Fabrication of Ferroelectric Polymer Nanocrystals with Tunable

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Fabrication of Ferroelectric Polymer Nanocrystals with Tunable Morphologies Min Kyung Lee and Jonghwi Lee* Department of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, South Korea ABSTRACT: We developed a procedure for the reliable and reproducible formation of monodispersed polymer nanocrystals whose morphologies can be easily controlled to produce structures ranging from nanoparticles to nanorods. The directional cooling crystallization of ferroelectric polymer solutions under nanoconfinement allowed polyvinylidene fluoride to form single crystalline monodisperse nanocrystals without the use of surfactants or stabilizers. An important growth condition necessary to yield highly crystalline ferroelectric polymers was a regulated thermal gradient under nanoconfinement to induce uniform nucleation and controlled directional growth. While other methods involving the nanoconfined crystallization of polymers are restricted to yield nanofibers or nanotubes with a high aspect ratio, our unidirectional cooling method produced nanocrystals of ferroelectric polymers with nanosphere, nanocapsule, and nanorod structures. Field-emission scanning electron microscopy, transmission electron microscopy, dynamic contact electrostatic force microscopy, grazing incidence X-ray diffraction, and differential scanning calorimetry were used to characterize the morphologies and structures of the nanocrystals. The results suggest that the directional cooling method facilitates nucleation and directional growth by controlling diffusion and promoting molecular alignment from the preferred crystal orientations due to the unidirectional thermal gradient. This study provides a novel method of rational material design to fabricate polymer nanocrystals by directing nucleation and growth of molecular crystals from solution.

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INTRODUCTION Polymer nanocrystals have potential applications in many areas such as microelectronics, energy harvesting, optical devices, and drug delivery. Along with advances in nanotechnology, there has been continuous research to develop polymer nanocrystals with optimized properties. There are diverse fabrication methods available such as solvent evaporation, salting-out, dialysis, supercritical fluid, and polymerization techniques. Although these methods can produce polymer nanocrystals, each technique has drawbacks. For example, the solvent evaporation method1 is time-consuming and the possible coalescence of nanodroplets during evaporation may affect the final particle size and morphology. The main problem in utilizing supercritical fluid technology2 is the poor solubility or even nonsolubility of polymers. Emulsion polymerization3 requires a large ratio of surfactant to monomer. Among the requirements of the preparation methods, precise control over the particle size and morphology is generally the most demanding for most applications, which is difficult to satisfy using current preparation methods. Furthermore, the requirement that a polymeric system should be completely free from additives or reactants such as surfactants or traces of organic solvents, which is often critical for applications in biomedical or environmental fields, is also hard to satisfy using current methods. The recent studies of polymer crystallization in a confined space have suggested the possibility of controlling nanoscale crystallization.4 The nanotemplating method via wetting © XXXX American Chemical Society

cylindrical nanopores with polymer melts or solutions has been an active area of research due to its simplicity and versatility in generating 1D polymeric nanostructures and modifying polymer crystal structures by confinement.5 However, the nanotemplating method is restrictive in the scope in that most of the nanotemplating research has focused on nanorods and nanotubes with high aspect ratios since there is no effective way to control the penetration depth of wetting, partially due to the high processing temperatures needed during the melt wetting process. The templating method is essentially a type of molding technique in which the pore morphology is replicated, the versatility of which is basically limited by the morphology of the template pores. The ability to control polymer crystallization with respect to polymorphism and orientation is also a crucial challenge.6 Moreover, template wetting with polymeric solutions is poorly understood and difficult to reproducibly execute under well-defined conditions due to the difficulty in controlling solvent evaporation on the nanoscale. In this work, a novel method of polymer nanoparticle preparation without using additives or reactants was developed. We present a simple combination of controlled cooling crystallization (freezing)7 and nanoconfinement processes to fabricate polymeric nanocrystals, whose morphology can be Received: September 15, 2012 Revised: December 10, 2012

