Low Frequency Storage Research Articles

Ultrasoft silicone gels with tunable refractive index for traction force microscopy.

We formulate and characterize silicone gels near the gelation threshold with tunable refractive index, 1.4 < n < 1.49, and small viscoelastic moduli, G'∼1 Pa, for use in traction force microscopy. The near-critical gels have low-frequency storage plateau moduli between 50 Pa and 1 Pa, with loss moduli that are more than fifty times lower at low frequencies. The gels are linearly elastic up to strains of at least 50%. The refractive index of the gel is tuned to eliminate spherical aberrations during confocal imaging thereby minimizing signal loss when imaging through thick gel substrates. We also develop an index-matched colloidal particle, stabilized by a silicone brush, that can be dispersed throughout the gel. These particles can be used to determine the deformation of the gel. The combination of mechanical and optical properties of these near-critical gels extends the lower limit of stresses that can be measured with traction force microscopy to single mPa values, while minimizing optical aberrations.

Viscoelastic relaxation of fibroblasts over stiff polyacrylamide gels by atomic force microscopy

Cell viscoelasticity provides mechanistic insights into fundamental biological functions and may be used in many applications. Using atomic force microscopy in time and frequency domains, we find a peculiar behavior in the viscoelastic relaxation of L929 mouse fibroblasts that may help understand how cells perceive and adapt to distinct extracellular environments. They are stiffer when cultured over polyacrylamide gels (20-350 kPa) than over glass-bottom Petri dishes. The stiffness enhancement of cells over gels is attributed to a significant increase in the low-frequency storage shear moduli compared to the loss moduli, indicating that gels induce a remodeling of cytoskeleton components that store elastic energy. Morphological alterations are then expressed by the fractal dimension measured on confocal images of the f-actin cytoskeleton. We show a direct scaling between the fractal dimension and the substrate’s rigidity.

Biomolecules of Similar Charge Polarity Form Hybrid Gel

ABSTRACT We report intermolecular gelation of deoxyribonucleic acid (DNA) and folic acid (FA, 1% (w/v) and DNA, 0.1–0.5% (w/v)) in Dimethyl Sulfoxide (DMSO)-water solution despite both having similar net charge. Zeta potential ζ of the biopolymers was, ζDNA = −58 and ζFA = −18 mV, therefore, formation of a self-organized hybrid network structure to assign sufficient rigidity to the gel must happen in the environment of electrostatic repulsion persisting between the inter and intra polymer segments. Regardless, a stable supramolecular hydrogel was formed at room temperature of 25° C with the DNA–FA complex displaying a zeta potential of ζcomplex = −21 mV. The gel strength (low frequency storage modulus, G 0) was tunable from ≈ 500 to 50 Pa, and gelation temperature T gel from 70 to 60° C, decreasing with DNA content. These soft gels were systematically characterized over several decades of time (dynamic light scattering), and length scales (small angle neutron scattering), rheology and scanning electron microscopy to obtain a deeper understanding of the self-assembly of the DNA in the FA-dominant solution. These gels formed of same polarity biopolymers, and in a milieu of strong electrostatic repulsion, make these belong to a special class of soft matter not well studied in the past.

Semi-convertible Hydrogel Enabled Photoresponsive Lubrication

Summary A semi-convertible hydrogel with reversible photoresponsive supramolecular lubricating ability is demonstrated by integrating a responsive supramolecular system of α-cyclodextrins/polyethylene glycol (α-CD/PEG) and competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide (AzoPB) within a poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) framework. The competitive host-guest interaction between α-CD/PEG supramolecular network and AzoPB after UV and visible-light irradiation enables the appearance and disappearance of the sol-state layer on the hydrogel surface while the PVA and PAAm consistently keeps the framework. This semi-convertible process realizes the reversible photoresponsive lubricating ability and variable toughness. We envision that this finding will offer a possible way for fabricating novel smart devices far beyond lubrication, leading to an emerging branch of interdisciplinary materials science that integrates supramolecular chemistry, hydrogel, and surface science.

Percolation in Multi-Walled Carbon Nanotube/Acrylonitrile Butadiene Styrene Polymer Nanocomposites Prepared By Melt Mixing and Solution Casting: Electrical Conductivity and Linear Rheology

Recently, 3D printing technology has emerged as a promising approach to print electrical circuits [1], sensors [2] and other electrical devices [3]. For 3D printing electrical devices, in addition to the required conductivity, the material should possess sufficient fluidity. Due to the wide variety of available matrices and fillers, it is believed [4] that polymer nanocomposites with the required characteristics can be developed. However, large quantities of fillers are typically necessary to obtain the required conductivity. Such nanocomposites exhibit poor fluidity and cannot be easily 3D printed. One solution is the use of high aspect ratio fillers, such as carbon nanotubes [4]. In addition, as the physical properties of the nanocomposites are expected to depend on the dispersion of filler in the matrix, we compared the properties of nanocomposites prepared by melt mixing with those prepared by solution casting.As it is already widely used in 3D printing, Poly(acrylonitorile butadiene styrene) (ABS) was chosen as the matrix polymer. Multiwalled carbon nanotubes (MWCNT) were used as the conductive at volume fractions between 0 – 4.52 %. For melt mixing, the MWCNT were dispersed in ABS using an internal batch mixer at 200°C for 10 minutes using a roller speed of 50 rpm. For solution casting, ABS dissolved in dichloromethane was added to a suspension of MWCNT/chloroform and sonicated for a further 10 minutes.We measured the conductivity of the nanocomposites prepared by both methods. Fig. 1 [5] indicates that the addition of a small quantity of MWCNT (0.113 vol%) increased the conductivity by several orders of magnitude. Upon further addition of MWCNT, the increase in conductivity becomes more gradual and appears to saturate. The obtained values are comparable to that in the literature [6]. To analyze the origin of the conductivity, the obtained conductivity was fit to a power-law equation and the corresponding percolation threshold and the critical exponent were obtained. The obtained critical exponent was close to the theoretical value of 2 [4], which suggests that the rapid increase in the conductivity could be due to the formation of a percolating path comprising the filler. The values of the threshold and the exponent were similar for the two preparation methods indicating that the preparation method had little effect on the obtained conductivity.The linear rheology data in Fig. 2 [5] indicated that, at low MWCNT additions, the low frequency storage modulus, G’, of the solution cast samples was larger than that of the melt mixed samples. This suggests that the MWCNT were better dispersed in the melt mixed samples and is contrary to expectation. Upon further addition of MWCNT, the difference in the low frequency G’ gradually disappeared. In Fig. 3, the variation of G’(ω = 0.0215 rad/s) with MWCNT addition was fit to a power-law. While the difference between the exponents for the samples prepared by the two methods was larger than that seen in the conductivity exponent, both values were close to 2 in this case too. However, the rheology threshold was smaller than the conductivity threshold suggesting that the percolation-like phenomenon seen in the rheology data could be, at least partly, attributed to the interaction between the filler and the polymer, i.e., unlike in electrical percolation filler-filler contact is not essential.In summary, we have achieved an increase in conductivity of several orders of magnitude without significantly altering the linear rheology of the nanocomposite. While the conductivity obtained is still several orders of magnitude smaller than that required for applications such as circuits, the obtained conductivity is sufficient for applications such as electromagnetic shielding.[1] S. W. Kwok et al., Applied Materials Today, 9, 167 (2017).[2] S. Guo et al., Nanoscale, 7, 6451 (2015).[3] J F. Christ et al., Materials and Design, 131, 394 (2017).[4] T. Gkourmpis, in Controlling the Morphology of Polymers, edited by G. R. Mitchell and A. Tojeira, (Springer International Publishing, 2016), pp. 209.[5] S. K. Sukumaran et al., Journal of The Electrochemical Society, 166, B3091 (2019).[6] D. P. Schmitz et al., Materials Today Communications, 15, 70 (2018). Figure 1

Fluorescent complex coacervates of agar and in situ formed zein nanoparticles: Role of electrostatic forces

