DMTA

The results from DMTA appeared to be subject to considerable scatter of experimental points giving rise to particularly large uncertainty. 

The general trend was for glass transition temperature to change little or rise with time of ageing. Silicone X was an exception in that the glass transition temperature tended to fall. Where the rise was considerable it occurred at longer times and at the higher temperatures. 

Ageing of Rubber - Accelerated Heat Ageing Test Results 

The same general trend, as expected, was seen for T2 and TIO temperatures with most materials showing little change or a rise with time for these properties. T2 decreased at longer times for compound P6 and there were unexplained drops in T2 for materials N4 and P9 at the longest time at particular temperatures. These three materials showed similar behaviour for TIO temperature. No sensible results were obtained for P2. 

---

Dynamic Mechanical Thermal Analyzer (DMTA) c 10-" -2 X 10" 10 -10" Rheometric Scientific, Inc., Piscataway, N.J. 

Similar information can be obtained from analysis by dynamic mechanical thermal analysis (dmta). Dmta measures the deformation of a material in response to vibrational forces. The dynamic modulus, the loss modulus, and a mechanical damping are deterrnined from such measurements. Detailed information on the theory of dmta is given (128). 

Viscoelastic phenomena always involve the change of properties with time and, therefore, the measurements of viscoelastic properties of solid polymers may be called dynamic mechanical. Dynamic mechanical thermal analysis (DMTA) is a very useful tool for studying... 

DMTA is based on the response of a viscoelastic material to an oscillatory excitation. Considering a fixed frequency, while the temperature is changed, there will... 

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

Figure 7 DMTA scan for (a) normal, and (b) shear gradient processed polymer.

The relaxation methods employed are Dynamic Mechanical Thermal Analysis (DMTA) and Dielectric Thermal Analysis (DETA). Generally in both cases a single excitation frequency is used and the temperature is varied,... 

Dynamic mechanical techniques for studying polymers are described in detail in Chapter 7. For the moment we will restrict ourselves to a simple outline of the method of DMTA as it is applied to the determination of Tg. 

Experimentally DMTA is carried out on a small specimen of polymer held in a temperature-controlled chamber. The specimen is subjected to a sinusoidal mechanical loading (stress), which induces a corresponding extension (strain) in the material. The technique of DMTA essentially uses these measurements to evaluate a property known as the complex dynamic modulus, , which is resolved into two component parts, the storage modulus, E and the loss modulus, E . Mathematically these moduli are out of phase by an angle 5, the ratio of these moduli being defined as tan 5, Le. 

The numerical value of tan S varies with temperature and reaches a maximum at Tg, after which it rapidly falls, making the position of the maximum very distinct. Hence determining from a typical DMTA spectrum is fairly straightforward, as shown in the example in Figure 3.7. 

Figure 3.7 DMTA spectrum for an alkyd paint film attached to an aluminium substrate, Tg = 43.5 °C...

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Another study looked at the miscibihty of E-plastomer-polyisobutylene blends. Blends were prepared from linear E-plastomers and a polyisobutylenes in the entire composition range. Flory-Huggins interaction parameters were determined from DMTA and DSC measurements. The usual technique had to be modified in the case of DSC data, since the Tg of E-plastomers cannot be detected by this technique. The two methods yielded identical results and indicated good interaction of the components, which was supported also by a SEM study and the mechanical properties of the blends. 

Puskas, J.E. Dendritic (arborescent) pol3tisobutylene-polystyrene block copolymers DMTA analysis and swelling studies, Polym. Mater. Sci. Eng., 91, 875-876, 2004. 

Differential Scanning Calorimeter (DSC) thermograms were obtained on a Perkin Elmer DSC-2 run at 10°C per minutes. Dynamic Mechanical Thermal Analysis (DMTA) spectra were obtained on a Polymer Labs DMTA at a frequency of 1Hz with a temperature range from -150°C to +150°C at a scan rate of 5°C per minute. 

The physical properties of the acid- and ion-containing polymers are quite interesting. The storage moduli vs. temperature behavior (Figure 8) was determined by dynamic mechanical thermal analysis (DMTA) for the PS-PIBMA diblock precursor, the polystyrene diblock ionomer and the poly(styrene)-b-poly(isobutyl methacrylate-co-methacrylic acid) diblock. The last two samples were obtained by the KC>2 hydrolysis approach. It is important to note that these three curves are offset for clarity, i.e. the modulus of the precursor is not necessarily higher than the ionomer. In particular, one should note the same Tg of the polystyrene block before and after ionomer formation, and the extension of the rubbery plateau past 200°C. In contrast, flow occurred in... 

Figure 8. DMTA behavior for poly(styrene)-b-poly(isobutyl methacrylate-co-methacrylic acid) potassium salt.

The comparison of the 2D plot of a graft copolymer with the 2D plot of the precursor PEO shows clearly that the graft copolymer sample does not contain any free PEO. This result was also confirmed by MALDI-TOF mass spectrometry. Next to the requirement of being PEO free, the PEO-g-PVA copolymers showed a good combination of film-forming properties, a fast dissolution, and a low solution viscosity in water. The phase separated morphology, as demonstrated by TEM, DSC, DMTA, and WAXS experiments, provided the PEO-g-PVA copolymers with relatively constant mechanical properties. 

DMTA dynamic mechanical thermal analysis DOP dioctyl phthalate... 

Dynamic properties are more relevant than the more usual quasi-static stress-strain tests for any application where the dynamic response is important. For example, the dynamic modulus at low strain may not undergo the same proportionate change as the quasi-static tensile modulus. Dynamic properties are not measured as frequently as they should be simply because of high apparatus costs. However, the introduction of dynamic thermomechanical analysis (DMTA) has greatly widened the availability of dynamic property measurement. 

Although the apparatus is expensive, DMTA produces a lot of data in a short time and is hence a very efficient way of generating dynamic properties. Furthermore, it has the advantage of being non-destructive and uses a small test piece. 

The effect of degradation agents on low temperature behaviour must be relevant in many applications but is virtually never specifically measured. There are low temperature tests for flexible materials (ISO 458 [36] and ISO 974 [37]), but generally DMTA or impact methods might be more appropriate. 

One must remember that static methods like DSC and dynamic ones like DMTA lead to very different data for some characteristics such as glass transition. 

Thermal analysis techniques (DSC,TGA, DMTA...) operating on mini or micro samples can detect pinpoint heterogeneities in final parts that bulk analysis methods such as rheom-etry are unable to do. Transient variations of moulding parameters, local design mistakes, internal stresses that influence the properties of the final product, notably impact behaviour, dimensional stability, warpage. .. can be displayed. 

The glass transition temperature of PE by DSC measurements is -110°C. The glass transition temperatures by DMTA measurements can be higher, depending on the frequency. These results relate to some grades only and cannot be generalized. 

---