Douglas M. Shinozaki

EXPERIMENTAL METHODS 

MECHANICAL PROPERTIES OF POLYMERS

    The study of mechanical properties of polymers can be separated into small and large strain behaviour.  The small strain deformation of polymers is most simply described as linear elastic and is measured in straightforward uniaxial tests such as the tensile test.  The small displacements of the molecular segments can involve some dissipation processes which manifest themselves as viscoelastic response, measured experimentally in the dynamic mechanical test, for example.  With an increase in temperature the molecular segments can become more free to move, and the dynamic mechanical test detects the change in molecular response as a relaxation.

    At large strains,  the molecules as a whole move with respect to each other.  Although local segmental motion is involved, the bulk mechanical response of the solid is often controlled by the motion of the larger parts of the molecule.

      Crystalline polymers consist of two phases: amorphous and crystalline.  The crystalline phase is finely distributed on the scale of 10's of nanometers in the form of lamellae.  Deformation in these materials (polyethylene, polypropylene, syndiotactic polystyrene, for example) involves molecular motions both phases.  The composite behaviour of the polymer then is controlled by the mechanical interaction of the two phases.  The processing of these kinds of polymers can be optimized to produce highly oriented lamellae which impart anisotropy to the bulk properties.

    Other multi-phase polymers are commonly found in materials such as ABS (acrylonitrile-butadiene-styrene).  The rubber phase is distributed on a very fine scale, resulting in a improvement in mechanical properties of the microcomposite.

Microindentation of polymer thin films and coatings

       In the examples of materials  above, the inhomogeneity of the microstructure  necessarily results in inhomogeneity of mechanical properties.  Ultimate performance of plastic parts then depends on the weakest region.  The microstructure of these polymers is affected by their solidification, crystallization, and thermo-mechanical history.  In some obvious cases, the fast cooling of melt processed parts results in a variation of microstructure from point to point in the part. It makes sense therefore, to attempt to measure the elastic and plastic properties at these critical points in the part using a technique such as quantitative microindentation (as developed in the laboratory  here at UWO).

       The progressive, constant velocity penetration of the cylindrical, flat tipped indenter is associated with the elastic-plastic deformation zone in the region of the specimen close to the tip of the indenter.  Because the indenter is a right circular cylinder with a flat face, the elastic-plastic field ahead of the tip can be analyzed using continuum mechanics.  The measured response of the indenter (load-displacement) is directly related to the elastic-plastic field sampled by the tip.  The careful control of tip velocity, and changes in tip velocity can be used in a way analogous to precise tensile testing of materials.  The difference lies in the inhomogeneity of stress and strain  inherent in the indentation test.

        The spatial resolution of the probe can be adjusted by changing the tip diameter since the deformation zone reaches a steady state geometry at a depth equal to approximately one tip diameter.  The overall size of the zone sampled by the indenter scales with the tip diameter.  Hence the smaller the tip diameter, the higher the spatial resolution of the measurement.  Under steady state conditions, the measured parameters (load and displacement of the tip) reflect the plastic resistance of the material and so are directly related to the yield stress and work hardening characteristics of the material.  Recent work has shown that the quantitative interpretation of the microindenter results yields accurate local plastic properties, described in terms of parameters used in standard tensile testing.  Local microfracture characteristics related to environmental stress cracking have also been identified reliably.

        The viscoelastic properties have also been measured using microindentation.  Application of an oscillating (sinusoidal) small displacement results in a sinusoidal load response.  The amplitudes and phase shift have been measured for extremely small displacements, and the storage modulus and loss factor recorded.  The test is shown to be extremely sensitive, and the since the spatial resolution depends on the tip diameter, polymer coatings can be easily tested.  The constraint of the substrate can be detected using larger tips, while near surface properties measured using smaller tips.  Appropriate normalization of the microindentation results produces material properties directly comparable to bulk measurements.

