The melting of PbBr2 in sealed crucibles was investigated by means of DSC. Three factors were considered to affect melting point: i) impurities, ii) the bromine pressure over the PbBr2, and iii) photolysis. Both crystals and powders were investigated. The peak of the melting changed after sample grinding. The bromine pressure over the PbBr2 was found to cause a significant error in the determination of the melting point.Lead bromide melts at 370.6±0.2°C. The heat of melting is 42.9±1.8 J g–1.
The melting and crystallization of copolymers of tetrafluoroethylene with ethylene, synthesized in bulk and in suspension
by semi-flow method, were studied by DSC.
X-ray diffractions and infrared spectra of the copolymers were measured and new crystalline reflections different from those
of the homopolymers were observed.
The melting temperature of the copolymers synthesized in bulk depends strongly on the composition and exhibits several maxima.
A certain small decrease in the melting temperature within the range of the alternating composition is observed.
For alternating copolymers synthesized in suspension, the peaks are monomodal indicating a higher structural and chemical
homogeneity of the copolymer.
The nonisothermal crystallization kinetics in the temperature interval from 260 to 255°C of the alternating copolymer prepared
in suspension can be described by a modified Avrami equation. The mechanism of nucleation and nuclei growth during the nonisothermal
crystallization of the tetrafluoroethylene-ethylene copolymer is close to that of polyethylene.
Sintering of polymeric powders is a peculiar characteristic of many processing technologies, including rotational moulding
and selective laser sintering (SLS). During polymer sintering, viscosity reduction in the melt state promotes densification
of polymer powders, through a double stage mechanism, involving powder coalescence and bubble removal. In particular, sintering
of semi-crystalline polymers is strongly influenced by the melting behaviour. Nevertheless, melting itself in absence of pressure
is not necessarily accompanied by powder sintering, unless low viscosities are achieved. In this work, the melting and sintering
behaviour of recycled high density polyethylene (rHDPE) have been analysed through differential scanning calorimetry (DSC)
and Thermomechanical Analysis (TMA). Efficient models capable of describing the melting temperature distribution and rate
of sintering of rHDPE powders have been developed, highlighting the inherent differences between the two distinct processes.
The melting and crystallization of a sharply melting standard has been explored for the calibration of temperature-modulated
differential scanning calorimetry, TMDSC. Modulated temperature and heat flow have been followed during melting and crystallization
of indium. It is observed that indium does not supercool as long as crystal nuclei remain in the sample when analyzing quasi-isothermally
with a small modulation amplitude. For standard differential scanning calorimetry, DSC, the melting and crystallization temperatures
of indium are sufficiently different not to permit its use for calibration on cooling, unless special analysis modes are applied.
For TMDSC with an underlying heating rate of 0.2 K min−1 and a modulation amplitude of 0.5–1.5 K at periods of 30–90 s, the extrapolated onsets of melting and freezing were within
0.1 K of the known melting temperature of indium. Further work is needed to separate the effects originating from loss of
steady state between sample and sensor on the one hand and from supercooling on the other.
Fish oil which is characterised by important amounts of poly-unsaturated ω-3 fatty acids attach increasing importance within
functional foods. Recently attention is directed on physical methods that allow fast and relatively easy the identification
and discrimination of oils. DSC measurements yield in information on thermal effects, characterised by changes in enthalpy
and their temperature range such as melting and crystallisation. The aim of the investigation presented here was to take DSC
curves in the temperature range +20 to −40°C on several fish oils and fish oil capsules to visualise the crystallisation and
melting behaviour and to compare transition temperatures and enthalpies.
GV150526A is a novel 2-carboxyindole derivative, recently synthesized by GlaxoWellcome, which is used in treatment or prevention
of CNS disorders resulting from neurotoxic damage. It has been prepared in three forms, F1, F2 and F3, having significantly
different hydration/dehydration behavior and/or diffraction patterns. Here, we extend the thermal analysis of these polymorphs
above 200C, where all forms are fully dehydrated and the main thermal phenomena are decomposition and melting. Simultaneous
TG/DSC measurements have been repeated in wet and dry nitrogen atmospheres over a wide range of heating rates. Form F3 displays
a qualitatively different behavior relative to F1 and F2. This fact is interpreted as an evidence of a mechanism of decomposition
which sets F3 apart from F1 andF2. The thermal data are summarized by simple heuristic equations and few ‘apparent’enthalpies.
Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE,
HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale,
to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer
is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the
same value as during the cooling section. This is not the case for LDPE and LLDPE.
Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during
the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle
is the same.
The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron
radiation during a heating cycle at temperatures below the melting peak.
A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of
interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication
of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect
to that of folded chain lamellae.
This analysis of interface phenomena considers the alternative processes that may result from heating a crystal, particularly including thermal decomposition, involving chemical reactions, and melting, involving loss of long-range structural order. Such comparisons are expected to provide insights into the factors that determine and control the different types of thermal changes of solids. The survey also critically reviews some theoretical concepts that are currently used to describe solid-state thermal reactions and which provides relevant background information to models used in a recently proposed theory of melting. Probable reasons for the current lack of progress in characterizing the factors that control chemical changes and mechanisms of thermal reactions in solids are also discussed.
It is concluded that some aspects of the macro properties of reaction interfaces in crystal reactions have been adequately described, including geometric representations of interface advance during nucleation and growth processes. In contrast, relatively very little is known about the detailed (micro) processes occurring within these active, advancing interfacial zones: reactant/product contacts during chemical reactions and crystal/melt contacts during fusion. From the patterns of behaviour distinguished, a correlation scheme, based on relative stabilities of crystal structures and components therein, is proposed, which accounts for the four principal types of thermal changes that occur on heating solids: sublimation, decomposition, crystallographic transformation or melting. Identifications of the reasons for these different consequences of heating are expected to contribute towards increasing our understanding of each of the individual processes mentioned and to advance theory of the thermal chemistry of solids, currently enjoying a prolonged quiescent phase.
The non-isothermal crystallization and melting of ultra high molecular weight polyethylene (UHMWPE) were observed by means
of differential scanning calorimetry and compared with those of ordinary high-density polyethylene (HDPE). The crystallization
temperature (Tc) and melting point (Tm) of UHMWPE were found to be higher thanTc andTm of HDPE, and the latent heat of crystallization (δHc) and fusion (δHm) of UHMWPE are smaller thanδHc andδHm of HDPE. The results were explained in terms of the theory of polymer crystallization and the structure characteristics of
UHMWPE. The relationships between the parameters (Tc,TT,δHc andδHm) and the molecular weight (M) of UHMWPE are discussed. Processing of the experimental data led to the establishment of four expressions describing the
Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades
of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally
in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most
effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation
up to 140C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The
degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting
peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.