Setting Foot in Asymmetric Wetting Environments ... - ACS Publications

Subscriber access provided by Washington University | Libraries

Article

Setting Foot in Asymmetric Wetting Environments: Fabrication of Mushroom-Like Anisotropic Polymer Nanoparticles Mu-Huan Chi, Zhi-Xuan Fang, Hao-Wen Ko, Chun-Wei Chang, and Jiun-Tai Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10426 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 7, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Setting Foot in Asymmetric Wetting Environments: Fabrication of Mushroom-Like Anisotropic Polymer Nanoparticles Mu-Huan Chi, Zhi-Xuan Fang, Hao-Wen Ko, Chun-Wei Chang, and Jiun-Tai Chen* Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010.

ABSTRACT: To achieve multi-functional materials using a single kind of ingredient or particle, it is desired to develop simple methods to prepare anisotropic nanoparticles, such as mushroomlike nanoparticles, which can be applied to many fields. Here, we investigate the wetting of polymer nanospheres on the open ends of the nanopores of anodic alumina oxide (AAO) templates, which offer asymmetric wetting environments, to fabricate mushroom-like anisotropic polymer nanoparticles with controllable morphologies. These nanostructures have special dynamic behaviors in aqueous solutions. In addition, we prepare mushroom-like Janus nanoparticles by altering the fabrication processes. This work not only demonstrates a novel method to prepare anisotropic polymer nanoparticles, but also provides a detailed understanding in the wetting behaviors under asymmetric environments.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

INTRODUCTION Particle science is very important because it is highly connected to the normal life.1 In the past, particles have been widely applied to different fields, such as cosmetics, ink and painting, medicines, catalysts, and sensors.2-10 Especially, isotropic particles have been widely used because they can be easily fabricated by various methods in a great quantity.11-13 The applicability of isotropic particles, however, is still highly limited because of their symmetric shapes and low surface area/volume ratios. To achieve multi-functional materials using a single kind of ingredient or particle, it is desired to develop simple methods to prepare anisotropic particles, which are particles with asymmetric shapes or surface compositions. Owing to the special geometries and multicomponents of anisotropic particles, they have special properties distinct from isotropic particles and have been applied to many fields. In the past, various anisotropic particles with different shapes and compositions have been prepared by different methods.14-17 Among diverse anisotropic particles, mushroom-like particles, especially nanoparticles, have already been applied to sensors, catalysts, and drug carriers because of their unique morphologies.18-25 Preparing mushroom-like polymer nanoparticles with controllable morphologies, however, is still a huge challenge. Here, we provide a simple and feasible method to fabricate mushroom-like anisotropic polymer nanoparticles with controllable morphologies by wetting polymer nanospheres on the open ends of the nanopores of anodic alumina oxide (AAO) templates. AAO templates, porous membranes with hexagonally-packed nanopores, are made by anodization process of aluminum foils.26 Because AAO templates possess several advantages, such as high porosity, regular pore arrangement, and high stability, they have been widely used for preparing diverse nanostructures.27-31 Furthermore, the pore-to-pore distances, pore


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diameters, and pore lengths of the AAO templates can be controlled by changing the anodizing conditions, such as the types of the electrolyte, working potentials, anodization times, or etching times.32-33 Consequently, nanostructures with different sizes can be obtained. In general, the surface energy of AAO nanopores is relatively high. Therefore, flowing materials such as solutal, swollen, or melted materials can infiltrate into the AAO nanopores spontaneously by capillary force. After solidifying by drying, cooling, or curing, one-dimensional nanostructures can be fabricated in the AAO nanopores, such as nanorods or nanotubes.34-39 Special nanostructures can also be prepared by adjusting the fabrication processes.40-43 To obtain free nanostructures, the AAO templates can be selectively removed by weak acid or base. In this work, we use the thermal-induced wetting of polymer nanospheres on the open ends of AAO nanopores to fabricate mushroom-like anisotropic polymer nanoparticles. After the suspensions of polymer nanospheres are spin-coated on AAO templates, the samples are thermally annealed. Because the open ends of AAO nanopores provide asymmetric wetting environments, mushroom-like anisotropic polymer nanoparticles are formed by wetting of the polymer melts in two different directions, on the top surfaces and in the nanopores of AAO templates. The wetting behavior of the polymer melts can be controlled by different annealing conditions, resulting in the controllable morphologies of the nanoparticles. By adjusting the fabrication processes, mushroom-like Janus nanoparticles can also be prepared, which can be applied to self-assemblies, sensors, and drug deliveries.44-49 This work not only offers a simple method for preparing anisotropic nanoparticles, but also gives a detailed understanding of the wetting behaviors under asymmetric environments.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

