Abstract
The first direct evidence for second-order nonlinearity in partially cross-linked polyacrylonitrile is reported. The cross-linking in γ-ray-induced polymerized neat acrylonitrile is well supported by UV−vis spectroscopy, FTIR, and 13C CP/MAS solid-state NMR. The second harmonic generation (SHG) powder test was employed to access the preliminary information about the occurrence of second-order nonlinear effect. The second harmonic generation is accomplished without the application of an external electric field, thus illustrating the importance of structural changes imparted by high-energy photon flux. Second-order nonlinearity in the present case is expected due to the small π conjugation imparted by cross-linking through −CN across polymeric chains coupled with the noncentrosymmetric molecular structure. Second harmonic intensity as high as 80% of that for powder urea has been observed.
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