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Thermal Annealing-Induced Self-Stretching: Fabrication of Anisotropic Polymer Particles on Polymer Films Yu-Ching Lo, Yu-Jing Chiu, Hsiao-Fan Tseng, and Jiun-Tai Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02771 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017
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Thermal Annealing-Induced Self-Stretching: Fabrication of Anisotropic Polymer Particles on Polymer Films Yu-Ching Lo,1 Yu-Jing Chiu,1,2 Hsiao-Fan Tseng, 1 and Jiun-Tai Chen1,2* 1
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010
2
Sustainable Chemical Science and Technology, Taiwan International Graduate Program, Academia Sinica and National Chiao Tung University
ABSTRACT: Designing anisotropic particles of various shapes draws great attention to scientists nowadays. In this work, we develop a facile and simple method to fabricate anisotropic polymer particles from spherical polymer particles. Polyvinyl alcohol (PVA) films spin-coated with polystyrene (PS) microspheres are confined on both sides using binder clips and are heated above the glass transition temperatures of the polymers. During the thermal annealing process, the PS particles sink into the PVA films and transform to anisotropic particles. Depending on the distances to the bound regions, oblate spheroid PS particles or prolate spheroid particles with different aspect ratios can be obtained. The transformation of the particles is mainly driven by the stretching forces and the squeezing forces. The main advantage of this method is that anisotropic particles with different shapes can be fabricated simultaneously on a single film. We expect this novel method can be helpful to various fields including colloids science, suspension rheology, and drug delivery.
Introduction
also developed a new method to stretch polymer films in eight directions to fabricate oblate polymer spheroids.19
The preparation of anisotropic polymer particles draws great attention to scientists.1-5 Anisotropic polymer particles, such as prolate and oblate spheroids, have different physical and rheological properties comparing with isotropic polymer particles.6-9 For anisotropic polymer particles, interesting properties and behaviors have been demonstrated. For example, Yunker et al. discovered that anisotropic polymer ellipsoidal particles can suppress the coffee ring effect, a commonly seen phenomenon of isotopic particles.10 The shapes of the anisotropic particles can deform the air/water interfaces, resulting in the uniform deposition of the particles during evaporation. Various researches have been reported as new applications of anisotropic particles, such as drug delivery,11 medical imaging,12 microelectronic,13, 14 and cosmetics.15
Even though many methods have been developed to prepare anisotropic polymer particles, there are still limitations to these methods. First, custom-made tools are usually required for providing the external forces to stretch the polymer films. Second, heating mediums such as silicon oil are often used to anneal the polymer films uniformly; the removal of the heating mediums (silicon oil), however, is always problematic. Third, in most cases, anisotropic polymer particles with only one shape are prepared under a specific stretching condition.
Among all methods to fabricate anisotropic polymer particles, the thermal stretching method is one of the most efficient methods.16 Ho et al. first introduced this method to fabricate polymer ellipsoids.17 They embedded polystyrene (PS) microspheres in poly(vinyl alcohol) (PVA) films and heated them above the glass transition temperatures of both polymers. Biaxial forces were applied to stretch the PVA films by a custom-made tool, and the PS microspheres were stretched to become ellipsoids. Champion et al. further revised the method and successfully fabricated more than 20 different kinds of anisotropic polymer particles by changing the order of the heating and stretching processes.18 Recently, Kim et al.
To overcome these limitations, here, we develop a novel and simple method to prepare anisotropic polymer particles without applying external forces or using silicon oil. PS microspheres and PVA films are chosen as the model materials because of their well-known chemical and physical properties and commercial availability. Instead of embedding the PS microspheres in the PVA films, the PS microspheres are spin-coated on the PVA films, giving asymmetric interfaces (air/PS and PS/PVA) for the PS microspheres.20-22 The PS/PVA composite films are bound with metal clips to confine the PVA films on both sides. After thermal annealing in an oven, the PS microspheres sink into the PVA films and are stretched to form anisotropic particles because of the stretching and squeezing forces exerting on the PVA films and the PS particles. It should be noted that there is no external force applied to the composite films during the thermal annealing processes and a self-stretching condition is achieved. The surface morphologies of the PS particles and PVA films under different annealing and stretching
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conditions are examined by scanning electron microscopy (SEM) and optical microscopy (OM). Two models, the squeezing force model and the stretching force model, are proposed to explain the deformation mechanism during the thermal annealing and self-stretching processes. Depending on the locations of the PVA films, PS particles are subjected to different degrees of stretching. Near the bound regions of the PVA films, oblate spheroid PS particles can be obtained. Away from the bound regions, prolate spheroid PS particles with increased aspect ratios are observed. Near the center of the PVA films, the degree of stretching reaches a maximum and prolate spheroid PS particles with the aspect ratio of ~1.5 can be obtained. The novelty of this work is that external forces or using silicon oil are not required. Moreover, anisotropic particles with different shapes can be obtained within a single sample. This study not only provides a feasible approach to prepare anisotropic polymer particles but also offers a deeper understanding on the deformation behaviors of polymer particles.
