Structure and Morphology Control in Crystalline Polymer–Carbon

Perspective pubs.acs.org/Macromolecules

Structure and Morphology Control in Crystalline Polymer−Carbon Nanotube Nanocomposites Eric D. Laird and Christopher Y. Li* Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States ABSTRACT: Polymer nanocomposites have been an area of active research for the past 20 years. Of all potential fillers for polymer nanocomposites, carbon nanotubes (CNTs) are of particular interest due to their low mass density, high aspect ratio, and excellent mechanical, electrical, and thermal properties. In semicrystalline polymer CNT nanocomposites (PCNs), CNTs are viewed as nucleation agents that can affect polymer crystallization. However, it is challenging to quantify and compare results from different research groups, mainly due to the complexity of CNTs. Different chiralities, diameters, surface functional groups, surfactants used, and sample preparation processes can affect PCN crystallization. In this Perspective, we will focus on the structure, morphology, and related applications of semicrystalline PCNs. We will first present the introduction to semicrystalline PCNs followed by a brief discussion on transcrystallization and linear nucleation in polymers. The detailed interface structure and morphology are best revealed by using the solution crystallization approach; novel nanohybrid shishkebab structures have been observed. We will then discuss a few case studies with the focus on bulk crystallization, followed by polymer crystal-enabled applications, flow-induced crystallization in PCNs, and future outlook of the field. describes the so-called “chirality” of the CNT. If the chiral vector is (n, n + 3j) for integer values of j, the nanotube takes on the electronic character of a small-band gap (Eg) semiconductor with Eg ∝ 1/D2. For all other cases, the nanotube behaves as a semiconductor with Eg ∝ 1/D.25,26 Among the many promising applications offered by CNT, polymer−CNT nanocomposites (PCNs) represent one of the first realized major commercial applications of CNTs.7,27 It has been reported that mechanical properties such as tensile strength, modulus, compression modulus, and fracture toughness increased dramatically at very low CNT contents. Incorporation of CNTs into polymers also imparts attractive properties such as electrical and thermal conductivity, electromagnetic interference shielding, and sensing capability to the otherwise inert polymer matrix (Figure 1). Depending on the targeted properties, a variety of polymers have been explored to form PCNs. These include amorphous polymers such as atactic polystyrene (PS),28−37 poly(methyl methacrylate),38−44 semicrystalline polymers such as polyethylene (PE),45−52 isotactic polypropylene (iPP),53−63 poly(ethylene oxide) (PEO),64−68 polycaprolactone (PCL),67−73 poly(lactic acid) (PLLA),74−77 poly(vinyl alcohol) (PVA),78,79 polyacrylonitrile (PAN),80−82 and poly(vinylidene fluoride) (PVDF),83−87 thermoplastic elastomers such as polyurethane,88−90 thermosets such as epoxy, 91−95 and conducting polymers such as poly(3hexylthiophene) (P3HT),96 polyaniline,97 and polypyrrole.98,99 The four main features that CNTs bring into a PCN system include high mechanical performance, conductivity, remote actuation, and sensing. Other properties such as enhanced

1. INTRODUCTION: POLYMER−CARBON NANOTUBE NANOCOMPOSITES Polymer nanocomposites have been an area of active research for the past 20 years.1,2 Of potential fillers for polymer nanocomposites, carbon nanotubes (CNTs) are of particular interest.3−8 Similar to graphite, a CNT is comprised of interconnected hexagons of carbon atoms spanning the entire surface of the nanotube. The ends of the tubes are formed from half-dome-shaped fullerene molecules as a result of topological defects near the tube ends.9 Nanotubes with single-layer graphene shells are called “single-walled nanotubes” (SWCNTs), while multiple layers nested like matryoshka dolls are called “multiwalled nanotubes” (MWCNTs). A separate classification is sometimes made for “double-walled CNTs” (DWCNTs). The discovery of MWCNTs is frequently attributed to Iijima et al. in 1991.10 However, there is much evidence to suggest that others made the discovery independently before this.11−13 The field of CNT has attracted increasing attention during the past two decades; over 90 000 CNT-related papers have been published since 1991. CNTs intrigue scientists and hold high technological promise. From a structural standpoint, CNT can be considered a rigid macromolecule (with a persistence length of ∼26−174 μm) with rich configurations.14 They have extremely high aspect ratios (with length-to-diameter ratio ranging from single digits up to tens or even hundreds of millions),15,16 low density (∼1.4−1.6 g/cm3 for SWCNTs),14 and high specific surface area (∼100−1300 m2/g).17 Their high aspect ratio combined with their theoretical Young’s modulus on the order of terapascals18−21 and ultimate tensile strength around 100 GPa22,23 has earned them a reputation as outstanding fibers.24 SWCNTs have an electronic structure that can be metallic if the chiral vector is (n, n). This index © XXXX American Chemical Society

Received: January 6, 2013 Revised: March 11, 2013

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Figure 1. Applications of carbon-nanotube-containing PCNs.

