High molar mass amphiphilic block copolymer

Polymer 127 (2017) 15e27

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High molar mass amphiphilic block copolymer enables alignment and dispersion of unfunctionalized carbon nanotubes in melt-drawn thin-films* Matthias M.L. Arras a, b, Bojia He a, 1, Klaus D. Jandt a, c, * a

Chair of Materials Science, Department of Materials Science and Technology, Otto Schott Institute of Materials Research, Faculty of Physics and Astronomy, €bdergraben 32, 07743 Jena, Germany Friedrich Schiller University Jena, Lo Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 May 2017 Received in revised form 13 August 2017 Accepted 14 August 2017 Available online 22 August 2017

To extensively control the nanofiller arrangement (location, orientation, shape) is still a bottleneck for multi-wall carbon nanotube (MWCNT) nanocomposites. Here, we demonstrate simultaneous control of alignment (orientation) and dispersion (location) of pristine, i.e., unfunctionalized MWCNTs using a high molar mass (HMM) amphiphilic block copolymer (BCP). We tested whether a HMM BCP in a selective solvent (i) disperses MWCNTs (ii) disperses MWCNTs by similar mechanisms to low molar mass BCPs and (iii) is melt-drawable to align the well dispersed MWCNTs. The dispersibility of MWCNTs within poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) (M w z500 kg=mol) and its homopolymers in (non-)selective solvents was investigated by sedimentation experiments, transmission electron microscopy and visible/near-infrared spectroscopy. Through BCP micelle mediated steric stabilization, HMM PS-b-P2VP led to a highly stable MWCNT dispersion, which is explained in two simple graphical models. Using the melt-drawing technique, the well dispersed MWCNT/PS-b-P2VP dispersion was processed into a nanocomposite with a high degree of MWCNT alignment and dispersion. HMM BCPs are of significance for structural MWCNT/polymer nanocomposites, typically containing HMM polymers. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes (CNTs) Block copolymer (BCP) Alignment Dispersion Poly(2-vinylpyridine) (P2VP)

1. Introduction The high aspect and surface-area-to-volume ratio as well as the

* This manuscript has been co-authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy. gov/downloads/doe-public-access-plan). * Corresponding author. Department of Materials Science and Technology, Otto Schott Institute of Materials Research, Faculty of Physics and Astronomy, Friedrich €bdergraben 32, 07743 Jena, Germany. Schiller University Jena, Lo E-mail address: [email protected] (K.D. Jandt). URL: http://www.cms.uni-jena.de 1 Current address: Mesoscopic Transport Phenomena Group, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

http://dx.doi.org/10.1016/j.polymer.2017.08.030 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

outstanding properties of carbon nanotubes (CNTs), such as a high Young's modulus, remarkable electrical and thermal properties, render them excellent nanofillers for polymer nanocomposites [1,2]. To realize the desired nanocomposite's property improvements, the internal arrangement of the highly anisotropic CNTs is of utmost importance [3]. However, to extensively control the arrangement, i.e., (i) orientation (e.g., alignment), (ii) location (e.g., dispersion), and (iii) shape [4,5] of the individual CNTs in the polymer nanocomposite is still an unresolved challenge, especially when all three (i-iii) arrangement types should be addressed simultaneously. Here, first steps toward this ambitious goal are taken by achieving simultaneous control of two out of three types of controlled arrangement of unfunctionalized CNTs in nanocomposites: control over orientation and location. More precisely, we demonstrate a high degree of uni-axial alignment and dispersion of CNTs by means of a high molar mass (BCP) serving as loadtransferring matrix and as dispersant at the same time. Although gradual differences in dispersibility exist, unfunctionalized CNTs cannot be stably dispersed in the majority of organic

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M.M.L. Arras et al. / Polymer 127 (2017) 15e27

solvents [6,7] or solutions of commodity polymers in desired amounts [8e10]. Therefore, the current prevalent answer to CNT dispersion is their covalent [11,12] and non-covalent functionalization [13,14]. Covalent functionalization inevitably changes the physical properties of the CNTs. Covalent functionalization is especially problematic for single or few walled CNTs, as it modifies the outer layer, but can also promote failure in MWCNTs e.g., via the “sword-in-sheath” failure mechanism [15]. Therefore, a noncovalent CNT functionalization seems superior for functional nanocomposite application, since it may preserve the original CNT properties. Although non-covalent CNT functionalization can be accomplished by surfactants [16], biomolecules [17], acids [18] and ionic liquids [19], the non-covalent functionalization directly by specialty polymers is of particular interest for the creation of CNT/ polymer nanocomposites. 1.1. Carbon nanotube dispersion by block copolymers It has been reported that conjugated polymers [20], polyelectrolytes [21] and BCPs [8,16] can disperse CNTs. Among these, the use of BCPs is appealing for CNT/polymer nanocomposites because on the one hand they can self-assemble to versatile nanostructures and on the other hand they can be blended easily with low-cost commodity polymers [22,23]. Up to now, four different CNT/BCP dispersion morphologies/mechanisms have been reported for low molar mass BCPs [24,25]: (i) individual BCP chain adsorption of one block of the BCP on CNTs, (ii) adsorption of BCP hemi-micelles on CNTs, (iii) adsorption of BCP micelles on CNTs and (iv) encapsulation of CNTs in cylindrical BCP micelles. The CNT dispersion stability results from steric stabilization, due to the mutual steric repulsion of the second, non-adsorbed BCP block. Previous studies used amphiphilic low molar mass di- and triblock copolymers which are widely used as polymeric dispersants like poly(ethylene)-block-poly(ethylene oxide) (PE-b-PEO), poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) [16,24,26] and poly(propylene oxide)-block-poly(ethylene oxide)block-poly(propylene oxide) (PPO-b-PEO-b-PPO) [27] or diblock copolymers based on poly(styrene) (PS) [9,26,28]. In addition, some studies used hybrid BCPs where one block is bound to another group of the aforementioned specialty polymers, e.g., deoxyribonucleic acid [29,30] or conjugated polymers [31]. To our best knowledge, only low molar mass BCPs were used as polymeric surfactants for CNTs and the dispersion by high molar mass ( > 105 g=mol) BCPs has not been investigated so far. High molar mass BCPs may, however, be advantageous for CNT/polymer nanocomposites: they can be blended easily with homopolymers of technically relevant molar masses without macrophase separation. As has been widely established for homogenous BCP/homopolymer blending, the homopolymer should not exceed the molar mass of the compatible block copolymer's block [22,23]. In addition, where good mechanical properties are relevant the matrix of a composite shall be of sufficiently high molar mass, because the tensile strength of a polymer is a function of the molar mass [32,33].2 Furthermore, drawing the CNT containing matrix, a major technique used to align embedded CNTs [35], is only possible when the matrix has a molar mass considerably above the entanglement molar mass Me [35e37]. Although matrix assisted shearing for unfunctionalized CNTs alignment in homopolymer nanocomposites has been reported [35, and citations within], the The tensile strength s can be calculated according to s ¼ s∞ 1 Me =M n , with the entanglement molar mass (Me ), the number-average molar mass (M n ) and the tensile strength at infinite M n ðs∞ Þ [32]. For example the entanglement molar mass for a polymer of relevance for this study, PS, is Me;PS ¼ 16:6 kg=mol [34]. 2

