Thermal Analysis of Adsorbed Poly(methyl methacrylate) on Silica

McKenna, G. B. In Comprehensive Polymer Science; Allen, G., Bevington, J. C., Booth, C., Price, C., Eds.; Pergamon Press: Oxford, England, 1989; p 311...
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Langmuir 2006, 22, 4741-4744

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Thermal Analysis of Adsorbed Poly(methyl methacrylate) on Silica Frank D. Blum,*,† Erin N. Young,‡ Gregory Smith,† and Oliver C. Sitton‡ UniVersity of MissourisRolla, Rolla, Missouri 65409-0010. ReceiVed NoVember 16, 2005. In Final Form: March 6, 2006 Modulated differential scanning calorimetry has been used to quantify the glass transitions of small adsorbed amounts of poly(methyl methacrylate) (PMMA) on silica. While a relatively narrow, single glass transition was found for bulk PMMA, broader two-component transitions were found for the adsorbed polymer. A two-state model based on loosely bound polymer (glass transition similar to bulk) and more tightly bound polymer (glass transition centered around 156 °C) was used to interpret the thermograms. On the basis of this model, the amount of tightly bound polymer was found to be approximately 1.3 mg/m2, corresponding to a 1.1 nm thick layer. The change in heat capacity for the tightly bound polymer at the glass transition temperature was estimated to be about 16% of that of the bulk polymer.

Introduction The physical properties of adsorbed polymers are of both scientific interest and technological importance. The properties of adsorbed polymers are often different from those of bulk polymers, primarily because of the presence of interfaces and interactions across them. These differences in properties are often more apparent when the amount of adsorbed polymer is small due to the relatively greater proportion of material affected by the interfaces. For supported, thin polymer films, one should consider that there are two rather different interfaces and that each has a characteristic effective length. If the polymer layer is sufficiently thick, there may be bulklike material between the two interfacial regions. The modification of properties (in both magnitude and direction) and the length scale are also affected by the interaction of the polymer and with the adjoining species. In addition, the nature of the experiment can influence the results. These characteristics, taken together, make the nature of interfacial polymers rather complicated. One of the interesting characteristics of adsorbed polymers is their glass transition. Much of the reported work has been reviewed1,2 and does not need to be repeated here. A variety of techniques have been used to probe this effect, including ellipsometry,1 dielectric relaxation,3 probe diffusion/quenching,4 NMR,5 and thermal analysis.6-11 Again, the measured glass transition temperatures vary in a manner dependent on the interaction with the adjoining mating species as well as the nature * Corresponding author. E-mail address: [email protected]. † Department of Chemistry and Materials Research Center. ‡ Department of Chemical and Biological Engineering. (1) Forrest, J. A.; Dalnoki-Veress, K. AdV. Colloid Interface Sci. 2001, 94, 167-196. (2) Alcoutlabi, M.; McKenna, G. B. J. Phys.: Condens. Matter 2005, 17, R461-R524. (3) Fukao, K. Eur. Phys. J. E 2003, 12, 119-125. (4) Ellison, C. J.; Mundra, M. K.; Torkelson, J. M. Macromolecules 2005, 38, 1767-1778. (5) Blum, F. D.; Lin, W.-Y.; Porter, C. E. Colloid Polym. Sci. 2003, 281, 197-202. (6) Porter, C. E.; Blum, F. D. Macromolecules 2000, 33, 7016-7020. (7) Porter, C. E.; Blum, F. D. Macromolecules 2002, 35, 7448-7452. (8) Fryer, D. S.; Nealey, P. F.; de Pablo, J. J. Macromolecules 2000, 33, 6439-6447. (9) Fryer, D. S.; Peters, R. D.; Kim, E. J.; Tomaszewski, J. E.; de Pablo, J. J.; Nealey, P. F.; White, C. C.; Wu, W. L. Macromolecules 2001, 34, 5627-5634. (10) Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z.; Allen, L. H. Phys. ReV. Lett. 2003, 91, 085703. (11) Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z. S.; Allen, L. H. Macromolecules 2004, 37, 4607-4616.

