HIGHLIGHTS


branched polyethylenes acting as soft segments. A thermoplastic elastomer would then be formed when inter- conversion between rac and meso forms of the catalyst occurs at a rate that is slow with regard to the polymerisation but still happens several times during the time it takes to generate an entire polymer chain. This is the first time a thermoplastic elastomer has been obtained from ethylene as the only monomer. It also shows that there is more to polyethylene than just high- density polyethylene and low-density polyethylene.


Organic-inorganic perovskite ferroelectrics Barium titanate (BTO) has a piezoelectric coefficient of approximately 190picocoulombs(cC)/ Newton (N), which has made it the best ferroelectric material for over 130 years. Its only other rival is lead zirconate titanate. Few ferroelectrics come anywhere near in performance despite a lot of effort over the years to find alternatives. A recent paper has reported a new organic-inorganic perovskite ferroelectric with a piezoelectric coefficient of 185pC/N and a phase transition temperature of 406K, which is even 16K above that of BTO (Y.-M. You et al; Science, 2017, 357, 306). The perovskite is based on


trichloromanganese(II) units aligned along the crystallographic c axis with chloromethyltrimethylammonium cations arranged beside the inorganic chains (Scheme 2). Piezoelectric materials respond


to mechanical stress or pressure by creating an electric voltage response. The perovskite ferroelectric undergoes a reversible phase transition at 406K, which is easy to study. At elevated temperatures, the


considerable potential for pressure- sensitive devices particularly in medical and biomechanical applications where toxic metals need to be avoided.


Ultrapermeable polymer membranes Poly(trimethylsilylpropyne) has been known since 1983 and intensively studied due to its promise as a highly permeable polymer membrane for large-scale gas separation. The polymer forms glassy solids, which possess rigid voids of varying sizes that facilitate gas transport. Unfortunately, its selectivity for gas separation turned out to be limited. More recently, polymers of intrinsic microporosity (PIMs) have shown a substantial improvement in performance. The latest example by N. B. McKeown and co-workers offers enhanced selectivity and permeability that rivals commercial membranes for biogas purification and carbon capture (Nat. Mater., 2017, 16, 932).


Scheme 1 Structure of the new α-diimine nickel complex capable of producing thermo- plastic elastomers from ethylene monomer


crystal exhibits a centrosymmetric phase. At temperatures below the phase transition, the crystal no longer possesses an inversion centre, which leads to a spontaneous polarisation and the material becomes ferroelectric. The large ferroelectric response


makes the new organic-inorganic perovskite a serious competitor to BTO. Unlike inorganic ferroelectrics, such as BTO, which are formed at high temperature, the organic- inorganic perovskite is synthesised at room temperature and can even be processed from solution. The new material is anticipated to have


AB


Scheme 2 Crystal struc- ture of the chlorometh- yltrimethylammonium trichloromanganese(II) perovskite in its (a) low- temperature phase and (b) high-temperature phase


The latest PIM contains a sterically


hindered triptycene in its repeat unit (Scheme 3). One of the three arms of the triptycene is fused to a bulky tetramethylcyclohexane ring, the other two arms are part of the polymer chain. The polymer has a ribbon-like appearance and, despite its high content of aromatic units, is processible from solution. The 2D structure of the polymer together with the bulkiness of the triptycene units leads to inefficient packing of the polymer chains and the formation of interconnected pores up to 1nm in size that are ideal for gas separation. Initial results have shown that its thin films age ‘gracefully’ over one year and could approach, and maybe even improve, the permeabilities of current commercial polymer membranes for gas separation.


3D printing of aluminium alloys 3D printing is not a processing technique usually associated with metal works, and the vast majority of alloys has so far eluded this new technique due to their high melting temperature or poor solidification dynamics. A layer-by-layer deposition


=Mn =Cl =N =C


tends to make many alloys prone towards defects and cracking. That this doesn’t have to be the case is illustrated by the example of a high- strength aluminium alloy made by a


09 | 2017 45


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