MODULATED TEMPERATURE THERMOMECHANICAL

Price DM (2000) Modulated temperature thermomechanical analysis. Thermochimica Acta 357-358 23-29. 

Figure 20.4 Modulated force thermomechanical response for NR with frequency and temperature.

SThM was carried out in the laboratory of H. Pollock and A. Hammiche in the Physics Department of the University of Lancaster, Lancaster, UK using a modified Topometrix Explorer SPM (Topometrix Corporation, Santa Clara, CA). The microscope uses a small Wollaston wire, bent and etched to form a contact mode AFM tip with a nominal radius of about 200 nm. The tip is used both as a heat source and a heat sensor. A second, reference, tip is held in air in close proximity to the sample for differential measurements. The heat to the tip can be modulated and the material response to the modulated heating can be monitored during imaging via lock-in techniques. For the work described here the microscope was operated in three imaging modes (1) constant deflection (for topography) (2) constant temperature (DC) and (3) modulated temperature (AC). In an unscanned mode, the tip can be positioned on the surface for local differential thermal analysis (DTA) or local modulated temperature-DTA and local thermomechanical (TMA) measurements (4,22). 

Table 2.5 summarises the main applications of thermal analysis and combined techniques for polymeric materials. Of these, thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) provide only physical properties of a very specific nature and yield very little chemical information. DMA was used to study the interaction of fillers with rubber host systems [40]. Thermomechanical analysis (TMA) measures the dimensional changes of a sample as a function of temperature. Relevant applications are reported for on-line TMA-MS cfr. Chp. 2.1.5) uTMA offers opportunities cfr. Chp. 2.1.6.1). The primary TA techniques for certifying product quality are DSC and TG (Table 2.6). Specific tests for which these techniques are used in quality testing vary depending upon the type of material and industry. Applications of modulated temperature programme are (i) study of kinetics (ii) AC calorimetry (Hi) separation of sample responses (in conjunction with deconvolution algorithms) and (iv) microthermal analysis.

The thermal and thermomechanical properties of the polymer/HAp composites (glass transition temperature, melting and crystallization behaviour, thermal stability, crosslinking effects, phase composition, modulus, etc.) can be evaluated by thermal analysis methods, like TG, DSC and DMA. Recently, a modulated temperature DSC (MTDSC) technique has been developed that offers extended temperature profile capabilities by, for example, a sinusoidal wave superimposed on the normal linear temperature ramp [326]. The new capabilities of the MTDSC method in comparison with conventional DSC include separation of reversible and non-reversible thermal events, improved resolution of closely occurring and overlapping transitions, and increased sensitivity ofheat capacity measurements [92,327]. 

The Mettler TMA 40 thermomechanical analyzer is illustrated in Figure 11.3. A measuring sensor applies a user-definable force to the sample of -0.05-0.5 N. The position of the sensor is continuously monitored by a LVDT. TMA measurements can be made in the temperature range -100-1000 C. This module is part of the Mettler TA 3000 thermal analysis system. 

Thermal and thermomechanical properties. Silicones, as a class, are rated among the highest temperature stable polymers. They can withstand temperatures of200°C, almost continuously, without degradation of physical or electrical properties and have been used at temperatures as high as 300°C. Because of their high thermal stabilities, they are used as adhesives and encapsulants for electronic modules that are expected to perform in extreme temperature environments, such as near automotive engines and in deep-well sensors. Because of their low moduli of elasticity, silicones also fare well at very low temperatures. They are rated for continuous use at -80°C, but may be used at even lower temperatures. 

Chapter 4 Thermal Analysis Tools contains a detailed description of thermometry, calorimetry, temperature-modulated calorimetry (TMC), dilatometry, thermomechanical analysis (TMA), d)mamic mechanical analysis (DMA), and thermogravimetry (TGA). 

In terms of modulated thermal analysis techniques , TMDSC clearly dominates the group. However, there is limited literature on temperature modulated thermogravimetic analysis (TMTGA) and temperature modulated thermomechanical analysis (TMTMA). Modulation principles have been applied to some less common thermal analysis techniques such as DMA and thermally stimulated current analysis and these developments will be briefly addressed here. It appears that the major development in thermal analysis in the next decade will be in the temperature modulated domain. 

Triple-shape polymers can change on demand from a first shape (A) to a second shape (B) and from there to a third shape (C), when stimulated by two subsequent temperature increases [10, 26, 27]. Specific cyclic, thermomechanical tensile experiments were developed to characterize the triple-shape effect (Chapter Shape-Memory Polymers and Shape-Changing Polymers [101] and Sect. 2.2) quantitatively. Analogous to the experiments for dual-shape materials, each cycle of these tests consisted of a programming and a recovery module. A cycle started with creating the two temporary shapes (B and A) by a two-step uniaxial deformation, followed by the recovery module, where shape (B) and finally shape (C) were recovered. 

The water vapour absorbed in the polymeric components of a module can act as reaction partner in chemical (e.g. hydrolysis) or physical (delamination by thermomechanical stresses) degradation processes. A principal difference to energy transfer by heat conductivity (temperature) or radiation transfer (UV) is the need for mass transport of the water vapour molecules that is based on permeation, especially diffusion, processes. There are two possibilities for acceleration—increasing the moisture gradient and increasing the temperature. The second way uses the temperature dependence of the diffusion coefficient (mostly according to the Arrhenius law). 

In dynamic thermomechanometry the dynamic modulus and/or damping of a substance under an oscillatory load is measured as a function of temperature while the substance is subjected to a controlled temperature. The frequency response is then studied at various temperatures. Torsional braid analysis is a particular case of dynamic thermomechanometry in which the substance is supported on a braid. These are all sophisticated versions of thermomechanical methods. The word dynamic here, as noted above, means oscillatory and this term can be used as an alternative to modulation. In DMA the sample is oscillated at its resonant frequency, and an amount of energy equal to that lost by the sample is added in each cycle to keep the sample oscillation at a constant amplitude. 

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