Macroscopic Characterization of Polymer Films

Thermal characterization of an emulsion polymer essentially means the measurement of the glass transition temperature Tg, that is the temperature above which the hard, glass-like polymer film becomes viscous or rubber-like. Polymers whose Tg lies well above room temperature are designated as hard , those with a Tg much lower than room temperature as soft . Normally Tg is measured by differential scanning calorimetry (DSC [25]). In this technique, the difference between the heat absorbed per unit time by the polymer film to that absorbed by a thermally inert reference material is recorded during a linear temperature ramp. The sample and the reference are placed on a sensor plate of defined thermal resistance R, and the temperature difference AT between the sample and the reference is then recorded over the temperature ramp. Usually, the heat flow difference, which is the negative quotient of AT and R, is plotted as a function of temperature (Fig. 3-11). 

heat capacity difference of the polymer in the temperature regions below and above onset 3rid 7 pgg, different definitions of the melting point AHs, enthalpy of melting). 

Melting processes are uncommon in the emulsion polymers described in this book. Exceptions are the melting and crystallization phenomena observed with ethylene oxide chains when highly ethoxylated emulsifiers or protective coUoids are employed in the polymerization process. 

Tg values of several important homopolymers are hsted in reference [2], The values were determined on samples of non-crosslinked emulsion polymers. In crosslinked polymers, Tg is shifted to higher temperatures as a result of the restricted chain mobility. A number of approximations for calculating the Tg of copolymers have been proposed in the literature [26], The Gordon-Taylor equation usually produces reliable results  

Here mj and m2 are the mass fractions of the monomers 1 and 2 and oris defined as with Aj8 the difference in the coefficient of expansion of the molten 

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