J. Phys. Chem. B 2002, 106, 2867-2874
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Two-Dimensional Correlation Gel Permeation Chromatography (2D GPC) Study of 1H,1H,2H,2H-Perfluorooctyltriethoxysilane Sol-Gel Polymerization Process Kenichi Izawa,† Toshiaki Ogasawara,‡ Hideki Masuda,§ Hirofumi Okabayashi,*,§ Charmian J. O’Connor,| and Isao Noda⊥ Fuji Silysia Chemical Ltd., Nakatsugawa Technical Center, 1683-1880, Nakagaito, Nasubigawa, Nakatsugawa, Gifu 509-9132, Japan, Tokai Technical Center Foundation, 710, Inokoshi 2, Meito-ku, Nagoya, Aichi, 465-0021, Japan, Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan, Department of Chemistry, The UniVersity of Auckland, PriVate Bag 92019, Auckland, New Zealand, and The Procter and Gamble Company, 8611 Beckett Road, West Chester, Ohio 45069 ReceiVed: July 25, 2001; In Final Form: NoVember 5, 2001
The novel analytical technique, two-dimensional (2D) correlation analysis, has been applied to a set of timeresolved gel permeation chromatography (GPC) profiles of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) to investigate intricate details of the process of its polymerization, catalyzed by 1 M HCl‚H2O, over a thirty minute period. In particular, the splitting behavior of the correlation cross-peaks and their intensity variation have been examined in detail, leading to the conclusion that the mechanism of the growth process of polymeric aggregates is strongly reflected in the patterns of the observed spectra. The 2D correlation GPC method, which has been used to follow the time-dependence of the split components, has proved to be a powerful tool in elucidation of the reaction mechanism.
Introduction The sol-gel polymerization process of tetraalkoxysilane or silane-coupling agents has been studied as a representative macroscopic sol-gel transition phenomenon.1-6 Thus far, and in order to assist application of this process in the industrial arena, discussion has been mainly focused on the property of the gel state rather than on the sol itself. However, since solution processing involves various intermediates or precursors, elucidation of the nature of these precursors may lead to the development of further applications. Accordingly, the solution processing of silane-coupling agents should be further investigated.
tion for the mechanism involved during the initial stage of PFOTES polymerization was not given. Size exclusion chromatography has been employed8,9 in order to examine the initial stage of the acid-catalyzed PFOTES-ethanol polymerization system. Time-resolved gel permeation chromatography profiles were combined effectively with the generalized two-dimensional correlation analysis used extensively in the field of molecular spectroscopy.10,11 In fact, this powerful combination technique, known as two-dimensional correlation gel permeation chromatography (2D GPC), has been successfully applied in the study of the sol-gel polymerization process of octyltriethoxysilane.8,9 In the present study, the time-dependent behavior of randomly polymerizing precursors in the PFOTES-ethanol-1 M HCl‚ H2O system has been examined in detail. In particular, emphasis has been placed on the first thirty minutes of this sol-gel polymerization reaction. Background
In our previous study,7 the growth process of polymerized precursors in the 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES)-ethanol-1 M HCl‚H2O system was investigated by use of time-resolved SAXS spectra, and it was shown that the growth of the polymeric precursors, prior to the macroscopic phase separation, proceeds in two steps. However, an explana* Corresponding author. † Fuji Silysia Chemical Ltd. ‡ Tokai Technical Center Foundation. § Nagoya Institute of Technology. | The University of Auckland. ⊥ The Procter and Gamble Company.
Generalized 2D correlation analysis, as proposed by Noda,10,11 has been applied to IR and Raman studies.12-16 We have used this original concept of 2D spectral analysis to expand this versatile theory of 2D correlation to GPC elution profiles.8,9 The time-resolved GPC trace intensity I(E, t) can be obtained as a function not only of the chromatographic elution time E but also of the sampling time t for each aliquot, collected during the polymerization reaction over the time period between Tmin and Tmax. The dynamic GPC trace intensity y˜ (E, t) of a timeresolved GPC profile is expressed by
y˜ (E, t) )
{
I(E, t) - hI (E) for Tmin e t e Tmax 0 otherwise
(1)
where hI(E) is the reference GPC trace profile of the system. The reference trace hI(E) is set to be the time-average of the
10.1021/jp012878y CCC: $22.00 © 2002 American Chemical Society Published on Web 02/26/2002
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trace profiles obtained over the observation period.
