Exploring the Sensitivity of Thermal Analysis Techniques to the Glass Transition

Five thermal analysis techniques (DSC, modulated DSC™, TMA, DMA, and DEA) are used to characterize the glass transition temperature (Tg) for amorphous polymers. Each of these thermal techniques detects the Tg based on changes in a different material property during the glass transition. Hence, the relative sensitivities of the different techniques for detecting the Tg vary depending on the nature of the material being evaluated as well as on experimental variables such as the heating rate. This study compares the Tg information obtained from the five common thermal techniques on a typical amorphous thermoplastic. The results illustrate the types of issues that can arise. INTRODUCTION The glass transition is the temperature region where an amorphous material changes from a glassy phase to a rubbery phase upon heating, or vice versa if cooling. The glass transition is very important in polymer characterization as the properties of a material are highly dependent on the relationship of the polymer end-use temperature to its Tg. For example, an elastomer will be brittle if its Tg is too high, and the upper use temperature of a rigid plastic is usually limited by softening at Tg. Hence an accurate and precise measure of Tg is a prime concern to many plastics manufacturers and end use designers. Thermal analysis, which is a generic term used to describe a family of analytical techniques that measure changes in the physical properties of a material with temperature, provides a convenient means of measuring the glass transition. Each of the thermal analysis techniques senses the glass transition based on changes in a specific material property. Table 1 summarizes the different thermal techniques, the property change(s) measured for each during the glass transition, and an indication of the relative sensitivity (relative signal change) of each for detecting the Tg. TABLE 1 Properties Measured by TA Techniques The polycarbonate used was TUFFAK®A from Rohm & Haas Co. The polystyrene was Aldrich Chemicals Catalog Number 18242-7. Polycarbonate samples for DMA evaluation were prepared by cutting bars 12mm wide by 60mm long from the 3mm thick sheets received. Samples for evaluation by the other techniques were prepared by pressing 0.15mm films from the original 3mm sheets in a 155oC hot press. Polystyrene samples for DSC and TMA evaluation were prepared by pressing the polymer beads originally received into 0.2-0.5mm films in a 200oC hot press. In this study, a typical amorphous thermoplastic polymer is evaluated by all of the techniques shown in Table 1, as well as modulated DSC™, to illustrate some of the experimental trade-offs and considerations commonly encountered. A user’s guide based on the results, and those for other types of polymers, is included as a summary. EXPERIMENTAL Materials Two unfilled amorphous thermoplastic materials were used in this study, polycarbonate and polystyrene. Unfilled amorphous thermoplastics undergo larger property changes at the glass transition than any other polymer type or morphology. In materials where the amorphous content is reduced, such as highly crystalline or highly filled thermoplastics, or where crosslinking reduces the size of mobile polymer chains, the property changes at the glass transition will generally be reduced. The smaller signal changes decrease the sensitivities of the respective thermal techniques to the glass transition, though each technique is affected to a differing degree, as will be discussed. Relative Signal Technique Property Measured Change at Tg Differential Scanning Calorimetry Heat flow (heat capacity) 0.2 Thermomechanical Analysis Expansion coefficient or softening 3 Dielectric Analysis Permittivity and dielectric loss 100 Dynamic Mechanical Analysis Mechanical strength and energy loss 200 Instrumentation All experiments were performed on the following thermal analysis equipment from TA Instruments: DSC 2910 with autosampler and MDSC™ upgrade, TMA 2940, DMA 983, and DEA 2970. All specimens were run under a nitrogen atmosphere. The results reported in this study were obtained from heating experiments only. Results from cooling experiments, although similar, are the topic for another study and hence are not included here. RESULTS AND DISCUSSION Sensitivity to Transition Representation There are two basic representations of the glass transition which are commonly used. They are onset or step change and peak maximum. In general, the measurement based on onset or step change is subject to greater uncertainty than the measurement based on a peak maximum because the former measurement relies on the ability to accurately define baselines and tangents surrounding the transition. This is illustrated in Figures 1 and 2. Figure 1 shows the correct cursor placement for onset and peak determinations. Figure 2 shows similar determinations where cursor placement for drawing the baseline tangent has been shifted 3oC. Note that this shift results in a 1.5oC decrease in the Tg determined from the onset while no change occurs in the measured peak maximum. In general, the glass transition temperature is less subject to operator interpretation when the property measured by the specific thermal analysis technique relies on a signal peak (e.g. DMA damping, DSC heat flow derivative). Figure 1 ONSET AND PEAK DETERMINATIONS WITH CORRECT CURSOR PLACEMENT 70 80 90 100 110 120 Temperature ( C) o 84.00 C o 101.1C o 106.2 C o 84.00 C o 107.0 C o 120.0 C o DSC Sensitivity DSC, which measures heat flow to and from a specimen relative to an inert reference, is the most common thermal analysis method used to measure the glass transition. The heat capacity step change at the glass transition yields three temperature values: onset, midpoint and endset. The midpoint is usually calculated as the peak maximum in the first derivative of heat flow (see Figure 3) although it can also be calculated as the midpoint of the extrapolated heat capacities [1] before and after the glass transition. The DSC thermal curve for polycarbonate heated at 20oC/minute is shown in Figure 3. Most DSC experiments are performed at 5 or 10oC/minute. A higher heating rate is beneficial in detecting Tg, however, because the heat flow signal associated with heat capacity change during the glass transition is enhanced with very little corresponding increase in noise, thereby increasing sensitivity. Nevertheless, this increase in sensitivity with increased heating rate does have penalties. Both the temperature and breadth of the glass transition are affected by increased heating rates. Figure 4 shows the glass transition of polystyrene (18 mg specimen) by DSC at various heating rates. Notice that Tg shifts to higher temperatures and the transition broadens as the heating rate is increased, especially at 20 and 50oC/min. It is therefore important to report heating rate along with Tg values. 70 80 90 100 110 120 Temperature (oC) 87.00oC 99.61oC 106.2oC 87.00oC 104.0oC 117.0oC Figure 2 ONSET AND PEAK DETERMINATIONS WITH INCORRECT CURSOR PLACEMENT -0.54 -0.56 -0.58 -0.60 -0.68 -0.66 -0.64 -0.62 120 130 140 150 160 170 -0.1 0.0 0.1 0.2 0.3 144.03oC 148.03oC (I) 149.11oC H ea t F lo w ( W /g ) Temperature (oC) Figure 3 [----] D er iv . H ea t F lo w ( W /g /m in ) GLASS TRANSITION OF POLYCARBONATE (13mg) by DSC AT 20oC/min 0.2 0.5 1 2oC/min 0.0