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Variable Crystal Orientation of Poly(ethylene oxide) Confined within the Tubular Space Templated by Anodic Aluminum Oxide Nanochannels Chien-Liang Liu† and Hsin-Lung Chen*,‡ †

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan Department of Chemical Engineering and Frontier Center of Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan

‡

S Supporting Information *

ABSTRACT: The use of nanoscale templates to confine the crystallization process is an effective approach for creating large-scale crystal orientation. Here we incorporated poly(ethylene oxide) (PEO) into anodic aluminum oxide (AAO) nanochannels by a solution infiltration process to generate PEO nanotubes confined in the channels. The preferred crystal orientation of the PEO crystallites developed in the tubular space was then studied as a function of crystallization temperature (Tc), PEO molecular weight (MPEO), and AAO channel diameter (DAAO = 23 and 89 nm). Two distinct types of crystal orientation, i.e., perpendicular and tilt orientation with the (120) plane aligning along and tilting 45° away from the channel axis, respectively, were identified. Crystallization in the nanotubes templated by the AAO with DAAO of 23 nm led predominantly to perpendicularly oriented crystallites except for the high molecular weight PEO (MPEO = 95 000 g/mol), where a significant fraction of the crystallites showed tilt orientation. In the nanochannels with DAAO = 89 nm, however, perpendicular orientation only dominated at the higher Tc (≥20 °C), as most crystallites developed at the lower Tc adopted tilt orientation with the invariant tilt angle of the (120) plane of 45°. On basis of the observed effects of the three parameters, the preferred crystal orientation attained was proposed to be governed by the strength of confinement to the crystal growth prescribed by the confinement geometry and the nucleation density of crystallization.

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INTRODUCTION

It has been established that, as long as the confinement effect is sufficiently strong, the crystallites developed in microdomains adopt the preferred orientation characterized by the alignment of the crystalline stems (i.e., the crystallographic c-axis) with respect to the microdomain interface. In most cases, the crystallites exhibit only one characteristic type of orientation. For instance, the crystallites formed by poly(L-lactide) (PLLA) blocks in the lamellar microdomains always show perpendicular orientation with the crystalline stems aligning perpendicularly to the lamellar interface irrespective of the counterpart amorphous block and crystallization condition.1−3 On the other hand, polyethylene (PE) blocks tend to form the crystallites with parallel orientation in both lamellar2,4−9 and cylindrical microdomains.7−9 A limited number of bcp systems have been found to display variable crystal orientation in the microdomain, where the orientation was tunable by the confinement size or crystallization temperature (Tc). Sun et al. have studied the crystal orientation of poly(ε-caprolactone) (PCL) in the lamellar

Attaining and controlling the large-scale orientation of polymer crystallites has received significant interests for the directional control of polymer properties. Among the various methods having been reported, confining the crystallization process within the long-range arrays of pores or microdomains comparable to (or not much larger than) the chain dimension is an effective strategy to produce large-area crystal orientation. In this case, the boundaries of the confining space may exert strong frustration to the crystal growth, such that the growth is directed toward the direction with minimum restriction, which eventually leads to a preferred orientation of the crystallites in the individual pore or domain. A number of nanoscale templates with well-defined geometries have been exploited to create the confined space for polymer crystallization.1−3 The microdomains generated by the microphase separation of block copolymer (bcp) are the most common type,4 where the lamellar, cylindrical and spherical domains formed by the crystallizable block in the melt state may impose the spatial confinement in one, two, and three dimensions, respectively, to the crystallization below the melting point. © XXXX American Chemical Society

Received: October 29, 2016 Revised: December 15, 2016

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DOI: 10.1021/acs.macromol.6b02347 Macromolecules XXXX, XXX, XXX−XXX

