TMA experiments

DTA experiments were carried out by Elspeth C. Eberlin and Joan Z. Whiting. TMA experiments were carried out by Joan K. Lucas and Marie T. Borghetti. 

DMA. bars (l/8"x "x5") were molded from each formulation. Samples for the DMA runs were cut from the center of the molded bars and the end pieces were used for TMA. In most cases, duplicate bars were molded and run (14 replicates, 29 observations in all), and all measurements were run in a randomized order. Both the DMA and TMA experiments were run at 5°C/min. The CTE below Tg was determined between 80-100 °C the CTE above Tg was determined between 220-240 °C and the Tg was determined by the intersection of the extrapolations from these two expansion regions. Tg was also determined from the peak of the DMA-loss modulus curve. The DMA storage modulus was determined at 40°C. This temperature was chosen because a more stable reading can be obtained than at room temperature. 

An old but still properly working Perkin Elmer IMS-1 was adapted to perform TSD/TMA experiments. The TMS-1 is schematically drawn in Figure 6.1 together with some of the important dimensions. The sample, between the probe and the quartz glass sample holder is placed in the furnace. The furnace temperature is programmed to increase linearly with the time. The thermal expansion of the sample is measured via the probe by the linear variable displacement transducer (LVDT). A thermocouple, placed as close as possible to the sample is giving the sample tenperature information. 

Note For example in a Thermomechanical Analysis (TMA) experiment the sample may be subjected to no force, a constant force, an increasing force or a modulated force - or any combination of the above - during a single experiment. The technique (TMA) has not changed, only the experimental variables for that technique. 

Thermomechanical analysis (TMA) measures the deformation of a material contacted hy a mechanical prohe, as a function of a controlled temperature program, or time at constant temperature. TMA experiments are generally conducted imder static loading with a variety of probe configurations in expansion, compression, penetration, tension, or flexime. In addition, various attachments are available to allow the instrument to operate in special modes, such as stress relaxation, creep, tensile loading of films and fibers, flexural loading, parallel-plate rheometry, and volume dilatometry. The type of probe used determines the mode of operation of the instrument, the manner in which stress is apphed to the sample, and the amount of that stress. 

The available free volume is one of the most important physical quantities influencing the glass transition and the thermal expansion. Changes in free volume (vt), can be measured by determining the volumetric change of the polymer, and this can be done with TMA experiments (Menard 1996 Bird... 

Figure 4.2. (a) The temperature dependence of free volume in an amorphous polymer (b) a typical TMA experiment of a cured epoxy thermoset (probe displacement is proportional to expansion) for determination of T, symbols and a, represent glassy and rubbery CLTEs, respectively [Turi et al. (1988) reprinted with permission of VCH Publishers]. 

One advantage of performing the dynamic load TMA experiment on a DMA is that we can also view the tan 8 values. Figure 4.30 shows these data. The immediate difference apparent from the two traces is the difference between the Tg as defined by the onset from the displacement measurement and the Tg as defined by tan 8. The onset Tg is essentially obtained from a static or low-fi equency test. This difference is due to the frequency dependence of the Tg. This is fully discussed in Sections 1.3 and 5.3. 

This type of TMA experiment can be valuable in screening out experimental formulations which are unlikely to succeed, prior to moving to other material evaluations and, more importantly, to production trials. Such screening can lead to time saving and considerable cost reductions for the manufacturing plant, since any failed trial produces only scrapped... 

Silicone rubber burns in air and, if the formulation includes appropriate fillers, will produce a hard insulating ash. A TMA experiment to simulate burning yields interesting results when carried out with a 1-mm-diameter probe, a load of 100 mN, air purge and a heating rate of 10 K/min. Figure 5.13 shows the result of one such experiment. 

The TMA experiments showed that the softening points of the PLA ionomers increased upon incorporation of the metal salt groups. The order of the softening points eorresponded to the increase in Tg values (i.e., the dependence on q/a), observed from DSC. Association of the ionic groups produces a physical network, which suppresses chain mobility, and, thus, deformation required higher temperatures. The greatest resistance to penetration was observed for the PLA ionomers with multivalent metal ions. 

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