Polystyrene Secondary relaxations

In contrast to polystyrene the observed intercepts for PMMA and PEMA in the glassy state remain high with values that are a substantial fraction of those observed in the equilibrium liquid state. Such a result should not be too surprising since it was shown above that a large part of the observed relaxation function above Tg was due to the secondary relaxation. The frequency of maximum dielectric or mechanical loss for the /9... 

TABLE 13.5. Glass-transitlon and secondary-relaxation temperatures of polystyrenes. 

Stress induced thermal events have been produced in all of the amorphous polymeric glasses examined to date. Surprisingly, these processes are always observable in a narrow temperature range above the ambient temperature, independent of the chemical structure of the polymer. Therefore, these effects are not, as might be expected, associated with the materials sub Tg secondary relaxations. The evidence for this assertion is presented in Figure 7 which shows the response of a variety of similarly stressed polymers approximately one hundred days after compaction. From top to bottom, in order of their respective glass transition temperatures are polystyrene (MW=37,000) ... 

Figure 13.8 Storage and loss moduli G oi)loP- and G"(

Some secondary relaxations of the components in thermoplastic AIPNs have been investigated by thermally stimulated depolarization current (TSDC) techniques and thermally stimulated conductivity (TSC) measurements [10,11]. It was found that upon addition of S-co-AA to CPU, the secondary and 3 CPU peaks (at ca. -140 °C and ca. -100 °C, respectively) shift slightly to lower temperatures, i.e., the corresponding relaxations become faster, these shifts being more pronounced at low S-co-AA contents. The shifts can be related to physical interactions between the IPN components and to their partial miscibility. Rizos et al. [15] have shown that as a result of such interactions, changes in the local free volume may occur, affecting the secondary relaxation times. The same changes in the [3 relaxation of PU have been found in polyurethane/polystyrene IPNs by Pandit and Nadkarni [16]. 

This general question of observing secondary transitions by thermal means was considered by one of us (RFB) on a prior occasion. We came to the conclusion, from existing litraature data, that Cp - T plots exhibited not a discontinuity in Cp but a change in slope at these secondary relaxations specifically, at T < Tg for atactic polystyrene and PVC, and at Tn for several elastomers. In each case precision measmements of heat content, AH, were available. A change in slope of a - r curve is a discontinuity in d HIdT, and hence a third order transition in the sense of Ehrenfest.22... 

This chapter discusses the dynamic mechanical properties of polystyrene, styrene copolymers, rubber-modified polystyrene and rubber-modified styrene copolymers. In polystyrene, the experimental relaxation spectrum and its probable molecular origins are reviewed further the effects on the relaxations caused by polymer structure (e.g. tacticity, molecular weight, substituents and crosslinking) and additives (e.g. plasticizers, antioxidants, UV stabilizers, flame retardants and colorants) are assessed. The main relaxation behaviour of styrene copolymers is presented and some of the effects of random copolymerization on secondary mechanical relaxation processes are illustrated on styrene-co-acrylonitrile and styrene-co-methacrylic acid. Finally, in rubber-modified polystyrene and styrene copolymers, it is shown how dynamic mechanical spectroscopy can help in the characterization of rubber phase morphology through the analysis of its main relaxation loss peak. 

Of the secondary factors to be considered in a study of this nature, a fundamental variable is the molecular weight. Detailed studies ( ) of relaxation rates for nearly monodisparse atactic polystyrenes (FS) of mrious molecular weights ranging from... 

It is tempting to relate the temperature at which the ductile-brittle transition takes place to either the glass transition or secondary transitions (Section 5.2.6) occurring within the polymer. In some polymers such as natural rubber or polystyrene Tb and Tg occur at approximately the same temperature. Many other polymers are ductile below the glass transition temperature (i.e. Tb < Tg). In this case it is sometimes possible to relate T to the occurrence of secondary low-temperature relaxations. However, more extensive investigations have shown that there is no general correlation between the brittle-ductile transition and molecular relaxations. This may not be too unexpected since these relaxations are detected at low strains whereas Tb is measured at high strains and depends upon factors such as the presence of notches which do not affect molecular relaxations. 

---