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PEO and poly(3-hexylthiophène) (P3HT). The crystalline stems of P3HT may align perpendicularly or in parallel to the channel long axis depending on how P3HT was incorporated into the AAO nanochannels.33−35 Maiz et al. have identified the parallel orientation for PEO crystallites developed in the nanotubes formed by the preferential wetting of PEO to the AAO channel wall.36 Guan et al. have incorporated low molecular weight PEO (Mw = 1998 g/mol) into AAO nanochannels; the crystallites were found to adopt perpendicular orientation under larger AAO diameter, but the preferred orientation transformed to parallel type as the diameter was decreased to 10 nm.37 The previous studies on the preferred crystal orientation in AAO nanochannels appear to indicate that the type of orientation attained is identical to that developed in bcp microdomains, thereby showing that the spatial confinement effect dominates the crystallization behavior in the microdomains. Nevertheless, no attempt has been made to systematically examine the accessibility of variable orientation with respect to the change of crystallization condition in AAO nanochannels. Inspired by the transformation of crystal orientation with respect to Tc of PEO in bcp microdomains, here we undertake a systematic study to resolve the crystal orientation of PEO developed in the AAO nanochannels as a function of Tc, PEO molecular weight (MPEO), and channel diameter (DAAO). Instead of filling the nanochannels homogeneously, PEO incorporated into the AAO nanochannels with DAAO of 89 and 23 nm by the solution infiltration method formed nanotubes with the shell thickness of 16.1−22.2 and 5.0−7.8 nm, respectively. It will be shown that irrespective of MPEO and DAAO, crystallization of PEO in the tubular space at higher Tc generated the crystallites with perpendicular orientation characterized by the perpendicular alignment of (120) plane with respect to the channel axis; however, when Tc was sufficiently low, a significant fraction of crystallites showed the tilt orientation with the angle between (120) plane and the channel axis of 45 o. In contrast to the bcp system, the tilt angle remained essentially constant with respect to the change of Tc. The observed variation of crystal orientation will be discussed in connection with the degree of constraint exerted on the crystal growth governed by the confinement geometry and the nucleation density.

microdomain of a series of poly(ε-caprolactone)-block-poly(4vinylpyridine) (PCL-b-P4VP) with different PCL domain thickness.10,11 It was demonstrated that decreasing the confinement size prescribed by the domain thickness transformed the PCL crystal orientation from perpendicular to parallel type when the thickness was sufficiently small. Poly(ethylene oxide) (PEO) is the polymer that exhibits the richest variation of crystal orientation in bcp microdomain. The seminal works of Cheng et al. have revealed that PEO crystallites developed in the lamellar microdomains of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) adopted perpendicular orientation at higher Tc (>35 °C); as the Tc was lower than 30 °C, tilt orientation characterized by the angle between the crystalline stems and the lamellar normal was observed. The tilt angle increased with decreasing Tc until the stems lay in parallel to the interface.12−17 Similar transformation of orientational order with Tc was identified for the cylinder-forming PEO-b-PS.18,19 Whether the crystal orientation is variable or not, it is a wellestablished fact that the crystallites developed in the bcp microdomains exhibit preferred orientation, and it is believed that the preferred orientation is a consequence of the confinement effect. Nevertheless, considering that the block chains in bcp are covalently connected at their ends, one end of the block chains is hence constrained in the microdomain interface as the crystallization occurs. Such a chemical junction constraint may also exert strong impact on the crystallization behavior in limiting the cross-sectional area allocated for each block chain and the chain mobility. As a result, the special crystallization behavior found in the bcp microdomain may arise from the effects of both spatial confinement and chemical junction constraint. Recently, Nakagawa et al. have studied the effect of the junction point constraint on the crystallization of PCL in the lamellar microdomains of PCL-b-PS bearing a photocleavable group at the block junctions. The junction point constraint was shown to greatly retard both the crystallization kinetics and crystallizability of PCL.20 To eliminate the junction effect, the nanochannel of anodic aluminum oxide (AAO) with well-defined diameter is a convenient template for imposing 2-D confinement to the crystallization process. The use of AAO template to study the confined crystallization involves first the incorporation of the polymer into the nanochannels followed by inducing the crystallization process in the channels. Many studies have revealed that the crystallites developed in AAO nanochannels also displayed preferred orientation. Polymer systems such as poly(vinylidene fluoride) (PVDF),21,22 PE,23,24 PLLA,25 poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)),26,27 syndiotactic polystyrene (sPS)28−31 and poly(trimethylene terephthalate) (PTT)32 were found to adopt perpendicular crystal orientation with respect to the long axis of the nanochannel (i.e., z-axis). Further studies on the crystal orientations of PVDF,22 PE,24 PLLA,25 and sPS28−31 in AAO nanochannels with different diameters showed that while all crystallites displayed perpendicular orientation, their orientational order increased with decreasing channel diameter. Recently, Shingen. et al. studied the orientation of P(VDFTrFE) crystallites in AAO and found that the degree of perpendicular orientation was better for the crystallites formed at the higher crystallization temperature.27 Although perpendicular crystal orientation was predominantly observed for polymer crystallites developed in AAO nanochannel; other types of orientation have been attained for

