Shape memory assisted self-healing polymers having load bearing structure

Abstract

A structural shape memory assisted self-healing polymer formed by laminating thin layers of an ionomer, such as a member of the poly(styrene sulfonate) (PSS) family of ionomers, with a WEGP-type SMP, such as atactic poly(styrene) (PS) with molecular weight in the 200 kDa range (or alternatively poly(methyl methacrylate) (PMMA)) in combination with polycyclooctene (PCO). The self-healing polymer may also comprise an interpenetrating, immiscible polymer network (IPN) based on a blend of polystyrene and polystyrene sulfonate (PSS).

Claims

1. A self-healing system, comprising: a structural layer consisting of a shape-memory polymer film selected from the group consisting of poly(styrene), poly(styrene-butadiene), poly(norbornene), poly(norbornene-cyclooctene), poly(tert-butyl acrylate-butyl acrylate), and crosslinked poly(cyclooctene) and having a glass transition temperature; and a heat-triggered self-healing layer consisting of a ionomeric polymer film having a self-healing temperature within 20 C. of said glass transition temperature of said shape-memory polymer film that is adjacent to and bonded to said structural layer.

2. The system of claim 1, wherein said ionomeric polymer is selected from the group consisting of poly(-caprolactone), poly(ethylene oxide), poly(ethylene glycol), poly(butylene terephthalate), poly(ethylene terephthalate), poly(ethylene-co-octene), nylon-11, nylon-12, nylon-6, poly(ethylene adipate), poly(n-butyl acrylate-co-methyl methacrylate), poly(t-butyl acrylate), poly(t-butyl acrylate co-n-butyl acrylate), poly(norbornene), poly(lactide), and glassy polymers having a glass transition temperature greater than room temperature and a molecular weight below 50,000 g/mol.

3. The system of claim 1, further comprising a plurality of said structural layers alternating with a corresponding plurality of said heat-triggered self-healing layers.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic of a lamination approach to forming a structural shape memory assisted self-healing polymer according to the present invention;

(3) FIG. 2 is a chemical diagram of a polymer useful for the embodiments of the present invention;

(4) FIG. 3 is a schematic of an IPN microstructure featuring interpenetration of the WEGP-type SMP (black) and the ionomeric rebonding polymer (gray) for use with a structural shape memory assisted self-healing polymer according to the present invention

(5) FIG. 4 is a series of photographs and a graph illustrating characterization tests for a structural shape memory assisted self-healing polymer according to the present invention.

(6) FIG. 5 is a series of photographs illustrating crack closure in an embodiment of the present invention.

(7) FIG. 6 is a series of photographs illustrating crack closure in another embodiment of the present invention.

(8) FIG. 7 is a series of photographs illustrating crack closure in an additional embodiment of the present invention.

(9) FIG. 8 is a schematic of a polymer for use with the present invention.

(10) FIG. 9 is a series of photographs illustrating crack closure in an embodiment of the present invention that includes the polymer of FIG. 8.

(11) FIG. 10 is a schematic of another polymer for use with the present invention.

(12) FIG. 11 is a series of photographs illustrating crack closure in an embodiment of the present invention that includes the polymer of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

(13) Referring now to the drawings, the present invention comprises a shape memory assisted self-healing polymer having load bearing structure. The present invention comprises several critical ingredients to achieve the desired effect in polymeric systems. First, reversible plasticity shape memory (RPSM) is needed to close cracks (from damage) and bring into near-molecular contact the surfaces created during crack formation. RPSM can be achieved in crosslinked (or high molecular weight) semicrystalline polymers whose Tg is below room temperature and Tm is above room temperature. It can also be achieved in crosslinked (alternatively, well-entangled) amorphous polymers whose Tg is no more than 15-20 C. above room temperature.

(14) Second, rebondable surfaces, such as surface-to-surface rebonding, may be required for effective self-healing. Combined with RPSM (described above) that closes the crack surfaces of damage, rebonding will enable development of some strength via molecular interactions. For example, non-covalent ionic interactions and/or polymer chain entanglements may be used for rebonding. The activation temperature for rebonding will need to roughly coincide with the activation temperature for RPSM so that a single heating event can cause both crack-closure and rebonding.

