increase with the increase in cooling rate. In Fig. 1b , there exist exothermic peaks of re-crystallization in the DSC traces of P4MP1 after 2.5, 5, 10, 20 K min −1 . At a same heating rate of 10 K min −1 , the re-crystallization makes the X c almost
The DSC characterisation of the morphology of the metastable a phase of stoichiometric nickel sulphide was carried out using
two calorimeters; a TA Instruments 2920 MDSC and a Perkin Elmer DSC-7, and two quenching histories. Based on these quenching
histories, significant differences were observed in the heat flow curves, including the observation of a second exothermic
peak which is tentatively assigned to be a metastable phase to metastable phase transformation. The kinetic constants for
the a to b recrystallisation were determined as a function of degree of conversion using a mechanism free isoconversional
model. Variations in the values of the kinetic constants were also ascribed to the quenching histories. Although the differences
in morphology observed were ascribed to the processing history, the shift in the position of the a to b recrystallisation
peak was partially attributed to the thermal resistances of the instruments used.
Thermal properties of food systems are important in understanding relationships between food properties and changes in food quality. Concentrated food systems (low-moisture and frozen foods) are seldom in an equilibrium state and they tend to form amorphous, non-crystalline structures. Several glass transition-related changes in such foods affect stability, e.g., stickiness and caking of powders, crispness of snack foods and breakfast cereals, crystallisation of amorphous sugars, recrystallisation of gelatinised starch, ice formation and recrystallisation in frozen foods and rates of non-enzymatic browning and enzymatic reactions. Relationships between glass transition, water plasticisation and relaxation times can be shown in state diagrams. State diagrams are useful as stability or quality maps and in the control of rates of changes in food processing and storage.
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: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.