       Another important area of study exploits the localized deformation field under the tip of the microindenter to examine adhesion between different materials. These include layered inhomogeneous structures such as polymer coatings on  materials with distinct mechanical properties (metals or ceramics for example).  The particular size range used in this microindenter is uniquely suitable to examine the micromechanics of fracture at interfaces found in commonly used polymer coatings.

Ultra-thin film in-plane tensile testing of polymers

       The quest to examine the inhomogeneity of mechanical properties on a microscopic scale is also the objective in the thin film testing.  Polymer molecules in near surface regions apparently relax somewhat, and the mechanical properties change because the molecules are not fully constrained.  This is manifested as a shift in glass transition temperature.  Conversely, the adhesion of polymer films to rigid substrates results in constraint of the polymer with consequent changes in effective properties.  Careful analysis of the measured tensile properties of extremely thin films of polymers, and of composite microstructures such as multilayers or nanocomposites has been the focus of this work.

experimental techniques summary

Mechanical testing

Large strain testing of bulk viscoelastic solids: temperature and environmental effects, rubber elastic phenomena.

Solid state forming of polymers: drawing, rolling, extrusion:  microstructural effects in multi-phase systems

Constant load tensile creep of polymers: environmentally assisted slow crack growth and service lifetime analysis.

Macroindentation: deep penetration of polymers

Microindentation: dynamic mechanical thermal analysis, large strain plastic properties, microfracture and delamination of thin layered composites.

Standard micro-hardness testing

Ultra-thin film tensile testing of materials: viscoelastic properties, delamination studies, in-situ microscopy.

Dynamic mechanical thermal analysis (sub-ambient to high temperature): microstructural effects in polymers.

Microstructural Characterization

    The microstructure of polymers and polymer based composites involves the sizes, shapes and distributions of the various phases.  These include crystalline-amorphous and polymer-polymer blends and phase segregated copolymers.  The techniques used here are to characterize these various materials.

Differential scanning calorimetry: sub-ambient to elevated temperatures.

Dielectric spectroscopy of thin films (frequency and temperature dependent measurements).  Layered structures, micro- and nano-composites.

Optical microscopy: transmitted light in birefringent thin films and reflected light. Digital image acquisition and analysis.

Transmission electron microscopy of thin film polymers: stained crystalline and copolymer systems 

Transmission electron microscopy of etched polymer microstructures

Wide angle x-ray scattering in crystalline polymers.

Laser light scattering from deformed polymer and rubbery composites.

Laser scanning confocal optical microscopy (three dimensional reconstructions of microstructures in bulk and suspension) (with Professor P.C. Cheng at the State University of New York at Buffalo)

Laboratory scale microtomography (instrument development with P.C. Cheng)

Instrument Development

    Modern materials engineering involves the control of instrumentation, data acquisition and microcomputer interfacing.  Measurement of properties and examination of microstructure often involves all of these aspects.  Various projects include the following.

Microcomputer interfacing

Instrument control and data acquisition

Data analysis

Microdeformation apparatus

In situ digital image analysis

Solid State Forming Processes

    Research in the fundamental aspects of large strain deformation of solid polymers leads also to a deeper understanding of plastic forming of polymers.  The reorganization of the crystalline phase by solid state forming can result in large changes in strength, modulus and fracture resistance.  Experiments in this area involve the development and characterization of such processes.

Solid state extrusion

Constrained forming processes

Thermoforming

Micro and nano studies

     The measurement of the influence of microstructure on properties of materials involves the development of instrumentation and methods for thin film preparation and testing.  Analytical methods so developed can also be useful in the study of nano-composite systems.  A variety of new kinds of materials in which microstructural dimensions are small is the subject of some recent work.  As the microstructural dimensions become small, molecular effects become dominant and continuum models which are valid in bulk materials begin to break down.  Experimental methods to examine these effects are being developed.

Nanocomposites

Layered ultra-thin films

Artificially fabricated microstructures

"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones "  Albert Einstein