Conformational Asymmetry and Quasicrystal Approximants in Linear Diblock Copolymers

Small angle x-ray scattering experiments on three model low molar mass diblock copolymer systems containing minority polylactide and majority hydrocarbon blocks demonstrate that conformational asymmetry stabilizes the Frank-Kasper $\sigma$ phase. Differences in block flexibility compete with space filling at constant density inducing the formation of polyhedral shaped particles that assemble into this low symmetry ordered state with local tetrahedral coordination. These results confirm predictions from self-consistent field theory that establish the origins of symmetry breaking in the ordering of block polymer melts subjected to compositional and conformational asymmetry.

Figure 1

(a) Molecular structure and (b) schematic representation of PEP-PLA, PI-PLA, and PEE-PLA diblock copolymers. The thickness and length of each block in (b) represents a common molecular volume ($f_L=½$) to accentuate the differences in space-filling characteristics imaged in cross section ($\sim R_g^2$). For PEP-PLA ($ϵ=1.06$), the blocks fill space nearly symmetrically. However, for PI-PLA ($ϵ=1.32$) and PEE-PLA ($ϵ=1.68$), conformational asymmetry becomes increasingly significant, driving concave interfacial curvature toward the PLA block. (c) Depiction of a spherical micelle and the resulting polyhedral shaped domain associated with the ordered structure of compositionally asymmetric diblocks [13].

Figure 2

Experimental phase portraits for compositionally asymmetric PEP-PLA, PI-PLA, and PEE-PLA block polymers. Points examined by SAXS are marked by open circles and ${(\chi N)}_{\mathrm{ODT}}$ identified by DMTA based on the mean-field theory is indicated by filled circles. The behavior of the PI-PLA samples has been previously reported [11, 12, 13, 29, 34].

Figure 3

Representative synchrotron SAXS data obtained from PEE-PLA (EL-44-23). A SAXS pattern consistent with the $\sigma$ phase is formed upon cooling below the $T_{\mathrm{ODT}}$ to $98°\mathrm{C}$ and annealing for 15 min. A direct $\sigma$-to-disorder transition is observed with subsequent heating at a ramp rate of $1°\mathrm{C}/\min$. Curves are shifted vertically for clarity.

Figure 4

(a) Experimental phase diagram for binary blends of PEP-PLA diblock copolymers centered about the bcc-hex phase boundary. The phase space points examined by SAXS are marked by open circles and squares for the PEP-PLA precursor block copolymers and associated blends, respectively. For blend samples, ${(\chi N)}_{\mathrm{ODT}}$ is identified by SAXS based on the mean-field theory and marked by filled squares. (b) SAXS patterns collected for a PEP-PLA blend of $f_L=0.20$, illustrating the direct hex-to-bcc and bcc-to-dis transitions. The relative peak positions of the hex phase at $50°\mathrm{C}$, ${(q/q*)}^2=1$, 3, 4, 7, 9 are marked by closed triangles. bcc is identified at $60°\mathrm{C}$ with ${(q/q*)}^2=1$, 2, 3, 4, 5, 6, 7, 8, 9 (open triangles). The transition is reversible as established by subsequently cooling to $35°\mathrm{C}$ to reform the hex structure. Curves are shifted vertically for clarity.