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Straight and Rod-like Core-Sheath Crystals of Solution-Crystallized Poly(#-caprolactone)/Multi-Walled Carbon Nanotube Nanocomposites Bing Zhou, Jun-Huan Li, Bin Fan, Ping Li, Jun-Ting Xu, and Zhiqiang Fan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00915 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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Crystal Growth & Design

Straight and Rod-like Core-Sheath Crystals of Solution-Crystallized Poly(εε-caprolactone)/Multi-Walled

Carbon

Nanotube

Nanocomposites

Bing Zhou, Jun-Huan Li, Bin Fan, Ping Li, Jun-Ting Xu,* and Zhi-Qiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

____________________________________________________________________ *Corresponding author. E-mail: [email protected]

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ABSTRACT The crystal morphology of poly(ε-caprolactone)/multi-walled carbon nanotube (PCL/MWCNT) blends and MWCNT-g-PCL grafting polymers crystallized in n-hexanol was investigated. Two typical morphologies are observed: straight and rod-like core-sheath structure with embedded MWCNTs as core and PCL poly-crystals of high crystallinity as sheath, and bent double-layer structure with MWCNTs covered by a PCL layer of low crystallinity. It is found that, thinner (outer diameter < 15 nm) and shorter (length < 2 µm) MWCNTs are easier to be straightened by PCL crystals, and the grafted PCL chains have weaker crystallizability due to structural confinement and thus weaker ability of straightening MWCNTs. Electron diffraction (ED) and high-resolution transmission electron microscopy (HR-TEM) reveal that the PCL crystals are randomly orientated with b-axis perpendicular to the MWCNT surface. The growth direction of the PCL crystals is not perpendicular to the axis of MWCNT, possibly due to the nucleation effect of the pre-adsorbed PCL chains in the solution, which helically wrap MWCNTs. This leads to wrapping and straightening of MWCNTs by rigid PCL crystals.

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INTRODUCTION Carbon

nanotubes

(CNTs)

have

tremendous

potential

applications

in

nano-engineering and nanotechnology due to their one-dimensional (1D) tubular structure with unique electronic, mechanical, thermal and chemical properties.1-3 However, due to the high aspect ratio, CNTs usually exhibit curled and entangled morphology, which is unfavorable to the electronic and mechanical properties.4-5 CNTs, including single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs), are frequently blended with polymers to prepare polymer nanocomposites with good mechanical and conductive properties.6-10 When the polymer is crystalline, CNTs have a great influence on polymer crystallization. Usually CNTs can act as nucleating agent for polymer crystallization,11 leading to spontaneous formation of a large number of heterogeneous nuclei, thus faster crystallization rate and smaller crystal size. The crystal morphology and orientation may also be greatly altered in the presence of CNTs. Due to dense nucleation on the surface of CNTs, transcrystallites are frequently observed in the bulk of polymer/CNT nanocomposites, in which the growth direction of the polymer lamellar crystals is perpendicular to the axis of CNTs and the polymer chains are either perpendicular or parallel to the axis of CNTs (Figure S1 in supporting information).12-17 Moreover, the so-called “nanohybrid shish kebab” (NHSK) structure was first observed in the polyethylene (PE)/CNTs blend by Li et al. and then by others (Figure S1 in supporting information).18-26 Such a structure was also prepared in many other polymer/CNT blends, including polytetrafluoroethylene (PTFE),27 poly(vinylidene fluoride) (PVDF),28 nylon,29-30 poly(butylene terephthalate) 3



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(PBT),31 and poly(vinyl alcohol) (PVA).32 Poly(3-hexyl thiophene) (P3HT)/CNT blend can form a centipede structure,33-35 which can also be viewed as a kind of NHSK. In the NHSK structure, the polymer chains are parallel to the axis of CNTs but the polymer lamellar crystals grow around and perpendicular to the axis of CNTs. Li et al. proposed a “soft epitaxy” mechanism for the formation of NHSK structure, in which the polymer chains are parallel to the axis of CNT without the need of crystallographic matching between the polymer crystals and CNTs.36 The crystal morphology in polymer/CNT composites is determined by many factors, such as polymer chain conformation, crystallization condition, CNT type and diameter, etc. It is found that the polymers that can form NHSK structure with MWCNT usually have planar zigzag chain conformation in the crystal. There are several examples dealing with the morphology of crystalline polymers with helical conformation in the crystal induced by CNTs in solution crystallization. For instance, Xu et al. found that poly(ethylene oxide) (PEO) crystal beads grew on the SWCNT surface in a helical way or PEO just covered SWCNT in amorphous state.37-38 Recently Jandt et al in-situ created the NHSK structure in poly(ε-caprolactone) (PCL)/MWCNT during electrospinning.39 Fu et al. also prepared the NHSK structure in the nanocomposites of SWCNT with isotactic polypropylene (iPP) and poly(L-lactic acid) (PLLA),40 which exhibit a helical conformation in the crystals. Nevertheless, the formation of NHSK structure is believed to be induced by the grooves among the aggregated SWCNTs,40-41 which is different from that in the nanocomposites of MWCNT with the polymers having zigzag conformation. 4


