Surface Functionality through Periodical Patterning on Carbon Nanotubes

From their foundational role in electronics to their pivotal contributions in the structural domain, carbon nanotubes (CNTs) exemplify a revolutionary nanomaterial with broad applications. The chemical inertness of the CNT surface is a primary limitation that restricts the full utilization of CNTs for mass use in several applications. Herein, works performed on the development of surface‐modified CNTs known as periodically patterned structures are focused. Different structures including shish kebab, nanofiber, nanoflower, and cheetah skin CNT have been studied. Periodic patterning on CNTs offers a chance to improve the nanotube's surface properties and roughness, facilitating better interaction with a matrix. Additionally, the fully controllable processing procedure opens significant opportunities for additional modification and attachment of phases onto specific areas of periodically patterned CNTs. The periodic patterning aspect holds promise for the mass production of surface‐modified CNTs, enhancing their surface multifunctionality. This advancement has the potential to cater to a diverse array of applications, offering improved surface functionality in a mass‐production setting.


Introduction
The history of carbon is richly adorned with the discovery and utilization of its various forms, from charcoal and carbon black to diamond and graphite among others.The application of carbon dates back to prehistoric times, evident in activities such as drawing, writing, or paintings found in caves of Lascaux, Altamira. [1]Carbon is one of the most studied elements in material science due to its impressive ability to form bonds with carbon and other atoms alike.Covalently bonded carbon atoms show ordered arrangements, which can be observed in various nanostructures such as graphene, nanotubes, fullerenes, and diamonds. [2,3]Among these nanostructured materials, carbon nanotube (CNT) is gleaned to be one the most promising material reinforcement due to their unique combination of thermal, mechanical, and electrical properties. [4,5]CNT discovery is credited to Ilijima in 1991, [6] who demonstrated the production of substantial quantity for the first time in DC arc discharge between graphitic electrode.Although, credit to the discovery of CNT is still debatable but this topic is exquisitely covered by Peter JF Harris in one of the sections in his work as "who discovered carbon nanotube." [1]Beyond such argument, CNT is regarded as a milestone that created "Nano Wave." [7] CNTs are seamlessly rolled graphene sheets of sp 2 -bonded carbon atoms of oriented hexagonal structure in a honeycomb lattice relative to the axis of the nanotube with termination (end cap), containing a hemisphere of fullerene.In the process of rolling graphite sheet, sp 2 hybrid orbital deformed to rehybridize toward sp 3 or σ-п bond mixing.J Han stated that the structural feature of rehybridization together with π electron confinement renders multifunctional attributes to CNT that includes electronic, mechanical, chemical, thermal, magnetic, and optical properties. [8]Dresselhause et al. coined CNT as "1D quantum wires" due to their small diameter (up to 0.7 nm) and relatively long length (up to several μm). [9]The first evidence of CNT possessed more than one graphitic layer was called multiwalled CNT (MWCNT).The central nanometric tube in MWCNT is surrounded by another graphitic layer with van der Waals force, distant at 0.34 nm.Primarily, CNTs are categorized as achiral (symmorphic) and chiral (nonsymmorphic) based on symmetry. [10]When CNT imitates the mirror image of its original structure then it is said to be achiral nanotube.Achiral nanotubes are further categorized as armchair or zigzag nanotube based on their degree of helical arrangement and chirality, as demonstrated in Figure 1.In contrast, chiral nanotube does not exhibit identical spiral symmetry and contains a mixture of cis and trans cross-sectional shape.Chirality is mainly defined as the orientation of the carbon atoms around the nanotube circumference. [11,12][20][21] In reality, 2/3 of all CNTs are semiconducting, and only 1/3 are metallic [22,23] Ebbesen et al. investigated the metallic and nonmetallic behavior of individual CNTs by lithographic deposition of tungsten by four-probe measurements. [24]hile numerous scholarly reviews have examined various CNT functionalization approaches, more recent works within the past few years have presented periodic patterning as an innovative CNT modification technique to regulate dispersion and interfacial interactions.Engineered periodic patterning imparted onto CNTs enables highly ordered nanotube-derived structures optimized for an expansive range of advanced applications.Although numerous researchers have utilized the method of periodic patterning of CNTs using semicrystalline polymers, to the best of our knowledge, there has been no comprehensive review to date examining the capabilities of this method to modulate CNT-polymer interfacial bonding.This review comprehensively examines research endeavors on periodical patterning on CNT for the first time.It explores various processing strategies, their impacts on structural and physicochemical properties, offering opportunities for enhanced understanding and new avenues for further research in the field.

