Hydrogel and method of making

Details Hydrogel and method of making Hydrogel and method of making

1999-09-30

2001-05-15

Lipman, Bernard (Department: 1713)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C524S827000, C524S829000, C524S839000, C524S831000, C525S329200, C525S354000

Reexamination Certificate

active

06232406

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hydrogels and, more particularly, to methods of making hydrogels utilizing controlled hydrolysis by self-termination.
2. Information Disclosure Statement
Polyacrylonitrile (PAN) polymers are frequently used for fabrication of membranes, for manufacture of textile fibers, for production of carbon fibers and hydrogels, or as engineering plastics. Their composition vary widely from pure PAN homopolymer to copolymers of acrylonitrile (AN) with to about 20% molar of various comonomers. AN is typically combined either with hydrophilic co-monomers (such as acrylamide, vinyl pyrrolidone, styrene sulfonic acid, vinylsulfonic acid etc.) or with hydrophobic comonomers (such as alkyl acrylates or methacrylates, styrene, vinylchloride, methylstyrene, vinylpyrridine etc.). Such copolymers are usually considered to be PAN as long as they still retain the main characteristics of PAN, namely, high crystallinity and high melting point in absence of PAN solvents. PAN is practically unmeltable because its melting temperature (theoretically over 320° C.) is higher than its decomposition temperature (PAN becomes discolored at temperatures above about 150° C. and above about 200° C. it turns into insoluble, non-meltable precursors of graphite).
PAN has a unique crystalline structure with main X-ray diffraction periodicity of about 5.2 Angstroms, insensitivity to stereo-regularity of the polymer and lateral organization of crystallites in oriented states. Other typical properties of PAN are excellent environmental stability and high tensile strength, particularly in oriented state.
As other non-meltable polymers (e.g., aramides), PAN have to be processed from solutions in suitable solvents (such as DMSO, DMF, concentrated solutions of ZnCl
2
and some other) using a suitable “wet” method (for instance, by coagulation, dry spinning etc.)
Some of the AN copolymers are rendered meltable by introduction of suitable comonomers. Such copolymers are processable by usual plastic-processing methods such as extrusion or injection molding. However, this cannot be achieved without a substantial destruction of crystalline structure and loss of certain valuable properties, such as high thermal stability. There is a substantial structural difference between meltable “modacrylic” copolymers and non-meltable PAN.
AN is sometimes combined with highly polar comonomers to increase its hydrophilicity and improve certain desirable properties such as dyeability of textile fibers, wettability of separation membranes, and the like. PAN copolymers with hydrophilic comonomers are sometimes used to form hydrogels, i.e. water-insoluble elastomers containing large amount of water. Hydrogels are particularly useful for biomedical products such as implants, wound dressings, contact lenses, bioseparation membranes and lubricious coatings.
Hydrogels can be either of a conventional “thermoset” type with a covalent network, or “thermoplastic” hydrogels with physical network formed by interactions between hydrophobic groups. Covalently crosslinked PAN-based hydrogels are sometimes synthesized by a combination of polymerization of AN monomer in aqueous solvents, such as concentrated solutions of ZnCl2 or HNO3. Hydrophilic comonomers are often derivatives of acrylic acid, such as salts, esters, amides, amidines, hydrazidines and the like. If such comonomers are copolymerized with AN, they are randomly distributed in the PAN chain. Copolymerization of AN with hydrophilic comonomers in concentrated zinc chloride solutions to form crosslinked hydrogels is described in U.S. Pat. No. 3,812,071 (A. Stoy). Modification of PAN properties by copolymerization with carboxylated co-monomers and subsequent treatment with alkalies is described in U.S. Pat. No. 4,272,422 (Tanaka). A relatively low concentration of randomly distributed hydrophilic monomers is needed to achieve solubility in water or very high swelling. This is because the crystallization capability requires a certain minimum length of the sequence of nitrile groups in 1,3 positions. As the length of continuous sequences of AN units (ie., the sequence of AN units between two non-AN units) decreases with increasing content of randomly distributed non-AN comonomers, so decrease crystallinity, thermal stability and other useful properties of PAN polymers. It was observed that properties of such hydrogels can be improved if polymerization or copolymerization of AN and aqueous inorganic solvents is followed by acid-catalyzed hydrolysis, as it is described in U.S. Pat. No. 4,123,406 (A. Stoy et al.), U.S. Pat. No. 4,172,823 (A. Stoy et al.) and U.S. Pat. No. 4,228,056 (A. Stoy).
It was postulated that improved properties in hydrolyzed PAN is due to a complementary physical network. In many cases, the physical network formed by a controlled partial hydrolysis of PAN is stable enough and additional covalent network is unnecessary.
Such a hydrolysis is typically carried out by using acid catalysis leading to formation of multi-block copolymers (MBC) with alternating sequences of acrylonitrile and acrylamide units. PAN solvents useful for acid hydrolysis are typically concentrated aqueous ZnCl
2
solutions, as described in U.S. Pat. No. 2,837,492 (Stanton et al.) and U.S. Pat. No. 3,987,497 (A. Stoy et al) or in concentrated inorganic acids, such as sulfuric acid, nitric acid, phosphoric acid and mixtures thereof. Such process is described, for example, in the U.S. Pat. No. 3,926,930 (Ohfuka et al.), U.S. Pat. No. 3,709,842 (A. Stoy), U.S. Pat. No. 4,026,296 (A. Stoy et al.), U.S. Pat. No. 4,173,606 (V. Stoy et al.) and U.S. Pat. No. 4,183,884 (Wichterle et al.).
The physical network in PAN-based hydrogels can be formed by clusters of polyacrylonitrile sequences. Physically crosslinked PAN hydrogels have their nitrile groups (AN units) and hydrophilic groups organized in alternating sequences, forming so-called “Multi-Block Copolymers” (MBC). If AN sequences (also called “Hard blocks”) are sufficiently long, they separate in presence of water from sequences of hydrophilic units (also called “Soft Blocks”) to form the network-forming crystalline clusters. A certain minimum length of the Hard block is necessary for the phase separation. Moreover, certain length of Hard Block is required to build a stable crystalline cluster. Actual minimum lengths of Hard Blocks are not known. According to some estimates, only hard blocks with 5 or more nitrile units are effective in building the network-forming clusters.
One can appreciate that the phase separation in the MBC is not clean or perfect. Each crystalline cluster will contain some hydrophilic groups that are disturbing its organization. Crystalline clusters will have also a broad distribution of sizes. Consequently, crystalline clusters will have a broad distribution of melting points and resistance of the cluster to the local stress. Such hydrogels typically show limited thermal stability and distinct creep behavior under stress. The longer the hard blocks and more uniform their length in a MBC, the better thermal and mechanical stability in the resulting hydrogel.
AN-containing MBCs are produced by suitable reactions of PAN (typically hydrolysis or aminolysis) converting pendant nitrile groups into hydrophilic derivatives of acrylic acid. The reactions leading to MBC are so called “Zipper reactions” in which nitrile groups adjacent to already reacted group (i.e., an acrylic acid derivative other than nitrile) are more reactive than nitrile groups adjacent to other nitriles (which is the basic arrangement in the original PAN homopolymer). Hydrophilic “Soft Blocks” are initiated by a relatively slow reaction of a nitrile flanked by other nitriles on both sides. Once a new pendant group is introduced, the “Soft Blocks” can grow by a faster propagation reaction. The propagation of the Soft Block continues until reaction consumes all available nitrile groups in the given polymer chain, unless the reaction is terminated before the nitrile consumption is completed. If all

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