Poly(ethylene-co-1-tetradecylacrylate) (EcoTDA) and poly(ethylene-co-1-octadecylacrylate) (EcoODA) copolymers were synthesized using poly(ethylene-co-acrylic acid) (EcoA) copolymers with 5 and 20% acrylic acid contents, respectively, and fatty alcohols (1-tetradecanol and 1-octadecanol) as novel polymeric solid-solid phase change materials. Chemical structure, thermal property, and crystalline morphology of the copolymers were characterized by using Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC), and polarized optical microscopy, respectively. Poly(ethylene-co-1-tetradecylacrylate) and poly(ethylene-co-1-octadecylacrylate) copolymers have two reversible phase transitions in DSC thermograms, that is, they have two morphologically different phases to transform from solid to amorphous connected each other. The change in surface morphology is only by color tone after the low-temperature phase transition as it was discrete to single color amorphous after the high-temperature phase transition according to microscopy. This property makes them resistant to flow above the first transition where thermal energy is stored for utility. Microindentation test proved that the hardness and reduced modulus values are considerably low after the solid-solid phase transition, but it is still measurable as a solid material.