New Double-Infiltration Methodology to Prepare PCLâPS CoreâShell

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New double–infiltration methodology to prepare PCL-PS core shell nanocylinders inside anodic aluminum oxide templates Belen Sanz, Iwona Blaszczyk-Lezak, Carmen Mijangos, Jordana K. Palacios, and Alejandro J. Müller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01258 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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New double–infiltration methodology to prepare PCL-PS core shell nanocylinders inside anodic aluminum oxide templates

Belén Sanzª, Iwona Blaszczyk-Lezakª, Carmen Mijangos*a,b, Jordana K. Palaciosc, Alejandro J. Müller*c,d

a

b

Instituto de Ciencia y Tecnología de Polímeros, Consejo Superior de Investigaciones Científicas (CSIC), Juan de la Cierva 3, Madrid 28006, Spain.

Donostia International Physics Center (DIPC) and Centro de Física de Materiales (CFM), CSIC-UPV, 20018 Donostia-San Sebastián, Spain. c

POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 Donostia-San Sebastián, Spain. d

IKERBASQUE, Basque Foundation for Science, Bilbao, Spain.

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ABSTRACT

Melt nano-molding of core-shell nanocylinders of different sizes, employing anodic aluminum oxide (AAO) templates, is reported here for the first time. The core-shell nanostructures are achieved by a new melt double-infiltration technique. During the first infiltration step, polystyrene (PS) nanotubes are produced by an adequate choice of AAO nanopore diameter size. In the second step, PCL is infiltrated inside the PS nanotubes, as its melting point (and infiltration temperature) is lower than the glass transition temperature of PS. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) measurements verified the complete double–infiltration of the polymers. Differential scanning calorimetry (DSC) experiments show that the infiltrated PCL undergoes a confined fractionated crystallization with two crystallization steps located at temperatures that depend on which surface is in contact with the PCL nanocylinders (i.e., alumina or PS). The melt double–infiltration methodology represents a novel approach to study the effect of the surrounding surface on polymer crystallization under confinement.


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INTRODUCTION

The emphasis of new technologies at nanoscale level demands a comprehensive understanding of how the physical properties of polymeric materials may change under such small environment.1-2 For that reason, there has been a growing interest in the morphology and crystallization behaviour of polymers under confinement, see for example a recent review on this subject and references therein.1 Confined crystallization is observed in any system where the number of isolated microdomains of a crystallizable polymer is several orders of magnitude larger than the typical number of active heterogeneities in the bulk polymer. Thus, confinement has been achieved in the past through several strategies that include droplets, polymers blends, segregated block copolymers, thin films, and inorganic templates. Among them, porous anodic aluminium oxide (AAO) templates have emerged as a very effective method to produce polymeric nanostructures by infiltration techniques that allow the study of confinement effects in the infiltrated polymeric chains. The AAO templates consist of a dense array of cylindrical nanopores arranged in wellordered domains. Masuda and co-workers first developed the so-called two step anodization process to achieve well-ordered porous AAO templates.3-4 By adjusting the synthesis parameters, the porous size and length can be modulated, with a very welldefined geometry.

2

Infiltration techniques of polymers inside AAO templates have been

well developed and nowadays, it is possible to infiltrate almost any polymer. Infiltration of different polymer or copolymer melts, solution or “in-situ” polymerization of monomers inside AAO templates can lead to polymer nanotubes, nanofibers, nanospheres, nanocapsules and more complex nanostructures, such as 3D architectures.2 3 ACS Paragon Plus Environment

