Temperature modulated differential scanning calorimetry (TMDSC) in the region of phase transitions : part 1: theoretical considerations

For temperature modulated differential scanning calorimetry (TMDSC) a simple model, the low pass filter, is presented which allows to see and calculate the influence of heat transfer into the sample on magnitude and phase shift of the modulated part of the measured heat flow rate and the heat capacity determined from it. A formula is given which enables to correct the measured magnitude of the periodic heat flow rate function and the calculated heat capacity in dependence on the thermal resistance and heat capacity of the sample. The correction becomes very important in regions where the heat capacity changes considerably as in the melting region. The approach is successfully tested with model substances with well-known excess heat capacity in the transition region

Temperature modulated differential scanning calorimetry (TMDSC) in the region of phase transitions : part 2: some specific results.

By means of four different examples (pressure crystallised, gel crystallised, nascent and highly stretched polyethylenes (PEs)) it is shown that temperature modulated DSC offers advantages against common DSC. It is possible to see dynamic processes inside the sample during melting. This way we found (i) that during melting of high pressure crystallised PE the so-called a2-process (known from DMA) takes place, (ii) the lamellae doubling in gel crystallised UHMWPE can be seen in TMDSC signals, though no balance heat flow rate is visible in the common DSC, (iii) the same is true for the recrystallisation in nascent and highly stretched PE many degrees before the melting peak appears. To separate these results from the measured curves the knowledge of the heat transport into and within the sample is needed. A simple low pass filter model has proved its worth for this purpose

Calorimetry on small systems—a thermodynamic contribution

Another thermodynamic approach to the Gibbs–Thomson equation, starting from an incremental composition of enthalpy and entropy of the chain molecule, is presented. This describes the melting temperature of (lamella) crystals of linear, folded and cyclic alkanes as well as polyethylenes (PEs) of different type with only one set of parameters. The essential variable turns out to be the number of repeat units (r.u.) ("beads") of the respective molecule, incorporated into the crystallite, rather than the crystallite size. The finding supports the melting being a dynamic process which starts at the surface (interface) of the crystallite. The approach helps to understand the melting behavior of semi-crystalline polymers, it enables the cyclic and normal alkanes to serve as model substances for polymer crystals although their crystals are nearly perfect and large by contrast to the situation in semi-crystalline polymer

High pressure differential scanning calorimetry on polymers

With the aid of a home-made high-pressure measuring cell, connected to a commercial power compensated DSC, it is possible to investigate the melting and crystallization behaviour of polymeric materials as functions of pressure (up to 500 MPa). Results from poly-4-methylpentene-1, syndiotactic polystyrene, and different types of polyethylene are presented and discussed. In addition the melting behaviour of alkanes and alkane mixtures has been investigated, and a thermodynamic description is given. These results are helpful for better understanding of phase transition behaviour under pressure. High pressure DSC is a powerful method which gives additional insight into polymer systems

Melt behaviour of poly(p-xylylene) (PPX) coated gel-spun UHMW-PE fibers as revealed by temperature modulated DSC

The method of temperature modulated DSC has been applied to obtain additional information about the effect of constraints on the melting behaviour of gel-spun ultra high molecular mass polyethylene (UHMW-PE) fibers coated with a high temperature stable poly(p-xylylene) (PPX) polymer. The underlying signal, corresponding to the normal DSC signal, reveals two endothermic peaks for the coated PE fibers. A shift in the underlying and magnitude signal from 142 to 145°C at 0.1 K min–1 , a relative small magnitude signal, together with a vanishing step-like change in the phase signal with increasing PPX coating layer thickness characterize the constraints in terms of a hindrance of the melting of the unconstrained orthorhombic crystal fraction. The time constant of the melting process can be estimated as larger than the reciprocal angular frequency 1/¿=5 s of the modulation

About models and methods to describe chip-calorimeters and determine sample properties from the measured signal

Different models used to describe chip-calorimeters and to simulate their thermal behavior are presented together with the physical basics of heat transfer. Different methods to deconvolute the measured signals are explained. One model example for a certain chip-calorimeter is given in more detail to show the procedur

Unusual pressure-induced phase behaviour in crystalline polymers : X-ray, calorimetric and spectroscopic results and further implications

We report some unusual phase behaviour, of general implication for condensed matter, on the polymers poly-4-methyl pentene-1 (P4MP1), syndiotactic polystyrene, poly-di-alkyl siloxane and polyethylene (copolymer) induced by changes in pressure (P) and temperature (T), as observed by in-situ X-ray diffraction and high pressure DSC. Upon increasing pressure beyond a threshold value, the polymer, crystalline at ambient conditions, looses its crystalline order isothermally. The process is reversible. To quote an example in P4MP1, this behaviour is observed in two widely separated temperature regions, one below the glass transition temperature (<50°C) and one close to the melting temperature (250°C), thus showing solid state amorphization and inversion in the melting temperature with increasing pressure. This further suggests inverse melting, i.e. re-entrant of the two widely separated liquid and amorphous phases along the T-axis at fixed P. This is confirmed experimentally as disordering in the crystalline structure on cooling. The inverse melting in P4MP1 raises the possibility of exothermic melting and endothermic crystallization as anticipated by Tammann (1903). The anticipated exothermic melting and endothermic crystallization is confirmed experimentally in the one component system P4MP1. We are observing similar features in a range of polymers. Similar observations have been observed in syndiotactic polystyrene, poly-di-alkyl siloxanes and polyethylene (copolymer)

The role of the amorphous phase in melting of linear UHMW-PE : implications for chain dynamics

In ultra-high molecular weight polyethylene (UHMW-PE), it is possible to obtain single chain forming single crystals, where chains are adjacently re-entrant. Depending on the heating rate, it is feasible to melt these crystals either by simple consecutive detachment of chain stems from the crystalline substrate or by cluster melting, where several chain stems are involved. The consecutive detachment of chain stems occurs at the melting point predicted from the Gibbs–Thomson equation, whereas the cluster melting at much higher temperatures. Melting by the consecutive detachment of chain stems from the crystal substrate and their diffusion in the melt ultimately result in a new melt state having a heterogeneous distribution of physical entanglements, which invokes differences in local mobility. With combined DSC, rheology and solid-state NMR studies, it is concluded that the disentangled domains present within the entangled matrix possess higher local mobility than the entangled domains, ultimately causing lower elastic modulus. The fraction of the entangled and disentangled domains is maintained at higher temperatures, leading to a thermodynamically non-equilibrium melt state. In contrast, in cluster melting, where several chain stems (initially disentangled) can simultaneously adopt the random coil state, entanglements that are formed are homogeneously distributed in the melt. The paper invokes the influence of the topological differences present in the amorphous phase of the semi-crystalline polymer on the melting kinetics of crystals. The reported findings have implications for the melting behaviour and the resulting melt state of polymers in general