hI (E) )
1 Tmax - Tmin
∫TT
max
I(E, t) dt
(2)
min
The reference trace could sometimes also be set simply equal to zero; in that case, the dynamic trace is identical to the observed variation of the GPC trace profile. We adopt the conventional form of reference trace defined by eq 2 for the 2D GPC analysis. The generalized 2D correlation function10,11 for time-resolved GPC analysis is defined as
Φ(E1, E2) + iΨ(E1, E2) ) 1 π(Tmax - Tmin)
∫0∞ Y˜ 1(ω) ‚ Y˜ 2*(ω) dω
(3)
The two orthogonal (i.e., real and imaginary) components of the 2D correlation function, Φ(E1, E2) and Ψ(E1, E2), are known, respectively, as the synchronous and asynchronous 2D correlation intensities. The synchronous 2D correlation intensity Φ(E1, E2) represents the overall similarity or coincidental trend between two separate concentration indicator (e.g., refractive index intensity) variations of the GPC trace measured at different elution counts, as the value of sampling (i.e., reaction) time t is scanned from Tmin to Tmax. On the other hand, the asynchronous 2D correlation intensity Ψ(E1, E2) may be regarded as a measure of dissimilarity or out-of-phase character of the GPC trace intensity variations. The term Y˜ 1(ω), which is the forward Fourier transform of the dynamic trace intensity variations y˜ (E1, t) observed at some given elution count E1 with respect to the sampling time t, is expressed by
Y˜ 1(ω) )
∫-∞∞ y˜ (E1, t)e-iωt dt
(4)
According to eq 1, the above Fourier integration of the dynamic spectrum is actually bound by the finite interval between Tmin and Tmax. The Fourier frequency ω represents the individual frequency component of the variation of y˜ (E1, t) traced along the sampling time t. The conjugate of the Fourier transform Y˜ 2*(ω) of the refractive index intensity variation y˜ (E2, t) observed at elution time E2 is provided by
Y˜ 2*(ω) )
∫-∞∞ y˜ (E2, t)e+iωt dt
(5)
Once the appropriate Fourier transformation of the dynamic trace y˜ (E, t), defined in the form of eq 1, is carried out with respect to the sampling time t, eq 3 will directly yield the synchronous and asynchronous correlation spectra, Φ(E1, E2) and Ψ(E1, E2). The basic properties of generalized 2D correlation spectra are provided in detail elsewhere.10,11 Further details of synchronous and asynchronous 2D GPC spectra are shown in the literature.8-12 Experimental Section Materials. 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) was purchased from Fluka Chemical Ltd. and was used without further purification. A PFOTES-ethanol-1 M HCl‚H2O (1:1:0.4 weight ratio) system was used as the acidcatalyzed reaction mixture at 298 K. Time-Resolved GPC Measurements. Gel permeation chromatography (GPC) measurements were carried out by using a SHIMADZU high-performance liquid chromatography system,
Figure 1. Time-resolved GPC elution profiles of PFOTES (a: monomer (t ) 0 s), b: 60 s, c: 300 s, d: 600 s, e: 1800 s). Assignment of bands A - F is listed in Table 1 and discussed in the text.
