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study range from tens of nanometers to 30 l mandwas analyzed by mercury-intrusion (for an example of the latter see supporting information). With respect to the adsorption of macromolecules on/in paper fibers, smaller pores in the cell walls were studied by various methods and values ranging from 10 to 100 nm were reported (Wu et al. 2009). Pores and grooves that are on the order of the size of an unperturbed polymer chain may not be trivial to be accessed by the macromolecules due to confinement effects (i.e. the polymer chain can only enter the pore if segments are stretching, which is thermodynamically not favored). However, polymers may easily enter pores that are larger than the molecular size of the macromolecule. With respect to this, it therefore makes a large difference, if fibers are pre-swollen in the solvent or if the polymer solution is transferred onto a dry paper sample. Submerging a dried paper sample in a given solvent leads to sudden imbibition into the sample, due to strong capillary forces. (Alava and Niskanen 2006) The work of El Seoud et al. (2008) allows for a quantitative comparison of the used solvents in this regard, by determining the extent of swelling using a simple gravimetric approach. The results show a significant difference of the ability to swell cellulose fibers: H 2 O * 62.7%, IPA * 4.7% and BuOH * 7.2%, respectively. From these values it can be inferred, that impregnating the paper with the copoly- mer dissolved in H 2 O, a significantly higher volume of the copolymer solution, i.e. around ten-fold more, is spontaneously pulled inside the fiber network and inside the fibers by capillary forces and fiber swelling, as compared to impregnation of paper with IPA and BuOH solutions. Without the possibility of fiber swelling, IPA and BuOH can only transport the copolymer to the outer surface of fibers, or inside the fiber lumen through larger defects and pinholes in the fiber wall. While drying, the solvent accumulates at fiber–fiber cross- ings, due to increased capillary forces, leading to precipitation of the copolymer at such spots. In addition, IPA has a lower boiling point and a higher vapor pressure [42.6 hPa (20 C) (Lide 2004)] com- pared to H 2 O and BuOH, leading to a more sudden evaporation of the solvent and hence yielding an inhomogeneous distribution of the copolymer across the paper width, which again is in accordance with our structure analysis.
imaging allows to state this. In contrast, when comparing the fluorescence channels of the fibers (cyan) and the copolymer (magenta) of the H 2 O- impregnated paper samples (Fig. 8a, b), it is difficult to see significant differences. It seems as though the fluorescence of the CW-labelled fibers is at the outermost part and the fluorescence of the copolymer is beneath it, which can be observed in the cross- sectional images in Fig. 7c, too. This suggests, that the copolymer is not outside of the fibers or on the surface, but rather inside the fiber wall and the lumen, respectively. The presence of the copolymer inside the fibers can also be observed when looking at the in- plane projection of the stacked images (see supple- mentary information). When comparing different solvents, the hydrody- namic properties, and, in particular, size exclusion effects as well as thermodynamic behavior of the polymers in solution can become critical and can impact the deposition and resulting spatial distribution of the adsorbed copolymers in/on the fibers. In order to learn more about the thermodynamic behavior of the polymers in dilute and concentrated solutions, we analyzed the polymer solutions by dynamic light scattering (at concentrations of 5 mg mL - 1 ) and additional turbidity measurements at higher concen- trations (5–45 mg mL - 1 ), in order to determine the phase behavior at variable temperatures (5–50 C). Data can be found in the supporting information. In brief, hydrodynamic radii of the copolymers in H 2 O, and IPA, show very similar values in the range of 7–11 nm, respectively. The turbidity experiments (see supplementary information ) do not show any aggregation for the solvents IPA and BuOH over a wide temperature- range even at highest chosen concentrations of up to 45 mg mL - 1 . However, for the copolymer-solution in H 2 O, clouding can be observed at around 13 Cwhich becomes more pronounced with increasing tempera- ture. Lowering the concentration to 25 mg mL - 1 (impregnation) and further to 5 mg mL - 1 (DLS), the temperature, where clouding is first observed, increases significantly to 21 and 36 C, respectively. In particular, in alcoholic solutions under impregna- tion-conditions chosen, no aggregates are present. To elucidate how accessible the adsorption sites are for the copolymer in the cellulose fiber network, information on the pore size (distribution) is impor- tant. The pore sizes of the paper sheets used in our
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