Phase-Separated Morphology and Resulting Electrical Properties of PαMSAN/PMMA Blends without and with Carbon Nanotubes: Compatibilization as a Tool

The demand for flexible conductive polymeric materials is on the rise in applications such as EMI shielding and supercapacitors. In this perspective, immiscible blends containing conductive nanoparticles offer tremendous possibilities to tune electrical properties by tailoring their biphasic morphology combined with selective particle localization. The research aims at unraveling the relations between the microstructure of polymer blends and the resulting dielectric properties. In particular, copolymers with suitable physical characteristics were employed to refine and stabilize the cocontinuous morphology of biphasic polymer blends containing carbon nanotubes. This further allowed to concurrently develop a percolated carbon nanotube network and a microcapacitor network of carbon nanotubes with enclosed dielectric polymer chains. As a result, an increase by several decades of magnitude in the electrical conductivity and dielectric interfacial capacitance of the blends were respectively achieved.status: publishe

Exploring the effects of compatibilizer on the morphology and interface of polymer blends by means of rheology and dielectric spectroscopy

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Dielectric Properties of Phase-Separated Blends Containing a Microcapacitor Network of Carbon Nanotubes: Compatibilization by a Random or Block Copolymer

© 2017 American Chemical Society. The mechanisms governing the dielectric blend properties at different length scales for phase separating blends with multiwall carbon nanotubes (MWNTs) are unravelled by tuning the microstructure. Thereto, compatibilization by interfacially segregated block copolymers (bcp) and random copolymers (rcp) of poly(styrene-random/block-methyl methacrylate) (PS-r/b-PMMA) was achieved in phase-separating blends of poly[(α-methylstyrene)-co-acrylonitrile] and poly(methyl methacrylate) (PαMSAN/PMMA) undergoing spinodal decomposition. In our recent work, we elucidated the effects of copolymer architecture and molecular weight on the percolating network of selectively localized MWNTs. Only short bcp and long rcp/bcp improved the connectivity and refinement of the PαMSAN phase laden with MWNTs and the resulting conductivity. In the present work, we study the effects of copolymer type, architecture, and concentration on the dielectric properties. We demonstrate a concurrent increase of the interfacial capacitance and decrease of the interfacial resistance of MWNTs with entrapped PαMSAN upon effective compatibilization. This is attributed to the increasing amount of connected parallel microcapacitor RC elements formed by the network of adjacent MWNTs enclosing a thin dielectric layer of PαMSAN. At high frequencies (above 1 MHz) the electrons hop between the neighboring MWNTs, whereas at intermediate frequencies, the electrons of the MWNTs tunnel through the barriers imposed by the entrapped PαMSAN. The physical characteristics of the microcapacitor network, namely the thickness of the microcapacitors and the volume fraction of entrapped PαMSAN contributing to the microcapacitor network, are estimated by describing the dielectric relaxation time and strength using the fluctuation induced tunneling model and the interlayer model, respectively. Combining the knowledge of the aforementioned parameters allows to describe the evolution of the total interfacial capacitance of the microcapacitor assembly as a function of copolymer type and concentration. Our robust and simple procedure to tune the MWNT microcapacitor network in polymer blends via the efficiency of the compatibilizer can be used to achieve a synergistic increase in the dielectric properties at different length scales.status: publishe

An unusual demixing behavior in PS-PVME blends in the presence of nanoparticles

The effect of silver nanoparticles (sNP) on the demixing and the evolution of morphology in off-critical blends of 90/10 and 10/90 (wt/wt) PS/PVME polystyrene/poly(vinyl methyl ether)] was probed here using shear rheology and optical microscopy. The faster component (PVME) has a higher molecular weight (80 kDa) than the slower component (PS, 35 kDa), which makes this system quite interesting to study with respect to the evolving morphology, as the blends transit through the binodal and the spinodal envelopes. An unusual demixing behavior was observed in both PVME rich and PS rich blends. Temperature modulated differential scanning calorimetry measurements showed that the T-g value for the blends with sNP was slightly lower than that of the neat blends. A decreased volume of cooperativity at T-g suggests confined segmental dynamics in the presence of sNP. Although, the addition of sNP had no influence on the thermodynamic demixing temperature, it significantly altered the elasticity of the minor component during the transition of the blend from the homogeneous to the heterogeneous state. This is manifested from energetically driven localization of the sNP in the PVME phase during demixing. As a direct consequence of this, the formation of the microstructures upon demixing was observed to be delayed in the presence of sNP. Interestingly, in the intermediate quench depth, the higher viscoelastic phase evolved as an interconnected network, which subsequently coarsened into discrete droplets in the late stages for the 90/10 PS/PVME blends. Similar observations were made for 10/90 PS/PVME blends where threads of PVME appeared at deeper quench depths in the presence of sNP. The interconnected network formation of the minor phase (here PVME), which is also the faster component in the blend, was different from the usual demixing behavior

Uniquely developing tunable morphologies and carbon nanotube localization in bi-phasic polymer blends by compatibilizer grafting