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Krishnamoorti et al. also prepared PCL lamellae vertically perforated by multiple SWCNTs.42 The diameter of CNTs affects the polymer crystal morphology as well. When the diameter of the CNT is increased to 100-300 nm, the geometric confinement effect may be weakened and the PE crystals will lose their uniform orientation on the CNT surface.43 So far, most of the crystalline polymers used for solution crystallization of polymer/MWCNT nanocomposites have planar zigzag conformation in the crystals, while the polymers with helical conformation are rarely reported. Moreover, in both transcrystallite and NHSK structure the growth direction of the lamellar crystals is perpendicular to the axis of MWCNT. If the growth direction of the polymer crystals is not perpendicular to the axis of MWCNT, it is expected that the polymer crystals will grow on the MWCNT surface and the MWCNTs may be covered by a layer of polymer crystals. Such crystal morphology would be interesting. On the one hand, since MWCNTs are tightly wrapped by the polymer crystals, the electronic properties of WMCNTs are greatly affected by polymer crystals. This is especially useful when polymer is conductive. On the other hand, because the polymer crystals are usually rigid, they may straighten the MWCNTs. This will further facilitate orientation of MWNCTs, which is beneficial to mechanical and photo-electronic properties. Therefore, herein we studied solution crystallization of PCL, which has helical conformation in the crystal, in the presence of MWCNTs.

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EXPERIMENTAL SECTION Materials. SWCNT and three types of MWCNT with different lengths and outer diameters (ODs) were used to study the effect of CNT structure on PCL crystallization. SWCNT and MWCNTs were purchased from Chengdu Organic Chemistry Co. Ltd., Chinese Academy of Sciences, with purity of > 90%. Figure 1 shows the TEM images of pristine SWCNT and MWCNTs. The data of length and OD are given in Table 1. SWCNTs have a smaller OD (1-2 nm) but a larger length (20-30 µm). However, SWCNTs tend to aggregate to form bundles. MWCNTs-1 and MWCNT-2 are shorter (length: 0.5-2 µm). The ODs of MWCNT-1 and MWCNT-2 are 3-8 nm and 8-15 nm, respectively. MWCNT-3 is much longer (20-30 µm) and also has a larger OD (20-30 nm). Due to the large dimension in length, severe entanglement occurs in MWCNT-3. ε-CL (ACROS) and stannous octoate (Aldrich) were used as received. Cyclopentadienyltitanium trichloride (CpTiCl3) was purchased from Alfa Aesar and used as received. 4-Bromobenzocyclobutene (BCB-Br, 97%) was kindly donated by Dow Chemical (USA) and used as received. All the other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. Toluene and tetrahydrofuran (THF) were refluxed over Na-benzophenone under N2 for 24 h, then distilled prior to use. The poly(ε-caprolactone) (PCL) homopolymer was synthesized using n-butanol as the initiator and stannous octoate as the catalyst.44 The number-average molecular weight Mn of PCL is 5220 g/mol, and the molecular weight distribution (Mw/Mn) is 1.22. The values of Mn and Mw/Mn of polyethylene (PE) used in the present work are 1.26×104 6


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g/mol and 2.17, respectively.

Table 1. Length and outer diameter (OD) of MWCNTs. CNTs

Length (µm)

OD (nm)

SWCNT

20-30

1-2

MWCNT-1

0.5-2.0

3-8

MWCNT-2

0.5-2.0

8-15

MWCNT-3

20-30

20-30

Figure 1. TEM images of pristine SWCNT (a), MWCNT-1 (b), MWCNT-2 (c), and MWCNT-3 (d).

