Triblock Copolymer Gels Research Articles

Ultrastretchable, cyclable and recyclable 1- and 2-dimensional conductors based on physically cross-linked thermoplastic elastomer gels

Stretchable conductors maintain electrical conductivity at large strains relative to their rigid counterparts that fail at much lower strains. Here, we demonstrate ultrastretchable conductors that are conductive to at least 600% strain and may be strain-cycled without significant degradation to the mechanical or electrical properties. The conductors consist of a liquid metal alloy injected into microchannels composed of triblock copolymer gels. Rheological measurements identify the temperature window over which these gels may be molded and laminated to form microchannels without collapsing the microscale features. Mechanical measurements identify the gel composition that represents a compromise between minimizing modulus (to allow the polymer to be stretched with ease) and maximizing interfacial adhesion strength at the laminated polymer–polymer interface. The resulting 2D stretchable conductors are notable for their ability to maintain electrical conductivity up to large strains, their mechanical durability, and their ability to be recycled easily with full recovery of the component species.

Phase behavior of SEBS triblock copolymer gels

AbstractDynamic density functional theory calculations were performed for thermoplastic elastomer gels composed of an ABA triblock copolymer immersed in a B‐attractive solvent. The triblock copolymer model was parameterized for poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS), while the solvent model was parameterized for the hydrocarbon oil tetradecane. The effect of the solvent concentration and S‐EB interaction on the morphology was investigated, where complementary experimental data was used to validate results at χABN≈ 100. Agreement was observed at solvent volume fractions of 0.2, 0.4, and 0.6, which correspond to the cylindrical, spherical, and spherical phases, respectively. Qualitative agreement was observed for 0.8 volume fraction solvent, where a core‐shell spherical micelle morphology was found. For a 50/50 vol % mixture of polymer/solvent, the effect of solvent molecular weight on the morphology was considered, where a transition between micro and macrophase separation was predicted at a critical solvent molecular weight. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1479–1491, 2011

Tailoring Micelle Formation and Gelation in (PEG−P(MA-POSS)) Amphiphilic Hybrid Block Copolymers

We demonstrate that hydrophobic POSS (polyhedral oligomeric-silsesquioxane) nanoparticles, added to block copolymer solutions of poly(ethylene glycol) (PEG) and poly(methacrylisobutyl polyhedral oligomericsilsesquioxane) P(MA-POSS) as the hydrophilic and hydrophobic blocks respectively, could be employed to tailor their micelle formation, gelation, and rheological performance. For example, the hydrodynamic size of the micelles, formed by PEG5k-b-P(MA-POSS)3.6, increased from 13.6 ± 1.0 to 56.9 ± 3.7, an increase of more than four times, with 0.1 wt % (with respect to block copolymer) POSS nanoparticles in solution. However, the micelles retain their spherical morphology with core−shell structure as evidenced by calculating the values of dimensionless ratios, Rg/Rh for micelles as a function of added POSS nanoparticles content. For P(MA-POSS)-b-PEG10k-b-P(MA-POSS) triblock copolymers, which is associative in nature, addition of POSS nanoparticles resulted in formation of more robust and stronger hydrogels with a significantly higher storage modulus, G′, yield strengths, σy, and lower critical gelation concentration, cg, as compared with those for the pristine triblock copolymer. The presence of eight vinyl groups, attached to the POSS nanoparticles under investigations, is also exploited for further enhancement of rheological properties of the hydrogels with UV treatment. Finally, gel formation is induced in aqueous solutions of PEG5k-b-P(MA-POSS)3.6 diblock copolymer by introducing P(MA-POSS)-b-PEG10k-b-P(MA-POSS) triblock copolymer chains, and the rheological performance of the produced hydrogels, with certain compositions, is even superior to that of pure triblock copolymer gel.

