Glassy polymers aging

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

Membranes from Glassy Polymers Physical Aging 69... 

Figure4.4 Effect of thickness on aging rate of glassy polymer films determined by change in oxygen permeability at 35 °C [46]. Reproduced with permission of Elsevier.

Figure4.6 Normalized refractive indices forthin films ( - 100 nm) of three glassy polymers as a function of aging time [38]. Reproduced with permission of the American Chemical Society.

The experimental results described above show that the gas-permeability properties of thin glassy polymer films (submicrometer in thickness) are more time- or history-dependent than much thicker films (the bulk state for example, 50 pm or thicker) seem to be. This is manifested in terms of physical aging over a period of 1 year and more. The observed permeability values for the current thin films are all initially greater than the reported bulk values but approach or become less than these values after a few days or weeks, depending on the thickness. After a year, the thin films may be as much as four times less permeable than the reported bulk values. Selectivity increases with aging time, as might be expected from a densification process. 

Thermal annealing at 140 °C in an inert dark atmosphere resulted in decreases in strength, ductility, and toughness, as seen in Fig. 3 and Table 1. These changes are attributed to physical aging processes occuring in the glassy polymer. As an additional... 

Hutchinson JM "Relaxation Processes and Physical Ageing", in Haward RN and Young RS (Eds) "The Physics of Glassy Polymers", Chapter 3, Chapmann and Hall, London, 2nd Ed, 1997. 

Here J is a constant, and T2 Tg — 50 K. The above equation is not valid for T < Tg and has an apparent singularity in x as T - T2. This basically prevents us from following this line of thought in order to determine the low temperature structural relaxation and physical aging in glassy polymers. 

Fig. 17. Calculated dependence of the physical aging rate on temperature for three glassy polymers...

We have reviewed the recent development of a nonequilibrium statistical mechanical theory of polymeric glasses, and have provided a unified account of the structural relaxation, physical aging, and deformation kinetics of glassy polymers, compatible blends, and particulate composites. The specific conclusions are as follows ... 

Rueda et al. (1995), have followed the physical ageing of another glassy polymer. They investigated the variation in microhardness of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) stored in an ambient atmosphere for different times and after annealing at different temperatures below Tg for different periods of time. This material has stiffer molecules than PET due to the presence of a naphthalene ring instead of the benzene ring in the backbone chain. 

J. G. Victor and J. M. Torkelson, Photochromic and fluorescent probe studies in glassy polymer matrices. 3. Effects of physical aging and molar weight on the size distribution of local free volume in polystyrene, Macromolecules 21, 3490-3497 (1988). 

Physical Aging of Glassy Polymers Effects of Subsidiary Relaxation Processes... 

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