Kinetic Monitoring of Block Copolymer Self-Assembly Using In Situ Spectroscopic Ellipsometry

Abstract

Understanding the kinetic pathways of self-assembly in block copolymers (BCPs) has been a long-standing challenge, mostly due to limitations of in situ monitoring techniques. Here, we demonstrate an approach that uses optical birefringence, determined by spectroscopic ellipsometry (SE), as a measure of domain formation in cylinder- and lamellae-forming BCP films. The rapid experimental acquisition time in SE (ca. 1 sec) enables monitoring of the assembly/disassembly kinetics of BCP films during solvent-vapor annealing (SVA). We demonstrate that upon SVA, BCP films form ordered domains that are stable in the swollen state, but disorder upon rapid drying. Surprisingly, the disassembly during drying strongly depends on the duration of solvent exposure in the swollen state, explaining previous observations of loss of order in SVA processes. SE thus allows for decoupling of BCP self-assembly and disordering that occurs during solvent annealing and solvent evaporation, which is difficult to probe using other, slower techniques.

From Lab to Fab: Enabling Enhanced Control of Block Polymer Thin-Film Nanostructures

“…The impacts of competing forces can be difficult to deconvolute, and large parameter spaces are challenging to investigate systematically. For example, preferential interactions between a polymer block and the substrate can influence the nanostructure formed, but the magnitude of this effect is not constant through the thickness of the film. , As such, in situ and gradient/combinatorial analyses are critical to determine underlying mechanisms of self-assembly and to optimize process parameters to improve final nanostructures. ,, In this section, several studies are described that use these tools to probe the morphological effects of interfacial interactions, solvent energetics, and DSA.…”

“…Fundamental studies of solvent/polymer interaction are essential for understanding self-assembly processes. , Experimental connections between the swollen- and dried-film nanostructures are nontrivial because of the difficulty associated with performing in situ measurements on films in a swollen state. However, carefully designed experiments that employ electron microscopy, grazing incidence small-angle X-ray scattering, neutron scattering, spectroscopic ellipsometry, or AFM can probe the swollen films in situ . ,,,− For example, environmentally controlled AFM can be used to examine the in situ swelling of BP thin films. With this method, direct investigation of swelling kinetics at the limit of single-layer polymer films was possible .…”

“…Although birefringence correlated to film thickness initially, ex situ birefringence measurements deviated with longer annealing times. Reproduced with permission from ref .…”

“…Another efficient tactic to study kinetic pathways of BP films during SVA uses spectroscopic ellipsometry to assess domain formation on the basis of optical birefringence (Figure b) . Fakhraai and co-workers employed this approach to investigate self-assembly of cylinder- and lamellae-forming BP films .…”

Annealing Process Dependence of the Self-Assembly of Rod–Coil Block Copolymer Thin Films

“…Solvent vapor annealing is often performed at room temperature, typically well below the bulk T g of both blocks, and relies on swelling of the BCP, which lowers the energy barrier for diffusion and the χ N value, increasing the chain mobility. − In addition, the selectivity of the solvent vapor toward different polymer blocks influences the chain conformation and the effective volume fraction in the swollen state, and a wide range of solvents for SVA provides a convenient method to tune morphology and periodicity. ,, A combination of solvent annealing and thermal annealing such as the solvothermal annealing process has also been applied to block copolymers. − The kinetics and ordering pathway also play a strong role in the control of the ordering and orientation of block copolymer thin films. The pathway dependence of the ordering process has been investigated during thermal annealing and solvent vapor annealing. ,− SVA can induce different initial morphologies in addition to the as-cast morphology, and this can affect the morphology resulting from subsequent thermal annealing. Here, our concern is to understand the pathway influence of a two-step annealing method on the ultimate morphology of the rod–coil BCP.…”