Study of Diffusion of Organic Dyes in a Triblock Copolymer Micelle and Gel by Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy (FCS) has been used to study translational diffusion of three fluorescent dyes in a micelle and a gel. It was demonstrated that a highly hydrophobic dye, DCM, remains confined to a particular micelle during the passage of the micellar aggregation through the confocal volume. As a result, DCM exhibits slow diffusion of the large micellar aggregate with a diffusion coefficient (D(t)) approximately 25 times slower compared with that of water. In contrast, a hydrophilic probe (C343 or C480) occasionally diffuses out of the micelle into bulk water and displays a large D(t) (twofold smaller in F127 and approximately six times smaller in the P123 micelle compared with that in bulk water). In a gel, diffusion of the individual micelles is completely arrested and hence, the autocorrelation in FCS arises solely from the diffusion of the dye in the gel. In this case, all the three dyes exhibit extremely slow diffusion (300, 45, and 20 times slower than that in water for DCM, C480, and C343 in F127 gel, respectively). In a P123 and F127 gel, diffusion of DCM is respectively, seven and 29 times slower compared with that of the ionic probe C343. The relatively small value of red-edge excitation shift (REES) of the emission maximum, suggests that DCM is confined within the core of the triblock copolymer micelles and gels. The hydrophilic probes (C343 or C480) exhibit fast diffusion in the micelles and gels. However, their REES is very different. The large REES of C480 suggests that it is distributed over a large region of the micelle, whereas the low REES of C343 indicates that it is located primarily in the peripheral corona region.

Fracture and large strain behavior of self-assembled triblock copolymer gels

The rate dependence of fracture has been studied in a series of physically associating triblock copolymer gels that have a well-defined molecular structure. Compressive experiments were performed to develop a strain energy function that accurately captures the strain hardening behavior of these materials. This same strain energy function was utilized in a finite element model of the crack tip stresses, which become highly anisotropic at stress values below the failure strength of the gels. The rate dependence of the energy release rate, G, is independent of the gel concentration when G is normalized by the small strain Young's modulus, E. The gels exhibit a transition from rough, slow crack propagation to smooth, fast crack propagation for a well-defined value of the characteristic length, G/E.

Physical gels of telechelic triblock copolymers with precisely defined junction multiplicity

We study transient networks formed by monodisperse telechelic polypeptides with collagen-like end blocks and a random-coil-like middle block. These artificial proteins are created using recombinant DNA techniques. Upon cooling, the end blocks associate reversibly into triple helices, leading to gels with a well-defined junction multiplicity of three. Both the storage modulus and the relaxation time of the gel increase very strongly as a function of concentration, and decrease with increasing temperature. All the experimental results are described quantitatively by an analytical model, based on classical gel theory, that requires no adjustable parameters, and accounts for the molecular structure of the gel, and the presence of loops and dangling ends.

Nanoparticle-reinforced associative network hydrogels.

ABA triblock copolymers in solvents selective for the midblock are known to form associative micellar gels. We have modified the structure and rheology of ABA triblock copolymer gels comprising poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) through addition of a clay nanoparticle, laponite. Addition of laponite particles resulted in additional junction points in the gel via adsorption of the PEO corona chains onto the clay surfaces. Rheological measurements showed that this strategy led to a significant enhancement of the gel elastic modulus with small amounts of nanoparticles. Further characterization using small-angle X-ray scattering and dynamic light scattering confirmed that nanoparticles increase the intermicellar attraction and result in aggregation of PLA-PEO-PLA micelles.

Titanium with controllable pore fractions by thermoreversible gelcasting of TiH 2

Thermoreversible gelcasting, a near-net-shape processing technique, is demonstrated here for titanium. The gelcasting system is composed of TiH 2 particles suspended in a triblock copolymer gel that behaves as a viscous liquid above 56 °C and an elastic solid at room temperature, a temperature-dependent transition that is fully reversible when solvent is present. Organic pyrolysis to remove the gel followed by vacuum sintering to densify the Ti powders (produced by decomposition of the hydride) results in titanium with near full density and low contamination. Incorporation of polypropylene and poly(methyl methacrylate) space-holder particles into the gel results in titanium with controlled porosities up to 44 vol.% and with low contamination. These foams exhibit tailorable stiffness and strength, together with excellent compressive ductility and energy absorption.