of the experiment itself. In experiments from our group,5,12-15 for example, we have found that one could clearly distinguish between the effects at the polymer-air, polymer-silica, and polymer-polymer interfaces. Calorimetry is perhaps the most widely accepted technique for the measurement of the glass transitions in polymers, although its interpretation is often far from simple.16 Previous studies from our lab have focused on studies of small amounts of poly(methyl methacrylate) (PMMA) adsorbed on silica. These studies showed that the PMMA glass transition broadened and shifted to higher temperatures for the adsorbed polymer.5,6 The amount of broadening and the shift in glass transition depends on the amount of polymer adsorbed. In the present study, we extend the use of modulated differential scanning calorimetry (MDSC) to study the thermal behavior of PMMA adsorbed on Cab-O-Sil silica at relatively low adsorbed amounts. We have been able to separate two distinct components in the thermograms and have attempted to quantify them in terms of a two-state model that yields information about the length scale for tightly bound PMMA and also provides an estimate for its change in heat capacity through the glass transition. Experimental Section High molecular mass PMMA was used as received (Aldrich Chemical Co., Milwaukee, WI). The molecular mass was determined to be 4.5 × 105 g/mol with a polydispersity index of 2.6 by using gel permeation chromatography with a Dawn EOS laser lightscattering instrument (LLS) and an Optilab refractive index detector (both from Wyatt Technology, Santa Barbara, CA). The PMMA tacticity was analyzed using 1H-NMR (Varian Unity 400, Varian Instruments, Palo Alto, CA), and the fractions of triads were found to be mm: 0.10, rm: 0.36, and rr: 0.54. Cab-O-Sil M-5P silica, with a specific area of 200 m2/g obtained from Cabot Corporation (Tuscola, IL), was used as the substrate. The specific surface area was previously verified by nitrogen adsorption. PMMA solutions in toluene (10 mL), of varying concentrations, were prepared and added to the silica (approximately 0.3 g) in a test tube. Prior to adsorption, the silica was dried in an oven at 400 °C for at least 4 h. The tubes were placed in a mechanical shaker for 48 h and then centrifuged at 2500 rpm for 1 h. The supernatant (12) Blum, F. D.; Xu, G.; Liang, M.; Wade, C. G. Macromolecules 1996, 29, 8740-8745. (13) Lin, W.-Y.; Blum, F. D. Macromolecules 1997, 30, 5331-5338. (14) Lin, W.-Y.; Blum, F. D. Macromolecules 1998, 31, 4135-4142. (15) Lin, W. Y.; Blum, F. D. J. Am. Chem. Soc. 2001, 123, 2032-2037. (16) Simon, S. L. Thermochim. Acta 2001, 374, 55-71.

10.1021/la053098+ CCC: $33.50 © 2006 American Chemical Society Published on Web 04/07/2006

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liquids were removed, and the adsorbed polymers were washed with toluene to remove excess polymer. The resulting adsorbed sample was dried in the tube, with air blown through a pipet at a low flow rate. Samples of higher adsorbed amounts were dried, without centrifugation, by bubbling nitrogen through the polymer/silica suspension. All samples were dried in a vacuum oven for 24 h under a 760 mm Hg vacuum at 70 °C. The adsorbed amounts were measured using thermal gravimetric analysis (TGA) with a TGA 2950 (TA Instruments, New Castle, DE). Samples of approximately 10-15 mg were placed in the sample tray and heated from an ambient temperature to 600 °C at a rate of 20 °C/min. Nitrogen was used as purge gas at 50 mL/min. From about 100 to 300 °C, there was a small mass loss. From about 350 to 400 °C, there was a rapid mass loss, and, by 600 °C, all of the organic material (polymer) was volatilized. The accuracy and validity of the technique was verified with TGA runs on bulk PMMA and PMMA/silica mixtures. The residual material contained only silica, and the adsorbed amount was calculated based on the masses of silica and PMMA and the specific surface area of the silica. The dependence of the thermal degradation on tacticity and adsorbed amount was previously reported.17 The glass transition region was probed using MDSC on a DSC 2920 modulated DSC (TA Instruments, New Castle, DE). The sample pans were referenced against empty pans and purged with a 50 mL/min nitrogen stream. The samples were held at 25.00 °C for 5 min, heated to 250 °C at a rate of 2.5 °C/min, held at 250 °C for 3 min, cooled to 25 °C at 2.5 °C/min, and then held at 25 °C for 5 min, to ensure that all samples had the same thermal history. After the initial cycle, the second MDSC scans to 250 °C were reported with a ramp rate of 2.5 °C/min and a modulation of (1.00 °C per 60 s. The thermograms show the derivative of the heat flow as a function of temperature (dQ/dT). The data were smoothed using a 10 °C running average to highlight the transition and reduce the high-frequency noise, without significantly distorting the thermogram. A sigmoidal baseline (TA Instruments software function) was used to estimate the area under the transitions in the dQ/dT plots. The choice of baseline can have a significant effect on the area, but only a small impact on the ratio of the transition intensities. The reported transition temperatures were taken at the peak of the derivative curve.

