BIMat :: Biologically Inspired Materials

BIMat investigations encompass four research themes:

And each theme is further defined by the following task areas:

High strength/high stiffness Sensors and actuators Self-healing materials Self-regulating systems
Carbon nanotubes (CNT)/biopolymer composites
Organometallic precursors for fiber synthesis/processing
Clay composites
Shear flow sensors (CNT/biopolymers)
PZT sensors and actuators
PZT microcantilever actuators
Non-covalent bonding
Sacrificial bonding ionomer
Phase separation driven by reaction
Coupling catalyst (stress- or electric field driven)

High Strength/High Stiffness Materials

CNT/Biopolymer Composites

Methods for coupling molecular mechanics models with continuum mechanics models have been developed based on an overlaid-domain decomposition with the energy in the overlaid sub-domain consisting of a blending of the two energies that vanishes at each of the respective sub-domains. The method has been applied to the study of the fracture of multiwalled carbon nanotubes. In these models, only a small portion of the outermost nanotube was modeled by molecular mechanics. The remainder of the model was treated by continuum mechanics. This enabled us to easily treat systems that contain on the order of 100 million atoms. One of the major studies carried out with this model concerned the effects of defects on the molecular structure on the fracture strength of nanotubes. By introducing defects that spanned two to eight atomic spacings in the nanotube, results that quantitatively agree with experimental studies have been achieved.

A study of the tensile strength of carbon nanotubes has been initiated. In the initial work, a semi-empirical electronic structure method known as PM3 was used to determine minimum energy structures for values of the stress between 0 and 25% for (10,0) and (5,5) tubes that contain up to 200 carbon atoms, and that are either perfect or which have a defect in the middle that consists of either a Stone-Wales defect or an adsorbed H2 molecule. The calculated Young''s modulus is about 15% higher than that obtained using molecular mechanics. In addition, stress to failure is about 20% for the defected tubes, also larger than that obtained from the molecular mechanics calculations. Car-Parinello methods are being developed so that these calculations can be redone within the framework of density functional theory.

Clay-based Nanocomposites

Other materials under investigation are clay/epoxy nanocomposites consisting of an epoxy matrix (diglycidyl ether of bisphenol A) and two different types of clay particles. One of these was Nanomer 1.28E, a natural montmorillonite modified with onium ion (Nanocor, Inc.). The other type of clay particle was Cloisite 30B (Southern Clay Products). The first task was the development of a suitable processing method for these materials. A three-roll mill was found to be an effective means of dispersion and exfoliation of the clay particles in the matrix. The compounding process was carried out with varying concentrations of clay particles (1 to 10 wt.%) and mixing times. The d-spacing between clay platelets was investigated using both X-ray diffraction (XRD) and TEM and was found to increase from 2.4 nm to 3.6 nm for the Nanomer particles and from 1.85 nm to 6.5 nm for the Cloisite particles. Compared to the conventional direct and solution mixing techniques, the compounding of clay/epoxy nanocomposites by a three-roll mill was found to be highly efficient in achieving higher levels of intercalation/exfoliation in a short period of time. Mechanical testing is underway to evaluate the nanoparticle induced stiffness enhancement.

Preliminary work on hybrid continuum and micromechanical modeling of nanoreinforced polymer systems has begun with promising results for nanotube shapes and the impact of geometry. Current modeling emphasizes impact of nanoinclusion shape on stiffness. Modeling for strength and effects of nonbulk polymer behavior in the vicinity of the inclusion will be incorporated. On the experimental side, work is ongoing to develop laboratory capabilities to functionalize nanotubes and disperse into polymer systems. Bulk mechanical testing on nonfunctionalized nanotube reinforced polymer samples has been demonstrated and results show significant impact of low volume fraction nanotubes on the relaxation spectra of the composite. Techniques will be applied to samples with functionalized nanotubes and nanoplatelets in the future.

Sensors and Actuators

A series of low-molar-mass, high aspect ratio ether-imide compounds have been synthesized and characterized. All ether-imides were obtained by terminating the appropriate dianhydride, i.e. pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetra-carboxylic dianhydride (NDA), 3,3?,4,4?-biphenyltetracarboxylic dianhydride (BPDA), and 3,3?,4,4?-oxydiphthalic dianhydride (ODPA), with three flexible aryl-ether tails of different chain length. Increasing the number of meta-substituted aryl-ether units reduces the melt transition temperatures and at the same time increases the solubility of the ether-imides. When the flexibility of the dianhydride moiety increases, the thermal behavior of the compounds becomes significantly more complex: The BPDA and ODPA based compounds form glasses and exhibit multiple crystal phases. Most compounds form isotropic melts upon heating, however, 2,7-bis-(-4-phenoxy-phenyl)-benzo[lmn][3,8]phenanthroline-1,3,6,8-tetraone (NDA-n0) displays a smectic A (SA) ?type texture when cooled from the isotropic phase, followed by what appears to be either a highly ordered smectic phase or a columnar phase. Single crystal X-ray diffraction analysis and cyclic voltammetry experiments indicate that the wholly aromatic ether-imides NDA and BPDA could be excellent candidates for n-type semiconductor applications.

