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Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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Reexamining the Crystallization of Poly(ε-caprolactone) and Isotactic Polypropylene under Hard Confinement: Nucleation and Orientation Guangyu Shi,†,⊥ Guoming Liu,*,† Cui Su,†,⊥ Haiming Chen,†,⊥ Yu Chen,‡ Yunlan Su,† Alejandro J. Müller,§,∥ and Dujin Wang†,⊥ †

CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China § 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 ∥ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain ⊥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: The crystallization of poly(ε-caprolactone) (PCL) and isotactic polypropylene (iPP) infiltrated in nanoporous anodic alumina oxide (AAO) templates was reexamined to demonstrate the importance of obtaining polymerfree, clean AAO surfaces on the nucleation, size dependence of crystallization temperature (Tc), and texture. The AAO pore diameters cover a broad range from 400 to 20 nm. When the AAO templates were completely free of any residual polymer on their surfaces, differential scanning calorimetry (DSC) experiments exhibited a single crystallization peak for all the samples with different AAO pore sizes. A drastic decrease in Tc with density of domains indicated a transition from heterogeneous to homogeneous/surface nucleation. A regular decrease of Tc with pore size was observed in the low Tc regime, as a result of the volume dependence of nucleation events. The chain alignment of the two polymers infiltrated in AAO was studied by two-dimensional wide-angle Xray scattering (WAXS). By comparing the experimental and simulated WAXS patterns, the orientation modes of the polymers were identified and compared with previous studies.

1. INTRODUCTION Polymer crystallization starts with an embryo (nuclei) in a disordered melt consisting of random coils. The embryo generally originates from random occurring fluctuations with different sizes. To satisfy the thermodynamic criteria, a nucleus needs to reach a critical size to overcome a free energy barrier. The homogeneous nucleation theory was originated from the work of Volmer, Becker, and Frenkel.1−3 If foreign surfaces exist in the system, such as dusts, catalysts residues, etc., a nucleus attached to this foreign surface would have lower free energy barrier as compared with a homogeneous nucleus. In practice, bulk polymers almost always crystallize via the extrinsic heterogeneous nucleation mechanism. To study the intrinsic homogeneous nucleation phenomenon, a procedure to get rid of the impurities must be implemented. Although this has been achieved in high purity water,4,5 the “impurity-free” strategy is not applicable to polymers because of their high-viscosity nature. One smart option is to split a polymer into microscopic regions. When the number density of the microdomains largely exceeds that of the impurities, naturally there will be impurity-free domains which will have to crystallize homogeneously. In the case of © XXXX American Chemical Society

immiscible polymer blends, the number density of microdomains is typically of the same order of magnitude as the density of active heterogeneities. Therefore, fractionated crystallization is generated, as different populations of droplets or microdomains can coexist in the sample. Some can contain heterogeneities (and crystallize at low supercoolings), and others are clean (and crystallize at higher supercoolings in one or more exotherms depending on the types of heterogeneities present in the sample).6−9 Homogeneous nucleation in microdomains has been demonstrated in block copolymers10,11 and droplets.12−15 In recent years, hard templates have attracted much attention, especially nanoporous anodic alumina oxide (AAO) templates because of their monodispersity, thermal stability, and ease of fabrication. Several review papers have been published about polymer infiltration and confinement within AAO templates.7−9 The transition from heterogeneous nucleation to homogeneous nucleation mechanism is generally claimed by the Received: October 27, 2017 Revised: November 7, 2017

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DOI: 10.1021/acs.macromol.7b02284 Macromolecules XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