(15) Third, it is preferred that the materials remain stiff (feature high Young's modulus 1 GPa) even during heat-triggered self-healing so that the materials can still bear some mechanical load during heating and not soften. In order that the self-healing materials meet this stiffness requirementeven during heat-triggered healingpolymers or polymer combinations that feature a glass transition temperature that exceeds room temperature are preferred.

(16) The preferred approach of the present invention involves combining an ionomer with a well-entangled glassy polymer in some form as more fully discussed below. The present invention also encompasses covalently crosslinked olefinic polymers, even though their modulus is below 1 GPa, given the RPSM behavior of such polymers. The ionomeric component offers rebonding capability, while the well-entangled glassy polymer offers RPSM shape memory. Thus, heating to a temperature within the breadth of Tg is expected to enable the combination of RPSM and rebonding events to occur nearly simultaneously. The present invention may be configured into: (i) laminated films (blending by assembly) or (ii) an interpenetrating polymer network (IPN) immiscible blend as seen in FIGS. 1 and 3, respectively.

(17) The present invention provides a platform for systematic study of the ionomeric and SMP layers for their rebonding and RPSM characteristics, respectively. FIG. 1 shows a schematic depiction of this approach to combining an ionomer and SMP, where thin layers of ionomer and WEGP-type SMP (each ca. 100 m) are compression molded as a stack 10-20 layers thick. Layers could be thinner and more numerous using microlayering extrusion technology.

(18) The present invention may use compression molding of individual films and then the multilayer stacking; however, it is important to note the existence of multilayer extrusion technology for mass-production of such structures such as that disclosed in Mueller, C D; Nazarenko, S; Ebeling, T; Schuman, T L; Hiltner, A; Baer, E, Novel structures by microlayer coextrusionTalc-filled PP, PC/SAN, and HDPE/LLDPE, Polym. Eng. Sci. 37 355-362 (1997), hereby incorporated by reference in its entirety.

(19) The SMP component of the present invention should be relatively stiff, processable by conventional melt processes (molding, extrusion, etc) and feature good shape memory. One SMP that be used in lamination in accordance with the present invention is atactic poly(styrene) (PS) with molecular weight in the 200 kDa range. This polymer features good shape memory properties owing to the long relaxation time for entanglements (related to high molecular weight), rendering those entanglements as physical crosslinks. Further, the polymer features a Young's modulus in excess of 2 GPa at room temperature and a Tg near 100 C. On the downside, such a high molecular weight features a high melt viscosity, when processed at a typical processing temperature, such as 180 C. Thus, optimization of the PS component will involve balancing of shape recovery with melt viscosity. A suitable substitute for PS here is poly(methyl methacrylate) (PMMA).

(20) Another SMP for lamination is polycyclooctene (PCO), which is semicrystalline and features a lower Young's modulus of about 0.3 GPa at room temperature. However, when crosslinked (thermally by peroxide cure or with radiation by exposure to electron beam), the SMP has excellent (near-100%) shape recovery and the uncured, precursor polymer has excellent flow characteristics. This polymer provides a model system that can help with the understanding of the combination of shape memory and rebonding with an ionomer as the molecular weight of the glassy PS-based SMP is optimized.

(21) Other SMPs that may be used include poly(styrene-butadiene) copolymers, polynorbornene, poly(norbornene-co-cyclooctene), acrylic copolymers (poly(t-butyl acrylate-co-n-butyl acrylate)), and all class III (physically cross-linked glassy copolymers as shape-memory polymers) as described in C. Liu, H. Qin, and P. T. Mather, J. Mater. Chem. V17 1543-1558 (2007), hereby incorporated by reference in its entirety, including but not limited to poly(vinyl acetate), poly(vinyl acetate)/PVDF melt-miscible blend, poly(vinyl acetate)/poly(lactide) melt-miscible blend, and polyurethanes utilizing amorphous soft segments with Tg greater than room temperature, including poly(lactide) and poly(ethylene adipate) and any diisocyanate and chain extender utilized commonly in the polyurethane industry.

