Interdiffusion and solid state structure of polyethylene microlayer composites

Chapter 1 describes how layer-multiplying devices permit two polymers to be combined into unique layered structures. These model microlayer structures are ideal for fundamental study of transport phenomena such as interdiffusion.;In Chapter 2, kinetics of interdiffusion of a miscible polymer pair, HDPE and LLDPE, was studied experimentally to characterize conditions required to construct microlayer gradient morphologies. High specific interfacial area of microlayers offset low diffusion mobility of polymeric chains so the system approached compositional homogeneity on a laboratory time scale. These polydisperse polymer pairs formed isomorphic blends upon crystallization from the melt. Thermal analysis of quenched microlayered pairs indicated systematic changes in melting behavior with composition gradient, allowing quantification of progress of interdiffusion without chemical labeling. Analysis with a diffusion model formulated especially for a polydisperse system revealed the role of different fractions, and allowed extraction of diffusion coefficients for elementary chains from the net kinetics of interdiffusion.;In Chapter 3, the compositional gradient was visualized by progressively melting the microlayer with increasing temperature, making it possible to monitor local kinetics of interdiffusion. Diffusion coefficients were obtained which correlated well with the data from bulk kinetics. The interlayer boundaries remained stationary during a characteristic time of interdiffusion of the component main fractions; the boundary moved at longer times as high molecular weight fractions became involved in interdiffusion.;This moving boundary phenomenon was exploited in Chapter 4 to substantially increase concentration of inorganic particles in one component of LLDPE/LDPE microlayers. When taken to the melt, the diffusion rate mismatch between linear LLDPE chains and branched LDPE chains caused layer boundaries to move in the direction of the slower-diffusing chains, resulting in shrinkage of the LLDPE layers. Adding a particulate to the LLDPE did not impede interdiffusion, and shrinkage served to increase particle concentration. The concentrating effect was demonstrated with nickel and talc platelets and TiO2 particles.;Chapter 5 describes breathable films developed by isothermal stretching of polymer microlayer composites. A system was produced with poly(ethylene oxide) and CaCO3-filled PP copolymer. The films' water vapor transmission rates were measured and related to microstructure via morphology and porosity.