Calorimetry deals with the energetics of atoms, molecules, and phases and can be used to gather experimental details about
one of the two roots of our knowledge about matter. The other root is structural science. Both are understood from the microscopic
to the macroscopic scale, but the effort to learn about calorimetry has lagged behind structural science. Although equilibrium
thermodynamics is well known, one has learned in the past little about metastable and unstable states. Similarly, Dalton made
early progress to describe phases as aggregates of molecules. The existence of macromolecules that consist of as many atoms
as are needed to establish a phase have led, however, to confusion between colloids (collections of microphases) and macromolecules
which may participate in several micro- or nanophases. This fact that macromolecules can be as large or larger than phases
was first established by Staudinger as late as 1920. Both fields, calorimetry and macromolecular science, found many solutions
for the understanding of metastable and unstable states. The learning of modern solutions to the problems of materials characterization
by calorimetry is the topic of this paper.
Phases may be smaller than visible to the human eye. In order to characterize a microphase, a phase smaller than 1 μm, one must consider surface area and free energy in addition to the standard thermodynamic functions.
As one approaches nanometer sizes, one also needs to know the changing thermodynamic functions within the phases. The Gibbs–Thomson
equation can be used to characterize microphases, but not nanophases. For the latter, the glass transition is needed to assess
the properties in the interior. In order to classify condensed phases as liquid, solid, mesophase, or crystal, one needs to consider the molecular motion in addition to the molecular structure. Most important are large-amplitude displacements in form of translation, rotation, and conformational motion. An operational definition based on experiments and an updated classification of the phases is given. The surprising result is the observation that
crystals, earlier assumed prime examples of solids, can have order–disorder transitions to more mobile mesophases, as well
as a glass transition without change in crystal structure, i.e., under certain condition, they cannot be identified as a solid.
To these observations, one has to add the fact that large-amplitude motion may start gradually to a more mobile phase without
abrupt changes in structure. These observations limit the usefulness of the 80-year-old classification of transitions as being
of first or second order. Quantitative thermal analysis is shown to be an important tool to identify the possible total of
57 different condensed states in terms of their macroscopic properties as well as molecular structure and motion.
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, Cp, in the temperature range from below the glass transitions to above the isotropization. Based on the contributions of the
vibrational motion to Cp, 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, Cp 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 Cp 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.
was earlier identified as a rigid-amorphous fraction, RAF [ 28 ]. The discussion of Fig. 3 suggests that this layer of noncrystalline PBT without a bulk-amorphous T g or latent heat, must be a different nanophase reaching its metastability by
Authors:Viorel Chihaia, Karl Sohlberg, M. Scurtu, S. Mihaiu, M. Caldararu, and M. Zaharescu
transmission variation in the UV–VIS and IR range [ 3 ]. Mixed CeO 2 /SnO 2 film electrodes were used as photoanodes in a new generation of nanophase solar cells [ 4 ]. Tin dioxide-doped ceria materials are potential electrolytes for solid oxide fuel cells
Authors:Wantinee Viratyaporn and Richard L. Lehman
recrystallization. Aluminum oxide (Al 2 O 3 ) and zinc oxide (ZnO) nanoparticles were obtained from Nanophase Technologies, Romeoville, IL, as either dry powder or dispersed in propylene glycol monomethyl ether acetate (PGMEA or PMA). Further details regarding the