Authors:R. Zelkó, Á. Orbán, J. Nagy, G. Csóka, and I. Rácz
In the course of the formulation of coated dosage forms, the selection of the suitable composition of the coating system is
of great importance in respect of the final dosage form. Since the applied coating systems are multicomponent, the possible
interactions between the components determine the physico-chemical stability of the formulated dosage form, the drug release
process, as well as the formulation parameters. In the present study, the influence of the applied plasticizer, dibutyl sebacate
on the enthalpy relaxation of casted Eudragit L 30D films was determined as a function of the plasticizer concentration. The
enthalpy relaxation was recorded by DSC during the applied isothermal recovery process of Eudragit films. The obtained results
indicate that enthalpy relaxation can be measured by DSC at 20 mass/mass% dibutyl sebacate concentration, which refers to
the increased molecular mobility consequently to the effect of the interaction between the polymer and plasticizer.
The volume and enthalpy relaxation rate of inorganic glasses and organic polymeric materials subjected to temperature jump
T has been analyzed. It is shown that the relaxation behavior in isothermal conditions can be compared on the basis of the fictive relaxation rate defined as Rf=(dTf/dlogt)i. No significant difference between volume and enthalpy relaxation rate has been found for all materials examined. A simple equation relating the Rf and parameters of Tool-Naraynaswamy-Moynihan (TNM) phenomenological model has been derived. This equation predicts increasing Rf with the magnitude of temperature jump. It seems that correct determination of TNM parameters might be problematic for slowly relaxing polymers as the effect of these parameters becomes comparable with experimental uncertainty.
the volume and enthalpy relaxation in a-PMMA subjected to temperature jumps in tg region has been analysed. The measured H and V data were compared with respect to aging time and proportionality between them as a slope of (∂H/∂V)T dependencies has been found. According to previous works the slope was identified as an apparent bulk modulus, Ka. This method is applied to aging following temperature up-jumps after consolidation periods of varying lengths. the main
finding is a marked increase of Ka with consolidation time, approaching a limiting value in an asymptotic fashion.
Summary Volume and enthalpy relaxation in polycarbonate subjected to double temperature jumps in the Tg region has been analysed. It concerns both initial Tdown-jump from equilibrium above Tg to consolidation temperature below Tg and fina1 Tup-jump to relaxation temperature, also below Tg. The measured H and V data after Tup-jump were compared with respect to aging time calculating (dH/dV) ratio denoted as aging bulk modulus, Ka. According this new methodology H and V relaxation response after Tup-jump demonstrates differences in relaxation responses.
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.
A new method of calculation of parameters of enthalpy relaxation models is proposed. Regression analysis treatment compares the experimental and calculated values of relaxation enthalpy. The experimental values of relaxation enthalpy are obtained by numerical integration of the difference between the two DSC curves. Contrary to the overall shape of the DSC curve the integral values are not affected by particular heat flow conditions during the DSC experiment. The Narayanaswamy's numerical model based on the Kohlrausch—William—Watts relaxation function was used to calculate the theoretical values of relaxation enthalpy. The application of the proposed method on the DSC experimental data of enthalpy relaxation of As2Se3 is shown.
We employ temperature
modulated DSC (TMDSC) to determine the dependence of the fictive temperature
on cooling rate for a series of polychlorinated biphenyls (PCB). From the
slopes of semi-logarithmic plots of cooling rate vs.
fictive temperature, the latter normalized by the fictive temperature for
an arbitrary cooling rate, we determine the enthalpic fragilities. Despite
significant differences in glass transition temperature and chemical structure
(specifically chlorine content), the PCB have the same fragility. The value
of the fragility determined using TMDSC is consistent with the fragility previously
determined using dielectric relaxation spectroscopy.
Structural relaxation for simple and more complex thermal histories is described by a phenomenological model based on a non-exponential
relaxation function, the reduced-time concept and the nonlinear structural contribution to the relaxation time. The history,
development of experimental techniques and data analysis is described. It is shown that the volume and enthalpy relaxation
response can conveniently be compared on the basis of a fictive relaxation rate, Rf. A simple equation relating Rf and the parameters of the phenomenological model is given. The calculated data for moderate departures from equilibrium are
in good agreement with our experiments and data previously reported in the literature.