Langmuir 2005, 21, 1143-1148
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Monitoring Surface Thermal Transitions of ABA Triblock Copolymers with Crystalline Segments Using Phase Contrast Tapping Mode Atomic Force Microscopy Wei Wu, Krzysztof Matyjaszewski, and Tomasz Kowalewski* Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213 Received June 22, 2004. In Final Form: October 25, 2004 Variable-temperature tapping mode atomic force microscopy was used to follow thermal transitions in nanoscale phase separated triblock copolymers containing partially crystalline poly(octadecyl methacrylate) or poly(docosyl methacrylate) and glassy (poly(tert-butyl acrylate)) segments. Melting/crystallization and devitrification/vitrification transitions in phase separated domains were followed with the aid of “phase shift thermograms” constructed from cantilever phase shift maps acquired at different temperatures. This type of analysis turned out to be particularly useful in following melting/crystallization and devitrification/ vitrification transitions occurring in the same temperature range and thus difficult or impossible to resolve using differential scanning calorimetry.
1. Introduction Heterogeneous nanopatterned surfaces are receiving considerable attention since they can exhibit interesting wettability,1 friction,2,3 or biocompatibility,4,5 possibly tunable in response to external stimuli.6,7 They have also been used as templates for creating nanoparticle arrays8-10 or surface modifiers to control fouling release.11,12 One especially attractive pathway to creating such surfaces is through the use of self-assembly in block copolymers containing incompatible segments.13 Among such systems, of particular interest are block copolymers containing crystallizable blocks, in which the final nanostructure is the result of the interplay between microphase separation, vitrification, and crystallization. This interplay may lead to unusual morphologies and complex hysteresis effects, which could be potentially employed in designing “smart” materials exhibiting complex stimulus responses.14,15 Variable-temperature atomic force microscopy (AFM) (hot stage AFM) has been established in recent years as a powerful tool to visualize the melting/crystallization of semicrystalline polymers and the devitrification/vitrification of glassy materials.16-20 So far, such studies have (1) Grundke, K.; Pospiech, D.; Kollig, W.; Simon, F.; Janke, A. Colloid. Polym. Sci. 2001, 279, 727-735. (2) Inoue, T.; Moritani, M.; Hashimot, T.; Kawai, H. Macromolecules 1971, 4, 500-507. (3) Iyer, K. S.; Luzinov, I. Langmuir 2003, 19, 118-124. (4) Li, L. M.; Beniash, E.; Zubarev, E. R.; Xiang, W. H.; Rabatic, B. M.; Zhang, G. Z.; Stupp, S. I. Nat. Mater. 2003, 2, 689-694. (5) Klok, H. A.; Hwang, J. J.; Hartgerink, J. D.; Stupp, S. I. Macromolecules 2002, 35, 6101-6111. (6) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (7) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (8) Hamley, I. W. Nanotechnology 2003, 14, R39-R54. (9) Ansari, I. A.; Hamley, I. W. J. Mater. Chem. 2003, 13, 24122413. (10) Roescher, A.; Moller, M. Nanotechnology 1996, 622, 116-127. (11) Emoto, K.; Nagasaki, Y.; Iijima, M.; Kato, M.; Kataoka, K. Colloids Surf., B 2000, 18, 337-346. (12) Sharma, S.; Johnson, R. W.; Desai, T. A. Appl. Surf. Sci. 2003, 206, 218-229. (13) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32-38. (14) Lendlein, A.; Schmidt, A. M.; Langer, R. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 842-847. (15) Osada, Y.; Matsuda, A. Nature 1995, 376, 219-219. (16) Schonherr, H.; Bailey, L. E.; Frank, C. W. Langmuir 2002, 18, 490-498.
