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et al. 2017; McKenzie and Higgins 1955), which further explains our results with the reference paper. Another factor is the extraction of fiber fragments known as ‘‘fines’’ of the fiber network, which is also known to decrease the tensile strength. Although the paper samples in this study haven’t been dry or wet pressed after paper making, the additional swelling and drying cycles during the treatment of paper samples can also lead to hornification, which could explain the observed decrease in measured tensile strength. In addition, it is safe to assume, that the intensive UV-exposure also has an influence on the tensile properties. It is known, that such UV-exposure significantly reduces the degree of polymerization (DP) of cellulose fibers (Kolar et al. 2000), and the group of Fang et al. (2020) observed that such a decrease of cellulose DP leads to a significant decrease of the tensile strength of nanocellulose films prepared thereof. Considering, that for the three reference experiments without any copolymer addition, the extraction process was carried out in H 2 O and the UV-treatment was identical, the observed similar values for dry and wet tensile index can be explained.
Considering, that the coefficients of variation are all below 5% (see supplementary information), these results are reproducible over many test samples. Only measurements for the wet tensile index of IPA- impregnated samples have a significantly higher coefficient of variation, indicating that IPA introduces a broader range of slightly different impregnation results, even though this can’t be observed for tensile tests in the dry state where the coefficient of variation is significantly lower. The amount of cross-linked copolymer inside the paper samples was evaluated by a gravimetric approach under norm climate conditions. The results (see supplementary information) show that more copolymer adsorbs during impregnation with H 2 O (11.4 wt%) if compared to IPA - (8.0 wt%) and BuOH- impregnation (7.3 wt%). However, given the low absolute difference of the impregnated amount com- bined with the similar dry tensile values, we do not believe that this parameter is the determining factor for the observed difference in tensile properties. In Fig. 6 the calculated values for relative wet strength of the different paper samples are shown. It is immediately evident, that applying the copolymer from H 2 O yields the highest relative wet strength paper samples. The high relative wet strength of 24.2% is directly related to the significant increase in wet tensile index, while at the same time the dry tensile index increases only by a significantly lower factor. Because the relative wet strength (Eq. 1) is calculated from the ratio of wet to dry tensile index, this overstates/exaggerates the wet strength of the paper samples, in comparison to the IPA- and BuOH- impregnated variants, where the increase in wet tensile strength was lower, while the increase in dry tensile strength was significantly higher. Therefore, the values for relative wet strength should always be treated with caution, when comparing paper samples and the data used for the calculation should be taken into account.
Impregnation using H 2 O v ersus IPA v ersus B uOH
We next investigated the stress–strain behavior of paper sheets modified with the copolymers in water, 2-propanol and 1-butanol, respectively. Examples of the data are shown in Fig. 3. It can be inferred that the dry tensile behavior for all solvents chosen are (almost) similar, whereas the stress–strain behavior of the respective samples in the wet state differs significantly. In particular, the maximum force at break progresses as BuOH \ IPA \ H 2 O. Dry and wet tensile index were further calculated from the measurements and are shown for all samples in Fig. 4. As can be inferred from the figure, the dry tensile index was increased by all three impregnation-treat- ments from 17.6 and 9.8 N m g - 1 for the Ref and RefSwell , respectively, to 36.8, 46.3 and 44.5 Nm g - 1 for H 2 O-, IPA- and BuOH-impregna- tion, respectively. Comparing the wet tensile indices, it becomes clear that the H 2 O-impregnated paper samples possessed significantly higher wet strength. The latter increased from 0.3 and 0.2 N m g - 1 for RefSwell and Ref , respectively, to 8.9 N m g - 1 , while IPA and BuOH-impregnation led to less increased values of 2.2 N m g - 1 and1.4 Nm g - 1 respectively.
Spatial distribution—impregnation in H 2 O versus IPA versus BuOH
Next, we addressed the question why different solvents lead to significant differences in tensile strength. To this, the spatial distribution of copolymer inside the cellulose fiber network was analyzed using CLSM. By incorporating a fluorescing group,
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