Functionalized Multi‐Walled Carbon Nanotubes Prepared by In Situ Polycondensation of Polyurethane

Treating MWCNTs with an acidic solution (concentrated H₂SO₄/HNO₃; 3:1) and subsequently with succinic acid peroxide leads to MWCNTs functionalized with carboxyl groups on their sidewalls (or surfaces). These MWCNT‐COOH were converted to MWCNT‐COCl and further to MWCNT‐NH₂. The PU was covalently coated onto the sidewalls (or surfaces) of the MWCNT by in situ polymerization of TDI under ultrasound in the presence of MWCNT‐NH₂. The results show that PU is chemically tethered to the MWCNT sidewalls (or surfaces) via the ～HN? CO? NH～ linkage, with a thickness in the range of approximately 2 to 5 nm, and the amount of grafted PU is about 34 wt.‐%.

     

A Scalable Route to Highly Functionalized Multi‐Walled Carbon Nanotubes on a Large Scale

A scalable and solvent‐free chemical process to obtain highly functionalized and dispersible multi‐walled carbon nanotubes is reported. Highly functionalized multi‐walled carbon nanotubes have been prepared using in situ generated aryl diazonium salts in the presence of ammonium persulfate and 2,2′‐azoisobutyronitrile by solvent‐free techniques. In the Raman spectra of the resulting materials, characteristic peaks, the D‐ and G‐bands, are shifted by about 10 cm^?1 to lower frequencies. At the same time, the relative intensity ratios between the D‐ and G‐bands increase in comparison to that in the spectrum of the purified product. Fourier‐transform infrared spectroscopy reveals the presence of the functional groups on the surface. Transmission electron microscopy images directly confirm the significant build‐up of sidewall organic moieties on the treated materials. The weight loss of various functional moieties determined by thermogravimetry–differential scanning calorimetry analysis is about 18–33%. The dispersibility of the functionalized materials in solvents, such as chloroform, tetrahydrofuran, and water, is obviously improved.

     

Enhancing the Colloidal Stability and Electrical Conductivity of Single‐Walled Carbon Nanotubes Dispersed in Water

In this article, the results of a focused systematic investigation of the nature, type, and concentration of cationic and anionic surfactants and proteins required to simultaneously disperse single‐walled carbon nanotubes (SWNTs) and enhance their electrical conductivities in water are presented. The dispersibility of SWNTs suspended in aqueous solutions is evaluated via light scattering to characterize the average particle size, particle size dispersion, electrophoretic mobility, and ζ‐potential of the dispersions. It is found that the colloidal stability of SWNT dispersions is influenced by the surfactant charge and concentration – i.e., above or below its critical micelle concentration and for proteins its charge. The amphiphilicity, concentration, and charge of the surfactant or protein determine their surface coverage on the SWNT and simultaneously increase electrostatic and steric repulsion and decrease surface chemical heterogeneity. It is found that the electrical conductivities of SWNT films stabilized with surfactant are as high as that without surfactants, with the added advantage of being homogeneously dispersed in water with significant enhancement in the long‐term stability of the nanotubes in water. These findings suggest that enhancement of the electrical properties of SWNTs requires selection of a surfactant that has strong adsorption, and thus strong interactions with the nanotube – i.e., π–π stacking – and using concentrations above the CMC. Overall, these results demonstrate the importance of understanding the structure/property relationships between SWNTs and their dispersants in order to achieve high colloidal stability and electrical conductivities.

     

Poly(propylene glycol)‐Grafted Multi‐Walled Carbon Nanotube Polyurethane

Summary: Poly(propylene glycol)‐grafted MWNT polyurethane was synthesized based on the hydroxyl functionalized MWNTs through a two‐step reaction. The grafted MWNTs can improve the rheological behavior of the polyol/MWNT dispersion, and have a better reinforcing effect on the mechanical properties of polyurethane and lower hysteresis compared to the raw carbon nanotubes. A comparison for energy dissipation in PU/carbon nanotube and exfoliated PU/organoclay nanocomposites was given. The energy dissipated in the grafted MWNT composite system is lower.

