Anal. Chem. 1989, 61, 1318-1321
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Determination of the Dissociation Temperature of Organic Micelles by Microcapillary Hydrodynamic Chromatography J a a p Bos,* Robert Tijssen, and M.Emile van Kreveld
KoninklijkelShell-Laboratorium,Amsterdam (Shell Research B. V.),Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
MlcrocapHlary hydrodynamic chromatography has been used to study the temperature behavlor of micelles of a poly(styrene-Isoprene) block polymer In the nonpolar solvent n -decane. A deactlvatlon scheme for the fused slllca column, necessary for working with nonpolar solvents, is glven. Results for the dissociation range of the micelle are compared wlth those of the iight-scattering technique used by Mandema et ai. on a comparable polymer. I n the transltlon range (for this polymer-solvent system, 20-30 "C) we have observed both the micellar and the molecular form of the polymer. Informatlon on the mlceile size was obtained. I n our study we have found no evidence for the exlstence of very large structures in the transltion reglon as reported by Mandema.
INTRODUCTION Hydrogenated poly(styrene-isoprene) two-block polymers in nonpolar solvents such as n-decane show a nonlinear relationship between inherent viscosity and temperature. The inherent viscosity is defined as qinh
= ( l / C ) In
(~/d
(1)
where T~ is the (dynamic) viscosity of the solvent, 7 is that of the polymer solution, and C is the polymer concentration. A typical case of nonlinear behavior for the polymer/solvent combination mentioned above can be found in the work of at Mandema e t al. ( I ) , who reported that high values of low temperatures suddenly drop to lower values at increased temperature. The common interpretation is that at low temperatures micellar aggregates exist with the polar polystyrene part of the molecules in the core, n-decane being a nonsolvent for polystyrene. At higher temperatures these micelles dissociate into a normal molecular solution. Using light-scattering techniques, Mandema showed the existence of the larger aggregates a t lower temperatures. However, with these techniques it is impossible to establish exactly a t what temperature the micelles start to dissociate or when the dissociation is complete. Also, in the transition range, after an initial decrease of the scattering intensity, corresponding to the presence of a smaller particle, the scattering increased again to high values in a small temperature interval. The latter phenomenon was interpreted as pointing to the existence of large conglomerates with dimensions of the order of the optical wavelength, Le. much larger than the micelle at lower temperature. We have studied the same type of polymer/solvent system by using microcapillary hydrodynamic chromatography (HDC). HDC is a separation technique that makes it possible to observe, in principle, both the micellar and the molecular species during dissociation. EXPERIMENTAL SECTION General Procedure. The instrumental setup for the experiments is similar to the one we used in our earlier publications
Table I S-2 polymer
used by Mandema wt-av mol wt
polydispersity polystyrene content, 70 (w/w)
80 000 1.16 38.0
polymer used in our work 92 000 1.05
35.8
on HDC (2,3). The only difference is that the injector and the major part of the column are immersed in a thermostated bath. An M6000 pump (Waters Associates, Milford, MA) was used to deliver the mobile phase, except during deactivation experiments where caustic and acid aqueous phases were delivered by a 114 M solvent system (Beckman, Berkeley, CA). The (high-temperature/high-pressure) injector was an M-4-C6W Vespel type (Valco, Schenkon, Switzerland) equipped with an external 180-pL loop. Between the pump and the injector, a aO-m-long, 0.25mm4.d. stainless steel tubing was placed in the thermostated bath to equilibrate the mobile phase to the required temperature. Eluting species were monitored by ultraviolet absorption at 215 nm with a Uvidex 100-VI detector (Jasco, Tokyo, Japan). The detector cell was equipped