Toughening fragile matter: mechanical properties of particle solids assembled from polymer-grafted hybrid particles synthesized by ATRP

• Published in: Soft matter Vol. 8; no. 15; pp. 472 - 482
• Main Authors: Choi, Jihoon, Hui, Chin Ming, Pietrasik, Joanna, Dong, Hongchen, Matyjaszewski, Krzysztof, Bockstaller, Michael R
• Format: Journal Article
• Language: English
• Published: 01.01.2012
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c2sm06915f

The effect of polymer-graft modification on the structure formation and mechanical characteristics of inorganic (silica) nanoparticle solids is evaluated as a function of the degree of polymerization of surface-grafted chains. A transition from hard-sphere-like to polymer-like mechanical characteristics of particle solids is observed for increasing degree of polymerization of grafted chains. The elastic modulus of particle solids increases by about 200% and levels off at intermediate molecular weights of surface-grafted chains, a trend that is rationalized as a consequence of the elastic modulus being determined by dispersion interactions between the polymeric grafts. A pronounced increase (of about one order of magnitude) of the fracture toughness of particle solids is observed as the degree of polymerization of grafted chains exceeds a threshold value that is similar for both polystyrene and poly(methyl methacrylate) grafts. The increased resistance to fracture is interpreted as a consequence of the existence of entanglements between surface-grafted chains that give rise to energy dissipation during fracture through microscopic plastic deformation and craze formation. Within the experimental uncertainty the transition to polymer-like deformation characteristics is captured by a mean field scaling model that interprets the structure of the polymer shell of polymer-grafted particles as effective two-phase systems consisting of a stretched inner region and a relaxed outer region. The model is applied to predict the minimum degree of polymerization needed to induce polymer-like mechanical characteristics and thus to establish design criteria for the synthesis of polymer-modified particles that are capable of forming mechanically robust and formable particle solid structures. Polymeric ligands are shown to enhance the toughness of particle array structures. Guidelines for the synthesis of nanoparticles capable of forming ordered yet robust array structures are established.