Dispersion stability of carbon nanotubes in polyether polyol after 40 min centrifugation (A: MWNT, B: MWNT‐OH, C: MWNT‐graft‐PU).     

Noncovalent Nonspecific Functionalization and Solubilization of Multi‐Walled Carbon Nanotubes at High Concentrations with a Hyperbranched Polyethylene

A hyperbranched polyethylene (HBPE) is employed herein for noncovalent nonspecific functionalization and solubilization of multi‐walled carbon nanotubes (MWCNTs) in organic solvents. Though constructed solely from ethylene without any specific functionality, this unique hyperbranched polymer has been found to effectively solubilize MWCNTs at surprisingly high concentrations (up to 1 235 mg · L^?1) in organic solvents such as chloroform and THF. These solubilities are comparable to and even better than the reported best values obtained through noncovalent specific functionalization with conjugated polymers capable of forming specific π‐π interaction with nanotubes in organic solvents. TEM and XRD results confirm that the nanotubes are completely exfoliated and debundled/de‐entangled upon functionalization with HBPE.

     

Interaction Between Polymer Chains Covalently Fixed to Single‐Walled Carbon Nanotubes

Summary: A single‐walled carbon nanotube (SWNT), which had been oxidized with a mixture of nitric acid and sulfuric acid to afford polar groups at its ends, was incubated with an azo‐type macroinitiator carrying dextran (DEX), poly(ethylene glycol) (PEG) or poly(N‐vinylpyrrolidone) (PVPy) chains at 70 °C. Similarly, the oxidized SWNT was incubated with 2,2′‐azoisobutyronitrile and acrylic acid (HAA) or N‐vinylpyrrolidone at 70 °C. Due to the large radical trapping ability of SWNT, the polymer chains corresponding to the cloven macroinitiator (PEG, DEX or PVPy) and the propagating polymer chains (poly(acrylic acid) (HPAA) or PVPy) were covalently fixed to the surface of the SWNTs. The hydrophilic polymer‐modified SWNTs could be stably dispersed in water. Furthermore, the SWNTs modified with PEG and DEX sedimented in the presence of free DEX and PEG, respectively, whereas there was no precipitation of the PEG‐ and DEX‐modified SWNTs in the presence of the same kind of free polymer. This seemed to be related to the phase separation phenomena in water soluble DEX and PEG systems induced by the repulsive interaction between PEG and DEX molecules. However, the mixture of two kinds of polymer‐modified SWNTs (PEG‐SWNT and DEX‐SWNT) did not show noticeable phase separation, probably due to steric hindrance for the efficient repulsive polymer‐polymer interaction by fixation to the gigantic SWNTs. Furthermore, upon mixing the dispersions of HPAA‐SWNT and PEG‐SWNT or PVPy‐SWNT, the turbidity of the dispersions gradually increased, while no increase in turbidity of the dispersion mixture was observed in the presence of dimethyl sulfoxide, indicating hydrogen bonding between the HPAA and PEG or PVPy chains on the surface of the SWNTs. The modification methods examined in this work would be promising to give various functions to SWNT.

Susceptible processes of radical trap on SWNT surface.     

Composites of Single‐Walled Carbon Nanotubes and Styrene‐Isoprene Copolymer Latices

Composites of a styrene‐isoprene copolymer and SWNTs were prepared by emulsion and miniemulsion polymerization in the presence of SWNTs, as well as by mixing dispersed SWNTs with a styrene‐isoprene copolymer latex after polymerization. For the former, the surfactant was displaced from SWNTs to monomer droplets leading to SWNT aggregation. Mixing dispersed SWNTs with latex after reaction was able to preserve SWNT dispersion and gave a polymer composite with an electrical percolation threshold of 0.2%. Dynamic mechanical measurements of film samples in tension showed that the composites had a measurable modulus above T_g, indicating entanglement formation. The modulus above T_g was strongly temperature dependent, confirming shear measurements that have shown entanglements in SWNT/polymer composites both polymer and nanotubes.

