Suppression of Segmental Chain Dynamics on a Particle's Surface in Well-Dispersed Polymer Nanocomposites.

The Rouse dynamics of polymer chains in model nanocomposite polyethylene oxide/silica nanoparticles (NPs) was investigated using quasielastic neutron scattering. The apparent Rouse rate of the polymer chains decreases as the particle loading increases. However, there is no evidence of an immobile segment population on the probed time scale of tens of ps. The slowing down of the dynamics is interpreted in terms of modified Rouse models for the chains in the NP interphase region. Thus, two chain populations, one bulk-like and the other characterized by a suppression of Rouse modes, are identified. The spatial extent of the interphase region is estimated to be about twice the adsorbed layer thickness, or ≈2 nm. These findings provide a detailed description of the suppression of the chain dynamics on the surface of NPs. These results are relevant insights on surface effects and confinement and provide a foundation for the understanding of the rheological properties of polymer nanocomposites with well-dispersed NPs.

Dynamically bonded cellulose nanocrystal hydrogels: Structure, rheology and fire prevention performance.

Flame retardant composite hydrogels offer many advantages over conventional flame retardants, such as high water-retention capacity, enhanced fire resistance, and mechanical tunability. Herein, we developed flame-retardant dynamic covalent hydrogels using wood-derived cellulose nanocrystals (CNCs) crosslinked with boronate ester bonds, addressing environmental and health issues associated with the presence of non-biodegradable synthetic polymer and/or inorganic nanoparticle components in the existing systems. Our rheological investigation shows a liquid-to-soft-solid transition of CNC dispersions with tunable network elasticity ranging between ≈ 0.2 kPa to 3.5 kPa and an immediate self-healing ability. Coating pine wood with these hydrogels delayed ignition by about 30 s compared to native wood, and achieved a remarkable limiting oxygen index of 64.5 %. Also, the increased borax content of the gels was found to decrease and delay the first peak of the heat release rate up to 40 s, causing an increase in the fire retardancy index by 277 %. We correlate the microstructure and rheological behavior with the fire prevention mechanisms for the rational design of sustainable fire-retardant materials, and the results showcased a circular use of plant-based dynamic gels to prevent wood fires, even after drying- a feature lacking in conventional hydrogels.

Cellulose nanocrystal and Pluronic L121-based thermo-responsive composite hydrogels.

Cellulose nanocrystal (CNC) is a promising sustainable material with its biocompatibility, high aspect ratio, and mechanical strength. CNC-based systems have potential applications in various fields including biosensors, packaging, coating, energy storage, and pharmaceuticals. However, turning CNC into smart systems remains a challenge due to the lack of stimuli-responsiveness, limitation in compatibility with hydrophobic matrices, and their agglomeration tendency. In this work, a thermo-responsive nanocomposite system is constructed with CNCs and polymersome forming Pluronic L121 (L121), and its phase behavior and mechanical properties are investigated in detail. Two different CNC concentration (4 % and 5 %) is studied by changing the L121 concentration (1-20 %) to understand the effect of unimers and polymersomes on the CNC network. At dilute L121 concentrations (1-5 %), the composite system becomes softer but more fragile below the transition temperature. However, it becomes much stronger at higher L121 concentrations (10-20 %), and a gel network is obtained above the transition temperature. Interestingly, the elastically reinforced CNC gels exhibit greater resistance to microstructural breakdown at large strains due to the soft and deformable nature of the large polymersomes. It is also found that the gelation temperature for hydrogels is tunable with increasing L121 concentration, and the nanocomposite hydrogels displayed thermo-reversible rheological behavior.

Enhanced ionic conductivity and mechanical strength in nanocomposite electrolytes with nonlinear polymer architectures.