simultaneous unfunctionalized CNT dispersion remains an unsolved challenge. 1.2. Carbon nanotube alignment by block copolymers In 2007 Park et al. [38] have reported the local orientation of covalently functionalized CNTs with reference to a lamellar BCP morphology for the first time, but they achieved no macroscopic alignment of the CNTs. Surprisingly, only two studies reported the attempt to align unfunctionalized CNTs via BCPs [39,40], but were not able to accomplish a high degree of CNT alignment. In addition, a larger number of studies on covalently functionalized CNT/BCP nanocomposites with mixed outcome emerged [41e45]. In most cases the CNTs were shown to template the surrounding BCP morphology and not vice versa [40,42,43,45]. One can extract thereof, that additional external measures may ultimately be required to ensure the joint orientation of CNTs and BCP as has been attempted by Wode et al. [39] who used oscillatory shear to introduce a preferential orientation of the BCP lamellae morphology and, in turn, of embedded CNTs - however, again, with limited success. The aim of the present study, therefore, was to create a CNT/BCP nanocomposite with a high degree of CNT alignment and dispersion using unfunctionalized CNTs, which we believe is an important step toward the extensive control of CNTs arrangement in CNT/ polymer nanocomposites. We tested the hypotheses that (i) a high molar mass amphiphilic BCP in a selective solvent can disperse MWCNTs, (ii) the dispersion mechanism is similar to the one in low molar mass BCPs and (iii) that the MWCNT/BCP dispersion can be melt-drawn to align the MWCNTs while maintaining a high degree of dispersion. To investigate the MWCNT dispersibility by a high molar mass BCP we used poly(styrene)-block-poly(2-vinylpyridine) (PS-bP2VP) with a total Mn of 509 kg=mol and performed a MWCNT dispersibility study. For the first time, it was shown that a high molar mass BCP dispersed MWCNTs by a similar mechanism to low molar mass BCPs [9], i.e., BCP micelle mediated steric stabilization. In a consecutive step, the stable MWCNT dispersion in PS-b-P2VP/ p-xylene was used to create a highly dispersed and highly aligned MWCNT/BCP nanocomposite film by melt-drawing. The overall strategy for the current study is displayed in Fig. 1. The high degree of MWCNT dispersion found in the BCP dispersion was maintained in the final nanocomposite after aligning the MWCNTs within. The degree of MWCNT dispersion was superior to previous reports on aligned MWCNT/polymer nanocomposites [35]. The novelty of our research lies in the simultaneous alignment and dispersion of pristine MWCNTs in a nanocomposite film by using only a BCP. High molar mass BCP dispersed CNTs have the potential to bring the outstanding properties of CNTs to commodity polymers by blending, as this route has the potential to preserve the original properties of the MWCNT. 2. Materials and methods 2.1. Materials The polymers used in this study are listed in Table 1. Chloroform, methanol and p-xylene were obtained from VWR (VWR International GmbH, Darmstadt, Germany) and Merck (Merck Chemicals GmbH, Darmstadt, Germany), see Table S1 in the supplementary data (SD) for additional data on the solvents. “Short and thin” MWCNTs produced by catalytical chemical vapor deposition were obtained from Sigma-Aldrich (Sigma-Aldrich Chemie GmbH, Schnelldorf, Germany). According to the manufacturer they feature an average outer diameter of 9:5 nm and an average length of 1 mm. All materials were used as received.


17

35 ultraviolet-visible spectrometer, Perkin Elmer, Cambridge, UK) was used. The samples for this were prepared in the same manner as described above and were allowed to settle for 1, 2, 4, 8 and 14 days, at room temperature (approximately 22 C), respectively. After sedimentation, the supernatant (approximately 30 % of the total dispersion volume) of each prepared sample was extracted and ultrasonicated for another 30 min at 60 to 80 C and its VISNIR transmittance spectrum was measured. The corresponding pure solvents were used as a reference. The transmittance spectrum was converted to an absorptance spectrum by assuming the reflectance to be negligible. The data were fitted to the empirical Equation (1).

½Aðl; tÞl¼660 nm ¼ A∞ þ Bexpðt=tÞ

Fig. 1. Strategy for a well aligned and dispersed MWCNT/BCP nanocomposite: using a high molar mass BCP to disperse MWCNTs well (top right) and using this to create a nanocomposite film with aligned and dispersed MWCNTs by melt-drawing (bottom). The high molar mass amphiphilic BCP provides a dual action: it is the MWCNT dispersant and provides load-transfer for aligning the MWCNTs within.

Table 1 Polymers used in this study. Polymer

M[kg/mol]

Ɖ

b

Supplier

c

PS

M w ¼ 350

2.05

Sigma Aldrich

P2VP

M n ¼ 152

1.05

Sigma Aldrich

PS-b-P2VP

M n;PS ¼ 380

1.25

Polymer Source

e

Sigma Aldrich

M n;P2VP ¼ 129 For comparison HDPE

660 nm was chosen in accordance with previous studies [6,47]. Here, A∞ is the base absorptance, i.e., the hypothetical absorptance after t ¼ ∞ which is a measure of the overall MWCNT dispersibility. The parameter B is proportional to the magnitude by which the absorptance and, thus, the MWCNT dispersibility decreases with time and t is the time constant, providing how fast the MWCNT dispersibility decreases with time, i.e., how stable the dispersion is. The following equation derived from the Beer-Lambert law was used to calculate the relative increase in amount concentration of the attenuating species, i.e., MWCNTs in the supernatant, x upon addition of polymer:

x¼ a

M v z100d

(1)

cMWCNT log10 ½1 Að660 nm; tÞ ¼ c log10 1 A ð660 nm; tÞ MWCNT

(2)

here A ðl; tÞ is the absorptance of the MWCNT/solvent dispersion without polymer. This equations holds when the optical path length and the attenuation cross section are the same with or without polymer. As the absorptance of the polymer is negligible in comparison with that of the MWCNTs in the spectral range under consideration both assumptions are valid.

a

Number-average molar mass (M n ), the mass-average molar mass (M w ) or viscosity-average molar mass (M v ). b Molar-mass dispersity (Ɖ). c Sigma-Aldrich (Sigma-Aldrich Chemie GmbH, Schnelldorf, Germany), Polymer Source (Polymer Source Inc., Dorval, Canada). d Coarse estimate based on the melt flow index of 2:2 g=min ð190 C; @ 2:16 kgÞ according to [46].

2.2. Preparation of MWCNT dispersions For each polymer, a mass fraction of 5 mg=g was added to the solvents methanol, p-xylene and chloroform, respectively. Dispersions without polymer were prepared as control samples. Each sample was stirred at 60 C for 3 h and ultrasonicated at 60 to 80 C (limited by the boiling point of the solvent) for 30 min (ultrasonic cleaner USC 300TK, 45 kHz, 80 W sonic power, VWR International GmbH, Darmstadt, Germany). A mass fraction of 50 mg=g of MWCNTs in the respective solvents were ultrasonicated at 60 to 80 C for 30 min. Subsequently, both, the polymer/solvent and MWCNT/solvent solutions/dispersions of corresponding solvents were mixed by equal aliquots and the resulting mixture was ultraconicated at 60 to 80 C for another 30 min. Finally, the samples were allowed to settle at room temperature (approximately 22 C) for 48 h, if not stated otherwise.