The Need for Surface Modification on CNT
The striking properties of CNTs have claimed a range of applications including polymer nanocomposites [25][26][27][28][29][30] for coatings, structural materials, chemical sensors [31] for environmental monitoring, food packaging, and agriculture, biological sensors, biomedical applications [32][33][34][35][36] for drug delivery, [37] wound healing, gene, and cancer therapy, energy storage [38] for fuel cells, lithium batteries, chemical, and biological separations, catalyst, [39] water treatment, [40] etc.The intriguing properties of CNTs are exploited by their assembling into a number of morphologies from individual tubes to macroscopic architecture. [41,42]It is interesting to note that the optimal performance of CNTs for all conceivable applications is generally possible only in monodispersed form relative to electronic type, diameter, length, and chirality handedness. [43]However, synthesized CNTs are obtained in polydispersed, highly entangled, bundled forms with varying diameters, lengths, and structures. [44,45]It is well documented that the effectiveness of CNTs mainly relies on their appropriate dispersion or adhesion with functional moieties relative to their purity, length, and orientation, [46][47][48][49] but entangled and bundled CNTs drastically limit these possibilities. [50]In addition, the CNT surface is principally proven to be chemically inert which further limits the full potential exploitation of CNT for a number of applications.For instance, Ma et al. elaborated that, for a given volume fraction (0.1%) in 1.0 mm 3 cube, 442 million pieces of CNTs are required compared to two pieces of Al 2 O 3 and 200 of carbon fiber for a similar volume fraction. [18]Such a large number of CNTs induces severe agglomeration as a result of van der Waals force and electrostatic force among them.Therefore, to exploit the full potential of CNT, chemical modification or functionalization is found to be an essential processing step based on its intended application.
In this context, chemical (covalent) or physical (noncovalent) functionalization is considered as one of the efficient pathways to render physical and chemical attributes to CNTs.The chemical modification or functionalization can be defined as mean in-lattice doping, intercalation, molecule or particle adsorption, encapsulation, etc. [51] As depicted in Figure 1, different routes are available for the functionalization of CNTs but principally covalent and noncovalent are considered as prime pathways. [52]oncovalent functionalization is conceived by van der Waals force, π-π, CH-π, or electrostatic interaction between polymer and CNT surface or physical adsorption of suitable molecules on CNT sidewall. [50]Such functionalization is achieved by polymer wrapping, surfactant, or small aromatic molecule adsorption and interaction with porphyrins and biomolecules.In contrast, the covalent functionalization method involves covalent bonding or grafting of polymer chains with CNTs.This method can achieve very strong bonding between the attached moieties Figure 1.Different functionalization routes for CNTs. [136]Reproduced with permission. [136]Copyright 2021, Wiley-VCH.
[55][56] In addition, covalent functionalization can be anchored from the reactive sites generated at the point of defect.It is acknowledged that synthesized CNTs contain a number of defects that later act as an anchor for chemical reactivity.For instance, the sidewall of CNT may contain five rings or seven rings or pair of both (Stone-Wales defect) enforcing bending or formation of curvature which may enhance the chemical reactivity. [57]Further reactive sites may be created when the presence of catalytic particles at the open end is removed by the oxidative method, creating oxygen-containing functional group decorated CNT. [57]In contrast to fullerene, sidewalls of CNTs are energetically less favorable, and therefore, addition reactivity is mainly influenced by пorbital misalignment compared to pyramidalization strain. [58]пorbital misalignment compared to pyramidalization is inversely related to the CNT diameter and hence, small diameter CNTs are more reactive than larger diameter.While both covalent and noncovalent functionalization methods have their respective advantages and disadvantages, it is noteworthy that noncovalent functionalization, for instance, preserves the intrinsic characteristics of individual CNTs without altering their structural and electronic properties. [59]Apart from that, it paves the way for CNTs to induce strong interfacial adhesion for polymer nanocomposites, conjugation of deoxyribonucleic acid, peptide nucleic acid, antineoplastic agents for biological and therapeutic application, [36,60] tethering of chemical moieties on CNTs surface for sensor application, [58,61] opening of hollow cavities at sidewall for storage application, [58] availability of inner cavity, peripheral groves, interstitial channels for water treatment, [40] and so on.Hence, the burgeoning field of chemistry contributes to an enhanced understanding of CNTs, paving the way for anticipated transition from laboratory trials to full-scale industrialization.

