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Inkjet printing as a possible route to study confined crystal structures
European Polymer Journal 49 (2013) 203–208
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Inkjet printing as a possible route to study confined crystal structures N. Sanandaji a, A. Oko b, D.B. Haviland c, E.A. Tholén d, M.S. Hedenqvist a, U.W. Gedde a,⇑ a
KTH Royal Institute of Technology, School of Chemical Science and Engineering, Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden YKI, Ytkemiska Institutet AB/Institute for Surface Chemistry, SE-114 86 Stockholm, Sweden KTH Royal Institute of Technology, School of Engineering Sciences, Nanostructure Physics, Albanova, SE-106 91 Stockholm, Sweden d Intermodulation Products AB, SE-169 58 Solna, Sweden b c
a r t i c l e
i n f o
Article history: Received 23 July 2012 Received in revised form 26 September 2012 Accepted 5 October 2012 Available online 23 October 2012 Keywords: Inkjet printing Crystallization Poly(e-caprolactone) Confinement
a b s t r a c t Inkjet printing is a technique for the precise deposition of liquid droplets in the pL-volume range in well-defined patterns. Previous studies have shown that inkjet printing is attractive in polymer technology since it permits the controlled deposition of functional polymer surfaces. We suggest that the technique might also be useful for studying crystallization, in particular confined crystallization. Inkjet printing is a non-contact deposition method with minimal risk of contamination, which allows the exact deposition of both polymer solutions and polymer melts. This paper demonstrates the possibility of utilizing the technique to create surfaces where polymer chains form isolated small structures. These structures were confined by both the low polymer content in each droplet and the time constraint on crystal formation that arose as the result of the rapid solvent evaporation from the pL-sized droplets. In theory, inkjet printing enables the exact deposition of systems with as few as a single polymer chain in the average droplet. With appropriate instrumentation, the versatile inkjet technology can be utilized to create whole surfaces covered with polymer structures formed by the crystallization of small, dilute and rapidly evaporating droplets. 110 pL droplets of a 10 6 g L 1 poly(e-caprolactone) solution in 1-butanol have been deposited and studied by atomic force microscopy. Small structures of ca. 10 nm thickness and ca. 50 nm diameter also seemed to exhibit crystalline features. Some of the small structures had unusual rectangular forms whilst others were interpreted to be early precursors to six-sided single crystals previously observed for poly(e-caprolactone). The unusual forms observed may have resulted from the entrapment of crystal structures into metastable phases, due to the limited amount of polymer material present and the rapid evaporation of the droplets. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Inkjet printing, a digital printing technology, is increasingly being applied in scientific fields such as nanotechnology, tissue engineering and polymer engineering [1,2] where it can be used for the precise deposition of pL volumes of suspensions or solutions in welldefined patterns. This ‘‘direct-writing’’ can be accurately controlled by micro-electro-mechanical ink-dispensers, ⇑ Corresponding author. Tel.: +46 8 7907640; fax: +46 8 208856. E-mail address: [email protected] (U.W. Gedde). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.10.001
which in turn are controlled by computers and highspeed electronics. An advantage of the technique is that inkjet printing offers non-contact deposition, which means that contamination is minimized [3]. Another advantage is that the precise deposition, in terms of both volume and placement, enables the efficient layer-bylayer assembly of polymer films. Typically, multiple rinsing steps are required during the creation of multi-layered assemblies in order to remove excess material. However, with inkjet printing, only the necessary quantity of polymer is deposited in each step, eliminating the need for rinsing [4].
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Recent studies show that inkjet printing is one of the most promising methods for the controlled deposition of functional polymer surfaces [5]. According to de Gans and Schubert [6], inkjet printing of polymers is a promising technique for the creation of polymer micro-arrays and libraries. Hoth et al. [7] have utilized the technique to produce heterojunction solar cells, by printing photoactive layers consisting of poly(3-hexylthiophene) blended with fullerene (6,6)-phenyl C61 butyric acid methyl ester. Several studies [8–11] have shown that inkjet printing can be used to manufacture polymer film transistors and light emitting diodes. Inkjet printing has also been used for the manufacture of polymer semiconductors, dielectrics and conductors [12–14]. Kawasea et al. [15] have inkjetprinted thin polymer films to create all-polymer transistors, active-matrix backplanes and inverters and Liu et al. [16] have inkjet-printed all-polymer capacitors. In this study, the application of inkjet printing is explored in another field of polymer technology, namely confined crystallization. As shown in this paper, inkjet technology permits the crystallization of dilute polymer solutions deposited in small and rapidly evaporating droplets. Crystal structures are formed under physical confinement, as they are not able to expand beyond a certain volume due to a limit of polymer chains in their vicinity. A number of studies [17–27] have focused on confined crystallization, showing that confinement tends to influence characteristics such as surface energy and crystallization kinetics. Studying confined crystallization is a way of gaining insight into metastable phases, which may precede a stable crystalline phase. These phases are inherently difficult to study due to their ephemeral nature. Since the presence of metastable phases has been shown to be size-dependent [28,29], confined crystallization represents a method for trapping the polymers in a metastable phase. Inkjet deposition not only makes it possible to ‘print’ structures where crystal growth is limited by the small amount of material present in each droplet, but also leads to an additional confinement effect due to the evaporation of the pL size droplets within a few seconds. This rapid evaporation constrains the process of crystal formation, limiting the ability of the polymer chains to assemble into crystalline structures while in solution, and it may therefore prove valuable for trapping the polymer in a metastable phase, a precursor to crystallization. Another advantage of the inkjet technique is that the non-contact deposition method is free from contaminants, which may otherwise influence crystal formation and particularly the initiation of crystal formation. For these reasons, we propose that inkjet printing can be used as a viable tool for studying the controlled crystallization of confined polymer systems.
