Thermomechanical analysis thermal expansion

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

Thermal expansion and contraction are reversible effects of temperature which may be very important in some applications. Usually expansion is measured using thermomechanical analysis (TMA) (see ISO 11359-2 [4]). 

ISO 11359-2, Plastics - Thermomechanical analysis (TMA) - Part 2 Determination of coefficient of linear thermal expansion and glass transition, 1999. 

The linear thermal expansion coefficient p calculated from these measurements are in excellent agreement with literature data obtained by the conventional method. For example, the values of P calculated from the thermal effects Q during stretching of PS and PET films agree well with conventional dilatometric results, i.e. for PS PQ = 6.8xKT5 -1, PdU = 7.0x 10-5 K 1 for PET PQ = 5.4x 10 5 K-1, Pdu = 5 0 x 10"5 K 1. The characteristic heat to work ratio q depends hyper-bolically on strain which is also in an excellent agreement with prediction following from the thermomechanical analysis (see Fig. 1). 

ASTM E831, 2003. Standard test method for linear thermal expansion of solid materials by thermomechanical analysis. 

Most of the physical properties of the polymer (heat capacity, expansion coefficient, storage modulus, gas permeability, refractive index, etc.) undergo a discontinuous variation at the glass transition. The most frequently used methods to determine Tg are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical thermal analysis (DMTA). But several other techniques may be also employed, such as the measurement of the complex dielectric permittivity as a function of temperature. The shape of variation of corresponding properties is shown in Fig. 4.1. 

Thermomechanical analysis (TMA). In this technique, information on changes in the size of a sample is obtained, e.g. thermal expansion and coefficient of thermal expansion, cure shrinkage, glass transition, thermal relaxations, any phase transformation involving volume change in the material. We describe the measurement of the coefficient of thermal expansion in detail later in this section. 

Thermomechanical analysis allows the calculation of thermal expansivity from the same data set as used to calculate the Tg. Since many materials are used in contact with a dissimilar material in the final product, knowing the rate and amount of thermal expansion helps design around mismatches that can cause failure in the final product. These data are only available when the Tg is collected by thermal expansion, not by the flexure or penetration method. This is in many ways the simplest or most essential form of TMA measurement. A sample is prepared with parallel top and bottom surfaces and is allowed to expand under minimal load (normally 5mN or less, ideally OmN) as it is slowly heated and/or cooled. The CTE is calculated by ... 

Thermomechanical Analysis. A thermomechanical analyzer (Perkln-Elmer TMS 2) was used to measure glass transition temperature (Tg) and coefficient of thermal expansion (a ) of completely cured samples In the penetration and expansion modes respectively. Experimental conditions are listed In Table I. 

Properties relating to performance of completely cured adhesive were determined by mechanical spectroscopy and thermomechanical analysis. Measurement of glass transition temperature and coefficient of thermal expansion was obtained from temperature scanning. 

Thermomechanical Analysis (TMA) can be defined as the measurement of a specimen s dimensions (length or volume) as a function of temperature whilst it is subjected to a constant mechanical stress. In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored. Many materials will deform under the applied stress at a particular temperature which is often connected with the material melting or undergoing a glass-rubber transition. Alternatively, the specimen may possess residual stresses which have... 

Plastics - Thermomechanical analysis (TMA) - Determination of linear thermal expansion coefficient and glass transition temperature Plastics - Thermomechanical analysis (TMA) - Determination of softening temperature Plastics - Determination of dynamic mechanical properties -General principles Plastics - Dynamic mechanical analysis - Determination of glass transition temperature Plastics - Dynamic mechanical analysis - Calibration... 

Thermomechanical-analysis (TMA) testing is used to measure a material s expansion coefficient above and below the glass-transition temperature or Tg. Thermomechanical analysis continuously monitors the expansion of a probe on a sample as a function of temperature. The standard test method for TMA is ASTM D3386, Coefficient of Linear Thermal Expansion of Electrical Insulating Materials.In addition to the glass-transition temperature or Tg, the expansion coefficients above and below Tg are reported. 

Preferably, thermomechanical analysis (TMA) is used to determine the linear thermal expansion coefficient of polymers according to ISO 11359. TMA uses a constant applied load (0.1 g to 5 g) and cylindrical or rectangular specimens with plane-parallel surfaces. The test is conducted with a low heating rate. An average or a differential coefficient of thermal expansion can be obtained, according to Eq. 3.5 and Eq. 3.6. 

This system is expanded to the thermomechanical analysis capability which requires the measurement of length in a penetration, expansion, or extension mode. The basis of this analysis has been discussed in the literature. The relationships of the thermal expansivity a and the length 1 of a sample with respect to temperature and tensile force f are expressed as follows... 

Thermomechanical analysis (TMA) is the measurement of the deformation of the phenolic material imder a constant strain as a function of temperature or time. TM A invesfigalion of crosslinked, modified phenolics gives the linear expansion coefficient and the glass tiansifion temperature of this material [33]. The linear expansion coefficient ( ) is measured by thermal expansion in one dimension. It is defined by equation 62 where dL/dT is the slope of the jdot of the longest sample dimension versus sample temperature and L is the original length of the crosslinked phenolic sample. 

ASTM E831-06 Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis... 

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