Scaling near glass transition

Another important characteristic aspect of systems near the glass transition is the time-temperature superposition principle [23,34,45,46]. This simply means that suitably scaled data should all fall on one common curve independent of temperature, chain length, and time. Such generahzed functions which are, for example, known as generalized spin autocorrelation functions from spin glasses can also be defined from computer simulation of polymers. Typical quantities for instance are the autocorrelation function of the end-to-end distance or radius of gyration Rq of a polymer chain in a suitably normalized manner ... 

It has been discussed in the previous section that the long-time part in the memory function gives rise to the slow long-time tail in the dynamic structure factor. In the case of a hard-sphere system the short-time part is considered to be delta-correlated in time. In a Lennard-Jones system a Gaussian approximation is assumed for the short-time part. Near the glass transition the short-time part in a Lennard-Jones system can also be approximated by a delta correlation, since the time scale of decay of Tn(q, t) is very large compared to the Gaussian time scale. Thus the binary term can be written as... 

The mechanical dispersion peaks in low-Tg epoxies such as Bisphenol-A based resin (Epon 828, products from Shell Development Company) have been the subject of numerous studies 143,145148,152 "155, l59>. The alpha-dispersion peak related to the glass transition can undoubtly be attributed to the large-scale cooperative segmental motion of the macromolecules. The eta-relaxation near —55 °C, however, has been the subject of much controversy 146,153). One postulated origin of the dispersion peak is the crankshaft mechanism at the junction point of the network epoxies (Fig. 17). The crankshaft motion for linear macromolecules was first propos-ed 163 166> as the molecular origin for secondary relaxations which involved restricted motion of the main chain requiring at least 5 and as many as 7 bonds 167>. This kind of... 

In Fig.2 we display the incoherent density-relaxation spectra 4>", q,Li) = iO) as a function of a in a logeirithmic scale. It is interesting to note that there is an additional quasi-elastic peak in "[q,u) at small tj (o /(i)o < 10" ) near the glass transition. This additional low-frequency intensity, which has not been detected for a one-component system, grows further as rj approaches tjb- Figure 3 shows the frequency-dependent diffusion constants for the same values of ij as in Fig.2. We note that a little bump... 

Fig. 8. Relaxation of the temperature, pressure, and structure for a sample of 864 U particles quenched from near the triple point (T 0.75) to a density (po —O.OT) and temperature (7 -0.33, r 39°K) in the glass transition region, see Fig. 7 (Ref. 12). To simulate the quench, which is seen to take place over 3 to 4 psec, particle velocities are scaled to the required temperature every SO time steps (0.5 psec). Configurational relaxation—reflected in the pressure decay—takes more than 100 psec. (From J. H. R. Clarke, to be published.)...

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