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dx.doi.org/10.1021/cg301350g | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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5 min of ultrasonic cleaning in deionized water. For solution wetting, the AAO template was immersed in a PVDF solution for 24 h and the AAO template was rapidly placed on filter paper to remove the residual solution outside the nanopores. The AAO template was subsequently put on the top of a copper plate (thickness of 0.5 mm), and the cooper plate was placed in contact with a liquid nitrogen bath. After the contact, freezing started from the bottom and spread to the top of the sample within 15 s, as the 1,4-dioxane (Tm of 11.8 °C) was allowed to freeze. In this freezing step, a sample was cooled to induce columnar 1,4-dioxane crystal growth along the vertical direction. To obtain reproducible results, all other experimental factors, such as the amount of sample and liquid nitrogen, were carefully kept constant. The frozen sample was then freeze-dried in a FD-1000 freeze-dryer (EYELA, Tokyo, Japan, trap chilling temperature of −45 °C, 5.6 Pa) for 24 h. Characterization. Field emission scanning electron microscopy (FESEM, SIGMA, Carl Zeiss, Korea) was used to analyze the AAO template with PVDF nanostructures. SEM images were obtained at an accelerating voltage of 2 kV and a working distance of 11 mm. A platinum layer was deposited onto the samples by ion sputtering (E1030, Hitachi, Japan). The number-average, short-axis diameter, and aspect ratios of the samples were measured using Scion Image software (Alpha 4.0.3.2, NIH, MD, USA). Three areas (1.5 × 1 mm) from each sample were analyzed. To detach the PVDF nanocrystals from the AAO template, the AAO template was removed by dissolving it in a 5 wt % NaOH solution. The detached PVDF nanostructures were ultrasonically deagglomerated and dispersed on an amorphous carbon film supported by a Cu grid for transmission electron microscopy (TEM). A JEOL-2000 TEM (JEOL, Peabody, MA, USA) operating at an accelerating voltage of 200 kV and equipped with a double tilt holder was used for imaging and electron diffraction. A CCD camera (DualView, Gatan, USA) was used to record the images and diffraction patterns. The camera length was calibrated using a similar TEM grid coated with 10 nm of aluminum. The beam current was limited to approximately 15 pAcm−2 to minimize damage of the sample. Diffraction calibration was performed using TiCl d spacings and their higher order diffractions. X-ray diffraction (XRD) was measured using a D/max2500 (Rigaku, Japan) with a q-2q method using a voltage of 40 kV and a Cu Kα source (λ = 1.541 Å). Grazing incidence X-ray diffraction (GIXD) measurements were performed using a four-circle diffractometer (SMARTLAB, Rigaku) with a rotating Cu anode X-ray generator. The incidence angle was fixed at 0.3° so that the X-ray beam could penetrate the entire thickness of the AAO surface containing the PVDF nanostructures (∼100 nm). To investigate the freezing behavior (freezing points, Tf) as a function of the PVDF concentration, differential scanning calorimeter (DSC) experiments were performed. Approximately 20 mg of the AAO templates filled with PVDF solutions of different concentrations were analyzed. The samples were cooled from 20 °C to −50 °C and reheated to 20 °C at a scanning rate of 10 °C/min in a Seiko Exstar 7000 (DSC 7020, Japan). Both the bulk and nanoconfined systems were tested in order to determine the effect of confinement on the freezing behavior. The piezoelectric response data was obtained using a cantilever with a conductive Ti−Pt-coated tip in the piezoelectric response spectroscopy mode of a dynamic contact electrostatic force microscope (DCEFM, XE-100). Silver paste was used to ensure good contact between the sample and metal substrate. DC (±50 V) and AC (17 kHz and 10 V) bias voltages were applied between the sample surface and tip. The theory of DC-EFM is to utilize the sustained vibration of the cantilever even in the contact state when an AC electrostatic modulation signal is applied to the tip. The piezoelectric effect causes vibration of the sample surface, which directly translates to the AC modulation of cantilever bending. Therefore, the ferroelectric response signal from the cantilever deflection can be measured using a lock-in amplifier.

