From Electrospun Polymer Core–Shell Fibers to Polymer

Nov 15, 2017 - From Electrospun Polymer Core–Shell Fibers to Polymer Hemispheres and Spheres: Two Types of Transformation Processes and Tearing ...
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From Electrospun Polymer Core−Shell Fibers to Polymer Hemispheres and Spheres: Two Types of Transformation Processes and Tearing Films with Linearly Arranged Cavities Yu-Jing Chiu,†,‡ Hsiao-Fan Tseng,† Yu-Ching Lo,† Bo-Hao Wu,† and Jiun-Tai Chen*,†,‡ †

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010 Sustainable Chemical Science and Technology, Taiwan International Graduate Program, Academia Sinica and National Chiao Tung University, Hsinchu, Taiwan 30010

‡

S Supporting Information *

ABSTRACT: Electrospun polymer core−shell fibers have gained much attention because of their promising applications in areas such as electronic devices, drug delivery, and tissue engineering. The morphology transformation of polymer core− shell fibers, however, has been rarely investigated. Here, we study the effect of thermal annealing on the morphology transformation of electrospun polystyrene (PS)/poly(methyl methacrylate) (PMMA) core−shell fibers on PMMA films. Two types of transformation processes are discovered. In the first type of the transformation process (type I), the PS cores transform to hemispherical particles after the annealing process; in the second type of the transformation process (type II), the PS cores transform to spherical particles after the annealing process. The measured sizes of the hemispherical and spherical PS domains fall into two classified regions, as predicted for the two different types of transformation processes. It is also observed that the growth rates of the undulated amplitude are similar for the two different types of transformation processes, but the type I fibers start to undulate at later annealing times than the type II fibers do. When the PS particles are selectively removed, the PMMA films with linearly arranged cavities are used for the tearing experiments, demonstrating a proof of concept on the potentials in studying the mechanical properties of cavity-containing films.

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routes.10,11 The first route is the coaxial electrospinning process using a dual-capillary spinneret. Polymer solutions or a combination of a polymer solution and melt are ejected from the core and the surrounding concentric nozzles; after the solvents are evaporated and the compound jets are solidified, core−shell fibers are formed.12 The second route is the emulsion electrospinning process using a single nozzle with polymer emulsions or blends. In most cases, the dispersed phases in the emulsions or blends become the cores of the core−shell fibers and the continuous phases become the shells of the core−shell fibers.13−15 Although the preparation of electrospun polymer fibers has been widely investigated, the effects of post-treatment on the morphology and property changes of electrospun polymer fibers have been rarely studied. In the past, different posttreatment techniques, such as thermal annealing and solvent annealing, have been utilized to control the morphologies and properties of polymer films and bulks.16,17 Previously, we prepared electrospun polystyrene (PS) fibers and studied their morphology transformation when the PS fibers are annealed on

INTRODUCTION In recent years, the electrospinning technique has gained much attention in academic and industrial communities because it can generate polymer fibers with diameters ranging from micrometers to nanometers. Because of electrostatic forces, polymer solutions can be ejected to form polymer jets, whipping under the applied fields.1−3 After the evaporation and the solidification processes, electrospun polymer fibers can be obtained. Electrospun polymer fibers have large surface area to volume ratios and flexibility in surface functionalization, which can be applied to various fields, such as filtration, sensors, and tissue engineering.4−7 The morphologies of electrospun polymer fibers are mainly controlled by adjusting the electrospinning parameters, such as the concentration, the solvent, the flow rate, the working voltage, the working distance, and the shape of the nozzle. For example, by replacing dimethylformamide (DMF) with tetrahydrofuran (THF) as the solvent, the morphologies of the electrospun poly(methyl methacrylate) (PMMA) fibers can change from cylinder-like morphology to ribbon-like morphology.8,9 To extend the functionalities, multicomponent polymer fibers, especially core−shell fibers, have gradually received more attention from researchers. For the fabrication of the polymer core−shell fibers using electrospinning, there are two main © XXXX American Chemical Society

