In situ grazing incidence wide-angle X-ray scattering (GIWAXS) is used to study the in situ thermal behavior of solution-processed, high mobility core-expanded naphthalene diimide thin films. A series of three different molecules is studied where the side-chain branching position is systematically varied through the use of 2-, 3- and 4-branched N-alkyl chains. For all molecules, a number of different phases and their associated phase transitions are observed with heating up to 200 degrees C. In situ GIWAXS measurements allow following significant variations of packing in each phase including crystalline coherence length, orientation, d-spacing, and paracrystallinity, as well as, for the first time, thin film thermal expansion coefficients in both the in-plane and out-of-plane direction. Relating these parameters with device measurements of quenched films, a striking correlation is found between high field-effect mobilities and low in-plane thermal expansion coefficients. This relationship indicates that high in-plane thermal expansion coefficients are detrimental to in-plane charge transport due to the formation of nanoscale defects in the critical first few monolayers upon quenching.