which consists of a column oven CTO-10Avp equipped with an Asahipack GF-310HQ column (30 cm) operated at 30 °C, a high pressure solvent delivery pump LC-10Advp, a DGU-14A degasser, and refractive index detector RID-10A. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.6 mL/ min. Time-resolved GPC profiles were obtained by the following method. Aliquots of 0.002 mL were sampled from the reaction mixture of the PFOTES-ethanol-1 M HCl‚H2O system every 60 s in the time range of 60-600 s and were quickly diluted by 1 mL of chilled THF at 273 K. Thus, fifteen time-sliced samples, reflecting the time-dependent compositional changes, were obtained. For each THF-diluted sample solution, normal GPC analysis was carried out. 2D Correlation Analysis. Synchronous and asynchronous 2D correlation GPC maps were derived from the time-resolved GPC elution profiles using the generalized 2D correlation scheme originally developed for spectroscopic analysis. The basic mathematical procedure for obtaining 2D GPC maps is described in recent publications.8,9 Computation was carried out using the 2D OGAIZA software developed at the Nagoya Institute of Technology and was based on an algorithm incorporating eqs 1-3 in the background section. Results Time-Resolved GPC Profiles. The GPC elution profiles, obtained by sampling during the thirty minutes of the polymerization process of PFOTES catalyzed by 1 M HCl‚H2O, are shown in Figure 1. The GPC profiles provide information on the time-dependence of the composition of the system and directly reflect the polymerization process. As shown in Figure 1, the monomer (profile a) has only one elution band at 11.44 min, but after 60 s of reaction (profile b), at least five distinguishable elution bands appear at 10.84 (band E), 11.16 (band D), 11.48 (band C), 12.12 (broad band B), and 13.00 (band A) min. Assignment of these bands, made on the basis of the GPC results for the amino-propyltriethoxysilane system,8 are listed in Table 1. Band A, at 13.00 min, may be identified as perfluorooctyltrihydroxysilane (PFOTHS) produced as a consequence of hydrolysis.8 Its appearance indicates that, under the reaction conditions, a hydrolysis reaction predominantly occurs during the initial 60 s. Bands B1 and B2, at 12.00 and 12.20 min, respectively, may be regarded as components that consist of partially hydrolyzed PFOTES monomers (monohydroxysilane (PFOMHS) and dihydroxysilane (PFODHS), respectively). The
2D GPC Analysis of PFOTES
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TABLE 1: Assignments of Elution Peaks Identified in Figure 1 elution counts
band no.
tentative assignment
13.0 12.2 12.0 11.5 11.2 10.8 10.4
A B1 B2 C D E F
Trihydrolyzed - PFOTES (PFOTHS) Dihydrolyzed - PFOTES (PFODHS) Monohydrolyzed -PFOTES (PFOMHS) PFOTES + PFOMHS Component I Component II Component III
band C at 11.48 min may be assigned to unhydrolyzed PFOTES. The band at 11.16 min and other bands with lower elution counts (bands D, E, and F) may come from polymerized precursors (dimer, trimer, and larger oligomers). Thus, the variation in the profiles clearly reflects the consumption of monomeric precursors (bands A, B, and C) and production of polymeric precursors (bands D, E, and F). To understand further the polymerization process, the twodimensional correlation GPC (2D GPC) analysis was carried out using a set of time-resolved GPC profiles. 2D GPC and Polymerization Process of PFOTES. The synchronous 2D correlation map, calculated from time-resolved GPC profiles observed during the period 60-180 s (stage I), is shown in Figure 2A. The synchronous 2D spectrum in this stage has eight cross-peaks, showing that every autopeak correlates with at least one other autopeak. The signs and coordinates of the correlation peaks are summarized in Table 2. A synchronous correlation square (CSq1) can be constructed by connecting the two autopeaks (E1 ) E2 ) 13.00, 10.84) and two cross-peaks ((E1, E2) ) (13.00, 10.84), (10.84, 13.00)). This correlation square implies that there exists a correlation between the two elution bands A and E. The very strong intensity of autopeak A at 13.00 min suggests that the PFOTHS monomers are probably produced following hydrolysis in the earlier step (0-60 s), and that they are rapidly consumed in the condensation reaction. The intensity of band E autopeak is smaller than that of the band A autopeak, indicating a smaller variation in the former peak. The two positive cross-peaks, corresponding to the bands B1 and B2, indicate that partially hydrolyzed monomers (PFODHS and PFOMHS) may be consumed in the subsequent condensation reaction. However, since the intensities of these cross-peaks are very weak, these components can play a minor role in the overall reduction in intensity. The signs of the two cross-peaks arising from bands E and F are negative, reflecting an increase in population of the precursors II and III present among three monomeric species (PFOTHS, PFODHS, and PFOMHS).
Figure 2. [A] Synchronous and [B] asynchronous correlation maps for stage I, 60-180 s (solid line: positive peak, broken line: negative peak).