(22) The ionomeric component of the present invention should feature an ability to disorder and re-order (thus allow rebonding) at a temperature near the SMP activation temperature. In this case, that should occur near the glass transition temperature (Tg) for PS or the glass transition temperature (Tm) for PCO. Fundamentally, the important temperature for healing is the temperature above which flow as a liquid (which means indefinitely under such forces as pressure, gravity, or capillary action) becomes possible, i.e., above Tg for amorphous polymers and above Tm for semicrystalline polymers. Thus, the present invention takes advantage what will be referred to as the self-healing transition temperature and this term shall mean the temperature at which a healing transition as described above will occur, which may be the glass transition temperature for amorphous polymers used as the healing component of the present invention, or the melting temperature for semicrystalline polymers used as the healing component of the present invention.

(23) For this purpose, the present invention may include the poly(styrene sulfonate) (PSS) family of ionomers, which allows for variation of sulfonation levels that will affect rebonding strength and kinetics, and the counter-anion composition, which will affect the order-disorder temperature for the ionic clusters. The polymers proposed for utilization are seen in FIG. 2. Other components that may serve as the healing element of the present invention include sulfonated polymers with the same backbone chemistry of the well-entangled glassy polymer. For example, such components may comprise partially sulfonated or completely sulfonated (hereafter sulfonated) polystyrene, sulfonated polycyclooctene, sulfonated polynorbonene, sulfonated polynorborne, and sulfonated poly(styrene-butadiene). In addition, low-melting thermoplastic polymers capable of flowing and entangling at the damage sight and then recrystallizing during cooling may serve as the healing element, including poly(-caprolactone), poly(ethylene oxide), poly(ethylene glycol), poly(butylene terephthalate), poly(ethylene terephthalate), poly(ethylene-co-octene), nylon-11, nylon-12, nylon-6, and poly(ethylene adipate). In addition, glassy polymers featuring low Tg greater than room temperature and low enough molecular weight (<50,000 g/mol) to allow adequate flow during healing may serve as the healing element, including poly(n-butyl acrylate-co-methyl methacrylate), poly(t-butyl acrylate), poly(t-butyl acrylate-co-n-butyl acrylate), poly(norbornene), and poly(lactide).

(24) Standard compression molding techniques may be used to make large area films of each SMP and ionomer (rebonding) polymers with thicknesses of 0.1 mm. These films may be cut into smaller rectangular pieces approximately 2 cm6 cm and made into stacks of 10-20 films for each component that are again compression molded to develop a strong bond between layers. For the case of PCO as the SMP, a final processing step of electron-beam irradiation or peroxide cure will be required. A peroxide cure may be used as the primary step here, with use of e-beam processing only if necessary.

(25) The present invention also includes a lamination process for combining one or more layers of shape memory polymers with one or more layers of healing components as described above. First, films of PCO compounded with a small percentage (approximately 3% by weight) of peroxide-based cros slinking agent (for example, dicumyl peroxide (DCP) or benzoyl peroxide) are prepared by blending in an twin-screw extruder or by other means and compression molded or extruded into film form above the melting point of PCO and below the crosslinking temperature, for example molding at T=70 C. Separately, PCO film is prepared by compression molding or extrusion at 70 C. In both cases, the films may feature similar thicknesses ranging from about 10 microns to 1 mm. The peroxide-containing PCO films are to become the shape memory portion of the layered composite, while the pristine PCO films are to become the healing portion of the layered composite. Having prepared the films, they are stacked in alternative format with a minimum number of layers being five (5) and with no maximum number of layers beyond practical manufacturing considerations and with peroxide-containing PCO located at the outer-most (top and bottom) surfaces. Finally, the assembled stack of films is cured by heating under pressure in a mold or autoclave to a curing temperature of about 140 C. for about 30 minutes. In doing so, a good bond between layers results and excellent SMASH behavior may be witnessed. An alternative process of curing the peroxide-containing PCO films before layered assembly and molding proved unable to yield layered composites with useful adhesion between layers.

