Dynamic Mechanical Analysis DMA

DMA provides the most significant information on the viscoelastic behavionr of polymers in addition to thermal transitions. In principle, a sinusoidal strain or stress is applied to a sample and the response is monitored as a function of frequency and temperature. A viscoanalyser is commonly used to apply a displacement d(w) at the 

Viscoelastic properties may be expressed in terms of a dynamic storage modulus (E ), dynamic loss modulus (E ) and mechanical damping factor (tan8). Mathematically, they are defined as follows  

Polyurethanes have a combination of elastic and viscous properties that can be explained in standard engineering terms using DMA methods. Information can be obtained on the properties of polyurethanes that relates to the storage and dissipation of energy applied during use. 

The equipment can operate in several different modes, over various frequencies, and over a wide temperature range. Typical modes that can be used are  

The manner in which these modes are carried out can further be modified by some of the following methods  

The instruments can operate from -150°C to well above the melting point of polyurethanes. 

Samples that are used must be free from any defects such as bubbles and must be uniform in size and measured accurately. The size of the samples is reasonably small, approximately 3 mm thick by 12 mm wide and 50 mm long. The exact size will depend on the instruments and the test being carried out. The basic results generated are. 

The dynamic mechanical analysis method deter-minesl l elastic modulus (or storage modulus, G ), viscous modulus (or loss modulus, G ), and damping coefficient (tan A) as a function of temperature, frequency or time. Results are usually in the form of a graphical plot of G, G , and tan A as a function of temperature or strain. DMA may also be used for quality control and product development purposes. 

The forced non-resonance techniquet l is one of the simpler dynamic mechanical methods. In the 

The above modulus definition does not take time into account. For materials that exhibit time-dependent deformation, such as polymers, the reported modulus must include, to be valid, a time factor. This 

For a completely elastic material, M = M, while for a perfectly viscous material, M = M . The factor A is the measured phase lag between the applied stimulus and the response. Tan A is given by the ratio M VM and is proportional to the ratio of energy dissipated to energy stored, called the loss tangent or damping factor, which is one of the key parameters in DMA. Loss tangent is observed to increase during transitions between different deformational mechanisms. 

Readers interested in a more in-depth understanding of data obtained from DMA measurements can refer to pol mier rheology textbooks. Some of the available helpful books published in this area as listed in Refs. 14-16. 

Polymers are viscoelastic materials, whose mechanical behavior exhibits characteristics of both solids and liquids. Thermal analysts are frequently called on to measure the mechanical properties of polymers for a number of purposes. Of the different methods for viscoelastic property characterization, dynamic mechanical techniques are the most popular, since they are readily adapted for studies of both polymeric solids and liquids. They are often referred to collectively as dynamic mechanical analysis (DMA). Thermal analysts often refer to the DMA measurements on liquids as rheology measurements. 

Thermal Analysis of Polymers Fundamentals and Applications, Edited by Joseph D. Menczel and R. Bruce Prime 

Detect transitions arising from molecular motions or relaxations 

Determine mechanical properties, i.e., modulus and damping of viscoelastic materials over spectrum of time (frequency) and temperature 

Phase separation (polymer blends, copolymers, polymer alloys) 

Theory [32-34]—A sinusoidally varying stress of frequency to, applied to a sample which is clamped into a rigid frame, produces a sinusoidally varying strain, where the stress proceeds the strain by a phase angle rV Thus, 

E (w) and E (w) denote the dynamic storage modulus and the dynamic loss modulus, respectively. The phase angle ri is calculated using 

The storage modulus of PP matrix, PP/KE (10 mm) biocomposites and PP/KE (1 mm) biocomposites at -30°C increased by 15, 3.5 and 4.7% with the addition of 5 wt% nanoclay, respectively, compared to those of the PP biocomposites without nanoclay. This results from a uniform distribution of nanoclay and the incorporation of the reinforcing KE restricting the mobility of the polymer molecules by good interactions among KE, PP matrix and nanoclay. On the contrary, storage modulus of nanobiocomposites with 10 wt% nanoclay decreased due to agglomeration of nanoclay in the PP matrix. 

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The principal techniques for determining the microstmcture of phenoHc resins include mass spectroscopy, proton, and C-nmr spectroscopy, as well as gc, Ic, and gpc. The softening and curing processes of phenoHc resins are effectively studied by using thermal and mechanical techniques, such as tga, dsc, and dynamic mechanical analysis (dma). Infrared (ir) and electron spectroscopy are also employed. 

