Deformation of oriented lamellar block copolymer films

Highly ordered, near-single-crystal lamellar films of a triblock copolymer (polystyrene-polybutadiene-polystyrene, PS/PB/PS) were used to study the de formation mechanism of a structure of alternating glassy-rubbery layers, at different orientations of the deformation axis relative to the layer norma l. Synchrotron radiation was used for simultaneous in-situ deformation and small-angle X-ray scattering measurements. These were augmented with direct imaging of the structure by transmission electron microscopy. The deformat ion mechanism depends on the orientation of the force with respect to the s tructure. Loading parallel to the lamellae results in yielding by propagati on of a stable macroscopic neck. The glassy PS layers break up at the neck front, releasing the rubbery layers to achieve high strain. The morphology that develops by deformation of the structure in other directions is an ens emble of new tilt boundaries oriented along the deformation axis. The lamel lar normals tilt away from the deformation axis with increasing strain, kee ping the lamellar spacing essentially constant. The effect of force applied perpendicular to the lamellae is to fold the layers into a "chevron" morph ology, similar to other layered systems such as smectic liquid crystals. At high strain, plastic deformation and fracture of the glassy PS hinges of t he "chevron" structure leads to symmetric kink boundaries parallel to the f orce axis. In addition, nucleation of kink band's around defects and propag ation of the kink boundaries into adjacent regions can lead to a similar mo rphology. The lamellar spacing remains constant during perpendicular stretc hing, and the tilt angle of the lamellar normal follows the macroscopic def ormation in an affine manner. Stretching at 45 degrees forms asymmetric kin k boundaries parallel to the force axis. The major limbs of the kink band t ilt with increasing strain so that the angle between the lamellar normal an d the force axis increases from its initial value of 45 degrees, while the lamellar period remains constant. The minor limbs tilt in the opposite dire ction and exhibit dilation of the lamellar spacing. Eventually the layers r upture, forming voids at the kink-boundary interfaces. The tilt angle of th e major-limb lamellae, as a function of strain, is less than predicted by t he affine model. This study suggests a general deformation mechanism for a lamellar structure of alternating glassy and rubbery layers. The layered st ructure responds to deformation, in any direction other then parallel;to th e layers, by creating new internal tilt-grain boundaries parallel to the de formation axis. At higher strain the layers yield and subsequently fracture at the kink-boundary interfaces. With increasing strain the lamellar stack s between the kink boundaries tilt toward the deformation axis until they a re nearly parallel to it. Since the main features of this mechanism are ind ependent of the initial orientation angle of the layers relative to the def ormation axis, it is relevant also to polygranular, globally unoriented lam ellar structures.