mer while it was being stirred. Solutions were subsequently diluted in several steps to obtain concentrations as low as 10 6 g L 1. The solutions were again filtered before inkjet dispensing, in order to ensure samples with a very high purity. Impurities would not only have affected crystal growth but could also have led to clogging of the nozzle. 2.2. Inkjet operating principles Fluid ejection was accomplished using a MD-K-140 piezoelectric dispenser obtained from Microdrop Technologies, Norderstedt, Germany. Inkjet printing is based on the creation of an acoustic pulse which ejects liquid droplets through a nozzle. The pulse can be generated by either thermal or piezoelectric means [5]. In the instrumental setup used in this work, the latter technology was used to eject droplets of precise size. The dispenser together with the fluid reservoir flask and the piezoelectric mechanism are sketched in Fig. 1. The core of the dispenser head consisted of a glass capillary surrounded by a tubular piezoelectric actuator. One end of the capillary was formed as a 51 lm nozzle. A voltage pulse caused a contraction of the piezoelectric actuator and built up a pressure wave that propagated through the capillary and pushed fluid outwards. The kinetic energy of the acoustic wave was partly converted to the surface energy of droplet formation, and partly to the kinetic energy of the free-flying droplet. When the amount of kinetic energy transferred outwards was greater than the energy needed for droplet formation, 110 pL droplets were ejected with a velocity of 2 m s 1. The ejected droplet flew freely through the air and in practice could be very accurately placed on a substrate located just below the dispenser head. The substrates were silica plates, cleaned by ethanol, Milli-Q water and plasma followed by vacuum drying. The sample stage on which the substrates were placed could be accurately moved in the x, y and z directions as necessary. Depending on the choice, either single droplets or arrays of multiple droplets were dispensed. Dispensing was carried out at a constant relative humidity of 45 ± 3%, and at constant temperature, 23 ± 1 °C. Both the fluid reservoir and the dispenser head were heated to 70 °C during dispensing. Fig. 2 sketches the droplet formation shortly after dispensing. As the droplet flew through the air it acquired a spherical shape. Each dispensing step, from the creation of the acoustic pulse to the formation of a spherical droplet, occurred so precisely that when stroboscopic images were taken at the same frequency of droplet formation, the viewer could almost be fooled to think that she or he were studying a single droplet hovering in mid-air. 2.3. Atomic force microscopy
2. Experimental 2.1. Materials and sample preparation PCL with M n = 10 kDa and M w = 14 kDa, and 1-butanol with greater than 99% purity, were obtained from Sigma Aldrich. The 1-butanol solvent was filtered before mixing with PCL and heated to 90 °C in order to dissolve the poly-
The solidified droplets on the substrate were studied by atomic force microscopy (AFM) in order to determine their mechanical response to forces applied by the AFM tip as it oscillated above the surface. The AFM (Digital Instruments, Dimension 3100) was equipped with a surface analysis upgrade for intermodulation AFM (Intermodulation Products AB), a multi-frequency dynamic force measurement
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Fig. 1. Sketch of the inkjet dispenser (right) and of the piezoelectric mechanism (left). The piezoelectric mechanism is based on the application of a voltage pulse (left, second figure from top) leading to the propagation of a pressure wave through the capillary (left, third and fourth image from top). The wave transfers the liquid to the nozzle (left, fifth image from top) and finally leads to formation of a droplet when the kinetic energy transferred outwards is greater than the surface energy required to form a droplet (left bottom image). A = Cord to heat control box. B = Vent. C = Cord to dispenser control box, controls dispensing and temperature at dispenser head. D = Dark grey area shows a protective shell including heating elements, which also extends around the flask containing the polymer solution.
Fig. 2. Sketch of the process of droplet formation as it appears shortly after dispensing. The droplets adopt a spherical shape during their flight.
method which rapidly and accurately extracted the nonlinear response of the oscillating cantilever. This response was analyzed using a contact-mechanics model for the nonlinear tip-surface force, in order to determine the model parameters that fitted the measured response [30]. In this way, parameter maps of the surface were generated, color-coded, and projected onto a two-dimensional rendering of the surface topography. The Derjaguin-MullerToporov (DMT) model [31] was used to determine the effective Young’s modulus E⁄ at each pixel of the image (256 256 pixels). It should be emphasized that this simple model neglects many effects, including the actual shape and geometry of the surface as well as the surface
energy. E⁄ therefore should not be compared with the bulk Young’s modulus of PCL, but rather considered as an effective measure of the surface stiffness, expressed as the equivalent bulk Young’s modulus of a material with an ideal planar interface, being indented with a rigid and perfectly round sphere of diameter 10 nm.
