Facilitation of interfacial dynamics in entangled polymer films

Abstract

In this article, we use cooling-rate dependent Tg measurements (CR-Tg) to indirectly probe the relaxation dynamics of supported polystyrene thin films of various molecular weights, all chosen to be above the entanglement molecular weight. We show that the dynamics in these films deviate from bulk dynamics below a temperature T(*) = Tg + 6 K = 380 K ± 1 K. We show that T(*) for films of all thicknesses and molecular weights is the same as the temperature at which the free surface dynamics deviate from the bulk dynamics. The apparent activation barrier of the glass transition in thin films decreases towards that of the free surface as the film thickness decreases. This provides strong evidence that thin film dynamics are facilitated by the enhanced mobility at the free surface. The observation of T(*) can help resolve some seemingly contradictory data by suggesting that studies performed at higher temperatures (T > T(*)), or which probe shorter relaxation times (τ < τ(*) ∼ 1 s) would not observe properties that deviate from bulk values. We also demonstrate that the relaxation dynamics of supported entangled polystyrene films slow down slightly as the molecular weight of polystyrene increases. An eight nanometer film of Mw =2240 kg/mol polystyrene shows a Tg reduction of 27 K at a cooling rate of 1 K/min, while a film of the same thickness made of Mw =45.8 kg/mol polystyrene has a 36 K reduction of Tg compared to the bulk film at the same cooling rate. We hypothesize this is either due to the density of a dynamically "dead" layer near the substrate increasing with molecular weight, or partial anchoring of long chains, which could hinder segmental diffusion near the free surface.

Disconnecting structure and dynamics in glassy thin films

“…This interpretation is supported by observations of probe molecules embedded in films [27,28]. Our results show that the two populations are indistinguishable from a structural point of view.…”

Long-range correlated dynamics in ultra-thin molecular glass films

“…It is important to note that all of these temperatures are well below bulk T g . The low apparent activation energy of ultra-thin films is consistent with previous studies of dynamics on polymeric thin films [14,15,29].…”

“…While a direct measure of absolute viscosity of thin films can not be obtained due to potential gradient in the dynamics induced by free interface, by relating the dewetting times with cooling rate-dependent T g (CR-T g ) measurements [14,29], we are able to measure the "effective viscosity" of ultra-thin films as function of film thickness and temperature. In the absence of gradients in the dynamics, the effective viscosity equals the film viscosity.…”

“…Recent experiments suggest that diffusion at the free surface of organic glasses can be several orders of magnitude faster [6,7], with weaker temperature dependence compared to bulk diffusion. Enhanced, weakly temperature-dependent dynamics on the surface of polymeric glasses [8,9] have been shown to significantly affect the properties of ultra-thin polymer films [9][10][11][12][13][14][15][16][17]. In polymeric systems, the molecular weight of the polymer [14], and the temperature range of the measurement [8,9,14] seem to also affect the observed properties, resulting in ambiguity in the relationship between enhanced dynamics at the free surface and properties of ultra-thin glass films.…”

“…4(c) (details in SI). CR-T g measurements were used to extend the dynamical range of the bulk measurements to temperatures below bulk T g [14,29,36]. CR-T g measurements were performed on films with a thickness range of 20 nm< h <100 nm as shown in Fig 4(b) (details in SI S8 and S9).…”