Background and Mechanism
Many review articles have covered various aspects of CNT functionalization.However, an emerging approach for CNT dispersion and interfacial bonding, commonly known as periodic patterning, has been introduced in the last few years.After Li et al. [62] found the method of periodic patterning of CNTs using semicrystalline polymers many researchers have demonstrated its usefulness in altering CNT/polymer bonding.The periodic patterning on CNTs makes these materials a suitable candidate to produce well-ordered structures with a wide variety of applications ranging from optical to electronic field. [63]Periodic functionalization of CNTs is challenging due to the difficulties in controlling the surface functional groups on CNTs on a nanometer scale.So far only a few methods have been reported for this purpose.One of the methods to achieve this structure is through the Bingel reaction, [64] where the long-range induced reactivity resulted in the formation of long-distance patterns (≈4.6 nm in periodicity) on the CNTs surface.The spatial fluctuation of the electron density has been shown as an important factor for obtaining high-regular-long-distance patterns. [64][71] Even though CVD technique could produce highly ordered nanostructures, the functionalization of CNTs at high temperatures usually damages the tubes. [72]NTs have been shown to act as nucleating agents and thus, to enhance the crystallization kinetics of polymers. [73]For example, periodically patterned CNTs with redox-active polyoxometalates were used as novel high-performance battery components. [74]he nucleation and orientation of the extended-chain crystals is a desirable structure for the formation of strong fibers. [75]rystalline polymers can be used to wrap the surface of CNT as a type of noncovalent functionalization possessing better mechanical properties compared to covalent methods.In addition, it prevents the aggregation of tubes in polymer matrix and in the solvent. [76]The noncovalent periodical patterning on CNT has the following advantages when compared to methods such as a surfactant or amorphous polymer wrapping: 1) crystalline polymers exhibit greater mechanical robustness compared to small molecules and amorphous polymers, making them a preferred choice for composite reinforcement; 2) the 3D structure of modified CNT positions it as an excellent candidate for reinforcement, attributed to the "nanoanchor effect"; [77] and 3) the potential to introduce functional groups to the ends of the polymer chain exists during crystallization. [78]1.1.Shish-Kebab Structure Nanohybrid Shish Kebab (NHSK): The shish-kebabs structure has generated a great deal of interest due to its potential to fabricate periodically patterned structures ranging from a few to hundreds of nanometers.[79][80][81] This structure closely resembles the traditional "shish-kebab" polymer crystals proposed in 1960s.[82,83] In a classic shish-kebab structure, a central fibril (shish) is surrounded by disc-shaped, folded-chain lamellae (kebabs), both composed of polyethylene (PE) crystals and oriented orthogonally to the shish.However, in the nanohybrid shish-kebab structure, the shish is composed of CNTs.The structural similarities include the diameter of the shish (central fibril core), approximately ranging from one to a few tens of nanometers, the shape and thickness of the disc-shaped lamellar single crystals (kebabs) enveloping the core, the orientation of the kebab to the shish, and the periodic arrangement of these lamellae along the onedimensional (1D) central cores.[79] Since the polymer chains act as kebabs, they can be side-or end-functionalized.Upon crystallization, due to the separation of functional groups on the surface of the polymer single crystals, the enrichment of the functional groups near to CNT surface occurs.[84,85] The functionalization degree can be tuned by controlling the important factors in crystallization process including the crystallization time and the cooling rate (Figure 2). [79] Th formation of NHSK structure with various types of polymers including PE block copolymers, poly(vinylidene fluoride) (PVDF), nylon, polybutylene terephthalate, poly(vinyl alcohol) (PVA), isotactic polypropylene, poly(l-lactide) (PLLA), PE-b-poly (ethylene oxide) (PEO), and polyoxymethylene (POM) have been investigated thoroughly in the literature.[86][87][88][89][90][91] These polymers can form a zigzag conformation in their crystal state resulting in NHSK structure.[76] NHSK structure is important as the origin of high-strength polymer fibers and films.[92] The NHSK structure comprised of PE crystals has shown a more regular pattern compared to nylon 66, PVDF, and poly-(L-lysine) NHSKs, likely due to the more flexible nature of PE chains.[93] Heeley et al. proposed a model for NHSK formation under different crystallization conditions. [94] Thy have proposed some small crystallites in the polymer amorphous matrix.In addition, the crystallinity, the size of crystallites, and the thickness of NHSK structures increased with an increase in isothermal crystallization temperature.[94] Two mechanisms have been reported for NHSK formation known as mode A and mode B. In mode A, the polymer chains are directly nucleated on the CNT surface.On the contrary, the polymer chains are wrapped around the CNT surface before the nucleation and crystal growth in mode B. [62] For instance, the transmission electron microscopy (TEM) images of the NHSK structure of PE block copolymers on CNT, have shown the coverage of CNT surface with PE layer and the growth of lamellar crystals subsequently.[86] The alignment of polymer chains along the CNT axis is required in both mechanisms.The oriented NHSK structure of poly (ethylene terephthalate) (PET) along MWNTs was observed using small-and wide-angle X-ray scattering (SAXS/WAXS).Thermal characterizations of MWNTs in PE matrix confirmed the increase in crystallization temperature of PE with an increase in MWNT content.However, adding a higher MWNT content lowered the crystalline perfection.The diffused SAXS patterns from neat polyethylene glycol (PEG) confirmed the amorphous structure of the polymer.Nevertheless, the crystalline structures with specific orientations were observed in PET-MWNT composites having NHSK structure.In addition, a transition from a well-oriented NHSK structure to a less ordered structure was observed with a reduction in crystallization temperature.The hot crystallization process led to more alignment than cold crystallization.It has been shown that the MWNT alignments during the extrusion act as prealigned nucleation sites which do not relax in polymer melt.[95]

Nanofiber Shish Kebab
In addition to the nanohybrid shish-kebabs structure, the nanofiber shish kebabs (NFSK) have also been fabricated using electrospunned nanofibers as the shish. [92]The utilization of the electrospinning technique enables the creation of hybrid composites with a uniform structure derived from polymer solutions or melts. [96]Similar to NHSK, in NFSK, the electrospunned nanofibers act as shish while kebab crystals form on them. [62]The single crystal structure has been decorated on nanofibers by either slow or fast crystallization techniques. [97]Wang et al. fabricated nanofibers from the PEO/dimethylformamide (DMF) solution.The nanofibers were incubated (slow crystallization) in a polymer solution and were dried after washing with DMF to remove the free polymers.In contrast, in the solvent evaporation technique, the PEO nanofibers were dried at room temperature. [97]HS-PEO decorated nanofibers were immersed in the golden nanoparticles solution for 60 min.Free AuNPs and ligands were removed by repeatedly washing with toluene.As shown in Figure 3, the AuNPs establish Au lines on the NHSK which are perpendicular to the nanofiber.As AuNPs are closely packed across the fiber and periodically located on the fiber(≈20 nm) which is consistent with the period of the HS-PEO NFSK in Figure 4, it is confirmed that the immobilization of the AuNP on the NFSK is because of the formation of the AuÀS bonds between HS-PEO and the AuNP.

