Phase Behavior Of Triblock Copolymers Research Articles

Rationalizing the Phase Behavior of Triblock Copolymers through Experiments and Molecular Simulations

In this paper, we develop a new coarse-grained model, under the MARTINI framework, for Pluronic block copolymers that is able to describe the self-assembly mechanism and reproduce experimental micelle sizes and shapes. Previous MARTINI-type Pluronic models were unable to produce realistic micelles in aqueous solution, and thus our model represents a marked improvement over existing approaches. We then applied this model to understand the effects of polymer structure on the cloud point temperature measured experimentally for a series of Pluronics, including both normal and reverse copolymers. It was observed that high polyoxypropylene glycol content leads to dominant hydrophobic interactions and a lower cloud point temperature, while high hydrophilic polyoxyethylene glycol content shields the micelles against aggregation and hence leads to a higher cloud point temperature. As the concentration increases, the effect of polymer architecture (normal versus reverse) starts to dominate, with reverse Pluronics showing a lower cloud point temperature. This was shown to be due to the increased formation of cross-links between neighboring micelles in these systems, which promote micelle aggregation. Our results shed new light on these fascinating systems and open the door to increased control of their thermal responsive behavior.

Thermodynamically controllable transition from 3D to 2D self-assembly of a hydrogelator induced by the phase behavior of triblock copolymers

Triblock copolymer PE6200 (PEO10.5–PPO30–PEO10.5) in aqueous solution can undergo a transition from an isotropic micellar phase to an anisotropic lamellar phase at a specific temperature. Based on this feature, the self-assembly behavior of a benzenetetracarboxylic acid based hydrogelator in aqueous solutions of PE6200 has been investigated. The results of small angle X-ray scattering (SAXS) measurements indicated that the hydrogelator self-assembled into stable three-dimensional network structures below 50°C. In the range of 50–60°C, a transition from three-dimensional to two-dimensional self-assembly was observed, which can be attributed to the fact that PE6200 undergoes a transition from an isotropic micellar phase to an anisotropic lamellar phase. Differential scanning calorimetry (DSC) and varying temperature laser Raman spectroscopy further confirmed the thermodynamically controllable transition of 3D to 2D self-assembly. Controlling the self-assembly by utilizing the phase behavior of triblock copolymers is a novel strategy.

Phase behavior of tri-block copolymers in solution: Mesoscopic simulation study

The microphase separation dynamics of tri-block copolymers Pluronic EO17PO60EO17 (P103), EO19PO43EO19 (P84) and EO19PO29EO19 (P65) was simulated in aqueous solution by a dynamic variant of mean-field density functional theory for Gaussian chains. For a Pluronic copolymer with smaller ratio of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) (P103, P84 and P65) at low concentration, with an increase of the concentration, the micellar phases change from spherical micelles and micellar clusters, to disk-like micelles for the P103 and P84 solutions, and from spherical micelles to worm-like micelles for the P65 solution. In the P103 and P84 systems, the spherical micelles and micellar clusters can be divided clearly by order parameter, and in the P65 solution the phase separation of worm-like micelle with simulation time is shown clearly by the 3D density field of the PO block. With a decrease of the ratio of hydrophobic PPO and PEO (from P103, P84 to P65), the critical micellar concentration (cmc) increases. It was found that the cmc were 4vol.% for P103, 7vol.% for P84 and 43vol.% for P65. It was concluded that mesoscopic simulation can be considered as an adjunct to experimentation and can provide mesoscopic information otherwise inaccessible (or, not easily accessible) from experimentation.

Phase Behavior of Triblock Copolymers Varying in Molecular Asymmetry

The transformation from A(1)B diblock copolymer to A(1)BA(2) triblock copolymers varying in molecular asymmetry is investigated as the A(2) end block is progressively grown via chemical synthesis. Dynamic rheological measurements show that the order-disorder transition (ODT) temperatures of two copolymer series differing in composition and molecular weight decrease when the A(2) block is short relative to the A(1) block, and then increase as the length of the A(2) block is increased further. The resultant ODT minimum, predicted by mean-field theory, is attributed to mixing between long B and short A(2) blocks.

Phase Behavior of Triblock Copolymers upon Incorporation of Non-Parent, Midblock-Associating Additives

ABSTRACTAddition of a block-selective homopolymer to a microphase-ordered block copolymer is known to result in preferential swelling of the chemically compatible microdomain. In this work, we examine the miscibility between a triblock copolymer and a relatively low-molecular-weight, chemically dissimilar, midblock-associating homopolymer and demonstrate that the homopolymer molecules residing in the swollen midblock matrix self-assemble to avoid repulsive interactions with neighboring microdomains. We extend this investigation to include systems composed of a very low-molecular-weight, midblock-associating additive (an oil). At high oil concentrations, the glassy copolymer endblocks micellize, resulting in the formation of a thermoplastic elastomer gel.