easily controlled from nanospheres to nanorods. How the controlled crystallization of a solvent by a temperature gradient affects the phase separation and crystallization of polymer chains (solute) was specifically investigated, where both crystallizations are constrained by cylindrical nanopores. Our process of particle formation comprises three stages: wetting, crystal nucleation, and growth. The main driving forces of these phenomena are supersaturation and a temperature gradient. Poly(vinylidene fluoride) (PVDF) was used in this work as a typical semicrystalline and ferroelectric polymer for developing novel ferroelectric nanocomponents useful for microelectronics, sensors, actuators, or energy harvesting systems. PVDF exhibits several polymorphisms (α, β, γ, and δ forms, which transform under certain conditions): the nonpolar TGTG′ α and δ phases, the polar TTTT β phase, and the TTTGTTTG′ γ phase, in which T and G are the trans and gauche chain conformations in a PVDF crystal, respectively.8 The preparation of PVDF for potential commercial applications relies heavily on the effective fabrication of polar ferroelectric crystalline nanostructures such as β and γ, which were conveniently obtained by the controlled nanoconfined cooling crystallization in this study.4a,9

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EXPERIMENTAL SECTION

Materials. Anodized alumina oxide (AAO) templates with a pore size of 200 nm were purchased from Whatman (Maidstone, U.K.). The membranes are freestanding disks with a diameter of 13 mm and a thickness of 60 μm. The poly(vinylidene fluoride) (PVDF, average Mw = 534 K g/mol) used in this work was purchased from Sigma Aldrich (USA) and used as received. 1,4-Dioxane (99%) was purchased from Samchun Pure Chemical (Korea). HPLC-grade water was purchased from J. T. Baker (Phillipsburg, NJ, USA). Nanocrystal Preparation by Nanoconfined Directional Cooling Crystallization. The experimental setup for confined directional cooling crystallization (Figure 1) was designed to obtain polymer nanocrystals. PVDF solutions with different concentrations in 1,4dioxane were prepared. Prior to the wetting process, each AAO template was ultrasonically cleaned in acetone for 15 min, followed by

Figure 1. Fabrication of PVDF nanostructures by nanoconfined, unidirectional cooling crystallization within an AAO template. B



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Figure 2. SEM images of (a) an AAO template, (b) PVDF particles prepared by nanoconfined, unidirectional cooling crystallization within an AAO template, (c) PVDF particles detached from the AAO template, and (d) a TEM image of PVDF particles detached from the AAO template.

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low, the amount of polymer was relatively small to fill the AAO nanopores. As the PVDF solution concentration increased, particles grew larger until a size comparable to the pore diameter was obtained (Figure 3c). Theoretical analyses have demonstrated that confinement inhibits the Ostwald ripening mechanism in which a coarsening process commonly follows in bulk and a plug−tube−capsule phase diagram is predicted.13 Tanaka observed that neighboring droplets merged once their size exceeded the confined pore size and eventually transformed into capsules.14 Similarly, the PVDF particles in this study could not continue to grow spherically when encountering nanopore confinement. Instead, the formation of nanoellipsoids along the pore axis occurred, as shown in Figure 3d. A further increase of the PVDF concentration led to the formation of nanorods with a high aspect ratio (Figures 3e,f). Until the short-axis diameter reached the size of the AAO pores, the aspect ratio remained at unity (spherical particles) and then increased nearly linearly with the increase in concentration while the short-axis diameter remained constant (Figure 4). This control of the particle morphology can allow for the formation of various structures, such as Janus particles and blocked nanorods, which have been reported to be difficult to produce using melt or solution wetting approaches.11 For comparison, a series of samples were prepared by solvent wetting and evaporation instead of the freezing method. In solution wetting methods, solvent was evaporated at the solvent/air interface, the rate of which is difficult to control. As the evaporation progressed, polymer chains concentrated at the interface, which induced a heterogeneous surface PVDF morphology (Figure 5). The poorly controlled solvent evaporation may also lead to a temporary fluctuation in the concentration gradient inside the pores. Because freezing prohibits significant solvent evaporation in our approach, the

