Transport kinetics of methanol in hydroxyethyl methacrylate homopolymer and its copolymers
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Tinh Nguyen National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 23 March 2004; accepted 6 August 2004)
The kinetics of methanol transport in 2-hydroxyethyl methacrylate (HEMA) homopolymer and 75/25 and 50/50 mol fraction HEMA/DHPMA (2,3-dihydroxypropyl methacrylate) copolymers at five different temperatures has been investigated using the sorption experiment technique. A combined case I and case II diffusion model was used to describe the transport processes. Four replicates for each temperature of each material having a nominal thickness of 0.1 mm were immersed in methanol maintained at 35, 40, 45, 50, and 55 °C, and the mass uptake as a function of time was measured gravimetrically. Experimental results are found to be in good agreement with model prediction at all temperatures and for all three materials. Both the diffusion coefficients of case I transport and velocity of case II transport increase with increasing temperature. D values at low temperatures (35 and 40 °C), which are in the 10−9 cm2/s range, of the HEMA homopolymer are less than those of the copolymers. On the other hand, the activation energies of case I transport of the copolymers are substantially higher than those of the HEMA homopolymer; however, the level of DHPMA loading in the copolymer does not seem to affect the activation energy. In addition, thermodynamic heat and free energy of mixing values indicate heat is released when HEMA/DHPMA copolymers are exposed to methanol and that the solvent/copolymer systems exist as a continuous phase. In contrast, the methanol/HEMA homopolymer system exists as separate phases.
I. INTRODUCTION
Hydroxyethyl methacrylate (HEMA) has a low reactivity with amine and is resistant to acid and alkaline hydrolysis. It has very high affinity toward water and good compatibility with biological systems. Because of these advantages, polymers made with HEMA have found applications in the medical and biological fields, including soft contact lens1,2 and controlled drug delivery.3,4 To further use these unique characteristics, copolymers of HEMA have been prepared for medical applications. For example, soft contact lens made of a copolymer material consisting of HEMA, ethylene glycol dimethacrylate, and methacrylic acid have been reported.5,6 This study investigates the transport kinetics of methanol (CH3OH) in HEMA homopolymer and its copolymers with 2,3-dihydroxypropyl methacrylate (DHPMA). Methanol was chosen for the study because it is a good solvent for HEMA, is commonly used to wash soft contact lens, and its solubility parameter DOI: 10.1557/JMR.2004.0443 J. Mater. Res., Vol. 19, No. 11, Nov 2004
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(29.7 MPa1/2)7 is close to that of the HEMA homopolymer (26.9 MPa1/2).8 Therefore, information on the sorption, diffusion, and swelling characteristics of methanol in HEMA polymer and its copolymers is of practical importance. Further, methanol is the lowest molecular mass, polar organic molecule, which maximizes its sorption a
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