Temperature-modulated calorimetry frequency dependence

Temperature-Modulated Calorimetry of the Frequency Dependence of the Glass Transition of Poly(ethylene terephthalate) and Polystyrene... 

B. Wunderlich and 1. Okazaki, Temperature-Modulated Calorimetry of the Frequency Dependence of the Glass Transition of Poly(Ethylene Terephthalate) and Polystyrene. In M.R. Taut and A.l. Hhl, Eds. Structure and Properties of Glassy Polymers, ACS Symposium Series 710, Am. Chem. Soc., Washington, DC (1998) 103 116. 

This is the reason why frequency-dependent measurements are of increasing interest, particularly in polymer science. A drawback of temperature-modulated calorimetry is the rather unclear complex frequency-dependent function and the theory behind it. 

The temperature-modulated mode of operation has been well known for many decades in calorimetry [33], but became well established only during the 1990s, when commercial DSC was modified this way [34], The idea is to examine the behavior of the sample for periodic rather than for isothermal or constant-heating-rate temperature changes. In this way it is possible to obtain information on time-dependent processes within the sample that result in a time-dependent generalized (excess) heat capacity function or, equivalently, in a complex frequency-dependent quantity. Similar complex quantities (electric susceptibility, Young s modulus) are known from other dynamic (dielectric or mechanical) measurement methods. They are widely u.sed to investigate, say, relaxation processes of the material. 

The linear dependence on ks is a strong advantage compared to the force-modulation SFM. Due to the inertia of the cantilever mass at high frequencies, the tip cannot follow completely the displacement of the sample. This results in smaller elastic deformations and low forces in the pN range, which are measured by accelerated mass md2zldt2. Recently, the SLAM technique has been advanced toward variable temperature experiments [138]. Before this development, temperature ramps have been used to perform local calorimetry [139]. 

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