Self-assembled spin-coated and bulk films of a novel poly(diacetylene) as second-order nonlinear optical polymers

The polymer chains of a novel asymmetric poly(diacetylene), namely poly{8-[[[butoxy-carbonyl]methyl]urethanyl]-1-(5-pyrimidyl)-octa-1,3-dyne} (BPOD), in the absence of nonlinear optically active chromophores and without recourse to poling assemble in an acentric stable polar organization upon spin coating or solution casting. Asymmetry of the side groups and the spontaneous alignment of the urethane side groups by means of hydrogen bonds appear to be responsible for this organization. An abrupt decrease of the second harmonic intensity at around 130 °C is found to be closely related to the breaking of noncentrosymmetry in the films in conjunction with the disruption of intramolecular hydrogen bonds. The changes observed were reversible below 130 °C but became irreversible above this temperature. Comparison of the absorption spectra of the poly(BPOD) films before and after thermal treatment indicates that only a slight change occurred in the absorption characteristics of the polymer backbone. FT-IR spectroscopy and dielectric spectroscopy confirm the relaxation of the stable hydrogen-bonded polar organization at higher temperatures. Activation energies involved in this process were found to be a little higher than hydrogen bonding dissociation energies calculated from polyamide and polyurethane. The larger activation energies observed here indicate the combined effect of the disruption of the hydrogen bonds followed by β-relaxation.