Materials and methods
PLA (NatureWork™ PLA 3001D) in pellet form with a density of 1.24 g cm-3 and melt flow index (MFI) of circa 15 g 10 min-1 (190°C 2.16 kg-1) was purchased from NatureWork® LLC (Minnetonka, MN USA). MCC obtained from cotton linter with an average particle size of 50 цш was supplied by Sigma-Aldrich (Avicel; type PH-101). Organo-modified MMT (Nanomer 1.30TC) was obtained from Nanocor Inc. (Arlington Heights IL, USA). Nanomer 1.30TC is organically modified with approximately 30 wt% of octadecylamine and has a mean dry particle size of 16—22 ^m. Chloroform, N,N-dimethylacetamide (DMAc) (99% purity), lithium chloride (LiCl) (99% purity), and sulfuric acid (H2SO4) (96% purity) were purchased from Merck, Malaysia.
Preparation of CNW
As described by Oksman et al. (2006) and Arjmandi et al. (2015b), CNW were prepared by swelling MCC and separating to whiskers by a sequence of chemical and ultrasonication treatments using DMAc and 0.5% LiCl solution as swelling agents. The initial concentration of MCC in DMAc/LiCl was 10 wt%. Then, MCC was vigorously agitated using a mechanical stirrer inside a water bath at 70°C for 12 h to swell the MCC particles. Subsequently, the slightly swelled particles were sonicated using an ultrasonic bath (Branson 2510) for 3 h over a period of 5 days with long intervals between each sonication treatment to separate the CNW. The gel-like suspension of CNW was repeatedly washed with distilled water, then refrigerated at 4°C and designated as CNW-CS.
Colloidal suspension of CNW in water was prepared as described in detail elsewhere (Bondeson et al., 2006; Arjmandi et al., 2016). 5 g of MCC was hydrolyzed in 64% of H2SO4 solution. The acid to MCC ratio was 8.75 mL g_1 and hydrolysis was carried out at 40°C for 60 min under strong agitation. The ongoing hydrolysis was stopped by adding 4—5 times cold distilled water based on the volume of the reacting mixture. The diluted suspension was centrifuged using a Universal 32 Hettich centrifuge (Newport Pagnell, England) at 5000 rpm for 15 min to get the precipitates. The precipitates were again suspended in distilled water, followed by a centrifugation. This process was repeated until the supernatant solution became turbid. The colloidal suspension was then collected and sonicated for 30 min. The sonication was carried out in a cold water bath to avoid heat-up. Subsequently, the produced suspension was stored in a refrigerator at 4°C and designated as CNW-AH.