Scientists recently reported industry-friendly synthesis of chemically modified SIS to improve the mechanical properties of styrenic block polymers and lead to a carbon-free society.
Polymers
Thermoplastic elastomers – also known as TPEs or thermoplastic rubbers – are a chemically-bonded combination of multiple polymers – or copolymer – typically a plastic and a rubber – that have both thermoplastic and elastomeric properties.
The thermoplastic property is useful in injection moulding, while the elastomeric property gives the object the ability to stretch and return to nearly its original shape. These materials are ubiquitous in the interiors and exteriors of vehicles. The best-known TPEs include “styrenic block polymers”, which contain molecular blocks of polystyrene, which is hard, and polydiene, which is rubbery.
Two important examples are polystyrene-b-polyisoprene-b-polystyrene (SIS) and polystyrene-b-polybutadiene-b- polystyrene (SBS). Styrenic block polymers were developed by the Shell Chemical Company in the 1960s and have since been further developed by many researchers in both academia and industry. While the annual global market for styrenic block polymer based TPEs is worth several billion dollars, elastomers with enhanced mechanical properties, especially toughness, also remain in great demand.
Styrenic block polymers improved
Scientists from both Nagoya University and the Zeon Corporation recently reported industry-friendly synthesis block polymers. To improve the mechanical properties of these polymers, the SIS, the hydrogen-bonded SIS, and ionically functionalised SIS were chemically modified. The cation, or positive ion, has one (monovalent) electron removed from the outer shell. Preliminary measurements revealed that the ionically functionalised SIS has an extremely high tensile toughness of 480 MJ/m3, which is the highest value of any known thermoplastic rubber material as far as we know.
This preliminary test was useful to scientists as it meant they could investigate the common mechanical properties of materials. However, the test did not reveal all the mechanical features of the material, particularly impact resistance that is crucially important in practical applications. Measuring the impact resistance is important for understanding the mechanism by which desirable mechanical properties arise in the material, and therefore how they can be achieved.
This study is the first to evaluate the impact resistance of the new elastomeric materials based on ionically functionalised SIS and, is the first to compare them to the impact resistance of a typical high-strength material based on glass-fiber-reinforced plastic (GFRP), which has a tensile strength of 330 MPa.
Drop weight impact tests demonstrated that ionically functionalised SIS, with monovalent or divalent cations, is three or four times more impact resistant than chemically unmodified SIS. Ionically functionalised SIS, with divalent cations, is found to be 1.2x more impact resistant than typical high-strength GFRP. In total, ionically functionalised SIS, especially with divalent ions, was found to be highly impact resistant, even though inorganic fillers – a typical additive for hardening polymers – are not incorporated into the polymer and the molecular structure of the polymer is not chemically cross-linked.
Carbon-free society
Carbon dioxide (CO2) is a greenhouse gas that contributes to climate change. This happens as CO2 soaks up infrared energy, vibrates and re-emits it back in all directions. Half the energy goes out into space, and the other half returns to Earth and contributes towards ‘the greenhouse effect.’ A large amount of this CO2 is released through the industrial production of automobiles, therefore a carbon-free manner of production using polymers would be significantly beneficial for climate change.
The team believe that their research achievements will contribute to the development of lightweight vehicles and the establishment of a carbon-free society. Automobile and other vehicle manufacturers are continually searching for lighter materials that are also resistant to damage. Since ionically functionalised SIS can be synthesised on an industrial scale, it has a great potential to become a next-generation elastomeric material for use not only in interior and exterior automobile parts, but also for automobile bodies, and even the outer panels of automobiles, trains, and other vehicles that require structural materials with high impact resistance as well as ease of manufacture. As well as this, the use of this newly developed ionically functionalised SIS means a greener planet, as production can take place without carbon being created.
