Determination of polymer molecular weight?

It's very simple. It's found in polymer physics experiments.

If conditions permit, you can consider molecular gel permeation chromatography, which is available from all major analytical instrument manufacturers. Not only can you obtain simple molecular weight, but also include the distribution range of molecular weight, etc.

The viscosity method requires preparing a solution, measuring the standby time t and the to of the standard solution, and then calculating the relative viscosity. According to the relationship between relative viscosity, intrinsic viscosity and molecular weight, the molecular weight can be calculated. But there is another problem: to determine the constants k and a, you need to refer to certain literature. Generally, you need to find a comprehensive system to determine.

Determination of viscosity average molecular weight of polymers by viscometry method 1. Experimental purposes 1. Master the basic principles of using viscometry to determine the molecular weight of polymers 2. Master the experimental technology and data processing methods for measuring the viscosity of polymer dilute solutions with an Ubbelohde viscometer 3. Analyze the relationship and impact of molecular weight on polymer properties and polymer processing properties. 2. Basic Principle The viscosity of a dilute polymer solution mainly reflects the internal frictional resistance caused by flow or relative motion between liquid molecules. Internal friction resistance is related to factors such as the structure of the polymer, the nature of the solvent, the concentration of the solution, temperature and pressure. The larger its value, the greater the viscosity of the solution. The change in viscosity of the polymer solution is generally described by the following viscosity. Relative viscosity, also known as viscosity ratio, is expressed by etar. It is the ratio of the solution viscosity eta to the pure solvent viscosity eta0 under the same temperature conditions, expressed as: etar=eta/eta0 (1) Relative viscosity is a dimensionless quantity that increases as the concentration of the solution increases. For polymer solutions at low shear rates, its value is generally greater than 1. Specific viscosity (relative increase in viscosity), expressed as etasp, is the fraction of increase in solution viscosity relative to the solvent.: ηsp = (η-η0)/η0 =ηr –1 (2) 3. Reduced viscosity (viscosity number). For polymer solutions, the relative increment of viscosity often increases with the increase in solution concentration. Therefore, the ratio of it to the concentration c is often used to express the viscosity of the solution, which is called the reduced viscosity or viscosity number, that is: etasp/c = (etar-1)/c (3) The dimension of viscosity is the reciprocal of concentration, generally expressed in ml/g. Inherent viscosity (logarithmic viscosity), which is defined as the ratio of the natural logarithm of the relative viscosity (viscosity ratio) to the concentration, that is: (lnηr)/c = /c (4) The unit is the reciprocal of concentration, commonly expressed in ml/g. Intrinsic viscosity (limiting viscosity), which is defined as the extrapolated value of the reduced viscosity (viscosity number) ηsp/c or the reduced logarithmic viscosity (logarithmic viscosity) lnηr/c at infinite dilution, expressed by: = lim(ηsp/c) = lim(lnηr/c) (5) c→0 c→0 is called intrinsic viscosity (or limiting viscosity number), its value has nothing to do with concentration, and its dimension is the reciprocal of concentration. Experiments have shown that for a given polymer, under a given solvent and temperature, the value of is determined only by the molecular weight Mη of the sample. The relationship with Mη is as follows: =K Mηα (6) The above equation is called the Mark-Houwink equation. in the formula: —— Expansion factor, related to the morphology of polymer molecules in solution ； Mη——viscosity average molecular weight (note: The K and α values ​​of some commonly used polymers are shown in Appendix Table 1) K and α are related to temperature, polymer type and solvent properties. The K value is significantly affected by temperature, while the α value mainly depends on the extent to which the polymer coils stretch in the solvent, generally