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Solvent-Induced Shape Recovery of Anisotropic Polymer Particles Prepared by a Modified Thermal Stretching Method Yu-Ching Lo, Hsiao-Fan Tseng, Yu-Jing Chiu, Bo-Hao Wu, Jia-Wei Li, and Jiun-Tai Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01479 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Solvent-Induced Shape Recovery of Anisotropic Polymer Particles Prepared by a Modified Thermal Stretching Method Yu-Ching Lo,

1

Hsiao-Fan Tseng, 1 Yu-Jing Chiu, 1,2 Bo-Hao Wu,

1

Jia-Wei Li, 1 and Jiun-Tai Chen

1,2,3*

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, Hsinchu, Taiwan 30010 3

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu, Taiwan

30010 *To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +886-3-5731631

ABSTRACT Anisotropic polymer particles have attracted great attention because of their unique properties and potential applications in various areas, such as microelectronics, drug delivery, and medical imaging. The fabrication and morphology control, especially the shape recovery, of anisotropic polymer particles, however, remains a challenging task. In this work, we develop a novel strategy to fabricate anisotropic polymer particles by thermally stretching poly(vinyl alcohol) (PVA) films embedding polystyrene (PS) microspheres using a weight. Depending on the preannealing condition, anisotropic PS particles with two different shapes, sharp-headed and blunt-headed PS particles, can be obtained. The PVA films can be selectively removed by IPA/water, releasing the anisotropic PS particles. By adding tetrahydrofuran (THF), a good solvent for PS, into the PS particle-containing solutions, the

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anisotropic particles gradually transform back to spheres to reduce the total interfacial energies. The shape recovery rates of the polymer particles can be controlled by the amount of the added THF. This work not only provides a simple and feasible route to fabricate anisotropic polymer particles but also contributes to a deeper understanding in the solvent-induced shape recovery process from anisotropic polymer particles to polymer spheres.

Keywords: anisotropic, polymer particle, selective removal, shape recovery, thermal stretching

INTRODUCTION Anisotropic polymer particles have drawn intensive interest because of their unique properties and potential applications in various areas, such as cosmetics,1 microelectronics,2, 3 drug delivery,4 and medical imaging.5 The physical and rheological properties of anisotropic polymer particles have been compared with those of isotropic polymer particles.6-9 For instance, Yunker et al. investigated that the coffee ring effect, a commonly observed phenomenon for isotropic polymer particles, can be eliminated by using ellipsoidal polymer particles.10 They discovered that the shape of anisotropic polymer particles deforms the air−water interfaces and produces strong interparticle capillary interactions, leading to uniform deposition of the ellipsoid particles during evaporation.11-14 So far, many methods have been developed to fabricate anisotropic polymer particles.15 For example, Ho et al. introduced the thermal stretching method to prepare polymer ellipsoids.16 Biaxial forces were applied to stretch the polymer particle-containing polymer films using a custom-made tool. This method was later modified by Champion et al. to prepare anisotropic polymer particles with 20 different shapes by changing the sequence of the heating and stretching steps.17 The thermal stretching method was also revised by Shin et al. using an eight-jaw extensional apparatus to generate anisotropic particles with various shapes.18 Recently, the thermal stretching method was modified by confining two sides of sphere-containing polymer films using binder clips.19 Oblate