RESULTS AND DISCUSSION The main concept of preparing mushroom-like polymer nanoparticles is thermal-annealinginduced wetting of polymer nanospheres under asymmetric environments, as shown in Figure 1. Polystyrene (PS) nanospheres with an average diameter of ~350 nm are used here, as shown in Figures 2a and b. First, the diluted suspensions of monodispersed polymer nanospheres are spincoated on the AAO templates with average pore diameters of ~229 nm (Figures 2c and d) and dispersive PS nanospheres on AAO templates can be obtained, as shown in Figure 2e. The advantage of using spin-coating is that the dispersity of PS nanospheres can be controlled by the concentrations of the suspensions and the rotation rates of the spin-coating process. During the spin-coating process, however, the polymer nanospheres are deposited randomly by centrifugal force instantaneously. The contacts between polymer nanospheres and the open ends of the AAO nanopores are poor, which may negatively affect the morphologies of the mushroom-like polymer nanoparticles in the following process. To improve the contacts and keep one nanosphere on top of one nanopore, the samples are dried slowly and subsequently sucked by a vacuum pump from another ends after the spin-coating process. Owing to the surface tensions induced by the solvent evaporation and the suction process by the vacuum system, PS nanospheres can attach to the open ends of the AAO nanopores closely, as shown in Figure 2f.


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Illustrative scheme for preparing mushroom-like polymer nanoparticles.

Figure 2. (a and b) SEM images of PS nanospheres with lower and higher magnifications. (c and d) SEM images of an AAO template with lower and higher magnifications. (e and f) SEM images of PS nanospheres spin-coated on an AAO template by 1000 rpm for 30 s with lower and higher magnifications.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

After the deposition of the polymer nanospheres, the samples are thermally annealed in an oven. Once the annealing temperature is higher than the glass transition temperature (Tg) of the polymers, the polymers are softened and the polymer chains possess enough mobilities to infiltrate into the AAO nanopores by capillary force. Because the surface tensions of AAO surfaces (~1340 mN m-1) are significantly higher than those of polymers (e.g., the surface tension of polystyrene is ~40.4 mN m-1 at 25 °C), polymer melts spontaneously wet the AAO surfaces to reduce the total surface and interfacial energies for reaching a thermodynamically favored state.50-51 Because only the bottom parts of the polymer nanospheres are in contact with the open ends of the AAO nanopores, only the bottom parts are infiltrated into the AAO nanopores and the upper parts still remain outside the AAO nanopores. Finally, the mushroomlike polymer nanoparticles are formed, resulting from the asymmetric wetting phenomenon. Figures 3a and b show the mushroom-like PS nanoparticles produced by thermally annealing samples at 130 °C for 25 min (the Tg of PS is ~100 °C). The shapes of these anisotropic nanostructures are similar to those of the commercial agaricus bisporus, composed of a hemispherical head (pileus) and a rod-like bottom (stipe), as shown in Figure 3c. From the magnified SEM image (Figure 3b), the pileus edges are found to be wave-like structures rather than smooth rims. Because each nanopore of the AAO template is surrounded by six other nanopores (hexagonally packing), there are six AAO walls perpendicular to the surface of the central nanopore, as shown in Figure 3d. When PS nanospheres are softened during the thermal annealing process, the upper parts of the nanospheres spread on the top surfaces of the AAO templates. While the circular spreading fronts encounter the perpendicular AAO walls, wave-like structures are formed on the pileus edges after the cooling process.


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Mushroom-like PS nanoparticles by thermally annealing PS nanospheres on the open ends of AAO nanopores at 130 °C for 25 min: (a and b) SEM images with lower and higher magnifications, (c) graphical illustration of each part of the nanostructure, and (d) graphical illustration of the formation of the wave-like pileus edge.

The main advantage of fabricating mushroom-like polymer nanoparticles by this method is that the morphologies of the nanostructures can be easily controlled by adjusting different annealing conditions. Because the wetting behaviors of polymers take place under asymmetric environments, the changes of the annealing conditions can strongly affect the sizes and shapes of the pilei and the stipes. Figures 4a-d show the various anisotropic PS nanoparticles made by different annealing temperatures (125, 130, 135, and 140 °C) for 30 min. When the annealing temperatures increase, the wetting behaviors of the PS melts on the top surfaces (spreading behavior) and in the nanopores (infiltrating behavior) of the AAO templates become more significant, resulting in larger pilei and longer stipes, respectively. Because the PS melts spread on the top surfaces and infiltrate into the nanopores of the AAO templates constantly, the


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

polymers in the upper parts of the PS nanospheres supply materials continuously to the wetting fronts and reduce the thicknesses of the pilei at higher annealing temperatures. Figure 4a shows the anisotropic PS nanoparticles prepared at 125 °C for 30 min with short stipes and small and thick pilei, whose shapes are similar to those of the agaricus bisporus. When the temperature increases to 130 °C, anisotropic PS nanoparticles with medium stipes and medium and thick pilei are produced, whose shapes are similar to those of the lentinula edodes, as shown in Figure 4b. At 135 °C, anisotropic PS nanoparticles with long stipes and large and thin pilei can be obtained, whose shapes are similar to those of the pleurotus eryngii (Figure 4c). Finally, all polymers infiltrate into the AAO nanopores and form the PS nanorods at 140 °C, as shown in Figure 4d. In addition to the annealing temperatures, the annealing times can also affect the morphologies of the mushroom-like polymer nanoparticles. Figures 4e-h show the various anisotropic PS nanoparticles made by annealing the samples at 130 °C for different times (15, 25, 35, and 65 min). Similar to changing the annealing temperatures, the spreading and infiltrating behaviors of the PS melts on the AAO surfaces become more obvious when the annealing times increase, resulting in larger pilei and longer stipes. The thicknesses of the pilei also become thinner at longer annealing times because of the volume conservation during the annealing process. Figure 4e shows the anisotropic PS nanoparticles prepared at 130 °C for 15 min with short stipes and small and thick pilei, whose shapes are similar to those of the agaricus bisporus. When the annealing times increase to 25 min, anisotropic PS nanoparticles with medium stipes and medium and thick pilei are produced, whose shapes are similar to those of the lentinula edodes, as shown in Figure 4f. For 35 min annealing, anisotropic PS nanoparticles with long stipes and large and thin pilei can be obtained, whose shapes are similar to those of the pleurotus eryngii (Figure 4g). Finally, all polymers infiltrate into the AAO nanopores and form PS nanorods after


