Phase behavior and dynamics of a micelle-forming triblock copolymer system

Dynamic Covalent Amphiphilic Polymer Conetworks Based on End-Linked Pluronic F108: Preparation, Characterization, and Evaluation as Matrices for Gel Polymer Electrolytes

We present the development of a platform of well-defined, dynamic covalent amphiphilic polymer conetworks (APCN) based on an α,ω-dibenzaldehyde end-functionalized linear amphiphilic poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (PEG-b-PPG-b-PEG, Pluronic) copolymer end-linked with a triacylhydrazide oligo(ethylene glycol) triarmed star cross-linker. The developed APCNs were characterized in terms of their rheological (increase in the storage modulus by a factor of 2 with increase in temperature from 10 to 50 °C), self-healing, self-assembling, and mechanical properties and evaluated as a matrix for gel polymer electrolytes (GPEs) in both the stretched and unstretched states. Our results show that water-loaded APCNs almost completely self-mend, self-organize at room temperature into a body-centered cubic structure with long-range order exhibiting an aggregation number of around 80, and display an exceptional room temperature stretchability of ∼2400%. Furthermore, ionic liquid-loaded APCNs could serve as gel polymer electrolytes (GPEs), displaying a substantial ion conductivity in the unstretched state, which was gradually reduced upon elongation up to a strain of 4, above which it gradually increased. Finally, it was found that recycled (dissolved and re-formed) ionic liquid-loaded APCNs could be reused as GPEs preserving 50-70% of their original ion conductivity.

Dynamic Light Scattering Based Microrheology of End-Functionalised Triblock Copolymer Solutions

Nano-sized particles functionalised with short single-stranded (ss)DNAs can act as detectors of complementary DNA strands. Here we consider tri-block-copolymer-based, self-assembling DNA-coated nanoparticles. The copolymers are chemically linked to the DNA strands via azide (N3) groups. The micelles aggregate when they are linked with complementary ssDNA. The advantage of such block-copolymer-based systems is that they are easy to make. Here we show that DNA functionalisation results in inter-micellar attraction, but that N3-groups that have not reacted with the DNA detector strands also change the phase behaviour of the tri-block polymer solution. We studied the triblock copolymer, Pluronic^® F108, which forms spherical micelles in aqueous solutions upon heating. We find that the triblock chains ending with either an N3 or N3-DNA complex show a dramatic change in phase behaviour. In particular, the N3-functionalisation causes the chain ends to cluster below the critical micelle temperature (CMT) of pure F108, forming flower-micelles with the N3-groups at the core, while the PPO groups are exposed to the solvent. Above the CMT, we see an inversion with the PPO chains forming the micellar core, while the N3-groups are now aggregating on the periphery, inducing an attraction between the micelles. Our results demonstrate that, due to the two competing self-assembling mechanisms, the system can form transient hydrogels.

Linking In Vitro Intrinsic Dissolution Rate and Thermodynamic Solubility with Pharmacokinetic Profiles of Bedaquiline Long-Acting Aqueous Microsuspensions in Rats

Pharmacokinetic (PK) profiles of a range of bedaquiline (BDQ) long-acting injectable (LAI) microsuspensions in rats after parenteral (i.e., intramuscular and subcutaneous) administration were correlated with the in vitro intrinsic dissolution rate (IDR) and thermodynamic solubility of BDQ in media varying in surfactant type and concentration to better understand the impact of different nonionic surfactants on the in vivo performance of BDQ LAI microsuspensions. All LAI formulations had a similar particle size distribution. The investigated surfactants were d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS), poloxamer 338, and poloxamer 188. Furthermore, the relevance of medium complexity by using a biorelevant setup to perform in vitro measurements was assessed by comparing IDR and thermodynamic solubility results obtained in biorelevant media and formulation vehicle containing different surfactants in varying concentrations. In the presence of a surfactant, both media could be applied to obtain in vivo representative dissolution and solubility data because the difference between the biorelevant medium and formulation vehicle was predominantly nonsignificant. Therefore, a more simplistic medium in the presence of a surfactant was preferred to obtain in vitro measurements to predict the in vivo PK performance of LAI aqueous suspensions. The type of surfactant influenced the PK profiles of BDQ microsuspensions in rats, which could be the result of a surfactant effect on the IDR and/or thermodynamic solubility of BDQ. Overall, two surfactant groups could be differentiated: TPGS and poloxamers. Most differences between the PK profiles (i.e., maximum concentration observed, time of maximum concentration observed, and area under the curve) were observed during the first 21 days postdose, the time period during which particles in the aqueous suspension are expected to dissolve.