2.3. Determination of temporal stability To investigate the temporal stability of the MWCNT/polymer dispersions, visible/near-infrared spectroscopy (VISNIR) (Lambda

2.4. MWCNT/polymer thin-films 2.4.1. Drop-casting Directly after ultrasonication (see above), the MWCNTs dispersed by PS-b-P2VP and poly(2-vinylpyridine) (P2VP) in pxylene and methanol were drop-cast on carbon-coated transmission electron microscope (TEM) grids, respectively. After the solvent evaporated, the obtained polymer thin-films were stained by I2 vapor for 3 h.

2.4.2. Melt-drawing The melt-drawing technique [37,48] has previously been used to align MWCNTs within semi-crystalline homopolymers [35]. To apply the melt-drawing technique to amorphous polymers was adapted from Keller et al. [36] in the following manner: Firstly, the MWCNT/PS-b-P2VP dispersion in p-xylene which had the same concentration as the drop-cast dispersions (see above), was heated and stirred at 60 C. Afterwards, approximately 1 ml MWCNT/ polymer dispersion was dropped onto and spread on a precision heating plate covered with a thin glass sheet. The temperature of the heating plate was 150 C. After the solvent evaporated, MWCNT/ polymer thin-films were obtained by drawing the visco-elastic MWCNT/polymer off the heating plate. This was done with tweezers at a speed of approximately 5 cm=s. The obtained MWCNT/PSb-P2VP thin-film was stained with I2 vapor for 3 h. For comparison, a MWCNT/high density poly(ethylene) (HDPE) film was created in the same way, except for the dispersions had been stirred at 120 C.

18


2.5. Transmission electron microscopy To investigate the morphology of the MWCNT/polymer thinfilms the TEM JEOL 3010 (JEOL Ltd., Tokyo, Japan) was used. The melt-drawn MWCNT/polymer thin-films were fixed on a TEM butterfly copper grid, while the drop-cast thin-films formed directly on the carbon-coated grid. The samples were I2 stained which adsorbs primarily on the P2VP block of the BCP and renders it dark on TEM bright field images. Specimen were mounted on a single-tilt TEM holder and imaged at 300 kV in the bright field. 2.6. Quantification of the degree of MWCNT dispersion To provide a measure of the degree of MWCNT dispersion, image processing was used to calculate the degree of dispersion as determined by TEM (DTEM ) in the following manner: At first, in the raw TEM images the MWCNTs were manually retraced in ImageJ [49]. The resulting image was processed by an edge detection filter to provide the outline of the MWCNTs only. Subsequently, the processed images were converted to binary and divided into a square main grid, each cell of the grid having an area of 150 nm 150 nm. In addition in each main grid's cell a subgrid3 of 25 nm 25 nm was created. For the main grid and each of the subgrids a corresponding zero matrix was created. When the MWCNT fell within a subgrid, the corresponding submatrix's entry was set to 1 by the algorithm. Afterwards, the number y of submatrix entries set to 1 was counted (possible values range from 0 to 36), and the value replaced the entry of the main zero matrix. Finally, the frequency distribution pðyi Þ (with yi ¼ ½1; 36) of the main matrix was calculated. The expected value of the main matrix EðyÞ was used to quantify the dispersion quality DTEM for each TEM image, which was calculated according to the following equation:

DTEM

E ð yÞ ¼1 ¼1 ymax

P

i yi pðyi Þ ¼1 ymax

P

i yi pðyi Þ 36

micrographs of MWCNT/polymer thin-films were manually retraced to result in an image showing only MWCNTs. The image of retraced MWCNTs is then further processed via ImageJ: (i) Binary / Make Binary, then (ii) Binary / Skeletonize to result in a one pixel wide representation of only the MWCNTs. Then the OrientationJ plugin is invoked via Plugins / OrientationJ / OrientationJ Distribution. We used a Gaussian window of sGW ¼ 1 pix and the Finite Difference Gradient option. Please see section S14 of the SD for details and also for a comparison with a fully automated procedure, which resulted in more noisy data but showed the same trend. Then the angle 4 between the MWCNTs segments as determined by OrientationJ and the director, i.e., the drawing direction was calculated. In addition, for both methods the Herman's orientation function f was calculated.

D E f ¼ 3=2 cos2 4 1=2

(4)

A perfect alignment of the MWCNTs parallel to the drawing direction corresponds to a vanishing FWHM and a f value of 1. Parameters based on the end-to-end (EE) method are denoted FWHMEE and fEE , those based on the segment-wise (SW) evaluation FWHMSW and fSW . 2.8. Hansen solubility parameters and distance in Hansen space The Hansen solubility parameters (HSPs) are used to explain dispersibility of various solutes in the solvents. The HSPs, which can be linked to the Flory-Huggins interaction parameter, have already been applied successfully to explain CNT dispersibility [6,52e55]. The distance Ra between two arbitrary solvents/solutes (denoted x and y) in Hansen space4 is defined as [56]:

2 2 2 R2a ðx; yÞ ¼ 4 ddx ddy þ dpx dpy þ dhx dhy :

(5)

(3)

For a perfect degree of dispersion DTEM is 1. Additional information on the calculation scheme for DTEM is presented in Fig. S1 in the SD. Please note, that this approach is generally unsuitable as an absolute measure of the degree of dispersion (for cross-study comparisons) because it suffers from the arbitrary choice of the grid size and does not take the geometry into account. We can use it here successfully for a relative comparison as we have kept key parameters (like film thickness and MWCNT concentration) constant.

where the HSP for a material is described by the vector ðdd ; dp ; dh Þ with the solubility parameter component for the dispersion forces (dd ), the solubility parameter component for the polar forces (dp ) and the solubility parameter component for the hydrogen-bonding forces (dh ). The values for a wide range of substances are tabulated [56,57]. The smaller Ra is between two solvents/solutes, the more likely they will mix. Given the HSP of a solute all solvents within a sphere of the interaction radius R0 are likely to be decent solvents. Here, the most relevant distance in Hansen space Ra was the distance of material y to the MWCNT x: Ra ðMWCNT; yÞ≡Ra ðMWCNTÞ, termed Ra ðMWCNTÞ in this study.

2.7. Quantification of the MWCNT alignment 3. Results & discussion The quantification of the MWCNT alignment was done in a twofold fashion: (i) based on the end-to-end distance vector of the MWCNT and (ii) segement-wise using the Fiji/ImageJ plugin OrientationJ [49e51]. The end-to-end method has previously been reported elsewhere [35]. In brief, the end-to-end distance vector of each MWCNT was determined manually with ImageJ. The angle 4 between the end-to-end distance vector and the drawing direction was calculated for each MWCNT. From these acquired angles 4, a Gaussian distribution can be fitted. The alignment degree can be evaluated by its full width at half maximum (FWHM) which is pffiffiffiffiffiffiffiffiffiffi FWHM ¼ 2 2ln2s, where s is the Gaussian's standard deviation. The segment-wise quantification of the MWCNT alignment was done via the ImageJ plugin OrientationJ. In brief, the TEM

The use of high molar mass BCP to disperse MWCNTs in dispersion was investigated by sedimentation experiments, TEM and VISNIR. The sedimentation experiment showed that PS-b-P2VP in p-xylene disperses MWCNTs, while the PS homopolymer did not. The P2VP homopolymer was found to disperse MWCNTs to an intermediate degree. TEM strongly suggests that the mechanism of MWCNT dispersion stabilization by PS-b-P2VP was micelle mediated steric hindrance. The VISNIR results confirmed that the PS-bP2VP containing dispersion was superior in dispersing the

3 This dimension is chosen to ensure that on average only one MWCNT can be found in the subgrid's cell.

dd direction is multiplied by a factor of two.