observation of drastic decreases in crystallization temperature (Tc). For example, PVDF infiltrated in 35 nm diameter AAO showed a supercooling of 60 °C,16 and the supercooling of poly(ethylene oxide) (PEO) in AAO was 60−70 °C.17−19 In several systems, including polyethylene (PE),20 syndiotactic polystyrene (sPS),21 isotactic polypropylene (iPP),22,23 poly(εcaprolactone) (PCL),24,25 and PEO−PCL block copolymer,26 more than one crystallization peak were observed (termed as “multiple nucleation events” in ref 24). Crystallization peaks close to the bulk were visible even when the diameter of templates was as small as 20−30 nm.21,22,24 The explanation for the multiple nucleation events was that domains with impurities crystallized heterogeneously at lower supercoolings, while clean domains crystallized homogeneously with larger supercoolings.21,22,24,26 Although quite straightforward, this explanation arises concerns about the density of heterogeneous nuclei in polymers. When the diameter of the templates is smaller than 100 nm, the density of nanodomains could be 3−5 orders of magnitude higher than the density of heterogeneous nuclei in typical polymers. In this respect, only a tiny fraction of the polymers would be influenced by the possible heterogeneities and the majority of the polymer nanodomains could be viewed as “impurity-free”. A proper study of the crystallization and other physical properties under confinement require “clean” environments, i.e., free of any residual surface that could act as interconnecting films and lead to “bulky” or unpredictable percolation behavior. Müller et al.7 and Floudas et al.27 have demonstrated that the residual surface film caused two additional high crystallization peaks of PEO within AAO templates. Then, it is natural to ask, what is the origin of the high-temperature crystallization peaks in those studies that claimed a cleaning procedure? Some previous studies did not discuss this issue.20,21,25 The multiple exotherms of infiltrated PCL were explained by the heterogeneous nucleation mechanisms induced by the AAO wall.24 The possibility of the influence of the residual surface polymer was mentioned to explain the fractionated crystallization of iPP.23 The highest exotherms of infiltrated iPP in AAO was ascribed to a small amount of iPP on the surface of AAO, which was believed to be irrelevant to the crystallization of polymers in the pores.22 In PEO-b-PCL block copolymers confined in AAO templates, the existence of high-temperature exotherms was attributed to the heterogeneous nuclei contained in PCL.26 Taking into account all these previous studies, in our opinion, the importance of the remaining polymer on the surface of the infiltrated AAO templates in confined crystallization studies is still underestimated. In the present work, we carried out a critical reexamination of the crystallization behavior of two infiltrated polymers that have been previously studied: PCL and iPP. We showed that all the high-temperature crystallization peaks disappeared after carefully cleaning the surface layer thoroughly. A regular decrease of Tc with nanodomain size was observed. With “clean” systems, we were able to carry out a thorough analysis of the chain orientation within the nanopores. By comparing the experimental patterns with simulated patterns, certain lattice directions were identified to be aligned preferentially parallel to the pore axis. This work emphasizes the importance of “clean systems” in the study of polymer crystallization under confinement. The methodology of the surface polishing procedure was described in detail to provide a guide for researchers in the field to obtain reliable results.