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spherical or prolate spherical polymer particles with different aspect ratios can be prepared depending on the locations of the embedded polymer particles in the stretched films. Despite the development on the research of anisotropic polymer particles, the fabrication and morphology control, especially the shape recovery, of anisotropic polymer particles is still challenging. In this work, we develop a novel method to fabricate anisotropic polymer particles by applying a modified thermal stretching strategy. Compared with the previously developed thermal stretching methods,16-18 this modified thermal stretching method does not require a special or expensive apparatus and heating mediums such as silicon oil are not necessary. In this method, polystyrene (PS) microspheres are first embedded in poly(vinyl alcohol) (PVA) films by using PS microsphere-containing PVA solutions. The samples are then stretched by a weight while they are thermally annealed in an oven. If the samples are preannealed before the thermal stretching process, the PS particles conform well with the deformation of the stretched PVA films and sharp-headed PS particles can be obtained; if the samples are not preannealed before the thermal stretching process, the PS particles do not follow the initial thermal stretching-induced deformation of the PVA films and blunt-headed PS particles can be obtained. By selectively removing the PVA films in IPA/water, anisotropic PS particles are released. Here, a solvent-induced shape recovery of the anisotropic polymer particles to spheres, driven by the reduction of the interfacial energies between the polymer particles and the surrounding medium, is also investigated by adding THF, a good solvent for PS. Furthermore, the shape recovery rates of the PS particles are studied to be controlled by the amount of the added THF. The advances and novelty of this work is that the anisotropic PS particles can transform back to PS spheres through a solvent-induced shape recovery. This work not only provides a simple and feasible route to fabricate anisotropic polymer particles but also contributes to a deeper understanding in the solvent-induced shape recovery process from anisotropic polymer particles to polymer spheres, which may find interesting applications in areas such as responsive devices or smart sensors.


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EXPERIMENTAL SECTION Materials Polystyrene (PS) microspheres with an average diameter of 10 µm (variation: 5 %) were synthesized by a modified emulsion polymerization method and bought from Polysciences. Poly(vinyl alcohol) (PVA, 86.7−88.7% hydrolyzed) with a weight-average molecular weight (Mw) of 130 kg mol-1 was purchased from Sigma-Aldrich. Deionized (DI) water was obtained from Milli-Q system. Tetrahydrofuran (THF) was bought from Macron Fine Chemicals. Ethanol, isopropanol (IPA), and toluene were purchased from Echo Chemical. Glass substrates were obtained from the FEA Company.

Preparation of PVA Films Embedding PS Microspheres A 2.6 wt % aqueous suspension of PS microspheres (diameter: 10 µm) was first diluted with ethanol and then dropped into a PVA solution (5 wt % in DI water). A 6 mL PVA solution containing the PS microspheres, which was stirred thoroughly, was poured into a Petri dish (diameter: 5.5 cm). After the solution was dried for 24 h and detached from the Petri dish, a PVA film (thickness: ∼100 µm) embedding the PS microspheres was obtained.

Solvent-Induced Shape Recovery of Anisotropic PS Particles Prepared by the Modified Thermal Stretching Method The PVA films embedding the PS microspheres were cut into pieces of films with sizes of 1 cm × 1.5 cm. For the experiments on samples with the preannealing step, a cut PVA film was clipped with two binder clips on both sides (top and bottom sides) of the film. Then the sample was placed in an oven, which had been preheated to 140 °C, and hung in the oven by connecting the clip on the top side of the film to a metal rod. After the sample was preannealed for 5 min, a weight of 300 g was connected


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to the bottom side of the film for the further annealing and stretching processes. The length of the stretched PVA film was measured constantly during the thermal stretching process. When the desired draw ratio of the PVA film was reached, the sample was removed from the oven and cooled. For the experiments on samples without the preannealing step, a cut PVA film was clipped with two binder clips on both sides (top and bottom sides) of the film and a weight of 300 g was connected to the bottom side of the film. The sample was then placed in an oven, which had been preheated to 140 °C, and hung in the oven by connecting the clip on the top side of the film to a metal rod. The length of the stretched PVA film was measured constantly during the thermal stretching process. When the desired draw ratio of the PVA film was reached, the sample was removed from the oven and cooled. To release the PS particles, the PVA films were selectively removed by using a 4 mL IPA/water (1/1 in volume) solution at 50 °C for 4 h. THF with different amounts (1 or 2 mL) was then added in the solution to induce the shape recovery. Finally, the PS particles were filtered and collected.

Structure Analysis and Characterization The morphologies of the PVA films and the 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 utilized to investigate the morphologies of the polymer samples. Before the SEM measurements, the samples were dried using a vacuum desiccator at room temperature and coated with 4 nm of platinum. ImageJ software was used to conduct the quantitative analyses of the OM and SEM data.

RESULTS AND DISCUSSION


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Figure 1. (a) Illustration of the thermal stretching process of the PVA films embedding the PS microspheres at 140 °C. (b) Illustration of the sharp-headed and blunt-headed PS particles embedded in the PVA films. (c) Illustration of the shape recovery process by adding THF.