The concentration of conducting fillers for rendering a continuous conductive pathway is referred to as the ‘electrical percolation threshold’. In this regard, the high aspect ratio of multiwalled carbon nanotubes (MWNTs) enables electrical percolation at lower concentrations. [1] Multiple successful attempts to reduce the electrical percolation threshold of MWNTs in insulating polymeric matrices are present in literature. We recently combined phase separation of polymer blends with selectively localized MWNTs [2] along with morphology refinement and stabilization by long random or block copolymers [3, 4] to reduce the electrical percolation threshold of MWNTs in PMMA/PαMSAN blends. We now present novel routes to simultaneously control the morphology and carbon nanotube network in phase separating 60/40 PMMA/PαMSAN blends. Hereto, different (co)polymers, including short PS polymers, long PMMA polymers and short PS-PMMA block copolymers with distinct molecular weight and asymmetry, are chemically grafted onto the surface of MWNTs. The intrinsic conductivity of the compatibilizer-grafted carbon nanotubes decreased by a decade due to the grafting procedure, thereby largely maintaining the beneficial electrical properties of MWNTs. In the blends, chemical grafting of (co)polymers onto the MWNTs simultaneously refined the blend’s morphology and steered the MWNT localization, as discerned from the dielectric relaxation spectra, thereby leading to some fascinating phenomena assisting long-ranged MWNT charge transfer at ultralow concentrations. In this regard, phase inversion, as verified by TEM images, leads to localization of MWNTs in the PαMSAN matrix with finely dispersed PMMA droplets, which facilitated charge transfer as connection or restriction points for the MWNT network in the matrix. The presence of interfacial MWNTs along the interface of small PMMA droplets further supports a continuous network of MWNTs in the blends. By chemical grafting, we could reduce the electrical percolation threshold of MWNTs from 0.5 wt% to 0.15 wt% in presence of very low compatibilizer concentrations (0.1 wt% instead of 2 wt% in the ungrafted case). We can thus conclude that a droplet-matrix morphology with tunable MWNT localization either in the matrix phase and/or at the blend interface is an effective alternative to bi-continuous morphologies for developing conductive blends. [1] I. Alig et al., Polymer, 53(1), 4-28 (2012) [2] S. Bose at al., Applied Materials and Interfaces, 2(3), 800-807 (2010) [3] A. Bharati at al., Polymer, 79, 271-282 (2015) [4] A. Bharati et al., Polymer, 108, 483-492 (2017)Conference talkstatus: publishe

Inducing electrical conductivity by tuning the phase-separated morphology and carbon nanotube network in bi-phasic polymer blends with free and grafted (co)polymers

We present novel routes to tune the phase-separated morphology and the percolated network of multiwalled carbon nanotubes (MWNTs) in 60/40 PMMA/PαMSAN LCST blends. The rheological and dielectric properties were monitored during phase separation to probe the morphological development and the MWNT network build-up. The steady-state microstructure of the phase-separated blends was verified by (S)TEM imaging. By altering the sample preparation method from melt to solution mixing, the improved dispersion of MWNTs reduced the rheological and electrical percolation threshold from 2 wt% in melt mixed blends to 0.5 wt% in solution mixed blends. Different novel compatibilizers were employed in order to stabilize and refine the blend morphology, including short PS-Br polymers, long SH-functionalized PMMA polymers and short PS-PMMA block-copolymers with distinct molecular weight and asymmetry. The interfacial segregation of the (co)polymers during phase separation resulted in different morphologies based on the ability of the (co)polymer to entangle with the blend components, the presence of steric hindrance and the (co)polymer asymmetry compared to the curvature of the blend interface. The kinetic competition between the migration of the (co)polymers to the blend interface and the migration of the MWNTs to the energetically preferred PαMSAN phase governed the MWNT localization. They ended up either in the continuous PαMSAN phase, at the blend interface or in the dispersed PMMA phase, thereby giving rise to different paths for charge transfer. Irrespective of the compatibilizer type, the electrical percolation threshold of solution mixed blends was further reduced from 0.5 wt% to 0.15 wt% in presence of 2 wt% of compatibilizer. As an alternative, chemically grafting the different compatibilizers onto the surface of the MWNTs further tuned the localization of the MWNTs and the formation of conductive pathways in one of the phases or along the blend interface. This allowed the formation of a percolated 0.15 wt% MWNT network at reduced compatibilizer concentrations of 0.1 wt% or less without severely affecting the intrinsic conductivity of the MWNTs. However, the limited amount of compatibilizer, combined with the interfacially localized MWNTs, exhibited less refinement ability as compared to 2 wt% of isolated compatibilizer (see Figure 1). Our novel routes could steer the development of polymer blends with tailor-made morphologies and highly specific percolated networks of conductive nanofillers at ultra-low concentrations.Posterstatus: publishe

Predicting the localization and interconnectivity of carbon nanotubes in compatibilized bi-phasic polymer blends