2.1. Materials and Sample Preparation. Poly(ε-caprolactone) (PCL) and isotactic polypropylene (iPP) were purchased from SigmaAldrich. The PCL had a number-average molecular weight (Mn) of 10 000 g/mol and a PDI of 1.4. The iPP had a Mn of 5000 g/mol and a PDI of 2.4. The samples were used as received without further purification. The AAO templates with pore diameters of 400, 200, 100, 60, 40, and 20 nm and a pore length of approximately 100 μm were prepared by a two-step electrochemical anodization of aluminum modified from previous methods.28,29 Because of the broad diameter range, different synthesis conditions were applied. AAO templates with 200 and 400 nm pore diameter were fabricated under 195 V in phosphoric acid. AAO templates with 100 nm pore diameter were fabricated under 50 V in oxalic acid, and AAO templates with 60 and 40 nm diameter were fabricated in the same electrolyte under 40 V. The 20 nm AAO templates were fabricated under 25 V in sulfuric acid. The depth of the pores was fixed to be 100 μm by monitoring the current−time curve and adjusting the anodizing durations.30 Before infiltration with polymers, the AAO templates were washed with acetone and ethanol to eliminate possible impurities. A piece of thin polymer film was placed on the surface of AAO and then annealed at 100 or 200 °C for several hours for PCL and iPP, respectively. The whole operation was carried out under a nitrogen atmosphere. The surface polishing method is nontrivial, and the best method may differ from polymer to polymer. Basically, the polishing method of this study included three steps. The first two steps were the same for PCL and iPP. Step 1: a sharp blade was used to remove most of the residual polymers on the AAO surface at room temperature. Step 2: the template was further cleaned with a soft polishing cloth at a temperature above the melting temperature of the polymer (100 °C for PCL and 200 °C for iPP). The above two steps have commonly been used in the literature (refs 18, 22, 24, 26, and 31); however, they were insufficient for cleaning the surface of infiltrated PCL or iPP templates. Therefore, a third step was applied. For PCL, the template was cleaned with a mixture of chloroform and ethanol (volume ratio 1:1). For iPP, a motor driven rotating polishing apparatus was applied. Finally, the template was washed in an ultrasonic water bath to remove the powders created by mechanical polishing. We found that the third step is crucial for breaking the polymer connectivity between the pores in different nanodomains, at least with the PCL and iPP samples employed here. 2.2. Characterization. A Hitachi SU-8020 scanning electron microscope (SEM), operated at 5 kV, was utilized to examine the surface morphology of AAO templates. To observe the PCL nanorods, the AAO template was etched out in a solution composed of 4.5 g of chromium trioxide (CrO3), 3.5 mL of phosphoric acid (H3PO4), and 96.5 mL of deionized water. All the SEM specimens were coated with gold to avoid charging. Thermal analysis was conducted with a differential scanning calorimeter (DSC Q2000, TA). The instrument was calibrated with indium before measurements. An amount of ∼5 mg was weighed for each infiltrated samples containing the aluminum base. The samples were scanned between −80 and 90 °C at a heating/cooling rate of 1, 2, 5, 10, and 20 °C/min for PCL. For iPP, the scan range was 0−200 °C with a rate of 10 °C/min. All DSC measurements were carried out under a nitrogen atmosphere. The density of spherulites was measured by using a polarized optical microscope (Olympus BX51, Japan) equipped with a high-temperature hot stage (Linkam THMS600, Linkam Scientific, UK). The samples were first heated to a temperature well-above the melting points to remove the thermal history and then quenched to different temperatures to crystallize isothermally. The number of spherulites was counted. The film thickness was measured by using a spiral micrometer. Therefore, the density of spherulites can be obtained by normalizing the number of spherulites with respect to volume. Wide-angle X-ray scattering (WAXS) experiments were carried out on a Huber 5-axis diffractometer at the beamline 1W1A of Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the B


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Macromolecules radiation was 1.549 Å. A two-dimensional image plate detector, MAR 345 with the pixel size of 100 × 100 μm2 (3450 × 3450 pixels), was used to collect the diffraction patterns. The sample-to-detector distance was 445 mm. The direction normal to the AAO surface is defined as the z-axis, while two lines in the AAO surface perpendicular to each other are defined arbitrarily as the x- and y-axis. The X-ray beam irradiates the sample with an incident angle of ∼2° (along the xaxis). The exposure time was 10 min for each sample.

indicate that the AAO templates provide an ideal environment to nanomold nanorods by infiltration. 3.2. Crystallization and Melting Behavior. Figure 2 shows the DSC cooling curves of bulk and infiltrated PCL and iPP. The bulk PCL shows a peak crystallization temperature (Tc) of ∼33 °C (Figure 2a). Within 400 and 200 nm AAO, the Tc of PCL decreases to 25 and 14 °C, respectively. When the diameter of AAO decreases to 100 nm, a drastic decrease of Tc to −37 °C is observed. With the diameter of AAO further decreasing, the Tc of PCL infiltrated in AAO decreases gradually. This result is completely different from a previous publication in which several exothermic peaks were observed for infiltrated PCL during cooling.24,25 Figure 2b shows the DSC cooling curves of the infiltrated iPP in AAO. The bulk iPP crystallizes at ∼112 °C. Within AAO, the Tc decreases drastically to ∼43 °C when the diameter of AAO is 200 nm. The Tc of iPP is almost the same when the diameter of AAO is between 200 and 60 nm. A clear decrease in Tc is only obvious when the diameter of AAO decreases to 20 nm. The lowest Tc in our experiment is 35.4 °C. Our result does not agree with the previous reports,22,23 where a crystallization peak close to the bulk crystallization temperature (above 100 °C) was observed when the AAO diameter was as small as 35 or 40 nm.22,23 Figure 3 shows the Tm and Tc of bulk and infiltrated PCL and iPP (the exact values are summarized in Table 2, and the DSC heating curves are shown in Figures S1 and S2 of the Supporting Information). The melting temperature (Tm) of all the PCL samples are roughly the same, independent of the size of the pores. The Tm of iPP decreased slightly from 157 to 149 °C, as pore size decreased. The lowest Tc achieved here for infiltrated PCL is −51.2 °C, which is significantly lower than the previous reports of infiltrated PCL within AAO (−35 °C24 and −25.7 °C25) and is comparable to the lowest value (−48.4 °C) observed in cylindrical domains in PCL-containing block copolymers with a similar molecular weight of PCL blocks.11 The lowest Tc in this work is also very close to the glass transition temperature of PCL (−60 °C41). By comparing the Tc with Tg,17 the crystallization of PCL in AAO smaller than 100 nm should proceed via homogeneous nucleation, as PCL chains are crystallizing at the maximum possible supercooling available to the material (just before vitrification). The drastic decrease of