Experimental Section Materials Poly(vinyl alcohol) (PVA, 86.7−88.7% hydrolyzed) was obtained from Sigma-Aldrich with weight-average molecular weights (Mw) of ~130 kg mol-1. Polystyrene (PS) microspheres were obtained from Polysciences with a mean diameter of 10 μm (variation: 5%). Ethanol, isopropanol, and toluene were purchased from Echo Chemical. Preparation of PVA films spin-coated with PS microspheres First, a 5 wt % PVA solution (2 g PVA dissolved in deionized water) was prepared. The PVA solution was then poured into a petri dish, and the sample was dried at room temperature on a flat table overnight. The average film thickness of the PVA film was ~50 µm. A suspension of PS microspheres in ethanol was spin-coated at 1200 rpm for 60 s on the PVA film. Annealing and selective removal processes of the PS/PVA composite films The PVA films spin-coated with PS microspheres were cut into pieces of 2×2 cm. The PS/PVA composite films were clipped with two binder clips on both sides, and they were annealed at 240 °C for 40 min. After the annealing process, the binder clips were removed from the films and the deformed PVA films with stretched PS particles were obtained. To confirm the structures of the PVA films and the PS particles after thermal annealing, the selective removal technique was used. By immersing the PS/PVA composite films in a 30% isopropanol/water solution at 65 °C for 24 h, the PVA films were removed and anisotropic PS particles were obtained; by immersing the PS/PVA composite film in toluene for 8 h, PS particles were removed and PVA films with cavities were obtained.23, 24
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Structure analysis and characterization The structures of the stretched PS particles were characterized by an optical microscope (OM, Zeiss). A scanning electron microscope (SEM, JEOL JSM-7401F) with an accelerating voltage of 5 kV was also used to investigate the morphologies of the PS particles and the PVA films. The samples were dried using a vacuum desiccator at room temperature and coated with 4 nm of platinum before investigation. Quantitative analyses of the OM and SEM data were conducted using ImageJ and Photoshop.
Results and discussion The schematic illustration of the thermal annealing and self-stretching process is shown in Figure 1. To generate anisotropic polymer particles, we first fabricate PVA films spin-coated with PS spheres. The PVA films are prepared by pouring PVA solutions into a petri dish, followed by spin-coating PS microspheres with average diameter of 10 μm. Subsequently, two binder clips are clipped on both sides of the PVA films and the samples are annealed at 240 °C for 40 min. The glass transition temperatures of PS and PVA are ~100 and ~85 °C, respectively. When the samples are annealed at 240 °C, far above the glass transition temperatures of both polymers, the PVA films and PS microspheres are softened and become deformable. Because the surface tension of PS is much lower than that of PVA, the PS microspheres sink into the PVA films during the annealing process. Meanwhile, the deformation of the PVA films to stretch the PS microspheres is induced by taking advantage of the high temperature annealing and bound edges. External forces are not applied to the PVA films during the deformation processes; therefore, a self-stretching process is considered. Residual water in the PVA films, a critical factor in this work, is evaporated because of the high temperature annealing, causing the contraction of the films and the deformation of the PS microspheres. The use of binder clips, another key factor in this study, can confine the rims of the PVA films, affecting the degrees of deformation of the PS particles at different locations of the PVA films. Consequently, anisotropic PS particles with different shapes can be obtained simultaneously on a single PVA film.
Figure 1. Schematic illustration of the thermal annealing and self-stretching processes.
