Model Surfaces for Paper Fibers Prepared from Carboxymethyl Cellulose and Polycations

Abstract

For tailored functionalization of cellulose based papers, the interaction between paper fibers and functional additives must be understood. Planar cellulose surfaces represent a suitable model system for studying the binding of additives. In this work, polyelectrolyte multilayers (PEMs) are prepared by alternating dip-coating of the negatively charged cellulose derivate carboxymethyl cellulose and a polycation, either polydiallyldimethylammonium chloride (PDADMAC) or chitosan (CHI). The parameters varied during PEM formation are the concentrations (0.1-5 g/L) and pH (pH = 2-6) of the dipping solutions. Both PEM systems grow exponentially, revealing a high mobility of the polyelectrolytes (PEs). The pH-tunable charge density leads to PEMs with different surface topographies. Quartz crystal microbalance experiments with dissipation monitoring (QCM-D) reveal the pronounced viscoelastic properties of the PEMs. Ellipsometry and atomic force microscopy (AFM) measurements show that the strong and highly charged polycation PDADMAC leads to the formation of smooth PEMs. The weak polycation CHI forms cellulose model surfaces with higher film thicknesses and a tunable roughness. Both PEM systems exhibit a high water uptake when exposed to a humid environment, with the PDADMAC/carboxymethyl cellulose (CMC) PEMs resulting in a water uptake up to 60% and CHI/CMC up to 20%. The resulting PEMs are water-stable, but water swellable model surfaces with a controllable roughness and topography.

Keywords: carboxymethyl cellulose; cellulose model surface; dip-coating; polyelectrolyte multilayers.

Figure 1

Molecular structures of the polyelectrolytes (PEs) used. The asterisk indicates on which position the functional group is attached to the polyelectrolyte, e.g., for CMC the asterisk represents the oxygen of the primary hydroxyl group. The negatively charged group of carboxymethyl cellulose (CMC) (a) is depicted in blue and the positively charged groups of chitosan (CHI) (b) and polydiallyldimethylammonium chloride (PDADMAC) (c) in red. For the weak PEs, the pK${}_a$-value and the degree of substitution (DS) are given.

Figure 2

Summary of the morphology results for PEMs PDADMAC/CMC (filled squares) and CHI/CMC (empty triangles) from ellipsometry (a,b) and AFM measurements (c–i): (a) Film thickness, (b) refractive index, and (c) roughness with an increasing number of bilayers (NoBL). (d–i) AFM images (5 × 5 µm²) for PDADMAC/CMC and CHI/CMC at 3, 5, and 7 BL. For all images, the height scale is set to 60 $\mathrm{n}$$\mathrm{m}$. The samples were prepared at pH 4 and c${}_{PE}$ = 1 g/L. All experiments were carried out at RH ≈ 40%. The first bilayer corresponds to the bilayer of the precoat of polyethylenimine (PEI) and CMC.

Figure 3

The change in frequency $\Delta$f (a,b) and dissipation $\Delta$D (c,d) measured by quartz crystal microbalance experiments with dissipation monitoring (QCM-D) for the PEMs PDADMAC/CMC and CHI/CMC. The gray region is the time period in which the polycation is adsorbed and the blue region in which the polyanion is adsorbed. The different curves in one plot represent different overtones of the measured intensity (5th, 7th, and 9th). All PE concentrations were set to 1 g/L, and the pH-value was maintained at pH = 4.

Figure 4

Summary of the morphology results for PEMS PDADMAC/CMC (filled squares) and CHI/CMC (empty triangles) from ellipsometry (a,b) and AFM measurements (c–k): (a) Film thickness, (b) refractive index, and (c) roughness with varying pH-values. (d–k) AFM images (5 × 5 µm²) for PDADMAC/CMC and CHI/CMC at pH = 2, 3, 4, and 5. For all images, the height scale is set to 60 $\mathrm{n}$$\mathrm{m}$. The samples were prepared at c${}_{PE}$ = 1 g/L and seven BL. All experiments were carried out at RH ≈ 40%.

Figure 5

(a,b) Change in thickness and refractive index for PDADMAC/CMC (filled squares) and CHI/CMC (empty triangles) with varying PE concentrations determined by ellipsometry (RH ≈ 40%). (c) Change in roughness, determined by AFM (RH ≈ 40%), with varying PE concentrations for PDADMAC/CMC (filled squares) and CHI/CMC (empty triangles). (d–i) AFM images (5 × 5 µm²) for PDADMAC/CMC and CHI/CMC with varying PE concentrations. For all images, the height scale is set to 60 $\mathrm{n}$$\mathrm{m}$. The NoBL was set to seven, and the concentration of all solutions was set to 1 g/L. The first bilayer corresponds to the bilayer of the precoat PEI and CMC.

Figure 6

The change in frequency $\Delta$f (a) and dissipation $\Delta$D (b) measured by QCM-D for the PEM PDADMAC/CMC at 5 g/L. The gray region is the time period during which the polycation is adsorbed, the blue region in which the polyanion is adsorbed, and white for the rinsing periods. The different curves in one plot represent different overtones of the measured intensity (5th, 7th, and 9th). The pH-value was maintained at pH = 4.

Figure 7

The swelling coefficient S dependent on the relative humidity RH of the PEMs PDADMAC/CMC and CHI/CMC with (a) varying NoBL (at pH = 4) and (b) different pH-values (seven BL). The thicknesses were measured by ellipsometry.

Figure 8

Proposed structures of the PEM systems PDADMAC/CMC and CHI/CMC, resulting from the high charge density of PDADMAC and the high effective persistence length. The polycations are depicted in black, the corresponding negatively charged counter ions in blue. The polyanions are depicted in red, the corresponding positively charged counter ions in orange.