between 0.5 and 1.0. At a certain temperature, for a given polymer-solvent system, K and α are constants within a certain molecular weight range, which are only related to the molecular weight. K and α values ​​can be found from relevant manuals (see Appendix Table 1), or measured using several standard samples and formula (6). The molecular weight of standard samples can be determined by absolute methods (such as osmotic pressure method and light scattering method). At a certain temperature, the viscosity of the polymer solution has a certain dependence on the concentration, which is usually described by the Huggins equation as: ηsp/c = – kˊ2c (7) or described by Kraemer’s equation as: (lnηr)/c = – β2c (8) For a given polymer, at a given temperature and solvent, kˊ and β should be constants, where kˊ is the Huggins constant, which represents the interaction between polymers in the solution and between polymers and solvent molecules. Generally speaking, kˊ is not sensitive to molecular weight. For linear flexible chain polymer good solvent system, kˊ=0.3 ~ 0.4, kˊ+β=0.5. The common intercept can be obtained by extrapolation, as shown in Figure 1. The equation obtained by the one-point method can be obtained from equations (7) and (8).: = (1/c) 1/2 (9) Figure 1 Plot c with ηsp/c and (lnηr)/c Figure 2 Ubbelohde viscometer As can be seen from the above, the key to measuring the molecular weight of polymers by viscometry is to obtain it. The most convenient one is to use a capillary viscometer to measure the relative viscosity (viscosity ratio) of the solution. The commonly used viscometer is the Ubbelchde viscometer (as shown in Figure 2). Its characteristic is that the volume of the solution has no effect on the measurement, so solutions of different concentrations can be obtained by gradually diluting the viscometer. Define η ρt(1-B/At2) based on relative viscosity (viscosity ratio) ηr = —— = —————— (10) η0 ρ0t0(1-B/At02) In the formula, ρ and ρ0 are the densities of the solution and the solvent respectively. Because the solution is very dilute, ρ=ρ0 ； A and B are viscometer constants ； t and t0 are the outflow time of the solution and solvent in the capillary tube respectively, that is, the time required for the liquid level to pass through the marked lines a and b. Under constant temperature conditions, use the same viscometer to measure the outflow time of the solution and the solvent. If the outflow time of the solvent in the viscometer is greater than 100 seconds, the kinetic energy correction term B/At2 is much less than 1 (for kinetic energy correction, please refer to the relevant information), so the viscosity ratio of the solution is ηr = t/t0. The sample solution concentration is generally below 0.01g/ml, so that the etar value is between 1.05 and 2.5. The maximum should not exceed 3.0. 3. Samples and instruments 1. The instruments are shown in Table 1. Table 1 List of viscosity measuring instruments Name Specifications Quantity Ubbelohde viscometer Solvent outflow volume greater than 100s 1 constant temperature water tank The temperature fluctuation is not greater than ±0.05℃ 1 set of volumetric flasks 25ml 2 pieces 100ml 2 pieces glass sand core funnel No. 3 2 pipettes 5ml 1 piece 10ml 2 stopwatches 1/10s 1 piece suction ball rubber 1 piece medical latex tube φ6 * 1501 Notes: The constant temperature water tank includes an electric mixer, a heater, a relay, a mercury contact thermometer, and a 50°C one-tenth scale thermometer. The main instruments used to measure molecular weight are viscometers and constant temperature baths. The constant temperature bath requires high temperature accuracy and small temperature distribution. 2. Drugs ① 1g of polystyrene or 1g of styrene-butadiene rubber to be tested ； ②Solvent: Toluene (AR) 100ml, propylene glycol (CP) 100ml. 4. Experimental steps 1. Adjust the temperature of the thermostatic bath to 25±0.05℃. 