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The annealing-induced self-stretching method is highly reproducible. The photographs of a PS/PVA composite film before and after the self-stretching processes are shown in Figure S1, in which the size changes of the entire film can be estimated. Before the annealing and stretching process, the size of the film is 2 cm × 2 cm. After the annealing and stretching process, the length of the film near the center is ~1.2 cm. In Figure S1, the shape changes of the drawn circles on the film after the annealing and stretching process give a rough idea about the deformation of the films. For example, the circle becomes an ellipse near the center of the film, implying the presence of the asymmetric stretching force. The morphologies of the PS particles before and after the selfstretching processes are investigated mainly by scanning electron microscopy (SEM), as shown in Figure 2. The corresponding anisotropic polymer particles are illustrated in Figure 2a. Before the thermal annealing and self-stretching processes, spherical PS microspheres on the PVA films can be observed (Figure 2b-d). During the annealing process at 240 °C, PS microspheres sink into the PVA films and deform simultaneously. For the PS particles collected from PVA films next to the binder clips, the samples are less stretched and oblate spheroid PS particles are observed, as shown in 2e-g. As the locations of the PS particles on the PVA films are further from the binder clips, the degree of stretching increases and prolate spheroid PS particles with aspect ratios of ~1.2 can be observed, as shown in Figure 2h-j. For the PS particles near the center of the PVA films, the degree of stretching is the highest and prolate spheroid particles with aspect ratios of ~1.5 can be observed (Figure 2k-m). The aspect ratio of an anisotropic particle is defined by the value of the major axis divided by the minor axis, as shown in Figure S2. To correctly obtain the values of the major and the minor axes, top view SEM images are used. The results indicate that the self-stretching forces are dependent on the positions of the PS particles on the PVA films, resulting in the formation of anisotropic PS particles with different shapes.
Figure 2. (a) Graphical illustrations of different anisotropic PS particles on PVA films. (b−m) SEM images of PS particles under different annealing and stretching conditions: (b−d) tilted-view and top-view SEM images with lower and higher magnifications of PS microspheres on PVA films before the annealing process, (e−g) tilted-view and top-view SEM images with lower and higher magnifications of oblate spheroid PS particles collected from PVA films next to the binder clips, (h−j) tilted-view and top-view SEM images with lower and higher magnifications of prolate spheroid PS particles with aspect ratios of ~1.2, and (k−m) tilted-view and top-view SEM images with lower and higher magnifications of prolate spheroid PS particles with aspect ratios of ~1.5.
To further confirm the structures of the PS particles and the PVA films after the thermal annealing processes, selective removal techniques are applied. By immersing the PS/PVA composite films at different regions in a 30% isopropanol/water solution at 65 °C for 24 h, the PVA films can be removed and the anisotropic PS particles can be collected, as shown in Figure 3a-c. In Figure 3c, the PVA film is not completely removed and an anisotropic PS particle on a PVA film with an oval cavity can be observed. The shapes of the anisotropic PS particles are different from those of the hemispherical PS particles that are prepared by the self-organized precipitation method.25 By immersing the PS/PVA composite films at different regions in toluene for 8 h, the anisotropic PS particles can be removed and PVA films with different shapes of cavities can be obtained (Figure 3d-f). The aspect ratios of the PS particles and the cavities after the selective removal processes agree well with those shown in Figure 2. Cavities with interesting shapes can also been obtained from the merging of multiple particles (Figure S3). It should be noted that the shapes of the anisotropic PS particles are also determined by the surface tensions of the polymers and the interfacial tension between the two polymers. Although the interfacial tension between PS and PVA at 240 °C could not be obtained, the shapes of the PS particles can be discussed qualitatively. From the experimental results, we speculate that the PVA/air interfaces are preferred to be replaced by the PS/air interfaces during the annealing and stretching process. As the PS particles are immersed into the PVA films, the additional interfacial energies between PS and PVA also play a critical role in the morphology transformation, as discussed in our previous work by annealing PS microspheres on PVA films.22
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the samples with four binder clips and without binder clips. The average diameter of the oblate spheroid PS particles fabricated with four binder clips is ~17 μm; the average diameter of the oblate spheroid PS particles fabricated without binder clips is ~12.5 μm. The larger diameters for the samples with four binder clips than those without binder clips elucidate that the stretching forces are induced by the binder clips and the directions of the stretching forces are affected by the relative locations of the binder clips.
