Effect of Symmetry of Molecular Weight Distribution in Block Copolymers on Formation of “Metastable” Morphologies

Polystyrene-b-poly(methyl acrylate) (PS−PMA) copolymers were synthesized using activators regenerated by electron transfer (ARGET) for atom transfer radical polymerization (ATRP). Polydispersity of the PMA block was varied by adjusting the amount of copper catalyst in ARGET ATRP, and the resulting molecular weight distributions were found to be approximately symmetric. At a composition of 35 vol % of PMA, the formation of a hexagonally perforated lamellar (HPL) morphology was observed for a polydisperse PS−PMA copolymer for short- and long-term solvent-casting conditions. No order−order transitions were observed at elevated temperatures or after prolonged thermal annealing. The observed stabilization of the HPL morphologythat is considered to be metastable in narrow-disperse diblock copolymers as well as diblock copolymers with selective block polydispersity given by a Schulz−Zimm distributionsuggests that the skewness of the distribution of block molecular weights is an important parameter for the structure selection during the microphase separation process. In particular, the results indicate that near-symmetric block molecular weight distributions (as realized by the ARGET ATRP technique) facilitate the stabilization of microdomain morphologies with increased standard deviation of mean curvature. The results point to the relevance of controlling both the width and symmetry of molecular weight distribution as a potential route toward the tailored synthesis of nonregular microstructures with particular topological properties that might be of future technological interest

Effect of Matrix Molecular Weight on the Coarsening Mechanism of Polymer-Grafted Gold Nanocrystals

A systematic evaluation of the effect of polymer matrix molecular weight on the coarsening kinetics of uniformly dispersed polystyrene-grafted gold nanoparticles is presented. Particle coarsening is found to proceed via three stages (i.e., atomic-diffusion-based Ostwald ripening (OR), particle-migration-based collision−coalescence, and the subsequent reshaping of particle assemblies). The relative significance of each stage and hence the evolution of particle size and shape have been found to depend sensitively upon time, temperature, and the molecular weight of the host polymer. At temperatures close to the matrix glass-transition temperature, Ostwald ripening has been observed to be dominant on all experimental timescales. With increasing annealing temperature, collision coalescence becomes the dominant mode of coarsening, leading to rapid particle growth. The onset of the latter process is found to be increasingly delayed with increasing molecular weight of the polymer host. Particle coalescence is observed to proceed via two fundamental modes (i.e., diffusion-limited aggregation and growth resulting in the formation of fractal particle clusters and the subsequent recrystallization into more spherical monolithic aggregate structures). Interestingly, particle coarsening in high-molecular-weight matrix polymers is found to proceed significantly faster than predicted on the basis of the bulk polymer viscosity; this acceleration is interpreted to be a consequence of the network characteristics of high-molecular-weight polymers by analogy to the phenomenon of nanoviscosity that has been reported in the context of nanoparticle diffusion within high-molecular-weight polymers

Effect of Chain Architecture on the Compatibility of Block Copolymer/Nanoparticle Blends

The effect of block copolymer chain connectivity on the structure formation in binary blends comprising block copolymer hosts and enthalpically neutralized particle fillers is investigated for linear diblock (AB) and triblock (ABA and BAB) as well as four-arm star copolymer architectures (AB3 and A3B). For particles with approximately constant effective size (defined here as the ratio of filler particle diameter to host polymer radius of gyration), miscibility was observed only within diblock copolymers and within the domains formed by the end blocks of triblock copolymers. The limitation of particle miscibility within the triblock mid-domain is interpreted as a consequence of the entropy loss associated with particle deposition due to the stretched configuration of bridged midblock chains. Particle aggregation was observed in both star copolymer samples irrespective of the architecture of the particle-loaded polymer domain. In the case of particle loading of the branched copolymer domain, this is rationalized as a consequence of the increased effective particle size, whereas the incompatibility of particle fillers in the linear block domain of miktoarm copolymer hosts is interpreted as a result of the coupling of dimensional changes within the microstructure along with the reduced axial compressibility of the particle-free branched domain. The sensitive dependence of the particle compatibility on the chain architecture of the polymer host illustrates a yet unexplored parameter space that will need to be taken into account if particle blends are to be designed with branched or multiblock host copolymer architectures