Grafting of multi-sensitive PDMAEMA brushes onto carbon nanotubes by ATNRC: tunable thickening/thinning and self-assembly behaviors in aqueous solutions

Abstract

Well-defined modification of multi-wall carbon nanotubes (MWNTs) with different molecular weight poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was high-efficiently realized by atom transfer nitroxide radical coupling (ATNRC), which was confirmed by ¹H NMR, TEM and thermogravimetric analyses (TGA). The multi-responsive behaviors of PDMAEMA modified MWNTs (MWNTs-g-PDMAEMA) suspensions were systematically investigated by rheology. Reversible thermo-thickening behavior was detected in MWNTs-g-PDMAEMA suspensions, attributing that PDMAEMA side chains aggregated to hydrophobic micro-domains physically cross-linking the polymer chains at temperatures above LCST. By the incorporation of sufficient free PDMAEMA chains (f-PDMAEMA), the suspensions with enough long grafted-chains exhibited obvious shear-thickening behaviors at high pH, which was absent in the suspensions with short grafted-chains due to steric hindrance of MWNTs as backbones. The primary mechanism of shear thickening was ascribed to the shear-induced transformation of intra-chain cross-linking to inter-chain cross-linking induced by the bridging effect of additional free-chains. Hydrophobic interaction, the amount of f-PDMAEMA chains and grafted-chain length were determined as the three vital factors of shear-thickening behaviours, which could be employed to fulfill tunable thickening/thinning responsive of MWNTs-g-PDMAEMA. In the study of self-assembly properties of MWNTs-g-PDMAEMA and the hybrid system of Au nanoparticles immobilized MWNTs-g-PDMAEMA dilute aqueous suspensions, typically dendritic fractal patterns with different details were formed in response to pH values and the concentration of NaCl, which have not been reported in the literature yet. The morphology of the self-assembled hybrid systems of Au nanoparticle immobilized MWNTs-g-PDMAEMA aggregates gradually transformed from rigid, straight and wide trunks with few side branches to soft, bent and slim trunks with many side branches as the pH value increased. The insights reported here can help to deeply understand the thickening/thinning essence and self-assembly principle of polymer brush grafted MWNTs suspensions, and guide the development of complex responsive materials with better performance.

---

Introduction

In the field of intelligent materials, stimuli-responsive polymers or copolymers have been considerably employed in recent years because their physical and chemical properties can be conveniently adjusted by external stimuli.^1–4 For example, hydrophilicity or hydrophobicity properties of representative polymers, such as poly(N-isopropyl acrylamide) (PNIPAM),^5,6 poly(acrylic acid) (PAA)^7,8 and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA),^9–11 can be switched by the variation of pH or temperature. According to their environment these macromolecular architectures are able to undergo a reversibly dramatic change of macroscopic properties like swelling/collapse, with important potentialities in macromolecular transducers, controlled permeation, drug release,^12–15 or thickening/thinning with control of flow properties in fluid technology.^16–18 Among these stimuli-responsive polymers with different structure, brush-shaped block polymers have recently attracted considerable attention due to their dense molecular architecture with moderate flexibility.^19,20 In our previous work,²¹ double hydrophilic brush copolymer of poly(ethylene oxide) as backbone and PDMAEMA as side chains (PEO-g-PDMAEMA) was successfully prepared and stimuli-responsive behaviors of the brush-shaped copolymer were investigated by rheology. An unexpected shear thickening behavior and dispersions fractal aggregates with different topology were obtained by adjusting the pH and NaCl concentration, which can supply a better understanding of the molecular self-assembling essence and be used to develop responsive materials with better performance.

Carbon nanotubes (CNTs),²² exhibiting unique structure and superior physical properties, have been widely applied in sensors,^23,24 supercapacitors,^25,26 hydrogen storage,^27,28 high-performance polymer composites^29–31 and so on. However, the lack of dispersion in solvents or polymer matrix due to strong π–π conjugation and van der Waals forces between CNTs causes a significant impediment to many potential commercial applications.^32,33 Generally, the linkage of polymer chains to CNTs by covalent bonding in the manner of “grafting onto” or “grafting from”^34,35 is deemed as an effective and stable strategy to improve dispersity, which can also show comprehensive properties combined with characteristics of polymers. Many different types of polymers have been successfully grafted onto CNTs, yet water soluble and responsive polymers^36–40 are of particular interest, showing the potential to assemble into ordered hierarchical structures due to strong hydrophilic/hydrophobic interactions. At present, there are already some examples of grafting dual responsiveness polymer PDMAEMA onto CNTs by ATRP^38,39 or RAFT⁴⁰ with “grafting from” approach, but low grafting amount and poor synthesis controllability impede the application expanding of this stimuli-responsive hybrid material. Furthermore, PDMAEMA modified CNTs can be approximately regarded as comb-like block copolymer, and the most significant difference is that rigid MWNTs with large size acted as main chain instead of flexible PEO chains by comparison with PEO-g-PDMAEMA,²¹ which must lead to unreported different multi-responsive and self-assembly properties compared with our previous report.

In our previous work,⁴¹ we put forward an efficient “grafting to” approach via combination of atom transfer radical polymerization (ATRP) and atom transfer nitroxide radical coupling (ATNRC) to successfully realize high density grafting of well-defined nonpolar polymer (polystyrene) onto multi-walled carbon nanotubes (MWNTs). Herein, MWNTs grafted by different molecular weight well-defined PDMAEMA with different molecular weight (MWNTs-g-PDMAEMA) were effectively prepared via our proposed strategy, extending its applicability. Rheological characterization was implemented to investigate the tunable responsive thickening/thinning of MWNTs-g-PDMAEMA. Some results about multi-responsive properties obtained contrary to what our expected were also discussed by compared with our previous report of PEO-g-PDMAEMA with comparable grafted amount. Self-assembly properties of MWNTs-g-PDMAEMA suspensions under the conditions of various pH value and ionic strength were also studied in order to obtain the fractal structures with various topological patterns.

Experimental

Materials

MWNTs with external diameter of 10–15 nm was purchased from Shenzhen Bill Scientific and Technical Corporation. 4-Hydroxy-2,2,6,6-tetramethylpiperidine (HO-TEMPO) was purchased from Beijing Tiangang Auxiliary Factory. 2,2′-Bipyridyl (bpy), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMEDTA) and ethyl α-bromoisobutyrate (EBiB) were purchased from Aldrich and used without further purification. N,N-(Dimethylamino)ethyl methacrylate (DMAEMA, purchased from Aladdin) was passed through a neutral alumina column to remove the inhibitor and then dried by CaH₂. CuBr (95%) was stirred overnight in acetic acid, filtered, washed with ethanol and diethyl ether successively, and dried in vacuum. 4-N,N-Dimethylaminopyridine (DMAP), N,N-dicyclohexyl carbodiimide (DCC), concentrated sulfuric acid (H₂SO₄), nitric acid (HNO₃), tetrahydrofuran (THF, 99%), dichloromethane (98%) and toluene (99%) were purchased from Sinopharm chemical Reagent Co. and purified by standard methods before using. Deionized water (18.25 MΩ cm) was prepared in a Sartorius Arium 611 system and used throughout the experiment.