Figure 3. SEM images of anisotropic PS particles and PVA films with cavities. (a) An oblate spheroid PS particle with an aspect ratio (AR) of ~1. (b) A prolate spheroid PS particle with an aspect ratio (AR) of ~1.2. (c) A prolate spheroid PS particle with an aspect ratio (AR) of ~1.5 and an oval cavity with an aspect ratio (AR) of ~1.5. (d) A PVA film with a round cavity. (e) A PVA film with an oval cavity with an aspect ratio (AR) of ~1.2. (f) A PVA film with an oval cavity with an aspect ratio (AR) of ~1.5.
To understand the mechanisms of the structure deformation of the PS/PVA composite films during the annealing processes, we propose two models: (1) the stretching force model and (2) the squeezing force model. In the stretching force model with two binder clips, the PS microspheres are stretched along the horizontal axis (X axis) parallel to the clip-clip direction, as shown in Figure 4a. Because the heat transfer coefficients of the binder clips are much higher than those of the PVA films, the temperatures of the PVA films increase faster and the molecular motions of the PVA chains are higher at the locations closer to the binder clips. As a result, the contraction of the PVA films near the binder clips causes the stretching of the middle PVA films toward the binder clips to release the residual strains, finally converting the PS microspheres to prolate spheroid PS particles, which can be confirmed from the OM image taken from the center of a PVA film (Figure 4c). To investigate the relationship between the binder clips and the directions of the stretching forces, we use four binder clips and perform the self-stretching experiments. With the new setup, the stretching forces are applied in four different directions toward the binder clips, as shown in Figure 4b. Similarly, the higher temperatures and the molecular motions of the PVA films closer to the binder clips cause the stretching of the middle PVA films toward the binder clips to release the residual strains. The stretching in the four different directions finally converts the PS microspheres to form oblate spheroid PS particles, which can be confirmed from the OM image taken from the center of a PVA film with four binder clips (Figure 4d). For reference purpose, we also anneal the PS/PVA composite films without using binder clips; therefore, there is no extra stretching force exerting on the PVA films. Similar to the conditions using four binder clips, oblate spheroid PS particles are observed for the samples without binder clips, as shown in Figure 4e. The diameters of the PS particles, however, are different for
Figure 4. Proposed stretching force models of the selfstretching processes for the samples with two and four binder clips. (a) Illustration of the self-stretching model with two binder clips. (b) Illustration of the self-stretching model with four binder clips. (c) OM image of the PS particles after self-stretching with two binder clips. (d) OM image of the PS particles after self-stretching with four binder clips. (e) OM image of the PS particles after self-stretching without binder clips. The OM images are taken at locations near the center of the PVA films.
The second proposed model is the squeezing force model caused by the contraction of the PVA films, as shown in Figure 5a. The contraction of the PVA films is probably caused by the surface tension of PVA and the evaporation of the residual water in the PVA films. For the contraction caused by the surface tension, the PVA films transform to reduce the total surface areas, resulting in lower surface energies. For the contraction caused by the evaporation of water during the high temperature annealing processes, the amounts of the residual water in the PVA films can be examined by the TGA analysis of the PS/PVA composite films before and after the annealing processes, as shown in Figure 5b,c, where ~0.2 mg of residual water in each PS/PVA composite film can be estimated. In the squeezing force model, the directions of the squeezing forces are perpendicular to the clip-clip
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direction and the PS microspheres are squeezed along the vertical axis (Y axis).
~1.2. At region c, prolate spheroid PS particles are obtained and the aspect ratios reach a maximum of ~1.5.
While the rims of the PVA films in the X axis is confined by the binder clips, the contraction of the PVA films in the Y axis is allowed, squeezing and deforming the PS particles. The degree of deformation of the PS microspheres increases as the distance to the binder clips increases.
As for the upper and lower 4 mm of the PVA films, PS prolate spheroid particles with aspect ratio of ~1.5 can also be obtained, as shown in Figure S4 in the Supporting Information. The OM and SEM images confirm that PS microspheres at the regions near the edges of the films are deformed to prolate spheroid particles. The squeezing forces are perpendicular to the major axes of the prolate spheroid particles, indicating that the deformation of the PS particles near the edges of the films is caused by the squeezing forces. The results also show that cutting the films before the particles are released is one of the possible ways to separate the particles with different aspect ratios. To separate the particles, another possible approach is by using centrifugation. It has been studied that particles with different shapes and sizes can be separated by the centrifugal force. For example, Akbulut et al. have used this concept to separate nanoparticles by considering the ratio of the sedimentation coefficient of two different shapes.26
Figure 5. Proposed squeezing force model of the selfstretching process. (a) Graphical illustration of the selfstretching model. (b) TGA curve of a PVA film before annealing. (c) TGA curve of a PVA film after annealing.