3.1. Carbon nanotube dispersion by high molar mass block copolymer

4

Per (semi-)empirical convention, in Hansen space the canonical unit vector in


MWCNTs and proved it to be the only dispersion stable for days. Since all dispersions contained MWCNTs, this fact is not always explicitly mentioned in the following sections. 3.2. Macroscopic and microscopic morphology The MWCNT dispersion by PS-b-P2VP was investigated by sedimentation experiments and TEM micrographs for different solvents. A selective solvent for the PS block (p-xylene) and for the P2VP block (methanol) as well as a non-selective solvent (chloroform) were used. The homopolymers PS and P2VP with a similar molar mass to the PS-b-P2VP blocks were tested as well, as a control and to investigate the impact of the individual blocks on the MWCNT dispersibility. The results of the sedimentation experiment after 48 h are shown in Table 2. Corresponding tables depicting these dispersions after a settling time of one day or two weeks are provided in the SD (Tables S3 and S5). Some of the presented discussion is based on the semi-empirical HSP theory, which has previously been successfully applied to the understanding of CNT dispersions [6,52e55]. Nevertheless at the end of the section, we will briefly address some potential shortcomings of the theory in the context of CNT dispersability. The HSPs for the materials used in this study are shown in Table 3. This discussion primarily focuses on the polymers showing a pronounced MWCNT dispersion effect, i.e., PS-b-P2VP and P2VP (the four samples highlighted in Table 2) which were also investigated more thoroughly by TEM and VISNIR. In addition, another amphiphilic system, poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO), was also tested (see Tables S3, S4, S5) , however this dispersion was inferior to PS-b-P2VP and not long term stable which hints to a special role of P2VP. The use of either pure chloroform, P2VP or PS-b-P2VP resulted in visible MWCNT dispersibility. This result is in line with the HSP theory: both, chloroform and P2VP (Ra ðMWCNTÞ of 5:1 ðMPaÞ1=2 and 5:7 ðMPaÞ1=2 ) show only a small distance to MWCNTs in Hansen space Ra ðMWCNTÞ (the smaller the distance the more likely CNTs will be dissolved or dispersed, see equation (5)). Detriche et al. [52] reported a maximum distance in HSP space of R0 ¼ 6 ðMPaÞ1=2

19

for good CNT dispersion, i.e., CNTs are expected to be dispersed when Ra ðMWCNTÞ < R0 ¼ 6 ðMPaÞ1=2 . Chloroform, however, was not beneficial for our study as adding polymer to it led to agglomeration which is counterproductive in creating a MWCNT/ polymer nanocomposite. So we are focussing on P2VP and PS-bP2VP in the following and discuss the impact of the solvent selectivity on the dispersions. Group III of Table 2 shows that samples of P2VP/methanol and P2VP/p-xylene featured MWCNT dispersibility. This is a remarkable result because it is uncommon that a synthetic homopolymer which does not belong to the expensive conjugated polymer family contributes significantly to the dispersion of MWCNTs. The Ra ðMWCNTÞ for P2VP is 5:7 ðMPaÞ1=2 (see Table 3) which approximately matches the value of N-methylpyrrolidone (HSP of ð18:0; 12:2; 7:2Þ ðMPaÞ1=2 [56] resulting in an Ra ðMWCNTÞ of 5:0 ðMPaÞ1=2 ). The latter has been reported as the “best” solvent for CNTs [6,7] and is also used for commercially available MWCNT dispersions (ORGACYL™ NMP0501 of Belgium Nanocyl SA, Sambreville). For 4-vinylpyridine it has been reported that the nitrogen lone pair of the pyridine interacts favorably with the p-system of the CNT. CNTs are good electron acceptors so a donor-acceptor interaction is highly likely. Giordani et al. [47] pointed out that the lone pair donicity is common among CNT dispersants. Also for C60, a strong interaction was found with poly(4-vinylpyridine) (P4VP) where the C60 accepts the lone electron pair of the P4VP [59]. On a fundamental note, it is known that dipoles are attracted by conductive surfaces which can be shown by the commonly known principle of image charges [60], accordingly it has to be expected that the slightly polar P2VP is attracted by the conductive MWCNT. This is also in line with the reported observations that deoxyribonucleic acid can disperse CNTs [29]. Thus, we assume that P2VP interacted favorably with and adsorbed to the MWCNTs. As a result, the strong intertube van der Waals interactions were screened at least partially. However, the adsorption of individual P2VP chains cannot prevent reagglomeration of the MWCNTs over longer periods of time. TEM micrographs of the drop-cast films of P2VP in methanol (Fig. S3a) and p-xylene (Fig. S3b) are shown in the SD.

Table 2 MWCNT dispersions 48 h after preparation [61]. The highlighted polymer dispersions showed a considerable degree of MWCNT dispersibility (red rectangle).

20


Table 3 Hansen solubility parameters (HSPs) for the materials used in this study. Material

Hansen solubility parameters

dd 1=2

½ðMPaÞ MWCNT Methanol p-Xylene Chloroform PS P2VP PS/P2VP HDPE

18.5 15.1 17.6 17.8 21.3 16.3 18.8 18.0

Ref.

dp

½ðMPaÞ 7.4 12.3 1.0 3.1 5.8 7.1 6.5 0.0

dh 1=2

1=2

½ðMPaÞ 8.0 22.3 3.1 5.7 4.3 11.6 8.0 2.0

Distance to MWCNT Ra ðMWCNTÞa

dt 1=2

½ðMPaÞ 21.5 29.7 18.0 19.0 22.5 21.2 21.4 18.1

½ðMPaÞ1=2

[52],b [57] c

[57] [56] d e

[56]

0 16.6 8.3 5.1 6.9 5.7 1.1 9.6

a

Ra ðMWCNTÞ≡Ra ðMWCNT; yÞ is the distance of the material y to the MWCNT in Hansen space (see Equation (5)). HSPs reported for CNTs in the literature vary. See Tables S6 and S7 in the SI and the body of the text for a discussion of these variations. Diversified Enterprises, 2016. Available From: http://www.accudynetest.com/solubility_table.html. [1st May 2017]. For comparison ð17:8; 1:0; 1:7Þ ðMPaÞ1=2 is given for oxylene in the literature [56]. d Estimated based on known P2VP solvents (see SD). Agrees well with determination of dt ¼ 21:3 ðMPaÞ1=2 for P2VP by Arichi et al. [58]. e Arithmetic mean was used which emulates a random block copolymer of equal volume fractions. b c

