ASSESSING THE VALIDITY OF THE UNIVERSAL CALIBRATION CONCEPTIN THF AND DMAC FOR A VARIETY OF POLYMERS

AND COLUMN SUPPORTS

C. Kuo and T. Provder

The Glidden CompanyMember of ICI Paints

Strongsville, Ohio, U.S.A.

R. A. Sanayei and K. F. O'DriScoll

University of WaterlooWaterloo, Ontario, Canada

INTRODUCTION

Size exclusion chromatography(SEC) analysisof low molecularweightpolymers (M < 10,000) can be complicated by at least two differentproblems, one relating to quantitative correctnessof using differentialrefractive index detector (DRI) as a surrogatevariable for concentration, andthe other relating to the applicabilityof the universalcalibration curve(UCC). In SEC the separation of macromoleculesis based on hydrodynamicvolume (HDV). The HDV of a polymer chain scales as intrinsicviscositytimes molecular weight ([_]i ii)" This is the basis of the UCC technique firstput forward by Benoit[1], and since validated for many different types of

, polymers having molecular weights usuallyabove 10,00012]. In contrast,the use of UCC in the low molecular weight region has not been extensivelyexplored. Viscosity measurements, necessary for application of UCC, arecommonly believed to be difficult for low molecularweight polymers.However, we have found an on-lineviscometer to be capable of accuratelymeasuring intrinsicviscositiesof polymerseluting from the SEC withmolecular weights as low as 200. An on-lineviscometer is therefore anideal supplementary detector for traditional SEC in order to investigate thevalidity of the UCC technique in the low molecularweight region. Anothercomplication ariseswhen polar solvent such as DMF or DMAC is used asthe SEC mobile phase due to the polymer-solvent-columnpackinginteractions. In this paper, the validity of the universalcalibration concept in

. THF and DMAC is exploredfor a variety of polymer types and columnpackings utilizingSEC -viscometry-DRI system. Use is made of the Sanayei-O'DriscolI-Rudin[3] singleparameter universal calibrationmodel based on K0to assess the validity of the universalcalibration curve,

193

//

PROCEDURE

On-line viscometry can be used in conjunction with conventional SEC tomeasure the specific viscosity of the polymer solution eluting through thedetector. At the very low concentrations used in SEC, the specific viscosityof the eluent can be expressed as the product of its concentration and

intrinsic viscosity (_i =Ci[//]i)" The currently available on-line viscometers aresensitive enough to be used for quantitative specific viscosity measurementsof low molecular weight polymers. The viscosity distribution of a polymer

sample, defined[4] as (wi[T/]i) v_.ss.([7/]iMp),can be obtained directly from theon-line viscometer measurements in SEC-viscometry analysis without anyneed to use a concentration detector. The number average molecularweight Mn, and [_/]of a polymer sample are attainable from this distribution.The method of doing so, using a differential viscometer alone, we shall termSEC/DV. The molecular weight statistics (Mn, Mv and Mw) and MWD of apolymer can be determined by combining the DRI with on-line viscometryinformation. We term this method SEC/DRI/DV. A Poster paper[5] in thisSymposium has shown that Mn values obtained from both methods shouldbe the same.

The Mark-Houwink-Sakurada equation relates the intrinsic viscosity tomolecular weight:

[_] = K M a (1)

For polymers with molecular weight below 10,OOO, the theoretical and also

the measured value of a is 0.5 and K is Ke regardless of the solvent type[6].Therefore, in the case of low molecular weight polymers an alternative formof the above equation can be used:

[_] = Ke M°'Sv (2)

and Mv = [_WiMi a]l/a (3)

The Mv of a polymer sample can be obtained from SEC/DRI/DV methodrelying on the validity of the UCC. The IT/]of the sample can be determined

independently of the UCC from SEC/DV alone. Therefore, the apparent K0 ofa sample can be estimated by dividing [17]by M °'s. If effects additional to

V

size exclusion were operating in the SEC separation ( e.g. adsorption on the

columns), the apparent K0 would be much larger than the literature value. Ifthe apparent Ke is within the error of reported values of Ke, the validity ofthe UCC technique is confirmed. For a highly branched polymer sample, it

is expected that the apparent K0 would be smaller than that of the linearpolymer.