thermal stability and flame retardancy have also been reported.38,39 Numerous review papers have been published a d d r e ss in g d i ff e r en t a s p e ct s o f C NT - c o n t ai n in g PNCs.8,14,100−105 CNT-induced polymer crystallization is a seemingly straightforward research field. It has been realized that CNTs promote polymer crystallization while CNT networks can impose physical confinement on polymer crystal growth. Typically, CNTs are viewed as nucleation agents that can accelerate polymer crystallization. Upshifted crystallization temperatures (Tc) during nonisothermal crystallization and reduced crystallization half-time (t1/2) are often observed. However, the complexity of CNTs makes it a challenge to quantify and compare results from different research groups. Crystallization behavior is sensitive to many factors, including sample preparation processes, differences in chirality, differences in diameter, presence of surface functional groups, and the use of surfactants. The structure, morphology, and tailored applications of semicrystalline PCNs will be the focus of this Perspective. We will first briefly discuss transcrystallization and linear nucleation in polymer. The structure and morphology of polymers at the CNT/matrix interface are best revealed by using a solution crystallization approach, which will be discussed in section 3, followed by a few case studies with the focus on bulk and flow-induced crystallization in PCNs and an outlook of the field.

Figure 2. Transcrystallization on CFs and CNT fibers. (a) Transmission-polarized optical micrographs of isolated fibers in thin iPP films. (b) Two-dimensional WAXD pattern of iPP reinforced with 28 vol % pitch-based CFs. Azimuthal-intensity variations reveal the oriented nature of the PP matrix. Fiber axis is vertical. (c) Idealized model for structure within TCL. Model incorporates daughter lamellae growing from an initially formed parent. Parts a, b, and c represent the unit cell axes of α-PP. (d) Optical micrograph of transcrystalline interphases for iPP surrounding the CNT fibers. (Parts a−c: Reproduced with permission from ref 108. Copyright 1998 Springer. Part d: Reproduced with permission from ref 109. Copyright 2008 Elsevier.)

apparently different from that of the bulk and approaches the latter at certain distance from the interface (ca. hundreds of micrometers). Figures 2b,c show a 2D wide-angle X-ray diffraction (WAXD) pattern and an idealized chain packing scheme of CF-induced iPP crystallization. The 2D WAXD pattern shows strong CF (002) reflections on the equator. Detailed analysis of azimuthal intensity variations of (110) and (040) diffractions of iPP led to a structural model of the chain orientation as shown in Figure 2c. Near the CF surface, iPP chains are parallel to the CF axis and the parent lamellae grow radially outward from the CF. The chain orientation is dictated by the epitaxial match between graphite lattice and the α-PP unit cell. As the parent lamellae grow, cross-hatched structures occur, and the daughter lamellae are then generated. Recently, Kumar et al. demonstrated that CNT fibers with an average diameter of ca. 40 μm can also induce similar transcrystallization of iPP as shown in Figure 2d.109 Detailed structure and morphology will be discussed in the following section.

2. POLYMER TRANSCRYSTALLIZATION From a geometric standpoint, CNTs are “downsized” carbon fibers (CFs) with a much smaller fiber diameter. It is then useful to start our discussion with CF−polymer systems, which have been extensively explored.106,107 CFs are known to be able to induce polymer crystallization in numerous systems, since heterogeneous nucleation is favored as opposed to homogeneous nucleation of the matrix polymers. In CF-induced polymer crystallization, although molecular epitaxy does not always occur, transcrystallization often takes place. Transcrystallization is typically characterized by textured crystalline regions developed from interfaces in polymer blends, composites, or at/near external surfaces. The textured crystalline region is often referred to as a transcrystalline layer (TCL). In the case of CF−polymer composites, TCL is concentric to the fiber axis. Figure 2a shows optical micrograph of an isolated, pitch-based CF in iPP.108 The formation of a TCL (dotted area in the figure) is attributed to dense nucleation at the interface, which confines the crystal growth to the direction orthogonal to the interface, e.g., the fiber axis in the CF−polymer composite case. The crystalline morphology is

3. INTERFACIAL EFFECT ON CNT-INDUCED POLYMER CRYSTALLIZATION 3.1. Nanohybrid Shish Kebabs, Structure, and Morphology. In order to clearly reveal the interface between CNTs and polymer crystals in the PNC, polymer solution crystallization has been employed to study CNT-induced polymer crystallization.110−113 The crystallization temperature was kept higher than PE’s homogeneous nucleation temperature to ensure that all the PE crystals formed were initiated via heterogeneous nucleation. Figure 3 shows a transmission electron microscopy (TEM) image of the PE-decorated MWCNTs obtained from the above procedure (Tc = 103 B