RESULTS AND DISCUSSION To prepare PVDF nanostructures by nanoconfined directional cooling crystallization, an AAO template with an average pore size of 200 nm (Figure 2a) was wetted with different concentrations of PVDF in 1,4-dioxane, which has good wetting characteristics for AAO.10 This first step, solution wetting, is the same as that described in previous AAO templating methods.11 However, the subsequent controlled nanoconfined cooling crystallization steps provide a completely novel means of controlling the nanostructure and molecular chain alignment. We used a liquid nitrogen reservoir to generate a temperature gradient, similar to our previous experiments in a freeze-drying system.7b,12 Temperature gradient-induced freezing grows the solvent crystals and, subsequently, the semicrystalline phase of the polymer molecules. As shown in Figure 2, the SEM images of the PVDF nanostructures after removing solvent crystals and subsequently dissolving the AAO templates demonstrate that nanoconfined directional cooling crystallization is not a simple nanomolding method and that the nanostructures were all relatively uniform. Although the pores of the commercial AAO used in this study were irregular (Figure 2a), the morphology of the PVDF particles (Figure 2b) did not exactly reflect the shape of the AAO nanopores as if formed via solvent crystallization. The obtained PVDF nanoparticles were confirmed by the fact that the individual particles can be detached (Figures 2c,d). The images in Figure 3 demonstrate how the aspect ratio can be engineered. An increase of the PVDF concentration was followed by nanostructural transformation from spherical nanoparticles to elliptical nanoparticles and further to nanorods. As can be seen in Figure 3a,b, the average size of the nanoparticles increased with increasing concentration of the polymer. When the PVDF solution concentration was relatively C



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Figure 3. SEM micrographs of PVDF nanostructures resulting from nanoconfined, unidirectional cooling crystallization of PVDF/1,4-dioxane solutions with different initial concentrations: (a) 1, (b) 3, (c) 5, (d) 6, (e) 7, and (f) 10 wt %.

correspond to the (020) and (110) reflections in the γ phase crystal, respectively.15 The γ phase diffraction peaks were well developed only under the 1D gradient. By contrast, the peaks were less pronounced in the other two cases with noticeable α phase peaks, indicating that PVDF chains were preferentially aligned by the cooling direction inside the AAO pores where the solvent crystallized (frozen) and aligned along the pore direction. The PVDF chains were directionally excluded from the solvent crystals. Crystallization of both the solvent and PVDF was governed by the directional freezing-induced alignment. TEM confirmed the morphology and crystal orientation of homogeneous nanometer-scale structures (Figure 7). In Figure 7a, monodisperse spherical nanocrystals with sizes ranging from 50 to 100 nm grown by controlled cooling crystallization under AAO confinement were observed. Direct evidence of the alignment of the PVDF chains was confirmed in the high magnification TEM image shown in Figure 7b. Electron diffraction patterns from individual nanocrystals showed an

morphologies of the nanostructures could be reliably controlled without a temporary concentration fluctuation or heterogeneous surface morphology. The formation of polymeric nanostructures inside the AAO pores was demonstrated in a reproducible manner based on the initial solution concentration and cooling direction. To further understand the effect of the 1D temperature gradient, cooling was applied in two ways: (1) unidirectional cooling following a 1D temperature gradient parallel to the AAO thickness direction and (2) nonunidirectional cooling imposed by dipping in liquid nitrogen. The PVDF nanostructures were evaluated by grazing incidence X-ray diffraction (GIXD) experiments that allowed X-ray scattering from the oriented samples in AAO. The analysis was performed on a PVDF-filled AAO template in the GIXD mode such that the scattering vector was near perpendicular to the template surface (parallel to the pore axis). The GIXD results (Figure 6) were compared to a vacuum drying case. The PVDF within the nanopores had two intense peaks at 18.5 and 20.1°, which D



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those in Figure 7 have rarely been reported. Indeed, from a crystallographic point of view it is very difficult to produce a spherical single crystal. In this system, the spherical particles must have amorphous-state chains allowing the formation of spherical morphology, as most polymeric crystals have significant amorphous contents. In general, spatial restriction causes anisotropic crystal growth.15 Only crystal growth in the unconfined direction, which advances in parallel with lateral crystal growth, can easily proceed and allows a unique crystallinity to be obtained. In this study, uniform nucleation and growth of PVDF single crystals, possibly aligned, occurred during anisotropic crystal growth of 1,4-dioxane. The properties of the polar ferroelectric γ-PVDF nanocrystals were confirmed by piezoelectric response spectroscopy. Figure 8 shows the hysteresis curve representing the polarization switching of the surface. When the bias voltage was ±50 V, the polarity of the nanoparticles ferroelectrically switched. A freezing front (solid−liquid interface) will move at a controlled speed in an AAO nanopore under directional cooling crystallization. As 1,4-dioxane unidirectionally freezes, the polymer chains are directionally excluded from the solvent crystals with the development of a concentration gradient in the unfrozen part of the system in front of the growing crystals. PVDF chains can be aligned by the influence of the growing direction of solvent crystals within nanopores and by the transfer of heat across the moving freezing front. In the early stage of PVDF nucleation, PVDF chains may rearrange themselves for the formation of the γ phase in the cryoconcentration region or at the freezing front. Subsequent

