Synthesis, Characterization and Properties of Aromatic PDMS Multiblock Copolymers

The main objective of the research described in this thesis is to explore the design, synthesis and (thermo)mechanical properties of a new family thermoplastic high-performance elastomeric (AB)n multiblock copolymers. The backbone is based on bismaleimide-functionalized all-aromatic liquid crystalline (LC) or amorphous (AM) precursors coupled with dithiol terminated PDMS oligomers. Thiol-ene click chemistry was used to prepare high molecular weight multiblock copolymers in high yield. The chemistry, phase behavior, and (thermo)mechanical behaviour will be described in detail. In Chapter 2, the synthetic details of the bismaleimide end-functionalized oligomers are described. Both LC and AM reactive precursors were synthesized using a standard solution polycondensation procedure, with target molecular weights (Mn) of 1, 5 and 9 kg·mol-1. All soluble samples showed unimodal molecular weight distributions and PDIs of ~2, which is consistent with step-growth polymerization. 1H NMR shows that the maleimide end-groups remain intact during synthesis, enabling further functionalisation of these oligomers towards multiblock copolymers. The temperature dependent properties and cure behavior of the LC- and AM- bismaleimide terminated oligomers are presented in Chapter 3. The two oligomer series show different behaviour with respect to their crosslinking chemistry, phase behavior and (thermo)mechanical properties. The cured thermosets show good thermal stabilities with Td5% > 390 °C and DSC, POM and XRD results confirm the liquid crystalline and amorphous nature of the different oligomers. The uncured LC oligomers and reference polymer shows Tgs of 136 – 157 ˚C, whereas the Tgs of the AM series are in the 130 – 134 ˚C range. After cure, the Tgs of the oligomers increased to 140 – 190 ˚C, depending on the concentration of reactive end-groups. Rheology and gel fraction test shows that the two series of oligomers with Mn of 1 and 5 kg·mol-1 are highly crosslinked, whereas those with an Mn of 9 kg·mol-1 are only partly crosslinked on mostly chain extended. The cured AM oligomer films show good mechanical properties with high tensile strengths (> 90 MPa), elastic moduli (~2 GPa), elongation at break (~10%) and toughness (~8 MJ·m-3). In Chapter 4, the synthesis and molecular weight characterization of the multiblock copolymers based on dithiol terminated PDMS and bismaleimidefunctionalized oligomers (LC and AM) are described. All thiol-terminated PDMS oligomers (Mn = 1, 5 and 10 kg·mol-1) could be successfully copolymerized with either LC- or AM-oligomers (Mn = 5 kg·mol-1), via thiol-ene click chemistry. 1H NMR confirmed that the multiblock copolymers exhibited high molecular weights, in the range of 22 – 58 kg·mol-1. The molecular composition, as calculated from 1H NMR experiments, are consistent with the theoretical values. The multiblock copolymers from Chapter 4 are characterized in terms of thermal stability, phase behavior, morphology and (thermo)mechanical properties, and are discussed in Chapter 5. DSC and DMTA experiments shows that the multiblock copolymers with PDMS segments with Mn of 5K and 10K show two glass transitions, indicating (micro)phase separation, due to the incompatibility of the aromatic ester units and PDMS units. In tensile test, the AM5K-b-PDMS multiblock copolymers show superior mechanical properties over their LC5K-b-PDMS analogs. The AM5K-b-PDMS1K film shows outstanding tensile strength of ~125 MPa, elastic modulus of 3.4 GPa and elongation at break higher than 30%. In Chapter 6, the (AB)n-multiblock copolymers based on all-aromatic polyester/PDMS as discussed in Chapter 4 and 5 were investigated as dual- and triple- shape memory polymers. The AM5K-b-PDMS1K film shows high Rf (100%) and Rr (>97%) in terms of dual- SME, while the LC5K-based analogue exhibits moderate shape memory performance with Rf of 97% and Rr > 80%. In triple- SME test, the AM5K-b-PDMS5K film shows high Rf (>95%) and high Rr (>96%), while the LC5K-based analogue exhibits slightly lower Rf of > 91% and Rr > 80%. In conclusion, we have demonstrated that thermoplastic (AB)n-multiblock copolymers can be prepared from all-aromatic oligomers and thiol-terminated PDMS oligomers via thiol-ene click chemistry. The best performing multiblock copolymer is AM5K-b-PDMS1K, which exhibits outstanding tensile strength (~125 MPa), elastic modulus (3.4 GPa) and elongation at break (>30%). These values surpass the mechanical test results of commercially available high-performance polymers such as PEKK, PPS and PEI.