Polymers enthalpy relaxation

Chung, H.-J., Lee, E.-J., and Lim, S.-T. (2002). Comparison in glass transition and enthalpy relaxation between native and gelatinized rice starches. Carbohydr. Polym. 48, 287-298. 

Chung, H. -J., Woo, K. -S., Lim, S. -T. (2004). Glass transition and enthalpy relaxation of eross-linked corn starches. Carbohydr. Polym., 55, 9-15. 

Hodge, I. M., and Huvard, G. S. (1983). Effects of annealing and prior history on enthalpy relaxation in glassy polymers. 3. Experimental and modeling studies of polystyrene. Macromolecules 16(3), 371-375. 

DSC has been amply demonstrated in the pat to be a useful instrument to follow the kinetics of enthalpy relaxation in polymers 41,42,74>. Figure 21 shows the DSC... 

Very important phenomena in polymer behaviour, such as viscoelasticity, stress, strain, volume and enthalpy relaxation, ageing, etc., are characterised by time-dependence of the polymer properties. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

Hutchinson, J.M., McCarthy, D., Montserrat, S., and Cortes, P. Enthalpy relaxation in a partially cured epoxy resin, /. Polym. Sci. Part B Polym. Phys., 34, 229, 1996. 

Kim, Y.J., Hagiwara, T., Kawai, K., Suzuki, T., and Takai, R. Kinetic process of enthalpy relaxation of glassy starch and effect of physical aging upon its water vapor permeability property, Carbohydr. Polym., 53, 289, 2003. 

Thus, the rate of enthalpy relaxation for each polymer can be expressed in terms of temperature and excess enthalpy according to a relationship of the form described previously ( ), l.e.. 

These data concerning the molecular-structure dependency of the enthalpy-relaxation process suggest that the f u tors that influence the onset of segmental mobility influence enthalpy relaxation, but that the presence of mobile groups in the main chain or the ciystal-ordering tendency of polymers does not seem to have a significant effect on the rate of enthalpy relaxation. 

The DSC is widely used to measure the glass-rubber transition temperature (Tg-value), which is an important parameter for polymer characterisation. The Tg-value represents the temperature region at which the (amorphous phase) of a polymer is transformed from a brittle, glassy material into a tough rubberlike liquid. This effect is accompanied by a step-wise increase of the DSC heat flow/temperature or specific heat/ temperature curve. Enthalpy relaxation effects can hamper the... 

Enthalpy relaxation studies have also been used to assess the aging of polyether ether ketone blends with polyetherimide, PEEJC/PEl = 50/50 [Hay, 1992]. The preparation of the blend produced an amorphous system with Tg 215°C, but crystallization of the PEEK occurred after raising the temperature above Tg. Enthalpic relaxation could only be observed in the temperature range Tg to (Tg - 50) and no aging could be detected at temperatures below 150°C (Table 14.15). The system was analyzed using the KWW Eq 14.7 that yielded values of P = 0.4, intermediate between those of the component polymers. 

Differential Scaling Calorimetry. The thermal analysis technique of differential scanning calorimetry has been amply demonstrated in the past to be a useful method to follow the kinetios of enthalpy relaxation in polymers (4,6, 1 ,11). Figure 2 shows the DSC scans of the fulTy-cros inked epoxy speoimens... 

Drozdov, A. D., Volume and enthalpy relaxation in glassy polymers, J. Non-Equilibrium Thermodynam., 24, 203-2X1 (1999). 

Agrawal, A., Effect of temperature and molecular weight on enthalpy relaxation in polystyrene, J. Polym. Sci. B, 27, 1449 1461 (1989). 

Cowie, J. M. G., and Ferguson, R., The ageing of poly(vinyl methyl ether) as determined from enthalpy relaxation measurements, Polym. Commim., 27, 258-260 (1986). 

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