Influence of initiator fragments as chain ends on the Tg-confinement effect and dewetting of thin films of ultralow molecular weight polymer

Abstract

Prior research on anionically polymerized polystyrene (PS) indicates that there is little or no influence of molecular weight (MW) from 2 to 3000 kg/mol on the Tg-confinement effect of supported PS films. Synthesis by free radical polymerization (FRP) with 2,2′-azobis(2-methylpropionitrile) (AIBN) or benzoyl peroxide (BPO) as initiator incorporates initiator fragments at chain ends that are distinct from PS repeat units and anionic initiator fragments as chain ends. While the influence of different chain ends on the Tg-confinement effect is negligible for high MW PS, we show that it is substantial for ultralow MW PS. As determined by spectroscopic ellipsometry and fluorescence measurements of films of ultralow MW PS (Mn = ∼4 kg/mol) supported on silicon wafers or glass with native oxide surfaces, the magnitude of Tg (21-nm-thick) – Tg (bulk) for PS made by FRP is reduced by 50% from that for anionically polymerized PS. Ellipsometry results indicate that the muted effect for ultralow MW PS made by FRP originates mainly from a weaker perturbation to Tg near the free surface. However, dewetting studies show that PS made by FRP results in enhanced film stability relative to anionically polymerized PS; this indicates that the nitrogen- or oxygen-containing FRP initiator fragments present as chain ends may undergo hydrogen bonding with hydroxyl groups on the substrate surface. In contrast to PS, supported films of ultralow MW poly(methyl methacrylate) show no significant influence of different chain ends on the Tg-confinement effect down to a thickness of 20 nm.