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BRANCHING AND CHAIN END EFFECTS ON SURFACE FLUCTUATIONS OF POLYSTYRENE MELT FILMS

Zhang, Fan, Mr.
http://orcid.org/0000-0002-8890-9221
http://rave.ohiolink.edu/etdc/view?acc_num=akron1542541224707819

Abstract Details

2018, Doctor of Philosophy, University of Akron, Polymer Science.
The first efficient method for synthesis of well-defined, tadpole-shaped polystyrene has been developed using anionic polymerization, silicon chloride linking chemistry and metathesis ring closure. The difunctional macromolecular linking agent, ω-methyldichlorosilylpolystyrene, was formed by reacting sec-butyllithium-initiated poly(styryl)lithium with excess (30× molar) methyltrichlorosilane to eliminate formation of linear dimer and three-arm star polystyrene. The asymmetric, three-arm, star precursor was formed by linking excess α-4-pentenylpoly(styryl)lithium (α-PSLi) with the macromolecular linking agent, and the excess a-PSLi was functionalized with ethylene oxide before termination with methanol to facilitate column chromatographic separation of unlinked arm. Cyclization of the asymmetric, three-arm, star precursor to form the tadpole-shaped polystyrene was effected in methylene chloride at high dilution using the Grubbs first generation catalyst, bis(tricyclohexylphosphine)benzylidene ruthenium(IV) chloride. The tadpole product was uniquely characterized by MALDI-ToF MS in terms of peaks that appeared characteristically 28 m/z units lower than those of the corresponding asymmetric, three-arm, star precursor and corresponding to the loss of an ethylene unit. The MALDI-ToF MS results showed that the tadpole-shaped polystyrene was of high purity. MD simulations find a smaller hydrodynamic volume for the tadpole-shaped PS as compared to the three-arm star precursor, in quantitative agreement with GPC results. Incorporating one cycle in the molecule, while leaving one chain end, leads to an increase in Tg of only 2.7 ± 0.8°C, much smaller than the increase of 13.6 ± 0.8 °C seen when going from the linear chain to cyclic analog with no ends at all. These changes in Tg with architecture results are consistent with self-plasticization by free chain ends. Thermally stimulated surface fluctuations in polymer films are important from a fundamental perspective. The surface fluctuations of melt films of linear polystyrene (LPS) and cyclic polystyrene chains (CPS) can be described by a hydrodynamic continuum theory (HCT) when the thickness, h, is sufficiently large. However, surface fluctuations can be slower than expected from the HCT when the viscosity enhancement manifests itself. To reveal branching and chain end effects on the surface fluctuations of melt films, the surface fluctuations of films of 15k, four-arm star polystyrene (SPS) and 6k TPS chains have been measured using X-ray Photon Correlation Spectroscopy (XPCS). For the star chains, the viscosity enhancement becomes evident at a film thickness between 112 nm (40Rg) and 72 nm (26Rg), which is remarkably larger than the critical thicknesses for LPS (< 7Rg) and CPS (10-14Rg) of the same molecular weight. This is true both for absolute thickness and thickness relative to chain size, Rg. XPCS data for the 15k star films can be rationalized using a two-layer model with a 17 nm (6Rg) thick, highly viscous layer at the silicon substrate, which is significantly thicker than the 1Rg thick “irreversibly adsorbed” layer. Here, the interpenetration of the chains accounts for this propagation of slow dynamics of the chains near substrate into the film. For a 29 nm (10Rg) thick film more striking viscosity enhancement occurs due to the overlapped viscosity enhancements from both interfaces. For TPS, all the surface fluctuation relaxation times are well above those anticipated by the HCT adopting the bulk viscosity experimentally measured in this work. This observation is very surprising and most readily explained as resulting from an inaccurate rheology measurement. Collapse of the data to a universal curve at a given temperature requires an increase in the highly viscous layer thickness with increasing temperature. This result suggests that these films were not fully equilibrated. The highly viscous layer was still growing during the measurement because the samples were effectively being annealed at the elevated temperature of the measurement. Rinsing films of TPS with good solvent yields an irreversibly adsorbed layer with thickness of 1Rg and scattering length density 20% larger than the bulk PS SLD value. The smaller relative thickness of the irreversibly adsorbed layer as compared to the irreversibly adsorbed layer of the CPS analog seems consistent with our anticipation that the linear tail should disrupt the compact packing. Again, the postulated highly viscous layer has been found to be much thicker than the irreversibly adsorbed layer after rinsing. We conjecture that threading of the cycles in some way provides a means for propagation of slow dynamics of the chains near the substrate.
Mark Foster, Professor (Advisor)
157 p.
Polymer Chemistry; Polymers
Synthesis, Thin film, Anionic Polymerization

Recommended Citations

Citations

  • Zhang, F. (2018). BRANCHING AND CHAIN END EFFECTS ON SURFACE FLUCTUATIONS OF POLYSTYRENE MELT FILMS [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1542541224707819

    APA Style (7th edition)

  • Zhang, Fan. BRANCHING AND CHAIN END EFFECTS ON SURFACE FLUCTUATIONS OF POLYSTYRENE MELT FILMS. 2018. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1542541224707819.

    MLA Style (8th edition)

  • Zhang, Fan. "BRANCHING AND CHAIN END EFFECTS ON SURFACE FLUCTUATIONS OF POLYSTYRENE MELT FILMS." Doctoral dissertation, University of Akron, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1542541224707819

    Chicago Manual of Style (17th edition)

akron1542541224707819
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© 2018, some rights reserved.Creative Commons License BRANCHING AND CHAIN END EFFECTS ON SURFACE FLUCTUATIONS OF POLYSTYRENE MELT FILMS by Fan Zhang Mr. is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Akron and OhioLINK.