Phase Behavior and Conformational Asymmetry near the Comb-to-Bottlebrush Transition in Linear-Brush Block Copolymers

Abstract

This study explores how conformational asymmetry influences the bulk phase behavior of linear-brush block copolymers. We synthesized 60 diblock copolymers composed of poly(trifluoroethyl methacrylate) as the linear block and poly[oligo(ethylene glycol) methyl ether methacrylate] as the brush block, varying the molecular weight, composition, and side-chain length to introduce different degrees of conformational asymmetry. Using small-angle X-ray scattering, we determined the morphology and phase diagrams for three different side-chain length systems, mainly observing lamellar and cylindrical phases. Increasing the side-chain length of the brush block from three to nine ethylene oxide units introduces sufficient asymmetry between the blocks to alter the phase behavior, shifting the lamellar-to-cylindrical transitions toward lower brush block compositions and transitioning the brush block from the dense comb-like regime to the bottlebrush regime. Coarse-grained simulations support our experimental observations and provide a mapping between the composition and conformational asymmetry. A comparison of our findings to strong stretching theory across multiple phase boundary predictions confirms the transition between the dense comb-like regime and the bottlebrush regime.

Scheme 1. RAFT Polymerization Scheme for the Synthesis of PTFEMA-b-POEM_x Copolymers

Figure 1

Room temperature SAXS patterns for a representative set of the PTFEMA-b-POEM_x copolymers (approximately 30 kg mol^–1) as a function of the volume fraction of the POEM_x block (f_POEMx). DIS, HEX, and LAM denote disordered, hexagonal (cylindrical), and lamellar phases, respectively.

Figure 2

Phase diagrams determined by SAXS for the PTFEMA-b-POEM_x copolymers as a function of volume fraction of the POEM_x block (f_POEMx) and the total degree of polymerization (N); x denotes the number or EO units in the side chain of the brush block. The star symbol in the PTFEMA-b-POEM₅ phase diagram denotes the possible coexistence of hexagonal and gyroid phases. Colors used for phase regions and dashed lines are exclusively intended as visual aids and should not be interpreted as delineating actual phase boundaries.

Figure 3

Crowding parameter Φ vs side-chain length for POEM. Circles represent the experimental systems chosen, and the green solid line represents the simulated systems. The blue solid line represents the value of Φ for other POEM side-chain lengths based on the experimental results. The dashed line is Φ = 1, bottlebrush regime. Φ values were calculated from eq 6. For the POEM system, b_s was assumed to be equal to 0.56 nm, b_BB = 0.65 nm, υ_BB = 0.14 nm³, and υ_s = 0.067 nm³. Discrepancy between simulation and experimental points is due to the discretization difference in the coarse-grained simulation.

Figure 4

Asymmetry phase diagram for the experimental PTFEMA-b-POEM_x systems listed in Table 1. The x axis is the relative volume fraction of bottlebrush block f_B, while the y axis is the asymmetry parameter. n_B denotes the number of repeat units in the side chains. Yellow and green solid lines represent the cylinder-to-lamella and lamella-to-cylinder transitions predicted by the SST.

Figure 5

Simulated asymmetry phase diagrams for the PTFEMA-b-POEM_x systems determined by using (a) bottlebrush and (b) comb scaling methods. The x axis is the relative volume fraction of bottlebrush block f_B, while the y axis is the asymmetry parameter. n_B denotes the number of beads in the side chains. Yellow and green solid lines represent the cylinder-to-lamella and lamella-to-cylinder transitions predicted by the SST. Note that these lines are the same in both plots, but the asymmetry parameter was calculated using either eq 1 (bottlebrush scaling) or eq 2 (comb scaling), respectively. The shaded area highlights the discrepancies observed in the lamella-to-cylinder transition for the comb-like scaling between the simulation data and SST for BCPs with side chains of 5 or more monomer units.