Anal. Chem. 1988, 60, 1125-1128
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Determination of Chemical Composition and Molecular Weight Distributions of High-Conversion Styrene-Methyl Methacrylate Copolymers by Liquid Adsorption and Size Exclusion Chromatography Sadao Mori Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514, Japan Copolymers having a broad chemical compodlion dlstrlbutlon (CCD) were separated according to composition by liquid adsorption chromatography (LAC). The stationary phase was silica gel and the moblle phase was a mixture of chloroform and ethanol. Linear gradient elution was employed for LAC In order ol increasing ethanol content at column temperatures of 30 or 40 OC. The copolymer molecules having more styrene eluted earlier. A CCD of the copolymers was obtained from a LAC chromatogram by converting the abscissa of the chfomatogram to percent methyl methacrylate (MMA % ) and the ordinate to d W/dMMA%. An example of the range of MMA content in a copolymer (average composition, MMA 67.2%) was between 54% and 85%. Fractions by size exclusion chromatography were subjected to LAC. A molecular welghl distrlbutlon (MWD) could be obtained from these data through a four-step conversion process with the a b s c h being converted to the copolymer molecular weight and the ordlnate to dW/d(log mol wt). These converslon procedures were proposed. CCD, MWD, and a three-dhnensional contour map (molecular weight, composition, and copolymer weight fraction) are demonstrated.
Synthetic random copolymers have a chemical composition distribution (CCD) and a molecular weight distribution (MWD). The accurate determination of these distributions is very important for the characterization of copolymers. Several attempts have been reported for obtaining a CCD and a MWD by using combinations of two techniques: size exclusion chromatography (SEC)/thin-layer chromatography (1-3),SEC/high-performance precipitation liquid chromatography (4,5), SEC/SEC (6),and column adsorption chromatography/SEC (3,7). Application of SEC with a dualdetector system such as a combination of an ultraviolet absorption (UV) detector and a differential refractometer (RI) is not adequate to obtain reliable information on both distributions (8). Separation by SEC is achieved according to the sizes of molecules in solution and not their molecular weights. Components eluted in the same retention volume might have different compositions and only the average composition could be detected at a specified retention volume. In previous papers (9-1 I), separation of low-conversion styrene-methyl methacrylate random copolymers, P(S-MMA), according to chemical composition by high-performance liquid adsorption chromatography (LAC) was reported. The copolymers containing more styrene eluted earlier and molecular weight dependence on retention volume was not observed in the separation of the copolymers by LAC. A combination of LAC and SEC gave information on the relationship between composition and molecular weight of the copolymers; i.e., the copolymer fractions having more styrene content had lower molecular weight averages. In the present work, high-conversion random copolymers of P(S-MMA) were prepared and characterized by a combination of LAC and SEC. Fractionation by LAC gave a CCD 0003-2700/88/0380-1125$01.50/0
of the copolymers. Fractionation by SEC followed by LAC gave a real MWD.
EXPERIMENTAL SECTION High-PerformanceLiquid Adsorption Chromatography. A Jasco TRIROTAR-VI high-performance liquid chromatograph (Japan Spectroscopic Co., Ltd., Hachioji, Tokyo 192, Japan) with a variable-wavelength ultraviolet (W)absorption detector, Model UVIDEC-100V1,was used at a wavelength of 254 nm. Silica gel with a pore size of 30 A and a mean particle diameter of 5 gm (Nomura Chemical Co., Seto, Aichi 489, Japan) was packed in a 4.6-mm4.d. X 50-mm-lengthstainless-steeltubing. This column was thermostated at a specified temperature by using a column jacket in which constant-temperature water was circulated. Elution was performed by the linear gradient elution method with mixtures of chloroform and ethanol. The initial mobile phase (A) was a mixture of chloroform and ethanol (99.O:l.O (v/v)) and the f d mobile phase (B)was a mixture of chloroform and ethanol (95.54.5 (v/v)). The compoaition of the mobile phase was changed from 100% A to 100% B in 15 min linearly and kept to 100% B for another 10 min. Sample copolymers were dissolved in A and a sample solution was injected 1min after from the start of gradient elution. A flow rate was 0.5 mL/min and an injection volume of a sample solution was 0.1 mL. High-PerformanceSize Exclusion Chromatography. A JascoTRIROTAR-V HPLC apparatus with a W detector, Model UVIDEC-lOOV, and a differential refractometer (RI),Model S E l l (Showa Denko Co., Minato-ku, Tokyo 105, Japan), was used. Columns for analytical SEC were two Shodex KF 80M HPSEC columns (each 25 cm X 8 mm i.d.) (Showa Denko Co.) packed with polystyrene gels for polymer fractionation. The number of theoretical plates of the columns was 13OOO plates/25 cm, which was obtained by injecting 0.05 mL of a 1% benzene solution. Columns for preparative fractionation were two Shodex A 80M HPSEC columns (each 50 cm length X 8 mm i.d.) packed with polystyrene gels for polymer fractionation. Chloroform containing 1%ethanol