Received: September 7, 2017 Revised: November 1, 2017

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DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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poly(methyl methacrylate) (PMMA) films.18,19 The PS fibers transform into PS hemispheres embedded in the PMMA films, driven by the reduction of the surface and interfacial energies. When the electrospun PS fibers are embedded in PMMA films and thermally annealed, spherical PS particles can be obtained.20 Recently, we also investigated the morphology evolution of electrospun PS/PMMA core−shell fibers annealed on PS films. The core−shell fibers transformed to core−shell hemispheres after annealing and then to microbowls after the PS domains are selectively removed.21 Although the morphology transformation of polymer fibers upon annealing has been studied, there are still several significant issues need to be resolved. First, the detailed mechanisms for the polymer fibers transforming to polymer particles are still unclear. Until now, either polymer hemispheres or spheres are observed after the transformation process; there is no report on observing both polymer hemispheres and spheres occurring under the same annealing conditions. Second, for annealing polymer core−shell fibers on polymer films, it remains a question whether or not both polymer hemispheres and spheres can be observed by using core−shell fibers of that the shell materials are the same as the underlying polymer films. Third, cavity-containing polymer films can be obtained after the transformation and selective removal processes, but the possible application for the cavitycontaining polymer films has not been demonstrated. To address these issues, here we study the morphology transformation processes of PS/PMMA core−shell fibers annealed on PMMA films. PS and PMMA, two of the most studied polymers, are chosen as model materials in this work because of their well-known chemical and physical properties. A main advantage of using PS and PMMA is that orthogonal solvents can be used to remove selectively either the PS or the PMMA components, enabling better characterization on the morphologies of the polymer composites. For example, the PS components in the PS/PMMA composites can be selectively removed by cyclohexane, leaving the PMMA domains essentially unaltered; the PMMA components in the PS/ PMMA composites can be selectively removed by acetic acid, leaving the PS domains essentially unaltered. For designing the annealing experiments, the PS/PMMA core−shell fibers are annealed on PMMA films, which are the same materials as the shell materials of the core−shell fibers, experiments that may induce the formation of both the polymer hemispheres and spheres after the transformation processes. During the thermal annealing processes, the PS cores are observed to transform to PS particles to decrease the interfacial energies between PS and PMMA while the PMMA shells melt together with the underlying PMMA films. By annealing core−shell fibers of that the shell materials are the same as the underlying polymer films, we successfully observe two different types of transformation processes (types I and II). For type I transformation process, the PS cores transform to hemispherical particles; after the selective removal process, PMMA films with hemispherical cavities are obtained. For type II transformation process, the PS cores transform to spherical particles; after the selective removal process, PMMA films with spherical cavities are obtained. Quantitative studies on the sizes of the PS particles and the growth rates of undulation amplitude are also conducted. Moreover, tearing experiments are conducted for the cavity-containing films after the selective removal process to demonstrate a proof of concept on the potentials in studying the mechanical properties of cavity-containing films.

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RESULTS AND DISCUSSION The schematic illustration of the experimental processes is shown in Figure 1. The PS/PMMA core−shell fibers are

Figure 1. Schematic illustration of the experimental processes to fabricate polymer core−shell fibers and to thermally anneal the fibers on polymer films.

prepared by a typical single axial electrospinning technique using PS/PMMA blend solutions. The PS/PMMA blend solutions are ejected from a capillary nozzle to form jets under high voltages. After the evaporation of the solvents, the jets are solidified and core−shell fibers are formed on the grounded collector. The formation of the core−shell fibers using the polymer blend solutions is caused by the phase separation of the polymers during the electrospinning process.3,22,23 There are requirements for choosing the polymer blend solutions. First, it was studied that incompatibilities and large solubility parameter differences of the two polymers in the blend solutions play a key role in the phase separation process. Moreover, the phase separation process is also controlled by the kinetic factors because of the rapid solvent evaporation in the electrospinning process, especially for polymers with lower molecular weights.24 For the thermal annealing experiments, the electrospun PS/ PMMA core−shell fibers are first placed on spin-coated PMMA films. The samples are then annealed at temperature higher than the glass-transition temperatures (Tg) of the polymers. During the thermal annealing processes, the PS/PMMA core− shell fibers sink and transform to hemispheres or spheres embedded in the PMMA films. The morphologies of the annealed samples can be further confirmed by the selective removal process. For the fabrication of the PS/PMMA core−shell fibers, a common single axial electrospinning setup, instead of a complicated coaxial electrospinning setup, is used. A PS (Mw: 166 500; PDI: 7.27)/PMMA (Mw: 49 300; PDI: 1.85) blend solution (weight ratio = 1:1) dissolved in dimethylformamide (DMF) is first prepared. DMF is chosen as the solvent for electrospinning because it has a low boiling point (153 °C) and is a good solvent for both PS and PMMA. Moreover, DMF has a proper dielectric constant (36.7 at 25 °C), which is essential for the electrospinning process. The PS/PMMA blend solution is ejected into a syringe, connected to a capillary nozzle. The morphologies of the electrospun fibers can be adjusted by the electrospinning parameters, such as the concentrations, the solvents, the flow rates, the working voltages, and the working distances.25 For example, the diameters of the electrospun fibers increase with the flow rates because of the increased ejected volumes per unit time.23 The optimized conditions for making the electrospun PS/PMMA core−shell fibers are B

DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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the OM images of the annealed samples at different annealing times. The whole transformation process can be observed from the video in the Supporting Information (Video S1). The annealing temperature is 220 °C, which is higher than the glasstransition temperatures (Tg) of PS (Tg: 107.8 °C) and PMMA (Tg: 104.4 °C). At longer annealing times, the PMMA shells of the fibers melt and sink into the PMMA films while the PS cores of the fibers gradually undulate and break into spherical shapes. During the thermal annealing process, the morphology transformation of the electrospun PS/PMMA core−shell fibers is mainly driven by the reduction of the surface and interfacial energies of the polymers. It has been studied that the surface tensions of PMMA (3 kg mol−1) and PS (44 kg mol−1) at 20 °C are 41.1 and 40.7 mJ/m2, respectively, and the interfacial tension between PMMA and PS at 20 °C is 3.2 mJ/m2.26,27 The morphology transformation during the thermal annealing process is related to the Plateau−Rayleigh instability.28 The Plateau−Rayleigh instability is a common phenomenon that can be seen in our daily lives, which is pioneered by Joseph Plateau.29 He found that a liquid cylinder deforms into a series of droplets to achieve lower surface energies when a liquid cylinder is falling down without any restriction from the column.29 Lord Rayleigh later used the fastest distortion mode to explain the wavelength of the undulation.28 The Plateau−Rayleigh instability was applied to solid cylinders by Nichols and Mullins.30 The mass transport of the solid cylinders was studied by calculating the wavelengths of maximum growth rates by considering either volume or surface diffusion. The perturbed surface of an infinitely long cylinder with infinitesimal longitudinal sinusoidal perturbation can be described by the following equation:

chosen as follows: the weight ratio (1:1), the solvent (DMF), the concentration (30 wt %), the working voltage (10 kV), the working distance (10 cm), and the flow rate (5 mL/h). The graphical illustration and SEM images of the electrospun PS/PMMA fibers are shown in Figure 2. From the SEM image

Figure 2. (a−c) Graphical illustrations of electrospun PS/PMMA core−shell fibers, a single PS/PMMA core−shell fiber, and a PMMA hollow fiber. (d, e) SEM images of the electrospun PS (Mw: 166 500)/ PMMA (Mw: 49 300) core−shell fibers at lower and higher magnifications. The concentration, the blend weight ratio, the solvent, the working voltage, the flow rate, and the working distance are 30 wt %, 1:1, DMF, 10 kV, 5 mL/h, and 10 cm, respectively. (f) SEM image of the PMMA hollow fibers, prepared by selectively removing the PS domains of the electrospun PS/PMMA core−shell fibers using cyclohexane.

(Figure 2d), the PS/PMMA core−shell fibers with smooth surfaces and an average diameter of ∼6.8 μm can be observed. The solid nature of the fibers can be confirmed by checking the edge of a broken fiber (Figure 2e). To examine the core−shell structures of the fibers, the selective removal process is conducted. The core PS domains can be removed selectively using cyclohexane, a good solvent for PS and a nonsolvent for PMMA. The remained PMMA hollow fibers can be seen from the SEM image (Figure 2f), confirming the PS/PMMA core− shell structures. After the electrospun PS/PMMA core−shell fibers are fabricated, they are placed on spin-coated PMMA films for the thermal annealing experiments. During the thermal annealing processes, OM is used to observe in situ the transformation processes of the fibers. Figure 3a shows the graphical illustration of the electrospun PS/PMMA core−shell fibers annealed on a PMMA film, and Figure 3b−3g displays