Nanoflower Structure
A periodic patterning on CNTs could also be achieved by generating flower-shaped nanomaterials called nanoflowers.b) TEM imaging of the PE/single wall CNT periodically patterned structure; scheme of the PE/CNT shish kebab and nanohybrid shish kebab. [62]Reproduced with permission. [62]Copyright 2006, American Chemical Society.
The flower-like structure has some advantages including large surface area, low packing density, and resistance to aggregation.They are constructed using inorganic or organic materials, or their combination (called a hybrid) and are commonly used in energy storage and biomedical applications. [98]Zhang et al. fabricated well-dispersed manganese oxide nanoflowers on a vertically aligned carbon nanotube array (CNTA) framework by electrodeposition technique. [99]In addition, nanoflowers could also be generated after polymer wrapping around CNTs.For example, Yang et al. reported an effective method of decorating Pt nanoflowers on the surface of CNTs after wrapping with poly(sodium 4-styrenesulfonate) or chitosan.Here, potassium tetrachloroplatinate (K 2 PtCl 4 ) was reduced L-ascorbic acid in an aqueous solution and Pt nanoflowers were deposited onto the surface of polymer-wrapped CNTs.Polymer wrapping facilitated the stable dispersion of CNTs in water and introduced functional groups capable of binding metal ions/nanoparticles.These Pt NFs/PSS-CNTs showed high electroactive specific surface making them ideal for oxygen reduction process and also find usage in catalysis, fuel cells, and biosensors. [100]sed on Bao et al.'s study, [101] in which the flower formation of crystallized PE terminated with ferrate complex (PR-Fe) having thin petals in Xylene has been reported, [101] Liang et al. produced organometallic polymer flowers along CNTs. [102]The formation of individual flowers in the absence of CNTs demonstrated the crucial role of the tubes and cyanoferrate in flower bundle formation.The differential scanning calorimetry (DSC) results confirmed the nucleation promotion in the presence of CNTs.The temperature effect on morphology and flower bundle formation mechanism has also been investigated using scanning electron microscopy (SEM) images.NHSK structure was observed at 50 °C, whereas random aggregates with CNTs were observed at 42 °C.The effect of CNT/PE-Fe ratio has shown the NHSK structure formation upon increasing this ratio.This has been attributed to the dominant effect of CNTs in heterogeneous nucleation.However, decreasing this ratio resulted in random aggregate formation due to homogenous nucleation of PE-Fe.These flowers have shown improved electrochemical activity and thermal stability with a wide range of applications from biosensing and nanodevices to the biological field.A higher  [97] Copyright 2008, American Chemical Society.
temperature for weight loss in PE-PB/ CNT bundles compared to pristine PE has shown the thermal stability of flower bundles using TGA.Furthermore, it has been shown that the electrochemical properties of the PE/PB-CNT hybrids can be tuned by varying crystallization temperatures.

Cheetah Skin CNT
The periodical patterning on CNT can be performed through the formation of the nanocarbon on the surface of CNT by controlling the size and distance between the nanocarbons.In our work that was published previously, [48] polyacrylonitrile (PAN) was employed as a starting material for the formation of the nanocarbon formation and poly(methyl methacrylate) (PMMA) as a substrate to disperse PAN onto the surface of the tubes.It is noteworthy that meticulous control of process parameters coupled with careful material selection are the key elements to achieve a distinctive periodical pattern.
A uniform dispersion of PAN particles (round) within the PMMA matrix is achieved by using a shearing process that is vital for creating various controlled morphologies onto the surface of CNTs.By altering manufacturing process parameters such as shear/stirring coefficients and thermal treatment/drying parameters, the final morphology can be altered.From Figure 5, it can be clearly seen that the round PAN particles were uniformly dispersed in the PMMA matrix in all cases, forming a tubular coating onto the surface of the CNTs, as the CNTs acted as a template for the polymerization process.The planar zigzag structure is thought to facilitate the formation of coatings on CNTs compared to helical polymers, due to the low helical symmetry and the presence of bulky side groups that reduce the regularity and contact density.Figure 5 demonstrates that under specific crystallization conditions, PAN with a helical structure has the capability to form both extended chains, represented by tubular coatings, and distinctive cheetah skin structures. [48] Case Studies of the Periodical Patterning on CNTs

