Review on biomedical and bioengineering applications of cellulose sulfate

Nanocelluloses, including nanocrystalline cellulose, nanofibrillated cellulose and bacterial cellulose nanofibers, have become fascinating building blocks for the design of new biomaterials. Derived from the must abundant and renewable biopolymer, they are drawing a tremendous level of attention, which certainly will continue to grow in the future driven by the sustainability trend. This growing interest is related to their unsurpassed quintessential physical and chemical properties. Yet, owing to their hydrophilic nature, their utilization is restricted to applications involving hydrophilic or polar media, which limits their exploitation. With the presence of a large number of chemical functionalities within their structure, these building blocks provide a unique platform for significant surface modification through various chemistries. These chemical modifications are prerequisite, sometimes unavoidable, to adapt the interfacial properties of nanocellulose substrates or adjust their hydrophilic-hydrophobic balance. Therefore, various chemistries have been developed aiming to surface-modify these nano-sized substrates in order to confer to them specific properties, extending therefore their use to highly sophisticated applications. This review collocates current knowledge in the research and development of nanocelluloses and emphasizes more particularly on the chemical modification routes developed so far for their functionalization.

When used as fillers in polymer composites, the thermostability of cellulose crystals is important. Sulfate groups, introduced during hydrolysis with sulfuric acid, are suspected to diminish the thermostability. To elucidate the relationship between the hydrolysis conditions, the number of sulfate groups introduced, and the thermal degradation behavior of cellulose crystals, bacterial cellulose was hydrolyzed with sulfuric acid under different hydrolysis conditions. The number of sulfate groups in the crystals was determined by potentiometric titration. The thermal degradation behavior was investigated by thermogravimetric analysis. The sulfate group content increased with acid concentration, acid-to-cellulose ratio, and hydrolysis time. Even at low levels, the sulfate groups caused a significant decrease in degradation temperatures and an increase in char fraction confirming that the sulfate groups act as flame retardants. Profile analysis of the derivative thermogravimetric curves indicated thermal separation of the degradation reactions by the sulfate groups into low- and high-temperature processes. The Broido method was used to determine activation energies for the degradation processes. The activation energies were lower at larger amounts of sulfate groups suggesting a catalytic effect on the degradation reactions. For high thermostability in the crystals, low acid concentrations, small acid-to-cellulose ratios, and short hydrolysis times should be used.