(26) The approach of the present invention is a brute force approach that integrates the shape memory and rebonding functionalities in a simple, stratified manner. The simplicity of the process holds a significant advantage in terms of making progress. Furthermore, this approach allows for independent variation of each component that will allow systematic determination of the best SMP and rebonding combinations. It will also reveal any synergistic compositions. As a disadvantage, the approach will be limited in form primarily to self-healing films; however, it may be possible to pelletize a micro-layered extrudate to yield a molding resin with self-healing characteristics.

(27) In an alternative embodiment, the present invention comprises an interpenetrating, immiscible polymer network to create a self-healing polymer that utilizes a microstructure that provides similar functionality to the inventor's prior work with epoxy/PCL blends, but in connection with thermoplastics. A schematic of an IPN microstructure featuring interpenetration of a WEGP-type SMP (black) and an ionomeric rebonding polymer (gray) is shown in FIG. 3. This embodiment involves the use of a polymer blend that can form such a structure, with one component serving as a WEGP-type SMP and the other serves as the ionomeric rebonding component. Further, the desired phase behavior would be of the upper-critical solution temperature (UCST) type, where in the two-phase, IPN microstructure would form upon cooling (not upon heating, as is the case for LCST blends) after extrusion or molding. Blends of polystyrene and polystyrene sulfonate (PSS) feature offer such UCST phase-separation behavior. The preparation of IPN microstructure featuring interpenetration of a WEGP-type SMP and an ionomeric rebonding polymer may be accomplished by electrospinning the SMP to create nanosize fibers and molding into the healing agent by compression molding. Alternatively, the preparation could be prepared by electrospinning both the SMP and the SH polymer at the same time to create an interpenetrating network.

(28) The present invention may use PS/PSS blends of the type reported by Winey (2007), hereby incorporated by reference in its entirety. Values for the sulfonation level and weight percentage of PSS incorporated may be selected so as to include three (3) levels of each for initial evaluation of properties, including thermal, mechanical, and microstructure. From among those nine (9) samples, three may be selected for more detailed analysis of tensile testing and self-healing.

(29) Polymer processing using a blend process may be employed to avoid lamination. A combination of solvent-casting (small-scale blending), extrusion (Randcastle; 50 gram scale), and compression molding may be used to prepare films. The IPN morphology will be achieved and examined by passing above the UCST (measured using rheology and differential scanning calorimetry, DSC; see below) to the single-phase region and then cooling into the two phase region and annealing. The annealing will proceed until the microstructure stops coarsening, as inspected with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and small-angle x-ray scattering (SAXS) measurements.

(30) The IPN approach of the present invention, while having more compositional variables, is attractive from the standpoint of processing. Indeed, this approach is quite amenable to the creation of a molding resin in pellet form that could be used by molding companies to manufacture self-healing, structural plastics. A disadvantage relative to the lamination approach appears at the research level since the shape memory and rebonding properties will be convoluted and may be challenging to decouple.

(31) For each group of shape memory polymers synthesized, the following characterization tests may be conducted, which will confirm the structures synthesized while also forecasting the expected shape memory properties to be characterized later on a subset of materials synthesized. Gel Permeation Chromatography (GPC) may be used to measure the molecular weights (Mn and Mw) of the polymers synthesized. As Mw is strongly correlated with strength and durability of a polymer, while also increasing viscosity, this test may be used to optimize the molecular weight of polymers selected.

(32) Thermal analysis based on differential scanning calorimetry (DSC) may be used to measure the melting transitions and glass transitions of the selected polymers, adjusting composition as needed to fine tune the Tg (or Tm) of the SMP. Further, DSC may be be used to ascertain, in conjunction with SAXS and rheology, the phase separation of PS and PSS.

(33) X-ray diffraction analysis may be used to characterize selected polymer systems and to measure the degree of crystallinity for crystalline phases within each material using wide-angle x-ray diffraction and associated standard methods. X-ray diffraction will be helpful in comparing degrees of crystallinity, degrees of orientation, and crystallization kinetics (complementing DSC) for those samples that are strong candidates for the present invention but for which there exists a need to fine-tune crystallization behavior. SAXS studies with the same instrument will be helpful in ascertaining the nature of the ionomeric associations pertinent to rebonding.

