Poly(methyl methacrylate) (PMMA) possesses excellent transparency and is often used as a substitute for glass. As PMMA is brittle, addition of rubber is often used for its toughening. Loss of transparency due to light scattering from the individual particles can be avoided by using nanoparticles. To prevent nanoparticle aggregation and thereby stabilize the dispersion, steric repulsion is introduced by grafting PMMA chains to the rubber nanoparticles. In spite of this, significantly reduced transparency is often observed. To elucidate the origin of the observed behavior, we investigated poly(methyl methacrylate) (PMMA) nanocomposites with core-shell nanoparticles. The core consisted of poly(butylacrylate) (PBA) rubber of diameter approximately 60 nm and the shell consisted of grafted PMMA chains. We began our investigations with two series of samples (7.5 weight% (A) and 12 weight% (B) rubber). The graft ratio, defined as the weight ratio of the grafted PMMA chains to the PBA particles, was varied from 0.8 to 1.5. For the A series, time-temperature superposition (TTS) was found to be valid over the entire range of investigated frequencies. For the B series, TTS was approximately valid at high frequencies but not at low frequencies. Failure of TTS at low frequencies in the B series could be traced to the occurrence of a sol-gel transition at the measurement temperature. At the sol-gel transition, the dependence of the critical gel moduli on frequency was a power-law. This suggested the formation of a self-similar, fractal structure of the grafted PBA nanoparticles. Transmission electron microscopy revealed that while the as-prepared A and B samples consisted of well-dispersed nanoparticles, the B samples after the viscoelastic measurements exhibited fragmented networks of the PBA particles. This offered supporting evidence for the existence of a gel at elevated temperatures. For the B series, the critical gel temperature and the critical power-law exponent were found to decrease with increasing graft ratio. However, upon varying the graft ratio over a wider range (from 0.1 to 1.5; D series), the dependence of the critical gel temperature and the critical power-law exponent on the grafting density (graft ratio) was found to be non-monotonic. The observed non-monotonicity will be discussed in terms of the interaction energies between the polymer grafted rubber nanoparticles in a polymer matrix suggested by theory.

Sol-gel transition in a nanocomposite with polymer grafted rubber nanoparticles

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.