Reversible Thermal Stiffening in Polymer Nanocomposites

Dynamics of poly(methyl acrylate)/poly(methyl methacrylate)-grafted-Fe3O4 nanocomposites

We investigated the dynamics of nanocomposites prepared through mixing poly(methyl methacrylate) grafted Fe3O4 nanoparticles (PMMA-g-Fe3O4) with poly(methyl acrylate) (PMA). A key feature here different from previous dynamics measurements of polymer nanocomposites is the different chemistry between the matrix polymer and the polymer grafts, which introduces chemical heterogeneity. Transmission electron microscopy shows clear evidence of nanoparticle clustering due to the poor miscibility between the bulk PMA and the bulk PMMA. At the same time, broadband dielectric spectroscopy measurements detect two leading relaxations, i.e. the α and α* processes, where the α process is associated with the bulk PMA and the α* process from the PMA interacting with the grafted PMMA in the nanoparticle clustering region. Interestingly, the characteristic time of α*, τα*, is slightly slower than that of the α, τα, at high temperatures, and exhibits near Arrhenius temperature dependence at low temperatures. As a result, τα* and τα cross each other in the activation plot upon cooling and τα* ≪ τα is observed at temperatures approaching the glass transition temperature of PMA. These observations suggest the presence of component dynamics and the dynamics confinement effect between PMA and PMMA in the nanoparticle clustering region, highlighting an active interaction between PMA and PMMA at the interface despite their poor miscibility. These results thus suggest new routes to control interface dynamics through immiscible polymer pairs.

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.

The effect of dynamically heterogeneous interphases on the particle dynamics of polymer nanocomposites

The entanglements of dynamically asymmetric polymer layers influence relaxations of nanoparticles in polymer nanocomposites. In this work, the dynamics of polymer-adsorbed and polymer-grafted nanoparticles in a poly(methyl acrylate) matrix polymer was investigated using X-ray photon correlation spectroscopy (XPCS) to understand the role of chain rigidity and chemical heterogeneities in particle dynamics. Locations of dynamic heterogeneities close to nanoparticles and away from particle surfaces were examined with the comparison of adsorbed and grafted nanoparticles. Our results show that the chemical heterogeneities around dispersed nanoparticles transitioned the particle dynamics from Brownian diffusion into hyperdiffusion, and moreover, the high rigidity of chains in the chemically heterogeneous interfacial layers slowed down the particle dynamics. The hyperdiffusion measured both in grafted particles and adsorbed particles was attributed to the dense interfacial mixing of dynamically heterogeneous chains.

Intensified Nonequilibrium Effect of Polymer Nanocomposites with Decreasing Nanoparticle Size

What are the most important and decisive parameters that determine the structure and the property of polymer nanocomposites (PNCs)? Previous studies answered that controlling the nanoparticle interface is critical, which can be achieved with a choice of a compatible nanoparticle, a proper surface modification, and a change in the polymer chain length. In addition to these parameters, the processing condition of PNCs has recently emerged as an influential parameter for controlling PNC properties, suggesting the existence of the nonequilibrium effect of PNCs. In this regard, we chose the solvent as a main change in the processing condition and investigated the initial solvent-driven nonequilibrium effect of PNCs with varied nanoparticle (NP) sizes. We found that the type of the initial solvent is indeed crucial in determining the ultimate properties of the PNCs, and this becomes more influential as the size of NPs decreases. The decreasing size of NPs causes a conformational change in the adsorbed polymers from tightly packed layers to loosely dangling chains. This results in much greater differences in NP microstructures and rheological properties of PNCs, indicating a stronger nonequilibrium effect with smaller NPs.

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.

Deformation of Chemically Heterogeneous Interfacial Layers of Polymer Nanocomposites

Dynamics of entangled interfacial polymer layers around nanoparticles determine the linear rheological properties of polymer nanocomposites. In this study, the nonlinear elastic properties of nanocomposites are examined under large-amplitude oscillatory shear (LAOS) flow to reveal the effect of interfacial chemical heterogeneity on the deformation mechanism of polymer-grafted and polymer-adsorbed nanoparticle composites. Adsorbed-poly(methyl methacrylate) (PMMA) layers presented stronger interfacial stiffening and reinforcement than PMMA-grafted layers. Chemical heterogeneities of interfacial layers, provided by polymer-adsorbed and low graft density particles, deformed at smaller strains than the poly(ethylene oxide) (PEO) matrix. Interfaces of loosely bound PMMA and PEO exhibited stiffening at low strains due to the enhanced chain mixing and entanglements. These results demonstrate that chemical and dynamic heterogeneities in interfacial layers have significant importance in designing adaptive polymer nanocomposites for large shear deformation.

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.

Chemical heterogeneity in interfacial layers of polymer nanocomposites

It is well-known that particle-polymer interactions strongly control the adsorption and conformations of adsorbed chains. Interfacial layers around nanoparticles consisting of adsorbed and free matrix chains have been extensively studied to reveal their rheological contribution to the behavior of nanocomposites. This work focuses on how chemical heterogeneity of the interfacial layers around the particles governs the microscopic mechanical properties of polymer nanocomposites. Low glass-transition temperature composites consisting of poly(vinyl acetate) coated silica nanoparticles in poly(ethylene oxide) and poly(methyl acrylate) matrices, and of poly(methyl methacrylate) silica nanoparticles in a poly(methyl acrylate) matrix are examined using rheology and X-ray photon correlation spectroscopy. We demonstrate that miscibility between the adsorbed and matrix chains in the interfacial layers led to the observed unusual reinforcement. We suggest that packing of chains in the interfacial regions may also contribute to the reinforcement in the polymer nanocomposites. These features may be used in designing mechanically adaptive composites operating at varying temperature.

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.