A study on the structural transition of a single polymer chain by parallel tempering molecular dynamics simulation
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The structural transition of a single polymer chain with chain length of 100, 200 and 300 beads was investigated by parallel tempering MD simulation. Our simulation results can capture the structural change from random coil to orientationally ordered structure with decreasing temperature. The clear transition was observed on the curves of radius of gyration and global orientational order parameter P as the function of temperature, which demonstrated structural formation of a single polymer chain. The linear relationships between three components of square radius of gyration Rgx2, Rgy2, Rgz2 and global orientational order P can be obtained under the structurally transformational process. The slope of the linear relationship between x (or y-axis) component Rgx2 (or Rgy2) and P is negative, while that of Rgz2 as the function of P is positive. The absolute value of slope is proportional to the chain length. Once the single polymer chain takes the random coil or ordered configuration, the linear relationship is invalid. The conformational change was also analyzed on microscopic scale. The polymer chain can be treated as the construction of rigid stems connecting by flexible loops. The deviation from exponentially decreased behavior of stem length distribution becomes prominent, indicating a stiffening of the chain arises leading to more and more segments ending up in the trans state with decreasing temperature. The stem length Ntr is about 21 bonds indicating the polymer chain is ordered with the specific fold length. So, the simulation results, which show the prototype of a liquid-crystalline polymer chain, are helpful to understand the crystallization process of crystalline polymers.
KeywordsA polymer chain Structural transition Parallel tempering molecular dynamics
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