Novel polyimide-b-polyurea supramacromolecule with remarkable thermomechanical and dielectric properties

A novel type of polyimide (PI)–polyurea (PU) block copolymer containing environmentally friendly aromatic polyether urea was successfully prepared by a two-step polymerization process that produces anhydride terminated poly(4,4′-oxydiphenylene-pyromellitamic acid) and isocyanate terminated poly(4,4′-oxydiphenylene-methylenebisphenylurea) polyurea in the first step followed by coupling of the two homopolymers in the second step to form poly(amic acid)-b-polyurea. The resulting polyimide-b-polyurea copolymer reorganized into a supramacromolecule by formation of hydrogen bonds between adjacent chains. The existence of three types of hydrogen bonds including, inter–urea, urea–methyl and imide–methyl hydrogen bonds was confirmed by using (i) X-ray diffraction (XRD) technique, (ii) Fourier Transform Infrared Spectroscopy (FTIR) and (iii) molecular dynamics simulation. The formation of hydrogen bonds in the copolymers is affected by post-curing temperature and mole fraction of polyurea. Free-standing thin films and coatings of the copolymer of about 70 μm thickness were prepared by solution casting from N-methyl pyrrolidone, NMP, followed by thermal treatment. The copolymers showed remarkable thermomechanical properties and ultra-low dielectric constant which makes them useful as ionic insulators and anti-corrosion coatings. Increasing polyurea concentration in the copolymers resulted in a significant increase in storage modulus (both in the glassy and rubbery plateau regions), glass transition temperature, Tg and a drastic decrease in dielectric constant. Two glass transition temperatures (Tg) associated with polyimide-rich phase and polyurea-rich phase, respectively, were observed. Damping ability of the copolymers, measured from the area under the α-transition peak from the tan δ versus temperature curve, decreased with increasing polyurea concentration.