The second network of soft-nanoparticles in linear polymers of the same chemistry

Replacing carbon black and silica with organic fillers can reduce the weight of polymer nanocomposites. Soft-nanoparticles (SNPs), a type of organic filler, can be produced in large quantities through microemulsion polymerization. Their elasticities and diameters can be tuned for a wide range. This study aimed to investigate the linear viscoelasticity of athermal all-polystyrene nanocomposites with different molecular weights of matrix chains. Polymer nanocomposites filled with soft SNPs do not show this second plateau. But the second plateau is observed with semi-rigid SNPs if the weight percentage (φ) of SNPs is above a critical value φc. Semi-rigid refers to the intermediate elasticity compared to phenol-capped silica nanoparticles and long-chain grafted silica nanoparticles/soft SNPs. In both cases of all-polystyrene nanocomposites, the entanglement plateau and higher frequency regions remain the same. This hints that polymer chains do not thread the network. Flory-Rehner theory further confirms negligible swelling of long polymer chains into dense crosslinking networks of SNPs. The modulus of this plateau scales to the 3.8 power of φ and the 0.26 power of Mw. The critical weight percentage φc for the onset of the second plateau corresponded with the compression of the polymer coil between two SNPs. As φ increases above φc, polymer chains would be depleted from neighboring SNPs, resulting in this network of semi-rigid SNPs. Thus, we believe that the second plateau originated from this network. This study analyzed the second plateau and highlighted the variety of viscoelastic features associated with the elasticity of SNPs filled in polymer nanocomposites.