Design of Heterogeneous Nuclei Composed of Uniaxial Cellulose

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Design of Heterogeneous Nuclei Composed of Uniaxial Cellulose Nanocrystal Assemblies for Epitaxial Growth of Poly(ε-caprolactone) Zihao Lu,†,§ Mingfeng Liu,†,§ Qinwei Gao,†,§ Danqin Yang,†,§ Zhisen Zhang,*,†,‡ Xiaopeng Xiong,*,§ Yuan Jiang,*,†,∥ and Xiang Yang Liu⊥,†,‡,∥ †

Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory of Soft Functional Materials Research, Department of Physics, §Department of Materials Science & Engineering, College of Materials, and ∥Department of Biological Materials, College of Materials, Xiamen University, Xiamen 361005, China ⊥ Department of Physics, Faculty of Science, National University of Singapore, 117542 Singapore ‡

S Supporting Information *

ABSTRACT: Epitaxial crystallization is the most prominent approach to achieving oriented thin films composed of semicrystalline polymers (SCPs). Nevertheless, current templates remain limited in fulfilling oriented SCPs with high-throughput coating processes. Herein, we report the first template for the epitaxial crystallization of SCPs based on a uniaxial assembly of shape-anisotropic nanocrystalscellulose nanocrystals (CNCs). The template was fabricated via a dip-coating method, leading to a uniaxial thin coating on both planar and nonplanar substrates. Such a thin coating functioned similarly to a laterally oriented SCP thin film in regulating the crystallization behaviors of poly(ε-caprolactone) (PCL). The orientational relationship between the CNC thin coating and the PCL overlayer was studied systematically by employing multiple characterization tools including scattering, diffraction, and microscopy. Moreover, the epitaxial match based on the crystallography-regulated hydrogen-bonding networks between the two layers was confirmed by molecular modeling, in agreement with the experimental results. Besides the orientational regulation, CNCs also promoted the crystallization kinetics of PCL effectively due to the nanoepitaxial effect provided by each CNC particle. We highlight this facile assembly approach to heterogeneous nuclei for the epitaxial crystallization of SCPs and its extendability of achieving thin coatings composed of 3D-oriented SCPs on ambient substrates.

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INTRODUCTION Thin films based on semicrystalline polymers (SCPs) have become increasingly ubiquitous in modern technologies and industrial applications due to their advantages including low fabrication cost, low density, and extraordinary properties. Moreover, the mechanical and physical properties of SCPs such as stiffness and strength,1 electrical conductivity,2,3 electroluminescence,4 and photoelectrical ones5 can be readily tailored by regulating their multiscale structures inclusive of crystallographic structures and crystal orientations. SCPs without enduring a special treatment nevertheless present spherulitic or liquid crystalline polydomains and therefore exhibit the inferior properties compared to those of their oriented and ordered counterparts. Among existing methods,6−17 epitaxial crystallization is an important and effective method for regulating crystal structures and orientations of SCP materials. A wide variety of epitaxial templates including inorganic facets,18−22 organic single crystals,23−25 and sheared polymer thin films6,7,25−27 have been explored to fulfill the increasing demands of oriented SCP thin films with arguable success. For instance, though facets of inorganic and organic crystals could template SCP thin films with high degrees of crystalline orientation and order, they are stiff substrates with only limited areas for epitaxial crystallization uses. Such stiff substrates are hardly adaptable to be integrated in flexible devices, hampering © XXXX American Chemical Society

the widespread applications of the epitaxial crystallization of SCPs in practice. It would be ideal to explore emerging templates for the scalable epitaxial crystallization of SCPs uniform across the macroscopic distance. Furthermore, it is desirable that the fabrication of these templates is compatible with high-throughput coating processes on flexible planar and nonplanar substrates. The success of achieving these templates will facilitate integration of oriented SCP thin films in functional devices, aiming at achieving excellent macroscopic performance. It is known that well-dispersed nanocrystalline dopants could function as individual heterogeneous nuclei in crystallization of synthetic28 and biogenic29,30 SCPs. An increasingly noticeable candidate is cellulose nanocrystals (CNCs), a kind of nanorods or nanofibrils dependent on their aspect ratios.31,32 For instance, silylated CNCs functioned as nucleation reagents, which could manifestly increase the crystallization kinetics of poly(L-lactic acid) (PLLA) and enhance the mechanical properties as well.28 Besides functioning as individual nanoitems, CNCs in either a concentrated dispersion or a thin film exhibit rich assembly behaviors in the presence of a specific Received: December 23, 2016 Revised: March 5, 2017

