A time resolved X-ray diffraction study following the thickening transition in ultra-long, strictly mono-disperse alkanes - a model polymer system - will be presented. Long chain alkanes crystallize into different integer folded forms, all of which are unstable with respect to the extended chain crystal. It will be shown that the re-organisation that accompanies thickening from one folded form to another results in a contraction of the crystal lattice - crystals with different numbers of folds have a subtly different structure. Prior to this contraction there is an increase in disorder, revealed by an increase in the peak width. In addition, by applying a sinusoidal temperature modulation with varying amplitude and period, on top of the linear heating ramp, the X-ray equivalent of a TMDSC experiment can be performed, utilising the high brilliance, high resolution capabilities of ID11 at the ESRF. This provides direct access to a wide range of additional information, and promises to be a technique of general applicability to polymer processes and transitions. Data relating to the reversible and irreversible parts of the transition between different folded forms will be presented, providing conclusive evidence of a reversible ordering and disordering within each folded crystal phase.

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