194

EXPERIMENTAL:

SEC/THF

Experimentalwork was carried out using an SEC system equipped with aDRI, a UV/VIS and an on-lineviscometer (Viscotek Model 110). The columneluent passedthrough the UV detector and then was split evenly betweenthe viscometer and the DRI detectors. One of two column sets was used,either a 4x30 cm PLgelcolumns (10s, 104, 500, and 100 A with 10/=rnbead size or 3x30 cm mixed bed columns. THF was utilized as solvent at 1ml/min., polymer concentrationswere .5 to 1.0% (w/v), and the systemwas operated at 30°C. Independent UCC'S were established for eachdetector using low molecularweight polystyrene standards from PolymerLaboratories, Shropshire,U.K. Detectors were interfaced with an IBMcompatible microcomputerfor data acquisition. The data were subsequentlyprocessed using a microcomputerwith a software package developed at theUniversity of Waterloo. Polymersamples were obtained from varioussources. Poly(isobutylene)(PIB) samples were purchased from AmericanPolymerStandards, Mentor, Ohio. Preparationdetails and characterizationof poly(methylmethacrylate) sample (PMMA) were previously reported[7].The M values, determined by SEC with DRI (directly calibrated with PMMA

It

standards) were 1485, 2314, and 3284 for PMMA-1, -2, and -3respectively. For PMMA-1, Mn was also determined by NMR to be 1502.The poly(ethylene oxide) (PEO)sample was obtained from Prof. J. Stevens,University of Guelph, Ontario, Canada.

SEC/DMAC

The instrument used in the SEC/DMAC work was a Waters 150 CVequipped with a single capillaryviscometer and a DRI detector [8]. Themobile phase, DMAC, had a flow rate of 1.0 ml/min, at 80°C. The columnsused were 3x30 cm Waters/_Styragel HT linear columnsplus lx30 cmShodex KD 802.5 column. Also used were 3x30 cm AsahiPak columnsobtained from Keystone Scientific. These columnswere packed withpolyvinyl alcohol with the following designations: GMS-700, GS-510, andGS-310. The data acquisitionwas done with bothWaters Expert Ease datasystem and LeSec multipledetectorGPC software version 3.13a. NarrowMWD polystyrene, PMMA, PEO/PEGwere used as the calibrants and wereobtained from both American Polymer Standardsand Toyo Soda company.Broad MWD PMMA samples were obtained from University of Waterloo andwere describedin the SEC/THF section. PMMA blend was a sample madein the Glidden laboratory from mixing three narrow MWD PMMA standards(6K:4K:2K/1:2:1). Polytetrahydrofuran(polyTHF) samplewas obtained fromBASF.

195

RESULTS AND DISCUSSION

SEC/THF

Three chemically different types of low molecular weight polymers,poly(isobutylene), poly(methylmethacrylate) and poly(ethylene oxide), wereanalyzed with PLgel columns in THF. A Viscotek model 110 viscometerwas used in conjunction with a DRI. The resultingmolecular weightstatistics are listed in Table I. It is observed that the Mn values obtained forall three types of polymersare in good agreement with the nominal values.

Using the [T/]and Mvvalues, it is possible to calculate the apparent K0foreach of the samples. Table II shows the Keresultsalong with theliterature[9] values for these polymers. It is seen that the Kevaluescalculated for PIB are in extremely good agreement with the literaturevalues, indicatingthat use of the UCC was valid for these low molecularweight PIB polymers. For the PMMA samples, the Kevalues areintermediate between the values reported for syndiotactic and isotacticPMMA, which is to be expected for PMMA prepared by free radicalpolymerization. The reasonableagreement with literature Kevalues suggestthe UCC is also valid for these low molecular weight PMMA samples. Forthe PEO sample, the Kevalue obtained is three-fold greater than theliterature value. It is concluded that when usingTHF as solvent, PEOprobably interacts by adsorptionon the column, and that separation occursnot only by size exclusion. Therefore, the UCC concept breakdown for lowmolecular weight PEO. This abnormality was also observed earlier byAmbler[10] when toluene was used as the eluent.