3. Results and discussion The polymer structures formed on the surfaces of the silica plates varied according to the concentration of the dispensed solution and the number of droplets placed on
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Fig. 3. A parameter map showing the surface stiffness, expressed as an equivalent modulus E⁄ as defined by the DMT contact mechanical model. E⁄ is color coded and projected on a 3D rendering of the surface topography. The structure shown was formed after dispensing 100 droplets of 10 5 g L 1 PCL solution. The edge of a spherulite is shown at the top right.
each spot. In Fig. 3a parameter map, based on an AFM height image, is shown for structures that have formed after deposition of 100 droplets of 10 5 g L 1 PCL solution. The amount of material deposited was relatively high in this sample, resulting in the formation of spherulites and thin films of branched finger-like structure. As expected, the silicon substrate showed a high E⁄-value (ca. 2.4 GPa). The thin PCL film was softer with an average recorded E⁄value of ca. 0.5 GPa. The plateaus of the thinner branching areas showed significantly higher E⁄-values than those of comparable locations on the spherulite. This is consistent with the interpretation that the measured stiffness is not a bulk stiffness, but that of a thin film, where the stiffness of the underlying substrate play a significant role. E⁄-values of regions with the same thickness were comparable and showed that locally harder regions existed both in the thin film and on top of the spherulite. In the regions outlined by white boxes in Fig. 3, E⁄ varied on the thin PCL film between 1.2 and 2.3 GPa, despite the fact that the film thickness was essentially the same through out this area. In the spherulitic region outlined by the red boxes1 (Fig. 3), E⁄ varied between 0.4 and 1.2 GPa. These local differences in E⁄ indicated that there were differences in molecular packing in both the film and the spherulite. Spots obtained from droplets which contained lower concentration of PCL did not form spherulitic textures, but instead round ‘island’ structures. Fig. 4 shows a height AFM image of such island structures formed in a sample where 10 droplets of 10 6 g L 1 PCL solution were deposited. The structures were uniformly dispersed and had a diameter of ca. 50 nm. The different ‘islands’ showed about the same E⁄, reflecting a uniform structure. Narrow bridges connecting the islands structures were observed. These bridges were aligned along a specific direction, and had formed in the process where the edge of the small pool formed by 10 droplets was moving, due to evaporation of solvent. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Fig. 5 shows structures with a diameter of ca. 60 nm formed after the deposition of single droplets of 10 6 g L 1 PCL. Calculation showed that each droplet contained approximately 4700 individual PCL molecules, whilst each island structure contained only ca. 100 molecules. The small structures were rounded, but our interpretation is that crystallinity could still be inferred from their structure. Due to rapid evaporation of droplets, and the low concentration deposited, the formed structures were expected to differ from developed single crystals. When studying structures with a diameter of 60 nm or smaller, it is important to note the limitations of the AFM method. The AFM measures a convulsion of the tip shape with the shape of the surface feature. The radius of a measured structure is enlarged by the radius of the tip, which in this study was 5 nm. The height on the other hand is more accurately preserved. It follows that the structures shown in Fig. 5 actually had a diameter of 50 nm, rather than the 60 nm observed. It also follows that the structures in actuality had sharper edges, which were rounded by the artifact effect created by the AFM tip. In Fig. 6, additional structures formed after deposition of single droplets of 10 6 g L 1 PCL are displayed. The smallest structures had diameters of ca. 20 nm whereas the largest ones had diameters of ca. 60 nm (translating to 10 nm and 50 nm respectively, when the effect of the AFM tip is taken into account). The smallest structures were only ca. 2 nm high, compared to the largest structures that had a height of 10–11 nm. We interpret that these structures as precursors to the six-sided single crystals previously observed for PCL [32–38]. Some of the facet angles found were close to the theoretical angles of the mature six-sided crystals, i.e. 113° (angle between the {1 1 0} and {1 1 0} sectors) and 124° (angle between the {1 1 0} and {1 0 0} sectors). In addition small structures with a rectangular shape were observed. The insert in Fig. 7 shows part of a structure formed after deposition of 10 droplets of 10 6 g L 1 PCL solution. Interestingly, the image shows features with a rectangular shape (marked by the white outline). This
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Fig. 4. A parameter map showing the surface stiffness, expressed as an equivalent modulus E⁄ as defined by the DMT contact mechanical model. E⁄ is color coded and projected on a 3D rendering of the surface topography. The structure shown was formed after dispensing 10 droplets of 10 6 g L 1 PCL solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Height AFM image of structures formed after deposition of a single droplet of 10 6 g L 1 PCL solution.
Fig. 7. Height AFM image of structures formed after deposition of 10 droplets of 10 6 g L 1 PCL solution. The inset image shows a close up of region marked with a dashed outline in the right-hand image.
unusual shape of a PCL crystal has recently also been observed in single crystals grown under extreme conditions (0–50 °C) from dilute solution in supercritical carbon dioxide [39]. The unusual lateral habit also suggested the presence of a different unit cell. Sanandaji et al. [40] present further details about this structure. 4. Conclusions
Fig. 6. Height AFM image of structures formed after deposition of a single droplet of 10 6 g L 1 PCL solution.