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Synthesis of PCLs grafted on MWCNT (MWCNT-g-PCLs). In order to compare with the PCL/MWCNT blends, the grafting polymers, i.e. PCLs grafted on different types of MWCNT, were also used for study. The details for synthesis of MWCNT-g-PCLs were described in our previous work.45 The brief procedure was as follow: 2-Hydroxyethyl benzocyclobutene (BCB-EO) was first grafted on MWCNTs via

a

[2+4]

cyclo-addition

reaction

to

yield

MWCNT-BCBEO,

then

MWCNT-BCBEO was reacted with CpTiCl3, and finally the reactant was used to catalyze the polymerization of ε-caprolactone to obtain MWCNT-g-PCL. The grafting degree (WPCL), which is defined as the weight percentage of PCL in MWCNT-g-PCLs, and the number-average molecular weight (Mn) of PCL in MWCNT-g-PCLs are given in Table 2.

Table 2. The grafting degree (WPCL) and number-average molecular weight (Mn) of PCL in MWCNT-g-PCLs. a WPCL Sample

Mn of PCL (%)

a

MWCNT-1-g-PCL

93.4

∼3140

MWCNT-2-g-PCL

96.0

∼5180

MWCNT-3-g-PCL

98.0

∼3550

Calculation of WPCL and Mn was based on the method described in our previous

work.45

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Solution Crystallization of PE/MWCNT Blends. Since PE exhibits zigzag planar conformation in crystal, which is different from the helical conformation of PCL, solution crystallization of PE/MWCNT blends was also studied to reveal the effect of polymer conformation on crystal morphology. Typically, 0.5 mg PE was dissolved in 4 mL o-dichlorobenzene, and the solution was kept in an oil bath at 150 °C for 30 min. 0.5 mg MWCNTs were dispersed in 1 mL o-dichlorobenzene in a test tube, and then submerged in an ultrasonic bath for 2 h to gain a uniform solution. Then the two solutions were mixed together vigorous stirring for 10 min at 150 °C. The test tube was transferred to another oil bath at 120 °C as soon as possible, and held for 3 h to complete crystallization. Solution Crystallization of PCL/MWCNT and PCL/SWCNT Blends. Typically, 0.5 mg MWCNTs (or SWCNTs) were dispersed in 1 mL n-hexanol in a test tube, and then submerged in an ultrasonic bath for 2 h to gain a uniform solution. The sonicated CNTs can be well dispersed in n-hexanol at least for several hours. 0.5 mg PCL was dissolved in 4 mL n-hexanol, and the solution was kept in an oil bath at 80 °C for 30 min to ensure dissolution of PCL. Then the two solutions were mixed under vigorous stirring for 10 min at 80 °C. The test tube was transferred to another oil bath at 40 °C as soon as possible, and held for 24 h to conduct isothermal crystallization of PCL. Growth of PCL crystals on CNTs surface can further enhance the dispersion stability of CNTs in n-hexanol, thus no precipitation occurs in several days. However, after one week, some PCL/CNTs composites of large size will precipitate from the solution. 9



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Solution crystallization of MWCNT-g-PCLs. In a typical experiment, 0.5 mg MWCNT-g-PCL was dispersed in 5 mL n-hexanol in a 25 mL test tube, and then the system was kept in an oil bath at 80 °C for 30 min under vigorous stirring to erase any thermal history. The test tube was then transferred to another oil bath at 40 °C as soon as possible, and held for 24 h to perform crystallization of the grafting polymers. When using THF as the solvent, 0.5 mg MWCNTs-g-PCL was dissolved into 5 mL THF to get a uniform solution with no further treatment. Transmission Electron Microscopy: The macroscopic morphology of PCL and PE crystals formed in the presence of CNTs was characterized by transmission electron microscopy (TEM). TEM observations were performed on a JEOL JEM-1230 electron microscope at an acceleration voltage of 80 kV, and the samples were prepared by dropping the sample solution onto carbon-coated copper grids. High-resolution transmission electron microscopy (HR-TEM) and electron diffraction (ED) were also used to characterize to the orientation of PCL crystals on MWCNTs. HR-TEM and ED characterizations were performed on a JEOL JEM-2100 electron microscope at an acceleration voltage of 200 kV. The sample preparation of HR-TEM is the same as that for common TEM.

