In this project, we showed that chemical heterogeneity of interfacial layers around particles governs the mechanical properties and reinforcement of polymer nanocomposites. Blends that have good miscibility and compatibility with nanoparticles and differences in their glass-transition temperatures (T_g’s) are chosen carefully to study reinforcement behavior of particularly low T_g matrix polymers. We measured the viscoelastic response of nanocomposites below and above T_g of the polymer that is adsorbed on particles and also has asymmetric composition relative to the matrix polymer of poly(ethylene oxide) (PEO) and poly(methyl acrylate) (PMA). Low glass-transition temperature composites consisting of poly(vinyl acetate) (PVAc) coated silica nanoparticles in PEO and PMA matrices; and of poly(methyl methacrylate) (PMMA) silica nanoparticles in PMA matrix are examined using rheology and X-ray photon correlation spectroscopy. We found that mixing and entanglements between the two miscible polymers in the interphase layer is a factor governing the collective particle relaxations and the hyper-diffusive mode of adsorbed particles, which in turn contribute to the unusual reinforcement measured in the short adsorbed chains (Fig. 1).

A general finding from these different systems is that good mixing of polymer blends in interfaces resulted in reinforcement of nanocomposites. In a semi-crystalline (PEO) matrix, miscibility of short chains was more feasible with the amorphous chains and the highest reinforcement was obtained. The faster particle relaxations in the reinforced composite led us to conclude that adsorbed chain mobility is an important parameter in the reinforcement mechanism. In an amorphous (PMA) system, dynamic coupling, that was dictated by entanglements and packing of chains in the interphase layer, was enhanced with the short matrix chains. Particle relaxations slowed down with the longer adsorbed chains, which in fact revealed the role of adsorbed chain length in the reinforcement mechanism. In summary, chemical heterogeneities at interphases control the reinforcement in low T_g polymer composites. Our results underpin the polymer bridging effect through the interfacial layers which is shown to be more effective with the short matrix and long adsorbed chains.

We collaborated with our colleagues on simulations of viscoelastic properties of polymer-grafted particles where brush conformations differ as collapsed and stretched. We demonstrate that glassy chains tethered onto nanoparticles dispersed within a soft (liquid–like) polymer matrix can significantly tune the overall viscoelastic and dynamic properties of the nanocomposite system. Using molecular dynamics, a composite system containing nanoparticles grafted with high–T_g polymer chains in a low–T_g matrix polymer is simulated. We consider two main conformational states for the high–T_g brushes: collapsed (onto the nanoparticle, and therefore, phase–separated from the matrix) and stretched. We found that in the case of stretched chains, the composite has significantly higher storage modulus than the system with collapsed brushes. Detailed analysis showed that the stretched conformational state of grafted chains reduced matrix chain mobility significantly. Finally, by simulating structures with a range of grafted chain conformations bounded by fully collapsed and fully stretched states, we demonstrated that grafted chain conformation could be used to control the viscoelastic response of the whole nanocomposite system. Chain conformation close to the particle surfaces then substantiates the intermixing of chains. Thus, this work led to better understanding on how chemical heterogeneity and chain architectures can be used to control the viscosity and viscoelastic moduli in polymer nanocomposites of bare particles.

In another work, the role of chain rigidity in this interfacially controlled reinforcement in PEO composites is investigated. We showed that particles adsorbed with less rigid polymers improved the mechanical properties of composites (Fig. 2). The storage modulus of PMMA with the lower segmental rigidity is higher than the P2VP-adsorbed samples at low frequencies. Fig. 3 shows that particles adsorbed with P2VP show a softening response, whereas PC with its lowest rigidity is more reinforced than interfaces with PMMA at 20 and 40 wt% loadings. These results emphasize that rheology results are sensitive to the molecular rigidity and dynamic heterogeneities of interfacial layers around nanoparticles.

The stability of adsorbed chains determines both the mechanical performance (Fig. 4) and the life-time of nanocomposites. We examined the stability of interfacial layers with large-amplitude oscillatory shear (LAOS) experiments. Our particles adsorbed with PMMA presented an overshoot in storage modulus in strain sweep tests. This behavior is attributed to the energy dissipation due to arrangement of chains in interfacial layers which was not observed with the PC or P2VP interfacial layers. Fig. 5 shows the strain sweep behavior of composites with different rigidity of adsorbed chains. It was shown that PMMA chains can be deformed and arranged themselves into a denser entangled state through strain-softening mechanism with e₃<0. PC and P2VP chains on the other hand present linear elastic behavior with e₃=0. We completed the LAOS analysis on the PMA and PEO composites and also with the bare particles and now in the process of writing the next manuscript.

Last Modified: 10/17/2018
Modified by: Pinar Akcora