Group IV in Table 2 shows that the best MWCNT dispersion was achieved for the PS-b-P2VP system in the PS-selective solvent pxylene. In methanol, selective for P2VP, the MWCNT dispersion was limited. Again this behavior can be understood in light of the HSPs. A mean value of the HSPs for PS and P2VP lead to a very low Ra ðMWCNTÞ ¼ 1:1 ðMPaÞ1=2 , see Table 3 which is far below the R0 ¼ 6 ðMPaÞ1=2 and thus is expected to disperse MWCNTs. This must be interpreted with caution, however, because this value cannot represent the HSP of a BCP, because we are fully neglecting the BCP nanostructure and its volume fractions by calculating the mean. The PS/P2VP mean can be understood as the HSP for a random copolymer of PS and P2VP with equal volume fractions, where a good mixing between the components is guaranteed. Although the main features of MWCNT dispersion observed in this study can be well understood by the HSPs of the components, this has only limited predictive power, because of the lack in consistent CNT surface chemistry and thus multiple HSPs for CNTs. Unlike polymers which can be already produced in a nearly monodisperse fashion with consistent configurations, MWCNTs are still highly heterogenous, in size and surface chemistry. In Tables S6 and S7 the effect of the variation in reported HSPs for CNTs is tabulated which shows that the explanation based on the HSPs values remains, nevertheless, valid for the majority of the reported HSPs of unfuctionalized MWCNTs, but not for all. In addition, even though the HSP is a semi-empirical concept, it is based on equilibrium. In cases where a dispersant is not dissolved in a thermodynamical sense, but kinetics play a role, it is unclear to which degree the HSP theory can be applied. However, as stated earlier, the HSP concept has already been successfully applied to the problem of MWCNT dispersion [6,52e55]. Ending the discussion on the solution state we move on to discuss the morphology of the drop-cast films. In methanol the P2VP was completely dissolved but the MWCNT dispersion in the drop-cast film was quantified by a DTEM of 48:3±19:1 % which is arguably inferior to that in p-xylene: The degree of MWCNT dispersion in the drop-cast P2VP/p-xylene dispersion was DTEM ¼ 58:5±7:1 %. This result may be explained by the prior discussed affinity of P2VP to the MWCNTs and a marginal solubility of P2VP in p-xylene (slight swelling was observed). The poor solvent most likely promoted the adsorption of P2VP onto the MWCNTs. Thus, the MWCNT dispersibility in P2VP/p-xylene was slightly better than the corresponding dispersion in methanol (see Table 4): the P2VP chains, due to the poor solvent collapsed, will cover the MWCNT surface

more completely and irreversibly in p-xylene. Both P2VP polymer solutions were, however, not effective to stabilize the MWCNT

Table 4 Evaluation of the degree of MWCNT dispersion for selected samples. MWCNT dispersant

Visual appearancea

DTEM ½%b

(Dispersion)

(Film)

PS-b-P2VP/p-xylene P2VP/p-xylene P2VP/methanol PS-b-P2VP/methanol

very dark gray dark gray gray light gray

71:2±8:2 58:5±7:1 48:3±19:1 44:9±13:1

a Qualitative evaluation of the visual appearance of the dispersion in the sedimentation experiment (see Table 2). b Degree of dispersion as determined by TEM (mean and standard deviation) as determined from drop-cast films in the TEM.

Fig. 2. TEM micrograph (montage of individual images) of the drop-cast MWCNT/ P2VP/p-xylene dispersion. P2VP blocks were selectively I2 vapor stained. White arrows point to elliptically deformed micelles, black arrows to micelles which attach to but do not envelop the MWCNT. The inset shows the corresponding dispersion from Table 2 [61].


dispersion on the long-term (see next subsection). The TEM micrograph of the PS-b-P2VP/p-xylene sample is shown in Fig. 2. Besides well dispersed MWCNTs, many spherical micelles were found in direct contact with the MWCNT. The micelles comprised of a P2VP core (stained dark by I2 ) and a PS corona (light gray region). The polymer micelle morphology was expected, considering the block length ratio and the use of the PS-selective solvent p-xylene. Taking the size of the micelles into account it is likely that those micelles were formed by multiple BCP chains. When drop-casting the PS-b-P2VP/p-xylene dispersion containing no MWCNTs, the mean diameter of the micelles’ cores was 70±10 nm (see SD, Fig. S5). In the MWCNT containing samples the diameter of the micelles’ cores without contact to the MWCNT remained the same (73±10 nm, see SD Fig. S6). The PS-b-P2VP micelles' cores seemed to attach to the MWCNTs (Fig. 2). Comparing this morphology with those obtained by low molar mass BCP used in previous studies [9] is challenging because the core and corona of the micelles of low molar mass BCP were too small to be clearly distinguished. However, Shin et al. [9] who used low molar mass poly(styrene)-block-Poly(4-vinylpyridine) (PS-bP4VP), chemically related to PS-b-P2VP, proposed that the P4VP block should have an affinity for and be in contact with the CNTs. For the high molar mass P2VP used in this study, the outlines of the large micelles' cores were easily identified. Thus, the speculated mechanism by Shin et al. [9] was verified here: P2VP was indeed in contact with the MWCNTs. From these TEM pictures alone it remains unclear whether the micellar attachment is due to unspecific hydrophobic solvent interactions or an effect of a favorable P2VPMWCNT interaction. Considering the observations for pure P2VP and the fact that PS-b-PEO (see SD) dispersed the MWCNT less well than pure P2VP, we think a favorable P2VP-MWCNT interaction must be present to explain the observations. Another information which is difficult to extract, due to the nature of the 2D imaging process, is the geometry of the micelle attachment, i.e., whether the MWCNT passes trough the bulk of the micelles' cores or envelops their surfaces. Although some micelles' cores deformed to ellipses to follow the MWCNTs which can be easily mistaken for MWCNT passing through the micelle in the 2D projection of a TEM image (see white arrows in Fig. 2), we propose that the micelles' cores only attach to the MWCNT. The reason is the observation of micelles' cores where only the core-corona interface is in contact with the MWCNTs which is unambiguous even in the 2D projection (see black arrows in Fig. 2). In addition, the elliptic form of the micelles' cores was not always as pronounced as in Fig. 2, e.g., visible in Fig. S8 in the SD. Thus, we conclude that the micelles' cores did not envelop the MWCNTs, rather the opposite may be true, as has been previously reported by us elsewhere [5]. Ibidem, you also find a discussion of the entropic advantage for an MWCNT to locate at the PS-b-P2VP interface. The mechanism behind the micelle mediated MWCNT dispersion is proposed to be the following: the well-dissolved PS chains of the attached micelles extend into the dispersion and screen the MWCNT from the attractive van der Waals interaction potential by steric repulsion, preventing the (re)agglormertation of individual MWCNTs. In addition the favorable P2VP interaction of the micelles' cores which attach to the MWCNT may even be relevant to the long-term stability of the PS-b-P2VP/p-xylene dispersion because a similarly processed PS-b-PEO dispersion did not yield a stable dispersion (see Table S5 in the SD) although the block length ratio was similar. We conclude that high molar mass BCP can stabilize MWCNTs by a similar mechanism to low molar mass BCP [24,25]. The drop-cast PS-b-P2VP/methanol dispersion showed a connected P2VP network surrounding areas of PS (see Fig. 3). The degree of MWCNT dispersion in this sample is the lowest of all P2VP

