the other relating to the applicabilityof the universal calibration curve. (UCC). In SEC the separation of macromoleculesis based on hydrodynamic volume (HDV).
ASSESSING THE VALIDITY OF THE UNIVERSAL CALIBRATION CONCEPT IN THF AND DMAC FOR A VARIETY OF POLYMERS AND COLUMN SUPPORTS C. Kuo and T. Provder The Glidden Company Member of ICI Paints Strongsville, Ohio, U.S.A.
R. A. Sanayei and K. F. O'DriScoll University of Waterloo Waterloo, Ontario, Canada INTRODUCTION
,
.
Size exclusion chromatography (SEC) analysis of low molecular weight polymers (M < 10,000) can be complicated by at least two different problems, one relating to quantitative correctnessof using differential refractive index detector (DRI) as a surrogate variable for concentration, and the other relating to the applicability of the universal calibration curve (UCC). In SEC the separation of macromoleculesis based on hydrodynamic volume (HDV). The HDV of a polymer chain scales as intrinsic viscosity times molecular weight ([_]i ii)" This is the basis of the UCC technique first put forward by Benoit[1], and since validated for many different types of polymers having molecular weights usually above 10,00012]. In contrast, the use of UCC in the low molecular weight region has not been extensively explored. Viscosity measurements, necessary for application of UCC, are commonly believed to be difficult for low molecular weight polymers. However, we have found an on-line viscometer to be capable of accurately measuring intrinsic viscositiesof polymers eluting from the SEC with molecular weights as low as 200. An on-line viscometer is therefore an ideal supplementary detector for traditional SEC in order to investigate the validity of the UCC technique in the low molecular weight region. Another complication arises when polar solvent such as DMF or DMAC is used as the SEC mobile phase due to the polymer-solvent-columnpacking interactions. In this paper, the validity of the universal calibration concept in THF and DMAC is exploredfor a variety of polymer types and column packings utilizing SEC -viscometry-DRI system. Use is made of the SanayeiO'DriscolI-Rudin[3] single parameter universal calibration model based on K0 to assess the validity of the universal calibration curve, 193
//
PROCEDURE On-line viscometry can be used in conjunction with conventional SEC to measure the specific viscosity of the polymer solution eluting through the detector. At the very low concentrations used in SEC, the specific viscosity of the eluent can be expressed as the product of its concentration and intrinsic viscosity (_i =Ci[//]i)" The currently available on-line viscometers are sensitive enough to be used for quantitative specific viscosity measurements of 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 the on-line viscometer measurements in SEC-viscometry analysis without any need to use a concentration detector. The number average molecular weight Mn, and [_/] of a polymer sample are attainable from this distribution. The method of doing so, using a differential viscometer alone, we shall term SEC/DV. The molecular weight statistics (Mn, Mv and Mw) and MWD of a polymer can be determined by combining the DRI with on-line viscometry information. We term this method SEC/DRI/DV. A Poster paper[5] in this Symposium has shown that Mn values obtained from both methods should be the same. The Mark-Houwink-Sakurada molecular weight:
equation relates the intrinsic viscosity to
[_] = 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 form of the above equation can be used: [_] = Ke M°'Sv and
Mv = [_WiMi
(2) a]l/a
(3)
The Mv of a polymer sample can be obtained from SEC/DRI/DV method relying 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 of a 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. If the apparent Ke is within the error of reported values of Ke, the validity of the UCC technique is confirmed. For a highly branched polymer sample, it is expected that the apparent K0 would be smaller than that of the linear polymer. 194