was used as the mobile phase with a flow rate of 1.0 mL/min. An injection volume of a sample solution was 0.1 mL for analytical SEC and 0.25 mL for preparative SEC. Samples. High-conversion random copolymers of P(S-MMA) were prepared by bulk polymerization in polymerization ampules. Styrene and MMA monomers were mixed in an ampule and AIBN (azobis(isobutyronitri1e))was added as an initiator at a concentration of 0.15% to the total monomers. After deaeration, the ampule was sealed and polymerization was performed at 60 O C for 23 h. The polymerization products were then dissolved in chloroform and precipitated in methanol. This purification process was repeated three times and the purified copolymers were dried and characterized by infrared spectrometry and SEC. The copolymers were cast on a KBr disk from a chloroform solution and the average composition of the copolymers was measured by using an infrared spectrophotometer Model FT/IR-3 (Japan Spectroscopic Co.). A calibration curve was constructed by using low-conversionrandom copolymers of P(S-MMA)which were the same as those in the previous reports ( S I I ) . An equation of the calibration curve is as follows: A,gg/A,,, = -1.36 X lo-' MMA wt % + 1.349 (1) where Am and A1,90are absorbances at 699 cm-' (a phenyl group) and at 1730 cm-' (a carbonyl group), respectively. The values of the absorbances were measured by a base-line method. This 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 1 1 , JUNE 1 , 1988
Table I. Composition and Molecular Weight Averages of High-Conversion Sytrene-Methyl Methacrylate Random Copolymers and Their Fractions"
composition,
styrene
recov-
sample
wt %
ery, %
unfractionated fraction 1 fraction 2 fraction 3
32.8
a
lo-*
lo-*
M,
36.2
83.8
2.31
28.7 47.6 42.8
50.0 84.2 88.2
1.74 1.77 2.06
34.3
78.9
2.30
23.9 40.6 20.2
49.3 83.3 54.3
2.06 2.05 2.69
n
b
mol wt average M,,X M , X M w /
H-3 36.6 31.7 29.4
32.1 42.0 25.9 H-4
unfractionated fraction 1 fraction 2 fraction 3
43.2 46.6 41.8 39.6
39.0 24.1 36.9
"Styrene wt % in feed: H-3, 31.6%; H-4, 42.0%. Degree of conversion: H-3, 97.6%; H-4, 97.2%. Recovery is corrected to 100% total. equation was effective for copolymers containing MMA between 30% and 95%. Composition of fractions of the copolymers by LAC and SEC was also measured by using eq 1. Styrene content and molecular weight of the copolymers are listed in Table I. Two samples, H-3 and H-4, were used in this experiment. Polystyrene standards having a narrow MWD were used for the construction of a calibration curve for SEC, and the values of molecular weights for the copolymers in the Table are all the polystyrene equivalent molecular weights.
RESULTS AND DISCUSSION LAC Chromatogram. A high-conversion P(S-MMA) copolymer, H-3, was subjected to LAC a t different column temperatures, and the chromatograms are shown in Figure 1. Column temperature was changed from 10 to 35 "C. With increasing column temperature, the elution of the copolymer retarded and peaks became broad, meaning higher resolution in this case. The change of the retention volume with column temperature was already observed in previous papers and the mechanism was discussed (10, 11). The optimal column temperature was dependent on the composition of the copolymers. For the present samples, it was between 30 and 40 "C. Even at lower column temperature, the copolymers cannot elute earlier than the specified retention volume. For example, the copolymer H-3 could not elute earlier than 4.75 mL at 10 OC of the column temperature. Because retention volume of the copolymer having the specified composition is controlled by not only column temperature but also ethanol content in the mobile phase. Therefore, peaks of the copolymer H-3 became sharp with the decrease of column temperature under the present gradient elution condition. A solution of a mixture of low-conversion P(S-MMA) copolymers VI (styrene content 42.1%), VI11 (26.5%), and IX (15.2%) was injected into a LAC system, and the chromatogram is shown in Figure If. Concentration of the copolymers was 0.05% each. A solution of a low-conversion P(S-MMA) copolymer V (styrene content 48.7%) was also injected in the system, and the chromatogram is shown in Figure lg. From these two chromatograms, it was possible to construct the calibration curve for composition of the copolymer versus retention volume to obtain a CCD for the high-conversion copolymers. The calibration curve is shown in Figure 2. A PMMA homopolymer might appear at about V , = 7.6 mL, which was obtained by extrapolation to 100% (=PMMA). Fractionation by LAC. In order to know the chemical heterogeneity of high-conversion copolymers H-3 and H-4,
0
1
2
3
6
5
6
7
8
VR (mL)
Flgure 1. LAC chromatogramsof P(S-MMA) copolymers at different column temperatures: sample, (a-e) H-3, (f) and (g), low-conversion copolymers; column temperature, (a) 35 OC, (b, f, g) 30 OC, (c) 25 "C, (d) 20 OC, (e) 10 OC; detector, UV; attenuation, (a-e), 0.64 AUFS (absorbance unit of full scale), (f, g), 0.32 AUFS; sample size, H-3, 0.12%,0.1 mL injection; low-conversion samples 0.05% each, 0.05 mL injection; composition of low-conversion P(S-MMA), V (styrene content 48.7%), V I (42.1%).VI11 (26.5%),and I X (15.2%). 10 I
/
9
,
I
8
f U
I 6
5
Figure 2. Calibration cwve for copotymer composklon versus retention volume at column temperature 30 OC.
