Glass transition temperature from viscosity

Since quench rates in simulations typically are artificially high, this leads to a special problem for comparison with experiment as well as to the question whether there is a more general way to determine the glass transition temperature from the structure of the system. The experimental definition of viscosity is certainly not apphcable to simulations. 

Multistep syntheses are required to manufacture such functionalized polymers from commercially available chemicals, and consequently, they are expensive. The exact control of polarity, molecular weight, glass transition temperature, and viscosity are rather difficult in such polymers, and reproducibility is a problem. The availability and variety of specifically functionalized structures is strongly limited. Moreover, the purification and the solubility of the polymers are difficult problems. Restricted solubility requires using environmentally nonfriendly organic solvents. Therefore, a number of activities to develop alternatives to glassy functionalized polymers were pursued. 

From the compiled vapor pressure and conductivity data, the evaporation enthalpy and the activation enthalpy for proton conduction were calculated as a function of composition. The critical temperature according the Vogel-Tammann-Fulcher law was determined from the viscosity data and compared with glass transition temperatures from other studies using NMR spectroscopy. A correlation between dynamic viscosity and molar conductivity was found. As expected, a considerable decoupling between ionic conduction and viscous flow can be determined from a Walden plot, which is based on proton-hopping mechanisms in phosphoric acid. 

Poly(8-caprolactone) (M =40,000) was blended with a series of ethylene terephthalate-PCL copolyesters of different composition 13 the polyesters were presumably random copolymers [123]. The copolymers, containing 18-87% caprolactone units, were prepared by copolymerisation of ethylene terephtha-late and 8-caprolactone molecular weights were not quoted but intrinsic viscosities of the copolymers were between 0.84 dl g" and 1.99 dl g" The pure copolymers exhibited composition-dependent glass-transition temperatures from -65 °C (87 wt % 8-caprolactone) to 33 °C (18 wt % 8-caprolactone). 

If we, however, consider that viscosity is inversely related to the fractional free volume, which increases from a small value at the glass transition temperature Tg linearly with temperature above this figure, then it is possible to derive an equation. 

Solid-state polycondensation (SSP) is thus a technique applied to thermoplastic polyesters to raise their molecular weight or IV. During solid-state polycondensation, the polymer is heated above the glass transition temperature and below the melt temperature of the polymer either under an inert gas or under vacuum. Increasing the intrinsic viscosity requires a residence time of up to 12 h under vacuum or under inert gas, at temperatures from 180 to 240 °C. 

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

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