Glass transition temperature shifts

The glass transition temperature shifted to lower temperatures with increasing moisture content. 

Note that subtracting an amount log a from the coordinate values along the abscissa is equivalent to dividing each of the t s by the appropriate a-p value. This means that times are represented by the reduced variable t/a in which t is expressed as a multiple or fraction of a-p which is called the shift factor. The temperature at which the master curve is constructed is an arbitrary choice, although the glass transition temperature is widely used. When some value other than Tg is used as a reference temperature, we shall designate it by the symbol To. 

By a variation of chemistry and/or chain length the different time regimes can be shifted. From a simulation point of view we are again faced by the decision what kind of information we want to get out of the simulation. If one wants to look at very local properties, depending on the local chemistry of the individual monomers, there is no way around a simulation with all chemical details. However, one should keep in mind that by such a technique it is impossible to equilibrate the system near the glass transition temperature. 

This behavior is in between that of a liquid and a solid. As an example, PDMS properties obey an Arrhenius-type temperature dependence because PDMS is far above its glass transition temperature (about — 125°C). The temperature shift factors are... 

The second consideration, anticipated in Sect. 3.1, is that whereas Sm and Sc increase little with temperature (see Fig. 8) so that in the literature the temperature dependence of Gc and Gm is often approximated by straight lines, Sl varies moderately up to the glass transition temperature Tg but will increase substantially between Tg and the isotropization temperature. The behavior of Sl is due to the fact that, as the temperature increases, polymer chains are progressively more likely to bend sharply in the melt, whereas they are forced to remain straight in the mesophase and in crystals. The downward curvature of Gl in Fig. 8 shifts to lower temperatures with inherently more flexible chains. With very flexible polymers Tml becomes therefore smaller than the crystal-melting temperature Tcl and a stable mesophase cannot form. 

The glass transition temperature is generally measured- by experiments that correspond to a time scale of seconds or minutes. If the experiments are done more rapidly, so that the time scale is shortened, the apparent Tg value is raised. If the time scale is lengthened to hours or days, the apparent Tg value is lowered. Thus, as generally measured, Tg is not a hue constant but shifts with the time scale of the experiment or observation. Moreover, Tg is masked by experimental difficulties, compounded by multiple and often inaccurate definitions of Tg in the literature. The least... 

Plasticizer and Copolymerization change the glass transition temperature as discussed in Chapter 1. Plasticixers have little effect on Copolymerization can change although less strongly than 7 x. As a result, the basic modulus-temperature and modulus-time curves are shifted as shown in Figure 8 for different compositions. The shift in the modulus-temperature curve is essentially the same as the shift in TK. The shift in the modulus-time curve includes this plus the effect of any change in ()jr... 

The method of relating the horizontal shifts along the log time scale to temperature changes as developed by Williams, Landel, and Ferry from equation (24) is known as the WI.F method. The amount of horizontal slut of (he log time scale is givvn by log a,-. If the glass transition temperature is chosen as the reference temperature, the temperature dependence ni the shift lactoi lor most amorphous polymers is... 

Several attempts have been made to superimpose creep and stress-relaxation data obtained at different temperatures on styrcne-butadiene-styrene block polymers. Shen and Kaelble (258) found that Williams-Landel-Ferry (WLF) (27) shift factors held around each of the glass transition temperatures of the polystyrene and the poly butadiene, but at intermediate temperatures a different type of shift factor had to be used to make a master curve. However, on very similar block polymers, Lim et ai. (25 )) found that a WLF shift factor held only below 15°C in the region between the glass transitions, and at higher temperatures an Arrhenius type of shift factor held. The reason for this difference in the shift factors is not known. Master curves have been made from creep and stress-relaxation data on partially miscible graft polymers of poly(ethyl acrylate) and poly(mcthyl methacrylate) (260). WLF shift factors held approximately, but the master curves covered 20 to 25 decades of time rather than the 10 to 15 decades for normal one-phase polymers. 

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

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