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Sample Preparation. Deposition of CNC Thin Films. The detailed CNC fabrication process can be found in our previous work.33 Uniaxial CNC thin films were fabricated by lifting a vertically aligned coverslip or capillary tube upward from a 5.6 wt % CNC aqueous dispersion at a constant rate by an electric motor. The temperature and humidity were 25 ± 1 °C and 40−60%, respectively. For XRD analyses, uniaxial CNC thin films 100 and 300 nm in thickness were prepared by repeating the dip-coating process for three and ten times, respectively. Nematic CNC thin films were deposited by dropping a 20 μL CNC dispersion (1 wt %) onto a glass slide 1.2 cm2 in size followed by a solvent-evaporation process in an ambient condition. The cholesteric CNC was prepared by dropping a 100 μL 8.1 wt % CNC suspension onto a glass slide followed by solvent evaporation. Preparation of SCP Thin Films. All polymers in the current study were dissolved in chloroform for preparation of stock solutions. They were filtrated with a 45 μm filter before use. PCL thin films with various thickness were prepared by spin-coating a volume of 20−100 μL stock solutions (1 wt %) onto a glass substrate. The thickness of all CNC and polymer thin films was measured with AFM. The SCP thin films about 30 nm in thickness were used for AFM and POM analyses. The samples with a thickness of 500 nm were used for SAXS/WAXS detection and the studies of crystallization kinetics. All PCL samples were heated to 90 °C for 10 min to eliminate thermal history and then slowly cooled to RT in an ambient condition before characterization. The PCL thin film on the surface of a glass capillary tube was deposited by lifting the tube from a 2 wt % stock solution. PLLA and PEO thin films were prepared by spin-coating a volume of 10 μL stock solution (0.5 wt %) on a uniaxial CNC substrate followed by an annealing procedure. PLLA thin films were heated up to 185 °C for 10 min and then cooled down to 90 °C in a period of 6 h. The asprepared PEO thin film was heated up to 80 °C for 10 min then cooled down to 30 °C in 6 h. Characterization. (Polarized) optical microscopy images were taken on an Olympus BX53 optical microscope equipped with a charge-coupled device (CCD) camera (Nikon DXM1200). Studies of isothermal PCL crystallization were performed on a Linkam hot stage (LTS120) coupled with the microscope. The topographic information on CNC and SCP thin films was recorded with a Nanoscope multimode V (Bruker Co.) in tapping mode. A commercial silicon cantilever tip was used for scanning. The resonance frequency was about 300 kHz, and the scanning density was 512 × 512 pixels per frame. A scanning electron microscopy (SU-70, Hitachi) was used to scan the edge of a PCL overlayer where the CNC substrate was exposed. The energy of the electron beam was 5 keV. SAXS/WAXS measurements were performed on a Xeuss 2.0 equipped with a GeniX3D X-ray generator. The wavelength used was 0.1542 nm of Cu Kα radiation. The detector was a “Pilatus” Hybrid CMOS Detector, and the accumulation time was 2 h. Prior to characterization, positions of peaks in SAXS and WAXS patterns were carefully calibrated with silver behenate and silicon powders, respectively. XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer with the Cu Kα radiation generated at 30 mA and 40 kV, and the step size was 0.008°. The experimental setups for SAXS/WAXS and XRD measurements are shown in Figure s2a−c. The experimental descriptions of WAXS and XRD are listed below Figure s2. Isothermal crystallization measurements were carried out on an optical microscope (Nikon, Eclipse Ci-L) equipped with an Ocean Optics USB 2000+ spectrometer. A NETZSCH DSC (204F1 Phoenix) was used with the heating rate of 10.0 K/min to obtain the melting thermograms of the PCL thin films peeled from a uniaxial CNC thin coating and a glass slide. The thin films were isothermally crystallized at 45 °C for 24 h before peeling.

