Thermal stability and crystallization behavior of cellulose nanocrystals and their poly(l-lactide) nanocomposites: effects of surface ionic group and poly(d-lactide) grafting

Construction of stereocomplexation on the interface of cellulose nanocrystal (CNC) and poly(l-lactide) (PLLA) has been proven to be an efficient way to strengthen interfacial interaction of the nanocomposite. However, the surface structure variation of CNC with different types of ionic group after grafting of poly(d-lactide) (PDLA) and its influence on the properties of their PLLA nanocomposites are still unrevealed. In this current work, the sulfonated CNC (SCNC) and phosphorylated CNC (PCNC) are prepared and grafted with PDLA by surface-initiated ring-opening polymerization. The removal of ionic groups is observed on the surface of SCNC and PCNC after modification, but it has an opposite effect on the thermal stability of the grafted CNC particles. The thermal stability of SCNC-g increases compared with SCNC while PCNC-g decreases compared with PCNC. Consequently, the large difference in thermal stability of the pristine CNCs disappears after grafting and PCNC-g only shows slightly higher thermal stability than SCNC-g. Although the average PDLA grafting content is similar, the number and length of grafted chains are different on SCNC-g and PCNC-g. The grafted chain length on SCNC-g is 4 times as large as that on PCNC-g, which facilitates the formation of interfacial stereocomplex (SC) crystallites in the SCNC-g/PLLA nanocomposites. The SC crystallites improve the thermal stability of the nanocomposites and accelerate the matrix crystallization kinetics. PCNC-g/PLLA samples contrarily show earlier decomposition and lower crystallinity due to the absence of SC crystallites. This study could provide some insights into the effects of ionic groups in the surface grafting process of CNC and the influences on interfacial structure and property of the PLLA nanocomposite.