3D Nanometer-Scale Study of Coexisting Bicontinuous Morphologies in a Block Copolymer/Homopolymer Blend

Revealing the 3D Structure of Block Copolymers with Electron Microscopy: Current Status and Future Directions

Block copolymers (BCPs) are considered model systems for understanding and utilizing self-assembly in soft matter. Their tunable nanometric structure and composition enable comprehensive studies of self-assembly processes as well as make them relevant materials in diverse applications. A key step in developing and controlling BCP nanostructures is a full understanding of their three-dimensional (3D) structure and how this structure is affected by the BCP chemistry, confinement, boundary conditions, and the self-assembly evolution and dynamics. Electron microscopy (EM) is a leading method in BCP 3D characterization owing to its high resolution in imaging nanosized structures. Here we discuss the two main 3D EM methods: namely, transmission EM tomography and slice and view scanning EM tomography. We present each method's principles, examine their strengths and weaknesses, and discuss ways researchers have devised to overcome some of the challenges in BCP 3D characterization with EM- from specimen preparation to imaging radiation-sensitive materials. Importantly, we review current and new cutting-edge EM methods such as direct electron detectors, energy dispersive X-ray spectroscopy of soft matter, high temporal rate imaging, and single-particle analysis that have great potential for expanding the BCP understanding through EM in the future.

In situ synthesis of a self-assembled AB/B blend of poly(ethylene glycol)-b-polystyrene/polystyrene by dispersion RAFT polymerization

A diblock-copolymer/homopolymer self-assembled blend was synthesized through dispersion RAFT polymerization, and its morphology changed with a decreasing ratio of diblock-copolymer/homopolymer.

Direct Visualization of Order-Order Transitions in Silicon-Containing Block Copolymers by Electron Tomography

Here, we aim to comprehend the mechanism of the order-order transition (OOT) from nonequilibrium, metastable phase to equilibrium phase. Polystyrene-block-polydimethylsiloxane (PS-PDMS) block copolymer (BCP) bulks with metastable cylinder (C) and double gyroid (G) phases can be obtained from lamellae (L) forming PS-PDMS by simply tuning the selectivity of casting solvent. The recovery of the intrinsic L phase can be achieved by thermal annealing through OOT. Time-resolved small-angle X-ray scattering (SAXS) experiments are carried out to reveal the variation of the structural evolution in reciprocal space during annealing. The structural evolution in real space is directly visualized by using electron tomography (i.e., 3D transmission electron microscopy (TEM)). As a result, combining the time-resolved scattering experiments and the morphological observations from electron tomography offers new insights into the phase behaviors of the OOT of BCPs.

Characterizing interface shape evolution in immiscible polymer blends via 3D image analysis

The coordinate transformation (CT) method was applied to measure the local curvature of the interface of an immiscible polymer blend made of fluorescently labeled polystyrene (FLPS) and styrene-ran-acrylonitrile copolymer (SAN). The CT method involves the local parametrization of the interface by a quadratic polynomial to compute the local values of the mean (H) and the Gaussian (K) curvatures. Distributions of the curvatures at different annealing times were obtained by measuring H and K at many (typically 10(7)) points on the interface. Coarsening of a symmetric (50/50 w/w FLPS/SAN) and a nonsymmetric (35/65 w/w) blend was monitored. For the symmetric blend, two regimes of surface evolution were identified: in the early stage, the probability densities of the curvatures at various times were successfully scaled by a time-dependent characteristic length, i.e., interface area per unit volume (Q). This behavior has been previously observed in blends with morphologies created by a different mechanism, namely spinodal decomposition. In the late stage, the dynamic scaling failed and the time evolution of the interface slowed down. For the nonsymmetric blend, the domains of the minor phase (FLPS) were more elongated and they eventually broke up producing a composite microstructure with islands of drops within cocontinuous domains. We defined a "scaled" genus (G) to quantify the topology evolution of the blends during coarsening. Loss of connectivity was evidenced by a decrease of G with time for the nonsymmetric blend, while a constant value of this parameter indicated no change in topology during coarsening for the symmetric blend.

Imaging of the 3D nanostructure of a polymer solar cell by electron tomography

Electron tomography has been used for analyzing the active layer in a polymer solar cell, a bulk heterojunction of an alternating copolymer of fluorene and a derivative of fullerene. The method supplies a three-dimensional representation of the morphology of the film, where domains with different scattering properties may be distinguished. The reconstruction shows good contrast between the two phases included in the film and demonstrates that electron tomography is an adequate tool for investigations of the three-dimensional nanostructure of the amorphous materials used in polymer solar cells.

AxBAx-Type Block–Graft Polymers with Soft Methacrylate Middle Segments and Hard Styrene Outer Grafts: Synthesis, Morphology, and Mechanical Properties

Novel copolymers with controlled architectures can function as new building blocks for well-defined nanostructures on the basis of microphase separation, unlike conventional ABA triblock copolymers. A series of well-defined A(x)BA(x)-type block-graft copolymers consisting of soft middle segments (dodecyl methacrylate (DMA)) and hard outer graft chains (styrene (St)) were synthesized by ruthenium-catalyzed living radical block and graft polymerization. NMR spectroscopy and size-exclusion chromatography combined with multiangle laser light scattering confirmed the well-defined structure of the A(x)BA(x) block-graft copolymers with backbones and graft chains of controlled lengths. Transmission electron microscopy and transmission electron microtomography revealed a series of morphologies for the copolymers. Morphological changes were observed from PSt "honeycomb" cylinders to lamellae and poly(DMA) cylinders with increasing PSt-graft content, whereby the phase diagram was shifted significantly to lower volume fractions of the larger-number component (St) relative to those of the corresponding ABA triblock copolymers. More specifically, poly(DMA) cylinders were observed even before the St content reached 50 wt %. The A(x)BA(x) and ABA copolymers with 17-30 wt % of St exhibited characteristics of a thermoplastic elastomer with tensile strengths of 1-6 MPa and elongations at break of 70-300 %. These mechanical properties can be related well to the microphase structures of the A(x)BA(x) and ABA copolymers.

Nanotomography in the chemical, biological and materials sciences

Nanotomography is a technique of growing importance in the investigation of the shape, size, distribution and elemental composition of a wide variety of materials that are of central interest to investigators in the physical and biological sciences. Nanospatial factors often hold the key to a deeper understanding of the properties of matter at the nanoscale level. With recent advances in tomography, it is possible to achieve experimental resolution in the nanometre range, and to determine with elemental specificity the three-dimensional distribution of materials. This critical review deals principally with electron tomography, but it also outlines the power and future potential of transmission X-ray tomography, and alludes to other related techniques.