Anal. Chem. 1999, 71, 4776-4780
Composition and Microstructure Determination of a Latex System by Pyrolysis Gas Chromatography Frank Cheng-Yu Wang
Analytical Sciences Laboratory, Michigan Division, The Dow Chemical Company, Midland, Michigan 48667
The composition and microstructure of polymers in a latex system have been studied by pyrolysis gas chromatography (Py-GC). Traditionally, when latex systems have been analyzed, all polymeric materials have been assumed to be in the emulsion phase, and the aqueous phase contains some additives, surfactants, initiators, and chain-transfer agents. The polymeric materials in the aqueous phase will easily be overlooked because most analytical approaches do not perform any detailed polymer analysis of the aqueous phase. In addition to that, the polymer in the aqueous phase generally contains acid or alcohol functional groups in order to be dissolved/stabilized in the aqueous phase. Since monomers containing acid or alcohol functional groups are not readily detected in most pyrolysis experiments, the Py-GC study of these types of polymers always has some degree of difficulty. However, the composition and microstructure analysis can be accomplished by prepyrolysis derivatization with an appropriate derivatization reagent to convert the acid functional group to its ester. After prepyrolysis derivatization followed by Py-GC, the composition of polymer can be obtained from calculations based on relative monomer intensities. The microstructure can be explored by comparing the pure and hybrid trimer peak pattern with appropriate standards. Water-based synthetic polymers have wide applications in industry. Polymers made by solution polymerization1 and emulsion polymerization1 are the two major classes.2 The applications are focused on the ink, graphic arts, adhesives, as well as architectural and industrial coatings.3 Especially in the architectural and industrial coating markets, the most important advantage of this water-based polymer coating is that there are no volatile organic solvents or minimum volatile organic compounds (VOCs) evolved during the production and application processes.3 In the architectural and industrial coating applications, most of the time, the base coating material is an emulsion polymer. However, in the aqueous phase, water-soluble polymers may be introduced in order to increase the necessary physical properties (such as viscosity and rheology) as well as the flexibility of (1) Stevens, M. P. Polymer Chemistry, 2nd ed.; Oxford University Press: Oxford, 1990; p 198. (2) Wicks, Z. W., Jr.; Jones, F. N.; Pappas, S. P. Organic Coating: Science and Technology, Vol. II: Applications, Properties, and Performance, John Wiley & Sons: New York, 1992; p 208. (3) Hepp, S. Adhes. Sealants 1999, 3, 38-39.
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blending and dispersing of pigments. This water-based coating system will not only have polymer in the emulsion phase but also have polymer in the aqueous phase. Because most pyrolysis approaches for emulsion polymer analysis more readily detect components in the emulsion phase, the polymer in the aqueous phase has usually been overlooked. Nonetheless, the polymer in the aqueous phase plays an important role in product performance. The analysis of polymer composition and microstructure in the aqueous phase as well as in the whole emulsion system has become an important and challenging task for the analyst. Pyrolysis gas chromatography (Py-GC)4 is one of the major techniques used for polymer analysis. Py-GC is a technique that uses thermal energy (pyrolysis) to break down a polymeric chain to monomers, oligomers, and other fragments, followed by the separation of pyrolysates with gas chromatography (GC) and detecting with appropriate detectors. Flame ionization detector (FID) is one of most commonly used detectors for pyrolysate detection, and mass spectrometry and mass-selective detector (MSD) are two of the most often used detectors for pyrolysate detection and identification after GC. The intensities of monomers or monomer-related fragments are commonly used to obtain composition data.5 The oligomers or oligomer-related fragments are used to elute microstructure information.6 The solution and emulsion polymers can be analyzed by PyGC. Most water-based synthetic polymers have monomer units that contain a carboxylic acid. A weak point of Py-GC analysis of these acid-containing polymers is the low efficiency of acid functional group preservation under pyrolysis. This is due to the conversion of carboxylic acids under pyrolysis conditions to carbon dioxide and aliphatic hydrocarbons.7 These aliphatic hydrocarbon fragments are hard to correlate back to their original acids when many other fragments are present in the pyrolysates. There are techniques available to improve the detection of carboxylic acids in polymer pyrolysis. The first one is a pyrolysis followed by a solvent-trapping technique.8 This technique is aimed at detecting low-level carboxylic acids in the polymer system. The other technique is a prepyrolysis derivatization technique.9 This (4) Wampler, T. P. Analytical Pyrolysis-An Overview. In Analytical Pyrolysis Handbook; Wampler, T. P., Ed.; Marcel Dekker: New York, 1995; pp 1-3. (5) Wang, F. C.-Y.; Smith, P. B. Anal. Chem. 1996, 68, 3033-3037. (6) Wang, F. C.-Y., Gerhart, B. B.; Smith, P. B. Anal. Chem. 1995, 67, 35363540. (7) Wang, F. C.-Y.; Green, J. G.; Gerhart, B. B. Anal. Chem. 1996, 68, 24772481. (8) Wang, F. C.-Y.; Gerhart, B. B.; Smith, C. G. Anal. Chem. 1995, 67, 36813686. (9) Wang, F. C.-Y. Anal. Chem. 1998, 70, 3642-3648. 10.1021/ac990464j CCC: $18.00
© 1999 American Chemical Society Published on Web 08/31/1999
technique can be considered as a general technique to explore the carboxylic acid in the polymer. Derivatization is a well-known technique in chromatographic analysis. It has been broadly used to enhance chromatographic separation and/or detection for those compounds not suitable for separation/detection originally. The same concept has been adapted to Py-GC analysis. For example, the pyrolysates can be derivatized “simultaneously”, “in situ”, or “on column” to reduce the difficulties of polar pyrolysates being separated in a nonpolar capillary column.10 In another example, unsaturated aliphatic alkenes can be derivatized by hydrogenation to simplify the number of fragments and increase the possibility of structural exploration.11 In general, these techniques modify the fragments produced by the pyrolysis process and can be viewed as “postpyrolysis” derivatization. Postpyrolysis derivatization implies that, during the course of pyrolysis, there is no attempt to altering the thermal degradation pathway through derivatization. The derivatization technique used in the pyrolysis experiment does not have to be postpyrolysis derivatization. The functional group in the polymer can be derivatized to the desired form (produce a stable monomer-related fragment after pyrolysis) before pyrolysis.12 Compared with other techniques described, this technique can be categorized as a prepyrolysis technique. In the prepyrolysis derivatization, the polymer backbone should be stable enough to resist the attack from the derivatization reagent. Pyrolysis is merely the mechanism to decompose the polymer to fragments. In both prepyrolysis and postpyrolysis, tetraalkylammonium hydroxide has been an important reagent widely used for the derivatization of acid and alcohol functional groups.9,13 In this study, the composition and microstructure of polymers in a latex system has been studied by Py-GC. The composition and microstructure of a polymer in the emulsion phase has been identified by direct pyrolysis analysis of the latex system followed by comparing the trimer peak pattern with appropriate microstructure standards. The polymer in the aqueous phase has been prepyrolysis derivatized with tetrabutylammonium hydroxide to convert the acid to its butyl ester. After that, similar procedures are followed to explore the composition and microstructure of the polymer in the aqueous phase. EXPERIMENTAL SECTION Reagent Sources. The SCX-2660 latex was obtained from S. C. Johnson & Son Inc. (Racine, WI). The poly(R-methylstyrene), styrene, and R-methylstyrene random copolymer as well as the tetrabutylammonium hydroxide solution was purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). All polymers and reagents were used without any further purification. All styrene butyl acrylate copolymers and styrene, R-methylstyrene, and butyl acrylate terpolymer were synthesized and blended in the laboratory with a textbook method.14 Methanol Extract of Aqueous Portion. Approximately 5.0 g of the SCX-2660 latex was weighed into a 50-mL centrifuge tube with 25 mL of 1% KBr/MeOH solution. Sample was placed on a (10) (11) (12) (13)
Challinor, J. M. J. Anal. Appl. Pyrolysis 1989, 16, 323-333. Ohtani, H.; Tsuge, S.; Usami, T. Macromolecules 1984, 17, 2557-2561. Wang, F. C.-Y. Anal. Chem. 1999, 71, 1131-1137. Wang, L.; Ishida, Y.; Ohtani, H.; Tsuge, S.; Nakayama, T. Anal. Chem. 1999, 71, 1316-1322. (14) Collins, E. A.; Bares, J.; Billmeyer, F. W., Jr. Experiments in Polymer Sciences; John Wiley & Sons: New York, 1973, pp 337-345.
