A Generalized Mechano-statistical Transient Network Model for Unravelling the Network Topology and Elasticity of Hydrophobically Associating Multiblock Copolymers in Aqueous Solutions

Abstract

In this contribution, we unravel the transient network topology and elasticity of micellar networks formed by hydrophobically associating multiblock copolymers in aqueous solutions. Unlike studies on conventional triblock copolymers bearing hydrophobic blocks as end groups, our research focuses on alternating poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) multiblock copolymers having multiple hydrophobic PPO blocks along the chain. We adopt a combinatorics approach to extend and generalize the mechano-statistical transient network model developed by Annable et al. for telechelic triblock copolymers [Journal of Rheology, 1993, 37, 695] to multiblock copolymers. The model allows one to predict the concentration dependent elasticity of networks formed by multiblock copolymers with known molecular characteristics by using knowledge of the micellar network microstructure. The spatial distribution of the hydrophobic nodes is inferred from Small-Angle X-ray Scattering (SAXS) by converting the structure factor to the radial distribution function. The number of closely neighboring micellar cores between which an elastic bridge can be formed (n m ) is calculated by spherical integration of the radial distribution function up to a distance of the radius of gyration of an intermediate soluble PEO block. Using the evolution of n m with concentration as an input for the model, the predictions show good agreement with experimental elasticity data, as inferred from the plateau modulus in linear shear rheology. The network evolves from loop-dominated, poorly elastic with cross-linking nodes with low functionality at low concentrations to bridge-dominated, highly elastic with higher node functionalities at more elevated concentrations. It is anticipated that our generalized mechano-statistical transient network model can also be used for equally spaced, multisticker associating polymers forming networks by multifunctional interactions other than micellar aggregation.

Phase behavior of medium-length hydrophobically associating PEO-PPO multiblock copolymers in aqueous media

“…G and G at a certain frequency (see Figure S6 in the ESI † , as well as our other publication [72]). Therefore, the microscopic picture giving rise to elasticity at more elevated concentrations must be significantly different for ExpertGel ® multiblock copolymers as compared to Pluronic ® triblock copolymers.…”

“…Therefore, at temperatures above the calorimetric endset temperature of concentrated solutions, an elastic network might already exist, but it is a dynamic network that is rheologically not detected at a frequency of 1 Hz due to the very labile hydrophobic interaction at that temperature (i.e. the time it takes for a hydrophobic block to detach from a micellar core is significantly shorter than 1 s) [72]. Finally, it is important to mention that the increase in G and G with increasing temperature is thermoreversible, without any hysteresis at a rate of 0.1°C/min, and that the rheological properties at all intermediate temperatures are stable, i.e.…”

“…With increasing concentration, the closer proximity of other micellar cores allows such bridges to form more easily, which transforms the material from a viscous solution of isolated micelles at low concentrations to a sample-spanning, three-dimensional network at higher concentrations. The dependence of the elastic network modulus at high frequency (independent of temperature at sufficiently high temperatures) on the multiblock copolymer concentration has been studied and modelled in our earlier work [72].…”

Effect of Chain Architecture on the Structure, Dynamics, and Rheology of Thermoresponsive Poloxamer Hydrogels and Associated Blends

“…Block polymers composed of two or more chemically distinct blocks can self-assemble into micelles when placed in a selective solvent for one of the blocks. − Characteristics such as the composition, block architecture (AB vs ABA vs BAB), and solvent selectivity dictate the micelle size and shape, as well as the critical micellization temperature (CMT) and concentration (CMC). − These effects are widely studied as micelles have broad industrial relevance, particularly as drug delivery vehicles and viscosity modifiers. , Symmetric triblock polymers composed of solvophobic end blocks and a solvophillic midblock (BAB), often referred to as telechelic or associative triblocks, are of particular rheological interest. − Due to the block arrangement, chains must form either loops (flower-like micelles) or bridges to neighboring micelles for both end blocks to be contained within a micelle core. As micelle bridges can serve as dynamic physical crosslinks which alter rheological properties such as modulus and relaxation time, the fraction of bridges versus loops formed within various BAB/solvent systems has been the focus of numerous computational and experimental works. − ,− …”

“…To minimize corona stretching while allowing for the formation of intermicelle bridges, dilute BAB formulations often phase-separate into a polymer-rich region composed of clusters of bridged micelles and a polymer-poor region. , In concentrated solutions above a critical volume fraction, the bridged clusters become system-spanning and a percolated bridged network phase is formed. − ,,,, Depending on the composition of the polymer blocks and solvent, such bridged networks display interesting rheological behaviors, such as time-dependent recovery following shear, relatively high dynamic moduli, and long relaxation times. ,, At high concentrations, the micelles in certain systems can order into periodic packingsmost commonly body-centered cubic (BCC)and this packing dominates the rheological properties within this region. ,, …”