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Crystallization inside AAO microdomains occurs at much larger supercoolings as compared to bulk samples. These higher supercoolings, which bring the crystallization temperature closer to the glass transition temperature, might be caused by changes in nucleation mechanisms. Also, first order crystallization kinetics have been observed, which means that the nucleation is the dominant step of the confined overall crystallization process. Thus, depending on the supercooling observed, the nucleation might go from heterogeneous, to surface induced or to homogeneous.2-3 The aforementioned phenomena have been explored in a wide range of semicrystalline polymers and copolymers5-14. The reduction in the crystallization temperature obeys to a variation in the character of the nucleation mechanism that can change from a heterogeneous to a homogeneous (or surface) nucleation. In addition, the overall crystallization kinetics of polymers is affected by the confinement inside AAO templates. In general, as the size of the nanoporous decreases, the overall crystallization rate is slower. Besides crystallization kinetics, other features related to polymer structure, such as crystal orientation and phase transitions inside the AAO template nanopores or in nanostructures molded using these templates, have also been in the focus of recent research.6-7, 12, 15-16 Since in many polymers the nucleation mechanisms change from heterogeneous to surface or homogeneous nucleation, it is interesting to study the effect of interphases or surfaces over the crystallization behavior. For instance, Casas et al.17 recently reported that PE-b-PS block copolymer formed a pseudo core-shell structure within the AAO template nanopores, in which the core correspond to PS and shell to PE. Thus, a double confinement effect is applied over the PE block, and the crystallization is affected by two different surfaces: the PS core and the AAO walls. In this case, surface nucleation is the preferential mechanism 4 ACS Paragon Plus Environment

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for PE nucleation.17 Besides this work, not other reports (for the specific case of crystallizable polymers) have been published with this type of morphology. Thus, the aim of the research was to develop a novel double–infiltration methodology in order to combine two different polymers in a nanocylinder core-shell like structure. In this way, the effect of the nature of the surrounding shell on the crystallization of the core (or viceversa) could be studied. Recently, a double and triple-solution wetting method inside AAO template nanopores has been reported by Chen et al.18 and Ko et al.19-20, respectively. In both cases, PS and poly(methyl methacrylate) (PMMA) were employed to obtain very interesting new morphologies like nanopeapods, and PMMA shells and encapsulated PS nanospheres (i.e., core-shell nanospheres). However, the authors did not obtain core-shell cylinders, since their final target morphologies after solvent evaporation were always nano-sphere based. Additionally, both materials employed for infiltration were amorphous and the authors did not study the confinement effects on polymer properties. Herein, we report for the first time the successful nanomoulding of core-shell cylindrical nanostructures from the melt, composed of a PS nanotube (shell) and a PCL core, employing AAO templates with different length and pore diameters. The originality of this work resides in the fabrication of novel core-shell structures from the melt, with a solution free method (greener and easy to implement). Furthermore, the objective of the work is to modify the surface of the alumina nanopores with a polymer coating (i.e., PS shell) to later study the crystallization of the semi-crystalline PCL core under confinement. In fact, our method provides an easier and more versatile way to study the effect of the surface on


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confined polymer crystallization in comparison with other possibilities such as chemical functionalization of AAO pores.

EXPERIMENTAL Materials Aluminium foils of 99.999 % were purchased from Goodfellow Cambridge Ltd. (United Kingdom), polycaprolactone (PCL) (Mw = 43 000 g mol-1, PDI = 1.38) (Capa 6430) was provided by Perstorp (Sweden) and polystyrene (PS) (Mw = 192 000 g mol-1, PDI=2.04) was supplied by Sigma-Aldrich (Spain). Template synthesis Four AAO templates of 350, 250, 140 and 60 nm pore diameter were synthesized. The AAO templates were prepared by a two-step electrochemical anodization process of aluminum, following the procedure reported previously by Masuda et al.3,4 Details about the synthesis process can be found in the Supporting Information. Single and double-infiltration procedure from the melt state Two PCL-PS core-shell structures of different pore diameter size were prepared. To that purpose, AAO templates of 350 and 140 nm pore diameter were employed. In order to achieve this particular nanostructure a novel double–infiltration approach from the melt was designed. First, a PS nanotube inside the AAO template was generated. In order to achieve this, a PS film of 0.8 mm thickness was prepared by casting, employing a 5% PS solution in 6 ACS Paragon Plus Environment

