Characteristic length of the glass transition in isochorically confined polymer glasses

We report the effect of isochoric confinement on the characteristic length of the glass transition (ε_α) for polystyrene (PS) and poly(4-methylstyrene) (P4MS). Utilizing silica-capped PS and P4MS nanoparticles as model systems, ε_α values are determined from the thermal fluctuation model and calorimetric data. With decreasing nanoparticle diameter, ε_α decreases, suggesting a reduction in the number of segmental units required for cooperative motion at the glass transition under confinement. Furthermore, a direct correlation is observed between ε_α and the isochoric fragility (m_v) in confined polymers. Due to a nearly constant ratio of the isochoric to isobaric fragility in confined polymer nanoparticles, a correlation between ε_α and m_v also implies a correlation between ε_α and the volume contribution to the temperature dependence of structural relaxation. Lastly, we observe that when the fragility and characteristic length are varied in the same system the relationship between the two properties appears to be more correlated than that of across different bulk glass-formers.