Predicting the graphitization and mechanical properties of pyrolyzed carbyne polymers

Abstract

Carbyne is a one-dimensional chain of carbon atoms that has high elastic modulus and thermal conductivity. However, its mechanical properties vary with temperature. We use molecular dynamics to investigate the bond structure of polymers formed from cumulenic and polyynic carbyne pyrolyzed at high temperatures after quenching and predict the polymers’ mechanical properties. We observe that nanostructures begin to form during pyrolysis at 1,000K, and there is a major transformation from sp-hybridized carbyne to sp²-, and sp³-hybridized polymers after heating the carbyne up to 3,000K. Pyrolyzed cumulene forms an amorphous carbon polymer with graphitic structures that become more crystalline and porous with an increase in temperature. However, pyrolyzed polyyne forms amorphous carbon polymer with no indication of graphitization and lower density than pyrolyzed cumulene when heated above 1,000K. We perform compression testing of the pyrolyzed carbyne polymers after quenching them to 300K and observe that the graphitization of the pyrolyzed cumulene leads to a significant increase in compression modulus at 2,000K. However, the less stable nanostructures and lower density of pyrolyzed polyyne at 2,000K result in a decrease of compressive modulus along all axes. We find that the pyrolysis-driven reorientation of bonds in the carbyne polymers contributes to the directional dependence of the compression modulus with respect to the initial axis of the carbyne at 300K. This is most notable after pyrolysis of the cumulene and polyyne at 3,000K; the modulus of the polymers decreases along the fiber axis and increases along axes perpendicular to the fiber axis as bonds are reoriented at high temperatures.