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EXPERIMENTAL SECTION

Sample Preparation. AAO membranes with DAAO of 89 and 23 nm were purchased from Synkera Inc., and were denoted as 89-nmAAO and 23-nm-AAO, respectively. PEO samples with the numberaverage molecular weights (Mn) of 3400, 10,000 and 95 000 g/mol were purchased from Polymer Source Inc., and they are denoted as PEO3.4k, PEO10k and PEO95k, respectively, in this study. Table 1 tabulates the Mn and the polydispersity indecies (PDIs) of these samples. PEO was infiltrated into AAO nanochannels by a solution method. The sample was first dissolved in chloroform to yield the solution with the polymer concentration of 7 wt %. The polymer solution was

Table 1. Molecular Weights and Polydispersity Indicies of the PEO Samples Studied

B

sample

Mn (g/mol)

PDI

PEO3.4k PEO10k PEO95k

3400 10 000 95 000

1.04 1.05 1.08 DOI: 10.1021/acs.macromol.6b02347 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules dropped on a glass slide, and the AAO membrane was immediately immersed on top of the solution to allow the liquid solution to flow into the nanochannels by the capillary force. The sample was then dried at 50 °C for 24 h in vacuum followed by annealing at 90 °C for 24 h. Finally, a blade was used to scratch out the PEO remained on the surface of the AAO membrane. Scanning Electron Microscope (SEM). The geometric identity (i.e., tube or rod) of PEO confined within the nanochannels was observed by a HITACHI SU8010 field emission SEM. The samples were coated with gold before the measurement. The AAO membranes that were observed by the SEM form the top view at 5−10 keV. Wide Angle X-ray Scattering (WAXS). The 2D WAXS patterns of the PEO crystallites formed within the AAO nanochannels were collected by the X-ray scattering Beamline 17A1 at of the National Synchrotron Radiation Research Center (NSRRC) located at Hsinchu, Taiwan. The energy of the X-ray source was 9 keV. A two-dimensional Mar345 imaging plate area detector at the sample-to-detector distance of 183 mm was used to record the 2-D WAXS patterns. The WAXS angular scale was calibrated using silver behenate and silicon powders. The incident X-ray beam was allowed to shine along the edge of the AAO membrane, such that the long-axis of nanochannel was along the z direction as shown in Figure 1. For the preparation of the crystalline

AAO membranes with the average channel diameter of 89 and 23 nm, respectively. The dark holes corresponding to the open ends of the nanochannels were observed for all samples, indicating that PEO did not fill the channels completely. The images of 10 holes in each SEM micrograph were scanned along their radial direction to obtain the average brightness profile for determining the average diameter of the dark holes. The brightness profiles of the AAO membrane with DAAO of 89 and 23 nm thus obtained are shown in Figure 2(e) and 3(e), respectively, and the average diameters of the holes determined from the profiles are listed in Table 2. Figures 2b−d and 3b−d show the top-view images of 89-nm-AAO and 23-nm-AAO containing PEOs with different MPEO, respectively; it can be seen that the nanochannels were not completely filled by PEO, as the holes were still visible. After the incorporation of PEO, the diameters of nanochannels became smaller than that of neat AAO membrane, as manifested clearly by the brightness profiles of the holes in Figures 2e and 3e. The reduction of the channel diameter attests that PEO incorporated into AAO nanochannels by the solution method formed tubes instead of rods. Table 2 tabulates the average diameters of the holes measured from the SEM image; the average shell thicknesses of the PEO tubes determined from the reduction of hole diameter are also listed in Table 2. The shell thicknesses were 16−22 and 5−7.8 nm in 89-nm-AAO and 23-nm-AAO, respectively. Tube morphology of PEO in AAO nanochannels has also been observed in a previous study by Maiz et al., where PEO was infiltrated into the nanochannels by the melt method.36 2. PEO Crystal Orientation in AAO Nanochannels. The 2-D WAXS patterns were collected to reveal the preferred orientation of PEO crystallites formed in the tubular space templated by the AAO nanochannels. Figures 4 and 5 display the edge-view 2D WAXS patterns of the samples having been crystallized at a series of Tcs in the 89-nm- and 23-nm-AAO, respectively. All samples exhibited arc pattern with the (120) diffraction pinpointed by the arrows, indicating that the PEO crystallites developed within tubular space in the AAO nanochannels adopted preferred orientation with respect to the channel axis (aligning along z direction). The observed 2-D patterns can be categorized into two types, namely, type I and type II. In type I pattern, a pair of (120) arcs were located at the meridian direction (or the channel axis). In the type II pattern, in addition to the pair of (120) arcs at the meridian (marked by the red arrow), two sets of (120) arcs (marked by the yellow arrows) were identified at ±45° and ±135° away from the meridian. The type of diffraction pattern observed was found to depend on DAAO, Tc, and MPEO. Under a given MPEO and DAAO, the WAXS pattern tended to transform from type I to type II with decreasing Tc. Type I pattern was predominantly observed for PEO confined in 23-nm-AAO, whereas type II was found prevalently for PEO in 89-nm-AAO. The WAXS patterns also depended on MPEO under a given DAAO and Tc. The results in Figures 4 and 5 thus demonstrate that the preferred orientation of PEO crystallites developed in the tubular space templated by the AAO nanochannels can be varied by the three variables studied. Below we first analyze pattern I and II to identify the corresponding mode of crystal orientation, then we will disclose the variation of crystal orientation with respect to the changes of Tc, DAAO and MPEO deduced from the corresponding changes of scattering pattern. Parts a and c of Figure 6 schematically illustrate the azimuthal scans of the (120) diffraction intensity in the upper half of type I and type II patterns (azimuthal angle Φ = 0−

