Effect of free volume on nonlinear optical characteristics of P(St-co-MMA)/CuO and PSAN/CuO polymer nanocomposites

•In P(St-co-MMA)/CuO and PSAN/CuO PNCs, reduced crystallinity and optical energy band gap favor NLO properties.•Amorphous regions are most favorable for the generation of bigger free volumes in these PNCs.•The bigger voids can enhance the NLO properties in PNCs by providing more space for the alignment of polarizable groups. Microstructural dependance of nonlinear optical (NLO) properties of Poly (Styrene-co-Methylmethacrylate) (P(St-co-MMA)/Cupric Oxide (CuO) and Poly(styrene-co-Acrylonitrile) (PSAN)/CuO polymer nanocomposites (PNCs) has been explored. The positron lifetime parameter viz., free volume hole size (Vf) and NLO parameters viz., third-order nonlinear absorption coefficient (β) and nonlinear refractive index (n2)) are maximum at 0.4 wt% and 1.0 wt% of CuO nanoparticles loadings in P(St-co-MMA)/CuO and PSAN/CuO PNCs respectively. The crystallinity and optical energy band gap decreased in P(St-co-MMA)/CuO and PSAN/CuO PNCs at 0.4 wt% and 1.0 wt% of CuO nanoparticles loading respectively. The reduced optical energy band gap and the formation of bigger free volumes promote the dipole polarization and therefore the NLO properties of P(St-co-MMA)/CuO and PSAN/CuO PNCs.