SH/T 0108-1992 Determination of Average Molecular Weight and Molecular Weight Distribution of Certain Polymeric Additives (Size Exclusion Chromatography)

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1. Method number / full name / English name / replacement relationship

• Standard number: SH/T 0108-92
• Chinese full name: Determination of Average Molecular Weight and Molecular Weight Distribution of Certain Polymeric Additives (Size Exclusion Chromatography)
• English name: not provided in the available source
• Replacement relationship: not provided in the available source
• Year confirmation: the source marks "(ZOO(, year confirmed)" as source-text extraction noise; the specific confirmation year requires confirmation against the source standard (source-text extraction)
• Approval/implementation: approved by China Petrochemical Corporation 1992-06-06, implemented on 1993-01-01
• Reference adoption: American Society for Testing and Materials standard ASTM D3593-80(86) "Liquid Exclusion Chromatography (Gel Permeation Chromatography—Determination of Average Molecular Weight and Molecular Weight Distribution of Certain Polymers by the GPC) Universal Calibration Method"

2. Scope of application and non-applicability

• Applicable to linear polymeric additives soluble in tetrahydrofuran.
• The determination range is the molecular weight of samples whose elution volume is equivalent to that of polystyrene standards with molecular weights from 5×10³ to 3×10⁶ g/mol (the source source-text extraction reads "5x1[garbled exponent and separator]3x1[garbled exponent and unit]mol", values require confirmation against the source standard (source-text extraction)).
• This standard is not an absolute method and requires calibration.
• Not applicable: no explicit non-applicability clause is provided in the available source; Figure 2 indicates that chromatograms in which the polymer peak and impurity peak are not completely separated are not suitable for determination by this standard.

3. Method principle

Inject the sample solution dissolved in tetrahydrofuran into a chromatographic column packed with special porous packing; in the column, the sample is separated according to molecular size, and its concentration is detected. From the molecular weight calibration curve of polystyrene standards and the viscosity parameters K and a of the polystyrene standards and sample in tetrahydrofuran at the same temperature, the molecular weight calibration curve of the sample is obtained. From the detected concentrations of the sample and their corresponding molecular weights, the average molecular weight and molecular weight distribution of the sample are calculated. The test temperature of this standard is 30℃; other temperatures may also be used if the K and a values at that temperature are available.

Universal calibration principle: the elution volume VR of a linear random coil polymer in size exclusion chromatography depends on the product of the polymer molecular weight M and the intrinsic viscosity [η]. For two polymers with different chemical properties, M₁·[η]₁ = M₂·[η]₂ (Equation 3). The intrinsic viscosity is calculated according to the Mark–Houwink equation [η] = K·Mᵃ (Equation 4). Substituting Equation (4) into Equation (3) and taking logarithms gives Equation (5), from which the molecular weight of polystyrene standards can be converted to the molecular weights of other polymers.

4. Reagents and materials (purity / preparation)

• 4.1 Tetrahydrofuran (THF): analytical reagent. It must be purified before use. Filter through a 0.5 μm filter and degas, or redistill; during distillation, ferrous sulfate or another reducing agent must be added to prevent the formation of peroxides. Tetrahydrofuran is flammable, and safety regulations for the use of flammable substances shall be observed.
• 4.2 Benzene or o-dichlorobenzene: analytical reagent.
• 4.3 Polystyrene (PS) standards: known molecular weight standard values, narrow distribution, symmetrical peak shape. The molecular weights of the standards shall cover the entire molecular weight range of the sample to be tested. Within each order of magnitude of the sample molecular weight, there shall be at least three standards.
• Standard solution concentration: 1 g/L (see 5.3.2).
• Reference range of sample solution concentration: 5～10 g/L (see 5.2.1).

