Dynamic Mechanical Analysis, elastic modulus

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

The physical properties of barrier dressings were evaluated using the Seiko Model DMS 210 Dynamic Mechanical Analyzer Instrument (see Fig. 2.45). Referring to Fig. 2.46, dynamic mechanical analysis consists of oscillating (1 Hz) tensile force of a material in an environmentally (37°C) controlled chamber (see Fig. 2.47) to measure loss modulus (E") and stored modulus (E ). Many materials including polymers and tissue are viscoelastic, meaning that they deform (stretch or pull) with applied force and return to their original shape with time. The effect is a function of the viscous property (E") within the material that resists deformation and the elastic property (E )... 

The recoverable deformation of the pellets was quantifled from the linear portion of the load/deformation profile obtained when measuring the strength of pellets by Aulton et al. (88). Alternatively, the elastic properties as a storage modulus" can be measured by the application of dynamic mechanical analysis (DMA) (89). The values obtained by this method for a series of pellet formulations were found to be considerably greater than those obtained by application of the former method (81). which must therefore be considered as an estimate of the real value. It did rank the pellet formulations in the same order as the DMA. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

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. 

In order to elaborate the relationship between stoichiometric imbalance of the epoxy-amine system and its resulting elastic properties, a series of reference samples of well-defined concentration ratios r was investigated using dynamic mechanical analysis (DMA). A clear increase in the glassy epoxy modulus (as measured at 20 °C) with an increasing excess of epoxy (r< 1) was observed [32]. Notably, the observation of modulus enhancement with increasing excess of epoxy is consistent with the concentration and modulus maps of the PVP/epoxy IP. 

Demonstration of Dual Cure. To demonstrate thermal cure after UV cure three sets of films were prepared for Dynamic Mechanical Analysis (DMA). In the first set, four formulations were based on AM 1, AH 2, the epoxy acrylate, amd the urethane acrylate. Formulations used the test resin at 65% in TPGDA, and included 3% Darocur 1173 as a source of free radicals. DMA plots for the epoxy acrylate and urethane acrylate are shown in Figure 9. The elastic or storage modulus (E ) in the rubbery region is often used as a measure of crosslink density. The maximum of the tan delta plot, not shown, is... 

Any polymer property that changes with temperature and has different values above and below Tg can be used, in principle, to determine Tg. For example, the change in specific volume, heat capacity, or elastic modulus may be used to measure Tg. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are two common methods for such determinations. An example of the results of DSC analysis Is presented In Fig. 3.46. It is common for different methods to yield slightly different values for Tg. 

Dynamic-mechanical analysis (DMA) is a versatile method for measuring viscoelastic values over a wide frequency range commonly the modulus of elasticity and the damping values are determined. Moreover the testing method is used to investigate material behaviour as a function of temperature (e.g. for determine the glass transition temperature). 

The analyses of several polymers by dynamic mechanical analysis, DMA, are described in Sect. 4.5 in connection with the brief description of the DMA equipment. It was observed in such experiments that neither the viscosity nor the modulus are constant, as is assumed for the discussion of energy and entropy elasticity, outlined in Sects. 5.6.4 and 5.6.5, respectively. One finds a stress anomaly when the elastic limit of a material has been exceeded and plastic deformation occurs. Other deviations have the stress depend both on strain and rate of strain. Finally, a time anomaly exists whenever the stress/strain ratio depends only on time and not on the stress magnitude. 

As noted in Subsection 24.1.2, viscoelasticity of polymers represents a combination of elastic and viscous flow material responses. Dynamic mechanical analysis (DMA, also called dynamic mechanical thermal analysis, DMTA) enables simultaneous study of both elastic (symbol ) and viscous flow (symbol ") types of behavior. One determines the response of a specimen to periodic deformations or stresses. Normally, the specimen is loaded in a sinusoidal fashion in shear, tension, flexion, or torsion. If, say, the experiment is performed in tension, one determines the elastic tensile modulus E called storage modulus and the corresponding viscous flow quantity E" called the loss modulus. 

Compliance, elastic n. Symbol S. An elastic constant, which is the ratio of a strain or strain component to a stress or stress component. For a perfectly elastic material it is the reciprocal of the elastic modulus. For a viscoelastic material the modulus and compliance are not reciprocally related due to their different time dependencies. Sepe MP (1998) Dynamic mechanical analysis. Plastics Design Library, Norwich, New York. 

Dynamic mechanical analyzer n. An instrument that can test in an oscillating-flexural mode over a range of temperature and frequency to provide estimates of the real , i.e., in-phase, and imaginary , i.e., out-of-phase parts of the complex modulus. The real part is the elastic component, the imaginary part is the loss component. The square root of the sum of their squares is the complex modulus. With polymers, the components and the modulus are usually dependent on both temperature and frequency. ASTM D 4065 spells out the standard practice for reporting dynamic mechanical properties of plastics. An example of a DMA thermogram of different Perkin-Elmer Inc., manufactures the Diamond DMA instrument. Polymer films is shown. Sepe MP (1998) Dynamic mechanical analysis. Plastics Design Library, Norwich, New York. 

It is well known that adhesives need to be low-modulus elastomers, so that the resin formulation has to be designed to give, after UV-curing, a soft material with a low glass transition temperature (Fg). Fig. 3 shows some typical elastic modulus E) and tan S profiles recorded by dynamic mechanical analysis for a UV-cured polyurethane-acrylate which is suitable for adhesives applications, based on its E and Tg values. It is quite soft, as shown by its Persoz hardness value of 50 s, on a scale that goes up to 400 s for glassy materials. 

Dynamic mechanical analysis (DMA) provides putative information on the viscoelastic properties - modulus and damping - of materials. Viscoelasticity is the characteristic behaviour of most materials in which a combination of elastic properties (stress proportional to strain rate) are observed. A DMA simultaneously measures both elastic properties (modulus) and viscous properties (damping) of a material. 

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