external field because of their shape anisotropy and the interfacial distribution of hydroxyl and sulfuric acidic groups.31,32 Our group has developed a reliable dip-coating approach to achieving uniaxial CNC thin coatings at the macroscopic scale by dealing with a concentrated CNC dispersion.33 It is hypothesized that such a uniaxial thin coating carrying regular crystallographic information on its exterior surface could behave in a similar way to the existing templates such as single crystalline facets and sheared polymer thin coatings for epitaxial crystallization uses. Our previous study showed that such a macroscopically sized uniaxial domain could template the oriented crystallization of organic compounds,34 evidencing the correctness of our assumption. Such a CNCbased uniaxial thin film could take advantage of each nanocrystalline template and the collective behaviors of CNCs across the macroscopic distance. In the current study, a uniaxial CNC thin coating was employed to template the epitaxial crystallization of a typical SCP candidatepoly(ε-caprolactone) (PCL). First, a dipcoating approach was introduced to obtain a uniaxial CNC thin film on either a planar glass substrate or a nonplanar one. Afterward, these uniaxial CNC thin films were employed to template the epitaxial crystallization of PCL. The structural information on the PCL overlayer, particularly the orientational relationship between the two layers, was characterized by multiple tools including (polarized) optical microscopy (OM and POM), scanning electron microscopy (SEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), wideangle X-ray scattering (WAXS), and X-ray diffraction (XRD). Interestingly, the orientation of PCL lamellae exhibits a fixed angle with respect to the long axes of CNCs in the layer underneath. Moreover, the molecular modeling revealed epitaxial match between CNCs and PCL. In the third section, the kinetic studies showed that the crystallization rate occurring on a uniaxial CNC thin film was significantly increased compared with that proceeded on a bare glass substrate. This difference in the crystallization rate can be attributed to the synergistic behavior of the nanoepitaxy effect provided by each CNC particle. Finally, the uniaxial CNC thin coatings were employed to template the oriented crystallization of two other typical SCP candidatespoly(ethylene oxide) (PEO) and PLLAconfirming that these emerging coatings based on nanoparticle assembly could be general templates for the oriented crystallization of multiple SCPs. To our understanding, the uniaxial CNC thin coating is the first heterogeneous nuclei of SCP crystallization based on assembly of shape-anisotropic nanoparticles. Compared with existing templates such as inorganic and organic crystalline facets, the emerging heterogeneous nuclei could be fabricated on both planar and nonplanar substrates via a simple dip-coating process, widening the application of oriented SCP thin films in integration of functional devices.

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EXPERIMENTAL SECTION

Materials. Chloroform and concentrated H2SO4 (96−98 wt %) were from Xilong Chemical. Ethanol absolute (EtOH) was from Huada Chemical. PCL (Mw = 200 000) was provided by Ji’nan Daigang Biomaterial. PLLA (viscosity-average molecular weight, Mv = 80 000) were kindly provided by Changchun SinoBiomaterials. PEO (Mv = 600 000) was from Xilong Chemical. All chemical reagents in this study were of analytical grade and used without any further purification unless otherwise stated. Deionized water (18 mΩ·cm−1) was used throughout the work. Glass slides and capillary tubes (500 μm o.d.) were cleaned with acetone and piranha solution before use.

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RESULTS AND DISCUSSION

The fabrication of an oriented PCL thin film on a planar glass substrate contained three steps. First, a uniaxial CNC thin coating was fabricated via a dip-coating process, according to our previous studies.38,39 Afterward, a PCL thin coating was B

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Figure 1. (a−c) Rotational POM (a, b) and AFM (c, the amplitude error mode) images of typical uniaxial CNC thin films. The inset in image c is the corresponding FFT image. The distribution of the average FFT amplitude along the azimuthal angle, Φ, indicates that the CNCs are aligned along the dip-coating direction. (d) XRD patterns of two uniaxial CNC film different in thickness (top red and middle green lines) and a cholesteric CNC thin coating (bottom black line). The XRD pattern of a cholesteric coating was used as a reference of the standard pattern. The white arrows in images a−c point to the lifting direction in the dip-coating process, and the white crossed ones in images a, b show the polarization direction.

orientational factor. As a comparison, a spontaneous solvent evaporation process in the absence of shearing force led to a CNC thin film composed of nematic polydomains (Figure s1a), consistent with previous studies.33,38 The quantitative analysis of the orientational order of CNCs in a uniaxial layer was conducted based on fast Fourier transformation (FFT) imaging.39 The average orientation angle, defined as ⟨cos2 Φ⟩, was calculated according to the following formula:

deposited on the CNC layer by using a spin-coating process. The above two-layered samples endured an annealing process to demolish structural defects caused by fast solvent evaporation, and hence, the oriented PCL thin films were obtained. In fabrication of a PCL thin film on a capillary glass tube, the dip-coating process was employed in replacement of the spin-coating process. Fabrication of Uniaxial CNC Thin Coatings. A uniaxial CNC thin coating could be obtained with the dip-coating method under proper experimental conditions introduced in our previous study.33,34 A pair of rotational POM images illustrate the uniform birefringent contrast under the polarizer, indicating that a uniaxial CNC domain was obtained continuously in an area as large as several square millimeters (Figure 1a,b). A localized AFM image provides the detailed structural information on this uniaxial layer, where the long axes of CNCs are roughly along the lifting direction (Figure 1c). A uniaxial CNC layer was obtained in the presence of the shearing gradient spontaneously created at the meniscus during a dip-coating process. When the glass substrate was lifted upward, continuous solvent evaporation caused the increase of the concentration of the CNC dispersion confined in the meniscus line. It is deemed that the localized CNCs tended to form a thixotropic hydrogel reversibly because such 1D nanoparticles with strong interparticulate interactions could possess very low percolation threshold volume fractions.35−37 The presence of continuous shearing force at the meniscus line hypothetically dissociated the thixotropic hydrogel to generate the uniaxial alignment of CNCs with their long axes along the lifting direction. The thorough evaporation of the remaining solvent finally stopped densely packed CNCs from further movement, leading to a CNC thin layer with a high