Properties of new nanocomposite triblock copolymer gels based on expandable graphite

AbstractIn this work, we investigated the effect of expandable graphite (EG) on the property of triblock copolymer prepared from a poly(styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene) (SEBS) imbibed with an EB‐compatible hydrocarbon oil. The rheological properties showed that at a temperature between 30 and 40°C below the gel point, the triblock copolymer gels had a dynamic storage modulus (G′) greater than loss modulus (G‵), thereby indicating that at ambient temperature, a physical network is still present in spite of the addition of nanoparticles. Dynamic rheological measurements of the resultant nanocomposite triblock copolymer gels confirmed that the addition of EG affects the linear viscoelastic properties and maximum operating temperature of the parent triblock copolymer gels. The mechanical properties showed only marginal increase, which can be attributed to the poor dispersion that leads to agglomeration of particle into micrometer size stacks, and thus the particles behave only as inorganic fillers. The morphology and X‐ray diffraction revealed that the EG used to generate nanocomposite triblock copolymer gels is dispersed generally within the swollen copolymer and/or solvent. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Structural Characterization of PLA−PEO−PLA Solutions and Hydrogels: Crystalline vs Amorphous PLA Domains

We have shown that we can significantly modify the nanoscale structure of solution and gels of ABA triblock copolymers in a solvent selective for the mid B block by making simple changes to the ste...

Autonomous Volume Transitions of a Polybase Triblock Copolymer Gel in a Chemically Driven pH‐Oscillator

AbstractSummary: A pH‐responsive ABA triblock copolymer, comprising poly(methyl methacrylate)‐block‐poly(2‐(diethylamino)ethyl methacrylate)‐block‐poly(methyl methacrylate) [PMMA‐b‐PDEA‐b‐PMMA], has been cast into thin films with a well‐defined microstructure. Small Angle X‐ray Scattering (SAXS) and Atomic Force Microscopy (AFM) studies confirm that this copolymer forms a hydrogel consisting of PMMA spheres embedded within a polybase PDEA matrix, with the PMMA domains acting as physical cross‐links. The hydrogel has a pH‐reversible coil‐globule transition at around pH 4.5. This responsive physical property was exploited by immersing a sample of copolymer hydrogel in an aqueous solution containing a cyclic pH‐oscillating reaction, whereby the pH was continuously oscillated above and below the transition pH so as to induce autonomous volume transitions. The changes in microscopic and macroscopic length scales correlate closely during (de)swelling cycles, with affine behaviour occurring over five orders of magnitude.

Self-Assembly and Stress Relaxation in Acrylic Triblock Copolymer Gels

The structure and relaxation behavior of thermoreversible gels made with poly(methyl methacrylate)−poly(n-butyl acrylate)−poly(methyl methacrylate) [PMMA−PnBA−PMMA] triblock copolymers in 2-ethylhexanol, a midblock selective solvent, were studied by small-angle X-ray scattering (SAXS) and rheology. Effects of endblock length, endblock fraction, and gel concentration on the gel properties were investigated. A dramatic decrease in SAXS intensity was observed over a 20 °C interval where the gel transitions smoothly from elastic to viscous behavior. SAXS patterns were fit with a Percus−Yevick disordered hard-sphere model from which aggregation number and average domain spacing were calculated. Aggregation number increases with increasing gel concentration and endblock length. Increasing the endblock length from 9K to 25K increases the relaxation time of a gel with a polymer volume fraction of 0.15 by a factor of 10^6. For a given triblock endblock fraction and molecular weight, the micelle aggregation number is strongly correlated to the gel relaxation time. Arrhenius behavior with an effective activation energy of ~550 kJ/mol was observed for all triblocks and concentrations. This very high effective energy barrier describes gels relaxation behavior over a 40 °C temperature range, where the relaxation times vary by a factor of 10^(10).

