Polymer Engineering Faculty Research


Spatial variation of structural hierarchy in injection molded PVDF and blends of PVDF with PMMA. Part II. Application of microbeam WAXS pole figure and SAXS techniques

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The influence of blend composition on the processing–structure–property relationships in injection molded PVDF and blends of PVDF with PMMA was investigated. Local crystalline order and chain orientation were studied using newly developed microbeam wide angle X-ray pole figure camera. The depth profiling of lamellar texture was investigated using SAXS combined with microtomy. These techniques provided a general picture on the macro and microstructural spatial gradients developed in the injection molded parts as influenced by the process condition and the composition of the blends.The injection molded PVDF samples were found to exhibit a typical three layer morphology. The skin layer is formed as a result of the extensional flow with dominant c-axis orientation along the flow direction (FD). The crystal lamellae in this region are mainly extended in the direction perpendicular to the FD. The “shear zone” formed under the shear flow possesses “shish–kebab” structure. The “shish” structure is formed under high shear stress with “kebab” lateral overgrowth that occurs at subsequent stages. The chains in the shear zone are highly oriented in the FD with the local symmetry axis slightly tilted inwards towards the core. This tilt angle tends to be the largest at the interior portions of the shear crystallized zone essentially following an approximate parabolic profile indicating that this feature of the structure is established by the spatial variation of the flow kinematics during injection. The microbeam X-ray pole figure results indicate that the a- and b-axes of the α crystals are distributed in a plane close to the ND–TD plane in the skin and shear zone. The level of orientation gradually decreases towards the mid symmetry plane at the interior of the samples from the slow cooling that takes place in these regions. The addition of small amount of diluent PMMA was found to enhance the chain orientation levels in the PVDF. This was attributed to the increase of melt viscosity as well as reduction of “self” entanglements of the crystallizable PVDF chains.

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