Dynamic Behavior of the Structural Phase Transition of Hydrogel Formation Induced by Temperature Ramp and Addition of Ibuprofen

Understanding the dynamic behavior of hydrogel formation induced by a temperature ramp is essential for the design of gel-based injectable formulation as drug-delivery vehicles. In this study, the dynamic behavior of the hydrogel formation of Pluronic F108 aqueous solutions within different heating rates was explored in both macroscopic and microscopic views. It was discovered that when the heating rate is increased, the gelation temperature window (hard gel region) shrinks and the mechanical strength of the hydrogel decreases. A given system at different heating rates would lead to different crystalline structural evolutions. The time-resolved small-angle X-ray scattering (SAXS) experiments at a heating rate of 10 °C/min disclose that the crystalline structure of micelle packing in the hydrogel exhibits a series of transitions: hexagonal close-packed (HCP) to face-centered cubic (FCC) and body-centered cubic (BCC) structures coexisting and then to the BCC structure along with the increasing temperature. For the system at equilibrium, the BCC structure exclusively dominates the system. Furthermore, the addition of a hydrophobic model drug (ibuprofen) to the F108 aqueous solution promotes hard gel formation at even lower temperatures and concentrations of F108. The SAXS results for the system with ibuprofen at a heating rate of 10 °C/min demonstrate a mixture of FCC and BCC structures coexisting over the whole gelation window compared to the BCC structure that exclusively dominates the system at equilibrium. The addition of ibuprofen would alter the structural evolution to change the delivery path of the encapsulated drug, which is significantly related to the performance of drug release.

Dynamics and Rheology of Soft Colloidal Glasses

The linear viscoelastic (LVE) spectrum of a soft colloidal glass is accessed with the aid of a time-concentration superposition (TCS) principle, which unveils the glassy particle dynamics from in-cage rattling motion to out-of-cage relaxations over a broad frequency range 10^-13 rad/s < ω < 10¹ rad/s. Progressive dilution of a suspension of hairy nanoparticles leading to increased intercenter distances is demonstrated to enable continuous mapping of the structural relaxation for colloidal glasses. In contrast to existing empirical approaches proposed to extend the rheological map of soft glassy materials, i.e., time-strain superposition (TSS) and strain-rate frequency superposition (SRFS), TCS yields a LVE master curve that satisfies the Kramers-Kronig relations which interrelate the dynamic moduli for materials at equilibrium. The soft glassy rheology (SGR) model and literature data further support the general validity of the TCS concept for soft glassy materials.

Probing of field-induced structures and their dynamics in ferrofluids using oscillatory rheology

We probe field-induced structures and their dynamics in ferrofluids using oscillatory rheology. The magnetic field dependence of the relaxation time and crossover modulus showed two distinct regions, indicating the different microstructures in those regions. The observed relaxation at various magnetic field strengths indicates that side chains are attached to the pinned single-sphere-width chains between the rheometer plates. Our results suggest that the ferrofluid under a magnetic field exhibits a soft solidlike behavior whose relaxation is governed by the imposed strain rate and the magnetic field. Using the scaling factors obtained from the frequency and modulus at the crossover point in the oscillatory rheological measurements, the constant strain-rate frequency sweep data is superimposed onto a single master curve. The frequency scaling factor increases with the strain rate as a power law with an exponent close to unity, whereas the amplitude scaling factor is almost strain-rate-independent at high magnetic field strengths. These findings are useful for a better understanding of field-induced ordering of nanoparticles in fluids and their optimization for practical applications.

Encapsulation of hydrophobic drugs in Pluronic F127 micelles: effects of drug hydrophobicity, solution temperature, and pH

Three drugs, ibuprofen, aspirin, and erythromycin, are encapsulated in spherical Pluronic F127 micelles. The shapes and the size distributions of the micelles in dilute, aqueous solutions, with and without drugs, are ascertained using cryo-scanning electron microscopy and dynamic light scattering (DLS) experiments, respectively. Uptake of drugs above a threshold concentration is seen to reduce the critical micellization temperature of the solution. The mean hydrodynamic radii and polydispersities of the micelles are found to increase with decrease in temperature and in the presence of drug molecules. The hydration of the micellar core at lower temperatures is verified using fluorescence measurements. Increasing solution pH leads to the ionization of the drugs incorporated in the micellar cores. This causes rupture of the micelles and release of the drugs into the solution at the highest solution pH value of 11.36 investigated here and is studied using DLS and fluorescence spectrocopy.

Thermogelling properties of triblock copolymers in the presence of hydrophilic Fe3O4 nanoparticles and surfactants

We investigate the supramolecular structure formed by thermogelation of a triblock polymer in the presence of nanoparticles and surfactant using rheometry and small-angle X-ray scattering (SAXS). The triblock copolymer, nanoparticle, and surfactant used in this study are poly(oxyethylene-oxypropylene-oxyethylene), Pluronic F108, Fe(3)O(4) nanoparticles, and sodium dodecyl surfactant, respectively. Addition of 1-5 wt % of Fe(3)O(4) nanoparticle, of average particle size ~10 nm, in a weak template of F108 (15 wt %) shows a decrease in the onset of gelation temperature and dramatic alteration in the viscoelastic moduli. The nanocomposite samples show a linear viscoelastic regime up to 5% strain. The SAXS measurement shows that the intermicellar spacing of the supramolecular structure of pure F108 is ~16.5 nm, and the supramolecular structure is destroyed when nanoparticles and surfactants are incorporated in it. Further, the addition of anionic surfactant to nanocomposites leads to a dramatic reduction in the viscoelastic properties due to strong electrostatic barrier imparted by the surfactant headgroup that prevents the formation of hexagonally ordered micelles. Our results show that the thermogelation is due to the clustering of nanoparticles into a fractal network rather than a close-packed F108 micelles, in agreement with the recent findings in Pluronic F127-laponite systems.