Synthesis of PDMAEMA

In a typical experiment, EBiB (0.12 mL, 0.67 mmol), CuBr (96.0 mg, 0.67 mmol), and bpy (105.0 mg, 0.67 mmol) were firstly dissolved in 20 mL distilled water. The solution was deoxygenated by freeze–pump–thaw cycles under argon. In the subsequent procedure, DMAEMA (23.0 mL, 134 mmol) was then added to the mixture under argon. The contents were further deoxygenated by three freeze–pump–thaw cycles. Then, the reaction was carried out at 30 °C under nitrogen atmosphere for 24 h, which was evidenced by attainment of high solution viscosities. Polymerizations were terminated by methanol dilution and exposed to air prior to copper removal on alumina columns. The crude polymer was collected by rotating evaporation from aqueous solution and extracted with n-hexane at room temperature three times to remove unreacted DMAEMA monomer. Finally, the products were dried at 60 °C under vacuum. In addition, PDMAEMA with different molecular weight were also obtained by variation of the DMAEMA monomer amount with the same polymerization procedure. The prepared PDMAEMA-Br homopolymers with different feed ratios were marked as PDMAEMA₅₀, PDMAEMA₁₀₀ and PDMAEMA₁₅₀, respectively, and the molecular weight (M_n) and the molecular weight distribution (M_w/M_n) was measured by GPC and summarized in Table S1.†

Synthesis of MWNTs-TEMPO

The synthetic procedure was carried out according to our previous report: carboxylic acid-functionalized MWNTs (MWNTs-COOH) was firstly prepared by oxidation of pristine MWNTs with concentrated HNO₃/H₂SO₄. Crude MWNTs (3.0 g), HNO₃ (67%, 40 mL), and H₂SO₄ (98%, 120 mL) were added into a flask equipped with a condenser. After ultrasonic treatment for 15 min, the mixture was stirred for 2 h under reflux at 120 °C. After cooling to room temperature, the mixture was diluted with massive deionized water and then filtered through a 0.22 μm PVDF membrane. Several times of washing with deionized water were implemented until the filtrates exhibit neutral. MWNTs-COOH powders were obtained after the sample dried in vacuum for 24 h at 60 °C. In the subsequent procedure, the prepared MWNTs-COOH (0.9 g), HO-TEMPO (0.432 g), DCC (0.519 g), DMAP (0.102 g), and anhydrous dichloromethane (20 mL) were mixed and stirred for 24 h at room temperature. Then, the products were washed with dichloromethane and separated via centrifugation. MWNTs-TEMPO was obtained after lower layer being dried overnight at 60 °C.

Synthesis of MWNTs-g-PDMAEMA

MWNTs-g-PDMAEMA was synthesized via typical “grafting onto” method. As-prepared MWNTs-TEMPO (0.12 g, 0.090 mmol, based on TGA data) and PDMAEMA-Br (0.126 mmol) were mixed with purchased PMDETA (0.053 mL) and anhydrous toluene (5 mL) in a schlenk flask associating with deoxygenated treatment, and then stirred at 90 °C for 24 h. The resulting sample was subsequently dialyzed against distilled water for 48 h to remove small molecules. The mixture was washed with distilled water followed by filtration through a 0.22 μm PVDF membrane, and repeated three times to adequately remove un-grafted homopolymer. Finally, MWNTs-g-PDMAEMA was obtained after being dried at 60 °C under vacuum. In addition, another two kinds of MWNTs-g-PDMAEMA were also obtained by variation of the molecular weight of as-prepared PDMAEMA homopolymer with the same polymerization procedure. The reaction is outlined in Scheme 1.

Scheme 1 Strategy for synthesis of MWNTs-g-PDMAEMA.

Characterization

Fourier transfer infrared (FT-IR) spectra were measured with a Nicolet Pro Instrument in transmission mode using the KBr disk method. Thermal gravimetric analyses (TGA) were conducted using a NETZSCHTG-209 instrument at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere. Gel permeation chromatography (GPC) was performed on a Waters 5010 system equipped with a Waters 515 pump, three styragel columns calibrated by narrow PS standards, and a Waters 410 RI detector with THF as the eluent at a flow rate of 1.0 mL min⁻¹ at 40 °C. ¹H NMR spectra were recorded using Bruker (500 MHz) spectrometers with D₂O as the solvent and tetramethylsilane as the standard at room temperature. Rheological measurements were performed on a MCR 301 rheometer (Anton Paar) equipped with parallel plate grippers of 25 mm in diameter. The gap distance was set as 0.5 mm, and solvent traps with low viscosity silicon oil were adopted to insulate heat loss from the solution for all tests. A steady sweep was conducted at 25 °C controlled by a Peltier temperature controller. A temperature ramp was implemented from 10 to 60 °C at a rate of 1 °C min⁻¹ and a constant angular frequency of 1 rad s⁻¹. The strain sweep was performed at 10 rad s⁻¹ to establish the linear viscoelastic region. Transmission electron microscopy (TEM) images were obtained using a JEM-1200EX instrument. Hybrid aqueous solutions of MWNTs-g-PDMAEMA with the variable pH and NaCl concentration were cast onto copper grids coated with carbon films followed by evaporation. Optical Microscope (OM) images were obtained using Leitz Laborlux 12 Pols. The suspensions were dropped onto abstersive slides.

Results and discussion

Synthesis and characterization of MWNTs-g-PDMAEMA

High density grafting of PDMAEMA onto MWNTs was achieved via the combination of ATRP and ATNRC with the method of “grafting to”, which was outlined in Scheme 1. Firstly, PDMAEMA-Br with well-defined structure was synthesized by ATRP in the presence of EBiB/CuBr/bpy, which was clarified by the results of GPC (Fig. S1†). Meanwhile, MWNTs-TEMPO was prepared by carboxylic acid-functionalized MWNTs with HO-TEMPO, which was confirmed by FT-IR (Fig. S2†). Then, MWNTs-g-PDMAEMA nanocomposites were obtained by grafting as-prepared PDMAEMA-Br side chains onto MWNTs-TEMPO via CuBr-catalyzed ATNRC. TEMPO is a radical scavenger in that it couples with carbon-centered radicals at diffusion-limited reaction rates. The reversibility of the reaction renders TEMPO an efficient mediator in controlled/“living” radical polymerization. When the mixture of PDMAEMA-Br and MWNTs-TEMPO was heated to 90 °C in the presence of CuBr/PMDETA, the macroradicals of PDMAEMA-Br were trapped by high-efficient nitroxide radicals of TEMPO groups on MWNTs to form the alkoxyamine, which constituent parameters were listed in Table S1† to quantitatively describe the well-defined structure by ATNRC.