Model While the thermal behavior of the adsorbed polymer is complicated, we can model the system to a first approximation, assuming that there are two distinct types of adsorbed polymer segments. The segments that are tightly bound (denoted as component i ) B) have a significantly elevated glass transition, and those that are more loosely bound (component i ) A) are similar to bulk polymer. The area under the peak (in some cases, more of a broad hump) in the dQ/dT curve is the heat flow through the transition, ∆Q. For either component (i ) A or B), ∆Qi for that component is given by

∆Qi ) mpi∆Cpi (dT/dt)

(1)

where mpi is the mass of the polymer component, ∆Cpi is the change in heat capacity at the glass transition, T is the temperature, and t is the time. In principle, the changes in heat capacity at the two transitions would be expected to be different from that of the bulk polymer. By mass balance, the total amount of polymer in the sample, mp, determined by TGA, is the sum of the masses for the two components or

mp ) mpA + mpB

(2)

We also define a normalized polymer mass, m′p, as the total mass (17) Zhang, B.; Blum, F. D. Thermochim. Acta 2003, 396, 211-217.

Figure 1. Adsorption isotherm for PMMA on silica from toluene. A fit to a Langmuir isotherm is shown.

of polymer divided by the mass of silica used, ms, or

m′p ) mp/ms

(3)

which can easily be measured with TGA. Normalized masses are also defined for both components as m′pA and m′pB. The ratio of the heat flow changes of components A and B, given by r, is related to the heat capacities of the components, or

r ) ∆QA/∆QB ) mpA ∆CpA/(mpB∆CpB)

(4)

Use of this ratio eliminates the time dependence of the measurements since both components have the same heating rates. Solving eq 2 for mpA, substituting the result into eq 4, and dividing both the numerator and the denominator by ms yields

r ) (m′p - m′pB)∆CpA/(m′pB∆CpB) ) [∆CpA/(m′pB∆CpB)] m′p - ∆CpA/∆CpB (5) On the basis of eq 5, a plot of the ratio of the areas of each transition in the thermograms should be a linear function of the total relative mass of polymer, m′p, for adsorbed amounts greater than m′pB. The intercept of the line is the ratio of the heat capacities, ∆CpA/∆CpB, and the slope is ∆CpA/(m′pB∆CpB). The quantity r is the ratio of the transition intensities and can be estimated from a single thermogram. It is often of interest to express the polymer behavior in terms of a “bound fraction”, fB, which is the ratio of the mass of bound polymer to the total amount of polymer. With the use of eq 5 and some rearrangement, the bound fraction can be expressed as a function of the experimental observable r as

fB ) m′pB/m′p ) mpB/mp ) 1/(1 + r∆CpB/∆CpA)

(6)

At adsorbed amounts greater than a certain critical amount, one expects that the relative amount of tightly bound polymer, m′pB, becomes constant. In that case, the relative amount of adsorbed polymer can be estimated from the ratio of the slope divided by the intercept. The fraction of “bound” polymer, fB, can be estimated from the model by using this value of m′pB determined from the linear regression data.

Results The adsorption isotherm for the PMMA on silica from toluene is shown in Figure 1. The isotherm roughly conforms to a Langmuir isotherm. The maximum adsorbed amounts found with this sample treatment (washing) were about 2 mg/m2. These adsorbed amounts are larger than those previously reported from our lab for a lower molecular mass polymer.6 The difference is

Thermal Analysis of Adsorbed PMMA on Silica

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Figure 3. Ratio (r) of the areas under the transitions in the A and B components of the thermograms as a function of the relative amount of polymer (m′p) for samples with tightly bound polymer only (squares) and those with both tightly and loosely bound polymer (circles).

Figure 2. Thermograms for bulk and adsorbed PMMA on silica at different adsorbed amounts. The derivative of the reversing heat flow is shown, and the vertical lines are shown for reference.