Solvent-free polymer-based electrolytes (SPEs) have gained significant attention to realize safer and flexible lithium-ion batteries. Among all polymers used for preparing SPEs electrolytes, poly(ethylene oxide), a biocompatible and biodegradable polymer, has been the most prevalent one mainly because of its high ionic conductivity in the molten state, the capability for the dissolution of a wide range of different lithium salts as well as its potential for the environmental health and safety. However, linear PEO is highly semicrystalline at room temperature and thus exhibits weak mechanical performance. Addition of nanoparticles enhances the mechanical strength and effectively decreases the crystallization of linear PEO, yet enhancement in mechanical performance often results in decreased ionic conductivity when compared to the neat linear PEO-based electrolytes; new strategies for decoupling ionic conductivity from mechanical reinforcement are urgently needed. Herein, we used lithium bis(trifluoromethane-sulfonyl)-imide (LiTFSI) salts dissolved in various nonlinear PEO architectures, including stars (4-arms and 8-arms) and hyperbranched matrices, and SiO2 nanoparticles (approximately equal to 50 nm diameter) as fillers. Compared to the linear PEO chains, the room temperature crystallinity was eliminated in the branched PEO architectures. The electrolytes with good dispersion of the nanoparticles in the nonlinear PEOs significantly enhanced ionic conductivity, specifically by approximately equal to 40% for 8-arm star, approximately equal to 28% for 4-arms star, and approximately equal to %16 for hyperbranched matrices, with respect to the composite electrolyte with the linear matrix. Additionally, the rheological results of the SPEs with branched architectures show more than three orders of magnitude enhancement in the low-frequency moduli compared to the neat linear PEO/Li systems. The obtained results demonstrate that the solvent-free composite electrolytes made of branched PEO architectures can be quite promising especially for irregularly shaped and environmentally benign battery applications suitable for medical implants, wearable devices, and stretchable electronics, which require biodegradability and biocompatibility.

Decoding Polymer Architecture Effect on Ion Clustering, Chain Dynamics, and Ionic Conductivity in Polymer Electrolytes.

Poly(ethylene oxide) (PEO)-based polymer electrolytes are a promising class of materials for use in lithium-ion batteries due to their high ionic conductivity and flexibility. In this study, the effects of polymer architecture including linear, star, and hyperbranched and salt (lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI)) concentration on the glass transition (T g), microstructure, phase diagram, free volume, and bulk viscosity, all of which play a significant role in determining the ionic conductivity of the electrolyte, have been systematically studied for PEO-based polymer electrolytes. The branching of PEO widens the liquid phase toward lower salt concentrations, suggesting decreased crystallization and improved ion coordination. At high salt loadings, ion clustering is common for all electrolytes, yet the cluster size and distribution appear to be strongly architecture-dependent. Also, the ionic conductivity is maximized at a salt concentration of [Li/EO ≈ 0.085] for all architectures, and the highly branched polymers displayed as much as three times higher ionic conductivity (with respect to the linear analogue) for the same total molar mass. The architecture-dependent ionic conductivity is attributed to the enhanced free volume measured by positron annihilation lifetime spectroscopy. Interestingly, despite the strong architecture dependence of ionic conductivity, the salt addition in the highly branched architectures results in accelerated yet similar monomeric friction coefficients for these polymers, offering significant potential toward decoupling of conductivity from segmental dynamics of polymer electrolytes, leading to outstanding battery performance.

Shear-Triggered Release of Lipid Nanoparticles from Tissue-Mimetic Hydrogels.

Shear forces are involved in many cellular processes and increase remarkably in the case of cardiovascular diseases in the human body. While various stimuli, such as temperature, pH, light, and electromagnetic fields, have been considered for on-demand release, developing drug delivery systems that are responsive to physiological-level shear stresses remains as a challenge. For this purpose, liposomes embedded in hydrogel matrices are promising as they can dynamically engage with their environment due to their soft and deformable structure. However, for optimal drug delivery systems, the interaction between liposomes and the surrounding hydrogel matrix, and their response to the shear should be unraveled. Herein, we used unilamellar  1,2-Dimyristoyl-sn-glycero-3phosphocholine (DMPC) liposomes as drug nanocarriers and polyethylene (glycol) diacrylate (PEGDA) hydrogels having different elasticities, from 1 to 180 Pa, as extracellular matrix (ECM)-mimetic matrices to understand shear-triggered liposome discharge from hydrogels. The presence of liposomes provides hydrogels with temperature-controlled water uptake which is sensitive to membrane microviscosity. By systematically applying shear deformation from linear to nonlinear deformation regimes, the liposome release under transient and cyclic stimuli is modulated. Considering that shear force is commonly encountered in biofluid flow, these results will provide fundamental basis for rational design of shear-controlled liposomal drug delivery systems.