This study has utilized inkjet technology to crystallize small quantities of PCL, deposited in small droplets with a precise size. Since the printing method allows for the precise deposition of thousands or even millions of small droplets, it can be used for creating surfaces where polymer chains form small isolated structures. The non-contact technique makes it possible to minimize contamination, which is valuable for gaining clean crystal systems. Utilizing this technology, it has been possible to study small
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structures consisting of only ca 100 polymer chains. Even though such small structures are inherently difficult to study due to their very limited size, crystallinity could be inferred from their structure. Some of the small structures formed were interpreted to be early precursors to the sixsided single crystals previously observed for PCL whereas others were rectangular. These for PCL atypical structures indicate unusual molecular packing, which in turn can be explained by the fact that the structures were formed with a limited amount of polymer material present and during the rapid evaporation of solvent from small droplets. It is possible to utilize inkjet technology to create even more confined systems. By using smaller nozzles and more dilute solutions, small droplets can be deposited which contain on average only a single polymer chain. Indeed, inkjet printing can be used to create whole surfaces consisting of single polymer chains deposited in rapidly evaporating droplets, where the individual chains are deposited in proximity to each other but have no direct contact. Acknowledgments This study was financed by the Swedish Research Council through Grants Nos. 2006-3559 and 2009-3188. References [1] de Gans B-J, Duineveld PC, Schubert US. Inkjet printing of polymers: state of the art and future developments. Adv Mater 2004;16:203–13. [2] Singh M, Haverinen HM, Dhagat P, Jabbour GE. Inkjet printing – process and its application. Adv Mater 2010;22:673–85. [3] Tekin E, Smith PJ, Schubert US. Inkjet printing as a deposition and patterning tool for polymers and inorganic particles. Soft Matter 2008;4:703–13. [4] Andres CM, Kotov NA. Inkjet deposition of layer-by-layer assembled films. J Am Chem Soc 2010;132:14496–502. [5] de Gans B-J, Schubert US. Inkjet printing of well-defined polymer dots and arrays. Langmuir 2004;20:7789–93. [6] de Gans B-J, Schubert US. Inkjet printing of polymer micro-arrays and libraries: instrumentation, requirements, and perspectives. Macromol Rapid Commun 2003;24:659–66. [7] Hoth CN, Choulis SA, Schilinsky P, Brabec CJ. High photovoltaic performance of inkjet printed polymer: fullerene blends. Adv Mater 2007;19:3973–8. [8] Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W, et al. High-resolution inkjet printing of all-polymer transistor circuits. Science 2000;290:2133–6. [9] Bharathan J, Yang Y. Polymer electroluminescent devices processed by inkjet printing: i polymer light-emitting logo. Appl Phys Lett 1998;72:2660–2. [10] Edwards C, Albertalli D. Application of polymer LED materials using piezo ink-jet printing. SID Symp Dig 2001;32:1049–51. [11] Kawase T, Sirringhaus H, Friend RH, Shimoda T. Inkjet printed viahole interconnections and resistors for all-polymer transistor circuits. Adv Mater 2001;13:1601–5. [12] Seamus EB, Cain P, Mills J, Wang J, Sirringhaus H. Inkjet printing of polymer thin-film transistor circuits. MRS Bull 2003;28:829–34. [13] Minemawari H, Yamada T, Matsui H, Tsutsumi J, Haas S, Chiba R, et al. Inkjet printing of single-crystal films. Nature 2011;475:364–7. [14] Yoshioka Y, Jabbour GE. Desktop inkjet printer as a tool to print conducting polymers. Synth Met 2006;156:779–83. [15] Kawasea T, Shimoda T, Newsome C, Sirringhaus H, Friend RH. Inkjet printing of polymer thin film transistors. Thin Solid Films 2003;438– 439:279–87. [16] Liu Y, Cui T, Varahramyan K. All-polymer capacitor fabricated with inkjet printing technique. Solid-State Electron 2003;47:1543–8.
[17] Zhu L, Mimnaugh BR, Ge Q, Quirk RP, Cheng SZD, Thomas EL, et al. Hard and soft confinement effects on polymer crystallization in micophase separated cylinder-forming PEO-b-PS/PS blends. J Macromol Sci, C: Polym Rev 2001;42:9121–31. [18] Loo YL, Register RA, Ryan AJ. Polymer crystallization in 25-nm spheres. Phys Rev Lett 2000;84:4120–3. [19] Huang P, Zhu L, Chen SZD, Ge Q, Quirk RP, Thomas EL, et al. Crystal orientation changes in two-dimensionally confined nanocylinders in a poly(ethylene oxide)-b-polystyrene/polystyrene blend. Macromolecules 2001;34:6649–57. [20] Zhu L, Cheng SZD, Calhoun BH, Ge Q, Quirk RP, Thomas EL, et al. Crystallization temperature-dependent crystal orientations within nanoscale confined lamellae of a self-assembled crystallineamorphous diblock copolymer. J Am Chem Soc 2000;122:5957–67. [21] Huang P, Guo Y, Quirk RP, Ruan J, Lotz B, Thomas EL, et al. Comparison of poly(ethylene oxide) crystal orientation and crystallization behaviors in nano-confined cylinders constructed by a poly(ethylene oxide)-b-polystyrene diblock copolymer and a blend of poly(ethylene oxide)-b-polystyrene and polystyrene. J Macromol Sci, C: Polym Rev 2006;47:5457–66. [22] Hu W. Molecular simulations of polymer crystallization under nanoconfinement. American Physical Society, APS March Meeting, March 15–19; 2010 [abstract #B18.008]. [23] Müller AJ, Balsamo V, Arnal ML. Nucleation and crystallization in diblock and triblock copolymers. Adv Polym Sci 2005;190:1–63. [24] Steinhart M. Supramolecular organization of polymeric materials in nanoporous hard templates. Adv Polym Sci 2008;220:123–87. [25] Müller AJ, Balsamo V, Arnal ML. Crystallization in block copolymers with more than one crystallizable block. In: Reiter G, Strobl G, editors. Lecture notes in physics: progress in understanding of polymer crystallization. Berlin: Springer; 2007. p. 229–59. [26] Hamley IW. Crystallization in block copolymers. Adv Polym Sci 1999;148:113–37. [27] Loo YL, Register RA. Crystallization within block copolymer mesophases. In: Hamley IW, editor. Developments in block copolymer science and technology. New York: Wiley; 