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Morphology Characterization in Multicomponent Macromolecular Systems Using Scanning Probe Phase Microscopy Ravi Viswanathan and Jing Tian Raychem Corporation, 300 Constitution Drive, Menlo Park, California 94025
David W. M. Marr* Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, Golden, Colorado 80401 Received August 7, 1996. In Final Form: January 20, 1997X A novel scanning probe microscopy, phase imaging, has been used to image blends of polyethylene (PE) and poly(dimethylsiloxane) (PDMS). By providing not only a topographical map of the surface but also information on material properties, this technique has allowed us to readily distinguish the individual components within the blend. We further illustrate its utility by investigating the role of interfacial modifier on the behavior of this incompatible blend system. With addition of a PDMS/PE copolymer, phase imaging readily shows greater interfacial widths.
Introduction Due to the high cost of synthesizing new and specifically designed polymers, there has been a great deal of recent interest in the blending of polymers for various material applications. Physical properties of the resulting blends depend heavily on the resulting morphology on submicrometer length scales, sizes which can be difficult to characterize by conventional microscopies. The need to cryofracture and microtome a sample often can be extremely difficult and can induce artifacts when examined. The ability to rapidly image a material at such magnifications without extensive sample preparation would be of great utility and is leading to the broad use of novel techniques such as atomic force microscopy. Over the last 10 years since its development, the field of atomic force microscopy (AFM) has undergone significant change. Initially touted as a tool to resolve atomic and molecular species across a surface,1,2 AFM/STM (scanning tunneling microscopy) has been incorporated into the new field of scanning probe microscopy. Now, the advantages of high-resolution topographic imaging through AFM/STM can be supplemented by examining other interactions. Material properties of systems such as frictional forces,3 electrostatic4,5 and magnetic forces,6 and moduli of elasticity7,8 can now be mapped. Despite its recent successes, criticisms of AFM remain and typically center around interpretation of data and artifacts that can (and do) result from imaging. From a topographic map, it is sometimes difficult to definitively state the location and composition of features. As such, * To whom correspondence should be addressed: dmarr@ mines.edu. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (2) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (3) Ruan, J. A.; Bhushan, B. Trans. ASME, J. Tribol. 1994, 116, 378. (4) Huber, C. A.; Huber, T. E.; Sadoqi, M.; Lubin, J. A.; Manalis, S.; Prater, C. B. Science 1994, 263, 800. (5) Viswanathan, R.; Heaney, M. B. Phys. Rev. Lett. 1996, 75, 4433. (6) Grutter, P. MSA Bull. 1994, 24, 416. (7) Maivald, P.; Butt, H. J.; Gould, S. A. C.; Prater, C. B.; Drake, B.; Gurley, J. A.; Elings, V. B.; Hansma, P. K. Nanotechnology 1991, 2, 103. (8) Radmacher, M.; Tillmann, R. W.; Gaub, H. W. Biophys. J. 1993, 64, 735.
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analysis of multicomponent and blend systems can be problematic. Lateral force microscopy (LFM) and force modulation microscopy (FMM) can give friction and compliance information; however, to gain useful information from these methods, the components must have different frictional or compliance characteristics. Also, because the technique relies on the response of the tip as it impinges upon the sample, force modulation can be harmful to the sample.9 To overcome many of these difficulties, we have used a novel facile and nondestructive scanning probe microscopy, phase imaging, to distinguish components in a polymer blend system. The technique is an extension of the Tapping mode AFM (TMAFM) where the cantilever is excited into resonance oscillation with a piezoelectric driver and the oscillation amplitude used as a feedback signal to measure topographic variations of the sample. In phase imaging, the phase lag of the cantilever oscillation relative to the signal sent to the cantilever’s piezo driver is simultaneously monitored and recorded. This lag is very sensitive to variations in material properties such as adhesion and viscoelasticity. Therefore, unlike LFM or FMM, differences in a number of material properties can give rise to phase image contrast. In addition, because phase imaging highlights edges and is unaffected by largescale height differences, it provides for clearer observation of fine features.10 Its ease of operation and the fact that a tapping mode image is simultaneously produced makes phase imaging an ideal tool for examining the blend systems and interfacial issues presented in this study. As a starting point for our investigations of the usefulness of this technique, we have begun with an incompatible blend of polyethylene (PE) and poly(dimethylsiloxane) (PDMS). To tune both the domain size and interfacial properties in the system, a block copolymer of PE and PDMS was synthesized and utilized as a compatibilizer in the PE/PDMS blend. Varying the system structure in such a fashion will provide a test of the phase imaging capabilities. (9) Application NotessLateral and Chemical Force Microscopy, Force Modulation Imaging; Digital Instruments: Santa Barbara, CA. (10) Babcock, K. L.; Prater, C. B. Application NotessPhase ImagingBeyond Topography; Digital Instruments: Santa Barbara, CA.