with the adjustable knives slit described in ref 3. The detector signal, after 8-Hz analog to digital conversion, was stored in an HP 3357 lab automation system; interactive graphics manipulations and calculationswere performed on an HP 300 computer system. Samples were injected as a block of 4-9 duration after 2 min of equilibrium time in the sample loop. The length of the restriction capillary was chosen such that in all experiments at different temperatures the elution time of toluene was between 21 and 25 min. Retention times were determined as the first statistical moment of the eluting peaks. Materials. Fused silica capillaries were supplied by SGE (North Melbourne, Australia). The restriction capillaries were of 100-pm i.d. The measuring capillary, bought as a nominally 5-pm-i.d. column, had an internal diameter of exactly 3.70 pm, as determined by the pressure drop-residence time method described in ref 3. The total length of the measuring capillary was 300 cm, with a flow cell created by burning off the polyimide coating at a position 28 cm before the column end. Of the chromatographic column length of 272 cm, a length of 15 cm next to the detector (at ambient temperature) was not thermostated. Solvents. All the solvents were used without further purification. Tetrahydrofuran (THF) (HPLC grade No. 9441), dimethylformamide (DMF') (Baker-analyzedNo. 7032), and toluene (Baker-analyzed No. 8077) were obtained from Baker (Deventer, Holland). n-Decane (Purum No. 30550) was supplied by Fluka (Buchs, Switzerland); N-(trimethylsily1)imidazole (No. 270403) was purchased from Chrompack (Middelburg, Holland). Polymers. To concur as closely as possible with the experiments of Mandema et al. (polymer designation S-2 in ref l), we used the same type of hydrogenated poly(styreneisoprene), which differed only in minor aspects from the S-2 polymer, as shown in Table I. Despite these differences the two polymers showed the same nonlinear relationship between inherent viscosity and temperature, as is shown in Figure 4 of ref 1, with the start of the nonlinear part, and so possibly the onset of the micelle dissociation, at 80 "C. In our experiments the polymer was dissolved (without stirring) in n-decane at 150 "C during 2 h. To prevent degradation of the polymer, the decane had been deaerated by bubbling a helium stream through it.
0003-2700/89/0361-1318$0~.50/0 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13,JULY 1, 1989
I
TOLUENE
I
1
1
1
0.900
0.925
0.950
0.975
1
1.000
1
1.025
DIMENSIONLESS RESIDENCE TIME, T
Figure 1. Hydrodynamic chromatography of the styrene-isoprene polymer in two different mobile phases at room temperature: (A) in ndecane, elution as the micellar aggregate; (B) in THF, elution as the molecular species. In both cases the injection concentration was 2 g/L polymer and 0.5 g/L toluene in the mobile phase. For column dimensions see Experimental Section.
THEORY In HDC the relative retention time 7,defined as the ratio of the residence time for a particle or a macromolecule to the holdup time for small molecules, can be described by the relation (3) 7
= (1
+ 2X - C X y
(2)
where X is the aspect ratio (particle size/column diameter = 2r/2R). For macromolecular species the size can be described by 2f where p represents the effective radius (3, 4 ) . C is a constant related to the relative (slip) velocity of the particles with respect to the surrounding liquid with a value that depends on the particles' geometry (C = 2.698 for free draining random macromolecular coils in a good solvent and C = 4.89 for solid spherical particles). The value of C for micelles is not known but is expected to be between the two extreme values mentioned. However, for not too low values of 7 (say 7 2 0.945) the exact value of C is not very important since the difference in X values calculated from the two extreme values above amounts to less than 4%. So measurement of r in this region will give a virtually model-free estimate of X. From this and the known diameter of the HDC column, the size of the eluting species is obtained.
