Material properties of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate hydrogels

The physical properties of four crosslinked polymers used for soft contact lenses were examined: the 2,3-dihydroxypropyl methacrylate (DHPMA) and the 2-hydroxyethyl methacrylate (HEMA) homopolymers, and two DHPMA-HEMA copolymers, with one-to-one and one-to-three molar ratios. The DHPMA-containing hydrogels were characterized to advance the design and development of new soft-contact lenses that are superior to materials currently available. In addition, little published information exists in the scientific literature regarding the material properties of DHPMA hydrogels. A study using differential scanning calorimetry (DSC) was conducted to determine the structure of water in these crosslinked hydrogels. The crystallization and melting enthalpies were determined for these hydrogels equilibrated in water, and at several states of partial hydration. The enthalpic information was used to quantify the fraction of nonfreezing water, freezing-bound and freezing water in the hydrogels. The calorimetric data suggests that increasing DHPMA content increases the amount and proportion of water available for transport processes, compared to HEMA hydrogels. Gravimetric desorption experiments were conducted to determine if DHPMA significantly changed the desorption rate of buffered saline solution or deionized water in HEMA. The early time desorption rates in HEMA and DHPMA were compared, since the initial rate is most significant for contact lens materials. The percentages of water lost during the initial desorption, and the evaporation rate, decreased as the DHPMA content increased. To complement the desorption experiments, the stifffness of the hydrogels was examined during desorption. The stiffness of the DHPMA hydrogels did not change during the initial desorption, but the stiffness of the HEMA hydrogels immediately increased upon desorption. A comparison of sodium and potassium ion transport rates across DHPMA and HEMA membranes was also conducted; the ion exchange rate increased as DHPMA content increased. The dynamic mechanical transitions present in these hydrogels were examined by analyzing the dry xerogels and hydrogels at various states of hydration. The temperatures of the primary α transition and the secondary β and γ transitions were determined in the tension mode; the storage modulus and loss modulus as a function of temperature and frequency were recorded. The results were compared to the results obtained from dielectric analysis at low hydration using tan δ. The frequency dependence of the dispersions was calculated for the dry and hydrated states, using mechanical and dielectric data. The information obtained was used to elucidate the interaction between the polymer and the sorbed water. Analysis of the low temperature secondary γ transition and secondary transitions resulting from polymer-water interactions was emphasized. The observed viscoelastic properties of DHPMA hydrogels were similar to the well-characterized viscoelasticity of HEMA hydrogels.