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


65 min annealing, as shown in Figure 4h. In contrast to changing the annealing temperatures, controlling morphologies using different annealing times can prevent the problems of thermal degradation or other side reactions because of the lower annealing temperatures. Annealing at lower temperatures, however, is more time-consuming to obtain the desired morphologies.

Figure 4. Mushroom-like PS nanoparticles by thermally annealing PS nanospheres on the open ends of AAO nanopores at different conditions: (a-d) SEM images of mushroom-like PS nanoparticles fabricated at 125, 130, 135, and 140 °C for 30 min and (e-f) SEM images of mushroom-like PS nanoparticles fabricated at 130 °C for 15, 25, 35, and 65 min.

To further study the wetting behaviors of polymer melts under asymmetric environments, quantitative analysis of the dimensions of nanoparticles prepared by different annealing


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

conditions are also performed. The definitions of different dimensions are shown in Figure 5a. Here, all dimensions are measured from the SEM images. The diameter of the pileus (‫ܦ‬୮ ) is the maximum measured length of the pileus edge. The thickness of the pileus (‫ܪ‬୮ ) is the length from the top of the pileus to the concave of the pileus edge. The length of the stipe (‫ܮ‬ୱ ) is the length from the concave of the pileus edge to the bottom of the stipe. By measuring these dimensions, we can observe the wetting phenomenon in different experimental conditions. At different annealing temperatures, the viscosity of the PS melts decreases exponentially when the annealing temperature increases.52 Therefore, the higher mobilities of the PS chains accelerate the spreading rates on the top surfaces and infiltrating rates in the nanopores of the AAO templates, resulting in the exponentially increased ‫ܦ‬୮ and ‫ܮ‬ୱ at constant annealing times, as shown in Figure 5b. Here, the initial point is assumed as 100 °C, which is equal to the Tg of PS, where the deformation at this temperature is small and can be ignored. At higher temperatures, however, ‫ܦ‬୮ decreases dramatically because the wetting fronts of the PS melts infiltrate into the surrounding AAO nanopores after serious spreading. Figure 5b also shows that ‫ܪ‬୮ decreases with the annealing temperatures.


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) SEM image and illustration of mushroom-like PS nanoparticles. The diameter and thickness of the pileus and the length of the stipe are indicated in the diagram. (b-d) Plots of the average dimensions of the nanoparticles, the wetting rates in different directions, and the volume ratios of each part of the nanoparticles versus the annealing temperature. The samples are annealed for 30 min. (e-g) Plots of the average dimensions of the nanoparticles, the wetting rates in different directions, and the volume ratios of each part of the nanoparticles versus the annealing time. The samples are annealed at 130 °C.

For comparing the different dynamic behaviors between the spreading process and the infiltrating process, the wetting rates under different environments are also calculated. The spreading rates of the PS melts on the top surfaces of the AAO templates are equal to the spreading lengths dividing the annealing times, where the spreading lengths are equal to the differences between the radii of the pilei and the radii of the AAO nanopores. Similarly, the infiltrating rates of the PS melts in the AAO nanopores are equal to the infiltrating lengths dividing the annealing times, where the infiltrating lengths are equal to ‫ܮ‬ୱ . When the annealing temperatures increase, both the spreading rates and the infiltrating rates increase exponentially, as shown in Figure 5c. These results are also caused by the temperature-dependent viscosities of


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

the polymer melts. In addition, the infiltrating rates are larger than the spreading rates at all annealing temperatures because the different geometries of the top surfaces and the nanopores of the AAO templates create asymmetric wetting environments. During thermal annealing, the PS melts are continuously stretched by the constant surface tensions, which can be seen as a creep process. Here, we assume that the thicknesses of the wetting fronts of the PS melts (‫ܪ‬୤ ) on the top surfaces and in the nanopores of the AAO templates are equal. Therefore, the stretching stress (σୱ୮ ) induced by the surface tensions on the top surfaces of the AAO templates can be described as the following equation: σୱ୮ =

గ஽౦ ஓ ୡ୭ୱ ஘ గ஽౦ ு౜

=

ஓ ୡ୭ୱ ஘

(1)