r = R 0 + δ sin(2π /λ)z

(1)

where r is the radius, R0 is the original radius of the cylinder, δ is the amplitude, λ is the wavelength of the perturbation, and z is the coordinate along the cylinder axis. From the equation, if the wavelength of the amplitude λ > 2πR0, the perturbations occur spontaneously. Furthermore, it can be calculated that the perturbations with λm = 8.89R0 have the highest growth rates, and the solid cylinder can eventually break into spheres with an average diameter d = 3.78R0. To further characterize the morphologies of the annealed PS/PMMA composite samples, selective removal processes are performed. Figure 4a shows the illustration of the selective removal process, in which the PS particles in the annealed samples are removed using cyclohexane, leaving PMMA films with cavities. The OM images of the annealed samples with lower and higher magnifications are displayed in Figure 4b,c, in which linearly aligned circular domains can be observed. The circular domains can be classified into two different types (types I and II), which are indicated by black and red arrows, respectively. The surface morphologies of the annealed samples before the selective removal process can be observed from the SEM image (Figure 4d). Flat top surfaces of the annealed samples can be seen, indicating that the circular PS domains displayed in OM images are embedded in the PMMA films. After the PS particles in the annealed samples are removed selectively using cyclohexane, the OM images of the PMMA films with linearly aligned circular domains can also be observed (Figure 4e,f). Similar to the samples before the selective removal process, the circular domains of the two different types (types I and II) in the samples after the selective removal

Figure 3. (a) Graphical illustration of an electrospun PS/PMMA core−shell fiber annealed on a PMMA film. (b−g) OM images of the electrospun PS/PMMA core−shell fibers annealed on a PMMA film (thickness: ∼26 μm) at 220 °C for different periods of time: 200, 409, 599, 659, 924, and 1454 s. C

DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a) Graphical illustration of the selective removal process, in which the PS domains are selectively removed using cyclohexane, leaving the PMMA film with cavities. (b−d) OM and SEM images of the annealed samples. (e−g) OM and SEM images of the samples after the selective removal process. (h) Graphical illustration of the selective removal process, in which the PS domains or the PMMA domains can be selectively removed using cyclohexane or acetic acid, respectively. (i, j) SEM images of the PMMA films with cavities at lower and higher magnifications. (k) SEM image of PS hemispheres. (l) SEM image of a PS sphere.

Figure 5. (a) Graphical illustration of the thermal annealing and selective removal processes. d0 is the diameter of the PS core; d1 and d2 are the diameters of the hemispherical and spherical cavities after the selective removal process, respectively. (b) Combined OM image of a PMMA film with cavities after the selective removal process. (c) Plot of the diameter ratio versus the cavity number shown in (b). (d, e) OM image and corresponding illustration of a PMMA film with cavities.

process are indicated by black and red arrows, respectively. The color contrasts of the OM images for the samples after the selective removal process (Figure 4e,f), however, become larger compared with those before the selective removal process (Figure 4b,c). This observed result is because the reflective index differences between PMMA and air are larger than those between PMMA and PS. Interestingly, the SEM images of the annealed samples after the selective removal process present only one type (type I) of the cavities, as shown in Figure 4g. It can be speculated that the other type (type II) of the cavities observed in the OM images (Figure 4e,f) are embedded below the surfaces of the PMMA films. To confirm the speculation, the cross sections of the annealed samples after the selective removal process are also examined by SEM, as shown in the graphical illustration (Figure 4h) and SEM images (Figure 4i,j). The two different types (type I and II) of the cavities shown in the OM images (Figure 4e,f) can both be observed in the SEM image (Figure 4i), indicated by the black and red arrows, respectively. The spherical shape of the cavities embedded under the surfaces of the PMMA films can be seen in the SEM image with a higher magnification (Figure 4j), indicated by the red arrows (type II). For the annealed samples, the PMMA films can also be removed selectively using acetic acid, leaving unsupported PS particles, as illustrated in Figure 4h. The SEM images of the PS samples after removing the PMMA films are shown in Figure 4k,l. The PS hemispheres (Figure 4k) and spheres (Figure 4l) correspond to the circular PS domains indicated by black (type I) and red (type II) arrows in Figure 4c, respectively. Therefore, we can concluded that hemispherical particles and cavities are belong to type I (the hemispherical type); spherical particles and cavities are belong to type II (the spherical type). To further understand the formation mechanisms of the two different types of PS particles, more quantitative analyses about their sizes and frequencies are also conducted. Figure 5a shows the graphical illustrations of a fiber before and after the thermal annealing and the selective removal processes with crosssectional views. Before the thermal annealing process, the diameter of the original PS core is assigned as d0; after the thermal annealing process, the diameters of the transformed PS