π /2

2

⟨cos Φ⟩ =

∫−π /2 ⟨cos2 Φ⟩I(Φ) dΦ π /2

∫−π /2 I(Φ) dΦ

(1)

where I(Φ) is the average intensity at a given azimuthal angle in an FFT image (based on the inserted image in Figure 1c), pointing to the maximum FFT amplitude of the halo. The FFT image displays a dumbbell-like pattern instead of a typical arclike one due to the polydispersity of CNCs in width and the inevitable appearance of structural defects. Hence, I(Φ) instead of the maximum intensity was used to calculate ⟨cos2 Φ⟩. The in-plane orientation factor f, defined as f = 2⟨cos2 Φ⟩−1

(2)

was at a value of 0.84 ± 0.04, where the values of 0.00 and 1.00 indicate the complete disorder and unidirectional orientation, respectively. In addition, Figure 1d depicts the XRD patterns of uniaxial CNC thin films different in thickness, together with that of a cholesteric one. All patterns show three typical diffraction peaks of cellulose Iβ,31 namely (1−10), (110), and (200). The pattern of the cholesteric sample is characteristic of the C

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Figure 2. (a−c) Rotational POM (a, b) and AFM (c, the amplitude error mode) images of an oriented PCL thin film crystallized on a uniaxial CNC substrate. The inset in image c shows the derived FFT pattern. (d) SEM image taken at the edge area of an oriented PCL overlayer, where the underneath CNC thin coating was visualized from the inset image. (e) 2D-SAXS pattern of an oriented PCL thin film, which was viewed from the direction normal to the film plane. The sample was peeled from CNC substrate prior to characterization. (f) Corresponding intensity plotted against the azimuthal angle Φ, according to the pattern e. The white crossed arrows in images a, b show the polarization direction. The dashed arrows in images a−e indicate the lifting direction of the CNC thin films, and the solid ones in images a−d show the orientation of the PCL overlayer.

indicating the formation of an oriented PCL thin film macroscopically (Figure 2a,b). The similarity in the oriented birefringent contrast of the CNC and PCL thin coatings confirms that the CNC layer underneath successfully templated the in situ oriented growth of the PCL overlayer. The birefringent pattern is reminiscent of those obtained on single crystals of inorganic and organic compounds and oriented SCPs.6,25 Next, AFM and SEM images provide the microstructural information on the PCL overlayer, showing uniaxial, edge-on lamellar superstructures (Figure 2c,d). After the structural information on the PCL overlayer was characterized with the microscopic tools, the orientational distribution of the lamellar superstructures was calculated by using both FFT imaging and SAXS. Derived from an FFT image (based on Figure 2c), the f value of an oriented PCL layer was 0.87 ± 0.02. Moreover, the f value of the same thin coating was 0.72 ± 0.06, which was calculated from a 2D SAXS pattern (Figure 2f) based on eq 1. When calculating the f value of the oriented PCL sample, the lower and upper limits of integration were adjusted according to the tilted direction. Though the lamellar architectures of PCL in both AFM and SEM images (Figure 2c,d) are not as well organized compared with those grown on single crystalline facets, p-terphenyl for example,25 they look very similar to those deposited on a rubbed polyethylene (PE) thin film in terms of the lamellar alignment. Moreover, the f