Figure 1. Schematic illustration of the configuration of the WAXS experiment; the incident beam traveled along the edge of sample. The long axis of the AAO nanochannels was along the z direction. samples, the PEO-containing AAO membrane was first annealed at 90 °C followed by rapid cooling to the desired crystallization temperature for crystallization for 24 h. The WAXS patterns were collected in the presence of the AAO template, because removing AAO involves the use of NaOH or H3PO4 aqueous solution which also dissolves PEO. Nevertheless, through the use of synchrotron radiation and careful subtraction of the background, we did not find any significant disturbance on the collected WAXS patterns from AAO.

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RESULTS AND DISCUSSION 1. Geometric Identity of PEO in AAO Nanochannels. Before presenting the preferred crystal orientations attained, it is important to resolve the geometric characteristic of PEO in the AAO nanochannels. It is known that the infiltration of a polymer into the AAO nanochannels may lead to two possible types of entity, namely, tube and rod. Rods are formed when the polymer fills the channel space homogeneously.38−40 If the polymer only wets the channel wall preferentially, tube morphology is formed.38−40 Melt infiltration with high melt viscosity normally results in rod entity,40 while tube may be created if the polymer is incorporated by the solution method with short infiltration time.38 SEM was used to identify the morphology of PEO in AAO nanochannels. Figure 2a and 3a show the top-view image of the C

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Figure 2. Top-view SEM images of (a) neat 89-nm-AAO and 89-nm-AAO infiltrated with PEO with molecular weights of (b) 3400, (c) 10000, and (d) 95000 g/mol. (e) Average brightness profiles of 10 holes in the SEM images for determining the average pore diameters.

Figure 3. Top-view SEM images of (a) neat 23-nm-AAO and 23-nm-AAO infiltrated with PEO with molecular weights of (b) 3400, (c) 10000, and (d) 95000 g/mol. (e) Average brightness profiles of 10 holes in the SEM images for determining the average pore diameters.

180°, with the channel axis corresponding to Φ = 90°), respectively. (120) plane is the growth plane of PEO; therefore, the orientation of PEO crystallites can be deduced from the orientation of this plane with respect to the channel axis. In the

type I, pattern, the (120) arcs are located at the meridian (Φ = 90°), indicating that the (120) plane of the crystallites tend to align along the z axis. The crystalline stems in this case align perpendicularly to the channel axis and this orientation mode is D