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tetrahydrofuran. The first melt infiltration step was done by placing the PS film on top of the AAO templates and then heating it in an oven under nitrogen atmosphere at 200 ºC for 1 h. The excess of bulk PS on the surface of the templates was removed using a sharp blade and acetone. A second infiltration step to form the PCL cores inside the PS nanotubes in both templates was performed. With that purpose, a PCL film of 0.3 mm thickness was first prepared by compression molding. Then, the infiltration procedure was similar to the one used for PS but with different conditions. The PCL film was placed on top of the previously infiltrated templates with PS nanotubes, and next, the PCL infiltration was carried out at 80 ºC and for 6 h, under vacuum. The infiltration was aided by application of mechanical force. The excess of bulk PCL on the surface of the template was removed using a sharp blade and acetone. Additionally, and for comparison purposes, a single infiltration of a PCL film inside AAO templates of 250 and 60 nm pore diameter was carried out following the aforementioned procedure. The infiltration was carried at 80 ºC for 2 h, under vacuum. Thus, PCL nanofibers with diameters of approximately ~250 and ~60 nm were produced inside AAO templates. Methods The morphology of AAO templates and infiltrated samples was observed by scanning electron microscopy (SEM) employing a Philips XL-30ESEM and FESEM Hitachi model SU8000 microscope. In order to perform the analysis over the polymer samples without the AAO template, first, the aluminum substrate was eliminated by treatment with a mixture of 7 ACS Paragon Plus Environment

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HCl, CuCl2 and H2O and then, the alumina was dissolved in 10% H3PO4 solution. Finally, the samples were coated with gold. To verify the infiltration of the polymers, the infiltrated AAO templates were examined by Fourier transform infrared spectroscopy (FTIR) employing a Perkin Elmer Spectrum One coupled with an attenuated total reflectance (ATR) accessory. To perform differential scanning calorimetry (DSC) measurements, the infiltrated templates were encapsulated in aluminum pans and tested in a previously calibrated DSC Perkin Elmer 8500 under ultra-high purity nitrogen atmosphere. A bulk PCL sample was also examined. The instrument was previously calibrated with an indium standard. All the samples were cool down until -80 ºC at 20 ºC min-1 after 3 min melting at 85 ºC. Then, they were heated up until 85 ºC at the same rate.

RESULTS AND DISCUSSION Despite alumina membranes are commercially available, we decided to prepare our own templates in the laboratory following the procedure of Masuda et al,3,4 since the commercial templates tend to have imperfections and sometimes their porous size and structure is not regular. Templates for crystallization studies under confinement must be consistent and defect free, both in nanodomains dimensions and distribution. The SEM micrographs depicted in Figure 1 confirm the dimensions of the synthesized AAO templates. The pore diameters of the templates correspond, almost perfectly, to the expected dimensions. After extensive measurement, the pore diameters of the AAO 8 ACS Paragon Plus Environment

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templates in Figures 1a and b were 253±15 nm and 348±14 nm, and in Figures 1d and e, were 145 ± 15 nm and 62 ± 4 nm, respectively. The lateral views of the templates of 80 and 25 µm lengths can be seen in Figure 1c and f. From the images, it is clear that the AAO templates exhibited high quality order pores, regularity in the size of the pores and invariance of the diameter through the length of template.

Figure 1. SEM micrographs of AAO templates with different pore diameters (nm) and lengths (µm): a) 250 nm / 80 µm (top view), b) 350 nm / 80 µm (top view), c) 250 nm / 80 µm (lateral view), d) 140 nm / 25 µm (top view), e) 60 nm / 25 µm (top view), f) 140 nm / 25 µm (lateral view).

From the templates shown above, those with 350 and 140 nm pore diameter were employed for the double–infiltration procedure, whereas the 250 and 60 nm pore diameter AAO templates were used for the single–infiltration of PCL. The effectiveness of the infiltration techniques was verified by FTIR. Figure 2 depicts the FTIR spectra of the single infiltrated PS in a 350 nm pore diameter template, the single infiltrated PCL in a 250 nm pore diameter template, and of the double infiltrated PS and PCL inside a 350 nm pore diameter template. 9 ACS Paragon Plus Environment

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1720

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Figure 2. FTIR-ATR spectra of PS nanotube infiltrated in a 250 nm pore diameter template, PCL nanofiber infiltrated in a 250 nm pore diameter template and PCL+PS double infiltrated nanostructure inside a 350 nm pore diameter template.