In the asynchronous 2D correlation map (Figure 2B), obtained from the time-resolved GPC profiles in stage I (60-180 s), we find a relatively strong cross-peak at coordinate (13.00, 11.52), which correlates elution band A with band C. The sign of both this cross-peak and the synchronous correlation at the same coordinate is positive. Therefore, variation in intensity of band
TABLE 2: Possible Correlation Squares and Correlations between the Elution Peaks in Stages I, II, and III of the Polymerization Process coordinates (E1, E2) correlation square stage I (60-180 s) CSq1 stage II (240-360 s) CSq2 CSq3 CSq4 CSq5 CSq6 CSq7 stage III (420-600 s) CSq8 CSq9 CSq10
autopeak
cross-peak
correlated elution peaks
+(13.00, 13.00), +(10.84, 10.84)
-(13.00, 10.84), -(10.84, 13.00)
A, E
+(13.00, 13.00), +(11.52, 11.52) +(13.00, 13.00), +(10.88, 10.88) +(13.00, 13.00), +(10.24, 10.24) +(11.52, 11.52), +(10.88, 10.88) +(11.52, 11.52), +(10.24, 10.24) +(10.88, 10.88), +(10.24, 10.24)
+(13.00, 11.52), +(11.52, 13.00) -(13.00, 10.88), -(10.88, 13.00) -(13.00, 10.24), -(10.24, 13.00) -(11.52, 10.88), -(10.88, 11.52) -(11.52, 10.24), -(10.24, 11.52) +(10,88, 10.24), +(10.24, 10.88)
A, C A, EH A, FL C, EH C, FL EH, FL
+(12.96, 12.96), + (11.44, 11.44) +(12.96, 12.96), +(10.24, 10.24) +(11.44, 11.44), +(10.24, 10.24)
+(12.96, 11.44), +(11.44, 12.96) -(12.96, 10.24), -(10.24, 12.96) -(11.44, 10.24), -(10.24, 11.44)
AL, CL AL, FL CL, FL
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TABLE 3: Signs of the Cross-Peaks in the Synchronous and Asynchronous Maps and the Order of Events during the Polymerization Process sign (E1, E2) stage I (13.00, 11.52) (13.00, 11.22) (13.00, 10.84) (13.00, 10.34) (11.52, 10.85) (10.85, 10.41) stage II (13.00, 12.26) (13.00, 11.52) (13.00, 11.16) (13.00, 10.88) (13.00, 10.36) (12.26, 11.50) (12.26, 10.86) (13.26, 10.37) (11.48, 11.05) (11.48, 10.88) (11.48, 10.24-10.68) (11.08, 10.84) (10.88, 10.26-10.68) stage III (13.08, 11.44) (12.96, 11.56) (12.96, 10.92) (12.96, 10.24) (11.56, 11.44) (11.44, 11.22) (11.56, 11.12) (11.50, 10.92) (11.56, 10.40) (11.44, 10.24) (11.16, 10.92) (10.92, 10.44) (10.40, 10.10) a
Φ syn. int.
Ψ asyn. int.
order of event
+a +a -a +
+ + + +
E1(A) f E2(C) E1(A) f E2(D) E2(E) f E1(A) E2(F) f E1(A) E2(E) f E1(C) E1(E) f E2(F)
+a + + + -a -a + +
+ + + + + + -
E1(A) f E2(B1) E1(A) f E2(C) E1(A) f E2(DH) E2(EH) f E1(A) E2(F) f E1(A) E1(B1) f E2(C) E2(EH) f E1(B1) E2(F) f E1(B1) E1(C) f E2(DL) E2(EH) f E1(C) E2(F) f E1(C) E2(EH) f E1(DL) E1(EH) f E2(F)
+ + -a + + + +a +
+ + + + + + + +
E1(AH) f E2(CL) E1(AL) f E2(CH) E1(AL) f E2(EH) E2(FL) f E1(AL) E1(CH) f E2(CL) E1(CL) f E2(DH) E1(CH) f E2(DL) E2(EH) f E1(C) E2(FH) f E1(CH) E2(FL) f E1(CL) E2(EH) f E1(DL) E1(EH) f E2(FH) E1(FH) f E2(FL)
Signs at high contour level.
A occurs before that of band C. This order of events may imply that consumption of PFOTHS monomers, produced by the hydrolysis reaction in the earlier step (0-60 s), occurs during this stage (60-180 s) before a further hydrolysis reaction of PFOTES. Signs and the order of events for other cross-peaks in the asynchronous map are summarized in Table 3. Since the asynchronous correlation between band E or F and band A provides the negative cross-peak at coordinates (13.00, 10.84) or (13.00, 10.34), the variation in intensity of band A occurs after that for bands E or F. Therefore, in stage I, consumption of components II and III occurs before that of PFOTHS. The 2D correlation spectra for stage II (240-360 s) are shown in Figures 3A and 3B. In the synchronous correlation map (Figure 3A), new autopeaks and cross-peaks appear, in addition to the auto- and cross-peaks found in the synchronous map for stage I. The connection of two autopeaks with two cross-peaks provides at least six correlation squares (listed in Table 2). We may classify these correlation squares into three classes: class 1 (CSq2, CSq3, and CSq4), class 2 (CSq5 and CSq6), and class 3 (CSq7). In class 1, the correlation square CSq2 indicates correlation between bands A and C. Furthermore, two correlation squares (CSq3 and CSq4) in class 1 imply correlation between band A and another band, either E or F. In particular, we note that the signs of the two cross-peaks, which constitute the CSq3 and CSq4 squares, are negative, indicating that the intensity of band
Figure 3. [A] Synchronous and [B] asynchronous correlation maps for stage II, 240-360 s (solid line: positive peak, broken line: negative peak).