EXPERIMENTAL: SEC/THF Experimentalwork was carried out using an SEC system equipped with a DRI, a UV/VIS and an on-line viscometer (Viscotek Model 110). The column eluent passedthrough the UV detector and then was split evenly between the viscometer and the DRI detectors. One of two column sets was used, either a 4x30 cm PLgelcolumns (10 s, 104, 500, and 100 A with 10/=rn bead size or 3x30 cm mixed bed columns. THF was utilized as solvent at 1 ml/min., polymer concentrationswere .5 to 1.0% (w/v), and the system was operated at 30°C. Independent UCC'S were established for each detector using low molecular weight polystyrene standards from Polymer Laboratories, Shropshire, U.K. Detectors were interfaced with an IBM compatible microcomputerfor data acquisition. The data were subsequently processed using a microcomputerwith a software package developed at the University of Waterloo. Polymer samples were obtained from various sources. Poly(isobutylene)(PIB) samples were purchased from American Polymer Standards, Mentor, Ohio. Preparation details and characterization of poly(methylmethacrylate) sample (PMMA) were previously reported[7]. The M It values, determined by SEC with DRI (directly calibrated with PMMA standards) were 1485, 2314, and 3284 for PMMA-1, -2, and -3 respectively. 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 CV equipped with a single capillary viscometer and a DRI detector [8]. The mobile phase, DMAC, had a flow rate of 1.0 ml/min, at 80°C. The columns used were 3x30 cm Waters/_Styragel HT linear columns plus lx30 cm Shodex KD 802.5 column. Also used were 3x30 cm AsahiPak columns obtained from Keystone Scientific. These columns were packed with polyvinyl alcohol with the following designations: GMS-700, GS-510, and GS-310. The data acquisitionwas done with bothWaters Expert Ease data system and LeSec multipledetector GPC software version 3.13a. Narrow MWD polystyrene, PMMA, PEO/PEGwere used as the calibrants and were obtained from both American Polymer Standards and Toyo Soda company. Broad MWD PMMA samples were obtained from University of Waterloo and were described in the SEC/THF section. PMMA blend was a sample made in the Glidden laboratory from mixing three narrow MWD PMMA standards (6K:4K:2K/1:2:1). Polytetrahydrofuran (polyTHF) sample was obtained from BASF. 195
RESULTS AND DISCUSSION SEC/THF Three chemically different types of low molecular weight polymers, poly(isobutylene), poly(methyl methacrylate) and poly(ethylene oxide), were analyzed with PLgel columns in THF. A Viscotek model 110 viscometer was used in conjunction with a DRI. The resulting molecular weight statistics are listed in Table I. It is observed that the Mn values obtained for all three types of polymers are in good agreement with the nominal values. Using the [T/]and Mv values, it is possible to calculate the apparent K0 for each of the samples. Table II shows the Ke results along with the literature[9] values for these polymers. It is seen that the Ke values calculated for PIB are in extremely good agreement with the literature values, indicating that use of the UCC was valid for these low molecular weight PIB polymers. For the PMMA samples, the Ke values are intermediate between the values reported for syndiotactic and isotactic PMMA, which is to be expected for PMMA prepared by free radical polymerization. The reasonable agreement with literature Ke values suggest the UCC is also valid for these low molecular weight PMMA samples. For the PEO sample, the Ke value obtained is three-fold greater than the literature value. It is concluded that when usingTHF as solvent, PEO probably interacts by adsorptionon the column, and that separation occurs not only by size exclusion. Therefore, the UCC concept breakdown for low molecular weight PEO. This abnormality was also observed earlier by Ambler[10] when toluene was used as the eluent. In a recent report[11] it is claimed that the breakdown of the UCC at very low molecular weights is "obvious given that intrinsic viscosities become negative'. While there have been reports of negative intrinsicviscosities being measured for some polymers in unique solvents[12], we are not aware of any such reports for polymers in THF. Negative intrinsic viscosities are regarded as arising from specific interactions between solute and solvent molecules such that a liquid structure of some kind existing in the solvent is destroyed in the vicinity of a solute polymer molecule[12]. In our experience, SEC-viscometry data (such as those in Figures 1 and 2) consistently show positive values for [_/]iin THF for low molecular weight samples of PSTY, PMMA, PIB and PEO. The scatter in values of [_]i at very low molecular weight (such as seen in Figure 2) is a consequence of the extremely low concentrations present in the SEC eluent. If measurements of [7/]for the whole sample are desired at extremely low molecular weight (ca. 200 to 1000), this can be accomplished by using higher concentrations in the SEC. 196