Multiscale mechanical characterization of biomimetic physically associating gels

The mechanical response of living tissue is important to understanding the injury-risk associated with impact events. Often, ballistic gelatin or synthetic materials are developed to serve as tissue surrogates in mechanical testing. Unfortunately, current materials are not optimal and present several experimental challenges. Bulk measurement techniques, such as compression and shear testing geometries, do not fully represent the stress states and rate of loading experienced in an actual impact event. Indentation testing induces deviatoric stress states as well as strain rates not typically available to bulk measurement equipment. In this work, a ballistic gelatin and two styrene-isoprene triblock copolymer gels are tested and compared using both macroscale and microscale measurements. A methodology is presented to conduct instrumented indentation experiments on materials with a modulus far below 1 MPa. The synthetic triblock copolymer gels were much easier to test than the ballistic gelatin. Compared to ballistic gelatin, both copolymer gels were found to have a greater degree of thermal stability. All of the materials exhibit strain-rate dependence, although the magnitude of dependence was a function of the loading rate and testing method.

Aqueous Gels of Triblock Copolymers of Ethylene Oxide and 1,2‐Butylene Oxide (Type BEB) Studied by Rheometry

AbstractSummary: A triblock copolymer of ethylene oxide and 1,2‐butylene oxide with hydrophilic central block and hydrophobic end blocks, denoted B10E227B10, was prepared and characterised. The frequency dependence of dynamic modulus was determined for micellar solutions in the concentration range of 8–20 wt.‐% and the temperature range of 5–70 °C, and the fluid/gel diagram was established. The results were combined with those reported previously for copolymers B12E227B12 and B12E114B12 to obtain insight into the effect of B‐block and E‐block length on the rheological properties of systems of this type. The results are discussed in terms of micelles forming bridged networks and, at higher concentrations, packing in structured gels.Temperature dependence of the logarithm of storage modulus for aqueous solutions of copolymer B10E227B10 at the concentrations (wt.‐%) indicated. Conditions: f = 1 Hz, A ≈ 0.5%.magnified imageTemperature dependence of the logarithm of storage modulus for aqueous solutions of copolymer B10E227B10 at the concentrations (wt.‐%) indicated. Conditions: f = 1 Hz, A ≈ 0.5%.

Silica reinforced triblock copolymer gels

The effect of silica and polymer coated silica particles as reinforcing agents on the structural and mechanical properties of polystyrene–poly(ethylene/butylene)–polystyrene (PS–PEB–PS) triblock gel has been investigated. Different types of chemically modified silica have been compared in order to evaluate the influence of the compatibility between gel and filler. Time-resolved SANS and small-angle X-ray scattering (SAXS) shows that the presence of silica particles affects the ordering of the polystyrene domains during gelsetting. The scattering pattern of silica-reinforced gels reveals strong scattering at very low q, but no structure and formfactor information. However, on heating above the viscoelastic to plastic transition, the ‘typical’ scattering pattern of the copolymer gel builds-up. All reinforced gels are strengthened by the addition of the reinforcing agent. The transitions from a viscoelastic rubber to a plastic fluid and from a plastic fluid to a viscoelastic liquid are shifted to more elevated temperatures when silica is added to the triblock copolymer gel.

Crack blunting and the strength of soft elastic solids

When a material is so soft that the cohesive strength (or adhesive strength, in the case of interfacial fracture) exceeds the elastic modulus of the material, we show that a crack will blunt instead of propagating. Large–deformation finite–element model (FEM) simulations of crack initiation, in which the debonding processes are quantified using a cohesive zone model, are used to support this hypothesis. An approximate analytic solution, which agrees well with the FEM simulation, gives additional insight into the blunting process. The consequence of this result on the strength of soft, rubbery materials is the main topic of this paper. We propose two mechanisms by which crack growth can occur in such blunted regions. We have also performed experiments on two different elastomers to demonstrate elastic blunting. In one system, we present some details on a void growth mechanism for ultimate failure, post–blunting. Finally, we demonstrate how crack blunting can shed light on some long–standing problems in the area of adhesion and fracture of elastomers.

