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In Situ Annealing and Thickening of Single Crystals of C294H590 Observed by Atomic Force Microscopy N. Sanz,† J. K. Hobbs,* and M. J. Miles University of Bristol, H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, United Kingdom Received December 17, 2003. In Final Form: April 1, 2004 The annealing behavior of twice-folded crystals of the long-chain alkane C294H590 is examined in situ, in real time, by atomic force microscopy (AFM). AFM is capable of following processes in real time provided that the time scale is sufficiently long for several images to be collected during the process. In this paper, we focus on the temperature dependence and the thickened morphology. We are able to investigate where the thickening starts and how this depends on temperature and how melting is influenced by morphology. By following the motion of holes within the crystal, a lower limit for the rate of diffusion of crystalline polyethylene is estimated. We also focus on the substrate effect on the crystal morphology and thickening, using mica, glass, and graphite.
Introduction Synthetic polymers crystallize to form platelike lamellae with a typical thickness of around 10 nm, in which the chain axes lie in the thin dimension of the crystal so the chains must fold back many times into the crystal. This is the ubiquitous “chain-folded” polymer crystal.1 The equilibrium crystal form contains extended chains, and it is generally accepted that the reason for this chainfolding is kinetic: the fastest way to consume the available free energy is to form fast-growing thin crystals rather than slow-growing thick ones. Thus, under almost all conditions, polymer crystals are metastable and if heated will reorganize to form thicker crystals with a lower free energy. The thickening process, although it has been known to occur for more than 40 years,2 is still not well understood. Questions that are still outstanding are the following: Does thickening occur through a solid-state transformation or through melting and recrystallization? What is the relationship between the underlying crystallography of the crystal and the thickening route taken? What effect do external factors such as a substrate in contact with the thickening crystal have? During the past 15 years, ultralong alkanes with chain lengths of several hundred carbons have been used as model systems for synthetic polymers.3-5 Although comparable in length to low molecular weight polymers, the precise synthesis route leads to strict monodispersity,6 removing one of the complicating factors that can confuse * Corresponding author. Now at the University of Sheffield, Department of Chemistry, Sheffield, S3 7HF, U.K. E-mail: Jamie.hobbs@ sheffield.ac.uk. † Now at Laboratoire de Physique de la Matie ` re Condense´e Unite´ Mixte de Recherche 7643 CNRS - Ecole polytechnique, 91128 Palaiseau Cedex, France. (1) Keller, A.; O’Connor, A. Discuss. Faraday Soc. 1958, 25, 114. (2) Wunderlich, B. Macromolecular Physics, Vol. 2: Crystal Nucleation, Growth, Annealing; Academic Press: New York, 1976; Chapter 7. (3) Ungar, G.; Steyny, J.; Keller, A.; Bidd, I.; Whiting, M. C. Science 1985, 229, 386. (4) Ungar, G.; Zeng, K. B. Chem. Rev. 2001, 101 (12), 4157-4188. (5) Alamo, R. G.; Mandelkern, L.; Stack, G. M.; Krohnke, C.; Wegner, G. Macromolecules 1994, 27, 147. (6) Brooke, G. M.; Burnett, S.; Mohammed, S.; Proctor, D.; Whiting, M. C. J. Chem Soc., Perkin Trans. 1 1996, 13, 1635.