RESULTS AND DISCUSSION Morphology of Solution-Crystallized PCL/MWCNT and PE/MWCNT Blends. Figure 2 shows the TEM images of the PCL/MWCNT blends after crystallization in n-hexanol. It is found that, for the blends of PCL with short MWCNTs 10


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(PCL/MWCNT-1 and PCL/MWCNT-2), rod-like crystals are formed (Figures 2a and 2c). It is also worth mentioning that the crystals are straight, which is in contrast to the bent morphology of MWCNTs (Figure 1). The ED images are also presented in Figure 2. ED can provide information about orientation and crystallinity of crystals. If polymer crystals are randomly orientated, diffraction rings corresponding to different crystal planes can be observed, while diffraction arcs appear for partly orientated crystals. For single crystals, diffraction spots with a specific pattern related to the unit cell are observed. One can see from Figures 2b and 2d that many diffraction spots are scattered in several rings. As compared with diffraction arcs in the ED pattern of MWCNT (Figure S2 in supporting information), these diffraction spots are produced by PCL crystals. Since MWCNTs are embedded in the rod-like crystals and covered by the PCL crystals of high density, MWCNTs can hardly be discerned from the TEM image, and the diffraction pattern of MWCNT is not obvious either. The ED patterns of the rod-like composite crystals also show that the PCL crystals on the surface of MWCNTs are poly-crystals with different orientations, instead of single crystals. Solution crystallization of PCL in the absence of MWCNTs was carried out under the same crystallization condition. It is found that typical single crystals of PCL with a hexagonal shape are formed (Figure S3 in supporting information). The different crystal morphologies of PCL and PCL/MWCNT blend after solution crystallization imply that MWCNT play an important role in formation of the rod-like composite crystals. The straight and rod-like crystal morphology has never been reported for polymer/MWCNT nanocomposites previously. However, Yan et al. grew 11



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ZnB2O4 single crystals on the surface of CNTs and obtained rod-like and straight composite crystals of CNT and ZnB2O4.46 Usually the NHSK structure is observed for most semicrystalline polymer/MWCNTs blends after solution crystallization. In order to explore the formation mechanism of such rod-like composite crystals, we also blended PE with three different types of MWCNTs and solution crystallization was carried out in o-dichlorobenzene. As shown in Figure 3, the NHSK structure is observed for all three solution-crystallized PE/MWCNT blends, as reported in literature.18-24 This shows that, the different crystal morphologies of PCL/MWCNT and PE/MWCNT originate from the polymer, instead of MWCNT. This is possibly due to their difference in chain conformation and will be further discussed later. On the other hand, when we use MWCNT-3, which is longer and thicker, to induce crystallization of PCL in n-hexanol, a quite different morphology is observed. We found that there existed both short and long CNTs in MWCNT-3. As a result, MWCNT-3 was separated into two parts by centrifuge. For a small amount of short CNTs (< 5 µm) in MWCNT-3, we still yield straight and rod-like crystals (Figure 2e), like the PCL/MWCNT-1 and PCL/MWCNT-2 blends. However, most of the CNTs in MWCNT-3 are quite long. For the long MWCNT-3, the MWCNTs are well dispersed in a thin layer of PCL, and the structure of MWCNT can be clearly seen in the TEM image, possibly due to the lower crystallinity of PCL (Figure 2g). As compared with the diffraction spots in the ED images of the rod-like composite crystals, only diffused diffraction rings are observed in the ED image of PCL/long MWCNT-3 (Figure 2h). Since the crystallinity of polymer crystals is usually less than 100%, the presence of 12


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the amorphous region may lead to diffusion of the diffraction rings or arcs. More diffused diffraction rings or arcs indicate lower crystallinity. Figure 2h reveals that the low crystallinity of PCL crystals in the PCL/long MWCNT-3 blend is quite low. Above results show that the crystal morphology of PCL/MWCNTs blends after solution crystallization strongly depends on the structure of MWCNTs, especially the length. It should be emphasized that at least three parallel experiments of TEM characterization were carried out for each sample, and the morphologies in majority were reported, thus the observed morphologies are repeatable.

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Figure 2. TEM (left) and ED (right) images of PCL/MWCNT-1 (a, b), PCL/MWCNT-2 (c, d), PCL/short MWCNT-3 (e, f) and PCL/long MWCNT-3 (g, h) after crystallization in n-hexanol.

Figure 3. TEM images of PE/MWCNT-1 (a), PE/MWCNT-2 (b) and PE/MWCNT-3 (c) after crystallization in o-dichlorobenzene.

