Joseph L. Keddie

Size-Dependent Depression of the Glass Transition Temperature in Polymer Films

The glass transition temperature of thin polystyrene films has been measured as a function of film thickness. It is found that the glass transition decreases in temperature as the thickness of the film is reduced. The effect is not strongly molecular-weight dependent, ruling out chain confinement as the major cause; instead we suggest that at the surface of the glassy film a liquidlike layer exists whose size diverges as the glass transition temperature is approached from below.

Interface and surface effects on the glass-transition temperature in thin polymer films

We have measured the thickness dependence of the glass-transition temperature (Tg) of thin films of poly(methyl methacrylate)(PMMA) by using spectroscopic ellipsometry to detect the discontinuity in thermal expansivity occurring at Tg. We studied films on two surfaces: the native oxide of silicon, and evaporated gold. The Tg of PMMA on a gold surface decreases with decreasing film thickness, in accordance with previous results for polystyrene on silicon. We suggest that at the air surface a liquid-like layer exists whose size diverges as Tg is approached from below. For films of PMMA on the native oxide of silicon, however, we find a slight increase in Tg with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility and leads to an increase in Tg, outweighing the effect of the free surface.

Film formation of latex

Latex film formation, the process by which an aqueous dispersion of polymer particles is transformed into a continuous material, has a direct bearing on the final film morphology and properties. Each of the primary stages of film formation (evaporative drying and ordering; particle deformation; and polymer interdiffusion) have been studied experimentally. Recently-developed non-invasive techniques, including direct nonradiative energy transfer, atomic force microscopy, small angle neutron scattering, environmental scanning electron microscopy, and various optical techniques have greatly enhanced the study of these stages. Even so, the exact cause (or causes) of particle deformation is still a topic of considerable debate. Several factors are experimentally known to influence latex film formation: the ambient conditions; the presence of surfactants, plasticizers and pigments; and latex particle structure. An important aim in the study of latex film formation is to understand the mechanisms by which these and other factors affect the process. New challenges lie in the study of film formation of core-shell latices, latex blends and new compositions.

Fundamentals of Latex Film Formation

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Waterborne Polyurethane−Acrylic Hybrid Nanoparticles by Miniemulsion Polymerization: Applications in Pressure-Sensitive Adhesives

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Design and fabrication of colloidal polymer nanocomposites

Waterborne polyurethane-acrylic hybrid nanoparticles for application as pressure-sensitive adhesives (PSAs) were prepared by one-step miniemulsion polymerization. The addition of polyurethane to a standard waterborne acrylic formulation results in a large increase in the cohesive strength and hence a much higher shear holding time (greater than seven weeks at room temperature), which is a very desirable characteristic for PSAs. However, with the increase in cohesion, there is a decrease in the relative viscous component, and hence there is a decrease in the tack energy. The presence of a small concentration of methyl methacrylate (MMA) in the acrylic copolymer led to phase separation within the particles and created a hemispherical morphology. The tack energy was particularly low in the hybrid containing MMA because of the effects of lower energy dissipation and greater cross-linking. These results highlight the great sensitivity of the viscoelastic and adhesive properties to the details of the polymer network architecture and hence to the precise composition and synthesis conditions.

The dependence of the rate of crosslinking in poly(dimethyl siloxane) on the thickness of coatings

It is well established that colloidal polymer particles can be used to create organised structures by methods of horizontal deposition, vertical deposition, spin-casting, and surface pattern-assisted deposition. Each particle acts as a building block in the structure. This paper reviews how two-phase (or hybrid) polymer colloids can offer an attractive method to create nanocomposites. Structure in the composite can be controlled at the nanoscale by using such particles. Methods to create armored particles, such as via methods of hetero-flocculation and Pickering polymerization, are of particular interest here. Polymer colloids can also be blended with other types of nanoparticles, e.g. nanotubes and clay platelets, to create nanocomposites. Structure can be controlled over length scales approaching the macroscopic through the assembly of hybrid particles or particle blends via any of the various deposition methods. Colloidal nanocomposites can offer unprecedented long-range 2D or 3D order that provides a periodic modulation of physical properties. They can also be employed as porous templates for further nanomaterial fabrication. Challenges in the design and control of the macroscopic properties, especially mechanical, are considered. The importance of the internal interfacial structure (e.g. between inorganic and polymer particles) is highlighted.

Locking carbon nanotubes in confined lattice geometries--a route to low percolation in conducting composites.

Abstract We consider reasons why the crosslinking reaction rate in poly(dimethyl siloxane) (PDMS) network coatings might differ from the rate found in the bulk and specifically examine the influence of coating thickness. Infrared spectroscopic ellipsometry (IRSE) is employed as an in situ probe of the reactions between vinyl (–CHCH2) end groups on PDMS and SiH groups in a crosslinker and between unreacted SiH groups and hydroxyl/silanol groups within PDMS coatings, all on silicon substrates. Measurements of the concentrations of SiH groups (using the characteristic vibration at 2160 cm−1) were obtained from coatings between 1 and 27 μm in thickness, over temperatures ranging from 25 to 120 °C. First-order kinetics are exhibited in the consumption of SiH groups. The reaction rate constant is found to decrease with increasing coating thickness. Although there is evidence that the Pt catalyst segregates to the interface with the substrate, this phenomenon does not appear to have an impact on the thickness dependence. The diffusion of water into the silicone might be the rate-limiting step in the reactions, however, and therefore lead to the observed thickness dependence of the reaction rate.

Structural analysis of organic interfacial layers by ellipsometry

A significant reduction in the electrical percolation threshold is achieved by locking carbon nanotubes (CNTs) in a predominantly hexagonally close-packed (HCP) colloidal crystal lattice of partially plasticized latex particles. Contrary to other widely used latex processing where CNTs are randomly distributed within the latex matrix, for the first time, we show that excluding CNTs from occupying the interior volume of the latex particles promotes the formation of a nonrandom segregated network. The electrical percolation threshold is four times lower in an ordered segregated network made with colloidal particles near their glass transition temperature (T(g)) in comparison to in a random network made with particles at a temperature well above the T(g). This method allows for a highly reproducible way to fabricate robust, stretchable, and electrically conducting thin films with significantly improved transparency and lattice percolation at a very low CNT inclusion which may find applications in flexible and stretchable electronics as well as other stretchable technologies. For instance, our technology is particularly apt for touch screen applications, where one needs homogeneous distribution of the conductive filler throughout the matrix.

Soft polymer and nano-clay supracolloidal particles in adhesives: synergistic effects on mechanical properties

Abstract Ellipsometry has ‘come of age’ as a technique for the analysis of problems related to colloid and interface science. It has advanced far beyond applications of measuring film thickness or optical constants — although these remain important uses. Studies of the structure of polymers at the solid/liquid interface have been advanced significantly by the realisation of Fourier transform ellipsometry. Another important achievement has been the calibrated measurement of the dynamic surface excess at the flowing surface of a liquid jet. The uses of ellipsometry to study critical adsorption in binary liquids and to measure the width of liquid/liquid interfaces are also noteworthy. An important development is the use of infrared — rather than visible — light, which opens up numerous possibilities for the simultaneous structural and chemical interrogation of interfaces non-invasively.