hemispheres and spheres are assigned as d1 and d2, respectively. The values of d1 and d2 are assumed to be maintained after the selective removal process. For conducting the quantitative analyses, OM images of the samples after the selective removal process are used because of the higher contrasts between PMMA and air than those between PMMA and PS; using the samples after the selective removal process, it is also easier to distinguish and classify the cavities as type I (the hemispherical type) or as type II (the spherical type). Figure 5b shows a combined OM image of a PMMA film with cavities after the selective removal process, and Figure 5c presents the plot of the diameter ratio versus the cavity number for the cavities shown in Figure 5b, in which 84 cavities are identified and measured. The diameter ratios (d1/d0 and d2/d0), plotted as orange and green dots, respectively, are derived from the measured diameters of the cavities (d1 for hemispherical (type I) cavities and d2 for spherical (type II) cavities) divided by the measured diameters of the original PS cores before annealing (d0). For studying the measured values of the diameters of the transformed PS hemispheres and spheres, two theoretical values can be predicted. The first predicted value (d2/d0) is the theoretical diameter ratios of the spherical particles (type II). From the theory of the traditional Rayleigh instability and the calculations by Nichols and Mullins, the theoretical diameter ratio of the sphere (d2/d0) to the fiber is ∼1.89, which is depicted as the green dashed line in Figure 5c, assuming that the volume is conserved from the fibers to the spheres.30 The second predicted value (d1/d0) is the theoretical diameter ratios of the transformed hemispheres (type I). Assuming that the hemispheres are transformed from the spheres while the transformation obeys the conservation of volume, the relationship between the radii of the spheres and the radii of the hemispheres can be described by the following equation:31 4/3π (d 2/2)3 = 1/2(4/3π (d1/2)3 )

(2)

where d2 is the diameters of the spheres and d1 is the diameters of the transformed hemispheres. Equation 2 can be rewritten as the following: D

DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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d1 =

3

2 d2

hemispherical type (type I) particles; the results are similar to those by annealing PS homopolymers on PMMA films. For regions which contain thicker PMMA shells, the larger distances between the PS cores and air cause the PS cores to remain in the PMMA films, resulting in the formation of the spherical type (type II) particles. In addition to the thicknesses of the PMMA shells, the residual air domains in the PS/PMMA core−shell fibers after the electrospinning process may affect the types of the transformed PS particles. Although two different types of the transformation processes have been observed, it is curious whether or not the types of the transformed particles can be determined or predicted at early annealing stages. Therefore, we study the morphologies of the PS/PMMA composites at different annealing stages by taking out the samples from the annealing stages at different annealing times and selectively removing the PS domains using cyclohexane to enhance the contrasts. The results show that the types of the transformed particles are determined at the early annealing stages, as shown in the graphical illustration and the corresponding OM images (Figure 6a−e). In the initial stage,

(3)