dominant (200) peak, in accordance with previous studies.40 As a comparison, the intensities of the (1−10) peak in the XRD patterns of both uniaxial CNC thin films are manifestly increased. In the XRD pattern of the sample 100 nm in thickness, the intensity of the (1−10) peak becomes almost as strong as that of the (200) one (Figure 1d, top). It is deducible that in uniaxial CNC thin films the decrease in thickness leads to the relative increase of the (1−10) peak over others, as the corresponding facet lying preferentially parallel to the substrate starts to be visualized.41,42 This is probably because increasing the thickness of the uniaxial CNC thin coating requires the increment of dip-coating times, resulting in the appearance of cholesteric polydomains and other structural defects. Hence, it is deduced that in a single layer of uniaxial CNC thin film CNCs preferably assemble with their (1−10) facet aligning in parallel to the receiver substrate.31,43,44 This piece of information will be useful for understanding the orientational relationships between the CNC and PCL thin layers, which will be introduced shortly by employing the (1−10) face of cellulose in molecular modeling to study the PCL adsorption behavior. Deposition of Oriented PCL Thin Films. The uniaxial CNC thin coating was next employed to template the oriented crystallization of PCL thin films. The uniform birefringent contrast in a pair of rotational POM images is manifested, D

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Figure 3. (a, b) Rotational POM images of a glass capillary (the upper one), a uniaxial CNC thin film deposited on a same capillary (the lower one). (c, d) Rotational POM images of an oriented PCL crystallized onto the uniaxial CNC substrate in images a, b. The white arrows in a−d indicate the capillary direction, and the white crossed ones in images a, c show the polarization direction.

shown by intersected dashed and solid arrows). Two independent tools, namely SEM and SAXS, were employed to address the orientational relationship between the PCL lamellae in the overlayer and the long axes of CNCs underneath quantitatively. The deliberate introduction of an interface between the CNC layer underneath and the PCL overlayer provided a direct evidence of the relative angle between the two layers (Figure 2d). This value of the angle around 12° is in agreement with those depicted in POM and AFM images (Figure 2a−c). A 2D-SAXS pattern of a PCL overlayer peeled off from the CNC template displays the orientational preference of lamellar superstructures, as indicated by the presence of a pair of symmetric luminous arcs (Figure 2e). An integral 1D-SAXS plot of the same data shows the maximum intensity at q = 0.37 nm−1, corresponding to the long period of the PCL lamellae about 16.7 nm (Figure s2d). The calculation from the azimuthal integration at q = 0.37 nm−1 (Figure 2f) confirms that the preferential orientation of the PCL lamellae has a fixed angle of 12 ± 3° relative to the lifting direction in a dip-coating process (i.e., the long axes of CNCs in the uniaxial template). In short, multiple tools, namely POM, AFM, SEM, and SAXS, unambiguously evidenced the presence of an angle between the PCL lamellae in the overlayer and the long axes of CNCs in the uniaxial template underneath. Assuming that each CNC particle functioned as a nanoepitaxial template, a uniaxial CNC thin film could regulate the crystallization behaviors of a PCL thin film in a similar way to an oriented SCP substrate. Next, the crystallographic match between the PCL and CNC layers will be discussed to understand the orientational relationship in between. Three major reflections appear in the 2D-WAXS pattern, namely two adjacent inner arcs and a weak outer ring (Figure 4a). The corresponding 1D-WAXS plot shows that the three reflections are at 2θ = 21.4°, 22.0°, and 23.8° (Figure s2e), distinctive of the (110), (111), and (200) facets of a PCL single crystal, respectively. The edge-on lamellar structures (Figure 2c,d) indicate that the c-axis (i.e., the direction of PCL chains) is parallel to the underneath CNC substrate. The (200) facet is

value between 0.7 and 0.9 revealed by quantitative analyses also indicates that the order degree of the current PCL thin film is comparable to that grown on an oriented SCP substrate.26 As a comparison, a nematic CNC thin film led to the typical Schlieren texture45 in the PCL overlayer, while direct crystallization of PCL on a bare glass caused spherulitic polydomains (Figure s1b,c). It is interesting that CNCsa kind of nanorod exhibiting the inherent crystallographic information on cellulosecould template the oriented crystallization of a typical SCP candidate, PCL. This is the first study of the oriented crystallization of SCPs templated by an oriented assembly of shape-anisotropic nanocrystals, though individual nanoparticles have been used extensively as heterogeneous nuclei in previous studies of crystallization of SCPs.28 The success can be convincingly attributed to the exposed crystallographic information inherited in each CNC particle31 and the controllable macroscopic assembly of CNCs under shear force. As a comparison, traditional templates for the oriented crystallization of SCPs rely on fabrication of large single crystals or engineering of a SCP candidate from its melted or solution phase. A dip-coating approach to a continuous uniaxial CNC thin coating could proceed on the exterior surface of a capillary glass tube, extending epitaxial crystallization of SCP thin coatings on curved substrates. Unlike stiff crystalline epitaxial templates, CNCs with hundreds of nanometers in length and several nanometers in thickness could adapt to the curvature of a glass capillary and hence align uniaxially along the long axis of the capillary glass tube (Figure 3a,b). This thin CNC coating could be employed for deposition of an oriented PCL thin film on a curved substrate (Figure 3c,d). As a comparison, typical spherulitic polydomains could be observed when the substrate was a bare capillary tube (Figure s3a,b). Orientational and Crystallographic Analyses. Interestingly, the POM and AFM images of oriented PCL thin films evidenced that there is a fixed angle between the long axes of CNCs in the uniaxial template underneath and the elongation direction of the lamellae in the PCL overlayer (Figure 2a−c, E

