Poly , polymer networks

A wide range of polymer networks are constructed in this manner. Poly(vinyltrichloacetate) was used as the coinitiator with styrene, MMA and chloroprene as cross-linking units. Polycarbonates, polystyrene, N-haloge-nated polyamide, polypeptides, and cellulose acetate, suitably functionalized, have been used as a coinitiator... 

Reactions of this type are quite popular and widely used to introduce hydrophilic and ionogenic groups into linear polymers as well as directly into polymer networks. These reactions include hydrolysis (PAAm, PAAc and their analogs from PAN, PVA from poly (vinyl acetate), oxyethylation and oxymethylation of starch and cellulose, sulfurization, and other reactions. These processes are of industrial importance, well studied and widely reviewed. 

Another class of silicon-containing polymers that have great potential to be extremely useful precursor materials are poly(chlorocarbosilanes).14f 46 Poly (chlorocarbosilanes) are not useful without modification because of the rapid hydrolysis of Si—Cl bonds, forming HC1 and an insoluble crosslinked polymer network. However, nucleophilic substitution of these Si—Cl bonds with various reagents produces materials widi a broad range of properties that are determined by the nature of the nucleophile used.47 Poly(chlorocarbosilanes) can be easily synthesized by ADMET (Fig. 8.18) without any detrimental side reactions, since the Si—Cl bond is inert to both catalysts 12 and 14. Early studies produced a polymer with Mn = 3000.14f... 

Davis, P. A. Nicolais, L., Ambrosio, L., and Huang, S. J., Synthesis and characterization of semi-interpenetrating polymer networks of poly(2-hydroxyethyl methacrylate) and poly(capro-lactone), Polym. Mater. Sci. Eng., 56, 536-540, 1987. 

Subsequent work by Johansson and Lofroth [183] compared this result with those obtained from Brownian dynamics simulation of hard-sphere diffusion in polymer networks of wormlike chains. They concluded that their theory gave excellent agreement for small particles. For larger particles, the theory predicted a faster diffusion than was observed. They have also compared the diffusion coefficients from Eq. (73) to the experimental values [182] for diffusion of poly(ethylene glycol) in k-carrageenan gels and solutions. It was found that their theory can successfully predict the diffusion of solutes in both flexible and stiff polymer systems. Equation (73) is an example of the so-called stretched exponential function discussed further later. 

Figure 11 Aqueous equilibrium swelling of cross-linked poly(A-isopropyl acrylamide-co-butyl methacrylate) as a function of temperature. Ww, Wp are the weight of water and polymer of swollen polymer network respectively. (From Ref. 32.)...

This process involves the suspension of the biocatalyst in a monomer solution which is polymerized, and the enzymes are entrapped within the polymer lattice during the crosslinking process. This method differs from the covalent binding that the enzyme itself does not bind to the gel matrix. Due to the size of the biomolecule it will not diffuse out of the polymer network but small substrate or product molecules can transfer across or within it to ensure the continuous transformation. For sensing purposes, the polymer matrix can be formed directly on the surface of the fiber, or polymerized onto a transparent support (for instance, glass) that is then coupled to the fiber. The most popular matrices include polyacrylamide (Figure 5), silicone rubber, poly(vinyl alcohol), starch and polyurethane. 

Philippova and Starodubtzev have also extensively studied the complex-ation behavior of polyacids and PEG, especially, the system of crosslinked of poly(methacrylic acid) and linear poly(ethylene glycol) (Philippova and Starodubtzev, 1995 Philippova et al., 1994). They observed that decreasing the molecular weight of PEG from 6000 to 1500 resulted in its slower diffusion into the swollen network of PMAA, and a drastic decrease in both the stability and equilibrium composition of the intermacromolecular complex. Analysis of dried polymer networks of PMAA with absorbed PEG chains by FT-IR spectroscopy revealed the presence of two types of hydrogen bonded structures (1) dimers of methacrylic acid at absorption frequency of 1700 cm-1 and (2) interpolymer complexes of PMAA and PEG at 1733 cm-1. In addition, they also suggested as a result of their studies, that the hydrogen bonded dimer of PMAA forms preferentially to the intermacromolecular complex between the PMAA network and PEG chains. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Hydrogels are 3D cross-linked polymer networks. They can withstand acid conditions and release the entrapped drug molecules. Purdue University researchers have used a poly[methacrylic acid-g-poly(ethylene glycol)] hydrogel to encapsulate insulin, which could be released by pH trigger. 