2004. p. 213–44. [28] Nandan B, Hsu J-Y, Chen H-L. Crystallization behavior of crystallineamorphous diblock copolymers consisting of a rubber amorphous block. J Macromol Sci, C: Polym Rev 2006;46:143–72. [29] Rastogi S. Role of metastable phases in polymer crystallisation; early stages of crystal growth. In: Reiter G, Sommer J-U, editors. Polymer crystallisation: observations, concepts and interpretations. Berlin: Springer-Verlag; 2003. p. 17–46. [30] Cheng SZD. Phase transitions in polymers. Amsterdam: Elsevier; 2008. [31] Forchheimer D, Platz D, Tholén EA, Haviland DB. Model-based extraction of material properties in multifrequency atomic force microscopy. Phys Rev B 2012;85:195449. [32] Garcia R. Dynamic atomic force microscopy methods. Surf Sci Rep 2002;47:197–301. [33] Organ SJ, Keller A. Solution crystallization of polyethylene at high temperatures. J Mater Sci 1985;20:1602–15. [34] Bittiger H, Marchessault RH, Niegisch WD. Crystal structure of polycaprolactone. Acta Crystal B: Struct Crystal Crystal Chem 1970;26:1923–7. [35] Brisse F, Marchessault RH. Contribution of electron diffraction on single crystals to polymer determination. ACS Symp Ser 1980;141:267–77. [36] Iwata T, Furuhashi Y, Su F, Doi Y. Single crystal morphologies of biodegradable aliphatic polyesters. RIKEN Rev 2001;42:15–8. [37] Núñez E, Gedde UW. Single crystal morphology of star-bransched polyesters with crystallisable poly(e-caprolactone) arms. J Macromol Sci, C: Polym Rev 2005;16:5992–6000. [38] Núñez E, Vancso GJ, Gedde UW. Morphology, crystallization, and melting of single crystals and thin films of star-branched polyesters with poly(e-caprolactone) arms as revealed by atomic force microscopy. J Macromol Sci, B: Phys 2008;47:589–607. [39] Beekmans LGM, Vancso GJ. Real-time crystallization of poly(ecaprolactone) by hot-stage atomic force microscopy. J Macromol Sci, C: Polym Rev 2000;25:8975–81. [40] Sanandaji N, Ovaskainen L, Klein Gunnewiek M, Vancso GJ, Haviland DB, Hedenqvist MS, et al. Morphology, unit cell structure and melting behaviour of single crystals of poly(e-caprolactone) prepared by the RESS technique. Polymer; submitted for publication .
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Inkjet printing as a possible route to study confined crystal structures N. Sanandaji a, A. Oko b, D.B. Haviland c, E.A. Tholén d, M.S. Hedenqvist a, U.W. Gedde a,⇑ a
KTH Royal Institute of Technology, School of Chemical Science and Engineering, Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden YKI, Ytkemiska Institutet AB/Institute for Surface Chemistry, SE-114 86 Stockholm, Sweden KTH Royal Institute of Technology, School of Engineering Sciences, Nanostructure Physics, Albanova, SE-106 91 Stockholm, Sweden d Intermodulation Products AB, SE-169 58 Solna, Sweden b c
a r t i c l e
i n f o
Article history: Received 23 July 2012 Received in revised form 26 September 2012 Accepted 5 October 2012 Available online 23 October 2012 Keywords: Inkjet printing Crystallization Poly(e-caprolactone) Confinement
a b s t r a c t Inkjet printing is a technique for the precise deposition of liquid droplets in the pL-volume range in well-defined patterns. Previous studies have shown that inkjet printing is attractive in polymer technology since it permits the controlled deposition of functional polymer surfaces. We suggest that the technique might also be useful for studying crystallization, in particular confined crystallization. Inkjet printing is a non-contact deposition method with minimal risk of contamination, which allows the exact deposition of both polymer solutions and polymer melts. This paper demonstrates the possibility of utilizing the technique to create surfaces where polymer chains form isolated small structures. These structures were confined by both the low polymer content in each droplet and the time constraint on crystal formation that arose as the result of the rapid solvent evaporation from the pL-sized droplets. In theory, inkjet printing enables the exact deposition of systems with as few as a single polymer chain in the average droplet. With appropriate instrumentation, the versatile inkjet technology can be utilized to create whole surfaces covered with polymer structures formed by the crystallization of small, dilute and rapidly evaporating droplets. 110 pL droplets of a 10 6 g L 1 poly(e-caprolactone) solution in 1-butanol have been deposited and studied by atomic force microscopy. Small structures of ca. 10 nm thickness and ca. 50 nm diameter also seemed to exhibit crystalline features. Some of the small structures had unusual rectangular forms whilst others were interpreted to be early precursors to six-sided single crystals previously observed for poly(e-caprolactone). The unusual forms observed may have resulted from the entrapment of crystal structures into metastable phases, due to the limited amount of polymer material present and the rapid evaporation of the droplets. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Inkjet printing, a digital printing technology, is increasingly being applied in scientific fields such as nanotechnology, tissue engineering and polymer engineering [1,2] where it can be used for the precise deposition of pL volumes of suspensions or solutions in welldefined patterns. This ‘‘direct-writing’’ can be accurately controlled by micro-electro-mechanical ink-dispensers, ⇑ Corresponding author. Tel.: +46 8 7907640; fax: +46 8 208856. E-mail address: [email protected] (U.W. Gedde). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.10.001
which in turn are controlled by computers and highspeed electronics. An advantage of the technique is that inkjet printing offers non-contact deposition, which means that contamination is minimized [3]. Another advantage is that the precise deposition, in terms of both volume and placement, enables the efficient layer-bylayer assembly of polymer films. Typically, multiple rinsing steps are required during the creation of multi-layered assemblies in order to remove excess material. However, with inkjet printing, only the necessary quantity of polymer is deposited in each step, eliminating the need for rinsing [4].