Thermal analysis iavolves techniques ia which a physical property of a material is measured agaiast temperature at the same time the material is exposed to a coatroUed temperature program. A wide range of thermal analysis techniques have been developed siace the commercial development of automated thermal equipment as Hsted ia Table 1. Of these the best known and most often used for polymers are thermogravimetry (tg), differential thermal analysis (dta), differential scanning calorimetry (dsc), and dynamic mechanical analysis (dma). 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

Thermal and thermomechanical analyses44 are very important for determining die upper and lower usage temperature of polymeric materials as well as showing how they behave between diose temperature extremes. An especially useful thermal technique for polyurethanes is dynamic mechanical analysis (DMA).45 Uiis is used to study dynamic viscoelastic properties and measures die ability to... 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

Dynamic DSC, 409. See also Differential scanning calorimetry (DSC) Dynamic mechanical analysis (DMA), 138, 163, 241-242, 407, 409... 

Mechanical properties. See also Dynamic mechanical analysis (DMA) of polyamides, 138 of polyester LCPs, 52 of polyurethanes, 242-244 of semicrystalline aromatic-aliphatic polyesters, 45 Mechanical recycling, 208 Medical applications, for polyurethanes, 207... 

Glass transition temperature (Tg), measured by means of dynamic mechanical analysis (DMA) of E-plastomers has been measured in binary blends of iPP and E-plastomer. These studies indicate some depression in the Tg in the binary, but incompatible, blends compared to the Tg of the corresponding neat E-plastomer. This is attributed to thermally induced internal stress resulting from differential volume contraction of the two phases during cooling from the melt. The temperature dependence of the specific volume of the blend components was determined by PVT measurement of temperatures between 30°C and 270°C and extrapolated to the elastomer Tg at —50°C. 

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

Thermo-mechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA)... 

An associated technique which links thermal properties with mechanical ones is dynamic mechanical analysis (DMA). In this, a bar of the sample is typically fixed into a frame by clamping at both ends. It is then oscillated by means of a ceramic shaft applied at the centre. The resonant frequency and the mechanical damping exhibited by the sample are sensitive measurements of the mechanical properties of a polymer which can be made over a wide range of temperatures. The effects of compositional changes and methods of preparation can be directly assessed. DMA is assuming a position of major importance in the study of the physico-chemical properties of polymers and composites. 

Electron irradiation causes chain scission and crosslinking in polymers. Both of these phenomena directly affect the glass transition temperature (Tg) of the materials. Thermomechanical (TMA) and dynamic-mechanical analysis (DMA) provide information about the Tg region and its changes due to radiation damage. Therefore, DMA and TMA were performed on all irradiated materials. 

Figure 8.1 shows dynamic mechanical analysis (DMA) data for an unfilled and 30 % glass-filled PBT. Note the sharply higher modulus ( ) in the glass-filled blend at all temperatures. 

Bauer, Denneler, and Wilert-Porada also studied the influence of temperature (30-120°C) and humidity (0 - 100%) on the mechanical properties of Nation 117 membrane via dynamic mechanical analysis (DMA). The mechanical behavior of Nation membranes in a humid atmosphere was observed to differ significantly from that in dry atmosphere, and the influence of water on the mechanical properties of the acid form of Nation was found to be complex. The maximum of the storage modulus ( ) as a function of humidity was shifted to higher humidity values with increasing temperature. 

Miura and Yoshida also investigated the changes in the microstructure of 1100 EW Nafion sulfonate membranes, in alkali, ammonium, and alkylammonium cation forms, that were induced by swelling in ethanol using DSC, dynamic mechanical analysis (DMA), SAXS, and electron probe microanalysis (EPMA). These studies were performed within the context of liquid pervaporation membranes that could potentially be used to separate ethanol from water... 

Dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA) provides a method for determining elastic and loss moduli of polymers as a function of temperature, frequency or time, or both [1-13]. Viscoelasticity describes the time-dependent mechanical properties of polymers, which in limiting cases can behave as either elastic solids or viscous liquids (Fig. 23.2). Knowledge of the viscoelastic behavior of polymers and its relation to molecular structure is essential in the understanding of both processing and end-use properties. 

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