3. RESULTS AND DISCUSSION 3.1. Pore Morphology of AAO Templates. Figure 1 shows the PCL nanorods and the cross sections of typical

Figure 1. SEM micrographs of the PCL nanorods in the 400 nm AAO template (a) and the cross section of pristine AAO templates with a diameter of 200 (b), 100, (c) and 40 nm (d). Scale bars: 1 μm.

nanoporous templates. The PCL nanofibers can be seen in Figure 1a (after partial dissolution of the alumina template). It can be observed that the nanorods are homogeneous in diameter. The cross-sectional images of the templates demonstrate that the pores are straight, homogeneous in diameter, and with no interconnection among them. The average diameters of the AAO templates are Φ = 400, 200, 100, 60, 40, and 20 nm. The pore depth is ∼100 μm. These results

Figure 2. DSC cooling curves of infiltrated PCL (a) and iPP (b) in AAO templates with different pore diameters. The cooling rate is 10 °C/min. C


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Figure 3. Melting temperature (Tm) and crystallization temperature (Tc) of infiltrated PCL (a) and iPP (b) in AAO templates as a function of domain densities. The density of domains was estimated according to the number density of pores of the template by assuming it was fully filled with polymer (values shown in Table 1).

Tc is observed in domain densities ∼1012 cm−3. This value is 6 orders of magnitude larger than the measured nucleation density (Table 1). Note that the crystallization temperature for PCL infiltrated in AAO gradually decreases with cooling rate, as depicted in Figure S3.

Duran et al.22 reported a Tc of 28.9 °C for infiltrated iPP within in 25 nm AAO, and Reid et al.23 observed a Tc of 41.7 °C in 15 nm AAO. For iPP-b-PS diblock copolymers, only one crystallization peak was observed, and the lowest Tc was ∼18 °C in cylindrical domains with a radius of 10.3 nm.42 Schawe reported a Tc value for bulk iPP as low as ∼13.5 °C by fast scanning DSC with a cooling rate of 500 K/s,43 which is probably the lowest value reported so far. However, in this case, the influence of cooling rate on the value of Tc should also be taken into account, when comparing with standard DSC cooling rates, as they can differ by at least an order of magnitude. The abrupt decrease in Tc of iPP occurs between 1011 and 1012 cm−3, which is 6 orders of magnitude higher than the measured heterogeneous nucleation density (Table 1). The transition point occurs at a lower domain density in iPP as compared with PCL, which is probably be related to the lower heterogeneity density contained in iPP. We do observe a cold crystallization process of iPP during heating, especially when it is infiltrated in small pores, as shown in Figure S2. This could be explained by the decreased crystallinity obtained during cooling in confined environments. Note that Duran et al.22 pointed out that the crystallization of iPP should be completed suppressed when the diameter of AAO is smaller than 20 nm. Our results do not agree with such estimation as we do find that iPP can crystallize when it is infiltrated in 20 nm pores. Reid et al. have also reported iPP crystallization when it is infiltrated within 15 nm pores.23 According to the simulation of Feng et al.,44 the critical nuclei are fibrillar-like with a radius of ∼6 Å (interchain direction) and a height of ∼27 Å (along the chain) for iPP at 300 K. Therefore, the AAO templates with nanopores larger than those sizes are able to provide enough space for the nucleation of iPP.

