Search Results
Abstract
The dynamic heat capacity of polyethylene was measured in the heating process over two decades of the modulating frequency using the light heating modulated temperature DSC. The dynamic heat capacity exhibited clear frequency dependence from 95°C to the end of the melting of the crystals. Frequency dependence of this work was compared with that of the quasi-isothermal measurement. The relaxation time estimated in this work was much shorter than that of the quasi-isothermal measurement. It was found that notable heat exchange between the sample and reference sides occurred between 120 and 135°C. Frequency dependence of the heat exchange was studied.
Abstract
Light heating dynamic DSC was used to study the melting transition of polyethylene. The results show that melting and crystallization are different phenomena from each other in terms of the complex heat capacity. Frequency dependence of the complex heat capacity was examined from 0.01 Hz to 0.2 Hz. It is found that at the lowest frequency the phase of the complex heat capacity exceeds π/2 radians. Thermodynamic considerations were made for the large phase of the complex heat capacity.
Modulated Differential Scanning Calorimetry
The effect of experimental variables
Abstract
The reproducibility and reliability of the TA Instruments Modulated Differential Scanning Calorimeter (MDSC) was tested over a range of conditions. The equipment base line was found to be fairly constant with a very small fluctuation (10 μW), which means a 0.1 % fluctuation on the scale of a normal polymer MDSC curve. The excellent stability of the base line and the reasonable reproducibility of the curves (5%) suggest that frequent calibration is not required. The heat capacities calculated from the modulated response to the variable temperature depend on the frequency for a given cell constant. The heat capacity cell constant is a unique function of the modulation frequency:k c =K c o p/(p−6.3) wherep is the time of the periodicity expressed in seconds and Kc o is the heat capacity cell constant measured on a standard material and reduced to zero frequency. The cell constants depend on the flow rate of the helium according to:K(He)=K o(1.298−0.004424He+1.438·10−5 He 2) whereHe is the flow rate of helium in ml min−1 andK o represents a constant at 100 cm3 min−1. There is a strong dependence of cell constant on the flow rate ranges from 10 to 80 cm3 min−1, while above this rate (up to 135 ml min−1) the cell constant approaches a plateau.
Abstract
The processes of vitrification and devitrification that occur in an epoxy resin when it cures non-isothermally with a hardener are studied in terms of their frequency dependence and as a function of the heating rate. A novel modulated DSC technique, TOPEM, has been used which permits the evaluation of the frequency dependence for a single sample in a scan at constant underlying heating rate, thus avoiding errors arising from the composition of the sample. The effects of both frequency and heating rate on vitrification and devitrification are investigated. Some advantages of this technique are observed and discussed.
Abstract
We measure the frequency dependences of complex heat flows for isothermally crystallized isotactic polypropylene (iPP) by the quasi-isothermal TMDSC. Regarding the quasi-isothermal melting processes as a kind of the single relaxation process, we analyze them by the Debye model. The resultant heat capacity of iPP is larger (about 11%) than usual thermodynamic heat capacity. We also found that the excess of the heat capacity, C p (excess), has non-monotonous temperature dependence. A simple model introducing some kinetic modes into amorphous producing after and during temperature modulation can reproduce the temperature dependence of C p (excess) very well.
Abstract
The frequency dependences of the complex-specific heat of the sodium borate glasses, xNa2O·(100 − x)B2O3, where x denotes molar concentration of Na2O, have been investigated by temperature-modulated DSC. The temperature dependences of α-relaxation time have been analyzed in Angell plot, and the fragility index has been determined. The composition dependence of the fragility index has been discussed on the basis of the variations of the structural units of the borate network. The origin of the fragility of the borate system relates to the distribution of the coordination number of boron atom.
Can one measure precise heat capacities with DSC or TMDSC?
A study of the baseline and heat-flow rate correction
Summary
During a prior study of gel-spun fibers of ultrahigh-molar-mass polyethylene, a substantial error was observed on calculating the heat capacity with a deformed pan, caused by the lateral expansion of the fibers on shrinking during fusion. In this paper, the causes of this and other effects that limit the precision of heat capacity measurements by DSC and TMDSC are explored. It is shown that the major cause of error in the DSC is not a change in thermal resistance due to the limited contact of the fibers with the pan or the deformed pan with the platform, but a change in the baseline. In TMDSC, the frequency-dependence is changed. Since irreversible changes in the baseline can occur also for other reasons, inspections of the pan after the measurement are necessary for precision measurements.
Vitrification during the isothermal cure of thermosets
Part I. An investigation using TOPEM, a new temperature modulated technique
Abstract
The process of vitrification that occurs during the isothermal cure of a cross-linking system at temperatures below T g∞, the glass transition temperature of the fully cured resin, has been studied by TOPEM, a new temperature modulated DSC (TMDSC) technique based upon the use of stochastic temperature pulses. A comparison is made between TOPEM and another TMDSC technique, and some advantages of TOPEM are considered. The TOPEM technique is used to show that the mobility factor is not always a reliable approach to predicting the cure rate during vitrification, in view of its frequency dependence. Also, the dependence of the apparent vitrification time on frequency is examined. There appears to be a non-linear relationship between the apparent vitrification time and log(frequency), which is further discussed in the second part of this series.
Abstract
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.
