Secondary crystallization of PEA, occurring during DSC scan and performed after isothermal primary crystallization has been
investigated. The high temperature exotherm peak has been attributed to the crystallization of amorphous fraction included
between crystallites formed at the primary crystallization. The temperature position of the highest rate of the secondary
crystallization depended on the temperature of the isothermal primary crystallization.
The crystallinity of poly(ethyleneterephthalate) has been determined by differential scanning calorimetry and by density.
The results obtained by calorimetry show that the increment in the crystallinity due to the heatsetting treatment is produced
by the increase of the crystallinity corresponding to the premelting endothermic peak.
We report the results of an investigation by differential scanning calorimetry (DSC) of two mobility controlled processes
in the amorphous phas e of semicrystalline PEEK — enthalpy relaxation below the glass transition (Tg) and secondary crystallization aboveTg. Both result in the observation of an endothermic peak just above the annealing temperature in the DSC scan of the polymer
— the enthalpy recovery peak and the low temperature melting peak, respectively. There is a striking similarity in the time
and temperature dependence of the endothermic peak for these two processes. These results are reminiscent of those obtained
from small strain creep studies of “physical aging” of semicrystalline PEEK below and aboveTg.
calorimetry (DSC) technique. Unusual fourfold melting endotherms were observed. A new and comprehensive explanation on the origin of the complex multiple melting behavior particularly the origin of the secondarycrystallization, which is different from
lower temperature. So that secondarycrystallization appears obvious at cooling rate 40 K min −1 and little at cooling rate 2.5 K min −1 , which suggests that the secondarycrystallization of P4MP1 was the re-arrangement of loose packed chains during
Authors:J. Suñol, J. Saurina, R. Berlanga, D. Herreros, P. Pagès, and F. Carrasco
A crystallization kinetics analysis of several polypropylene-polyethylene (PP-PE), PP-rich copolymers was made by means of
differential scanning calorimetry. The crystallization was studied via calorimetric measurements at different cooling rates.
Several additives were added to the base material. Some test samples were subjected to artificial ageing processes. A modified
isoconversional method was used to describe the crystallization process under non-isothermal conditions. The value of the
Avrami parameter was determined for primary and secondary crystallization.
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.
Authors:C. Schick, M. Merzlyakov, A. Minakov, and A. Wurm
Quasi-isothermal temperature modulated DSC (TMDSC) were performed during crystallization to determine heat capacity as function of time and frequency. Non-reversible and reversible phenomena in the crystallization region of polymers were distinguished. TMDSC yields new information about the dynamics of local processes at the surface of polymer crystals, like reversible melting. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for polycaprolactone (PCL). This shows that also after long crystallization times the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are living crystals. A strong frequency dependence of complex heat capacity can be observed during and after crystallization of polymers.
melting and crystallisation behaviour of poly(m-xylene
adipamide) (MXD6) are investigated by using the conventional DSC, X-ray diffraction
and polarised light microscopy. Triple, double or single melting endotherms
are obtained in subsequent heating scan for the samples after isothermal crystallisation
from the melt state at different temperatures. The lowest melting peak can
be ascribed to the melting of secondary crystals. The melting of primary crystals
causes the medium melting peak and the highest melting peak is attributed
to the melting of recrystallised species formed during heating. Following
the Hoffman–Weeks theory, the equilibrium melting temperature is equal
to 250C and the equilibrium melting enthalpy ΔHm0
to 175 J g–1. Then, using the Lauritzen–Hoffmann
theory of secondary crystallisation, the analyse of the spherulitic growth
shows that the temperature of transition between the growing regimes II and
III is equal to 176C. Finally the Gibbs-Thomson relationship allows the
determination of the distribution function of crystalline lamellae.