Table 1. Nucleation Density of PCL and iPP and Number Density of AAO Pores nucleation density (cm−3)

polymer

4 × 108 2 × 107 107−108 1.0 × 106 5.7 × 106 2.3 × 107 iPP 104−105 104−106 7 × 104 105−106 105−106 106−107 3.7 × 105 9.7 × 105 1.2 × 106 pore depth AAO diameter (nm) (μm) PCL

400 200 100 60 40 20

100 100 100 100 100 100

conditions cryst at 54 °C cryst at 47 °C cryst at 40 °C cryst at 47 °C cryst at 43 °C cryst at 40 °C cryst at 130−150 cryst at 120−147 cryst at 140 °C cryst at 121−125 cryst at 124−136 cryst at 133−140 cryst at 135 °C cryst at 133 °C cryst at 130 °C volume (nm3) 1.26 3.14 7.85 2.83 1.26 3.14

× × × × × ×

1010 109 108 108 108 107

ref 32 33 34 this work

°C °C

35 36 37 38 39 40 this work

°C °C °C

pore density (cm−3) 8 3.2 1.3 3.5 8 3.2

× × × × × ×

1010 1011 1012 1012 1012 1013

Table 2. Data Summary of DSC Results of Infiltrated PCL and iPP

Tm,PCL (°C) Tc,PCL (°C) Tm,iPP (°C) Tc,iPP (°C)

bulk

400 nm

200 nm

100 nm

60 nm

40 nm

20 nm

54.1 33.2 157.0 111.8

53.0 25.1 156.9 110.8

54.0 14.0 155.4 42.3

54.4 −37.1 153.1 43.9

54.5 −37.1 152.7 42.8

52.8 −46.0 152.4 38.8

53.6 −51.2 148.6 35.4

D


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Figure 4. (a) DSC cooling curves of the infiltrated PCL in 100 nm AAO templates. (b) DSC cooling curves of infiltrated iPP in 40 nm AAO templates. Samples 1 and 2 were cleaned by step 1 and step 2 procedures (see Experimental Section), while samples 3 and 4 were further cleaned by step 3.

Figure 5. WAXS geometry of the experiments (a) and WAXS patterns of infiltrated PCL in different AAO templates (b).

Duran et al.22 and Reid et al.23 assigned the drastic decrease of Tc to be a result of the transition from heterogeneous nucleation to homogeneous nucleation. However, the Tc of iPP is still remarkably higher than its Tg (∼−10 °C45). A more rigorous judgment should be made on the basis of the dependence of nucleation rate on the volume or surface area, which could be probed by studying the isothermal crystallization kinetics. The differences in DSC results between our work and previous reports22−24 are fundamental. An intuitive explanation for the observation of multiple crystallization peaks is the existence of residual polymers on the template surface. Our own experience during sample preparation indicates that PCL and iPP are very difficult to remove from the surface of the nanopores, possibly because of the high toughness and/or high melt viscosity. The DSC curves of infiltrated PCL and iPP within AAO templates with different degree of polishing are shown in Figure 4. Fractionated crystallization with multiple crystallization peaks is observed for samples cleaned by a sharp blade and soft polishing paper (steps 1 and 2). Those hightemperature peaks finally vanished when the third polishing step was properly applied. It is important to observe that the Tc of the low-temperature peak is influenced by the existence of high-temperature peaks. Typical clean and unclean AAO surfaces are shown in Figure S4. It is evident that the presence of remaining polymer layers or fragments will lead to irregular

and unpredictable fractionated crystallization peaks, possibly as a consequence of different degree of pore percolation. 3.3. Crystal Orientation in Nanodomains. According to previous reports, the presence of remaining polymer film on the template surface,16 pore size,18 crystallization temperature,31,42 etc., have a significant influence on the chain orientation within AAO templates. Since we obtained two clean systems of PCL and iPP, it is interesting to study the orientation of crystallites within AAO nanopores. Figure 5 shows the WAXS patterns of PCL infiltrated in AAO templates. The samples for WAXS measurements were first heated above their respective melting temperatures (100 °C for PCL and 200 °C for iPP) and then cooled slowly to crystallize at a cooling rate of 1 °C/min (down to −60 °C for PCL and to 0 °C for iPP). The 1D intensity profiles are plotted in Figure 6. The (110), (111), and (200) reflections can be observed, which correspond to the PCL orthorhombic unit cell, as reported by Chatani and co-workers.46 The d-spacings of those reflections are summarized in Table 3. We can conclude that the crystal form does not change upon confinement for PCL. It is noticed in Figure 5 that the (110) reflection exhibits several maxima at different azimuthal angles. Figure 7 shows the azimuthal intensity profiles of (110) reflection of PCL. For PCL infiltrated within 400, 200, 100, and 60 nm AAO templates, a single maximum is located at the meridian (defined as 0°). An additional maximum is observed at an azimuthal angle of ∼32° for infiltrated PCL in 40 and 20 nm AAO templates. This E


