How to determine the molecular weight of cellulose by light scattering method

Light scattering is an analysis technology based on the interaction between light and matter, and is widely used to determine the molecular weight and molecular size of polymers (such as cellulose). The light scattering method is mainly divided into two types: static light scattering (SLS) and dynamic light scattering (DLS). Among them, static light scattering is used to measure the weight average molecular weight (Mw), root mean square rotation radius ( Rg ) and the second Vary coefficient (A2) of the polymer; dynamic light scattering is used to measure the diffusion coefficient and hydraulics of particles. Radius (Rh). The following will describe in detail how to determine the molecular weight of cellulose using light scattering method .

1. Basic Principles

1.1 Static Light Scattering (SLS)

When a monochromatic, parallel light irradiates into the polymer solution, the polymer in the solution will cause light scattering. The intensity of scattered light is related to parameters such as molecular weight, concentration and molecular size of the polymer. According to Rayleigh scattering theory, for small particles (particle size is much smaller than the light wavelength), the scattered light intensity is proportional to the molecular weight. By measuring the scattered light intensity at different concentrations and drawing a Zimm plot , it can be extrapolated to zero concentration and zero angle to obtain Mw and A2.

1.2 Dynamic Light Scattering (DLS)

Dynamic light scattering is based on the fluctuation of the scattered light intensity over time. The diffusion coefficient D of the particle is calculated using the autocorrelation function, and then the hydraulic radius Rh is calculated:

However, DLS is mainly used to measure the size distribution of particles, and the direct determination of molecular weight is not as accurate as SLS.

2. Experimental steps

2.1 Sample Preparation

Dissolve cellulose

Cellulose has a high degree of crystallinity and hydrogen bonding, and has poor solubility, so it is necessary to choose an appropriate solvent system. Commonly used solvents include:

• LiCl/DMAc system : Adding 8-10% lithium chloride (LiCl) to dimethylacetamide (DMAc) can effectively dissolve cellulose。

• Cupriethylenediamine (Cuen) : a solution of cupric ammonia complex, soluble in cellulose.

• NaOH/urea aqueous solution : At low temperature (such as -12°C), NaOH/urea aqueous solution can dissolve cellulose.

Things to note

• Avoid aggregation and gelation : The dissolution process should be fully stirred, and ultrasonic treatment should be used if necessary to ensure that the cellulose is completely dissolved and avoid aggregation between molecules.

• Solution filtration : Filter the solution using a filter membrane with a pore size of 0.45μm or less to remove undissolved particles and impurities.

2.2 Light scattering measurement

Instrument preparation

• Light Scattering Meter : A static light scattering Meter equipped with a multi-angle detector, such as a multi-angle laser light scattering Meter (MALLS).

• Temperature control system : Maintain a constant temperature, generally 25°C.

Measurement steps

1. Calibrate the instrument

oCalign the zero point and light intensity of the instrument using standard samples (such as pure solvents or standard substances with known scattering coefficients).

2. Measure the scattered light intensity of the solvent

oThe scattered light intensity I0 of the pure solvent is used as the background and needs to be deducted from the scattered light intensity of the sample.

3. Measure different concentrations of cellulose solutions

o Prepare a series of cellulose solutions at known concentrations (usually 4-5 concentration gradients).

oThe scattered light intensity III of each concentration solution is measured at different scattering angles (such as 30°, 45°, 60°, 75°, 90°, etc.).

4. Issues that need to be paid attention to in the experiment

4.1 Solvent selection

• Solvent mass : The solvent must be highly pure and impurities will affect the measurement of scattered light intensity.

• Interaction between solvent and cellulose : Choose solvents that are compatible with cellulose to avoid aggregation and gelation.

4.2 Dissolution of samples

• Full Dissolve : The cellulose must be completely dissolved, otherwise the undissolved particles will seriously affect the measurement results.

• Light-proof operation : Some solvents and cellulose may be sensitive to light and must be avoided during operation.

4.3 Temperature control

• Constant temperature : The temperature has an impact on the viscosity and refractive index of the solution, and a constant temperature must be maintained.

4.4 Selection of measurement angle

• Multi-angle measurement : In order to accurately extrapolate to zero angles, measurements need to be performed at multiple scattering angles.

• Avoid high angle measurements : The scattered light intensity at high angles is weak, the signal-to-noise ratio is low, and data reliability is reduced.

4.5 Data Reliability

• Repeat measurements : Repeat the measurements of each concentration and angle multiple times, take the average value, and improve the reliability of the data.

• Instrument calibration : Calibrate the instrument regularly to ensure the sensitivity and linearity of the detector.

5. Special considerations and challenges

5.1 Aggregation of cellulose

• Aggregation problem : The hydrogen bonding between cellulose molecules is strong, and it is easy to form aggregates, affecting the accuracy of the scattered light intensity.

• Solution :

oUltrasound treatment : Use ultrasonic treatment solution to break the aggregates.

oAddants : Add a small amount of deaccumulator, such as salt or surfactant to the solution, but be sure to not affect the measurement results.

5.2 Molecular conformation influence

• Rigid chain characteristics : Cellulose is a semi-rigid chain, and the conformation of the polymer chain will affect the scattering result.

• Correction method :

o Consider conformation factor : In data analysis, consider the chain conformation of cellulose and fit using an appropriate model.

oUse appropriate theoretical models : such as the improved Zimm model or Berry method.

5.3 High concentration effect

• Concentration dependence : At high concentrations, the interaction between molecules is enhanced, affecting the intensity of scattered light.

• measure :

oLow concentration measurement : Try to measure within the low concentration range to ensure linear relationship.

o Correction interaction : Correction of concentration effect by the second Vili coefficient A2.

6. Summary

Determining the molecular weight of cellulose using light scattering is a direct and effective method, and the weight average molecular weight and molecular size information of The key lies in the full dissolution of the sample, strict experimental conditions control and correct data analysis. It is important to note that the special structure and properties of cellulose can have an impact on measurements and appropriate measures must be taken to overcome these challenges.cellulose can be obtained.