Douglas M. Shinozaki

    RESEARCH IN POLYMERS

The central theme of research relates microstructure of polymers and polymer based composites to physical properties. The most recent research deals with the examination of mechanical properties on a local scale using novel techniques. From the viewpoint of mechanical testing, experimental methods to measure time and temperature dependent mechanical properties on very small regions in solid samples are being developed. Dedicated instrumentation has been constructed and the new capabilities which have resulted are being exploited to study traditional polymers, including polyolefines, and new materials such as blended polymer-polymer composites. It is apparent from recent work on syndiotactic polystyrene that applications in processed polymer parts, thin film properties, and adhesive mechanics can be usefully addressed with these new methods. An important part of this work is an attempt to optimize processing of polymers to obtain the best mechanical properties.

A new subject of study  is that of dielectric properties of polymers and microcomposite structures, which are related to the basic molecular motions and hence to the microstructure of the material.  A large part of this work is to develop methods to measure molecular effects in movement of segments of large molecules.

Fundamental molecular processes can also be examined using careful mechanical testing.  The time and temperature dependent deformation behaviour of polymers is revealed in the measurement of strain rate and cyclic loading experiments.  Polymers are mechanically anisotropic and inhomogeneous on a local scale, and these characteristics affect bulk engineering properties significantly.  Interlamellar cracking which develops in polyolefines subjected to low tensile stresses has been shown to lead to premature and catastrophic fracture under particular testing conditions.

Earlier research in the area of ultra-high resolution soft x-ray lithography examined the limits of microscopic patterning using thin polymer film resists.  The techniques developed to prepare and examine specimens are currently being applied to the study of thin film mechanics and properties described above.  The use of resists to create ultra-small patterned structures opens up new areas  in which microstructure and nanostructure can be fabricated artificially.  Ongoing work already deals with nanostructural effects seen in block copolymers in which phase segregation occurs on extremely small scales.

The subject of scale of microstructure on bulk properties is also being examined in polymer matrix nanocomposites.  Extremely small, rigid particles can be distributed in a soft matrix to engineer specific physical properties.  The relationship between processing, properties and microstructure is the focus of this work.  Ongoing experimental studies in polypropylene-montmorillonite systems are designed to understand the role of the nanoscale clay particles in changing the composite mechanical properties.  Theoretical modeling of nanomechanical interactions between stiff, small particles and low modulus, ductile matrices is applied to examine a variety of unique small scale effects.  

Research projects combine advanced experimental techniques with  modelling of various kinds. Recent projects have included the application of finite element analysis to microindentation processes using ABAQUS.  This has led to methods to measure interfacial shear strength in coatings and thin surface layers.  The quantitative relationship between polymer coating properties and interfacial strength has been revealed by using a combination of experimental measurement and numerical calculation.  

Microtensile testing has been used to measure the mechanical properties of thin viscoelastic films.   Multilayer composites consisting of ultrathin metal-polymer layers have been tested to study fracture and interfacial delamination processes. Large strain properties of very small grain size vacuum evaporated metal films have been experimentally measured.  Testing of single corn stalk fibers has been related to microstructure.

Methods to measure the  time and temperature dependence of mechanical behaviour in viscoelastic solids have been developed to understand fundamental molecular processes which control yield, irreversible and reversible deformation.  Work hardening and cyclic mechanisms have been analyzed in this way.

In addition to using novel mechanical testing methods, a number of methods to study microstructure in polymers have been developed. These include three dimensional optical microscopy of polymers and polymer composites (confocal optical microscopy); and transmission electron microscopy of  thin polymer films, and two stage replicas of etched polymer surfaces. X-rays have been applied to the examination of both macro and microstructure in composites.  Periodic structures at various scales have been examined using diffraction in a variety of modes.