Acrylic hydrogels based on 2-hydroxyethyl methacrylate, H, have
been synthesized by
copolymerization reaction of this monomer with 2-ethylhexyl acrylate,
E, using AIBN as initiator.
Reactivity ratios were estimated from copolymerization reactions
carried out in solution and at low
conversion, by using both linearization and nonlinearization methods.
They were found to be r
E =
0.29
and r
H = 2.54. The swelling behavior of
the hydrogels was studied by immersion of copolymer films
prepared by bulk copolymerization, in water at different temperatures.
Equilibrium water uptake was
strongly dependent on composition and decreased with the E content in
the copolymer. Fickian behavior
was observed in all cases for reduced sorption coefficients lower than
0.4. The diffusion coefficient was
found to decrease with increasing E content in the copolymer. A
value of apparent activation energy for
the diffusion process of 8.8 kJ/mol was obtained for
poly(2-hydroxyethyl methacrylate); however, H/E
copolymers did not obey the Arrhenius behavior over the short
temperature interval studied. Differential
scanning calorimetry has been used to study the organization of water
in the copolymer hydrogels. The
amounts of nonfreezable water for the copolymers with 5 and 10 wt % of
E content were found to be 28
and 26 wt %, respectively. The surfaces of the copolymer films
were characterized by contact angle
measurements. The surface free energy decreased noticeably with
the presence of 10 wt % of the flexible
hydrophobic monomer, the reduction of the polar component being higher
than that of the dispersive
component. Segregation of the components in microdomains, the H
segments being the continuous phase,
is postulated tentatively in the light of the results
obtained.
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