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Recent studies show that inkjet printing is one of the most promising methods for the controlled deposition of functional polymer surfaces [5]. According to de Gans and Schubert [6], inkjet printing of polymers is a promising technique for the creation of polymer micro-arrays and libraries. Hoth et al. [7] have utilized the technique to produce heterojunction solar cells, by printing photoactive layers consisting of poly(3-hexylthiophene) blended with fullerene (6,6)-phenyl C61 butyric acid methyl ester. Several studies [8–11] have shown that inkjet printing can be used to manufacture polymer film transistors and light emitting diodes. Inkjet printing has also been used for the manufacture of polymer semiconductors, dielectrics and conductors [12–14]. Kawasea et al. [15] have inkjetprinted thin polymer films to create all-polymer transistors, active-matrix backplanes and inverters and Liu et al. [16] have inkjet-printed all-polymer capacitors. In this study, the application of inkjet printing is explored in another field of polymer technology, namely confined crystallization. As shown in this paper, inkjet technology permits the crystallization of dilute polymer solutions deposited in small and rapidly evaporating droplets. Crystal structures are formed under physical confinement, as they are not able to expand beyond a certain volume due to a limit of polymer chains in their vicinity. A number of studies [17–27] have focused on confined crystallization, showing that confinement tends to influence characteristics such as surface energy and crystallization kinetics. Studying confined crystallization is a way of gaining insight into metastable phases, which may precede a stable crystalline phase. These phases are inherently difficult to study due to their ephemeral nature. Since the presence of metastable phases has been shown to be size-dependent [28,29], confined crystallization represents a method for trapping the polymers in a metastable phase. Inkjet deposition not only makes it possible to ‘print’ structures where crystal growth is limited by the small amount of material present in each droplet, but also leads to an additional confinement effect due to the evaporation of the pL size droplets within a few seconds. This rapid evaporation constrains the process of crystal formation, limiting the ability of the polymer chains to assemble into crystalline structures while in solution, and it may therefore prove valuable for trapping the polymer in a metastable phase, a precursor to crystallization. Another advantage of the inkjet technique is that the non-contact deposition method is free from contaminants, which may otherwise influence crystal formation and particularly the initiation of crystal formation. For these reasons, we propose that inkjet printing can be used as a viable tool for studying the controlled crystallization of confined polymer systems.
mer while it was being stirred. Solutions were subsequently diluted in several steps to obtain concentrations as low as 10 6 g L 1. The solutions were again filtered before inkjet dispensing, in order to ensure samples with a very high purity. Impurities would not only have affected crystal growth but could also have led to clogging of the nozzle. 2.2. Inkjet operating principles Fluid ejection was accomplished using a MD-K-140 piezoelectric dispenser obtained from Microdrop Technologies, Norderstedt, Germany. Inkjet printing is based on the creation of an acoustic pulse which ejects liquid droplets through a nozzle. The pulse can be generated by either thermal or piezoelectric means [5]. In the instrumental setup used in this work, the latter technology was used to eject droplets of precise size. The dispenser together with the fluid reservoir flask and the piezoelectric mechanism are sketched in Fig. 1. The core of the dispenser head consisted of a glass capillary surrounded by a tubular piezoelectric actuator. One end of the capillary was formed as a 51 lm nozzle. A voltage pulse caused a contraction of the piezoelectric actuator and built up a pressure wave that propagated through the capillary and pushed fluid outwards. The kinetic energy of the acoustic wave was partly converted to the surface energy of droplet formation, and partly to the kinetic energy of the free-flying droplet. When the amount of kinetic energy transferred outwards was greater than the energy needed for droplet formation, 110 pL droplets were ejected with a velocity of 2 m s 1. The ejected droplet flew freely through the air and in practice could be very accurately placed on a substrate located just below the dispenser head. The substrates were silica plates, cleaned by ethanol, Milli-Q water and plasma followed by vacuum drying. The sample stage on which the substrates were placed could be accurately moved in the x, y and z directions as necessary. Depending on the choice, either single droplets or arrays of multiple droplets were dispensed. Dispensing was carried out at a constant relative humidity of 45 ± 3%, and at constant temperature, 23 ± 1 °C. Both the fluid reservoir and the dispenser head were heated to 70 °C during dispensing. Fig. 2 sketches the droplet formation shortly after dispensing. As the droplet flew through the air it acquired a spherical shape. Each dispensing step, from the creation of the acoustic pulse to the formation of a spherical droplet, occurred so precisely that when stroboscopic images were taken at the same frequency of droplet formation, the viewer could almost be fooled to think that she or he were studying a single droplet hovering in mid-air. 2.3. Atomic force microscopy
2. Experimental 2.1. Materials and sample preparation PCL with M n = 10 kDa and M w = 14 kDa, and 1-butanol with greater than 99% purity, were obtained from Sigma Aldrich. The 1-butanol solvent was filtered before mixing with PCL and heated to 90 °C in order to dissolve the poly-
The solidified droplets on the substrate were studied by atomic force microscopy (AFM) in order to determine their mechanical response to forces applied by the AFM tip as it oscillated above the surface. The AFM (Digital Instruments, Dimension 3100) was equipped with a surface analysis upgrade for intermodulation AFM (Intermodulation Products AB), a multi-frequency dynamic force measurement
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Fig. 1. Sketch of the inkjet dispenser (right) and of the piezoelectric mechanism (left). The piezoelectric mechanism is based on the application of a voltage pulse (left, second figure from top) leading to the propagation of a pressure wave through the capillary (left, third and fourth image from top). The wave transfers the liquid to the nozzle (left, fifth image from top) and finally leads to formation of a droplet when the kinetic energy transferred outwards is greater than the surface energy required to form a droplet (left bottom image). A = Cord to heat control box. B = Vent. C = Cord to dispenser control box, controls dispensing and temperature at dispenser head. D = Dark grey area shows a protective shell including heating elements, which also extends around the flask containing the polymer solution.
