Hydrophilic cross linked co-polymers
As a mechanical engineer I have spent the last 20 years at least wishing I had listened more diligently to my chemistry teacher when at school.
You may ask the same as I did some 20 years ago. What on earth is a hydrophilic cross linked co-polymer?
Most polymers will absorb some water. Even polymers considered to be hydrophobic can absorb up to 5% water. Hydrophilic polymers can absorb between 5% and 90% whilst still remaining structurally stable. All reach an equilibrium with their environment. In many environments the air has significant relative humidity and thus provides a water source for a hydrophilic polymer to reach a significant but not large water content.
Suitable polymers are addition polymers or co-polymers of monoethylenically unsaturated monomers and, if the polymer is to be water swellable, the monomers should be hydrophilic or the monomer system should contain a sufficient proportion of hydrophilic monomer that the final product is hydrophilic. Examples of monomers having hydrophilic properties are amino-alkyl acrylates and methacrylates, especially those in which the alkyl group is one containing 1 to 4 carbon atoms, e.g. methyl or ethyl. The amino group may be mono- or di- substituted and any substituent is preferably a C1-4 alkyl group e.g. methyl or ethyl. Specific examples are aminoethylacrylate, dimethylaminoethyl acrylate, methylaminoethyl methacrylate and diethylaminoethyl methacrylate. Hydroalkyl acrylates and methacrylates may also be used as the hydrophilic monomer and in these the alkyl group is preferably of 1 to 4 carbon atoms e.g. methyl or ethyl. A specific example is hydroxymethyl methacrylate.
Preferred hydrophilic monomers include N-vinyl pyrrolidone and other vinyl lactams and acrylamide and methacrylamide and N-substituted derivative thereof. Substituted acrylamide and methacrylamide derivatives may be mono- or di- substituted and preferred substituents are alkyl, hydroxylalkyl and aminoalkyl (including mono- and di-substituted aminoalkyl e.g. di-alkyl aminoalkyl groups. Preferably any alkyl group present contains 1 to 4 carbon atoms, methyl and ethyl being especially preferred. Examples of such derivatives are N-methylacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-dimethylaminomethylacryamide, N,N-di-methylaminoethylacrylamide and N-methyl-aminoisopropyl acrylamide.
The polymers are preferable copolymers, two or more hydrophilic monomers, e.g. as described above, being copolymerised together or one or more hydrophilic monomers being copolymerised with one or more other monomers. Preferably a hydrophilic monomer is copolymerised with an alkyl acrylate or methacrylate, especially one in which the alkyl group has from one to four carbon atoms e.g. methyl or ethyl, or acrylonitrile. Styrene may also be used as a comonomer. Specific examples of suitable copolymers are copolymers of N-vinyl pyrolidone and methyl methacrylate, of N-vinyl pyrolidone and hydroxymethyl methacrylate and of N-vinyl pyrolidone, styrene and acrylonitrile. Co-polymers of terephthalic acid and N-vinyl pyrolidone may also be suitable.
It is greatly preferred that the polymers should be cross-linked and this may be achieved by incorporating di- or poly- functional cross-linking agents in the monomer system. Examples of suitable cross-linking agents are compounds containing two or more ethylenically unsaturated groups e.g. allyl methacrylate, divinylbenzene, ethylene glycol dimethacrylate and trimethylol propane trimethacrylate and trimethylol propane trimethacrylate. Usually it is desirable that the polymers should be relatively lightly cross-linked, the cross-linking agent being used, for example, in an amount of about 1% by weight of the monomer system.
In order to provide a high strength polymer system, cross-linked copolymers composed of acrylonitrile or methacrylonitrile and vinyl pyrolidone may be used.