SEC/DMAC//_HT A previous SEC study[8] with DMAC as eluant 3/_HT linear columns and a Shodex KD-802.5 column set (styrene-divinylbenzene packings), demonstrated that the universal calibration was applicable for high molecular weight PS, PMMA, and PEO/PEG samples. Shown in Figure 3 is the primary molecular weight 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 and PMMA. However, when the data are plotted onto a universal calibration curve, the data points fall into a common curve except in the very low molecular weight regions as shown in Figure 4. The current study focuses on the low molecular weight regions. Figures 5 and 6 show the primary MW calibration curve and the UCC respectively, in the low MW regions. It is seen that PS deviates from the common I-IDV curve based on PEO/PEG standards. Using this common HDV curve, molecular weight statistics were obtained for the PMMA samples used in the SEC/THF study. The molecular averages in Table III obtained from the SEC/DMAC system are comparable to those obtained from the SEC/THF system. Also in agreement within experimental error are the K0 values. This data confirms that the Ke value is independent of solvent type. Results also are obtained for a synthetic polymer blend from mixing known amounts of narrow MWD PMMA standards. The MW statistics obtained from the 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 also in agreement with the results obtained for those broad MWD PMMA studied. When the HDV curve generated from PS standards was used in the calculation, the MW averages were over estimated while the Ke value underestimated indicating the incorrect HDV was used casting doubt on the validity 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 to explore the effect of different packings (AsahiPak GSM-700, GS-510, GS310). Figure 7 shows the experimental chromatogram of a low MW polystyrene sample (MW-500). The chromatogram shows the excellent resolution for this sample as well as the complete elution of all components free of interference from the solvent trash peaks. Figure 8 shows the primary MW calibration curves for low MW PS, PMMA and PEO. It is seen again that the PEO did not fall into the same curve as those of PS and PMMA. When plotted 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 the styrene-divinylbenzene packings where PEO and PMMA form a common HDV curve. A series of samples were run to generate the MW averages and
•
apparent K0 values. Table VII shows the results obtained for a polyTHF sample with 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 K0 value is in good agreement with the literature value. Similar conclusionswere obtained that use of the incorrect HDV curve leads to wrong MW statistics and Ke values. CONCLUSIONS For the three types of polymers studied with styrene-divinylbenzene packings in THF, UCC is valid for PMMA and PIB, but is not valid for PEO. In the case of SEC/DMAC, no common hydrodynamic volume(HDV) calibration curve exists either with styrene-divinylbenzene or polyvinyl alcohol column packings for low molecular weight polymers over wide range of polarity. PMMA and PEO/PPG form a common HDV curve with styrene-divinylbenzene packings while PMMA and PS form a common HDV curve with polyvinyl alcohol packings. Overall, the apparent Ke values appear to be indicative of the validity of UCC (HDV Curve) for solvent/column packing systems for a wide range of polymer types. If Ke is higher than the literature value, the sample is interacting with the column and if Ke is lower than the literature value, the calibrants are not generating a UCC. ACKNOWLEDGMENT Support of this research related to the SEC/THF study by the Natural Sciences and Engineering ResearchCouncil of Canada and by the Ontario Center for Materials Research is gratefully acknowledged. One of us, RAS, acknowledges a discussionwith Dr. N. Weeks of the Polaroid Corp. which stimulated 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 Molecular Weights 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) and references 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 DRI Detectors
Figure 2.
Intrinsic Viscosity vs. Molecular Weight for PIB-3 from Figure 1. Data points calculated are [_]i of eluent; solid line is plot of Equation 2 using literature values of Ke[9].
Figure 3.
Primary Molecular Weight Calibration Curves for PMMA, PS and PEO in DMAC.
Figure 4.
Universal CalibrationCurve in DMAC.
Figure 5.
Primary Molecular Weight Calibration Curves for PMMA, PS, and PEO in DMAC the low MW Regions.
Figure 6.
Hydrodynamic Volume Calibration Curves for PMMA, PS and PE0 in DMAC the low MW Regions.
Figure 7.
SEC Chromatogramof Polystyrene (MW= 500) sample in DMAC with AsahiPak Columns.
Figure 8.
Primary Molecular Weight Calibration Curves for PS, PMMA and PEO in DMAC with AsahiPak Columns.
Figure 9.