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7 shows the azimuthal scans of the (120) diffraction of the crystallites formed in the 89-nm-AAO at various Tcs. For PEO3.4k, type I pattern was observed for Tc= 20 °C, showing that the crystallization led mainly to perpendicular crystal orientation (Figure 7a). When Tc was lowered to 0 °C, type II pattern emerged as the two peaks at Φ= 45 and 135° were discernible, signaling that tilt orientation developed and coexisted with the perpendicular orientation. Tilt orientation became the dominant mode as Tc was further decreased to −20 and −40 °C. The effect of Tc on the crystal orientation of PEO3.4k was thus resolved from the azimuthal intensity distribution in Figure 7a: PEO crystallites predominantly adopted perpendicular orientation at Tc ≥ 0 °C, but when Tc was sufficiently low (≤-20 °C) tilted crystallites became dominant. It is interesting that the tilt orientation was characterized by the persistent tilt angle of 45°, which was in contrast with the scenario observed in PEO-b-PS, wherein the tilt angle of PEO crystallites tended to decrease with decreasing Tc18,19 The crystal orientation behavior of PEO10k was analogous to that of PEO3.4k, where the crystallites formed at Tc= 20 °C displayed predominantly perpendicular orientation, whereas tilt orientation became dominant at Tc ≤ 0 °C. For the system with the highest molecular weight studied, i.e., PEO95k, tilt orientation already set in obviously at Tc = 20 °C, and it again became the dominant mode at Tc ≤ 0 °C. For the crystal orientation in 23-nm-AAO, the 2-D WAXS patterns collected at various Tcs displayed in Figure 5 revealed again the presence of type I and II patterns, such as those found for the 89-nm-AAO system. Figure 8 shows the corresponding azimuthal scans of the (120) diffraction. It is interesting that

Table 2. Average Diameters of the Nanochannels and the Shell Thicknesses of PEO Tubes Formed in the AAO Nanochannels

89-nm-AAO

23-nm-AAO

sample

average diameter (nm)

PEO tube thickness (nm)

neat AAO PEO3.4k in AAO PEO10k in AAO PEO95k in AAO neat AAO PEO3.4k in AAO PEO10k in AAO PEO95k in AAO

88.9 56.7 56.1 44.4 23.2 13.1 7.6 8.5

0.0 16.1 16.4 22.2 0.0 5.0 7.8 7.4

termed as the “perpendicular orientation”, as schematically illustrated in Figure 6a.18,19 In the pattern shown in Figure 6b, the (120) arcs are identified at the four quadrants and located at ±45° and ±135° with respect to the z axis, signaling that the (120) planes tilt 45° from the channel axis. This orientation mode is thus called “tilt orientation”, as schematically illustrated in Figure 6b.41Among the 2-D WAXS patterns shown in Figures 4 and 5, the pattern in Figure 6b was hardly observed. The type II pattern observed experimentally is indeed the superposition of type I and the pattern in Figure 6b, indicating that the type II pattern corresponds to the coexistence of perpendicular and tilt orientation. Now we examine the effects of various parameters on the preferred orientation of the PEO crystallites developed within the tubular space templated by the AAO nanochannels. Figure

Figure 4. 2-D WAXS patterns of PEO crystallites developed in 89-nm-AAO. The arrows mark the (120) diffraction arcs. Two types of scattering pattern, i.e., types I and II, were identified, as denoted in the 2D pattern. E

DOI: 10.1021/acs.macromol.6b02347 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. 2-D WAXS patterns of PEO crystallites developed in 23-nm-AAO. The arrows mark the (120) diffraction arcs. Two types of scattering pattern, i.e., types I and II, were identified, as denoted in the 2D pattern.

all crystallites adopted perpendicular orientation. f tilt of PEO95k increased from ca. 0.35 at Tc= 20 °C to ca. 0.45 at Tc= 0 °C and then leveled off with further decrease of Tc. In the case of 89nm-AAO, the crystallites of the two samples with the lower molecular weight did not show clear tilt orientation at Tc = 20 °C, but f tilt increased abruptly as Tc was lowered to 0 °C and it then saturated at the value of ca. 0.65 with further reduction of Tc. Tilt orientation however already set in at Tc = 20 °C for PEO95k, and f tilt increased with decreasing Tc to the saturated value ca. 0.9 at Tc ≤ 0 °C. It is noted that the saturated extent of tilt orientation in 89-nm-AAO system was larger than that (ca. 0.45) associated with 23-nm-AAO, signaling that increasing DAAO tended to promote the formation of tiled crystallites. On the basis of the results of Figure 10, the effects of Tc, MPEO and DAAO on the preferred orientation of PEO crystallites developed in the tubular space templated by AAO nanochannel were revealed. Under a given MPEO, decreasing Tc tended promote the formation of tilt-oriented crystallites. The increase of f tilt with decreasing Tc appeared to be quite abrupt and f tilt tended to saturate at Tc≤ ∼ 0 °C. The PEO with lower MPEO (= 3400 and 10000 g/mol) showed lower extent of tilt orientation, whereas the high MPEO (=95000 g/mol) system formed tilt-oriented crystallites at the Tc ≅ 20 °C. The effect of DAAO was also evident, where the crystallites developed in 23nm-AAO predominantly exhibited perpendicular orientation, while tilt orientation appeared under almost all conditions studied in 89-nm-AAO. It is emphasized that the tilt orientation was always characterized by the tilt angle of 45° for the (120) plane. 3. Mechanism of Crystal Orientation Development in the Confined Space. It was clear that the orientation of PEO

