How to determine the molecular weight of cellulose by gel permeation chromatography

Gel Permeation Chromatograph (GPC), also known as size exclusion chromatography (SEC), is an analysis method based on molecular volume differential separation of polymers. It is widely used to determine the molecular weight and molecular weight of polymers. distributed. GPC is suitable for determining the molecular weight of polymer materials such as The following are the steps and detailed principles for determining the molecular weight of cellulose using GPC.cellulose .

1. Basic Principles

GPC separates molecules of different sizes by using a column of porous fillers. Since macromolecules cannot enter small pores, they will pass through the column first, while small molecules can enter pores, thus delaying their outflow. By analyzing the retention time of the molecules in the chromatographic column, the molecular weight of the molecules can be calculated.

The results of GPC usually include:

• Number average molecular weight (Mn ): represents the average molecular weight of all molecules in the sample.

• Weight average molecular weight (Mw ): The average molecular weight weight weight weighted according to mass.

• Molecular weight distribution (PDI, Mw / Mn): measures the distribution width of molecular weight in the sample. PDI is close to 1, indicating that the molecular weight distribution is narrow. The larger the PDI, the wider the distribution.

2. Instruments and Equipment

• GPC system : includes solvent pumps, samplers, chromatographic columns, detectors (such as refractive index detectors, ultraviolet detectors or light scattering detectors) and data processing systems.

• Chromatographic column : A column filled with porous material, the pore size determines the separation ability of the molecules. Commonly used fillers include polystyrene gels, etc.

• Solvent : A solvent that dissolves cellulose. Common solvents include dimethyl sulfoxide (DMSO), tetrahydrofuran (THF) or chloroform, etc. The specific choice depends on the solubility of the cellulose .

• Standard substances : Polymers with known molecular weight (such as polystyrene standard samples) are used to establish calibration curves.

3. Experimental steps

(1) Sample preparation

• Dissolve the cellulose sample in an appropriate solvent. To ensure that the cellulose is completely dissolved, it usually needs to be carried out under heating or stirring.

• Remove undissolved impurities or large particles by filtration to avoid clogging the column.

• Sample concentration is usually between 0.1-1.0 mg/mL and is appropriately adjusted according to the nature of the sample.

(2) Chromatographic calibration

• Select suitable polymer standard samples (such as polystyrene) and their molecular weight and molecular weight distribution are known.

• Use these standard samples to perform GPC experiments, record their retention time (or volume) versus molecular weight, and draw calibration curves. The calibration curve is usually a linear relationship between retention time and molecular weight logarithm.

(3) Sample determination

• Inject the prepared cellulose solution into the GPC system. The column will separate the cellulose molecules according to the size of the molecule.

• Cellulose molecules of different molecular weights flow out of the column in turn, and the detector records their retention time or volume.

• The data processing system draws a chromatogram based on detection signals (such as refractive index or ultraviolet absorbance) to display the distribution of different molecular weights.

(4) Molecular weight calculation

• Using calibration curves, convert cellulose retention time into molecular weight information. By analyzing the peak shape of the chromatogram, the number average molecular weight (Mn ), weight average molecular weight (Mw ) and molecular weight distribution (PDI) of cellulose can be calculated.

• Common molecular weight calculation software can automatically process these data and output relevant molecular weight indicators.

4. Data analysis and interpretation

• Number average molecular weight Mn : Calculated by the following formula, it represents the average molecular weight of all molecules in the sample:

Mn =∑(Ni ⋅ Mi ) / ∑Ni

Among them, Ni is the number of each type of molecules, and Mi is the molecular weight of this type of molecules.

• Weight average molecular weight Mw : reflects the influence of larger molecules in the sample, and the calculation formula is:

Mw=∑(Ni ⋅ Mi 2) / ∑ (Ni ⋅ Mi)

Weight average molecular weight is more sensitive to macromolecules and is usually higher than the number average molecular weight.

• Molecular weight distribution PDIPDIPDI : represents the width of the molecular weight distribution in the sample. The calculation formula is:

PDI=Mw / Mn

Ideally, a PDI of 1 means that the molecular weights of all molecules in the sample are exactly the same. When the PDI is greater than 1, it indicates that there is a molecular weight distribution in the sample.

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

• Solvent selection : Cellulose has poor solubility, so it is very important to choose the right solvent. Commonly used solvents include DMSO, DMAc/LiCl (lithium chloride/dimethylacetamide), etc. Ensure that the cellulose is completely dissolved and that the solvent is compatible with the column.

• Column selection : Select a chromatographic column with an appropriate pore size range to ensure good resolution of cellulose samples within the target molecular weight range.

• Sample concentration : Too high concentration may cause the chromatographic peak to broaden or tail, and too low concentration may cause the detection signal to be too weak.

• Solvent effect : Different solvents have a greater impact on the extension or contraction of cellulose chains, which may lead to errors in molecular weight determination. Pay attention to the comparison between experimental conditions and literature or standard experiments.

6. Advantages and limitations

advantage:

• Molecular weight distribution information : In addition to measuring the number average and weight average molecular weight of cellulose, it can also provide the molecular weight distribution of the sample, revealing the diversity of molecular size in the sample.

• Automation and Efficiency : Modern GPC devices are usually highly automated and can process large numbers of samples quickly and efficiently.

limitation:

• Calibration Problem : GPCs need to be calibrated using standard samples of known molecular weights, and the molecular structure of cellulose is different from commonly used standards such as polystyrene, which may lead to certain deviations. Therefore, the result is often relative molecular weight, not absolute molecular weight.

• Solvent Selection : The solubility of cellulose limits the range of solvents that can be used, and some solvents may cause damage to the column.

• Measurement accuracy : Due to the rigid structure and dissolution characteristics of cellulose, the measurement results may be greatly affected by solvents and experimental conditions, and attention should be paid to the optimization of conditions.

7. Summary

Gel permeation chromatography is a technique for accurately and quickly determining the molecular weight and molecular weight distribution of Detailed molecular weight information of cellulose samples can be obtained by rationally selecting solvents, calibration standards and chromatographic columns. Although GPC has certain calibration problems, its widespread use in cellulose and other polymer molecular weight determination shows its importance as an analytical tool.cellulose .