To prepare the anisotropic PS particles, the modified thermal stretching method is used, as shown in Figure 1a. In this method, PVA solutions are first mixed with PS microspheres and dropped onto substrates, followed by a drying process, forming the PVA films embedding the PS microspheres. The composite films are then stretched by a weight while the samples are annealed at 140 °C. During the thermal stretching process, the PVA films are elongated along the stretching direction and the embedded spherical PS particles also transform to anisotropic PS particles, driven by the gravitational force of the weight. Depending on the preannealing and stretching conditions, anisotropic PS particles with two different shapes, sharp-headed and blunt-headed particles, can be obtained, as illustrated in Figure 1b. After the PVA films are selectively removed by IPA/water, the PS particles can be released. The shapes of the PS particles can be further recovered by adding THF in the particle-containing solution, as shown in Figure 1c. THF is miscible with IPA/water and can


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defuse into the PS particle domains in the solution. Swollen by THF, a good solvent for PS, the polymer chains in the PS particles can rearrange to reduce the total interfacial energies between the PS particles and the surrounding medium (IPA and water), resulting in the shape recovery from anisotropic PS particles to PS spheres.

Figure 2. (a) Illustration of the experimental process to stretch the PS spheres/PVA films with the preannealing step. (b) OM image of a sharp-headed PS particle embedded in a PVA film. (c,d) SEM images of sharp-headed PS particles with different magnifications. (e) Illustration of the experimental process to stretch the PS spheres/PVA films without the preannealing step. (f) OM image of a blunt-headed PS particle embedded in a PVA film. (g,h) SEM images of blunt-headed PS


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particles with different magnifications.

In the thermal stretching processes of the PVA films embedding the PS microspheres, anisotropic PS particles with two different shapes, sharp-headed or blunt-headed particles, can be obtained depending on whether or not the preannealing step is applied. For the experiments with the preannealing step, before the thermal stretching process, the samples are annealed at 140 °C, higher than the glass transition temperatures (Tg) of PVA (∼80 °C) and PS (∼100 °C). When the samples are thermally stretched by a weight, the polymer chains in the preheated PS microspheres can rearrange with the stretched PVA films, forming the sharp-headed PS particles, as illustrated in Figure 2a. During the thermal stretching process, the transformation of the PS particles conforms well with that of the PVA films in the stretching direction; the transformation of the PS particles, however, does not conform with that of the PVA films in the direction perpendicular to the stretching direction, resulting in the formation of voids, as shown in the illustration (Figure 2a) and optical microscopy (OM) image (Figure 2b). The shapes of the sharp-headed PS particles can be further confirmed by selectively removing the PVA films using IPA/water to release the PS particles, as shown in the SEM images (Figure 2c and d). For the thermal stretching experiments without the preannealing step (Figure 2e), the samples connected with a weight are directly moved into the oven, which is preheated to ∼140 °C. During the thermal stretching process, the samples are gradually heated in the oven. When the samples are heated to ∼80 °C, near the Tg of PVA, the PVA films are initially stretched while the PS particles still maintain the spherical shapes because of the lower Tg of PVA (∼80 °C) than that of PS (∼100 °C), leaving void spaces around the PS particles. At longer thermal stretching times, when the samples are heated to ∼100 °C, near the Tg of PS, the polymer chains in the PS particles start to wet the walls of the void spaces in the PVA films, resulting in the formation of blunt-headed PS particles embedded in the PVA films, as shown in the illustration (Figure 2e) and OM image (Figure 2f). The shapes of the


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blunt-headed PS particles can be further confirmed by selectively removing the PVA films using IPA/water to release the PS particles, as shown in the SEM images (Figure 2g and h).

Figure 3. (a−c) SEM images of sharp-headed PS particles at different draw ratios of the PVA films: (a) 2 (b) 3, and (c) 4. (d) Plot of the aspect ratio of PS particles versus the draw ratio of PVA films.

