ASTM D6474-1999(2006) Standard Test Method for Determining Molecular Weight Distribution and Molecular Weight Averages of Polyolefins by High Temperature Gel Permeation Chromatogra.pdf

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1、Designation: D 6474 99 (Reapproved 2006)Standard Test Method forDetermining Molecular Weight Distribution and MolecularWeight Averages of Polyolefins by High Temperature GelPermeation Chromatography1This standard is issued under the fixed designation D 6474; the number immediately following the desi

2、gnation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the d

3、etermination of molecularweight distributions and molecular weight averages of linearpolyolefins by high temperature gel permeation chromatogra-phy (GPC). This test method uses commercially availablepolystyrene standards and equipment and is applicable topolyethylenes (excluding high pressure low de

4、nsity polyethyl-eneLDPE) and polypropylenes soluble in 1,2,4-trichlorobenzene (TCB) at 140C. This test method is notabsolute and requires calibration.NOTE 1Size exclusion chromatography (SEC) often is used as analternative name for gel permeation chromatography (GPC).NOTE 2Specific methods and capab

5、ilities of users may vary withdifferences in columns, instrumentation, applications software, and prac-tices between laboratories.NOTE 3One general method is outlined herein; alternative analyticalpractices can be followed and are attached in notes where appropriate.NOTE 4There is no similar or equi

6、valent ISO standard.1.2 The values stated in SI units, based on IEEE/ASTM S1-10, are to be regarded as the standard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate

7、 safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 883 Terminology Relating to PlasticsD 3016 Practice for Use of Liquid Exclusion Chromatogra-phy Terms and RelationshipsD 5296 Test Method for Molecular We

8、ight Averages andMolecular Weight Distribution of Polystyrene by HighPerformance Size-Exclusion ChromatographyE 685 Practice for Testing Fixed-Wavelength PhotometricDetectors Used in Liquid ChromatographyE 691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Metho

9、dIEEE/ASTM S1-10 Standard for Use of the InternationalSystem of Units (SI): The Modern System (replaces ASTME 380 and ANSI/IEEE Standard 268-1992)3. Terminology3.1 DefinitionsDefinitions of terms applying to plasticsappear in Terminology D 883.3.2 Definition of Term Specific to This Standard:3.2.1 p

10、olyolefin, nused in this context, refers to PE(except LDPE) and PP thermoplastics.4. Summary of Test Method4.1 In this test method, a polyolefin sample is dissolved in asolvent and injected onto a chromatographic column(s) packedwith a solid substrate, which separates the molecules accordingto their

11、 size in solution. The separated molecules are detectedand recorded as they elute from the column according toconcentration. Through calibration, retention times are con-verted to molecular weights. Average molecular weight param-eters and molecular weight distribution are determined fromthe molecul

12、ar weight concentration data.5. Significance and Use5.1 This test method measures the molecular weight distri-bution and molecular weight averages of polyethylene (exceptLDPE) and polypropylene resins. Differences in molecularweight and molecular weight distribution significantly affectphysical prop

13、erties, such as morphology, strength, melt flowetc., and as a result, the final properties of products made fromthese resins.6. Interferences6.1 A major interference is the presence of insoluble, highlyentangled, high molecular weight material that may be linear orcross-linked. A successful outcome

14、of the test requires that thesample be dissolved completely prior to the chromatographic1This test method is under the jurisdiction of ASTM Committee D20 on Plasticsand is the direct responsibility of Subcommitttee D20.70 on Analytical Methods.Current edition approved March 15, 2006. Published April

15、 2006. Originallyapproved in 1999. Last previous edition approved in 1999 as D 6474992For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary

16、page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.separation. The presence of the above-described material oftenprecludes the necessary dissolution step.6.2 A mismatch in the antioxidant level in the dissolvedsa

17、mple and that of the TCB eluent (see 8.1).6.3 The accuracy of the molecular weight results decreases,that is, they become increasingly underestimated, as thea-olefin comonomer content increases in linear low densitypolyethylene (LLDPE). For example, the results for an octenecopolymer containing 30 b

18、ranches per 1000 carbon atoms willbe about 6 % low.7. Apparatus7.1 Essential ComponentsThe essential components ofthe instrumentation are a solvent reservoir, a pump, a solventdegasser, a sample injection system, packed columns, and asolute mass detector.NOTE 5Complete high temperature GPC units wit

