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U.S. Patent Feb. 20,2007 Sheet 4 of 4 US 7,179,864 B2
This application is a divisional of U.S. Ser. No. 10/345, 498, filed on 16 Jan, 2003 now U.S. Pat. No. 6,875,818.
BACKGROUND OF THE INVENTION
The present invention relates to polymer nano-strings, methods for their preparation, and their use as, for example, additives for rubber, including natural and synthetic elastomers. The invention advantageously provides several mechanisms for surface modifications, functionalization, and general characteristic tailoring to improve performance in rubbers, elastomers, and thermoplastics.
Tires are often subjected to rough road conditions that produce repetitive, localized high-pressure pounding on the tire. These stresses can cause fatigue fracture and lead to crack formation and growth. This degradation of the tire has also been referred to as chipping or chunking of the tread surface or base material.
In an attempt to prevent this degradation, it is known to add reinforcements such as carbon black, silicas, silica/ silanes, or short fibers. Silica has been found advantageous due to its ability to deflect and suppress cut prolongation, while silanes have been added to bind the silica to unsaturated elastomers. The fibers that have been added include nylon and aramid fibers.
It is also known that the addition of polyolefins to rubber compositions can provide several beneficial properties. For example, low molecular weight high density polyethylene, and high molecular weight, low density polyethylene, are known to improve the tear strength of polybutadiene or natural rubber vulcanizates. In the tire art, it has also been found that polyethylene increases the green, tear strength of carcass compounds and permits easy extrusion in calendaring without scorch. Polypropylene likewise increases the green strength of butyl rubber. Polypropylene has also been effective in raising the static and dynamic modulus of rubber, as well as its tear strength.
Although the addition of polyolefins to rubber compositions is known to provide several beneficial effects, the addition of polyolefin to tire recipes may also have a deleterious effect on other mechanical and wear properties of tires, as well as handling and ride of the tire.
Polymer nano-particles have attracted increased attention over the past several years in a variety of fields including catalysis, combinatorial chemistry, protein supports, magnets, and photonics. Similarly, vinyl aromatic (e.g. polystyrene) microparticles have been prepared for uses as a reference standard in the calibration of various instruments, in medical research and in medical diagnostic tests. Such polystyrene microparticles have been prepared by anionic dispersion polymerization and emulsion polymerization.
Nano-particles preferably are monodisperse in size and uniform in shape. However, controlling the size of nanoparticles during polymerization and/or the surface characteristics of such nano-particles can be difficult. Accordingly, achieving better control over the surface composition of such polymer nano-particles also is desirable.
Nano-particles can serve as discrete particles uniformly dispersed throughout a host composition. Rubbers may be advantageously modified by the addition of various polymer compositions. The physical properties of rubber moldability and tenacity are often improved through such modifications. Of course, however, the simple indiscriminate addition of
nano-particles to rubber is likely to cause degradation of the matrix material, i.e., the rubber characteristics. Moreover, it is expected that the selection of nano-particles having suitable size, material composition, and surface chemistry, etc,
5 will improve the matrix characteristics. Polymer nanostrings may also serve as a reinforcement material for rubber compositions in order to overcome the above-mentioned drawbacks of polyolefin and silica reinforcement. Polymer nano-strings are capable of dispersing evenly throughout a
10 rubber composition, while maintaining a degree of entanglement between the individual nano-strings, leading to improved reinforcement.
In this regard, development of polymer nano-strings having a surface layer which would be compatible with a wide variety of matrix materials is desirable because discrete strings could likely disperse evenly throughout the host to provide a uniform matrix composition. However, the development of a process capable of reliably producing acceptable nano-strings has been a challenging endeavor. Moreover, the development of a solution polymerization process producing reliable polymer nano-strings advantageously employed in rubber compositions, has been elusive.
25 SUMMARY OF THE INVENTION
A polymer nano-string composition including a poly (alkenylbenzene) core and a surface layer of poly(conjugated diene) is provided. The nano-strings have a mean
30 average diameter of less than about 100 nm and a length of between about 1 and 1000 (jm, the length being greater than the diameter. Preferably, the nano-strings have a length of at least about 10 urn.
A polymer nano-string including polyalkylene is pro
35 vided. According to the embodiment, these nano-strings include a poly(alkenylbenzene) core and a polyalkylene surface layer including at least one alkylene monomer unit. The nano-strings have a mean average diameter less than about 100 nm and a length between about 1 and 1000 urn.
40 A process for forming polymer nano-strings is also provided. The process includes polymerizing alkenylbenzene monomer and conjugated diene monomer in a hydrocarbon solvent to form a block copolymer. After formation of the block copolymer, a polymerization mixture including worm
45 like micelles of the block copolymer is formed by adjusting the concentration of the polymerization mixture until the solid content is between about 0.01 to 10% or between about 18 to 60%. At least one crosslinking agent is then added to the polymerization mixture to form crosslinked, polymer
50 nano-strings having a rope-like structure and including an alkenylbenzene core and a conjugated diene surface from the micelles. The poly(conjugated diene) layer is optionally hydrogenated to form nano-strings containing a poly(alkenylbenzene) core and a polycrystalline outer layer.
55 A rubber compound composition containing the inventive nano-strings is provided. Such compound shows its relatively high hysterisis, good tensile strength, strong resistance to creep, and high temperature resistance. A process of making the rubber compound is similarly provided. Herein throughout, unless specifically stated otherwise:
i. "vinyl-substituted aromatic hydrocarbon" and "alkenylbenzene" are used interchangeably; and
ii. "rubber" refers to rubber compounds, including natural 65 rubber, and synthetic elastomers including styrene-buta
diene rubber, ethylene propylene rubber, etc, which are known in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of the design structure of a nano-string and a cross-section view thereof, according to the present invention. 5
FIG. 2 is an atomic force microscopy photograph of polymer nano-strings, on a mica-surface, produced according to the present invention.
