Advanced materials based on montmorillonite modified with poly (ethylenimine) and poly(2-methyl-2-oxazoline): Experimental and DFT study

This work reports for the first time the ability of inorganic matrix Na-montmorillonite (Mt) to interact with two structurally similar but in term of charge different polymers: linear cationic poly(ethylenimine) (PEI) and nonionic poly(2-methyl-2-oxazoline) (PMeOx). The effect of various polymer loadings on the interlayer structure and physico-chemical characteristics of composite materials was examined using experimental (XRD, IR, TA analysis) and ab initio Density Functional Theory (DFT) methods. The different charge characteristics of the polymers affected their intercalation into Mt. For PEI-Mt, almost complete exchange of Na+ with PEI cations was found, implying that the PEI chains interacted with the Mt interlayer surface electrostatically. In PMeOx-Mt, the content of Na+ remained the same as in Na-Mt, indicating that PMeOx chains adhered to the Mt surface by hydrogen bonds and/or van der Waals forces. The results showed that PEI could only be intercalated into Mt interlayers up to 100% of the cation exchange capacity (CEC) of Na-Mt; the resulting chains were adsorbed on the Mt outer surface. In contrast, unexpected and very high adsorption of PMeOx on Mt interlayer and outer surfaces, markedly exceeding the CEC of Na-Mt, was discovered for the first time. DFT calculations indicated that both polymers are anchored onto the basal surface of montmorillonite through moderate to weak Cp-Hp & BULL;& BULL;& BULL; Ob hydrogen bonds. Additional Np-Hp & BULL;& BULL;& BULL;Ob hydrogen bonds in the PEI-Mt structure support the stability of this hybrid structure. Both the PMeOx-Mt and PEI-Mt models showed high stability.