5. Instruments and equipment

• 3.1 Gel permeation chromatograph: composed of a solvent reservoir, constant-flow pump, injection system, chromatographic column, detector, recorder, etc.
• 3.1.1 Solvent reservoir: shall be able to withstand corrosion by tetrahydrofuran, be dustproof, moistureproof, and lightproof; the tubing connected to the pump shall have a filtration device to ensure that no suspended particles enter the pump. The solvent storage volume shall be sufficient to ensure use for multiple tests.
• 3.1.2 Constant-flow pump: flow adjustment range 0.1～5.0 mL/min, flow stability ±1% (the source source-text extraction reads "11%o", requires confirmation against the source standard (source-text extraction)).
• 3.1.3 Injection system: no leakage or sample carryover during operation, ensuring injection repeatability.
• 3.1.4 Chromatographic columns: use four stainless steel columns of length 300 mm and inner diameter 7 mm (the source source-text extraction reads "inner diameter 7,", units require confirmation against the source standard (source-text extraction)), respectively packed with crosslinked polystyrene microparticles having average pore sizes of 50 nm, 10³ nm, 10⁴ nm, and 10⁵ nm (the source source-text extraction reads "50nm, 1[garbled exponent]nm, 1[garbled exponent]nm and 1[garbled exponent]nm", specific values require confirmation against the source standard (source-text extraction)). Connect these four columns in series in order of packing pore size from small to large to form a combined column. Other equivalent columns may also be used instead.
  • 3.1.4.1 Number of plates: the number of plates of the chromatographic column shall not be less than 4800.
  • 3.1.4.2 Resolution (R₁,₂): the resolution of the chromatographic column must be greater than or equal to 1.30.
• 3.1.5 Detector: mainly differential refractive index and ultraviolet absorption. Their sensitivities are respectively 5×10⁻⁷ refractive index units (RIU) and 2×10⁻⁴ ultraviolet absorption units (ODU), and baseline drift is respectively less than 1×10⁻⁶ RIU/h and 4×10⁻⁴ ODU/h (the source source-text extraction reads "1 x 10-0RIU/h and 4 x 10-a0DU/h", values require confirmation against the source standard (source-text extraction)).
• 3.1.6 Recorder: a recorder whose sensitivity and signal-to-noise ratio match the detector shall be selected so that the concentration signal has no obvious interference.

6. Recommended operating conditions

• Test temperature: 30℃ (other temperatures may also be used if the K and a values at that temperature are available).
• Flow rate: 1 mL/min ± 0.1 mL/min (the source source-text extraction reads "lml/mi n[unreadable sign]0. In[garbled unit]/[unreadable character]n", requires confirmation against the source standard (source-text extraction)).
• The baseline must be stable before injection; the baseline drift within 3 h shall not exceed 5% of the recorder full scale.
• The sample peak height shall be adjusted to 0.33～0.50 of the recorder full scale.
• The paper speed shall be such that the peak width for a molecular weight distribution of 1.1 is not less than 30 mm (the source source-text extraction reads "not less than 30}", units require confirmation against the source standard (source-text extraction)).
• The amount of polymer injected is generally 1～4 mg. For samples with a narrower distribution, the injection amount is smaller.
• For polymeric additive sample solutions, the injection solution volume generally does not exceed 300 μL (the source source-text extraction reads "3001ALo", requires confirmation against the source standard (source-text extraction)).

7. Sampling and sample preparation

• 5.2.1 Preparation of sample solution: for polymeric additive samples, the reference range for preparing the solution concentration is 5～10 g/L. The sample shall be dissolved in tetrahydrofuran freshly discharged from the solvent reservoir. Shake appropriately to aid dissolution. Use only after complete dissolution. High-molecular-weight samples shall avoid vigorous shaking to prevent degradation. Before injection, the sample solution shall be filtered through a membrane filter with a pore size not less than 0.4 μm and not greater than 5 μm to remove fibers and other impurities that may clog the column.
• 5.2.2 Sample solution concentration test: size exclusion chromatography for determining molecular weight distribution generally does not require an accurate sample concentration, but for high-molecular-weight samples, a high concentration can cause uneven flow in the column and affect the peak elution volume. Therefore, it is best to dilute the sample solution by a factor of two and inject it again to determine whether the peak position of the chromatogram is reproducible. If it changes, re-inject at a lower concentration until the position of the chromatogram no longer changes.

