Measurement of length and thickness of cellulose nanocrystals using asymmetrical flow FFF and multi-angle light scattering. By Dr Thorsten Klein, Dr Robert Reed & Dr Soheyl Tadjiki
Cellulose is the most abundant natural polymer on Earth and can be found in plants, marine animals, algae and bacteria. Cellulose nanocrystals (CNCs) are mostly isolated from native cellulose using acid hydrolysis. CNCs have several attractive properties such as low thermal expansion, optical transparency, renewability, biodegradability, low toxicity and low cost. They have been used widely in industrial applications such as polymer reinforcing, photonic crystals, drug delivery, water treatment membranes and biomedical devices.
Thorough size characterisation of CNCs is important since their rheological and control processing properties can be affected by their size, shape and surface area. Asymmetrical flow field-flow fractionation (AF4) coupled to multi-angle light scattering (AF4-MALS) is an alternative to bulk characterisation techniques such as transmission electron microscopy (TEM), atomic force microscopy (AFM) and dynamic light scattering (DLS). The main advantages of AF4-MALS over the bulk characterisation techniques are high resolution and fast analysis time.
In this study a CNC sample isolated from softwood sulphite pulp was analysed by AF4-MALS to measure the length and thickness of the crystals as a function of size.
Fig. 1 shows an AFM image of the bulk CNC sample. The sample had a broad length distribution with the crystal length ranging from 20 to 550nm. The fractogram of the CNC sample obtained from AF4-MALS analysis is illustrated in Fig. 2, where the scattered light intensity measured by MALS (90° angle, solid black line) was plotted against retention time. The root mean square (RMS) and hydrodynamic radii profiles were also plotted as a function of retention time and shown in Fig. 2. The RMS radius (Rg, red solid circles) calculated from MALS suggested a radius range of 40-60nm across the CNC distribution. Assuming a thin rod structure for the crystals the RMS radius can be converted to rod length and rod volume using the expressions below:
Rod Length = 2√3 Rg (1)
Rod Volume = π/2 √3 Rg τ2 (2)
where τ is rod thickness
Using FFF theory, the retention time can be converted to hydrodynamic radius (Rh, blue solid circles in Fig. 2).
The Rh profile of the CNC sample showed an equivalent spherical radius range of 5-30nm. The CNC hydrodynamic volume can be calculated from Rh using the expression:
Hydrodynamic volume = 4π/3 Rh3 (3) Using expressions (2) and (3), CNC thickness can be calculated from: τ Rod thickness τ ≈ 1.24√(Rh3/Rg) (4)
Fig. 3.1 compares measured CNC length distributions obtained from AFM and AF4-MALS techniques. The AF4-MALS distribution was obtained from the RMS radius data using expression (1). Both techniques showed similar distributions with an average length of 170nm.
Fig. 3.2 illustrates the profile of CNC thickness calculated from expression (4), across the CNC distribution. The CNC thickness ranged from 1nm to 20nm with an average value of 4.3±3.4nm. Comparing the trends of CNC thickness and RMS radius may suggest aggregation (bundling) of single CNCs for the upper end of the distribution.
POSITIVE RESULTS
AF4-MALS was successfully employed to characterise a polydisperse CNC sample for measurement of size and thickness in a relatively short analysis time of 40 min. The CNC length distribution was in a good agreement with the result of the AFM analysis. CNC Rh (obtained from AF4) and RMS radii (obtained from MALS) are used to calculate CNC thickness. The CNC thickness profile showed a broad distribution suggesting aggregation of single crystals in the larger end of the distribution.
