1、Designation: D 4001 93 (Reapproved 2006)Standard Test Method forDetermination of Weight-Average Molecular Weight ofPolymers By Light Scattering1This standard is issued under the fixed designation D 4001; the number immediately following the designation indicates the year oforiginal adoption or, in t
2、he 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 describes the test procedures fordetermining the weight-average mol
3、ecular weight Mwof poly-mers by light scattering. It is applicable to all nonionichomopolymers (linear or branched) that dissolve completelywithout reaction or degradation to form stable solutions.Copolymers and polyelectrolytes are not within its scope. Theprocedure also allows the determination of
4、 the second virialcoefficient, A2, which is a measure of polymer-solvent interac-tions, and the root-mean-square radius of gyration (s2)1/2,which is a measure of the dimensions of the polymer chain.1.2 The molecular-weight range for light scattering is, tosome extent, determined by the size of the d
5、issolved polymermolecules and the refractive indices of solvent and polymer. Arange frequently stated is 10,000 to 10,000,000, but this may beextended in either direction with suitable systems and by theuse of special techniques.1.2.1 The lower limit to molecular weight results from lowlevels of exc
6、ess solution scattering over that of the solvent. Thegreater the specific refractive increment dn/dc (difference inrefractive indices of solution and solvent per unit concentra-tion), the greater the level of solution scattering and the lowerthe molecular weight that can be determined with a givenpr
7、ecision.1.2.2 The upper limit to molecular weight results from theangular dependence of the solution scattering, which is deter-mined by the molecular size. For sufficiently large molecules,measurements must be made at small scattering angles, whichare ultimately outside the range of the photometer
8、used.1.3 The values stated in SI units are to be regarded asstandard.1.4 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 safety and health practices and determine the
9、applica-bility of regulatory limitations prior to use.NOTE 1There are no similar or equivalent ISO standards.2. Referenced Documents2.1 ASTM Standards:2IEEE/ASTM SI 10 American National Standard for Use ofthe International System of Units (SI): The Modern MetricSystem3. Terminology3.1 DefinitionsUni
10、ts, symbols, and abbreviations are inaccordance with IEEE/ASTM SI 10.4. Significance and Use4.1 The weight-average molecular weight is a fundamentalstructure parameter of polymers, which is related to manyphysical properties of the bulk material, such as its rheologicalbehavior. In addition, knowled
11、ge of the weight-average mo-lecular weight, together with knowledge of the number-average molecular weight from osmometry, provides a usefulmeasure of the breadth of the molecular-weight distribution.4.2 Other important uses of information on the weight-average molecular weight are correlation with
12、dilute-solutionor melt-viscosity measurements and calibration of molecular-weight standards for use in liquid-exclusion (gel-permeation)chromatography.4.3 To the extent that the light-scattering photometer isappropriately calibrated, light scattering is an absolute methodand may therefore be applied
13、 to nonionic homopolymers thathave not previously been synthesized or studied.5. Apparatus5.1 Volumetric Flasks, 100-mL, or other convenient size.5.2 Transfer Pipets.5.3 Photometer, whose major components, described inAppendix X1, are a light source, a projection optical system, asample-cell area, a
14、 receiver optical system, a detector system,and a recording system. Typical photometers are described andsummarized (1)3in the literature.1This test method is under the jurisdiction ofASTM Committee D20 on Plasticsand is the direct responsibility of Subcommittee D20.70 on Analytical Methods(Section
15、D20.70.05).Current edition approved March 15, 2006. Published April 2006. Originallyapproved in 1981. Last previous edition approved in 1999 as D 4001-93 (1999).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book o
16、f ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis test method.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,
17、United States.5.4 Differential Refractometer, with sensitivity of approxi-mately 3 3 106refractive-index units, capable of measuringthe specific refractive increment dn/dc at the wavelength andtemperature of the scattering measurements (2).NOTE 2Specific refractive increments are tabulated (2,3) for
18、 manypolymer-solvent systems.5.5 Refractometer, Abb type or equivalent, capable ofmeasuring the refractive indices of solvents and solutions at thewavelength and temperature of the scattering measurements.5.6 Spectrophotometer, capable of measuring the absor-bance of solutions at the wavelength of t
19、he scattering measure-ments.5.7 Laminar-Flow Clean-Air Station, to provide a dust-freearea for preparing and cleaning solutions and filling thescattering cell.5.8 Filters and Filter Holders, for cleaning solvents andsolutions. Membrane filters with pore sizes from 0.10 to 0.45m, used in glass or pla
20、stic filter holders, are recommended.5.8.1 For water and aqueous solutions, and for organicsolvents that do not attack the material, the use of polycarbon-ate (Nucleopore) filters is recommended. These filters have theadvantages of high flow rate without the use of gas pressure,minimal retention of
21、solute on the filter, and efficient cleaningaction. For other solvents, the use of cellulosic filters (Milli-pore or equivalent) is recommended.NOTE 3Sintered-glass filters may be used, but these are relativelyexpensive and difficult to clean between uses. Centrifugation may be used,but this step re
22、quires special care and techniques, or special scattering celldesign, to be satisfactory.6. Reagents and Materials6.1 Solvents, as required. Since dn/dc is a function ofcomposition, solvents should be of high purity. Significanterrors in molecular weight, which depends on the square of dn/dc, may be
23、 incurred if literature values of dn/dc are employedand the actual value of this quantity is different because ofimpurities in the solvent.6.2 12-Tungstosilicic Acid, as standard for calibration ofphotometer.7. Sample7.1 The sample must be homogeneous, and must be thor-oughly free of all foreign imp
24、urities. If at all possible, samplesto be used for light-scattering measurements must be speciallytreated from synthesis on to minimize exposure to or contami-nation with particulate impurities. Gels, which may consist ofvery high-molecular-weight particles, are sometimes formedduring synthesis and
25、will interfere with the analysis. All suchparticulate matter must be removed, sometimes with consider-able difficulty. It should be understood that when this is done,the remaining sample is no longer truly representative of theentire polymer. The extent of the difference from the originalsample will
26、 depend on the removal techniques employed.NOTE 4Reduction of sample particle size in a clean Spex or Wileymill speeds solution and, with slow-dissolving materials, may be essentialif the measurements are to be made in a reasonable time. Overheating withconsequent sample degradation must be avoided
27、during the millingprocess. Hard, tough samples or those with low melting points can behandled by mixing with clean dry ice, milling the mixture, and thenallowing the dry ice to sublime. Clean dry ice may be obtained by openinga tank of carbon dioxide to the atmosphere. Commercial dry ice is usuallyc
28、ontaminated.8. Preparation of Dust-Free Cell and Contents8.1 Clean all glassware, including the scattering cell, with asuitable detergent to remove grease and other contaminants.Use of an ultrasonic cleaning bath is recommended. Rinseglassware at least four times with distilled water to remove alltr
29、aces of detergent, and dry in a clean, dust-free drying oven.NOTE 5A laminar-flow clean-air station is recommended for provid-ing a dust-free area for solution preparation and filtration. If a clean-airstation is not used, a closed area in a location free of drafts and ofsufficient size to hold the
30、filter unit, scattering cell, and other glasswareshould be provided.8.2 Filter solvent directly into the scattering cell. First rinsethe cell several times with 5 to 10 mL of filtered solvent each,to remove dust particles. Be sure upper surfaces of the interiorof the cell are well washed down. Close
31、 the cell with a capsimilarly rinsed with filtered solvent. After rinsing, fill the cellwith the minimum amount of solvent required to bring theliquid level above the point where the light beam in thephotometer passes through the cell.NOTE 6Use of a small filter holder fitting between a hypodermicsy
32、ringe and needle is convenient where only small quantities of liquidsneed be filtered. A cell cap, with a hole just large enough to insert theneedle, may conveniently be used.8.3 Place the scattering cell in the photometer, or in anequivalent strong light beam, and examine it in the dark,viewing at
33、small scattering angles. Bright specks of dust shouldnot be visible; if they are, the cell was not rinsed completely orthe filtration procedure is inadequate.8.4 Subsequent use of the clean cell for adding incrementsof filtered solution or for replacing solvent with solutionrequires no further rinsi
34、ng, except to ensure that residualsolvent remaining, after the cell is emptied, is removed andreplaced with solution.9. Procedure9.1 Calibrate the light-scattering photometer. This calibra-tion is required to convert measurements of scattered lightintensity from arbitrary to absolute values, an esse