A decreases while those of bands E and F increase. Class 2 contains two squares, CSq5 and CSq6, which reflect a reduction in intensity of band C and an increase in intensity of bands E or F. The correlation square CSq7, in class 3, reveals that the intensities of bands E and F increase together as a function of sampling time. The asynchronous 2D correlation spectrum, obtained from the GPC data in stage II, is shown in Figure 3B. When we compare band A on the E1 axis with bands C and D on the E2 axis, the signs of both the asynchronous and synchronous crosspeaks are positive. Therefore, in stage II, the decrease in intensity of band A occurs first followed by a decrease in intensity of bands C and D. For the contour maps constructed from band A on the E1 axis and bands E and F on the E2 axis, the signs of both the synchronous and asynchronous cross-peaks are negative. We therefore conclude that the increase in intensity of bands E and F occurs first and is followed by the reduction in intensity of band A. The correlation and order of events between bands B1 and C, B1 and E, F, C and D, and C and E are summarized in Table 3. The correlation of band C with the polymeric components
2D GPC Analysis of PFOTES provides at least five positive cross-peaks, demonstrating the resolution-enhancing characteristic of the asynchronous correlation map. We suggest that band C decreases in intensity as a consequence of hydrolysis and that this decrease is then followed by the variation in intensity of the polymeric component bands. This resolution-enhancement characteristic of 2D GPC analysis provides further details of subtle changes in GPC profiles in terms of the presence of cross-peaks at coordinates (11.12, 10.88) and (11.20, 10.24-10.49). The existence of these crosspeaks might be considered to imply the existence of another band between bands D and E and its correlation with the polymeric components. However, the former cross-peak can be assigned to the correlation peak between the low-elution component (DL) of band D and the high-elution component (EH) of band E, while assignment of the latter cross-peak is to correlation between the high-elution component (DH) of band D and the polymeric component (discussed in stage III). Furthermore, we note that splitting of bands E and F also occurs during this stage, thus providing the high- and low-elution bands (coordinates: EH (E1 ) 10.92), EL (E1 ) 10.76), FH (E1 ) 10.44), and FL (E1 ) 10.24)). The synchronous and asynchronous correlation maps, derived from the GPC data in 420-600 s (stage III), are shown in Figures 4A and 4B, respectively. In the synchronous map, we find the correlation square (CSq10), which can be constructed by connecting two autopeaks at 11.44 and 10.24 min and two correlated negative cross-peaks. The correlation implies that the strong bands C and F are dominant during this stage and that the intensity of band C rapidly decreases while that of band F increases. We note that there is no autopeak coming from band A in the synchronous map, probably because of its very weak variation in intensity. However, we may assume the existence of an autopeak at 12.96 min, leading to constitution of two correlation squares, CSq8 and CSq9. Since the four cross-peaks at (12.96, 11.44), (11.44, 12.96) and (12.96, 10.24), (10.24, 12.96) are also weak in intensity, we assume that there is little correlation between bands A and C or between bands A and F. In the asynchronous map (Figure 4B), obtained from the GPC data during stage III, band A consists of two components, arising from enhancement of the spectral resolution. The signs of the coordinates at (12.96, 11.56) and (13.08, 11.44) (Table 3) imply that the intensity decrease of the elution band at 12.96 counts occurs before that of the elution band at 11.56 counts, and that the elution band at 13.08 counts decreases in intensity before the elution band at 11.44 counts. Furthermore, the cross-peaks in the region of coordinates (10-12, 10-12) are relatively strong in intensity, reflecting the dominant correlation between band C and the polymeric component bands. Enhancement of the spectral resolution also splits band C into two components (CH and CL), which appear at coordinates CH (E1 ) 11.56) and CL (E1 ) 11.56) in the asynchronous map. The signs and the order of events (Table 3) indicate that the elution band at 11.56 counts decreases in intensity before that at 11.44 counts. Band D consists of two components (DH (E1 ) 11.24) and DL (E1 ) 11.12)), a splitting caused, as before, by enhancement of the spectral resolution. We must use the strong positive cross-peaks at (11.56, 10.44) and (11.44, 10.24) to interpret the correlation between bands C and F, as well as between the polymeric component bands. It seems that the decrease in intensity of the component at 10.44 occurs prior to that of the component at 11.56, and that variation in intensity of the elution band on the E2 axis (10.24) occurs before that of the E1 elution band (11.44). Moreover, from the
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2871
Figure 4. [A] Synchronous and [B] asynchronous correlation maps for stage III, 420-600 s (solid line: positive peak, broken line: negative peak).
