Effects of Hydrostatic Pressure on the Deformation and Fracture of Polymers
Accurate observations under controlled conditions of the effects of increased environmental pressure on the mechanical behavior of polymers are relatively recent. From a critical analysis of these various pressure observations, phenomena that appear to be characteristic of such effects for the major polymer classes are identified, and the validity of hypotheses advanced for particular phenomena in specific polymers, together with their generality, is examined.
For the modulus (i.e., the preyield region of the stress-strain curve), the larger pressure dependence for semicrystalline compared with amorphous glassy polymers is associated with the pressure-induced increase in the temperature of subambient relaxation processes in the disordered component of the structure. In an analogous manner, increases in the glass transition temperature with pressure cause discontinuous increases in modulus for elastomers. For the yield stress, the measured pressure dependence — both from hydrostatic and from biaxial stress experiments — conforms to a modified von Mises yield criterion. However, unlike the modulus, there is no clear differentiation in the behavior of crystalline and amorphous polymers. Although the “volume-change equivalence” hypothesis is found to be invalid, the temperature and pressure dependence of yield stress can be correlated in a manner analogous to the time-temperature superposition concept. Applications of rate theory to the pressure, strain rate, and temperature dependence of yielding appear promising, but have not yet elucidated the specifics of the molecular mechanisms involved in polymer yielding.
The brittle fracture and crazing of amorphous glassy polymers can be suppressed by pressure and shear yielding induced. For polystyrene, there is evidence that this brittle-ductile transition may result from changes in crack-propagation characteristics rather than from simply a suppression of crazing. In some normally ductile polymers, decreases in strain to fracture occur with increases in pressure; such effects appear to be associated with pressure-induced changes in the temperature of relaxation processes.
It is concluded that the development of experimental techniques for studying polymer behavior under pressure has reached the stage where substantial contribution can be made to the elucidation of the factors controlling the mechanical behavior of this class of materials.
KeywordsPolymethyl Methacrylate Hydrostatic Pressure Pressure Dependence Amorphous Polymer Glassy Polymer
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