     

Functionalization of Shortened Single‐Walled Carbon Nanotubes with Poly(p‐dioxanone) by “Grafting‐From” Approach

Summary: It has been a real challenge to form carbon nanotube (CNT)/polymer composites where CNTs are well‐dispersed in the polymer matrix and the interactions between CNTs and polymers are effectively strong. In this paper, we applied surface‐initiated, ring‐opening polymerization (SI‐ROP) of p‐dioxanone (PDX) to shortened single‐walled carbon nanotubes (s‐SWCNTs) and successfully formed s‐SWCNT/PPDX composites (see Figure). Due to intimate interactions between s‐SWCNTs and PPDX, we observed dramatic changes in PPDX properties upon the formation of the composites: 10%‐weight‐loss‐temperature of PPDX increased by 20 °C (measured by thermogravimetric analysis) and the patterns of T_g and T_m were greatly altered. We did not observe any noticeable peaks from the composite up to 120 °C in differential scanning calorimetry (DSC), while DSC data of PPDX itself showed T_g and T_m at ?13.4 and 103 °C respectively.

Schematic representation of the procedure for formation of s‐SWCNT/PPDX composites.     

Active Poly(4‐chloromethyl styrene)‐Functionalized Multiwalled Carbon Nanotubes

Concentrated nitric acid‐treated multiwalled carbon nanotubes (MWCNTs) are functionalized with active poly(4‐chloromethyl styrene) (PCMS) through the esterification reaction of the carboxyl groups of the former and the p‐benzyl chloride groups of the latter in the presence of a phase‐transfer catalyst. Characterization using Raman spectroscopy, Fourier transform infrared spectroscopy, and hydrogen nuclear magnetic resonance spectroscopy demonstrates that the active PCMS chains are chemically tethered onto the side walls (or surfaces) of the MWCNTs. The core‐shell nanostructure of active PCMS‐modified MWCNTs (MWCNT‐PCMS) can be observed by high resolution transmission electron microscopy, and the amount of PCMS present is 31.3 wt% by thermogravimetric analysis. Solubility testing shows that MWCNT‐PCMS dissolves well in tetrahydrofuran, chloroform, toluene, and N,N‐dimethylformamide, and the maximum nanotube concentration in toluene is 413 mg L^?1.

     

Synthesis of Poly(L‐lactide)‐Functionalized Multiwalled Carbon Nanotubes by Ring‐Opening Polymerization

MWNTs are modified to possess hydroxy groups and are used as coinitiators to polymerize L ‐lactide by the surface‐initiated ring‐opening polymerization. FT‐IR and TEM observations reveal that the PLLA is covalently attached to the MWNTs (MWNT‐g‐PLLA), and the weight gain as a result of the functionalization is determined by TGA analysis. Two kinds of solvents, namely DMF and toluene, are used to carry out the two series of polymerizations at 140 and 70 °C, respectively, for 2–20 h. The amount of grafted PLLA increases with the reaction time either in DMF or in toluene, but it increases more significantly in DMF at 140 than in toluene at 70 °C, with the reaction time being the same. The grafted PLLA layer on the MWNT is more uniform when the reaction is performed in DMF than in toluene, and some bare surfaces are observed in the TEM image of the MWNT‐g‐PLLA prepared in toluene. The MWNT‐g‐PLLAs are well dispersed in the organic solvents as well as in the PLLA matrix. Incorporation of MWNT‐g‐PLLA greatly improves the tensile modulus and strength without a significant loss of the elongation at break. The specific interaction between the MWNT‐g‐PLLA and the polymer matrix is quantified by way of the Flory‐Huggins interaction parameter, B, which is determined by combining the melting point depression and the binary interaction model.

     

Modification and Grafting of Multi‐Walled Carbon Nanotubes with Bisphenol‐A‐polycarbonate

Multi‐walled carbon nanotubes (MWNT) were oxidized with nitric acid and resulting carboxy groups were transformed to amino groups with a diamine under microwave irradiation. Grafting of bisphenol‐A‐polycarbonate onto carboxy‐ and amino‐functional MWNT through transesterification or aminolysis resulted in weight increase up to 300%. The morphology of functionalized and grafted MWNTs was investigated with scanning force microscopy (SFM) and transmission electron microscopy (TEM). Depending on the type of functionalisation and the extent of grafting different morphologies were observed. Aminofunctional MWNT‐g‐PCs showed more “ball‐like” or irregular protrusions of grafted PC. Carboxyfunctional MWNT‐g‐PCs were more smoothly covered with PC.