shaker for 1 h at room temperature and then centrifuged (model Marathon 21K, Fisher Scientific, Pittsburgh, PA) at 13 000 rpm for 20 min. The latex/methanol mixture produced a slightly cloudy methanol layer. Approximately 20-25 mL of the methanol was removed with a pipet to another 50-mL centrifuge tube and centrifuged at 13 000 rpm for 20 min. The liquid was still slightly cloudy and was transferred to a 50-mL vial and dried under a nitrogen stream at room temperature for 4 h. Derivatization Reaction. The derivatized polymer was prepared by adding 0.1 g of a methanol-extracted latex portion with 2.0 g of tetrabutylammonium hydroxide solution (40% in water). The mixed solution was heated to 90 °C for approximately 2 h to ensure the completion of the derivatization reaction. The resulting solution was frozen first and then put into a freeze-drier (model 75034, Labconco Corp., Kansas City, MO) to remove the water. The final solid was used as derivatized polymers for this study. Py-GC Conditions. Samples of polymer were carefully deposited into a quartz tube. The quartz tube was put into an offline pyrolysis interface for 5 min at 300 °C to evaporate any nonpolymeric material (water, unreacted reagents). After this cleaning procedure, the quartz tube was equilibrated for 5 min in a 300 °C interface connected to the injection port of a HewlettPackard (HP) model 6890 gas chromatograph equipped with a FID. The samples were pyrolyzed (CDS 2000 Pyroprobe, Pt coil) at a calibrated temperature of 700 °C. The coil was heated to the calibrated temperature at 20 °C/ms and held at the set temperature for a 20-s interval. The pyrolysis products were split in the 300 °C injection port, with 250:1 split ratio, a fast-flow program (15 psi/0.2 min, 75 psi/min, to 90 psi/8.8 min), and separated on a fused-silica capillary column (J & W Scientific DB-5, 10 m × 0.10 mm i.d., 0.4-µm film) using a fast temperature ramping program (50 °C/0.2 min, 100 °C/min, to 100 °C/0 min; 80 °C/ min, to 140 °C/0 min; 60 °C/min, to 200 °C/0 min; 50 °C/min, to 280 °C/0 min; 40 °C/min, to 320 °C/5.2 min) and then detected by a FID. Py-GC/MS Conditions. The sample preparation and pyrolysis in the Py-GC/MS experiments were the same as with the Py-GC experiments. The GC used is a HP model 5890 gas chromatograph. The pyrolysis products were split in the 300 °C injection port, with 10 psi head pressure, 30:1 split ratio, separated on a fused-silica capillary column (J & W Scientific, DB-5, 30 m × 0.25 mm i.d., 1.0-µm film) using a linear temperature program (40 °C/4 min, 10 °C/min, to 320 °C/18 min), and detected by a HP 5791 MSD. The GC output region to the MSD was kept at 300 °C. An electron ionization mass spectrum was obtained every second over the mass range of 15-650 Da. The results of Py-GC/MS are used mainly for component identification. Safety Considerations. The derivatization reagents are strong bases and are corrosive. Personal protective devices should be used when the derivatization reaction is performed. The experiment should be conducted inside a vent hood. The polymers are considered nonhazardous, but skin contact or inhalation of vapor/ powder should be avoided. RESULTS AND DISCUSSION Figure 1 shows the pyrogram of the SCX-2660 latex system. All the major pyrolysates have been identified and listed in Table 1. On the basis of the major components identified, the latex may be a terpolymer of styrene, methyl methacrylate, and butyl Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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Figure 1. Pyrogram of the SCX-2660 latex system. Table 1. Peak Assignment in the Pyrograms of the SCX-2660 Latex System (Figure 1) as Well as in All Other Pyrograms peak no.