The first spectrum exhibits signals at 3025 cm-1 (C-H stretch, aromatic), 1601cm-1 (C=C stretches in the aromatic ring), 1451 cm-1 (CH2 symmetric stretch) and 694 cm-1 (C-H bending) that correspond to PS vibrations.21 In the second spectrum the distinctive signals belonging to PCL can be seen, located at 1720 cm-1 (C=O stretch), 1238 cm-1 (C-H bending) and 1167 cm-1 (C-O stretch).22-23 In the third spectrum the bands of both PS and PCL appear, thus confirming the infiltration of PCL inside the PS nanotubes. In order to provide undeniable evidence of the nanostructures formed inside of the AAO templates, exhaustive SEM observations were carried out, as well as TEM. Figure 3 illustrates that PS nanotubes and PCL-PS core-shell nanocylinders were successfully achieved with the single and double–infiltration approach, respectively.


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Figure 3. SEM micrographs of a) PS nanotubes after infiltration inside AAO templates with nanopores of 350 nm diameter, b) PCL-PS core-shell nanostructure of 350 nm diameter (PCL core 220 nm diameter), c) PCL-PS core-shell nanostructure of 350 nm diameter (PCL core 220 nm diameter) inside the AAO template (lateral view), d) Colored figure of c) to guide the eye, blue for PCL core and orange for PS shell, e) PS nanotubes after infiltration inside AAO templates with nanopores of 140 nm diameter and f) PCL-PS core-shell nanostructure of 140 nm diameter (PCL core 60 nm diameter).


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Figure 3a shows the PS nanotubes obtained employing an AAO template with 350 nm pore diameter and Figure 3b and c, the PCL-PS core-shell nanocylinders achieved after the subsequent infiltration of the PCL. As can be seen from Figure 3a, the thickness of the walls of PS nanotubes was around 70 nm. Thus, the internal diameter (cavity) of PS nanotube filled with PCL was ~220 nm. A proof of the successful double–infiltration and of the PCL-PS core-shell nanostructures formed is given in Figure 3b and c. The nanostructures shown in Figure 3b were obtained and observed after dissolving the AAO template with NaOH. One PS nanotube (circled in orange) that was not filled with PCL can be clearly seen while all others exhibit fully infiltrated PCL cores. To confirm the core-shell nanostructure, lateral view observations were made after a cryogenic fracture of the AAO template (see Figure 3c). The SEM micrograph shows a PCL nanofiber (smaller diameter) covered by a PS shell (larger diameter), coming out of a nanopore inside the AAO template (in Figure 3d, the PS shell and the PCL core were colored in orange and blue, respectively, to guide the eye). Since the PS is fragile, the PS shell was broken, confirming that a complete PCL nanofiber constitutes the core of the nanostructure. Additionally, Figure 3e and f demonstrate that PS nanotubes and PCL-PS core-shell nanocylinders of smaller dimensions can also be successfully obtained. In this case, the PS nanotube wall was approximately 40 nm and the PCL core ~60 nm. The reduction of the PS wall thicknesses in the smaller AAO templates might obey to a stronger influence of the surface tension over the PS as the size of the pore is reduced. A TEM micrograph of this sample can be observed in Figure 4. The darker core constitutes the PCL nanofiber inside


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the PS nanotube (i.e., light gray shell). Furthermore, the dimensions of the core shell cylinders correspond exactly with the expectation for complete double infiltration.

Figure 4. TEM micrograph of PCL-PS core-shell nanostructure of 140 nm diameter (PCL core 60 nm diameter).

As demonstrated in the preceding figures, PS can be successfully infiltrated into AAO template nanopores to create PS nanotubes. These nanotubes can be filled with PCL in a second infiltration step. The result of this double – infiltration technique is a core-shell nanostucture: a nanotube of PS filled with a PCL nanofiber. In other words, PCL nanofibers are surrounded by a PS surface and not by the AAO template surface. Therefore, the nature of the surface surrounding the PCL nanofibers and its effect on the nucleation and crystallization can be potentially study. The ability to use this nanomoulding technique to prepare core-shell nanofibers opens up new possibilities to study confined nucleation and crystallization phenomena. The technique is easy to implement as long as the infiltration temperatures of the core material 13 ACS Paragon Plus Environment