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the crystallization conditions such as CNT concentration, polymer concentration, Tc, and crystallization time. There are two possible factors that control NHSK growth: the epitaxial growth of PE on CNT and geometric confinement. Epitaxial growth of PE on the surface of highly ordered pyrolytic graphite (HOPG) dictates the PE chain direction or the ⟨001⟩ of the PE crystal to be parallel with the ⟨21̅1̅0⟩ directions of the graphite below.117−119 However, because of their small diameters, CNTs can be considered as rigid macromolecules; hence, the polymer chains prefer to align along the tube axis regardless of the lattice matching between the polymer chain and the graphitic sheet. “Soft epitaxy” is then the natural choice to describe the mechanism: strict lattice matching is not required while there is a cooperative orientation of the polymer chains and CNT axis. The growth mechanism for CNT-induced PE crystallization involves both, and it is size-dependent. First, on the surface of the carbon nanofibers (CNFs) with a diameter much larger than the polymer size, the polymer behaves as if it was on a flat surface; epitaxy is the main growth mechanism. Second, when the fiber/ tube diameters decreased to the order of the polymer size, as a polymer chain starts to crystallize onto this surface, geometric confinement is the major factor and the polymer chain is parallel to the CNT axis, disregarding the CNT chirality. As a consequence, the PE crystal lamellae are perpendicular to the CNT axis and orthogonal orientation is obtained. Selective area electron diffraction (SAED) experiments have also confirmed this orientation.120 Figure 4 shows the schematic representation of the size-dependent, soft epitaxy mechanism in the PE/CNT system. Similar molecular orientation has also been confirmed by Wei using molecular dynamics simulation.121−123 PE with 100 methylene repeating units was shown to have selective conformations on the CNT surface. While PE molecules prefer 0° wrapping on small radius armchair CNT (5, 5) at low temperatures, their configurations are shifted to larger wrapping

Figure 3. PE NHSK. (a) TEM image of enlarged PE/MWCNT NHSK structures. (b) Schematic representation of the PE/CNT NHSK structure. For clarity, SWCNT was used. PE forms folded lamellar single crystals on CNT surface. (Reproduced with permission from ref 110. Copyright 2005 John Wiley and Sons.)

°C) using p-xylene as the solvent. The tube is decorated with polymer single crystal lamellae which are uniform in size and semiperiodically located along the CNT. This morphology is similar to the classic “shish-kebab” polymer crystals formed under an extensional field, observed in the 1960s by Geil, Reneker (“Hedgerow”),114 and Pennings.115 The CNT/ polymer system in this case was not under extension/shear flow during crystallization; it is the MWCNT that induces nucleation of polymer chains on its surface. Figure 3b is the schematic representation of this nanohybrid structure. CNT forms the central stem and PE periodically grows on the CNT. Figure 3 therefore resembles “nanohybrid shish kebabs” (NHSKs). Along the MWCNT, the single crystal lamellae (kebabs) are perpendicular to the MWCNT axis. Comparing Figures 2 and 3, the fiber-induced transcrystalline and shishkebab morphologies are similar as both involve orthogonal growth of polymer crystals to the nucleating fiber/CNTs. The main difference is that in the shish-kebab case single crystals are formed while closely packed 2D spherulites are formed in fiberinduced transcrystalline morphology, due to the impingement of the adjacent spherulites. A CNT can be considered as a 1-D nucleation agent that consists of numerous nucleation sites, which is similar to a polymer linear nucleation system.116 The molecular origin of NHSK periodicity might be related to the concentration gradient and the heat dissipation at the lamellar growth front. As the nucleation starts to occur (during the induction time), there is a polymer concentration gradient at the crystal growth front. This concentration gradient possesses a periodic profile along the CNT axis, rendering the NHSK periodicity. Furthermore, the crystal growth is an exothermic process; hence, the temperature at the growth front is higher than the solution temperature. Since CNT has a higher thermal conductivity, it is anticipated that most of the heat generated by crystal growth is dissipated along the CNT, resulting in a possible temperature gradient along the CNT. Combining these two effects might lead to the formation of the periodically patterned kebab crystals. A number of parameters are needed to quantitatively understand the NHSK structure, e.g., the kebab period n, the CNT diameter r, polymer lamellar thickness l, polymer lamellar lateral size a, and CNT chirality, as shown in Figure 3b. The variables l, a, and n can be controlled by varying

Figure 4. Size-dependent soft epitaxy. As the diameter of the carbon nanofiber is relatively large, PE lamellae follows the crystal lattice orientation on the fiber surfaces (a, c). As the diameter decreases, PE chains are compelled to orient exclusively parallel to the fiber axis (b, d). (Reproduced from ref 111.) C