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in the overlayer points to (010) and (100) planes along the lateral and vertical directions, respectively, as confirmed by the edge-on superstructures observed in both AFM and SEM images (Figure 2c,d). Hence, a model was established to demonstrate the crystallographic relationship of the PCL lamellae in the overlayer and the uniaxial CNC template underneath (Figure 4d). Meanwhile, the relatively weak diffraction of the (200) facet suggests that the a-axis of PCL crystals could also be parallel to the substrate. The appearance of the PCL (200) facet can be attributed to the inevitable presence of flat-on structural defects (Figure s4d), which might be induced by the exposed crystal facets other than (1−10) in the CNC substrate. Furthermore, both SAXS and WAXS patterns (Figures 2e and 4a) confirm the presence of a tail in each luminous arc, which is positioned on the clockwise side of each main reflection arc. This intriguing phenomenon shows that there is a divergence in the PCL lamellae on the left-handed side of the dominant orientation. This might be induced by the intrinsic chirality of CNC nanorods or the residual cholesteric phases.46,47 Representative structural defects in a PCL overlayer are depicted in Figure s4a−c. Additionally, the nanoconfinement effect11,15 was also excluded in induction of the oriented growth of a PCL thin film because the surface roughness of a uniaxial CNC template is equivalent to that of a glass substrate (Figure s5a,b). Subsequently, molecular dynamic simulations were conducted to illuminate the crystallization behavior of PCL on the uniaxial CNC thin film, especially the peculiar angle divergence between the two crystal orientations. From our results (XRD profiles in Figure 1d) and previous reports,31,43,44 the (1−10) surface of cellulose crystals is regarded as the most favorably exposed face in the experiments of this work. Thus, molecular dynamic simulations were conducted to explore the oriented crystallization mechanism of PCL on the (1−10) surface of cellulose crystals (see Supporting Information for the simulation details). The radial density distribution function profiles are extracted (Figure 5b): cellulose and PCL (black line, CEL_PCL), cellulose and O atoms of PCL (blue line, CEL_O(PCL)), and the surface hydroxyl group of cellulose and O atoms of PCL (red line, OH(CEL)_O(PCL)). The first

Figure 4. (a) Upper half of a 2D-WAXS pattern of an oriented PCL thin film viewed from the direction normal to the film plane. The sample was peeled from CNC substrate prior to characterization. (b) Intensity of the (200) reflection plotted against the azimuthal angle Φ. (c) Schematic diagram combining 2D-SAXS (Figure 2e) and WAXS information (a). (d) Schematic illustration showing the crystal orientation of the PCL lamellae on a uniaxial CNC substrate. The angle between the b-axis of PCL crystals (blue dashed line) and the long axis of the CNC nanorods (namely c-axis, red solid line) is about 12°. The dashed arrow in image (a) indicates the lifting direction of the uniaxial CNC thin film.

normal to the incident beam, which causes the minimal scattering at 23.8° (Figure 4a). Meanwhile, the same facet shows an intensive peak in the XRD pattern (Figure s2f), which confirms the preferential orientation of the a-axis of PCL crystals along the vertical direction of the substrate. The strong reflection arc showing the dominance of the (110) peak in an oriented PCL thin film suggests that the lateral growth is along the b-axis of PCL crystals. The azimuthal integral of the (110) reflection further shows the intersection angle between the baxis of PCL crystals (i.e., the preferential growth direction of PCL in the oriented overlayer) and the long axes of CNC nanorods is about 15° (Figure 4b). This result is in good agreement with the aforementioned findings based on multiple microscopic tools. The overall 2D-SAXS/WAXS pattern is illustrated in Figure 4c, highlighting strong reflections with intensive arcs. The schematic pattern unambiguously reveals the 3D-oriented crystallographic information on an oriented PCL overlayer on a uniaxial CNC template. Each PCL lamella