the interpretation of data in common polymers. As will be seen in this paper, the precise uniformity of the material is of particular importance when considering a process such as crystal thickening where the structure at a nanometer scale is of paramount importance. These materials have been used extensively as model systems for polymer crystallization7,8 and crystal thickening,9-11 and many new insights into polymer behavior have been gained. Here we present a series of experiments in which single crystals grown from the ultralong alkane C294H590 are allowed to thicken after depositing onto different substrates. The thickening process is monitored in real time using high-temperature atomic force microscopy (AFM),12 a technique that is becoming widely used in studies of polymer crystallization,13-16 crystal thickening,10-11 and melting.17,18 Several studies of the thickening behavior of single crystals of polyethylene have been published recently,19-21 as well as a study of the thickening behavior of the ultralong alkane C390H582.11 In this study, we concentrate on the influence of temperature, substrate, (7) Zeng, X. B.; Ungar, G. Polymer 1998, 39 (19), 4523. (8) Bassett, D. C.; Olley, R. H.; Sutton, S. J.; Vaughan, A. S. Polymer 1996, 37 (22), 4993. (9) Hobbs, J. K.; Hill, M. J.; Barham, P. J. Polymer 2000, 41 (25), 8761. (10) Winkel, A. K.; Hobbs, J. K.; Miles, M. J. Polymer 2000, 41 (25), 8791. (11) Magonov, S. N.; Yerina, N. A.; Ungar, G.; Reneker, D. H.; Ivanov, D. A. Macromolecules 2003, 35 (15), 5637. (12) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56 (9), 930-933. (13) Hobbs, J. K.; Humphris, A. D. L.; Miles, M. J. Macromolecules 2001, 34, 5508. (14) Pearce, R.; Vancso, G. J. Polymer 1998, 39, 1237. (15) Ivanov, D. A.; Pop, T.; Yoon, D. Y.; Jonas, A. M. Macromolecules 2002, 35 (26), 9813. (16) Hobbs, J. K. In Polymer Crystallization: Observations, Concepts and Interpretations; Sommer, J.-U., Reiter, G., Eds; Springer-Verlag: Berlin, 2003; pp 82-97. (17) Pearce, R.; Vancso, G. J. J. Polym. Sci., Part B 1998, 36, 2643. (18) Hobbs, J. K.; Winkel, A. K.; McMaster, T. J.; Humphris, A. D. L.; Baker, A. A.; Blakely, S.; Aissaoui, M.; Miles, M. J. Macromol. Symp. 2001, 167, 1. (19) Dubreuil, N.; Hocquet, S.; Dosiere, M.; Ivanov, D. A. Macromolecules 2004, 37 (1), 1. (20) Hocquet, S.; Dosiere, M.; Thierry, A.; Lotz, B.; Koch, M. H. J.; Dubreuil, N.; Ivanov, D. A. Macromolecules 2003, 36 (22), 8376. (21) Tian, M. W.; Loos, J. E. Polymers 2003, 51.
10.1021/la036385r CCC: $27.50 © 2004 American Chemical Society Published on Web 06/11/2004
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Sanz et al. taneously. Imaging conditions were maintained so as to just allow the surface to be tracked while maintaining the fast scan rates necessary to follow the process in real time. All images were taken at 256 × 256 pixels.
Results
Figure 1. An AFM topographic image showing a single crystal of C294H590 typical of those used in the study. The insert shows a line profile taken across the crystal, showing the uniform height. The scale bar represents 5 µm. Black to white represents a change in height of 60 nm.
and time on the thickening behavior and in particular on the slow evolution of the thickened morphology during annealing. Experimental Section G. Brooke6 and the EPSRC kindly provided the materials used in this study. Samples of C294H590 were weighed into glass tubes, and toluene was added to give a concentration of ∼0.1% (w/w). The glass tubes were then flame sealed to prevent the volatile solvent from escaping during crystallization and placed into a preheated oil bath at 105 °C for 10 min. The sample was quenched at 72 °C, held for 1 h, and then placed rapidly in an oil bath at 85 °C for 15 h. Initially all the sample was crystallized at 72 °C, and then the majority of the crystals dissolved at 85 °C, leaving the most stable fragments to act as seeds for further crystal growth on cooling. This complicated “self-seeding”22 technique is required in these materials as their strict monodispersity gives a very sharply defined dissolution temperature. Slow cooling at a rate of 0.25 °C/min resulted in twice-folded crystals of C294H590 as shown in Figure 1, the self-seeding method leading to a population of crystals with uniform size and shape. The dilute crystalline suspensions were then pipetted onto freshly cleaved mica or graphite, which was glued to a stainless steel sample disk. Glass substrates were carefully cleaned and then rinsed with distilled water and finally with ethanol, before depositing the crystals. AFM experiments were performed using a Digital Instruments/ Veeco Dimension D3100 microscope with a modified cantilever holder as described in ref 16. This allowed the use of a Linkam Scientific Instruments heating stage. By using this device, sample temperatures of 140 °C can be safely accessed. The accurate measurement of the temperature of the sample surface is influenced by the distance from the heat shield and the alignment of the probe relative to its holder, which vary from experiment to experiment, making a reliable calibration problematic. The temperature difference between the sample surface and the heater was estimated to be 3.5-7 °C in a previous study.13 The temperatures quoted in the text are the temperatures given on the Linkam and are therefore ∼4 °C higher than the estimated sample temperature. The atomic force microscope was operated in tapping mode, and phase, height, and amplitude images were collected simul(22) Blundell, D. J.; Keller, A. J. Macromol. Sci., Phys. 1968, B2 (2), 301.