resolution-enhancing characteristics of the asynchronous correlation, it is evident that elution band F splits into H- and L-components. The synchronous and asynchronous correlation maps, calculated from the GPC data time-resolved during stage IV (7801140 s) and stage V (1200-1800 s), are shown in Figures 5 and 6, respectively. In the synchronous and asynchronous maps, we note that the peaks arising from the correlation between band A and other bands disappear and that the correlation peaks between band C and lower-elution bands are concentrated within the region E1 ) E2 ) 10-12. This fact implies that most of the PFOTHS monomers which had been hydrolyzed in an earlier step may be consumed by a subsequent condensation reaction in stages IV and V and that the resulting components I and II participate in further reactions to form both polymeric aggregates and PFOTES monomers. In particular, in stage V, autopeak A reappears and a pair of positive and negative cross-peaks, arising from the EH- and EL-components, respectively, is found, thus reflecting the rapid variation in intensity of the EH- and ELcomponents.
2872 J. Phys. Chem. B, Vol. 106, No. 11, 2002
Figure 5. [A] Synchronous and [B] asynchronous correlation maps for stage IV, 780-1140 s (solid line: positive peak, broken line: negative peak).
Discussion The 2D GPC correlation maps, derived from the time-resolved GPC profiles, directly represent the reaction and interaction mechanisms taking place during the polymerization process. In Scheme 1 we present a summary of the sequence of these intricate reactions or interactions. This sequence is further discussed below. [Stage I]. PFOTHS monomers, produced during rapid hydrolysis at the very beginning (0-60 s) of the reaction, produce mainly components II and III, as a consequence of condensation between the PFOTHS monomers. Although there may exist some other reaction routes (II f III, II or III f highly polymerized component (IV)) during this stage, their total contribution to the whole process is small. [Stage II]. Following further hydrolysis of PFOTES, formation of PFOTHS monomers occurs. Furthermore, the trihydroxide (PFOTHS) monomers are consumed to form components II and III. The polymeric components thus produced also contribute to formation of higher components (e.g., II f III and III f IV). In particular, it should be noted that a strong correlation exists between PFOTES and component II (or component III).
Izawa et al.
Figure 6. [A] Synchronous and [B] asynchronous correlation maps for stage V, 1200-1800 s (solid line: positive peak, broken line: negative peak).
[Stage III]. Further hydrolysis of PFOTES results in formation of PFOTHS. The monomeric molecules thus produced are mostly consumed to form component III, which contributes to formation of highly polymerized components. During this stage, there exists a strong correlation between PFOTES and component III (or IV). [Stages IV and V]. Most of the PFOTHS monomers produced at the very beginning of the reaction finally disappear. The condensation reaction between smaller aggregates progresses further, to form higher-polymeric aggregates. Thus, two polymerization processes (II f III, and III f IV) occur in parallel during the five stages. However, each stage has its own distinct characteristics. In stage I, PFOTHS monomers are predominantly consumed to produce lower molecular weight components (II and III). In stage II, however, very few PFOTHS monomers are consumed and the reaction is dominated by hydrolysis of PFOTES to PFOTHS. In stage III, it is the hydrolysis of PFOTES which is dominant. A few PFOTHS monomers, thus produced, are consumed to form component III. In the later stages (IV and V), most of PFOTHS monomers are consumed and the reaction is dominated by condensation of smaller aggregates.
