Grafting Bimodal Polymer Brushes on Nanoparticles Using Controlled Radical Polymerization

Abstract

RAFT (reversible addition–fragmentation chain transfer) polymerization has been widely used to synthesize different polymer architectures such as polymer brushes on nanoparticles for incorporation into polymer nanocomposites. It is believed that these polymer brushes, with the same chemistry as the matrix polymer, can be employed to improve filler dispersion by compatibilizing unfavorable enthalpic interactions between the inorganic nanoparticles and their organic host matrices. However, monomodal brush graft nanoparticles are found to aggregate into a range of isotropic and anisotropic morphologies, formed due to a delicate balance between enthalpic and entropic interfacial interactions. This coupling of enthalpy and entropy leaves only a small window of graft densities and molecular weights to obtain randomly dispersed filler morphologies. These issues can be countered by using a bimodal polymer brush that contains a small number of long homopolymer chains that can entangle, and a high density of short brushes that screens the particle/particle attraction, thereby aiding in decoupling the interfacial enthalpic and entropic interactions. In the present work, we demonstrate a robust step-by-step technique using RAFT polymerization to synthesize these bidisperse/bimodal polymer brush-anchored nanoparticles. A layer of dense brush of the first population was initially prepared using surface-initiated RAFT polymerization from colloidal silica nanoparticles. After cleavage of the chain transfer agent from the first population of chain ends, a second RAFT agent was attached onto the silica nanoparticles and then a monomer, which may be the same or different from the first brush, was polymerized. This versatile and widely applicable route enables us to independently control the molecular variables of the attached chains, such as composition, molecular weights and graft densities of the individual populations. The bimodal brush-grafted colloidal silica nanoparticles show superior dispersion and interaction with a homopolymer matrix when compared to monomodal brush-grafted particles.

Polymer grafted nanoparticles: Effect of chemical and physical heterogeneity in polymer grafts on particle assembly and dispersion

“…In agreement with these predictions, recent experiments by Rungta et al, 2 where silica particles with bimodal polystyrene grafts, synthesized using step-by-step RAFT polymerization, were found be better dispersed in a monodisperse polystyrene matrix as compared to silica particles with monodisperse polymer grafts of length comparable to the long polymers in the bidisperse distribution. These promising results from theory and experiments should direct more groups to treat polydispersity in homopolymer ligands as a design parameter for stabilizing dispersions of polymer grafted particle in a polymer matrix.…”

Extraordinary toughening and strengthening effect in polymer nanocomposites using lignin-based fillers synthesized by ATRP

“…(14) Kadla and coworkers were the first to demonstrate the application of ATRP to the synthesis of polymer-modified lignin derivatives. In particular the authors grafted Nisopropylacrylamide from kraft lignin to impart temperature-dependent solubility.…”

“…(8)(9)(10)(11)(12) Parallel progress in surface-initiated controlled radical polymerization has established a range of synthetic methodologies to facilitate the tethering of polymer chains with precisely controlled degree of polymerization, grafting density and even modality of polymeric tethers. (8,13,14) Recently our group demonstrated that the application of surfaceinitiated atom transfer controlled radical polymerization (SI-ATRP) facilitates the efficient synthesis of polymer tethered kraft lignin with high molecular polymeric tethers. (15) SI-ATRP is a particularly interesting technique in the context of filler modification because of its applicability to a wide range of monomer chemistries, the high level of control over molecular characteristics of tethered chains as well as its proven economic efficacy and scalability.…”

The effect of polymer grafting in the dispersibility of alumina/polysulfone nanocomposites

“…[22][23][24] Homopolymer grafted to surface nanoparticles have been widely used to enhance dispersion and mixing into the polymer matrix by creating a chemical compatible layer between the nanoparticle and the polymer matrix. [25][26][27][28][29] Besides, variations in graft density () and graft chain length (N) provide different chain conformations of the shell, which affects the dispersion/aggregation of nanoparticles in the matrix. 30 was selected as the matrix.…”