Herein, the complex coacervation between in situ formed spherical fluorescent zein nanoparticles and polyanion agar as function of mixing ratio (R=[Agar]/[Zein]) was investigated. This interaction yielded two distinguishable regions (at pH 5.4): Region I (R < 0.2), where fully charge neutralized soluble complexes with protein denaturation was noticed, and Region II (R > 0.2), where overcharged complexes were formed, with R = 0.2 defining the optimum binding. Small angle neutron scattering studies demonstrated that in the low-q region, nanoparticles formed the crosslink junctions and in the persistence regime of high-q region, the data captured the cross-sectional radius ( = 3.5 nm) for agar-zein complexes. The coacervates became more viscoelastic in salt-free samples because both the low frequency storage modulus and crosslink density were found to decrease with mixing ratio. Systematic decrease in storage modulus with ionic strength (0-0.01 M) implied screened Coulomb interaction was responsible for the observed coacervation. Further, we seek to find universality in complex coacervation of zein nanoparticle with biopolymers, and polysaccharides in particular.

Electrical Conductivity and Linear Rheology of Multiwalled Carbon Nanotube/Acrylonitrile Butadiene Styrene Polymer Nanocomposites Prepared by Melt Mixing and Solution Casting

Polymer nanocomposites are potential materials for three dimensional (3d) printing of functional components. For effective 3d printing, besides ensuring high conductivity of the nanocomposites, their flow characteristics also need careful adjustment. To satisfy the contradictory requirements, multiwalled carbon nanotubes (MWCNT) have emerged as an attractive option. As the dispersion of the MWCNT is believed to strongly influence the properties of the nanocomposites, we compared nanocomposites prepared using melt mixing and solution casting. The MWCNT endow the nanocomposites with appreciable conductivity even at relatively low loadings. At these loadings, the low frequency storage modulus indicates that the MWCNT impart solid-like character to the nanocomposites. While the conductivity percolation threshold of the nanocomposites prepared by melt mixing was lower, at higher loadings the conductivity of the nanocomposites prepared by the two methods were similar. Fitting the electrical conductivity and the linear viscoelastic storage modulus to power-laws, the obtained critical exponents for the two methods were close to each other and to the percolation theory predictions for the conductivity exponent. Our findings suggest that for MWCNT/ABS polymer nanocomposites, simple melt mixing can yield conductivities that are similar to that obtained by solution casting and close to the highest values reported in the literature.

Polylactide/organoclay nanocomposites: The effect of organoclay types on the structure development and the kinetic of cold crystallization

Polylactide (PLA)/organoclay composites were prepared by melt compounding with 4 phr of two different types of organoclays (cloisite 20A and cloisite 30B). Structure development and nonisothermal kinetic of cold crystallization of PLA/organoclay nanocomposites were examined by using of X‐ray diffraction technique, transmission electron microscopy, melt viscoelastic measurements, and differential scanning calorimetry. XRD results demonstrated that the melt intercalation of PLA chains into the cloisite 30B and cloisite 20A galleries was achieved to the same extent. However, it was shown that, PLA/cloisite 20A sample exhibited a significant viscosity upturn and a pronounced nonterminal low frequency storage modulus whose values were greater than those of PLA/cloisite 30B nanocomposite. A detailed analysis of the linear melt viscoelastic properties for the filled and unfilled samples at low frequencies was conducted by fitting the complex viscosity and storage modulus data with Carreau–Yasuda and Fractional Zener models, respectively. The glass transition, cold crystallization, melting temperature, and degree of crystallinity of virgin PLA and PLA/organoclay nanocomposites were inspected. Subsequently, the cold crystallization kinetics was analyzed by Avrami, Jezioney, and Lauritzen–Hoffman kinetic models. It was shown that, the crystallization rate followed Avrami equation with the exponent n around 2.4. From Lauritzen–Hoffman equation and Kissinger model, the nucleation parameter Kg and activation energy were estimated, respectively. J. VINYL ADDIT. TECHNOL., 25:48–58, 2019. © 2018 Society of Plastics Engineers

Imidazolium based ionic liquid induced DNA gelation at remarkably low concentration

