Secondary transitions polystyrene

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. 

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... 

Other transition metal compounds able to catalyze the selective oxidation of secondary alcohols include VO(acac)2 with t-BuOOH as secondary oxidant,40 a polystyrene-supported (catecholato)oxorhenium complex in the presence of DMSO 41 and a mixture of ferric nitrate and ferric bromate that catalyzes the oxidation of secondary alcohols with air.42... 

Further, data of Nishi and Kwei (68) show that the LCST for the polystyrene-poly(vinyl methyl ether) system is constant to within 10°C when the polystyrene Mw lies between 50,000 and 1,000,000. This result again suggests that entropic contributions to the phase transition are of secondary importance when the component molecular weights are high and that the phase instability at LCST is governed by enthalpic consider-... 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Monocyclopentadienyl complexes of titaninm (Cp TtXs) perform poorly as catalysts for ethylene or propylene polymerization, bnt in the presence of MAO, they polymerize styrene to stereo- and regioregnlar syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a Ti complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

Secondary spectral features such as shake up satellites are frequently observed for polymeric materials containing unsaturated hydrocarbons. Theoretical studies have shown that these are associated with ti-ti transitions in aromatic materials. The shake up peak is clearly visible in the spectra of an oligomeric polyalkylthiophene that is being subjected to attack by and polystyrene (Figure 9.15). It should be noted that the difference between the binding energy for aliphatic carbons in the backbone of polystyrene and the... 

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) ... 

---