been based on observation of changes of surface morphology. Herein we present results which demonstrate that thermal transitions can be studied in a spatially resolved fashion in the absence of immediately recognizable changes in morphology. Understanding the relationship between surface morphology and physical properties of such systems could greatly benefit from the availability of techniques capable of simultaneously characterizing both aspects. The presented simple variable-temperature AFM method accomplishes this task by providing spatially resolved information about thermal transitions in nanoscale phase separated domains in block copolymers.21 With tapping mode atomic force microscopy (TMAFM), particularly useful information can be obtained through monitoring the cantilever phase shift (“phase contrast imaging”), which is sensitive to energy dissipation, in particular through mechanical interaction between the tip and the sample.22-29 This study demonstrates how quantitative analysis of phase shift maps acquired at different temperatures can be used to construct “phase shift thermograms” useful in the spatially resolved analysis of thermal transitions resulting in variation of the viscoelastic properties of a material. Interestingly, the thermal transition temperatures determined by TMAFM in this study coincided very well with the values determined by differential scanning calorimetry (DSC). (17) Godovsky, Y. K.; Papkov, V. S.; Magonov, S. N. Macromolecules 2001, 34, 976-990. (18) Dinelli, F.; Buenviaje, C.; Overney, R. M. J. Chem. Phys. 2000, 113, 2043-2048. (19) Pearce, R.; Vancso, G. J. Macromolecules 1997, 30, 5843-5848. (20) Tsui, O. K. C.; Wang, X. P.; Ho, J. Y. L.; Ng, T. K.; Xiao, X. D. Macromolecules 2000, 33, 4198-4204. (21) Reiter, G. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 18691877. (22) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H. J.; Whangbo, M. H. Langmuir 1997, 13, 3807-3812. (23) Brandsch, R.; Bar, G.; Whangbo, M. H. Langmuir 1997, 13, 63496353. (24) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1997, 71, 2394-2396. (25) Bar, G.; Brandsch, R.; Whangbo, M. H. Langmuir 1998, 14, 73437348. (26) Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B. Appl. Phys. Lett. 1998, 72, 2613-2615. (27) Garcia, R.; Tamayo, J.; Calleja, M.; Garcia, F. Appl. Phys. A 1998, 66, S309-S312. (28) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1998, 73, 2926-2928. (29) Balantekin, M.; Atalar, A. Phys. Rev. B 2003, 67, 193404.
10.1021/la048460j CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005
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Langmuir, Vol. 21, No. 4, 2005 Chart 1. pODMA-ptBA-pODMA
As shown in Chart 1, the crystallizable A blocks in the ABA triblock copolymers studied here comprised poly(octadecyl methacrylate) (pODMA, Tm ) 30 °C) or poly(docosyl methacrylate) (pDSMA, Tm ) 55 °C), which are examples of main chain atactic polymers with crystallizable side chains; the amorphous B blocks comprised poly(tert-butyl acrylate) (ptBA, Tg ∼ 45 °C). Crystallization of the A blocks was subjected to boundary conditions imposed by phase separated domains; in addition, depending on the system, the boundary walls were in a glassy or molten state. Confined crystallization and melting in main chain21,30-38 and side chain39-42 crystallizable polymers have already received considerable attention in the literature. The work described here demonstrates the usefulness of TMAFM phase shift thermograms in simultaneously tracking the morphology and phase transitions in such systems. 2. Experimental Section Materials and Sample Preparation. Triblock copolymer pODMA-ptBA-pODMA (OBO) was synthesized through atom transfer radical polymerization (ATRP)43,44 as described elsewhere.45 pDSMA-ptBA-pDSMA (DBD) was synthesized in a similar way. Difunctional bromo-terminated macroinitiator Br-p(tert-butyl acrylate)-Br was first synthesized through ATRP at 70 °C using CuBr/PMDETA46,47 as a catalyst and DMDBH as an initiator, followed by the chain extension of difunctional ptBA macroinitiator with pODMA or pDSMA blocks using halogen exchange with CuCl/dNbpy as a catalyst48 leading (30) Dorenbos, G.; Sommer, J. U.; Reiter, G. J. Chem. Phys. 2003, 118, 784-791. (31) Opitz, R.; Lambreva, D. M.; de Jeu, W. H. Macromolecules 2002, 35, 6930-6936. (32) Hamley, I. W.; Fairclough, J. P. A.; Bates, F. S.; Ryan, A. J. Polymer 1998, 39, 1429-1437. (33) Ryan, A. J.; Fairclough, J. P. A.; Hamley, I. W.; Mai, S. M.; Booth, C. Macromolecules 1997, 30, 1723-1727. (34) Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J.; Bates, F. S.; Towns-Andrews, E. Polymer 1996, 37, 4425-4429. (35) Hamley, I. W.; Fairclough, J. P. A.; Terrill, N. J.; Ryan, A. J.; Lipic, P. M.; Bates, F. S.; Towns-Andrews, E. Macromolecules 1996, 29, 8835-8843. (36) Rangarajan, P.; Register, R. A.; Adamson, D. H.; Fetters, L. J.; Bras, W.; Naylor, S.; Ryan, A. J. Macromolecules 1995, 28, 1422-1428. (37) Loo, Y. L.; Register, R. A.; Ryan, A. J. Phys. Rev. Lett. 2000, 84, 4120-4123. (38) Loo, Y. L.; Register, R. A.; Ryan, A. J.; Dee, G. T. Macromolecules 2001, 34, 8968-8977. (39) Moment, A.; Miranda, R.; Hammond, P. T. Macromol. Rapid. Commun. 1998, 19, 573-579. (40) Zheng, W. Y.; Hammond, P. T. Macromolecules 1998, 31, 711721. (41) Hammond, P. T.; Rubner, M. F. Macromolecules 1995, 28, 795805. (42) Alig, I.; Jarek, M.; Hellmann, G. P. Macromolecules 1998, 31, 2245-2251. (43) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (44) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-5615. (45) Qin, S. H.; Saget, J.; Pyun, J. R.; Jia, S. J.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2003, 36, 8969-8977. (46) Xia, J.; Matyjaszewski, K. Macromolecules 1997, 30, 7697-7700. (47) Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 40394047.