perpendicular orientation became almost the sole orientation mode over the Tc range studied for PEO3.4k and PEO10k. A small fraction of crystallites was found to adopt tilt orientation for PEO95k. Through the systematic examination of the type of crystal orientation attained under different conditions, we constructed the orientation diagrams to depict the type of orientation attained under a given combination of MPEO and Tc, as shown in Figure 9. It can be seen that in the PEO nanotubes with stronger confinement (i.e., the nanotubes formed within 23nm-AAO), the crystallites predominantly adopted perpendicular orientation except for the PEO with the highest MPEO studied (i.e., MPEO = 95000 g/mol), where a small fraction of tilted crystallites was observed. In the tubes with the smaller confinement (i.e., the tubes formed in 89-nm-AAO), coexistent orientation with mainly tilt orientation and a small fraction of perpendicular orientation became dominant except for the PEO with the smaller molecular weights (MPEO = 3400 and 10000 g/ mol) crystallized at the higher Tc (= 20 °C). In the case that the two types of crystal orientation coexisted, the areas of the peaks in the azimuthal intensity distribution of (120) diffraction were used to calculate the relative fraction of the crystallites with tilt orientation, viz. ftilt = sum of the areas of the peaks at Φ = 45, 135, 225, and 315° total area

Figure 10 shows the calculated f tilt as a function of Tc. For 23nm-AAO (Figure 10a), f tilt of PEO3.4k and PEO10k remained virtually zero over the Tc range studied, indicating that nearly F

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Figure 6. Schematic illustration of the types of PEO crystal orientation developed in the nanotubes in AAO (the direction of the (120) plane is marked by the red arrows), the corresponding (120) diffraction arcs in the 2D WAXS patterns, and the azimuthal scans of (120) diffraction intensity: (a) perpendicular orientation (b) tilt orientation, and (c) coexistence of perpendicular and tilt orientation.

(Figure 11c). Nevertheless, perpendicular orientation was still the preferred type for maximizing the distance of crystal growth under the prescribed confinement geometry. As has been discussed in our previous work,2,4 once the nucleation density in the confined space was high, the growth of a given crystallite was also restricted by the neighboring crystallites (Figure 11d), which would limit its growth along z axis. As a result, the crystallite may experience the confinement exerted by tube thickness, tube curvature and the neighboring crystallites, as schematically illustrated in Figure 11d. When the space available for the growth along z direction was not much larger than the tube thickness, the crystallite may attempt to maximize the growths of the two (120) planes under the confined space. In this case, the two (120) growth planes competed with each other for growth, which in turn led to 45° of tilt angle, because in this case the two (120) growth planes may reach equal and longest growth distance under the prescribed confined space (see Figure 11e).

crystallites in the nanotube templated by AAO nanochannels depended on Tc, MPEO, and DAAO. Here, we propose a model to depict how the three parameters could affect the preferred crystal orientation. Since PEO formed tube in the AAO nanochannel, its crystal growth was constrained by the tube thickness in r direction and the curvature in θ direction. The direction that did not impose strong confinement to the growth is the long axis of the nanochannel, i.e., z axis (Figure 11a). At the beginning of crystallization, the nucleation density was low; the crystallites developed were too small to feel the confinement set by the tube geometry, so they were randomly oriented.4 As the crystallites grew in size, they started to feel the constraint in r and θ direction (Figure 11b); PEO crystallites then attempted to align toward the perpendicular orientation to allow the growth front to advance along the z axis. In this case, only one of the two (120) growth planes was allowed to grow over a larger distance, while the growth of the other (120) plane was still constrained by the tube thickness and curvature G

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Figure 7. Azimuthal scan of (120) diffraction of PEO crystallites developed in 89-nm-AAO: (a) PEO3.4k, (b) PEO10k, and (c) PEO95k. The crystallization temperatures are indicated in the figures.