2. Prepare the polymer solution. Accurately weigh 100-500 mg of the sample into a 100 ml clean and dry volumetric flask. Pour in about 80 ml of toluene to dissolve it. After the sample is completely dissolved, put it into an adjusted constant temperature bath and place the solubilizing flask into the constant temperature bath. Add solvent to the mark, take out and shake well, use a No. 3 glass sand core funnel to filter into another 100ml volumetric flask, put it into a constant temperature bath and set it aside for use. Wash the volumetric flask and glass sand core funnel immediately after use. The glass sand core funnel should be washed with a sulfuric acid solution containing 30% sodium nitrate, filtered with distilled water, and dried for later use. 3. Cleaning the viscometer The cleaning of the viscometer and the liquid to be measured is one of the keys to the success of the experiment. If the viscometer is new, it should be washed with lotion first, then washed three times with tap water, three times with distilled water, and dried before use. For a used thermometer, first pour toluene (solvent) into the viscometer and soak it to remove the polymers left in the viscometer. Especially the capillary part must be repeatedly cleaned with solvent. After washing, pour the toluene liquid (pour into the recovery bottle), then wash with lotion, tap water, distilled water, and finally dry. 4. Measurement of solvent outflow time: Place the clean and dry Ubbelohde viscometer vertically into the constant temperature water tank so that the water surface completely submerges the ball. Use a pipette to suck 10 ml of toluene, inject it from tube A into ball E, keep it in a constant temperature bath at 25°C for 3 minutes, and then perform the measurement. Put a latex tube on the C tube and hold it with your hands to make it airtight. Use a suction ball in tube B to suck up the solvent in ball E, and suck it into ball G through the capillary tube and ball F. Then release the suction ball first, and then loosen the rubber tube of tube C to let the C tube vent to the atmosphere. Then, the sucked up solvent begins to flow back to ball E. At this time, the operator should concentrate, watch the falling liquid level with eyes horizontally, and use a stopwatch to accurately measure the time it takes for the liquid level to flow between line a and line b, and record it. Repeat the above operation three times, and the difference between each measurement is no more than 0.2 seconds. Take the average value of three times as t0, which is the outflow time of solvent toluene. 5. Measurement of the solution outflow time (1) After measuring t0, pour the toluene in the viscometer into the recovery bottle and dry the viscometer. Use a clean pipette to absorb 10ml of the solution to be measured that has been constant-temperatured. Move it into the viscometer (be careful not to stick the solution on the wall of the tube). Keep the temperature constant for 2 minutes. Follow the previous steps to measure the outflow time t1 of the solution (concentration c1). (2) Use a pipette to add 5 ml of toluene that has been thermostatically heated to dilute the above solution. The diluted solution concentration (c2) is 2/3 of the initial concentration c1. Then use the same method to measure the outflow time t2 of the solution with concentration c2. In the same way, add 5ml, 10ml, and 10ml of toluene in sequence to make the solution concentration 1/2, 1/2, and 1/4 of the initial concentration. Measure the outflow time and record it (note that after each addition of pure reagent, be sure to mix evenly, and wait until the temperature is constant before measuring) 6. After the viscometer is washed and measured, take out the viscometer, pour the solution into the recovery bottle, wash it repeatedly with solvent several times, dry it, and fill it with hot solution. After soaking for several hours, pour out the washing liquid, rinse it with tap water and distilled water, and dry it for later use. 5. Precautions 1. The viscometer must be clean, and the solvent and solution must be filtered and pure. ； 2. The viscometer is made of glass and is easily damaged, especially the B and C tubes, so be particularly careful when operating them. ； The temperature of the thermostatic bath must be strictly controlled within the required range ； The viscometer must be installed vertically and the readings must be accurate. 6. Data processing 1. The record format is shown in Table 2. For the convenience of drawing, the relative concentration c′ is used for calculation and drawing. 