Reducing the Bulk Fragility and Suppressing the Fragility-Confinement Effect in Polystyrene with Very Low Levels of 2-Ethylhexyl Acrylate Comonomer

Abstract

The effects of nanoconfinement on the glasstransition temperature (T g ) and fragility index (m) of thin polymer films, e.g., polystyrene (PS), can be substantial. We recently demonstrated the elimination of the T g -confinement effect in supported PS films by including 2−6 mol % 2-ethylhexyl acrylate comonomer (Macromolecules 2022, 55, 9601). Here, we use spectroscopic ellipsometry and differential scanning calorimetry to characterize how m is affected by inclusion of low levels of 2ethylhexyl acrylate (EHA), n-butyl acrylate (nBA), or n-octyl acrylate (nOA) in styrene (S)-based random copolymers. Relative to PS, the inclusion of 1 mol % EHA or 5 mol % nBA or nOA in Sbased copolymers has no effect within uncertainty on bulk fragility, m bulk . In contrast, 98/2 mol % and 94/6 mol % S/EHA copolymers exhibit ∼21% and ∼30% reductions in m bulk relative to PS. In thin films supported on silicon, PS, 99/1 mol % S/EHA, 95/5 mol % S/nOA, and 95/5 mol % S/nBA exhibit similar, strong confinement effects on T g and m. In contrast, 98/2 mol % S/EHA exhibits weak m-confinement effects in sub-100 nm thick films, and 94/6 mol % S/EHA shows no effect down to 22 nm. The strengths of the T g -confinement effect for various systems are in accord with the m-confinement effects. Thus, the role of the EHA comonomer in suppressing or eliminating T g -and m-confinement effects cannot be generalized to other acrylates. These results support previously reported correlations between the susceptibilities of T g -and m-perturbations caused by nanoconfinement and provide a very simple route to suppress or eliminate confinement effects in linear polymers.

Effect of intermolecular hydrogen bonding strength on the dynamic fragility of amorphous polyamides

“…S1a (ESI†) according to eqn (3). The fragility obtained from enthalpy relaxation has been verified to be very close to the value determined by dielectric relaxation at relaxation time τ α = 100 s. 44,45 Therefore, enthalpy relaxation was used here to determine the m values.…”