Simulation of self-associating polymer systems. I. Shear-induced structural changes

Abstract

We present the results of nonequilibrium molecular dynamics (NEMD) studies of self-associating polymer systems composed of flexible telechelic chains with associating end-groups (“stickers”). Formation of micellar aggregates, their structure and structural characteristics of associative polymer network (micellar gel) are studied under the influence of external shearing forces. When the association energy εc is quite large (εc⩾εc*, where εc* is a critical association energy corresponding to the gelation transition at rest), the spatial organization of the system as a whole is characterized by a typical network architecture with bridging chains connecting different micellar aggregates. The shearing forces cause only a slight perturbation in the structural properties of the sol (at εc<εc*). However, when the association energy εc becomes quite large (εc≳εc*), we observe sharp structural variations as the shear force is increased. At sufficiently strong attraction between stickers, the shear flow facilitates development of the aggregation process: even at rather weak shearing forces, the cluster-size distribution functions become considerably wider when the association energy is close to εc*. Thus, aggregation processes become considerably more pronounced under shear, as compared to the same system at rest. As the shear force increases, the content of looplike chains decreases while the fraction of bridgelike chains rises. This process is accompanied by stretching of the chains. However, under a condition of high shear, many chains belonging to the associative network tend to be coiled up. This leads to the redistribution of structural elements in favor of loop chains. This exchange process can decrease the internal stress in the associative network which is mainly determined by the number and spatial topology of elastically active bridgelike chains.