Letter pubs.acs.org/macroletters
“Patchy” Carbon Nanotubes as Efficient Compatibilizers for Polymer Blends Thomas Gegenhuber,†,‡ Marina Krekhova,† Judith Schöbel,† André H. Gröschel,*,§,# and Holger Schmalz*,† †
Makromolekulare Chemie II, Universität Bayreuth, 95440 Bayreuth, Germany Department of Applied Physics, Aalto University School of Science, 00076 Aalto, Finland
§
S Supporting Information *
ABSTRACT: Surface-modified carbon nanotubes (CNTs) have become well-established filler materials for polymer nanocomposites. However, in immiscible polymer blends, the CNT-coating is selective toward the more compatible phase, which suppresses their homogeneous distribution and limits harnessing the full potential of the filler. In this study, we show that multiwalled CNTs with a patchy polystyrene/poly(methyl methacrylate) (PS/PMMA) corona disperse equally well in both phases of an incompatible PS/PMMA polymer blend. Unlike polymer-grafted CNTs with a uniform corona, the patchy CNTs are able to adjust their corona structure to the blend phases by selective swelling/collapse of respective miscible/immiscible surface patches. Importantly, the high interfacial activity of patchy CNTs further causes a significant decrease in PMMA droplet size with increasing filler content. The combined effect of compatibilization and homogeneous distribution makes patchy CNTs interesting materials for polymer blend nanocomposites, where next to the compatibilization, a homogeneous filler distribution is important to gain the desired materials property (e.g., reinforcement).
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accumulate in one of the phases. The preparation of CNTbased polymer blends with a homogeneous distribution of CNTs still remains challenging and asks for more versatile and adaptive coatings, yet would allow the functional design of polymer blend nanocomposites. Recently, we have developed an ultrasound-assisted approach for the noncovalent functionalization of multiwalled CNTs with polystyrene-block-polyethylene-block-poly(methyl methacrylate) triblock terpolymers in organic solvents.46 The polyethylene middle block of the SEM triblock terpolymer selectively adsorbs to the CNTs’ surface and the immiscible PS and PMMA end blocks microphase-separate into a patchy PS/PMMA corona. This patchy corona provided excellent dispersibility and stability of the functionalized CNTs in various organic media. Beyond that, studies on worm-like micelles with a patchy PS/PMMA corona revealed exceptional interfacial activity comparable to that of PS/PMMA Janus cylinders, which was attributed to the patchy corona being able to adapt to its surrounding by expansion/collapse of the respective soluble/insoluble patch.47 Patchy CNTs should thus be very attractive multifunctional additives for polymer blends unifying several benefits: The high interfacial activity together with the adaptive properties of the patchy corona should improve
he blending of polymers is a versatile and well-established concept to create materials with tailor-made properties, often combining the benefits of both blend components.1−3 The inherent immiscibility of most polymers necessitates their compatibilization with the aim to control the domain size and blend morphology.4 Compatibilization can be achieved by reactive blending5−8 or additives such as graft9,10 and block copolymers,11−16 or nanoparticle fillers.17−20 In this context, Janus particles (JPs), having two opposing sites (faces) of different chemistry and/or polarity,21,22 have received increasing attention as highly efficient compatibilizers in polymer blends due to their outstanding interfacial properties.23−25 Carbon nanotubes (CNTs), on the other hand, are excellent filler materials that add new properties to nanocomposites and blends, such as electrical or thermal conductivity,26 or greatly enhance mechanical strength27−30 as compared to nonreinforced systems. The location of pristine CNTs in polymer blends depends on their affinity to the blend components, the CNT fraction in the blend, mixing sequence, and processing conditions.31−34 However, the poor interfacial adhesion and strong tendency of CNTs to bundle make their dispersion in polymer matrices challenging and bring about the need for proper surface modification.27,29,35−41 One elegant way to enhance dispersibility is the grafting of CNTs with polymers that are compatible with one of the blend components.28,42−45 However, such homopolymer coatings usually show unequal affinity toward the blend components, and CNTs selectively © XXXX American Chemical Society
Received: January 15, 2016 Accepted: February 10, 2016
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DOI: 10.1021/acsmacrolett.6b00033 ACS Macro Lett. 2016, 5, 306−310
Letter
ACS Macro Letters
Figure 1. Patchy SEM@CNTs and uniform PS-coated SES@CNTs. (A) Schematic representation of SEM@CNT with a patchy, compartmentalized PS/PMMA corona and SES@CNT with a uniform PS shell obtained by ultrasound-assisted, noncovalent grafting of multiwalled CNTs with PS-bPE-b-PMMA and PS-b-PE-b-PS, respectively. (B, C) TEM micrograph and tomography of the patchy SEM@CNT hybrid. (D) TEM micrograph of the SES@CNT.
