Simulation of diffusion of asymmetric diblock and triblock copolymers in a spherical domain structure
Simulation of diffusion of asymmetric diblock and triblock copolymers in a spherical domain structure
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球形域结构中不对称二嵌段和三嵌段共聚物扩散的模拟
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发表时间：
2000
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影响因子：
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通讯作者：
G. Fredrickson
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作者：
Hideaki Yokoyama;E. Kramer;G. Fredrickson
采用朗之万动力学数值模拟了不对称二嵌段和三嵌段共聚物在体心立方对称的三维周期势场中的扩散。为了模拟无纠缠共聚物熔体，引入了一种弹簧和熔头模型（Rouse）。为了模拟纠缠的聚合物熔体，共聚物链只允许沿着其轮廓移动，遵循严格的重复算法，具有固定的段长度，没有轮廓长度波动。对于具有短a嵌段的AB二嵌段共聚物，我们计算了归一化扩散系数D/ d0，其中Do为没有势场时的扩散系数。我们发现，对于相当大的αN A，无论扩散机制如何（Rouse或严格重复），D/D 0根据exp（-αN A）表示，其中A为势场的振幅，N A为A块中的段数。在非对称区（少数嵌段分数，f = 0.1），产物αN A明显地决定了二嵌段共聚物的扩散。与二嵌段共聚物一样，ABA三嵌段共聚物在Rouse机制下的扩散也与αN A成比例，其中三嵌段共聚物的N A是每个A嵌段的段数。这表明三嵌段共聚物的扩散一次只激活一个A嵌段（行走扩散），这与实验结果一致。然而，严格的重复计算表明，在bcc势场中，三嵌段共聚物的扩散系数降低幅度较大。显然，缠结抑制了这种行走扩散机制，三嵌段共聚物比二嵌段共聚物更强烈地降低了扩散。
Diffusion of asymmetric diblock and triblock copolymers in a three-dimensional periodic potential field with body centered cubic (bcc) symmetry is simulated numerically using Langevin dynamics. To simulate unentangled copolymer melts, a spring and bead model (Rouse) is introduced. To simulate entangled polymer melts, a copolymer chain is only allowed to move along its contour, following a strict reptation algorithm, with fixed segment length and no contour length fluctuations. For an AB diblock copolymer with a short A block, we compute a normalized diffusion coefficient, D/D 0 , where Do is the diffusion coefficient in the absence of potential fields. We find that D/D 0 scales according to exp(-αN A ) for fairly large αN A regardless of the mechanism of the diffusion (Rouse or strict reptation), where a is the amplitude of the potential field and N A is the number of segments in the A block. In this asymmetric regime (fraction of minority block, f = 0.1), the product αN A evidently determines the diffusion of the diblock copolymer. The diffusion of ABA triblock copolymers by the Rouse mechanism also scales with αN A in the same way as for diblock copolymers, where N A for the triblock copolymer is the number of segments in each A block. That suggests that the triblock copolymer diffuses by activating only one A block at a time (walking diffusion), which is consistent with experiments. However, the strict reptation calculations show larger reduction of diffusion coefficients for triblock copolymers in the bcc potential field. Apparently, entanglements inhibit such a walking diffusion mechanism and reduce the diffusion more strongly for triblock copolymers than for diblock copolymers.