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are lower than the melting point or glass transition temperature (or more generally speaking the heat distortion temperature) of the shell material. Therefore, the major interest with this technique is to achieve very small nanodomains in which the PCL could crystallize. It is reported here that this was indeed possible in core-shell nanocylinders of very small dimensions. Some preliminary exploratory DSC experiments were performed to the PCL-PS core-shell nanostructures depicted in Figure 3f, which is the one with the smallest dimensions (PS nanotube, ~140 nm diameter, PCL core, ~60 nm diameter). As it was mentioned earlier, an AAO template with ~60 nm pore diameter was infiltrated with PCL for comparison purposes. Both samples were tested inside their AAO templates. Figure 5 presents the cooling and second heating scan of the three samples.

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-80 -60 -40 -20 0 20 40 60 80 Temperature (°C)


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-80 -60 -40 -20 0 20 40 60 80 Temperature (°C)

Figure 5. a) DSC cooling scans and b) subsequent heating scans at 20 ºC min-1.

The crystallization and melting temperatures of the bulk sample are typical for a high molecular weight PCL sample, 19.4 and 56.4 ºC, respectively. Confinement is expected to reduce the crystallization temperature of infiltrated polymers. Nevertheless, it can be seen that the PCL infiltrated inside the AAO template (PCL-60) exhibits two crystallization peaks. The main one, located at 31.9 ºC and the other more weak, at -17.1 ºC. If the PCL is now confined inside a PS nanotube of similar dimensions within the AAO template (PCLPS-60 core-shell nanostructure), the low crystallization temperature peak was reduced to even lower temperatures, until -25.7 ºC while the other remained almost unchanged, at 30.3 ºC (PCL-PS-60). Similar fractioned crystallization behavior was observed for the PCL inside the core-shell nanocylinders of larger dimensions (PS nanotube, ~350 nm diameter, PCL core, ~220 nm diameter), results not shown (a high temperature crystallization was observed at 31.2 ºC and a low one at -18.2 ºC). 15 ACS Paragon Plus Environment

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The higher crystallization peaks exhibit an increase of approximately 10 ºC when compared to bulk PCL. This unexpected behavior was observed, regardless of the type of surface: AAO or PS, and requires further research in order to understand it. A similar observation was reported by some authors11 for infiltrated PCL inside AAO templates with nanopore diameters between 25 and 65 nm. However, the increment reported was only 2 ºC. The low temperature small peaks present a dramatic reduction of the crystallization temperature (36.5 and 45.1 ºC, respectively) that accounts for a confinement effect over the PCL crystallization. These extreme supercoolings are probably due to a change in the nucleation mechanism from heterogeneous to homogeneous (or surface induced nucleation). The higher crystallization peaks (31.9 and 30.3 ºC) correspond to the crystallization starting from heterogeneous nuclei. But, as the volume per active heterogeneous nuclei estimation of bulk PCL is 3 to 5 orders of magnitude higher than the volume per pore, only a fraction of nanopores will contain heterogeneous nuclei1, 11 and the rest can be considered as heterogeneity free nanodomains, in which the crystallization takes place from homogeneous nuclei (or from surface induced nucleation). Therefore, the lower crystallization temperature peaks (-17.1 and -25.7 ºC) are due to confined crystallization of clean microdomains (without heterogeneity). Suzuki etal.1,11 reported a homogeneous crystallization temperature at around -20 ºC for PCL infiltrated in AAO templates of 65 nm pore diameter. The increase in the supercooling is higher for PCL in the core-shell nanocylinders as compared to the single infiltrated PCL, even though both PCL core and PCL nanofibers have similar dimensions. The nature of the surface (alumina versus PS) in contact with PCL may be the origin of this difference. This observation requires deeper examination to 16 ACS Paragon Plus Environment

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ascertain the exact origin of the heterogeneity free nucleation in each case (i.e., surface versus homogeneous nucleation). The in depth study of the nucleation and crystallization of double infiltrated systems will be the subject of future investigations. However, it is important to point out that although the nature of the surface had an effect that needs to be studied further, the PS wall thickness has no influence on the crystallization behavior, since crystallization only depends on the size of the nanodomain. Therefore, for comparison purposes, the infiltration conditions have been carefully controlled in order to obtain PCL core nanodomains of similar dimensions in the PCL nanofibers (single infiltration) and core-shell nanocylinders (double infiltration). Regarding the melting temperatures, the infiltrated PCL in both samples did not exhibit significant changes as compared to bulk PCL.

