Modulus, dynamic mechanical, glassy

Figure 1. Typical dynamic mechanical modulus and loss tangent data as a junction of temperature. Key a, glassy region b, transition region and c, rubbery...

Plasticization, whose main manifestation is the decrease of the glass transition temperature (a transition in dynamic mechanical spectra), is generally accompanied by an increase of the glassy modulus in the temperature interval between Tp and T. , an effect is known as antiplasticization. 

It is clear that the determination of such a modulus temperature curve takes an awful lot of time. Moreover, the transitions in the glassy region are difficult to determine, because the time needed for such a transition will be very small it may be of the order of or even much faster than the time in practice to apply an instantaneous deformation. For that reason in general use is made of dynamic mechanical measurements as a function of frequency to elucidate the modulus temperature curves, in particular in the glassy region. An additional advantage is that elastic and viscous forces are separated in this kind of measurements. 

Dynamic thermal mechanical analysis indicates significant broadening of glass transition temperature at or around PU/EP composition of 70/30. This is true for all three groups studied. At this composition, storage modulus showed much less steep variations with temperature during the transition from glassy to rubbery state. 

The glass transition temperature, Tg, can also be calculated from the stress-temperature profile. It can be taken as the intersection of two lines drawn from the linear regions of the glassy and rubbery regimes, in a manner similar to Tg determination from TMA. Here, however, the measured Tg corresponds more with those derived from dynamic mechanical methods because of the strong dependence of stress on the modulus transition between the rubbery and glassy states. Other ways to measure Tg would be to take the derivative of the a(T) curve to obtain Ea(T) the Tg could then be determined as either the inflection point or the midway point in the transition between glassy Ea and rubbery Ea. 

Previous studies of ionomers have shown that dynamic mechanical tests are most sensitive to the presence of large clusters. For this reason, dynamic mechanical test was performed using a computerized torsion pendulum (28). The frequencies varied from ca. 3 Hz for the glassy region to ca. 0.1 Hz for the low modulus region. The heating rate was usually 0.6°C/min with a temperature control of 1°C. 

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. 

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