Microscopic Mechanisms of Electric-Field-Induced Alignment of Block Copolymer Microdomains

We investigate the microscopic mechanisms responsible for microdomain alignment in block copolymer solutions exposed to an electric field. Using time-resolved synchrotron small-angle x-ray scattering, we reveal two distinct processes, i.e., grain boundary migration and rotation of entire grains, as the two dominant microscopic mechanisms. The former dominates in weakly segregating systems, while the latter is predominant in strongly segregated systems. The kinetics of the processes are followed as a function of polymer concentration and temperature and are correlated to the solution viscosity.

Figure 1

(A) Experimental setup for in situ SAXS. (B)–(D) Two-dimensional SAXS pattern of a 35 wt. % solution of a SI diblock copolymer in toluene taken at 25 °C prior to (B) and after application of an electric field ($E=1\mathrm{k}\mathrm{V}/\mathrm{m}\mathrm{m}$) (C),(D). The color scale in C is exaggerated for clarity [see Fig. 2a].

Figure 2

Time development of the scattering intensity as a function of the azimuthal angle $\phi$ in the presence of an electric field of strength $E=1\mathrm{k}\mathrm{V}/\mathrm{m}\mathrm{m}$ for different polymer concentrations and temperatures. (A) $w_{\mathrm{p}\mathrm{o}\mathrm{l}}=35\mathrm{w}\mathrm{t}.\%$, $T=25°\mathrm{C}$; (B) $w_{\mathrm{p}\mathrm{o}\mathrm{l}}=50\mathrm{w}\mathrm{t}.\%$, $T=25°\mathrm{C}$; (C) $w_{\mathrm{p}\mathrm{o}\mathrm{l}}=47.5\mathrm{w}\mathrm{t}.\%$, $T=27°\mathrm{C}$; (D) $w_{\mathrm{p}\mathrm{o}\mathrm{l}}=47.5\mathrm{w}\mathrm{t}.\%$, $T=80°\mathrm{C}$.

Figure 3

Sketch of the two dominating microscopic mechanisms: (A) grain boundary migration and (B) grain rotation, eventually leading to an alignment parallel to the electric field (C). (D) TEM picture of a typical kink band defect observed after electric-field alignment. Scale $\mathrm{\text{bar}}=400\mathrm{n}\mathrm{m}$.

Figure 4

(A) Angular shift of rotational component at different concentrations ($\square =35\mathrm{w}\mathrm{t}.\%$, $◯=47.5\mathrm{w}\mathrm{t}.\%$, $△=50\mathrm{w}\mathrm{t}.\%$) at field strength of $E=1\mathrm{k}\mathrm{V}/\mathrm{m}\mathrm{m}$ at $T=25°\mathrm{C}$. The solid lines represent least-squares fits to the data yielding the rotational time constant ${\tau }_{\mathrm{r}\mathrm{o}\mathrm{t}}$; (B) bilogarithmic plot of ${\tau }_{\mathrm{r}\mathrm{o}\mathrm{t}}$ versus solution viscosity, $\eta$.