Chiyoko Tomita

Melting of polymer crystals observed by temperature modulated d.s.c. and its kinetic modelling

Abstract Irreversible melting of poly(ethylene terephthalate) crystals on heating has been examined by temperature modulated differential scanning calorimetry (t.m.d.s.c.). The apparent heat capacity of complex quantity obtained by t.m.d.s.c. showed a strong dependence on frequency and heating rate during the melting process. In order to explain this behavior, a kinetic modelling of melting has been presented. The modelling considers the melting of an assembly of fractions having a continuous distribution of non-equilibrium melting points. Three cases of the superheating dependence of melting rate coefficient have been examined: constant rate coefficient, linear dependence and exponential dependence. The modelling predicts frequency response functions similar to Debyes type with a characteristic time dependent on heating rate. The response function successfully explains the dependence on frequency and heating rate of the apparent heat capacity obtained experimentally. The characteristic time of melting of crystallites has been evaluated as a fitting parameter of the response function, and the superheating dependence of melting rate coefficient has been distinguished by the heating rate dependence of the characteristic time. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is further suggested that the ‘reversing’ heat flow is related to the pure endothermic heat flow of melting and the ‘non-reversing’ heat flow corresponds to the exothermic heat flow of re-crystallization and reorganization when extrapolated to ω → 0. The behavior of the apparent heat capacity will be an important characteristic feature of the melting kinetics, and hence the modelling will develop a new applicability of t.m.d.s.c. to the melting of polymer crystals.

An application of temperature modulated differential scanning calorimetry to the exothermic process of poly(ethylene terephthalate) crystallization

Abstract We have examined the applicability of a new analysing method of temperature modulated differential scanning calorimetry to the exothermic process of poly(ethylene terephthalate) crystallization. The method utilizes the change in the phase lag between modulation components of sample temperature and of heat flow, to introduce an apparent heat capacity of complex quantity. The phase lag showed a peak and a dip during the isothermal crystallization, above and below the temperature at which the growth rate of crystals becomes a maximum, respectively. The present method incorporates the change, and predicts negative and positive temperature dependence of crystal growth rate, for the peak and dip in the phase lag, respectively. The temperature dependence of crystal growth rate agreed well with the literature values obtained from the direct measurements of growth rate of spherulites by optical microscopy.

Thermo-mechanical coupling and self-excited oscillation in the neck propagation of PET films

Abstract The self-excited oscillation of neck propagation during cold drawing of polymer films has been examined experimentally. On the basis of Barenblatts model considering a thermo-mechanical coupling at the neck, the temperature rise at the neck has been studied with an infrared camera. The temperature began to rise in a range showing a negative velocity dependence of the applied load. The behavior is consistent with the view of thermo-mechanical coupling. The temperature rise was up to 80°C (>Tg) and explains the occurrence of crystallization for faster drawing rates. It has also been confirmed that the temperature rise follows the oscillation of stress due to the coupling.

Computer simulation of the melting kinetics of polymer crystals under condition of modulated temperature

Computer simulation has been applied to the modeling of the melting kinetics of polymer crystals, which we have recently presented to predict the response of the kinetics to a sinusoidal modulation in temperature on heating. The frequency and heating-rate dependencies have been examined with a Gaussian or uniform distribution of the melting points. For both of the distributions, the details of the dependence have been examined on the basis of the analytical results of the modeling. It has also been confirmed that the response of the kinetics has higher harmonics as expected from the formulation of the modeling. This behavior corresponds to the experimental results of temperature-modulated DSC (T-MDSC) in the melting region of polymer crystals.

Temperature modulated d.s.c. study of poly(ethylene terephthalate) crystallization : 2. Applicability to non-isothermal process

The non-isothermal crystallization of poly(ethylene terephthalate) has been examined by temperature modulated differential scanning calorimetry (TMd.s.c.). A new analytical model of TMd.s.c. has been applied to the process, taking account of the response of exothermic heat flow to temperature modulation in an apparent heat capacity of complex quantity. By examining the frequency dependence of the apparent heat capacity, the applicability has been successfully examined for the non-isothermal process. The method is capable of determining the temperature dependence of crystal growth rate from TMd.s.c. data analysis. The results agree well with the dependence determined from literature values of spherulite growth rate measured by optical microscopy.

A calibration of complex heat capacity obtained by temperature-modulated DSC in the melting region of polymer crystals ☆

Abstract Complex heat capacity obtained in melting region of polymer crystals by temperature-modulated differential scanning calorimetry of heat flux type has been calibrated with a method based on a model proposed by Hatta. The calibration method corrects for the effect of thermal conductivity of the DSC apparatus on the magnitude and phase angle of the heat capacity. The validity of the correction has been confirmed by examining the reversible melting and crystallization of indium under quasi-isothermal conditions. For the irreversible melting of polymer crystals analyzed with an additional underlying heating rate, the calibrated heat capacity becomes a complex quantity with a frequency dependence roughly approximated by Debyes type, the characteristic time of which depends on the underlying heating rate. This behavior qualitatively agrees with the previous results obtained by the calibration of baseline-subtraction from the phase angle. The applicability of the “baseline-subtraction” has also been discussed.

Periodically modulated driving force applied with tmdsc to the crystallization and melting kinetics of ice crystals confined in a porous silica gel

The application of a periodically modulated driving force has been examined in the melting and crystallization kinetics of ice crystals confined in a porous media. The kinetic response of transformation gives the real and imaginary parts of the ‘apparent’ heat capacity obtained with a temperature modulated differential scanning calorimetry (TMDSC). Based on a modelling of the kinetics, the detailed examination of the frequency dispersion and its dependence on underlying heating/cooling rate enables us to evaluate the transformation rate and the dependence of the rate coefficient on the driving force, i.e. the degree of supercooling or superheating. The experimental results indicate that the transformation processes are limited by heat diffusion from the growth interface of each crystallite to surroundings.