Polymer architecture effect on rheology and segmental dynamics in poly (methyl methacrylate)-silica nanocomposite melts.

Architecturally different polymer chains lead to fundamentally different rheological responses and internal dynamics, which can be utilized to rationalize advanced thermoplastic nanocomposites with tunable mechanical behavior. In this work, three model poly (methyl methacrylate) (PMMA) polymers with linear, bottlebrush, and star architectures with the same total molar mass were investigated in their neat form, and nanocomposites with well-dispersed silica nanoparticles using rheology and broadband dielectric spectroscopy (BDS). The master curves of the dynamic moduli obtained by time-temperature superposition (TTS) over the entire range from the Rouse regime to the terminal flow and a sequence of significantly different relaxation modes were observed for the samples with linear and branch chains. While linear chains form an entangled polymer network, the branched bottlebrush, and star chains show a viscoelastic response with no sign of rubbery entanglement plateau and a weak arm relaxation regime between Rouse and terminal flow, akin to other branched polymers. Moreover, branched chains showed a higher fragility index (m = 3.46 for the bottlebrush and 5.36 for the star) compared to linear chains (m = 3.29) due to dynamical heterogeneities induced by arm relaxation. The addition of nanoparticles affects only the terminal relaxation regime, where the whole chain motion is hindered by the attractive nanoparticles. The dynamics of the polymer segment were investigated by performing broadband dielectric spectroscopy (BDS) at a frequency range from 10-2 Hz to 107 Hz. The results revealed more than 10 times slower segmental relaxation for the star homopolymers and a slowdown in the α-relaxation process for all three architectures in their composite form. The dynamical slowdown in the composites is temperature dependent and more pronounced at low temperatures (leading to approximately equal to 80 times slower dynamics for nanocomposite with bottlebrush PMMA at 150 °C) due to prolonged relaxation of the interfacial polymer compared to the matrix chains. The results from this study have practical applications in fields such as gas separation and polymeric electrolyte membranes, where simultaneous improvement of segmental mobility and mechanical moduli is highly desired.

Multiscale Dynamics of Lipid Vesicles in Polymeric Microenvironment.

Understanding dynamic and complex interaction of biological membranes with extracellular matrices plays a crucial role in controlling a variety of cell behavior and functions, from cell adhesion and growth to signaling and differentiation. Tremendous interest in tissue engineering has made it possible to design polymeric scaffolds mimicking the topology and mechanical properties of the native extracellular microenvironment; however, a fundamental question remains unanswered: that is, how the viscoelastic extracellular environment modifies the hierarchical dynamics of lipid membranes. In this work, we used aqueous solutions of poly(ethylene glycol) (PEG) with different molecular weights to mimic the viscous medium of cells and nearly monodisperse unilamellar DMPC/DMPG liposomes as a membrane model. Using small-angle X-ray scattering (SAXS), dynamic light scattering, temperature-modulated differential scanning calorimetry, bulk rheology, and fluorescence lifetime spectroscopy, we investigated the structural phase map and multiscale dynamics of the liposome-polymer mixtures. The results suggest an unprecedented dynamic coupling between polymer chains and phospholipid bilayers at different length/time scales. The microviscosity of the lipid bilayers is directly influenced by the relaxation of the whole chain, resulting in accelerated dynamics of lipids within the bilayers in the case of short chains compared to the polymer-free liposome case. At the macroscopic level, the gel-to-fluid transition of the bilayers results in a remarkable thermal-stiffening behavior of polymer-liposome solutions that can be modified by the concentration of the liposomes and the polymer chain length.

Tissue-Like Optoelectronic Neural Interface Enabled by PEDOT:PSS Hydrogel for Cardiac and Neural Stimulation.

