This review traces the development of thermal analysis over the last 40 years as it was experienced and contributed to by the author. The article touches upon the beginning of calorimetry and thermal analysis of polymers, the development of differential scanning calorimetry (DSC), single run DSC and other special instrumentations, up to the recent addition of modulation to calorimetry. Many new words and phrases have been introduced to the field by the author and his students, leaving a trail of the varied interests one can have over 40 years. It began with “cold crystallization” and most recently the term “oriented, intermediate phase” was coined, creating in-between: “extended chain crystals,” the “irreversible thermodynamics of melting of polymer crystals,” “dynamic differential thermal analysis” (DDTA), “the rule of constant increase ofCp per mobile bead within a molecule at the glass transition temperature,” “superheating of polymer crystals,” “melting kinetics,” “crystallization during polymerization,” the “chain-folding principle, “molecular nucleation,” “rigid amorphous phase,” a “system of classifying molecules,” “macroconformations,” “amorphous defects,” “rules for the entropy of fusion based on molecular shape and flexibility,” “single-molecule single-crystals,” “a system of classifying phases and mesophases,” and “condis phase.”
Polymer molecules have contour lengths which may exceed the dimension of microphases. Especially in semicrystalline samples
a single molecule may traverse several phase areas, giving rise to structures in the nanometer region. While microphases have
properties that are dominated by surface effects, nanometer-size domains are dominated by interaction between opposing surfaces.
Calorimetry can identify such size effects by shifts in the phase-transition temperatures and shapes, as well as changes in
heat capacity. Specially restrictive phase structures exist in drawn fibers and in mesophase structures of polymers with alternating
rigid and flexible segments. On several samples shifts in glass and melting temperatures will be documented. The proof of
rigid amorphous sections at crystal interfaces will be given by comparison with structure analyses by X-ray diffraction and
detection of motion by solid state NMR. Finally, it will be pointed out that nanophases need special attention if they are
to be studied by thermal analysis since traditional ‘phase’ properties may not exist.
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.
Thermal analysis and polymers are two subjects in the field of chemistry and materials sciences that have not developed to the level commensurate with their importance. In this paper the reasons for this deficiency are traced to the history of the development of these subjects which led to only limited availability of courses of instruction. A first remedy to this problem is suggested, teaching via the internet. The attempt by the author to generate such a course is described in this paper. The course contains an up-to-date store of basic information. It is divided into 36 lectures and displayed in about 3000 computer screens filled with graphs, text, and hypertext. All lectures are downloadable. Including presentation software, each lecture requires only 1–3 Mbyte of computer memory. The inclusion of color,movies, and sound would exceed the capacity of most presently available personal computers,but might point the way to future of teaching the ever increasing number of subjects.
Modulated differential scanning calorimetry (MDSC) uses an abbreviated Fourier transformation ≼r the data analysis and separation of the reversing component of the heat flow and temperature signals. In this paper a simple spread-sheet analysis will be presented that can be used to better understand and explore the effects observed in MDSC and their link to actual changes in the instrument and sample. The analysis assumes that instrument lags and other kinetic effects are either avoided or corrected for.
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.
Contributions of modern, temperature-modulated calorimetry
are qualitatively and quantitatively discussed. The limitations are summarized,
and it is shown that their understanding leads to new advances in instrumentation
and measurement. The new thermal analysis experiments allow to separate reversing
from irreversible processes. This opens the irreversible states and transitions
to a description in terms of equilibrium and irreversible thermodynamics.
Amorphous systems can be treated frommacroscopic to nanometer sizes with weak
to strong coupling between neighboring phases. Semicrystalline, macromolecular
systems are understood on the basis of modulated calorimetry as globally metastable,
micro-to-nanophase-separated systems with locally reversible transitions.
Structure sensitive thermal analysis results on linear macromolecules can be obtained when correcting measurements with heat capacity instead of “baseline” data. The ATHAS (Advanced Thermal Analysis) effort in heat capacities of crystals, glasses and liquids is described, and applied to the interpretation of microphase separated samples. Semicrystalline homopolymers, block-copolymers, and blends are to be discussed.