The polymer poly(2,6-diimidazo [4,5-b:4＇,5＇-e]pyridinylene-1,4(2,5-dihydroxy)phenylene (PIPD) is spun from a liquid crystalline solution in polyphosphoric acid. The as-spun fibre is a crystal hydrate, which transforms into a bidirectional hydrogen-bonded structure during heat treatment. This transition results in an increase in crystalline modulus along the chain direction due to a decrease of the cross-sectional area per chain, and in an increase in shear modulus due to stronger interaction between the chains caused by the hydrogen bonds. Heat-treated PIPD has a tensile modulus up to 360 GPa, a tensile strength well above 4 GPa, an internal shear modulus of 7 GPa, and a compressive strength of 1.7 GPa.