Understanding structure-dynamics-property relationship of polymer nanocomposites: effect of size
Date
2022
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Publisher
University of Delaware
Abstract
Polymer nanocomposites (PNC), materials consisting of polymers with nano-sized fillers dispersed in the polymer matrix, exhibit significant changes to the overall system properties such as the mechanical, optical, and electrical properties relative to the host polymer. These materials are already widely applied and studied because of their potentials in tunability, responsiveness, and functionality. However, sufficient understanding of the structure-dynamics-property relationships of these materials is lacking. This work seeks to quantify molecular level physicochemical properties, such as the nanoscopic structure and dynamics, of PNCs and their relationship to the macroscopic rheological behavior. The fundamental understanding of this relationship will aid in developing a map of the effects in the vast parameter space of PNC on its bulk properties. ☐ Here, silica nanoparticles (SiO2) in polyethylene oxide (PEO) of 20 nm in diameter are investigated as a model attractive PNC using scattering methods and rheology to connect the molecular level structure and dynamics to the macroscopic properties. First, small angle scattering techniques are employed to investigate the size, shape, polydispersity, and the dispersion of the nanoparticles. Moreover, the conformation of the polymer chains is also investigated using small angle neutron scattering via contrast matching SiO2 with hydrogenated and deuterated PEO. The Gaussian nature of the structure of PEO has been confirmed independent of the loading of SiO2, even in the presence of aggregation. ☐ Bulk rheological properties are investigated for the nanocomposite in the linear viscoelastic regime. The viscosity and moduli behaviors are compared with previous studies on PNC. The samples approach the transition from liquid to gel-like behavior with increase in the loading of the nanoparticle. The viscosity behavior with the loading of the NP is also observed and is connected to the standard theories developed for bulk polymers to connect it to the microscopic dynamics. ☐ Dynamic neutron scattering techniques, such as quasi elastic neutron scattering and spin echo, are used to measure the effect of particle loading on the Rouse dynamics and reptation tube diameter. Data are interpreted in terms of standard theories developed for bulk polymers to understand the effect on the microscopic dynamics of the polymer from the nanoparticle. Raising the loading of the SiO2 slows down the Rouse dynamics of PEO; this effect is enhanced with smaller NPs which increase the interfacial surface for the same particle loading. ☐ In these PNCs, we observe deviations from the predicted Einstein theory on viscosity for dispersion of particle in a fluid. A remarkable fluidification takes place for small loading before a reinforcement of the PNC is observed with the increase in the NP volume fraction. This behavior is mirrored by the segmental dynamics of the polymer chains. However, this latter microscopic observation cannot fully explain the observed rheological behavior. The observed fluidification could be however rationalized by the small NP acting as solvent molecules in the polymer matrix. Finally, the behavior of the parameters describing the chain dynamics as a function of the particle loading, when compared among different NP sizes, demonstrates that the surface to volume ratio, or similarly the confining parameter, rather than NP volume fraction, is a key parameter for predicting the dynamical properties of the matrix.
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Keywords
Neutron scattering, Polymer nanocomposites, Rheology, Soft matter