Characterization of injection molded polymers – from conventional to wood-based thermoplastics

Licentiate thesis, 2022

The success of polymers products is associated with the melt processability, which allows to create products with complex shapes at a low cost. One of the most widely used processing techniques utilizing melt processability is injection molding, where a polymer is heated until it flows into a mold under pressure. Due to varying shear- and cooling rates during processing, injection molding creates a multilayered structure, consisting of complex hierarchical morphologies. In addition to process conditions, the structures formed are dependent on the molecular architecture including chemical environment and branching of the polymer chain. The resulting morphology defines the mechanical properties of the injection molded parts and consequently, understanding the correlation between material, processing parameters, and resulting morphology is an important challenge. Furthermore, to expand the use of injection molding to renewable cellulosic materials, intrinsic limitation in cellulose that impede melt processing must be overcome. This can be achieved by chemically modifying the cellulose, however chemical modifications impact the morphology formed during processing. This thesis focuses on using advanced scanning small- and wide-angle X-ray scattering as main characterization techniques, to unfold the nature of the complex semicrystalline structures in injection molded synthetic and cellulose-based polymers. By varying material parameters, processing conditions and using complementary techniques, such as computational simulations and mechanical testing, the underlying factors for formation of hierarchical morphologies is further studied. This thesis brings us one step closer to understanding and predicting the polymer microstructures and resulting mechanical properties of injection molded materials.