Effect of density on the physical aging of pressure-densified polymethylmethacrylate

TL;DR

This study shows that the physical aging rate of pressure-densified PMMA is unaffected by density, emphasizing local packing as the key factor in glassy dynamics and revealing pressure densification's impact on the glass transition temperature.

Contribution

It demonstrates that the aging behavior of PMMA's secondary relaxation is independent of density, challenging existing models of glassy dynamics and highlighting local packing as the dominant variable.

Findings

Aging rate is independent of density in pressure-densified PMMA.

Pressure densification reduces the glass transition temperature.

Different pressure-induced structures do not alter aging behavior.

Abstract

The rate of physical aging of glassy polymethylmethacrylate (PMMA), followed from the change in the secondary relaxation with aging, is found to be independent of the density, the latter controlled by the pressure during glass formation. Thus, the aging behavior of the secondary relaxation is the same whether the glass is more compacted or less dense than the corresponding equilibrium liquid. This equivalence in aging of glasses formed under different pressures indicates that local packing is the dominant variable governing the glassy dynamics. One consequence is that pressure densification yields a reduction in the glass transition temperature. The fact that pressure densification yields different glass structures is at odds with a model for non-associated materials having dynamic properties exhibited by PMMA, such as density scaling of the relaxation time and isochronal superposition of the relaxation dispersion.