DNA dissolved in 1.0% (w/v) aqueous solution of ionic liquid (IL, 1-octyl-3-methylimidazolium chloride, [C8mim][Cl]) showed gelation transition at a remarkably low concentration of [DNA]≈0.3% (w/v) unlike in hydrogels where gelation occurs≥2.0% (w/v). Thus, the IL induced gelation was observed at 85% lower concentration. These ionogels were probed in the 0.1%–1.0% (w/v) concentration region of DNA where the hydrogels never form. The gelation temperature Tgel (and gel strength G0, G0 is low frequency storage modulus) increased from Tgel=52±2°C (G0=10±1Pa) to 72±2°C (G0=40±2Pa) with increase in DNA content. The dynamic structure factor of gelling sol revealed ergodic to non-ergodic transition at the onset of gelation and the corresponding ergodicity breaking time τEB was found to be identical to gelation time, tgel. Interestingly, the characteristic parameters defining gelation transition, such as tgel, τEB and Tgel revealed linear dependence with DNA content. Remarkably, the gelation (also melting) temperature could be increased by as much as 2.4°C when the [DNA] was raised by a mere 0.1%, which will impart considerable advantage in terms of variables, and ability to process DNA gels.

Growths of mechanical elasticity and electrical conductance of graphene nanoplatelet/poly(lactic acid) composites under strong electric field: correlation with time evolution of higher order structure of graphene nanoplatelets

For a composite of poly(lactic acid) containing graphene nanoplatelets (GNPs) at a low concentration (0.34 vol%), this study examined growths of mechanical and electrical properties under an alternating current (AC) electric field, focusing on field-induced GNP structures governing those properties. The composite was subjected to the AC field (60 Hz) of various intensities E for various times t E at 190 °C. A fraction of randomly oriented GNPs was aligned by the field and then connected into columns, as suggested from optical microscopy. This structural evolution led to qualitatively similar growths of low-frequency storage modulus and static electrical conductivity. The key quantity for understanding this growth was a time t E* for occurrence of short circuit that detected formation of GNP columns conductively bridging the electrodes. The growths of both modulus and conductivity for various E were summarized as functions of a reduced variable, t E/t E*, confirming the growths commonly reflected the evolution of the GNP columns. However, the modulus grew fast and leveled off by t E/t E* ~ 1, whereas the conductivity kept growing gradually even at t E/t E* > 1. This difference was discussed in relation to the matrix chains and leftover GNPs out the column.

Structural changes in HDPE produced by in-line plasma-pretreated reactive extrusion

Structure of high density poly ethylene (HDPE) was changed after Plasma-induced graft polymerization of methyl methacrylate (MMA) monomer onto HDPE. Continuous polymerization of monomer wetted HDPE pellets was carried out sequentially via in-line plasma pretreatment followed by reactive extrusion. It was influenced by plasma pretreatment times between 15 s and 120 s on degree of grafting and molecular structure of the extrudate. Maximum degree of grafting was limited to 6% after 30 s plasma pretreatment. The addition of dicumyl peroxide (DCP) increased the degree of grafting to 10% after 15 s of plasma treatment. Gel formation to levels in the range 3–5% was detected, increasing with plasma pretreatment time. The formation of long chain branched (LCB) architecture, of the extruded PMMA-g-HDPE, was confirmed by rheological test in oscillatory shear, which results in increases in low frequency storage and loss moduli and melt viscosity than that of neat HDPE, especially after DCP addition. Finally, formation of a fully crosslinked structure was observed after annealing at 210 °C for 1 h as evidenced by a change in rheological properties at lowest frequencies to solid-like behaviour. Such extensive crosslinking could be useful in producing high strength adhesive joints between HDPE and other materials.

Smoluchowski aggregation kinetics, gelation, ergodicity breaking and aging dynamics of (1:1) Laponite-Montmorillonite mixed clay dispersions

Abstract Herein, we report clay concentration dependent hierarchical self-assembly, ergodicity breaking with the aging dynamics for (1:1) Laponite-Montmorillonite mixed dispersions which turned into colloidal gels after a characteristic gelation time tg. Growth dynamics could be universally described by, η r ∼ ( 1 − t t g v i s c ) − ∝ ; t ≪ t g v i s c and G 0 ∼ ( t t g r h e o − 1 ) γ ; t ≫ t g r h e o where η r is relative viscosity of gelling dispersion, and G0 is low frequency storage modulus of the gel. Growth exponents decreased from α = 1.0 to 0.3 and γ = 1.5 to 1.2, when clay concentration was increased to 1.25% (w/v). Above this concentration, there was increase in value of exponent α from 0.3 to 0.8 while γ remained invariant (≈1.3). For [Clay] ≤ 1.25% (w/v), t g v i s c ≫ t g r h e o implying early network formation, when [Clay] > 1.25% (w/v), t g v i s c ≈ t g r h e o . Dynamic structure factor data revealed presence of two relaxation modes: the fast mode relaxation time t1 was independent of aging time tw while the slow mode relaxation time showed τ 2 ∼ t w k s . Nucleation time tn determined from slow mode relaxation, scattered intensity and ergodicity parameter were consistent. At low clay concentration nucleation and growth occurred over a time scale tn ∼ 103 s, while at higher concentration (>1.25%), this was instantaneous leading to faster gelation and dynamic arrest. Difference between the gelation time tg and tn was large at low clay concentration which converged with increase in clay concentration. The transition from ergodic to non-ergodic phase occurred at τEB ∼ 105 s (ergodicity breaking time) and τEB decreased with clay concentration. Smoluchowski aggregation model was used to describe the diffusion limited nucleation and growth of aggregates that eventually self-assembled to form entropic gels.

Rheological Study of Order-to-Disorder Transitions and Phase Behavior of Block Copolymer–Surfactant Complexes Containing Hydrogen-Bonded Small Molecule Additives

Dynamic mechanical measurements were used to investigate the effect of small molecule additives on the order-to-disorder transitions (ODTs) of Pluronic, poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–PEO triblock copolymer surfactant melts. The small molecule additives contain multiple functional groups (carboxyl or hydroxyl), which selectively interact with the PEO component of Pluronic via hydrogen bonding, thereby effectively increasing χ of the system and leading to microphase separation in otherwise disordered melts. The ODTs of these Pluronic/small-molecule-additive complexes can be detected by rheology since, upon increasing temperature, crossing the order-to-disorder transition temperature (TODT) results in a sharp decrease in the low frequency storage and loss moduli (G′ and G″, respectively). The crystallization of the PEO component is suppressed with increasing additive loading due to strong hydrogen bond interactions. The TODT is strongly composition dependent and increases up to 145 °...

Hierarchical self-assembly, relaxations and ergodic–non-ergodic transition in laponite ionogels

We report on the formation of entropic ionogels (gels in IL solutions) of laponite platelets in ionic liquid (IL), 1-ethyl-3-methyl imidazolium chloride, solutions at room temperature (0<[IL]<0.05% (w/v)). Self-assembled colloidal networks were formed following a growth mechanism given by, ηr∼(1−t/tgvis)−p for t<tgvis; and G0∼(t/tgrheo−1)β for t>tgrheo where ηr, G0 and tgvis,rheo represent relative viscosity, low frequency storage modulus and gelation time (determined from viscosity and rheology) respectively. The exponent p was found to increase from 0.23 to 0.85 with increase in IL concentration implying slower network growth kinetics, which helped in the formation of stronger ionogels (β decreased from 2.40 to 0.83). It was found that tgvis>tgrheo which indicated that the network, after formation, required considerable amount of equilibration time (longer for low IL concentration) to reorganize and attain a solid-like structure capable of sustaining shear deformation. Reorganization of clay platelets caused reduction in the inter platelet repulsion due the IL-double layer screening and facilitated formation of stronger ionogels for samples with [IL]≤0.03% (w/v). Dynamic structure factor data revealed two primary relaxation modes: (i) the anomalous slow mode that was frozen at all IL concentrations, and (ii) the diffusive fast mode that contained two diffusion components both strongly dependent on IL concentration. Neutron scattering data could be split into two distinct scattering regions: (i) 0.001<q<q# Å−1, I(q)∼q−α with α=0.97–1.67 (Power–law region) and (iii) q#<q<0.35Å−1, I(q)∼e−(q2Rc2/2) (Guinier–Porod region) with Rc=4.6–5.0Å−1. This implied existence of rod-like structures in homogeneous and rarefied clusters in fragile gels. In summary, it is shown that laponite gels could be systematically customized in IL solutions to generate a range of soft materials with properties solely dependent on IL concentration which may not exceed 0.05% (w/v).

