Interfacial self-assembly of cellulose-based polyelectrolyte complexes: pattern formation of fractal trees

A two-step procedure was adopted to prepare cellulose-based polyelectrolyte complexes (PECs) nanoparticles dispersed in water. First, an aqueous solution of a weak anionic polyelectrolyte of sodium carboxymethyl cellulose (CMCNa) was mixed with four types of cationic polyelectrolytes (poly 2-methacryloyloxy ethyl trimethylammonium chloride (PDMC), cationic cellulose, poly diallyldimethylammonium chloride (PDDA), chitosan (CS)) in HCl aqueous solutions. Four types of CMCNa-based PEC solids (CMCNa-PDMC, CMCNa-cationic cellulose, CMCNa-PDDA and CMCNa-CS), were obtained, purified and dried. Second, these PECs were re-dispersed in NaOH aqueous solutions. PEC solids and their aqueous dispersions were characterized by FT-IR, wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and dynamic light scattering (DLS), respectively. It was found that well grown fractal patterns (referred to as fractal trees) with diameters ranging from 5-300 mm for the four PECs were obtained after their aqueous dispersions were dried on silicon wafers or glass slides at 30 degrees C. This PECs interfacial self-assembly phenomenon is interesting but not shown in literature, even though various other PECs dispersions have also been dried in the similar way. Fractal dimensions of these fractal trees were calculated and their structures were characterized by polarized light microscopy (PLM), filed emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The formation mechanism of these fractal trees was tentatively examined by time-dependent FESEM. Moreover, effects of drying temperature, PEC concentration and solvent composition on fractal trees formation were studied. Potential applications of the fractal pattern formation in fields such as fast bottom-up nanofabrication, surface patterning and membrane separation were discussed.