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Research Activities

The primary objective of the research work performed in the context of the proposal “Effect of Asymmetric Block Polydispersity on the Structure Formation and Mechanical Properties of Diblock Copolymers” was to understand the implications of block polydispersity on the near-equilibrium microstructure formation and the order-disorder transition of amorphous diblock copolymers. The key hypotheses to be tested in this project revolved about the effect of asymmetric block polydispersity on the stability regions of the traditional microdomain structures and whether asymmetric block polydispersity can facilitate the stabilization of morphologies that are otherwise only observed as ‘metastable phases’ in monodisperse block copolymers.

Summary

Two primary activities were pursued under the funded effort, i.e.

(1) Synthesis and characterization of poly(styrene-b-methyl acrylate) block copolymers with block-selective polydispersity.

(2) Elucidation of the effect of macroinitiator impurities on the structure formation of polydisperse block copolymer systems.

Other Research Activities

Several secondary (and complementary) research projects have been pursued with the objective to better understand of the relevance of select parameters on block copolymer self assembly or to leverage the understanding of the statistical characteristics of molecular weight distributions to understand the statistical properties of functionalized particle systems. Secondary research activities during the past two year funding cycle encompass: (i) the distribution of functional sites in partially modified particle systems, (ii) the effect of particle surface modification on the coarsening behavior of embedded particle systems, and (iii) structure formation in ternary block copolymer/particle blend systems.

In total four publications have been published about research that was supported by ACS-PRF, one manuscript is currently under review.

In the following, the major findings associated with the studies outlined above as well as the impact of the project on student training will briefly be described.

Effect of Asymmetric Block on the Structure Formation in Amorphous Diblock Copolymers

While the original plan was to only test the effect of chain molecular weight distribution on neat copolymer systems it has been extended to encompass the effect of homopolymer addition in order to elucidate the relevance of macroinitiator impurities on the structure formation process. Two significant insights of this program are (1) the observation that chain polydispersity can stabilize (classically) metastable morphologies (in the present case the hexagonal perforated lamellae microstructure); and (2) the presence of macroinitiator induces complex changes to the stability regions of polydisperse block copolymers due to concentration-dependent chain segregation. In all cases the system under study comprised poly(styrene-b-methacrylate) (PS-PMA) copolymers with block selective polydispersity of either the PS or the PMA domain.

Stabilization of the Hexagonal Perforated Lamellar Microstructure in PS-PMA

Two important insights resulted from this study: (i) the effect of chain length disparity on the stabilization of morphologies with increased variance of the interfacial curvature. (ii) For the first time, the relevance of skewness in the context of molecular weight distributions has been highlighted. The latter is important because the common notion is to describe molecular weight distributions in terms of only the first and second moment while higher moments are necessary to distinguish symmetric from asymmetric distributions.

Listak, J., et al., Macromolecules, 2008. 41(15): p. 5919-5927.

Effect of Block Molecular Weight Distribution on the Structure Formation in Block Copolymer/Homopolymer Blends

Understanding the implication of homopolymer addition on the structure formation in polydisperse block copolymer materials is relevant because popular synthetic techniques to synthesize block copolymers with controlled chain polydispersity often result in macroinitiator impurities. This contribution aims to elucidate the implications of macroinitiator impurities and thus contribute to the better interpretation of documented results in the literature but also to expand the parameter space for the engineering of block copolymer microstructures.

Other Research Activities

Distribution of Functional Sites in Partially Modified Particle Systems

This is a side project with the aim to establish preliminary results for a continuation proposal. The motivation of this project is that while the concept of ‘distributed molecular characteristics’ is common in the area of polymer materials it is a largely unknown concept in other areas of chemistry and materials science in which it might be of equal (or even greater) relevance. In particular in the synthesis of surface-modified nanocrystals the common approach is to consider each particle (after the modification) to carry an identical number of functional sites. In this project a kinetic analysis was applied to establish the distribution of functional sites in particle and particle-like systems. We believe that these results will provide a major contribution to understanding the physicochemical and biochemical properties of functionalized particle systems.

Hakem, I. F. et al. J. Am. Chem. Soc. 2010, 132, 16593-15598.

Elucidation of the effect of thermal annealing on the coarsening mechanism of polymer-embedded gold nanocrystals

A long term goal of the present project was to establish the effect of block polydispersity on the capacity of block copolymers to solubilize particle fillers. Gold nanocrystals are (almost) universally applied as model particle fillers in order to study the structure formation in heterogeneous block copolymer systems. This is because of (1) the established chemical procedures to control the surface chemistry of gold nanocrystals, (2) the ability to stabilize small particle sizes that typically support compatibilization of the particle fillers and (3) the electron band structure of gold that enables the use of electron optical and various X-ray scattering techniques to study the structure formation process. In this project we systematically evaluated the effect of matrix polymer molecular weight on the coarsening mechanism of embedded PS-coated gold nanocrystals of various particle diameter.

X. Jia et al. Langmuir, 2010, 26, 12190-12197.

Bimetallic particle superstructures in ternary block copolymer/nanoparticle blends

This side project aimed at understanding if the parameter relations observed in binary polymer/particle systems also apply to more complicated ternary systems in which a block copolymer is admixed with two different particle species. This side project bears direct relevance to the primary research objective since most particle systems used in block copolymer blends systems are size polydisperse.

M. R. Bockstaller, et al. J. Mater. Chem. 2010, 20, 9339-9341.