¹H NMR is an effective tool to investigate chemical structure of MWNTs-g-PDMAEMA, which was shown in Fig. S2.† The characteristic methyl proton signals of PDMAEMA chains were located at chemical shifts of 0.9–1.2 ppm and 2.3 ppm, corresponding to the methyl protons C–CH₃ (d) and N–CH₃ (a), respectively. The methylene protons peaks located at chemical shifts of 1.8 ppm, 2.6 ppm and 4.1 ppm were attributed to N–CH₂– (b), C–CH₂– (e), –(C=O)–O–CH₂– (c), respectively. In addition, FT-IR spectrum of MWNTs-g-PDMAEMA was shown in Fig. S3,† representative characteristic absorption peaks of PDMAEMA chains at 2950 cm⁻¹ (C–H stretching of methyl and –CH₂– groups), 1726 cm⁻¹ (C=O stretching) and 1150 cm⁻¹ (C–C–N bending) were clearly detected. Therefore, analysis results of ¹H NMR and FT-IR confirmed that PDMAEMA chains had been successfully grafted onto the surface of MWNTs.

Morphological evidence for the successful grafting of polymer chains onto the surface of MWNTs was further provided by TEM images. Fig. 1 showed the TEM images of MWNTs-COOH and MWNTs-g-PDMAEMA. The surfaces of pristine MWNTs were very smooth, and aggregated together due to strong π-conjugation among the tubes. Polymer-grafted MWNTs were coated with a polymer layer with the uniform thickness of 15 nm, which was a direct representative of the covalent grafting of PDMAEMA. In addition, the inserts in Fig. 1 showed photographs of aqueous dispersions of pristine MWNTs and MWNTs-g-PDMAEMA. It was clear that the dispersibility of pristine MWNTs in H₂O was very poor, and there was much sedimentation at the bottom of vial. However, MWNTs-g-PDMAEMA was easily dispersed in H₂O, forming a homogeneous dispersion, and there was no sedimentation observed even after several months. All the evidences indicated that polymer chains have been successfully grafted onto the surface of MWNTs.

Fig. 1 TEM images of pristine MWNTs (A), and MWNTs-g-PDMAEMA (B). The insets show photographs of aqueous dispersions of pristine MWNTs (a) and MWNTs-g-PDMAEMA₁₀₀ (b) with a concentration of 2 mg mL⁻¹.

To quantitatively determine the grafting rate of polymers in MWNTs-g-PDMAEMA, TGA measurements of pristine MWNTs and modified MWNTs were performed as presented in Fig. S4.† Pristine MWNTs exhibited excellent thermostability during TGA measurement, manifested by negligible weight loss even at 800 °C. MWNTs-TEMPO showed weight loss of about 9.4 wt% from 150 to 350 °C, which is higher than that of MWNTs-COOH, ascribed to the weight gain (about 7.3%) from introduced TEMPO groups on MWNTs surface and the degree of esterification is estimated as 45%. As shown in TGA curves of MWNTs-g-PDMAEMA (Fig. S4(d–f)†), the major weight loss occurred in the temperature range 220–430 °C, corresponding to thermal decomposition of the grafted PDMAEMA chains on the surface of MWNTs, which could be readily clarified by comparison with TGA curve of PDMAEMA (Fig. S4g†). Taking into account the residue at 800 °C, the grafted weight fractions of polymers with increasing M_n for MWNTs-g-PDMAEMA were estimated as 75.4%, 67.2% and 37.4%, respectively, which were significantly higher than the reports about preparation of MWNTs-g-PDMAEMA by the method of “grafting from” with poor synthesis controllability, including that of Fan's work⁴² (30.7%) and Lou's work⁴³ (30%) using nitroxide radical initiation, that of Lim's work (46.1%) via RAFT⁴⁰ and Kim's work (53.9%) via ATRP,⁴⁴ determined by outstanding coupling efficiency between pending-TEMPO groups and halogen-containing polymer for ATNRC reaction.^45,46 In addition, in terms of PDMAEMA content and TEMPO group content of MWNTs-TEMPO, the average grafting densities of PDMAEMA chains with respect to the MWNTs-g-PDMAEMA samples were estimated and listed in Table S1.† It can be seen that the grafting density decreased as the molecular weight of PDMAEMA increased, ascribing to the slowdown of diffusion induced by steric hindrance for high molecular weight polymer side chains.⁴⁷ In another word, MWNTs-g-PDMAEMA₅₀ had plenty of short branched chains, while MWNTs-g-PDMAEMA₁₅₀ possessed a spot of long branched chains. For MWNTs-g-PDMAEMA₁₀₀, grafting chain length and grafting density both lied between those of the other two. From the aforementioned results, it is clear that ATNRC reaction is a highly-efficient approach to attach well-defined polymers chains with certain architecture onto MWNTs with high grafting amount and grafting density under mild aqueous conditions.

Multi-responsive behaviors of MWNTs-g-PDMAEMA suspensions

The dependences of steady viscosity (η) on the shear rate ([small gamma, Greek, dot above]) of MWNTs-g-PDMAEMA suspensions with different grafting status (chain length and density) of PDMAEMA at a concentration of 20 wt% under pH = 9 were shown in Fig. 2. All the samples are typical pseudoplastic fluids, consisting of the Newtonian region and the shear thinning region. The three kinds of MWNTs-g-PDMAEMA suspensions possessed nearly equivalent viscosity in Newtonian region, but exhibited enormously different shear thinning behaviors. MWNTs-g-PDMAEMA₁₀₀ suspension showed rather wide Newtonian plateau at <200 s⁻¹, followed by rapid shear thinning, while MWNTs-g-PDMAEMA₁₅₀ suspension showed wild shear thinning over the whole experimental shear rate range, absence of the Newtonian region even at extremely small shear rate due to the high MWNTs content (62.6%) calculated from TGA. Remarkable shear thinning behaviors have been wildly reported in some composites embedded with nano-fillers, resulting from the breakage of transient network induced by orientation of fillers under shear.^48,49 Given that the Newtonian plateau represented anti-shearing stability of PDMAEMA chains/MWNTs hybrid network, it can be inferred that suitable grafting chain length and grafting density were both vital factors for the formation of stable hybrid network.