Polyethylene on CNT
Various methodologies and starting materials have been employed to develop periodical patterned CNT with the assistance of polyethylene.To investigate the effect of different types of PE on chain mobility during the crystallization and kinetics of the reactions, crystallization kinetics of high-density and lowdensity polyethylene were investigated on CNTs via interfacial crystallization. [103]To allow polymers to crystallize on welldispersed sinlgle wall CNT (SWCNT) and thus to eliminate agglomeration of SWCNTs in subsequent steps, polymer-xylene solutions, and SWCNT-xylene dispersions were first precipitated and then flocculated in ethanol.It was found that high density polyethylene (HDPE) crystallized faster than low-density polyethylene (LDPE) on SWCNTs due to its linear chains enabling increased polymer density and chain mobility.A simple approach for generating high-performance composites together with NHSK structure was shown in Shi et al.'s study. [104]They melt-blended CNT with linear LDPE (LLDPE) for 4 cycles and then molded by microinjection molding (MIM) to prepare LLDPE/CNTs nanocomposites.Unexpectedly, a kind of NHSK structure was detected in the samples, leading to the mechanical enhancement.LLDPE nanocomposite containing 1 wt% CNTs increased the tensile strength, modulus, and toughness by 64.1%, 100%, and 134.6%, respectively.Apparently, repeated shear force in melt-blending together with the strong shear force (as high as 7.8 Â 103 s À1 ) in MIM process enabled efficient dispersion of CNTs in LLDPE and resulted in a remarkable reinforcement effect.According to the results, it would be possible to generate high-performance composites together with NHSK  [99] Reproduced with permission. [99]Copyright 2008, American Chemical Society.
NHSK structure of PE.In the case of CNTs and CNFs, polymer crystals formed classical hybrid shish-kebab structures by following the "soft epitaxy" mechanism.However, CFs did not induce PE chains to form hybrid structures, although CFs and CNFs have the same graphitic planes (002), and CFs are anticipated to form hybrid structures if the crystallization patterns obey the lattice matching mechanism.In the case of MWCNTs and CNFs, the van der Waals attractive forces were sufficient to drive PE chains to deposit on the surface of the fibers, forming PE underlayer.After decreasing the temperature, ordered subglobules evolved into typical hybrid shish-kebab structure.However, in the case of CFs having larger diameters, the attractive van der Waals interactions between CFs and PE chains were insufficient from PE underlay, which resulted in the absence of shish-kebab structure.Accordingly, the most plausible explanation for the formation mechanism of hybrid shish-kebab structures would be the "size-dependent soft epitaxy" for CNTs and CNFs (Figure 6).
Free-standing CNT films or buckypapers showed exceptional mechanical and electrical properties and gained lots of interest in the last decades.Fabrication of free-standing CNT paper with separated CNTs and tuneable pore size has been very attractive for various applications.Laird et al. [106] reported the fabrication of uniform, free-standing CNT films by vacuum filtration of NHSK suspension by using polyethylene single crystal-decorated CNTs as the precursor.Both superhydrophobicity and high adhesion were achieved thanks to hierarchical roughness created by intraand inter-NHSK nanostructure, which can find applications in sensors, electrochemical devices, and coatings.

PEG/PE on CNT
Le et al. [107] compared the effect of copolymer composition on nanohybrid NHSK architecture on CNTs.The researchers used a semicrystalline amphiphilic di-block copolymer, polyethyleneb-polyethylene glycol (PE-b-PEG), and varied the copolymer composition based on molecular weight and the PE to PEG ratio.The mobility of PEG is influenced by its chain length, which significantly influences the shish kebab development process.Higher PEG chain length provided better stability to copolymer micelles.The stability of the NHSK architecture was found to change over time at the same crystallization temperature.This work sheds light on the mobility of PEG in the copolymer and its impact on the formation and stability of disk-shaped crystals during crystallization with CNTs.The study offers valuable insights for mechanically tunable NHSK with adjustable copolymer crystal architecture by modifying PEG's molecular weight (Figure 7).
The selectivity of the solvent to a segment in a block copolymer determines the self-assembled structure of the polymer on CNT surface.For example, the periodic structure of PE-b-PEO has been obtained in 1,2-dichlorobenzene (DCB) or p-xylene as selective solvents for PE segments.However, the periodic patterned structure was not observed in N,N-dimethylacetamide (DMAc) as a selective solvent for PEO segments. [87]In this study, the doublecrystalline di-block PE-b-PEO was successfully modified on CNTs using a supercritical carbon dioxide (SC CO 2 ) antisolvent-induced polymer epitaxy method.TEM observation showed that the unique PE-b-PEO copolymer decorates CNTs periodically, resulting in a novel amphiphilic nanohybrid structure.The choice of solvent played a decisive role in the morphological assembly of PE-b-PEO on CNTs.Solvents like DCB or p-xylene led to periodic patterns due to nanotube-induced PE crystallization, whereas DMAc favored thin polymer coatings.Increasing SC CO 2 pressure significantly enhanced the decorating degree of PE-b-PEO on CNTs.This work suggests a controllable method using SC CO 2 to fabricate functional CNTs-based nanocomposites with various micro morphologies in different organic solvents.
Le et al. [108] employed a statistical model for the first time to study the effects of different parameters such as polymer concentration, crystallization time, and undercooling temperature on the NHSK architecture of CNTs and polyethylene-b-polyethylene glycol (PE-b-PEG) block copolymer.They reported the usefulness of response surface methodology-central composite design to predict various NHSK structural features such as diameter, periodicity, and thickness.This approach provides a better understanding of how to control and create tunable hierarchical structures on CNTs, which is crucial for their unique applications in hierarchically ordered polymer nanocomposites.

Nylon on CNT
Nylon wrapping on CNT was achieved through noncovalent functionalization, utilizing MWNT and Nylon 66 single crystals in a controlled solution crystallization process. [78]In this method, Nylon 66 lamellar crystals formed kebabs on the CNT shish.As MWNT content increased, the heat of fusion at lower temperatures and the associated endothermic peaking also increased, indicating that the network of MWNT nanotubes may impede the lamellar thickening process.The addition of CNT in Nylon 66 slightly decreased its growth dimension.This study showed that the effect of MWNTs on Nylon 66 crystallization is twofold: MWNTs provide heterogeneous nucleation sites for Nylon 66 crystallization while the tube network structure hinders large crystal growth. [78]The coverage of CNT surface with polymers can improve the mechanical properties of the system.For instance, the interfacial load transfer and mechanical properties of CNT/nylon-11 NHSK structure have been investigated using Raman spectroscopy.It has been found that the thermal coefficient mismatch between the CNT and polymer chains leads to the CNT straining in the absence of external force.The improvement in strains exhibited by CNTs leads to the enhanced load transfer in NHSKs structure.Periodic kebab crystals consisting of the aligned polymer chains made the CNT surface rougher and enhanced the interaction between the CNT and polymer matrix. [109]Therefore, increasing the crystalline polymer interphase resulted in an enhancement in CNT/matrix load transfer. [110,111]