Figure 4. Average short-axis diameter and aspect ratio of the PVDF nanostructures as a function of the PVDF solution concentration in 1,4-dioxane.

unexpected single crystal structure, which allowed us to determine the crystal symmetry and lattice parameters (Figure 7c). A monoclinic phase was observed with lattice parameters of a = 4.97 Å, b = 9.67 Å, c = 9.24 Å, and β = 93°, consistent with the monoclinic space group C32.16 The electron diffraction results confirm the existence of highly oriented PVDF nanostructures with the molecular chain aligned in the cooling direction. The nanocrystals were spherical (Figure 3a and Figure 7a), but they are highly anisotropic and have a single crystal γ form. Single crystal, polymeric nanoparticles such as

Figure 5. SEM image of PVDF nanostructures produced from vacuum drying of PVDF/1,4-dioxane solutions with different initial concentrations: (a) 1, (b) 3, (c) 5, and (d) 7 wt %. E



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Figure 8. Local phase response to DC bias between the tip and sample surface of the PVDF nanospheres prepared by unidirectional cooling crystallization.

Freezing a solution is a complex phenomenon at the micro and molecular levels. The interactions between solvent and polymer and their molecular mobility can widely alter the results of the freezing phenomenon, particularly in a confined system. Furthermore, there are numerous other parameters that cannot be neglected, such as the freezing stress, desiccation stress, heat transfer, heat extraction from the moving freezing front into the remaining unfrozen solvent, supersaturation, freezing rate, interface instability, viscoelastic properties of cryoconcentrates, and partial solidification in cryoconcentrated regions. As the heat transfer rates are typically much greater than the mass transfer rates, polymer diffusion tends to control the growth of the solvent crystals. The observed single crystal nanoparticles at concentrations less than 3 wt % are related to the diffusion-limited growth. In the confined space, significant shifts in the freezing temperature due to intermolecular forces can be expected. Two intermolecular potentials are then involved in a simple approach, based on the pair of potentials of uff(r) = eff f(r/sff) and ufw(r) = efw f(r/sfw), where e and s are the surface energy and molecular diameter parameters, respectively, and f and w denote fluid and wall molecules, respectively.17 The phase transition temperature, Ttr, is a function of three dimensionless variables, Ttr* = f (H*,a,sfw/sff), where H* = H/sff, a = Cefw/eff, and α is the ratio of the fluid−wall (fw) to fluid−fluid (ff)

Figure 6. GIXD results for (a) PVDF nanostructures produced from unidirectional cooling crystallization, nonunidirectional cooling crystallization, and solvent evaporation of 10 wt % PVDF/1,4-dioxane solutions. (b) PVDF confined within AAO templates resulting from unidirectional cooling crystallization of PVDF/1,4-dioxane solutions with different initial concentrations.

controlled growth of the polymer phase within the nanoconfined space would follow, and the growth will be restricted by the number of available polymer chains. The preferred γ phase demonstrated in this study, which is uncommon in simple confined templating, supports this conjecture about the chain alignment induced by directional cooling crystallization.

Figure 7. TEM images of (a) PVDF nanospheres formed by unidirectional cooling crystallization of a 3 wt % PVDF/1,4-dioxane solution, (b) magnified image showing the periodic arrays of crystalline lamellae, and (c) electron diffraction (ED) pattern of the PVDF nanospheres in the specimen at a normal incidence. F



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Figure 9. (a,b) Freezing temperatures (Tf1 and Tf2) of confined (w/AAO) and unconfined (w/o AAO) PVDF solutions defined by the respective peak maximum temperatures of the DSC traces. The freezing temperature of pure 1,4-dioxane, Tf1, decreases with nanoconfinement or with increased PVDF solution concentration.

