Model amphiphilic polymer conetworks in the Bulk: dissipative particle dynamics simulations of their self-assembly and mechanical properties

Abstract

Dissipative particle dynamics (DPD) simulations were performed on bulk melts of model amphiphilic polymer conetworks (APCN), a relatively new macromolecular architecture comprising two types of mutually incompatible polymer segments, and provided their morphological behavior and mechanical properties under uniaxial elongation. The simulated APCN systems were based on four-armed star diblock copolymers covering the composition range from φA = 0.05 to 0.50, end-linked via tetra-functional cross-links. The corresponding (uncross-linked) bulk melts of linear diblock and four-armed star diblock systems were also simulated using a recent DPD reparametrization (the same reparametrization was also the one employed for the conetworks) for comparison of the morphology results with those for APCNs, and for validation of our DPD methodology by comparison with the results of the original reparametrization study on linear diblocks. Our simulations provided APCN morphologies similar to those exhibited by their linear and star diblock counterparts, differing mainly at the polymer composition of φA = 0.35 where the conetworks organized into perforated lamellae, while both the linears and stars self-assembled into gyroids. Expectedly, at that composition, the shape parameters (asphericity, prolateness, and acylindricity) displayed the largest differences between the conetworks, on the one hand, and the linears and stars, on the other. Interestingly, when the segregation strength was sufficiently lowered from χN = 60 and 80 down to 40, the APCN with composition φA = 0.35 self-assembled into a gyroid morphology, suggesting that this morphology is also accessible to the present materials. The prevalence of the gyroid or the perforated lamellar morphology in APCNs with φA = 0.35 is the result of a delicate balance of forces, where the competition between the minimization of the interfacial energy and the minimization of elastic energy, commonly known as “packing frustration”, also plays a major role. Uniaxial tension eventually transformed all originally unstretched APCNs, both self-assembled (φA = 0.15 to 0.50) and those in the disordered state (φA = 0.05 and 0.10), into lamellae normal to the direction of extension, with the APCNs possessing compositions of φA = 0.15 and 0.20, acquiring four different morphologies during the elongation process. When the present model APCN systems of equimolar composition (φA = 0.50) are sufficiently randomized by cleaving a high enough percentage (50%) of cross-links and either (again randomly, but in a different random way from the initial random cleavage) reconnected (partially or totally) or not, uniaxial stretching led to the formation of tilted lamellae. This latter finding reconciles, to some extent, earlier, apparently contradicting, results from previous studies, some of which yielded normal lamellae and some other parallel. We expect that the results of the present simulations would facilitate the design and development of next-generation APCNs.