Dynamic mechanical study of the transition from swollen particles to hydrogel caused by neutralization

The copolymer of 2-(2-carboxybenzoyloxy)ethyl methacrylate (CEM) with butyl methacrylate (BMA) (BMA/CEM = 40/60 wt.) and terpolymers CEM/BMA/2-hydroxy ethyl methacrylate (HEMA) ((BMA + HEMA)/CEM = 40/60 wt.; HEMA/BMA = 35/5, 3 0/10, 20/20 and 10/30) were prepared by emulsion radical copolymerization i n water in the presence of sodium dodecyl sulfate and their dynamic mechani cal behaviour was investigated as a function of the degree of neutralizatio n a. Main attention was devoted to the transition from swollen particles to physical gel with increasing degree of neutralization and to the structure of formed hydrogels. From the results it followed: (a) the transition from swollen particles to the gel state occurs in a narrow neutralization inter val at alpha similar to 0.45 for BMA/CEM copolymer; increasing the HEMA con tent shifts the transition to lower ex values; (b) with increasing shear st rain gamma, the hydrogels passed from the gel to liquid state and this tran sition at the critical strain gamma(c), was reversible; (c) junctions in th e gel state are probably formed by the hydrophobic interactions of the ends of CEM units which form clusters and the junction concentration is indepen dent of the HEMA content and degree of neutralization alpha (d) increasing degree of neutralization alpha and the HEMA content (increasing polarity of the system) stabilizes the junctions and the critical gamma(c) values incr ease: (e) the values of the low-strain storage G'(0) and loss G "(0) moduli together with critical strains gamma(c) did not depend on angular frequnec y omega in the interval 10(-1) - 10 rad/s.