DUTCHER, J.R. PIN: STATUS AND RECENT PROGRESS OF RESEARCH During the period of the last grant, our work has focused on achieving an

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1 STATUS AND RECENT PROGRESS OF RESEARCH During the period of the last grant, our work has focused on achieving an understanding of molecular mobility, self-assembly and pattern formation in thin polymer films in which the film thickness h is comparable to the overall size of the polymer molecules. This work has resulted in a number of very significant results. Much of our work has involved the fabrication and study of unsupported or freelystanding polymer films. This thin film geometry is appealing because the film is symmetric about its midplane and because it eliminates physical and chemical interactions with an underlying substrate. Glass transition: Our measurements of the glass transition temperature T g in freely-standing polystyrene (PS) films have revealed very large T g reductions (up to 80 o C) for very thin films [1-3]. By characterizing in detail the dependence of T g on film thickness h and molecular weight M w [3], we observed a beautiful scaling dependence of T g on h that describes all of the measured reduced T g values (> 40) by only 4 parameters. These results have attracted the attention of Nobel Laureate P.-G. de Gennes, who has proposed a mechanism by which enhanced mobility of the segments at the free surfaces can result in enhanced mobility to depths comparable to the overall size of the polymer molecules [4]. This has led to a collaboration that has resulted in a joint publication [5]. We have recently measured the glass transition and hole growth in freely-standing films of linear PS and cyclic PS for films of different film thickness and two different, relatively low M w values [6]. We find that there are no measurable differences between the results obtained for the two molecular architectures. More interestingly, following the annealing of the films on mica substrates at high temperatures for an extended time, we observe small, irreversible changes in the thickness h and index of refraction n of the film using ellipsometry and scanning probe microscopy during the initial stages of annealing the films in the freely-standing state. With repeated thermal cycles, the h and n values approach constant values at a given temperature, indicating that the film evolves toward a stable state and that the initial annealing of the PS films on mica is insufficient to relax the polymer chains. Very recently, Michael Wübbenhorst (TU Delft) has constructed a dielectric spectroscopy experiment in my laboratory to measure dielectric relaxation processes in thin polymer films. Preliminary results on poly (methyl methacrylate) (PMMA) films of different tacticities have shown that there are shifts and broadenings to both local and cooperative relaxation processes as the film thickness is reduced [7]. Chain diffusion: At elevated temperatures, freely-standing polymer films are unstable to the formation of holes which grow to ultimately destroy the film. Using optical microscopy, we have measured the growth of the radius of isolated holes in very thin freely-standing PS films as a function of film thickness [8]. In agreement with previous work on thick films of a much less viscous polymer [9], we find that the holes grow exponentially with time, with uniform thickening of the films. The characteristic growth time decreases with decreasing film thickness, which can be understood quantitatively in terms of nonlinear viscoelastic effects (shear thinning). We have also developed a new differential pressure experiment (DPE) which allows us to obtain a sensitive measure of the onset of hole formation upon heating of freely-standing polymer films [10]. The DPE has been used to probe hole formation and growth in very thin PS films that have dramatically reduced values of T g. We find that hole formation occurs only at temperatures comparable to the bulk value of T g. Because holes can form only if there is sufficient chain mobility across the entire film, the combined T g and hole growth results provide evidence for a variation in mobility across the thickness of the films. Self-assembly and pattern formation: Pattern formation on micrometer and nanometer length scales has direct technological application and is of fundamental interest. These patterns can be achieved using sophisticated lithography techniques, but soft materials such as polymers can self-assemble into regular patterns by merely changing e.g. the temperature of the system. Our work on self-assembly and pattern formation has focused on two different polymer trilayer systems. For freely-standing trilayer films, in which the central layer is much softer than the two capping layers, we have observed the self-assembly of a novel, periodic, in-plane morphology for trilayer films consisting of a wide variety of materials 5

