Polyolefin blends with immiscible polymers; weld line strength, impact properties, microlayer morphology, and barrier

In Chapter 1, stress-strain behavior coupled with fractography was used to investigate the weld line strength of 30/70 w/w poly(vinyl chloride)/high density polyethylene (PVC/HDPE) blends. The weld line strength depended upon the domain shape of the PVC phase, with elongated domains causing weld line weakness. To alter the PVC domain shape, the viscosity ratio (η_PVC /η_HDPE) of the blend was varied by changing the PVC molecular weight. The domain shape at the fracture initiation site was used in conjunction with a modified Nielsen approach to predict the ductile to brittle transition at the weld line. For the composition studied, a critical aspect ratio of the PVC phase of 1.24 was determined. The calculations predicted that a viscosity ratio of 21 would produce a particle with an aspect ratio of 1.24. The observed weld line strength confirmed this prediction: blends with a viscosity ratio below 21 were brittle and those with a viscosity ratio above 21 had ductile weld lines.; Chapter 2 also consisted of PVC/polyethylene blends. In this chapter, an ultra low density polyethylene copolymer was dispersed in the PVC to act as an impact modifier. Good impact properties were achieved in quiescient systems, but were subsequently lost due to coalescence of the polyethylene during injection molding.; In Chapter 3, microlayer coextrusion was used as a tool to create structures with microplatelets of high aspect ratio. Polypropylene was combined with polyamide 66 (PA66) by microlayering. A high volume fraction of PA66 microplatelets dispersed in the PP was achieved by injection molding the microlayer materials between the melting temperatures. The PA66 remained in the solid state, and resulted in good barrier enhancement due to the microplatelet structure.; In Chapter 4, the gas barrier properties of injection molded structures prepared in Chapter 3 were investigated further. The resulting material had significantly reduced permeability to oxygen and carbon dioxide compared to the conventional melt blend. Structural models for permeability indicated that enhanced barrier arose primarily from increased tortuosity of the diffusion pathway provided by the oriented, flat platelets of high aspect ratio in the skin region of the complex injected molded structure. A modified Maxwell approach in conjunction with the skin core morphology adequately modeled the experimental data.