8. Analytical procedure

• 5.1 Instrument preparation
  • 5.1.1 Testing of the chromatographic column
    • 5.1.1.1 Plate number test: use o-dichlorobenzene or benzene as the sample and perform the determination under the same test conditions as the sample. The plate number N is calculated according to Equation (1): N = 16(VR/W)², where VR is the peak elution volume of the chromatogram (mL), and W is the chromatographic peak width expressed in volume units (mL).
    • 5.1.1.2 Resolution test: use two narrow molecular weight distribution polystyrene standards whose molecular weights differ by more than about a factor of ten. The standard concentration is 1 g/L. The resolution R₁,₂ is calculated according to Equation (2).
  • 5.1.2 Check flow rate: the flow rate shall be checked each time the instrument is started, and its value must match the flow rate used when preparing the calibration curve.
  • 5.1.3 Constant temperature: the instrument is kept at constant temperature at the test temperature.
  • 5.1.4 Stabilize baseline: the baseline must be stable before injection.
• 5.3 Calibration
  • 5.3.2 Calibration test: rinse the syringe with the eluent tetrahydrofuran, prepare a polystyrene standard solution at a concentration of 1 g/L according to the requirements of clause 5.2, and inject it into the chromatographic column. During injection, record the injection volume and time and note them on the chromatogram. Bubbles are not allowed during injection, and the next sample may be injected only after the previous sample has completely eluted. Measure the elution volume of the recorded standard chromatographic peak.
  • 5.3.3 Draw the molecular weight calibration curve of polystyrene standards: plot the elution volume VR at the peak top of the standard as the abscissa and the logarithm of the square root of the product of the corresponding weight-average molecular weight (Mw) and number-average molecular weight (Mn) (Mw·Mn)^1/2 as the ordinate; draw a smooth curve through the points, which is the molecular weight calibration curve of polystyrene standards. This curve is also used to calculate the spreading effect correction.
  • 5.3.4 Determine the molecular weight calibration curve of the sample: substitute the (Mw·Mn)^1/2 values, i.e., the M₁ values, of each polystyrene standard covering the sample range, as well as the K and a values of the polystyrene standards and the sample, into Equation (5) to calculate the M₂ value of the sample. Draw the relationship curve between the logarithm of M₂ and VR in the same way as for 5.3.3, which is the molecular weight calibration curve of the sample.
• 6.1 Determination of the sample: inject the sample solution prepared according to clause 5.2, record the chromatogram, and follow the same procedure as in 5.3.2.
• 6.2 Chromatogram processing
  • 6.2.1 Determine the baseline: the starting peak point Ve and the ending peak point Vf should be selected properly (the original source-text extraction is "ending peak point pit", requires confirmation against the source standard (source-text extraction)), then draw a straight line through these two points, which is the baseline. Figure 1 is a typical chromatogram of a polymer suitable for determination by this standard. The polymer peak returns to the baseline; any of the 40th readings may be selected, and the 41th, 39th, 38th and 37th readings may also be selected. Figure 2 is a typical chromatogram of a polymer not suitable for determination by this standard (the polymer peak and impurity peak are not completely separated, and this standard cannot determine the correct baseline and cutting range of the chromatogram).
  • 6.2.2 Divide the entire chromatographic peak from the starting peak point to the ending peak point into at least 20 equal parts, draw vertical lines from the baseline to the recorded curve at each equal division point, and measure the height of each spectral line; the measurement error should be less than 0.25 mm.
  • 6.2.3 From the 5.3.4 calibration curve, find the molecular weight corresponding to the elution volume at each equal division point.