2 combinations [11]. We have developed a simple theory, based on linear stability analysis, which correctly predicts the dependence of the wavelength of the morphology on the individual film thicknesses, as determined by the interplay between the energy decrease associated with the attractive dispersion force acting across the trilayer film and the energy increase associated with the bending of the solid capping layers. For an all-polymer system, PS/PI/PS, we have shown that the morphology can be removed by altering the dispersion interaction [11], and can be reformed with a larger periodicity merely by decreasing the temperature [12]. Above the T g value of the PS capping layers, holes form and grow in the central PI layer, forming a distinct rim around the edge of the hole [12]. Line defects, e.g. cracks, in the films can lead to the self-assembly of isolated PI dots, which consist of only ~10 6 PI chains, creating microencapsulated PI on micrometer (in-plane) and nanometer (out of plane) length scales [12]. We have also studied supported trilayer films, consisting of alternating layers of two polymers with similar mechanical properties (PS/PMMA/PS) [13]. At elevated temperatures, the trilayer films undergo an instability that is qualitatively different from that observed for the PS/PI/PS freely-standing trilayer films. By combining optical and scanning probe microscopy together with the use of an exclusive solvent for PS, we have identified a two-stage dewetting process in which the top layer dewets first, exposing the central polymer layer that subsequently dewets. This process can be understood in terms of the similarity of the mechanical properties and the surface and interfacial tensions of the two polymers. The resulting morphologies are very complex, with very different morphologies obtained as the thickness and M w values of the different layers are varied. We have developed an image analysis method based on the use of dark field optical microscopy and Voronoi tessellation to quantify a transition in the morphology as the capping layer thickness is increased. We have also studied the self-assembly of polystyrene microspheres on both hydrophilic glass substrates and hydrophobic polymer substrates and developed an image analysis procedure for characterizing the changes to the morphology produced by annealing below and above the glass transition temperature of the PS microspheres [14]. Experimental facilities: During the period of the last grant, our experimental facilities have been improved dramatically due to funding received from NSERC and the Canada Foundation for Innovation. In particular, this has resulted in the purchase of several new, sophisticated instruments (scanning probe microscope, imaging ellipsometer, quartz crystal microbalance) as well as dramatic improvements to our sample preparation facilities and improved control of our existing experiments (Labview, vibration control). In addition, we have continued to develop specialized experiments, such as the DPE. LONG TERM OBJECTIVES AND SHORT TERM PROJECTS Our research program has several well-defined long term objectives, with many shorter term projects in progress or planned in the near future in support of the long term objectives. This work will be accomplished by applying our well-developed experimental techniques and materials physics approach to achieve a deeper understanding of thin films of a variety of soft materials. We have found that to carry out internationally-competitive research, especially interdisciplinary research, it is necessary to collaborate with theory groups and other experimental groups (see list of collaborators in the principal investigator s NSERC Form 100). In addition, to achieve advances in fundamental knowledge in problems that are of direct relevance to industry, we collaborate with and have funding provided by two industrial partners, 3M Canada and Dow Chemical. Because of space restrictions, the applied research program will not be described in this proposal. 1) The non-equilibrium nature of thin polymer films. Objectives: (a) determine the effect of the solvent quench during spincoating on the properties of thin polymer films; (b) understand physical ageing effects in thin polymer films; (c) develop a reliable protocol for the production of thin polymer films with a well-defined thermal and solvent history. 6

3 Projects: (a) vary the solvent content of atmosphere during spincoating; (b) measure changes in the film properties upon annealing. Personnel: Chris Murray (PhD), Connie Roth (PhD), and an undergraduate student, in collaboration with Greg McKenna (Texas Tech) 2) Molecular mobility in freely-standing polymer films. Objectives: (a) obtain a more fundamental understanding of the glass transition in thin polymer films; (b) characterize the variation in mobility across the thickness of thin polymer films; (3) understand the hole growth process in very viscous thin polymer films Projects: (a) study the crossover from the domination of chain confinement effects for large M w values [3] to the domination of finite size effects for small M w values [15]; (b) measure the scaling behavior of the T g reductions on