ு౜

where γ is the resultant interfacial tension on the contact line and θ is the contact angle. The stretching stress (σ୧୬ ) induced by the surface tensions in the AAO nanopores can be described as the following equation: ஠஽ ஓ ୡ୭ୱ ஘ ౜ ఽ ିு౜ ሻ

σ୧୬ = గு ఽሺ஽

= ቂሺ஽

஽ఽ

ቃቀ

ఽ ିு౜ ሻ

ஓ ୡ୭ୱ ஘ ு౜

ቁ

(2)

where ‫ܦ‬୅ is the average diameter of the AAO nanopores, which is equal to the diameter of the stipes. From eq. (1) and eq. (2), σ୧୬ is slightly larger than σୱ୮ . Consequently, a higher deformation rate is obtained, causing a higher wetting rate in the AAO nanopores. Moreover, the local temperatures in the AAO nanopores are larger than those on the top surfaces of the AAO templates because the heat conductivity of AAO is much better than that of air. Therefore, PS melts with lower viscosities are obtained, causing higher wetting rates in the AAO nanopores.


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


By assuming that the pilei and stipes are perfect spherical caps and cylindrical rods, respectively, the volume of each part can also be calculated. The volume of the pileus (ܸ୮ ) can be expressed by the following equation: ܸ୮ = π‫ܪ‬୮ ሺ3‫ܦ‬୮ଶ + 4‫ܪ‬୮ଶ ሻ/6

(3)

and the volume of pileus (ܸୱ ) can be described by the following equation: ܸୱ = π‫ܦ‬୅ଶ ‫ܮ‬ଶୱ /4

(4)

Figure 5d shows the calculated volume ratios of each part at different temperatures. The volume of pileus decreases exponentially (1 − ݁ ்/௞ ) with the temperature, while the volume of stipe increases exponentially (݁ ்/௞ ) with the temperature. For constant annealing times, the larger infiltrating rates lead more PS melts to infiltrate into the AAO nanopores and form stipes. In addition to the kinetic factors, thermodynamic factors also affect the results. Because the total interfacial energies of the stipes are larger than those of the pilei, the PS melts in the AAO nanopores are more stable and energetically favorable. When the annealing temperatures increase, the PS melts can reach the equilibrium state faster. Therefore, the volumes of the stipes are larger than those of the pilei at high temperatures. The effects of the annealing times on the dimensions of the PS nanoparticles at constant temperatures are also investigated, as shown in Figure 5e. At fixed temperatures, the viscosities of the PS melts are constant, and the PS melts are stretched by a constant stress. Therefore, the stretching strain increases with the annealing time, resulting in exponentially decay (increasing form) of ‫ܦ‬୮ and ‫ܮ‬ୱ . These results can be expected from the Voigt-Kelvin model.53 Similar to changing the annealing temperatures, considerable decrease of ‫ܦ‬୮ for long annealing times is


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

also observed. Figure 5e also shows that ‫ܪ‬୮ decreases with the annealing times. In addition, the different dynamic behaviors between the spreading process and the infiltrating process for different annealing times are also examined. The wetting rates are calculated by differentiating the curves of the wetting lengths on different directions. When the annealing times increase, both the spreading rates and infiltrating rates decrease exponentially, as shown in Figure 5f. These results are also caused by the creep behavior of the polymer melts, where the strain rates change exponentially. Similar to changing the annealing temperatures, the infiltrating rates are also larger than the spreading rates at all annealing times because of the different stretching stresses and local temperatures under asymmetric wetting environments. The volume ratios of pilei and stipes are also calculated, as shown in Figure 5g. The volumes of pilei and stipes decrease and increase linearly with the annealing times, respectively. Similar to changing the annealing temperatures, the volume ratios of the stipes are larger than those of the pilei for longer annealing times because both the kinetic and thermodynamic factors affect the equilibrium state. In order to demonstrate the universality of this method, we also apply this method to poly(methyl methacrylate) (PMMA) nanospheres with average diameters of ~300 nm. After the samples are thermally annealed at 160 °C for 30 min, mushroom-like PMMA nanoparticles can also be obtained, as shown in Figure 6a. Comparing to PS, the more ill-defined morphologies of the mushroom-like PMMA nanoparticles are due to the weaker electron resistances of PMMA, which lead to the damage of the PMMA nanoparticles under the electron beams in the SEM measurement. Here, we also apply PS nanospheres with a larger average diameter (~500 nm). To prepare mushroom-like nanoparticles (one pileus connecting to one stipe), the diameters of the polymer nanospheres cannot be much larger than the diameters of the AAO nanopores. Therefore, PS nanospheres with an average diameter of ~350 nm are used above. In contrast,


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


because of their larger melting domains, the PS melts infiltrate to multiple nanopores during the annealing process when PS nanospheres with much larger average diameters are used. After the cooling process, jellyfish-like PS nanoparticles are produced, which are composed of one pileus and multi-foot, as shown in Figure 6b. The results are similar to those reported previously.54

Figure 6. (a) Mushroom-like PMMA nanoparticles by thermally annealing PMMA nanospheres on the open ends of AAO nanopores at 160 °C for 30 min. (b) Jellyfish-like PS nanoparticles by thermally annealing PS nanospheres on the open ends of AAO nanopores at 130 °C for 25 min.