     

α,ωn‐Heterotelechelic Hyperbranched Polyethers Solubilize Carbon Nanotubes

The synthesis of novel linear‐hyperbranched (linhb) polyether block copolymers based on poly(ethylene oxide) and branched poly(glycerol), bearing a single pyrene or myristyl moiety at the α‐position of the linear chain is described. The polymers exhibit low polydispersity ( < 1.3) and controlled molecular weights ( = 5 000 g · mol^?1). The mainly hydrophilic block copolymers with multiple hydroxyl end groups readily dissolve multiwalled carbon nanotubes (MWCNTs) in water by mixing and subsequent sonification, resulting in noncovalent attachment of the linhb hybrid structure to the carbon nanotubes (CNTs). Transmission electron microscopy (TEM) was employed to visualize the solubilized nanotubes; after sulfation of the multiple hydroxyl groups the polymer layer was detected in the TEM images.

     

Selective Localization Behavior of Carbon Nanotubes: Effect on Transesterification of Immiscible Polyester Blends

The selective localization of carbon nanotubes (CNTs) in an immiscible polymer blend has attracted much attention. If the two component polymers could react with each other, do selectively located CNTs affect those reactions? Here, an immiscible polyester blend based on polycarbonate/poly(trimethylene terephthalate) (PC/PTT) is studied. CNTs introduced during melt mixing are selectively located in the PTT phase and on the phase interface during the middle stage of melt mixing. The interface‐located CNTs can act as additional substrate to catalyze or even participate in the transesterification themselves, homogenizing the phase morphology of the matrix blend. The degree of randomness of the composite systems is increased, accompanied by a reduced number‐average length of the copolymer sequences.

     

The Use of Solution Viscosity to Characterize Single‐Walled Carbon Nanotube Dispersions

Summary: A recent report by Agarwal and coworkers (I. Cotiuga, F. Picchioni, U. S. Agarwal, D. Wouters, J. Loos, P. J. Lemstra, Macromol. Rapid Commun. 2006 , 27, 1073) has shown that solution viscosity measured in a capillary rheometer can be used to monitor the dispersion quality of SWNTs. They used solution viscosity to determine the optimal sonication time. Their method is compared and contrasted here with other methods used to monitor dispersion quality, and it is argued that it has some advantages over other methods. Specifically, the method of Agarwal and coworkers is quick to apply, and gives a reliable single‐parameter measure of the relative quality of a SWNT dispersion.

Change in solution viscosity with sonication time and the corresponding nature of the dispersed carbon nanotubes.     

A Unique Meta‐Form Structure in the Stereocomplex of Poly(D‐lactic acid) with Low‐Molecular‐Weight Poly(L‐lactic acid)

The crystalline structure and crystallization behavior of PLLA crystals in a 1:1 w/w mixture of low‐MW PLLA with high‐MW PDLA were analyzed using WAXD, DSC, and SAXS. Under cold crystallization, homopolymeric PLLA, appearing as a meta crystal, was discovered in the PDLA/LMW‐PLLA blend. The meta‐ and α′ crystal forms of PLLA were found to form on crystallization at a T_cc of 85–95 °C and the α crystal PLLA formed at 100 ≤ T_cc < 120 °C. The meta‐crystal PLLA may be incorporated in the stereocomplexed PDLA/LMW‐PLLA lamellar region. During heating, the meta‐crystal PLLA first partially melted and then repacked directly into the α crystal PLLA without going through the less‐stable α′ form.