Letters to the final block copolymers, with molecular weights Mn ) 75 900 (OBO) and Mn ) 74 000 (DBD). In both cases, the polymerization was well controlled as indicated by low polydispersity indices of the final products (Mw/Mn ) 1.14 for OBO and 1.29 for DBD). (See Supporting Information for additional materials information and GPC data.) Thin film (∼1 µm) samples for AFM characterization were prepared by drop-casting from 1.0 mg/mL solutions in chloroform onto precleaned silicon wafers with a native oxide layer, followed by overnight vacuum-drying at room temperature. Atomic Force Microscopy. TMAFM experiments were carried out with the aid of a Multimode Nanoscope III system (Digital Instruments, Santa Barbara, CA) equipped with an Extender Module, vertical engage J-scanner, and hot stage. Measurements were performed under dry N2 atmosphere using commercial Si cantilevers (MPP-11100, NanoDevices, Santa Barbara, CA) with a nominal spring constant and resonance frequency, respectively, equal to 40 N/m and 300 kHz and a nominal tip radius below 10 nm as specified by the manufacturer. All analyzed images were acquired at 2 Hz scan rate and at set-point ratio A/A0 ) 0.7 with A0 ) 1.0 V, where A0 and A denote respectively the “free” and “tapping” cantilever amplitude. For consistency of further analysis, in all cases the cantilever was oscillated at the frequency at which its free amplitude was equal to 70% of its amplitude on resonance (this required retuning the cantilever at each temperature). For the same reason, the cantilever free oscillation phase shift was zeroed each time before taking any images. All the AFM samples were subjected to at least 3 heating/cooling cycles before the final phase images were acquired. All phase shift values ∆φ reported in this paper were calculated from the measured values ∆φm using the equation given by manufacturer: ∆φ ) arcsin(2 ∆φm/π). AFM images were displayed and analyzed quantitatively using custom procedures written in MATLAB 6.5 (Mathworks, Inc., Natick, MA). Differential Scanning Calorimetry. The DSC experiments were performed with ∼5 mg samples placed in sealed Al pans, using a Seiko SSC/5200 instrument operated under N2 purge (30 mL/min) and cycled at 10 °C/min between -50 and 100 °C. To minimize the effects of previous thermal and rheological history, data from the third heating/cooling cycle were used in the analysis.
3. Results and Discussion DSC traces recorded for both copolymers are shown in the Supporting Information. For OBO, only the melting of pODMA at 30.1 °C and no glass transition of ptBA was observed. For DBD, both the Tg of the tBA block and the Tm of the pDSMA block were observed at 42 and 58 °C, respectively. In both cases, the height and phase contrast TMAFM images of thin films of copolymers prepared by solvent casting on silicon wafers revealed the characteristic meandering morphology consistent with phase separation between immiscible blocks (Figure 1, cf. Supporting Information for phase images of DBD). Areas of lower height corresponded to darker areas in phase shift images, which in turn indicated regions of stronger (more negative) phase shift of the cantilever with respect to the drive. In the case of the OBO copolymer containing 44.0 wt % of pODMA, the area of the image corresponding to domains exhibiting the stronger phase shift was close to the area of regions of less pronounced phase shift, thus preventing domain assignment based on their surface fraction. In the case of DBD, containing 21.6% of pDSMA, stronger phase shift areas accounted for ∼25% of the image surface, suggesting that they corresponded to the minority phase (pDSMA). Variable-temperature experiments described later confirmed this assignment. Phase contrast plays an important role in AFM imaging of polymers, since it allows one to visualize regions of materials characterized by different mechanical and (48) Matyjaszewski, K.; Shipp, D. A.; Wang, J.-L.; Grimaud, T.; Patten, T. E. Macromolecules 1998, 31, 6836-6840.
