Thermal properties of aliphatic nylons and their link to crystal structure and molecular motion

Abstract

The phase behavior of semicrystalline, aliphatic nylons is analyzed on the basis of differential scanning calorimetry, DSC, and quasi-isothermal, temperature-modulated DSC, TMDSC. The data of main interest are the apparent heat capacities, C _p, in the temperature range from below the glass transitions to above the isotropization. Based on the contributions of the vibrational motion to C _p, as is available from measurements in our laboratory, the ATHAS Data Bank, and multifaceted new TMDSC results, as well as on information on the crystal structures, NMR, molecular dynamics simulation of paraffin crystals, and quasi-elastic neutron scattering, the following observations are made: (a) In semicrystalline nylons the glass transition of the mobile-amorphous phase is broadened to higher temperature. The additionally present rigid-amorphous phase, RAF, undergoes a separate, broad glass transition at somewhat higher temperature. (b) The transition of the RAF, in turn, overlaps usually with an increase in large-amplitude motion of the CH₂-groups within the crystals and latent heat effects due to melting, recrystallization, and crystal annealing. (c) Above the glass transitions of the two non-crystalline phases, C _p of the crystals approaches and exceeds that of the melt. This effect is due to additional entropy contributions (disordering) within the crystals, which may for some nylons lead to a mesophase. In case a mesophase is formed, the C _p drops to the level of the melt as is common for mesophases. (d) Some locally reversible melting is present on the crystal surfaces, but seems to be minimal for the mesophase. (e) The increasing amount of large-amplitude motion in the crystals is described as a third glass transition, occurring over a broad temperature range below the melting or disordering transition from crystal to mesophase.

The assumption of a separate glass transition in ordered phases was previously discovered on analyzing aliphatic poly(oxide)s such as poly(oxyethylene), POE, and in the broad class of mesophase-forming small and large molecules. To attain a full description of the globally-metastable, semicrystalline phase-structure of nylons and to understand its properties, one needs quantitative information about the glass transitions of the two non-crystalline phases and that of the crystal, as well as the various irreversible and locally reversible order/disorder transitions and their kinetics. Finally, with different substitutions in the α-position of the backbone structure of nylon 2, one describes poly(amino acid)s which on proper copolymerization yield proteins. This links the present study to the earlier thermal analyses of all naturally occurring poly(amino acid)s, a number of copoly(amino acid)s, and globular proteins in their dehydrated states. It will be of importance to check by quantitative thermal analysis if similar glass transitions and phase structures as seen in the aliphatic nylons are also present in the poly(amino acid)s to possibly gain new information about the nanophase structure of proteins.