(34) TEM may be used to evaluate laminate and blend microstructures. Samples can be microtomed (or cryo-microtomed, if necessary) and then evaluated using TEM equipment (such as the Delong LVEM-5 instrument) to see the length scale and degree of phase separation in the materials for correlation with self-healing performance.

(35) Melt rheology may be performed, such as with a TA Instruments AR-G2 melt rheometer in a steady shearing mode, to study the flow characteristics of individual components and blends pertinent to processing. Furthermore, the AR-G2 may be used in an oscillatory mode to ascertain the linear viscoelastic properties during cooling through the UCST phase boundary, confirming the results of Winey as applied to the present invention. Such results may be corroborated with SAXS results.

(36) Dynamic Mechanical Analysis (DMA) may be used to determine the stiffness of the laminates and blends as a function of temperature, such as by using a Q-800 DMA (TA Instruments).

(37) Tensile testing may be used to determine the durability of the polymers and may be assessed using ultimate tensile testing such as with a Linkam TST-350 apparatus. Thus, for each composition, it is possible to measure the Young's modulus, yield stress, elongation at break, and engineering toughness. Those materials made in accordance with the present invention that combine good reversible actuation with high toughness will be highly ranked from among the candidates for transition to further development.

(38) The present invention may also be evaluating using self-healing characterization tests on materials produced using both approaches. In this testing, deeply double-edge notched (DDEN) test specimens that feature two opposing cracks may be used to simulated damage, as seen in FIG. 4. More specifically, self-healing testing wherein film specimens are prepared with a custom deeply double-edge notched (DDEN) punch (a) to yield specimens of the type shown in (b). Testing will involve tensile loading to incomplete failure (c; blue line) and recovery of mechanical properties (c; red lines). Importantly, the healing temperature may be varied for comparison with the material stiffness as ascertained with DMA. The trade-off (if any) may be quantfied between extent of healing and stiffness during healing, noting that stiffness is preferably to be maintained during healing. Rebonding and RPSM may be evaluated separately if necessary; however, if initial self-healing experiments reveal excellent performance, this separate testing will be obviated.

Example 1

(39) As described above, the SMP component can comprise polystyrene. Polystyrene having a molecular weight of 283 kDa and a glass transition temperature of 106.2 degrees C. was formed into a strip and tested for crack closure by punching notches in the strip, stretching to propagate the cracks, and then heated to initiate crack closure at 130 degrees C. As seen in FIG. 5, polystyrene was capable of restoring its original shape after physical damage. The composition had a storage modulus higher than 1 GPa at room temperature.

Example 2

(40) Poly(styrene-butadiene) having a molecular weight from 620 to 830 KDa and a glass transition temperature between 25 and 39 degrees C. was tested as set forth in Example 1. As seen FIG. 6 poly(styrene-butadiene) was capable of restoring its original shape after physical damage. The composition had a storage modulus around than 1 GPa at room temperature.

Example 3

(41) Polynorbornene having a molecular weight from 340 to 411 KDa and a glass transition temperature between 55 and 57 degrees C. was tested as set forth in Example 1. As seen FIG. 7, polynorbornene was capable of restoring its original shape after physical damage. The composition had a storage modulus close to 1 GPa at room temperature.

Example 4

(42) Poly(tBa-BA), as seen in FIG. 8, having a molecular weight of 236 KDa and a glass transition temperature of 38.5 degrees C. was tested as set forth in Example 1. As seen FIG. 9, poly(tBa-BA) was capable of restoring its original shape after physical damage. The composition had a storage modulus close to 1 GPa at room temperature.

Example 5

(43) Crosslinked polycyclooctene (PCO), as seen in FIG. 10, having a molecular weight of 90 KDa and a melting point of between 50 and 53 degrees C. was tested as set forth in Example 1. As seen FIG. 11, crosslinked polycyclooctene was capable of restoring its original shape after physical damage. The composition had a storage modulus close to 100 MPa at room temperature and up to 600 MPa at 60 degrees C.