Enzymatic engineering of nanometric cellulose for sustainable polypropylene nanocomposites

In this work, the influence of enzymatic modification on the properties of nanocellulose reinforced polypropylene nanocomposites was investigated in detail. Methods such as high-performance liquid chromatography coupled with a refractometric detector, differential scanning calorimetry, X-ray diffraction, dynamic light scattering, and optical microscopy were used in this study. In addition, the tensile properties of polymer nanocomposites were tested. Enzymatic modification of nanocellulose was carried out using cellulases from microorganisms Trichoderma reesei and Aspergillus sp. (Viscozyme®L). It was found that controlled, enzymatic hydrolysis of nanocellulose resulted in significant changes in the morphology, super molecular structure, as well as nucleating behaviour of transcrystalline structures. Because of the modification, nanocellulose was obtained with an increased amount of the crystallinity phase (70 %) and the content of nanometric particles with a size below 100 nm at the level of about 94 %. In addition, an interesting correlation between kinetic parameters (crystallization temperature, degree of phase conversion, crystallization half times) and creation of different crystal phase of the polymer matrix was established. Composites containing nanocellulose modified in the enzymatic process have been characterized by the presence of a significant amount of additional β-PP polymorph at the level of 40 %, as well as very good nucleation properties, e.g. an increase in the crystallization temperature by 5 °C, a reduction of crystallization half times and induction times of transcrystalline structures by approx. 30 %. Besides, tensile tests of nanocomposites proved that types of enzymatic modification have an important impact on the morphology of nanocellulosic fillers, and as a result, determines the polypropylene nanocomposite’s mechanical performance. It is worth mentioning that the introduction of nanocellulose modified with the Trichoderma reesei enzyme into the polypropylene matrix resulted in the production of composites with a significantly increased value of strength at break 36 MPa and high values ​of elongation at break (over 80 %). Furthermore, the use of specific enzymes enables procurement of a nanocellulose with strictly defined dispersive characteristics that also affects the supermolecular structure of nanocomposites, and consequently the mechanical properties. Depending on the content of individual polymorphs, it is possible to control the flexibility of produced nanocomposite materials.