Interactions between Cellulose Nanocrystals and Polymer Chains in Aqueous Solutions

• Author / Creator

  Oguzlu, Hale

• Cellulose nanocrystal (CNC), a new breathtaking nanomaterial, is of scientific and commercial interest because of its low density, renewability, biodegradability, non-toxity and wide range of application potential such as personal care, paints, pharmaceuticals, coatings, drilling fluids, etc (1). Besides, non-isotropic orientation or percolation of rod-shaped CNC suspension results in self-assembly, elasticity, thixotropy and birefringence at low concentration. With the presence of electrolytes, polymer solutions, micelles, foams and other particles, the degree of ordering of CNCs can be controlled in mesoscopic level, leading to certain macroscopic properties useful for a wide range of products. Therefore, in this research, interactions between CNCs particles with neutral and anionic polymer chains in aqueous solutions were investigated in terms of rheological measurements, structure formation, stability of particles, and Nuclear Magnetic Resonance (NMR) measurements. Regarding rheological measurements, studies were conducted on the steady-state shear flow and linear viscoelastic properties of CNC suspensions with/without carboxylmethyl cellulose (CMC) and polyethylene oxide (PEO). An addition of a low amount of CNC concentration in water does not generate significant changes in the steady shear viscosity. This behavior is reasonable because dilute and semi-dilute CNC suspensions were in the isotropic state. By contrast, adding both dilute (0.5%) and semi-dilute entangled (1.00%, 2.00% and 3.00%) CMC solutions in CNC suspensions, within the concentration range of 0.33-2.00 vol%, increased the viscosity drastically. When linear viscoelastic properties was measured, it was found that the structure formation in CNC-CMC mixtures led to G’ >G’’ and gel-like behavior. Adding polyethylene oxide (PEO) in CNC suspensions showed an unexpected shear viscosity trend than CMC solutions and didn’t cause any significant change in the steady-shear viscosity and viscoelasticity. Therefore, the presence of anionic CMC and non-ionic PEO in CNC suspensions led to different phase behaviors due to different interaction mechanisms. The structure formation was investigated with the use of a Scanning Transmission Electron Microscopy (STEM) and a Polarized Optical Microscopy (POM). Ordered CNC structures, as nematic flocs, were observed in STEM images of dilute CMC-CNC solutions. POM images of CNC-CMC solutions showed phase transitions from the isotropic state to nematic and chiral nematic phases, indicating depletion interactions. In the case of dilute PEO-CNC solutions, there were no specific alignments of CNC particles in PEO matrix. However, birefringence structures, which are a sign of the nematic order of CNC particles, were observed in the case of 2.00 vol% CNC suspensions in 5.0 wt% PEO solutions. Stability of particles was studied in terms of dynamic light scattering and zeta potential measurements. The diffusion coefficient of CNC decreased in the presence of CMC chains since the depletion flocculation was more probable. Moreover, the values of electrophoretic mobility didn’t change in the presence of CMC chains. The adsorption of PEO chains onto the surface of CNCs increased the zeta potential values from -56 mV to the range between -29 and -22 mV. 1H spin-lattice relaxation NMR data direct techniques for understanding interactions of CMC and PEO chains with CNC particles. The minima of the specific relaxation rate, (R2), constants with respect polymer concentration showed depleted CNC nanoparticles in CMC and PEO solutions. In the case of CNC-PEO solutions, obtaining two different spin-spin relaxation time (T2) values indicates the adsorption of PEO chains onto CNC. Consequently, the depletion flocculation was the main interaction mechanism between CMC chains and CNC particles in an aqueous medium. This mechanism created nematic flocs of CNC particles. The presence of blob structures from nematic flocs in CNC-CMC solutions resulted in a dramatic increase in the steady-steady viscosity and gel-like behavior. The PEO adsorption onto CNC surfaces weakened depletion interactions so viscosity and viscoelasticity of CNC suspensions didn’t change significantly in the presence of PEO chains. This study explained that the different interaction mechanisms between polymers and CNC nanoparticles can generate different structure and rheological behaviours.

• Subjects / Keywords

  • polyethylene oxide
  • carboxymethyl cellulose
  • rheology
  • nanocrystal
  • depletion
  • cellulose
  • adsorption

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3W669R1B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.