film thickness for several different polymers and different molecular architectures; (c) measure the in-plane versus out-of-plane anisotropy in the mobility of the polymer molecules; (d) measure the dielectric relaxation dynamics of supported and freely-standing polymer films; (e) characterize the effects produced by differences in thermal expansion for freely-standing and supported polymer films; (f) determine effect of crystallinity on hole growth in freely-standing films of semicrystalline polymers. Personnel: Connie Roth (PhD), Chris Murray (PhD), Jason Thomas (MSc), PDF from Objective 4 below, in collaboration with James Forrest (Waterloo), Michael Wübbenhorst (TU Delft), Greg McKenna (Texas Tech) and Pierre-Gilles de Gennes (College de France) 3) Self-assembly and pattern formation in polymer multilayer films. Objectives: (a) obtain quantitative agreement between the calculated and measured dispersion-driven morphology wavelength and dynamics; (b) exploit instabilities, patterning and crystallinity to achieve unique self-assembled morphologies Projects: (a) measure dynamics of instabilities in polymer multilayer films; (b) vary M w values of polymers in supported PMMA/PS/PMMA trilayer films to achieve qualitatively different morphologies; (c) measure effect of non-equilibration of films on resulting morphology Personnel: Jason Thomas (MSc), Chris Schultz-Nielsen (PhD), PDF from Objective 4 below, an undergraduate student, in collaboration with Bernie Nickel (Guelph) and Kari Dalnoki-Veress (McMaster) 4) Interdisciplinary collaborations in soft materials science. Objective: to apply our expertise in thin polymer film physics to related problems in other disciplines. Projects: (a) measure viscoelastic properties of biofilms; (b) study instabilities and barrier properties in multilayer active packaging systems; (c) investigate microencapsulation of biomaterials Personnel: Chris Schultz-Nielsen (PhD), Chris Murray (PhD), in collaboration with Terry Beveridge (Microbiology, Guelph) and Ian Britt (Food Science, Guelph). I would hire two PDFs, one for each project, with money from NSERC and the Ontario Research & Development Challenge Fund. PROPOSED APPROACH AND ANTICIPATED SIGNIFICANCE OF RESEARCH PROPOSAL The proposed research will be performed primarily using the wide range of state-of-the-art equipment designed specifically for the study of thin films of soft materials that is available in the principal investigator s laboratory, and within the Centre for Food & Soft Materials Science and the Electrochemical Technology Centre at the Univ. of Guelph (e.g. ellipsometers, scanning probe microscopes (SPM), optical microscopes, quartz crystal microbalance (QCM), differential pressure experiment (DPE), Brillouin light scattering (BLS) spectrometer, dielectric relaxation spectrometer). To take full advantage of the facilities available, and to address all of the soft materials physics projects listed above, additional research personnel are required, as detailed in the Proposed Expenditures. 1) Non-equilibrium nature of thin polymer films: Spincoating of dilute polymer solutions onto substrates is a powerful technique that allows the deposition of thin polymer films of uniform thickness 7

4 ranging from nanometers to micrometers. However, the glassy polymer film produced by rapid evaporation of the solvent (solvent quench) differs from that produced by a temperature quench in that there are unwanted solvent molecules present during the solvent quench. As the solvent content is reduced during spincoating, during a time determined by the volatility of the solvent, the glassy state freezes in below a critical concentration, which for polystyrene in toluene at 25 o C is 14% by mass toluene [16]. This means that the material undergoes a large shrinkage in the glassy state as the remaining solvent molecules are removed. Annealing of the films at temperatures greater than the bulk value of T g is typically performed after spincoating to allow the chains to relax, but stress relaxation can be inhibited by attractive interactions between the polymer molecules and the substrate. In addition, there is a tendency for the film to break up or dewet at elevated temperatures due to nucleation of holes at defects, e.g. dust or density inhomogeneities, or by amplification of thermal fluctuations of the free surface of the film for the cases in which the dispersion interaction acting across the film is attractive. Therefore the annealing process is a tradeoff between relaxing the chains and preventing the dewetting of the film, and the criteria for producing a glassy polymer film is not unambiguously defined. Solvent quench: We will investigate the nature of the solvent quench by flowing a mixture of air and solvent vapour across the sample during spincoating. This will allow us to vary in a controlled fashion the solvent partial pressure in the surrounding air and therefore the solvent quench time, i.e. the time available to the polymer molecules to equilibrate during spincoating, for a single solvent. We will