Immiscible Polymer-Filler Systems for Tunable Mechanical Properties

Abstract A model system consisting of a 50/50 immiscible mixture of polyethylene glycol (PEG) and polytetrahydrofuran (PTHF), with covalently bound nano-silica reinforcing particles, was used to investigate how the distribution of particles affects viscoelastic properties. The rubbery polymers were generated by chain extending from their respective oligomers through hydroxyl end groups with 1,6-diisocyanato hexane. The viscosities of the resulting PEG and PTHF polymers were 32 and 1000 Pa-s, respectively. Setting the overall SiO2 concentration to 10 phr, we explored three different silica distributions: (1) “all-in-PEG” ‒ 20 phr in PEG and neat PTHF; (2) “uniform” ‒ 10 phr in both PEG and PTHF; and (3) “all-in-PTHF” ‒ neat PEG and 20 phr in PTHF. When compared to the uniform system, the storage and loss shear moduli, and the viscosity of the all-in- PTHF system showed a 20-fold increase of the stiffness at low strain, yet nearly the same viscosity at high strain. Similarly, the storage and loss moduli of all-in-PEG mixtures were roughly 1/10 that of the uniform system. The surprisingly high modulus and high viscosity of the all-in-PTHF system can be understood as the entire PTHF regions themselves becoming solid filler.