Thermal stress, physical

In addition to high permselectivity, the membrane must have low-elec trical resistance. That means it is conductive to counterions and does not unduly restrict their passage. Physical and chemical stabihty are also required. Membranes must be mechanically strong and robust, they must not swell or shrink appreciably as ionic strength changes, and they must not wrinkle or delorm under thermal stress. In the course of normal use, membranes may be expec ted to encounter the gamut of pH, so they should be stable from 0 < pH < 14 and in the presence of oxidants. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

Note The organism recovered from production environments may be highly stressed due to physical factors, contact with chemicals, and thermal stress. It may be difficult to obtain typical biochemical reactions with these isolates. The databases for commercial test kits and ID systems are often designed for clinical isolates and may be incomplete with regard to industrial isolates. Thus, interpretation of such microbial data requires experienced judgment. 

Thermal stresses are more difficult to imagine than physical stresses, but we can observe what causes them anytime we look at a liquid thermometer. When materi-... 

We also see thick and thin glass in the laboratory. Because their concave bottoms could not otherwise withstand the force of a vacuum, filter flasks are made of thick glass. However, do not place a filter flask on a heating plate—it cannot tolerate the (heat) stress. The standard Erlenmeyer, by comparison, is thin-walled, designed to withstand thermal stress. However, a standard Erlenmeyer flask cannot tolerate the physical stresses of a vacuum The flask s concave bottom will flex (stress) and is likely to implode in regions of flaws. 

In order to study the effect of physical aging on the carbon-fiber reinforced epoxy, the freshly quenched materials were then sub-Tg annealed at 140 °C. After annealing for only 10 minutes at that temperature, the toughness of the composite was restored to a level comparable to that of the postcured material (see Fig. 7). It is likely that residual thermal stresses resulted from the quenching were annealed away during this 10 minutes thermal aging at 140 °C. 

Thus, although it is possible to construct a clear physical picture of sphere contraction during aging, the amount of thermal stress reduction which can be achieved over reasonable time scales translates into immeasurably small changes... 

The thermal stresses were computed by the finite element code ANSYS [29], taking full advantage of the axial symmetry of the filter see Fig. 16. Both the temperature-dependent physical properties of the EX-54 filter (Section V) and the time-dependent thermocouple data were used as inputs to stress analysis. The maximum stresses in the axial and tangential directions at the midsection are summarized in Table 14. It should be noted in Table 14 that the radial temperature gradient is the major contributor to thermal stresses those due to axial gradient are less than 20%. 

---