In the past, anisotropic particles have been proven to have special properties distinct from isotropic particles. Anti-coffee-ring effect is one of the most common phenomenon. Normally, when a droplet of suspension of isotropic particles, such coffee powder, is dried on a substrate, particles deposit on the edge of the dried droplet domain and form coffee-ring. On the contrary, uniform particle layers can be obtained when a suspension of anisotropic particles is used.16 The formation of the coffee-ring-effect is suppressed because of the special dynamic behaviors of anisotropic particles in the suspension droplet during the drying process. Here, we also conduct anti-coffee-ring experiments to demonstrate the special dynamic behaviors of mushroom-like PS nanoparticles in aqueous solution. Figure S1a shows the OM image of a dried droplet domain of PS nanospheres. Seriously-stacked particles rim is observed on the edge of the dried droplet domain. Some particles remaining inside the dried droplet domain may be caused by the content


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

of the surfactants or the stronger interactions between those small nanospheres. In contrast, more agglomerations of nanoparticles are observed inside the dried droplet domain when the suspensions of mushroom-like PS nanoparticles are used (Figure S1b). These results show the different dynamic behaviors between nanospheres and mushroom-like PS nanoparticles in aqueous solutions. To demonstrate another practical application of this method, the processes of this method are also adjusted to prepare mushroom-like Janus nanoparticles. Janus particles, named after a Roman ancient god, are composite particles composed of two distinct materials. Because of their multi-functional surfaces, Janus particles have been applied to many fields, such as selfassembly, sensor, and drug delivery.44-49 Here, we provide a simple route for preparing mushroom-like Janus nanoparticles, as shown in Figure 7. In this route, pre-treatment is applied to create asymmetric surface compositions on PS nanospheres (Figure 7a). After depositing PS nanospheres onto the AAO templates, the samples are sputtered with platinum (Pt). Because the plasma sputter provides an anisotropic coating process, only the top surfaces of the PS nanospheres are coated with Pt layers. The bottom part of the PS nanospheres are sheltered and remain as PS surfaces. After the samples are thermally annealed, the PS melts infiltrate into the AAO nanopores and form mushroom-like Janus nanoparticles, as shown in Figure 7b. Because the spreading behaviors of the PS melts are prevented on the top surfaces of the AAO templates by the Pt caps, mushroom-like Janus nanoparticles with smaller pilei and longer stipes can be obtained, whose shapes are similar to those of the cultivated flammulina velutipes (Figure 7c). The Pt caps also largely prevent the adhesion between the adjacent nanospheres.


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Mushroom-like Janus nanoparticles by pre-treatment of the samples. The nanospheredeposited AAO templates are first sputtered with Pt for 100 s and then annealed at 170 °C for 30 min. (b and c) SEM images of the mushroom-like Janus nanoparticles with lower and higher magnifications.

In addition to pre-treatment, post-treatment is also a possible method for producing mushroomlike Janus nanoparticles (Figure S2). After the mushroom-like PS nanoparticles are formed, the samples can be sputtered with Pt before the removal of the AAO templates. Because the stipes are protected by the AAO nanopores, only the pilei are covered with Pt. The most significant advantage of this approach is that the morphologies of the Janus nanoparticles can be controlled. In the past, the portions between two components in a Janus particle are hard to be modulated.55 Here, the portions between the stipes and the pilei can be controlled by using different annealing conditions, as shown in Figures 5d and g. After the mushroom-like polymer nanoparticles on the AAO templates with different morphologies are fabricated, mushroom-like Janus nanoparticles with controllable morphologies can be obtained by the post-treatment processes.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

CONCLUSIONS We provide a novel method to prepare mushroom-like polymer nanoparticles. After thermally annealing, the polymer nanospheres on the open ends of the nanopores of the AAO templates transform to mushroom-like polymer nanoparticles by the asymmetric wetting behaviors of the polymer melts. By changing the annealing conditions, the morphologies of the mushroom-like polymer nanoparticles can be controlled. In addition to mushroom-like PS nanoparticles, mushroom-like PMMA nanoparticles and jellyfish-like PS nanoparticles can be prepared by this method, demonstrating the universality of this method. Comparing to isotropic polymer nanospheres, these nanostructures have special dynamic behaviors in aqueous solution. Moreover, we prepare mushroom-like Janus nanoparticles by changing the fabrication processes. The morphologies of the composite nanostructures can be controlled. In the future, we will apply the mushroom-like Janus nanoparticles to fields such as sensors, drug delivery, and selfassembly. EXPERIMENTAL SECTION Materials Poly(methyl methacrylate) (PMMA) nanospheres with an average diameter of ~300 nm, polystyrene (PS) nanospheres with average diameters of ~350 nm and ~500 nm were purchased from Polysciences. Ethanol (EtOH) and acetone were obtained from Echo Chemical. Phosphoric acid was obtained from Showa. Anodic aluminum oxide (AAO) templates (Anodisc 13) with an average pore diameter of ~0.2 µm were purchased from Whatman. Polycarbonate membrane


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


filters with an average pore diameter of ~0.1 µm were purchased Merck Millipore. Deionized water was produced by Milli-Q Millipore.

