Heat capacities (Cp) of solid benzene, biphenyl,p-terphenyl,p-quaterphenyl, and poly-p-phenylene were analyzed using the ATHAS Scheme of computation. The calculated heat capacities based on approximate vibrational
spectra of solid benzene and the series of oligomers containing additional phenylene groups were compared to experimental
data newly measured and from the literature to identify possible additional large-amplitude motion. The skeletal heat capacity
was fitted to the Tarasov equation to obtain the one- and three-dimensional vibration frequencies Θ1 and Θ3 using a new optimization approach. Their relationship to the number of phenylene groupsn is: Θ1=426.0−150.3/n; and Θ3=55.4+81.8/n. Except for benzene, the quantitative thermal analyses do not show significant contributions from large-amplitude
motion below the melting temperatures.
of experimental techniques are employed to characterize physical and thermal
properties of poly(lactic acid), PLA. To characterize PLA in terms of molecular
mass and molecular mass distribution, size exclusion chromatography was used.
The value of the specific refractive index increment was measured by differential
refractometry. The thermal properties of semicrystalline PLA were measured
by standard and temperature-modulated differential scanning calorimetry. The
thermal stability of PLA was monitored by measuring the changes of mass using
thermogravimetric analysis. The mechanical properties of amorphous PLA were
measured by dynamic mechanical analysis and the results were discussed and
compared with DSC in the glass transition region.
Authors:J. Pak, W. Qiu, M. Pyda, E. Nowak-Pyda, and B. Wunderlich
During a prior study of gel-spun fibers of ultrahigh-molar-mass polyethylene, a substantial error was observed on calculating
the heat capacity with a deformed pan, caused by the lateral expansion of the fibers on shrinking during fusion. In this paper,
the causes of this and other effects that limit the precision of heat capacity measurements by DSC and TMDSC are explored.
It is shown that the major cause of error in the DSC is not a change in thermal resistance due to the limited contact of the
fibers with the pan or the deformed pan with the platform, but a change in the baseline. In TMDSC, the frequency-dependence
is changed. Since irreversible changes in the baseline can occur also for other reasons, inspections of the pan after the
measurement are necessary for precision measurements.
Authors:M. Pyda, Manika Varma-Nair, W. Chen, H. S. Aldrich, R. H. Schlosberg, and B. Wunderlich
Quantitative thermal analysis was carried out for tetra[methyleneoxycarbonyl(2,4,4-trimethyl)pentyl]methane. The ester has a glass transition temperature of 219 K and a melting temperature of 304 K. The heat of fusion is 51.3 kJ mol−1, and the increase in heat capacity at the glass transition is 250 J K−1 mol−1. The measured and calculated heat capacities of the solid and liquid states from 130 to 420 K are reported and a discussion of the glass and melting transitions is presented. The computation of the heat capacity made use of the Advanced Thermal Analysis System, ATHAS, using an approximate group-vibration spectrum and a Tarasov treatment of the skeletal vibrations. The experimental and calculated heat capacities of the solid ester were compared over the whole temperature range to detect changes in order and the presence of large-amplitude motion. An addition scheme for heat capacities of this and related esters was developed and used for the extrapolation of the heat capacity of the liquid state for this ester. The liquid heat capacity for the title ester is well represented by 691.1+1.668T [J K−1 mol−1]. A deficit in the entropy and enthalpy of fusion was observed relative to values estimated from empirical addition schemes, but no gradual disordering was noted outside the transition region. The final interpretation of this deficit of conformational entropy needs structure and mobility analysis by solid state13C NMR and X-ray diffraction. These analyses are reported in part II of this investigation.
Authors:W. Chen, A. Habenschuss, M. Pyda, Monika Varma-Nair, H. S. Aldrich, and B. Wunderlich
The symmetric neopolyol ester tetra[methyleneoxycarbonyl(2,4,4-trirnethyl)pentyl]methane (MOCPM) has been studied by variable-temperature solid-state13C NMR and X-ray powder diffraction and compared to molecular mechanics calculations of the molecular structure. Between melting and glass transition temperatures the material is semicrystalline, consisting of two conformationally and motionally distinguishable phases. The more mobile phase is liquid-like and is, thus attributed to an amorphous phase (≈16%). The branches of the molecules in the crystal exhibit two conformationally distinguishable behaviors. In one, the branches are well ordered (≈56%), in the other, the branches are conformationally disordered (≈28%). Different branches of the same molecule may show different conformational order. This unique character of the rigid phase is the reason for the deficit of the entropy of fusion observed earlier by DSC. In the melt, solid state NMR can identify two bonds that are rotationally immobile, even though the molecules as a whole have liquid-like mobility. This partial rigidity of the branches accounts quantitatively for the observed increase in heat capacity at the glass transition. The reason for this unique behavior of MOCPM, a small molecule, is the existence of one chiral centers in each of the four arms of the molecule. A statistical model assuming that at least two of the chiral centers must fit into the order of the crystal can explain the crystallization behavior and would require 12.5% amorphous phase, 28.1% conformational disorder, and 59.4% crystallinity, close to the observed maximum perfection.