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considered as the nucleus of the kebab crystal. Moreover, the chain rotation suggested in the PE coating layer is speculative, and it is not clear whether a “homogeneous” PE coating layer is needed for subsequent kebab crystal growth. NHSK structures have recently been observed in a number of laboratories for a variety of polymers using solution, thin film, and bulk crystallization methods. Zhang et al. used watersoluble, sodium dodecyl sulfate-coated SWCNTs to spray on the surface of nascent ultrahigh-molecular-weight PE (UHMWPE).126 The mixture was then dissolved in xylene at a relatively high concentration. Upon cooling, gel was formed, and the system was allowed to dry/crystallize for a week. Scanning electron microscopy (SEM) showed that NHSK structure was formed.126 Uehara et al. used dichlorobenzene (DCB) as the solvent to solution crystallize UHMWPE, and similar NHSK was also observed.113 These authors also suggested that SWCNTs prevented the thickness doubling of PE lamellae during the heating process. While most NHSK exhibit toroid-like kebab crystals, P3HT showed different morphology. Zhai et al. demonstrated that using similar solution crystallization approach, controlled P3HT nanofibers can be grown onto CNT surface, as shown in Figure 6a,b.96 In

angles on a similar radius zigzag CNT (10, 0), as shown in Figure 5. In addition, Wei reported an isotropic-to-nematic

Figure 5. Snapshots of PE at the interface of CNT (5, 5) at T = 50 K (a) and 600 K (b). (c) PE at the interface of CNT (10, 0) at 50 K. (d− f) show snapshots of atomic structures of decane CNT (5, 5) composite at T = 500 K: (d) right after the cooling process at T = 300 K (e) and the final configuration at the end of an additional 7 ns equilibrium run at T = 300 K, after the cooling process (f). (Reproduced from refs 121 and ref 122. Copyright 2007 American Physical Society.)

transition with molecules aligned with embedded nanotubes. Smectic transition of the alkane molecules was discovered with small radius CNT (5, 5) as a nucleation site (Figure 5). Interestingly, it was found that alkane molecules form lamellar layers along the nanotube axis and have two-dimensional structure ordering in planes perpendicular to the tube axis, which is extremely similar to the previously discussed NHSK structures. In addition, simulation suggests that molecular crystallization is strongly dependent on CNT chirality: in the case of a CNT (10, 0), two-dimensional ordering of molecules was not observed, which was attributed to the preferred tilted wrapping angles at the interface. Note that such CNT-sensitive crystallization was not observed experimentally except that when relatively large diameter CNFs were used while simulation suggests that even for small ∼1 nm diameter CNTs, crystallization behavior is chirality-dependent. This discrepancy may be attributed to factors such as different PE molecular weights used for simulation and experiments, possible defects on CNTs, and limited crystallization conditions that have been explored. Recently, Zhang et al. suggested that NHSK was formed following three steps:124 First, PE chains near the MWCNTs are adsorbed onto the nanotube surface as soon as they are mixed in solution. Second, increasing numbers of wrapping chains slide along tubes and change into extended chain conformation, forming a homogeneous coating around MWCNTs with few “subglobules”. Third, as temperature decreases, PE chains epitaxially grow from these “subglobules” of the homogeneous coating and formed the crystal lamellae. They also used molecular dynamics simulation work by Yang et al. as the further evidence.125 However, it should be noted that formation of an adsorbed polymer layer depends on polymer molecular weight, annealing temperature, and time. Simulation by Yang et al. also suggested folded chain instead of extended chain conformation for the crystallites. Epitaxy growth from a “subglobule” is rather confusing as the “subglobule” should be

Figure 6. TEM images of P3HT NHSK (a: P3HT/MWCNT mass ratio = 7; b: P3HT/SWCNT mass ratio = 22) (scale bar: 100 nm). (c) Crystallization process of P3HT on MWCNTs monitored by in situ UV−vis spectroscopy at room temperature. (d) UV−vis absorbance change at 600 nm of the P3HT suspension with (squares) and without (circles) CNTs during the crystallization process. (Reproduced with permission from ref 96. Copyright 2009 Wiley.)

their study, pristine CNTs (SWCNTs or MWCNTs) were added to a hot P3HT anisole solution. The solution was then quickly cooled down to room temperature and kept overnight for crystallization. Because of the low solubility of P3HT in anisole at room temperature, P3HT molecules precipitated from the supersaturated solution and formed fibril-shaped crystals on CNT surfaces. The crystallization process was monitored using in situ UV−vis spectroscopy (Figure 6c,d), and the growth process was found to follow first-order kinetics. This hybrid structure mimics “centipedes”, and it is similar to twodimensional NHSK formed using physical vapor deposition (PVD) process, which will be discussed in the following section. D

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3.2. Nanohybrid Shish Kebabs via Different Preparation Methods. A variety of methods have been used to prepare NHSKs. In addition to solution crystallization, other methods include supercritical CO2-induced polymer crystallization, PVD, thermally initiated chemical vapor deposition, thin film growth, and bulk crystallization. Zhang et al. used pxylene or DCB as the solvent and supercritical CO2 as the nonsolvent to induce polymer growth on CNTs.127 Relatively uniform PE NHSK was formed. A similar method was used to produce PVA and poly(ethylene glycol) (PEG) NHSK.128 While PVA formed uniform NHSK, PEG/CNT hybrids are less well-defined. Thermally initiated chemical vapor deposition can be used to grow polytetrafluoroethylene (PTFE) single crystals and form PTFE NHSKs.129 Another facile method to study the CNT-induced NHSK formation is using PVD.130,131 Using PE as an example, upon heating under vacuum, PE degraded into oligomers, which crystallized into rod-shaped single crystals (Figure 7a,b). While in contact with CNTs, these PE rod