Deposition of Polymer Nanospheres on AAO Templates First, the AAO templates were cleaned with acetone and dried by heating. Then, the AAO templates were fixed on glass substrates. Few suspension droplets of polymer nanospheres diluted by ethanol (1:60 in volume) were dropped onto the AAO templates. The samples were spin-coated with 1000 rpm for 30 s. Subsequently, the samples were dried slowly under ambient condition. Finally, the samples were sucked by a vacuum pump to enhance the contacts between polymer nanospheres and AAO templates. The AAO templates can be selectively removed by 15 wt % phosphoric acid in aqueous solution. After filtrating by polycarbonate membranes, free polymer nanospheres can be obtained for SEM measurement.

Fabrication of Mushroom-Like Anisotropic Polymer Nanoparticles After the polymer nanospheres were deposited on AAO templates, the samples were thermally annealed by an oven at 125-140 °C for 15-65 min. Then, the samples were cooled at ambient condition, and the AAO templates were selectively removed by 15 wt % phosphoric acid in aqueous solution. Free nanoparticles for SEM measurement can be obtained by filtrating the samples with polycarbonate membranes.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Structure Analysis and Characterization The surface morphologies of nanoparticles were observed by a scanning electron microscope (SEM) (JEOL, JSM-7401F) with an accelerating voltage of 5 kV. All samples were dried and sputtered with 4 nm of platinum before the SEM measurements. To examine the morphologies further, a transmission electron microscope (TEM) (JEOL, JEM-2100) with an accelerating voltage of 200 kV was also used. For observing the coffee-ring effect, an optical microscope (OM) (ZEISS, Axiophot) was used.


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. OM images of the dried aqueous suspension droplets on clean wafers.

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +886-35731631 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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

REFERENCES (1)

Contado, C. Nanomaterials in Consumer Products: A Challenging Analytical Problem.

Front. Chem. 2015, 3, 20. (2)

Papakostas, D.; Rancan, F.; Sterry, W.; Blume-Peytavi, U.; Vogt, A. Nanoparticles in

Dermatology. Arch. Dermatol. Res. 2011, 303, 533-550.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

Page 22 of 28

Martins, I. M.; Barreiro, M. F.; Coelho, M.; Rodrigues, A. E. Microencapsulation of

Essential Oils with Biodegradable Polymeric Carriers for Cosmetic Applications. Chem. Eng. J. 2014, 245, 191-200. (4)

McDonald, C. J.; Devon, M. J. Hollow Latex Particles: Synthesis and Applications. Adv.

Colloid Interface Sci. 2002, 99, 181-213. (5)

Allen, N. S.; Edge, M.; Ortega, A.; Sandoval, G.; Liauw, C. M.; Verran, J.; Stratton, J.;

McIntyre, R. B. Degradation and Stabilisation of Polymers and Coatings: Nano versus Pigmentary Titania Particles. Polym. Degrad. Stab. 2004, 85, 927-946. (6)

De Jong, W. H.; Borm, P. J. A. Drug Delivery and Nanoparticles: Applications and

Hazards. Int. J. Nanomed. 2008, 3, 133-149. (7)

Marambio-Jones, C.; Hoek, E. M. V. A Review of the Antibacterial Effects of Silver

Nanomaterials and Potential Implications for Human Health and the Environment. J. Nanopart. Res. 2010, 12, 1531-1551. (8)

Thorley, A. J.; Tetley, T. D. New Perspectives in Nanomedicine. Pharmacol. Ther. 2013,

140, 176-185. (9)

Aiken, J. D.; Finke, R. G. A Review of Modern Transition-metal Nanoclusters: Their

Synthesis, Characterization, and Applications in Catalysis. J. Mol. Catal. A: Chem. 1999, 145, 144. (10)

Brigger, I.; Dubernet, C.; Couvreur, P. Nanoparticles in Cancer Therapy and Diagnosis.

Adv. Drug Del. Rev. 2002, 54, 631-651. (11)

Kawaguchi, H. Functional Polymer Microspheres. Prog. Polym. Sci. 2000, 25, 1171-

1210.


22

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12)

Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Monodispersed Colloidal Spheres: Old

Materials with New Applications. Adv. Mater. 2000, 12, 693-713. (13)

Freiberg, S.; Zhu, X. Polymer Microspheres for Controlled Drug Release. Int. J. Pharm.

2004, 282, 1-18. (14)

Yang, S. M.; Kim, S. H.; Lim, J. M.; Yi, G. R. Synthesis and Assembly of Structured

Colloidal Particles. J. Mater. Chem. 2008, 18, 2177-2190. (15)

Lee, K. J.; Yoon, J.; Lahann, J. Recent Advances with Anisotropic Particles. Curr. Opin.