FIG. 3 is a transmission electron microspcopy photograph of a single polymer nano-string produced according to the present invention.
FIG. 4 is a transmission electron microscopy photograph of multiple tangled nano-strings produced according to the present invention.
DETAILED DESCRIPTION OF THE
General Nano-Particle Process of Formation 20
This application incorporates by reference U.S. Pat. No. 6,437,050, U.S. Ser. No. 10/038,748 (filed Dec. 31, 2001) and Ser. No. 10/223,393 (filed Aug. 19, 2002).
One exemplary polymer nano-string of the present invention is formed from diblock copolymer chains having a 25 poly(conjugated diene) block and a poly(alkenylbenzene) block. The poly(alkenylbenzene) blocks may be crosslinked to form the desired nano-strings. The nano-strings have diameters—expressed as a mean average diameter—that are preferably less than about 100 nm, more preferably less than about 75 nm, and most preferably less than about 50 nm. The nano-strings have a rope-like shape, with a length of between about 1 and 1000 urn, more preferably between about 2 and 100 urn. The nano-strings preferably retain their discrete nature with little or no polymerization between strings. The nano-particles preferably are substantially monodisperse and uniform in shape.
The nano-stings preferably have a high glass-transition temperature (Tg), contributing to the improved reinforce- 4Q ment capabilities. Preferably, the Tg is between about 50 and 220° C, more preferably between about 90 and 200° C.
The nano-strings are preferably formed by dispersion polymerization, although emulsion polymerization is also contemplated. Hydrocarbons are preferably used as the 45 dispersion solvent. Suitable solvents include aliphatic hydrocarbons, such as pentane, hexane, heptane, octane, nonane, decane, and the like, as well as alicyclic hydrocarbons, such as cyclohexane, methyl cyclopentane, cyclooctane, cyclopentane, cycloheptane, cyclononane, cyclode- 50 cane, and the like. These hydrocarbons may be used individually or in combination. However, as more fully described herein below, selection of a solvent in which one polymer forming the nano-particles is more soluble than another polymer forming the nano-particles is beneficial to 55 micelle formation.
With respect to the monomers and solvents identified herein, nano-strings can be formed by maintaining a temperature and concentration that is favorable to polymerization of the selected monomers in the selected solvent(s). 60 Preferred temperatures are in the range of about -40 to 250° C, with a temperature in the range of about 0 to 150° C. being particularly preferred. As described in more detail below, the interaction of monomer selection, temperature, and solvent facilitates the formation of diblock polymers 65 which form worm-like micelles and ultimately the desired polymer nano-strings.
According to one embodiment of the invention, a diblock copolymer is formed of vinyl aromatic hydrocarbon monomers and conjugated diene monomers in a hydrocarbon solvent. The diblock copolymer contains at least a first block that is substantially soluble in the hydrocarbon solvent, preferably a conjugated diene monomer, and a second block which is less soluble in the hydrocarbon solvent, preferably a vinyl-substituted aromatic hydrocarbon monomer. Moreover, in one preferred embodiment, a vinyl-substituted aromatic hydrocarbon monomer is chosen, the polymer of which is insoluble in the dispersion solvent.
As is known in the art, such a diblock copolymer may be formed by living anionic polymerization, in which a vinylsubstituted aromatic hydrocarbon monomer is added to a completely polymerized conjugated diene monomer. Another method of forming substantially diblock copolymers is the living anionic copolymerization of a mixture of a conjugated diene monomer and a vinyl-substituted aromatic hydrocarbon monomer in a hydrocarbon solvent, particularly, in the absence of certain polar additives, such as ethers, tertiary amines, or metal alkoxides which could otherwise effect the polymerization of the separately constituted polymer blocks. Under these conditions, the conjugated diene generally polymerizes first, followed by the polymerization of the vinyl-substituted aromatic hydrocarbon. Of course, certain advantages, as described below, may be achieved via a random polymerization of at least one block of the polymer.
Nonetheless, it is generally preferred that a conjugated diene block polymerize first, followed by a vinyl-substituted aromatic, positioning the living end of the polymerizing polymer on the vinyl aromatic block to facilitate later crosslinking.
Such copolymers, formed by either method, are believed to aggregate to form micelle-like structures, with for example, the vinyl-substituted aromatic blocks directed toward the center of the micelle and the conjugated diene blocks extending as tails therefrom. By maintaining a relatively higher ratio of vinyl-substituted aromatic block to conjugated diene, a worm-like micelle shape is formed. The micelle formation may also be controlled by the maintenance of favorable solids content and favorable polymerization temperatures. For example, a further hydrocarbon charge may be made to control the solids content of the polymerization mixture. A preferred solids content is between about 0.01 and 50%, with a solids content between about 0.1-10% or about 18 to 60% being more preferred. The control of the solids content within the desirable range is believed to help achieve formation of the desired wormlike shape of the micelles. Moreover, these steps may be used to take advantage of the general insolubility of the vinyl-aromatic blocks. An exemplary temperature range for micelle formation is between about 40 and 150° C, more preferably between about 40 and 120° C, and most preferably between about 50 and 100° C.
After the micelles have formed, additional conjugated diene monomer and/or vinyl-substituted aromatic hydrocarbon monomer can be added to the polymerization mixture as desired.
After formation of the micelles, a cross-linking agent is added to the polymerization mixture. Preferably, a crosslinking agent is selected which has an affinity for the vinylsubstituted aromatic hydrocarbon monomer blocks and migrates to the center of the micelles due to its compatibility with those monomer units and initiator residues present in the center of the micelle and its relative incompatibility with the dispersion solvent and monomer units present in the