Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light; We show that upon self-assembly in a thin layered film they act as "molecular zippers" that can be opened and closed with UV- and blue light, respectively. Simultaneously in situ recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced E -> Z isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the E Z isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity.