Expanding The GPC/SEC Experiment Beyond Conventional .

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Expanding the GPC/SEC ExperimentBeyond Conventional Limits usingViscometry and Light ScatteringDetectionApplication NoteAuthorAbstractAnthony GriceThe Agilent 1260 Infinity Multi-Detector GPC/SEC System is a powerful tool forWarwick Polymer Grouppolymer characterization. The signals from a concentration sensitive detectorUniversity of Warwick, UKsuch as refractive index (RI) or ultra-violet (UV) coupled with light scattering andBen MacCreathAgilent Technologies, Inc.Craven Arms, UKviscometry can be used to determine the absolute molecular weight, size, andshape of the polymer in solution. This Application Note shows the value of thetechnique applied to the analysis of three very structurally different materials andillustrates the advantages of using multi-detection GPC/SEC to study a range ofpolymers.Verified for Agilentlent1260 Infinity II LCGPC/SEC System

IntroductionGPC/SEC is an essential tool forthe analysis of polymers, providinginformation required to understand how apolymer will process and perform. Whenusing a concentration detector such as RI,the process is known as ConventionalGPC/SEC. The chromatographicmechanism separates by the size ofthe polymer coil in solution, and isuseful to determine the distribution ofcomponent chain lengths within a givensample. The information obtained canbe increased considerably by addingviscometry and light scattering detectionto create a multi-detector system. Highperformance modules are required tocontrol temperature and maintain preciseflow rates to ensure reproducible results.As with any chromatographic system,minimal band-broadening is critical toensure data integrity is maintained.This Application Note highlights theadvantages multi-detection brings to theGPC/SEC experiment with a range ofpolymer examples.Experimental8.Agilent 1260 Infinity MDS LightScattering (G7803A)SoftwareAll instrument control, data collectionand data analysis was performed withthe Agilent GPC/SEC Software CDSconsisting of: Agilent GPC/SEC Software(G7850AA) Agilent GPC/SEC Multi-DetectorUpgrade (G7852AA) Agilent GPC/SEC InstrumentControl (G7854AA)Column Set and Calibrants PS nominal Mp 100 k(PL2013-5001) EasiVial PS-H 4 mL(PL2010-0200)Reagents, samples, and materialsAll chemicals and reagents were of HPLCgrade and passed through a 0.22 µm filterprior to use. Samples were prepared bydissolution over a 16 hour period withgentle agitation in the eluent at theconcentrations stated in Table 1.Chromatographic parametersGPC/SEC parameters used with the 1260Infinity Multi-Detector GPC/SEC Systemare shown in Table 2.3 PLgel 10 µm Mixed-B, 300 7.5 mm (PL1110-6100)Table 1. Sample preparation details.Sample typeStructureConcentration (mg/mL)Polyacrylate comb2.020Synthetic entationThe Agilent 1260 Infinity Multi-DetectorGPC/SEC System used in the experimentsconsisted of the following individualmodules:1.Agilent 1260 Infinity StandardDegasser (G1322A)2.Agilent 1260 Infinity Isocratic Pump(G1310B) 1260 Infinity StandardAutosampler (G1329B)Table 2. Chromatographic parameters used for GPC/SEC.ParametersConditionsEluentTetrahydrofuran (stabilized with 250 ppm BHT)Agilent 1260 Infinity ThermostattedColumn Compartment (G1316A)TCC Temperature35 CDetectors temperature35 CAgilent 1260 Infinity GPC/SEC MultiDetector Suite (G7800A)Injection Volume100 µLAgilent 1260 Infinity MDS RID(G7801A)Flow Rate1.0 mL/minStop Time35 minutesPost Time12 minutesDetectionRefractive Index at 660 nmViscometer Inlet PressureViscometer Differential PressureLight Scattering 15 at 660 nmLight Scattering 90 at 660 nmAgilent 1260 Infinity MDSViscometer (G7802A)2

Results and DiscussionSystem calibrationThe system was first calibrated using anarrow dispersity polystyrene calibrant of113,000 g/mol, dispersity 1.03 (Figure 2).This single standard determines essentialdetector constants and calculates theinter-detector delay (IDD) between thedetectors, which are connected in series.The unique small volume flow celldesigns result in market-leading lowdispersion technology, minimizing theband-broadening commonly seen whenemploying multiple detectors. This isclearly illustrated in the separation of thenarrow polystyrene standard (Figure 2).50454035302520151050-5-10-15-20RILS 15LS 90Visc DPResponse (mV)The polymer samples were analyzedusing multiple methods to highlightthe differences between the differenttechniques. Stable baselines andexcellent signal-to-noise (S/N) wereachieved by precise temperature controlfor the column oven and each individualdetector (Figure 1).24681012141618Retention time (min)20222413012011010090RILS 15LS 90Visc DP80Response (mV)In Conventional GPC/SEC, the molecularweight results are relative to thestandards used to generate the columncalibration. For further information onConventional GPC, refer to the Agilentprimer1.28Figure 1. Raw data chromatogram for the polymethacrylate sample.70Molecular weight Retention time (min)222426Figure 2. Chromatogram showing normalized responses for all detectors highlighting low dispersion andexcellent S/N.3

