Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1999-10-28
2002-04-23
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S035000, C526S160000, C526S201000, C526S279000, C526S264000, C526S242000
Reexamination Certificate
active
06376636
ABSTRACT:
BACKGROUND OF THE INVENTION
Adhesion can be defined as the state in which two surfaces are held together by interfacial forces which may consist of valence forces, molecular bonding, or mechanical, interlocking action. Failure of the adhesive can result from the bond between the adhesive and one or both substrates being insufficient to withstand force applied to the adhered substrates so that the adhesive strips away from one or both substrates. Cohesive failure of the adhesive can occur when the internal strength of the adhesive is not as great as the force applied to it. In such failure, the adhesive may remain bonded to both substrates. As an adhesive is subject to increasing force, its behavior depends on the elasticity of the adhesive.
Gluing materials together with conventional adhesives has traditionally involved either relatively stiff adhesives such as epoxies, or elastic adhesives such as silicon adhesives. As illustrated in the force vs. extension curves of
FIG. 1
, when pulling two surfaces glued together with a short molecule, the pulling force increases rapidly with only a little extension of the molecule. A perfect stiff adhesive would be a short molecule bound to each surface by strong (that is, covalent or ionic) bonds and the molecules of the adhesive itself would be held together with strong bonds. Thus the break strength of each adhesive molecule would be the force required to break a strong bond: of the order of one nanoNewton, which is estimated by dividing one electron volt by an extension of one angstrom. For a material with many strongly bound molecules in parallel, the microscopic tensile strength is expected to be of the order of several gigapascals. This is the order of magnitude for the breaking force of strong polymers such as Kevlar [AI-Hassani, S. T. et al, Strain rate effects on GRP, KRP and CFRP composite laminates.
Key Eng. Mater
. 141, 427-452 (1998); Greenwood, J. H. et al, Compressive behavior of Kevlar 49 fibres and composites.
J. Mater. Sci
. 9, 1809-1814 (1974)]. The fracture toughness of such materials is rather small, however, even though the forces are large. This can be understood by considering the area under the force vs. extension curve shown in
FIG. 1
, which is the energy required to break the material. Because those stiff materials have a small elastic strain, the extension over which the force must be exerted until it breaks is small. Therefore, the area under the curve, or the energy required to break the material, is small.
In contrast to this behavior, the idealized curve for an elastic fiber made of long molecules shows that the force increases slowly as the elastic material is stretched to the point at which the elastic limit is reached, as also illustrated in FIG.
1
. [Lu, H. et al, Unfolding of titin immunoglobulin domains by steered molecular dynamics simulation.
Biophysics, J
. 75, 662-671 (1988); Slater, G. W. et al, Construction of approximate entropic forces for finitely extensible nonlinear elastic (FENE) polymers.
Macromol. Theory Simulat
. 3, 695-704 (1994).] Then the force increases rapidly for further extension until it breaks. As with stiff adhesives, this break will also occur at a force of the order of one nanoNewton per molecular chain, assuming each chain is bound to each surface with a strong bond and is itself held together by strong bonds. Contrary to the case of short, inelastic molecules, the pulling force must be applied over much larger extensions. Therefore, the area under the force vs. extension curve would be larger, as shown in
FIG. 1
, and thus more energy would be needed to break the material. Unfortunately, the technology does not at present exist for making such an idealized elastic material. Real elastic materials such as rubbers have tensile strengths that correspond to breaking forces per molecule of the order of 0.1% of the theoretical maximum.
Natural materials are renowned for strength and toughness not possessed by man made materials. [Qin, X., Coyne et al, A novel natural copolymer: A collagenous molecule from mussel byssus contains silk fibroin-like domains.
Am. Zool
37, 125A (1997)] Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel. [Hinman, M. et a, in
Biomedical Materials
(eds Viney, C. et al 25-34 (Materials Research Soc., Pittsburgh, 1993); Heslot, H. Artificial fibrous proteins: A review.
Biochimie
(Paris) 80, 19-31) (1998)] and is representative of many other strong natural fibers. [Waite,J. H. et a, The peculiar collagens of mussel byssus.
Matrix Biol
. 17, 93-106 (19998); Vollrath, F. et al, Modulation of the mechanical properties of spider silk by coating with water.
Nature
340, 305-307 (1989); Qin, X. X. et al, Tough tendons, Mussel byssus has collagen with silk-like domains.
J. Biol. Chem
. 272, 32623-32627 (1997)]
Titin, the giant sarcomeric protein of striated muscle, is capable of massive length gains, allowing the muscle to be overstretched without irreversible damage to the sarcomere. Rief et al used a single-molecule atomic force microscope to repeatedly stretch individual titin molecules to elongate them. [Rief, M. et al, Reversible unfolding of individual titin immuno-globulin domains by AFM.
Science
, 276, 1109-1112 (1997)] Rief et at found that for larger extensions, the force vs. extension curves typically exhibited a sawtooth-like discontinuity which they hypothesized might reflect the successive unraveling of individual domains of a single titin molecule, unfolding one domain at a time.
The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3000 times more fracture resistant than a single crystal of the pure mineral. [Currey, J. D. Mechanical properties of mother of pearl in tension.
Proc. R. Soc. Lond. B
196, 443-463 (1997); Jackson, A. P. et al, The mechanical design of nacre.
Proc. R. Soc. Lond. B
415-440 (1988)] The organic component, comprising just a few percent of the composite by weight [Watable, N. et al (eds.)
The Mechanisms of Bimineralization in Invertebrates and Plants
(Univ. South Carolina Press, Columbia, SC., 1976)], is thought to hold the key to nacre's fracture toughness. [Weiner, S. Organization of extracelluarly mineralized tissues: a comparative study of biological crystal growth.
CRC Crit. Rev. Biochem
. 20, 365-408 (1986); Jackson, A. P. et al, A physical model of nacre.
Composites Sci. Technol
. 36, 255-266 (1989)]
Nacre is the scientific name given the “mother of pearl” on the inside of the abalone shell. A matrix protein, named Lustrin A, from the nacreous layer of the shell and pearl produced by the abalone,
Haliotis rufescens
, was cloned and its cDNA coding characterized. [Shen, X. et al, Molecular cloning and characterization of Lustrin A, a matrix protein from shell and pearl nacre of
Haliotis rufescens. J. BioL Chem
. 272, 32472-32481 (1997)] This protein is found between the mineral plates in the abalone shell and participates in holding these plates together. The complete amino acid sequence of Lustrin A was reported by Shen et al, revealing a shell matrix protein with a repeating modular structure. A schematic representation of the modular structure as elucidated by Shen et al is shown in
FIG. 2
where cysteine-rich modules (C
1
-C
9
) and proline-rich modules (P
1
-P
8
) are arranged in tandem and repeated nine and eight times, respectively, in the N-terminal two-thirds of Lustrin A.
The reason for nacre's fracture resistance does not appear to be simply its lamination with an organic material, but the modular nature of the organic material. Ceramics laminated with organic material are more fracture resistant than non-laminated ceramics [Jackson, A. P. et al, supra; Clegg, W. J. et al, A simple way to make tough ceramics.
Nature
347, 455-457 (1990)], but synthetic materials made of interlocking ceramic tablets bound by a few weight per cent of ordinary adhesives d
Deming Timothy J.
Hansma Paul K.
Kindt Johannes
Morse Daniel E.
Stucky Galen D.
Fulbright & Jaworski L.L.P.
Harlan R.
The Regents of the University of California
Wu David W.
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