Figure 1 shows an AFM image of the typical crystal morphology of C294H590 samples involved in this study. The somewhat unusual shape of these crystals most probably resulted from the self-seeding approach used. The experimental angle found between two (110) planes is 114°, in good agreement with that expected for lozengeshaped alkane crystals, 112°. The unusual (100) curved truncation is most probably due to growth in the vicinity of the growth rate minimum, as discussed in ref 23. The inset in Figure 1 shows a cross section taken of one of the crystals, from which the crystal thickness of 12.4 nm can be measured, which is close to the expected value of 12.54 nm for a twice-folded chain lying perpendicular to the crystal surface, as is the case for crystallization of long alkanes from dilute solution.4 Figure 2 shows a series of AFM topography images taken consecutively during the heating of a 2F sample dried down onto a mica substrate. We heated at a rate of 10 °C/min until the temperature of 100 °C was reached with the scan tube and the cantilever far from the heater. Then we slowly approached the cantilever above the heat shield, and we continued heating at the rate of 1 °C/min to 110 °C. When the AFM probe is initially brought into contact with the surface, it takes several minutes for temperature equilibrium to be reached between the sample and the cantilever system. To allow for this, we held at 110 °C for 10 min before taking the first image. Before the heating experiment was started, the crystal edges were fairly straight (Figure 1). Figure 2a,b shows crystal edges starting to change their shape at 110 °C. We held for 20 min at this temperature, and no thickened material was observed within the crystal. Then we heated continuously at a rate of 0.5 °C/min to 120 °C (Figure 2c-e). The brighter regions in images c-e are the thickened material. A thickening front can be observed progressing along the crystal edges. A feature is the preferential thickening along crystallographic directions corresponding to (110), (100), and (010) (see thick arrows on Figure 2f-h). Angles close to the expected values of 56°3 and 33°7 are very often found in thickening directions. The height of the thickened region is in the range of 14.518.7 nm, which is consistent with once-folded chains tilted at an angle of between 0° and 35° to the lamellar normal (the expected value for once-folded chains at 35° is 15.40 nm). A final chain tilt angle of 35° is commonly found both on melt crystallization and on high-temperature annealing of solution-grown mats of the long alkanes24 and has been confirmed to occur on thickening from twice-folded to oncefolded crystals in C294H590 as part of an extensive X-ray study of this transition, preliminary results from which are published in ref 25. After thickening had started, the crystal was annealed for 15 min at 120 °C. Figure 2e-i shows the time evolution of the thickening process during this period. Several features are of particular interest. The thick circle shows the behavior of a thickened region at the edge of the crystal, in which, after initially thickening, part of the crystal breaks off and appears to move across the surface of the (23) Putra, E. G. R.; Ungar, G. Macromolecules 2003, 36 (14), 5214. (24) de Silva, D. S. M.; Zeng, X. B.; Ungar, G.; Spells, S. J. Macromolecules 2002, 35 (20), 7730-7741. (25) Terry, A. E.; Phillips, T. L.; Hobbs, J. K. Macromolecules 2003, 36 (9), 3240.