19、h a maximumoperating temperature of 210C are commercially available.7.2 Solvent ReservoirThe solvent reservoir shall holdsufficient TCB to ensure consistency of composition for anumber of runs. The TCB should be protected from exposureto water in the air and the reservoir material shall be inert to

20、thesolvent.7.3 PumpThe principal requirement of the pump is pro-duction of a relatively constant flow, with minimum pulsations,of solvent through the columns. In general, the rate should beadjustable between 0.1 and 5.0 cm3/min and back pressuresshould not exceed limits specified by the column manuf

21、acturer.Flow rate precision shall be at least 60.3 % as measured underthe conditions and time interval for a typical analysis.7.4 Sample Injection SystemThe purpose of the injectionsystem is to introduce the solution containing the sample intothe flow stream as a sharply defined zone. Either a six-p

22、ortvalve with an attached sample loop or a variable volumeinjector can be used for this purpose in conjunction with anautosampler. Requirements include minimal contribution toband spreading, injector ability to operate at the back pressuregenerated by the columns, repeatability of injection volume,a

23、nd no carryover.7.5 ColumnsStainless steel columns with uniform andhighly polished inside walls are recommended for high tem-perature GPC. Columns with lengths ranging from 20 to 50 cmwith fittings, frits, and connectors designed to minimize deadvolume and mixing are recommended. Generally, the pack

24、ingmaterials, typically styrene divinylbenzene copolymers, havenarrow particle size distributions in the 3 to 20 m range.Packing materials are available in a variety of shapes and poresizes. Columns may be packed with particles of relativelyuniform pore size or with a “mixed bed” of particles to pro

25、ducea broad range of pore sizes. If a set of columns with uniformpore size is used, it is recommended that the columns beconnected in order of increasing pore size towards the lowpressure detector side.NOTE 6Packed high temperature GPC columns are available from anumber of manufacturers.7.6 Detector

26、The detector provides a continuous measureof the concentration of solute eluting from the column(s). Thedetector shall be sufficiently sensitive and respond linearly tothe solute concentration, independently of molecular weight,and shall be of low internal volume so as not to distort theconcentratio

27、n gradient during elution. For this test method, thedetector cell volume should be 30 L or less. The mostcommonly used concentration detectors for high temperatureGPC are refractive index or infrared. The former has moderatesensitivity and general utility. When testing detector perfor-mance, follow

28、the recommendations of the instrument manu-facturer.NOTE 7For polyolefins, the refractive index (dn/dc) increment isessentially constant above molecular weights of 5 000 g/mol. Theresponse of the components below 5 000 g/mol should be corrected priorto the molecular weight calculations using a pre-e

29、stablished dn/dcmolecular weight calibration. The principal disadvantage of the differen-tial refractometer is that the temperature within the detector cell shall becontrolled to within 0.0001C.7.7 Tubing and FittingsAll tubing between the sampleinjector and the detector should be no greater than 0.

30、25 mm(0.01 in.) internal diameter and rated for pressures up to 42MPa. Connecting column tubing should be kept as short aspossible and all fittings and connectors shall have low deadvolumes to prevent mixing.7.8 Data Acquisition/Handling SystemMeans shall beprovided for determining chromatographic p

31、eak heights orintegrated area segments at prescribed time intervals and forhandling and reporting data. This is best accomplished using acomputer with appropriate software.NOTE 8Data acquisition and handling systems for high temperatureGPC have not been standardized. However, a number of differentma

32、nufacturers provide GPC specific computer software.8. Reagents and Materials8.1 Solvent1,2,4-trichlorobenzene (TCB) is recom-mended as the solvent for this test method; however, anysolvent that has a boiling point higher than the operatingtemperature, is considered a good solvent for polyolefins, an

33、dis compatible with the GPC components, may be used. With arefractive index detector, the solvent shall have a refractiveindex different than that of the polyolefins analyzed. Solventpurity and consistency shall be considered when choosing asolvent. For example, unless freshly distilled, and subse-q

34、uently, kept in a glass container under an inert gas, TCB willreact with water to form hydrochloric acid that will attacktubing walls and degrade column packing. The TCB reservoirshould be protected against exposure to moisture, or bereplaced frequently with fresh solvent, or both. An antioxidant,su

35、ch as 2,6-di-tert-butyl-4-methylphenol (BHT) should beadded to the solvent reservoir at the same concentration, that is,about 250 mg/L, as in the solvent used to dissolve the polymerto minimize any interference due to an antioxidant mismatchpeak.NOTE 9Several laboratories are successfully recycling

36、TCB usingpartial vacuum distillation.8.2 Polymer StandardsNarrow MWD (MW/Mn0.3 %) in the flow ratebetween the time of calibration and sample analysis will causesignificant, systematic errors in MW values calculated. Usersshould determine the average flow rate of their system bymeasuring the volume o

37、f solvent eluted over a specified timeperiod. When flow rate variations in excess of 0.3 % areobserved, replace or service the pump or correct for flowvariations by addition of a flow measuring device or use a flowrate marker such as for example the air peak or toluene, that is,internal standard.14.

