Abstract Phase change materials (PCMs) are promising thermal energy storage materials due to their high specific latent heat. Conventional PCMs typically exploit the solid–liquid (s–l) transition. However, leakage and leaching are common issues for solid–liquid PCMs, which have to be addressed before usage in practical applications. In contrast, solid–solid (s–s) PCMs would naturally overcome these issues due to their inherent form stability and homogeneity. In this study, we report a new type of s–s PCM based on chemically linked polyethylene glycol (PEG, the PCM portion) with polylactic acid (PLA, the support portion) in the form of a block co‐polymer. Solid‐solid latent heat of up to 56 J/g could be achieved, with melting points of between 44 °C and 55 °C. For comparison, PEG was physically mixed into a PLA matrix to form a PEG:PLA composite. However, the composite material saw leakage of up to 9% upon heating, with a corresponding loss in thermal storage capacity. In contrast, the mPEG/PLA block co‐polymers were found to be completely homogeneous and thermally stable even when heated above its phase transition temperature, with no observable leakage, demonstrating the superiority of chemical linking strategies in ensuring form stability.
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