425670 Substantial Spatial Heterogeneity and Tunability of Glass Transition Temperature Observed with Dense Polymer Brushes Prepared By Arget ATRP

Tuesday, November 10, 2015: 1:45 PM
251B (Salt Palace Convention Center)
Tian Lan, Department of Materials Science and Engineering, Northwestern University, Evanston, IL and John M. Torkelson, Depts of Chemical and Biological Engineering and of Materials Science and Engineering, Northwestern University, Evanston, IL

Dense polystyrene (PS) brushes were synthesized on silica wafers via a

“grafting from” technique using activators regenerated by electron transfer atom transfer

radical polymerization (ARGET ATRP). Spectroscopic ellipsometry and fluorescence

spectroscopy were used to investigate the effect of nanoscale confinement on the glass

transition temperature (Tg) and its distribution in dense PS brushes and bilayer constructs

of brushes and freely deposited PS films. Single-layer PS brushes exhibit an overall

average Tg that is nearly invariant with thickness down to 11 nm, with Tg values of the

dense brushes differing by no more than 2 °C from those of bulk PS of the same

molecular weight as that of the brush. These results differ greatly from those obtained

with freely deposited PS films that show major Tgreductions with confinement.

Distributions of Tgs were also studied by fluorescence in dense, ∼70-nm-thick PS brushes.

Relative to Tg,bulk, chain segments within 10 nm of the substrate exhibit a ∼36 °C increase

inTg while segments within 5 nm of the free surface exhibit a 14 °C decrease in Tg.

Fluorescence was also used to characterize the tunability of Tg within a single layer of

bilayer constructs. As brush thickness increases from 13 to 94 nm, the Tg of a 15-nm-

thick PS overlayer film decreases from 100 °C to 87 °C; in contrast, as the overlayer PS

film thickness increases from 0 to 101 nm, the Tg of an 11-nm-thick underlayer PS brush

increases from 98 °C to 126 °C. These results are compared with the few previous

experimental reports of Tg-confinement effects of densely grafted PS brushes and bilayer

constructs of brushes and films.

Extended Abstract: File Not Uploaded
See more of this Session: Nanoscale Structure in Polymers
See more of this Group/Topical: Materials Engineering and Sciences Division