In a recent report[11] it is claimedthat the breakdown of the UCC at verylow molecular weights is "obvious given that intrinsicviscosities becomenegative'. While there have been reports of negative intrinsicviscositiesbeing measured for some polymers in unique solvents[12], we are not awareof any such reports for polymers in THF. Negative intrinsicviscosities areregarded as arising from specific interactions between solute and solventmolecules such that a liquid structure of some kind existing in the solvent isdestroyed in the vicinity of a solute polymer molecule[12]. In ourexperience, SEC-viscometry data (such as those in Figures 1 and 2)consistently show positive values for [_/]iin THF for low molecular weightsamples of PSTY, PMMA, PIB and PEO. The scatter in values of [_]i at verylow molecular weight (such as seen in Figure 2) is a consequence of theextremely low concentrationspresent in the SEC eluent. If measurementsof [7/]for the whole sample are desired at extremely low molecular weight(ca. 200 to 1000), this can be accomplished by using higher concentrationsin the SEC.

196

SEC/DMAC//_HT

A previous SEC study[8] with DMAC as eluant 3/_HT linear columns and aShodex KD-802.5 column set (styrene-divinylbenzene packings), demonstratedthat the universal calibration was applicable for high molecular weight PS,PMMA, and PEO/PEG samples. Shown in Figure 3 is the primary molecularweight calibration curves for the narrow MWD, PS, PMMA and PEO standards.The PEO/PEG standards did not fall onto the same curve as those for PS andPMMA. However, when the data are plotted onto a universal calibrationcurve, the data points fall into a common curve except in the very lowmolecular weight regions as shown in Figure 4. The current study focuses onthe low molecular weight regions. Figures 5 and 6 show the primary MWcalibration curve and the UCC respectively, in the low MW regions. It is seenthat PS deviates from the common I-IDV curve based on PEO/PEG standards.Using this common HDV curve, molecular weight statistics were obtained forthe PMMA samples used in the SEC/THF study. The molecular averages inTable III obtained from the SEC/DMAC system are comparable to thoseobtained from the SEC/THF system. Also in agreement within experimental

error are the K0 values. This data confirms that the Ke value is independent ofsolvent type.

Results also are obtained for a synthetic polymer blend from mixing knownamounts of narrow MWD PMMA standards. The MW statistics obtained fromthe common HDV curve are in good agreement with those calculated from the

known mixture component amounts as shown in Table IV. The Ke value is alsoin agreement with the results obtained for those broad MWD PMMA studied.When the HDV curve generated from PS standards was used in thecalculation, the MW averages were over estimated while the Ke valueunderestimated indicating the incorrect HDV was used casting doubt on thevalidity of the UCC. Tables V and VI summarize the effects of the HDV curve

on the apparent Ke values for PS and PMMA samples.

SEC/DMAC/AsahiPak

A set of columns packed with polyvinyl alcohol was also used for this study toexplore the effect of different packings (AsahiPak GSM-700, GS-510, GS-310). Figure 7 shows the experimental chromatogram of a low MWpolystyrene sample (MW-500). The chromatogram shows the excellentresolution for this sample as well as the complete elution of all componentsfree of interference from the solvent trash peaks. Figure 8 shows the primaryMW calibration curves for low MW PS, PMMA and PEO. It is seen again thatthe PEO did not fall into the same curve as those of PS and PMMA. Whenplotted as an HDV curve in Figure 9, PEO still deviates from the common HDV

197

curve formed by PS and PMMA. This is different from the behavior of thestyrene-divinylbenzene packings where PEOand PMMA form a common HDVcurve. A series of samples were run to generate the MW averages and

apparent K0values. Table VII shows the resultsobtained for a polyTHF samplewith nominal Mn of 2000. It is seen that using PEO calibration curve, the Mn

• value is in excellent agreement with the nominalvalue. Also the apparent K0value is in good agreement with the literature value. Similar conclusionswereobtained that use of the incorrect HDV curve leads to wrong MW statistics and

Kevalues.

CONCLUSIONS

For the three types of polymersstudied with styrene-divinylbenzene packingsin THF, UCC is valid for PMMA and PIB, but is not valid for PEO. In the caseof SEC/DMAC, no common hydrodynamic volume(HDV) calibration curveexists either with styrene-divinylbenzeneor polyvinyl alcohol column packingsfor low molecular weight polymersover wide range of polarity. PMMA andPEO/PPG form a common HDV curve with styrene-divinylbenzenepackingswhile PMMA and PS form a common HDV curve with polyvinyl alcohol

packings. Overall, the apparent Kevalues appear to be indicative of thevalidity of UCC (HDV Curve) for solvent/column packing systems for a widerange of polymer types. If Ke is higher than the literature value, the sample isinteracting with the column and if Ke is lower than the literature value, thecalibrants are not generatinga UCC.

