Processing and Properties of Nanostructured Biocomposites

Abstract: In this work, nanostructured biocomposite fibers and films with cellulose nanofibers (CNF), cellulose nanocrystals (CNC) and chitin nanocrystals (ChNC) were prepared using solutions mixing followed by electrospinning and melt compounding. The main processing challenges for these materials were to find parameters for: 1) fiber alignment in electrospinning, 2) feeding the nanomaterials into the extruder and 3) dispersion and distribution of the nanomaterials in the polymeric matrix. This thesis consists of three publications, which are summarized below.The first study was about random and aligned cellulose fibers prepared by electrospinning. Cellulose acetate (CA) was used as a matrix and a mixture of acetic acid and acetone (1:1) was used as a solvent. CNC with different concentrations (0–5 wt-%) were used as reinforcement. Microscopy studies showed fibers with smooth surfaces, different morphologies and diameters ranging between 200 and 3300 nm. It was found that the fiber diameters decreased with increased CNC contents. The microscopy studies also indicated well-aligned fibers. Results from dynamic mechanical thermal analysis indicated improved mechanical properties with the addition of CNC. The storage modulus of electrospun CA fibers increased from 81 to 825 MPa for fibers with 1 wt% CNC at room temperature. X-ray analysis showed that the electrospun CA fibers had a crystalline nature and that there was no significant change in crystallinity with the addition of CNC.In the second study, polylactic acid (PLA) and its nanocomposite based on CNF and glycerol triacetate (GTA) were prepared using a co-rotating twin-screw extruder. GTA was used as a plasticizer, a processing aid to facilitate nanofiber dispersion and as a liquid medium for feeding. The optical, thermal and mechanical properties were characterized and the toughening mechanism was studied. The addition of GTA (20%) and CNF (1%) resulted in increased degree of crystallinity and thus decreased optical transparency. Furthermore, these additives showed a positive effect on the elongation at break and toughness, which increased from 2 to 31% and from 1 to 8 MJ/m3, respectively. A combination of nanofiber-matrix interfacial slippage and a massive crazing effect is suggested for PLA toughening. CNF were expected to restrict the spherulite growth and therefore enhance the craze nucleation. In the third study, triacetate citrate plasticized poly lactic acid and its nanocomposites based on cellulose nanocrystals (CNC) and chitin nanocrystals (ChNC) were prepared using a co-rotating twin-screw extruder. The materials were compression molded to films using two different cooling rates. The cooling rates and the addition of nanocrystals (1 wt%) had an impact on the crystallinity as well as the optical, thermal and mechanical properties of the films. The fast cooling resulted in more amorphous materials, increased transparency and elongation to break, (approx. 300%) when compared with slow cooling. Chitin nanocomposites were more transparent than cellulose nanocomposites; however, microscopy study showed presence of agglomerations in both materials. The mechanical properties of the plasticized PLA were improved with the addition of a small amount of nanocrystals resulting in PLA nanocomposites suitable for use in film blowing and thus packaging applications. Summing up, this thesis shows that solution mixing followed by electrospinning can be used to produce reinforced green nanocomposite fibers with random or aligned orientation with, probably, potential to be used in membranes, filters or even in medical applications. It was also shown that PLA-CNF nanocomposites can be prepared using extrusion and liquid feeding and that small amounts of CNF changed the fracture mechanism, resulting in increased toughness. In addition, the cooling rate of the plasticized PLA and its nanocomposite films was found to significantly impact the film properties.