Structure, permeability, and rheology of supercritical CO2 dispersed polystyrene-clay nanocomposites

Improvement in clay dispersion and clay-polymer interfacial interactions are keys to producing superior nanocomposites. A supercritical CO2 (scCO2) processing method was utilized to pre-disperse commercial organic clays, for further solvent mixing with polystyrene (PS) to form nanocomposites with significant dispersion and interfacial enhancement. The effect of scCO2 processing on clay pre-dispersion, and clay dispersion and interfacial interaction in nanocomposites were investigated. SEM and WAXD of the clays indicated that after scCO2 processing the clays lose their long region ordered layer structure appreciably, associated with reduction in particle size. WAXD and TEM of the PS/clay nanocomposites showed that the polymer penetrated into the pre-dispersed clay, leading to a disordered intercalated/exfoliated structure with improved interfacial interaction rather than a disordered intercalated structure as seen with as-received clays. Relationships between those structures, rheological and barrier properties were investigated. The scCO2-processed nanocomposites showed a plateau in the low-frequency storage modules and increased complex viscosity, each associated with significant clay dispersion in the nanocomposite. With only 1.09% volume fraction of clay, significant reduction (∼49%) of oxygen permeation was achieved.

Effect of solvent hydrophobicity on gelation kinetics and phase diagram of gelatin ionogels.

We present a systematic investigation of the effect of solvent hydrophobicity (alkyl chain length) on the gelation kinetics and the phase states of the polypeptide gelatin in imidazolium based ionic liquid (IL) solutions. We have observed that IL concentration and hydrophobicity had dramatic influences on the thermal and viscoelastic properties of gelatin ionogels. Gelation concentration cg was observed to increase from 1.75 to 2.75% (w/v) while the gelation temperature Tg was found to decrease from 32 to 26 °C with increase in 1-octyl-3-methyl imidazolium chloride [C8mim][Cl] (most hydrophobic) concentration as compared to the case of the least hydrophobic IL 1-ethyl-3-methyl imidazolium chloride [C2mim][Cl], where the corresponding changes were marginal. Gradual softening of the gel with increase in hydrophobicity and concentration of IL was clearly noticed. The viscosity of the gelling sol diverged as ηr ∼ ε(1)(-k) and storage modulus of gel grew as G0 ∼ ε(1)(t) where ε1 = |1 - c/cg| with the exponents having values k = 1.2-1.8 ± 0.08 and t = 1.2-1.6 ± 0.08, close to but not exactly the same as predicted by the percolation model: k = 0.7-1.3 and t = 1.9. Thus, the gelation kinetics involved in the growth of interconnected networks could be conceived to follow an anomalous percolation model. The temporal growth of self-assembled structures followed a power law dependence given by: ηr ∼ ε(2)(-α) and Rh ∼ ε(2)(-β) where ε(2) = t > tg (α = 1-2.9 ± 0.08 and β = 1-2.7 ± 0.08). The low frequency storage modulus G0, gelation temperature Tg, gelation concentration cg and gelation time tg adequately defined the sol-gel phase diagram. Results clearly revealed that by adjusting the hydrophobic chain length and concentration of IL it was possible to customize both thermal and mechanical properties of these ionogels to match specific application requirements.

Effects of Nanoclay on Morphology, Rheological, and Mechanical Properties of Polyvinyl Chloride/Acrylonitrile-Butadiene-Styrene

In this work, PVC/ABS matrix reinforced with several organoclay concentrations were prepared by melt mixing process. The SEM and XRD tests were presented in conjunction with the mechanical and rheological properties of these nanocomposites. The XRD spectra showed an intercalated microstructure in the nanocomposites. Addition of up to 5 wt% organoclay to PVC/ABS blends led to nanocomposites with the rigidifying effect of organoclay and the toughening effect of ABS. The optimum mechanical properties were found in the blends with 5 wt% organoclay. A remarkable increase in the viscosity and non-terminal low frequency storage modulus was found for the nanocomposites.

