AZO POLYMERS FOR REVERSIBLE OPTICAL STORAGE .4. COOPERATIVE MOTION OF RIGID GROUPS IN SEMICRYSTALLINE POLYMERS

Polarized light induces a preferred orientation of azobenzene groups through photochemical trans-cis-trans processes. Stronger orientation tendencies exist if the azo-containing polymers are crystalline or liquid crystalline and the cooperative reorientation of nonactive mesogens of similar shape has been demonstrated. This paper shows that cooperative reorientation is a more general phenomenon and can take place even for a nonactive rigid group of a different shape which is bonded in the main chain while the azobenzene groups are in the side chains. Comparison is made between a disperse red 1-containing amorphous azo polymer (pDR1A) and a disperse red 19-containing semicrystalline azo polymer (pDR19T) with phenylene diacrylate (PD) groups within the main chain. Dichroic ratios are obtained from infrared and electronic spectra and are correlated with a writing/erasing/rewriting sequence performed with an argon laser. A maximum birefringence of 0.08 can be obtained on pDR1A, while on pDR19T the birefringence is 0.27 in similar conditions. This is the result of a combination of the semicrystallinity of pDR19T and of the cooperative orientation of PD groups along with the azobenzene groups. The drawback in terms of optical storage is the longer time required to write or erase the induced birefringence in pDR19T.