Chain-length and tacticity effects on the conformational behavior of MMA-oligomer thin films on an Au (1 1 1) substrate

• Published in: Computational materials science 2009-06, Vol.45 (4), p.867-874
• Main Authors: Ju, Shin-Pon, Liao, Ming-Liang, Cheng, Ching-Ho, Lee, Wen-Jay, Chang, Jee-Gong
• Format: Article
• Language: English
• Subjects: Applied sciences; Conformations; Density distribution; Exact sciences and technology; Mean square radius of gyration; Molecular dynamics; Organic polymers; Physicochemistry of polymers; Poly(methyl methacrylate); Polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0927-0256; 1879-0801
• DOI: 10.1016/j.commatsci.2008.12.008

Chain-length and tacticity effects on the conformational behavior, including density distributions and conformations, of MMA-oligomer thin films deposited on an Au (1 1 1) substrate at room temperature were investigated using molecular dynamics (MD) simulations. Three kinds of MMA-oligomer thin film with the oligomers containing m repeat units with m = 5, 10, and 20, respectively, were selected to examine the chain-length effects. Three types of MMA-oligomer thin film (made of isotactic, syndiotactic, and atactic PMMA isomers, respectively) were utilized to inspect the tacticity effects. For ease of investigation, the thin film was divided into three regions (the contact, the bulk, and the surface regions) according to the through-thickness distribution of density of the thin film. For short-chain thin films (with m = 5 and 10), the density distribution had a remarkable peak in the contact region of the thin film. The density peak, however, was not evident for the long-chain thin film (with m = 20). Regarding tacticity, the isotactic thin films had a prominent peak density in the contact region as compared with the syndiotactic and atactic thin films. Moreover, the MMA oligomers were found to exhibit a flattened conformation parallel to the Au substrate in the contact region and a slightly flattened conformation in the surface region for both the short-chain and the long-chain thin films. The flattened conformation remained in the bulk region of the long-chain thin film. However, it was not present in the bulk region of the short-chain thin film, which had a randomly orientated conformation–a characteristic similar to bulk MMA. It was also observed that the tacticity of the MMA-oligomer thin films did not have obvious effects on the conformations of the thin films for both the short-chain and the long-chain cases.