Universal Polymeric-to-Colloidal Transition in Melts of Hairy Nanoparticles

Two different classes of hairy self-suspended nanoparticles in the melt state, polymer-grafted nanoparticles (GNPs) and star polymers, are shown to display universal dynamic behavior across a broad range of parameter space. Linear viscoelastic measurements on well-characterized silica-poly(methyl acrylate) GNPs with a fixed core radius (R-core) and grafting density (or number of arms f) but varying arm degree of polymerization (N-arm) show two distinctly different regimes of response. The colloidal Regime I with a small N-arm (large core volume fraction) is characterized by predominant low-frequency solidlike colloidal plateau and ultraslow relaxation, while the polymeric Regime II with a large N-arm (small core volume fractions) has a response dominated by the starlike relaxation of partially interpenetrated arms. The transition between the two regimes is marked by a crossover where both polymeric and colloidal modes are discerned albeit without a distinct colloidal plateau. Similarly, polybutadiene multiarm stars also exhibit the colloidal response of Regime I at very large f and small N-arm. The star arm retraction model and a simple scaling model of nanoparticle escape from the cage of neighbors by overcoming a hopping potential barrier due to their elastic deformation quantitatively describe the linear response of the polymeric and colloidal regimes, respectively, in all these cases. The dynamic behavior of hairy nanoparticles of different chemistry and molecular characteristics, investigated here and reported in the literature, can be mapped onto a universal dynamic diagram of f/[R-core(3)/nu(0))1/4] as a function of (N-arm nu 0f)/(R-core(3)), where nu(0) is the monomeric volume. In this diagram, the two regimes are separated by a line where the hopping potential Delta Uhop is equal to the thermal energy, kBT. Delta U-hop can be expressed as a function of the overcrowding parameter x (i.e., the ratio of f to the maximum number of unperturbed chains with N-arm that can fill the volume occupied by the polymeric corona); hence, this crossing is shown to occur when x = 1. For x > 1, we have colloidal Regime I with an overcrowded volume, stretched arms, and Delta U-hop > k(B)T, while polymeric Regime II is linked to x < 1. This single-material parameter x can provide the needed design principle to tailor the dynamics of this class of soft materials across a wide range of applications from membranes for gas separation to energy storage.

Automated summary

The study reveals universal dynamic behavior of hairy self-suspended nanoparticles in the melt state, including polymer-grafted nanoparticles and star polymers. Different response regimes are observed, with one characterized by low-frequency solidlike colloidal plateau and ultraslow relaxation, and the other dominated by starlike relaxation of partially interpenetrated arms. Transition between these regimes is marked by a crossover where both polymeric and colloidal modes are discerned.