Effects of composition on the water uptake and hydroplasticisation of the glass transition temperature of methacrylate copolymers

The effects of composition on the glass transition of dry and hydroplasticised copolymers of methyl methacrylate (MMA), butyl methacrylate (BMA) and 2-hydroxypropyl methacrylate (HPMA) were studied by differential scanning calorimetry. Results from the hydroplasticisation studies showed that a high PHPMA content (>75 wt.%) was required for high water absorption and that the amount of water uptake was not linear with HPMA content but increased in an accelerating manner with increasing HPMA content. This behaviour was attributed to the increase in the hydrophilic character of the copolymer due to the strong hydroxyl–hydroxyl interactions at high PHPMA content. The Tgs of the dry copolymers were successfully predicted by all three equations used (rule of mixtures, Fox and Gordon–Taylor) but were poorly predicted for the hydroplasticised copolymers. This failure was attributed to the inadequacy of the equations in accounting for the specific interactions between the different segments of the copolymer chains. HPMA depressed the Tg of the water-saturated copolymers but enhanced the Tg of the dry system and this behaviour has particular relevance to its use in water-based latex paints.