Search Results
Abstract
40% w/w sucrose/water solutions were analyzed by Modulated Differential Scanning Calorimetry [1] in the sub-ambient temperature region. At these temperatures, the solutions exhibit a complex, two-step thermal event. The lower-temperature event is believed to be the glass transition of the amorphous sucrose phase. The nature of the higher-temperature event is the subject of controversy. This event has been shown to have distinct second-order characteristics, and as such is believed to be a second T g. Others feel that this event is the onset of melting. The temperature region between these events contains a devitrification exotherm. Through the use of MDSC, both in scanning and stepwise quasi-isothermal modes, improved sensitivity and resolution of MDSC provides new insight into the nature of these transitions.
Summary
Modulated differential scanning calorimetry (MDSC) and dielectric analysis (DEA) have been used to characterize the cure process of the system diglycidyl ether of bisphenol A (DGEBA(n=0)/1,2 diaminocyclohexane (1,2 DCH). The trans isomer and a mixture cis/trans(30-70% respectively) of 1,2 DCH were used to find their different behaviour. The study allowed to check the influence of the cisisomer on the thermoset curing process. Gelation times were obtained through the equation proposed by Johari and vitrification times from the point of inflection of the complex calorific capacity modulus.
The application of modulated differential scanning calorimetry to the glass transition
Theoretical analysis using a single parameter model
The modulated differential scanning calorimetry (MDSC) technique superimposes upon the conventional DSC heating rate a sinusoidally varying modulation. The result of this modulation of the heating rate is a periodically varying heat flow, which can be analysed in various ways. In particular, MDSC yields two components (‘reversing’ and ‘non reversing’) of the heat flow, and a phase angle. These each show a characteristic behaviour in the glass transition region, but their interpretation has hitherto been unclear. The present work clarifies this situation by a theoretical analysis of the technique of MDSC, which introduces a kinetic response of the glass in the transition region. This analysis is able to describe all the usual features observed by MDSC in the glass transition region. In addition, the model is also able to predict the effects of the modulation variables, and some of these are discussed briefly.
Measurements of Conductive Epoxy Adhesives by MDSC . In: Thermal Library Application Brief TA312 , TA Instruments New Castle , Delaware . [14]. D. Schrader
scanning calorimetry (MDSC) is one of the easier and more accurate methods for determining heat capacity, and this method has been greatly developed for directly determining heat capacities for various materials isothermally and non-isothermally [ 8 – 13
Abstract
Reading and co-workers introduced a new technique a few years ago called Modulated Differential Scanning Calorimetry or MDSC. Here the first part of a theoretical analysis for this technique is given. A simple mathematical model for modulated differential scanning calorimetry in the form of an ordinary differential equation is derived. The model is analysed to find the effect of a kinetic event in the form of a chemical reaction. Some possible sources of error are discussed. A more sophisticated version of the model allowing for spatial variation in a calorimeter is developed and it is seen how it can be reduced to the earlier model. Some preliminary work on a phase change is also presented.
Abstract
The calorimetric glass transition and dielectric dynamics of 
-relaxation in propylene glycol (PG) and its five oligomers (polypropylene glycol, PPG) have been investigated by the modulated differential scanning calorimetry (MDSC) and the broadband dielectric spectroscopy. From the temperature dependence of heat capacity of PPGs, it is clarified that the glass transition temperature (T
g) and the glass transition region are affected by the heating rate. The kinetic changes of PG and PPGs near T
g strongly depend on the underlying heating rate. With increasing the molecular mass of PPGs, the fragility derived from the relaxation time against temperature also increases. The PG monomer is stronger than its oligomers, PPGs, because of the larger number density of the —OH end group which tends to construct the intermolecular network structure. Adam-Gibbs (AG) theory could still hold for MDSC results due to the fact that the dielectric relaxation time can be related to the configurational entropy.
Abstract
PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition is studied by MDSC which allows to access the glass transition temperature T g and the variations of ΔC p=C p1−C pg (difference between thermal capacity in the liquid-like and glassy states at T=T g). Variations of T g with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed with regard to the structure engaged in these materials. The increments of ΔC p at T g are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’ of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase.
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 Modulated Differential Scanning Calorimeter (MDSC) technique, using TA Q1000 instrument, has been applied as a tool to study the reversible and non-reversible heat flow characteristics of a wide range of polyethylenes. It was found that the heat flow characteristic is dependent upon the heating rates and modulation period used in the test. By using a set of standard test conditions, MDSC was found to be useful in studying the effect of previous thermal processing conditions, additive effects, and also the density, MI, type of comonomer, and molecular architecture.
