Time‐dependent heat capacity in the glass transition region

DSC analysis was performed on uniaxially oriented and unoriented atactic polystyrene samples prior to and after annealing at 80 °C. With increasing annealing times, an endothermic peak appeared, whose area increased with the duration of the annealing period. No difference was found between the endotherm areas for the oriented and the unoriented polymer. The DSC curve of the unannealed oriented polystyrene exhibited a relaxation exotherm. DSC and relaxation studies indicated that this relaxation exotherm was independent of the main chain orientation; it may rather be due to sample densification

Section: John Wiley and Sons Limited Chichestermentioning

confidence: 95%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

On the sub-Tg annealing of uniaxially oriented and unoriented atactic polystyrene

Carfagna

Nicodemo

Nicolais

1986

“…Amplitude calibrations were based on heats of fusion, and the areas of the recording evaluated by planimetry. The use of a modified DTA cell in conjunction with the DuPont 900 thermal analyzer for heating rates up to 400 K/rain by installation of a more powerful heater was described earlier [8,9].…”

Section: Sample and Instrumentationmentioning

confidence: 99%

Fast DSC applied to the crystallization of polypropylene

Dann

Cheng

et al. 1988

In an effort to find the limit of crystallization of polypropylene, a series of quantitative and semiquantitative DSC experiments at rates up to 10,000 deg/min are described. Even at these fast rates polypropylene crystallized on cooling between 350 + 15 K and 280 :t: 6 K. No "fully amorphous" polypropylene was produced. No initial stage crystallization to the condis state could be proven by quenching after partial crystallization.The present status of scanning calorimetry permits measurements at heating rates between one and 100 K/rain without undue temperature gradient within the sample [1]. Analysis of metastable substances and the study of superheating need, however, faster thermal measurements [2]. It would also be extremely helpful if fast industrial processes such as injection molding, fiber spinning, film blowing and the various conditioning cycles could be duplicated in the scanning calorimeter. Up to the present there have been only the efforts of Hager to develop a fast foil calorimeter [3] and some experiments by us [4] to reach up to and beyond 1000 K/min. In this paper we want to discuss the problem of fast calorimetry, describe several methods for fast measurement and quenching, and apply these to the problem of polypropylene crystallization. The problem of fast calorimetryCalorimetric data can be obtained by a simple temperature measurement only as long as the temperature gradient within the sample is experimentally negligible, i.e. is less than perhaps + 0.5 K. With larger temperature gradients, measurements will only yield thermal diffusivity information. The time-dependent heat flux dQ/dt into a disc of thickness L under steady-state conditions, from the underside is given, for example, by

“…Further, Boyer [2] mentions in his review the influence of the heating rate on the value of the glass transition temperature. The dependence of T~, on the heating rate has also been investigated by Wolpert et al [3] with DTA measurement, and by Prud'homme et ah [4] when measuring heat capacity with DSC. Nielsen [5] states that the glass transition is generally measured in experiments which correspond to a time scale of seconds or minutes.…”

mentioning

confidence: 96%

Glass transition temperatures of polymer materials, measured by thermomechanical analysis

Schwartz

1978

The linear expansions of two materials have been measured, a double-base propellant and a carboxyl-terminated polybutadiene. The glass transition temperature, T~ and expansion coefficients below and above T, have been calculated. The influence of the heating and cooling rates and sample thickness has been investigated. The resull~s show that the value of Tg is dependent on the rates of heating and cooling but not on the sample thickness. Extrapolating to zero rate gives the same Tu for both heating and cooling. The expansion coefficients are not influenced by the rates of heating and cooling or by the sample thickness.In measurements of the glass transition temperatures T~ of polymers it has been found that the value of T~, is dependent on the rate of heating. Strella [1 ] has made an investigation of Tg for two polymers by DTA. He found an increase of Te with increasing heating rate. Further, Boyer [2] mentions in his review the influence of the heating rate on the value of the glass transition temperature. The dependence of T~, on the heating rate has also been investigated by Wolpert et al. [3] with DTA measurement, and by Prud'homme et ah [4] when measuring heat capacity with DSC. Nielsen [5] states that the glass transition is generally measured in experiments which correspond to a time scale of seconds or minutes. If the time scale in the experiments is shortened, the apparent T~ is raised, and if the time scale is lengthened to hours or days, Tx is lowered. Shen and Eisenberg [6] have come to the same conclusion in their review.The reason for this investigation was that in measurements of the expansion coefficients and Tg of polymer materials the value of Tg varied with the rate. In order to have a reliable value of Te it was necessary to establish the influence of different rates of heating and cooling with this particular apparatus. At the same time it was of great interest to know the influence of the sample thickness on the results. Experimental MaterialsThe investigation was made on two materials. One is a double-base propellant, that is the propellant is mainly made of nitrocellulose and nitroglycerine. The nitroglycerine functions as a plasticizer for the polymer nitrocellulose.