3. Use the extrapolation method to plot and calculate Mη. Plot ηSP/c′ and lnηt/c′ against the concentration c′ to get two straight lines. Extrapolate to c′→0 to get the intercept. After conversion, the intrinsic viscosity is obtained. By substituting into formula (6), the molecular weight Mη of the polymer can be calculated. 4. Calculate the molecular weight of the polymer using the "one point method". In actual work, we hope to simplify the operation and quickly obtain the molecular weight of the product. “"One-point method" only needs to measure the viscosity ratio at one concentration, and its molecular weight can be calculated using formula (9). 7. Experiment report requirements 1. Briefly describe the experimental principle. 2. Clarify the operating steps and precautions. 3. Keep original records and data processing. 4. Record in detail the phenomena observed during the stretching process, and analyze the causes of the phenomena (including deformation, surface and color changes, fractures and cross-section damage, etc.) based on the theoretical knowledge learned. 8. Preliminary * Requirement 1. Understand the experimental principle ； 2. Understand the steps and precautions for determining the viscosity average molecular weight of polymers using viscometry. 3. Write a good preview * Report and prepare recording forms. 9. Experiment record reference form Table 2 Viscosity measurement record form __ _ _ _ _ Year__ _ _ _ _ _ _ _ moon__ _ _ _ _ _ _ day, room temperature__ _ _ _ _ _ _ ℃, relative humidity__ _ _ _ _ _ % sample name--------------- ； Solvent--------------- ； Viscometer number--------------- ； Thermostatic bath temperature--------------- ； Solution concentration c1--------------- ； Solvent outflow time (1) ---------- (2) ---------- (3)---------- ； Average value t0----------Amount of added solvent (ml) Relative concentration c′ Solution outflow time (s) Average value (s) ηtηSPηSP/clnηt/ c(1)(2)(3)01 52/3 51/2 101/3 101/4 Appendix Table 1 K and α values ​​of some common polymers Polymer polymerization methods Molecular weight range, * 103 Solvent temperature (°C) K value, * 102α value polystyrene (PS) solution polymerization 3～1700 toluene 251.70.691～11 benzene 254.170.605.9～5.2 benzene 201.230.72330～toluene 301.10.73 polymethyl methacrylate (PMMA) Bulk polymerization 70～6300 Benzene 250.4680.77240～4500 Benzene 250.380.70 Emulsion polymerization 410～3400 Acrylonitrile 250.960.69410～3400 Toluene 250.710.73410～3400 Chloroform 250.340.83 Styrene-butadiene rubber (SBR) Emulsion polymerization 50℃ 25～500 Toluene 255.250.6626～1740 Toluene 301.650.735℃ 55～1000 Toluene 302.950.7525～1000 Benzene 251.30.55 Natural rubber (NR) 0.4～1500 Benzene 255.020.17 Butadiene rubber (BR) 20～1300 Toluene 252.150.6526～660 Butong 304.80.55 Polyacrylonitrile (PAN) 48～270 Dimethylformamide 251.660.813～370 Dimethylformamide 252.330.75 Polyester (PET) 12～28 Phosphorus chlorobenzene 253.00.775～25 Phenol/tetrachloroethane 252.10.82 Polyvinyl alcohol (PVA) 11.6～195 water 255.950.6344～1100 water 505.90.6730～120 water 306.60.64

Indeed, several constants are sometimes difficult to determine and difficult to find in the literature.

Can I use a rotational viscometer to measure the molecular weight of PTFE? What is the specific operation?

What material do you want to measure? I need to explain it clearly before I can answer you.

Viscosity method, I am* Ubbelohde viscometer is commonly used. Someone has already made it clear before. The parameters can usually be obtained by searching the literature. It's not difficult. You'd better be more detailed. GPC is everywhere now and it is not difficult. Usually this is more convincing when writing articles. I only use the rotational viscometer to measure viscosity, and it should be converted using empirical formulas. However, your solid content has a significant impact on the viscosity and may have a great impact on the calculation. Ubbelohde viscometer is the first choice for measuring molecular weight by viscometric method. I recommend gpc, it is convenient for writing papers. . . After all, molecular weight distribution is critical and makes it easier to discuss.

Yes, if possible, you can use GPC to measure it, but Ubbelohde viscometer is also more convenient.

Ubbelohde viscometer requires skillful techniques to measure accurately, while GPC does not require high requirements and is simple to operate.