Based on the solution property of poly (DMAEMA-co-AAm) in response to temperature, the temperature dependence of equilibrium swelling of poly (DMAEMA-c6>-AAm) gel as a function of chemical composition was observed as shown in Figure 6. The transition temperature of copolymer gel between the shrunken and swollen state was shifted to the lower temperature with increases in AAm content in the gel network. This is attributed to the hydrogen bond in the copolymer gel network and its hydrophobic contribution to the LCST Copolymer II gel was selected as a model polymer network for permeation study because it showed the sharp swelling transition around 34°C. 

The temperature-sensitive poly(A-isopropyl acrylamide) and pH-sensitive poly(methacrylic acid) were used as the two component networks in the IPN system. Since both A-isopropyl acrylamide (NIPAAm) (Fisher Scientific, Pittsburgh, PA) and methacrylic acid (MAA) (Aldrich, Milwaukee, Wl) react by the same polymerization mechanism, a sequential method was used to avoid the formation of a PNIPAAm/PMAA copolymer. A UV-initiated solution-polymerization technique offered a quick and convenient way to achieve the interpenetration of the networks. Polymer network I was prepared and purified before polymer network II was synthesized in the presence of network I. Figure I shows the typical IPN structure. 

Katono, H., Maruyama, A., Sanui, K., Ogata, N., Okano, T., and Sakurai, Y. Thermo-responsive swelling and drug release switching of interpenetrating polymer networks composed of poly(acrylamide-co-hutyl methacrylate) and polyfacrylic acid). J. Controlled Release 1991,16, 215-228. 

Lim, Y. H., Kim, D., and Lee, D. S. Drug releasing characteristics of thermo- and pH-sensitive interpenetrating polymer networks based on poly(A-isopropylacry-lamide). J. Appl. Polym. Sci. 1997, 64, 2647-2655. 

For bioadhesive applications, anionic polymers appear to provide the most effective balance between adhesiveness and toxicity, with carboxylic materials preferred over sulfonic polymers [400]. Polyfacrylic acid) microparticles have been identified as particularly effective bioadhesive materials [402]. Studies with poly(acrylic acid) microparticles have indicated that, while water-swollen particles exhibit good bioadhesion, dry polymer particles give no adhesion at all. In addition, adhesive strength increases as the degree of ionization of the polymer is increased [402]. Thus the expanded nature of the polymer network is important to mucoadhesion, probably via polymer interdiffusion and entanglement with mucin [403],... 

With controllable strength of the self- and hetero-associated hydrogen, a molecular level penetrated semi-interpenetrating polymer networks (semi-IPNs) of ST-VPDMS with poly(N-vinylpyrrolidone) (PVPr) were prepared by condensation of the self-associated silanols.13... 

Stress-strain properties and structure of poly (dimethylsiloxane) networks. Microsymposium on macromolecules Polymer gels and concentrated solutions. Inst, of Macromolecular Chemistry, Prague 1967, E6. 

The existence of two T2 relaxation times was also observed in the radiation cured photopolymers 99), and studies of tri- and tetra-functional network organosilicon polymers with rings at the network points 100). The effects of crystallization of poly-dimethylcarbosiloxane networks (PDMCS) I01), and water sorption and stoichio-metrical composition of the cured DGEBA/DETA resins on phase structure and mobility 102) were studied. 

The density of the polymer network of the gel fluctuates with time and space in microscopic scale. Such fluctuations are frozen in the gel under certain gelation conditions. As a result of the frozen density fluctuations, opacity develops in the gel. Although quantitative studies have not been made so far, the qualitative studies of the opacity development in poly(acrylamide) gel have been reported... 

Fig. la, h. Chemical structure of poly N-isopropylacrylamide (a) and of trisodium salt of copper chlorophyllin molecule (b). The chlorophyllin molecule has a double bond which can be covalently connected to the polymer networks... 

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