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Notes Atomic Force Microscopy Visualization of Morphology and Nanostructure of an Ultrathin Layer of Polyethylene during Melting and Crystallization Yu. K. Godovsky† and S. N. Magonov*,‡ Karpov Institute of Physical Chemistry, 10 Vorontsovo Pole, Str., Moscow 103064, Russia, and Digital Instruments/ Veeco Metrology Group, 112 Robin Hill Road, Santa Barbara, California 93110 Received June 29, 1999. In Final Form: December 20, 1999
Introduction Ultrathin (less than 100 nm in thickness) polymer layers are of increasing scientific and practical interest, because of their potential applications in nanotechnology.1,2 In the extensive literature on polymer crystallization,3-9 there are only a few papers concerning crystallization of polymers in constrained environments, in general, and in the ultrathin polymer layers, in particular.10,11 The crystalline structure of poly(di-n-hexylsilane) in ultrathin films and the kinetics of crystallization were examined with UV and FTIR. A critical thickness of 15 nm, which is close to the typical lamellar thickness of crystalline polymers, is needed for crystallization.10 The crystallization rate is initially slow but increases rapidly as the thickness of the film approaches 50 nm. Atomic force microscopy (AFM) is extremely useful for studies of polymer surfaces,12,13 including ultrathin polymer films, † ‡
Karpov Institute of Physical Chemistry. Digital Instruments/Veeco Metrology Group.
(1) Swalen, J. D.; Allara, D. I.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelashvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Ulman, A. An Introduction to Ultrathin Organic films. From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (3) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1973; Vol. 1; 1976; Vol. 2; 1980; Vol. 3. (4) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I. In The Rate of Crystallization of Linear Polymers with Chain Folding; Hanny, N. B., Ed.; Treatise in Solid State Chemistry; Plenum Press: New York, 1976; Vol. 3. (5) Lotz, B.; Witmann, J.-C. In Structure of Polymer Single Crystals. Cahn, C. W., Haasen, P., Kramer, E. J., Thomas, E. L., Eds.; Materials Science and Technology, Structure and Properties of Polymers; VCH Publishers: 1993; Vol. 12, pp 79-150. (6) Barham, P. Crystallization and Morphology of Semicrystalline Polymers. In Cahn, R. W., Haasen, P., Kramer, E. J., Thomas, E. L., Eds.; Materials Science and Technology, Structure and Properties of Polymers; VCH Publishers: 1993; Vol. 12, pp 153-212. (7) Phillips, P. J. Rep. Prog. Phys. 1990, 53, 549. (8) Mandelkern, L. The Crystalline State. In Physical Properties of Polymers; Mark, J. E., Eisenberg, A., Graessley, W. W., Samulski, E. T., Koenig, J. L., Wignall, G. D., Eds.; American Chemical Society: Washington, DC, 1993; pp 145-200. (9) Keller, A.; Hikosaka, M.; Rastogi, S.; Toda, A.; Barham, P. J.; Goldbeck-Wood, G. The size factor in phase transition: its role in polymer crystal formation and wider implications. In Self-Order and Form in Polymeric Materials; Keller, A., Warner, M., Windle, A. H., Eds.; Chapman & Hall: London, 1995; pp 1-15. (10) Despotopoulou, M. M.; Frank, C. W.; Miller, R. D.; Rabolt, J. F. Macromolecules 1996, 29, 5797. (11) Reiter, G.; Sommer, J.-U. Phys. Rev. Lett. 1998, 80, 3771. (12) Magonov, S. N.; Reneker, D. Annu. Rev. Mater. Sci. 1997, 27, 175.