not form as shown in Figure 7c because no additional PE oligomers diffused to the CNT surface to form PE rod crystals. The PVD method also allows us to directly investigate the surface chemistry effect on CNT-induced polymer crystallization. For example, alkane modification of the MWCNT surface prohibited the PE single-crystal growth while bare MWCNTs induce NHSK formation using PVD. 3.3. Block Copolymer Nanohybrid Shish Kebabs. Another unique CNT-containing NHSK has been observed in CNT−semicrystalline block copolymer (BCP) blends. BCPs typically are used as macromolecular surfactants to wrap CNTs and to improve the dispersibility of the latter. On the other hand, if CNTs induce the crystallization of one block of the BCP, this crystallization process can then facilitate BCP phase separation. Block copolymers that have been studied include PE-b-PEO,132 PE-b-SBR,133 and triblock copolymer poly(vinylcyclohexane)-b-poly(ethylene)-b-poly(vinylcyclohexane) (PVCH−PE−PVCH).134 In the case of PE-b-PEO, a simple sequential spin-coating method has been used to prepare the sample. Figure 8 shows a TEM image of the resultant BCP/

Figure 7. (a) TEM and (b) AFM image of PE-decorated MWCNTs. (c) shows an HRTEM image of a PE-decorated MWCNT. The MWCNT was located in the hole region of a lacey carbon grid. This image captured the intermediate state of the PE decoration process. A thin layer of PE molecules was deposited on the MWCNT surface. PE crystals did not grow on the CNT because the MWCNT was detached from the substrate; diffusion of PE oligomers onto the CNT surface was thus prohibited. (d) shows that PE oligomers are parallel to the CNT axis disregarding the chirality of the latter. (Reproduced from ref 130.)

Figure 8. Alternating pattern of PE-b-PEO formed on SWCNTs. (a) 0.005 wt % BCP/chloroform solution was spin-coated on SWCNTs (scale bar 200 nm). The inset shows an enlarged area (scale bar 20 nm). (b) Schematic representation of the arrangement of the PE-bPEO molecules along a SWCNT. (c−g) show TEM images of the five different morphologies of the BCP/SWCNT hybrid at various BCP concentrations. Concentration of the BCP solution: (c, d) 0.001, (e) 0.005, (f) 0.020, and (g) 0.050 wt %. The scale bars are 50 nm. (Reproduced with permission from ref 132. Copyright 2009 Nature Publishing Group.)

crystals aligned perpendicular to CNTs. The formation mechanism can also be attributed to soft epitaxy previously discussed: the orientation of PE oligomer crystal is dictated by alignment of PE chain parallel with CNTs, as shown in Figure 7d. This method also provides a facile approach to study the growth process of the PE oligomers, which include two steps. First, PE oligomers deposit onto and near the CNTs, and second, these oligomers diffuse to CNT surfaces and crystallize. When the CNT was detached from the substrate, NHSK did

SWCNT hybrid. Alternating patterning of PE (gray) and PEO (dark) stripes are formed, and they are aligned perpendicular to the CNTs. Compared with the crystal patterns formed in the CNT-induced homopolymer crystallization, the alternating pattern formed by the BCP is far more uniform with a period of ∼11.9 ± 0.9 nm with the width of the PE domain ca. 5.9 ± 0.7 nm. PE blocks form one layer of an interdigitated extended chain crystal. During spin-coating, the BCP molecules adsorb onto the SWCNT surface due to the favorable interaction E

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crystals form a transcrystalline layer of aligned iPP lamellar crystals around the nucleating CNT.61 A similar nucleation effect and the formation of a transcrystalline layer were also observed for ultrathin film CNT/iPP samples. SAED studies showed that only α phase of iPP exists. More interestingly, they found that transcrystalline layer is highly oriented around the nucleating CNTs, and the crystallographic c-axes of the lamellae are oriented perpendicular to the long axis of the nucleating CNT. This peculiar chain orientation is in contradiction to the case of PE NHSK as well as the nanofiber transcrystallization study. Figure 9 shows the bright-field TEM image and the

between PE segments and SWCNTs, leading to heterogeneous nucleation. After a stable nucleus forms, the PE crystal starts to grow following the soft epitaxy mechanism. This process is driven by CNT-induced PE crystallization. At an extremely low BCP concentration, the BCP molecules crystallize only along the initially formed nuclei. Because of the local concentration gradient generated at the crystal growth front, the stripes grow laterally on the carbon film with a perpendicular orientation to the tube axis. As the BCP concentration increases, more crystals start to appear. The tethered PEO chains at the crystal edges, dangling in the solution, can attract PEO segments of the free BCP molecules to the nearby region and facilitate the BCP to crystallize at a certain distance away from the initially formed nuclei on the SWCNT. Repetition of this process leads to the formation of patches of alternating BCP stripes along the SWCNT. As the BCP concentration reaches a critical point, the patches are long enough to connect with each other, leading to the continuous, alternating stripes spanning the entire SWCNT.

