Nonlinear optical anisotropy and molecular orientational distribution in poly(p-phenylene benzobisthiazole) Langmuir–Blodgett films

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The orientational distribution and packing of polymer chains were investigated in poly(p-phenylene benzobisthiazole) (PBT) Langmuir–Blodgett (LB) films by nonresonant third-harmonic generation measurement at a wavelength of 1907 nm. The tensor components of the third-harmonic susceptibility on the PBT LB film with a surface pressure of 50 mN/m were determined to be χ(3)XXXX=(16.6±2.5)×10−12 and χ(3)YYYY=(2.0±0.3)×10−12. The large nonlinear optical anisotropy can be explained as a result of highly oriented packing of the polymer chains induced by a flow orientation. A Gaussian distribution function with a standard deviation of σ=0.40 gives a practical description of the orientational distribution of PBT polymer chains. A maximum χ(3) value of (26.8±4.4)×10−12 esu is predicted assuming a perfect alignment of polymer chains. The χ(3)XXXX value increased by factor of 2 with the surface pressure from 30 to 50 mN/m mainly due to the packing density of the polymer chains, while the orientational degree did not change.