Breadth of the glass transition region

Glass transition temperatures were determined using a Perkin-Elmer DSC-II or DSC-IV at a heating rate of 10 °C/min. Several scans were run at 20 °C/min on a DSC-IV to gain information on the breadth of the glass transition region. 

During heating or cooling, the breadth of the glass transition region within the framework of Cooper-Gupta model satisfies the constraint [100,101]... 

To date, many supposedly miscible polymer pairs [5,6,13,14,42] have been reported in the literature. However, in some cases [13,14], the breadth of the glass transition region, ATg, taken as the difference between the onset and completion temperatures, is quite broad. For some blend systems, ATg values approach 100°C [13,14]. The transition region may also be... 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

The glass transition is accompanied by a precipitous decrease in the modulus of several decades, as indicated in Figure 4-1. The breadth of this transition region ranges from 5 °C to more than 20 °C, depending on the nature of the polymer in question. 

Besides T, the breadth of the ttansition region, and ACp, partial crystallization affects also the hysteresis behavior (enthalpy relaxation) of the amorphous fraction. Figure 6.129 in its bottom curve depicts a typical enthalpy relaxation in amorphous PET that can be used to study the thermal history, as outlined for polystyrene in Figs. 4.125-127. The DSC curves for the samples with increasing crystallinity show that the hysteresis disappears faster than the step in ACp at the glass transition. The top sample in Fig. 6.129 of 31% crystallinity shows a smaller change in heat capacity than expected for an amorphous content of 69%, due to some rigid-amorphous fraction, but the hysteresis peak seems to have disappeared completely. 

Measurements section for further DSC details). The onset to the ass tran on (Tonei) and the end of the transition region (Tnd) were also assessed for the blmd system in order to determine values of glass tranation breadth (Tcad-Toaei). Denaty measurements were made at 23 C using a pycnometer manufoctured by hficromeritics (Model AccuPyc 1330). 

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