Colloid Interface Sci. 2011, 16, 195-202. (16)

Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the Coffee-Ring Effect

by Shape-Dependent Capillary Interactions. Nature 2011, 476, 308-311. (17)

Zhu, X. X.; Pack, D. W.; Braatz, R. D. Modelling Intravascular Delivery from Drug-

Eluting Stents with Biodurable Coating: Investigation of Anisotropic Vascular Drug Diffusivity and Arterial Drug Distribution. Comput. Methods Biomech. Biomed. Eng. 2014, 17, 187-198. (18)

Gorb, S.; Varenberg, M.; Peressadko, A.; Tuma, J. Biomimetic Mushroom-Shaped

Fibrillar Adhesive Microstructure. J. R. Soc. Interface 2007, 4, 271-275. (19)

Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust Omniphobic

Surfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 18200-18205. (20)

Shiigi, H.; Morita, R.; Yamamoto, Y.; Tokonami, S.; Nakao, H.; Nagaoka, T. Self-

organization of an Organic-Inorganic Hybrid Nanomushroom by a Simple Synthetic Route at the Organic/Water Interface. Chem. Commun. 2009, 3615-3617. (21)

Hsieh, H. Y.; Huang, T. W.; Xiao, J. L.; Yang, C. S.; Chang, C. C.; Chu, C. C.; Lo, L.

W.; Wang, S. H.; Wang, P. C.; Chieng, C. C., et al. Fabrication and Modification of Dual-Faced


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Nano-Mushrooms for Tri-Functional Cell Theranostics: SERS/Fluorescence Signaling, Protein Targeting, and Drug Delivery. J. Mater. Chem. 2012, 22, 20918-20928. (22)

Kumar, R.; Kumar, G.; Umar, A. ZnO Nano-Mushrooms for Photocatalytic Degradation

of Methyl Orange. Mater. Lett. 2013, 97, 100-103. (23)

Yamagami, T.; Kitayama, Y.; Okubo, M. Preparation of Stimuli-Responsive "Mushroom-

like" Janus Polymer Particles as Particulate Surfactant by Site-Selective Surface-Initiated AGET ATRP in Aqueous Dispersed Systems. Langmuir 2014, 30, 7823-7832. (24)

Shen, J. L.; Su, J.; Yan, J.; Zhao, B.; Wang, D. F.; Wang, S. Y.; Li, K.; Liu, M. M.; He,

Y.; Mathur, S., et al. Bimetallic Nano-Mushrooms with DNA-Mediated Interior Nanogaps for High-Efficiency SERS Signal Amplification. Nano Res. 2015, 8, 731-742. (25)

Li, W. B.; Jiang, X. Q.; Xue, J. C.; Zhou, Z. K.; Zhou, J. H. Antibody Modified Gold

Nano-Mushroom Arrays for Rapid Detection of Alpha-Fetoprotein. Biosens. Bioelectron. 2015, 68, 468-474. (26)

Masuda, H.; Fukuda, K. Ordered Metal Nanohole Arrays Made by A 2-Step Replication

of Honeycomb Structures of Anodic Alumina. Science 1995, 268, 1466-1468. (27)

Poinern, G. E. J.; Ali, N.; Fawcett, D. Progress in Nano-Engineered Anodic Aluminum

Oxide Membrane Development. Materials 2011, 4, 487-526. (28)

Martin, J.; Maiz, J.; Sacristan, J.; Mijangos, C. Tailored Polymer-Based Nanorods and

Nanotubes by "Template Synthesis": From Preparation to Applications. Polymer 2012, 53, 11491166. (29)

Jani, A. M. M.; Losic, D.; Voelcker, N. H. Nanoporous Anodic Aluminium Oxide:

Advances in Surface Engineering and Emerging Applications. Prog. Mater. Sci. 2013, 58, 636704.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30)

Lee, W.; Park, S. J. Porous Anodic Aluminum Oxide: Anodization and Templated

Synthesis of Functional Nanostructures. Chem. Rev. 2014, 114, 7487-7556. (31)

Mijangos, C.; Hernandez, R.; Martin, J. A Review on the Progress of Polymer

Nanostructures with Modulated Morphologies and Properties, Using Nanoporous AAO Templates. Prog. Polym. Sci. 2016, 54-55, 148-182. (32)

Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Highly

Ordered Nanochannel-Array Architecture in Anodic Alumina. Appl. Phys. Lett. 1997, 71, 27702772. (33)

Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. Hexagonal Pore Arrays with a

50-420 nm Interpore Distance Formed by Self-Organization in Anodic Alumina. J. Appl. Phys. 1998, 84, 6023-6026. (34)

Martin, C. R. Nanomaterials: A Membrane-Based Synthetic Approach. Science 1994,

266, 1961-1966. (35)

Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.;

Choi, J.; Gosele, U. Polymer Nanotubes by Wetting of Ordered Porous Templates. Science 2002, 296, 1997-1997. (36)

Zhang, M. F.; Dobriyal, P.; Chen, J. T.; Russell, T. P.; Olmo, J.; Merry, A. Wetting

Transition in Cylindrical Alumina Nanopores with Polymer Melts. Nano Lett. 2006, 6, 10751079. (37)

Chen, J. T.; Lee, C. W.; Chi, M. H.; Yao, I. C. Solvent-Annealing-iInduced Nanowetting

in Templates: Towards Tailored Polymer Nanostructures. Macromol. Rapid Commun. 2013, 34, 348-354.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38)

Page 26 of 28

Mei, S. L.; Feng, X. D.; Jin, Z. X. Polymer Nanofibers by Controllable Infiltration of

Vapour Swollen Polymers into Cylindrical Nanopores. Soft Matter 2013, 9, 945-951. (39)

Chang, C. W.; Chi, M. H.; Chu, C. W.; Ko, H. W.; Tu, Y. H.; Tsai, C. C.; Chen, J. T.

