2.2. Methods
2.2.1. Hydrophobic modification of NCF
the modification of NCF was conducted via a free radical polymerization. The initial NCF suspension in water (5.4%,w/w) was washed and centrifuged with acetone and anhydrous ethanol several times in order to remove the water as much as possible. After the centrifugation at 5000 rpm and 20◦C for 5 min and the removal of the supernatant, the NCF was then re-dispersed in 200 ml of anhydrous ethanol in a three-necked round-bottomed flask equipped with a magnetic bar. After stirring and nitrogen purging to reduce the inhibiting effect of the oxygen in the radical polymerization reaction, a certain amount of hydrophobic monomer BA and initiator CAN were added via a dropping funnel. The system was maintained at 50◦C under constant stirring for 5–6 h as well as nitrogen purging until the end of the reaction. After termination of the reaction, the mixture was filtrated and the sediment was purified by Soxhlet extraction in the mixture of acetone and ethanol at 60◦C for 24–48 h to remove the residual monomers and ungrafted copolymers. The grafting ratio was calculated by the weight change of the paper sheets before and after the modification.And the formula used is shown in Eq. (1).