Remote activation of thermo-reversible dry adhesives

The technical field generally relates to polymer adhesives and more specifically to thermo-reversible dry adhesives that may be remotely activated.

Thermo-reversible dry adhesives are made of at least one layer of an elastomeric dry-adhesive and at least one layer of a shape memory polymer (SMPs). SMPs represent responsive polymers that can fix to deformed temporary shapes and recover to their permanent (original) shapes only upon external stimuli such as heating. By heating and imposing a load during subsequent cooling to transform the dry adhesive to a temporary shape, adhesive strength may be increased. By subsequently heating the dry adhesive in the absence of load, therein transforming the dry adhesive back to its permanent shape, adhesive strength may be decreased. These dry adhesives may thus be used to reversibly couple together substrate materials.

One embodiment includes a thermo-reversible dry adhesive material, having shape memory polymer characteristics, introduces one or more particles of micro- or nano-particles to at least one layer of the at least one layer of shape memory polymer. These particles may be remotely activated to induct heat sufficient to aid in transforming the dry adhesive material from its temporary shape to its permanent shape, or form its permanent shape to its temporary shape, to be coupled or uncoupled from a substrate material.

In one exemplary embodiment, the thermo-reversibly dry adhesive material may be formed by mixing one or more electrically conductive particles of micro- or nano-meter diameter are mixed into at least one layer of the shape memory polymer. When eddy currents are applied to the electrically conductive particles, heat is generated that may aid in transforming the dry adhesive between its temporary shape and its permanent shape.

Another exemplary embodiment discloses a thermo-reversible dry adhesive material in which magnetic particles of micro- or nano-meter diameter are mixed into at least one layer of the shape memory polymer. Magnetic hysteresis heating is the result of energy absorption that results due to friction of magnetic micro-volumes or domains with the shape memory polymer rotate to align themselves with the periodic re-orientation of the external magnetic field that may aid in transforming the dry adhesive between its temporary shape and its permanent shape.

Thus, the exemplary embodiments may allow the dry adhesive to be remotely activated by inductive heating (through the use electricity or magnetism), as opposed to directly heating the dry adhesive in an oven or similar heating device, to transform the dry adhesive.

Such a remote mechanism may be desirable for use in systems wherein direct heating to return the dry adhesive to its permanent shape may be problematic, costly or may adversely affect the substrates to which the dry adhesive has been previously applied.

Other exemplary embodiments will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a dry adhesive material according to one exemplary embodiment having a shape memory polymer;

FIG. 2 is the dry adhesive material of FIG. 1 transformed from its permanent shape to a temporary shape;

FIG. 3 is the dry adhesive material of FIG. 2 coupled to a substrate material; and

FIG. 4 is the dry adhesive material of FIG. 2 coupled between two substrate materials.

The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses.

Shape memory polymers (SMPs) represent responsive polymers that can fix to deformed temporary shapes and recover to their permanent (original) shapes only upon external stimuli. SMPs may be available exhibiting a dual shape memory effect (DSME), wherein the SMP can only memorize one temporary shape in addition to its permanent shape in each shape memory cycle. It is also contemplated that SMPs may be available exhibiting a triple shape memory effect (TSME) or greater, wherein the SMP can memorize two distinct temporary shapes (for a TSME) or more in addition to its permanent shape in each memory cycle.

In general, to transform an SMP from its permanent shape to its temporary shape, the permanent shape may be heated to a first elevated temperature and then deformed under stress to yield the first temporary shape, a shape which may be different in visual appearance from the permanent shape. By definition, the first elevated temperature is a temperature sufficiently high to ensure a phase transition of the SMP (i.e. is a temperature above the glass transition temperature (T g ) of SMP). The SMP may then be cooled under stress to a temperature below the glass transition temperature of one SMP, wherein the stress may be relieved while maintaining the first temporary shape. To recover the permanent shape from the first temporary shape, the SMP may be reheated to the first elevated temperature in the absence of stress.

An exemplary embodiment creates a dry adhesive including at least one layer of a shape memory polymer (SMP) that takes advantage of the SMP's ability to transform between its permanent shape and a temporary shape and further allows remote activation of the SMP adhesive material (i.e. not through direct heating of the entire dry adhesive) to actuate such a transformation.

This remote activation may be particularly desirable when direct heating is not readily available, or inconvenient, or costly. This remote activation may also be particularly desirable for the subsequent removal of a previously applied dry adhesive to one or more substrate materials.

In one exemplary embodiment of the present invention, as shown in FIGS. 1 and 2 , the dry adhesive 10 may include at least one layer of an elastomeric adhesive material 12 and one or more layers of a shape memory polymer material 14 . At least one layer of the one or more layers of the shape memory polymer material 14 may include particles 18 of micro- or nano-meter diameter. The dry adhesive 10 may also include one or more functional groups FG that form a portion of either the adhesive material 12 or the shape memory polymer material 14 , or both.

In order to utilize the shape memory properties of the dry adhesive 10 , the dry adhesive 10 may be hot-pressed under a load and cooled under load. The hot pressing process may transform the layers of the shape memory polymer 14 from its permanent shape to its temporary shape. In conjunction therewith, this transforms the dry adhesive 10 from its permanent shape 10 A, as shown in FIG. 1 , to its temporary shape 10 B, as shown in FIG. 2 .

