Block Copolymer Blends
Block copolymers as surfactants
Block copolymers are amphiphiles, analogous to the surfactants used to compatibilize oil and water. Each block is miscible with the homopolymer of the same species, so diblock copolymers can be used to compatibilize immiscible homopolymers. Thermodynamic parameters can be used to adjust the morphology of nanostructures in the blend, enabling control over bulk physical properties in the material.
The most basic use of a diblock copolymer is in a ternary blend comprising two immiscible homopolymers, A and B, and the diblock copolymer A-block-B. Linear diblock copolymers are good surfactants, but cost much more to manufacture than homopolymers on account of the more challenging chemistry required. Increasing the surfactancy of a diblock copolymer is a cost saving measure because it decreases the amount of copolymer needed, as well as increasing the composition range in which the surfactant is effective. A simple way to improve the surfactancy of a diblock copolymer is to choose at least one block that has a negative χ with one of the homopolymers, resulting in highly attractive interactions. This differs from the A/B/A-block-B system in which all the interactions are either neutral (χ = 0 for A:A and B:B interactions) or repulsive (χ > χcrit for A:B interactions). A single attractive interaction can dramatically change the phase behavior of a ternary blend.
A/B/A-block-C ternary blends
We studied A/B/A-block-C polymer blends in which the A homopolymer was neutrally miscible with the A-block of the copolymer and the B homopolymer was highly miscible with the C-block of the copolymer. This diblock copolymer demonstrated the ability to compatibilize polymer blends at concentrations as low as 1 vol%1, forming a disordered microemulsion. Higher concentrations of the diblock copolymer surfactant led to ordered lamellae.2 An isopleth was studied in which the concentration of diblock copolymer was held constant at 40 vol% and the ratio of homopolymers A:B was varied from 0.2 – 5. The chemical species represented by A was present in both homopolymer A and the A-block of the diblock copolymer, so the total fraction of A, including both these contributions, is useful for understanding the asymmetry of the blends. Small-angle neutron scattering (SANS) was used to measure the blend structure. Surprisingly, lamellae were found in all blends studied, for which the total fraction of A species varied from 32 – 72 vol%. Generally, lamellae form at near-symmetric conditions, and other structures, like cylinders, spheres, and microemulsions form under asymmetrical conditions. This finding has ramifications for the robustness of diblock copolymer surfactant phase behavior under fluctuating manufacturing feed stocks.
The phase behavior and lamellar spacings were well predicted by the random phase approximation and self-consistent field theory (SCFT), respectively, as shown in Fig. 2. No adjustable parameters were used, only previously measured expressions for χ for each of the three polymer pairs. The lamellar spacing, interfacial width, and the width of a single lamellar sheet were all predicted using SCFT. Experimental results were measured by fitting a lamellar structure factor to SANS data, taking care to incorporate a background of random phase mixing for the deuterium-labeled block. Results agreed remarkably well with predictions, indicating a thorough understanding of the thermodynamics relevant to this system. The stable, predictable phase behavior of block copolymer blends enables manufacturing flexibility, particularly in feedback control.