9. Qualitative and Quantitative Calculation Formulas

• 7.1 The apparent number-average molecular weight Mn(u) (g/mol) of the sample is calculated according to formula (6): Mn(u) = ΣHi / Σ(Hi/Mi), where Hi is the spectral line height (mm), Mi is the molecular weight corresponding to Hi (g/mol), and i is the number of data points into which the chromatogram is equally divided.
• 7.2 The apparent weight-average molecular weight Mw(u) (g/mol) of the sample is calculated according to formula (7): Mw(u) = Σ(Hi·Mi) / ΣHi.
• 7.3 The apparent molecular weight distribution D(u) of the sample is calculated according to formula (8): D(u) = Mw(u) / Mn(u) (the original formula (8) source-text extraction is incomplete, requires confirmation against the source standard (source-text extraction)).
• The apparent average molecular weight and molecular weight distribution obtained from formulas (6), (7), (8) are not corrected for spreading effect. The spreading effect correction of the sample shall be processed according to Appendix A to obtain the number-average molecular weight Mn(t), weight-average molecular weight Mw(t) and molecular weight distribution D(t) of the sample.
• Appendix A Correction of spreading effect (supplementary document):
  • A1 Calculate the spreading effect correction factors: using the molecular weight standard value of the polystyrene standard sample as the true value, according to the polystyrene standard sample chromatogram and calibration curve obtained from 5.3.2 and 5.3.3, process the standard sample chromatogram according to clause 6.2, then use the calculation formulas in Chapter 7 to calculate the apparent average molecular weight of the polystyrene standard sample without spreading effect correction. The spreading effect correction factors X₁ and X₂ are calculated according to formulas (A1), (A2) and (A3).
  • A2 Correction of spreading effect of the sample:
    • A2.1 When X₂ is greater than 0.15, it indicates that the experimental spectrum is seriously distorted, and the experimental conditions should be reconsidered.
    • A2.2 When X₂ is less than 0.05 and X₁ is less than 1.05, it indicates that the distortion of the experimental spectrum can be ignored (the correction values of number-average molecular weight and weight-average molecular weight are less than 5%), and the apparent average molecular weight and molecular weight distribution without spreading effect correction can be directly used.
    • A2.3 When X₂ is less than 0.05 and X₁ is greater than 1.05, it indicates that the experimental spectrum has non-negligible symmetric spreading. In this case, the number-average molecular weight Mn(t), weight-average molecular weight Mw(t) and molecular weight distribution D(t) of the sample after spreading effect correction are calculated according to formulas (A4), (A5) and (A6).
    • A2.4 When X₂ is greater than 0.05 and X₁ is greater than 1.05, it indicates asymmetric spreading, and the degree of skewness cannot be ignored, and it also needs to be calculated according to formulas (A4), (A5) and (A6).
• Appendix B Values of K and a in tetrahydrofuran for polymers commonly used in petrochemicals (reference document): lists polystyrene, T602 viscosity index improver, T603 viscosity index improver, T613 viscosity index improver, T814 pour point depressant, T631 viscosity index improver, poly(ethyl methacrylate), ethyl methacrylate-styrene copolymer, poly(butyl methacrylate), poly(methacrylate)s, poly(octadecyl methacrylate), octyl methacrylate-styrene copolymer, poly(vinyl octadecyl ether), styrene-monoethyl maleate copolymer, isopropyl-1,4phenylene copolymer, polybutadiene (with different vinyl contents), poly1,4-butadiene, cis-polyisoprene (natural rubber), polyisoprene, poly(vinyl acetate), etc., their K×10³ mL/g, a values, molecular weight ranges and temperatures (25℃ or 30℃). For specific values, see the table in Appendix B of the original text (many source-text extraction values require confirmation against the source standard).

10. Precision

For polymethacrylates, polyisobutylene and ethylene-propylene copolymers, the precision was determined by tests; the reliability of test results is judged by the following provisions (95% confidence level).

• 8.1 Repeatability
  • 8.1.1 Number-average molecular weight: the difference between two results of repeated determinations by the same operator shall not exceed 12% of the arithmetic mean.
  • 8.1.2 Weight-average molecular weight: the difference between two results of repeated determinations by the same operator shall not exceed 12% of the arithmetic mean.
• 8.2 Reproducibility
  • 8.2.1 Number-average molecular weight: the difference between determination results from two different laboratories shall not exceed 24% of the arithmetic mean.
  • 8.2.2 Weight-average molecular weight: the difference between determination results from two different laboratories shall not exceed 22% of the arithmetic mean.

11. Report

The report shall include the following contents:

• 9.1 Column plate number and resolution.
• 9.2 Source, K, a values, (Mw·Mn)^1/2 value and peak elution volume of the polystyrene standard sample.
• 9.3 K and a values of the sample.
• 9.4 Spreading effect correction factor X₁ and X₂ values.
• 9.5 Take the arithmetic mean of the number-average molecular weight Mn(t), weight-average molecular weight Mw(t) and molecular weight distribution D(t) from two repeated determination results as the determination result. Express with three significant figures.

12. Reference Documents

• ASTM D3593-80(86) "Determination of Average Molecular Weight and Molecular Weight Distribution of Certain Polymers by Liquid Size Exclusion Chromatography (Gel Permeation Chromatography—GPC) Universal Calibration Method)" (referenced adoption)
• Other reference documents: not provided in the available source

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick Reference for Questions and Answers