RESULTS AND DISCUSSION In a good solvent such as T H F in which both the polystyrene and polyisoprene parts of the molecule are soluble, the polymer is supposed to elute as the molecular species, while in the nonpolar solvent decane the polymer is expected, at room temperature, to elute as the larger micellar aggregate. The experimental results as shown in Figure 1confirm that this is actually the case. In both chromatograms solutions of 2 g / L polymer in the mobile phase were injected. The injection solution also contained 0.5 g/L toluene, which was used as the holdup time indicator. In T H F the polymer elutes at a relative retention time T = 0.9855, indicating an effective radius P = 13.8 nm. In decane the polymer elutes earlier, viz. a t r = 0.9496, indicating a larger species with P = 52.8 nm, clearly the micelle. The values of the effective radii reported
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above and in the further part of the paper were calculated by using C = 2.698 (random coil value) for the molecular species, while for the micelle the value C = 4.89 (solid spheres) has been used. Increasing the temperature of the decane experiment should in principle reveal the dissociation of the micelle by a shift in the relative retention toward the higher value for the molecular species, as we found in the T H F experiment. However, a complication arises due to the polarity of the fused silica column surface. This surface contains polar silanol groups, which can interact with eluting polar species. In polar solvents such as T H F these silanol groups are sufficiently deactivated, but they play an important role with nonpolar mobile phases. In decane, as long as the polymer is present as a micelle with the nonpolar polyisoprene chains on the outside, there is no interaction with the column wall. However, as the micelle starts to dissociate, the more polar polystyrene part of the molecule will come into contact with the column wall and interact strongly with the silanol groups. So, upon increasing the temperature in the decane experiments, we find, up to 75 "C, a micelle peak that disappears completely at higher temperatures. Only at temperatures of 140 "C or higher does the polymer elute as the molecular species, but it does so with the typical tailing profile that shows that it has been subject to an adsorptive process. This is in line with results of gel permeation experiments on related polymers (5) in weak mobile phases where also only elution of the micelle was observed, but the free polymer molecules were retained on the column packing material. While it is virtually impossible to completely deactivate the tremendously large surface area of a gel permeation column packing material, we succeeded in deactivating our fused silica column by silylation. We found, however, that after direct silylation (N-(trimethylsily1)imidazolein DMF at 150 "C) the column still showed a small degree of adsorption, probably due to epoxide-type oxygen that is known to be present on the silica surface. To convert these epoxide groups, which are difficult to silylate, to the more reactive silanol group we used as a first step the procedure of Tock et al. (6). For a complete deactivation of the column we used the following treatment: (1)At room temperature we converted the epoxide groups by successively pumping through the following aqueous mobile phases: 1 M potassium hydroxide (2 h), water (10 min), and 0.03 M hydrochloric acid (2 h), followed by water (1 h). (2) For the silylation we changed the mobile phase to DMF, increased the column temperature to 150 "C, and deactivated the column by repeated injection of a 1/1 (v/v) solution of the strong silylation agent N-(trimethylsily1)imidazolein DMF. (3) The column temperature was lowered to 60 "C, the DMF was replaced by THF, and the column temperature was lowered to ambient. Column internal diameter determination after this treatment showed no difference from the 3.70 pm found earlier for the untreated column. (4) The T H F was replaced by the n-decane mobile phase. After this treatment the polymer could be detected a t all temperatures between ambient and 140 "C, whether as the micelle or the molecular species or as both species if present. An example of the transition range is given in Figure 2. In this experiment the mobile phase contained 1 g / L polymer to provide a background micelle concentration. Polymer injections of 2 g/L in this mobile phase showed the micelle being present from ambient temperature up to 80 "C. Between 80 and 95 "C there is a gradual decrease in the concentration of the micelle, with material eluting at the retention time of the molecular species and also with some material eluting in between these peak positions. This effect can be interpreted as being due to polymer that exists partly as
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
TOLUENE
