Processing, structure, and properties of polyacrylonitrile-nanocellulose composite films and fibers

Abstract

Cellulose is the most abundant biopolymer on earth with an estimated production of 1.5 x 1012 tons per year. With the high production, cellulose could be utilized to increase our sustainability and reduce our dependence on synthetic polymers made from oil. Recently nanocellulose in the form of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) as potential fillers have been gaining significant interest. This is due to nanocellulose’s high mechanical properties, high surface area, biodegradability, biorenewability, and low toxicity.Polyacrylonitrile (PAN) is a synthetic polymer and has uses in clothing, home furnishings, and filtration membranes. PAN is also the predominant precursor for carbon materials such as carbon fiber. The effect of CNC loading on the mechanical, thermal, optical, and structural properties of PAN-CNC films was studied. Films were made successfully with up to 40 wt% CNC loading. These composite films demonstrated the same optical transparency, and an increase in mechanical properties when compared to neat PAN. The effect of CNC loading on rheology was also looked at, and it was found that the addition of CNCs increases the viscosity at low shear rates. It was also found that the viscosity of the PAN-CNC suspensions increased with time. The stabilization kinetics of the PAN-CNC films, and structural changes of the films were investigated. It was found that the addition of CNCs reduced the cyclization activation energy. It was also found that the reaction rate of cyclization and oxidation could increase with the addition of CNCs. The thermal stability of CNCs was determined to be higher in the composite films than in neat form, which was examined by wide angle x-ray diffraction and Fourier transform infrared spectroscopy. A method to make sulfonated CNFs that has the same surface chemistry as CNCs made by sulfuric acid hydrolysis is developed. This sulfonation of CNFs improves the dispersibility of CNFs in solvent. The method developed in this study can produce sulfonated CNFs faster the other reported sulfonation method. This technique is also faster than the most common CNF functionalization, TEMPO oxidation. The chemical stability of TEMPO CNFs and sulfonated CNFs in water and dimethylformamide was also investigated. The characterization of PAN-CNF fibers and the effect of processing on the resulting mechanical properties was studied. Two different methods of functionalization were performed on the CNFs, TEMPO oxidation and amination, and the reinforcement effect of the different functionalizations were studied. The aminated CNFs provided more mechanical reinforcement than the TEMPO CNFs. This is attributed to the difference between surface chemistry, and the better interaction between the amine groups on the aminated CNFs and PAN than the carboxylic acid groups on the TEMPO CNFs.