Optoelectronic biointerfaces have made a significant impact on modern science and technology from understanding the mechanisms of the neurotransmission to the recovery of the vision for blinds. They are based on the cell interfaces made of organic or inorganic materials such as silicon, graphene, oxides, quantum dots, and π-conjugated polymers, which are dry and stiff unlike a cell/tissue environment. On the other side, wet and soft hydrogels have recently been started to attract significant attention for bioelectronics because of its high-level tissue-matching biomechanics and biocompatibility. However, it is challenging to obtain optimal opto-bioelectronic devices by using hydrogels requiring device, heterojunction, and hydrogel engineering. Here, an optoelectronic biointerface integrated with a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), PEDOT:PSS, hydrogel that simultaneously achieves efficient, flexible, stable, biocompatible, and safe photostimulation of cells is demonstrated. Besides their interfacial tissue-like biomechanics, ≈34 kPa, and high-level biocompatibility, hydrogel-integration facilitates increase in charge injection amounts sevenfolds with an improved responsivity of 156 mA W-1 , stability under mechanical bending , and functional lifetime over three years. Finally, these devices enable stimulation of individual hippocampal neurons and photocontrol of beating frequency of cardiac myocytes via safe charge-balanced capacitive currents. Therefore, hydrogel-enabled optoelectronic biointerfaces hold great promise for next-generation wireless neural and cardiac implants.

Thermoresponsive and Injectable Composite Hydrogels of Cellulose Nanocrystals and Pluronic F127.

Thermoresponsive amphiphilic Pluronic F127 triblock copolymer solutions have been widely investigated in smart biomaterial applications due to the proximity of its critical gel temperature to human body temperature. Meanwhile, cellulose nanocrystals (CNCs) have quickly become the focus of many drug delivery and tissue engineering applications due to their biocompatibility, abundance, ability to conjugate with drug molecules, and superior rheological properties. Herein, we investigate the phase behavior and thermo-rheological properties of the composite hydrogels containing cellulose nanocrystals (up to 5% by weight) and the temperature responsive Pluronic F127. Our results revealed an unprecedented role of CNC network formation on micellization and gelation behavior of the triblock copolymer. Linear and nonlinear rheological analysis suggest that at low and moderate nanocrystal loadings (1-3% by weight), the composite gel remarkably becomes softer and deformable compared to the neat Pluronic F127 gels. The softening effect results from the disruption of the close packed micelles by the rodlike CNCs. At high concentrations, however, the nanocrystals form their own network and the micelles are trapped within the CNC meshes. As a result, the original (neat F127) hard-gel modulus is recovered at 4 to 5% nanocrystal loading, yet the composite gel is much more deformable (and tougher) in the presence of the CNC network. Our temperature sweep experiments show that the CNC addition up to 3% does not change the rapid thermal gelation of the F127 solutions; therefore, these composites are suitable for smart drug delivery systems. On the other hand, at higher CNC concentrations, abrupt viscosity transition is not observed, rather the composite gels smoothly thicken with temperature in contrast to thermal thinning of the aqueous neat CNC. Thus, they can be used as smartly adaptive biolubricants and bioviscostatic materials.

Influence of Kosmotrope and Chaotrope Salts on Water Structural Relaxation.

The structural relaxation in water solutions of kosmotrope (structure maker) and chaotrope (structure breaker) salts, namely sodium chloride, potassium chloride, and cesium chloride, were studied through quasielastic neutron scattering measurements. We found that the collective dynamics relaxation time at the structure factor peak obtained using heavy water solutions shows a distinctively different behavior in the kosmotrope as opposed to the chaotrope solutions, increasing with the salt concentration in the former and decreasing in the latter. In both cases the trends are proportional to the concentration dependence of the relative viscosity of the solutions. These results indicate that kosmotropes and chaotropes influence the solution's viscosity by impacting in opposite ways the hydrogen bond network of water, strengthening it in one case and softening it in the other.

Surfactant Driven Liquid to Soft Solid Transition of Cellulose Nanocrystal Suspensions.