ACKNOWLEDGMENT

Support of this research related to the SEC/THF study by the Natural Sciencesand EngineeringResearchCouncil of Canada and by the Ontario Center forMaterials Research is gratefully acknowledged. One of us, RAS,acknowledges a discussionwith Dr. N. Weeks of the Polaroid Corp. whichstimulated some of the ideas in this paper.

REFERENCES

1. Benoit, H.; Rempp, P.; Grubisic, Z.; J. Polym. Sci., Polym. Phys. Ed., 5,753 (1967).

2. Yau, W. W.; Chemtracts- Macromol. Chem., 1, 1 (1990).

3. Sanayei, R. A.; O'Driscoll, K. F.; Rudin, A., ACS Symposium Series, 521,103 (1993).

198

4. Sanayei, R. A.; Suddaby, K. G., Rudin, A.; Makromol. Chem., 194, 1953(1993).

5. Schulz, W. W.; Baniukiewicz, S.; Chance, R. R.; "Absolute MolecularWeights by GPCNiscometry, Poster Paper #11, this Symposium.

6. Flory, P. J. Fox, T. G.; J. Am. Chem. Soc., 73, 1909 (1951).

7. Sanayei, R. A.; O'Driscoll, K. F.; J. Macromol. Sci. Chem. A23, 1137(1989).

8. Kuo, C.; Provder, T.; Koehler, M. E.; ACS Symposium Series, 521, 231(1993).

9. Flory, P. J.; "Statistical Mechanics of Chain Molecules", Hanser Publisher,New York (1989).

10. Ambler, M.; Mate, R. D.; Chromatogr. Sci., Series, 8, 93 (1976).

11. Chance, R. R.; Baniukiewicz, S.; Verstrate, G.; Hadjichristidis, N.;Abstracts of 6th Intl. Symp. Polym. Anal. Chrtzn., Crete, July, 1993.

12. Abe, F.; Einaga, Y.; Yamakana, H.; Macromol. 24, 4423 (1991) andreferences therein.

13. Kurata, M.; Utiyama, H.; Kamada, K.; Macromol. Chem., 88, 281 (1965).

14. Kurata, M.; Stockmayer, W. H.; Fortsch. Hochpolym. Forsch. 3, 198

t (1963).

199

I

List of Figures

Figure 1. SEC Chromatogramsfor PIB-3 Using On-line DV and DRIDetectors

Figure 2. IntrinsicViscosity vs. Molecular Weight for PIB-3 from Figure 1.Data points calculated are [_]i of eluent; solid line is plot ofEquation 2 using literature values of Ke[9].

Figure 3. Primary Molecular Weight CalibrationCurves for PMMA, PS andPEO in DMAC.

Figure 4. Universal CalibrationCurve in DMAC.

Figure 5. PrimaryMolecular Weight CalibrationCurves for PMMA, PS, andPEOin DMAC the low MW Regions.

Figure 6. Hydrodynamic Volume CalibrationCurves for PMMA, PS and PE0in DMAC the low MW Regions.

Figure 7. SEC Chromatogramof Polystyrene (MW= 500) sample in DMACwith AsahiPak Columns.

Figure 8. Primary Molecular Weight Calibration Curves for PS, PMMA andPEO in DMAC with AsahiPak Columns.

Figure 9. HydrodynamicVolume CalibrationCurves for PS, PMMA and PEOin DMAC with AsahiPak Columns.

200

Table I: Results of SEC/DV and SEC/DKI/DV Analysis of PIB, PMMA, and PEO in THF

i,

SEC/DRI/DV SEC/DV

III

PIB-1 935 1756 1469 925 4.09PIB-2 1408 3316 2649 1317 5.52PIB-3 2499 7432 6076 2473 8.38PIB-4 1146 1205 1175 1146 3.62

PMMA-1 1574 3606 3068 '1575 4.10PMMA-2 2419 5870 4853 2339 4.52PMMA-3 3455 8114 6771 3265 5.22

PEO-1 2352 8481 7283 2125 29.05i

201

Table II: Comparison of Ke Estimated From SEC-Viscometry Techniquesfor PIB, PMMA, and PEO with Literature Values

PIB PMMA PEOim

IX) 1 2 3 4 1 2 3 1

I_ 10.67 10.73 10.75 10.56 7.40 6.48 6.34 34.04xlO_

i II

I_ 10.7 ± 0.5 4.8 ± 0.2' - 7.5 ± 0.22 11.5± 1.5xlO2"