Fig. 2 Steady viscosity (η) as a function of shear rate ([small gamma, Greek, dot above]) for MWNTs-g-PDMAEMA suspensions (C = 20 wt%) with pH of 9 at 25 °C.

Taking MWNTs-g-PDMAEMA₁₀₀ for example, effect of pH on the steady rheology of suspension was shown in Fig. 3. PDMAEMA is a representative pH-sensitive polymer, which is attributed to protonation/deprotonation of the tertiary amine side groups, and the pKa for protonated PDEAEMA is around pH 7–8 (ref. 50 and 51), which means that this polyelectrolyte deprotonates almost completely above pH 8. As pH increased from 7 to 14, a zero-shear viscosity of enhancement of two times and significant decrease of the critical shear rate that shear thinning took place were observed in Fig. 3. Inter-chain hydrophobic aggregation of deprotonated PDMAEMA blocks in the suspension with increasing pH led to the formation of a gradual physical network structure, which could explain the enhancement of zero-shear viscosity. Compared with our previous reports about PEO-g-PDMAEM,²¹ relatively small enhancement of viscosity was attributed to the inertia to pH value of MWNTs as backbones with high content (62.6%). More compact network structure at high pH together with easy-orientation of MWNTs induced lower critical shear rate of shear thinning. The pH dependence of steady rheology flows for MWNTs-g-PDMAEMA suspensions with different grafted-chain molecular weight at the same concentrations were also provided in Fig. S5,† which showed similar variation tendency to that of MWNTs-g-PDMAEMA₁₀₀. Molecular weight and concentration of polymer chains were both key factor to hydrophobic association. MWNTs-g-PDMAEMA₅₀ with plenty of short branched chains and MWNTs-g-PDMAEMA₁₅₀ with a spot of long branched chains both exhibited lower magnitude of viscosity enhancement and critical shear rate of shear thinning with pH increase than MWNTs-g-PDMAEMA₁₀₀, showing suitable grafting chain length and grafting density were both vital factors determining pH sensitiveness of rheology behaviors.

Fig. 3 η as a function of [small gamma, Greek, dot above] for MWNTs-g-PDMAEMA₁₀₀ suspensions with different pH at 25 °C (C = 20 wt%).

In addition, other steady flowing curves of MWNTs-g-PDMAEMA suspensions with various concentrations under different temperatures and pH were provided in Fig. S6.† It is noteworthy that adjusting grafting density and chain length, concentration, pH and temperature over the wide range cannot change the shear thinning tendency at all and wishful shear thickening behavior did not occur in rheology curves, which was distinctive from our previous work about double hydrophilic brush copolymer PEO-g-PDMAEMA. The charming shear thickening behaviors for PEO-g-PDMAEMA concentrated aqueous solution were interpreted by the proposed mechanism that PDMAEMA grafting chains with a collapsed conformation in the initial solution were sufficiently stretched to form elastically active bridging network due to hydrophobic association at high pH upon application of large stress. By comparison of the chemical structure for the two similar systems, the most significant difference was that rigid MWNTs with large size acted as backbone instead of flexible PEO chains for MWNTs-g-PDMAEMA. Based on the above discussion, we can infer that the grafted PDMAEMA chains in MWNTs-g-PDMAEMA suspension cannot readily interact with each other limited by sterically hindered effect⁵² of the large dimension of MWNTs overwhelming its hydrophobic contribution, and thus result fewer intermolecular hydrophobic associations of PDMAEMA chains during shear and alignment of MWNTs under shear field, which let hybrid suspensions show shear thinning property.

In fact, shear-thickening as a key issue with wild application prospect in many fields has been extensively investigated in recent years. Among these, the studies with respect to associating polymers showed that the vital factors determining shear-thickening behaviors were association interactions^17,53,54 (hydrogen bonding or hydrophobic interaction), molecular weight and concentration of polymer chains.^17,55 In recent work, the hydrophobic nature of MWNTs was considered to be beneficial to hydrophobic association, but it was dominant that the large dimension of MWNTs could retard the inter-chain associations of PDMAEMA chains due to steric hindrance resulting in “weak” gels mainly composed of the entangled, spaghetti-like CNTs network with less of polymer chains connectivity in the associated network, according to Hietala' report.⁵⁶ Since MWNTs acting as main chain impaired hydrophobic association, molecular weight and concentration of PDMAEMA chains perhaps can be employed to adjust shear thickening behaviors. Inspired by the argument of Wang⁵⁷ that free polymer chains can firstly join the transient network as dangling chains and eventually transform to elastically active bridging chains under high shear, free PDMAEMA homopolymers (f-PDMAEMA) was introduced into MWNTs-g-PDMAEMA suspensions to explore the occurrence possibility of shear thickening behaviors in the following discussion.

A certain amount of f-PDMAEMA with unified molecular weight was incorporated into the three kinds of MWNTs-g-PDMAEMA suspensions, respectively, and the flow behaviors were systematically investigated again. Taking the hybrid suspension with the concentration of MWNTs-g-PDMAEMA₁₀₀ of 15 wt% and 3.0 wt% f-PDMAEMA for an example, the steady flow curves at different pH were shown in Fig. 4. The solution of pH 7 showed a nearly Newtonian viscosity over the entire experimental shear rate range. Samples of higher pH (9, 12 and 14) also showed a Newtonian behavior at low shear rates, but this was followed by a smooth shear thickening behaviors of significant magnitude at critical shear rates around 200 s⁻¹, associated with a sharp decrease at the higher shear rate. It is noteworthy that the critical shear rates that shear thickening takes place decreased with increasing of pH, which implied that higher pH values do favor to the occurrence of shear thickening due to the stronger hydrophobic association of PDMAEMA chains.

Fig. 4 η as a function of [small gamma, Greek, dot above] for MWNT-g-PDMAEMA₁₀₀/f-PDMAEMA suspension with different pH at 25 °C.

Fig. 5 showed the steady flow curves of MWNTs-g-PDMAEMA₁₀₀/f-PDMAEMA suspensions with different concentrations of f-PDMAEMAM at pH = 14. As the amounts of f-PDMAEMAM increased, overall viscosities were gradually elevated owing to the contribution of f-PDMAEMAM to physical networking structure. When the concentrations of f-PDMAEMA were 1.0 and 2.0 wt%, the hybrid suspension exhibited typical pseudoplastic fluid-like behaviors, consisting of the Newtonian region and the shear thinning region. It was worth noting that an expecting shear-thickening phenomenon was observed as the concentration of f-PDMAEMA increased to 2.5 wt%, and the hybrid suspension with 3.0 wt% f-PDMAEMA showed more pronounced shear-thickening with characteristics of the smaller critical shear rate and larger shear-thickening magnitude. The critical concentration of PDMAEMA including free and grafted chains with respect to total solid contents in hybrid suspension was calculated as 74.2 wt% (corresponding to 2.5 wt% f-PDMAEMA), comparable to that of PEO-g-PDMAEMA (78 wt%). It was inferred that enough f-PDMAEMA chains with high-motility can bridge adjacent grafted MWNTs by inter-chain associations to form new polymer-dominant network under high shear, that is to say, critical amount of f-PDMAEMA chains is an essential factor to the occurrence of shear thickening behaviours in MWNTs-g-PDMAEMA suspensions.