PET on CNT
There are various studies related to PET/CNT nanocomposites.However, these studies deal mainly with properties after processing.Few studies investigated the in situ morphology evolution during processing.The WAXS and SAXS observations showed the role of CNT in the orientation, crystallinity, and structural  a-d). [107]Reproduced with permission. [107]Copyright 2019, Wiley-VCH.
development of PET/MWCNT composites.The PET/MWCNT composites revealed an NHSK morphology with reduced orientation and crystallinity.Uniaxially deformation of PET-MWCNT films resulted in improved mechanical properties compared with the neat PET films.Here, MWCNTs acted as shish for the epitaxial growth of PET crystallites.Nucleation and crystal growth occurred in the PET matrix.However, MWCNTs hindered a full lamellar structure by inhibiting the crystallite development of PET.Using the cheetah skin CNT as a Bisphenol A (BPA) removal agent/catalyst a) kinetics, dark BPA adsorption studies, and total organic carbon (TOC) changes.The kinetic and TOC changes in H 2 O 2 condition.(N0: pure CNT; N9, N10, N11, and N12: different types of cheetah skin CNTs). [42]Reproduced with permission. [42,112]Copyright 2018, Elsevier.

Polyacrylonitrile on CNT
The load-transfer and interfacial morphological structure of PAN were investigated by adding SWCNTs, and a nonuniform distribution of PAN kebabs with varying lamellar size was observed. [75]This phenomenon was attributed to the large steric hindrance effect induced by the bulky nitrile side groups.For the formation of cheetah skin CNT, an innovative approach was Reproduced with permission (Bottom: a-f ). [119]Copyright 2015, American Chemical Society.
employed based on the concept that immiscible polymer blends can form matrix-disperse morphology, effectively modifying the surface of the CNT.As discussed extensively in previous works, [48,112] PMMA works as a matrix and PAN as a dispersed phase.Following a carefully controlled mixing process and subsequent thermal treatment, the PMMA underwent evaporation, leaving behind carbon droplets adhering to the nanotube surface.This resulted in the formation of a distinct cheetah skin structure.The mechanism relies on the polymer chains being attracted to each other, leading to the formation of a long-range three-dimensional order, resulting in a solid mass. [113]The crystallization process in the CNT/PAN system is regulated by both heat treatment and solvent evaporation (Figure 8).
Utilization of the cheetah skin CNT and comparing its influence as a reinforcement for PE-based composite have been shown in Figure 9. Due to better interfacial bonding with the matrix being translated into the change of mode of crack initiation and propagation during the loading. [112,114]The cheetah skin-CNT/PE showed 51% increase in elastic modulus compared to 35% for pure CNT/PE.The cheetah skin CNT has been used as a removal agent for BPA removal too. [42]The cheetah skin CNT, acting as a coating or film, demonstrates a synergistic effect for efficient removal due to its unique combination of a great substrate for removal purposes and a nanocarbon surface capable of attaching to mBPA molecules.This coating comprises surfacemodified nanotubes in each plane, forming an effective removal plane.The results indicate a significantly higher removal capacity for cheetah skin CNT compared to pure CNT, which also actively engages in the photooxidation of BPA.Additionally, the porosity of CNT, not just within the tubes but also between CNT bundles and the modified surface, contributes to the overall removal process". [42]6.Poly(L-Lactic Acid) On CNT In an attempt to prepare conductive nanoporous polymeric materials, [115] Ye et al. [116] simply incorporated MWCNTs into melt-miscible poly(L-lactic acid) (PLLA)/poly(oxymethylene) (POM) blends.It was found that the POM component first crystallized onto the MWCNTs and formed the NHSK structure at high temperatures, while poorly crystallizable PLLA chains were expulsed into the intra-NHSK regimes simultaneously.Afterward, crystallization of PLLA transformed the morphology into "ternary-hybrid shish kebab" superstructure, which is called "block-assembling."It is proposed this novel structure as a new and simple template to prepare conductive nanoporous polymeric materials by selectively removing PLLA component.

Isotactic Polypropylene on CNT
Lu et al. [117] used combined DSC and TEM to analyze bulk composite materials and ultrathin film CNT/iPP samples.In the study, the crystallization behavior of isotactic polypropylene (iPP) near SWCNTs and MWCNTs was investigated.They found that CNTs act as nucleation sites for iPP during crystallization, leading to wrapping of iPP around the CNTs.The transcrystalline layer is highly oriented around the CNTs, with the c-axes of the lamellae perpendicular to the CNT's long axis, contrary to previous assumptions.The study suggests that iPP macromolecules wrap around the CNTs before nucleus formation, providing an explanation for this unique crystallization behavior.

