Subject physical ageing

One might anticipate that, in favorable circumstances, the physical age determined by this procedure would equal the chronological age of the sample, that is, the time elapsed since the textile originated would equal the time elapsed since the Tg of the sample was exceeded. However, in determining the physical age of fibers and claiming their chronological age to be the same, one must assume that the fibers have been subjected to a fairly constant temperature between Tp and Tg and that no other conditions have erased their physical aging. 

Large specimens of amorphous polymers that can be subjected to large loads and can produce large displacements in creep generally have been the subject in creep studies of physical aging. Electronic sensing of displacements with sensitive oscillators and amplifiers have been employed in measuring displacements when loads of 1-50 kg were applied. 

Figure 3.1. Schematic illustration of temperature dependences of the specific volumes of amorphous materials. This figure also illustrates the effects of the nonequilibrium nature of glass structure, which results from kinetic factors. Glass 1 and Glass 2 are specimens of the same polymer, but subjected to different thermal histories. For example, Glass 1 may have been quenched from the melt very rapidly, while Glass 2 may either have been cooled slowly or subjected to volumetric relaxation via annealing ( physical aging ) in the glassy state.

To demonstrate the "Thermoreversibility of physical aging ( ), the follov/ing requenching procedure was csr ried out. Some of the specimens aged for 10 rain, were heated to 260°C, v/hich is above the epoxy Tg, for 20 min. and then air-quenched to room temperature inside a desiccator. Five of these requenched specimens were tested immediately, while the rest were subjected to "re-aging" at 1402c in.nitrogen for the time increments of 10, 10 , 10, 10, and up to lO min. At least five specimens were tested for each decade of re-aging time. 

As stated in a preceding section, physical aging starts when a polymer is quenched to a temperature below its T, irrespective of the mechanical loading on the material. Therefore, when a sub-Tg polymer is subjected to mechanical load, two processes occur simultaneously (1) physical aging and (2) stress relaxation or creep associated with mechanical load or deformation. Each of these processes has its own unique time and temperature dependence and hence, both effects must be accounted for in the long... 

WAXS measurement of crystallinity involves taking the ratio of the areas under the individual diffraction peaks to the total area under the peaks. However, it implies that the amorphous polymer should be devoid of significant order and this has been the subject of much speculation. Pechhold and Grossman invoked a cubic mesophase for the parallel packing of chains in the melt, justifying it by a cluster-entropy hypothesis. Nodular structures, of 5—10 nm in diameter, have been observed particularly in physical aged glasses (these have been reviewed by... 

Time-dependent creep can be accurately modelled using the viscoelastic theory, which inherently assumes that all deformation is eventually recovered. However, when considering the long-term deformational behaviour of polymers it is important to realise that all polymers are subject to physical ageing, which not only affects the polymer s stiffness but has a profound influence on its creep deformation. Physical ageing of the matrix material should therefore be considered in order to make the investigation of the delayed failure of the composite meaningful. 

Golovchak R., Jain H., Shpotyuk 0., Kozdras A., Saiter A., and Saiter J. M., Experimental verification of the reversibility window concept in binary As-Se glasses subjected to a long-term physical aging, Phys. Rev. B, 78, 014202-6 (2008). 

Guo, Y., Bradshaw, R.D. Long-term creep of polyphenylene sulfide (pps) subjected to complex thermal histories The effects of nonisothermal physical aging. Polymer 50, 4048 (2009)... 

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