The pressure dependence of the melting and crystallization temperatures of poly(vinylidene fluoride) is determined by high-pressure differential thermal analysis (DTA). These results are used in the investigation of the polymorphic crystal form obtained by pressure quenching molten poly(vinylidene fluoride): The resulting crystal form depends on both the initial and final pressures. The pressure-quenching experiments were performed in a high-pressure piston-cylinder system and in a high-pressure DTA system; a comparison is made of the results obtained by both methods.
Piezoelectric properties and ferroelectric hysteresis effects in uniaxially stretched nylon11 filmsAt the present time, only poled, drawn poly (vinylidene fluoride) (PVF 2 ) films give evidence of sufficiently high piezoelectric response to be useful in device applications, and for this reason the great majority of research has centered around this polymer. As in the case of PVF 2 , many odd nylons crystallize in a polar space group with a large net dipole moment in the unit cell. On the basis of the understanding now reached of the properties of poled PVF 2 films, it would appear that the odd nylons have the potential to make films with high piezoelectric activity. Piezoelectricity and pyroelectricity in nylon 11 films have been studied previously. The piezoelectric strain constants found were larger than most polymers, but still two orders of magnitude less than the corresponding activity found in poled oriented PVF 2 (d 31 -20 pC/N). Studies carried out in this laboratory have shown that by appropriate variation of poling conditions and sample microstructure, relatively large piezoelectric constants can be obtained for nylon 11 films (d 31 -3 pC/N). The dependence of the piezoelectric strain constant d 31 on poling temperature, poling field, and crystal structure will be discussed.
The torsional shear stress-strain behavior of a graphite/epoxy composite material and also the epoxy matrix material alone has been determined at various hydrostatic pressures up to 6 kbar, using a newly constructed high pressure torsion apparatus. The composite samples were machined into thin walled hollow cylinders from press molded and oven cured, laminated panels. Graphite fibers of the samples were continuous, (0°) uniaxial, and 60% by volume.The normally linear elastic shear stress-strain curve of the epoxy matrix material at atmospheric pressure shifted upwards with pressure and showed nonlinearity at 2 kbar and higher. The shear modulus (G ) and the fracture strength ( τ f ) and strain (γ f ) all increased markedly and bilinearly with a break occurring at 2 kbar. The shear stress-strain curves of the composite material showed dramatic changes from an almost linear curve with γ f of 8% at atmospheric pressure to a nonlinear curve, exhibiting yielding, work-hardening, and extensive drawing with γ f of 57 % at 6 kbar. The G increased bilinearly with pressure from 10.7 x 10 8 at atmospheric pressure to 12 x 10 8 N/m 2 at 6 kbar with a break also occurring at 2 kbar. The work hardening parameter, x, determined at 1% offset strain increased significantly with pressure. The τ f and γ f also increased linearly with an abrupt jump occurring between 4 and 5 kbar as the mode of fracture changed from delamination to shearing.
An x-ray diffraction study of Nylon 11 was carried out at high pressures and high temperatures. Careful measurements made on both wet and dry samples at atmospheric pressure indicated that the structure of Nylon 11 suggested by Slichter was not quite correct. Some distortion of the planar conformation causes a shortening of the c repeat distance from the value required for a fully extended conformation. The compressibility of the crystal lattice at higher pressures up to 20 kbar was studied and related to the anisotropic bonding present in the structure. A crystal transition at atmospheric pressure previously unreported was observed to occur at 95 °C, the triclinic structure (α phase) changing to a pseudohexagonal structure. The influence of pressure on this transition was studied. It was found that the increase in transition temperature with pressure dTt/dP (∼15 K/kbar) was greater than the melting temperature increase with pressure dTm/dP (∼9 K/kbar). A pressure-temperature phase diagram for Nylon 11 was obtained and a triple point was observed at 310 °C and 14.5 kbar. At pressures above 14.5 kbar the α phase is stable until melting.
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