Gel permeation chromatography (GPC)
Gel Permeation Chromatography (GPC) was first studied by J. C. Moore in 1964. Not only for the separation and identification of small molecules, but also for the analysis of different molecules of the same chemical nature of different polymer homologues. (The polymer is separated on a separation column according to the molecular hydrodynamic volume)
1.1 separation principle
The polymer solution being measured is passed through a column of different pore sizes. The path through which the molecules are available in the column is the gap between the particles (larger) and the through hole (smaller) in the particles. As the polymer solution flows through the column, the larger molecules are excluded from the pores of the particles and pass only through the gaps between the particles at a faster rate; smaller molecules can enter the pores in the particles, Passed much slower. After a certain length of the column, the molecules are separated according to the relative molecular mass, with relatively high molecular mass in the front (ie, the elution time is short) and the relative molecular mass is low (ie, the elution time is long). Since the sample into the column to be rinsed out, the total volume of the received leaching solution is called the leaching volume of the sample. When the instrument and experimental conditions are determined, the leaching volume of solute is related to its molecular weight. The larger the molecular weight, the smaller the leaching volume.
(1) Volume exclusion
(2) limited diffusion
(3) Flow separation
1.2 correction principle
Using a monodisperse standard polymer of known relative molecular mass, a correlation curve of rinse volume or rinse time and relative molecular mass is made in advance. This line is called "calibration curve". Almost no monodisperse standard can be found in the polymer, which is usually replaced by a narrowly distributed sample. Under the same test conditions, a series of GPC standard spectra were made, corresponding to the retention times of different relative molecular mass samples, plotted as lgM against t, and the resulting curve is the "calibration curve." Through the calibration curve, the information on the various required relative molecular masses and relative molecular mass distributions can be calculated from the GPC spectra. There are not many kinds of polymers in the polymer that can be prepared for the standard sample. There is no calibration curve for the polymer without the standard sample, and it is impossible to obtain the relative molecular mass and relative molecular mass distribution of the polymer by the GPC method. For this you can use the universal calibration principle.
1.3 universal correction principle
Because GPC separates the polymer based on the molecular hydrodynamic volume, ie, for the same molecular hydrodynamic volume, it flows out at the same retention time, ie, the hydrodynamic volume is the same.
The hydrodynamic volumes of the two flexible chains are the same:
[Η] 1M1 = [η] 2M2
K1M1α1 + 1 = k1M2α2 + 1
Take the logarithms on both sides: lgk1 + (α1 + 1) lgM1 = lgk2 + (α2 + 1) lgM2
That is, if the k and α values ​​of the standard sample and the measured polymer are known, the relative molecular mass M2 of the sample to be tested can be calibrated by the standard sample M1 of known relative molecular mass.
Direct Method: The viscosity or light scattering is measured while measuring the concentration of the leachate to determine its molecular weight.
Indirect method: using a group of molecular weight range, monodisperse sample as a standard sample, respectively, to determine their leaching volume and molecular weight, you can determine the relationship between the two.
2.1. Instruments
The GPC instrument consists of a pump system, an (automatic) injection system, a gel column, a detection system and a data acquisition and processing system.
2.1.1. Pump System: Includes a solvent reservoir, a degassing device and a high pressure pump. Its job is to flow the mobile phase (solvent) into the column at a constant flow rate. Pump working conditions directly affect the accuracy of the final data. The more sophisticated equipment, the more stable the working condition of the pump is required. Required flow error should be less than 0.01mL / min.
2.1.2. Column: The core component of GPC instrument separation. In a hollow stainless steel tube with different pore size as a filler. Each column has a certain relative molecular mass separation range and penetration limit, the column has the upper and lower limits of use. The upper limit of column use is when the smallest polymer has a larger size than the largest gel in the column, and the polymer can not enter the pore size of the gel, all flowing from outside the gel particles The purpose of separating high molecular weight polymers with different relative molecular masses has not been achieved. But also blocked the gel hole may affect the separation effect of the column, reducing its service life. The lower limit of column usage is when the largest size molecular chain in the polymer is smaller than the smallest pore size of the gel pores and at this time it does not achieve the purpose of separating different relative molecular masses. Therefore, when using gel chromatography to determine the relative molecular mass, you must first select a column that is compatible with the polymer's relative molecular mass range.
2.1.3. Filler (According to the choice of solvent used, the most basic requirement for the filler is that the filler should not be dissolved by the solvent.): Crosslinked polystyrene gel (suitable for organic solvents, high temperature resistance), crosslinked poly Vinyl ester gel (up to 100 ° C, suitable for polar solvents such as ethanol and acetone) Porous silica balls (for water and organic solvents), porous glass, porous alumina (for water and organic solvents)
2.1.4. Pillar: glass, stainless steel
2.1.5. Detection System: Universal Detector: Suitable for the detection of all polymers and organic compounds. Refractive index detector, UV absorption detector, viscosity detector.
2.1.6. Differential Refractometer Detector: The refractive index of the solvent is as large a difference as possible from the refractive index of the sample under test.
2.1.7. UV Absorption Detector: The solvent is not strongly absorbed near the characteristic absorption wavelength of the solute.
2.1.8. Selected Detector: Suitable for polymers and organic compounds that have a special response to this detector. A UV, infrared, fluorescence, conductivity detector.
2.2. Operation
2.2.1. Solvent Selection: Can Dissolve Multiple Polymers; Does Not Corrode Instrument Parts; Matches Detector.
2.2.2. The laser light scattering and gel chromatography combined with the concentration spectrum obtained at the same time, you can get the scattering intensity of the leaching volume spectrum, so as to calculate the molecular weight distribution curve and the entire sample each The average molecular weight
2.2.3. In laser light scattering experiment, the sample must be strictly dedusted. The dust in the solution will produce strong light scattering, which seriously interferes with the light scattering measurement of the polymer solution. Solution dust is the key to the success or failure of light scattering. The first is the solvent dedusting, the configuration of the test sample solvent should be distilled, and after 0.2μm ultrafiltration membrane before use. With a good solution should also use 0.2μm ultrafiltration membrane filtration. In addition, the test equipment used, such as: syringes, soaked before use to use lotion, water rinse.
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