Figure 5. Results of molecular dynamic simulation: (a) Lattice match between a PCL molecule and the (1−10) facet of a cellulose crystal. The main chain of PCL is shown in the white Licorice mode with the O atoms being highlighted in the green VDW mode. For cellulose, the main chain is shown in the dark gray licorice mode, and the surface O atoms are shown in the red CPK mode. (b) The top, middle, and bottom rows are radial density distribution function (rdf) between cellulose and PCL (black), cellulose and the O atoms of PCL (blue), the surface hydroxyl group of cellulose and the O atoms of PCL (red), respectively. F

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Figure 6. (a−h) A series of in situ POM images showing the nucleation and growth of the PCL overlayers isothermally crystallized at 45 °C occurring on a uniaxial CNC substrate (a−d) and on a glass slide (e−h). (i, j) (1 − χt) of the PCL overlayer on a uniaxial CNC substrate (open circles) and on a glass slide (closed squares), plotted against time (i) and the corresponding Avrami analysis (j). The white arrow in image a points to the lateral orientation of the PCL overlayer, and the white crossed ones in images a and e show the polarization direction. The red lines in image j are best linear least-squares fit.

peak in the rdf(CEL_PCL) profile shows up at ∼0.17 nm. Combining with the information on rdf(CEL_O(PCL)) and rdf(OH(CEL)_O(PCL)), it could be concluded that the peak at the vertical dash purple line indicates the H···O distance in hydrogen bonds, the second peak (∼0.26 nm) in rdf(CEL_O(PCL)), and the rdf(OH(CEL)_O(PCL)) profiles corresponding to the O···O distance in hydrogen bonds. The above information suggests that hydrogen bonding is the major interactions between PCL molecules and the (1−10) surface of cellulose crystals (also refer to Figure s6b for the hydrogenbonding interactions). The additional analysis reveals good lattice match between PCL molecules and the (1−10) surface of cellulose crystals (Figure 5a). For visualization, the lattice match between the oxygen atoms in the hydroxyl groups of the (1−10) surface of cellulose crystals and the PCL crystal is shown in the red VDW mode. The spacing of 1.71 nm is very close to the c-axis lattice in the PCL crystal, which could result in the oriented adsorption of PCL molecules along the same lattice. The angle between the direction marked with the dashed yellow line and the c-axis of cellulose crystals (solid yellow line) is 79.96°. In other words, the molecular simulation results suggest an angle about 10.0° between the b-axis of the PCL crystal and the c-axis of the cellulose one. Thus, it is the hydrogen bonding that guides the oriented adsorption of PCL molecules on the (1−10) surface of cellulose crystals. Isothermal Crystallization Kinetics. Epitaxial growth of PCL on a uniaxial CNC thin film not only led to an oriented thin film but also accelerated the crystallization kinetics effectively. Theoretically, the large number of heterogeneous nuclei in the CNC template could highly increase the

crystallization rate of PCL by shortening the traveling distance of each growing PCL crystalline domain. Ideally, nucleation of each PCL lamella in the oriented overlayer could be an independent affair, irrelevant of the adjacent ones. POM was hence employed to evidence the kinetics of a pair of isothermal crystallization processes of a melted PCL layer on a uniaxial CNC template (Figure 6a−d) and on a glass substrate (Figure 6e−h). The crystallization kinetics in the PCL overlayer was recorded by the appearance and strengthening of the birefringent contrast under polarizers. The crystallization was characteristic of the simultaneous appearance of the large number density of crystal nuclei, each growing in the moderate distance to form a continuous crystalline PCL overlayer (Figure 6a−d). As a comparison, the low number density of heterogeneous nuclei led to sporadic spherulitic polydomains, which extended across tens of micrometers to meet with adjacent ones (Figure 6e−h). Furthermore, the light depolarization technique was applied to quantify the kinetic difference between the PCL crystallization processes on a uniaxial CNC template and on a glass slide, respectively, shedding light on the mechanistic understanding of the SCP crystallization.48,49 The measurement of the intensity of the transmitted light with time was translated to the relative crystallinity at a given time (χt) of the PCL overlayer. In the light depolarization technique, χt is expressed as

