Speaker
Dr
Francis Pratt
(STFC)
Description
`Intensive studies of the properties of polymer thin films have followed the discovery of significant suppression of the glass transition temperature $T_g$ for freestanding films of nanoscale thickness (reviewed in [1]). A local $T_g$ that depends on distance from a free surface has often been invoked to explain these results, but there has previously been a lack of experimental techniques able to measure directly such dependence in an individual sample. Low energy muons (LEM) can be used to clarify this issue by making depth-resolved measurements of the local $T_g$ near the surface of a polymer, which can be used to identify the mechanisms responsible for reducing $T_g$. LEM data obtained for polybutadiene (PBD) and polystyrene (PS) across a range of molecular weights have been analysed against possible models for the transition between surface and bulk behaviour and a consistent picture emerges [2] in which the transition is governed by a mechanism of diffusing conformational kinks first proposed by de Gennes [3]. This diffusive mechanism operates over a length scale determined by the size of the polymer chain, crossing over at longer distances to a capillary wave mechanism first proposed by Herminghaus [4]. A detailed study of the characteristic length scale against molecular weight for PBD shows good agreement with random coil length scales at low molecular weight but a crossover in the behaviour is seen around 50 kDa, suggesting breakdown of the random coil assumption for larger molecules [2]. In addition, when the muon implantation depth is increased beyond 100 nm we find evidence for a buried region containing inhomogeneous nanostructure [5], most likely nanopores formed as the solvent is evacuated in the spin coating process. Studies on the polymer polydimethysiloxane (PDMS) have also been made, showing some basic similarities as well as some clear differences with the PBD/PS results [5].`
References
1. M. Alcoutlabi and G.B. McKenna, `$\textit{J. Phys. Condens. Matter}$` `$\textbf{17}$`, R461 (2005).
2. F.L. Pratt et al, (unpublished).
3. P. G. de Gennes, `$\textit{Eur. Phys. J. E}$` `$\textbf{2}$`, 201 (2000).
4. S. Herminghaus, `$\textit{Eur. Phys. J. E}$` `$\textbf{8}$`, 237 (2002).
5. F.L. Pratt et al, `$\textit{Polymer}$` `$\textbf{105}$`, 516 (2016).
Primary author
Dr
Francis Pratt
(STFC)