The stretching forces and squeezing forces are not independent and are believed to affect the shapes of the anisotropic PS particles at different locations on the PVA films. To compare the anisotropic PS particles at different locations more quantitatively, the shapes and the aspect ratios of the particles are measured, as shown in Figure 6. Here, the aspect ratios are defined by the ratios of the diameters along the major and minor axes from the topview SEM images of the anisotropic PS particles. Some regions near the binder clips that are severely deformed by the squeezing forces are eliminated for the quantitative analysis. At regions a and a’, there are neither the stretching forces nor the squeezing forces because of the confinement of the binder clips; during the annealing processes, the PS microspheres gradually sink into the PVA films and transform to oblate spheroid PS particles because the surface tensions of the PS microspheres are much lower than those of the PVA films. At regions b and b’, both the stretching forces and the squeezing forces start to exert on the PS particles; during the annealing processes, the PS microspheres transform to prolate spheroids PS particles. At region c, both the stretching forces and the squeezing forces have stronger effects than those at regions b and b’; during the annealing processes, the PS microspheres transform to prolate spheroids PS particles with larger aspect ratios. In Figure 6b, the aspect ratios of the PS particles are plotted versus the locations of the PS particles on the PVA films. At regions a and a’, oblate spheroid PS particles are obtained and the aspect ratios are ~1. At regions b and b’, prolate spheroid PS particles are obtained and the aspect ratios increase to
Figure 6. (a) Illustration of the PS particles collected from different positions on a PVA film. (b) Plot of the aspect ratios of the PS particles versus the positions on the PVA films. One practical issue of this method is the productivity because the deformed area is restricted to the central part of the polymer film. To overcome this problem, we propose three possible solutions. The first solution is to increase the sizes of the polymer films; therefore, the deformed areas can be larger. The second solution is to increase the densities of the polymer particles on the polymer films by spin-coating polymer particles with higher concentrations or at lower spin-coating rates; more polymer particles can be deformed for the preparation of the anisotropic polymer particles. The third solution is to use multi-layer polymer films; more films can be stretched using the same setup. Despite the productivity issue, the main advantage of this method is that anisotropic particles with different shapes can be obtained within a single sample. The thermal annealing-induced self-stretching method can be applied to other polymers. The applicability of this method, however, depends on the availability of the polymer microspheres. In addition, the glass transition temperatures (Tg) of the polymers are critical for forming the anisotropic particles; polymers with suitable glass transition temperatures are necessary.
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Conclusion In this work, we develop a facile and versatile method to prepare anisotropic polymer particles from microspheres using an annealing-induced self-stretching strategy. Prolate and oblate spheroid PS particles with aspect ratios up to ~1.5 can be obtained. The surface morphologies of the PS particles and PVA films under different annealing and stretching conditions are characterized by SEM and OM. Two models, the stretching force model and the squeezing force model, are proposed to account for the deformation mechanism of the PS particles. The use of four binder clips instead of two binder clips confirms the presence of the stretching forces. In addition, the squeezing forces induced by the evaporation of the residual water in the PVA films are proved by the TGA analysis. Comparing with the traditional methods to prepare anisotropic polymer particles, this approach does not require external forces or using silicon oil. This efficient and practical method has great potential in making particles for applications such as drug delivery, microelectronics, and sensors.
Associated content Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX Photograph of a PS/PVA composite film, illustrations of anisotropic PS particles, and SEM images of cavity-containing PVA films.
Acknowledgement This work was supported by the Ministry of Science and Technology of the Republic of China.
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SYNOPSIS TOC
Thermal Annealing-Induced Self-Stretching: Fabrication of Anisotropic Polymer Particles on Polymer Films *
Yu-Ching Lo, Yu-Jing Chiu, Hsiao-Fan Tseng, and Jiun-Tai Chen
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