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Figure 2. A series of AFM topographic images of a single crystal of C294H590 on a mica substrate. The broad vertical bars are an artifact caused by the interference in the optical lever cantilever detection system. (a) 110 °C, 0 s. (b) 110 °C, 444 s. (c) 112 °C, 1914 s. The arrow shows a region where thickening has started along the edge of the crystal. (d) 117 °C, 2606 s. The arrow shows a region where thickening has started along the edge of the crystal. (e) 120 °C, 3249 s. The arrow indicates the further evolution of the thickening. (f) 120 °C, 3338 s. The arrow shows the growth of the thickened region across the substrate. The ring shows a thickened area growing across the substrate. (g) 120 °C, 3405 s. The thick arrows indicate areas that are thickening along particular crystallographic planes. The arrow labeled A indicates a thickened area moving across the substrate. (h) 120 °C, 3512 s. The arrows indicate areas that are thickening along particular crystallographic planes. (i) 120 °C, 3843 s. The insert is a higher magnification image of the area indicated with a thin arrow, showing the dendritic morphology of the thickened material when thickening occurs within the body of the crystal. The scale bar represents 1 µm.
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Figure 3. A graph showing the variation in speed of the ringed crystal block shown in Figure 2. Speeds were measured over a 40 s period, the time it took to collect one image.
mica. Height measurements indicate that the material is the same thickness as the other once-folded regions of the crystal, so it appears that this moving object is crystalline. The arrowed area labeled A indicates a region where similar motion of crystalline material is observed. The direction of motion seems to be related to the underlying crystallography of the crystal rather than to the scan direction, implying that the motion is not caused by simple pushing with the AFM probe. The rate at which these crystal blocks move was measured over a series of several images. It was found that the speed was not a constant but rather varied around an average value of 2.2 nm s-1. Figure 3 shows the variation in speed with time for the crystalline block circled in Figure 2g. The inset in Figure 2i shows a higher magnification image of a partially thickened region within the crystal in which a dendritic morphology, characteristic of thickening in the ultralong alkanes,10 can be seen. The width of the “arms” of the dendritic structure is approximately 20 nm, although it is difficult to make a precise measurement considering the steepness of the edges of the structures and the shape of the imaging AFM probe. Although most of the large-scale motion is limited to the edges of the crystals, substantial reorganization also occurs within the body of the crystal, as can be seen by the formation of holes, which start to occur in large numbers once the temperature is raised to 120 °C. Careful examination shows that these holes move around within the crystal, usually leaving a trail of thickened material behind them. Figure 4 shows a series of AFM topography images taken consecutively during the heating of a 2F sample deposited on a glass substrate. The same procedure as above was used to reach the temperature of 100 °C, and then the sample was heated at a rate of 1 °C/min to 120 °C. Figure 4a,b shows the crystal edge starting to become irregular as temperature is increased, but no thickened crystal is seen. The temperature had to be increased to 120 °C (Figure 4c,d) before thickening was observed. Initially, small regions of thickened material form at intermittent positions along the edge of the crystal, the unthickened portion of the crystal between these regions forming an indentation. Over time, the indentation becomes deeper and material along the edges starts to migrate to close off the gap, forming a hole within the crystal. The process appears visually somewhat similar to a two-dimensional bubble moving into a liquid, the final shape of the hole in the crystal being approximately circular except in the cases where the edge is disrupted by thickened material. The rate of progression of the indent/hole into the crystal was measured along a line from the edge of the crystal to the furthest point into the crystal that the indent reaches before becoming a hole. This gives an approximate average
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speed of motion of 0.44 nm s-1, taking the average over the five main indentations that occur in the figure and over several different time periods for each indentation. Figure 5 shows the variation in speed with time for the indentation marked A in Figure 4e. The measurements of speed are taken along the dotted line marked in the figure. Ideally a more sophisticated approach would be taken, perhaps measuring the motion of the “center of mass” of the hole. However, as the images are collected line by line rather than as a snapshot, it is difficult to make such an analysis in a meaningful way. Figure 6 shows a series of AFM phase images, each image collected simultaneously with the topographic images shown in Figure 4. Under the imaging conditions used here, the phase image