Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel.

Ville Hynninen,Sami Hietala,Lasse Murtomäki,Jason R Mckee,Orlando J Rojas,Nonappa Nonappa,Olli Ikkala

doi:10.1021/acs.biomac.8b00392

Ville Hynninen, Sami Hietala + Show 5 more

Open Access

https://doi.org/10.1021/acs.biomac.8b00392

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Abstract

We show that composite hydrogels comprising methyl cellulose (MC) and cellulose nanocrystal (CNC) colloidal rods display a reversible and enhanced rheological storage modulus and optical birefringence upon heating, i.e., inverse thermoreversibility. Dynamic rheology, quantitative polarized optical microscopy, isothermal titration calorimetry (ITC), circular dichroism (CD), and scanning and transmission electron microscopy (SEM and TEM) were used for characterization. The concentration of CNCs in aqueous media was varied up to 3.5 wt % (i.e, keeping the concentration below the critical aq concentration) while maintaining the MC aq concentration at 1.0 wt %. At 20 °C, MC/CNC underwent gelation upon passing the CNC concentration of 1.5 wt %. At this point, the storage modulus (G′) reached a plateau, and the birefringence underwent a stepwise increase, thus suggesting a percolative phenomenon. The storage modulus (G′) of the composite gels was an order of magnitude higher at 60 °C compared to that at 20 °C. ITC results suggested that, at 60 °C, the CNC rods were entropically driven to interact with MC chains, which according to recent studies collapse at this temperature into ring-like, colloidal-scale persistent fibrils with hollow cross-sections. Consequently, the tendency of the MC to form more persistent aggregates promotes the interactions between the CNC chiral aggregates towards enhanced storage modulus and birefringence. At room temperature, ITC shows enthalpic binding between CNCs and MC with the latter comprising aqueous, molecularly dispersed polymer chains that lead to looser and less birefringent material. TEM, SEM, and CD indicate CNC chiral fragments within a MC/CNC composite gel. Thus, MC/CNC hybrid networks offer materials with tunable rheological properties and access to liquid crystalline properties at low CNC concentrations.

Highlights

Cellulose nanocrystals (CNCs) are rodlike, sustainable nanoparticles that can be extracted from wood and plant-based materials by strong acid hydrolysis.[1]
Methylcellulose (MC, Figure 1b) has attracted considerable interest due to its biocompatibility and ability to form thermosensitive gels.[40−46] It has been shown that, upon heating, the methyl cellulose (MC) chains in aqueous solutions aggregate into persistent fibrils with a hollow, ring-like lateral structure of ∼14 nm diameter and length of several hundreds of nanometers.[47−52] In water at room temperature (20−22 °C), MC chains exist in the form of random coils (Figure 1c,d).[52]
The morphology, surface charge, and aggregation behavior of freshly prepared cellulose nanocrystal (CNC) dispersed in aqueous media were characterized with Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS), and polarized optical microscopy (POM)

Summary

INTRODUCTION

Cellulose nanocrystals (CNCs) are rodlike, sustainable nanoparticles that can be extracted from wood and plant-based materials by strong acid hydrolysis.[1] Because of the negative surface charges caused by sulfate half-ester residues from the sulfuric acid hydrolysis (Figure 1a), CNCs form stable aqueous colloidal suspensions that are isotropic at low concentrations.[2−4] because of their high aspect ratio (typically 10−50), lyotropic liquid crystalline (LC) order emerges when a critical concentration is exceeded, ≥4−5 wt % in the case of cottonbased CNCs.[3,5,6] The LC assembly is left-handedly twisted and driven by the stacking of the inherently right-handed CNCs and can be detected by optical birefringence via polarized optical microscopy (POM) and optical probes.[3,7−9] The chiral nematic pitch and critical concentration can be tuned, for example by adjusting the electrostatic environment of the CNC or through polymer grafting.[10−14] The cholesteric structure is retained in dried CNC films, which broadens the scope for possible CNCbased photonic, plasmonic, and composite material applications.[8,15−21] Because of their excellent mechanical properties, aspect ratio, low density, tunable surface chemistry, and biocompatibility, CNCs have been studied as fillers and reinforcing additives in a variety of compositions ranging from cement paste to composite fibers and hydrogels.[22−32] pristine CNCs form gels at sufficiently high concentrations or when suitably modi-. The incorporation of CNC rods in a MC hydrogel above a certain concentration, and especially upon heating to 60 °C, would modify the rheological properties and optical birefringence due

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES