Anal. Chem. 2000, 72, 2490-2498
Characterization of Polyether and Polyester Polyurethane Soft Blocks Using MALDI Mass Spectrometry John T. Mehl,† Renata Murgasova,‡ Xia Dong,§ and David M. Hercules*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 Hartmut Nefzger
Bayer Corporation, New Martinsville, West Virgina 26155
Selective degradation reactions combined with MALDI analysis have been applied for molecular weight (MW) determination of polyether and polyester polyurethane (PUR) soft blocks. Selective degradation allows recovery of the polyols, and direct observation of the soft block oligomer distribution is possible for the first time by using MALDI. Ethanolamine is applied for polyether PUR degradation. MALDI analysis indicates that the recovered polytetrahydrofuran (pTHF) MW distribution is nearly identical to the unreacted pTHF material. Reduction in the ethanolamine reaction time allows observation of oligomer ions containing the diisocyanate linkage, which provide identification of the diisocyanate. Ethanolamine is not used for polyester PUR’s degradation because the ester bonds will be cleaved. Therefore, phenylisocyanate is applied for polyester PUR degradation. Polybutylene adipate (pBA) oligomers were directly observed in the MALDI spectra of the degraded pBA-PUR samples. Comparison of the degraded pBA-PUR oligomer distribution with the unreacted pBA material indicates that lowmass oligomers are less abundant in the degraded pBAPURs. Oligomer ions containing the diisocyanate linkage are also observed in the spectrum, providing a means for identifying the diisocyanate used for PUR syntheses. Sizeexclusion chromatography (SEC) was combined with MALDI to provide accurate MW determination. Narrow MW fractions of the degraded and unreacted polyols were collected and analyzed by MALDI. This method allows precise calibration of the SEC chromatogram. The SECMALDI results provide significantly larger Mw and PD values than MALDI alone. Using SEC-MALDI, it was determined that the PD indexes of the pTHF and pBA samples are larger than the assumed values, which are based on the polyol synthesis reactions. The combination of selective degradation with SEC-MALDI, using either ethanolamine or phenylisocyanate, is a viable method for polyurethane polyol characterization. Polyurethane (PUR) elastomers are multiblock copolymers with an alternating sequence of hard- and soft-block locks. The soft blocks are usually polyether or polyester polyols of varying 2490 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000
molecular weight. Urethane linkages are formed through reaction of diisocyanate with the polyol and with a short chain diol (chain extender). The chain extender and diisocyanate form the hard segments. Because of thermodynamic incompatibility of hard and soft segments, microphase separation occurs upon cooling. The polyol soft segments form an amorphous matrix in which the hard crystalline segments become embedded. Hard segments act as thermohard blockreversible cross-links and fillers. The soft blocks have easy segmental rotations at the temperature of use (in many cases ambient temperature), thus imparting desired elastomeric properties. Both the chemical composition and molecular weight distribution (MWD) of the incorporated soft block influence the macroscopic properties of the resulting PUR. This is also true for the hard blocks. Thus, variation of the PUR formulation covers a wide range of properties and applications (tailor-made elastomers). A variety of conventional instrumental techniques such as nuclear magnetic resonance spectroscopy (NMR),1,2 infrared spectroscopy (IR),3 and size exclusion chromatography (SEC)4 have been applied to characterize PURs. Information about PUR terminal groups, diol or diamine extenders, polyol soft blocks, and diisocyanates can be obtained. IR and NMR techniques based on end-group analysis can provide accurate determination of number average molecular weight (Mn). However, end-groupbased methods do not provide weight average molecular weight (Mw); hence, valuable information about polydispersity (PD) is not obtained. SEC provides both Mn and Mw; however, because of a lack of narrow molecular weight elution standards for polyethers and polyesters, large errors in Mn and Mw result. What is required for PUR characterization is a method which provides reliable determination of both Mn and Mw for polyol soft blocks. † Present address: Division of Bioengineering and Environmental Health, Massachusetts Institute of Technology, 77 Massachusetts Ave., Room 56-738, Cambridge, MA 02139. ‡ Permanent address: Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, 842 36 Bratislava, SK. § Present address: Evans East, 104 East Windsor Center, Suite 101, East Windsor, NJ 08520. (1) Yeager, F. W.; Becker, J. W. Anal. Chem. 1977, 49, 722-724. (2) Cambell, D.; White J. R. Polymer Characterization: Physical Techniques; Chapman and Hall: London, 1989. (3) Corish, P. J. Anal. Chem. 1959, 31, 1298-1306. (4) Haken, J. K.; Burford, R. P.; Vimalsiri, P. A. D. T. J. Chromatogr. 1985, 349, 347.
10.1021/ac991283k CCC: $19.00
© 2000 American Chemical Society Published on Web 05/04/2000
Mass spectrometry can be used for polymer characterization. Time-of-flight secondary ion mass spectrometry (TOF-SIMS),5-9 field-desorption (FD-MS),10 and fast-atom bombardment (FABMS)11,12 have all been applied for polymer analysis. Several investigations of PURs using TOF-SIMS 13,14 have been reported, including average molecular weight (MW) estimation of PURs.15 Though these techniques are highly informative in terms of identification of PUR components, low desorption/ionization of higher masses and fragmentation limit the utility of these methods for average MW determination. New ionization techniques such as matrix-assisted laser desorption/ionization (MALDI)16 and electrospray (ESI) allow larger MW’s to be determined. Poly(ethylene glycol) (PEG) oligomer ions in excess of 5 000 000 MW have been measured by ESI.17 However, problems with multicharged oligomer ions have limited most ESI polymer analysis to molecular weights below 2000 Da.18 MALDI, in contrast, generally produces singly charged oligomer ions, and MW’s up to 1 500 000 for polystyrene (PS) have been measured.19 MALDI has been applied for MWD determination of a wide range of polymer types including PEGs,16 poly(methyl methacrylate)s (PMMA),20 poly(dimethylsiloxane)s (PDMS),21,22 polybutadienes (PBD), 23 and poly(acrylic acid)s (PAA).24 However, MW determination is generally reliable only for narrow MWDs (PD < 1.2).25 MALDI MW determination of high PD polymers typically results in lower values compared with SEC. Reasons for this discrepancy originate from mass bias during the MALDI desorption/ionization process and sample preparation26 and instrumental effects.27,28 Small oligomers typically desorb/ ionize more easily than large oligomers.18 Instrumental bias can occur due to detector saturation and velocity-dependent detector (5) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Hagenhoff, B.; Niehuis, E.; Benninghoven, A. Anal. Chem. 1991, 63, 1953-1960. (6) Hercules, D. M. J. Mol. Struct. 1993, 292, 49-64. (7) Dong, X.; Procter, A.; Hercules, D. M. Macromolecules 1997, 30, 63-70. (8) Hittle, L. R.; Hercules, D. M. Surf. Interface Anal. 1994, 31, 217-225. (9) Xu, K.; Proctor, A.; Hercules, D. M. Mikrochim. Acta 1996, 122, 1-15. (10) Lattimer, R. P.; Schulten, H. Anal. Chem. 1989, 61, 1201A-1214A. (11) Montaudo, G.; Montaudo, M.; Scamporrina, G.; Vitalini, D. Macromolecules 1992, 25(5), 5099-5107. (12) Ballistreri, A.; Garozza, D.; Giuffrida, M,; Montaudo, G. Anal. Chem. 1987, 59, 2024-2027. (13) Bletsos, I. V.; Hercules, D. M.; van Layen, D.; Benninghoven, A.; Karakatsanis, C. G.; Rieck, J. N. Anal. Chem. 1989, 61, 2142-2149. (14) Bletsos, I. V.; Hercules, D. M.; van Layen, D.; Benninghoven, A.; Karakatsanis, C. G.; Rieck, J. N. Macromolecules 1990, 23, 4157-4163. (15) Cohen, L. R.; Hercules, D. M.; Karakatsanis, C. G.; Rieck, J. N. Macromolecules 1995, 28, 5601-5608. (16) Bahr, U.; Deppe, A.; Karas, M.; Hillencamp, F.; Giessman, U. Anal. Chem. 1992, 64, 2866. (17) Nohmi, T.; Fenn, J. B. J. Am. Chem. Soc. 1992, 114, 3241-3246. (18) Hunt, S. M.; Derrick, P. J.; Sheil, M. M. Eur. Mass Spectrom. 1998, 4, 1-12. (19) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 2721. (20) Larsen, B. S.; Simonsick, W. J.; McEwen, C. N. J. Am. Soc. Mass Spectrom. 1996, 7, 287-292. (21) Belu, A. M.; DeSimone, J. M.; Linton, R. W.; Lange, G. W.; Friedman, R. M. J. Am. Soc. Mass Spectrom. 1996 7, 11-24. (22) Williams, J. B.; Gusev, A. I.; Hercules, D. M. Macromolecules 1996, 29, 8144-8150. (23) Yalcin, T.; Schriemer, D. C.; Li, L. J. Am. Soc. Mass Spectrom. 1997, 8, 1220-1229. (24) Danis, P. O.; Karr, D. E.; Mayer, F.; Holle, A.; Watson, C. H. Org. Mass Spectrom. 1992, 27, 843-846. (25) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Rapid Commun. Mass Spectrom. 1995, 9, 453-460. (26) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4169-4175. (27) Jackson, C.; Larsen, B.; McEwen, C. Anal. Chem. 1996, 68, 1303-1308. (28) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4176-4183.
response.27,28 To overcome these limitations, several researchers have used SEC to separate the polymer into narrower MW fractions. MALDI analysis of the fractions provides Mn, which, in turn, is used to calibrate the SEC trace without the need for external elution standards. Montaudo25 and others 29 have shown that this method can provide reliable molecular weight determination for wide polydispersity polymers. MALDI has been applied for the analysis of polyethers and polyesters30-32 and more recently in combination with SEC.18,30 The purpose of the present research is to extend this work for the analysis of PUR soft blocks. Through the use of selective chemical degradation reactions, polyether and polyester soft blocks can be liberated from the PUR. Upon liberation, the soft blocks are amenable to characterization by MALDI and SECMALDI. Ethanolamine and phenylisocyanate are selective chemical reagents which are used to analyze polyether and polyester PURs. In addition to identification and MW determination of the constituent polyol, the method described here also identifies the diisocyanate linkage. EXPERIMENTAL Synthesis of PURs. All polymer materials were supplied by Bayer Corp. (New Martinsville, WV). These included a series of model PURs based on 4,4′-diphenylmethane-diisocyanate (MDI) and either polytetrahydrofuran (pTHF) or polybutylene adipate of various MWs. Premixed polyol and chain extender (butanediol) were heated to 60 °C and reacted with an equimolar amount of diisocyanate (60 °C) using a high speed stirrer. After approximately 1 min of intense mixing, the reacting mixture was cast onto a preheated tray and annealed for 18 h at 110 ×bcC. Finally the PUR was allowed to cool to room temperature. End-Group Titration. Number average molecular weights (Mn) of the polyols were determined using the phthalic anhydride method with pyridine as solvent.34 After being hydrolyzed, the remaining excess anhydride was titrated using normalized KOH solution. The effective anhydride concentration was determined, as well, and taken into account. The resulting OH numbers are converted into number average molecular weight using
Mn )
(56.1 g/mol)(1000 mg/g)(2 mol KOH/mol polyol) (OH# mg KOH/g polyol)
where 56.1 g/mol is the formula weight of KOH, and the conversion factor of 2 is used for bifunctional polyol samples. The acid number of the polyesters was lower than 0.6 mg KOH/g and was not taken into account for the above determination. MALDI-MS Analysis. All MALDI-MS spectra were acquired using a Voyager-DE STR, MALDI-TOF mass spectrometer from (29) Nielen, W. F.; Malucha, S. Rapid Commun. Mass Spectrom. 1997, 11, 11941204. (30) Blais, J. C.; Tessier, M.; Bolback, G.; Remaud, B.; Rozes, L.; Guittard, J.; Brunot, A.; Mare´chal, E.; Tabet, J. C. Int. J. Mass Spectrom. Ion Processes 1995, 144, 131-138. (31) Guittard, J.; Tessier, M.; Blais, J. C.; Bolbach, G.; Rozes, L.; Mare´chal, E.; Tabet, J. C. J. Mass Spectrom. 1996, 31, 1409-1421. (32) Williams, J. B.; Gusev, A. I.; Hercules, D. M. Macromolecules 1997, 378113787. (33) Montaudo, G.; Garozzo, D.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Macromolecules, 1995, 28, 7983-7989. (34) Sorensen, W. R.; Cambell, T. M. Preparative Methods of Polymer Chemistry, 2nd ed.; Interscience: New York, 1968; p 134.
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Perkin-Elmer Biosystems, (Forest City, CA). The instrument is equipped with a N2 laser emitting at 337 nm. Spectra were acquired in the positive-ion mode using the reflectron. The acceleration voltage ranged from 20 to 25 kV. Typically, 48-128 single-shot mass spectra were summed to give a composite spectrum. Polymer samples were dissolved in CHCl3 at 10 mg/ mL. The dithranol (Aldrich Chemical Co., Milwaukee, WI) matrix solution was prepared by dissolving 30 mg in 1 mL of CHCl3 (HPLC grade, Fisher Scientific, Pittsburgh, PA); matrix and polymer solutions were mixed in a 10:1 ratio. The MALDI target was pre-spotted with 1-2 µL of NaI solution (1 mg/mL in methanol) and allowed to air-dry. One to two microliters of matrix/polymer solution was deposited onto the sample target and air-dried. Size Exclusion Chromatography (SEC). SEC was performed using a preparative PLgel, mixed-D, linear PS/DVB from Polymer Laboratories, (Amherst, MA) with a refractive index detector from Knauer (Berlin, Germany). One hundered microliters of a 10% (wt/vol) polymer solution in THF (HPLC grade, Fisher Scientific, Pittsburgh, PA) was injected. THF was used as the eluent at a flow rate of 2 mL/min. For SEC fractionation, 20-30 fractions (0.8-1.0 mL) were collected manually in test tubes and evaporated to dryness overnight. Fifty to five hundred microliters of dithranol solution (30 mg/mL in CHCl3) was added prior to MALDI-MS analysis. MALDI was used to obtain a spectrum for each fraction. Using the Voyager software package, the Mn value was determined for each spectrum. The software uses the peak height in the Mn calculation. An SEC calibration curve was generated by plotting log Mn for each fraction vs elution volume. The SEC peak was integrated using the SEC software (PL LogiCal version 6.01, Polymer Laboratories, Amherst, MA), and the elution volumes for Mn and Mw were determined. Finally, the Mn and Mw elution volumes were converted to Mn and Mw using the SEC-MALDI calibration curve. Chemical Degradation. pTHF-PUR. Two and two tenths milliliters of ethanolamine (Aldrich Chemical Co., Milwaukee, WI) was added to 2.25 g of PUR in a three-neck flask. The reaction proceeded under N2 (g) atmosphere at 150 °C for 3.5 h. After the reaction was complete, ethanolamine was removed under vacuum. Purification was carried out by washing the residue three times with ether. From each wash a transparent ether phase was separated from an insoluble yellow phase by filtration. The product was recovered from the ether by evaporation under vacuum. pBA-PUR. Four and four-tenths of phenylisocyanate (Aldrich Chemical Co., Milwaukee, WI) was added to 4.0 g of PUR in a three-neck flask. The reaction proceeded under N2 (g) atmosphere at 150-185 °C for 3.5-12 h. After the reaction mixture was cooled to room temperature, 6.8 mg of dibutylamine (Aldrich Chemical Co., Milwaukee, WI) in 20 mL of THF were added dropwise, not allowing the temperature to exceed 25 °C. The solution was poured into a dish and allowed to evaporate overnight. Purification was carried out by washing the solid raw product with ether. The ether was removed from the desired solid product by filtration, and the product was air-dried. Ethanolamine, dibutylamine, and phenylisocyanate are toxic corrosives. Avoid inhalation of vapors and direct skin contact. 2492 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000
Figure 1. Spectrum of pTHF 1000 (a), and spectrum of pBA 3560 (b). Open circles indicate cyclic oligomers, c1 indicates a monocarboxyl-terminated ester, and c2 indicates a dicarboxyl-terminated ester.
Chloroform is a suspected carcinogen. Tetrahydrofuran can form explosive peroxides. Work with these substances was conducted in a chemical ventilation hood wearing chemical-resistant gloves and safety goggles. RESULTS AND DISCUSSION MALDI Analysis. Successful MALDI analysis is highly dependent upon matrix selection and sample preparation. Several matrixes have been reported for use with polyethers and polyesters: IAA, HABA, DHB, 5-CSA, HCCA, 30,31 and dithranol.18 Each of these matrixes was tested with the present polymer samples, and DHB and dithranol were found to provide the highest signal intensity. Dithranol was more reproducible than DHB; therefore, this matrix was used for the analysis of the samples examined in this study. Figure 1 shows positive-ion MALDI spectra of polytetrahydrofuran (pTHF) (Mn ) 1000) and polybutylene adipate (pBA)(Mn ) 2000), two polyols which are commonly used to make PUR elastomers. For both polymers, intense [M + Na]+ oligomer ion peaks are observed. The samples were prepared by first prespotting the target with NaI solution. Without additional sodium salt, only very weak signal intensity for oligomer ions was observed. It has been found by others that addition of extra sodium salt can result in enhanced signal intensity of Na+-adduct ions.16,35 It should be noted, however, that other groups have indicated that addition of salt is not always necessary, depending upon the particular matrix and polymer used.30,33 In the spectrum of pTHF (Mn ) 1000), linear oligomer peaks range from n ) 3 (m/z ) 257) up to typically n ) 59 (m/z ) (35) Pasch, H.; Gores, F. Polymer 1995, 36, 1999.
4292). The spacing between the main peak series is 72 Da, characteristic of the THF repeat unit. The wide mass range of the ion peaks is typical for broad dispersity polymers such as polyethers and polyesters. Oligomers below n ) 3 are not observed. Absence of oligomer ions below n ) 3 has been reported by others18,20,30 and is thought to be due to a decrease in complex formation with sodium for small oligomers (n < 4).36,37 In addition to the linear oligomer ion peak series, there is a series of weak peaks located 18 Da below the linear oligomer ion peaks(0). These peaks correspond to cyclic oligomer ions. ESI-TOF analysis also revealed the presence of cyclic oligomer ions.44 Electrospray is a gentle ionization process, imparting very little internal energy to the ionized molecules. Therefore, the presence of cyclic oligomer ions in both MALDI and ESI-TOF spectra suggests that the cyclic oligomers originate from the sample. In the spectrum of pBA (Mn ) 2000), the main peak series corresponds to linear dihydroxy-terminated oligomer ions ranging from n ) 3 (m/z ) 713) up to typically n ) 39 (m/z ) 7922). The spacing between the main peak series is 200 Da, characteristic of the polybutylene adipate monomer. Also present in the spectrum, at low signal intensity, are oligomers with different terminal groups. Located at 28 and 72 Da below the dihydroxyl-terminated oligomer ion peaks are the dicarboxyl- (c2) and monocarboxyl(c1) terminated oligomers, respectively. Additionally, cyclic oligomer ions (0) are present at 90 Da below the linear dihydroxyl terminated oligomer ions. Williams et al.32 also reported the presence of dicarboxyl- and monocarboxyl-terminated and cyclic oligomer ions in MALDI and ESI spectra of polyesters. Selective Degradation. Degradation reactions are useful for the analysis of insoluble or complex polymer systems. Various methods have been used. Partial transesterfication using triflouroacetic acid (TFA) can cleave ester and urethane bonds and has been used in combination with TOF-SIMS for the analysis of polyester-PURs.15 In addition, TOF-SIMS analysis of TFA esterfied OH terminal groups was used to determine the branching number.38 Hydrolysis with sodium hydroxide enables the analysis of PUR diols and polyols.3 Thermal degradation using pyrolysis yields the constituent diol or polyol.39 MALDI analysis of pyrolysis residues enabled the identification of linear polyester oligomers.40 In the present research, two chemical degradation methods which have not been previously reported in conjunction with MALDI analysis will be applied to enable determination of the polyol identity and MWD. Ethanolamine will be applied for polyetherPUR analysis, and phenylisocyanate will be applied for polyesterPUR analysis. Ethanolamine has been reported for the recycling of PURs by enabling recovery of polyether polyols.41 The recovered polyol can (36) Bu ¨ rger, H. M.; Mu ¨ ller, H.-M.; Seeback, D.; Bo¨rnsen, K. O.; Scha¨r, M.; Widmer, H. M. Macromolecules 1993, 26, 4738-4790. (37) Reinhold, M.; Meier, R. J.; de Koster, C. G. Rapid Commun. Mass Spectrom. 1998, 12, 1962-1966. (38) Kim, Y. L.; Hercules, D. M. Macromolecules 1994, 27, 7855-7871. (39) Berenbaum, M. B. In Chemical Reactions of Polymers; Fettes, E. M., Ed.; Interscience: New York, 1969; pp 983-984. (40) Lattimer, R. P.; Polce, M. J.; Wesdemiotis, C. J. Anal. Appl. Pyrolysis 1998, 48, 1-15. (41) Yamamura, H. U.S. Patent 721,595, 1995. (42) Bunge, W. Angew. Chem. 1960, 72, 1002. (43) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 95. (44) Mehl, J. T. Unpublished results.
Scheme 1. Ethanolamine Degradation Reaction of PTHF-PUR
then be reacted with fresh diisocyanate to form new PUR. Scheme 1 shows the degradation reaction of pTHF-PUR using ethanolamine. Ethanolamine cleaves only the urethane bonds, producing hydroxy-terminated polyethers. The amino group of ethanolamine is involved in the reaction because it is more nucleophilic than the hydroxyl group of ethanolamine. Ethanolamine cleaves ester bonds, so therefore was not used for the analysis of polyester-PURs because the soft block would degrade. For this reason, phenylisocyanate was used as a selective chemical reagent for polyester PURs. The reaction of isocyanate with urethane bonds is well-known in PUR chemistry.42 Scheme 2 shows the equilibrium reaction of phenylisocyanate with pBAPUR. The presence of an equilibrium has consequences with respect to the yield which can be achieved. Under conditions of excess phenylisocyanate, the major product is the N-phenylurethane-terminated polyester. Phenylisocyanate cleaves only the urethane bond, leaving the polyol ester bonds intact. Minor products are the MDI-terminated, and mixed N-phenylurethane/ MDI-terminated polyester. Deactivation of reactive isocyanate groups is achieved using dibutylamine. Figure 2a shows a MALDI spectrum of an ethanolamine degraded pTHF-PUR. The main peak series corresponds to oligomer [M + Na]+ ions of pTHF. Oligomer ions extend out to n ) 55 (m/z ) 4008), which is nearly identical to the unreacted pTHF starting material (see Figure 1a). Inspection of the expanded spectrum (see inset) reveals that there are no intermediate peaks between the oligomer ion peaks. This indicates that the ethanolamine reaction is complete, cleaving all of the urethane bonds. No evidence is found for cyclic oligomers formed during the degradation reaction. Since the repeat pattern shows only pTHF oligomer ions, information about the type of diisocyanate used is not available from the spectrum. However, by stopping the reaction prior to completion, it is possible to obtain an additional reaction product which contains the MDI segment. Figure 2b shows a MALDI spectrum of an incompletely degraded pTHF-PUR; the reaction was stopped after 1 h. An additional peak series is observed. This series is due to species containing the PUR diisocyanate linkage. The peaks are labeled A and B in Figure 2b, and the structures of these ions Analytical Chemistry, Vol. 72, No. 11, June 1, 2000
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are illustrated in Figure 3a. This demonstrates that, by controlling reaction conditions, valuable information about the PUR structure can be obtained. In this case, the type of polyether and type of diisocyanate used to synthesize the PUR can be identified. Figure 4 shows a MALDI spectrum of a phenylisocyanatedegraded pBA-PUR. The separation between peaks of the main series is 200 Da, consistent with the polybutylene adipate repeat unit. The ions consist of a pBA chain with N-phenylurethane terminal groups. In addition to this series, there are two other series present. These peaks correspond to the expected minor products of the phenylisocyanate degradation reaction. The additional peak series are labeled Y and Z, and the ion structures are shown in Figure 3b. These ions provide identification of the diisocyanate linkage. Attempts to drive the phenylisocyanate degradation reaction further toward completion were not successful. Even after 12 h at 190 °C, degradation products containing the MDI linkage were still present in the MALDI spectra. The phenylisocyanate reaction is reversible and establishes system equilibrium and so it is therefore unlikely that all the urethane bonds will remain uncleaved; some urethane bonds will be reformed. In some cases the peaks corresponding to the minor products are more intense than shown in Figure 4 due to batchto-batch variability of the degradation reaction. The number average molecular weight (Mn), the weight average molecular weight (Mw), and the polydispersity index (PD) can be determined by using the masses of the oligomer ions and the corresponding signal intensity. Mw and Mw are calculated using
Mn ) Mw )
∑ N M /∑ N i
i
i
∑ N M /∑ N M 2
i
i
i
i
PD ) Mw/Mn
where Ni is the measured peak intensities (peak height) of a molecular ion with degree of polymerization i and Mi is the mass of the ith oligomer. Table 1 gives Mn and Mw values determined by MALDI for the pTHF and degraded pTHF-PUR samples. End-group titration values are also listed for the pTHF samples. There is very good agreement between the MALDI Mn and Mw values for the pTHF and degraded pTHF-PUR samples. This indicates that the degradation reaction allows recovery of a representative pTHF oligomer distribution. The polydispersity (PD) indexes determined by MALDI are reasonable for pTHFs, which are expected to have PD values of approximately 1.5, based on the polymerization conditions. Though the PD values are not known for the pTHF samples, the results suggest that MALDI analysis may be sufficient for MW determination of small polyethers. In general it has been shown that MALDI does not provide accurate MW and PD values for polymers with polydispersities greater than 1.2.18,25 However, the results shown here indicate that for low MW polyethers MALDI appears to provide reasonable values of Mn and Mw. The Mn values determined by MALDI for the two smaller pTHF samples (pTHF 650 and 1000) are higher than the endgroup titration values. This indicates that MALDI analysis is biased against smaller oligomers (n e 3). The monomer and dimer peaks were not observed in the spectra of the pTHF samples. This could 2494
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Scheme 2. Phenylisocyanate Degradation Reaction of PBA-PUR
be due to either decreased cationization of small oligomers with sodium, loss of the more volatile smaller oligomers to the vacuum system, or detector saturation from matrix and lower MW contamination peaks in the low mass spectral region. For the pTHF 2000 sample, smaller oligomers are less abundant in the oligomer distribution, and therefore, mass bias does not influence the Mn determination. Table 2 gives the Mn and Mw values determined by MALDI for the pBA and degraded pBA-PUR samples. Mn values determined by end-group titration are included for the pBA samples. For the lowest MW sample (pBA 1000), the MALDI value is higher than the titration value. This reflects a mass bias against smaller oligomers during MALDI analysis. The same behavior is observed for the lower MW pTHF samples, as mentioned above. For the pBA 2000 sample, the MALDI and titration values are reasonably close. However, for the pBA 4000 sample, the titration Mn value is higher than the MALDI value. This is also the case for the pBA 3650 sample, which is a more narrowly distributed sample than the pBA 4000 sample. This indicates that for the larger MW samples, MALDI under-represents the larger oligomers. There is a transition which occurs with MW for Mn determination by MALDI. For lower MW pBA samples, the lower mass oligomers are discriminated against; however, for higher MW pBA samples, the higher mass oligomers are discriminated against. The pBA 2000 sample coincides with an intersection of the two methods, where the Mn values determined by end-group titration and MALDI are similar. Comparing the pBA and degraded pBA-PUR samples, there are obvious differences between the PD indexes. The PD indexes for the degraded samples are consistently lower, indicating that the oligomer distributions are different. The Mn values for the
Figure 2. Spectrum of pTHF(1000)-PUR degraded using ethanolamine; reaction time is 3.5 h (a). Spectrum of degraded pTHF(1000)-PUR; reaction time is 1 h (b). The ion structures for the A and B series are shown in Figure 3a.
degraded PUR samples are greater than the corresponding unreacted pBA samples, indicating that smaller oligomers are less abundant in the degraded material. The purification procedure is the most likely cause for the observed difference in the oligomer distribution of the degraded pBA-PUR samples. Presently, simple washing of the solid raw product with ether is used to remove the degradation reaction byproducts. Small pBA oligomers are soluble in ether and might be selectively removed during the washing step. Additionally, the MALDI analysis could be influenced by the presence of degradation reaction byproducts not completely removed during the purification procedure. The appearance of the MALDI crystals are different when comparing the pBA and degraded pBA samples. It has been shown that sample preparation can introduce serious
mass biasing in MALDI spectra of polymers.26 Further investigation of these possible effects on the recovered pBA oligomer distributions are necessary to determine the accuracy of this degradation method for MW and PD determination. However, the results shown here indicate that the phenylisocyanate reaction allows recovery of a polyester soft-block oligomer distribution from PURs. Therefore, phenylisocyanate degradation in combination with MALDI analysis is a promising method for soft-block characterization. SEC-MALDI. SEC was used to separate the polymers into narrower molecular weight fractions. MALDI analysis of the fractions provides determination of Mn and Mw for each fraction. Figure 5 shows SEC chromatograms for unreacted pTHF (2000) and ethanolamine degraded pTHF(2000)-PUR. The large peak is Analytical Chemistry, Vol. 72, No. 11, June 1, 2000
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Figure 3. Structure of ions observed in spectra of ethanolamine-degraded pTHF-PUR (a). Ion peaks are marked in Figure 2. Structure of ions observed in spectra of phenylisocyanate-degraded pBA-PUR (b). Ion peaks are marked in Figure 4.
Figure 4. Spectrum of pBA(2000)-PUR degraded using phenylisocyanate. Ion structures for the X, Y, and Z series are shown in Figure 3b.
due to pTHF. A second peak, at longer elution time, is present in the degraded pTHF-PUR sample. Most likely this peak belongs 2496
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to the degradation reaction product N,N′′-ethylenedi-4,1-phenylene)bis[N′-(2-hydroxyethyl)]-urea, which is partially soluble in
Table 1. Molecular Weights for Polytetrahydrofuran (pTHF) and Degraded Polyether Polyurethanes Determined by MALDI and SEC-MALDI titrationc
MALDI
SEC-MALDI
sample
Mn
Mn
Mw
PD
Mn
Mw
PD
pTHF 650a pTHF(650)-PURb pTHF 1000a pTHF(1000)-PURb pTHF 2000a pTHF(2000)-PURb
660
1059 1104 1234 1304 1914 1981
1638 1599 1944 1906 2739 2786
1.55 1.45 1.58 1.46 1.43 1.41
1168 1205 1433 1348 1873 1779
2194 2169 2675 2831 4765 4082
1.88 1.80 1.87 2.10 2.54 2.29
984 2051
a Unreacted polytetrahydrofuran (pTHF). b Degraded polyether polyurethane; values in parentheses are the nominal weights of polyether used to make original polyurethane. c End-group titration performed only on unreacted pTHF.
Table 2. Molecular Weights for Polybutylene Adipate (pBA) and Degraded Polyester Polyurethanes Determined by MALDI and SEC-MALDI titrationd
MALDI
SEC-MALDI
sample
Mn
Mn
Mw
PD
Mn
Mw
PD
pBA 1000a pBA(1000)-PURb pBA 2000a pBA(2000)-PURb pBA(2000)-PUR-Ec pBA 3650a,e pBA 4000a pBA(4000)-PURb
1022
1757 1817 1872 2731 2893 3048 2327 2875
2713 2381 2897 3447 3567 3783 3541 3877
1.55 1.31 1.56 1.26 1.23 1.24 1.53 1.35
1295 1943 1910 2604 3456 3078 2980 3515
3303 3069 4748 5196 5253 6200 7322 8513
2.55 1.58 2.48 2.00 1.52 2.01 2.46 2.42
2011 3650 3650
Figure 5. Size-exclusion chromatograms of pTHF 2000 (a), and ethanolamine-degraded pTHF(2000)-PUR (b). Selected fractions are marked in (b), and the corresponding MALDI spectra are shown in Figure 6.
a Unreacted polybutylene adipate. b Degraded polyester polyurethane; values in parentheses are the nominal weight of polyesters used to make original polyurethane. c Same as footnote b; however, polyurethane included butanediol chain extender. d End-group titration performed only on unreacted polybutylene adipate. e Narrow distributed polybutylene adipate, assumed PD of approximately 1.5. Obtained by extracting smaller oligomers from normally distributed pBA 2000.