4. STRUCTURE AND MORPHOLOGY OF SEMICRYSTALLINE POLYMER−CNT NANOCOMPOSITES Most of the studies in semicrystalline polymer PCNs are focused on the influence of CNTs on the bulk crystallization behavior of the polymer. Solution blending, melt blending, and in-situ polymerization methods have been used to fabricate PCNs.8 In general, CNTs can (1) enhance, (2) impede, or (3) have no effect on polymer crystallization. Müller et al. recently reported a fourth, the so-called “supernucleation”, effect in PCNs.67,68,135−137 Cases 2 and 3 were observed in PEO systems. Case 4 has been observed in PE- and PCL-grafted CNTs and PCL PCNs. Most of semicrystalline polymers, including PVA, PLLA, PCL, iPP, PE, PA, and PVDF, upon forming PCNs, show case 1 behavior: crystalline morphology changes from spherulites to “granular” structures as clearly revealed by polarized light microscopy. Differential scanning calorimetry (DSC) experiments show that, with addition of CNTs, Tc increases during cooling and t1/2 decreases in isothermal crystallization. The degree of the Tc and t1/2 change depends on polymer chemical structures. Crystallization behavior also depends on CNT surface chemistry as well as CNT content. Low CNT content typically induces crystallization while high CNT content may impede crystallization. In the following section, we will discuss a few selected PCN systems that are related to CNT−polymer interaction. 4.1. iPP−CNT Nanocomposites. One of the most extensively studied systems is iPP PCN. Grady et al. first investigated nonisothermal and isothermal crystallization behavior of iPP PCNs.54 They used CNTs functionalized with octadecylamine, and the PCNs were prepared using solution mixing. They noted that the rate of crystallization was substantially higher in the PCN system than the pristine polymer. More interestingly, on the basis of fitting of the melting peak of DSC thermograms, they reported that CNTs promoted growth of β form iPP at the expense of the α form. A number of other authors, combining DSC and WAXD/TEM tests, reported the formation of only α form or the mixture of α and γ phases. For instance, Assouline et al. showed that, in an iPP/MWCNT system, MWCNT nucleated α phase and no β or γ phase were observed.138 Lu et al. used DSC and TEM to study similar systems, and they confirmed that CNTs nucleate iPP when crystallizing from the quiescent melt and that iPP

Figure 9. Bright-field TEM image (a) and the corresponding SAED pattern (b) originating from the same area of transcrystalline grown iPP around SWCNTs in an ultrathin film. (c) shows a TEM brightfield image of ultrathin film samples of transcrystalline organization of iPP around MWCNTs. (d) shows the possible nucleation mechanism of iPP on the surface of a CNT. (Reproduced from ref 61.)

corresponding SAED pattern of the hybrids. The authors argued that the protruding methyl groups of the iPP interact with the graphite layer of the CNT. This confinement of the macromolecules is maintained during cooling of the sample from quiescent melt and causes enhanced and oriented nucleation of iPP lamellar crystals, which have their crystallographic c-axis tangential to the graphite shell of the CNTs. Most recently, Ning et al. reported the formation of iPP NHSK structure by using SWCNT bundles.139 They showed that these bundles apparently have “grooves”, which can promote iPP chain alignment along the CNT axis. A similar NHSK structure with parallel chain orientation can also be found in flowinduced iPP crystallization in PCN, which will be discussed in section 5. Another interesting observation was reported by Zhang et al., who showed that when iPP is infiltrated into nanotube aerogel fibers to form a composite, both α and γ iPP are formed with γ structure as the major component (as high as 80% γ phase was reported).140 This observation was supported by both DSC and WAXD experiments. The high γ form content of iPP obtained from quiescent crystallization and atmospheric pressure was attributable to the high CNT loading: because of the infiltration method the authors used, the CNT contents were ca. 30−80 wt %. The authors then suggested that α form iPP dominates the overall interphase if the nanotube loading is low while CNTs F

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preferably nucleate γ form iPP when the CNT contents are high. The latter is because geometric confinement dominates over the crystal growth rate. However, it is not yet clear why geometric confinement would favor γ phase growth. 4.2. PE−CNT Nanocomposites. Bulk crystallization of PE PCNs was investigated using different sample preparation methods. Kodjie et al. used the previously mentioned NHSK as the precursor to prepare the composite in order to prevent CNT agglomeration.45 Haggenmueller et al. developed a hot coagulation method to incorporate SWCNT into polymer matrix with SWCNT loadings as high as 30 wt % with relatively uniform distribution on the 10 μm scale.47 Zhang et al. fabricated UHMWPE CNT nanocomposites by spraying an aqueous solution of SWCNTs onto a fine UHMWPE powder directly obtained from synthesis.126 In all these cases, accelerated PE crystallization has been observed as evidenced by shortened t1/2, higher Tc upon cooling, decreased Avrami exponent n, increased Avrami growth rate K, and/or faster developed elastic modulus. Another interesting observation has been recently reported in surface-induced crystallization of high-density PE (HDPE) in vertically aligned multiwalled carbon nanotube arrays.141 In this case, ca. 1 mm long nanotube arrays were infiltrated by PE solutions, and then the system was allowed to crystallize under controlled conditions. The weight fractions of CNTs in the composites were determined to be 30−80 wt %. SEM showed that NHSKs were formed with the PE single crystals grown perpendicularly to the aligned nanotubes but do not completely fill the intertube spacing (Figure 10a), forming oriented 3D