MW
structure
1 2 3 4 5 6 7 8 9 10 11
56 74 100 92 128 104 118 384 360 336 312
butene butanol methyl methacrylate toluene butyl acrylate styrene R-methylstyrene butyl acrylate trimer one-styrene and two-butyl acrylate trimer two-styrene and one-butyl acrylate trimer styrene trimer
acrylate. Further examination of the trimer peaks pattern indicated this is a styrene and butyl acrylate copolymer. The methyl methacrylate may come from the secondary reaction during pyrolysis of the latex system. The accurate composition can be calculated by monomer intensities,5 and the composition as well as microstructure may be elucidated from trimer intensities.8 For the microstructure exploration, instead of working on the numberaverage sequence length with all trimer peak intensities, the microstructure can be quickly appraised by examining the trimer peak pattern and their relative intensities. Figure 2 shows the pyrograms of styrene and butyl acrylate polymer systems of different microstructures. The top one is a random copolymer; the middle one is a core-shell copolymer; and the bottom one is a blend polymer system. When a copolymer has monomers randomly distributed in the copolymer chain, the probability of having mixed trimer fragments is larger than the probability of having pure trimers. The pure trimer peak intensities will be much less than mixed trimers in the random copolymer. In the other case, such as in a core-shell or block copolymer, because the domain of each monomer is separated, the interface area is limited; the probability of having pure trimer fragments is larger than the probability of having mixed trimers. The pure trimer peak intensities will be much higher than mixed trimer peak intensities. In the case of blend polymers, in theory, there is no interface area of different monomers; there will be no peak for the mixed trimers in the pyrogram. The insets of Figure 2 have been clearly reflected the relationship between microstructure and relative trimer peak intensities. On the basis of the trimer area of SCX-2660 latex system (as the inset in the Figure 1), the pyrogram shows high intensities 4778 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
Figure 2. Pyrograms of different microstructures of styrene and butyl acrylate polymer systems.
of pure butyl acrylate and styrene trimers and minimum intensities of all other mixed trimers. The trimer peak pattern indicated that the copolymer has a nonrandom monomer distribution in the copolymer chain. In other words, styrene and butyl acrylate monomer units are arranged in a separated domain in the copolymer. The copolymer is a block copolymer. In the emulsion polymer system, the most probable configuration of separated domain will be as a core-shell-type copolymer. The R-methylstyrene peak intensity is unusually high in the SCX-2660 latex system. R-Methylstyrene is a typical pyrolysis fragment for polymers containing styrene.15 The mechanism that produces R-methylstyrene has been well studied.16 When a set of experimental conditions and a fixed sample amount are used in the pyrolysis, the amount of R-methylstyrene produced should be able to be predicted. This means the peak intensity ratios of styrene and R-methylstyrene should be fixed. Figure 3 shows a comparison of R-methylstyrene (labeled as peak 7A) produced (15) Bouster, C.; Vermande, P.; Veron, J. J. Anal. Appl. Pyrolysis 1980, 1, 297314. (16) Sousa Pessoa de Amorim, M. T.; Bouster, C.; Vermande, P.; Veron, J. J. Anal. Appl. Pyrolysis 1981, 3, 19-34.
Figure 3. Comparison of R-methylstyrene (labeled as peak 7A) produced from a 50/50 wt % styrene and butyl acrylate core-shell latex with R-methylstyrene (labeled as peak 7B) produced from an SCX-2660 latex system studied.
Figure 4. Pyrogram of the methanol extracts of the SCX-2660 latex system. Because the methanol extraction cannot completely remove the emulsion phase, there are low-intensity peaks in the trimer area contributed by the polymer in the emulsion phase. Figure 6. Pyrograms of styrene, R-methylstyrene, and butyl acrylate terpolymer with different microstructures.
Figure 5. Pyrogram of the methanol extract portion of SCX-2660 and derivatized with tetrabutylammonium hydroxide.