Broadband dielectric spectroscopy (BDS) is often used to probe the electrical percolation threshold (EPT) of multiwalled carbon nanotubes (MWNTs) in polymeric systems. In this regard, phase separation of polymer blends with selectively localized MWNTs and stabilization of the cocontinuous morphology by block and random copolymers are valuable tools to reduce the EPT. We now present novel routes to simultaneously tune the phase-separated morphology and MWNT network in 60/40 PMMA/PαMSAN blends and thereby reduce the EPT. We developed a method based on BDS to successfully predict the localization and interconnectivity of MWNTs in the resulting conductive blends and validated our predictions by (S)TEM images. An improved dispersion and hence overall connectivity of MWNTs in solution mixed blends compared to melt mixed blends was discerned by comparing the electrical properties in the blends to that in equivalent PMMA and PαMSAN monophasic nanocomposites, thereby decreasing the EPT from 2 wt% in melt mixed blends to 0.5 wt% in solution mixed blends. Morphology stabilization and refinement were achieved by employing novel types of compatibilizers in solution mixed blends, including short PS-Br polymers and long PMMA-SH polymers, which further reduced the EPT from 0.5 wt% MWNTs to 0.15 wt% MWNTs in presence of 2 wt% of compatibilizer. The kinetic competition between the migration of the compatibilizers to the blend interface and the migration of the MWNTs to their energetically preferred PαMSAN phase during phase separation, gave rise to different MWNT localization, either in the PMMA phase, in the PαMSAN phase or at the interface, depending on the polymer compatibilizer. This in turn resulted in disparate interfacial polarization peak characteristics. The amount of entrapped polymer between adjacent MWNTs in the microcapacitor assembly was estimated by the dielectric interlayer model. The gap spacing of the microcapacitors, on the other hand, was deduced from the relaxation time of charge migration by fluctuation-induced tunnelling. Both these parameters allow to model the interfacial capacitance of the various MWNT microcapacitor networks in the compatibilized bi-phasic blends.status: publishe

A strategy to achieve enhanced electromagnetic interference shielding at ultra-low concentration of multiwall carbon nanotubes in P alpha MSAN/PMMA blends in the presence of a random copolymer PS-r-PMMA

A unique strategy was adopted to achieve an ultra-low electrical percolation threshold of multiwall carbon nanotubes (MWNTs) (0.25 wt%) in a classical partially miscible blend of poly-alpha-methylstyrene-co-acrylonitrile and poly(methyl methacrylate) (P alpha MSAN/PMMA), with a lower critical solution temperature. The polymer blend nanocomposite was prepared by standard melt-mixing followed by annealing above the phase separation temperature. In a two-step mixing protocol, MWNTs were initially melt-mixed with a random PS-r-PMMA copolymer and subsequently diluted with 85/15 P alpha MSAN/PMMA blends in the next mixing step. Mediated by the PS-r-PMMA, the MWNTs were mostly localized at the interface and bridged the PMMA droplets. This strategy led to enhanced electromagnetic interference (EMI) shielding effectiveness at 0.25 wt% MWNTs through multiple scattering from MWNT-covered droplets, as compared to the blends without the copolymer, which were transparent to electromagnetic radiation

Dielectric properties of phase separated blends containing a microcapacitor network of carbon nanotubes : compatibilization by a random or block copolymer

The mechanisms governing the dielectric blend properties at different length scales for phase separating blends with multiwall carbon nanotubes (MWNTs) are unravelled by tuning the microstructure. Thereto, compatibilization by interfacially segregated block copolymers (bcp) and random copolymers (rcp) of poly(styrene-random/block-methyl methacrylate) (PS-r/b-PMMA) was achieved in phase-separating blends of poly[(α-methylstyrene)-co-acrylonitrile] and poly(methyl methacrylate) (PαMSAN/PMMA) undergoing spinodal decomposition. In our recent work, we elucidated the effects of copolymer architecture and molecular weight on the percolating network of selectively localized MWNTs. Only short bcp and long rcp/bcp improved the connectivity and refinement of the PαMSAN phase laden with MWNTs and the resulting conductivity. In the present work, we study the effects of copolymer type, architecture, and concentration on the dielectric properties. We demonstrate a concurrent increase of the interfacial capacitance and decrease of the interfacial resistance of MWNTs with entrapped PαMSAN upon effective compatibilization. This is attributed to the increasing amount of connected parallel microcapacitor RC elements formed by the network of adjacent MWNTs enclosing a thin dielectric layer of PαMSAN. At high frequencies (above 1 MHz) the electrons hop between the neighboring MWNTs, whereas at intermediate frequencies, the electrons of the MWNTs tunnel through the barriers imposed by the entrapped PαMSAN. The physical characteristics of the microcapacitor network, namely the thickness of the microcapacitors and the volume fraction of entrapped PαMSAN contributing to the microcapacitor network, are estimated by describing the dielectric relaxation time and strength using the fluctuation induced tunneling model and the interlayer model, respectively. Combining the knowledge of the aforementioned parameters allows to describe the evolution of the total interfacial capacitance of the microcapacitor assembly as a function of copolymer type and concentration. Our robust and simple procedure to tune the MWNT microcapacitor network in polymer blends via the efficiency of the compatibilizer can be used to achieve a synergistic increase in the dielectric properties at different length scales