Stability of multifunctional lignin-g-PLGA films as a function of lignin type and lignin : PLGA ratio

Abstract

Biodegradable films were synthesized from lignin(LN)-grafted-PLGA polymers, and their stability was tracked over 12 months. The impact of the type of lignin, alkaline LN (ALN) and sodium lignosulfonate (SLN), and the LN : PLGA w/w ratio (1 : 4 and 1 : 6 w/w) on the mechanical, chemical, thermal and surface properties of the films was assessed. Films were made using an interphase formation process and were stored at two relative humidities (RH; 30% or 70%) for one year. Mechanical characterization revealed that ALN-g-PLGA films were stiffer than SLN-g-PLGA and control (PLGA) films. LN-grafted films had a glass transition temperature (T_g) of approximately 50.9 ± 4.6 °C, which remained consistent over 12 months at both RHs. For the LN-grafted films, the contact angles (CAs) and roughness coefficients (R_c) of the aqueous and organic sides differed. The aqueous side showed a lower CA and higher R_c, while the organic side had a higher CA and lower R_c, suggesting a direct correlation between wettability and roughness. The CA and R_c of most films showed no significant changes over time. UV (UV-A/B/C) shielding was above 95% for ALN-g-PLGA, and above 75% for SLN-g-PLGA films; in comparison, control PLGA films only blocked 67% of UV radiation at time zero. The UV-blocking by LN-g-PLGA films did not change over time, but for PLGA films it decreased from 67% to 23%. Our study shows that, compared with PLGA films, LN-g-PLGA films maintained their integrity for a longer period at both high and low RHs. Because of their higher durability, UV absorption properties and potential for tunability of mechanical and surface properties of the films, it is concluded that LN-g-PLGA films have a high versatility for applications ranging from packaging to coating materials.