Additive manufacturing (AM) is a broad definition of various techniques to produce layer-by-layer objects made of different materials. In this paper, a comprehensive review of laser-based technologies for polymers, including powder bed fusion processes [e.g. selective laser sintering (SLS)] and vat photopolymerisation [e.g. stereolithography (SLA)], is presented, where both the techniques employ a laser source to either melt or cure a raw polymeric material. The aim of the review is twofold: (1) to present the principal theoretical models adopted in the literature to simulate the complex physical phenomena involved in the transformation of the raw material into AM objects and (2) to discuss the influence of process parameters on the physical final properties of the printed objects and in turn on their mechanical performance. The models being presented simulate: the thermal problem along with the thermally activated bonding through sintering of the polymeric powder in SLS; the binding induced by the curing mechanisms of light-induced polymerisation of the liquid material in SLA. Key physical variables in AM objects, such as porosity and degree of cure in SLS and SLA respectively, are discussed in relation to the manufacturing process parameters, as well as to the mechanical resistance and deformability of the objects themselves. Graphic abstract

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Liquid crystal elastomers (LCEs) exhibit some remarkable physical properties, such as the reversible large mechanical deformation induced by proper environmental stimuli of different nature, such as the thermal stimulus, allowing their use as soft actuators. The unique features displayed by LCE are originated from their anisotropic microstructure characterized by the preferential orientation of the mesogen molecules embedded in the polymer network. An open issue in the design of LCEs is how to control their actuation effectiveness: the amount of mesogens molecules, how they are linked to the network, the nematic order degree, the cross-link density are some controllable parameters whose spatial distribution, in general, cannot be tuned except for the last one. In this paper, we develop a theoretical micromechanical-based framework to model and explore the effect of the network cross-link density on the mechanical actuation of LCE elements. In this context, the light-induced polymerization (photopolymerization) for obtaining the elastomers’ cross-linked network is of particular interest, being suitable for precisely tuning the cross-link density distribution within the material. This technology enables to obtain a molecular-scale architected LCEs, allowing the optimal design of the obtainable actuation. The possibility to properly set the cross-link density arrangement within the smart structural element (LCE microstructure design and optimization), represents an intriguing way to create molecular-scale engineered LCE elements having a material microstructure encoding the desired actuation capabilities.

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