Fig. 5 η as a function of [small gamma, Greek, dot above] for MWNT-g-PDMAEMA₁₀₀/f-PDMAEMA suspensions with different concentration of f-PDMAEMAM at pH = 14.

The dependences of f-PDMAEMA concentration on steady flow behaviours for the other two systems with different grafted chain length were also shown in ESI.† For MWNTs-g-PDMAEMA₁₅₀ hybrid suspensions (Fig. S7†), the same tendency as MWNTs-g-PDMAEMA₁₀₀ was detected, and the critical amount of PDMAEMA including free and grafted chains that shear-thickening began to take place was calculated as 57.0 wt%, corresponding to 3.0 wt% f-PDMAEMA. However, shear-thickening behaviors did not appear at all in f-PDMAEMA/MWNTs-g-PDMAEMA₅₀ suspensions even if 5.0 wt% f-PDMAEMA was added (Fig. S8†), corresponding to extremely high total amount of PDMAEMA (81.2%), which could be explained by that additional free-chains were hard to bridge the adjacent short grafted-chains due to steric hindrance of MWNTs as backbones. It was inferred that sufficient grafted-chain length, supplying handy bridging sites of f-PDMAEMA chains, was also an essential factor for shear thickening of MWNTs-g-PDMAEMA suspensions.

On basis of the “structure-forming” mechanism proposed by Witten⁵³ and “free chains-bridging” reported by Wang,⁵⁷ we suggested a mechanism for formation of inter-chain associations bridged by imposed polymer chains under strong shear, which was shown in Scheme 2. The grafted PDMAEMA chains on MWNTs surface exhibited the collapsed conformation in the initial suspensions due to increased hydrophobicity caused by the deprotonation of tertiary amino groups at high pH. Upon application of stress, the orientation of MWNTs along shear direction induced the large-scale stretch of grafted PDMAEMA chains to break original the transient network. In the absence of additional free-chains, stretched grafted-chains cannot move freely to interact with each other limited by steric hindrance of the large dimension of MWNTs overwhelming its hydrophobic contribution, and thus created fewer intermolecular hydrophobic associations, inducing the shear thinning behaviors. After the incorporation of free PDMAEMA chains, stretched grafted-chains with enough length were easily bridging by stretched free-chains to construct new hybrid physical network with stronger association, leading to the observed shear thickening effect in the MWNTs-g-PDMAEMA₁₀₀ and MWNTs-g-PDMAEMA₁₅₀ suspensions, that is, the primary mechanism of shear thickening is ascribed to the shear-induced transformation of intra-chain cross-linking to inter-chain cross-linking induced by the bridging effect of additional free-chains. However, shear thickening MWNTs-g-PDMAEMA₁₅₀ suspensions was absent in spite of excessive addition of free chains, attributing to deficient bridging between free-chains and short grafted-chains limited by steric hindrance of MWNTs as backbones.

Scheme 2 Schematic drawing of the hybrid aqueous solution under shear filed MWNTs-g-PDMAEMA (a), f-PDMAEMA (M_n = 4770) add in MWNTs-g-PDMAEMA₅₀ (b), f-PDMAEMA (M_n = 8420) add in MWNTs-g-PDMAEMA₁₀₀ (c), f-PDMAEMA (M_n = 11170) add in MWNTs-g-PDMAEMA₁₅₀.

In a word, combination of elevating pH value with introducing adequate f-PDMAEMA could lead to the occurrence of shear thickening behaviours under the precondition of enough long grafted-chain on the surface of MWNTS, that is to say, hydrophobic interactions, amount of f-PDMAEMA chains and grafted-chain length are the three vital factors determining shear-thickening behaviours. Compared with our precious report about PEO-g-PDMAEMA, the fundamental feature lay in indispensable addition of enough f-PDMAEMA chains for MWNTs-g-PDMAEMA suspensions, attributing to limited grafted amount induced by watered-down coupling efficiency of ATNRC reaction and restricted bridging actions between f-DMAEMA and grafted chains both due to the steric hindrance of MWNTs as backbones. The observations reported here can effectively guide the regulation of shear thickening behaviors for complex responsive materials.

Fig. 6 displayed the temperature dependence of complex viscosity (η*) for MWNTs-g-PDMAEMA₁₀₀ aqueous suspension with a concentration of 20 wt% at pH = 9, and inserted digital pictures showed the states of the suspension during heating and cooling. Under a temperature below 40 °C, a continuous thinning in viscosity was observed upon heating, following the usual Arrhenius-type behavior of most fluids due to decreasing friction between chain segments and the solvent as temperature increased. While the temperature exceeded 40 °C, a significantly reversible thermo-thickening behaviour was detected, attributing that PDMAEMA side chains aggregated to form hydrophobic micro-domains physically cross-linking the polymer chains, leading to the formation of a high-viscosity network at the temperature above lower critical solution temperature LCST. The suspension at higher pH (12) and lower pH (7) both exhibited similar reversible thermo-thickening behaviours (Fig. S9†) with subtle divergences in LCST (35 and 42 °C), increasing with reduced pH. Compared with the previous results about PEO-g-PDMAEMA,²¹ the higher LCST were owing to steric hinderance of MWNTs as backbone. As may be seen in the inserts Fig. 8, the suspension was a clear viscous liquid at 25 °C but further viscosities upon warming to 60 °C, behaved as a hydrogel which did not flow in the inverted tube and recovered to a homogeneous solution upon cooling, revealing the excellent reversible sol–gel transition.

Fig. 6 Temperature dependence of complex viscosity (η*) for MWNTs-g-PDMAEMA₁₀₀ aqueous solution (C = 20 wt%) at pH = 9 monitored at 1 rad s⁻¹ with a temperature rate of 1 °C min⁻¹. The inserts show digital pictures of MWNTs-g-PDMAEMA₁₀₀ suspension at given temperatures.