Cellulose nanocrystals (CNCs) have recently attracted wide interest due to their abundance, biocompatibility, and extraordinary physical properties. In particular, easy manipulation of their surface properties, hydrophilicity, and high aspect ratio make them ideal rheology modifiers; yet, the gelation mechanisms and microscopic origin of the complex rheological behavior in the presence of secondary components, such as polymers and surfactants, are far from well understood. In this work, we used light scattering, small-angle neutron scattering, and bulk rheology to study the phase behavior and mechanical behavior of aqueous CNC solutions in the presence of cationic 1-decyl trimethyl imidazolium chloride and 1-decyl trimethyl imidazolium ferric tetrachloride. The micelles of these surfactants form at similar cmc's (about 50 mM) and adopt identical hydrodynamic sizes (on the order of a few nanometers) and prolate-shaped ellipsoids but vary in their intermicelle interactions (charged vs neutral), thus allowing us to clarify the unprecedented effect of the surfactant micelle charge on the gel behavior of the aqueous CNC-surfactant complexes. Our results show that the positively charged micelles greatly strengthen the gel network while excessive free micelles weaken the gels due to repulsive micelle-micelle interaction. In the meantime, analysis of the transition from linear to nonlinear deformation regimes suggests that the gels gradually become more fragile with surfactant concentrations due to electrostatic repulsion of the charged micelles. Such a surfactant concentration-dependent gel fragility was not observed in the presence of the neutral micelles. These results provide a great step further in our understanding of the phase behavior and rheology of complex CNC-surfactant mixtures and obtaining biocompatible hydrogels with tunable mechanical properties.

Nanoscale Particle Motion Reveals Polymer Mobility Gradient in Nanocomposites.

Polymer mobility near nanoparticle surfaces has been extensively discussed; however, direct experimental observation in the nanocomposite melts has been a difficult task. Here, by taking advantage of large dynamical asymmetry between the miscible matrix and surface-bound polymers, we highlighted their interphases and studied the resulting effect on the nanoparticle relaxation using X-ray photon correlation spectroscopy. The local mobility gradient is signified by an unprecedented increase in the relaxation time at length scales on the order of polymer radius of gyration. The effect is accompanied by a transition from simple diffusive to subdiffusive behavior in accord with viscous and entangled dynamics of polymers in the matrix and in the interphase, respectively. Our results demonstrate that the nanoparticle-induced polymer mobility changes in the interphases of nanocomposite melts can be extracted from the length-scale-dependent slow particle motion.

Multiscale Polymer Dynamics in Hierarchical Carbon Nanotube Grafted Glass Fiber Reinforced Composites.

Carbon nanotube (CNT) grafted glass fiber reinforced epoxy nanocomposites (GFRP) present a range of stiffnesses (MPa to GPa) and length scales (µm to nm) at the fiber-matrix interface. The contribution of functionalized CNT networks to the local and bulk polymer dynamics is studied here by using a combination of torsion dynamical mechanical thermal analysis (DMTA), positron annihilation lifetime spectroscopy (PALS), and neutron scattering (NS) measurements. DMTA measurements highlight a reduction in the storage modulus (G') in the rubbery region and an asymmetric broadening of the loss modulus (G″) peak in the α-transition region. NS measurements show a suppressed hydrogen mean-square displacement (MSD) in the presence of glass fibers but a higher hydrogen MSD after grafting functionalized CNTs onto fiber surfaces. PALS measurements show greater free volume characteristics in the presence of the functionalized CNT modified composites, supporting the view that these interface layers increase polymer mobility. While NS and DMTA are sensitive to different modes of chain dynamics, the localization of functionalized nanotubes at the fiber interface is found to affect the distribution of polymer relaxation modes without significantly altering the thermally activated relaxation processes.

Dynamics of Architecturally Engineered All-Polymer Nanocomposites.

We present nanocomposite materials formed by using glassy star-shaped polymers as nanofillers and dispersing them in soft matrices. The resulting "architecturally engineered" polymer nanocomposites structurally reside between the linear homopolymer blends and the conventional polymer nanocomposites with inorganic fillers, inducing reinforcement, which can be as strong as that of solid nanoparticles, or softening depending on the compactness and concentration of the nanoparticles. Such behavior can be traced back to the dynamical features at the local segmental and the chain level, which we investigated using neutron scattering over a wide range of time and length scales in the glassy and melt states of the nanocomposites. The local and segmental dynamics as well as the degree of chain-chain entanglements are all modified by the star-shaped fillers. The presented approach to tuning the physical properties of all-polymer-based nanocomposites is readily adaptable to other polymer architectures with immediate applications in numerous areas including gas separation membranes, tissue engineering, drug delivery, and functional coatings.

Nanoscale Particle Motion in Attractive Polymer Nanocomposites.