2D GPC Analysis of PFOTES SCHEME 1: Schematic Growth Process of PFOTES Polymeric Aggregates during Stages I, II, and IIIa
a Solid lines indicate the extent of reaction (thick arrow: high extent; intermediate arrow: intermediate extent; and thin arrow: small extent) and hollow double-headed arrows imply correlation between two components.
In particular, it should be emphasized that, in the 2D GPC correlation maps for stages II and III, a strong correlation peak appears between the bands for PFOTES monomers and a polymeric component (II or III or IV). We may assume that the perfluorooctyl chains self-assemble during the process of polymerization within the PFOTES-ethanol-1 M HCl‚H2O system, since the perfluorooctyl portion has both hydrophobic and lipophobic characteristics. Thus, the self-assembling behavior of the perfluorooctylsilane chains should affect the microstructure of the polymerized components. It is likely that the perfluorooctyl chains of PFOTES monomers are incorporated into the aggregated portions of the perfluorooctyl chains of the polymerized component II or III or IV and of the oligomer-oligomer complexes. Such complex formation should furnish a strong correlation between PFOTES monomeric and polymeric bands. We may now summarize the splitting behavior of the crosspeaks, as confirmed in the asynchronous correlation spectra, as follows (Table 4). The thick arrows and a pair of thin arrows indicate a single cross-peak and a pair of cross-peaks, respectively.
J. Phys. Chem. B, Vol. 106, No. 11, 2002 2873 TABLE 4: Schematic Expression of Splitting Behavior of Elution Bands
In stages I and II, the correlation cross-peak between bands A and C appears as a single cross-peak, while in stage III, the cross-peaks due to the AH-CL correlation appear as a pair of positive and negative cross-peaks. However, such cross-peaks disappear in later stages (IV and V). It seems, therefore, that in stages I and II, elution band A either consists only of monomers that differ very little in reactivity or it consists of L- and H-components with identical reactivity. Thus, only a single cross-peak appears. In stage III, band A probably consists of L- and H-components with different reactivities, leading to the appearance of a pair of cross-peaks. In fact, the intensity of the AL-CH correlation cross-peak is larger than that of the crosspeak for the AH-CL correlation (Figure 4B), indicating that there exists a difference in reactivity of the two components. The reactivity of the AH component may be higher, since there is a rapid variation in intensity of the AH band, compared with that of the AL band. Moreover, in stage III, a cross-peak from the AL-FL correlation appears, while that from the AH-F correlation disappears, following a decrease in intensity due to a fast reaction, thus indicating a probable difference in reactivity of these two components. In stages I and II, band C provides a single cross-peak as a consequence of its correlation with other bands. However, we see the appearance of the CL- and CH-components in the asynchronous maps, (in particular, the cross-peak coming from the CH-CL and CL-CH correlations in the later stages (III IV). Furthermore, for the CH-DL, CL-DH, CH-EL, and CLEH correlations, we see a pair of positive and negative crosspeaks. We emphasize that the cross-peak coming from the C-E correlation appears as a single peak in stages II and III. Moreover, the intensity of this cross-peak is stronger in stage III than it is in stage II. Finally, in stages IV and V, and as a consequence of splitting, CH-EL and CL-EH correlation peaks appear. The splitting behavior for band E indicates that the contribution of such a component-component correlation becomes greater as the reaction progresses. We highlight the appearance of a pair of cross-peaks during stage II, arising from the FH-FL correlation, although their intensity is, at first, very small. As the reaction proceeds in the later stages, the intensities of these cross-peaks increase, clearly reflecting the contribution of the H- and L-components to the growth process of high molecular weight components. The splitting behavior of the cross-peaks and their variation in intensity, observed over time, evidently reflect the mechanism of the growth process of polymeric aggregates. Importantly, the resolution-enhancing characteristics of the asynchronous correlation allow us to confirm the existence of the L- and H-components and of the component-component correlations.