IC-
llooc 105 O C
100 O C
0.900 0.925 0.950 0.975 1.000 1.025 DIMENSIONLESS RESIDENCE TIME, 7 Flgure 2. Temperature range of the micelle dissociation: mobile phase, n-decane contalning 1 g/L polymer; injectlon concentration, 2 g/L polymer and 0.5 g/L toluene in the mobile phase. micelle, partly as molecular species during the time of the chromatographic process. At 105 "C no material is eluting a t the micelle retention time, and a t this point the peak area of the molecular species equals, within the experimental error, that of the micelle peak at lower temperatures (a 4-fold determination at low temperature gives a value for the peak area and its standard deviation of 7.15 f 0.25 mVs; at 105 "C these values are 7.00 f 0.15 mVs). This indicates that the conversion from micelles into the molecular species is complete, assuming the two forms to have the same ultraviolet absorption response. From 110 "C up to 140 "C no change in retention time of the molecular species was found, which at r = 0.9856 yielded the same molecular size as in the good solvent T H F at room temperature. Although the transition interval found above is in agreement with the viscometric experiments, we must bear in mind that so far we have used a lower polymer concentration than that used by Mandema et al. In their experiments the concentration was 7 g/L, while in our experiments, due to the dilution by chromatographic dispersion as the injection pulse is transported through the column, the concentration will, on the average, be lower than 3 g/L. Experiments with a background concentration of 7 g/L were, however, impossible due to the high viscosity of the mobile phase, which leads to a very poor injection quality. We therefore kept the mobile phase concentration at 1 g/L but increased the injection concentration from 2 g/L to 10 g/L (with 5 g/L toluene). As shown in Figure 3, this high concentration gives a badly distorted peak shape from which reliable size information can no longer be obtained. Qualitatively, however, the micelle transition can be studied, as is shown in Figure 4. Again, as in Figure 2, the transition is complete at about 110 "C. Due to the high chromatographic efficiency of the microcapillary column, the top concentration in the polymer elution profile is still as high as 7 g/L. So, even a t this concentration the polymer appears to be incapable of forming a stable micelle above 110 "C. We have shown that microcapillary HDC is a powerful tool for studying the dissociation behavior of organic micelles. Both the micelle and the molecular species can be detected in the same experiment. Compared to viscometric and light-scattering studies, HDC also has the advantage that the exact temperature can be determined at which micelle for-
0.900
0.925 0.950 0.975 1.000 1.025 DIMENSIONLESS RESIDENCE TIME, r Figure 3. Peak distortion at higher polymer concentration at room temperature. In both cases the mobile phase was ndecane containing 1 g/L polymer. Injection concentrations in the mobile phase: (A) 2 g/L polymer and 0.5 g/L toluene; (B) 10 g/L polymer and 5 g/L toluene. (Attenuationof chromatogram B is 5 times higher than that of A , )
TOLUENE
: L 115OC
11ooc
105 O C
L 100 O C
0.900 0.925 0.950 0.975 1.000 1.025 DIMENSIONLESS RESIDENCE TIME. 7 Flgure 4. Temperature range of the micelle dissociation at higher polymer concentrations: mobile phase, n decane containing 1 g/L polymer; injection concentration, 10 g/L polymer and 5 g/L toluene in the mobile phase. mation is no longer possible. Since, in contrast to gel permeation chromatography, HDC has no need of calibration standards, direct information on the micelle size can be obtained. As the precision of the T value determination is high (a 10-fold determination for the micelle gives a standard deviation of 3 X r units), the sole uncertainty in the micellar size we report is in the unknown C factor of eq 2. For the extreme values of C we find sizes of 51.0 (C = 2.698, random coil model) and 52.8 nm (C = 4.89, solid sphere model), so a difference exists of less than 4%. In our experiments we have found no evidence for the existence of very large particles in the transition range as reported by Mandema et al. ( I ) .
Anal. Chem. 1989, 6 1 , 1321-1325
LITERATURE CITED (1) Mendema, W.; Zeldenrust, H.; Emels, C. A. Mekromol. Chem. 1979,
180, 1521-1538. (2) Tijssen, R.; Bleumer, J. P. A.; van Kreveld, M. E. J . Chromafogr. 1983, 260, 297-304. (3) Tijssen, R.; Bos, J.; van Kreveld, M. E. Anal. Chem. 1988, 58, 3036-3044. (4) van Kreveld, M. E.; van den Hoed, N. J . Chromatogr. 1973, 83, 111.
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(5) Price. C.; Hudd, A. L.; Booth, C.; Wright, B. Po/ymer 1982, 23, 650-653. (6) Tock, P. P. H.; Stegeman, G.; Peereboom, R.; Poppe, H.; Kraak, J. C.; Unger, K. K. Chromatograph& 1987, 24, 617-624.
RECEIVED for review November 17, 1988. Accepted March 10, 1989.