Figure 2. Compatibilization of PS/PMMA blends (80/20 w/w) with (A) adaptive, patchy SEM@CNTs and (B) uniform, PS-coated SES@CNTs.
compatibilization of the blend interfaces and simultaneously allow homogeneous distribution of the CNTs in both phases; the stiff CNT “backbone” could further serve as reinforcing filler to enhance structural stability of the blend. In this contribution, we study the compatibilization of PS/ PMMA (80/20 w/w) blends using multiwalled carbon nanotubes (CNTs, l ≈ 3−6 μm) that feature a chemistrymatched and patchy PS/PMMA corona. The CNTs were physically grafted with a polystyrene-block-polyethylene-blockpoly(methyl methacrylate) triblock terpolymer (S140E690M160, subscripts denote the number-average degree of polymerization of the respective block), as described in detail in our previous publication.46 The schematic in Figure 1A shows the employed SEM triblock terpolymer and the patchy CNT, denoted in the following as SEM@CNT (details on used materials and
equipment as well as the grafting procedure can be found in the Supporting Information). After the physical grafting process, the incompatible and microphase-separated PS/ PMMA patches are clearly visible in transmission electron microscopy (TEM, Figure 1B), where PS appears dark (RuO4 staining) and PMMA appears bright. Since TEM is merely a 2D projection of 3D objects, we performed TEM tomography to better visualize the spatial distribution of the patches. The electron density map of the 3D reconstruction (Figure 1C) illustrates the patch volume distribution, where segments colored in cyan correspond to high contrast of PS and “empty” segments to low (or no) contrast of PMMA. Frequently, the PS patches cover one-half of the CNT surface, while the PMMA patches cover the other, and the PS and PMMA patches are arranged in an almost 307
DOI: 10.1021/acsmacrolett.6b00033 ACS Macro Lett. 2016, 5, 306−310
Letter
ACS Macro Letters
Figure 3. TEM micrographs of PS/PMMA blends (80/20 w/w) compatibilized with different amounts of patchy SEM@CNTs: (A) 1, (B) 2, and (C) 9 wt % and magnification in (D). (E) Histograms of PMMA domain areas in PS/PMMA (80/20 w/w) blends in dependence of the SEM@ CNT content. At least 200 PMMA domains were evaluated for each sample.
alternating fashion along the CNT “backbone”. This is inline with the morphology of patchy worm-like micelles formed by the neat SEM triblock terpolymer, as deduced from TEM and SANS.48,49 The tomography further reveals the typical bellshaped distribution of corona brushes after collapse on a surface,50 which originates from denser chain packing close to the CNT surface and the collapse of the coronal chains on the underlying carbon-coated TEM grid. The multiwalled CNT resides within the center of the hybrid nanostructure and is entirely covered (shielded) by the SEM triblock terpolymer, where the patchy PS/PMMA corona structure should provide favorable interactions with the employed blend components PS and PMMA. To be able to compare the compatibilization efficiency of patchy SEM@CNTs with that of CNTs with a uniform PS coating, multiwalled CNTs were functionalized with a polystyrene-block-polyethylene-block-polystyrene (S90E320S90) triblock copolymer in analogy to the procedure described for the SEM triblock terpolymer (Supporting Information).46 The TEM image in Figure 1D shows the obtained CNTs with a continuous and uniform PS corona surrounding the CNT core (SES@CNTs).
First, we studied the efficiency of patchy SEM@CNTs as compatibilizer in PS/PMMA (80/20 w/w) blends at a filler content of 5 wt % (Figure 2). We deliberately chose a blend composition with a PMMA minority phase for easier assignment of the phases: The PS matrix appears dark (inherent contrast) and PMMA droplets bright. The blends were solution cast from chloroform, microtomed into ultrathin slices (
