Authors:P. Šimon, P. Thomas, J. Okuliar, and A. Ray
An incremental integral isoconversional method for the determination of activation energy as a function of the extent of conversion
is presented. The method is based on the treatment of experimental data without their transformation so that the resulting
values of activation parameters should not be biased. The method was tested for recovering the activation energies from simulated
data and employed for the treatment of experimental data of the NiS recrystallisation.
Authors:Eszter Vozáry, Kinga Ignácz, and Bíborka Gillay
structure of honey can change, the mobility of ions and charged groups can increase causing a decrease of electrical impedance of honey. The recrystallization of honey, the recovery to the original state presumably takes a relatively long time – some weeks
Authors:Z. Malek, V. Balek, D. Garfinkel-Shweky, and S. Yariv
The dehydration and dehydroxylation of the smectites, beidellite and montmorillonite (dioctahedral) and saponite and laponite (trioctahedral), were investigated by ETA, DTA, TG and X-ray diffractions. There are differences between the behaviors of di- and trioctahedral clays which are due to the differences in the ability of the exchangeable cations to migrate into hexagonal holes of the SiO4 network and due to the fact that the dehydroxylation and recrystallization of dehydroxylated trioctahedral smectites occur simultaneously, whereas there is a temperature gap of about 300°C between the dehydroxylation and the recrystallization of dioctahedral smectites.
Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC)
measurements. As in TMDSC TMDMA allows the investigation of reversible and non-reversible phenomena during crystallisation
of polymers. The advantage of TMDMA compared to TMDSC is the high sensitivity for small and slow changes in crystallinity,
e.g. during re-crystallisation. The combination of TMDMA and TMDSC yields new information about local processes at the surface
of polymer crystallites. It is shown that during and after isothermal crystallisation the surface of the individual crystallites
is in equilibrium with the surrounding melt.
Summary The thermal behavior of poly(L-lactic acid) (PLLA) was studied with differential scanning calorimetry (DSC) and polarized optical microscopy. For amorphous PLLA samples, double cold crystallisation peaks were observed in the DSC traces during heating process, being strongly dependent on heating rates. The observation was discussed based on the assumption that the quenched PLLA sample presented some remaining metastable or a precrystalline phase. A small exothermal peak was observed before the main melting peak at low heating rates. The probable reason was discussed through melt-recrystallisation mechanism. Influence of thermal history on the cold crystallisation and melting behavior was also performed on heating process for PLLA samples.
Authors:Elisana Moura, Lidiane Correia, Márcia Pinto, José Procópio, Fábio de Souza, and Rui Macedo
This article had studied the thermal characterization of the raw material and different fluconazole crystals, obtained through
recrystallization with different solvents using thermoanalytical techniques (TG, DTA, DSC-50, DSC Photovisual, DSC-60) and
Pyr-GC/MS. The results confirmed that the fluconazole volatilizes without decomposition until 250 °C. Pyr-GC/MS showed hexachlorobenzene
like impurities in fluconazole raw material.
The crystallisation kinetics, melting behaviour and
morphology, of bacterial poly(3-hydoxybutyrate) (PHB) have been investigated
by using differential scanning calorimetry (DSC), step-scan DSC (SDSC), wide
angle X-ray diffraction (WAXRD) and hot stage polarised optical microscopy
(HSPOM). DSC imparted isothermal crystallisation thermal history. The subsequent
melting behaviour revealed that all PHB materials experienced secondary crystallisation
during heating and the extent of secondary crystallisation varied depending
on the crystallisation temperature. PHB samples were found to exhibit double
melting behaviour due to melting of SDSC scan-induced secondary crystals,
while considerable secondary crystallisation or annealing took place under
the modulated heating conditions. The overall melting behaviour was rationalised
in terms of recrystallisation and/or annealing of crystals. Interestingly,
the PHB materials analysed by SDSC showed a broad exotherm before the melting
peak in the non-reversing curve and a multiple melting peak reversing curve,
verifying that the melting-recrystallisation and remelting process was operative.
HSOM studies supported the conclusions from DSC that the radial growth rate
of the PHB spherulites was significantly varied upon the crystallisation conditions.
One form of crystals was shown by WAXRD from isothermally crystallised PHB.
The nucleating efficiency and selectivity of different
β-nucleating agents was characterised and compared by differential scanning
calorimetry, (DSC) and temperature-modulated DSC (TMDSC). The nucleating agents
were the calcium salts of pimelic and suberic acid (Ca-pim and Ca-sub), linear trans-γ-quinacridone (LTQ), a commercial nucleator
NJ Star (NJS) and an experimental product (CGX-220). The efficiency and the
selectivity of Ca-sub and Ca-pim are extremely high. NJS is efficient above
a critical concentration, which is connected with its partial dissolution
in polypropylene melt. LTQ and CGX-220 possess strong overall nucleating ability
and moderate selectivity. Using TMDSC, we found that three consecutive processes
take place during the heating of β-nucleated samples cooled down to room
temperature: reversible partial melting of the β-form, irreversible βα-recrystallisation,
and the melting of the α-modification formed during βα-recrystallisation
or being present in samples prepared with non-selective β-nucleators.
Melting of the α-phase contains both reversible and irreversible components.
Authors:S. H. Murphy, G. A. Leeke, and M. J. Jenkins
The infrared spectrum of polycaprolactone has been recorded as a function of temperature in the range where melting and crystallisation of the polymer can occur. Examination of the carbonyl band of the spectra reveals a clear morphological sensitivity; heating the semi-crystalline polymer through the melting region results in a decrease in the intensity of the crystalline component of the carbonyl band. Accordingly, there was a subsequent increase in intensity of the crystalline carbonyl band on cooling. To enable comparison of these findings with a more conventional method of thermal analysis, similar experiments were conducted using a differential scanning calorimeter. The heated ATR accessory adopted for use in the FTIR spectrometer imposed significant limitations in the range of possible heating and cooling rates, but when these rates were carefully matched between FTIR and DSC, close correlation between the melting point and onset of re-crystallisation was observed. The results confirm that FTIR can be used as an alternative, if more laborious, way of investigating melting and re-crystallisation.
Authors:Shmuel Yariv, Isaak Lapides, and Mikhail Borisover
peaks do not appear in the diffractograms. Trace amounts of spinel and cristobalite are obtained from thermal recrystallization. Quartz is present as an impurity in the Wyoming bentonite and after the amorphization of MONT its X-ray peak becomes very