conformations and packing.142 PVD has been used to show that PVDF oligomers can form single crystals on CNTs to form NHSK.131 In the reported PVDF PCN study, much attention has been focused on the polymorphism of PVDF in the composites. Levi et al. reported PCNs comprised of SWCNTs (HiPco and Arc grown)/MWCNTs (Arc grown) and PVDF, poly(vinylidene fluoride−tetrafluoroethylene) (PVDF−TFE), or poly(vinylidene fluoride−trifluoroethylene) (PVDF− TrFE).143 In all cases, they observed that the piezoelectric β phase is significantly enhanced over α phase using WAXD. Furthermore, although all types of CNTs (HiPco SWCNT, ARC SWCNT, fluorinated HiPCo SWCNT, and MWCNTs) induced the formation of β phase, for HiPCo SWCNTs the maximum β:α ratio occurs at a weight loading of 0.01 wt %; for ARC SWCNTs the maximum occurs at a loading of 0.05 wt %. Fluorinated HiPco SWCNTs have a maximum just above 0.001 wt %, and MWCNTs show a maximum β:α phase ratio above 0.8 wt %. This observation suggests that surface chemistry and the size of CNTs play an important role for PVDF structural formation. Yu et al. prepared two PVDF PCNs using solution sonication and mechanical mixture methods.144 They found that the α phase coexists with the β phase in the composite prepared by sonicating the PVDF/CNT mixture solution, while no β phase can be observed in the composite prepared from the mechanical mixture route. They further used density functional theory calculations to show that a large amount of energy is required for transforming trans−gauche−trans−gauche′ (TGTG′) into trans−trans (TT) conformations, and the TT molecular chain can be bound on the CNT surface tightly. Because of sonication, some TGTG′-type polymer chains can be transformed into TT-type chains in the mixture solution. The TT-type molecular chains adsorbed on the CNTs surface act as nucleating agents, which leads to the formation of β phase during PVDF crystallization. Interestingly, they also showed that the emergence of β phases is independent of zigzag carbon atoms on the CNT surface. Recently, Manna and Nandi examined phase structure of PVDF PCNs with the focus on the CNT surface chemistry and sample preparation effect.145 They used two types of MWCNTs: unfunctionalized and −COOC2H5-functionalized MWCNT. Both solution casting and melt-cooling were used to prepare the composites. In the solution casting method similar to that previously discussed, PCNs do not experience temperatures higher than the melting temperature of PVDF. They showed that the solvent-cast films have the β phase for functionalized MWCNT concentration ≥1% (w/w) and have a mixture of α and β polymorphs below that concentration. In the melt-cooled specimen, there occurs a mixture of α and β phases, and the latter is totally absent in the corresponding unfunctionalized MWCNT PVDF PCNs. Their results suggest that (1) the surface chemistry of CNTs, in this case the specific interaction of the ⟩CO group in functionalized MWCNTs and the ⟩CF2 group of PVDF, can significantly affect polymer chain conformation, and (2) melting PVDF PCNs can (at least partially) convert β to α structures. Note that in both Levi and Yu’s work, solution casting was used to prepare PCNs. On the other hand, Gregario and Borges found that solutions of PVDF using a number of different solvents cast into neat films produced spherulites with a thermodynamic preference for β phase PVDF (α phase spherulites were kinetically favored).146 The α phase spherulites can also undergo α → γ phase transition as a solid−solid phase transition,147−149 and both of

Figure 10. (a) SEM image of surface structures of the CNT−PE composites prepared from infiltrating PE solution into aligned CNTs after 12 h of crystallization. (b) Integrated wide-angle X-ray diffractions of the composites prepared from 1.0 wt % solution for 0.5 h of crystallization. (Reproduced from ref 141. Copyright 2010 Wiley.)

porous structures. WAXD results showed that both orthorhombic and monoclinic phases were formed, and the orthorhombic crystal phases dominate over the monoclinic ones (Figure 10b). Upon heating, the monoclinic form of PE is gradually transformed into the orthorhombic form. Furthermore, they observed three endotherms upon DSC heating. Zero-heating-rate melting temperatures of 123.8, 134.6, and 175.1 8 °C are obtained. Combined with thermo-WAXD approaches, these transitions were attributed to the bulk PE melting, orthorhombic-to-nematic and nematic to isotropic transitions of PE at the CNT surface, respectively. 4.3. Poly(vinylidene difluoride). PVDF has been extensively studied for their unique piezoelectric properties, chemical inertness, and mechanical properties.142 Five different polymorphisms have been reported based on different chain G

