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Chapter 14
An Investigation into the Influence of Crosslinker Type and Solvent Composition on Physical Properties and Phase Transition Behavior of Poly(N-isopropylacrylamide) Hydrogels Candan Erbil,* Ezgi Toz, Özgür Akdemir, and Nurseli Uyanık Istanbul Technical University, Chemistry Department, 34469 Maslak, Istanbul, Turkey *
[email protected] Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels have been synthesized by free radical polymerization in 1,4Dioxane using N,N′-methylenebisacrylamide (BIS) and α,ω-acryloxyorganofunctional poly(dimethylsiloxane) (VTPDMS) as crosslinkers. The disc samples equilibrated by swelling in water, water-methanol mixtures and methanol were examined by volumetric and stress-compression measurements to afford the values of swelling ratio (V/ Vo), polymer-solvent interaction parameter (χ), compression modulus (G) and effective crosslinking density (νe). It was observed that the incorporation of hydrophobic VTPDMS chains, instead of hydrophilic BIS molecules, as crosslinker into the structures of neutral NIPAAm hydrogels improved their mechanical strengths. The values of νe, being greater than νt (theoretical crosslinking density) also indicated the presence of hydrophobic physical interactions, i.e., physical crosslinkings, between dimethyl siloxane groups in the PNIPAAm hydrogels crosslinked with VTPDMS. Further, the G-, χ- and νe-solvent composition curves of both NIPAAm/VTPDMS and NIPAAm/BIS hydrogels swollen to equilibrium in water/methanol mixtures (80/20, 50/50 and 20/80, in v/v %) at 23°C showed maxima corresponding to © 2010 American Chemical Society In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
0.20 and 0.50 of methanol volume fractions, respectively. It was assumed that the maxima on G, νe and χ versus solvent composition curves resulted from the competitions between interaction of methanol and water together versus their individual interactions with PNIPAAm/VTPDMS and PNIPAAm/BIS polymers.
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Introduction Synthetic polymer hydrogels have been used extensively in the development of the smart drug delivery systems (1). The smart hydrogels show a phase transition accompanying drastic discontinuous or continuous volume change with variation of environment conditions such as temperature, pH, ions or aqueous - organic solvent compositions (2–4). Poly (N-isopropyl acrylamide) (PNIPAAm) hydrogel, which is one of the most widely studied examples of the hydrogel systems that undergo a temperature-controlled volume phase transition, is attracting interest in biomedical applications because it exhibits a well-defined lower critical solution temperature (LCST) in water around 32°C, which is close to the body temperature (5, 6). Pure NIPAAm hydrogels crosslinked by using hydrophilic tetrafunctional N,N′-methylene bisacrylamide (BIS) as crosslinker have low mechanical strength in the swollen state, i.e., below LCST (7, 8). The combination of large swelling and high mechanical performance within the same gel structure is important for both industrial and biomechanical applications. The formation of nanocomposite gels, hydrophobically modified copolymers and double networks by incorporation of hydrophilic/hydrophobic inorganic filler or comonomer into the hydrogel structure can be given as the main examples of the methods which are used to improve the mechanical properties (9–12). Using the hydrogel as drug delivery device and load it with hydrophilic or hydrophohic drug molecules, its volume change in low molecular weight solventwater mixtures and the nature of the interactions between the network and solvent molecules should also be studied. In this study, hydrogels composed of NIPAAm, BIS and vinyl terminated poly(dimethyl siloxane) (VTPDMS) (commercial product) as monomer, hydrophilic crosslinker and hydrophobic crosslinker, respectively, were prepared to investigate the effect of hydrophobic component, i.e., VTPDMS on the compression moduli of the samples attained equilibrium swollen state in water, methanol and water/methanol mixtures (80/20, 50/50 and 20/80 in v/v %) at 23°C and 37°C. The most important parameters used to characterize the network structure of hydrogels are the polymer volume fraction in the swollen state (υ2s) and crosslinking density (νe). These two parameters, which are related to one another, were determined through the use of equilibrium-swelling and uniaxial-compression experiments.
168 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Experimental Materials
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N-isopropylacrylamide (NIPAAm); α,ω-acryloxyorganofunctional poly(dimethylsiloxane) (VTPDMS; Tegomer V-Si 2250 (n~20, 5