monitor the thinning of the film during spincoating by reflecting laser light from the film surface and observing the characteristic signal of the solvent quench. The above approach using a single solvent has advantages over the use of solvents of different volatility to achieve different partial pressures [17]. SPM will be used to measure the roughness of the films. Changes in film properties upon annealing: We can measure very small changes in the film thickness h and index of refraction using our ellipsometer, and these changes can be used to monitor equilibration of the films [6]. We will monitor the removal of residual solvent during annealing using QCM [18]. We will also use the polarization modulation infrared reflection-absorption spectroscopy experiment, which is ideal for thin films, in J. Lipkowski s laboratory to detect the orientation of the benzene ring side groups and the backbone of the PS molecules as a function of h and sample annealing in very thin PS films on Au substrates. We will also use joint solvent- and temperature-annealing to study changes to the film properties in the molecular mobility and self-assembly projects listed below. 2) Molecular mobility: Glass transition and chain mobility: There are important outstanding issues related to the glass transition in freely-standing PS films, as detailed in the list of projects above. These crucial experiments require the unique sensitivity of our ellipsometer and BLS spectrometer. Several mechanisms have been proposed to describe enhanced mobility near polymer free surfaces: (1) segregation of chain ends at the free surfaces [19,20]; (2) enhanced orientation due to chain confinement in thin films of high M w polymers which reduces the intermolecular coupling [21]; (3) near-surface cooperative motion of the chain segments [15]; (4) enhanced surface melting due to coupling of the capillary waves at the free surfaces to the bulk flow of chains [22]; and (5) the sliding of portions of chains between segments in contact with the free surfaces [4]. Mechanisms (1,3,4) cannot explain the M w -dependence observed for high M w freely-standing PS films. Mechanism (2) is expected to depend on both film thickness h and M w. Mechanism (5) accounts qualitatively for the h- and M w -dependence of the T g reductions, but quantitative agreement is poor. The proposed experiments should allow a more critical analysis of the available theories and serve to motivate refinements to the theories. We will use patterning techniques to attempt to create freely-standing films for dielectric relaxation (DR) measurements. Although a crude measure of relaxation dynamics has been obtained previously using photon correlation spectroscopy [23], the DR experiment will allow the first comprehensive measurement of relaxation dynamics in freely-standing polymer films. We will combine the DPE with ellipsometry to obtain the first 8

5 measurement of segmental and chain mobility, and in-plane and out-of-plane thermal expansion, for the same freely-standing film. Using optical microscopy, SPM and the DPE, we will investigate the effect of crystallinity on the hole growth process in freely-standing polymer films. The crossover between exponential and linear hole growth as the hole diameter increases [24] will be explored in detail. 3) Self-assembly & pattern formation: For freely-standing polymer trilayer films, we applied a linear stability analysis to calculate the dependence of the morphology wavelength λ on the individual film thicknesses. Although the calculated scaling dependence agreed well with that measured, the calculated λ values were larger than those measured by a factor of 7. This large discrepancy between the predictions of the calculation and experiment may be due to several approximations in the original calculation (effective Hamaker constant for the trilayer film, unretarded form of the dispersion interaction). We will investigate this discrepancy by measuring the morphology as well as the dynamics using optical and scanning probe microscopy. Recent work by Herminghaus group [25,26] has shown for the first time that it is possible to get quantitative agreement between calculation and experiment in the simpler case of dewetting of PS on SiO/Si, and this is encouraging. We will examine the effects of varying the M w and non-equilibration of the individual layers on the resulting morphology. 