Viscoelastic relaxations of four poly(tert-butyl acrylate-g-styrene) copolymers are studied over a wide range of temperatures. The temperature location and apparent activation energy of the distinct relaxations observed under tension mode are discussed. As grafted polystyrene content increases in the copolymer, microdomains of both components occur and, consequently, two relaxations associated with cooperative motions of either the acrylate backbone or the graft are observed. A bending deformation mode is also analyzed in the region of the glass transition of the two components to study the phase separation. Moreover, the glass transition temperature of the acrylic backbone is estimated by microhardness measurements. The relationship between microhardness and storage modulus is established either below or above the glass transition of the tent-butyl acrylate backbone.