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Morphology of Solution-Crystallized MWCNT-g-PCLs. In our previous work, we synthesized the PCLs grafted on the surface of MWCNTs (MWCNT-g-PCLs).45 In the MWCNT-g-PCLs the grafting degrees are quite high (> 90 wt%). Herein we also investigated the morphology of the MWCNT-g-PCLs after crystallization in n-hexanol. The TEM and ED images of solution-crystallized MWCNT-g-PCLs are illustrated in Figure 4. As we can see from Figure 4a, for MWCNT-1-g-PCL, in which the MWCNTs are thin and short, straight and rod-like crystals with embedded but invisible MWCNT-1 are formed. By contrast, for the other two grafting polymers (MWCNT-2-g-PCL and MWCNT-3-g-PCL), the bent CNTs can be clearly seen, and the CNTs are wrapped with a polymer layer (Figures 4c and 4e). We call it as double-layer structure. Such morphology is similar to that of PCL/long MWCNT-3 blend as well as that of CNTs-grafting amorphous polymers. This shows that the grafted PCL can only straighten the MWCNTs with a shorter length and a smaller outer diameter. Since MWCNT-1 and MWCNT-2 have a similar average length but the outer diameter of MWCNT-1 is smaller than that of MWCNT-2, the bent structure of MWCNT-2 in MWCNT-2-g-PCL indicates that the outer diameter of MWCNT is another important factor affecting the crystal morphology of PCL on the surface of MWCNTs. ED was also used to investigate the crystallinity of PCL in MWCNT-g-PCLs after crystallization from n-hexanol. One can see from Figure 4b that, diffraction spots from PCL crystals are observed for the straight and rod-like composite crystals of MWCNT-1-g-PCL, while only diffused diffraction rings appear in the ED images of 15



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MWCNT-2-g-PCL and MWCNT-3-g-PCL, which form a double-layer structure. This shows that the crystallinity of PCL in the straight and rod-like composite crystals is higher, whereas the crystallinity of PCL is relatively lower in the double-layer structure.

Figure 4. TEM (left) and ED (right) images of MWCNT-1-g-PCL (a, b), MWCNT-2-g-PCL (c, d), and MWCNT-3-g-PCL (e, f) after crystallization in n-hexanol. 16


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Combining the results of PCL/MWCNT blends and MWCNT-g-PCL grafting polymers, one can see that two typical crystal morphologies can be formed for the PCL/MWCNT composites after solution crystallization in n-hexanol. One is straight and rod-like morphology, and the other is bent double-layer morphology. The PCL crystals can straighten MWCNTs, possibly due to the rigid characteristics of polymer crystals. The related mechanism will be further explored in the next section. The formation of the straight and rod-like composite crystals is dependent on both the structure of MWCNT and the crystallizability of PCL. On the one hand, when MWCNTs are thin and short, they can be straightened more easily by crystallized PCL. On the other hand, more perfect PCL crystals exhibit a stronger ability of straightening MWCNTs. The crystallizability of the grafted PCL chains is weaker than that of the free PCL chains due to structural confinement,47-49 and thus MWCNT-2 with a larger diameter cannot be straightened by the grafted PCLs. However, it should be noted that, even in MWCNT-2-g-PCL and MWCNT-3-g-PCL of low crystallinity, the PCL crystals still influence the structure of MWCNTs to some extent. As shown in Figure S4 in supporting information, when the MWCNT-g-PCLs are prepared from THF, in which PCL cannot crystallize due to good solubility, the MWCNTs exhibit a bent and twisted structure and the PCL layer grafted on the surface of MWCNTs can hardly be observed by TEM, irrespective of the diameter and length of MWCNTs. Moreover, we speculate that the structure of MWCNT and crystallization of PCL may affect each other. The bent structure of MWCNTs is disadvantageous to formation of perfect polymer crystals and may hinder 17



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crystallization of PCL, leading to lower crystallinity of PCL and visible structure of MWCNT in TEM images. This agrees with the report that SWCNTs with a larger curvature exhibit weaker nucleation ability toward polymer crystallization.50 Orientation of PCL Crystals on MWCNT Surface. Our result shows that the crystal morphology of PCL/MWCNTs blends and MWCNT-g-PCLs is different from those reported for the blends of other semicrystalline polymers with MWCNTs. To understand such a difference at the level of microscopic mechanism, orientation of the PCL crystals on the surface of MWCNTs should be first determined. Therefore, the composite crystals of the PCL/MWCNT blends and MWCNT-g-PCLs were characterized

with

HR-TEM.