attractive intermolecular potentials. The parameter C depends on the pore geometry and the nature of the wall material and accounts for the arrangement and density of the wall atoms. The relative strength of the attractive/repulsive interaction at the interface can be used to classify porous materials with regards to freezing and melting of nanoconfined fluids. Materials with α > 1 can be considered as strongly attractive, while those with α < 1 are weakly attractive.17 Many researchers have reported experimental evidence that the adsorbed molecular layers adjacent to the pore wall have different structures than the inner adsorbed layers.18 The high affinity of polymers to the interfaces decreases the mobility of polymers near the interfaces. This enthalpy effect will slow down or freeze polymer reorganization near the interfaces.19 However, for weakly attractive walls, a local minimum in the free energy of a contact layer phase can be expected. In our experiments, the low affinity of the polymers (PVDF) for the interfaces (α = 0.32, calculated based on the surface energy20) produced a relatively homogeneous solution state and subsequent homogeneous crystal growth without significant contact layer development, allowing the preparation of monodisperse nanoparticles. The relatively high affinity of 1,4-dioxane further assisted this mechanism. The 1,4-dioxane in the polymer solutions nanoconfined in the AAO pores freezes at lower temperatures compared to bulk materials (Figure 9). The relative decrease of the freezing temperature as a function of the confinement size affects the PVDF nanoparticle formation. The shift of the freezing temperature, ΔTf, is related to the pore width, D, on the basis of the Gibbs−Thomson thermodynamic equation (eq 1). ΔTf = Tf,pore − Tf,bulk = −2

solution in confined systems is unknown, the sequence of the phase development could be surmised by the thermal characterization of the PVDF solutions (Figure 9). Dioxane freezing was dependent on the concentration and confinement. PVDF solutions confined in the AAO template had lower dioxane freezing points (Tf1) than their bulk counterparts. In addition to Tf1, confined solutions had secondary freezing points (Tf2). Therefore, PVDF solutions freeze in two steps: pure 1,4-dioxane freezes at Tf1 and then the residual cryoconcentrated solution freezes at Tf2 < Tf1. Both freezing points depend on the PVDF concentration. However, Tf2 is not observed without AAO confinement and its peak intensity increases with increasing PVDF concentration. The second freezing phenomenon is PVDF crystallization in cryoconcentrates under the strong influence of dioxane crystallization or possibly vitrificaiton of cryoconcentrates. In addition to the crystallization, the phase separation phenomena of cryoconcentrate can impede the sequence of phase development. All of the events propagate following the direction of the temperature gradient, thereby producing controlled nanostructures. Currently, additional polymer crystals have been examined, and the data combined with the results of the current study proves that the successful nanoconfined directional cooling crystallization is not ordinary nanomolding, but a novel, versatile method to control the formation of nanostructures with a desired morphology and molecular assembly.

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CONCLUSIONS Monodispersed particles with morphologies ranging from spheres to nanofibers were reliably prepared with a unique ferroelectric crystalline phase using the simple modification of commercially available freeze-drying technology. Single crystalline spherical particles with ferroelectric properties were also obtained. The obtained PVDF nanocrystals showed a γ form and the properties of the polar ferroelectric γ-PVDF nanocrystals were confirmed. The efficiency of this process depends on the regulated thermal gradient under nanoconfinement to induce uniform nucleation and controlled directional growth. We expect that this novel technique will allow the stepwise preparation of more complex structures, such as Janus particles and blocked nanofibers, as well as the preparation of

(γws − γwl)T HρD

(1)

In eq 1, γws and γwl are the interfacial free energies per unit area of the wall−solid and wall−liquid interfaces, respectively,18 H is the melting enthalpy of the solid, ρ is its density, and D is the nanopore diameter.21 As the concept of surface tension (energy) is not well-defined, a deviation from the Gibbs− Thomson equation based on classical thermodynamics is expected. Additionally, the repulsive or attractive interactions of liquids with the pore walls lead to changes of the freezing point depression. Although the phase boundary of the polymer G



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ferroelectric nanostructures for applications based on microand nanosystems with piezoelectric properties.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 2 8165269. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-014107) and a grant from the Korea Healthcare technology R&D Project, Ministry of Health & Welfare, Republic of Korea (Grant No. A103017).

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