Shape memory polymers

Details Shape memory polymers Shape memory polymers

2002-05-08

2004-04-13

Gorr, Rachel (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C528S076000, C528S080000, C528S176000, C528S272000, C528S193000, C528S196000, C528S271000, C525S415000, C525S440030, C525S450000, C525S903000

Reexamination Certificate

active

06720402

ABSTRACT:

BACKGROUND OF THE INVENTION
This application is generally in the area of shape memory polymers, and more particularly to shape memory polymers having enhanced performance characteristics and more than one shape in memory.
Shape memory is the ability of a material to remember its original shape, either after mechanical deformation (FIG.
1
), which is a one-way effect, or by cooling and heating (FIG.
2
), which is a two-way effect. This phenomenon is based on a structural phase transformation.
The first materials known to have these properties were shape memory metal alloys (SMAs), including TiNi (Nitinol), CuZnAl, and FeNiAl alloys. The structure phase transformation of these materials is known as a martensitic transformation. These materials have been proposed for various uses, including vascular stents, medical guidewires, orthodontic wires, vibration dampers, pipe couplings, electrical connectors, thermostats, actuators, eyeglass frames, and brassiere underwires. These materials have not yet been widely used, in part because they are relatively expensive.
Scientists are actively developing shape memory polymers (SMPs) to replace or augment the use of SMAs, in part because the polymers are light, high in shape recovery ability, easy to manipulate, and economical as compared with SMAs. In the literature, SMPs are generally characterized as phase segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition temperature of the hard segment.
When the SMP is heated above the melting point or glass transition temperature of the hard segment, the material can be shaped. This (original) shape can be memorized by cooling the SMP below the melting point or glass transition temperature of the hard segment. When the shaped SMP is cooled below the melting point or glass transition temperature of the soft segment while the shape is deformed, that (temporary) shape is fixed. The original shape is recovered by heating the material above the melting point or glass transition temperature of the soft segment but below the melting point or glass transition temperature of the hard segment. In another method for setting a temporary shape, the material is deformed at a temperature lower than the melting point or glass transition temperature of the soft segment, resulting in stress and strain being absorbed by the soft segment. When the material is heated above the melting point or glass transition temperature of the soft segment, but below the melting point (or glass transition temperature) of the hard segment, the stresses and strains are relieved and the material returns to its original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabilities of a material are the shape recovery of the original shape and the shape fixity of the temporary shape.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress, particularly at the melting point or glass transition temperature of the soft segment. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above the melting point or glass transition temperature of the soft segment. Also, the hardness of the material changes dramatically when the soft segment is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature of the soft segment, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles, and can be heated above the melting point of the hard segment and reshaped and cooled to fix a new original shape.
The shape memory effect exists for polymers (e.g. heat-shrinkable films). However, it is not a specific bulk property, but results from the polymer's structure and morphology. The effect is persistent in many polymers, which might differ significantly in their chemical composition. However only a few shape memory polymer systems have been described in the literature (Kim, et al., “Polyurethanes having shape memory effect,”
Polymer
37(26):5781-93 (1996); Li et al., “Crystallinity and morphology of segmented polyurethanes with different soft-segment length,”
J. Applied Polymer
62:631-38 (1996); Takahashi et al., “Structure and properties of shape-memory polyurethane block copolymers,”
J. Applied Polymer Science
60:1061-69 (1996); Tobushi H., et al., “Thermomechanical properties of shape memory polymers of polyurethane series and their applications,”
J. Physique IV
(Colloque C1) 6:377-84 (1996)).
Examples of polymers used to prepare hard and soft segments of SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; U.S. Pat. No. 5,665,822 to Bitler et al.; and Gorden, “Applications of Shape Memory Polyurethanes,”
Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, SMST International Committee
, pp. 115-19 (1994). The SMPs that have been developed thus far appear to be limited to being able to hold only one temporary shape in memory. It would be advantageous to provide SMPs that are able to form objects which are able to hold more than one shape in memory.
It is therefore an object of the present invention to provide SMPs that are able to form objects which are able to hold more than one shape in memory.
It is another object of the present invention to provide SMPs with physical and chemical properties and chemical structures which are different than those in conventional SMPs.
It is still another object of the present invention to provide SMPs with shapes in memory that are elicited by a stimulus other than temperature.
SUMMARY OF THE INVENTION
Shape memory polymer compositions, articles of manufacture thereof, and methods of preparation and use thereof are described. In a preferred embodiment, the shape memory polymer composition can hold more than one shape in memory. For example, the composition can include a hard segment and at least two soft segments. The T
trans
of the hard segment is at least 10° C., and preferably 20° C., higher than the T
trans
of one of the soft segments, and the T
trans
of each subsequent soft segment is at least 10° C., and preferably 20° C., lower than the T
trans
of the preceding soft segment. A multiblock copolymer with a hard segment with a relatively high T
trans
and a soft segment with a relatively low T
trans
can be mixed or blended with a second multiblock copolymer with a hard segment with a relatively low T
trans
and the same soft segment as that in the first multiblock copolymer. Since the soft segments in both multiblock copolymers are identical, the polymers are miscible in each other when the soft segments are melted. The resulting blend has three transition temperatures: one for the first hard segment, one for the second hard segment, and one for the soft segment. Accordingly, these materials are ab

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