From eq 3, it can be known that when a sphere is transformed into a hemisphere, the diameter of the hemisphere is 21/3 (∼1.26) times larger than the diameter of the sphere. Using the predicted diameters of the spheres (d2 = 1.89d0), thus, the theoretical value of the diameter ratio between hemispheres (d1) and the original PS cores before annealing (d0) is calculated to be ∼2.38, which is depicted as the orange dashed line in Figure 5c. For ideal cases, the diameters of the hemispheres (d1) should be ∼1.26 times larger than those of the spheres (d2), which is exemplified in Figure 5e; the diameters of the illustrated spheres (d2) are taken from the measured diameter of a selected sphere (d2′) shown in the corresponding OM image (Figure 5d), in which the selected sphere is indicated by a red arrow. The relationship between the sizes of the fibers (d0), the diameters of the hemispheres (d1), and the diameters of the spheres (d2) is also illustrated in Figure S4. From Figure 5c, it can be seen that the measured values of the diameter ratios of the hemispherical (type I) and spherical (type II) cavities fall into two regions, as expected for the two different types of the cavities. The measured values of the diameter ratios for both types of the cavities, however, are generally lower than the predicted values (orange and green dashed lines). The deviations can be attributed to the following reasons. First, the hemispheres are not transformed from the spheres; the hemispheres are observed to transform directly from some parts of the fibers, as demonstrated in Video S1. Second, the measured values of the diameters of the original PS cores before annealing (d0), the hemispheres (d1), and the spheres (d2) are obtained from the OM images, which should contain measurement errors because of the focusing issue during the operation process. Third, the hemispheres and spheres described above are not perfect spheres and hemispheres, respectively. Fourth, and perhaps most importantly, the measured values of the diameters of the original PS cores before annealing (d0) are overestimated concerning the loose packing of the polymer chains after the electrospinning process; when the diameters of the transformed hemispheres and spheres are measured, the values are lower than those from the theoretical calculations. Although two different types of PS particles and cavities can be identified from the experimental results for the annealed samples before and after the selective removal processes, we would like to study the origins of the formation of the each types of the PS particles. Previously, we studied the morphology transformation of electrospun PS fibers thermally annealed on PMMA films.18,19 The PS fibers were observed to transform into PS hemispheres, and PMMA films with hemispherical cavities were obtained after the removal of the PS hemispheres. In those works, however, only PS particles and cavities with hemispherical type (type I) were observed. In this work using PS/PMMA core−shell fibers, PS particles and cavities with both the hemispherical type (type I) and the spherical type (type II) are observed. The PMMA shells covering the PS cores are considered to play a critical role in determining the types of the transformed PS particles. During the thermal annealing process, the PS/PMMA core−shell fibers gradually sink into the PMM films. For regions of the fibers which contain thinner PMMA shells, the shorter distances between the PS cores and air cause the PS cores to move to the polymer/air interface, resulting in the formation of the

Figure 6. (a) Graphical illustration of the morphology transformation process by annealing an electrospun polymer core−shell fiber on a polymer film. (b−e) Corresponding OM images of the annealed samples. In (c−e), the PS domains are selectively removed using cyclohexane. (f) Graphical illustration of an undulated cylinder. (g, h) OM images and corresponding plots of log α/D0 versus time for the electrospun PS/PMMA fibers annealed on PMMA films with different times.

the PS/PMMA core−shell fibers sink into the PMMA films. The PS domains that expose to air at this stage transform to hemispherical (type I) particles; the PS domains that are still embedded in the PMMA films at this stage transform to spherical (type II) particles. For the two different types of the transformation processes, the transformation kinetics are also discussed by considering the growth rates of the undulated amplitude (q) of the PS core. The growth rates of the undulated amplitude are affected by the interfacial tension and the viscosity ratio between the polymer fiber and the polymer matrix. Previously, Elemans et al. have studied the growth rates of the undulated amplitude of an annealed fiber embedded in a polymer film to calculate the interfacial tensions between the two polymer materials.32 For a polymer fiber annealed in a polymer film, the amplitudes (α) of perturbation of the fiber during the transformation process can be described by the following equation: α = α 0eqt E

(4) DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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different from those by annealing PS/PMMA core−shell fibers on PMMA films, which might be due to the differences of the interfacial tensions between PS/PMMA and air/PMMA. After the selective removal process, the cavities-containing polymer films may be applied to study the mechanical properties of cavity-containing films. For example, perforation lines are commonly used on polymer label films for soft drink bottles. The holes of the perforation lines provide spots with weaker mechanical strengths for easy tearing along specific directions, as shown in Figure 7a. Inspired by the perforation

where α0 is the origin amplitude.19,20,32 The amplitude of the perturbation can be calculated by the following equation: α = (b − a)/4

(5)

where a and b are the thinner and thicker diameters of the undulated fiber, respectively, as shown in Figure 6f. The values of a and b at different annealing times can be obtained by measuring the sizes of the undulated fibers in the OM images, and the values of α can thus be calculated. Equation 4 can be rewritten as below: log(α /D0) = qt /2.303 + log(α0/D0)