38、 Calibration14.1 Selection of Polystyrene StandardsPrepare solutionsof polystyrene calibration standards as outlined in 12.2.Aminimum number of three standards per decade of MW need tobe used to adequately define the calibration curve over the MWrange covered by the columns.14.2 Injection of Polysty

39、rene StandardsMake injectionswith the instrument autosampler at the same temperature as thecolumn oven temperature, that is, typically 140C. Add theinternal standard, if used, to the solution prior to injection. Theinjection volumes of all standards shall be identical regardlessof concentration. The

40、 recommended volume is #300 L forcolumns with internal diameters of 0.8 to 1.0 cm. For diametersof 0.8 cm, a smaller volume should be chosen.14.3 Data AcquisitionDetermine the elution peak maximaand the corresponding elution volumes (or times) for thevarious polystyrene standards (and internal stand

41、ard). Usually,this is accomplished with a data acquisition software package.14.4 Generation of Calibration CurveConvert the poly-styrene peak molecular weights to polyolefin molecularweights. This is accomplished using the following equation:log10M25 $1/1 1a2!% 3 log10K1/K2! 1 $1 1a1!/1 1a2!% 3 log1

42、0M1(3)where:M2= the molecular weight of the polyole-fin,a2,K2and a1,K1= Mark-Houwink constants for thepolyolefin and polystyrene, respec-tively, andM1= molecular weight of the polystyrene.The above equation is derived fromthe empirical Mark-Houwink equa-tion:h 5 K 3 Ma(4)where:h = intrinsic viscosit

43、y and the universal calibration con-cept4which states that the product of intrinsic vis-cosity and molecular weight for any polymer isproportional to its hydrodynamic volume, which inturn defines the elution volume of the polymer.Values for Ks and as may be determined with anon-line viscometer or ob

44、tained from the literature.The following values are recommended for polysty-rene (PS), polyethylene (PE), and polypropylene (PP)in TCB at 140C:KPS5 19 3 1023mL/g aPS5 0.655 (5)KPE5 39 3 1023mL/g aPE5 0.725KPP5 19 3 1023mL/g aPP5 0.72514.4.1 Generate the polyolefin GPC calibration curve byplotting th

45、e logarithm of the peak molecular weight valuesversus the measured peak elution volumes (or times). Thecalibration curve generally assumes an s-shape that asymptoti-cally approaches total permeation at low MW and total exclu-sion at high MW. It is recommended that a third or fifth orderpolynomial be

46、 used to fit the calibration data. A number ofsoftware packages are available to do this.15. Procedure15.1 Preparation for AnalysisPrepare polymer samplesolutions as described in Section 12. An internal standard maybe added to each sample solution before injection. Alterna-tively, a “stock” solution

47、 containing an internal standard formonitoring eluent flow rate may be used to prepare the samplesolutions. Prepare the apparatus and complete the performancerequirements in Section 13.4Z. Grubistic, R. Rempp, and H. Benoit, J. Polym. Sci., Part B, Vol 5, 753(1967).D 6474 99 (2006)415.2 Injection of

48、 Sample SolutionsFollowing guidelinesdescribed in 14.2, the injection volume shall be identical to thatselected for calibration. A sharp increase or “pulse” in backpressure upon injection indicates a serious problem in the GPCsystem that shall be remedied before continuing. When oper-ated unattended

49、, the system should possess the ability to shutdown when a specified maximum pressure, for example 250MPa, is reached.15.3 Baseline DeterminationSatisfy baseline criteria dis-cussed in 13.4. The baseline is assumed to be linear. It isestablished by averaging the baseline noise before and after thechromatographic envelope and connecting the two with astraight line. If the actual baseline deviates from the generatedline due to drift, shift of excessive noise, the analysis should bediscarded.15.4 Integration LimitsThe establishment of the l