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Figure 8. WAXS patterns of iPP infiltrated in different AAO templates.

anisotropy with decreasing pore diameter is observed. In 400 nm AAO, the reflections are broad arcs; in contrast, clear reflection spots are seen in AAO templates with the diameter smaller than 100 nm. To compare the crystal forms of different samples, 1D intensity profiles are plotted in Figure 9.

Figure 6. 1D intensity profiles of bulk and infiltrated PCL in different AAO templates.

Table 3. Lattice Spacing of Infiltrated PCL in AAO Templates d110 (Å) d111 (Å) d200 (Å)

bulk

400 nm

200 nm

100 nm

60 nm

40 nm

20 nm

4.17 4.05 3.77

4.17 4.06 3.79

4.16 4.04 3.76

4.17 4.05 3.77

4.17 4.06 3.78

4.18 4.06 3.79

4.17 4.05 3.78

Figure 9. 1D intensity profiles of iPP infiltrated in different AAO templates.

Reflections with characteristic spacings of 6.26, 5.29, 4.77, 4.18, and 4.05 Å can be assigned to the (110), (040), (130), (111), and (041)/(131̅) planes of the monoclinic α phase, respectively. Again, the crystal form does not change with confinement strength for iPP. It is noted that no mesophase phase was detected for iPP under confinement, which agrees with previous reports of iPP/AAO (with fractionated crystallization)22,23 and iPP/PS blends.48 On the other hand, mesophase was observed in iPP nanodroplets embedded in PS matrix prepared by layer-multiplying coextrusion.49,50 We do not have a good explanation for the discrepancies reported in the literature and prefer to leave this question open for further studies. 3.4. Orientation Simulation. The degree of orientation could be quantitatively characterized by Hermans’ orientation function. However, the signal/noise ratio of the azimuthal intensity profiles is not high enough, especially for the (200) plane of PCL and the (110) plane of iPP, which hinders a complete orientation function analysis. Instead, since all the diffraction features are visible in the 2D patterns, we feel it is convenient to compare experimental patterns with simulated patterns of specific orientations. To make it directly comparable to the experimental pattern, we only calculated the strongest reflections manifested in the experimental patterns. The details

Figure 7. Azimuthal intensity profiles of the (110) reflection of infiltrated PCL in AAO templates.

indicates that stronger confinement may lead to new chain orientation. The angle between the normal of (110) plane with respect to the AAO normal can be calculated from the azimuthal angle by the use of the Polanyi equation47 (ignoring the small tilted/incidence angle): cos φ = cos θ cos ϕ

where θ is the half Bragg angle and ϕ the azimuthal angle. Plugging θ = 10.7° and ϕ = 32° into the equation yield a φ value of 33.6°, which agrees with the angle between the (110) normal with b*-axis. We will address the orientation in more detail in the following section. Figure 8 shows the WAXS patterns of infiltrated iPP in different AAO templates. A gradual increase of the degree of F


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Figure 10. (a) Experimental and simulated pattern of PCL infiltrated in 40 nm AAO. The simulated pattern is a combination of c ⊥ pore axis and b* || pore axis with a Gaussian misorientation angle of 5°. (b) Intensity profiles of infiltrated PCL (110) reflection as a function of azimuthal angle. (c) Experimental and simulated pattern of iPP infiltrated in 40 nm AAO. The simulated pattern is a combination of a* || pore axis and b* || pore axis with a small misorientation angle (Gaussian distribution, derivation = 5°). (d) Intensity profiles of iPP (110) reflection as a function of azimuthal angle.