2874 J. Phys. Chem. B, Vol. 106, No. 11, 2002 We may use the following assumptions to account for the origin of the difference in reactivity between the L- and H-components. (a) For the PFOTHS monomers with three silanol groups, which were the products of hydrolysis, association between the silanol groups may be possible16 and will result in the difference in reactivity. Most of the SiOH groups may form a hydrogen-bonding network with the silanol itself, or with ethanol or water molecules.16 The silanols associated with SiOH may be more reactive than those associated with ethanol or water. (b) A PFOTES molecule easily forms the solvated type complex (R1Si(OEt)3(OHEt)m-3) in ethanol (EtOH).17 Since small aggregates containing unreacted ethoxy groups may be formed in the reaction mixtures, formation of solvated-type complexes of small aggregates, with differing degrees of solvation, may also be possible in ethanol. We may assume that a solvated-type complex is less reactive than an unsolvated species (or unsolvated groups). Therefore, the greater the number of unreacted ethoxy groups, the smaller is the reactivity of small aggregates, thereby accounting for the difference in reactivity between the L- and H-components. In particular, steric hindrance caused by the bulky and rigid perfluorooctyl chain may induce this difference in behavior of the PFOTES monomers. (c) Steric hindrance of the bulky perfluorooctyl chain probably promotes isolation of any silanol groups which may be incorporated into the grooves of the self-assembled aggregates of the perfluorooctyl chains. This effect may result in formation of weakly associated silanols,16 thereby rendering the aggregates less reactive. In summary, assumption (a) accounts for splitting of band A into AL and AH components. Component AL probably consists of PFOTHS monomers solvated by ethanol, while unsolvated PFOTHS monomers constitute component AH. Assumption (b) accounts for formation of the CL and CH components, and assumptions (b) and (c) may be used to account for the greater contribution to the splitting of other bands (D, E, and F). Conclusions Generalized 2D correlation analysis has been applied to a set of time-resolved GPC profiles obtained over the first thirty minutes of the sol-gel polymerization reaction of PFOTES, catalyzed by 1 M HCl‚H2O in ethanol. It has been found that the 2D GPC correlation maps directly represent details of the
Izawa et al. polymerization process. The behavior of cross-peaks in the synchronous and asynchronous maps obtained throughout five stages (I (60-180 s), II (240-360 s), III (420-600 s), IV (7801140 s), and V (1200-1800 s)) of the polymerization process has been examined in detail with emphasis being placed on their time-dependence. The results directly indicate that the splitting behavior of the correlation cross-peaks, and in particular their time-dependence, strongly reflect the mechanism for growth of polymeric aggregates in the reaction mixture. The appearance of the L- and H-components occurs, not in the initial stage of the reaction, but after the reaction has progressed for at least 3-4 min. References and Notes (1) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (2) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995. (3) Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1982, 48, 31. (4) Nakanishi, K.; Soga, N.; Matsuoka, H.; Ise, N. J. Am. Ceram. Soc. 1992, 75(4), 971. (5) Lee, K. J.; Tien, T. Y.; Gulari, E. J. Non-Cryst. Solids 1994, 171, 46. (6) Huo, H.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (7) Ogasawara, T.; Izawa, K.; Hattori, N.; Okabayashi, H.; O’Connor, C. J. Colloid Polym. Sci. 2000, 278, 293. (8) In a previous paper (Izawa, K.; Ogasawara, T.; Masuda, H.; Okabayashi, N.; Noda, I. Macromolecules 2002, 35, 92), we have shown that the elution times of (aminopropyl)triethoxysilane and (aminopropyl)trihydroxysilane are 14.96 and 15.85 min, respectively, reflecting the difference in the quantity of ethoxy moieties remaining unreacted. (9) Izawa, K.; Ogasawara, T.; Masuda, H.; Okabayashi, H.; Noda, I. Phys. Chem. Commun. 2001, 12, 1-3. (10) Noda, I. Appl. Spectrosc. 1993, 47, 1329. (11) Noda, I.; Dowrey, A. E.; Marcott, C.; Story, G. M.; Ozaki, Y. Appl. Spectrosc. 2000, 54, 236A. (12) Noda, I. Appl. Spectrosc. 1990, 44, 550. (13) Sefara, N. L.; Magtoto, N. P.; Richardson, H. H. Appl. Spectrosc. 1997, 51, 536. (14) Noda, I.; Liu, Y.; Ozaki, Y.; Czarnecki, M. J. Phys. Chem. 1995, 99, 3068. (15) Ren, Y.; Shimoyama, M.; Ninomiya, T.; Matsukawa, K.; Inoue, H.; Noda, I.; Ozaki, Y. J. Phys. Chem. B 1999, 103, 6475. (16) Ogasawara, T.; Nara, A.; Okabayashi, H.; Nishio, E.; O’Connor, C. J. Colloid Polym. Sci. 2000, 278, 1070. (17) Einaga, H. In Inorganic Synthesis in Solution as a Reaction Field; Baifukan: Tokyo, 2000; p 169.
