| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 96 === Dissipative particle dynamics (DPD) was employed to perform three- dimensional simulations to investigate the phase separation dynamics of polymer solutions with different polymer chain length and bond strength. How the chain length and bond strength influenced the concentration fluctuation, morphology evolution and coarsening mechanism of polymer-rich regions during the phase separation were presented. The results show that phase separation dynamics was suppressed by the chain length effects and the hindrance effect resulted from enhanced spring between polymer segments. On morphology evolution, the chain length effect sustained the bicontinuous structure, and the bond strength effect due to the increase of spring constant resulted in local aggregation of polymer-rich regions..
The growth of polymer-rich domains was analyzed to study the effects of chain length and bond strength on coarsening mechanism. For the polymer solution composed of shorter and more flexible chains, a two–stage coarsening was observed with the crossover of the domain growth exponent from 1/3 to 2/3 during the course of phase separation. The crossover reflected that the growth mechanism altered from diffusion to interfacial-tension driven flow. When the chain flexibility was kept the same but the chain length increased, the growth exponent was reduced to 1/4 in the diffusion-dominating coarsening regime, while effects on the growth exponent in the flow–dominating regime were absent. Besides, the concentration fluctuation as well as the dissipation of interfacial energy during phase separation was suppressed. When the chain length was kept short but the bond strength was enhanced by increasing the spring constant between the polymer segments, the growth exponent approached 1/5 in the diffusion-dominating regime. Nevertheless, the entanglement effects in the flow-dominating coarsening regime were not remarkable. The chain length effect slowed down the domain growth, which could be explained by that polymer chains could only perform reptation when chain entanglements occurred. Moreover, when both the effects of chain length and bond strength were enhanced, polymer networks composed of longer chains with stronger bond strength imposed an energy barrier for phase separation to occur, which corresponded to the Frenkel-Flory-Rehner hypothesis. As a result, the polymer solution with longer chains with stronger bond strength can only undergo phase separation when a larger quench depth was employed to initiate the phase separation, reflecting that the entanglement effects derived from the increase of chain length and bond strength were alleviated with the deeper quench.
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