In its temporary shape 10 B, any functional groups FG located on the surface portion of the dry adhesive 10 , in either the elastomeric layer 12 or the shape memory polymer layer 14 , may be more accessible for interaction to a single substrate material 22 , as shown in FIG. 3 , or individually to one or both of the two substrate materials 22 and 24 , as shown in FIG. 4 . This may allow for increased adhesion of the dry adhesive to the substrate materials 22 , 24 in the temporary shape 10 B than in the permanent shape 10 A.

The functional groups FG may interact with the substrate materials 22 , 24 to create increased reversible adhesion from a number of different non-covalent bonding adhesion mechanisms. While FIGS. 3 and 4 illustrate lines between FG and the substrate 22 or 24 that are typically associated with covalent bonds, the lines illustrated are merely illustrative of non-covalent bonding adhesion interaction between the functional group FG and respective substrate 22 , 24 and are not intended to represent chemical bonding therebetween. A non-exhaustive list of potential non-covalent bonding adhesion mechanisms that may be contributed by the functional groups FG being brought into closer proximity to the respective substrate 22 or 24 include hydrogen bonding, acid-base interaction, ionic bonding, aromatic interaction (or π-π interaction), van der Waal forces, increased dry adhesive surface roughness, or combinations of two or more of these non-covalent bonding adhesion mechanisms.

The introduction of the particles 18 may provide a different method for transforming the shape memory polymer layer 14 , and hence the dry adhesive 10 , from the temporary shape 10 B back to the permanent shape 10 A that does not require the direct heating of the entire dry adhesive 10 , and any coupled substrate 22 or 22 , 24 , above the T g of the shape memory layer or layers 14 . In the exemplary embodiments provided herein, the particles 18 may be capable of being remotely activated to enable inductive heating of the shape memory polymer material 14 sufficient to transform, or aid in transforming, the dry adhesive 10 from its temporary shape (shown as 10 B in FIG. 2 below) to its permanent shape 10 A. This, in turn, may increase the distance between the functional groups FG and the respective substrate 22 or 24 , and hence decreases the amount of interaction between the functional groups FG and the respective substrate 22 or 24 , which allows the dry adhesive 10 to be easily uncoupled from the respective substrate 22 or 24 .

In one exemplary embodiment, the particles 18 may be electrically conductive particles of micro- or nano-meter diameter that are mixed into one or more layers of the shape memory polymer 14 . When eddy currents are applied to the electrically conductive particles 18 , heat is generated that may aid in transforming, or alone be sufficient to transform, the dry adhesive 10 from its permanent shape 10 A to its temporary shape 10 B for increased adhesion to a substrate 22 or substrates, 22 , 24 , or from its temporary shape 10 B to its permanent shape 10 A for subsequent removal from a substrate 22 or substrates 22 , 24 . Examples of electrically conductive particles that may be used include ceramic particles, ceramic wires, metallic particles and metallic wires.

Another exemplary embodiment discloses a thermo-reversible dry adhesive material in which the particles 18 may be magnetic particles of micro- or nano-meter diameter that are mixed into the shape memory polymer 14 . Magnetic hysteresis heating is the result of energy absorption that results due to friction of magnetic micro-volumes or domains with the shape memory polymer 14 rotate to align themselves with the periodic re-orientation of the external magnetic field that may aid in transforming, or alone be sufficient to transform the dry adhesive 10 from its permanent shape 10 A to its temporary shape for increased adhesion to a substrate 22 or substrates 22 , 24 , or from its temporary shape 10 B to its permanent shape 10 A for subsequent removal from a substrate 22 or substrates 22 , 24 . Examples of magnetic particles that may be used include metallic particles.

In another exemplary embodiment, a combination of electrically conductive and magnetic particles may be used as the particles 18 . Also, some of the particles 18 may function as both electrically conductive and magnetic particles, including particular metal particles, and thus may be activated remotely by two separate mechanisms.

Exemplary embodiments of shape memory polymer 14 materials include, but are not limited to epoxy polymers, polyurethanes or polyacrylates.

Exemplary embodiments of electrically conductive or magnetic particles 18 include, but are not limited to, organic materials, metallic materials, or ceramic materials. Specific exemplary embodiments include copper, iron, or Fe 3 O 4 .

The substrate materials 22 and 24 may be formed of the same material or separate materials. Non-limiting examples of substrate materials 22 and 24 include glass substrates, polymer substrates, metal substrates and other non-metal substrates.

By increasing the likelihood of such interaction with the substrate 22 or 24 , which occurs when the adhesive material 10 is transformed to the temporary state 10 B and the functional groups are moved such that there is more possible interaction with the substrate 22 or 24 , the adhesive strength may be increased as compared with when the adhesive material 10 is in its permanent state 10 A.

Weighted copper nanoparticles (electrically conductive) were added into Jeffamine D-230 and the mixture was sonicated for 30 minutes. EPON 826 (a bisphenol A-type epoxy resin pre-melted at 75 degrees Celsius for 15 minutes), NGDE (a flexible polyether-type epoxy), and Jeffamine D-230 (a polyether diamine) (with copper particles) at a mole ratio of 1.6/0.4/1 were introduced into a glass vial, which was shaken vigorously by hand for about ten seconds to mix the components. The mixture was poured into an aluminum mold, cured at 100 degrees Celsius for 1 hour, and postcured at 130 degrees Celsius for 1 hour to yield the composite shape memory polymer (SMP). Based on this process, composite samples with weight percentages of copper content from 1% to 90% can be produced. Alternatively, magnetic field responsive particles such as Fe 3 O 4 may replace the copper particles in the formulation to result in magnetically responsive SMP.

The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.