Hydrodynamic Volume Calibration Curves for PS, PMMA and PEO in 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 II
I
PIB-1 PIB-2 PIB-3 PIB-4
935 1408 2499 1146
1756 3316 7432 1205
1469 2649 6076 1175
925 1317 2473 1146
4.09 5.52 8.38 3.62
PMMA-1 PMMA-2 PMMA-3
1574 2419 3455
3606 5870 8114
3068 4853 6771
'1575 2339 3265
4.10 4.52 5.22
PEO-1
2352
8481
7283
2125
29.05
i
201
Table II: Comparison of Ke Estimated From SEC-Viscometry for PIB, PMMA, and PEO with Literature Values
PIB
Techniques
PMMA
PEO
im
IX) I_ xlO_
1
2
3
4
1
2
3
1
10.67
10.73
10.75
10.56
7.40
6.48
6.34
34.04
i
II
I_ xlO2"
10.7 ± 0.5
2
Syndiotactic PMMA Isotactic PM]VIA
*
Literature Value,reference [9]
4.8 ± 0.2' - 7.5 ± 0.22
Table 1TI: Results of SEC/DV and SEC/DRI/DV
11.5± 1.5
Analysis of PMMA in DMAC//xHT
SEC/DRI/DV
SEC/DV I
Fan
h]nd/g
im
PM.M.A.-I i|l
PM24A-2
x 1o' o
1701
4331
3048
1667
4.38
7.9
(1574)
0606)
(3068)
(1575)
(4.10)
(7.4)
2579 (2419)
7126 (5870)
5307 (4853)
2441 (2339)
5.07 (4.52)
7.0 (6.48)
3220
9940
7089
3147
5.81
7.0
0445)
(8114)
(6771)
0265)
(5.22)
(6.34)
ml
PM]vLE-3
* Values in parenthesis
are Waterloo results obtained in THF.
202
i
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
PMMA CALIBRATION
_
_
_
_ , [_
2881
4635
3989
2947
_ _ lo_ 5.02
7.95
i
PS CALIBRATION
6754
7627
CALCULATED
5185
3825
7518
6435
5.00
5.77
Table V: Effect of HDV Curves on the Apparent I_ for Polystyrene Samples
1_
HDV/PS
HDV/PMMA
9100
9.09
10.0
5570
8.99
13.6
4000
8.65
15.0
LITERATURE
8.2 4- 0.5 03)
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 inDMAC/AsahiI'ak
SEC/DRI/DV
of PolyTHF 2000
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
204
1164
11.9
23.3
...
12
'
"
1200
Yiscotek
,i D_ 8 --
-- 800
4--
-- 400
t
-
0"
DO
1400
1600
1800
2000
Elution Time (s)
FIGURE 1 205
2200
2400
100.0
o.1 i
'
' ' '''"1
100
' 1000
10000
Molecular
FIGURE. 2
' '' ''"1
206
Weight
'
' ' ' '"'
10C
-d
r-.
t_ t==I _'
.
3 pHT
Linear & KD 802.5 in DMAC
(9 A
POLY(METHYLMETHACRYLATE) POLY(ETHYLENEOXIDE) POLYSTYRENE
"X_
0
%_
'_1'-
-26
FIGURE 3
30
32 RETENTION
34 TIME (minutes)
36
38
40
o _a
o
3 pHT Linear _..
m O
& KD 802.5
In DMAC
_[_/_
POLY(METHYLMETHACRYLATE) POLY(ETHYLENEOXIDE) POLYSTYRENE
""-, A .,.. _ G o
"26
I 28
" I 30
'I I 32 34 RETENTION TIME (minutes)
I 36
-
I 38
"
40
3 p.HT+ KD 802.5/DMAC 4
I
I
•
I
I
I
I
4.2•
.4
! B
3.8-
\
log M '"
' °
-
3.4-
3.2-
2.s
-
.
!
42
1
I
44
46
RT " PEO -- PMMA =- PS 'FtGURE-5
209
I
411
,
50
,
3 pHT + KD 802.5/DMAC 4
I
I
I
I
@
log [filM
I
1.5,-
t
!
t
I
42
44
46
48
RT
°- _£0 - p.$ FIGURE 6
2]0
5U
PS_eBJBJ_ EX (. 8) '
i**A
]nj
....
J Ch ! I
FIGURE 7
.........
I
.........
I
......
|.,I..l_a...||..
_
......
|*-A*----¢
ASAHIPAK/DMAC 4.2
4
,,I,
log M "
*
.3.4
2.6 I$
19
20
21
RT
° PEG
" PMMA *" PS FIGURE 8 212
22
23
24
ASAHIPAK/DMAC
1-
Ig
19
20
21
RT
° PEG PMMA ° PS FIGURE 9
213
22
23
24
