Nanoindentation of biodegradable cellulose diacetate-graft-poly(L-lactide) copolymers:: Effect of molecular composition and thermal aging on mechanical properties

Nanoindentation of cellulose diacetate-graft-poly(lactide)s (CDA-g-PLLAs) synthesized by ring opening graft copolymerization of L-lactide in bulk onto the residual hydroxyl positions on CDA were conducted to investigate the effect of the molecular composition and thermal aging on mechanical properties and creep behavior. Continuous stiffness measurement (CSM) technique was used to obtained hardness and elastic modulus. These material properties were expressed as a mean value from 100 to 300 nm depths and an unloading value at final indentation depth. The hardness and elastic modulus in all CDA-g-PLLAs were higher than those in pure CDA, indicating that the introduction of PLLA increases the hardness and elastic modulus. With an increase of crystallinity by thermal aging, the hardness and elastic modulus were increased in both CDA-g-PLLA and PLLA. The creep test performed by CSM showed that the creep strain of CDA was decreased by the grafting of PLLA. Thermal aging decreased the creep strain of CDA-g-PLLA and PLLA. With an increase of holding time, hardness was decreased, whereas elastic modulus was kept almost constant. (c) 2007 Wiley Periodicals, Inc.