Effect of the Reduction Degree of Graphene Oxide on the Cyclization of Polyacrylonitrile in PAN/GO Composite Membranes Prepared by Phase Inversion

This study evaluates the effect of the reduction degree of graphene oxide on the cyclization of polyacrylonitrile (PAN) in composite PAN/GO membranes prepared by phase inversion. All composite membranes contained the same amount of graphene-based additive, namely 5 wt.% relative to dry PAN, which made it possible to separate the effect of additive loading from the effect of surface chemistry. The additives were obtained from a single batch of graphene oxide and subjected to controlled thermal reduction. Their oxygen contents, determined by EDS, were 7.2, 15.3, 24.6, 35.4 and 48.1 wt.% for PAN/GO-1, PAN/GO-2, PAN/GO-3, PAN/GO-4 and PAN/GO-5, respectively. DSC analysis showed that, in the PAN/GO-2-PAN/GO-5 series, the maximum temperature of the exothermic PAN cyclization effect decreased from 294 to 224 °C as the oxygen content increased, while the cyclization enthalpy decreased from 506.7 to 404.2 J/g. PAN/GO-1, containing the most strongly reduced additive with 7.2 wt.% oxygen, showed a different behavior, with the highest cyclization maximum, approximately 316 °C, and the highest enthalpy, 628.5 J/g. FTIR spectra confirmed the disappearance of the nitrile band at approximately 2240 cm−1 and the growth of C=N/C=C bands assigned to the developing PAN ladder structure. SEM observations showed that all selected membranes retained an asymmetric, macrovoid-containing architecture, while local differences in surface topography and cross-sectional heterogeneity were present. The results demonstrate a non-monotonic response across the complete series and indicate an association between reduction treatment, total oxygen content, and PAN cyclization; changes in dispersion, morphology, and polymer–filler interfacial organization may also contribute.