Using x-ray photon correlation spectroscopy, we examined the slow nanoscale motion of silica nanoparticles individually dispersed in an entangled poly (ethylene oxide) melt at particle volume fractions up to 42%. The nanoparticles, therefore, serve as both fillers for the resulting attractive polymer nanocomposites and probes for the network dynamics therein. The results show that the particle relaxation closely follows the mechanical reinforcement in the nanocomposites only at the intermediate concentrations below the critical value for the chain confinement. Quite unexpectedly, the relaxation time of the particles does not further slow down at higher volume fractions-when all chains are practically on the nanoparticle interface-and decouples from the elastic modulus of the nanocomposites that further increases orders of magnitude.

Chain dynamics and nanoparticle motion in attractive polymer nanocomposites subjected to large deformations.

The effect of large deformation on the chain dynamics in attractive polymer nanocomposites was investigated using neutron scattering techniques. Quasi-elastic neutron backscattering measurements reveal a substantial reduction of polymer mobility in the presence of attractive, well-dispersed nanoparticles. In addition, large deformations are observed to cause a further slowing down of the Rouse rates at high particle loadings, where the interparticle spacings are slightly smaller than the chain dimensions, i.e. in the strongly confined state. No noticeable change, however, was observed for a lightly confined system. The reptation tube diameter, measured by neutron spin echo, remained unchanged after shear, suggesting that the level of chain-chain entanglements is not significantly affected. The shear-induced changes in the interparticle bridging reflect the slow nanoparticle motion measured by X-ray photon correlation spectroscopy. These results provide a first step for understanding how large shear can significantly affect the segmental motion in nanocomposites and open up new opportunities for designing mechanically responsive soft materials.

Small Particle Driven Chain Disentanglements in Polymer Nanocomposites.

Using neutron spin-echo spectroscopy, x-ray photon correlation spectroscopy, and bulk rheology, we studied the effect of particle size on the single-chain dynamics, particle mobility, and bulk viscosity in athermal polyethylene oxide-gold nanoparticle composites. The results reveal a ≈25% increase in the reptation tube diameter with the addition of nanoparticles smaller than the entanglement mesh size (≈5 nm), at a volume fraction of 20%. The tube diameter remains unchanged in the composite with larger (20 nm) nanoparticles at the same loading. In both cases, the Rouse dynamics is insensitive to particle size. These results provide a direct experimental observation of particle-size-driven disentanglements that can cause non-Einstein-like viscosity trends often observed in polymer nanocomposites.

Microscopic Chain Motion in Polymer Nanocomposites with Dynamically Asymmetric Interphases.

Dynamics of the interphase region between matrix and bound polymers on nanoparticles is important to understand the macroscopic rheological properties of nanocomposites. Here, we present neutron scattering investigations on nanocomposites with dynamically asymmetric interphases formed by a high-glass transition temperature polymer, poly(methyl methacrylate), adsorbed on nanoparticles and a low-glass transition temperature miscible matrix, poly(ethylene oxide). By taking advantage of selective isotope labeling of the chains, we studied the role of interfacial polymer on segmental and collective dynamics of the matrix chains from subnanoseconds to 100 nanoseconds. Our results show that the Rouse relaxation remains unchanged in a weakly attractive composite system while the dynamics significantly slows down in a strongly attractive composite. More importantly, the chains disentangle with a remarkable increase of the reptation tube size when the bound polymer is vitreous. The glassy and rubbery states of the bound polymer as temperature changes underpin the macroscopic stiffening of nanocomposites.

Structure and Entanglement Factors on Dynamics of Polymer-Grafted Nanoparticles.

Nanoparticles functionalized with long polymer chains at low graft density are interesting systems to study structure-dynamic relationships in polymer nanocomposites since they are shown to aggregate into strings in both solution and melts and also into spheres and branched aggregates in the presence of free polymer chains. This work investigates structure and entanglement effects in composites of polystyrene-grafted iron oxide nanoparticles by measuring particle relaxations using X-ray photon correlation spectroscopy. Particles within highly ordered strings and aggregated systems experience a dynamically heterogeneous environment displaying hyperdiffusive relaxation commonly observed in jammed soft glassy systems. Furthermore, particle dynamics is diffusive for branched aggregated structures which could be caused by less penetration of long matrix chains into brushes. These results suggest that particle motion is dictated by the strong interactions of chains grafted at low density with the host matrix polymer.