Criteria governing rod formation vs. wormlike micelle growth in self-assembled polymers

Poloxamers are widely used in cosmetics and drug delivery due to their tunable structure and properties via molecular weight, block length, and block ratio. The sphere-to-rod transition in aqueous poloxamers is well-characterized; however, the molecular parameters and solution conditions controlling further growth and elongation into wormlike micelles (WLMs) are unknown. Here, we systematically evaluate rod formation and growth in a series of aqueous poloxamers using linear rheology and SANS to identify parameters critical to WLM formation. Important molecular and microstructural features include amphiphile size, micelle cross-section curvature, and solvent penetration. Despite vastly different amphiphile molecular weights, all poloxamers forming rods have nearly identical cross-sectional areas. However, a high relative curvature reduces the likelihood of further growth and WLM formation. Salt type and concentration also play a key role, where increasing salinity decreases the temperature for rod formation, but also narrows the temperature range over which WLMs are formed. This research sheds light on the fundamental role of poloxamer subunit and solution conditions on assembly and growth processes, providing a comprehensive dataset for validating thermodynamic models.