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individual fibres from each other (the repeating pattern, that can be mistaken for individual fibres, are imprints from the drying mesh). Upon hot pressing, the uneven structure of the modified paper is more or less completely transformed to a smooth film surface without distinct features (at higher magni- fication, however, nanofibrils can be seen; results not shown), which largely explains the significant decrease in haze upon pressing since haze is in many materials largely due to surface roughness. 27 – 29 Quite remarkably, if the non-pressed or pressed, modified papers are soaked in water and strained until failure, fibres become clearly visible as they protrude from the failure surface (Fig. 6g and h), showing that the modi- fication and hot pressing do not significantly a ff ect the macro- scopic fibre structure but makes possible a very close contact between individual fibres, still leaving the fibres as discrete entities. The last row in Fig. 6 shows cross-section images and agrees well with the densities and transmittance values reported in Fig. 4 and 5, showing a porous reference sheet and a basically non-porous modified paper. Barrier properties High density (Fig. 4), high transparency (Fig. 5) and a highly consolidated network structure (Fig. 6) are typical traits of a polymer-based gas barrier. Therefore, and since non-porous
during drying. Fig. 5b shows that the total light transmittance increased significantly upon chemical modification; the three most modified samples had a total transmittance of about 80% at a sample thickness of about 120 μm. However, most of the transmitted light, about 90%, was measured as di ff use light. After hot pressing the total transmittance increased to nearly 90%, and the transmittance mode was changed into mainly direct transmittance, with a haze value of only 20 – 40%. This possibility of tailoring the optical haze indeed open the way for applications outside the traditional pulp, paper and packaging arena. Recently, hybrid materials between highly beaten fibres and CNFs were identified as interesting materials in solar cell devices due to their combination of high total transparency and high haze. 25 The structure of the papers was further studied by SEM. Fig. 6 shows micrographs of reference papers and papers made from the most modified fibres and, as can be seen, the pressing had little e ff ect on the structure of the reference paper (first and second column). However, if the reference and modified paper, both before and after pressing, are compared, remarkable di ff erences can be observed (first row): the refer- ence fibres form an expected ribbon-like structure where indi- vidual fibres can easily be identified, whereas the modified fibres are merged together making it di ffi cult to distinguish
Fig. 6 Micrographs of the reference paper and the most modi fi ed paper; the fi rst and second columns are non-pressed and pressed reference papers, respectively, the third and last columns are non-pressed and pressed modi fi ed papers, respectively. (a – d) Top-view images, (e – h) top-view images of the failure surface of wet-strained papers, (i – l) cross-section images.
Green Chem. , 2016, 18 , 3324 – 3333 | 3329
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