Hydrogen-bond density controlled sub-Tg annealing peaks in fast-scanning-chip calorimeter heating scans of non-crystallized aliphatic polyamides

A series of aliphatic polyamides (PAs) including PA 6, PA 6.6, PA 6.10, PA 6.12, PA 11, and PA 12 has been subjected to physical aging and analyzed regarding changes of structure during annealing at temperatures around 50 to 100 K below the glass transition temperature ($T_g$) for periods up to few hours, employing fast scanning chip calorimetry (FSC). Annealing the amorphous glass of non-crystallized samples at such low temperatures produces endothermic sub-$T_g$ annealing peaks at slightly higher temperature in subsequently recorded FSC heating scans, scaling in area with the time of annealing for a given annealing temperature. Variation of the annealing temperature furthermore indicates that the underlying process of structure-change slows down with decreasing annealing temperature but not disappears even 100 K below $T_g$. Most important, comparison of the behavior of the various PAs investigated reveals a distinct effect of the chemical structure such that the amount of structure-reorganization (glass relaxation and/or ordering) in a pre-defined period of time reduces with decreasing methylene-group sequence-length between amide groups. It appears that the amide-group concentration in the chains, which determines the density of the hydrogen-bond network to neighbored molecular segments, controls/restricts the re-arrangements of sub-nm sized molecular units.