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Figure 1. TMAFM phase images of OBO thin film acquired at different temperatures. All the images are shown in a common gray scale ranging from -20° to 5°.
surface properties. Analysis of dynamics of a tapping cantilever indicates that for a given set-point ratio s ) A/A0, the phase shift primarily depends on the amount of energy dissipated through tip-sample interactions per oscillation cycle.24,25 According to the approximate relationship derived by Cleveland et al.,26 in the steady state, when the average energy supplied to the cantilever per period is equal to the average energies dissipated via hydrodynamic viscous interaction with the environment and by inelastic interactions at the tip-sample interface,
sin φ )
QEdis ω A + ω0 A0 πkA0A
where φ is the phase angle, ω and ω0 are respectively the excitation and natural frequencies of the cantilever, A is the tapping (set-point) amplitude, Q is the quality factor, k is the cantilever spring constant, and Edis represents the amount of energy dissipated per cycle. The dependence of phase shift on energy dissipation opens the way to the use of TMAFM as a tool to study thermal transitions associated with major mechanical relaxation transitions in polymers (melting, devitrification).24 One of the ways to accomplish this is to analyze AFM images acquired over a range of temperatures around the transition temperature. As shown below, a particularly convenient mode of such analysis involves the construction
of phase shift thermograms, in which a succession of normalized phase shift histograms of images acquired at consecutive temperatures is presented in the form of a two-dimensional color map, with temperature as the abscissa, phase shift as the ordinate, and the color corresponding to the fraction of image pixels with phases within (φi, φi + ∆φi). Phase shift thermograms for both copolymers under study constructed according to this procedure are shown in Figure 2 (OBO, heating) and in Figure 3 (DBD heating (a) and cooling (b)). In all cases, phase shift histograms of images acquired at room temperature were unimodal with asymmetric tails extending toward more pronounced (more negative) phase shifts. In phase shift thermograms, this was reflected as a relatively narrow band of intensity centered around the phase shift representing the histogram mode. In the case of OBO, images acquired just a few degrees above room temperature (25 °C) produced a broad, distinctly bimodal phase shift histogram (Figure 2). Traces 1 and 2 map the positions of modes corresponding respectively to less pronounced and more pronounced phase shifts. After the first temperature increment, trace 2 shifted abruptly by about 8° toward more negative values. Upon further increase of temperature, the position of trace 1 initially remained relatively unchanged, whereas trace 2 shifted further by about 7° and then remained relatively unchanged. Trace 1 started to move its position toward more
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Figure 2. 2D Phase shift thermogram of OBO in the heating cycle. The inset illustrates how the thermogram was constructed by arraying the histograms of consecutive phase images acquired at different temperatures.
pronounced phase shifts in images acquired at 45 °C and above. In contrast with the abrupt drop of trace 2, the position of trace 1 moved gradually by about 1° per temperature step (5 °C). More pronounced (more negative) values of cantilever phase shift indicate more extensive energy dissipation in tip-sample interactions. Thus, displacement of the phase shift distribution mode toward more negative values indicates that material undergoes a transition accompanied by an increase in energy dissipation. For viscoelastic materials such as polymers, the most straightforward mechanism of energy dissipation is through the viscous component of tip-induced sample deformation. Significant contributions of topography to phase images of our block copolymers were ruled out on the basis of the absence of changes in phase images upon reversal of scanning direction (for more extensive discussion of “topographic crosstalk”, see Supporting Information). Thus, any increase in the magnitude of phase shift with the increase of temperature may be an indication of the onset of viscous damping associated with thermal transitions such as melting and/or devitrification (glass transition). Indeed, the broad downturn in trace 1 coincides well with the glass transition of ptBA observed by DSC for ptBA macroinitiator (see Supporting Information). Accordingly, the abrupt downturn in trace 2 correlates well with the melting transition of pODMA (cf. Supporting Information). Thus, traces 1 and 2 can be used to follow devitrification and melting of phase separated domains of the copolymer, illustrating the usefulness of TMAFM phase shift thermograms to monitor thermal transitions in a spatially resolved fashion. Interestingly, due to the proximity of Tm of pODMA and Tg of ptBA these two transitions could not be resolved by DSC, which monitors the global thermal response of the sample, which in this case is dominated by the melting transition. Phase shift thermograms of DBD constructed from images acquired upon heating and cooling (Figure 3a,b, respectively) resembled those obtained for OBO, showing
