Glass transition regions

The transition from the glass to the rubberlike state is an important feature of polymer behavior, marking as it does a region where dramatic changes in the physical properties, such as hardness and elasticity, are observed. The changes are completely reversible, however, and the transition from a glass to a rubber is a function of molecular motion, not polyma- structure. In the rubberhke state or in the melt, the chains are in relatively rapid motion, but as the temperature is lowered, the movement 

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Fig. 19. Generalized modulus—temperature curves for polymeric materials showing the high modulus glassy state, glass-transition regions for cured and uncured polymers, plateau regions for cross-linked polymers, and the dropoff in modulus for a linear polymer.

Fig. 5. Typical DSC-traces for the specific-heat jumps at the glass transition regions of iron-epoxy particulates, or E-glass fiber-epoxy composites and the mode of evaluation of ACp s...

Glass transition temperature (Tg) for pure NR is —63.43°C, while for the nanocomposite it increases to —61.92°C. NR-rectorite nanocomposite shows a higher glass transition temperature, lower tan 8 peak, and slightly broader glass transition region compared to pure NR. 

Fig. 3. Schematic of Chambon-Winter gel spectrum. The longest relaxation time diverges to infinity. The relaxation time X0 marks the crossover to the short-time behavior, which depends on the material. The depicted case corresponds to a low-molecular-weight precursor (crossover to glass transition region)...

Overall Diffusion Control in the Glass Transition Region... 

The temperature range of T5 to Ti is referred as the glass transition region. The calorimetric glass transition temperature (Tg) may be defined as the temperature... 

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. 

These differences in modulus may be at least partially explained by DSC data such as that in Fig. 10. It is seen that in general the glass transition regions of the ATBN-and CTBN-modified epoxies are broader and have a lower midpoint than those of the control and two siloxane-modified materials. This thermal data suggests that the butadiene oligomers are relatively more miscible with the epoxy and may act as plasticizers. As an additional point, it is likely that the higher molecular weight of the... 

Fig. 5. Schematic of the heat capacity on cooling and heating at largely different rates in the glass transition region. Top Curves Cooling at 5.0 K/min, heating at 0.25 K/min. Bottom Curves Cooling at 5.0 K/min, heating at 150 K/min. (Hypothetical polymer, time dependence as in polystyrene)...

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