In the thermal stretching process, the particle-containing PVA films are thermally stretched by a weight of 300 g, which is connected to the bottom side of the film. In the thermal stretching process, the force to stretch the particle-containing PVA films is the gravitational force. At longer stretching times, the lengths of the stretched PVA films, which are measured constantly during the thermal stretching process, are longer. For the sharp-headed PS particles prepared by the thermal stretching process with the preannealing step the aspect ratios of the particles can be controlled by the draw ratios of the PVA films. The draw ratios of the PVA films (n) are defined by the lengths of the stretched PVA films (l) divided by the original lengths of the unstretched PVA films (l0); the draw


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ratios of the PVA films (n) are controlled by adjusting the annealing and stretching times at which the desired lengths of the PVA films are reached. Figure 3a−c shows the SEM images of sharp-headed PS particles with different aspect ratios by changing the draw ratios of the PVA films (draw ratio: 2, 3, and 4). It can be seen that the aspect ratios of the PS particles increase with the draw ratios of the PVA films (Figure 3d), which is mainly due to the thermal stretching-induced shape transformation. Assuming that the PS microspheres transform to ellipsoids and their volumes are conserved before and after the thermal stretching processes, the following relationship is obeyed:

π = ( × )

(1)

where r is the original radius of the PS microspheres before the thermal stretching process, R1 is the radius of the transformed PS ellipsoids along the stretching direction, and R2 is the radius of the transformed PS ellipsoids perpendicular to the stretching direction. When the PVA films are stretched for n times (draw ratio = n), the radius of the transformed PS ellipsoids along the stretching direction is assumed to be n times of the original radius of the PS microspheres (R1 = nr) and Equation 1 can be rewritten as the following:

π = ( × )

(2)

By rewriting Equation 2, the following relationship between the radius of the transformed PS particles perpendicular to the stretching direction (R2) and the original radius of the PS microspheres (r) can be obtained:

= /√


(3)

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Therefore, the aspect ratio of the transformed PS particles can be calculated as the following:

= / = √

(4)

In the ideal case, the aspect ratio is controlled by the draw ratio of the PVA films (n). For the draw ratio of the PVA films (n) equals to 2, 3, and 4, the ideal aspect ratio of the transformed PS particles can be calculated as 2.8, 5.2, and 8, respectively. It can be seen that the measured values of the aspect ratio of the transformed PS particles at different draw ratios are all larger than the calculated values, mainly due to the formation of the voids during the thermal stretching processes. From the experimental results (Figures 2 and 3), it can be seen that the preannealing step is critical for determining the shapes of the anisotropic PS particles. In addition to preannealing the samples by heating the particle-containing PVA films before the stretching processes, the preannealing step is also conducted by dipping the particle-containing PVA films in toluene, a good solvent for PS. As shown in Figure S1, sharp-headed PS particles can also be obtained for the samples dipped in toluene in the preannealing process. Still, the shape control of the samples by heating the particle-containing PVA films is better than that by dipping the particle-containing PVA films in toluene. For the following shape recovery experiments, the sharp-headed PS particles are obtained from those using the preannealing step by heating the particle-containing PVA films.


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Figure 4. (a−c) SEM images of PS particles by adding the solution containing the sharp-headed PS particles with 2 mL THF for different times: (a) 0, (b) 5, and (c) 120 min. (d) Plot of the aspect ratio of PS particles versus time for the sharp-headed PS particles using 2 mL THF. (e−g) SEM images of PS particles by adding the solution containing the sharp-headed PS particles with 1 mL THF for different times: (e) 0, (f) 24, and (g) 96 h. (h) Plot of the aspect ratio of PS particles versus time for the sharp-headed PS particles using 1 mL THF. (i−k) SEM images of PS particles by adding the solution containing the blunt-headed PS particles with 2 mL THF for different times: (i) 0, (j) 10, and (k) 20 min. (l) Plot of the aspect ratio of PS particles versus time for the blunt-headed PS particles using 2 mL THF.