Other Case Studies
It is known that polyferrocenyldimethylsilane (PFS) homopolymer and PFS-based block copolymers form plate-like crystals in solvents such as decane or hexane.In addition, PFS and its block copolymer micelles can nucleate seeded growth and form uniform elongated structures with controlled length. [118]Jia et al. [119] investigated the possibility of preparing such kind of structures by coating the surface of MWCNTs with PFS homopolymer crystals and PFS block copolymers.To that end, they used PFS 31    d). [121]Reproduced with permission. [121]Copyright 2019, Wiley-VCH.
CNT generated elongated micelles, which resulted in structures resembling hairy caterpillars.However, TEM characterization clearly showed that these structures are very similar to NHSKs and share aspects in common with the NHSKs.Further decoration of these structures with the PFS-b-PI and PFS-b-P2VP micelles led to an unusual wavy kinked structure, which is very different from their uniform smooth structures.Zhang et al. [120] investigated the effect of block length on PE crystallization behavior on CNT, CNF, and graphite by solution crystallization.To that end, they synthesized ethylene/functional cyclic olefin block copolymers (PE-b-HPNBMA) with different noncrystallizable HPNBMA block lengths.Previously prepared block copolymer-xylene solutions and carbonaceous nanofillerxylene dispersions were stirred at 110 °C for 30 min and then temperature was decreased to 60 °C for 5 h for the isothermal Reproduced with permission. [122]Copyright 2015, Elsevier.
crystallization.The usage of CNTs and CNFs induced PE chains to crystallize in the form of NHSK.The length of HPNBMA block changed not only the nanocrystal (kebab) size but also the surface chemical structure.The NHSK films exhibited superhydrophobicity (contact angle > 150 o ) with tuneable adhesion.In contrast, the graphite usage led to a honeycomb structure instead of the NHSK.
Another facile method to achieve an NHSK structure on CNTs could be the combination of polymerization and crystallization in situ in a single step.There are some attempts to achieve this goal, ending up with NHSK structures in a few cases but with limited control.Xu et al. [121] demonstrated a one-step large-scale in situ synthesis of a polycyclopentene (PCP) with NHSK structure in the presence of MWCNTs.Films made of PCP-crystaldecorated MWCNTs showed the characteristic lotus-leaf-like superhydrophobicity (contact angle > 150°), while being electrically conductive.The contact angle measurement of the surfacemodified CNTs (lotus-leaf-like) is superhydrophobic(152-155°) confirming the effectiveness of NHSK formation using PCPs (Figure 10).
[124] Looking at classical polymer solution crystallization, polymer single crystals can be formed in the interface, in the case of polymer crystallization happens after liquid/liquid separation.There are already some studies showing the formation of SWCNT nanoring (100 nm to ≈ microns) when used as a pickering agent. [125]Considering these facts, Wang et al. prepared oil in water (O/W) pickering emulsions by using SWCNTs and showed ring-shaped NHSK structure at L/L interface.They used a dilute polymer solution (polyethylene (PE), poly(L-lactic acid) (PLLA), and poly(3-hexylthiophene) (P3HT) as the oil phase, which crystallizes onto the SWCNT nanoring upon cooling.Despite the small diameter of the ring and the curved nature of the interface, they showed the formation of NHSK structure due to the domination of heterogeneous nucleation in the crystallization process.They also used polyethyleneblock-polyethylene oxide (PE-b-PEO) to decorate the SWCNT rings.They showed the assembly of AuNP nanowires in the block copolymer (BCP) NHSK nanoring by selectively functionalizing the PEO block (Figure 11). [122]genhuber et al. [126] developed a noncovalent functionalization method using SEM triblock terpolymers to modify MWCNTs in organic media.The PE middle block of SEM strongly adsorbs to the CNTs' surface, resulting in stable and well-separated 1D hybrid structures with up to 3 wt% CNT content.The incompatible polystyrene (PS) and PMMA end blocks form alternating corona patches, providing excellent steric stabilization for the CNTs.The method preserves the sp 2 -structure of CNTs and offers a simple, efficient, and time-saving approach for preparing polymer-stabilized CNTs.The hybrids demonstrate potential as intelligent fillers in polymer blends, serving as both compatibilizers and reinforcing agents (Figure 12).
Zhou et al. [127] investigated the crystal morphology of poly (ε-caprolactone) (PCL)/MWCNT blends and MWCNT-g-PCL grafting polymers that were crystallized in n-hexanol.They observed two main structures: a straight and rod-like core-sheath structure, where MWCNTs acted as the core and PCL polycrystals formed the sheath with high crystallinity, and a bent doublelayer structure, where MWCNTs were covered by a PCL layer with low crystallinity.Thinner and shorter MWCNTs were more easily straightened by PCL crystals.Electron diffraction and TEM showed that PCL crystals had random orientation with the b-axis perpendicular to the MWCNT surface.The researchers concluded that the crystallizability of PCL and the dimensions of MWCNTs influenced the formation of different structures, and PCL chains' crystallizability affected their ability to straighten MWCNTs.
Uchida et al. [128] studied the crystallization of Poly(p-phenylene benzobisthiazole) (PBZT) from dilute solutions in the presence of SWCNT or MWCNT as nucleating agents.They found that both SWCNT bundles and MWCNT acted as nucleating agents for PBZT crystallization, but the nucleating effect of SWCNT bundles was higher than that of MCWNT.The morphology of PBZT crystals formed on the CNT could be controlled by varying the crystallization conditions, such as PBZT molecular length and the type of CNT.Furthermore, PBZT crystals formed on SWCNT bundles exhibited improved thermal stability, and subsequent heat treatment in the solvent further enhanced their thermal stability through dissolution and recrystallization.
and SC/homocrystal (HC) kebabs.They combined electrospinning and controlled polymer crystallization to create hierarchically ordered PLA NFSKs.The SC-PLA nanofibers served as the shish, and PLA in the form of HC or SC was decorated on the nanofiber surface to form single-crystal kebabs.The NFSKs were formed through a soft epitaxy mechanism, demonstrating the capability of SC crystals to nucleate PLA SC and HC.
The study provides insights into the controlled modification of nanofiber surfaces, enabling the introduction of multifunctionalities in an ordered fashion.
Arras et al. [92] demonstrated the creation of a hierarchical NHSK structure during electrospinning.MWCNTs were aligned within PCL nanofibers, forming the NHSK structure in situ.Additionally, a NFSK structure was achieved by incubating the NHSK-containing nanofibers in a supersaturated PCL solution.This process was confirmed using TEM.The study introduced a novel polymer system and proposed a hierarchical structure based on shish-kebab morphology, offering a potential avenue for future high-performance materials bridging nanofillers like CNTs with the macroscopic world.The findings showcase the potential of creating advanced materials through controlled morphologies (Figure 13).