Origins of Mechanical Strength and Elasticity in Thermally Reversible, Acrylic Triblock Copolymer Gels

Thermoreversible gels were formed by dissolving a poly(methyl methacrylate)−poly(tert-butyl acrylate)−poly(methyl methacrylate) triblock copolymer in a variety of alcohols, including ethanol, 1-butanol, 2-ethylhexanol, and 1-octanol. The gels exhibit an ideal and reversible solid/liquid transition in each of these solvents, behaving as strong elastic solids at room temperature and as freely flowing liquids above the gel transition. The time-dependent elastic properties of the gels are governed by two transition temperatures. The first transition is the critical micelle temperature (cmt) near 60 °C, below which the PMMA blocks aggregate to form a physically cross-linked network. As the gels are cooled to room temperature, differential scanning calorimetry (DSC) reveals a second transition where the PMMA domains undergo a glass transition. The glass transition temperature of the PMMA domains increases when the gels are aged at room temperature. Time−temperature superposition can be applied below the cmt to give master curves describing the relaxation behavior of the gels in the vicinity of the glass transition of the PMMA domains. These relaxation times increase by 1 decade for every 8 K decrease in temperature, a result that is consistent with previous measurements of polymer relaxations in the vicinity of the glass transition temperature.

On the Mechanism of Electrophoretic Migration of DNA in Pluronic Gels

Capillary electrophoresis of nucleic acids has recently employed gels of self-assembled uncharged triblock copolymers as sieving media. Pluronic F127 contains poly(ethylene oxide) (EO) and poly(propylene oxide) (PO) with the block structure (EO)106(PO)70(EO)106. Aqueous solutions of 30% w/w of this polymer are liquids at low temperatures, but above 11 °C the polymers assemble to micelles that pack into a locally cubic lattice forming a gel-like lyotropic liquid crystal phase. Here we use linear dichroism spectroscopy to study the orientation dynamics of double-stranded DNA molecules during the electrophoresis. In 30% Pluronic F127, a 5400 bp DNA migrates with substantial perpendicular orientation of the helix axis, which is in contrast to electrophoresis in agarose gels where the helix axis of DNA is aligned parallel to the field direction. Comparison between linear and circular DNA indicates that neither DNA form enters the cubic microcrystals at low fields, and when combined with velocity measurements the kinetics of alignment buildup and relaxation suggests that migration instead occurs in grain boundaries between domains of microcrystals.

ABA Triblock Copolymer Gels Modified with an A-Compatible Semicrystalline Homopolymer

In the presence of a midblock-selective solvent, ABA triblock copolymers form physical gels in which bridged and entangled B-chains establish a swollen network stabilized by A-microdomains. Here, we seek to improve the properties of an ABA gel through the addition of an A-compatible, high-molecular-weight semicrystalline homopolymer (shA). Dynamic rheology indicates that the elastic modulus increases substantially, and far beyond that achievable with an inert filler, with increasing shA content at constant solvent concentration. Transmission electron micrographs reveal the existence of nanoscale shA filaments and sheets dispersed in a micelle-stabilized gel network. The shape of the nanoscale shA objects, which are partially crystalline according to differential scanning calorimetry, and their apparent interaction with the A-rich micelles enhance network development and are responsible for the pronounced modulus increase.

Polymer-assisted formation of giant polyoxomolybdate structures.

A highly ordered, porous nanoscale network structure of polyoxomolybdate was prepared from the slow decomposition of an unstable precursor compound MoO(2)(OH)(OOH) in the presence of PEO-containing triblock copolymer gels or semidilute/concentrated poly(ethylene oxide) (PEO) homopolymer solutions. Small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements of the 1-microm size crystals revealed an extremely ordered primitive cubic (pc) structure made of polyoxomolybdates. Scanning electron microscopy (SEM) measurements showed that the overall size of these single crystals was close to 1 microm, in agreement with estimates from the scattering peak widths. Evidence from both scattering measurements and transmission electron microscope (TEM) measurements suggested that the system formed a highly porous primitive cubic network similar to that of certain zeolite structures. However, the observed lattice constant of 5 nm was much larger. The function of the PEO-containing polymer network was found to be very subtle and complex. Presumably, PEO acted simultaneously as a weak reducing agent and as a viscous matrix to ensure sufficient time for the formation of long-range ordered structures, resulting in the growth of extremely uniform but probably hollow "nanospheres". We suggest that this new material may find unique applications as a high-efficiency absorbent or catalyst. Furthermore, the current synthetic method may open up new pathways to prepare similar functional nanomaterials.