SIMULATION OF SELF-ASSOCIATING POLYMER SYSTEMS - I - SHEAR-INDUCED STRUCTURAL-CHANGES

We present the results of nonequilibrium molecular dynamics (NEMD) stu dies of self-associating polymer systems composed of flexible telechel ic chains with associating end-groups (''stickers''). Formation of mic ellar aggregates, their structure and structural characteristics of as sociative polymer network (micellar gel) are studied under the influen ce of external shearing forces. When the association energy epsilon(c) is quite large (epsilon(c) greater than or equal to epsilon(c)(), wh ere epsilon(c)() is a critical association energy corresponding to th e gelation transition at rest), the spatial organization of the system as a whole is characterized by a typical network architecture with br idging chains connecting different micellar aggregates. The shearing f orces cause only a slight perturbation in the structural properties of the sol (at epsilon c < epsilon c). However, when the association en ergy epsilon c becomes quite large (epsilon(c) greater than or similar to epsilon(c)()), we observe sharp structural variations as the shea r force is increased. At sufficiently strong attraction between sticke rs, the shear flow facilitates development of the aggregation process: even at rather weak shearing forces, the cluster-size distribution fu nctions become considerably wider when the association energy is close to epsilon(c)(). Thus, aggregation processes become considerable mor e pronounced under shear, as compared to the same system at rest. As t he shear force increased, the content of looplike chains decreases whi le the fraction of bridgelike chains rises. This process is accompanie d by stretching of the chains. However, under a condition of high shea r, many chains belonging to the associative network tend to be coiled up. This leads to the redistribution of structural elements in favor o f loop chains. This exchange process can decrease the internal stress in the associative network which is mainly determined by the number an d spatial topology of elastically active bridgelike chains. (C) 1998 A merican Institute of Physics. [S0021-9606(98)51843-0].