Fig. 2. Sketch of the process of droplet formation as it appears shortly after dispensing. The droplets adopt a spherical shape during their flight.
method which rapidly and accurately extracted the nonlinear response of the oscillating cantilever. This response was analyzed using a contact-mechanics model for the nonlinear tip-surface force, in order to determine the model parameters that fitted the measured response [30]. In this way, parameter maps of the surface were generated, color-coded, and projected onto a two-dimensional rendering of the surface topography. The Derjaguin-MullerToporov (DMT) model [31] was used to determine the effective Young’s modulus E⁄ at each pixel of the image (256 256 pixels). It should be emphasized that this simple model neglects many effects, including the actual shape and geometry of the surface as well as the surface
energy. E⁄ therefore should not be compared with the bulk Young’s modulus of PCL, but rather considered as an effective measure of the surface stiffness, expressed as the equivalent bulk Young’s modulus of a material with an ideal planar interface, being indented with a rigid and perfectly round sphere of diameter 10 nm.
3. Results and discussion The polymer structures formed on the surfaces of the silica plates varied according to the concentration of the dispensed solution and the number of droplets placed on
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Fig. 3. A parameter map showing the surface stiffness, expressed as an equivalent modulus E⁄ as defined by the DMT contact mechanical model. E⁄ is color coded and projected on a 3D rendering of the surface topography. The structure shown was formed after dispensing 100 droplets of 10 5 g L 1 PCL solution. The edge of a spherulite is shown at the top right.
each spot. In Fig. 3a parameter map, based on an AFM height image, is shown for structures that have formed after deposition of 100 droplets of 10 5 g L 1 PCL solution. The amount of material deposited was relatively high in this sample, resulting in the formation of spherulites and thin films of branched finger-like structure. As expected, the silicon substrate showed a high E⁄-value (ca. 2.4 GPa). The thin PCL film was softer with an average recorded E⁄value of ca. 0.5 GPa. The plateaus of the thinner branching areas showed significantly higher E⁄-values than those of comparable locations on the spherulite. This is consistent with the interpretation that the measured stiffness is not a bulk stiffness, but that of a thin film, where the stiffness of the underlying substrate play a significant role. E⁄-values of regions with the same thickness were comparable and showed that locally harder regions existed both in the thin film and on top of the spherulite. In the regions outlined by white boxes in Fig. 3, E⁄ varied on the thin PCL film between 1.2 and 2.3 GPa, despite the fact that the film thickness was essentially the same through out this area. In the spherulitic region outlined by the red boxes1 (Fig. 3), E⁄ varied between 0.4 and 1.2 GPa. These local differences in E⁄ indicated that there were differences in molecular packing in both the film and the spherulite. Spots obtained from droplets which contained lower concentration of PCL did not form spherulitic textures, but instead round ‘island’ structures. Fig. 4 shows a height AFM image of such island structures formed in a sample where 10 droplets of 10 6 g L 1 PCL solution were deposited. The structures were uniformly dispersed and had a diameter of ca. 50 nm. The different ‘islands’ showed about the same E⁄, reflecting a uniform structure. Narrow bridges connecting the islands structures were observed. These bridges were aligned along a specific direction, and had formed in the process where the edge of the small pool formed by 10 droplets was moving, due to evaporation of solvent. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Fig. 5 shows structures with a diameter of ca. 60 nm formed after the deposition of single droplets of 10 6 g L 1 PCL. Calculation showed that each droplet contained approximately 4700 individual PCL molecules, whilst each island structure contained only ca. 100 molecules. The small structures were rounded, but our interpretation is that crystallinity could still be inferred from their structure. Due to rapid evaporation of droplets, and the low concentration deposited, the formed structures were expected to differ from developed single crystals. When studying structures with a diameter of 60 nm or smaller, it is important to note the limitations of the AFM method. The AFM measures a convulsion of the tip shape with the shape of the surface feature. The radius of a measured structure is enlarged by the radius of the tip, which in this study was 5 nm. The height on the other hand is more accurately preserved. It follows that the structures shown in Fig. 5 actually had a diameter of 50 nm, rather than the 60 nm observed. It also follows that the structures in actuality had sharper edges, which were rounded by the artifact effect created by the AFM tip. In Fig. 6, additional structures formed after deposition of single droplets of 10 6 g L 1 PCL are displayed. The smallest structures had diameters of ca. 20 nm whereas the largest ones had diameters of ca. 60 nm (translating to 10 nm and 50 nm respectively, when the effect of the AFM tip is taken into account). The smallest structures were only ca. 2 nm high, compared to the largest structures that had a height of 10–11 nm. We interpret that these structures as precursors to the six-sided single crystals previously observed for PCL [32–38]. Some of the facet angles found were close to the theoretical angles of the mature six-sided crystals, i.e. 113° (angle between the {1 1 0} and {1 1 0} sectors) and 124° (angle between the {1 1 0} and {1 0 0} sectors). In addition small structures with a rectangular shape were observed. The insert in Fig. 7 shows part of a structure formed after deposition of 10 droplets of 10 6 g L 1 PCL solution. Interestingly, the image shows features with a rectangular shape (marked by the white outline). This
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Fig. 4. A parameter map showing the surface stiffness, expressed as an equivalent modulus E⁄ as defined by the DMT contact mechanical model. E⁄ is color coded and projected on a 3D rendering of the surface topography. The structure shown was formed after dispensing 10 droplets of 10 6 g L 1 PCL solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Height AFM image of structures formed after deposition of a single droplet of 10 6 g L 1 PCL solution.
