Elastic properties of certain polymer matrices at low temperatures

Abstract

A study is made of the viscoelastic behavior of the polyimide DFO and polybenzoxasol (PBZ) Some relatively new polymers --polyimides and polyoxymethylenes --have recently been used as matrices in composites. These polymers are stable at elevated temperatures: However, an analysis of their chemical structure shows (especially due to the presence of large low-mobility groups in the main chain) that these materials should also have a large free volume at very low temperatures as well. Thus, polyimides and polyoxymethylenes should have good service properties at both low and high temperatures. Nevertheless, there has been very little study of the viscoelastic behavior of these polymers at very low (helium) temperatures.In light of this, we measured the velocities of longitudinal and transverse ultrasonic waves at frequencies of 1 and 5 MHz in polyimide DFO and polybenzoxasol (PBZ) within the temperature range 4.2-240 K. As is known [1], the results of such measurements can be used to calculate parameters that characterize the viscoelastic behavior of polymers: dynamic elastic modulus E', shear modulus G', compressive bulk modulus Ks', Poisson's ratio and the corresponding loss moduli.The experimental unit, cryostat, iustnunentation, and method of measuring ultrasonic velocity were described in [2]. The relative error of the velocity measurement was 1%. Temperature was measured with a copper--comtantin thermocouple to within 0.2 K. Specimens of DFO and PBZ were used for the acoustic measurements. The specimens were in the form of cylinders with a diameter of 22 ram, height of 5 ram, and densities of 1.392 and 1.308 g/cm 2, respectively. The chemical strucutre of the polymers was as follows:Polyimide DFO Polybenzoxazolr /o~ c

Correlation between the High-Temperature Local Mobility of Heterocyclic Polyimides and Their Mechanical Properties

“…In simulations, the Poisson’s ratio remains practically unchanged upon changing the values of the deformation or cooling rates. This very weak dependence justifies the direct comparison of the simulated and experimental data; the conclusion is that the simulated Poisson’s ratio of both samples with and without partial charges correlate with the experimental result, μ = 0.34–0.40. − …”