Conventional GPC1e6MW (g/mol)In conventional GPC, the concentrationdetector response alone is used, usuallya refractive index detector. A calibrationcurve is generated using narrowcalibrants, in which the apex of the peak,Mp, is related to the retention time, as inFigure 3. Polystyrene EasiVials, PS-H vialswere used in this study which with theMixed-bed column technology creates afirst order calibration curve.1e71e51e41e31e216171819202122Retention time (min)23242526Figure 3. Conventional calibration curve.GPC/SEC viscometryA viscometer measures the solutionviscosity of materials as they elute fromthe column, this property is used todetermine molecular weights using theUniversal Calibration. This is an approachthat permits the calculation of accuratemolecular weights regardless of thechemistry of the standards employed inthe calibration.The same polystyrene EasiVial-Hstandards were used to generate aUniversal column calibration (Figure 4).Table 3. Conventional calibration data.Retention time (min)MWlog MW15.506,035,0006.78Percent 3.11–5.94–0.025GPC light scattering1e81e71e6MW * IVBy measuring the Raleigh scattering,molecular weights are determined fromfirst principles negating the requirementfor a column calibration. Importantly,by simultaneously measuring at morethan one angle, angular dependence isovercome. For further information onmulti-detector GPC, refer to the ention time (min)23242526Figure 4. Universal calibration curve.Table 4. Universal calibration data.Retention time (min)MWIVlog(MW IV)Percent 0.01025.971,2800.0341.64–0.99–0.0044

The samples were analyzed using threedifferent calculation methods. The firstapplies a conventional column calibrationwhich was generated using narrowdistribution polystyrene standards. Thesecond utilizes a viscometer detectorto generate a “Universal calibration”from the same set of polymer standards.The third method uses a light scatteringdetector, which does not require acolumn calibration. The results calculatedusing the three different methods are,as expected, quite different as can beseen in Table 5. The main reason beingthat polymers of differing chemistryand topology have different sizes whenin solution. Because the polystyrenecalibrant is structurally different fromthe polymer being characterized, for agiven molecular weight the two polymershave a different size in solution. Asthe GPC/SEC mechanism separates byhydrodynamic volume, not molecularweight, the retention time for a givenmolecular weight of synthetic rubberfor example, is quite different than fora polystyrene standard of the samemolecular weight. The effect can be quitedramatic, for example, synthetic rubber inTHF forms a much larger coil in solutionto that of the equivalent molecular weightpolystyrene, resulting in calculatedmolecular weights of almost twice theactual true value.Table 5. Molecular weight results from the different analysis methods.SampleConventional calibrationUniversal calibrationLight scatteringMnMwMnMnPolyacrylate tic 074,000188,900MwMw2018GPC/ViscGPC/LSConventional GPC161412dW/dLogMMolecular weight comparison10864201e41e5Fitted MW (g/mol)1e6Figure 5. Overlaid molecular weight distribution plots for the synthetic rubber.By introducing a viscometer and usingthe intrinsic viscosity to determine theuniversal calibration curve, chemicaland structural differences between thepolystyrene standard and the sampleof interest are compensated for. Thecalculated molecular weights are nolonger relative, providing much moreaccurate results. When using lightscattering, the detector response isdirectly proportional to the molecularweight, and is considered to give anabsolute, accurate, measurement ofmolecular weight, without the need tocalibrate the columns.5