Fig. 7 showed the temperature dependence of storage modulus (G′) and loss modulus (G′′) for 20% MWNTs-g-PDMAEMA₁₀₀ solutions with pH value of 9 subjected to an increasing temperature ramp followed by a decreasing temperature ramp, and additional dynamic temperature ramp curves with different pH were shown in Fig. S10.† It was evident that all the three suspensions exhibited a pronounced abrupt reversible thermo-thickening effect. The viscoelastic response changed above a critical temperature that corresponded to the LCST of suspensions with different pH. Moreover, G′′ values were always higher than G′ at a frequency of ω = 1 rad s⁻¹ in the detected temperature range for three systems, implying that the viscous contribution was still predominant in the 20 wt% hybrid aqueous solution even after phase separation and molecular aggregation.

Fig. 7 Dynamic temperature ramp for MWNTs-g-PDMAEMA₁₀₀ suspension (C = 20 wt%) at pH = 9 monitored at 1 rad s⁻¹ with a temperature rate of 1 °C min⁻¹.

Self-assembly of MWNTs-g-PDMAEMA in diluted aqueous solution

The self-assembled patterns of these stimuli-responsive copolymers on solid substrates are particularly attractive because the micro- and nano-engineered materials and devices with responsive surfaces mimic the cooperativity encountered in responsive systems found in nature.⁵⁸ PDMAEMA is a weak polybase, which pH-sensitive property was endowed by protonation/deprotonation of the tertiary amine.^59,60 Theoretically, the associative amphiphilic polymers can form diffusion-limited aggregations under proper conditions to develop fractal structures in solution. So the self-assembly behaviors of PDMAEMA brushes haven been widely reported in several copolymer with flexible chains. However, in the MWNTs-g-PDMAEMA system, sterically hindered effect and hydrophobic contribution of the large dimension of MWNTs as main chain instead of flexible polymer chains could lead to complex and versatile self-assembly properties compared with previous reports about PDMAEMA brushes. Herein, we studied the effect of external environment (pH and salt concentration) on the self-assembly behaviours of MWNTs-g-PDMAEMA.

The morphology of the MWNT-g-PDMAEMA copolymer patterns obtained by casting the diluted solution onto the TEM copper grids under different pH values were shown in Fig. 8. For the samples at pH = 4.0, regular fractal patterns were found in the TEM image. Upon closer inspection, it can be observed that the tree fractal pattern was composed of rigid, straight and wide trunks with few side branches (Fig. 8a), and the diameter of the trunks was over the range of 2–5 μm, which could be attributed to hydrophobic nature of MWNTs and the competition in the diffusion field among the original protrusions with high density. For the sample at pH = 7.0, a perfect dendritic morphology was also formed, however, the dendritic pattern was composed of soft, bent and slim trunks with many side branches, and the diameter of the trunks was about 1 μm (Fig. 8b). It is because that clusters were more tightly bound at neutral pH, resulting in the less globule spreading (thinner and narrower patterns). When the pH was increased to 9, typical dendritic fractal patterns with more bent trunks and multi-furcation were detected, and some black circles of dendritic self-assemble could also be found (Fig. 8d). At a higher pH (12.0), the branches connected together in the beginning shrunk, and converted to some little isolated islands, forming many micelles with retentive overall dendritic structure (Fig. 8e). In this situation, grafted PDMAEMA chains aggregated and formed fractal patterns more easily due to the deprotonation of tertiary amine groups and the random moving ability becomes difficult and there is not enough space for rearrangement and aggregation of a large number of the primitive aggregates due to slowdown random motion,⁶¹ resulting in shuttle-like morphologies.

Fig. 8 Typical TEM images of the 2 wt% MWNT-g-PDMAEMA₁₀₀ aggregates formed in the aqueous dispersions with different pH values in the presence of NaCl (0.5 wt%): pH = 4 (a), pH = 7 (b), pH = 9 (c) and pH = 12 (d).

Fig. 9 showed optical microscopy (OM) images of diluted MWNTs-g-PDMAEMA aqueous suspensions with different NaCl concentrations, which exhibited fractal patterns of dendrites at the submicrometer size range merely in the presence of NaCl solutions and the denser growth of fractal patterns with increasing salt concentration. OM micrographs in the presence of 0.5 wt% NaCl revealed the tree-like fractal patterns, composed of spherical or crescent-shaped clusters with the average dendrite branching length of 6 μm, and distinct trunks and veins. As shown in Fig. 9c, the trunk length of the dendritic structure obviously increased and the average side branching length increased to 130 μm with addition of 1 wt% NaCl, yielding more closely dendritic structure and fuzzy branched patterns. It is evidenced that the additional salt promoted the aggregate formation, the cluster association, and the directional growth. The salt can act as the counter ions around the polymer aggregates, which can screen out the electrostatic repulsions between the protonated PDMAMEA blocks, and promote the polyelectrolyte aggregates formation. Moreover, the deposited crystals can act as nucleation sites for the polyelectrolyte chains in the presence of NaCl. In a word, compared with our previous reports about PEO-g-PDMAEMA system,²¹ the onset of fractal patterns for MWNTs-g-PDMAEMA occurred at lower pH, which was attributed to hydrophobic nature of MWNTs. And the motion of MWNTs drove by diffusion of PDMAEMA chains together with aggregations of PDMAEMA brushes limited by steric hindrance of MWNTs led to more abundant fractal structures with various topological patterns via regulating the external environment (pH and salt concentration), which could combined the properties of PDMAEMA and MWNTs.

Fig. 9 Optical images (OM) of the self-assembly MWNTs-g-PDMAEMA₁₀₀ aqueous suspensions at different concentration of NaCl: C_NaCl = 0 (a), C_NaCl = 0.5 wt% (b), C_NaCl = 1.0 wt% (c) and C_NaCl = 2.0 wt% (d).

It is reported that Au nanoparticles (NPs) could be facilely loaded onto the thermo-sensitive PDMAEMA brushes through in situ-reduction of HAuCl₄ via the coordination of the amino group with a gold atom.^62–64 In this paper, the hierarchical self-assembly of the hybrid system of Au NPs immobilized MWNTs-g-PDMAEMA on the solid substrate was finally investigated (Fig. 10). With increasing the pH value, the morphology of the self-assembled aggregates changes from rigid, straight and wide trunks with few side branches to soft, bent and slim trunks with many side branches. The major tendency observed in the dendritic patterns shown in Fig. 10 is the gradual increase of side branching density with the increase of pH values. The multi-responsive fractal patterns for CNT and CNT/Au hybrid system have not been reported in the literature yet. Tam and his co-workers^65,66 reported that no fractal pattern was observed for PDMAEMA-b-C₆₀ in NaCl solution, while in the present work, this is a common feature for all obtained images. The tunable self-assembly of the hybrid system through the external stimuli will provide an intriguing possibility to fabricate intelligent devices with better performance.⁶⁷

Fig. 10 Typical TEM images of the 1 wt% MWNTs-g-PDMAEMA₁₀₀-Au NPs aggregates formed in the aqueous dispersions with different pH values in the presence of NaCl (0.1 wt%): pH = 4 (a), pH = 9 (b), pH = 12 (c, d).