because it provides real-space polymer morphology and nanostructure. In particular, crystallization of ultrathin layers of poly(ethylene oxide) (PEO), which have been deposited by spin-casting on Si wafers, has been examined recently with AFM.11 The layer morphology is defined by fingerlike branched patterns, which result from a competition between the material transport on the surface of the crystalline fingers and a tendency of the material to incorporate into growing crystals. Recent developments in the AFM characterization of polymers involve measurements at different temperatures.14-18 An AFM temperature study of ultrathin films of PEO and -caprolactone (PCL) in the -30 °C to +70 °C temperature range provides a real-space visualization of structural changes during their melting and crystallization, as well as kinetic estimates of the spherulites’ growth.18 To extend this approach, we applied AFM for the first time for structural characterization of ultrathin layers of low-density polyethylene (LDPE) in order to examine structural changes accompanying its melting and crystallization. Experimental Section LDPE (density 0.921 g/cm3, melt flow rate 2.3 g/10 min) is a generous gift of Dr. G. Capaccio (BP Chemicals Ltd.). DSC characterization of this polymer (DSC 30 scanning calorimeter of TA 3000 system with STAR software, Mettler-Toledo) gave Tm ) 112.3 °C, heat of fusion Qf ) 88.5 J/g, and the corresponding degree of crystallinity of about 30%. During cooling (20 °C/min) of the melt, its crystallization started at ca. 95 °C. The ultrathin polyethylene film on a Si substrate (a piece of Si wafer) was prepared by dipping the substrate into a hot (110 °C) PE solution in xylene (0.1% w/w). After 10 min of exposure, the substrate was pulled out of the hot solution with a speed of ca. 1 cm/min. AFM measurements show that the “dip-crystallized” layer of 20 nm in thickness with reproducible morphology can be obtained in such procedures. AFM studies were performed with a Nanoscope IIIa MultiMode scanning probe microscope (Digital Instruments, Santa Barbara, CA). The results were obtained in tapping mode AFM. A verticalengage J-scanner and Si probes (220 µm in length, resonant frequency in the 150-200 kHz range, stiffness of ca. 40 N/m) were applied in all experiments. The driving frequency in tapping mode was chosen at the resonant frequency of the free-oscillating cantilever in the immediate vicinity of the sample surface. For imaging, we have chosen an amplitude of the free-oscillating cantilever, A0, in the 20-30 nm range and a set-point amplitude, Asp, of 0.8-0.9A0. Height and phase images were recorded simultaneously. Surface corrugations are presented in height images, whereas phase images emphasize fine structural details. AFM measurements in the temperature range between room temperature (RT) and 130 °C were performed with a thermal accessory designed for the Nanoscope microscope described elsewhere.19 This accessory includes a ceramic block with an embedded Pt heater, which is positioned underneath a sample (13) Magonov, S. N.; Heaton, M. Am. Lab. 1998, May, 30, 9. (14) Magonov, S. N.; Elings, V.; Papkov, V. S. Polymer 1997, 38, 297. (15) Pearce, R.; Vansco, J. Polymer 1998, 39, 1237. (16) Sikes, H. D.; Schwartz, D. K. Science 1997, 278, 1604. (17) Magonov, S. N.; Godovsky, Y. K. Am. Lab. 1998, 30, 15. (18) Gullerud, S. Ph.D. Thesis, Stanford University, Stanford, CA, 1999. (19) Daniels, B.; et al. Rev. Sci. Instrum. 1999, submitted for publication.
10.1021/la990846k CCC: $19.00 © 2000 American Chemical Society Published on Web 02/10/2000
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Figure 1. (a and b) Height images of the dip-crystallized LDPE layer obtained at room temperature. The contrast covers height variations in the 0-50 nm range (a) and in the 0-30 nm range (b). (c) Phase image of the dip-crystallized LDPE.
Figure 2. Height images of the dip-crystallized LDPE layer as in Figure 1a at different temperatures: (a) after the steplike heating to T ) 100 °C; (b) after further heating to T ) 115 °C; (c) after cooling to T ) 100 °C (immediately after cooling); (d) at T ) 100 °C, 20 min after the cooling; (e) the same conditions as in part d but larger area; (f) after cooling to RT. The images in parts a-d were recorded in the same area. The contrast covers height corrugations in the 0-50 nm range in all images. puck of 0.6 cm in diameter. The sample was fixed on the puck with small traces of glue and with a metallic clamp from above. The metallic clamp contains a thermocouple, which touches the sample surface close to the scanning area and measures the sample temperature. The latter is recorded by an electronic control unit, which is also used for powering the heater. Stability of the sample temperature is ca. (0.1 °C. Heating of the sample was performed with a 5 °C/s rate, which becomes substantially slower when the sample temperature is 5 °C below the target temperature. For tapping mode imaging at elevated temperatures, an additional heater was applied to heat the AFM probe close to the sample temperature. This helps to prevent the condensation of moisture on the backside of the cantilever and thus to avoid instabilities of the cantilever resonant frequency.19 Changes of the cantilever’s temperature lead to the shift of the resonant frequency of the Si cantilever; therefore, retuning of the driving frequency of tapping is needed. In our study of LDPE melting and crystallization, we have obtained AFM images in two different heating procedures. In steplike heating, the sample was heated from RT in 5 °C increments, and the images were recorded at each temperature until the melting point. In the T-jump procedure, the sample was quickly heated from RT to a preselected temperature with a 5 °C/s rate, and then images were recorded. Crystallization can
be performed differently: in steplike cooling or during rapid quenching. The initial rate of the temperature drop, after the power to the heater is disabled, is ca. 10 °C/s, and it slows as the sample temperature gets within 5 °C of the target temperature. Temperature variations cause an inevitable shift of the probe position with respect to the surface region of interest because of expansion/contraction of different components of the microscope and the sample. Therefore, the vertical position of the sample with respect to a piezoscanner may need to be adjusted in some thermal studies. When temperature increments are small (510 °C), this can be done with a stepper motor. When temperature increments are larger, a complete withdrawal of the tip from the surface is needed until the sample temperature is stabilized. As a result of thermal drift, the probe re-engagement might bring the tip 1-2 µm away from the place it was before.