Work on the fabrication of tunable piezoresponsive composites from PZT whiskers embedded in polyimide matrices currently focuses on the parallel paths of whisker synthesis, polyimide matrix fabrication, and the incorporation of oriented whiskers in a polyimide thin layer. PZT whiskers are fashioned by infiltrating sol-gel precursor solutions into rectangular microchannel molds placed on a suitable substrate, curing in-situ, removing the mold, and converting the precursor to PZT. We have found that rounding the corners within the micromold ensures uniform wetting of the precursor in the mold, and encourages the retention of channel shape in the subsequent whiskers. Molds of parallel channels yields oriented whiskers on the substrate, onto which polyimide resin is cast and polymerized to fashion a composite skin of whiskers in resin. Whisker adhesion to the substrate is prevented by using release agents (self-assembled monolayers of alkanethiols) on gold-coated surfaces. The dimensions of the ultimate whiskers can be tailored through design of channel dimensions and choice of precursor chemistry.

Another sensor technology uses microchannel infiltration by sol-gel precursors to fabricate PZT microcantilevers. Improved sensitivity in piezoelectric cantilevers has been demonstrated in millimeter-scale cantilevers when compared to comparable scaled silicon-based cantilevers and quartz crystal microbalance (QCM). BIMat research has reduced the scale of PZT whiskers to the micrometer, as noted above, and seeks to further enhance sensitivity by coating PZT microcantilevers with nanoporous silica. Proof-of-concept research has utilized QCMs coated with an isotropic nanoporous silica formed by templating tetraethoxysilane (TEOS) onto an isotropic liquid crystal (the L3 phase). The absorption of water vapor by such coatings is 3-4 times higher (on a weight-to-weight basis) than in coatings formed by standard sol-gel techniques.

Self-Healing Materials

Ionomers

One branch of this research focuses on the use of commercially available materials (React-A-Seal, Surlyn 8920), starting with ionomers studied at VPI for the evaluation of existing experiments. Critical experiments are under design that will lead to an understanding of the scaling properties of these materials. Recognizing that current materials are not optimal for the desired characteristics, new systems are being evaluated, especially fluoropolymer-based, self-healing ionomers, such as DuPont?s Nafion. Nafion is very hygroscopic, which does not limit its potential application in space structures but requires special processing and characterization techniques within the laboratory.

Application of ionomers in a self-healing composite combines alumina tapes to form layered ceramic/polymer composites, a step in the emulation of nacre structure. Alumina tapes are made by tape-casting powder suspensions onto a flexible substrate, followed by heat treating to remove processing aids and partially sinter the tape. Up to 15 tapes have been stacked and sintered to 58% density, left porous for the subsequent infiltration by ionomer. Several procedures for introducing the polymer into the layered ceramic tapes are under trial, including melt infiltration, solution infiltration, and in-situ polymerization. High viscosity limits the use of melt infiltration. Solvent infiltration of ionomer has reached 25% by volume, with significant retained porosity in the composite. In-situ polymerization are in the early stages but shows promise in significantly reducing pore volume. Initial mechanical testing has demonstrated that the introduction of polymer into the layered structure acts to protect the composite from damage during high-speed machining. The contribution to the overall mechanical properties of the composite is under study.

Electron micrographs of abalone nacre, emphasizing the presence of pillars and asperities on surfaces of the ceramic component. In the lower schematic, asperities are added to the surfaces of stacked tabulated ceramic layers. The synthetic layered structure is infiltrated with a self-healing ionomer, forming a synthetic analog to the natural shell.

Bioinspired Sacrificial Bonding Agents

The self-healing, elastomeric adhesive biopolymer lustrin is being used to create biomimetic self-healing polymer networks. Polysiloxane-organic hybrid copolymers have been constructed in an ABA triblock configuration in which the two components are (A) oligopeptides of alanine or glycine and (B) polydimethylsiloxane (PDMS), some with side-grafted polypeptides or oligopeptides. These materials are transparent and have unique properties, which are presently being characterized in terms of toughness and self-healing properties. Thus far, the synthesized products range from viscous oils to tough and tacky elastomers and powders. Mechanical testing and models for these materials are under collaborative development.

Self-Regulating Systems


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