CONCLUSIONS The melt double–infiltration technique proposed here has successfully produced PCL-PS core-shell nanocylinders, in which the PCL core can crystallize under confined conditions. The proposed methodology is easy to carry out and allows obtaining polymer nanostructures through nano-molding using AAO templates of different pore diameters. SEM and TEM observations clearly show PS nanotubes, and PCL-PS nanocylinders composed of a PCL core surrounded by a PS shell. The double–infiltration was also verified by FTIR measurements. DSC experiments proved that the infiltrated PCL undergoes fractionated crystallization with two crystallization steps, one which can be attributed to the crystallization from heterogeneity containing domains and a second for


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clean or heterogeneity free domains. The low crystallization temperature characteristic of clean PCL nanocylinders depends on the nature of the surface that surrounds them (i.e., alumina or PS). Further investigations will be pursued in order to gain more knowledge on the nucleation and crystallization behavior of polymers inside nanoscale environments with different types of surfaces produced by this novel double–infiltration technique.

AUTHOR INFORMATION Corresponding Authors [email protected], [email protected]

ACKNOWLEDGEMENTS Financial support from the Spanish Ministerio de Economia y Competitividad (MINECO) under projects MAT2014-53437-C2-1P and MAT2014-53437-C2-2P is acknowledged. The authors thank D. Gómez for SEM experiments. REFERENCES (1) Michell, R. M.; Müller, A. J. Confined Crystallization of Polymeric Materials. Progress in Polymer Science, 2016, 54-55:183-213


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(2) Mijangos, C.; Hernández, R.; Martín, J. A Review on the Progress of Polymer Nanostructures with Modulated Morphologies and Properties, Using Nanoporous AAO Templates. Progress in Polymer Science, 2016, 54-55:148-182.

(3) Masuda, H.; Yada, K.; Osaka, A. Self-Ordering of Cell Configuration of Anodic Porous Alumina with Large-Size Pores in Phosphoric Acid Solution. Jpn. J. Appl. Phys. 1998, 37, L1340.

(4) Masuda, H.; Fukuda, K. Ordered Metal Nanohole Arrays Made by a Two-Step Replication of Honeycomb Structures of Anodic Alumina. Science 1995, 268, 1466-1468.

(5) Dai, X.; Niu, J.; Ren, Z.; Sun, X.; Yan, S. Effects of Nanoporous Anodic Alumina Oxide on the Crystallization and Melting Behavior of Poly(Vinylidene Fluoride). J. Phys. Chem. B 2016, 120, 843-850.

(6) Suzuki, Y.; Duran, H.; Steinhart, M.; Butt, H. J.; Floudas, G. Homogeneous Crystallization and Local Dynamics of Poly(Ethylene Oxide) (PEO) Confined to Nanoporous Alumina. Soft Matter 2013, 9, 2621-2628.

(7) Shin, K.; Woo, E.; Jeong, Y. G.; Kim, C.; Huh, J.; Kim, K. W. Crystalline Structures, Melting, and Crystallization of Linear Polyethylene in Cylindrical Nanopores. Macromolecules 2007, 40, 6617-6623.

(8) Woo, E.; Huh, J.; Jeong, Y. G.; Shin, K. From Homogeneous to Heterogeneous Nucleation of Chain Molecules under Nanoscopic Cylindrical Confinement. Phys. Rev. Lett. 2007, 98, 136103/1– 4.

(9) Reid, D. K.; Ehlinger, B. A.; Shao, L.; Lutkenhaus, J. L. Crystallization and Orientation of Isotactic Poly(Propylene) in Cylindrical Nanopores. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1412–1419. 19 ACS Paragon Plus Environment

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(10) Duran, H.; Steinhart, M.; Butt, H. J.; Floudas, G. From Heterogeneous to Homogeneous Nucleation of Isotactic Poly(Propylene) Confined to Nanoporous Alumina. Nano Lett. 2011, 11, 1671-1675.

