Thermodynamics and Mechanical Properties of Diblock Copolymers Containing Polyethylene and Polynorbornene Derivatives

Abstract

A major theme within materials science is the development of structure-property relationships: understanding how small-scale details of chemistry and morphology give rise to observable behaviors. The ideal approach employs a model system in which some aspect of chemistry or morphology can be systematically varied across a wide range, to observe the corresponding changes in macroscopic properties. An excellent system for this type of study in hydrocarbon polymers is the family of hydrogenated, substituted polynorbornenes synthesized by ring-opening metathesis polymerization (ROMP). This series shares a common backbone structure of five-membered hydrocarbon rings linked by two-carbon bridges, and a substituent group can be appended to each ring to tune the thermodynamic and mechanical properties.

This dissertation makes use of eight polynorbornene species, including variants bearing n-alkyl, branched alkyl, cycloalkyl, and aromatic substituents. In much of this work, linear polyethylene (PE) is employed as a model against which the properties of the polynorbornene derivatives are contrasted. PE is a hugely important synthetic polymer, and a major interest of this thesis is the identification of polynorbornene-based species that can be combined with PE to exhibit useful properties.

This work is organized into several sections, each of which informs and motivates the next. First, ROMP of cyclopentene is examined as a crucial synthetic precursor to PE. The influence of “acyclic metathesis” side-reactions is quantified, methods for preparing well-defined, narrow-distribution polymer are developed, and a kinetic model is derived.

Second, the mixing thermodynamics of polynorbornene-containing diblock copolymers are investigated. Commonly employed thermodynamic models are tested and sources of deviations are explored. Several polynorbornene derivatives are identified that exhibit both high glass transition temperatures (Tg) and substantial miscibility with PE, an extraordinarily rare combination of properties. Finally, the unusual phase behavior of one polymer pair, which de-mixes upon heating, is examined in detail.

Third, one of the high-Tg polynorbornene derivatives is selected for incorporation into majority-PE diblock copolymers. With judicious choices of block lengths and thermal history, the low-strain mechanical properties of these “glassy-PE” diblock copolymers can be dramatically improved over those of the PE homopolymer.