likely due to the effect of molecular mass and its distribution on the adsorption.18 The thermograms for some representative samples of PMMA adsorbed on silica and a bulk sample are shown in Figure 2. For reasonable comparisons, a 10 °C smoothing was done for all samples, the vertical scales were divided by the amount of polymer in each sample, and the thermograms were shifted vertically for clarity. The glass transition temperature (Tg) for the bulk polymer was found to be 119 °C (at 2.5 °C/min) with a full width of the derivative curve (no smoothing) of about 12 °C. This value is higher than that previously reported by us for a PMMA sample with different molecular mass and tacticity.6 The ∆Cp for bulk PMMA was found to be 0.29 J/(g‚°C), consistent with literature values.19,20 The thermograms for the adsorbed polymers show two distinct peaks that we label the A component (loosely bound, transition temperature close to bulk) and the B component (tightly bound, higher transition temperature). It is apparent that, as the adsorbed amount increases, the relative intensity under transition A increases, while that of transition B decreases slightly. At the lowest adsorbed amount shown, the transition intensity is roughly the size of the noise. The peak of the transition for the A component systematically increased slightly from 126 °C for bulk and to 135 °C for the lower adsorbed amounts (e.g., the shoulder for the 1.4 mg/m2 sample). There was not enough A component to observe in the 0.6 mg/m2 sample. The position of the B component was relatively constant at 156 ( 1 °C (SD). A plot of the ratio of intensities of the A and B thermogram components, r, as a function of the relative mass of polymer (m′p) is shown in Figure 3. We focused our studies on the region where the r value varies significantly with the adsorbed amount. As suggested by eq 5, above a critical value of m′pB, the data are linear, with a positive slope. A least-squares fit of the data in this region yields a slope (∆CpA/(m′pB∆CpB)) of 23.5 ( 2.1 (SD) and an intercept (-∆CpA/∆CpB) of -6.11 ( 0.75 (SD). Below the critical value of m′pB, the value of r is roughly equal to 0, a verification that there is little of component A (loosely bound) in those samples. (18) Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 559-573. (19) McKenna, G. B. In ComprehensiVe Polymer Science; Allen, G., Bevington, J. C., Booth, C., Price, C., Eds.; Pergamon Press: Oxford, England, 1989; p 311. (20) Andreozzi, L.; Faetti, M.; Giordano, M.; Zui, F. Macromolecules 2005, 38, 6056-6067.

Figure 4. Estimate of the bound fraction of PMMA on silica as a function of the adsorbed amount. The smooth curve is based on the model with a fixed amount of bound material.

The bound fraction, based on a fixed amount of bound polymer, was calculated from eq 6 for each of the samples and shown in Figure 4. The bound fraction decreases relatively smoothly with increases in the adsorbed amounts.

Discussion It is clear that two distinct transitions are found in the MDSC curves. While the MDSC peaks may not always be totally resolved, their resolution is much greater than that of most techniques in which little or no separation is observed.1,2 Since the A transition is close to that for the bulk polymer, it can be assigned to the adsorbed polymer and can be considered loosely bound, almost bulklike, as far as the thermal data suggest. However, since this transition is somewhat sensitive to the adsorbed amount, it is not entirely equivalent to bulk polymer. Nevertheless, its behavior is more like the bulk polymer than that of component B (tightly bound polymer). Glass transition temperatures for PMMA on solid supports have shown increases in Tg when there was a favorable interaction between PMMA and the substrate from techniques such as ellipsometry,21 neutron reflectometry,22 local thermal analysis,8 or thin film DSC.10 In those cases, the measurements did not resolve the presence of different components, although broadening of the glass transition was observed. A very clear distinction of how the nature of the surface of the solid substrate affected the direction of the change in Tg was made through local thermal analysis,8 where hexamethyldisilizane-treated silicon oxide surfaces caused a decrease in the Tg of thin PMMA films as the thickness of the films decreased. (21) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 219230. (22) Wu, W.-L.; van Zanten, J. H.; Orts, W. J. Macromolecules 1995, 28, 771-774.