Figure 8. Azimuthal scan of (120) diffraction of PEO crystallites developed in 23-nm-AAO: (a) PEO3.4k, (b) PEO10k, and (c) PEO95k. The crystallization temperatures are indicated in the figures.

degree of supercoiling at a given Tc, which in turn led to a higher nucleation density and hence a higher population of tilted crystallites. Finally, as DAAO became small (e.g., 23 nm), the confinement imposed by the tube thickness and the curvature became more serious. In this case, the crystallites developed in the nanotube might start to feel the confinement effect at the earlier stage of crystallization where the nucleation density was still low. The crystallites then aligned toward perpendicular orientation to maximize growth distance without the interference from other crystallites. Consequently, the window of coexistent orientation in the orientation diagram was largely suppressed for 23-nm-

Now we apply the proposed mechanism to explain qualitatively the experimental observations on the effects of Tc, MPEO, and DAAO on PEO crystal orientation. At the lower Tc (or large undercooling), the nucleation density was higher, such that the probability of a given crystallite to experience the crowding effect that resulted in tilt orientation was higher. As a consequence, the preferred crystal orientation transformed from perpendicular type to coexistent orientation as Tc was sufficiently low. Regarding the MPEO effect, it is known that the equilibrium melting point of a polymer increases with increasing molecular weight. This means that the higher molecular weight PEO (e.g., PEO95k) experienced a larger H

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Figure 9. Orientation diagram of PEO crystallites in the nanotube templated by (a) 23-nm-AAO and (b) 89-nm-AAO.

Figure 10. Tc dependence of f tilt of PEO in (a) 23-nm-AAO and (b) 89-nm-AAO.

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CONCLUSION PEO was infiltrated into AAO nanochannels by a solution method to yield the confined space with tube geometry for crystallization. The effects of crystallization temperature, PEO molecular weight and AAO channel diameter on the crystal orientation attained have been systematically examined in this study. The preferred crystal orientation was revealed from the azimuthal scan of the (120) diffraction arcs in 2-D WAXS patterns. Two modes of crystal orientation were identified here, namely, perpendicular orientation with the (120) plane aligning along the channel axis and tilt orientation with the (120) planes tilted 45° away from the channel axis. Decreasing Tc tended to transform the orientation from perpendicular type to tilt configuration with 45° in tilt angle, whereas increasing MPEO and DAAO suppressed the population of crystallites with perpendicular orientation. The crystal orientation was postulated to be governed by the strength of confinement to the crystal growth in AAO nanochannels prescribed by the tube thickness and curvature and the nucleation density. Higher nucleation density at the larger degree of supercooling created a crowded environment where the growth along the channel axis was highly restricted; in this case, the crystallites tilted 45° from the channel axis to maximize the growth distance of the two (120) planes. On the other hand, the smaller tube thickness and larger curvature prescribed by smaller DAAO tended to force

AAO, where only PEO95k which exhibited the highest nucleation density showed the coexistence of tilt and perpendicular orientation. In principle, it is possible to control the nanotube thickness by varying the initial concentration of the solution used for infiltration, such that the effect of tube thickness on the crystal orientation may be studied. Indeed, we have also infiltrated PEO into the AAO nanochannels by the melt infiltration method, which can be considered as the extreme scenario of zero solvent concentration for generating the maximum tube thickness under the prescribed nanochannel geometry. In this case, nanorods with PEO homogeneously filling the channels were obtained. The characterization of the orientation of the crystallites developed in the nanorod revealed that the window of the perpendicular orientation was much smaller than that associated with the nanotube reported here (see Supporting Information). The result is consistent with our postulate that perpendicular crystal orientation is favored under strong confinement. That is, increasing the tube thickness to the maximum value represented by the nanorod reduced the degree of confinement, so as to narrow down the window of perpendicular crystal orientation. I