from a 50/50 wt % styrene and butyl acrylate core-shell latex with R-methylstyrene (labeled as peak 7B) produced from an SCX2660 latex system. It proved that there is extra R-methylstyrene monomer in the SCX-2660 latex system in which the source of R-methylstyrene monomer needed to be determined. Because of the extra amount of R-methylstyrene found in the pyrogram, there is a good possibility that there is more than one polymer in this SCX-2660 latex system. The pyrogram may reflect a combination of pyrolysates from two polymers. On the basis of the previous microstructure study, this latex system is a core-
shell type of styrene and butyl acrylate copolymer. It is unlikely that another polymer can be inserted into this core-shell system. Therefore, the only other place this extra polymer may reside is the aqueous phase. Figure 4 shows the pyrogram of the methanol extracts of SCX2660 latex system. The major peaks in the pyrogram are styrene and R-methylstyrene. It is easy to draw the tentative conclusion that the second polymer is a copolymer of styrene and R-methylstyrene. Because the methanol extraction cannot completely remove the emulsion phase, there are low-intensity peaks in the trimer area contributed by the polymer in the emulsion phase. The microstructure of this copolymer is unknown because there is no clear peak pattern in the trimer area. In a further consideration, both styrene and R-methylstyrene are nonpolar monomers. There are no literature reports that a copolymer of two nonpolar monomers could be dissolved into water. To have a polymer containing styrene and R-methylstyrene dissolve into water, there must be a third monomer, which is polar or highly water-soluble. In the styrene and acrylate latex system, the most commonly used polar monomer is acrylic acid. A way to prove there is an acid functional group containing monomer in the second polymer is by derivatization. The Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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methanol extract portion of SCX-2660 was derivatized with tetrabutylammonium hydroxide. The pyrogram of the derivatized polymer is shown in the Figure 5. On the basis of the monomer peaks, this is a styrene, R-methylstyrene, and butyl acrylate terpolymer after derivatization. In other words, before being derivatized, this polymer is a styrene, R-methylstyrene, and acrylic acid terpolymer. The trimer area of Figure 5 reflects the microstructure of this terpolymer. As the inset shows, the trimer peak pattern is similar to the styrene and butyl acrylate random copolymer. The difference between these two trimer areas is a broadened peak, or maybe two peaks overlapped (labeled as A) and an extra peak (labeled as B). It is also necessary to note that because of the low ceiling temperature of R-methyl styrene, the pyrolysis of an R-methylstyrene-containing polymer will favor the production of an R-methylstyrene monomer unit.17 This results in a high intensity of R-methylstyrene monomer peak and low peak intensities in the R-methylstyrene-containing oligomers. It will be hard to determine the microstructure of R-methylstyrene-containing copolymer or terpolymer through direct peak intensity observation in the trimer area. Figure 6 shows a set of pyrograms of styrene, R-methylstyrene, and butyl acrylate terpolymer with different microstructures. The top one is a styrene acrylate copolymer blended with a poly(Rmethylstyrene). The trimer area is different from Figure 5 with two trimer peaks (labeled with *) produced from poly(R-methylstyrene) as well as peaks A and B. The bottom one is a styrene and R-methylstyrene copolymer blended with butyl acrylate. The lack of all mixed styrene and butyl acrylate trimers as well as peaks A and B disqualified the possibility of this being the microstructure. The middle one is a random terpolymer of styrene,
R-methyl styrene, and butyl acrylate. The complete match of peaks in the trimer area in addition to a more than 95% match in ion/ mass intensities for mass spectra of peaks A and B between those two polymers leads to the conclusion that the microstructure of the second polymer of SCX-2660 is a random terpolymer.
(17) Tsuge, S.; Ohtani, H. Pyrolysis Gas Chromatography of High Polymers Fundamentals and Data Compilation; Techno-System: Tokyo, 1989, pp 126-127.
Received for review May 3, 1999. Accepted July 22, 1999.
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CONCLUSIONS The composition and microstructure of polymers in a latex system, which is used in inks, graphic arts, and coatings, has been studied by Py-GC. Traditionally, when a latex system is analyzed, all polymeric materials are assumed to exist in the emulsion phase; the aqueous phase will only contain some additives, surfactants, initiators, and chain-transfer agents. If there is any polymeric material in the aqueous phase, it will be easily overlooked because conventional analytical approaches do not perform any detailed polymer analysis in the aqueous phase. The best way to prevent this mistake is not to ignore any unusual pyrolysate in the pyrogram. In addition to that, the polymer in the aqueous phase will have acid or alcohol functional groups. If a polymer contains only nonpolar monomers, it will not dissolve in water. Because the acid or alcohol functional group containing monomers do not show their identity under pyrolysis, the Py-GC study of this type of polymer always has some degree of difficulty. However, the composition and microstructure analysis still can be accomplished by prepyrolysis derivatization with an appropriate derivatization reagent to convert the acid functional group to its ester. After prepyrolysis derivatization followed by Py-GC, the composition of polymers can be obtained by calculations based on relative monomer intensities. The microstructure can be explored by comparison of the pure and hybrid trimer peaks with appropriate standards.
AC990464J