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Figure 11. TEM micrographs of PA66 NHSKs formed by crystallizing PA66 on MWCNTs in glycerin at (a) 185 °C and (b) 172 °C. (c) SEM micrograph of 0.5 wt % CNT/PA66 nanocomposites (negative PA66 spherulites). (Reproduced from ref 49. Copyright 2007 Elsevier.)

these species can be formed through solvent annealing.150 Given the complex phase transformation of pristine PVDF, the origin of PVDF phase structure in PCNs should be tempered with caution. 4.4. Polyamide. Crystallization of a number of polyamide PCNs has been studied.49,151−153 In particular, earlier studies showed that polyamide 66 (PA66) NHSK can be formed using solution crystallization (Figure 11a,b).49 These NHSKs were used to prepare PA66 PCNs using a solution-casting method. Excellent dispersion was revealed by optical and electron microscopy. Interestingly, PA66 negative spherulites formed in the PCN during solution crystallization. Nitric acid etching of the nanocomposites showed that MWCNT formed a robust network in PA 66. Observation of the NHSK network within a PA66 spherulite indicates that, upon forming NHSK, the network structure of NHSK is relatively robust. As PA66 spherulites grew, the diffusion and growth of PA66 failed to repel the adjacent NHSK; instead, the spherulite engulfed the NHSKs, which leads to the observation of negative PA66 spherulites with closely packed NHSK network inside (Figure 11c). Nonisothermal DSC results showed multiple melting peaks, which can be attributed to lamellar thickness changes upon heating. Isothermal DSC results showed that crystallization kinetics increased first (0−0.1 wt %) and then decreased (0.1− 2 wt %) with increasing MWCNT contents, suggesting that the effect of MWCNTs on PA66 crystallization is twofold: MWCNTs provide heterogeneous nucleation sites for PA66 crystallization while the tube network structure hinders large crystal growth. 4.5. Poly(ethylene oxide). PEO PCN has shown case 2 and 3 crystallization behaviors; i.e., CNTs have little or negative effects on PEO crystallization. To this end, Goh et al. synthesized double-C60-end-capped PEO and blended it with acid-treated MWCNTs.64 They showed that while the storage modulus of PEO was significantly enhanced, the crystallization behavior was not significantly affected upon the incorporation of MWCNTs. Geng et al. used a roll-casting method to prepare fluorinated SWCNTs (F-SWCNTs)-containing PCN with the F-SWCNT contents varying from 1 to 10 wt %.154 They reported ∼7 deg melting point depression upon adding FSWCNTs; no crystallinity changes were observed. On the other hand, Kim et al. showed that magnetic CNT (m-CNT) may have moderate impact on PEO crystallization.155 m-CNT-

containing PEO PCNs were readily aligned parallel to the direction of a magnetic field under a relatively weak magnetic field. Magnetic particles (γ-Fe2O3) provided nucleation sites for the PEO crystal growth, while maintaining the spherulitic crystal morphology. Chatterjee et al. took a different approach.156−158 Inspired by studies on PEO electrolytes, they used lithium-containing surfactants such as lithium dodecyl sulfate (LDS) as the dispersing agents for SWCNTs and then prepared PEO PCNs. While LDS reduces PEO crystallization due to the favorable interaction between lithium ion and the EO groups, the effect is much more significant in the PCN system. By comparing the crystallization behaviors of pristine PEO, PEO−LDS blends, PEO−CNT−LDS nanocomposites, and PEO−CNT nanocomposites with sodium dodecyl sulfate and dodecyltrimethylammonium bromide as the surfactants, they concluded that the synergistic effect of LDS and SWCNTs significantly reduces PEO crystallization. 4.6. Supernucleation. One extreme case in PCN is grafting polymer brushes onto CNTs so that all the chains are tethered on CNTs. To this end, Trujillo et al. prepared nanocomposites of HDPE, with CNT of different geometries (SWCNT, DWCNT, and MWCNT) by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system.135 Following the method proposed by Fillon et al.,159,160 they estimated the efficiency of the CNT as nucleating agents and compared it with selfnucleation. The nucleation efficiency (NE) is calculated using the following equation: NE =

Tc,NA − Tc,neat Tc,max − Tc,neat

× 100%

where Tc,NA is the peak crystallization temperature of the polymer with the nucleating agent, Tc,neat is the peak crystallization temperature of neat polymer, and Tc,max is the maximum crystallization temperature after the polymer has been self-nucleated and annealed. In the case of HDPE PCNs, it was shown that in all PCNs (with different CNT contents) NE is higher than 100% (151−440%), which implies that CNTs are more efficient in nucleating HDPE than its own crystals. They therefore refer to this nucleating effect as “supernucleation”. Figure 12 shows the NE of CNTs as nucleating agent for several polymer matrices. H

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rate >50 s−1 for 1 s) are requisite to form shish-kebab structure in the neat polymer. However, for the low shear (shear rate