Morphological Diversity in Diblock Copolymer Solutions: A Molecular Dynamics Study

Abstract

Coarse-grained molecular dynamics simulations that incorporate explicit water-mediated hydrophilic/hydrophobic interactions are employed to track spatiotemporal evolution of diblock copolymer aggregation in initially homogeneous solutions. A phase portrait of the observed morphologies and their quantitative geometric features such as aggregation numbers, packing parameters, and radial distribution functions of solvent/monomers are presented. Energetic and entropic measures relevant to self-assembly such as specific solvent accessible surface area (SASA) and probability distribution functions (pdfs) of segmental stretch of copolymer chains are analyzed. The simulations qualitatively capture experimentally observed morphological diversity in diblock copolymer solutions. Topologically simpler structures predicted include spherical micelles, vesicles (polymersomes), lamellae (bilayers), linear wormlike micelles, and tori. More complex morphologies observed for larger chain lengths and nearly symmetric copolymer compositions include branched wormlike micelles with Y-shaped junctions and cylindrical micelle networks. For larger concentrations, vesicle strands, held together by hydrogen bonds, and “giant” composite aggregates that consist of lamellar, mixed hydrophobic/hydrophilic regions and percolating water cores are predicted. All structures are dynamic and exhibit diffuse domain boundaries. Morphology transitions across topologically simpler structures can be rationalized based on specific SASA measurements. PDFs of segmental stretch within vesicular assemblies appear to follow a log-normal distribution conducive for maximizing configuration entropy.

Energetic and Entropic Motifs in Vesicle Morphogenesis in Amphiphilic Diblock Copolymer Solutions

“…Second, experimental observations of shape transitions are often rationalized based on elegant yet simplified geometric arguments, such as the one based on the packing parameter, which was developed to explain spherical, cylindrical, and vesicular aggregates in amphiphilic surfactants [27]. However, experiments [7][8][9][10][11][12]28,29] and a recently reported systematic coarse-grained molecular dynamics (CGMD) study by the authors [30] have shown that the self-assembly of BCPs in a solution generates extraordinarily diverse and topologically complex morphologies. The quantitative characterization of structure evolution in such systems requires a detailed understanding of the dynamics of molecule exchange between a self-assembled structure and solvent as well as the internal reorganization of molecules that is often associated with morphology transitions.…”

Vesicle Morphogenesis in Amphiphilic Triblock Copolymer Solutions

“…Shape transitions in BCP systems are often interpreted using simple geometric models based on the elegant packing parameter concept [13]. However, experimental [16,17,[20][21][22][23][24][25] and molecular/mesoscopic simulation [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] studies have revealed an extraordinary diversity of BCP morphologies with complex topological and interfacial characteristics. The self-assembly of BAB triblock copolymer solutions, in which B and A represent the solvophobic and solvophilic blocks, respectively, was investigated by Kotaka et al [43], who reported that poly(methylmethacrylate)-polystyrene-poly(methylmethacrylate) BCPs in a toluene-p-cymene (pCY) solvent mixture with high pCY content form highly branched structures.…”

“…The CGMD simulations reported in this work are adapted from previous studies on self-assembly, shape transitions, and the rheology of surfactant solutions [26,27,[77][78][79][80][81][82][83][84]. They utilize the MARTINI coarse-graining approach and force fields [85].…”

Vesicle Morphogenesis in Amphiphilic Triblock Copolymer Solutions

“…Shape transitions in BCP systems are often interpreted using simple geometric models based on the elegant packing parameter concept [13]. However, experimental [16,17,[20][21][22][23][24][25] and molecular/mesoscopic simulation [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] studies have revealed an extraordinary diversity in BCP morphologies with complex topological and interfacial characteristics. copolymers dispersed within a solvent are brought together by thermodynamic forces to form a molecular assembly, their individual configurations undergo significant changes due to inter-chain interactions.…”

“…CGMD simulations reported in this work are adapted from previous studies on self-assembly, shape transitions, and rheology of surfactant solutions [26,27,[77][78][79][80][81][82][83][84]. They utilize the MARTINI coarse-graining approach and force fields [85].…”