Conclusions

Three kinds of well-defined MWNTs-g-PDMAEMA with different molecular weight were successfully prepared by ATNRC reaction. ¹H NMR, FT-IR, TEM and TGA were employed to confirm the successful grafting of polymer chains onto the surface of MWNTs. The multi-responsive MWNTs hybrid aqueous solution was systematically investigated by rheology. The MWNTs-g-PDMAEMA suspensions with various concentration, pH, temperature, grafting density and chain length all exhibited obvious shear-thinning behaviors, which was distinctive from our previous work about PEO-g-PDMAEMA, attributing to sterically hindered effect of the large dimension of MWNTs as backbones overwhelming its hydrophobic contribution. By the incorporation of sufficient free f-PDMAEMA chains, expecting shear-thickening behaviors were detected in the MWNTs-g-PDMAEMA₁₀₀ and MWNTs-g-PDMAEMA₁₅₀ suspensions at high pH, while was absent in the MWNTs-g-PDMAEMA₅₀ suspensions with short grafted-chains due to steric hindrance of MWNTs. Hydrophobic interactions, amount of f-PDMAEMA chains and grafted-chain length as the three essential factors significantly influenced shear-responsive behaviours, conveniently regulating the shift of shear thinning and thickening responsive of MWNTs-g-PDMAEMA. The primary mechanism of shear thickening was ascribed to the shear-induced transformation of intra-chain cross-linking to inter-chain cross-linking induced by the bridging effect of additional free-chains. The MWNTs-g-PDMAEMA suspensions also exhibited obvious thermo-thickening behaviors, because PDMAEMA side chains aggregated to form hydrophobic microdomains leading to formation of a high-viscosity network at the temperature above LCST higher than that of PEO-g-PDMAEMA, the owing to steric hinderance of MWNTs. In the study of self-assembly properties of MWNTs-g-PDMAEMA and the hybrid system of Au nanoparticles immobilized MWNTs-g-PDMAEMA dilute aqueous suspensions, typically dendritic fractal patterns with different details were formed responsive to pH values and the salt addition, which have not been reported in the literature yet. The morphology of self-assembled both hybrid systems aggregates gradually transformed from rigid, straight and wide trunks with few side branches to soft, bent and slim trunks with many side branches as pH value increased. The current results can supply a theoretical guide for the convenient regulation of responsive behaviors to better meet the demands of applications in drug and gene carriers, protective materials or flooding agent in tertiary oil recovery for novel intelligent materials.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (No. 51403113 and 51573080), Program of Science and Technology in Qingdao City (14-2-4-3-jch and 14-2-4-122-jch), Natural Science Foundation for Distinguished Young Scientists of Shandong Province (BS2014CL007), Postdoctoral Science Foundation of China and Shandong Province (2016T90610, 2015M571994 and 201501007), Project of Shandong Province Higher Educational Science and Technology Program (J14LA19), and Program for Taishan Scholar of Shandong Province.