LAC chromatograms were divided into three fractions as shown in Figure 3. Column temperature for fractionation of the copolymer H-4 was increased to 40 "C to obtain a wider chromatogram. Sample concentration was 0.12% and 0.1 mL of the solution was injected. Fractionation was repeated 50 times and each fraction was combined. One portion of the fractions was used for the determination of composition and the other for the determination of molecular weight averages. Composition of fractionated copolymers was measured by using an infrared spectrophotometer and molecular weight averages were by SEC. The results are listed in Table I. The difference of composition between fraction 1 and fraction 3
ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
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d W l d MMA"lo
0
I
I
I
I
I
I
1
I
I
1
2
3
4
5
6
7
8
9
I
h
I
,
,
, ,
I I
I
-
i\c,
. I
I
I
I
I
I
I
I
I
I
0
1
2
3
4
5
6
7
8
9
V~(mi-1 Flguro 3. Fractionationof high-conversh copolymers K 3 (a) and K4 (b) by LAC, column temperatures (a) 30 'C and (b) 40 'C.
Figure 5. Chemical composition distribution (a) and LAC chromatograms of the SEC fractions (b) for the copolymer H-3.
I
I
I
I
I
I
I
I
19
18
17
16
15
14
13
12
(weight) (obtained by a UV detector) in the eluate at VR. To obtain a CCD from any of the chromatograms in Figure 3, one must first divide the chromatogram into equal parts, obtain the composition at each division from Figure 2, correct the height of the chromatogram to the copolymer content (weight) in the eluate, and finally normalize the chromatogram. The ordinate is converted into dW/dV, by this treatment, where dW/dVR means weight fraction of the copolymer per unit retention volume. The abscissa and the ordinate for a CCD should be MMA% (or styrene %) and dW/dMMA% (or dW/d(styrene %)), respectively. The former is converted by using Figure 2 and the latter is converted by
dW
VR(mL) Flgue 4. SEC chromatograms of the high-conversion copolymers and their fractions by LAC: (a-d), H-3; (e-h), H-4; (a, e), unfractionated ones; (b, f), fraction 1; (c, g), fraction 2; (d, h), fraction 3; (0),styrene weight fraction ( W s ) .
for both copolymers is about 7%, but this is the difference of average composition and the composition difference in the copolymers might be larger than this value, which will be discussed later. The fractions eluted earlier contained more styrene and had lower molecular weight, which were similar to the results found in the previous studies (9, IO). The relative standard deviation of the compositional analyses was between 0.3% and 0.5% by five repeated analyses. Fractionation by SEC. SEC chromatograms of highconversion copolymers H-3 and H-4 and their fractions by LAC are shown in Figure 4. A differential refractometer was used as a detector. In general, fractions eluted later by LAC had higher molecular weight averages and appeared earlier in SEC. The analysis by using an SEC/UV-RI system shows that unfractionated samples H-3 and H-4 had heterogeneous composition that styrene content increased with increasing retention volume. Fractions by LAC had almost uniform composition, so the results are not shown in Figure 4. Determination of a CCD and a MWD. The axis of abscissa of chromatograms in Figure 3 represents retention volume VR and the ordinate shows the styrene content
dMMA%
= -d W x dVR
d VR dMMA%
(2)
where d V R / d m % is the reciprocal slope of the calibration curve in Figure 2. The CCD for copolymer H-3 thus obtained is shown in Figure 5a. The MMA content of the copolymer H-3 was between 54% and 85%. SEC chromatograms of unfractionated H-3 and H-4 shown in parts a and e of Figure 4 are expressed as the abscissa by V, and the ordinate by polymer concentration obtained by an RI detector. As a MWD is expressed conventionally by log molecular weight for the abscissa and by dW/d(log mol wt) for the ordinate, next four steps are necessary for the conversion from chromatograms to a MWD. Step 1. A preparative SEC chromatogram was divided into equal parts (here 11 parts) and several fractions were obtained. LAC chromatograms of these fractions (corrected by eq 2) are shown in Figure 5b. Average composition of each fraction was determined by infrared spectrometry and the polystyrene equivalent molecular weight averages by SEC. Step 2. Height of the preparative SEC chromatogram was corrected by knowing composition. The ratio of the response coefficients of styrene and MMA for an RI detector in the mobile phase of chloroform was 2.18 and the correction is made by using Hcor,i
= Hobed,i(WS,i + 2*18WMMA,i)
(3)
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
M M A , w tolo Flgwe 7. A contour map of the three-dimensional distribution of molecular weight and chemical composition for the copolymer H-3. The figures show the relatlve height.
