Methods of Measuring Transitions in Polymers

There are two ways of characterizing polymers via dUatometry. The most 

Dilatometric data agree well with modulus-temperature studies, especially if the heating rates and/or length of times between measurements are controlled. (Raising the temperature l°C/min roughly corresponds to a 10 s mechanical measurement.) Besides being a direct measure of Tg, dilatometric studies provide free volume information, of use in theoretical studies of the glass transition phenomenon (see Section 8.6.1). 

Volume-temperature Polymer confined by mercury (home made) 

Torsional braid analysis TBA (Plastics Analysis Instruments) 

Two closely related methods dominate the field—the older method, differential thermal analysis (DTA), and the newer method, differential scanning calorimetry (DSC). Both methods yield peaks relating to endothermic and exothermic transitions and show changes in heat capacity. The DSC method also yields quantitative information relating to the enthalpic changes in the polymer (26-28) (see Thble 8.5). 

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The GC route is particularly attractive for it requires no a priori information on the polymer. With the exception of X-ray measurements, most methods of measurement involve a comparison of some property of the polymer, such as density, with that of the totally amorphous or crystalline material. Furthermore neither the mass of polymer in the column nor the flow rate of carrier gas need to be measured since a ratio of retention volumes is computed in Eq. (21). It should be added, however, ttiat for the successful application of the method it is essential that the measured retention volumes correspond effectively to equilibrium bulk sorption, both above and below. Low molecular weight compounds are known to exhibit apparently similar discontinuities in retention diagrams at their melting points but this is to be ascribed to a change in retention mechanism, from surface adsorption for the solid to bulk sorption for the liquid stationary phase. For a detailed discussion of retention characteristics of low molecular weight substances near their transition temperatures the reader is referred to a recent review by McCrea (8J). 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

Generally, it is possible to obtain data from broad-band NMR spectroscopy that compares favorably with dynamic-mechanical measurements of polymers. The two types of data, NMR and dynamic-mechanical, complement each other in that, for some physical or chemical changes that occur in the polymer, both methods check each other or, alternatively, one method gives results which the other method cannot give. For example, both amorphous and crystalline phase changes can be detected by NMR and dynamic-mechanical measurements. Some crystalline melt transitions cannot be detected by NMR. In addition some secondary amorphous transitions in polymers cannot be detected by NMR. On the other hand, NMR is a more sensitive technique for determining side chain ro-... 

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