The

HR-TEM

images

of

PCL/MWCNT-1,

PCL/MWCNT-2 and PCL/short MWCNT-3 blends are shown in Figure 5. Because of the low crystallinity, the crystal structure of PCL in the PCL/long MWCNT-3 blend cannot be determined by HR-TEM and the HR-TEM image of this blend is not presented. Crystal fringes with different orientations can be observed in the HR-TEM images of the rod-like composite crystals of PCL/MWCNT blends. Fast Fourier transform (FFT) was conducted for some selected crystal fringes in the HR-TEM images. By applying FFT, the arrangement and order of the structures can be revealed more clearly. The FFT images in Figure 5 show that the crystal fringes corresponding to different crystal planes of PCL can be observed. The d-spacings of the crystal planes can be calculated from the crystal fringes in the HR-TEM images. The measured d-spacings (dm) and the standard d-spacings (ds) of different crystal planes are listed in Table 3. The values of dm are the averages of at least ten measures from 18


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different TEM images. One can see that the deviation of dm is quite small, indicating that dm is precise enough for assignment of different crystal planes. By comparing the values of dm and ds, these crystal planes can be assigned. The assignments of the crystal planes are indicated in the HR-TEM images as well. One can see that the crystal planes (101), (203), (210), (006) and (301) are frequently observed in different samples. Particularly, the crystal plane (101) seems to be parallel to the axis of the MWCNTs, as shown in Figure 5a. The HR-TEM images of the grafting polymers MWCNT-g-PCLs after solution crystallization in n-hexanol are presented in Figure 6. It is found that, for the grafting polymers with a double-layer structure (MWCNT-2-g-PCL and MWCNT-3-g-PCL), only the structure of MWCNTs can be observed (Figures 6b and 6c), thus no information of PCL crystals can be retrieved from the HR-TEM images. By contrast, crystal fringes of PCL crystals appear in the HR-TEM image of the rod-like crystals of MWCNT-1-g-PCL. The HR-TEM image of MWCNT-1-g-PCL is analyzed with the same method applied to PCL/MWCNT blends. As shown in Figure 6a, the crystal planes of (101), (203), (210) and (006) are observed for MWCNT-1-g-PCL, which is similar to those in the PCL/MWCNT blends. In MWCNT-1-g-PCL, the (101) crystal plane is also parallel to the axis of MWCNT-1. In order to further ascertain the assignments of these crystal planes, the angles between different crystal planes are also measured from the HR-TEM images and compared with the standard values, as summarized in Table 4. It should be noted that the PCL crystals on the surface of MWCNTs are indeed poly-crystals instead of single 19



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crystals, as can be evidenced by the ED images. Therefore, the observed crystal fringes may come from different crystals. To avoid any possible mistake, only the angles between the crystal planes that are close in position and may come from the same crystals are measured. The data in Table 4 show that, in most cases the measured angles agree well with the standard ones, verifying the correction of our assignments for these crystal planes. However, if the two crystal planes come from different crystals, the measured angle between them will evidently deviate from the standard value. For example, we notice that the measured angle between the (006) and (301) crystal planes in PCL/MWCNT-3 is 65.5° (Table 4 and Figure 5c), which is quite different from the standard value, 81.7°. Crystals are composed of unit cell, which is the smallest repeating unit of crystal structure with parallelepiped shape. The arrises of the unit cell along three different directions refer to a-, b- and c-axis, respectively. The parameters h, k and l are related to the inverse of the intercepts for the (hkl) crystal plane along a-, b- and c-axis, respectively. The HR-TEM characterization reveals that most of the observed (hkl) crystal planes have a common feature: the index k is 0. This means that the b-axis of the PCL crystals in both the PCL/MWCNT blends and MWCNT-g-PCLs is basically perpendicular to the surface of MWCNTs, which is in accordance with the result reported in our previous work for MWCNT-1-g-PCL.47 Moreover, for the MWCNT-1 with a short length and a small outer diameter, the (101) PCL crystal plane is parallel to the axis of MWCNT in the straight and rod-like composite crystals.

20


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Figure 5. HR-TEM images of PCL/MWCNT-1 (a), PCL/MWCNT-2 (b), and PCL/short MWCNT-3 (c) after crystallization in n-hexanol. The inserts show the FFT images corresponding to the framed areas. The red lines indicate the borders of MWCNTs.

21



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Figure 6. HR-TEM images of MWCNT-1-g-PCL (a), MWCNT-2-g-PCL (b), and MWCNT-3-g-PCL (c) after crystallization in n-hexanol. The inserts in (a) show the FFT images corresponding to the framed areas.