21

Fig. 3. TEM micrographs (montage of individual images) of the drop-cast MWCNT/PSb-P2VP/methanol dispersion. P2VP blocks were selectively I2 vapor stained. Black arrows point to MWCNTs at the interface between both blocks (regime I), white arrows point to MWCNTs which may be in the center of a cylindrical P2VP phase (regime II). The inset shows the corresponding dispersion from Table 2 [61].

polymer containing samples (see Table 4), however, the morphology had interesting features: virtually all MWCNTs were in the minority P2VP phase. This by itself is a very interesting result because researchers are trying to achieve a selective sequestering of CNTs to individual BCP blocks by a selective covalent functionalization of the CNTs [38,39] and, here, this was accomplished by simply changing the solvent. A MWCNT/BCP nanocomposite where one block selectively contains MWCNTs may be attractive, e.g., for enhanced charge extraction in future organic solar cells. Two different regimes of MWCNT sequestering were distinguished: regime I showed MWCNTs within large P2VP areas but mostly in close vicinity to the BCP interface (see black arrows in Fig. 3). In regime II the MWCNTs were only covered by a thin P2VP layer which resembles a cylindrical P2VP phase enveloping the MWCNT (see white arrows in Fig. 3). The morphology of the second regime was similar to the morphology obtained by Li et al. [62], with the distinct difference that covalently functionalized MWCNTs were used by them to achieve the morphology. Although in methanol, the BCP efficiently dispersed the MWCNTs in the polymer morphology, the dispersion is not stable and sediments rapidly over time (see Table S5). Sluzarenko et al. [28] demonstrated that depending on the block length ratio of a PS-block-poly(isoprene), the BCP either dispersed MWCNTs in a selective solvent for the PS or the poly(isoprene) block. The longer block should be selectively dissolved for a good MWCNT dispersion [28]. Their and our results are understood when considering that three of four reported BCP mechanisms involve (hemi-/spherical or cylindrical) BCP micelles [24,25]. As a result, the MWCNTs in the PS-b-P2VP/methanol dispersion were only dispersed to a limited degree because the block length ratio (see Table 1) did not allow for the formation of free BCP micelles in methanol. These TEM investigations (Figs. 2 and 3) confirmed the results obtained from the liquid phase, both, visually and quantitatively, with a calculated DTEM value of 71:2±8:2 % for PS-b-P2VP/p-xylene, by far the highest degree of dispersion obtained in this study and a

22


DTEM value of 44:9±13:1 % for PS-b-P2VP/methanol. For comparison, we also performed sedimentation experiments for PS-b-PEO, poly(ethylene oxide) (PEO) and HDPE homopolymer. The results are presented in the SD in Table S4. Of these samples only PS-b-PEO in p-xylene showed limited MWCNT dispersibility. In contrast to the MWCNT dispersion in PS-b-P2VP/p-xylene, the MWCNT dispersion in PS-b-PEO/p-xylene was not long-term stable (SD Table S5), which indicates that a preferential interaction of the attached block, as seen with P2VP (or P4VP [9]), may be fundamental for long-term stability. Table 4 summarizes the visual appearance classification according to the sedimentation experiment and the DTEM values calculated from the TEM micrographs (the higher the DTEM value the better is the dispersion of MWCNTs). It is notable how well the qualitative visual appearance classification corresponds to the quantitative DTEM values (DTEM values continuously decrease as the visual appearance classification goes from very dark gray to light gray). Two simple graphical models of the MWCNT/PS-b-P2VP which summarize the, based on the experimental results, conceived interactions are presented in Table 5. It should be kept in mind that during the initial preparation of the MWCNT/polymer dispersion elevated temperatures and ultrasonication were used. The initial state of the polymer may, thus, be different from the postpreparation morphology as analyzed in the sedimentation experiments and the TEM. In p-xylene (Table 5, left; abstracted from Fig. 2), the PS block was dissolved very well but the P2VP block collapsed which led to the micellar morphology for PS-b-P2VP. The preferential interaction of P2VP and the MWCNT as well as the selective solvent promoted the adsorption of the P2VP to the MWCNT. Because many micelles attached to one individual MWCNT, individual MWCNTs were stabilized in the dispersion due to the steric repulsion between PS blocks of micelles adherent to different MWCNTs, which led to a long-term stability of this dispersion. In methanol which only solves P2VP (Table 5, right; induced from Fig. 3), the MWCNTs were driven to the interface with PS to reduce the area of non-favorable interaction (regime I) or to the center of P2VP cylinders (regime II, not displayed). This was probably effective in dispersing the MWCNTs but due to the large

Table 6 Parameters obtained by the VISNIR experiment. MWCNT dispersant

xa

A∞ b

Bb

tb

P2VP/methanol P2VP/p-xylene PS-b-P2VP/p-xylene

4.8 4.3 14.3

0.09 0.25 0.61

0.35 0.22 0.21

38:3 h 34:5 h 51:8 h

a Relative increase in the amount concentration of MWCNTs upon adding polymer to the solvent in comparison to the solvent only. b Fit parameter of Eq. (1) to the data of Fig. 4b. For a detailed description of the parameters see Eq. (1).

network nature of the PS block, these very large MWCNT/BCP structures did not stay in the dispersion for long. As a consequence, the majority of the dispersed MWCNT/PS blocks sedimented within a day. Corresponding models for P2VP are presented in Table S8 in the SD. 3.3. Temporal stability As the goal of the study was to use the well dispersed MWCNT/ polymer dispersion for the processing to aligned nanocomposites the temporal stability of those dispersions was critical. Thus, VISNIR was used to quantitatively investigate especially the temporal stability of the top three MWCNT dispersions containing P2VP or PS-b-P2VP (Group III and IV, Table 2): Fig. 4a shows the VISNIR spectra of the respective MWCNT dispersions measured 48 h after preparation. The spectra recorded at different times after preparation were qualitatively similar, i.e., the whole curves merely shifted to lower absorptance over time. Spectra recorded after 1, 2, 4, 8 and 14 days are displayed in the SD in Fig. S4. Table 6 lists the relative increase in the amount concentration of the MWCNTs x (see Eq. (2)) by adding the polymer to the dispersion in relation to the solvent only samples. After two days of settling, the amount of dispersed MWCNTs was increased by more than 14 times for the PS-b-P2VP/p-xylene dispersion. For the P2VP samples the dispersed MWCNT concentration was increased approximately 5fold. It is expected that these values increase with time as the sedimentation progresses faster in the solvent only samples. Fig. 4b

Table 5 Simple graphical model of the MWCNT dispersion via P2VP and PS-b-P2VP in p-xylene and methanol. P2VP chains; PS chains; undissolved P2VP block; undissolved PS block. Compare with Figs. 2 and 3.


23

Fig. 4. VISNIR of MWCNT dispersions [61]. (a) Shows the absorptance spectra of MWCNT dispersions 48 h after preparation, (b) the temporal evolution of the absorptance at 660 nm. The data was fitted by Eq. (1). Table 6 lists the obtained parameter of the fit. The inset in (b) shows the corresponding dispersions after two weeks, i.e., 336 h. The legend color applies to both subfigures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

presents the absorptance-time dependence at a constant wavelength which represents the temporal stability of the MWCNT dispersions. The data was fitted to the empirical Equation (1) and the obtained fit-parameters are shown in Table 6. The most important result is that, although all P2VP polymers showed an initial effect on the MWCNT dispersibility, only the BCP PS-b-P2VP in p-xylene showed significant long-term MWCNT dispersion stability. PS-b-P2VP/p-xylene showed the highest time constant t ¼ 51:8 h and the highest base absorptance with A∞ ¼ 61 %. This was just a decrease of 25 % in comparison to the initial absorptance. In contrast, the two P2VP samples showed roughly half the initial absorptance of PS-b-P2VP/p-xylene and a decrease by 45 % (p-xylene) and by 80 % (methanol) from the initial absorptance. The different temporal stabilities are understood when taking the proposed mechanisms of MWCNT dispersion into account (see above).