References

1. M. A. C. Stuart, W. T. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk and M. Urban, Nat. Mater., 2010, 9, 101–113.
2. F. D. Jochum and P. Theato, Chem. Soc. Rev., 2013, 42, 7468–7483.
3. P. Theato, B. S. Sumerlin, R. K. O'Reilly and T. H. Epps III, Chem. Soc. Rev., 2013, 42, 7055–7056.
4. H. Meng and G. Li, Polymer, 2013, 54, 2199–2221.
5. J. Chen, M. Liu, C. Gao, S. Lü, X. Zhang and Z. Liu, RSC Adv., 2013, 3, 15085–15093.
6. J. Y. Li, Z. J. Zhang, X. J. Zhou, T. Q. Chen, J. J. Nie and B. Y. Du, Phys. Chem. Chem. Phys., 2016, 18, 519–528.
7. C. Lu, L. Chen, K. Huang and G. Wang, RSC Adv., 2014, 4, 43682–43690.
8. J. Liu, C. Detrembleur, A. Debuigne, M.-C. De Pauw-Gillet, S. Mornet, L. Vander Elst, S. Laurent, C. Labrugère, E. Duguet and C. Jérôme, Nanoscale, 2013, 5, 11464–11477.
9. J. Chen, P. Xiao, J. Gu, Y. Huang, J. Zhang, W. Wang and T. Chen, RSC Adv., 2014, 4, 44480–44485.
10. H. Rosilo, J. R. McKee, E. Kontturi, T. Koho, V. P. Hytönen, O. Ikkala and M. A. Kostiainen, Nanoscale, 2014, 6, 11871–11881.
11. A. A. Steinschulte, B. Schulte, S. Rutten, T. Eckert, J. Okuda, M. Moller, S. Schneider, O. V. Borisov and F. A. Plamper, Phys. Chem. Chem. Phys., 2014, 16, 4917–4932.
12. S. Mura, J. Nicolas and P. Couvreur, Nat. Mater., 2013, 12, 991–1003.
13. R. Cheng, F. Meng, C. Deng, H.-A. Klok and Z. Zhong, Biomaterials, 2013, 34, 3647–3657.
14. F. Liu and M. W. Urban, Prog. Polym. Sci., 2010, 35, 3–23.
15. Z. Wang, Y. Wang and G. Liu, Angew. Chem., 2016, 128, 1313–1316.
16. D. Hourdet, J. Gadgil, K. Podhajecka, M. V. Badiger, A. Brûlet and P. P. Wadgaonkar, Macromolecules, 2005, 38, 8512–8521.
17. A. Lele, A. Shedge, M. Badiger, P. Wadgaonkar and C. Chassenieux, Macromolecules, 2010, 43, 10055–10063.
18. F. A. Plamper, A. Schmalz, M. Ballauff and A. H. Müller, J. Am. Chem. Soc., 2007, 129, 14538–14539.
19. A. Filippov, E. Belyaeva, N. Zakharova, A. Sasina, D. Ilgach, T. Meleshko and A. Yakimansky, Colloid Polym. Sci., 2015, 293, 555–565.
20. S.-D. Xiong, L. Li, J. Jiang, L.-P. Tong, S. Wu, Z.-S. Xu and P. K. Chu, Biomaterials, 2010, 31, 2673–2685.
21. K. Sui, X. Zhao, Z. Wu, Y. Xia, H. Liang and Y. Li, Langmuir, 2011, 28, 153–160.
22. S. Iijima, Nature, 1991, 354, 56–58.
23. P. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. Dai, S. Peng and K. Cho, Nano Lett., 2003, 3, 347–351.
24. S. F. Liu, L. C. Moh and T. M. Swager, Chem. Mater., 2015, 27, 3560–3563.
25. Y. Chen, Y. Zhang, Y. Hu, L. Kang, S. Zhang, H. Xie, D. Liu, Q. Zhao, Q. Li and J. Zhang, Adv. Mater., 2014, 26, 5898–5922.
26. M. Zhi, C. Xiang, J. Li, M. Li and N. Wu, Nanoscale, 2013, 5, 72–88.
27. A. D. K. Jones and T. Bekkedahl, Nature, 1997, 386, 377.
28. H.-M. Cheng, Q.-H. Yang and C. Liu, Carbon, 2001, 39, 1447–1454.
29. Z. Zeng, H. Jin, L. Zhang, H. Zhang, Z. Chen, F. Gao and Z. Zhang, Carbon, 2015, 84, 327–334.
30. M. Chen, L. Zhang, S. Duan, S. Jing, H. Jiang, M. Luo and C. Li, Nanoscale, 2014, 6, 3796–3803.
31. G. H. Kim, S. M. Hong and Y. Seo, Phys. Chem. Chem. Phys., 2009, 11, 10506–10512.
32. E. Nativ-Roth, R. Shvartzman-Cohen, C. Bounioux, M. Florent, D. Zhang, I. Szleifer and R. Yerushalmi-Rozen, Macromolecules, 2007, 40, 3676–3685.
33. L. Hu, D. S. Hecht and G. Gruner, Chem. Rev., 2010, 110, 5790–5844.
34. H. Y. Ma, L. F. Tong, Z. B. Xu and Z. P. Fang, Adv. Funct. Mater., 2008, 18, 414–421.
35. G. Guo, D. Yang, C. Wang and S. Yang, Macromolecules, 2006, 39, 9035–9040.
36. C.-Y. Hong, Y.-Z. You and C.-Y. Pan, Chem. Mater., 2005, 17, 2247–2254.
37. M. Müllner, T. Lunkenbein, J. Breu, F. Caruso and A. H. Müller, Chem. Mater., 2012, 24, 1802–1810.
38. W. Li, C. Gao, H. Qian, J. Ren and D. Yan, J. Mater. Chem., 2006, 16, 1852.
39. L.-P. Yang, P. Zou and C.-Y. Pan, J. Mater. Chem., 2009, 19, 1843.
40. M. R. Islam, L. G. Bach, T.-S. Vo, T.-N. Tran and K. T. Lim, Appl. Surf. Sci., 2013, 286, 31–39.
41. K. Sui, C. Yang, S. Gao, X. Shan, Y. Xia and Q. Zheng, J. Appl. Polym. Sci., 2009, 114, 1914–1920.
42. D.-Q. Fan, J.-P. He, W. Tang, J.-T. Xu and Y.-L. Yang, Eur. Polym. J., 2007, 43, 26–34.
43. X. Lou, C. Detrembleur, C. Pagnoulle, R. Jérôme, V. Bocharova, A. Kiriy and M. Stamm, Adv. Mater., 2004, 16, 2123–2127.
44. Y. T. Joo, K. H. Jung, M. J. Kim and Y. Kim, J. Appl. Polym. Sci., 2013, 127, 1508–1518.
45. Q. Fu, C. Liu, W. Lin and J. Huang, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 6770–6779.
46. Q. Fu, G. Wang, W. Lin and J. Huang, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 986–990.
47. R. C. Chadwick, U. Khan, J. N. Coleman and A. Adronov, Small, 2013, 9, 552–560.
48. Q. Zhang, F. Fang, X. Zhao, Y. Li, M. Zhu and D. Chen, J. Phys. Chem. B, 2008, 112, 12606–12611.
49. Y. S. Song, Rheol. Acta, 2006, 46, 231–238.
50. F. Liu and M. W. Urban, Macromolecules, 2008, 41, 6531–6539.
51. J.-T. Sun, C.-Y. Hong and C.-Y. Pan, J. Phys. Chem. C, 2010, 114, 12481–12486.
52. M. Majumder, X. Zhan, R. Andrews and B. J. Hinds, Langmuir, 2007, 23, 8624–8631.
53. T. Witten Jr and M. Cohen, Macromolecules, 1985, 18, 1915–1918.
54. D. Xu, J. L. Hawk, D. M. Loveless, S. L. Jeon and S. L. Craig, Macromolecules, 2010, 43, 3556–3565.
55. F. Bossard, V. Sfika and C. Tsitsilianis, Macromolecules, 2004, 37, 3899–3904.
56. S. Hietala, M. Nuopponen, K. Kalliomäki and H. Tenhu, Macromolecules, 2008, 41, 2627–2631.
57. S. Q. Wang, Macromolecules, 1992, 25, 7003–7010.
58. X. Han, Z. Xiong, X. Zhang and H. Liu, Soft Matter, 2015, 11, 2139–2146.
59. X. J. Loh, S. J. Ong, Y. T. Tung and H. T. Choo, Polym. Chem., 2013, 4, 2564–2574.
60. F. Chen, X. Jiang, T. Kuang, L. Chang, D. Fu, J. Yang, P. Fan and M. Zhong, Polym. Chem., 2015, 6, 3529–3536.
61. T. Witten Jr and L. M. Sander, Phys. Rev. Lett., 1981, 47, 1400.
62. S. Gupta, M. Agrawal, M. Conrad, N. A. Hutter, P. Olk, F. Simon, L. M. Eng, M. Stamm and R. Jordan, Adv. Funct. Mater., 2010, 20, 1756–1761.
63. J. Chen, P. Xiao, J. Gu, D. Han, J. Zhang, A. Sun, W. Wang and T. Chen, Chem. Commun., 2014, 50, 1212–1214.
64. H. Hu, X.-J. Hou, X.-C. Wang, J.-J. Nie, Q. Cai and F.-J. Xu, Polym. Chem., 2016, 7, 3107–3116.
65. C. H. Tan, P. Ravi, S. Dai and K. C. Tam, Langmuir, 2004, 20, 9901–9904.
66. V. Salgueiriño-Maceira, C. E. Hoppe and M. A. Correa-Duarte, J. Phys. Chem. B, 2007, 111, 331–334.
67. S. Sahoo, S. Husale, S. Karna, S. K. Nayak and P. M. Ajayan, J. Am. Chem. Soc., 2011, 133, 4005–4009.

---

Footnotes

† Electronic supplementary information (ESI) available: GPC traces of grafted PDMAEMA, FT-IR spectrum of MWNTs-g-PDMAEMA, steady flow curves and dynamic temperature ramp of MWNTs-g-PDMAEMA suspensions. See DOI: 10.1039/c6ra20088e

‡ These authors contributed equally to this work.

---