is dark in areas that are molten and light in areas that are crystalline. The regions marked with an arrow in Figure 6b-e are therefore most probably molten alkane. These areas can be matched up with higher regions in the corresponding topography images and indicate areas that have melted within the body of the crystal, prior to thickening. Such melting is not observed elsewhere within the crystal as part of the thickening process and in particular is not seen when thickening occurs along the edges of the crystal. Figures 7 and 8 show a series of height representations of AFM images taken consecutively during the heating of a 2F sample dried down to a graphite substrate. We heated the sample continuously at the rate of 1 °C/min to 125 °C. Figures 7a-d show images collected during heating to 120 °C, from which the markedly different behavior from that observed on the other substrates can be seen. Initial reorganization of the crystal has already started at a temperature of 105 °C, approximately 15° below the temperature at which thickening begins on glass and mica. During heating, the crystal breaks up into a fine, radial texture, maintaining the shape of the underlying crystal but presumably with a significant reorientation of the molecules. From the AFM data, it is not possible to determine the actual orientation at a molecular scale. Figure 8 shows a series of images of a different area of the same sample as shown in Figure 7, collected during isothermal annealing at 125 °C. In this area, two crystals were initially deposited so they overlapped each other. During the first stage of heating, the crystal lying in contact with the substrate reorganized, leaving a clear boundary where the double thickness layer began. Figure 8 shows the gradual retraction of this double thickness layer. Figure 9 is a lower magnification image taken at 133 °C showing the area surrounding the region imaged in Figure 8; the area corresponding to Figure 8 is indicated by the black square. The outline of one of the original crystals can be seen as a faint outline to the reorganized alkane. The remnant of the double layer is visible in the middle of this reorganized region. Discussion From the data presented, it is immediately apparent that there is a significant difference in behavior between the thickening that occurs on a glass or mica substrate and that observed when the crystals are annealed on graphite. In the following, we will discuss first the thickening behavior on mica and glass and then the reasons for the different behavior on graphite. Thickening on Mica and Glass Substrates. Several striking and unexpected features that occur during thickening are observed on both these substrates, as well as some subtle differences in behavior. In both cases, there
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Figure 4. A series of AFM topographic images of a single crystal of C294H590 on a glass substrate. (a) 100 °C, 0 s. (b) 115 °C, 1045 s. (c) 120 °C, 1558 s. (d) 120 °C, 1907 s. (e) 120 °C, 2265 s. The dotted line indicates the line along which the speeds in Figure 5 were measured. The area labeled A indicates the indentation referred to in the text. (f) 120 °C, 2504 s. (g) 120 °C, 2859 s. (h) 120 °C, 3399 s. The arrow indicates a region of thickened material within the body of the crystal that has a dendritic texture. The scale bar represents 1 µm.
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Figure 5. A graph showing the variation of speed of the motion of the indent marked A in Figure 4e, along the dotted line marked in that figure. Speeds were measured over a 58 s period, the time it took to collect one image.
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is an initial roughening of the edges of the crystal, followed by the formation of isolated areas of thickened material along the edges, the reduction in top surface area that occurs during thickening being accommodated by a migration of material along the edges of the crystal. This migration of material leads to the formation of indentations in the edges of the crystal. The rate of diffusion and the degree of mobility at the surface of the crystal are likely to be higher than in the bulk, so thickening is expected to start at the edges. What is surprising even in this initial stage is that the migration of material appears to happen over long distances, or at least so as to minimize the degree of curvature of the surface of the crystal. That is, it might be expected that once a small patch of thickened material has formed, it would grow through the reorganization of the chains immediately adjacent to it, which
Figure 6. A series of AFM phase images collected simultaneously with the images shown in Figure 4. Image a corresponds to Figure 4c, image b corresponds to Figure 4d, image c corresponds to Figure 4f, image d corresponds to Figure 4g, and image e corresponds to Figure 4h. The dark regions indicated by arrows are most probably molten material. The scale bar represents 1 µm.
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Figure 7. A series of AFM topographic images showing a single crystal of C294H590 on a graphite substrate. (a) 105 °C, 0 s. (b) 110 °C, 416 s. (c) 115 °C, 764 s. (d) 120 °C, 1029 s. The scale bar represents 1 µm.
