How to determine the molecular weight of cellulose by viscosity method

The viscosity method is a simple and effective method to determine the molecular weight of polymers, and is often used to determine the relative molecular weight of polymers such as cellulose. The specific process is based on the relationship between the intrinsic viscosity of the solution and the molecular weight of the polymer. The following are by viscosity :the general steps and principles for determining the molecular weight of cellulose

1. Basic Principles

In the viscosity method, the average molecular weight of cellulose can be calculated by measuring the viscosity of the cellulose solution and combined with empirical formulas such as the Mark- Houwink equation. The viscosity determination method depends on the relationship between the viscosity and molecular weight of the polymer solution, i.e.: [η]= K ⋅ Mv a where:

• [η] is the intrinsic viscosity (unit: dL/g) of cellulose, which is a dimensionless value derived from the viscosity data of the solution.

• Mv is the viscosity average molecular weight of cellulose.

• K and a are constants associated with polymer and solvent systems, usually determined by experiments.

2. Experimental steps

(1) Prepare cellulose solution

To dissolve cellulose in a suitable solvent, commonly used solvent systems include:

• Dimethylsulfoxide (DMSO)/tetraethylamine chloride (TEAC) solution;

• Cupric ammonia solution;

• Zinc phosphate solution.

It should be noted that cellulose has poor solubility, so pretreatment should be carried out before dissolution, such as treatment with alkali solution, or the appropriate dissolution temperature and time should be selected.

(2) Measure the viscosity of the solution

The viscosity of the cellulose solution is determined using a capillary viscometer (such as Upper Viscometer or Oswald Viscometer). Measurements are required during the experiment:

• The flow time t0 of the solvent (i.e. the flow time of the pure solvent when cellulose is not added).

• The flow time t of the solution containing cellulose.

The viscosity value can usually be calculated by flow time and the density of the solution.

(3) Calculate the intrinsic viscosity

By measuring the viscosity of cellulose solution at different concentrations, the relative viscosity is calculated using the following formula: ηrel =t / t0 where t is the flow time of the cellulose solution and t0 is the flow time of the pure solvent.

Next, calculate the incremental viscosity: ηsp =ηrel −1

Then, by linear fitting the specific viscosity and concentration relationship of cellulose solutions of different concentrations, extrapolated to zero concentration, the intrinsic viscosity [η]: [η]= lim ⁡ C → 0η sp / C where C It is the mass concentration of the solution (g/dL).

(4) Calculate molecular weight by Mark- Houwink equation

According to the Mark- Houwink equation: [η]= K ⋅ Mva Using known constants K and a (usually obtained by literature or experiments), the viscosity average molecular weight Mv of cellulose can be calculated.

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

• Solvent selection : Cellulose is difficult to dissolve in ordinary solvents. It is very important to choose an appropriate solvent system. Different solvents may affect the experimental results.

• Temperature control : Viscosity measurement is very sensitive to temperature and should be maintained during the experiment, usually around 25°C.

• Solution concentration : When measuring the viscosity of solutions at different concentrations, the concentration range should be appropriate and should not be too high or too low to ensure the accuracy of the linear relationship.

4. Summary

Viscosity method to determine the molecular weight of cellulose is an indirect determination method based on the flow characteristics of polymer solutions. By measuring the viscosity of cellulose solutions at different concentrations and combining with the Mark- Houwink equation, the viscosity average molecular weight of cellulose can be calculated. In the experiment, it is necessary to select the appropriate solvent, accurately control the temperature, and ensure the rationality of the solution concentration to obtain accurate measurement results.