Glass transition specific heat measurement

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

In agreement with the correlations discussed above we see once again that the glass transition as detected by the specific heat measurements occurs at a lower temperature than Te as detected by measurements having a much shorter time scale. Although the molecular frequencies that determine the specified heat are far greater... 

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

The glass-transition temperatures and the corresponding specific heats were measured three times for each sample in order to enable the calculation of the standard deviations, which were in the range of 3% or lower. Apparently, the kind of substituent greatly influences the Tg values, and rigid substituents (phenyl or methyl) or flexible substituents (ethyl or nonyl) cause an increase or decrease in corresponding Tg values, respectively. The measured Tg values are plotted in Fig. 19. 

Frequency-Dependent Specific Heat. We mention measurements of volume relaxation through the frequency-dependent specific heat Cn(co) as in fluids near the glass transition [52]. This is feasible when the experimental frequency co is of the order fi0 in small gels. The deviations of the entropy, temperature, and volume are related by SS = CV5T + (dH/dT)v8Vand the relaxation equation reads... 

Perhaps the best comparison is that of Alford and Dole (1955) who measured the specific heat of a sample of polyvinyl chloride from the same source as the polyvinyl chloride used by Fuoss (1941) in his extensive dielectric loss studies. The comparison between c and e", the dielectric loss factor measured at 60 cps, is shown in Fig. 15, and covers the glass transition range. Note that the peak of the dielectric loss curve is at 100° C, about 20 degrees higher than the inflection point of the c — T curve. 

In DSC the measured energy differential corresponds to the heat content (enthalpy) or the specific heat of the sample. DSC is often used in conjunction with TA to determine if a reaction is endothermic, such as melting, vaporization and sublimation, or exothermic, such as oxidative degradation. It is also used to determine the glass transition temperature of polymers. Liquids and solids can be analyzed by both methods of thermal analysis. The sample size is usually limited to 10-20 mg. 

While the dilatometer method is the preferred method of determining the glass transition temperature, it is a rather tedious experimental procedure and measurements of Tg are often made in a differential scanning calorimeter (DSC). In this instrument (18), the heat flow into or out of a small (10-20 mg) sample is measured as the sample is subjected to a programmed linear temperature increase (typically 10 C/min). The heat flow is proportional to the specific heat of the sample. At the glass transition, there is an increase in the heat flow into the sample due to the increase in specific heat at this point. Values obtained in this manner are only a few degrees higher than the dilatometer values. 

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