Glass transition measurement

An analysis of the glass transition of polystyrene by TMDSC is illustrated in Fig. 4.125. These measurements can be compared to the DSC results in Sect. 4.3.7. The left half of the figure shows besides the modulated heat-flow rate, AT(t), the three 

Quasi-isothermal Analysis of the Glass Transition of Poly(ethylene Terephthalate) 

The simplest model for the representation of the glass transition, perhaps, is the hole theory. With it the larger expansivity of liquids and the slower response to external forces is said to be due to changes in an equilibrium of holes. These holes are assumed to be all of equal size, and their number depends on temperature. The equilibrium number of holes at a given T is N, each contribnting an energy to the enthalpy. The hole contribution to Cp is then given under equilibrium conditions by  

Data Analysis of the Quashisothermal Measurements of FET at Different Frequencies in the Vicinity of 

Creation, motion, and destraction of holes are cooperative kinetic processes and may be slow. This leads to deviations from Eq. (1) if the measurement is carried out faster than the kinetics allows. Applied to the glass transition, one can write and solve a simple, first-order kinetics expression [40] based on the equilibrium expression, above  

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Dinelli, F, Buenviaje, C. and Ovemey, R.M., Glass transition measurements on heterogeneous surfaces. Thin Solid Films, 396(1-2), 138-144 (2001). 

Bair, H.E. 1994. Glass transition measurements by DSC. In Assignment of the Glass Transition (R J. Seyler, ed.), pp. 50-74. American Society for Testing and Materials, Philadelphia, PA. 

Schawe, J. E. K. (1998). A description of the glass transition measured by temperature modulated differential scanning calorimetry. Colloid Polym. Sci. 276(7), 565-569. 

Figure 3. Elastic and loss moduli of four soy products. SSF = Soy Spent Flakes, DSF = Defatted Soy Flour, SPC = Soy Protein Concentrate, SPl = Soy Protein Isolate. Arrows indicate the regions of glass transition. Measured at 0.05% strain and 0.16 Hz. (Reproduced from reference 17.)...

Huth H, Minakov AA, Serghei A, Kremer F, Schick C (2007) Differential ac-chip calorimeter for glass transition measurements in ultra thin polymeric films. Euro Phys J Special Topics 141 153-160... 

The fact that the sample mass is continuously monitored means that DTA peak measurements can be referred back to a true sample weight, after allowing for prior lower temperature weight losses such as drying. Confidence in the quality of peak area measurements, especially at high temperatures, is increased when the TG data show that no sublimation or decomposition is taking place. Glass transition measurements can be markedly influenced by the moisture content in the case of many important polymers. With STA, the moisture content at the time of measurement is known exactly. 

Figure 4.126 illustrates that the reversing glass transition measurement is almost independent of the annealing (thermal) history. The still little-explored, small differences are an indication of the changes in relaxation kinetics on annealing of glasses. The corresponding nonreversing components are shown in Fig. 4.127. They permit the quantitative characterization of the thermal history of the glass as mentioned above, the differences, however, between the curves of Fig. 4.126 are not considered in this separation.

The possibilities arising from the advent of MTDSC will now be discussed Complex thermal histories affect the ease with which it is possible to make determinations of the increment of heat capacity, ACp, at Tg because of structure relaxation. If a thermal analysis apparatus that can separate the structure relaxation part from the total heat flow signal can be developed ACp could be determined accurately. It is well known that ACp is related to the weight fraction of each component in a heterogeneous system such as a polymer blend. In multi-phase polymeric materials, each phase has its own characteristic glass transition temperature and ACp. Thus, important information may be obtained from ACp and glass transition measurements, allowing such materials to be analysed quantitatively. 

In order to eliminate the thermal history of the sample, heat it at a rate of 20 K min to 395 K and maintain this temperature for 10 min. Then cool the sample at a rate of 10 K min to 335 K and maintain for 10 min. This procedure is the same as that used in the glass transition measurement. 

Glass Transition Measurements of Ultrathin Polystyrene Films... 

Figure 2.117. Cure of the reactive blend DGEBA + aniline (r = l)/20wt% PES at 100°C (a) change in heat capacity (reversing signal) during isothermal cure at 100°C, with cloud point from OM (A), onset of heat flow phase relaxation (O) (b) glass transitions measured after different isothermal cure times at 100°C (heating rate = 2.5°C/min) numbers indicate cure times on (a) (Swier, unpublished results).

DSC (differential scanning calorimetry) Heat capacity versus temperature or time allows measurement of heats of fusion, identification of crystalline and liquid crystalline phases, degrees of crystallinity, etc. Glass transition measurement allows characterisation of ageing, blend compatibilities. Heats of reaction allow cure and degradation studies. 

The Tg determination, while not a thermodynamic criteria for misdbility, is an easy and effective method for determination of the phase behavior. The author, having conducted literally thousands of these measurements (primarily by dynamical mechanical methods), has found this to be a viable screening method that is highly reliable. In the borderline cases (xu 0), the experimental protocol is critical and with proper consideration of the experimental details, exceptions to these observations are rare. In the cases of specific interactions xn < 0), the literature is even clearer as glass transition measurements agree well with the results of other characterization methods, including those which meet the criteria of thermodynamic misdbility. 

The glass transition measured by DMA also showed similar results. An infi ared spectrum taken from the sample showed that the sample taken fi-om the flashing was neoprene-based. These thermoanalytical techniques... 

Huth, H., Mlnakov, A.A., Schick, C. Differential ac-chip calorimeter for glass transition measurements in ultrathin films. J. Polym. Sci. Pt. B-Polym. Phys. 44(20), 2996-3005 (2006)... 

TMA has already been considered in conjunction with glass transition measurements in applications such as degree of cure or heat-distortion temperatures of epoxy resin castings. The technique has proved useful for many other applications, some of which are described below. 

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