Grafting density dominant glass transition of dry polystyrene brushes.
basic_science · Level V
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- Record sourced from PubMed, PMID 28150841.
- Also identified by DOI 10.1039/c6sm02790c.
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Abstract
The effects of the grafting densities (σ<sub>p</sub>), molecular weights (M<sub>n</sub>) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σ<sub>p</sub> > 0.30 increases with decreasing M<sub>n</sub> (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σ<sub>p</sub> < 0.30 decreases with decreasing M<sub>n</sub> (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σ<sub>p</sub> = 0.30), T becomes independent of M<sub>n</sub> or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.