In this paper, we provide an overview of techniques for determining the molecular weight of polymers, and discuss methods such as chromogenic assay, dynamic light scattering, mass spectrometry, and reactive liquid chromatography. These techniques are used to accurately measure the molecular weight and distribution of polymers, which are of great significance for emulsion polymerization and materials research. Through these methods, researchers can gain insight into the properties of polymers and provide reliable data for relevant applications.
1. Determination of molecular weight of polymers
End-based analysis
End group analysis estimates the molecular weight of polymers by measuring the functional groups at the end of the polymer chain. The method relies on the chemical changes resulting from the reaction of a specific group at the end of the polymer chain to determine their number and derive the molecular weight accordingly.
method
Carboxyl Determination – Determination of the number of carboxyl groups at the end of a polymer using acid-base titration. This method calculates the amount of carboxyl groups by adding a certain concentration of lye (e.g., NaOH) to the sample and determining the titration result after the reaction.
Hydroxyl determination – samples are chemically treated with acetic anhydride and then the resulting esters are analyzed by titration. The acetic anhydride-treated sample was titrated to determine the hydroxyl content.
Reagents & Instruments
Carboxyl determination – chloroform, NaOH solution
Hydroxyl determination – acetic anhydride, pyridine, benzene
Analytical balances: for accurate weighing of samples and reagents
Grinding Erlenmeyer flasks: for titration reactions
Pipettes: for precise measurement of liquid volumes
Experimental Procedure
Determination of carboxyl groups
1. Dissolve the polymer sample in an appropriate amount of chloroform to prepare a sample solution.
2. React the sample solution with a known concentration of NaOH solution.
3. The remaining amount of NaOH after the reaction is determined by titration, and the content of the carboxyl group is calculated.
Determination of hydroxyl groups
1. The polymer sample is treated with acetic anhydride and pyridine to generate the reaction product.
2. The amount of ester generated is measured using titration and the amount of hydroxyl is calculated from the titration results at the end of the reaction.
data processing
Calculate the amount of carboxyl and hydroxyl groups, and use the appropriate formula to convert the molecular weight of the polymer.
Analyze sources of error in experimental data, which may include errors in reagent concentration, instrument calibration issues, or operational errors. Repeat experiments and comparative analyses were performed to improve the accuracy and reliability of the results.
2. Determination of polymer molecular weight and Huggins parameters by membrane permeation method
Osmotic pressure is the pressure in the solution due to solute particles, and the properties of the polymer solution can be estimated based on the osmotic pressure calculation formula of a reliable solution. The Flory-Huggins theory is used to describe the behavior of polymer solutions, calculating osmotic pressure as a function of the molecular weight of polymers by considering the interaction between molecules.
Measurement of osmotic pressure – both static and dynamic. The static method measures the rate at which the liquid level rises or falls, while the dynamic method determines the osmotic pressure by monitoring the real-time changes of the liquid, which improves the measurement accuracy.
reagent
Polymethyl methacrylate (PMMA)
Acetone (for dissolving polymers)
instrument
Bruss membrane permeameter: used to measure the osmotic pressure difference between the two sides of the membrane
Thermostatic Water Tank: Maintain the stability of the experimental temperature and ensure the accuracy of the measurement
Experimental Procedure
Measurement of pure solvents
1. Use a thermostatic sink to keep the temperature of the pure solvent stable.
2. Record the dynamic equilibrium point of the pure solvent in the membrane permeameter, i.e., the change of the liquid level.
Measurement of sample solutions
1. Prepare a polymer sample solution and treat it using a Thermostatic Water Bath.
2. Measure the osmotic pressure of the sample solution in the Bruss membrane permeameter, using the dynamic method to improve the accuracy.
data processing
The height difference of the liquid column is calculated for the osmotic equilibrium, which is used to determine the osmotic pressure.
According to the Flory-Huggins theory and measurement data, the molecular weight of the polymer and the Huggins parameters were calculated, and the density correction term was considered to improve the accuracy of the calculation.
3. Determination of molecular weight by vapor pressure permeation method
principle
This method estimates the molecular weight of a polymer by measuring its effect on solvent vapor pressure. The decrease in the vapor pressure of a polymer in a solvent varies depending on how much it dissolves the solvent vapor, and the molecular weight of the polymer can be calculated from these changes.
merit
It is suitable for polymer materials and can provide accurate molecular weight data. This method can be applied over a wide range of molecular weights and is particularly suitable for high molecular weight polymers.
In conclusion, the molecular weight determination of polymers is an important task in materials science, and common methods include end group analysis, membrane permeation, and vapor pressure permeation. End-group analysis estimates molecular weight by analyzing reactive groups at the end of polymer chains and is suitable for polymers with smaller molecular weights. The membrane permeability method uses the osmotic pressure of a solution to calculate the molecular weight, is suitable for high molecular weight polymers, and is able to provide information on intermolecular interactions in solution. Vapor pressure permeation provides an alternative means of molecular weight determination by measuring the effect of polymers on solvent vapor pressure. Each of these methods has its own advantages and disadvantages, and by combining different techniques, it is possible to fully understand the properties and applications of polymers.
