In Situ Measurement of Block Copolymer Ordering Kinetics during the Drying of Solution-Cast Films Using Small-Angle X-ray Scattering

Abstract

The ordering of a poly(styrene-b-butadiene) block copolymer solution into hexagonally packed cylinders during the extraction of a neutral solvent from a solution-cast film was studied in real time using in situ small-angle X-ray scattering (SAXS). By tracking changes in the scattering intensity as solvent is continually removed from the film, information on the ordering rate was obtained as the film progresses from the dilute to the concentrated regime. The ordering rate was found to be controlled by thermodynamics below a critical concentration and by limited chain mobility at higher concentrations. The effect of drying temperature on the ordering kinetics was investigated by extracting solvent at three different temperatures: 30, 40, and 60 °C. The net effect of drying temperature on the ordering rate was found to depend on a competition between the thermodynamics of the drying film and the mobility of the copolymer chains. An intermediate temperature, 40 °C, provided a balance between these competing phenomena, resulting in improved overall ordering in the films. The effect of drying rate was considered by drying films in the presence of a sweep gas. The introduction of a sweep gas to remove solvent resulted in appreciable concentration gradients that hindered ordering in the films.

Kinetics of Block Copolymer Phase Segregation during Sub-millisecond Transient Thermal Annealing

“…Other studies have examined the ODT 36−41 or the order−order transition (OOT) 42−45 kinetics in solvent/polymer systems at comparable quench depths while inducing the phase transformation by controlling temperature 37−45 or changing the solvent volume fraction. 36 Direct measurements of the segregation process, however, are difficult at quench depths beyond ∼10 °C33−35 due to the fast kinetics, and little work has been reported in this deeply metastable regime. Small-angle X-ray and neutron scattering has been extensively used to probe BCP segregation.…”

“…Previous kinetic studies − have followed the ODT in pure polymer systems for shallow quench depths on the order of ∼10 °C. Other studies have examined the ODT − or the order–order transition (OOT) − kinetics in solvent/polymer systems at comparable quench depths while inducing the phase transformation by controlling temperature − or changing the solvent volume fraction . Direct measurements of the segregation process, however, are difficult at quench depths beyond ∼10 °C − due to the fast kinetics, and little work has been reported in this deeply metastable regime.…”

“…As the BCP blocks mix, the X-ray scattering intensity discontinuously decreases across the ODT by ∼2 orders of magnitude to a residual intensity arising from concentration fluctuations of similar length scale. This coincides with a discontinuous broadening of the scattering signal. , Consequently, this scattering intensity has been used as an estimate of a segregation order parameter − Γ. Depending on composition and driving forces, phase segregation can potentially occur by either (i) a nucleation and growth mechanism or (ii) a spinodal decomposition.…”

Block copolymer thin films: Characterizing nanostructure evolution with in situ X-ray and neutron scattering

“…WAXS typically measures scattering from a q of approximately 10 -2 Å -1 to 10 1 Å -1 (nm to Å size scale analysis) [97][98][99]. Using in situ SAXS and WAXS during thermal annealing, BCP domain rearrangement has been studied on chemically-patterned substrates [100,101], during SVA [102], during electric field exposure [103,104], and upon drying after film casting [54,[105][106][107][108]. Notably, SAXS and WAXS examinations of BCP thin films are limited by the relatively small scattering volumes, the potential for radiation damage to the film, and inadvertent structural reorganization from sample interactions with X-rays [111][112][113].…”

“…For example, sample chambers with precise temperature control have facilitated in situ investigations of thermal annealing processes [160,174], while sample chambers with controlled solvent atmospheres and inline monitoring have been built to permit in situ SVA studies (see Figure 8a) [139,153,155]. Similar sample chambers and configurations have been constructed to probe the effects of electrical/magnetic fields [103,175], solvent drying from the film after SVA or casting [105,106], and UV exposure [176]. For example, Stegelmeier et al built a custom blade-casting apparatus to track the structural evolution of domains immediately after film casting via SAXS (see Figure 8b) [106].…”