3.4. Carbon nanotube alignment and dispersion by high molar mass block copolymer Now, that the MWCNT were well and stably dispersed in PS-bP2VP/p-xylene, we used this dispersion to add the second arrangement control step, i.e., alignment, to the nanocomposite. Due to the high molar mass nature of the used BCP for dispersion, it is possible to use the very effective melt-drawing technique for the alignment of the MWCNTs within the BCP matrix [35]. This allows us to create a nanocomposite with highly aligned and dispersed unfunctionalized MWCNTs, so far unreported in the literature. As the use of the melt-drawing technique for MWCNT alignment in semi-crystalline homopolymers and its mechanisms have already been reported by us elsewhere [35], we adopt the thin-film of HDPE/MWCNT, which resulted in the highest MWCNT alignment in our previous study [35], as a benchmark for the MWCNT/BCP films. We want to clarify that no BCP as a dispersant was added to fabricate the HDPE/MWCNT films. Also it shall be stressed again that, the high molar mass of the PS-b-P2VP is of utmost importance because it renders the BCP matrix drawable, attempts to draw thinfilms with low molar mass BCP were unsuccessful. We are convinced that other drawing techniques would also benefit from high molar mass matrices as an increase in the shear forces should be beneficial to MWCNT alignment. Fig. 5 shows the TEM micrographs of the melt-drawn MWCNT thin-films and the corresponding MWCNT angle distributions with reference to the drawing direction. We used two different methods

to determine the MWCNT angle after the alignment process. One employs an end-to-end (EE) approximation of the MWCNT and is not sensitive to any meandering, but only the overall direction, each MWCNT is ascribed one orientation angle. In contrast, OrientationJ is used to evaluate the MWCNT orientation segment-wise (SW), here meandering is taken into account as is the length of the MWCNT. In Fig. 5a the melt-drawn PS-b-P2VP/p-xylene sample is shown. Individual well dispersed MWCNTs were observed mostly aligned parallel to the drawing direction (white arrow). They located between deformed PS-b-P2VP micelles. We estimate these thin-films to be in the range of 100 200 nm in thickness [35e37]. At thicker regions the thin-film is composed of stacked micelles which becomes apparent when compared to Fig. 2 in the SD. The micelle cores are only elongated to a small degree. Keller et al. [36], who were the first to process BCPs into thin-films by the meltdrawing technique found a much higher degree of micelle deformation towards a needle-like structure, but processed the polymer at a higher temperature of 200 C. For comparability reasons we chose a common heating plate temperature of 150 C for both the MWCNT/PS-b-P2VP and the MWCNT/HDPE. This in addition with the additional rheological changes due to the MWCNT addition may explain the observed difference in the matrix morphology. The resulting MWCNT alignment in PS-b-P2VP/p-xylene is summarized by the angle distribution in Fig. 5a (top). It features a FWHMEE of approximately 80 and a fEE of 0:52±0:14 (see Table 7) when looking at the overall MWCNT orientation and a fSW of 0:55±0:13 for the segment-wise evaluation. Both Herman's orientation functions reasonably agree which means that the overall shape of the aligned MWCNTs is straight with only a small degree of waviness, which is consistent with the visual impression seen in Fig. 5a and c. For comparison, a drop-cast film of MWCNT/PS-bP2VP shows a fSW of 0:20±0:19 (see SD S3b) which is reasonably close to the expected value of f ¼ 0 when considering the margin of error. The degree of MWCNT dispersion is high which is indicated by a DTEM of 65:8±4:1 %, although this value is slightly lower than for the unaligned, drop-cast MWCNT/PS-b-P2VP dispersion in pxylene. However within the error margins it can be concluded that the degree of MWCNT dispersion was maintained after MWCNT alignment. In contrast, Wode et al. [39] who used a lamellae forming PS-b-P4VP and oscillatory shear to align the MWCNTs in the nanocomposite cast from the non-selective solvent chloroform did not find any aligned MWCNTs (compare with Fig. 10ii in Reference [39]) when using unfunctionalized MWCNTs. This might have been a result of the very poor MWCNT dispersion in that particular

24


Fig. 5. TEM micrographs of melt-drawn thin-films [61]. (a) Shows the MWCNT/PS-b-P2VP thin-film, melt-drawn from the p-xylene based dispersion (group IV sample 2, Table 2). The MWCNT were manually contrast enhanced to increase clarity, for the original image see Fig. S9 in the SD. (b) Shows the corresponding melt-drawn MWCNT/HDPE thin-film (see Table S4 for the dispersion experiment). The corresponding orientation distribution of the aligned MWCNTs with respect to the drawing axis is shown above the micrographs. (c) and (d) show the results of the OrientationJ analysis which give a segment-wise (SW) orientation of the MWCNTs. For illustration, the end-to-end (EE) method for MWCNT angle determination is depicted exemplarily in (c). The legend in (d) applies to both (c) and (d) where the absolute angle of the MWCNT segments is shown without reference to the drawing axis.

Table 7 Dispersion and alignment properties of melt-drawn samples. FWHM[ ]c

MWCNT dispersant

DTEM [%]a

fb fSW

fEE

FWHMSW

FWHMEE

PS-b-P2VP/p-xylene HDPE/p-xylene

65:8±4:1 45:8±9:8

0:55±0:13 0:89±0:03

0:52±0:14 0:82±0:09

1:57±0:07 1:63±0:05

80:0±18:9 28:3±1:65

a

Degree of dispersion as determined by TEM. Herman's orientation function. SW is based on segment-wise, EEpon end-to-end evaluation of the MWCNT angle. ffiffiffiffiffiffiffiffiffiffi Full width at half maximum of the MWCNT angle distribution (2 2ln2s). SW is based on segment-wise, EE on end-to-end data. For SW, the parameter is given for the Gaussian fit to the center-peak (dashed gray lines in Fig. 5). b c