The effect of mixing process on linear viscoelastic and electrical properties of ABS/MWNT nanocomposites

AbstractThe aim of this article was to study the microstructure development and the linear melt viscoelastic properties of ABS/MWNT nanocomposites. The nanocomposite samples varying in nanotube concentration (1, 2, 4, and 8 wt %) were prepared using two different methods; solution method and direct melt compounding. Results of the melt linear viscoelastic measurements performed on the nanocomposite samples prepared by both methods showed a pronounced low frequency nonterminal behavior in storage modulus along with the viscosity upturn with greater extent for the samples prepared by the solution method. The values of low frequency storage modulus for the solution processed samples were found to be increased with increasing the nanotube content up to 3 wt % above which it is increased more significantly. These results could be related to a 3D microstructure development, the extent of which was found to be enhanced in the samples prepared by the solution method due to the improving interfacial interaction and promoting the nanotube dispersion in ABS matrix. This was supported by the SEM results of the samples. Dynamic mechanical thermal analysis results of the melt‐mixed and solution‐processed samples showed that Tg of the SAN is increased about 20°C at the presence of nanotube. The results are related to the chain mobility restrictions caused by physical interaction between carbon nanotubes (CNT) and polymer chains. The electrical conductivity versus CNT content for the nanocomposite samples prepared by solution method showed a percolation behavior with CNT concentration above 4 wt % which attributed to the 3D network between nanotubes. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Study on Morphology, Rheology and Mechanical Properties of Thermoplastic Elastomer Polyolefin (TPO)/Carbon Nanotube Nanocomposites with Reference to the Effect of Polypropylene-grafted-Maleic Anhydride (PP-g-MA) as a Compatibilizer

The structure–property relationships of polypropylene/ethylene-propylene-diene (PP/EPDM) (80/20) nanocomposites containing single-walled carbon nanotubes (SWCNTs) by melt-mixing process were investigated, the main focus being on the effect of SWCNTs concentration and compatilizer content. Morphological observations by scanning electron microscopy (SEM) are presented in conjunction with the mechanical, thermal, and rheological properties of these nanocomposites. The tensile modulus of nanocomposites was enhanced by increasing the SWCNTs concentration. A high level of toughness in the thermoplastic elastomer polyolefin (TPO)/SWCNTs nanocomposite was achieved with 0.5 wt% of SWCNTs and 1 wt% of polypropylene-grafted maleic anhydride (PP-g-MA). Differential scanning calorimetry (DSC) experiments confirmed the nucleation effect of nanotubes on the crystallization process of the TPO/SWCNTs composites. An appreciable viscosity upturn and a non-terminal low frequency storage modulus were observed in nanocomposites containing SWCNTs whose values increased in the presence of compatibilizer.

Investigation of clay modifier effects on the structure and rheology of supercritical carbon dioxide‐processed polymer nanocomposites

AbstractSuperior property enhancements in polymer–clay nanocomposites can be achieved if one can significantly enhance the nanoclay dispersion and polymer–clay interactions. Recent studies have shown that nanoclays can be dispersed in polymers using supercritical carbon dioxide (scCO2). However, there is need for a better understanding of how changing the clay modifier affects the clay dispersability by scCO2 and the resultant nanocomposite rheology. To address this, the polystyrene (PS)/clay nanocomposites with “weak” interaction (Cloisite 93A clay) and “strong” interaction (Cloisite 15A clay) have been prepared using the supercritical CO2 method in the presence of a co‐solvent. Transmission electron microscopy images and small‐angle X‐ray diffraction illustrate that composites using 15A and 93A clays show similar magnitude of reduction in the average tactoid size, and dispersion upon processing with scCO2. When PS and the clays are coprocessed in scCO2, the “dispersion” of clays appears to be independent of modifier or polymer–clay interaction. However, the low‐frequency storage modulus in the scCO2‐processed 15A nanocomposites is two orders of magnitude higher than that of 93A nanocomposites. It is postulated that below percolation (solution blended composites), the strength of polymer–clay interaction is not a significant contributor to rheological enhancement. In the scCO2‐processed nanocomposites the enhanced dispersion passes the percolation threshold and the interactions dictate the reinforcement potential of the clay–polymer–clay network. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 823–831, 2010