22


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Table 3. The d-spacing calculated from HR-TEM images (dm) and standard d-spacing (ds) for different crystal planes for PCL/MWCNT blends and MWCNT-g-PCL grafting polymers. No.

hkl

dm (nm)

ds (nm)

1

101

0.6687 ± 0.0067

0.6905

2

203

0.3106 ± 0.0016

0.3146

3

210

0.3006 ± 0.0016

0.3004

4

006

0.2730 ± 0.0030

0.2834

5

301

0.2440 ± 0.0030

0.2459

Table 4. The angles between different PCL crystal planes of the PCL crystals in PCL/MWCNT blends and MWCNT-g-PCL grafting polymers. Measured

Standard

angles (°)

angles (°)

(101)/(006)

64.5 ± 0.8

66.3

(101)/(301)

28.5 ± 0.7

15.4 or 32.0

PCL/MWCNT-2

(006)/(301)

84.3 ± 1.0

81.7

PCL/MWCNT-3

(006)/(301)

65.5 ± 0.9

81.7

MWCNT-1-g-PCL

(101)/(203)

62.1 ± 0.8

13.6 or 61.0

Samples

Crystal planes

PCL/MWCNT-1

23



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Possible Mechanism for Straightening MWCNT by PCL Crystals. There already exists interaction between polymer chains and CNTs in the solution or melt before polymer crystallization, and some polymer chains may be adsorbed on the surface of CNTs, driven by non-covalent π-π, CH-π or van der Waals interactions etc.51-54 It is reported in literature that polymer chains may interact with CNTs in two modes: coaxial interaction34,

55-56

or helical wrapping57-61. These two interaction

modes have also been confirmed by molecular dynamic simulations, respectively.62-73 When polymer chains are coaxially attached to the CNT surface and align along the axis of CNT, maximal interaction between polymer chains and CNT can be achieved. However, the gauche conformers in the polymer chains must be transformed into trans ones in such an interaction mode, which will lead to loss in conformational entropy.74 Moreover, the repulsion between the bulky side groups should be overcome.55 As a result, the coaxial interaction usually occurs for polymer chains with high rigidity or with small side groups, such as PE, nylon and P3HT, etc. These polymers also exhibit zigzag planar conformation, i.e. all-trans, in the crystals, indicating that such conformation can be readily realized at a higher temperature with the aid of the interaction between polymer chains and CNTs. On the other hand, the polymers exhibiting helical conformation in the crystals tend to helically wrap the CNTs in the solution as well, since it represents the thermodynamically stable conformation of polymer chains. The adsorbed polymer chains on the CNT surface can act as nuclei upon polymer crystallization and affect the crystal morphology and orientation.75-77 The coaxially 24


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attached polymer chains may induce the crystals to adopt a similar orientation, which means that the c-axis (usually the polymer chain direction) of the unit cell in polymer crystals is also parallel to the axis of CNT. As a result, the polymer crystals grow along the direction perpendicular to the CNT, leading to formation of nano-discs around the CNT, i.e. kebab structure. This situation is shown in Figure 7a. Moreover, due to the limited thickness of the nano-discs, which is determined by the lamellar crystal thickness, the nano-discs scatter along CNTs and have no straightening effect on CNTs. By contrast, if the polymers are helically attached on the CNT surface, the segments in a single polymer chain may have different orientations, which nucleate polymer crystallization to form small crystals with different orientation as well, as shown in Figure 7b. The c-axis of the polymer crystals is approximately on MWCNT surface, but not parallel to the axis of MWCNT. Accordingly, the growth direction of the polymer crystals is not perpendicular to the axis of MWCNT either, and thus the polymer crystals can grow on the MWCNT surface. This will result in being wrapped of MWCNT by polymer crystals, which form a sheath around MWCNT. Since polymer crystals are usually rigid, MWCNTs may be straightened by the sheath of polymer crystals. This is the possible reason why straight and rod-like composite crystals of PCL/MWCNT are formed. As is well known, CNTs are generally curled and entangled due to the high aspect ratio, which is disadvantageous to the electronic and mechanical properties.4-5 By contrast, straightened and aligned CNTs may exhibit improved properties. However, straightened and aligned CNTs are usually in-situ prepared. Otherwise, CNTs may recover their curled morphology upon transfer. In the 25



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present work, we discover a new method to straighten MWCNTs. The straight and rod-like morphology of the polymer/MWCNT composite crystals can facilitate free transfer without recovery of curvature and further alignment, and thus is very important and useful for application of MWCNT. Moreover, the in the straight and rod-like polymer/MWCNT crystals there exist tight interaction between polymer and MWCNT, since the polymer crystals grow on the MWCNT surface and wrap MWCNTs. This is also beneficial to the properties of the polymer/MWCNT nanocomposites.

