Role of Tie Molecules in Ductility and Chain Deformation of Polyethylene

Abstract

Understanding structure-property relationships is vital, as they help in building a framework for designing materials with tailored properties. One of the most important microstructural elements in semicrystalline polymers is a tie molecule, a chain that connects two different crystal lamellae across the amorphous phase. It works as a stress transmitter, and without tie molecules, semicrystalline polymers are brittle and have limited commercial applications. This thesis aims to investigate the role of tie molecules in the ductility and chain deformation of semicrystalline polymers, using model linear polyethylenes (PEs). These PEs are made by ring-opening metathesis polymerization (ROMP) of cyclopentene (CP) followed by catalytic hydrogenation. In contrast to commercially available PEs, this synthesis method yields narrow dispersity (Ɖ) and allows precise control over molecular weight.The first section of the thesis focuses on perhaps the most obvious but yet unanswered question: what is the threshold tie molecule content for PE to be ductile? Model PEs with varying tie molecule contents are made by varying molecular weights and crystallization histories, and their extensibilities are measured by uniaxial tensile tests. Despite having the same chemical structure, the threshold tie molecule content was found to depend on crystallization history, rather than being constant across PEs. Incorporation of literature data for hydrogenated polybutadiene (short-branched PE) showed that the fraction of tie molecules required for ductility rapidly decreases with increasing crystallinity. This strong dependency on crystallinity is explained by the fact that tie molecule content increases the brittle fracture stress, but does not affect the yield stress, while both the fracture and yield stresses are influenced by crystallinity. In the second half of the thesis, the chain dimensions of isotropic and deformed PEs are measured by small-angle neutron scattering (SANS). Two homogeneous catalysts were tested to deuterate polycyclopentene to produce partially deuterated linear PE. 13C NMR spectroscopy was used to quantify the level of deuteration as well as the distribution of deuterium (D) along the backbone. While homogenous catalysts incur side reactions – double bond migration and H/D exchange – D atoms were randomly distributed along the polymer backbone, producing ideal PE samples for SANS experiments. SANS profiles from the labeled PEs confirmed that there is no excess scattering induced by non-uniform distribution of D. Employing one of these homogeneous catalysts, linear PE SANS samples were made by blending hydrogenated and deuterated linear PE. The radius of gyration (R_g) of isotropic samples in the melt and solid states, as well as a uniaxially drawn sample, were measured. The values of R_g in the melt and quenched states were identical, as reported previously for PEs with broader Ɖ. On the other hand, slowly cooled PE samples showed isotopic segregation, which hindered direct measurement of R_g. Nonetheless, the chain stiffness in the melt, quenched, and slowly cooled states was found to be invariant, supporting the ‘solidification’ model for crystallization. For the drawn sample, changes in R_g relative to the isotropic sample indicated that the chains deform in a sub-affine manner. The sub-affine behavior is attributed to the limited content of tie molecules, which are the chain elements expected to deform affinely throughout the extension.