Mechanical properties of hydrophilic copolymers of 2-hydroxyethyl methacrylate with ethyl acrylate, n-butyl acrylate, and dodecyl methacrylate.

Copolymers were prepared of 2-hydroxyethyl methacrylate, which is strongly hydrophilic, with hydrophobic comonomers having a low glass transition temperature, i.e., ethyl acrylate, n-butyl acrylate, and dodecyl methacrylate. The glass transition temperature, T alpha (1 Hz), of polymers in the dry state (xerogels) was determined by means of dynamic mechanical measurements. The dependence of T alpha on composition in all the three series of copolymers was described in terms of a one-parameter equation. The equilibrium swelling of copolymers in water decreases more steeply than the weight fraction of 2-hydroxyethyl methacrylate. When swollen in water to equilibrium, all copolymers with ethyl acrylate or n-butyl acrylate are in the rubberlike state; their tensile modulus E assumes values in the range 0.17-0.50 MPa. While poly(2-hydroxyethyl methacrylate) has the modulus E = 0.39 MPa, the tensile strength sigma u = 0.32 MPa, and the strain-at-break epsilon u = 1.81, for the weight fraction of the comonomer in the range 0.36 less than or equal to omega 2 less than or equal to 0.56 it is possible to achieve sigma u and epsilon u lying in the respective ranges 0.7-0.9 MPa and 5-7. Copolymers with the dodecyl methacrylate content omega 2 less than 0.2 or omega 2 greater than 0.8 are rubberlike, and at 0.30 less than or equal to omega 2 less than or equal to 0.62 they become leathery; the latter have the modulus E and strength sigma u within the ranges 12-32 and 3-4 MPa, respectively. The stress-strain curves of these copolymers evidence yielding and orientation hardening which have been tentatively explained by the nonhomogeneous composition and structure of the copolymers.