© 1997 American Chemical Society
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Table 1. Polymer Properties polyethylene poly(dimethylsiloxane)
Mw
Mw/Mn
Tm (°C)
69 000 185 600
2.0 1.4
140
Experimental Section Blends of polyethylene and poly(dimethylsiloxane) (see Table 1) were prepared in a Brabender Plasti-Corder blender at a temperature 30 °C above the polyethylene melting point. The polymer mixture was mixed at 50 rpm for 5 min in a 30 cm3 bowl. Blends including compatibilizer were prepared in a similar fashion: the desired amount of PE was mixed first with the compatibilizer for 2 min followed by the addition of PDMS, and then the mixture blended for an additional 5 min. The blends unloaded from the mixer were compression molded into 1mm thick sheets at 160 °C in a hydraulic press and cooled in a cold press to room temperature. A commercial Digital Instruments Nanoscope Multimode AFM with a Phase Extender Box was used for imaging the blends in Tapping and Phase modes. The films were imaged in air at ambient conditions. “Height” mode (tip-to-sample distance is adjusted by feedback to keep the force constant) was used for imaging with microfabricated single-crystal silicon tips (5 nm radius of curvature). A 128 µm × 128 µm scanner was used for imaging.
Results and Discussion Figure 1a shows a 20 µm TMAFM image of a blend of polyethylene/PDMS (90/10 weight ratio). The image quality is not ideal though the lighter circular-shaped areas do appear to be the dispersed PDMS phase. To convince ourselves that the small domains are indeed PDMS, we imaged another blend at a PE/PDMS ratio of 60/40. As expected, the dispersed particles are much larger in size due to the increased amount of PDMS in the 60/40 sample. The contrast in the TMAFM image (Figure 1a) arises from topographic variations on the surface of the sample, as the tip “sees” small, circular-shaped domains that are raised relative to the surrounding area. A significant amount of information however is revealed when examining the corresponding phase image, shown in Figure 1b. The image quality is far superior to that obtained through TMAFM, as SPM is extremely sensitive to phase lags caused by differences in material properties.
For this particular system, the material properties having the greatest influence in the phase image contrast are primarily mechanical properties. The high crystallinity of PE compared with the fluid-like structure of PDMS gives rise to an enormous difference in moduli of elasticity between the components (1 × 106 psi for high-density PE and 50 psi for PDMS). Hence, the phase lag produced from a tip traversing these two surfaces gives rise to the significant amount of contrast as viewed in Figure 1b. Also, the free volume, and hence the mobility of the PDMS, is substantially higher than that for PE. This mobility difference between the two components will also contribute to the phase image contrast. The darker regions in the phase image correspond to the dispersed PDMS domains, labeled as A in the image, with the surrounding lighter regions representing the PE matrix. Though the phase image confirms many of the features observed in the TMAFM image, the utility of the technique can be verified by several subtle features highlighted in the phase image. First, the phase image is able to resolve smaller-sized PDMS domains than appear in the topographic image. Also, some of the raised (lighter) features, labeled as B in the TM image, initially thought to be PDMS regions, actually are shown to be clumps of PE through phase imaging. Hence, phase imaging can be a powerful complement to normal tapping mode AFM, and the two techniques can simultaneously map out the morphology and component behavior of blends and/or composite systems. Having demonstrated the utility of TMAFM/phase imaging to study polymer blend morphology, we introduced a third component, a compatibilizer, to observe its effects on polymer blend morphology. This compatibilizer, a copolymer of PE and PDMS, migrates to the interface during processing, mitigates interfacial energetics, and results in lower domain size and different interfacial structure. Parts a and b of Figure 2 are TMAFM and phase images, respectively, of a 20 µm area of a blend of PE/PDMS/compatibilizer with the PE/PDMS ratio kept constant at 90/10. Similar to the without compatibilizer case (Figures 1), the TM image is difficult to interpret as the image quality is not ideal. The phase image once again
Figure 1. (a) 20 µm × 20 µm TMAFM image of 90/10 blend of PE/PDMS. Lighter (higher) circular-shaped areas appear to be dispersed PDMS phase. (b) Corresponding phase image, showing much sharper detail. Darker regions are PDMS phases and lighter regions are PE regions. Note that some of the lighter, raised regions in the TM image, thought to be PDMS, are actually PE as revealed by the phase image.
