Crystallization in diblock copolymer thin films at different degrees of supercooling

The crystalline structures in thin films of polystyrene-$b$-poly(ethylene oxide) (PS-$b$-PEO) diblock copolymers were studied in dependence on the degree of supercooling. Atomic force microscopy showed that the crystalline domains (lamellae) consist of grains, which are macroscopic at low and intermediate degrees of supercooling, but of submicrometer size for strong supercooling. Using grazing-incidence wide-angle x-ray scattering, we could determine the grain orientation distribution function which shows that the chain stems are perpendicular to the lamellae at low supercooling, but tilted at intermediate and strong supercooling. These results suggest that, at intermediate and strong supercooling, the crystalline PEO lamellae do not grow homogeneously, but by the formation of small crystallites at the growth front.

Figure 1

(Color online) AFM topography images of films crystallized at (a) $T_x=50°\text{C}$, (b) $40°\text{C}$, and (c) $25°\text{C}$, all measured at room temperature. The height scales are given in nm. The insets show optical micrographs with a size of $100\times 100\mu {\text{m}}^2$.

Figure 2

(Color online) 2D GISAXS images of thin films crystallized at (a) $T_x=50°\text{C}$, (b) $T_x=40°\text{C}$, and (c) $T_x=25°\text{C}$. ${\alpha }_i=0.39°$. The logarithmic intensity scale runs from 30 to 6000. The arrows indicate (from bottom to top) the position of the Yoneda peak and the specularly reflected beam as well as the diffuse Bragg sheets. (d)–(f) Peak positions of the diffuse Bragg sheets as a function of incident wave vector, $k_{\text{iz}}=2\pi /[\lambda \times \text{sin}\left({\alpha }_i\right)]$, at the respective temperatures. Symbols: experimental values of the diffuse Bragg sheets (circles). Lines: model curves (see text).

Figure 3

(Color online) Experimental 2D GIWAXS maps of thin films crystallized at (a) $T_x=50°\text{C}$ and (c) $25°\text{C}$, interpolated to reciprocal coordinates. The strong intensities close to the origin are due to parasitic scattering. (b,d) Corresponding simulated 2D patterns of the diffraction from PEO crystals for (b) $T_x=50°\text{C}$ and $\theta =0°$ with a HWHM of $3°$,and (d) $T_x=25°\text{C}$ and $\theta =35°$ with a HWHM of $13°$.

Figure 4

Schematics of the structures of the film crystallized at (a) $50°\text{C}$ and (b) $25°\text{C}$. Approximate length scales are indicated.