(6)

where D0 is the origin diameter of the fiber and 2.303 comes from ln(10). By plotting log(α/D0) versus time, the growth rates of the undulated amplitude can be obtained from the slopes of the plots.19,20 For example, the sizes of a fiber transforming to hemispherical (type I) particles, indicated by a green arrow, can be measured and plotted, as shown in Figure 6g; the sizes of a fiber transforming to spherical (type II) particles, indicated by a blue arrow, can be measured and plotted, as shown in Figure 6h. The growth rates of the undulated amplitude for fibers transforming to hemispherical (type I) and spherical (type II) particles are calculated to be 0.071 and 0.076 (s−1), respectively. Although the growth rates of the undulated amplitudes are similar, it can be seen that the type I fibers start to undulate at later annealing times than the type II fibers do. For example, in Figure 6g,h, the type I fiber starts to undulate at ∼300 s while the type II fiber starts to undulate at ∼180 s. The longer starting times are probably due to the effect of the polymer/air interface. In addition to the electrospun PS/PMMA core−shell fibers with the weight ratio of PS/PMMA = 1:1, we have prepared electrospun PS/PMMA fibers using two other weight ratios (PS/PMMA = 7:3 and 1:9). The thicknesses of the PMMA shells and the diameters of the PS cores can be determined by selectively removing the PS domains using cyclohexane (Figure S1c,d). The results indicate that the core diameters of the electrospun PS/PMMA fibers are larger by increasing the weight ratio of the PS component, as shown in Figure S1c for the weight ratio of PS/PMMA = 7:3; the shell thicknesses of the electrospun core−shell fibers are larger by increasing the weight ratio of the PMMA component, as shown in Figure S1d for the weight ratio of PS/PMMA = 1:9, but multicore structures are formed. The multicore PS/PMMA fibers (weight ratio of PS/PMMA = 1:9) are also annealed on PMMA films. The OM and SEM images of the annealed samples before and after the selective removal process are shown in Figure S2. Similar to the annealing results from the core−shell fibers (weight ratio of PS/PMMA = 1:1), the two types of transformation processes are observed for the multicore fibers (weight ratio of PS/ PMMA = 1:9). To further understand the transformation process, we also anneal PMMA hollow fibers, instead of PS/PMMA core−shell fibers, on the PMMA films, as shown inFigure S3. The hollow fibers are prepared by selectively removing the PS cores of the PS/PMMA core−shell fibers using cyclohexane. When the PMMA hollow fibers are annealed on a PMMA film, the PMMA hollow fibers first sink into the PMMA film, and the hollow cores can be regarded as air cylinders inside the PMMA shells. After longer annealing times, the air cylinders undulate and break into air bubbles, embedded in the PMMA film, driven by the Plateau−Rayleigh-type instability. The result is

Figure 7. (a) Photos of tearing a portion of the polymer label film of a soft drink bottle along the perforation lines. (b) Graphical illustration of the thermal annealing and selective removal processes, in which the tearing position is indicated by a red arrow. (c, d) Photos of the tearing experiment and a torn PMMA film.

lines, here we study the tearing experiments of the annealed PS/PMMA composites. After small bundles of the PS/PMMA core−shell fibers are placed on PMMA films, the samples are thermally annealed and dipped in cyclohexane to remove the PS particles, leaving the PMMA films with linearly arranged cavities, as illustrated in Figure 7b. The mechanical strengths of those regions with cavities are weaker than those without cavities, allowing easier tearing along the linearly arranged cavities. After tearing by hands, the films are observed to be torn along the linearly arranged cavities, as shown in Figures 7c and 7d.

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CONCLUSIONS In conclusion, we prepare PS/PMMA core−shell fibers using a single axial electrospinning setup and study the transformation processes by annealing the core−shell fibers on the PMMA films. During the thermal annealing process, the PS cores in the fibers undulate and transform into particles, driven by the reduction of the surface and interfacial energies of the polymers, similar to the Plateau−Rayleigh instability. The selective removal techniques are also used to confirm the morphologies of the transformed PS/PMMA composites. Two different types of transformation processes are identified. For the first type of the transformation process (type I), the PS cores transform to hemispherical particles after the annealing F