In this work, we also develop a novel strategy to recover the shapes of the anisotropic PS particles back to PS spheres. Anisotropic PS particles are first released by selectively dissolving the PVA films in IPA and water. Subsequently, THF is added in the IPA/water solution containing the anisotropic PS particles. Here, THF is found to be a suitable solvent for the shape recovery process of the anisotropic PS particles. THF has the following unique features other than other common organic solvents. First, THF is a good solvent for PS; the solubility parameter (δ) of THF (9.5 cal1/2 cm-3/2) is close to that of PS (9.1 cal1/2 cm-3/2). Second, THF is miscible with water; therefore, THF can defuse through water and swell the PS chains in the particles. Third, the vapor pressure of THF


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(173 hPa at 20 °C) is high, so THF can evaporate quickly from the PS particles after the shape recovery process. Otherwise, the residual solvents may cause the PS particles to fuse together upon aggregation. Although solvents such as toluene (δ = 8.9 cal1/2 cm-3/2), benzene (δ = 9.2 cal1/2 cm-3/2), chloroform (δ = 9.2 cal1/2 cm-3/2), and cyclohexane (δ = 8.2 cal1/2 cm-3/2) are good solvents for PS, they are not miscible with water. In addition to THF, acetone might be another choice for the purpose for the shape recovery. The solubility parameter (δ) of acetone (9.9 cal1/2 cm-3/2) is also close to that of PS (9.1 cal1/2 cm-3/2), and acetone is fully miscible with water. Besides, the vapor pressure of acetone (240 hPa at 20 °C) is high and can evaporate quickly after the shape recovery process. Because THF is a good solvent for PS and miscible with water, THF can defuse into water and swell the polymer chains in the PS particles. The chain fluidity of PS is enhanced by THF, and the swollen PS chains can rearrange and delocalize to decrease the interfacial energies between the swollen PS particles and the surrounding medium. Considering that spheres possess the minimum interfacial areas and interfacial energies to the surrounding medium, the anisotropic PS particles transform back to the PS microspheres driven by the interfacial tensions between the swollen PS particles and the surrounding medium. Sharp-headed PS particles with an aspect ratio of ∼7, prepared by using the thermal stretching method with a draw ratio of 3, are used as a model example. When a 4 mL solution containing the sharp-headed PS particles is added by 2 mL THF, the concentration of THF in the mixture is 33%, the sharp-headed PS particles gradually change back to the spherical particles after THF is added for different lengths of time, as shown in Figure 4a−d. The original aspect ratio of the sharp-headed PS particles before adding THF is ∼7 (Figure 4a). When THF is added for 5 min, the aspect ratio of the PS particles decreases to ∼3 (Figure 4b). After THF is added for 120 min, the PS particles have returned back to spheres with an aspect ratio of ∼1 (Figure 4c). The aspect ratio versus time after THF are added is also plotted in Figure 4d. It can be observed that the aspect ratio of the sharp-headed PS particles decreases rapidly in the initial 10 min and continuously decreases at a


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slower rate for longer times. After THF is added for 100 min or longer times, the PS particles return back to spheres with an aspect ratio of ∼1 (the ideal plateau value). When a 4 mL solution containing the sharp-headed PS particles is added by 2 mL THF the sharp-headed PS particles gradually change back to the spherical particles after THF is added for different lengths of time, as shown in Figure 4a−d. The data shown in Figure 4a−d are obtained by adding 2 mL THF into the 4 mL PS particle-containing solutions. To further study the effect of THF, the amount of THF is decreased to 1 mL, the concentration of THF in the mixture is 20%, and similar experiments are conducted, as shown in Figure 4e−h. After THF is added in the PS particle-containing solutions for 24 h, the aspect ratio of the PS particles decreases from ~7 (Figure 4e) to ~2.5 (Figure 4f). After THF is added for 96 h, the PS particles have returned back to spheres with an aspect ratio of ~1 (Figure 4g). The aspect ratio versus time after 1 that the aspect ratio of the sharp-headed PS particles decreases rapidly in the initial 1 h and continuously decreases at a slower rate for longer times. After THF is added for 100 h or longer times, the PS particles return back to spheres with an aspect ratio of ~1 (the ideal plateau value). From the results, we know that when the amount of THF is decreased from 2 to 1 mL, the shape recovery of the PS particles occurs at a longer time scales. For 2 mL THF, the time required for shape recovery is ~10 min; for 1 mL THF, by comparison, the time required for shape recovery is largely increased to ~100 h. By adding THF, the chain fluidity of the PS chains in the particles is enhanced. At relatively higher amounts of THF, the chain fluidity of the PS chains in the particles is higher, resulting in shorter kinetic response times in the shape recovery process. In addition to the shape recovery of the sharp-headed PS particles, the shape recovery of the blunt-headed PS particles is also investigated. Blunt-headed PS particles with an aspect ratio of ~3, prepared by using the thermal stretching method without the preannealing step, are used. When a 4 mL solution containing the blunt-headed PS particles is added by 2 mL THF, it can also be observed that the blunt-headed PS particles gradually change back to the spheres after THF is added for