Fig. 7. Height AFM image of structures formed after deposition of 10 droplets of 10 6 g L 1 PCL solution. The inset image shows a close up of region marked with a dashed outline in the right-hand image.
unusual shape of a PCL crystal has recently also been observed in single crystals grown under extreme conditions (0–50 °C) from dilute solution in supercritical carbon dioxide [39]. The unusual lateral habit also suggested the presence of a different unit cell. Sanandaji et al. [40] present further details about this structure. 4. Conclusions
Fig. 6. Height AFM image of structures formed after deposition of a single droplet of 10 6 g L 1 PCL solution.
This study has utilized inkjet technology to crystallize small quantities of PCL, deposited in small droplets with a precise size. Since the printing method allows for the precise deposition of thousands or even millions of small droplets, it can be used for creating surfaces where polymer chains form small isolated structures. The non-contact technique makes it possible to minimize contamination, which is valuable for gaining clean crystal systems. Utilizing this technology, it has been possible to study small
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structures consisting of only ca 100 polymer chains. Even though such small structures are inherently difficult to study due to their very limited size, crystallinity could be inferred from their structure. Some of the small structures formed were interpreted to be early precursors to the sixsided single crystals previously observed for PCL whereas others were rectangular. These for PCL atypical structures indicate unusual molecular packing, which in turn can be explained by the fact that the structures were formed with a limited amount of polymer material present and during the rapid evaporation of solvent from small droplets. It is possible to utilize inkjet technology to create even more confined systems. By using smaller nozzles and more dilute solutions, small droplets can be deposited which contain on average only a single polymer chain. Indeed, inkjet printing can be used to create whole surfaces consisting of single polymer chains deposited in rapidly evaporating droplets, where the individual chains are deposited in proximity to each other but have no direct contact. Acknowledgments This study was financed by the Swedish Research Council through Grants Nos. 2006-3559 and 2009-3188. References [1] de Gans B-J, Duineveld PC, Schubert US. Inkjet printing of polymers: state of the art and future developments. Adv Mater 2004;16:203–13. [2] Singh M, Haverinen HM, Dhagat P, Jabbour GE. Inkjet printing – process and its application. Adv Mater 2010;22:673–85. [3] Tekin E, Smith PJ, Schubert US. Inkjet printing as a deposition and patterning tool for polymers and inorganic particles. Soft Matter 2008;4:703–13. [4] Andres CM, Kotov NA. Inkjet deposition of layer-by-layer assembled films. J Am Chem Soc 2010;132:14496–502. [5] de Gans B-J, Schubert US. Inkjet printing of well-defined polymer dots and arrays. Langmuir 2004;20:7789–93. [6] de Gans B-J, Schubert US. Inkjet printing of polymer micro-arrays and libraries: instrumentation, requirements, and perspectives. Macromol Rapid Commun 2003;24:659–66. [7] Hoth CN, Choulis SA, Schilinsky P, Brabec CJ. High photovoltaic performance of inkjet printed polymer: fullerene blends. Adv Mater 2007;19:3973–8. [8] Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W, et al. High-resolution inkjet printing of all-polymer transistor circuits. Science 2000;290:2133–6. [9] Bharathan J, Yang Y. Polymer electroluminescent devices processed by inkjet printing: i polymer light-emitting logo. Appl Phys Lett 1998;72:2660–2. [10] Edwards C, Albertalli D. Application of polymer LED materials using piezo ink-jet printing. SID Symp Dig 2001;32:1049–51. [11] Kawase T, Sirringhaus H, Friend RH, Shimoda T. Inkjet printed viahole interconnections and resistors for all-polymer transistor circuits. Adv Mater 2001;13:1601–5. [12] Seamus EB, Cain P, Mills J, Wang J, Sirringhaus H. Inkjet printing of polymer thin-film transistor circuits. MRS Bull 2003;28:829–34. [13] Minemawari H, Yamada T, Matsui H, Tsutsumi J, Haas S, Chiba R, et al. Inkjet printing of single-crystal films. Nature 2011;475:364–7. [14] Yoshioka Y, Jabbour GE. Desktop inkjet printer as a tool to print conducting polymers. Synth Met 2006;156:779–83. [15] Kawasea T, Shimoda T, Newsome C, Sirringhaus H, Friend RH. Inkjet printing of polymer thin film transistors. Thin Solid Films 2003;438– 439:279–87. [16] Liu Y, Cui T, Varahramyan K. All-polymer capacitor fabricated with inkjet printing technique. Solid-State Electron 2003;47:1543–8.
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