log M Flgure 6. Molecular weight distribution for the copolymer H-3.
where H, and HOMare height of the chromatogram, corrected and observed, at retention volume i. W Sand W u are weight fractions of styrene and MMA in the copolymer eluted at retention volume i, respectively. The SEC chromatogram is normalized, so that the ordinate of the chromatogram is expressed as dW/dVR, where d W is the weight fraction of the copolymer per unit retention volume. Step 3. The molecular weight of each fraction is converted from the polystyrene equivalent molecular weight to the copolymer molecular weight from (12) mol wt of the copolymer a t retention volume
i=
mps,iMps,i + ~ P M M A 1.967M~s,/'.~~~ ,~ (4) where MPMM= 1.967Mps0.918,which was obtained in the mobile phase of tetrahydrofuran (13) and was also confirmed to be valid in chloroform. The terms mps and mPu are mole fraction of styrene and MMA in the copolymer and the suffix i means the fraction number in this case. The value of Mps is obtained by using the calibration curve constructed with polystyrenes. The values mps and mPu are obtained by IR analysis, and when the LAC chromatogram of each fraction has a broad distribution, then the value m should be divided into several parts and the sum of m,M, should be applied. Equation 4 is an empirical equation and includes assumptions that molecular weights of random copolymers obey the rule of additivity and that the same viscosity-molecular weight parameters as their respective homopolymers can be applied. Step 4. A calibration curve for the copolymer is reconstructed by eq 4. The ordinate of the normalized SEC chromatogram is converted by
dW d(1og mol wt)
= -d W x
dVR
d VR d(1og mol wt)
bration curve. A MWD is shown in Figure 6. Fractions having lower molecular weight showed rather narrow CCDs and had lower MMA content. Fractions having middle molecular weight had broad CCDs. Fractions having larger molecular weight had narrow CCDs in comparison with those having middle molecular weight and relatively large MMA content. A contour map could be constructed from Figures 5 and 6 and is shown in Figure 7. A ridge of three-dimensional distribution shown by a dotted line is parallel with the molecular weight axis at the range of low molecular weight and turn toward the MMA% axis with increasing molecular weight. It was found that lower MMA components in the copolymer having higher molecular weight and higher MMA components having lower molecular weight were not existent in the copolymers. Registry No. (S)(MMA) (copolymer), 25034-86-0.
LITERATURE CITED Belenkii, B. G.; Gankina, E. S. J. Chromafogr. 1977, 141, 13-90. Teramachi, S.; Hasegawa, A.: YoshMa, S. Macromolecules 1983, 16. 542-545. Inagaki, H.; Tanaka, Y. Pure Appl. Chem. 1982, 5 4 , 309-322. Glockner, G.; van den Berg, J. H. M.; Meijerink, N. L. J.; Scholte, T. G.; KoningsveM, R. Macromolecules 1984, 17, 962-967. Qockner, G.; van den Berg, J. H. M.; Meijerink, N. L. J.; Scholte, T. G.; Konlngsvekl, R. J. Chromafogr. 1984, 317, 615-624. Balke, S. T.; Patel, R. D. Adv. Chem. Ser. 1983, No. 203, 281-310. Tanaka, T.; Omoto, M.; Donkai, N.; Inagaki, H. J. Macromol. Sci., fhyS. 1980, 8 1 7 , 211-228. Mori, S . J. Chromatogr. 1987, 41 1 , 355-362. Mori, S.; Uno, Y.; Suzuki, M. Anal. Chem. 1986, 5 8 , 303-307. Mori, S.; Uno, Y. Anal. Chem. 1987, 5 9 , 90-94. Mori, S.; Uno, Y. J. Appl. folym. Sci. 1987, 3 4 , 2689-2699. Mori, S. J. Chromatogr. 1978, 157, 75-84. Mori, S. Anal. Chem. 1981, 5 3 , 1813-1818.
(5)
where dVR/d(log mol wt) is the reciprocal slope of the cali-
RECEIVED for review October 20, 1987. Accepted February 11, 1988.
