Polymer Swelling Hydrophobic polymers

Osmotic Pressure Controlled Oral Tablets. Alza Corp. has developed a system that is dependent on osmotic pressure developed within a tablet. The core of the tablet is the water-soluble dmg encapsulated in a hydrophobic, semipermeable membrane. Water enters the tablet through the membrane and dissolves the dmg creating a greater osmotic pressure within the tablet. The dmg solution exits at a zero-order rate through a laser drilled hole in the membrane. Should the dmg itself be unable to provide sufficient osmotic pressure to create the necessary pressure gradient, other water-soluble salts or a layer of polymer can be added to the dmg layer. The polymer swells and pushes the dmg solution through the orifice in what is known as a push-pull system (Fig. 3). The exhausted dmg unit then passes out of the body in fecal matter. 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

Freger s model treats swelling as a nonaffine inflation of the hydrophobic polymer matrix by small aggregates of water molecules (droplets) separated by polymer films. This model results in a relation. 

The parameters obtained from the fit are consistent with theoretical expectations. Based on the LCST s of the linear precursor polymers we would order these polymers in terms of increasing hydrophilidty as PNIPAAm > HPC > MC HPMC-E > HPMC-K [50, 125], This is the same order in which the values of P are ordered. We also see that the sharper volume transitions correspond to more negative values of Zsg (which is the theoretical prediction [53, 125]), which occur for the more hydrophobic polymers. This makes physical sense from the standpoint that for the more hydrophobic polymers to swell substantially, as observed, they must have significant favorable interactions with the solvent in other words, Zsg must be significantly negative (hydrogen bonding is the important specific interaction known to exist between these polymers and water). 

Studies on glucose-induced polymer swelling have focussed on developing membranes that could serve in systems for controlled delivery of insulin to diabetics (3,4). It has been shown that hydrophobic methacrylate copolymers undergo a sharp swelling transition as the pH is decreased from 7 to 6 (3-7). However, the kinetics of the transition are too slow for the proposed application to glucose delivery. 

Polymer Swelling. Swelling phenomena are generally encountered in both the hydrophilic and hydrophobic polymer matrices during the release of entrapped water soluble drug in an aqueous environment. 

Type III is the sorption isotherm of Flory-Huggins. Here the solubility coefficient increases continuously with pressure. It represents a preference for formation of penetrant pairs and clusters it is observed when the penetrant acts as a swelling agent for the polymer without being a real solvent. An example is water in relatively hydrophobic polymers containing also some polar groups. 

For the past several years, our laboratory has studied the swelling properties of weakly basic, hydrophobic polymer gds. The gels are formed by bulk polymerization of hydro diobic n-alkyl methacrylates (n-AMA) with N,N-dimethylaminoethyl methacrylate (D( ), tte latter comonomer bearing a tertiary amine sidediain. To form a three-dimensional polymer network that will not dissolve upon swelling, small amounts of the crosslinker divinylbenzene (DVB) are added. These gds, when placed in aqueous solutions, absorb minimal amounts of water in neutral and alkaline media, but become swollen in addic media. Hence, they might be considered for applications in which drug release in the stomach is desirable. 

Paschek D, Nonn S, Geiger A. Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution. Phys. Chem. Chem. Phys. 2005 7 2780-2786. 

Important parameters for the characterization of ion-exchange membranes are the density of the polymer network, hydrophobic and hydrophilic properties of the matrix polymer, the distribution of the charge density, and the morphology of the membrane itself. All these parameters do not only determine the mechanical properties, but they also have a considerable influence on the sorption of the electrolytes and the non-electrolytes and therefore on the swelling [13]. 

---