Patrick T. Mather

Review of progress in shape-memory polymers

Shape-memory polymers (SMPs) have attracted significant attention from both industrial and academic researchers due to their useful and fascinating functionality. This review thoroughly examines progress in shape-memory polymers, including the very recent past, achieved by numerous groups around the world and our own research group. Considering all of the shape-memory polymers reviewed, we identify a classification scheme wherein nearly all SMPs may be associated with one of four classes in accordance with their shape fixing and recovering mechanisms and as dictated by macromolecular details. We discuss how the described shape-memory polymers show great potential for diverse applications, including in the medical arena, sensors, and actuators.

Implantable medical devices

A medical device includes a balloon catheter having an expandable member, e.g., an inflatable balloon, at its distal end and a stent or other endoprosthesis. The stent is, for example, an apertured tubular member formed of a polymer and is assembled about the balloon. The stent has an initial diameter for delivery into the body and can be expanded to a larger diameter by inflating the balloon.

Reinforcement and environmental degradation of nylon-6/clay nanocomposites

Hybrid organic/inorganic nanocomposites are being developed to improve the physical and mechanical properties of polymeric materials without adversely effecting their processing characteristics. One such nanocomposite developed by Toyota and commercialized by Ube Industries is the nylon-6/montmorillonite clay nanocomposite. The mechanism of reinforcement in nylon-6/clay nanocomposite materials is investigated through tensile experiments, infrared absorption spectrography, and dynamic mechanical analysis. 200% improvements in modulus and 175% improvements in yield stress are attributed to the complexation of mid-chain carbonyl groups with the exfoliated clay lamellae. Because of the initial use of these materials in automotive components, and the known deleterious effects of the air pollutant NOx on nylon-6, the degradation of the nanocomposites in NOx was examined through post-exposure tensile experiments. It was found that NOx degrades the mechanical performance of the nanocomposites regardless of the constraining effect of clay lamellae.

POSS Polymers: Physical Properties and Biomaterials Applications

Research into polymers incorporating polyhedral oligomeric silsesquioxane (POSS) has intensified during the past several years, revealing new fundamental polymer physics, new synthetic routes, and unexpected applications. The present review article critically examines the recent scientific literature on POSS polymers with an emphasis on structure-property relationships. We conclude that it is an exciting time to work on such materials and we expect the field to continue to grow in the foreseeable future.

Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

A unique polyurethane (PU) elastomer containing inorganic polyhedral oligomeric silsesquioxane (POSS) molecules as molecular reinforcements in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. The mechanical properties of POSS modified polyurethane (POSS-PU) were also compared to those of polyurethane without POSS. The crystal structures of two different POSS molecules were first determined by X-ray powder diffraction analysis, yielding a rhombohedral cell with aa 11.57 A ˚, aa 95.58 for octacyclohexyl-POSS (1,3,5,7,9,11,13,15octacyclohexylpentacyclo[9.5.1.13,9.15,15.17,13] octasiloxane) and aa 11.53 A ˚, aa 95.38 for hydrido-POSS (1-[hydridodimethylsiloxy]3,5,7,9,11,13,15-heptacyclohexylpentacyclo [9.5.1.13,9.15,15.17,13] octasiloxane). WAXD results showed that reflection peaks distinct to POSS crystal diffraction were seen in POSS-modified polyurethane, which suggests that POSS molecules formed nanoscale crystals in the hard domain. During deformation, the average size of POSS crystals in POSS-PU was found to decrease while elongation-induced crystallization of the soft segments was observed at strains greater than 100%. The SAXS results showed microphase structure typical of segmented

