DMA experiments

If an experiment works, something has gone wrong. (Murphy s quote) 

While I agree it is always good to be realistic, I never like to be pessimistic. Certainly the wellknown Murphy s quote does not apply to the experiments that are described in this book. There is no study described here that would count on the success of a single experiment  

The complex Young modulus ( ) consisting of contributions of a storage modulus ), and of a loss modulus E ), is measured (eq. 3.1). The complex modulus reflects the inherent viscoelastic nature of the polymer, where stress and strains will be out of phase with one another. 

Ptisacana, Polyurethane Elastomers, DOI 10.1007/978-3-709T0514-6 3, Springer-Verlag/Wien 2011 

Information on the PUs different flexible and rigid blocks can be obtained by means of DMA spectra in which the various segments are respectively observed to melt in sequence as the temperature is raised. The storage modulus E, provides information regarding the stiffness of the material, while the loss factor tan 5 measures the degree of molecular motion. The glass transition temperature (To) of the SS is defined as the tr transition in the tan curve. 

---

Fig. 24. Variation in tan3 as a function of the temperature of polychloroprene-aromatic hydrocarbon resin blends containing different resin content. Frequency = 1 FIz. Target strain = 0.005. DMA experiments.

Figure 34 Left (a) equilibrium storage modulus G as a function of y-irradiation dose determined from DMA experiments on exposed PU samples (b) 1/T2ave as a function of G. Right BC CPMAS NMR measurements on non-irradiated PU sample (a) 3 ms, (b) 0.5 ms, and (c) 100 pis. SS denotes soft segment HS denotes hard segment. Reprinted from Maxwell [87], Copyright 2003, with permission from Elsevier.

The sample is clamped into a frame in the DMA experiment. An applied sinusoidally varying stress is applied to a polymer sample at a frequency co with a small amplitude, which can be represented as [1,2,8,10-13]... 

For the measurements, the single cantilever flexure mode was chosen. For this, rectangular polymer samples of 32 mm x 6 mm x 2 mm had to be produced. This was done by injection molding of the original polymers. Fig. 8 shows the setup of a DMA-experiment in the single cantilever configuration. 

Despite the potential of DMA as a tool to characterize humic materials, challenges remain with respect to sample preparation and data interpretation. Unlike conventional DSC experiments, samples in DMA experiments require particular... 

This is because although 0 = (10), in general, cr(10) oQ (it will usually be less). In principle, the quantities we have defined, E(t), Dit), Gif), and J(i), provide a complete description of tensile and shear properties in creep and stress relaxation (and equivalent functions can be used to describe dynamic mechanical behavior). Obviously, we could fit individual sets of data to mathematical functions of various types, but what we would really like to do is develop a universal model that not only provides a good description of individual creep, stress relaxation and DMA experiments, but also allows us to relate modulus and compliance functions. It would also be nice to be able formulate this model in terms of parameters that could be related to molecular relaxation processes, to provide a link to molecular theories. 

One of the biggest choices made in selecting a DMA is to decide whether to choose stress (force) or strain (displacement) control for applying the deforming load to the sample. Because most DMA experiments run at very low strains ( 0.5% maximum) to stay well within a polymers linear region, both analyzers give the same results. 

Figure 9. Correlation between ageing rates determined by (a) DRS and PALS experiments, (b) DP. S and DMA experiments, (c) DMA and PALS experiments. Ageing rates in each case are determincxi from gradients of log fits to data.

A various number of procedures are available for performing DMA experiments. They differ on the type of vibrational excitation (forced vibrations, free damped vibrations and resonant vibrations). 

In a DMA experiment under dual cantilever setup (see Figure 2.4 with typical geometry and configuration), strain, e, is imposed as ... 

DMA experiments on a pultruded glass fiber-reinforced polyester laminate (sampled from DuraSpan deck) [12], with a glass-transition temperature T of... 

Substituting the theoretical results of and into Eq. 5.6, and taking Eg = 12.3 GPa as the original modulus (modulus of glassy state) and Bj. = 3.14GPa as the modulus at approximately 250 °C (modulus of leathery or rubbery state) from DMA experiments, the temperature-dependent E-modulus can be obtained. 

Figure 4.153 shows a schematic drawing of a torsion pendulum. It was used for some of the first DMA experiments that were carried out as a function of temperature [43]. The pendulum is set into vibrations of small amplitude ( 3°) and continues to... 

Dynamic mechanical analyzers can be divided into resonant and defined frequency instruments. The torsion pendulum just described is, for example, a resonant instrument. The schematic of a defined-frequency instrument is shown in Fig. 4.155. The basic elements are the force generator and the strain meter. Signals of both are collected by the module CPU, the central processing unit, and transmitted to the computer for data evaluation. The diagram is drawn after a commercial DMA which was produced by Seiko. At the bottom of Fig. 4.155, a typical sample behavior for a DMA experiment is sketched. An applied sinusoidal stress, o, is followed with a phase lag, 6, by the strain, e. The analysis of such data in terms of the dynamic moduli (stress-strain ratios, see Fig. 4.143) at different frequencies and temperature is the subject of DMA. 

---