We report the observation of the smectic A F , a liquid crystal phase of the ferroelectric nematic realm. The smectic A F is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 10 5 V / m is observed. The SmA F phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)–smectic Z A (SmZ A )–ferroelectric nematic (N F )–SmA F phase sequence, and 7N/DIO, exhibiting an N–SmZ A –SmA F phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.

We have structurally characterized the liquid crystal (LC) phase that can appear as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-y-old void in the taxonomy of smectic LCs and which we term the “smectic Z A ,” is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer with a 180 Å period. A Landau free energy, originally derived from the Ising model of ferromagnetic ordering of spins in the presence of dipole–dipole interactions, and applied to model incommensurate antiferroelectricity in crystals, describes the key features of the nematic–SmZ A –ferroelectric nematic phase sequence.

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Thermoplastics encompass the majority of commercial plastics but are limited to manufacturing techniques that require heat and/or solvent to enable material reprocessing and reshaping. Photopolymerization offers a readily accessible in situ alternative to conventional processing techniques but has typically been restricted to cross-linked networks, thus removing the potential for recycling/reprocessing of the materials. In this work, the paradigm of photopolymerizable thermoplastics consisting of semicrystalline thiol–ene polymers with repeat units structures mimicking those of poly­(ethylene terephthalate) is explored. These materials are rapidly photopolymerized (<10 s) under mild irradiation conditions by using commercially available, unpurified dithiol and diene monomers to give polymeric materials with distinctive mechanical properties. The complex relationships between monomer structure, crystallization, and material properties (optical, thermal, and mechanical) were investigated by modifying the dithiol alkyl chain length (xDT, x = 2–10). Though subtle, these monomeric structural perturbations significantly impacted both the rate (∼0.3–19 MPa min–1) and extent of crystallization as well as the corresponding material properties (e.g., Young’s modulus and elongation to break ranging from 80–280 MPa and 500–840%, respectively). To confirm the expansive applicability of this facile photopolymerization method, substitutions to the diene functionalities that participate in crystallization were explored to understand the relationship between the monomer structure, the extent of crystallinity, and the mechanical performance. The photopolymers reported herein provide a basic framework to expand access to photocurable step-growth linear polymers with well-controlled polymerization-induced crystallization for a variety of applications, including for the rapid manufacture of prototypical or sacrificial components in 3D printing.

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