Method for determining molecular weight of cellulose

Determination of cellulose molecular weight is a key step in cellulose chemistry research. Since cellulose is a linear polymer and different sources and treatment methods will affect its molecular weight, scientific determination of its molecular weight is crucial for the processing and application of cellulose materials. Commonly used cellulose molecular weight measurement methods include viscosity method, gel permeation chromatography (GPC), light scattering method and terminal group analysis. The following are detailed descriptions of these methods:

1. Viscosity method

Viscosity method is the most commonly used indirect method to determine the molecular weight of cellulose. This method is based on the empirical relationship between the viscosity of a polymer solution and its molecular weight.

• Basic principle : By measuring the intrinsic viscosity [η] of the cellulose solution and combining with the Mark- Houwink equation, the viscosity average molecular weight Mv of cellulose is calculated:

[η]= K ⋅ Mv a

Among them, K and a are the empirical constants of solvent and polymer systems.

• step :

1. Dissolve cellulose in an appropriate solvent.

2. Use a capillary viscometer to measure the viscosity of solutions at different concentrations.

3. By calculating the specific viscosity and intrinsic viscosity, the molecular weight is finally calculated by the Mark- Houwink equation.

• Advantages : Simple operation and low instrument requirements.

• Disadvantage : The measurement result is viscosity-average molecular weight (only reflects the average value of the overall distribution), and no molecular weight distribution information can be obtained.

2. Gel Permeation Chromatography (GPC)

GPC is a chromatography technology based on the principle of size exclusion, used to determine the molecular weight and molecular weight distribution of polymers.

• Basic Principle : GPC uses gel columns to separate molecules of different sizes. Large molecules flow out of the column first, and small molecules flow out later. Polymers of different molecular sizes can be determined by a detector (usually a refractive index detector or an ultraviolet detector) and molecular weights can be calculated by comparison with known standard substances such as polystyrene.

• step :

1. Dissolve cellulose in a suitable solvent.

2. Inject the solution into the GPC system and separate the molecules through a chromatographic column.

3. Obtain the distribution curve of the effluent through the detector and calculate the molecular weight based on the calibration curve.

• Advantages : The molecular weight distribution can be obtained, including number average molecular weight ( MnM_nMn ), weight average molecular weight ( MwM_wMw ), distribution index and other information.

• Disadvantages : Precision instruments and appropriate solvents are required, and the operation is complicated.

3. Light scattering method

Light scattering method is a technique that directly determines the molecular weight of a polymer by analyzing the scattered light intensity in a polymer solution.

• Basic principle : The molecules in polymer solution will scatter light, and the scattered light intensity is related to the molecular weight. By analyzing the scattered light intensity, the molecular weight can be calculated.

• step :

1. Dissolve cellulose in solvent.

2. Use a multi-angle laser light scattering meter to measure the scattered light intensity of the solution.

3. Data fitting is performed through Zimm graphs to obtain the number average molecular weight and weight average molecular weight.

• Advantages : The molecular weight of a polymer can be directly measured and is suitable for high molecular weight samples.

• Disadvantages : The instrument is expensive, complex in operation, and the measurement accuracy is greatly affected by the solvent and sample concentration.

4. Terminal group analysis method

This method calculates the molecular weight of the polymer by analyzing the number of terminal groups of the polymer.

• Basic Principle : For oligomers or polymers with lower molecular weight, each molecule usually has two terminal groups. The molecular weight of the polymer can be calculated by measuring the concentration of terminal groups by chemical or spectroscopy.

• step :

1. Quantitative analysis of the terminal groups of cellulose samples (such as by NMR or chemical titration).

2. Calculate the number of terminal groups per mole of polymer, and then calculate the number average molecular weight MnM_nMn of the polymer。

• Advantages : It is suitable for polymers with lower molecular weight, and the method is simple.

• Disadvantages : Not suitable for high molecular weight polymers, and the accuracy is limited by the accuracy of terminal group analysis.

5. Precipitation Titration Method

Precipitation titration is often used to determine the degree of substitution of cellulose (such as the degree of substitution of carboxymethylcellulose) to calculate the molecular weight.

• Basic principle : By titrating the functional groups in the cellulose solution, determining its molar content, and then calculating the molecular weight.

• step :

1. Dissolve the sample in water or appropriate solvent.

2. Use standard solution for titration until a complete reaction occurs.

3. Calculate the molecular weight by calculating the volume consumed by titration.

• Advantages : Suitable for the molecular weight determination of functionalized cellulose.

• Disadvantages : Low accuracy and can only be used for specific types of cellulose (such as carboxymethylcellulose).

6. Ultracentrifugation

This method separates polymers of different molecular weights by efficient ultracentrifugation and calculates the molecular weight based on the sedimentation rate.

• Basic principle : Molecules settle according to their molecular weight and shape under the action of centrifugal force of high-speed rotation, and the settlement rate is proportional to the molecular weight.

• step :

1. Dissolve the cellulose in the solvent and put it in an ultracentrifuge.

2. Measure the sedimentation rate of different molecules through centrifugal force.

3. Calculate molecular weight by analyzing the settlement data.

• Advantages : Suitable for high molecular weight polymers.

• Disadvantages : High-precision equipment is required and the operation is complicated.

7. Nuclear magnetic resonance (NMR) method

NMR can infer the molecular weight of cellulose by measuring the characteristic signals in the polymer structure.

• Basic Principle : NMR can analyze the chemical environment in cellulose molecules and calculate the molecular weight by analyzing the proportion of terminal group signals.

• step :

1. Dissolve cellulose in a suitable solvent.

2. Determine its structure by NMR and analyze the terminal group and repeat unit signals.

3. Calculate the number of terminal groups and calculate the molecular weight.

• Advantages : The molecular structure and molecular weight can be analyzed simultaneously.

• Disadvantages : Only suitable for lower molecular weight samples.

Summarize

Different assay methods are suitable for different cellulose types and molecular weight ranges. The viscosity method is simple and practical, suitable for routine measurement; GPC can provide detailed molecular weight distribution information; light scattering method is suitable for measuring high molecular weight cellulose. The terminal group analysis method and NMR method are more suitable for samples with lower molecular weight. The choice of suitable assay methods depends on the specific structure, molecular weight range and experimental requirements of the cellulose.