Figure 7. Schemes for the arrangements of adsorbed polymer chains and the induced crystal morphologies on MWCNT for PE (a) and PCL (b).

Nevertheless, there is another possibility for the helically attached polymer chains. Since the helical conformation is highly like that in the solution or melt, which is less ordered than the zigzag planar conformation and thus has weaker nucleation ability. 26


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When CNTs are covered with a large amount of less ordered helical polymer chains, CNTs may lose their nucleation ability toward polymer crystallization. For example, Xu et al. also observed a layer of amorphous PEO on the MWCNT surface.38 The polymer layer with no or low crystallinity is less rigid, thus has no straightening effect on MWCNTs and only a double-layer structure is observed. Although we believe that the PCL chain wrapping MWCNT induce crystallization of the dissolved PCL, the epitaxial effect of MWCNT on PCL crystallization cannot be completely excluded. We also notice that in Figures 5a and 6a the crystal plane is parallel to the axis of MWCNT-1 and is longer than the and crystal planes. This may be ascribed to the epitaxial effect of CNTs. Li pointed out that such an epitaxial effect was more evident when the diameter of the CNTs is smaller.41 This is the reason why longer (101) crystal plane is observed in the composites containing MWCNT-1. It should be also pointed out that the crystal morphology and orientation mechanism of polymers on SWCNTs may be different from those on MWCNTs. SWCNTs tend to aggregate to form bundles due to their small diameter and large specific surface area, as shown in Figure 1a. There exist grooves among the aggregated SWCNTs. Fu et al. revealed that polymer chains with helical conformation, like PLLA and PP, may be adsorbed in grooves.40 Because of the spatial confinement, the polymer chains are parallel to the axis of SWCNTs, which can induce formation of NHSK structure. We also investigated the crystal morphology of PCL/SWCNT blend after solution crystallization in n-hexane. As shown in Figure S5 in supporting 27



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information, the NHSK structure, in which the aggregated SWCNTs form the shish, is observed. If SWCNTs also have a large diameter like MWCNTs, it is expected that they will not aggregate to form bundles and the PCL crystals will also grow on the SWCNT surface. Moreover, recently the NHSK structure was also in-situ prepared by electrospinning PCL/MWCNT.39 We believe that the PCL chains may undergo extension during electrospinning and thus coaxially interact with MWCNT, leading to formation of NHSK structure.

CONCLUSIONS In summary, the solution-crystallized PCL/MWCNT blends and MWCNT-g-PCL grafting polymers can form straight and rod-like core-sheath crystal or double-layer structure, depending on crystallizability of PCL, diameter and length of MWCNT. PCL chains of higher crystallizability exhibit stronger ability of straightening MWCNT. MWCNT with a smaller diameter (< 15 nm) and shorter length (< 2 µm) can be straightened by PCL crystals more easily. ED and HR-TEM show that PCL poly-crystals with random orientation form sheath around MWCNT, possibly nucleated by the helical PCL chains wrapping MWCNT, but the b-axis of the PCL crystals is always perpendicular to the MWCNT surface. Since the growth direction of the PCL crystals is not perpendicular to the axis of MWCNT, the PCL crystals also exert a straightening force on MWCNT.

28


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Supporting Information HR-TEM and ED images of MWCNTs, TEM images of the solution-crystallized PCL crystals

from

n-hexanol,

and

MWCNT-1-g-PCL,

MWCNT-1-g-PCL

and

MWCNT-1-g-PCL prepared from THF solution, the nanohybrid shish-kebab structure formed by PCL/SWCNT blend. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China (21574116) and Zhejiang Provincial Natural Science Foundation of China (LY15B040002).

29



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For Table of Content use only

Straight and Rod-like Core-Sheath Crystals of Solution-Crystallized Poly(εε-caprolactone)/Multi-Walled

Carbon

Nanotube

Nanocomposites Bing Zhou, Jun-Huan Li, Bin Fan, Ping Li, Jun-Ting Xu,* Zhi-Qiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

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Straight and Rod-like CoreâSheath Crystals of ... - ACS Publications

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