χt = G

I∞ − It I∞ − I0

(3) DOI: 10.1021/acs.macromol.6b02775 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules where I∞ and I0 represent the intensity of the final and the initial stages, respectively, and It denotes the light intensity at the time t.50 Figure 6i depicts the (1 − χt) value as a function of time, highlighting the substrate effect in the PCL crystallization. The half-crystallization time t1/2 obtained from Figure 6i on the uniaxial CNC template and on the glass substrate was 4.4 and 19.2 min, respectively. This kinetic difference clearly indicates that the crystallization process occurring on the CNC template was faster than that on the glass substrate. Though the crystallization rates differ clearly, the crystallinities of the PCL thin films obtained on the above CNC and glass substrates were 62.9 and 62.5 wt %, respectively, according to the heat of fusion based on the DSC thermograms (Figure s7; the calculation was based on the enthalpy of fusion (135.44 J/g) for the fully crystallized PCL).51,52 The increase of the crystallization rate and the unchanged crystallinity of PCL on a CNC template plausibly exclude the involvement of the nanoconfinement effect, which otherwise causes the dramatic decrease both in crystallization rate and in the crystallinity of PCL.13,14 The Avrami plots of both crystallization case studies can be fitted into the linearized form of the Avrami equation log[− ln(1 − χt )] = log(K ) + n log(t )

substrates at the macroscopic scale. Microscopically, the AFM images (Figure 7b,d) show that the lamellae of both polymer candidates form edge-on structures, aligning approximately parallel to the long axes of CNC nanorods. It is noted that the thickness of PLLA and PEO films needs to be as low as 20 nm to inhibit the spherulitic crystallization occurring in the bulk phase. This thickness issue can be attributed to the strong intermolecular interactions between polymer chains of each kind, which deteriorate their interactions with the CNC substrate underneath. In short, the uniaxial CNC thin coating could template the oriented crystallization of PEO and PLLA via hydrogen-bonding interactions. More details will be provided in subsequent separate studies.

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CONCLUSIONS This study reports the design of a new type of epitaxial templates for SCP crystallization based on a nanoparticle assembly route. CNCsa kind of shape-anisotropic nanocrystalswere fabricated as a uniaxial thin coating via a dip-coating process. Such a uniaxial thin coating with the statistic crystallographic information functioned similarly to existing oriented polymer templates in induction of the epitaxial growth of PCL and another two SCP candidates, where the analyses of orientational relationships between the CNC template and PCL were highlighted. Such a template based on cellulose would be a particularly interesting template for the epitaxial crystallization of SCPs via hydrogen-bonding interactions. Compared with the existing templates, the current one based on the assembly of polymer nanoparticles is unique for its availability of deposition on nonplanar/flexible substrates and the potential applications in the high-efficiency industrial production. We therefore deem that such a controllable, solution-route approach to epitaxial templates on planar, nonplanar, and flexible substrates could be readily extended to the integration of functional SCPs into flexible nonplanar devices, where the oriented growth of SCPs is key to their performance.

(4)

where K and n are the temperature-dependent crystallization rate constant and the Avrami index related to the dimension of crystal growth, respectively. The n values were calculated from the curves in Figure 6j. For crystallization occurring on a glass slide, the n value is 2.8, indicating the 3D crystal growth. Meanwhile, the PCL thin film crystallized onto a uniaxial CNC substrate has an n value of 2.4, indicative of the approximate 2D crystal growth.13 Deposition of Other SCPs. The oriented CNC template was successful in regulating the oriented crystallization of another two typical SCP candidates: PEO and PLLA. Both candidates could interact with the CNC template through hydrogen-bonding interactions53−57 in a similar way as PCL. The POM images (Figure 7a,c) clearly show that both PLLA and PEO can form oriented thin films on the uniaxial CNC

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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.6b02775. Simulation details, the supporting POM images of CNC and PCL thin films, the experimental setups, details, and 1D profiles of SAXS, WAXS, and XRD detections of an oriented PCL thin film peeled from the CNC substrate; AFM images showing structure defects in an oriented PCL thin film; AFM images of the surface of a uniaxial CNC substrate and a glass slide; DSC thermograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (X.X.). *E-mail: [email protected] (Y.J.).

Figure 7. Rotational POM images of PLLA (a) and a PEO (c) thin films crystallized on uniaxial CNC substrates. Zoom-in AFM images (the amplitude error mode) of oriented PLLA (b) and PEO (d) thin films correspond to images a and c, respectively. The dashed arrows in images a−d point to the lifting direction in the dip-coating process, while the crossed solid ones show the polarization direction.

ORCID

Yuan Jiang: 0000-0002-1669-8023 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.macromol.6b02775 Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This research work was supported by National Natural Science Foundation of China (No. 21303144 and No. 51273166), Natural Science Foundation of Fujian Province, China (No. 2014J01207 and No. 2013J01206), China Postdoctoral Science Foundation (No. 2016M592090), the “111” Project (B16029), National Natural Science Foundation of China (No. U1405226), Fujian Provincial Department of Science & Technology (2014H6022), and the 1000 Talents Program from Xiamen University. Special thanks are given to Mr. Ming Li and Ms. Wei Zheng for taking SEM and the DSC analyses, respectively.

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