Shape memory and nanostructure in poly(norbornyl-POSS) copolymers

The microstructure and shape-memory properties of norbornyl-POSS hybrid copolymers having either cyclohexyl corner groups (CyPOSS) or cyclopentyl corner groups (CpPOSS) were investigated by transmission electron microscopy and thermomechanical analysis. Here, POSS refers to the polyhedral oligomeric silsesquioxane macromer. Samples containing 50wt% of POSS macromer have been mechanically drawn at temperatures above their glass transition temperatures, followed by rapid quenching in LN2 . Shape-memory properties of such drawn samples were explored by measuring recovered strain while heating above the T g using thermomechanical analysis. Incorporation of POSS comonomers within PN is found to slightly reduce the percentage recovery, while improving thermal stability significantly. Interestingly, the types of corner groups in the POSS macromer affect the shape-memory behaviour, with the CyPOSS copolymer showing lower percentage recovery than the CpPOSS copolymer due to enhanced aggregation of CyPOSS macromers. # 2000 Society of Chemical Industry

Nanoscale reinforcement of polyhedral oligomeric silsesquioxane (POSS) in polyurethane elastomer

A unique class of polyurethane (PU) elastomer containing inorganic molecules (polyhedral oligomeric silsesquioxane, POSS) as molecular reinforcement in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. WAXD results indicate that POSS molecules form nanoscale crystals showing distinct reflection peaks. The formation of POSS crystals is probably prompted by the microphase separation between solid-like hard segments and rubbery soft segments in PU. The microphase separation of hard and soft segments was observed by SAXS, which shows a long period of 111 A for 34 wt% POSS-PU and 162 A for 21 wt% POSS-PU, and hard segment domains with sizes of about 34 A for both of them. WAXD results from a series of POSS compounds with a corner substituted by a functional group of varying length were compared with POSS-PU, which also confirms the presence of nanoscale POSS crystals in the polymer matrix. © 2000 Society of Chemical Industry

Conductive shape memory nanocomposites for high speed electrical actuation

A new shape memory nanocomposite that exhibits rapid electrical actuation capabilities is fabricated by incorporating continuous, non-woven carbon nanofibers (CNFs) into an epoxy based SMP matrix. The fiber morphology and nanometre size provide a percolating conductive network with a large interfacial area. This not only gives high electrical conductivity but also simultaneously enhances heat transfer and recovery stress.

Dynamic cell behavior on shape memory polymer substrates

Cell culture substrates of defined topography have emerged as powerful tools with which to investigate cell mechanobiology, but current technologies only allow passive control of substrate properties. Here we present a thermo-responsive cell culture system that uses shape memory polymer (SMP) substrates that are programmed to change surface topography during cell culture. Our hypothesis was that a shape-memory-activated change in substrate topography could be used to control cell behavior. To test this hypothesis, we embossed an initially flat SMP substrate to produce a temporary topography of parallel micron-scale grooves. After plating cells on the substrate, we triggered shape memory activation using a change in temperature tailored to be compatible with mammalian cell culture, thereby causing topographic transformation back to the original flat surface. We found that the programmed erasure of substrate topography caused a decrease in cell alignment as evidenced by an increase in angular dispersion with corresponding remodeling of the actin cytoskeleton. Cell viability remained greater than 95% before and after topography change and temperature increase. These results demonstrate control of cell behavior through shape-memory-activated topographic changes and introduce the use of active cell culture SMP substrates for investigation of mechanotransduction, cell biomechanical function, and cell soft-matter physics.

Shape memory polymers with built-in threshold temperature sensors

The design, fabrication and characterization of a new shape memory polymer (SMP) with built-in temperature sensing capabilities are reported. The material was prepared by incorporating a fluorescent, chromogenic oligo(p-phenylene vinylene) dye into a cross-linked poly(cyclooctene) (PCO) matrix via guest diffusion. The dye concentration was chosen to allow for self-assembly of the dye upon drying, resulting in the formation of excimers. Exposure of these phase-separated blends to temperatures above the melting point (Tm) of the PCO leads to dissolution of the dye molecules, and therefore causes a pronounced change of their absorption and fluorescence color. The optical changes are reversible; i.e., the aggregate absorption and emission are restored upon cooling below Tm. The color is dictated by the phase behavior and is independent of the mechanical state of the SMP. Thus the effect allows one to monitor reaching of the set/release temperature of the polymer.