would tend to form a relatively sharp indentation directly adjacent to the thickened material. Instead, the shape of the crystal edge adjusts so as to maintain an approximately constant angle of curvature. This implies that the rate of diffusion is considerably greater than the rate of thickening, so thickening at the edges of the crystal is not controlled by material transport. The fact that the crystal edge has a smooth curvature, rather than crystallographic facets, shows that there is little surface tension anisotropy at these temperatures close to the melting point and that the surface tension is sufficiently high to influence the reorganization. The initiation of thickening at the edges is, perhaps, expected, due to the lower barrier to chain motion at the surface compared to within the complete lattice in the bulk. What is particularly surprising is the subsequent formation of indents that appear to “grow” or migrate into the body of the crystal. This resembles the formation of liquid droplets when a dense liquid is placed on top of a less dense liquid and, following an instability at the surface, starts to fall through the less dense liquid.26 It would be interesting to explore this analogy in more depth, although currently the technically demanding nature of the data collection makes obtaining sufficient sample statistics problematic. What is clear is that there is largescale motion of material within the bulk of the still 2F crystal and that this motion is driven not only by the instability of the 2F crystal with respect to the 1F crystal but also by the (apparently isotropic) surface tension of the 2F crystal. This leads to motion of material as the hole (26) Nye, J. F.; Lean, H. W.; Wright, A. N. Eur. J. Phys. 1984, 5, 73-80.
is driven to adopt an approximately circular shape. Measuring the rate of motion of the hole into the crystal gives a lower bound for the rate at which individual crystalline molecules are moving along the surface. This is the first time that a direct measurement has been made of a surface crystalline diffusion rate within a polymeric system and can be used to inform our understanding of the rates of reorganization within such a material during crystallization, as well as during annealing. The other point that is clearly made by the high level of mobility within the crystalline phase is that there is not large-scale melting prior to thickening in these relatively short molecules. There are multiple examples where an area is repopulated by 2F crystal, rather than 1F crystal, even when adjacent to a 1F area. This again re-emphasizes the observation that the thickening process is limited by something other than material transport when it happens at the edge of a crystal, as in this case new material has moved into contact with an existing 1F area but deposited as 2F crystal. The above has considered thickening at the edges of the main crystal. There are also examples where the thickening process occurs within the bulk of the crystal. As expected, considering the reduction in top surface area that must occur, this thickening is accompanied by the formation of holes within the crystal. However, in a manner similar to that previously observed in C162H326 10 the morphology of the thickened material is dendritic, implying a diffusion-controlled process. This is in contrast to the conclusion drawn above from observations where the thickening occurred at the outer surfaces of the crystal. In ref 10, it was suggested that the diffusion of “holes”
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Figure 8. A series of AFM topographic images showing a single crystal of C294H590 on a graphite substrate at 125 °C. (a) 0 s. (b) 697 s. (c) 5263 s. (d) 5500 s. The scale bar represents 1 µm.
Figure 9. An AFM topographic image showing the same crystal as that shown in Figure 8, after heating to 133 °C. The box indicates the region shown in Figure 8. The scale bar represents 2 µm.
caused by the unfolding, away from the thickening front, determined the rate of growth of the thickened region. Here we invoke the same explanation, with the addition
that once a visible hole is formed, this tends to travel along ahead of the thickening front and possibly facilitate thickening by the increased rate of diffusion of material around its perimeter. Occasionally, small patches of molten material appear within the body of the crystal, as seen from the phase images. This is surprising, as crystals usually melt from the edges in, rather than spontaneously in the middle. Also, these areas are stable for relatively long times (minutes). The reason for this rare behavior is unclear. The small patches of molten material are relatively stable with respect to both the 2F and 1F crystallization and in some cases eventually recrystallize into the thinner (and less stable) 2F form. In the above discussion, we have concentrated on the surprising degree of mobility of the apparently crystalline material as part of the bulk crystal. On the mica substrate, we see the motion of small chunks of crystal across the surface, detached from the main parent crystal. These blocks of material are the same height as the 1F crystal and maintain a rodlike shape, so it must be assumed that they are still crystalline. The obvious question is, are the chunks being moved by the action of the AFM tip, or, if not, what is responsible for the motion? There are two compelling pieces of evidence to suggest that the motion is not due to the AFM tip. First, the direction of motion of the chunks is not along the scan direction of the probe but rather seems to be along relatively straight lines at some arbitrary angle to this direction. Second, careful examination of the area around partially thickened crystals that have not been imaged during thickening reveals the presence of similar blocks of material, which
