End-Tethered Chains Increase the Local Glass Transition Temperature of Matrix Chains by 45 K Next to Solid Substrates Independent of Chain Length

TL;DR

This study shows that end-tethered polystyrene chains grafted onto silica substrates significantly raise the local glass transition temperature of nearby matrix chains by about 45 K, regardless of chain length or grafting density.

Contribution

It provides experimental evidence that covalently bonded tethered chains can substantially increase local Tg, independent of chain length and grafting density.

Findings

End-tethered chains increase local Tg by approximately 45 K.

The Tg increase is independent of chain length from 8.6 to 212 kg/mol.

The Tg elevation occurs across a wide range of grafting densities.

Abstract

The local glass transition temperature Tg of pyrene-labeled polystyrene (PS) chains intermixed with end-tethered PS chains grafted to a neutral silica substrate was measured by fluorescence spectroscopy. To isolate the impact of the grafted chains, the films were capped with bulk neat PS layers eliminating competing effects of the free surface. Results demonstrate that end-grafted chains strongly increase the local Tg of matrix chains by $\approx$45 K relative to bulk Tg, independent of grafted chain molecular weight from Mn = 8.6 to 212 kg/mol and chemical end-group, over a wide range of grafting densities $\sigma$ = 0.003 to 0.33 chains/nm$^2$ spanning the mushroom-to-brush transition regime. The tens-of-degree increase in local Tg resulting from immobilization of the chain ends by covalent bonding in this athermal system suggests a mechanism that substantially increases the local activation energy required for cooperative rearrangements.