Engineering cellulose nanofibril-polylactide composite films for high water vapor barrier food packaging.

While biopolymer films, such as polylactide (PLA) and cellulose nanofibril (CNF) films are attractive alternatives to synthetic plastics in food packaging, their poor water vapor barrier properties limit their commercial applications for moisture-sensitive products. Here, we demonstrate that triazabicyclodecene (TBD) catalyzes the ring-opening polymerization (ROP) of lactide into PLA and subsequent esterification of cellulose hydroxyl groups by PLA (i.e., grafting). Pure CNF films were immersed in a tetrahydrofuran (THF) solution containing lactide and TBD for 5 s, 2 h, and 24 h, followed by thermal treatment at 105 °C for 5 h. The procedure reduced the pore size distribution, moisture sorption, and water contact angles of the CNF films. While immersion time had minimal impact on physio-chemical properties, 24-h immersion resulted in higher PLA uptake, forming an additional PLA layer that sealed surface pores and reduced bulk pore connectivity. Films immersed for 24 h achieved water vapor transmission rates (WVTRs) as low as 5.4 g/m2·day, which approaches the WVTR of some synthetic plastics such as polypropylene (2.8 g/m2·day). These findings demonstrate the potential of biopolymer films as synthetic plastic alternatives in moisture-sensitive food packaging applications.