Green and Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications

Various research has been conducted on combination of cellulose nanocrystals with either thermoplastic or thermoset matrices to produce high performance nanocomposites. Poly (L-Lactide) despite its advantages and promises on production of bio-based polymer on industrial scale suffer shortcomings such as poor oxygen barrier properties and low heat distortion temperature. Further research done on this has led to significant improvements in Poly (L-Lactide) thermochemical properties.

Reinforcing Poly (L-Lactide) with cellulose nanocrystals provide a good avenue for fully bio-based material with enhanced performance including heat distortion temperature. Despite these good performance, the aggregation and degradation of cellulose nanoparticles during high temperature has limited their use. Improvement of surface modification of cellulose nanoparticles to reduce its aggregation and enhance their dispersion can however be achieved by silylation and polymer grafting reactions.

Spinella et al. (2016) modified cellulose nanocrystals (CNC) with poly (methyl methacrylate) PMMA in order to take advantage of its miscibility with various bio-based polymers including poly (L-Lactide) PLLA when melt-blended. The work published in ACS Sustainable Chemistry & Engineering, paid attention on two grafting techniques in water medium using two different redox initiators: Fenton’s reagent and ceric ammonium nitrate.

The authors demonstrated that poly (methyl methacrylate) grafts modifications on cellulose nanoparticle surface improves interfacial interactions between poly (L-Lactide) and corresponding modified cellulose nanoparticles. Efficient redox-initiated free radical polymerization or grafting of (meth) acrylic monomers or polysaccharides were easily performed under water with common initiators such as Fenton’s reagent and ceric ammonium nitrate for grafting poly (glycidyl methacrylate) and poly (2-hydroxyethyl methacrylate) on cellulose fibers.

They performed graft polymerization of methyl methacrylate on cellulose nanoparticles using Fenton’s reagent while graft polymerization of methyl methacrylate on cellulose nanoparticles using ceric ammonium nitrate .

Methyl methacrylate grafting conversions achieved by Fenton’s reagent and ceric ammonium nitrate were 60% and 45% respectively. However, ceric ammonium nitrate gave a much higher grafting efficiency of 77% when compared to Fenton’s reagent which gave a low grafting efficiencies of 12% corresponding to previous studies on vinyl polymer of starch, cellulose nanofibers and nanocrystals.

After purification, poly (methyl methacrylate) grafted on surface of cellulose nanoparticles by Fenton’s reagent and ceric ammonium nitrate is 15% and 50% by weight respectively. Cellulose nanoparticles modified by Fenton’s reagent is known as CNC-g-PMMA₁₅ while that of ceric ammonium nitrate is CNC-g-PMMA₅₀.

Transmission electron microscopy images showed unmodified cellulose nanoparticles of a typical rod-like morphology with large aggregates of individual cellulose nanoparticles due to hydrophilic nature of cellulose nanoparticles and corresponding strong interactions between particles. Transmission electron microscopy images of CNC-g-PMMA₁₅ had similar appearance and features to that of unmodified cellulose nanoparticles. In contrast, morphology of CNC-g-PMMA₅₀ was dramatically different from unmodified cellulose nanoparticles and CNC-g-PMMA₁₅ with images showing spread apart, individualized and embedded cellulose nanoparticles with clear reduction if its aggregates. The result depicts drastic reduction in amount of free hydroxyl groups on the surface of cellulose nanoparticles available for inter-particle interactions.

Addition of unmodified and poly (methyl methacrylate)-modified cellulose nanoparticles significantly increased storage modulus . The amount of poly (methyl methacrylate) grafted on cellulose nanoparticles surface and related dispersion state of cellulose nanoparticles allow dramatic increase in poly (L-Lactide) storage modulus both at ambient and elevated temperatures. It was also discovered that CNC-g-PMMA₅₀ affording a 3D network in poly (L-Lactide) nanocomposites displayed the best thermomechanical performance. The high dispersion state also allowed a positive effect on oxygen permeability of poly (L-Lactide) and strong beneficial effect on heat deflection temperature reaching outstanding temperature (>130⁰C).

The authors’ findings on green grafting method in water showed that it could efficiently increase cellulose nanoparticles dispersion in various bio-based polymers via melt dispersion.