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different lengths of time, as shown in Figure 4i−l. After THF is added in the PS particle-containing solution for 10 min, the aspect ratio of the PS particles decreases from ∼3 (Figure 4i) to ∼1 (Figure 4j). After THF is added for 20 min, almost all the PS particles recover back to spheres with an aspect ratio of ∼1 (Figure 4k). The aspect ratio of the blunt-headed PS particles at different times after 2 mL THF is added is also plotted in Figure 4l. It can be observed that the aspect ratio of the blunt-headed PS particles decreases from ∼3 to ∼1 in 10 min after THF is added. By comparing the results of the sharp-headed and blunt-headed PS particles, it can be observed that the shape recovery of the blunt-headed PS particles occurs at similar time scales as that of the sharp-headed PS particles. Besides, the IPA added in the water is also found to play a critical role in the shape recovery processes. Figure S2 shows the SEM images of the PS particles after the shape recovery processes using IPA/water mixtures with different ratios (IPA/water = 0/1, 1/3, 1/2, and 1/1). From the results, it can be seen that broken PS films rather than microspheres are obtained without adding IPA (Figure S2a). When the IPA/water mixtures with the ratios of 1/3 and 1/2 are used, aggregated PS microspheres are observed (Figure S2b,c). PS microspheres without aggregation can be obtained when the IPA/water mixtures with the ratios of 1/1 are used (Figure S2d). The results indicate that the addition of IPA can effectively avoid the aggregation of the transformed PS particles. In this work, it is also interesting to discuss the states of the polymer chains. When the PS particles embedded in the PVA films are thermally stretched, the PS microspheres are transformed to anisotropic particles, in which the PS chains remain in the compact coil state. When the PS particles are swollen by THF, the PS chains possess a more open coil state. Driven by the interfacial tensions between the swollen PS particles and the surrounding medium, the anisotropic PS particles transform back to the swollen PS microspheres, in which the PS chains also possess a more open state. When the solvents are evaporated, the swollen PS microspheres return to the unswollen PS microspheres, in which the polymer chains change back again to the compact coil state.


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CONCLUSION In this work, we successfully fabricate anisotropic PS particles by a modified thermal stretching method. The PVA films embedding the PS microspheres are thermally annealed and stretched, forming anisotropic PS particles embedded in the PVA films. Anisotropic PS particles with two different shapes, the sharp-headed and the blunt-headed PS particles, can be prepared depending on whether the PS particle-containing PVA films are preannealed or not. After the PVA films are selectively removed by IPA/water, the anisotropic PS particles can be released. Moreover, by adding THF, the anisotropic PS particles dispersed in the IPA/water solution can transform back to PS spheres to reduce the total interfacial energies between the PS particles and the surrounding medium. THF, which swells the polymer chains in the PS particles, is found to play a critical role in determining the shape recovery rates from anisotropic PS particles to PS spheres. In the future, we will apply this strategy to fabricate anisotropic polymer particles and to study their shape recovery using other polymers, in an attempt to extend the potential applications of anisotropic polymer particles.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX SEM images of the PS particles.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was financially supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This work was also supported by the Ministry of Science and Technology of the Republic of China.

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Solvent-Induced Shape Recovery of Anisotropic Polymer Particles Prepared by a Modified Thermal Stretching Method Yu-Ching Lo, Hsiao-Fan Tseng, Yu-Jing Chiu, Bo-Hao Wu, Jia-Wei Li, and Jiun-Tai Chen *


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Solvent-Induced Shape Recovery of Anisotropic ... - ACS Publications

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