two traces corresponding to modes of phase shift histograms. Trace 1, exhibiting less negative phase shift, showed more gradual downturn between 35 and 45 °C, which can be associated with the glass transition of ptBA. Trace 2, corresponding to more negative phase shift, exhibited a characteristic abrupt downturn between 45 and 55 °C which can be related to melting of pDSMA. In addition, upon heating, trace 1 exhibited an additional minimum at 75 °C, which was not observed upon cooling. This feature does not appear to be a mere irregularity, e.g., due to incorrect phase shift zeroing, etc. Importantly, it had been reproduced in another set of experiments (see Supporting Information). Closer inspection of AFM images acquired at this temperature in a heating cycle revealed that meandering elongated domains of pDSMA started to lose continuity and were partially fragmented, as shown in Figure 4a,b. This behavior is not surprising, given the fact that in the DBD system the docosyl block constitutes a distinctly minor fraction (20 wt %). Typical phase diagrams of block copolymers show multiple phase boundaries in this composition range, and the fragmentation of elongated docosyl domains could be an indication that the system crossed the boundary between continuous (e.g. cylindrical) and dispersed (e.g. spherical) phases with the increasing temperature. Thus, the observed enhanced energy dissipation in the ptBA phase in the temperature range corresponding to the fragmentation of the pDSMA phase could be explained as a result of increased energy loss due to coupling of the tip-induced deformation of the ptBA matrix with the displacement of the newly mobile fragments of pDSMA domains being “freed up” in the process of their transition to a more dispersed state. One of the ways of lending more evidence to this assertion would be to “freeze in” pDSMA domains in their putative fragmented state. However, experiments with samples quenched from 75 to 50 °C did not produce an expected result, leaving this issue unresolved. The phase shift thermogram recorded upon cooling of DBD (Figure 3b) showed an almost exactly similar, but
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Figure 4. TMAFM phase images of DBD acquired at 65 °C (a) and 70 °C (b). Note the apparent fragmentation of DBD domains at 70 °C. The scale bar in the images is 200 nm. Figure 3. 2D Phase shift thermogram of DBD constructed from phase images acquired at sequentially increasing (a) and decreasing (b) temperatures.
reverse, transition. However, two major differences were observed in the cooling cycle: (a) the absence of the additional phase shift due to the fragmentation of pDSMA domains and (b) the upturn of trace 2 due to the crystallization of pDSMA domains did not occur until the sample was supercooled to 45 °C. The most likely reason for the supercooling effect was the highly dispersed nature of DSMA domains, which made crystallization reliant on homogeneous nucleation, as observed in other block copolymer systems34,37 and polymer blends.50 At room temperature, after crystallization of pDSMA blocks, trace 1 and trace 2 eventually merged, indicating the absence of pronounced difference in energy dissipation in tipsample interactions with domains corresponding to different blocks. To obtain insights into the extent to which thermal transitions in block copolymers were affected by their nanoscale phase structure, we have carried out a (49) Stark, M.; Moller, C.; Muller, D. J.; Guckenberger, R. Biophys. J. 2001, 80, 3009-3018. (50) Kowalewski, T.; Ragosta, G.; Martuscelli, E.; Galeski, A. J. Appl. Polym. Sci. 1997, 66, 2047-2057.
Table 1. Samples Used for Phase Shift Imaging
sample
DP of DP of tBA ODMA
pODMA-ptBA-pODMA 331 pDSMA-ptBA-pDSMA 446
103 43
Mn
wt % of crystalline Tm PDI segments (°C)
75 900 1.14 74 000 1.29
44.0 21.6
31.6 57.2
series of control experiments with relevant homopolymers (for a detailed description see Supporting Information). In all cases, AFM experiments revealed the presence of analogous transitions (devitrification/vitrification of ptBA, melting/crystallization of pODMA and pDSMA). Notably, thermal transitions in homopolymers occurred within a few degrees from their counterparts in copolymers. This result indicates that in the systems that are the subject of this study, the phase structure of block copolymers did not significantly affect their thermal transitions in the immediate vicinity of the surface. 4. Conclusions The presented analysis of TMAFM phase shift images acquired at varied temperatures provides a basis for the use of phase shift thermograms to follow thermal transitions in (sub)surface regions of thin films of microphase-
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separated block copolymers. TMAFM phase shift thermograms may be particularly useful to spatially resolve thermal transitions occurring at similar temperatures and thus difficult or impossible to monitor by “bulk” techniques, such as differential scanning calorimetry. The use of phase shift thermograms, constructed in this way, does not have to be limited to monitoring the thermal transitions on surfaces of polymer thin films and could be extended to discrete nano-objects with complex structures. Acknowledgment. This work has been supported by the National Science Foundation (DMR 0210247, DMR
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0090409). W.W. thanks Dr. Shijun Jia for his kind help with AFM experiments and Jinyu Huang and Chuanbing Tang for their help with the synthesis of copolymers. Comments from Professor Guy C. Berry are also greatly appreciated. Supporting Information Available: Materials synthesis and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. LA048460J