Results and DiscussionAdvanced detectors increase thepower of the experiment and allowdetermination of other useful parametersof the polymer in solution whichconventional GPC cannot provide.Molecular sizeThe molecular size can be determined byusing advanced detection. The viscometermeasures the intrinsic viscosity of apolymer, which is directly related to thehydrodynamic radius, Rh. Light scatteringdirectly determines a different sizemeasurement, the radius of gyration, Rg.This measurement requires that datafrom two angles (15 and 90 degrees) iscollected simultaneously and the polymeris large enough to observe an angulardependence (above approximately 10nm). This difference between Rh andRg can be observed by comparing theconformation plots for a sample ofsynthetic rubber, shown in Figure 6. Thesize measurement determined usinglight scattering begins at approximately200,000 g/mol, which is the molecularweight at which angular dependence isfirst observed for this sample. However,as the molecular distribution extendsdown to lower molecular weights theviscometer has a larger range for thisparticular sample, since the detectorresponse simply relies on a change insolution viscosity.Rh (from viscometer)Rg (from LS)1e2Rg (nm)It is not only chemistry that has an effecton the hydrodynamic size, for examplecertain types of polymer have extendedside chains (also known as branching).One such example being a polyacrylategraft polymer, where a large relativedifference is observed between molecularweights calculated using conventionalGPC and using light scattering and/orviscometry. The Agilent GPC Softwarecontains powerful sub routines to beable to identify and quantify thesebranched polymer structures3. The resultsfor a polymethacrylate homopolymershow less of a difference in molecularweight between the different calculationmethods. This is because, whendissolved in THF, it has only a slightlysmaller hydrodynamic volume than thatof polystyrene standard of the samemolecular weight.1e11e5Fitted MW (g/mol)Figure 6. Conformation plot for synthetic rubber.61e6

Molecular shapeThis plot illustrates the differences inconformation of the different types ofpolymer. The polyacrylate has an alpha(slope) of 0.59, whereas the syntheticrubber and polymethacrylate have alphasof 0.63 and 0.64 respectively. A value of0.7 suggests the polymer is behaving likea random coil in a good solvent whereas0.5 suggests a tightly coiled polymer.The offset is related to the density of thepolymer which is quite different for thepolymethacrylate, which indicates a muchdenser coiling in solution compared to theother two polymers.GPC triple detectionThis technique uses all of the availabledetectors to provide a comprehensiveunderstanding of the polymer throughGPC/SEC. The RI detector providesaccurate concentration informationand polymer profile, the light scatteringdetector provides an accurate molecularweight and size, and the viscometerprovides information on conformation andsize across the molecular weight range.ConclusionGPC/SEC is a unique, importanttechnique to help understand howpolymers will behave during processingand their physical properties underapplication as a finished product. TheConventional column calibration haslimitations which can be addressed byusing light scattering and/or viscometry.These advanced detectors also furtherthe ability to detect subtle but significantdifferences between sets of polymers.1e1IV (dL/g)The slope of the conformation plotprovides information on the shape ofthe polymer in solution. When usinga viscometer a similar plot, the MarkHouwink plot (Figure 7) is generated byplotting the intrinsic viscosity against themolecular weight.PolyacrylateSynthetic rubberPolymethacrylate1e01e-11e51e6Fitted MW (g/mol)Figure 7. Mark-Houwink plot showing all samples overlaid.Molecular weight results from GPC/SECcan be quite different depending onthe method used in the calculations.Concentration only data, for examplejust a RI or UV, are relative to thestandards used to generate the columncalibration and therefore the molecularweight results are often not accurate.The advanced detectors are sensitiveto molecular weight and overcome therelative nature of GPC. Through the useof the Universal Calibration, a viscometerprovides this by producing molecularweights independent of the calibrantsused. The most accurate molecularweight data is determined using a lightscattering detector, which respondsdirectly to molecular weight and thereforerequires no column calibration.The advantages of advanced detectorsis not limited to just more accuratemolecular weights. Molecular sizes canbe calculated from both viscometry andLight Scattering detection, each havingits own strengths. The light scatteringdetector directly calculates the size7but a viscometer has a lower operatingrange. The shape, or conformation of thepolymer can also be determined, a crucialparameter to understand how a polymerbehaves in solution.The Agilent 1260 Infinity Multi-DetectorGPC/SEC System can have anycombination of RI, UV, light scatteringand viscometry to expand the GPC/SECexperiment beyond conventional limits.The efficiency of separation is maintainedwith low dead volume detectors whichexhibit excellent signal-to-noise andstable baselines for confidence inanalysis.

References1. “An Introduction to GPC/SEC”, Agilentpublication, 5990-6969EN.2. “An Introduction to Multi-DetectorGPC/SEC”, Agilent publication,5990-7196EN.3. “Analysis of Star Polymers Using theAgilent 1260 Infinity Multi-DetectorGPC/SEC System”, Agilent ApplicationNote 5991-2887EN.www.agilent.com/chemThis information is subject to change without notice. Agilent Technologies, Inc., 2013-2016Published in the USA, May 1, 20165991-2891EN

GPC/SEC System used in the experiments consisted of the following individual modules: 1. Agilent 1260 Infi nity Standard Degasser (G1322A) 2. Agilent 1260 Infi nity Isocratic Pump (G1310B) 3. Agilent 1260 Infi nity Standard Autosampler (G1329B) 4. Agilent 1260 Infi nity Thermostatted

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