Yielding and structural relaxation in soft materials: evaluation of strain-rate frequency superposition data by the stress decomposition method

Rheological properties of soft materials are often investigated in oscillatory shear and characterized by the storage and loss modulus, G' and G'', respectively. Unfortunately, the relaxation dynamics of most soft materials is too slow to be directly probed by commercial rheometers. Recently, it was shown by Wyss et al. [Phys. Rev. Lett. 98, 238303 (2007)] that the application of an oscillating strain-rate drives such soft materials and shifts the structural relaxation to higher times. They called this experimental technique strain-rate frequency superposition (SRFS). The great benefit of SRFS is the extremely extended frequency range. As viscoelastic measures, Wyss et al. proposed the familiar storage and loss modulus. Using these moduli results in a serious drawback: When the material yields, nonlinearities appear and the physical interpretation of the storage and loss modulus breaks down. Thus, SRFS as proposed by Wyss et al. is limited to the linear regime and the benefit of the extended frequency regime vanishes. In the present work, we validate an alternative data analysis technique, recently established as the stress decomposition method [K. S. Cho et al., J. Rheol. 49, 747 (2005); R. H. Ewoldt et al., J. Rheol. 52, 1427 (2008)], for combination with SRFS. Use of the stress decomposition method provides a physical interpretation of linear and nonlinear SRFS data in terms of strain stiffening and softening as well as shear thickening and thinning.

Experimental studies of the jamming behaviour of triblock copolymer solutions and triblock copolymer-anionic surfactant mixtures

Photon correlation spectroscopy and rheological measurements are performed to investigate the microscopic dynamics and mechanical responses of aqueous solutions of triblock copolymers and aqueous mixtures of triblock copolymers and anionic surfactants. Increasing the concentration of triblock copolymers results in a sharp increase in the magnitude of the complex moduli characterising the samples. This is understood in terms of the changes in the aggregation and packing behaviours of the copolymers and the constraints imposed upon their dynamics due to increased close packing. The addition of suitable quantities of an anionic surfactant to a strongly elastic copolymer solution results in a decrease in the complex moduli of the samples by several decades. It is argued that the shape anisotropy and size polydispersity of the micelles comprising mixtures cause dramatic changes in the packing behaviour, resulting in sample unjamming and the observed decrease in complex moduli. Finally, a phase diagram is constructed in the temperature-surfactant concentration plane to summarise the jamming-unjamming behaviour of aggregates constituting triblock copolymer-anionic surfactant mixtures.

Phase diagram of the Pluronic L64-H2O micellar system from mechanical spectroscopy

The linear viscoelastic properties of aqueous Pluronic L64 solutions have been investigated at high copolymer concentrations (25-62 wt%) using our modified low-frequency mechanical spectroscopy. The concentration-temperature phase diagram of the L64/H(2)O system was constructed by studying the evolution of the loss modulus and loss tangent as temperature is increased at a fixed frequency. A particular attention was focused on the dynamics approaching the beginning and ending points (39% and 60%) of the fusiform gel region in the phase diagram. The dynamics is found to have a similar viscoelastic behavior at the low and high concentrations, where a frequency scaling expected for a static percolated network is exhibited. Moreover, with increasing temperature, the system above the critical gel concentration undergoes a transition from a viscoelastic liquid to a solid gel through a percolated particle network. Therefore, our results suggest that the formation of the gel is dominated by the percolation of the particle clusters.

Strain-rate frequency superposition in large-amplitude oscillatory shear

In a recent work, Wyss [Phys. Rev. Lett. 98, 238303 (2007)] have noted a property of "soft solids" under oscillatory shear, the so-called strain-rate frequency superposition. We extend this study to the case of soft solids under large-amplitude oscillatory shear (LAOS). We show results from LAOS studies in a monodisperse hydrogel suspension, an aqueous gel, and a biopolymer suspension and show that constant strain-rate frequency sweep measurements with soft solids can be superimposed onto master curves for higher harmonic moduli with the same shift factors as for the linear viscoelastic moduli. We show that the behavior of higher harmonic moduli at low frequencies in constant strain-rate frequency sweep measurements is similar to that at large strain amplitude in strain-amplitude sweep tests. We show surface plots of the harmonic moduli and the energy dissipation rate per unit volume in LAOS for soft solids and show experimentally that the energy dissipated per unit volume depends on the first harmonic loss modulus alone, in both the linear and the nonlinear viscoelastic regime.