Gelling water-soluble polymers

Gels can be divided into two groups, depending on the nature of the bonds between the chains of the network. Gels of type 1 are irreversible systems with a three-dimensional network formed by covalent bonds between the macromolecules. They include swollen networks which have been formed by polymerisation of a monomer in the presence of a crosslinking agent. 

Type 11 gels are heat-reversible, being held together by intermolecular bonds such as hydrogen bonds. Sometimes bridging by additive molecules can take place in these type 11 systems. Polyfvinyl alcohol) solutions gel on 

Gelation can occur either with a fall (as with poly(vinyl alcohol)) or a rise in the temperature depending on the type of temperature variation of solubility. While gels of type 11 are commonest in pharmacy, with the interest in polymers as drug delivery adjuvants some type 1 materials are being used. 

If water-soluble polymer chains are covalently crosslinked, gels will be formed when the dry material interacts with water. The polymer swells in water but cannot dissolve as the 

Hydrophilic contact lenses (such as Soflens) are made from crosslinked poly(2-hydrox-yethyl methacrylates). These have also been utilised as drug carriers. Conventional eye medication has been modified over the years through the addition to formulations of a variety of viscosity-enhancing agents, polymers such as hydroxypropylmethyl cellulose, poly(vinyl alcohol) and silicones. These all prolong contact of dmg with the cornea by 

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The process of gelling water soluble polymers with metal ions can be broken down Into 3 basic steps 1) production of active crosslinker, 2) attachment of active crosslinker to the polymer, and 3) crosslinking of two polymer segments. ... 

Examples of the components of the gelling composition are a water-soluble polymer such as polyacrylamide, an iron compound such as ferric acetyl-acetonate or ammonium ferric oxalate, and a ketone such as 2,4-pentanedione [1247]. The composition forms a temporary gel that is useful for the temporary plugging of a formation. The temporary gels that are formed will disappear after 6 months. 

Simple coacervation involves the use of either a second more-water soluble polymer or an aqueous non-solvent for the gelatin. This produces the partial dehydration/desolvation of the gelatin molecules at a temperature above the gelling point. This results in the separation of a liquid gelatin-rich phase in assocation with an equilibrium liquid (gelatin-poor) which under optimum separation conditions can be almost completely devoid of gelatin. 

Gelled and Cross-Linked Polymers. By themselves, water-soluble polymers are unlikely to prove suitable for improving gas-flood mobility control since these agents viscosify the aqueous phase, making the gas-to-aqueous phase mobility ratio even more adverse. 

Polyacrylic acid (PAA] is a biodegradable water-soluble polymer with a wide range of medicinal applications and is considered pharmaceutically safe. They are used for oral and mucosal contact applications in the form of controlled release tablets, oral suspensions, and bioadhesives. The unique property of PAA is that it exists as a liquid at pH 5 and as a gel at pH 7. Permeation of cations into the gelled polymer converts the gel back to a liquid. It is an ideal polymer used for ocular delivery of ribozymes to the corneal epithelium as a drug delivery vehicle. 

The exact chemical mechanism of the third basic step in gelation, metal ion crosslinking of two water-soluble polymer molecules, is not well known. However, the finding that shearing a gelling solution increased the gelation rate implied that some step of the process was diffusion controlled . It is sensible to assume that as shear increases the number of polymer/polymer collisions, the number of collisions resulting in crosslinks is also increased. 

In all encapsulation methods the polymer solution must be gelled by crosslinking the polymer chains after dispersing living cells in solution. Many water-soluble polymers possess high reactivity hydroxyl, carboxyl... 

The ceramics industry is accustomed to handling water based systems and therefore there is considerable interest in water soluble polymer compositions for oxide ceramics. The thermo-gelling properties of methyl cellulose solutions have produced a successful moulding process for metal powders which could be adapted to ceramic suspensions. There is some interest in moulding processes which use chilled mould conditions below ambient temperatures to produce solidification in a range of liquids. The moulded article is ejected and the liquid component removed without remelting, by sublimation under freeze-drying conditions. 

The addition of an a-hydroxycarboxyhc acid to a tetraethylene, propylene, diethjiene, or hexylene glycol titanate gives water-soluble complexes suitable for gelling aqueous solutions of hydroxyl polymers, such as poly(vinyl alcohol) (PVA), or cellulose (qv) derivatives. These are useful as binding agents for glass fibers, clays (qv), and paper coatings (85). 

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