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Figure 2. (a) 20 µm × 20 µm TMAFM image of 90/10 blend of PE/PDMS with compatibilizer added. (b) Corresponding phase image showing sharper features and enhanced edge effects.
Figure 3. (a) Identical image shown as Figure 2b. (b) Random line scan of image shown as 3a showing relative phase lag jumps in traversing from one component to another. The phase units (y-axis) are only a relative measure and cannot be analyzed absolutely. (c) Identical image shown as Figure 1b. (d) Random line scan of image shown as 1a. Note the sharper transitions and deeper wells, both caused by the absence of compatibilizer. The line scans cross feature edges as well as the features themselves.
clearly reveals features corresponding to PE (the lighter areas comprising the matrix) and the PDMS. The effect of the compatibilizer can also be quantified by comparing one-dimensional line scans taken across phase images of the with and without compatibilizer systems. The sharpness of the transitions in traversing from one component to another can be compared, as this transition should be lessened through addition of the
compatibilizer. Figure 3 shows phase images and line scans of the systems with and without compatibilizer. Figure 3a is the identical image shown as Figure 2b; Figure 3b is a random horizontal line scan taken through the image. The phase units shown on the y-axis cannot be interpreted absolutely as they are not tied to any specific material property or effect. The line scans can only be compared relatively. Parts c and d of Figure 3 are phase
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transition (interfacial) width via SPM doubles in size when compatibilizer is added (see Figure 4). The line scans from the two systems also show that the transition heights measured between phases are not as great with the addition of the compatibilizer. Though the exact cause of this is unclear, it does indicate that the physical properties of the two phases have become more similar with the addition of compatibilizer. One expects a certain miscibility of the copolymer in each phase which could certainly lead to this. Though it is clear from this that the properties are different in each case, it does emphasize the current nonquantitative nature of this technique. Summary
Figure 4. Autocorrelations of the “phase unit” line scans of Figure 3b and Figure 3d indicating broader transitions in the blend with added compatibilizer. Fitting these to simple exponentials of the form y ) const + (1.0 - const) exp(-x/width) provides interfacial widths of 1.8 ( 0.5 and 0.51 ( 0.04 µm, indicating a significant increase in interfacial width with the addition of a compatibilizing agent.
image and line scans of the system without compatibilizer (shown as Figure 1b) where it can be seen that the line scan of the without-compatibilizer case shows abrupt transitions between PE and PDMS and deep wells in going from PE to PDMS. Figure 3b, which represents the withcompatibilizer scenario, shows distinct differences, in particular, a more gradual transition between phases. By fitting the decay of the autocorrelation function of these line scans to a single exponential, one can estimate the
This study establishes the utility of SPM techniques in examining complicated macromolecular systems. Using phase imaging together with tapping mode, one can now readily establish the surface morphology of multicomponent systems. We have examined blends of PE/PDMS and quantified the effects of adding an interfacial modifier. From SPM data, it is clear that the compatibilizer is migrating to the interface between the immiscible PE and PDMS. In addition, by comparing line scans of images taken from with- and without-compatibilizer systems, one can see the interfacial width has clearly increased upon addition of copolymer. Acknowledgment. The authors wish to acknowledge Clayton Henderson for useful discussions. LA9607823