     

Pyrene Containing Polymers for the Non‐Covalent Functionalization of Carbon Nanotubes

Pyrene containing diblock copolymers based on poly(methyl methacrylate) were synthesized and investigated regarding their adsorption on carbon nanotubes (CNT). The pyrene units were introduced using a reactive ester monomer for the build up of the second block which later on was reacted polymer‐analogously with amine functionalized pyrene derivatives. As we started from the same reactive ester intermediate, full block length identity is given. We varied the length of the anchor block to find an optimal block length and used pyren‐1‐yl‐methylamine as well as 4‐pyren‐1‐yl‐butylamine as anchor units. For both anchor units a maximal adsorption was found for 13 and 20 anchor units, respectively. The absolute adsorption was best for the 4‐pyren‐1‐yl‐butylamine anchor units as the longer spacer enhances the mobility of the anchor unit. The dispersion diagram of CNTs and diblock copolymer in terms of dispersion stability was investigated and a stable dispersion of 2.5 mg · ml^?1 CNTs in THF was found.

     

Separate Crystallization and Cocrystallization of Poly(L‐lactide) in the Presence of L‐Lactide‐Based Copolymers With Low Crystallizability,Poly(L‐lactide‐co‐glycolide) and Poly(L‐lactide‐co‐D‐lactide)

Poly(L ‐lactide) (PLLA) and poly(L ‐lactide‐co‐glycolide) (78/22)0 [P(LLA‐GA)] were phase‐separated in PLLA/P(LLA‐GA) blends, forming independent domains and thence crystallizing separately. The crystallization of PLLA was not disturbed or delayed by the presence of P(LLA‐GA) and vice versa. PLLA and poly(L‐lactide‐co‐D ‐lactide) (77/23) [P(LLA‐DLA)] were miscible in the PLLA/P(LLA‐DLA) blends, overall crystallization was delayed, and the growth of crystallites was disturbed in the presence of P(LLA‐DLA). In isothermal crystallization, the originally noncrystallizable P(LLA‐DLA) became crystallizable in the presence of PLLA, with which it cocrystallized. The disturbance effect on periodical lamella twisting of PLLA was larger for P(LLA‐GA) than for P(LLA‐DLA).     

Multi‐Arm Star Polyglycerol‐block‐poly(tert‐butyl acrylate) and the Respective Multi‐Arm Poly(acrylic acid) Stars

Summary: Well‐defined multi‐arm star block copolymers, polyglycerol‐block‐poly(tert‐butyl acrylate) (PG‐b‐PtBA), with average arm‐numbers of 17, 27, 36, 66 and 90 arms, respectively, have been prepared by atom transfer radical polymerization (ATRP) of tBA in acetone, using a core‐first strategy. After hydrolysis with excess concentrated HCl in refluxing dioxane, full hydrolysis of the tert‐butyl ester groups was achieved, resulting in multi‐arm star polyelectrolytes, polyglycerol‐block‐poly(acrylic acid) (PG‐b‐PAA). The hyperbranched macroinitiators employed were prepared on the basis of hyperbranched polyglycerols via esterification with 2‐bromoisobutyryl bromide. Both CuBr/PMDETA and CuBr/Me₆TREN catalyst systems have been employed for ATRP of tBA. CuBr/PMDETA was found to permit good control. Polydispersity indices for the new multi‐arm stars were mainly in the range of 1.22 to 1.4, and the absolute data were in agreement with the calculated values. Moreover, kinetic curves show a linear dependence of ln([M]₀/[M]_t) on time, confirming that the polymerizations are controlled.

Relationship between arm numbers of the multi‐arm stars and maximum conversion achieved.     

Isotactic Polypropylene/Multi‐Walled Carbon Nanotube Nanocomposites: The Effect of Modification of MWCNTs on Mechanical Properties and Melt Crystallization

Nanocomposites of isotactic polypropylene (iPP) containing modified multi‐walled carbon nanotubes (MWCNTs) are prepared by melt mixing. MWCNTs with alkyl groups (alkyl‐MWCNTs) and with methyl‐PEG (mPEG‐MWCNTs) are used, as well as unmodified MWCNTs, to prepare the three series of nanocomposites. The surface treatment and the content of MWCNTs affect the mechanical properties of the iPP. In all cases, theoretical models are used to estimate the effect of surface treatment. The crystallization rates of nanocomposites in both isothermal and non‐isothermal crystallization conditions are measured. The nanocomposites containing unmodified MWCNTs crystallize faster than those with modified MWCNTs.