Title

High performance thermosets: Liquid crystalline networks and reactive hyperbranched additives

Date of Completion

January 2004

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

To develop novel thermosets with exceptional properties, two efforts were attempted. While the first one involves the development of new liquid crystalline thermosets, which will include the synthesis, crosslinking and processing of the prepolymer as well as mechanical properties of the cured resin, the second focuses on the modification of conventional thermosets to improve the fracture toughness without any compromise on its modulus, high glass transition temperature and processability. ^ Via Acyclic Diene Metathesis (ADMET) polymerization of 2-t-butyl-1,4-phenol(bis(4-pentenyloxybenzoate)), a family of main chain, thermotropic, unsaturated nematic polyesters have been prepared. Under the promotion of three different Ruthenium based catalysts, polymers featuring substantially different transition temperatures are obtained, implying an influence of the catalyst on the resulting polymer chain microstructure, specifically, in the trans/cis ratio of the double bonds and the degree of isomerization. By peroxide crosslinking of the resultant unsaturated polymer, a nematic network with a broad window of liquid crystallinity and unique thermomechanical behavior was obtained. In addition to high damping and soft elasticity, this material also exhibits two distinct shape memory effects, which have different deformation mechanisms (by stress and by temperature) as well as different recovery triggering temperatures (glass transition (T g) and nematic-isotropic transition (TNI) temperature). Through the two sequential and independent deformation thermomechanical events, the sample can be deformed. At different temperature regimes (TNI > T > Tg and T > TNI), the strain by different deformation mechanism can be selectively recovered. During a simple continuous heating, two tandem shape recovery events are observed. ^ Expecting the improvement in the processability as well as the mechanical properties, we used an allyl terminated hyperbranched polyimide (AT-PAEKI) to modify Bisphenol-A based bismaleimide resin (BPA-BMI). By rheological characterization and dynamic mechanical analysis, AT-PAEKI is found capable of decreasing the processing viscosity and increasing the glass transition temperature, modulus as well as hardness of the cured resin. The study on fracture toughness reveals a modest toughening effect (from 0.48 to 0.55 MPam0.5) upon the addition of AT-PAEKI. ^