(11) Suzuki, Y.; Duran, H.; Akram, W.; Steinhart, M.; Floudas, G.; Butt, H. J. Multiple Nucleation Events and Local Dynamics of Poly(ε-Caprolactone) (PCL) Confined to Nanoporous Alumina. Soft Matter 2013, 9, 9189-9198.

(12) Mi, C.; Zhou, J.; Ren, Z.; Li, H.; Sun, X.; Yan, S. The Phase Transition Behavior of Poly(Butylene Adipate) in the Nanoporous Anodic Alumina Oxide. Polym. Chem. 2016, 7, 410 417.

(13) Michell, R. M.; Lorenzo, A. T.; Müller, A. J.; Lin, M. C.; Chen, H. L.; Blaszczyk-Lezak, I.; Martín, J.; Mijangos, C. The Crystallization of Confined Polymers and Block Copolymers Infiltrated within Alumina Nanotube Templates. Macromolecules 2012, 45, 1517-1528.

(14) Suzuki, Y.; Duran, H.; Steinhart, M.; Butt, H. J.; Floudas, G. Suppression of Poly(Ethylene Oxide) Crystallization in Diblock Copolymers of Poly(Ethylene Oxide)-b-Poly(ε-Caprolactone) Confined to Nanoporous Alumina. Macromolecules 2014, 47, 1793-1800.

(15) Guan, Y.; Liu, G.; Gao, P.; Li, L.; Ding, G.; Wang, D. Manipulating Crystal Orientation of Poly(Ethylene Oxide) by Nanopores. ACS Macro Letters 2013, 2, 181-184.

(16) Li, X.; Han, P.; Song, G.; Peng, Z.; Yu, Y.; Feng, S.; Zhou, C.; Li, M. Assembly of Polyamide 6 Nanotube Arrays with Ordered Patterns and the Crystallization Behavior. Mater. Lett. 2015, 141, 157-160.


Page 21 of 22

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

Langmuir

(17) Casas, M. T.; Michell, R. M.; Blaszczyk-Lezak, I.; Mijangos, C.; Puiggalí, J.; Lorenzo, A. T.; Müller, A. J. Self-Assembly of Semicrystalline PE-b-PS Diblock Copolymers within AAO Nanoporous Templates. Polymer 2015, 70, 282-289.

(18) Chen, J.-T.; Wei, T.-H.; Chang, C.-W.; Ko, H.-W.; Chu, C.-W.; Chi, M.-H.; Tsai, C.-C. Fabrication of Polymer Nanopeapods in the Nanopores of Anodic Aluminum Oxide Templates Using a Double-Solution Wetting Method. Macromolecules 2014, 47, 5227-5235.

(19) 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.

(20) Ko, H.-W.; Chi, M.-H.; Chang, C.-W.; Chu, C.-W.; Luo, K.-H.; Chen, J.-T. Fabrication of Core−Shell Polymer Nanospheres in the Nanopores of Anodic Aluminum Oxide Templates Using Polymer Blend Solutions. ACS Macro Letters 2015, 4, 717-720.

(21) Liang, C. Y.; Krimm, S. Infrared Spectra of High Polymers. 6. Polystyrene. J. Polym. Sci. 1958, 27, 241-254.

(22) Bassi, A. K.; Gough, J. E.; Zakikhani, M.; Downes, S. The Chemical and Physical Properties of Poly(ε-Caprolactone) Scaffolds Functionalised with Poly(Vinyl Phosphonic Acid-co-Acrylic Acid). J. Tissue Eng. 2011, 2, 615328.

(23) Elzein, T.; Nasser-Eddine, M.; Delaite, C.; Bistac, S.; Dumas, P. FTIR Study of Polycaprolactone Chain Organization at Interfaces. J. Colloid Interface Sci. 2004, 273, 381-387.


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GRAPHICAL TABLE OF CONTENTS


New Double-Infiltration Methodology to Prepare PCLâPS CoreâShell

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