Browsing by Author "Neto, C"

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    Competition between dewetting and cross-linking in poly(N-vinylpyrrolidone)/polystyrene bilayer films
    (American Chemical Society, 2011-12-06) Telford, AM; Thickett, SC; James, M; Neto, C
    We investigated the dewetting of metastable poly(N-vinylpyrrolidone) (PNVP) thin films (45 nm) on top of polystyrene (PS) thin films (58 nm) as a function of annealing temperature and molecular weight of PS (96 and 6850 kg/mol). We focused on the competition between dewetting, occurring as a result of unfavorable intermolecular interactions at the PNVP/PS interface, and spontaneous cross-linking of PNVP, occurring during thermal annealing, as we recently reported (Telford, A. M.; James, M.; Meagher, L.; Neto, C. ACS Appl. Mater. Interfaces2010, 2, 2399?2408). Using optical microscopy, we studied how the dewetting morphology and dynamics at different temperatures depended on the relative viscosity of the top PNVP film, which increased with cross-linking time, and of the bottom PS film. In the PNVP/PS96K system, cross-linking dominated over dewetting at temperatures below 180 °C, reducing drastically nucleated hole density and their maximum size, while above 180 °C the two processes reversed, with complete dewetting occurring at 200 °C. On the other hand, the PNVP/PS6850K system never achieved advanced dewetting stages as the dewetting was slower than cross-linking in the investigated temperature range. In both systems, dewetting of the PNVP films could be avoided altogether by thermally annealing the bilayers at temperatures where cross-linking dominated. The cross-linking was characterized quantitatively using neutron reflectometry, which indicated shrinkage and densification of the PNVP film, and qualitatively through selective removal of the bottom PS film. A simple model accounting for progressive cross-linking during the dewetting process predicted well the observed hole growth profiles and produced estimates of the PNVP cross-linking rate coefficients and of the activation energy of the process, in good agreement with literature values for similar systems.© 2011, American Chemical Society
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    On the composition of the top layer of microphase separated thin PS-PEO films
    (American Chemical Society, 2009-07-14) Neto, C; James, M; Telford, AM
    The topography and surface composition of thin films (ca. 100 nm) of a polystyrene-b-poly(ethylene oxide) (PS-PEO) block copolymer are investigated using a suite of complementary techniques, namely tapping mode atomic force microscopy (AFM), optical microscopy, X-ray photoelectron spectroscopy (XPS), neutron reflectometry, and wettability measurements. The copolymer films separate into lamellar structures oriented parallel to the silicon substrate, and bicontintious and island/hole morphologies characteristic of this arrangement appear. Even though the crystalline topography of the film's surface and its wettability properties suggest the presence of 11130 oil the top surface, XPS and neutron reflectometry data point undoubtedly to the presence of a top layer of PS at the air/film interface. Tapping mode AFM images unequivocally demonstrate that in air only one block is present at the air/film interface. Neutron reflectometry data identify the nature of each phase-separated layer within the film. This finding differs from a model of domain arrangement proposed in a classic and much-cited paper oil these systems (Macromolecules 1979, 12, 323). After exposure to water, PEO blocks rearrange and access the top surface of the film. After many hours of thermal annealing, both PS and PEO blocks can be made to appear at the film/air interface, within isolated droplets formed upon Film dewetting. © 2009, American Chemical Society
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    Ordered microphase separation in thin films of PMMA-PBA synthesized by RAFT: effect of block polydispersity
    (American Chemical Society, 2009-04-28) Sriprom, W; James, M; Perrier, S; Neto, C
    The microphase separation of diblock copolymers synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization, containing one monodisperse block (poly(methyl methacrylate), PMMA) and one polydisperse block (poly(butyl acrylate), PBA), was investigated in thin films (<100 nm). The formation of ordered microphase-separated domains was observed by atomic force microscopy (AFM) and resulted in four morphologies, depending on composition and film thickness: parallel lamellae, hexagonally packed perforated lamellae (PL), parallel cylinders (C∥), and hexagonally packed spheres, and in C∥-to-PL-to-C∥ transitions. Polydispersity of the PBA block shifts the phase boundaries toward higher PBA volume fraction values with respect to those expected for monodisperse block copolymers and stabilizes the perforated lamella morphology. Neutron reflectivity data confirmed that lamellae parallel to the substrate form at a very low PBA volume fraction, fPBA = 0.23. Polydispersity of the PBA block also has the effect of stabilizing each microphase domain over a film thickness regime larger than expected for monodisperse blocks. For the first time RAFT-polymerized block copolymers are shown to microphase separate with high reproducibility and with excellent degree of order, hence proving to be ideal systems to test the effect of polydispersity on microphase separation. © 2009, American Chemical Society
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    Thermally cross-linked PNVP films as antifouling coatings for biomedical applications
    (American Chemical Society, 2010-08-25) Telford, AM; James, M; Meagher, L; Neto, C
    Protein repellent coatings are widely applied to biomedical devices in order to reduce the nonspecific adhesion of plasma proteins, which can lead to failure of the device. Poly(N-vinylpyrrolidone) (PNVP) is a neutral, hydrophilic polymer with outstanding antifouling properties often used in these applications. In this paper, we characterize for the first time a cross-linking mechanism that spontaneously occurs in PNVP films upon thermal annealing. The degree of cross-linking of PNVP films and their solubility in water can be tailored by controlling the annealing, with no need for additional chemical treatment or irradiation. The physicochemical properties of the cross-linked films were investigated by X-ray photoelectron spectroscopy, infrared spectroscopy, neutron and X-ray reflectometry, ellipsometry, and atomic force microscopy, and a mechanism for the thermally induced cross-linking based on radical formation was proposed. The treated films are insoluble in water and robust upon immersion in harsh acid environment, and maintain the excellent protein-repellent properties of unmodified PNVP, as demonstrated by testing fibrinogen and immunoglobulin G adsorption with a quartz crystal microbalance. Thermal cross-linking of PNVP films could be exploited in a wide range of biotechnological applications to give antifouling properties to objects of any size, essentially making this an alternative to high-tech surface modification techniques. © 2010, American Chemical Society