sudden appearance of the b* || pore axis orientation for a moderate change of the pore diameter (from 60 to 40 nm). 2D WAXS patterns with a specific orientation of iPP are shown in Figure S7. Different from PCL, the degree of isotropy of iPP infiltrated in AAO templates with a diameter smaller than 100 nm is quite high. This does not fit the loose constraint of the c-axis ⊥ pore axis. The off-meridian (110) reflection agrees with the a* || pore axis, and the meridian (040) peak fits the misoriented b* || pore axis (the (040) reflection should be absent for the perfect condition of b* || pore axis). Figure 10c indicates that a combination of a* || pore axis and b* || pore axis fits the experimental pattern well. The simulated azimuthal intensity profile of (110) reflection agrees nicely with experiments (Figure 10d). The a* || pore axis orientation could be explained by the fact that the preferential growth direction of iPP α crystal is the a*-axis.54 On the other hand, as the b*-axis grows much slower, one could only explain those crystallites are formed with a population of initial nuclei with b* || pore axis orientation. The orientation feature agrees with the previous report on iPP/AAO system, although multiple crystallization peaks were observed in that study.23 The population of the nuclei formed in AAO templates may not be a totally random process for iPP. The surface nucleation has been observed in the PLLA/AAO system.51 The possibility of surface nucleation in iPP/AAO system need to be explored further. The relationship between polymer/AAO interface interaction and surface nucleation is still an open question for future studies.

of the simulation could be found in the Supporting Information. 2D WAXS patterns with a specific orientation of PCL are shown in Figure S6. The relative broad maxima on the meridian of the (110) and (200) reflection of infiltrated PCL in AAO with pore diameter larger than 40 nm agree with the complex orientation with only one constraint; i.e., the c-axis preferentially aligns perpendicular to the pore axis. It is found that the ∼32° off-meridian peak of the (110) reflection comes from those crystallites with b*-axis align parallel to the pore axis (Figure S6b). By comparing the experimental pattern with the simulated patterns, the most likely structure of the infiltrated PCL in 40 and 20 nm is a combination of c ⊥ pore axis and b* || pore axis (Figure 10a). Figure 10b shows the comparison of the azimuthal intensity profiles of the (110) peak, indicating the hypothesis fits experiments well. Our results are different from a previous publication on PCL/AAO system with multiple crystallization peaks,24 where a ⟨110⟩* || pore axis orientation was proposed for slow-cooling PCL in 65 nm AAO. The c ⊥ pore axis orientation in our study follows the criteria of “kinetic selection growth” mechanism proposed by Steinhart.16 A similar low anisotropic feature has been observed in PLLA confined in AAO templates crystallized from the glassy state.51 The b* preferentially orientation may be originated from the highest growth rate of the crystal along b*-axis.52 The b* || pore axis orientation has been observed in PCL crystallized within 13 nm cylindrical domains of PCL-b-PS block copolymers.53 We do not have a good explanation for the G


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4. CONCLUSIONS Crystallization studies within nanoporous alumina require very clean surfaces. Any residual polymer on the surface would significantly influence the nucleation and growth process and may lead to fractionated crystallization and irregular changes of crystallization temperatures. In the present research, with a great effort on surface polishing, a single exothermic peak was observed during cooling from the melt for both PCL and iPP infiltrated within AAO templates in a wide nanopore range (i.e., 400 to 20 nm). The crystallization temperature decreases regularly with decreasing AAO pore size. The lowest Tc achieved was −51.2 °C for PCL and 35.4 °C for iPP, which are close to the lowest values reported in the literature. The packing of PCL chains in AAO templates with the smallest pores was best described as a combination of two type of orientations: c-axis perpendicular to the wall while keeping other axis align arbitrarily and a uniaxial orientation with b* parallel to the pore axis. For iPP, the diffraction pattern was best fitted by a combination of two type uniaxial orientations, i.e., a* or b* aligned along the pore axis. Our study emphasizes that great caution should be taken on sample preparation in using AAO as templates in order to avoid experimental artifacts and obtain reliable results.

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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.7b02284. Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.L.). ORCID

Guoming Liu: 0000-0003-2808-2661 Yunlan Su: 0000-0002-3036-0053 Alejandro J. Müller: 0000-0001-7009-7715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, 21274156 and 51203170). G.L. is grateful for Youth Innovation Promotion Association, CAS (2015026). A.J.M. gratefully acknowledges the fundings received through the Mineco MAT2014-53437-C2-P project and the CAS President's International Fellowship Initiative (PIFI) (No. 2016VMA010). The BSRF is acknowledged for kindly providing the beam time. G.S. thanks Dr. Yu Guan for help with the sample preparation.

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REFERENCES

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