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Pulp and handsheet preparation
chemical analysis of the molar composition of the copolymer was done using 1 H-NMR (see supporting information), which showed that the copolymer con- sists of about 96.5 mol% of the matrix DMAA, and 3.5 mol% of MABP and RhBMA, respectively. Using size exclusion chromatography (SEC) with a GRAM VS/GRAM linear 10 HS/100 l L (DMF 0.002 LiCl) column and a narrow dispersed poly(methyl methacry- late) standard, the molar mass was determined to be around M n = 27 000 g mol - 1 ( Ð * 5.7). The copolymer was stored inside a plastic container in a refrigerator at 6 C until further use.
All paper samples, modified with the copolymer as described below, where lab-engineered using bleached eucalyptus-sulfate pulp [median fiber length (length-weighted): 0.76 mm; curl: 15.9%; fibrillation degree: 5.1%; fines content: 9.1%]. The paper samples having a grammage of 80 ± 1.6 g m - 2 where pre- pared using a Rapid-Ko¨then sheet former according to DIN 54358 and ISO 5269/2. In order to prevent any influence on the physical properties, no additives or filler materials other than the copolymer were used. Prior to impregnation, the paper was conditioned for at least 24 h under standard conditions (23 C, 50% r.h.).
Copolymer treatment of the paper samples
Polymer synthesis
Before impregnation, the lab-engineered paper was cut into samples with the dimensions of 15 9 120 mm. Those dimensions were chosen because tensile testing was carried out according to DIN ISO 1924-2, where a gauge length of 100 mm is specified. To determine the amount of cross-linked copolymer in the samples, a gravimetric approach was chosen. Before each weighing/measurement on a scale under climate controlled conditions (23 C and 50% r.h.), the samples were conditioned for at least 24 h. Three measurements were carried-out, once before impregnation (m before), after drying the impregnated samples (m after) and finally after the extraction and consecutive drying (m extraction), in order to calcu- late the amount (wt%) of copolymer in relation to the paper weight as well as the amount of extractable (un- bound) copolymer. For the three solvents the amount of copolymer in the paper samples was 11.4 (H 2 O), 8.0 (IPA) and 7.3 wt% (1-BuOH). For the reference experiments the paper samples were impregnated in H 2 O without any copolymer, but otherwise the procedure was carried out in the same way as described before. Here, the measurement after swel- ling and drying (m delta) was used to calculate the wt% change induced by the treatment without copoly- mer. Complete details of these calculations and results can be found in the supplementary information. The copolymer was dissolved in H 2 O, in 2-propanol , or in 1 -butanol , respectively, at a concentration of 25 mg mL - 1 and stored in flasks in a refrigerator at 6 C under exclusion of light until use. The polymer- modification of the paper samples is illustrated in Fig. 2 and was conducted as follows.
The photo-cross-linking monomer 4-benzoylphenyl methacrylate (MABP) was synthesized according to the procedure developed in one of our groups and described by Toomey et al. (2004). First, the fluores- cent monomer rhodamine B methacrylamide (RhBMa) was prepared in two steps from the fluorescing rhodamine B base according to previously published literature (Scha¨fer et al. 2013). The copoly- mer poly(dimethylacrylamide-co-4-benzoylphenyl-2- methacrylate-co-rhodamine B-methacrylic acid (P(DMAA-co-MABP-co-RhBMA)) carrying the flu- orescent monomer, the photo-cross-linking monomer and the matrix monomer, respectively, was synthe- sized using free radical copolymerization according to the procedure described by Janko et al. (2015). The structure is schematically illustrated in Fig. 1. The
Fig. 1 Molecular structure of the synthesized copolymer poly(dimethylacrylamide-co-4-benzoylphenyl-2-methacrylate- co-rhodamine B-methacrylic acid (P(DMAA-co-MABP-co- RhBMA)) carrying the fluorescent monomer (m), the photo- cross-linking monomer (o) and the matrix monomer (n), respectively. Molar composition as determined by 1 H-NMR (see supporting information): m ? o = 3.5%; n = 96.5%
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