The development of sustainable polymeric materials has been receiving increasing attention largely due to the rising environmental awareness and stringent governmental regulations. Biorenewable resource based biodegradable polylactic acid (PLA) is one of the most promising alternatives to petroleum-based polymers. Cellulose nanofibers (CNFs), derived from renewable resources, are being regarded as the next generation renewable reinforcements for the production of high performance biocomposites. The combination of PLA and CNFs not only endows the resulting composites with fully biodegradable feature, but also gives them significantly improved performance properties. In this context, the objective of this thesis is to develop uniform fine-cell PLA/CNF composite foams with improved mechanical properties. The material properties of PLA/CNF biocomposites, foaming mechanisms, and the mechanical properties of the composite foams were investigated.

During the isothermal and non-isothermal crystallization process, CNFs, acting as effective crystal nucleating agents, enhanced the crystal nucleation density, decreased the crystallite sizes, and accelerated the crystallization of PLA. Pressurized CO₂ also affect the crystallization kinetics of PLA/CNF composites significantly. The addition of CNFs increased the melt viscosity of PLA dramatically due to the formation of 3D network structure of CNFs and a good interaction between PLA and CNFs. In-situ observation revealed that CNFs were effective cell nucleating agents during foaming. Compared to the neat PLA and PLA/micro-size fiber composite foams, PLA/CNF composite foams showed a much finer and uniform cell structure. Different void fractions of PLA/CNF composite foams were obtained by altering the processing conditions. The flexural strength and modulus of the composite foams decreased with increased void fraction. Comparable specific flexural strength to the neat PLA was achieved in PLA/CNF composite foams and the toughness was increased up to 3.5 times. Notched Izod impact strength was improved by 55 % at 70 % void fraction. The addition of CNFs also improved foams' dynamic mechanical properties.