Isotherms physical aging

Fig. 7. DSC scans of heat capacity vs temperature for poly(ether imide) after isothermal physical aging at 201.3°C for the various aging times indicated. Scans were made on heating at 10°C/min. The data were corrected to agree with absolute Cp measurements in the liquid state. Data are replotted from Ref 23.

Guo, Y. and Bradshaw, R.D. Isothermal physical aging characterization of polyether-ether-ketone (PEEK) and polyphenylene sulfide (PPS) films by creep and stress relaxation, Mechanics of Time-Dependent Materials, 2007, 11(1), p. 61-89. 

Wang, N., Isothermal Physical Aging Characterization and Test Methodology for Neat and Nanotube-Reinforced Poly(Methyl Methacrylate) (PMMA) Near the Glass Transition Temperature, MS Thesis, University of Louisville, 2007... 

Struik has introduced an exponent p to characterize the physical aging observed in his isothermal creep experiments [2] ... 

Various thermal and rubbing processes were imposed on the test samples, and their effects on the RIB relaxation behaviors of the samples were studied by using the following standard tests. The first was to measure the normalize birefiingence (NB) as a function of temperature when the sample temperature was raised continuously at 2 K/min. The results are referred to as the Continuous Curves (7). The second was the isothermal relaxation oiNB, i. e., the NB of a sample was measured as a function of time at a fixed temperature. Note that the isothermal relaxation process also served as physical aging as both are expected to take place. The third was the combination of the two, i. e., a continuous temperature increase at 2 K/min after an isothermal process at a given temperature for a given period of time. 

There are two TGA techniques to determine the degradation temperature of a plastic. The first is called the isothermal method and is based on setting the sample on a temperature that is between 50 and 150C above its melting temperature and letting the instrument measure the weight loss with time. This is one technique used to determine physical aging of a sample under extreme conditions. A typical decomposition curve is shown in Fig. 6. 

Fig. 7.27 Illustration of the concept of physical aging for PMMA. Material is first rejuvenated above Tg then quenched to 15°C below Tg for isothermal aging. Creep compliance curves are obtained at each aging time and these can be shifted as shown to provide a momentary master curve. A Kohlrausch fitting function is also shown through the shifted data. (Data from Wang (2007).)...

Due to the universality in all glasses, physical aging can be theoretically explained in a straightforward way based on the free-volume concept. As proposed by Struik, This is the basic and rather obvious idea that the transport mobility of particles in a closely packed system is primarily determined by the degree of packing of the system or by its inverse measure, viz. the free volume [2]. The idea could date back to 1943 when Alfrey et al. proposed that the isothermal aging below Tg can be attributed to the diffusion of free volume holes from the interior of polymers into the surface [34]. This free volume diffusion model (FVDM) was developed by Curro et al. [35] to quantitatively analyze the volume relaxation experiments of poly(vinyl acetate) [36, 37]. The motion of free volume holes can be described by a diffusion equation ... 

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