sample. When they used functionalized MWCNTs much better dispersion and some degree of MWCNT alignment was visible, however, the limited number of MWCNTs in their samples (often only one MWCNT per 1 1 mm micrograph) did not allow them nor us to compute a meaningful degree of MWCNT alignment of their samples for comparison. The MWCNT/HDPE melt-drawn thin-film which is shown in Fig. 5b, showed the pronounced shish-kebab morphology of the melt-drawn HDPE matrix [35,48], i.e., lamellae crystals (kebas) perpendicular to the drawing direction. The alignment of the MWCNTs is higher (FWHMEE of 28:3 , fEE of 0:82±0:09, see Table 7) than for the PS-b-P2VP sample. The segement-wise orientation evaluation gives fSW ¼ 0:89±0:03 even a slightly better value, however similar within the uncertainty of the measurement to the end-to-end approximation. This superior alignment effect of MWCNTs in the HDPE sample can be due to three reasons: (i) processing an amorphous BCP with the melt-drawing technique, which was originally conceived for semi-crystalline polymers, was challenging. Due to the absence of shear-induced crystallization in the amorphous BCP, the transition between melt-flow and solid film was not as sharp and, thus, the mediated shear rate was not as high as in the HDPE sample [35,37]; (ii) since the MWCNTs interact with the micelle interface which results in micelle core wrapping [5], MWCNT were bent to follow the shape of the core (exemplarily depicted in Fig. 5a and c, see term “core wrapping”), the degree of orientation was systematically lowered for PS-b-P2VP; (iii) the presence of a small subgroup of considerably shorter MWCNT which seemed to be oriented perpendicular to the drawing direction (labelled “short MWCNT” in Fig. 5a and c), those may be more prominent in the P2VP samples due to the improved dispersion. The MWCNT dispersion was, however, far inferior for the MWCNT/HDPE sample which is among the lowest DTEM values encountered in this study of 45:8±9:8 %. In addition, as discussed in our previous publication [35], the MWCNTs in the HDPE sample appeared localized in bands5 because the MWCNTs in HDPE are just partially distributed during the disentanglement process of large MWCNT nests. This resulted in regions where MWCNT cannot be found on the whole TEM micrograph, yet these regions were not taken into account while calculating DTEM . This was different for the MWCNTs/PS-b-P2VP sample, where virtually any region looked like the one presented in Fig. 5b. Atomic force microscopy and field emission scanning electron microscopy show that MWCNTs are in the bulk of the film and not on its surface (data not shown). It is concluded that using high molar mass PS-b-P2VP opens a route to simultaneous dispersion and alignment of MWCNTs in a nanocomposite as has been demonstrated here for the first time. Investigations of the properties of these remarkable MWCNT/BCP thin-films are currently in progress. 4. Conclusion We show that a high molar mass PS-b-P2VP can be successfully used to simultaneously obtain highly aligned and dispersed unfuctionalized MWCNT in a nanocomposite film. The high molar mass BCP performs a dual-action and acts as dispersant and loadtransferring matrix during shear-alignment. The steric stabilization of PS-b-P2VP micelles in p-xylene lead to a long term stable (for weeks) MWCNT dispersion. We attribute this effect to a preferential electrostatic (dipole-conductor type) interaction of the polar P2VP with the conductive MWCNT wall, which is due to the lone pair donacity of the P2VP and is also underpinned by a small

5 The MWCNT density at the upper right and lower left corner of Fig. 5b is much lower.

25

distance in Hansen space between P2VP and CNTs [5:7 ðMPaÞ1=2 ]. The importance of P2VP in the micelles' cores was emphasized by the comparison with PS-b-PEO which resulted in poor long-term stability of the MWCNT dispersion. The outstanding degree of unfunctionalized MWCNT dispersion was maintained after the alignment of the MWCNTs by the melt-drawing technique. The alignment and dispersion of MWCNTs is one of the main current bottlenecks in (unfunctionalized) MWCNT/polymer nanocomposite application, because it is a key to unleash the outstanding anisotropic properties of the MWCNT nanofillers to the macroscale. Our results are of special importance since the presented method worked without covalent MWCNT modification and, thus, no deterioration of MWCNT properties has to be expected. The long-time stable dispersion via high molar mass PS-bP2VP micelles is a significant result for the application of MWCNT/ polymer nanocomposites, because it helps to create nanocomposites reproducibly. This may be an important step towards their successful and beneficial application. Acknowledgments The research at Oak Ridge National Laboratory's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Science, U.S. Department of Energy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2017.08.030. References [1] X. Sun, H. Sun, H. Li, H. Peng, Developing polymer composite materials: carbon nanotubes or graphene? Adv. Mater 25 (37) (2013) 5153e5176. http://dx.doi. org/10.1002/adma.201301926. [2] C. Roman, M. García-Morales, J. Gupta, T. McNally, On the phase affinity of multi-walled carbon nanotubes in PMMA: LDPE immiscible polymer blends, Polymer 118 (2017) 1e11. ISSN 00323861, http://dx.doi.org/10.1016/j. polymer.2017.04.050. [3] M.R. Bockstaller, R.A. Mickiewicz, E.L. Thomas, Block copolymer nanocomposites: perspectives for tailored functional materials, Adv. Mater 17 (11) (2005) 1331e1349. http://dx.doi.org/10.1002/adma.200500167. [4] W. Wang, E.D. Laird, Y. Gogotsi, C.Y. Li, Bending single-walled carbon nanotubes into nanorings using a pickering emulsion-based process, Carbon 50 (5) (2012) 1769e1775. ISSN 00086223, http://dx.doi.org/10.1016/j.carbon.2011. 12.024. [5] M.M.L. Arras, C. Schillai, K.D. Jandt, Enveloping self-assembly of carbon nanotubes at copolymer micelle cores, Langmuir 30 (47) (2014) 14263e14269. ISSN 0743-7463, http://dx.doi.org/10.1021/la502298j. [6] S.D. Bergin, Z. Sun, D. Rickard, P.V. Streich, J.P. Hamilton, J.N. Coleman, Multicomponent solubility parameters for single-walled carbon nanotubesolvent mixtures, ACS Nano 3 (8) (2009) 2340e2350. http://dx.doi.org/10. 1021/nn900493u. [7] K.D. Ausman, R. Piner, O. Lourie, R.S. Ruoff, M. Korobov, Organic solvent dispersions of single-walled carbon nanotubes: toward solutions of pristine nanotubes, J. Phys. Chem. B 104 (38) (2000) 8911e8915. http://dx.doi.org/10. 1021/jp002555m. [8] E. Nativ-Roth, R. Shvartzman-Cohen, C. Bounioux, M. Florent, D. Zhang, I. Szleifer, R. Yerushalmi-Rozen, Physical adsorption of block copolymers to SWNT and MWNT: a nonwrapping mechanism, Macromolecules 40 (10) (2007) 3676e3685. http://dx.doi.org/10.1021/ma0705366. [9] H.-I. Shin, B.G. Min, W. Jeong, C. Park, Amphiphilic block copolymer micelles: new dispersant for single wall carbon nanotubes, Macromol. Rapid Commun. 26 (18) (2005) 1451e1457. ISSN 1022-1336, http://dx.doi.org/10.1002/marc. 200500290. [10] Z. Li, Q. Li, S. Shang, L. Li, X. Yang, X. Yu, G. Yan, Sensing behaviors of polymer/ carbon nanotubes composites prepared in reversed microemulsion polymerization, J. Appl. Polym. Sci. 119 (3) (2011) 1842e1847. http://dx.doi.org/ 10.1002/app.32894. [11] S. Banerjee, T. Hemraj-Benny, S.S. Wong, Covalent surface chemistry of singlewalled carbon nanotubes, Adv. Mater 17 (1) (2005) 17e29. ISSN 0935-9648, http://dx.doi.org/10.1002/adma.200401340. [12] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chemistry of carbon nanotubes, Chem. Rev. 106 (3) (2006) 1105e1136. ISSN 0009-2665, http://dx.doi.org/10. 1021/cr050569o.

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