Glass transition, interval

Fig. lib. Change of the thermal expansion coefficient /3th at the glass transition interval (b)... 

In some cases dissolution without a gel layer is found, especially at low temperatures. It appears that dissolution by stress cracking is the cause of this phenomenon. Cracks are observed which run into the polymer matrix and combine to form small blocks of the polymer, which leave the surface in a kind of eruption process. Large amounts of stored stress energy, frozen in the glass transition interval and concentrated along the wider... 

A comparison of neutron spectra with theory was made by Danner et al. (7). These workers obtained data for Marlex 6050 at temperatures below and above the glass-transition interval and the melting point, and for samples of branched, irradiated, and quenched polyethylene at room temperature. The spectrum at 100 K (Fig. 3) showed two peaks with shapes characteristic of acoustic modes at 550 and 200 cm" (peaks C and E in Fig. 3). Five additional peaks were observed at 1360,750,340,... 

It is to be noted that is quite differently from N T) in Eq. 9), if the glass attains more than its uilitaium number of conformations. Here is a parameter to describe various frozen in states on cooling a liquid through its glass transition interval. 

Change in heat capacity in glass transition interval, AC is average value. Molar cohesive energy as calculated from Ref. (J62). 

The absolute need for a distribution of relaxation times can be illustrated by a more complicated thermal history. The sample is annealed to equilibrium above the glass-transition interval and then quenched rapidly to a temperature deep within the glass-transition region. It is isothermally annealed until its volume achieves a value consistent with a glass with a fictive temperature in the middle of the glass-transition interval. The partially annealed glass is then rapidly heated to this fictive temperature, where the sample achieves a volume exactly equal to its equilibrium value. The volume then increases spontaneously to a maximum value and finally relaxes back to its equilibrium volume. This trajectory is called the "memory effect" and is shown in Figure 8.2. 

Figure 6.39 The dependence of the glass transition interval width AT = T Tg on the curing agent oligomer ratio K for epoxy polymers EP-1 (1) and EP-3 (2) [94]...

Obvioudy, tte presence of a loose temperature glass transition interval can be explained by tte fact that various mechanisms participate in the process of glass transition. Of interest from this point of vfew is the examination of the effect of fillers on such relaxation processes in which sufficiently large structural elements take part. Those phenomena were studied (33. 45-47) by calculating the average relaxatirai time proceeding from the data on the isothermic contraction of the volume of various filled systems by the method proposed in (48). 

Glass transition temperature (Tg) measurements were carried out on a Perkin Elmer DSC2 differential scanning calorimeter. A heating rate of 10°C/min was used with the Tg being taken as the midpoint of the temperature interval over which the discontinuity took place (75). 

Analysis of flow curves of these polymers has shown that for a nematic polymer XII in a LC state steady flow is observed in a broad temperature interval up to the glass transition temperature. A smectic polymer XI flows only in a very narrow temperature interval (118-121 °C) close to the Tcl. The difference in rheological behaviour of these polymers is most nearly disclosed when considering temperature dependences of their melt viscosities at various shear rates (Fig. 20). 

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. 

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