ether. MALDI analysis of the sharp peak indicated the absence of pTHF oligomers. For lower MW pTHF-PUR soft blocks, the sharp peak partially overlaps with the polymer SEC peak, because small oligomers have longer elution times. Peak interference in SEC can cause difficulties for peak integration because the baseline and integration end points are more difficult to establish. MALDI spectra of selected SEC fractions of an ethanolaminedegraded pTHF-PUR sample are shown in Figure 6. The polydispersity of each fraction is narrower than that of the unfractionated sample. Fractionation of the sample allowed detection of larger oligomer ions, up to n ) 174 (m/z ) 12580), compared with n ) 79 (m/z ) 5736) for the unfractionated sample. Accurate Mn determination of each fraction is possible using MALDI, and this method was used to calibrate the SEC trace. Mn and Mw values for each polymer were then determined from the SEC-MALDI calibration curve and the respective elution times calculated through integration of the chromatogram by the SEC software. Table 1 lists the results of SEC-MALDI analysis for the pTHF and degraded pTHF-PUR samples. There is reasonable agreement between the Mn values determined by MALDI and those determined by SEC-MALDI. However, both methods yield higher values than end-group titration, indicating that SEC-MALDI also discriminates against lower mass oligomers. This could be due
Figure 6. Spectra of selected SEC fractions from ethanolaminedegraded pTHF(2000)-PUR. The location of the fractions are shown in Figure 5.
to use of a refractive index (RI) SEC detector, which is less sensitive to lower mass oligomers. The Mw and PD values determined by SEC-MALDI are higher than the MALDI Mw and PD values. This indicates that SECMALDI is more sensitive to higher mass oligomers than MALDI. It is well recognized that MALDI sensitivity decreases with increasing MW and PD.25 In contrast, the sensitivity of an RI detector increases with MW. Therefore, more accurate determination of Mw and PD can be made using SEC-MALDI compared Analytical Chemistry, Vol. 72, No. 11, June 1, 2000
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with using MALDI analysis alone. The SEC-MALDI determined PD indexes range from 1.8 to 2.5, which is higher than the expected value of 1.5. The expected PD value is based on the reaction condition used to synthesize the pTHF polyethers. However, the PD indexes for these samples have not been previously determined, and it is only assumed that the PD indexes are 1.5. The SEC-MALDI results indicate that the PD indexes are higher than 1.5 for the pTHF samples examined. SEC-MALDI analysis was also performed on the pBA and degraded pBA-PUR samples, and the results are given in Table 2. The Mn values of the unreacted pBA samples determined by SEC-MALDI agree reasonably well with the end-group titration values, indicating that SEC is not suffering from significant discrimination against smaller pBA oligomers. The PD values determined by SEC-MALDI are higher than the MALDI determined values. The same behavior is observed for the pTHF samples shown in Table 1. Once again, MALDI is less sensitive to the larger oligomers. The combination of SEC with MALDI can overcome this problem, enabling more accurate determination of Mw and Mn. The PD values for pBA samples are close to 2.5, except for the pBA 3650, because of its preparation by extraction of smaller oligomers from a 2000 MW pBA sample. The PD indexes for these samples are expected to be close to 2.0, based on reaction theory;43 however, the PD indexes have not been previously determined for these samples. The SEC-MALDI results indicate that the pBA samples are more disperse than theory predicts. The pBA polyols recovered from the phenylisocyanate degradation reaction have lower PD indexes than the unreacted pBA samples, except for the largest MW sample. This means that the recovered pBA samples have different oligomer distributions than the corresponding unreacted pBA samples. The Mn values determined by SEC-MALDI for the degraded samples are larger than the unreacted pBA samples, indicating that smaller oligomers are less abundant in the degraded samples. CONCLUSIONS Selective chemical degradation combined with MALDI analysis was used for the characterization of polyether and polyester PURs.
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Two selective chemical degradation reagents, which have not been previously reported in combination with MALDI analysis, were utilized. Ethanolamine was applied for the recovery of pTHF soft blocks. MALDI analysis of the degraded polymers indicates that ethanolamine gives recovery of a representative oligomer distribution. Allowing only partial degradation enables identification of the diisocyanate using ions containing the diisocyanate linkage which appear in the MALDI spectrum. The polydispersity indexes determined by MALDI analysis are in reasonable agreement with the expected values based on the pTHF synthesis reaction. This suggests that, for small MW polyethers, MALDI analysis may be sufficient for Mn and Mw determination. Phenylisocyanate, which does not cleave ester bonds, was applied for the analysis of polyester PURs. MALDI analysis showed that the pBA oligomer distribution is recovered, along with minor degradation products containing the MDI linkage, providing identification of both the polyester and the diisocyanate. Comparison of the degraded pBA-PUR oligomer distributions with the unreacted pBA material indicates that smaller oligomers are less abundant in the degraded samples. SEC-MALDI was used to obtain more accurate MWD determinations than obtained with MALDI alone. The polydispersity indexes determined using SEC-MALDI are higher than the MALDI determined PD indexes. The results presented here indicate that selective degradation in combination with SECMALDI analysis is a viable means for polyether and polyester polyurethane soft-block characterization. ACKNOWLEDGMENT This research was supported by the National Science Foundation, Grant CHE-9520336. The authors wish to thank Dr. Costas G. Karakatsanis for critical review of the manuscript.
Received for review November 10, 1999. Accepted March 24, 2000. AC991283K