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must have moved there during the time that the crystal was not being imaged. This leaves the question of what is responsible for the motion? Two possibilities present themselves. It could be that the asymmetry of the crystalline blocks when placed on a surface, due to the presence of a tilt of the chain axis relative to the lamella surface, leads to one side of the block being favorable for the attachment of new molecules relative to the other side.27 However, with thermal energy only, we would expect this to eventually relax into some equilibrium shape where the amount of low-energy surface was maximized, and average motion would stop (otherwise perpetual motion would ensue). An alternative is that there is some reaction with the underlying mica substrate, possibly wetting of the surface by the alkane molecules. Motion could then occur in a manner similar to that suggested in ref 28, where a “reactive wetting” process is invoked. The latter suggestion might explain the observation of this behavior on mica but not on glass, due to the different chemistry of the two surfaces. However, large chunks of material do not move in this way, although substantial rearrangements at the edges of the crystals on a mica substrate are seen, including migration over distances of several hundreds of nanometers, but without breaking free from the parent crystal (data not shown). This perhaps favors the first suggestion, due to the more limited “fuel” for the motion in this scenario. To determine decisively between these different possibilities, more work is required. Thickening on Graphite. Graphite is an epitaxial nucleating agent for polyethylene and alkanes,29,30 so it is not surprising that the behavior on graphite is markedly different from that on the other substrates. As the temperature increases and the chains within the crystal become mobile, those around the edge will start to explore the substrate surface. It may be that as the graphite lattice is likely to be perfect over lengths considerably longer than the chain length, it is favorable for the molecules to lie down on the surface rather than on the 2F crystal substrate. This reorganization requires considerably less cooperative motion than thickening to the 1F form and so will dominate. Thus, in this case there is no thickening process as such, but rather a reordering so as to maximize the surface contact between the alkane and the graphite (akin to wetting, but with the alkanes in crystallographic (27) Keith, H. D.; Padden, F. J.; Lotz, B.; Wittmann, J. C. Macromolecules 1989, 22, 2230. (28) Santos, F. D. D.; Ondarcuhu, T. Phys. Rev. Lett. 1995, 75 (16), 2972. (29) Rabe, J. P.; Buchholz, S. Science 1991, 253 (5018), 424-427. (30) Tracz, A.; Jeszka, J. K.; Kucinska, I.; Chapel, J. P.; Boiteux, G.; Kryszewski, M. J. Appl. Polym. Sci. 2002, 86 (6), 1329-1336.
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register with the substrate). It may be that a closer examination of this process will give an insight into thickening in the bulk, however, as in this case there is also a contact between a thin crystal and a large epitaxial surface (the already thickened material). Finally, it is worth commenting on the slightly unusual shape of the as-grown crystals. As has already been suggested, this is most probably due to growth in proximity to the growth rate minimum, as described in ref 23. We observed no difference in behavior between the different “sectors” of these crystals during thickening, possibly because of the different origin of the shape when compared to crystals of polyethylene. Also, essentially identical behavior has been observed by ourselves in these materials when the crystals have strictly planar crystal surfaces. However, crystals formed in this way are not so suitable for AFM study, making a comprehensive exploration of the effect of substrate difficult and comparison between parallel experiments problematic because of the inability to say with confidence that crystallization occurred under the same conditions for the entire sample. Conclusions In situ, real-time, high-temperature imaging of the thickening of the ultralong alkane C294H590 has provided some new insights into how single crystals of polymers reorganize during high-temperature annealing. In particular, a surprisingly high degree of mobility within the crystalline phase, coupled with the influence of an essentially isotropic surface tension, results in a selfsustaining process in which thickening leads to the creation of holes which in turn facilitate further thickening because of the high degree of mobility around their perimeter. The rate of thickening at the perimeter of a crystal does not appear to be influenced by diffusion, while thickening within the body of a crystal leads to morphologies that are indicative of a diffusion-controlled process. Measurement of the rates of motion of holes within the crystals has given a lower bound for the rate of diffusion of crystalline chains along the crystal surface. Comparison between the behavior on different substrates has allowed substrate specific behavior, such as surface wetting (in the case of graphite) and motion of crystal blocks (in the case of mica), to be separated from the general observation of a high degree of mobility within the crystals. Acknowledgment. We thank the Engineering and Physical Sciences Research Council, U.K., for funding. LA036385R