SyndiotacticPMMA2 IsotacticPM]VIA

* LiteratureValue,reference[9]

Table 1TI: Results of SEC/DV and SEC/DRI/DV Analysis of PMMA in DMAC//xHT

SEC/DRI/DV SEC/DVI

Fan h]nd/g x1o'i m o

PM.M.A.-I 1701 4331 3048 1667 4.38 7.9

(1574) 0606) (3068) (1575) (4.10) (7.4)i|l

PM24A-2 2579 7126 5307 2441 5.07 7.0

(2419) (5870) (4853) (2339) (4.52) (6.48)ml

PM]vLE-3 3220 9940 7089 3147 5.81 7.0

0445) (8114) (6771) 0265) (5.22) (6.34)i

* Values in parenthesis are Waterloo results obtained in THF.

202

i Table IV: Results of SEC/DV and SEC/DRI/DV of PMMA Blend(6K/4K/2K:I/2/1):DMAC/uHT)

SEC/DRI/DV SEC/DV

V

_ _ _ _ , [_ _ _ lo_PMMACALIBRATION 2881 4635 3989 2947 5.02 7.95

i

PSCALIBRATION 6754 7627 7518 6435 5.00 5.77

CALCULATED 5185 3825

Table V: Effect of HDV Curves on the Apparent I_ for Polystyrene Samples

1_ HDV/PS HDV/PMMA LITERATURE

9100 9.09 10.0

5570 8.99 13.6 8.2 4- 0.5 03)

4000 8.65 15.0

Table VI: Effect of HDV Curves on the Apparent K0for PMMA Samples

SAMPLE HDV/PMMA/DMAC HDV/PS/THF I-IDV/PS/DMAC

PMMA-1 7.9 7.4 5.32

PMMA-2 7.0 6.48 5.42

PMMA-3 7.0 6.34 5.75

203

Table VII: Results of SEC/DV and SEC/DRI/DV of PolyTHF 2000inDMAC/AsahiI'ak

SEC/DRI/DV SEC/DV

HDV _ lil. Pi. Fin In] Iq x 10'

PEO 1913 4316 3695 2010 11.9 19.6

(18.0±

PS/PMMA 1063 3357 2630 1164 11.9 23.3

204

... 12 ' 1200

" Yiscotek

,iD_

8 -- -- 800

4-- -- 400

t -

0" DO

1400 1600 1800 2000 2200 2400

Elution Time (s)

FIGURE 1205

100.0

o.1 i ' ' ' '''"1 ' ' '' ''"1 ' ' ' ' '"'100 1000 10000 10C

Molecular Weight

FIGURE. 2 206

-d

r-. 3 pHT Linear & KD 802.5 in DMAC

POLY(METHYLMETHACRYLATE)POLY(ETHYLENEOXIDE)(9

A POLYSTYRENEt_t==I_'

. "X_0 %_

'_1'-

--

26 30 32 34 36 38 40RETENTION TIME (minutes)

FIGURE 3

o

_a

o

3 pHT Linear & KD 802.5 In DMAC _[_/_

_.. m POLY(METHYLMETHACRYLATE) ""-, AO POLY(ETHYLENEOXIDE) .,..

POLYSTYRENE _

G o

I " I 'I I I - I ""26 28 30 32 34 36 38 40

RETENTION TIME (minutes)

3 p.HT+ KD 802.5/DMAC

4 • I II I I I

4.2- !• .4 B

3.8-

\

log M '" ' ° -

3.4-

3.2- -

2.s . ! 1 I I , ,42 44 46 411 50

RT" PEO-- PMMA=- PS

209'FtGURE-5

3 pHT + KD 802.5/DMAC

4I I I I

@

log [filM

I

1.5,-

t ! t I42 44 46 48 5U

RT

°- _£0

- p.$

FIGURE 62]0

PS_eBJBJ_ EX (. 8) ]nj J Ch !' i**A .... I ......... I ......... I ...... |.,I..l_a...||.. _ ...... |*-A*----¢

FIGURE 7

ASAHIPAK/DMAC

4.2

4 ,,I,

log M " *

.3.4

2.6

I$ 19 20 21 22 23 24

RT° PEG" PMMA*" PS

FIGURE 8212

ASAHIPAK/DMAC

1-

Ig 19 20 21 22 23 24

RT

° PEGPMMA

° PS

FIGURE 9 213