科研之友
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摘要
Elastic behavior of 4-branched star polymer chain with different chain length N adsorbed on attractive surface is investigated using steered molecular dynamics (SMD) simulation method based on the united-atom (UA) model for branched alkanes. The simulation is realized by pulling up the chain via a linear spring with a constant velocity v = 0.005 nm/ps. At the beginning, the chain lies extensionally on adsorbed surface and suffers continuous deformations during the tensile process. Statistical parameters as mean-square radii of gyration < S-2 >(xy), < S-2 >(z), shape factor , describing the conformational changes, sectional density < den > which gives the states of the chain, and average surface attractive energy < U-a >, average total energy < U >, average force < f > probed by the spring, which characterize the thermodynamic properties, are calculated in the stimulant process. Remarkably, distinguishing from the case in linear chains that there only exists one long plateau in the curve of < f >, the force plateau in our study for star chains is multiple, denoting different steps of desorption, and this agrees well with the experimental results in essence. We find during the tensile process, there are three characteristic distances Z(c), Z(t) and Z(0) from the attractive surface, and these values vary with N. When Z = Z(c), the chain is stripped from the surface, but due to the form of wall-monomer interaction, the surface retains weak influence on the chain till Z = Z(c). From Z = Z(t), parameters < U-a >, < U > and < f > respectively reach a stable value, while the shape and the size of the chain still need adjustments after Z(t) till Z(0) to reach their equilibrium states. Specifically, for short chain of N = 41, Z(t) and Z(0) are incorporated. These results may help us to deepen the knowledge about the elastic behavior of adsorbed star polymer chains.