Conclusion
The agglomeration of nanoparticles including CNT poses a significant challenge for industries aiming to achieve widespread use of CNT.This enhancement improves the interlocking capabilities with a matrix.The development of periodically patterned CNT presents an opportunity to address this challenge by establishing fully controlled surface properties.This in turn enhances interlocking capabilities with a matrix, offering a promising solution for overcoming the issues associated with nanoparticle agglomeration.Periodical patterning of CNT presents an opportunity to improve the surface properties and roughness of the nanotube, enabling better interaction with a matrix or facilitating the incorporation of a second phase.Despite significant efforts dedicated to the development of periodically patterned CNT, only a limited number of studies have explored their potential industrial applications.Several factors contribute to the limited industrial adoption of this technology.The cost associated with pristine CNT, the complexity and expenses related to the periodical patterning process, and concerns regarding health and safety associated with CNT usage are key considerations.Addressing these challenges is crucial to realizing the broader industrial potential of periodically patterned CNTs.The periodic patterning observed on CNTs can be extended to other templates and further combined with various materials and nanostructures to create hybrid structures.A recent work that focused on a hybridization of the boron nitride and CNT in a shish-kebab structure, represents an enhanced thermal conductivity and tensile strength in a HDPE matrix that referred to the synergy of the hybridization and periodical patterning process. [130]he synergistic combination of hybridization with advanced materials like boron nitride, graphene, liquid metal, and MXENE, along with the periodic patterning on CNTs, presents novel possibilities for advanced applications. [130,131][134] It is crucial to highlight the potential for utilizing these patterns in additional modifications such as drug delivery or attaching new phases for targeted dosing.Consequently, the versatility inherent in this structured approach suggests its capability to contribute to various cutting-edge technologies including medical applications, electronics, food packaging, biomedical sensors, drug delivery, and structural and composite applications [48,135] making it a valuable candidate for future advancements across multiple industries.

Figure 2 .
Figure 2. a,b) TEM imaging of the PE/single wall CNT periodically patterned structure; scheme of the PE/CNT shish kebab and nanohybrid shish kebab.[62]Reproduced with permission.[62]Copyright 2006, American Chemical Society.

Figure 3 .
Figure3.Scheme of NFSK growth; NFSK developed using HS-PEO.TEM observation of gold nanoparticle-modified NFSKs using the incubation crystallization technique.The K-P concentration (0.05 and 5 wt%, for a and b, respectively).c and d) Developed using the solvent evaporation technique.d) An enlarged view of c) (-: PEO chains; •: AuNPs) Reproduced with permission.[97]Copyright 2008, American Chemical Society.

Figure 7 .
Figure 7.The formation of the copolymer micelle (PE-b-PEG) (scheme a,b) and the effect of crystallization time in the formation of the periodical pattern around CNT (electron microscopy images: a-d).[107]Reproduced with permission.[107]Copyright 2019, Wiley-VCH.

Figure 8 .
Figure 8.The effect of using cheetah skin CNT on enhancing the interfacial bonding with PE matrix and changing the failure mechanism in PE/CNT PE/cheetah skin CNT composites (Scheme and mechanical Graphs a,b and SEMS a-f ).[112]Using the cheetah skin CNT as a Bisphenol A (BPA) removal agent/catalyst a) kinetics, dark BPA adsorption studies, and total organic carbon (TOC) changes.The kinetic and TOC changes in H 2 O 2 condition.(N0: pure CNT; N9, N10, N11, and N12: different types of cheetah skin CNTs).[42]Reproduced with permission.[42,112]Copyright 2018, Elsevier.

Figure 10 .
Figure 10.In situ synthesis of a PCP with NHSK structure in the presence of MWCNTs (TEM images a-d); contact angle measurement of different samples (SEMs and inserted images of contact angles a-d).[121]Reproduced with permission.[121]Copyright 2019, Wiley-VCH.
Films formed with PFS-b-PI micelles were superhydrophobic (contact angle, 152 o ), while those of PFS-b-P2VP micelles were hydrophilic (contact angle, 54 o ).Manipulation of surface properties through changing the block length could open the door to interesting applications of MWCNT hybrid materials.

Figure 11 .
Figure11.a) Formation process of NHSK ring and the case of PE/DCB/CNT ring (Scheme).Structural imaging of the NHSK ring and associated data in case of SWCNT/PE ring (TEM images a-e).Reproduced with permission.[122]Copyright 2015, Elsevier.