4) Interdisciplinary collaborations: Biofilms: Bacteria will preferentially adhere to a solid surface, forming bacterial colonies called biofilms that are responsible for e.g. biofouling and metal corrosion [27]. To enhance its adhesion to the surface, each bacterium exudes extra polymer substance (EPS) that forms a hydrated polymer gel, encapsulating the bacteria and inhibiting removal of the biofilm. Although biofilms can be studied using a variety of microscopy techniques, the measurement of their viscoelastic properties is more elusive [28]. Terry Beveridge s laboratory has considerable experience in the preparation of Pseudomonas aeruginosa PAO1 biofilms [29,30]. We will combine SPM and QCM measurements to probe the onset of adhesion and the viscoelastic properties of the biofilms. QCM is particularly sensitive to the onset of adhesion of soft materials [31]. Scanning confocal laser microscopy will be used to monitor the growth of the biofilms. Also, we will use fluorescently-tagged nanoparticles embedded in the biofilms in combination with external fields to obtain a local probe of the mechanical properties of the EPS network. Active packaging: Multilayer food packaging films have been developed containing active components to, e.g. scavenge oxygen, control moisture and kill microbes [32]. An exciting recent development has been to integrate biosensing molecules specific to a particular type of bacteria in the packaging material, using luminescent chromaphores to achieve a visual indication of the contamination of the packaging film, as marketed by Toxin Alert. We will apply our expertise and techniques developed for the study of the stability of polymer multilayer films to multilayer films incorporating thin gel layers containing biologically active molecules, and this could lead to new types of active packaging. This may also lead to unique methods of producing microencapsulated biomaterials, as demonstrated by the self-assembly of PI dots in PS/PI/PS trilayer films [12]. Barrier layer properties of the packaging films will be studied using new, state-of-the-art equipment in Food Science. TRAINING OF HIGHLY QUALIFIED PERSONNEL The proposed research conducted in our laboratory will result in excellent training of students at all levels. The students will also benefit through interactions with other members of the Centre for Food & Soft Materials Science and they will receive unique interdisciplinary training on a wide variety of sophisticated equipment. This will provide the students with a broad range of research skills, allowing them to pursue careers in both industry and academia. Of my last four Ph.D. students and PDFs, two have pursued jobs in industry (JDS Uniphase & Newbridge Networks) and two have accepted faculty appointments (Waterloo & McMaster). I also involve many undergraduate students in my laboratory. It is important to involve undergraduates from physics and other disciplines to introduce them to materials physics research, and I have had very good success in using this to recruit excellent graduate students. 9

6 REFERENCES [1] J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens and J.R. Dutcher, Phys. Rev. Lett. 77, 2002 (1996). [2] J.A. Forrest, K. Dalnoki-Veress and J.R. Dutcher, Phys. Rev. E 56, 5705, (1997). [3] K. Dalnoki-Veress, J.A. Forrest, C.A. Murray, C. Gigault and J.R. Dutcher, Phys. Rev. E 63, (2001). [4] P.-G. de Gennes, Eur. Phys. J. E 2, 201 (2000). [5] K. Dalnoki-Veress, J.A. Forrest, P.-G. de Gennes and J.R. Dutcher, J. Phys. IV 10, Pr7-221 (2000). [6] C.A. Murray, J. Thomas, J.R. Dutcher and G.B. McKenna, Phys. Rev. E, submitted. [7] M. Wübbenhorst, C.A. Murray, J.R. Dutcher and J.A. Forrest, in preparation. [8] K. Dalnoki-Veress, B.G. Nickel, C. Roth and J.R. Dutcher, Phys. Rev. E 59, 2153 (1999). [9] G. Debrégeas, P. Martin, F. Brochard-Wyart, Phys. Rev. Lett. 75, 3886 (1995). [10] C. Roth, K. Dalnoki-Veress, B.G. Nickel and J.R. Dutcher, Rev. Sci. Instrum., submitted. [11] K. Dalnoki-Veress, B.G. Nickel and J.R. Dutcher, Phys. Rev. Lett. 82, 1486 (1999). [12] C.A. Murray, J. Thomas and J.R. Dutcher, Phys. Rev. E, submitted. [13] C. Schultz-Nielsen and J.R. Dutcher, in preparation. [14] C. Gigault, K. Dalnoki-Veress and J.R. Dutcher, J. Colloid Interf. Sci. 243, 143 (2001). [15] J.A. Forrest and J. Mattson, Phys. Rev. E 61, R53 (2000). [16] G.B. McKenna, J. Phys. IV 10, Pr7-53 (2000). [17] K.E. Strawhecker, S.K. Kumar, J.F. Douglas and A. Karim, Macromolecules 34, 4669 (2001). [18] A. Isherwood, M.V. Massa, C. Gigault and J.R. Dutcher, in preparation. [19] A.M. Mayes, Macromolecules 27, 3114 (1994). [20] K. Tanaka, A. Taura, S.-R. Ge, A. Takahara and T. Kajiyama, Macromolecules 29, 3040 (1996). [21] K.L. Ngai, J. Phys. IV 10, Pr7-21 (2000). [22] S. Herminghaus, K. Jacobs and R. Seemann, Eur. Phys. J. E 5, 531 (2001). [23] J.A. Forrest, C. Svanberg, K. Révész, M. Rodahl, L.M. Torell and B. Kasemo, Phys. Rev. E 58, R1226 (1998). [24] F. Brochard-Wyart, G. Debrégeas, R. Fondecave and P. Martin, Macromolecules 30, 1211 (1997). [25] K. Jacobs, S. Herminghaus and K.R. Mecke, Langmuir 14, 965 (1998). [26] R. Seemann, S. Herminghaus and K. Jacobs, Phys. Rev. Lett. 86, 5534 (2001). [27] J.W. Costerton, P.S. Stewart and E.P. Greenberg, Science 284, 1318 (1999). [28] V. Körstgens, H.-C. Flemming, J. Wingender, W. Borchard, J. Microbiol. Methods 46, 9 (2001). [29] S.A. Makin and T.J. Beveridge, Microbiology 142, 299 (1996). [30] S. Langley and T.J. Beveridge, Can. J. Microbiol. 45, 616 (1999). [31] M. Rodahl, F. Höök, C. Fredriksson, C.A. Keller, A. Krozer, P. Brzezinski, M. Voinova and B. Kasemo, Faraday Discuss. 107, 229 (1997). [32] Active Food Packaging, ed. M.L. Rooney (Chapman & Hall, London, 1995). 10