Microwave-Annealing-Induced Nanowetting: A Rapid and Facile Method for Fabrication of One-Dimensional Polymer Nanomaterials. RSC Adv. 2015, 5, 27443-27448. (40)

Lee, C. W.; Wei, T. H.; Chang, C. W.; Chen, J. T. Effect of Nonsolvent on the Formation

of Polymer Nanomaterials in the Nanopores of Anodic Aluminum Oxide Templates. Macromol. Rapid Commun. 2012, 33, 1381-1387. (41)

Ko, H. W.; Chi, M. H.; Chang, C. W.; Su, C. H.; Wei, T. H.; Tsai, C. C.; Peng, C. H.;

Chen, J. T. Fabrication of Multicomponent Polymer Nanostructures Containing PMMA Shells and Encapsulated PS Nanospheres in the Nanopores of Anodic Aluminum Oxide Templates. Macromol. Rapid Commun. 2015, 36, 439-446. (42)

Chi, M. H.; Kao, Y. H.; Wei, T. H.; Lee, C. W.; Chen, J. T. Curved Polymer Nanodiscs

by Wetting Nanopores of Anodic Aluminum Oxide Templates with Polymer Nanospheres. Nanoscale 2014, 6, 1340-1346. (43)

Chi, M. H.; Chang, C. W.; Ko, H. W.; Su, C. H.; Lee, C. W.; Peng, C. H.; Chen, J. T.

Solvent-Induced Dewetting on Curved Substrates: Fabrication of Porous Polymer Nanotubes by Anodic Aluminum Oxide Templates. Macromolecules 2015, 48, 6241-6250. (44)

Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E. B.; Duguet, E. Design and

Synthesis of Janus Micro- and Nanoparticles. J. Mater. Chem. 2005, 15, 3745-3760. (45)

Hurst, S. J.; Payne, E. K.; Qin, L. D.; Mirkin, C. A. Multisegmented One-Dimensional

Nanorods Prepared by Hard-Template Synthetic Methods. Angew. Chem., Int. Ed. 2006, 45, 2672-2692.


26

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46)

Walther, A.; Muller, A. H. E. Janus Particles. Soft Matter 2008, 4, 663-668.

(47)

Hu, J.; Zhou, S. X.; Sun, Y. Y.; Fang, X. S.; Wu, L. M. Fabrication, Properties and

Applications of Janus Particles. Chem. Soc. Rev. 2012, 41, 4356-4378. (48)

Walther, A.; Muller, A. H. E. Janus Particles: Synthesis, Self-Assembly, Physical

Properties, and Applications. Chem. Rev. 2013, 113, 5194-5261. (49)

Yi, Y.; Sanchez, L.; Gao, Y.; Yu, Y. Janus Particles for Biological Imaging and Sensing.

Analyst 2016, 141, 3526-3539. (50)

McHale, J. M.; Navrotsky, A.; Perrotta, A. J. Effects of Increased Surface Area and

Chemisorbed H2O on the Relative Stability of Nanocrystalline γ-Al2O3 and α-Al2O3. J. Phys. Chem. B 1997, 101, 603-613. (51)

Chen, G.; Soper, S. A.; McCarley, R. L. Free-standing, Erect Ultrahigh-Aspect-Ratio

Polymer Nanopillar and Nanotube Ensembles. Langmuir 2007, 23, 11777-11781. (52)

Kobayashi, H.; Takahashi, H.; Hiki, Y. Temperature Dependence of the Viscosity

through the Glass Transition in Metaphosphate Glasses and Polystyrene. Mater. Sci. Eng., A 2006, 442, 263-267. (53)

Schiessel, H.; Metzler, R.; Blumen, A.; Nonnenmacher, T. F. Generalized Viscoelastic

Models: Their Fractional Equations with Solutions. J. Phys. A: Math. Gen. 1995, 28, 6567-6584. (54)

Chen, J. T.; Chen, D.; Russell, T. P. Fabrication of Hierarchical Structures by Wetting

Porous Templates with Polymer Microspheres. Langmuir 2009, 25, 4331-4335. (55)

Deng, R. H.; Li, H.; Zhu, J. T.; Li, B. H.; Liang, F. X.; Jia, F.; Qu, X. Z.; Yang, Z. Z.

Janus Nanoparticles of Block Copolymers by Emulsion Solvent Evaporation Induced Assembly. Macromolecules 2016, 49, 1362-1368.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

TOC Graphic


28

Setting Foot in Asymmetric Wetting Environments ... - ACS Publications

Recommend Documents