DOI: 10.1021/acs.macromol.7b01916 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

small bundles of the PS/PMMA core−shell fibers were placed on PMMA films and thermally annealed. After the samples were dipped in cyclohexane and dried, they were torn by hands. Structure Analysis and Characterization. An optical microscope (OM), equipped with a charge-coupled device (CCD), was used to observe and record the thermal annealing processes in situ. A scanning electron microscope (SEM) (JEOL, JSM-7401F) at an acceleration voltage of 5 kV was used to examine the samples. Before the SEM measurements, the samples were dried by a vacuum pump and coated with platinum (thickness: ∼4 nm).

process and to hemispherical cavities after the selective removal process; for the second type of the transformation process (type II), the PS cores transform to spherical particles after the annealing process and to spherical cavities after the selective removal process. The sizes of the hemispherical and spherical cavities are measured, and the measured values fall into two regions, as predicted for the two different types of cavities. The formation of the two different types of the particles and cavities might probably due to different thickness of the PMMA shells at different regions of the fibers and the presence of air in the core−shell fibers. For the two different types of the transformation processes, the growth rates of the undulated amplitude are similar, but the type I fibers start to undulate at later annealing times than the type II fibers do. The PMMA films with linearly arranged cavities, in which the PS particles are selectively removed by cyclohexane, are also used for the tearing experiments. The weaker mechanical strengths of those regions with cavities allow easier tearing along the linearly arranged cavities, showing a proof of concept on the potentials in studying the mechanical properties of cavity-containing films. In the future, we will study the transformation of hydrophobic/ hydrophilic or hydrophilic/hydrophobic core−shell fibers instead of hydrophobic/hydrophobic core−shell fibers. After the transformation process, hydrophilic polymer surfaces with isolated hydrophobic domains or hydrophobic polymer surfaces with isolated hydrophilic domains may be obtained, which can be applied to biomedical applications, such as protein patterning.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01916. Figures S1−S4 (PDF) Video S1 (MPG)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +886-3-5731631 (J.T.C.). ORCID

Jiun-Tai Chen: 0000-0002-0662-782X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of the Republic of China.

EXPERIMENTAL SECTION

Materials. Poly(methyl methacrylate) (PMMA, Mw: 49 300; PDI: 1.85) and polystyrene (PS, Mw: 166 500; PDI: 7.27) were purchased from Scientific Polymer Products and Sigma-Aldrich, respectively. Toluene and dimethylformamide (DMF) were obtained from Tedia. Cyclohexane was purchased from J.T. Baker. Acetic acid was obtained from Sigma-Aldrich. Preparation of Electrospun PS/PMMA Core−Shell Fibers and PMMA Hollow Fibers. To fabricate electrospun PS/PMMA core−shell fibers, a blend solution (30 wt %) in DMF containing PS (Mw: 166 500) and PMMA (Mw: 49 300) with a weight ratio of 1:1 was first prepared. The blend solution was then injected into a syringe, which was connected to a capillary nozzle (inner diameter: 0.41 mm). A syringe pump (KD Scientific), which was also connected to the syringe, was used to control the flow rate. A high voltage (10 kV) was applied between the grounded collector and the syringe by a power supply (SIMCO). The working distance between the capillary nozzle and the grounded collector was 10 cm. A vertical spinning configuration was used in the electrospinning process at ambient temperature. For the selective removal processes, the electrospun PS/PMMA core−shell fibers were immersed in acetic acid or cyclohexane to selectively remove the PMMA domains or the PS domains, respectively. PMMA hollow fibers can be obtained after the PS domains were removed. Preparation of PMMA Films. A PMMA (Mw: 49 300) solution (30 wt %) in toluene was first prepared. The PMMA solution was spin-coated on glass substrates at spinning rates of 800 rpm for 90 s and 3000 rpm for 30 s. The spin-coated PMMA films were further annealed in an oven at 150 °C for 3 h to evaporate the residual solvents and to decrease the roughness of the films. Thermal Annealing and Selective Removal Processes and Tearing Experiments. The electrospun PS/PMMA core−shell fibers were first placed on PMMA films. The samples were then thermally annealed using a heating stage (Mettler, FP82), which was equipped with a temperature controller. After the thermal annealing process, the PMMA or PS domains of the samples can be selectively removed by acetic acid or cyclohexane, respectively. For the tearing experiments,

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