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Macromolecules

Figure 11. Schematic illustration of the development of crystal orientation in the tubular space templated by AAO nanochannels. (a) Tube shell imposes confinement in r and θ directions. (b) For low nucleation density, the originally randomly oriented crystallite feels the constraint in r and θ directions as it grows in size. (c) The crystallite then aligns toward perpendicular orientation to seek the free space to grow further. (d) For high nucleation density, the crystallite feels the constraint in r and θ directions set by the tube geometry and also the restriction imposed by the neighboring crystallites. (e) The two (120) growth planes hence compete with each other, giving rise to tilt orientation with the tilt angle of 45°. Segregated Plla-B-Pe and Plda-B-Pe Diblock Copolymers. Macromolecules 2008, 41 (16), 6154−6164. (4) Liu, C.-L.; Lin, M.-C.; Chen, H.-L.; Mű ller, A. J. Evolution of Crystal Orientation in One-Dimensionally Confined Space Templated by Lamellae-Forming Block Copolymers. Macromolecules 2015, 48 (13), 4451−4460. (5) Lin, M.-C.; Wang, Y.-C.; Chen, H.-L.; Müller, A. J.; Su, C.-J.; Jeng, U.-S. Critical Analysis of the Crystal Orientation Behavior in Polyethylene-Based Crystalline−Amorphous Diblock Copolymer. J. Phys. Chem. B 2011, 115 (11), 2494−2502. (6) Higa, T.; Nagakura, H.; Sakurai, T.; Nojima, S. Crystal Orientation of Poly(ε-Caprolactone) Blocks Confined in Crystallized Polyethylene Lamellar Morphology of Poly(ε-Caprolactone)-BlockPolyethylene Copolymers. Polymer 2010, 51 (23), 5576−5584. (7) Kofinas, P.; Cohen, R. E. Morphology of Highly Textured Poly(Ethylene)/Poly(Ethylene-Propylene) (E/Ep) Semicrystalline Diblock Copolymers. Macromolecules 1994, 27 (11), 3002−3008. (8) Hamley, l. W.; Patrick, J.; Fairclough, A.; Ryan, A. J.; Bates, F. S.; Towns-Andrews, E. Crystallization of Nanoscale-Confined Diblock Copolymer Chains. Polymer 1996, 37 (19), 4425−4429. (9) Hamley, I. W.; Fairclough, J. P. A.; Terrill, N. J.; Ryan, A. J.; Lipic, P. M.; Bates, F. S.; TownsAndrews, E. Crystallization in Oriented Semicrystalline Diblock Copolymers. Macromolecules 1996, 29 (27), 8835−8843. (10) Sun, Y.-S.; Chung, T.-M.; Li, Y.-J.; Ho, R.-M.; Ko, B.-T.; Jeng, U.-S. Crystal Orientation within Lamellae-Forming Block Copolymers of Semicrystalline Poly(4-Vinylpyridine)-B-Poly(Epsilon-Caprolactone). Macromolecules 2007, 40 (18), 6778−6781. (11) Sun, Y.-S.; Chung, T.-M.; Li, Y.-J.; Ho, R.-M.; Ko, B.-T.; Jeng, U.-S.; Lotz, B. Crystalline Polymers in Nanoscale 1d Spatial Confinement. Macromolecules 2006, 39 (17), 5782−5788. (12) Huang, P.; Zhu, L.; Guo, Y.; Ge, Q.; Jing, A. J.; Chen, W. Y.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Lotz, B.; Hsiao, B. S.; Avila-Orta, C. A.; Sics, I. Confinement Size Effect on Crystal Orientation Changes of Poly(Ethylene Oxide) Blocks in Poly(Ethylene Oxide)-B-Polystyrene Diblock Copolymers. Macromolecules 2004, 37 (10), 3689−3698. (13) Zhu, L.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Liu, L.; Lotz, B. Initial-Stage Growth Controlled Crystal Orientations in Nanoconfined Lamellae of

the crystallites to adopt perpendicular orientation at the early stage of crystallization at which the nucleation density was low.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02347. Orientation diagram of PEO crystallites in the nanorods templated by 23-nm-AAO and 89-nm-AAO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.-L.C.) E-mail: [email protected]. ORCID

Hsin-Lung Chen: 0000-0002-3572-723X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science of Technology under Grant Nos. MOST 103-2221-E-007-133 and MOST 103-2622-E-007-025.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b02347 Macromolecules XXXX, XXX, XXX−XXX