Results and Discussion The initial morphology of the dip-crystallized utrathin PE layer is characterized by “quasi-2D spherulites” (Figure 1a) with sheaflike patterns in the center, which spread as curved fibrils into areas of several microns in diameter. Some dark spots seen in the AFM height images are up to 20 nm in depth, which corresponds to the layer
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Figure 3. (a) Sequence of height images obtained during recrystallization at T ) 90 °C. The contrast covers height variations in the 0-50 nm range. (b) Growth of the lamellar aggregates during recrystallization.
thickness. Higher magnification images (Figure 1b,c) reveal lamellar organization in the spherulites. Most of the lamellae, which are up to 1 µm in length, are seen edge-on with the respect to the substrate, and only a few of them exhibit a flat-lying orientation. The phase image in Figure 1c shows the granular structure of the lamellar edges and the flat-lying surfaces. The grains of the lamellar edges are 10-15 nm in diameter, and those seen on the surface of flat-lying lamellae are 6-8 nm in diameter. The edges of neighboring lamellae are separated from each other by distances of 30 nm and larger. Therefore, the lamellar packing is not tight. The granular structure is similar to that reported in the recent AFM studies of a number of crystalline materials.20,21 The hypothesis that these grains with folded chain interiors and less-ordered exteriors are the primary building units of crystalline architecture is under discussion. Most likely, the architecture of the dip-crystallized layer is a result of the competition between the rate of crystallization and the speed of pulling. Though the metastable character of this morphology can be expected, only AFM provides a direct visualization of how this morphology transforms to a more stable one and how fast these changes occur on the micron and submicron scale. In steplike heating (with 5 °C increments) up to 80 °C, the spherulitic morphology remains unchanged, though structural details became less pronounced than at RT. Drastic morphologic changes were found after heating to a temperature range of 90-100 °C. Figure 2a shows that at 100 °C linear lamellar aggregates of several microns in length and up to 1 µm in width have appeared in several places instead of the initial spherulitic morphology at the same surface area as in Figure 1a. This is a result of polymer recrystallization that is directly visualized with AFM. Upon further heating above the melting point of the bulk sample, the morphology of the whole layer became featureless, indicating complete melting (Figure 2b). Cooling of this sample to 100 °C had induced a partial crystallization, with the lamellar aggregates growing within an amorphous matrix (Figure 2c-e). The image in Figure 2e was taken from a larger area than the previous images. There is no evidence that the smaller area, which was scanned many times during recording the images in Figure 2a-d, is different from other areas. Therefore, the AFM probe interference into the crystallization process (20) Magonov, S. N.; Godovsky, Yu. K. Am. Lab. 1999, 31, 52. (21) Hugel, T.; Strobl, G.; Thomann, R. Acta Polym. 1999, 50, 214.
Figure 4. Height images of the same area of the dip-crystallized LDPE layer before (a, c) and after T-jump to 70 °C (b, d). The images in parts c and d present the part of the area shown in parts a and b. The contrast covers height variations in the 0-30 nm range.
is minimal. Further cooling of this sample to RT induces a crystallization of the amorphous surrounding with a formation of small lamellae (Figure 2f). The ability of AFM to visualize polymer recrystallization can be applied for evaluation of the recrystallization rate. Figure 3a shows LDPE recrystallization, which was observed after steplike heating to 90 °C. The rate of recrystallization of 0.9 µm/min is determined from the initial slope of the “aggregate’s length vs time” curve (Figure 3b). This process slows down with time, and it is practically completed in 4 h. In some places, growing lamellae encountered each other, thus restricting their further growth. During recrystallization, a large number of holes (round black spots in Figure 3a) have appeared in amorphous regions of the layer. Holes grew with time,
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and some of them overlapped with each other. This is an indication of dewetting of the molten polymer. When the dip-crystallized layer was heated quickly from RT to 70 or 85 °C (T-jump with 5 °C/s rate), AFM images indicated structural changes which were less drastic than those observed during the recrystallization shown in Figures 2 and 3. The same surface region before and after a T-jump to 70 °C is shown in Figure 4a,b. After the T-jump the spherulitic morphology remained; however, some of the sheaflike patterns substantially changed their orientation (Figure 4b). This suggests local-scale nanostructure rearrangement which leads to the formation of differently oriented lamellae. A partial melting of LDPE had accompanied the morphology changes caused by the T-jump to 85 °C (Figure 4c). New morphology is characterized by sheaflike structures, which consist of extended lamellae. When this sample was cooled to RT, the amorphous material crystallized with a formation of smaller lamellae. Hence, the presented results show that AFM can be routinely applied for examination of structural rearrangements during melting and crystallization of polymer
Notes
samples, including kinetics of crystallization and recrystallization. We have characterized the morphology and nanostructure of the ultrathin LDPE layer, which had been crystallized in the confined geometry. Quasi-2D spherulites formed of the lamellae with the granular substructure are similar to the structural patterns found in the spin-cast ultrathin films of other polymers (PEO, PCL).17,18 Therefore, AFM at variable temperatures is an invaluable complementary tool to other methods (dilatometry, calorimetry, X-ray, etc.), which is applied to crystallization studies of polymer bulk. Acknowledgment. We are thankful to R. Daniels and K. Koski (both of Digital Instruments, Inc.) for the development of the thermal accessory. Yu.K.G. is thankful to Digital Instruments Inc. and University of California at Santa Barbara (Professor E. Kramer) for support of his research visit to Santa Barbara, where this work has been done. LA990846K