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In a previous study,6 a sample of silica-adsorbed PMMA with 0.48 mg/m2 had a compound transition centered around 158 °C that moved to lower temperatures with additional adsorbed polymer. The elevated transition for the bound component (B) is consistent with those previous results (although a different PMMA was used)6 and is clearly due to the hydrogen bonding interaction between the surface silanols and carbonyl groups.23-25 The identification of the high-temperature peak with tightly bound PMMA was also verified through MDSC studies of silicaadsorbed polystyrene (PS)-r-PMMA copolymers.26 In those studies, it was found that the glass transition, and the amount of a high-temperature component increased with the amount of MMA in the copolymer. Thus, the major moiety responsible for the increased temperature transition was the MMA. The ratio for ∆CpA/∆CpB can be estimated from the intercept as 6.11. The ∆Cp for the bulk polymer was estimated as 0.29 J/(g‚°C), and this value can be used as an approximation to ∆CpA. On the basis of this value, ∆CpB would be 0.047 J/(g‚°C), which likely represents an upper limit, as the ∆CpA is probably less than that of the bulk polymer. Given that the Tg of the A component is similar to that of the bulk polymer, the true value of ∆CpA is probably not very different from our estimate. The value of ∆CpB is considerably less than that of the bulk polymer. We believe that this is the first estimate of this quantity for an adsorbed polymer. The reduced value is consistent with the notion that the restriction of the polymers at the surface limits the freedom of the polymer above the glass transition. In other words, the segmental mobility in the tightly bond polymer above its glass transition is not nearly as much as that for the bulk polymer. The reasonableness of the assumption that the mass of the bound polymer is constant seems to be verified through the data analysis. The amount of polymer corresponding to this bound layer is 1.3 mg/m2, which, based on a bulk density of 1.2 g/cm3, corresponds to a thickness of 1.1 nm if the layer is flat and uniform. We recognize that the model used here is an oversimplification, that the adsorbed polymer is not simply divided up as two distinct components. However, this simplification is fairly consistent with the level of detail available from the MDSC experiment. The estimate of the relative amount of tightly bound polymer (bound fraction), given in Figure 4 shows that the value of fB goes from close to 1 to about 0.6 as a function of adsorbed amount over the range studied. In Chapter 5 of their book, Fleer et al.27 show how different experimental techniques yield estimates of bound fractions that may differ substantially. Techniques such as NMR and electron spin resonance (ESR) are sensitive to changes in segmental dynamics and are affected by the indirect binding of nearby segments. In contrast, IR shifts in carbonyl frequencies are sensitive to only those moieties that are directly bound, yielding lower estimates of the bound fraction. Few measurements of bound fraction have been made in solventless (dried) systems, but an interesting comparison can

be made with the PMMA/metal oxide systems in the presence of CCl4. ESR results28 (which are similar to those for NMR27,29) yield bound fractions between about 1 and 0.5 over a similar range of adsorbed amount. For IR, results on similar systems are considerably lower, roughly about half of that for ESR.25,28 Clearly, our present results are more consistent with the view for NMR and ESR in terms of the magnitude of the bound fraction. In our laboratory, it has previously been shown via deuterium NMR methods that, in the glass transition region, the behaviors of silica-adsorbed poly(vinyl acetate) (PVAc)12 and poly(methyl acrylate) (PMA)13 are quite heterogeneous. In those studies, a small amount of the adsorbed polymer had dynamics that were significantly faster than the bulk material. Through studies using unlabeled polymers on top of the silica-bound labeled polymer,15 it was verified that the material with enhanced mobility was from the polymer segments at the air-polymer interface. One might expect segments such as these to show thermal activity in the MDSC studies, but no definitive evidence for them was observed in this work. This is probably because the amounts of material with enhanced mobility were too small (approximately >5% for PVAc or PMA), and, consequently, their thermal signature might have been too weak for our instrumentation. On the other hand, the NMR studies also revealed that the majority of segments had mobilities that ranged from bulklike to quite rigid above the bulk Tg. Some of these segments were rigid well above the Tg for the bulk polymer. These segments with very restricted mobility likely belong to those closely associated with the silica surface, and these can be associated with the B transitions in the thermograms. The elevated and broad glass transition observed in both the MDSC and the NMR studies are clearly consistent.

(23) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961, 65, 480-487. (24) Berquier, J. M.; Arribart, H. Langmuir 1998, 14, 3716-3719. (25) Soga, I.; Granick, S. Colloids Surf., A 2000, 170, 113-117. (26) Zhang, B.; Blum, F. D. Macromolecules 2003, 36, 8522-8527. (27) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.

LA053098+

Conclusions Using MDSC, we have been able to identify thermal transitions that can be identified with tightly and loosely bound PMMA adsorbed on silica. The relative intensities of the transitions were used to estimate both the amount of tightly bound polymer and the ratio of the ∆Cp for both components. The amount of tightly bound polymer corresponds to a thickness of about 1.1 nm. Assuming that the value of ∆Cp for the loosely bound polymer is similar to that of the bulk polymer, the ∆Cp for the tightly bound polymer is about 6 times lower (0.047 J/(g‚°C) than that for the bulk. This lower value is consistent with a reduced mobility of the tightly bound polymer above its glass transition. These studies are consistent with other dynamics studies in terms of the increases and breadth of the glass transition in adsorbed polymers hydrogen bonded to oxide surfaces. Acknowledgment. The authors acknowledge the support of the National Science Foundation under DMR 0412340. The authors also thank Rakesh Nambiar for his help in measuring the ∆Cp for the bulk polymer.

(28) Sakai, H.; Imamura, Y. Bull. Chem. Soc. Jpn. 1980, 53, 1749-1750. (29) Barnett, K. G.; Cosgrove, T.; Vincent, B.; Cohen-Stuart, M.; Sissons, D. S. Macromolecules 1981, 14, 1018-1020.