35、ntial step inthe calculation of molecular weight. The calibration procedure,which is lengthy and requires great care to obtain accurateresults, is given in Appendix X2. The calibration constant ofmost photometers remains stable for long periods of time,however, so that the calibration procedure need
36、 be carried outonly infrequently.9.2 Prepare a stock solution of polymer, noting the precau-tions of Sections 7 and 8, at a concentration estimated asfollows: For a polymer of Mw= 100,000 in a solventsuch that dn/dc 0.2 mL/g (for example, polystyrene in2-butanone), the stock solution should be in th
37、e range from 10to 20 g/L. Since scattered intensity is proportional to Mwand tothe square of dn/dc, estimates of the stock-solution concentra-tion required for other samples and systems can be made.Prepare no more stock solution than is required by thefollowing procedure.D 4001 93 (2006)29.3 Select
38、one of the following measurement schemes:9.3.1 Where the volume of liquid required for measurementin the photometer can be varied by at least a factor of two, it isrecommended that the scattering from the minimum volume ofsolvent be measured first, followed by measurement of solu-tions prepared in t
39、he cell by the addition of weighed orvolumetrically measured aliquots of filtered stock solution.From four to six such solutions should be measured, the mostconcentrated consisting of approximately equal volumes ofsolvent and stock solution if its concentration is selected inaccordance with 9.2, and
40、 the least concentrated being aboutone fourth this concentration. A specific example is given inAppendix X3.9.3.2 If the volume of liquid in the scattering cell cannot bevaried as in 9.3.1, it will be necessary to prepare and filter intothe cell from four to six separate solutions covering the range
41、suggested in 9.3.1.9.3.3 A further alternative, which is felt to be undulycomplicated, is to measure the most concentrated solution first(for this purpose, the stock solution concentration estimated in9.3.1 should be reduced by a factor of two), followed bysuccessive dilutions with solvent. The scat
42、tering from the puresolvent must be measured in a separate step, and it may benecessary to start dilution sequences at two or more concen-tration levels to obtain the range specified in 9.3.1.9.4 Measure the scattering of the pure solvent, filtered intothe cell as described in Section 8, and of each
43、 of the series offiltered solutions described in 9.3, following the instructionsprovided with the photometer or in the literature (4), being surethat the following steps are included. (This procedure is basedon the scheme of 9.3.1.)9.4.1 Instrument CheckSee that the photometer is pre-pared for measu
44、rement, with the lamp lit, high voltage suppliedto the photomultiplier detector, and all components fullywarmed up and stabilized.9.4.2 Solvent PreparationFill the cleaned scattering cellwith filtered solvent as described in Section 8, insert it in theinstrument, and align it as required.9.4.3 Inten
45、sity LevelSelect the wavelength-isolating filterto be used. Turn the detector to the specified angle and set thelevel of high voltage, or adjust the slit openings, as called forto provide an appropriate solvent reading. In subsequent steps,do not readjust these variables, but change amplifier gain b
46、yknown factors or insert neutral filters of known transmittanceas required to maintain readings on scale.9.4.4 Solvent MeasurementAfter the cell has remainedundisturbed in the photometer for 10 to 15 min to allowresidual dust to settle out, read and record the scatteredintensity at angles of 30, 90,
47、 150, and at least three pairsbetween, symmetrically placed with respect to 90, as availableon the photometer used.9.4.5 ReferenceTurn the phototube to the specified refer-ence angle, adjust amplifier gain or insert neutral filters asrequired, insert the reference standard, and read and record thein
48、dicated reference intensity.9.4.6 Solution MeasurementPrepare and filter into the cellthe solutions required in 9.3. Mix thoroughly, allow a fewminutes for residual dust to settle out, and measure eachsolution as in 9.4.4.9.5 Determine solution concentrations. Since filtrationthrough membrane filter
49、s may result in retention of somepolymer on the filter, it is necessary to determine the solutionconcentrations after filtration.9.5.1 If successive concentrations are generated in the cellfrom a stock solution filtered under constant conditions, onlythe concentration of the filtered stock solution need be deter-mined; otherwise, the concentration of each solution measuredmust be determined.9.5.2 Determine the concentrations of solutions, as required,by one of the following methods. Use standard analyticaltechniques where applicable.9.5.2.1 Evaporate a portion of
