Studies on the properties and formation mechanism of flexible nanocomposite hydrogels from cellulose nanocrystals and poly(acrylic acid)

A novel series of nanocomposite hydrogels based on cellulose nanocrystals (CNCs) and poly(acrylic acid) (PAA) have been synthesized by in situ free radical polymerization within an aqueous medium. Rheological measurements were applied to monitor the gelation process and results indicated that the gelation took place as monomers (acrylic acid, AA) grafted from the CNC surface and PAA chains entangled to produce flexible CNC–PAA gels. By tailoring the concentration of CNC (CCNC) over a wide range of 0.02–1 wt%, two critical CCNC, C* and C**, were found which corresponded to polymer chains that occurred in overlapping entanglements and promoted conformational rearrangements on the basis of earlier gel precursors, respectively. The formation mechanism of CNC based nanocomposite hydrogels, in which the nanoparticles transformed from the isolated state below C* to the spatially continuous percolation structure above C**, was proposed. The CNC–PAA gels exhibited excellent, composition-dependent mechanical properties, such as a large elongation ratio (>1100%) and high tensile strength (>350 kPa). Transmission electron microscopy (TEM) revealed that the CNCs were surrounded by grafted chains and formed inter-connected network structures, where the CNCs acted as multifunctional cross-links with an average effective functionality of 75. The mechanical measurements indicated that the increase of CCNC led to an increase in the hydrogels viscous characteristics and contributed to the energy dissipating mechanism, which was responsible for CNC–PAA gels excellent flexibility. The swelling and partial dissolution behaviors of the hydrogels were examined, focusing on the effect of CCNC on the gels characteristic partial deswelling and gel-to-sol transition. Some new chain entanglements were formed under concentrated conditions after drying treatment above the glass transition temperature (Tg) which was verified by observation of the greater tensile strength and modulus. All the results corresponded to the self-consistent network structure model for CNC–PAA gels.