Size Exclusion Chromatography of Poly(ethy1ene terephthalate) Using Hexafluoro-2-propanol as the Mobile Phase Sadao Mori
Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514, Japan
A method for obtalnlng a callbratlon curve (CC) In hexafluoro-2-propanol (HFIP) uslng a characterlzed poly(methy1 methacrylate) (PMMA) as a secondary standard was proposed. Polystyrene (PS) standards were used as a prlmary standard and a PS CC In tetrahydrofuran (THF) was converted to a PMMA CC In THF by use of a ConvBcSJon equatbn. An Integral molecular welght dlstributlon curve of cumulative weight percent vs log molecular weight ( M ) for the PMMA secondary standard was constructed by uslng a PMMA chromatogram In THF and a PMMA CC. An Integral dlstrlbutlon curve of cumulatlve welght percent VS retention volume ( V , ) for PMMA In HFIP was obtained from a PMMA chromatogram In HFIP. Finally, a CC of log M of PMMA vs VR In HFIP was obtained by uslng these two Integral curves. PMMA equlvalent number average molecular weights of poly(ethy1ene terephthalate) (PET) samples characterlredby the end-group titration method were determlned and the ratios of two averages were obtalned. The ratlo Is designated as a correctlon factor, I, and an average value was 0.57. Therefore, corrected (true) molecular welght average of PET can be obtained from PMMA equivalent molecular weight by multlplylng f .
INTRODUCTION Solvents most commonly used for size exclusion chromatography (SEC) of synthetic polymers are tetrahydrofuran (THF), chloroform, and toluene, which are good solvents for polystyrene (PS) and other common polymers. The construction of a calibration curve of an SEC column system using these solvents as the mobile phase is easy for handling, because they can dissolve PS standards which are exclusively used for the calibration purpose. Polyacrylonitrile (PAN), for example, does not dissolve in these solvents and dimethylformamide (DMF) had to be used for SEC of PAN (I). However, DMF is a poor solvent for PS and other standard polymers such as poly(ethy1ene oxide) have been used for calibration standards. Poly(ethy1ene terephthalate) (PET) is one of the unmanageable polymers for SEC with respect to solvents and calibration standards. m-Cresol has been used as the mobile phase for P E T at 125 "C (2). However, rn-cresol is viscous and requires a high column temperature, which leads to polymer degradation. Several solvent systems for the SEC of P E T in which degradation was eliminated or kept to a 0003-2700/89/0361-1321$01.50/0
minimum have been reported. A mixture of nitrobenzene and tetrachloroethane has been used for SEC of P E T at room temperature (3) and degradation of P E T was not observed for several months. The mobile phase was 0.5% nitrobenzene in tetrachloroethane. A sample (0.2 g) of P E T was dissolved in 0.8 mL of nitrobenzene by heating at 180 "C and the solution was then diluted with 60 mL of hot tetrachloroethane. A mixture of phenol and tetrachloroethane (3:2, w/w) was also used for the SEC of PET (4). However, difficulties have been encountered in dissolving P E T in these solvents at times. Hexafluoro-2-propanol (HFIP), pentafluorophenol, and their mixtures were used as the mobile phase in an SEC-LALLS system for P E T ( 5 ) . The solvent HFIP has been found to be a superior solvent capable of dissolving P E T at room temperature (6). HFIP has been commercially available and can be obtained in Japan. There are two major drawbacks in the use of HFIP as an SEC solvent. One is the expensive price of HFIP as a SEC solvent. The other is the insolubility of PS standards, which prevents the construction of a PS calibration curve in an SEC HFIP system. To overcome the high cost, semimicro-SEC using a mixture of HFIP and chloroform (1:l)has been proposed (7). However, the flow rate reliability in conjunction with a conventional pumping system for semimicro-HPLC was not sufficient in general for the determination of molecular weights of polymers. For calibration of a SEC system, Provder et al. proposed a general method for obtaining a calibration curve in a SEC trifluoroethanol (TFE) system for polyamide analysis (8). If HFIP alone can be used as a SEC solvent, then the availability of an SEC HFIP system can be expanded to polymers that can be dissolved in HFIP only. In this paper methods to remove these two drawbacks have been proposed. Recycling use of the effluent from the column outlet by redistillation was employed to reduce the cost of the SEC measurement accompanied by the use of HFIP. A method to generate a calibration curve for a column system was similar to Provder's method (8),but the hydrodynamic volume concept and the Mark-Houwink constants were not employed here. PS standards were used as primary calibration standards and poly(methy1 methacrylate) (PMMA) of known molecular weight averages was a secondary standard. A PMMA calibration curve in THF was constructed by using a PS calibration curve and a conversion equation (9). Integral molecular weight distribution curves in both T H F and HFIP for the PMMA secondary standard were obtained from their respective SEC chromatograms and a PMMA calibration 0 1989 American Chemlcal Society
