Cellulose (2021) 28:5775–5791
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of tensile properties of the samples. Due to this, some images may show inhomogeneous staining (e.g. Fig. 7g), probably caused by interactions between the copolymer and the dye. In consideration of the ability of H 2 O to swell cellulose fibers, it can be assumed, that the dissolved copolymer is able to penetrate the cellulose fiber network as a whole, which includes the fiber wall and the lumen, respectively. Taking a closer look at the fluorescence-channel from the fluorescently-labelled copolymer in Fig. 7b, this hypothesis can be con- firmed. The fluorescence can be observed across the whole paper width, pointing towards the complete and homogeneous penetration on the scale of the fibers. Furthermore, we find copolymer inside the fiber lumen and upon higher magnification (see Fig. 7c) even within the fiber walls. In contrast, as both IPA and BuOH do not account for high degrees of swelling of the paper fibers, polymers dissolved in these solvents and brought into contact with the fiber are not expected to access all spaces within the non-woven sheets. Taking a closer look at the superimposed image of the fluorescence channels of the IPA- and BuOH-impreg- nated cellulose fibers (CW—cyan) and the copolymer (RhB—magenta) in Fig. 7f, i, respectively, this hypothesis can be confirmed. The copolymer seems to be mainly accumulated on the outside of the fibers and scattered rather inhomogeneously across the paper width. Upon higher magnification, Fig. 7f, i, respec- tively, show, that the copolymer is not able to penetrate inside the fiber or into the fiber walls and only a few of the fiber lumens are partially filled. To further analyze the spatial distribution between the fibers, image-stacks of the cellulose fiber network were taken with the CLSM, and combined to 3D- images. The fluorescent brightening agent CW was once again used to stain the cellulose fibers. The stacked images of the cellulose fiber network in Fig. 8 reveal the distribution of the copolymer in between the fibers. In analogy to the observations in the cross-sectional images, the overlay of the fluores- cence channels in Fig. 8c shows, that the copolymer dissolved in H 2 O seems to be distributed homoge- neously across the paper width. In comparison, applying the copolymer out of IPA and BuOH seems to lead to a more scattered, inhomogeneous distribu- tion across the paper width, as can be inferred from Fig. 8f, i, respectively. However, BuOH-impregnation seems to be more homogeneous, especially when
Rhodamine B, into the copolymer and subsequently staining the treated cellulose fibers with the fluores- cent brightening agent CW, before embedding them in an epoxy-resin and preparing thin cross-sections, the spatial distribution throughout the fiber network was analyzed. Cellulose fibers were stained after the copolymer- treatment, in order to enhance their visibility while fluorescence imaging. Although cellulose fibers from eucalyptus-sulfate pulp contain considerable amounts of lignin detectable with fluorescence imaging, the importance of the staining procedure can be seen in Fig. S4 in the supplementary information. Staining cellulose fibers after impregnation was done in order to rule out any effect of the staining on the spatial distribution of the copolymer and on the development Fig. 6 Relative wet strength of eucalyptus-sulfate paper samples with a grammage of 80 g m - 2 shown side by side for: pure cellulose paper samples not subjected to any treatment (Ref), samples subjected to the procedure for copolymer application, without any copolymer in the solution (RefSwell), samples where the copolymer was applied out of H 2 O, IPAand BuOH (for more results, see supplementary information)
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