Gradient IPNs

All of the compositions reported above are uniform on a macroscopic scale through the material. IPNs can be produced, however, in which the structure or composition of the macroscopic material is not homogeneous 

Gradient IPNs may easily be prepared by swelling polymer network I with monomer mix II on either one or both sides, and polymerizing before the swelling becomes uniform through diffusion. 

Sperling and Thomas employed gradient IPNs to obtain a hard exterior, a soft interior, and a composition-graded intermediate zone. Such materials were useful for noise and vibration damping as explored in Section 8.8. Predecki swelled hydroxyethyl methacrylate monomer mixes into silicone rubber to produce materials having hydratable surfaces. Such materials could replace surface coatings in arteriovenous shunts. 

Akovali et prepared crosslinked polystyrene and poly(methyl methacrylate) sheets, and dipped them into acrylonitrile or methylacrylate, respectively. After various periods of time, the materials were removed and 

Very recently. Berry et prepared a series of physical/chemical 

These structures differ from homogeneous sequential IPN s in that the macroscopic composition varies in a systematic way. For example, the ratio 

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Gradient IPN. In this case, the overall composition or crosslink density of the material varies from location to location on the macroscopic level. One way of preparing these materials involves partial swelling of polymer network I by the monomer II mix, followed by rapid polymerization before diffusional equilibrium takes place. Films can be made with polymer network I predominantly on one surface, and polymer network II predominantly on the other surface with a gradient composition existing throughout the interior. 

With the gradient IPN s ", the composition is varied within the sample at the macroscopic level. This is conveniently carried out by soaking a sheet of network I in monomer II for a limited period of time, and then polymerizing II rapidly, before diffusion equilibrium can occur. 

Figure 13.13. Proposed use of gradient IPN s as constrained layer damping materials. Left, A is the substrate to be damped, B is a gradient IPN with the plastic component increasing in concentration toward the outside, as shown on the right.

A series of castor oil polyurethane/poly(methyl methacrylate) interpenetrating polymer networks (IPNs) and gradient IPNs, cured at room temperature, were prepared by a simultaneous IPN method, and nanocomposites with BaTiOs superfine fibre were reported for the systems. A dose-dependent improvement in thermoelectric and mechanical properties was observed in the nanocomposites compared to the pristine systems. 

Other areas of IPN application cover circuit boards (Tate and Vamell 1989 Takahashi et al. 1996) and, via gradient IPN technology, improved optical fibers (Bukhbinder and Kosjakov 1996). This last works by having a gradient refractive index firom the axis to the surface, curving the light back away from the surface and... 

Amongst the above mentioned compatibilization methods, the obtaining of IPNs and SIPNs often proved to be a promising and very efficient route. An IPN is a polymer alloy comprised of two or more chemically crosslinked polymers. The difference between polymer blends and IPNs is that the latter ones swell instead of dissolving in solvents and do not creep or flow. Types of IPNs include sequential, simultaneous, latex and gradient IPNs and may also be thermoplastic (i.e. when physical crosslinks are imphed). Thermoplastic IPNs behave as thermosets at ambient temperature, but usually flow when heated at certain temperatures, possess IPN properties and often exhibit dual phase behavior [1]. 

Gradient IPN an IPN of nonuniform macroscopic composition, usually by nonequilibrium swelling in monomer II, and polymerizing rapidly. 

IPNs and gradient IPNs based on polyether-urethane-urea (PEUU) block copolymers and acrylamide, 2-hydroxyethyl methacrylate (HEMA), or N-vinyl-2-pyrrolidone. Intended for biomedical uses, such compositions created high-strength, water-absorbing hydrogel surfaces showing good blood compatibility. 

G. Akovali, K. Biliyar, and M. Shen, Gradient Polymers by Diffusion Polymerization, /. Appi. Polym. Sci. 20, 2419 (1976). Gradient IPNs mechanical behavior. 

G. C. Berry and M. Dror, Modification of Polyurethanes by Interpenetrating Polymer Network Formation with Hydrogels, Am. Chem. Soc. Div. Org. Coat. Plast. Chem. Pap. 38(1), 465 (1978). Polyether-urethane-urea block copolymers with crosslinked HEMA, NVP, or acrylamide. IPNs and gradient IPNs for biomedical purposes. Strength, water swellability, and good blood compatibility. 

AIChE Preprints, 87th National Meeting, Boston, Massachusetts, August 1979, paper No. 14d. Gradient IPNs and sequential IPNs. PMMA/poly(2-chloroethyl acrylate). Glass transition and mechanical studies. 

M. Shen and H. Kawai, Properties and Structure of Polymeric Alloys, AIChE J. 24(1), 1 (1978). Review of block copolymers and IPNs. Gradient IPNs. Thermodynamics of mixing. 

According to the mode of synthesis, IPNs are distinguished into five different types (i) sequential IPNs, (ii) simultaneous IPNs, (iii) interpenetrating elastomeric networks, (iv) thermoplastic IPNs and (v) gradient IPNs ... 

The average free energy of mixing of solvent with the individual components, with IPNs of various compositions, and with gradient IPN layer for solutions of different concentrations was estimated according to the equation ... 

Figure 5 shows the change of the free energy of mixing of benzene with individual networks and with IPNs. All the systems studied (PU-benzene, copolymer-benzene, gradient IPN-benzene) are thermodynamically stable systems, as for them the following condition is vahd ... 

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