Dynamic mechanical parameters

Users of DMA will be familiar with the typical outputs of such instruments, namely M (storage or real modulus, E or G ), M (loss or imaginary modulus, E or G ) and tan 8, which is the ratio M /M. One of the reasons that DMA is such a powerful technique for exploring the properties of polymeric materials and others that show time-dependent behaviour is that aU ofthe above parametersare influenced greatly by the materials relaxation 

Therefore the parameters M, M and tan 8 give a view of a material s relaxational behaviour, which in turn reveals its molecular structure. Subtle differences between grades of the same material and different structures arising from a variant polymerisation will all be visible if the molecular structure has been altered. 

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Figure 3. Effect of acid catalyst (HCl) content on the temperature dependence of the dynamic mechanical parameter tan 8 for a series of TEOS(48)-PDMS(1700)-50-X-80C materials. See text for sample nomenclature. (Reproduced from reference 4. Copyright 1987 American Chemical Society.)...

Bankman, I.N., Gruben, K.G., Halperin, H.R., Popel, A.S., Geurd, A.D., and Tsithk, J.E. Identification of dynamic mechanical parameters of the human chest during manual CPR. IEEE Trans. Biomed. Eng., 37, 211-217,1990. 

The determination of G and A (or any equivalent pair of dynamic mechanical parameters) is used very commonly both as a research tool and also as a quality control tool. The measurements are made at constant frequency over an extremely wide range of temperature, usually by torsion pendulum. 

Dynamic-mechanical parameters and their dependences on particular factors Fj (temperature T, frequency V, amphtude of tension of a varying... 

Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) followed by dynamic mechanical parameter tests were used for the investigation of the morphology of unsaturated polyester/diol-polyurethane hybrid polymer networks (UP/PU HPNs). The effect of phase separation on mechanical properties was described [142]. 

Using physical properties relating to performance parameters leads to the development of algorithms for predicting performance for laboratory screening of potential improvements. Many of these algorithms have been estabUshed. The two main categories of measurement criteria are quasi static and dynamic mechanical properties. 

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

In dynamic mechanical analysis of plastics, the material is subjected to a sinusoidal variation of stress and the strain is recorded so that 1, 2 and S can be determined. The classical variation of these parameters is illustrated in Fig. 2.55. 

Interest in the use of syntactic foam as a shock attenuator led to studies of its static and dynamic mechanical properties. Particularly important is the influence of loading rate on stiffness and crushing strength, since oversensitivity of either of these parameters can complicate the prediction of the effectiveness of a foam system as an energy absorber. 

Dynamic mechanical data near the gel point allow easy determination of the parameters of the critical gel, Eq. 1-1. Tan 8, as shown in Fig. 26, gives the relaxation exponent n... 

As the aromatic dicarboxylic acid BB has a much more rigid structure than Cl-PEC, the Tgs of the LCPs derived from BB are supposed to be much higher than those derived from Cl-PEC. Although, in the case of LCPs derived from BB, it is difficult to detect the Tg values by DSC measurements, these values have good correlation with the E"(max) parameters determined by dynamic mechanical analysis (Figure 19.11). According to this study, the Tg of Me-HQ/BB was... 

Dynamical mechanical spectroscopy and Izod impact results suggest that the glass transition temperature of the elastomer phase constitutes the most critical parameter in achieving impact resistance in these materials. 

Thermal analysis, moisture uptake and dynamic mechanical analysis was also accomplished on cured specimens. Thermal analysis parameters used to study cured specimens are the same as those described earlier to test resins. The moisture uptake in cured specimens was monitored by immersing dogbone shaped specimens in 71 C distilled water until no further weight gain is observed. A dynamic mechanical scan of a torsion bar of cured resin was obtained using the Rheometrics spectrometer with a temperature scan rate of 2°C/minute in nitrogen at a frequency of 1.6Hz. The following sections describe the results obtained from tests run on the two different BCB resin systems. Unless otherwise noted all tests have been run as specified above. 

Dynamic swelling parameters of bioactive substances into hydrogel matrices are summarized in Table 19.1. The values of n>0.5 indicate that the loading mechanism of bioactive substances deviates from Fickian diffusion. For NIPA-APSA and NIPA-AA/Richlocaine systems the diffusion mechanism is relaxation-controlled because their n values are close to 1. Temperature-dependent release of richlocaine from the NIPA-AA hydrogels is shown in Figs. 19.3 and 19.4. 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

Most conspicuous modifications of the dynamic mechanical response spectra of PHEMA and related polymers are brought about by incorporation of low-molecular weight compounds (Fig. 13). Along with alterations of parameters (temperature, height, shape) of the peaks characteristic of a dry polymer, usually a new diluent peak appears. (The relaxation patterns of various polymethacrylates are not modified by diluents in a unique way but several modes can be distinguished as mentioned before.) A remarkable feature... 

The dynamic mechanical response of a material can be characterised through the loss modulus, the loss tangent, tan S, or the loss compliance, However, as already mentioned for Ar-Al-PA (Sect. 6), the loss compliance can be considered the most relevant parameter for quantitatively comparing different materials, at least for additive purposes. For this reason, the semi-quantitative analysis and the comparison of viscoelastic data determined for different systems have been performed [63] in terms of /", whereas the determination of activation energies and entropies are based on loss modulus data. 

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