**Preface to "Polymer Processing"**

During the transformation of polymeric materials into final usable objects, polymers (usually viscoelastic fluids) undergo complex histories of deformation and temperature distributions. The morphology of each element of the final object is determined by the deformation and temperature history of the particle, which, at the end of a complex evolution, solidified in that position. Crystallization kinetics, when active, is deeply influenced by the molecular stretch acquired by the effect of the flow. The distribution of final morphology determines the properties of the final object and these can undergo remarkable changes (more than one order of magnitude) via the effect of morphology variations. These observations have been shared for some time within the scientific community; however, only in special cases is understanding clear or are phenomena described occuring along chain of processing–morphology–properties. On the basis of these considerations, the objective of this Special Issue was the collection of progression or reviews clarifying the relationships among processing conditions, as well as the resulting morphology and properties of the final objects. This objective included both experimental correlations and modelling both in relation to any polymer processing operation and to any polymer of technological interest. This Special Issue includes 11 research articles and 1 review. Hashimoto et al. experimentally studied ultra-high molecular weight polyethylene (UHMWPE) films stretched under different operating conditions by adopting different temperatures and stretching speeds, and with different operational configurations by adopting both uniaxial and biaxial stretching modes, in order to evaluate the stretching effect on the film final structure. Zhu et al. adopted a Lagrangian approach to obtain numerical results about the mixing mechanism and performance of a novel four-screw extruder. Sun et al. developed a 3D numerical simulation of reactive extrusion processes with the aim of better understanding the effect of operational and geometric parameters on both mixing and reaction processes in the preparation of PP/TiO2 nanocomposites. Speranza et al. accurately analyzed morphology via atomic force microscopy (AFM) and discussed, in relation to the operating conditions, the morphologies developed along the cross-sections of moldings, obtained by adopting a system able to rapidly change the cavity surface temperature during the process. The processes adopted to obtain those samples were numerically simulated to apply a criterion for the achievement of fibrillar morphology based on histories of molecular stretching and mechanical work. The results of the criterion were found to be consistent with the morphology distributions along the cross-section of each sample by Liparoti et al. The replication of micro- and nano-features on PLA molded samples, obtained by injection-molding tests performed with a modulated cavity surface temperature during the process, was analyzed by Liparoti et al. Li et al. constructed an adaptive optimization method to reduce stress and deformation exerted on a polymer stent produced with the micro-injection molding process. Nagato et al. investigated the replication of microlens arrays on polymethylmethacrylate (PMMA) films produced using a laser-assisted thermal imprinting process (LATI) under locally different processing conditions with a pressure and laser power heating mold surface. Hao et al. provided an innovative preparation method of insulation pressboards adopted in converter transformers; the method is based on the coating of the pressboards with a polytetrafluoroethylene (PTFE) functional film by radio frequency magnetron sputtering. The method was found to be effective in enhancing both the electrical insulation and the oil insulation of the prepared pressboards. Hamidi et al. investigated the processability and properties of silk-reinforced composites, produced with vacuum-assisted resin transfer molding (VARTM). Ruan predicted the effect of flow and temperature on the spherulitical and shish-kebab structures by adopting a simulation model based on a multiscale approach. Finally, Gonzalez-Gutierrez et al. reviewed the material extrusion additive manufacturing (MEAM) techniques: they focused the review on the techniques that adopt polymers filled with high contents of ceramic and metallic powders.

> **Giuseppe Titomanlio, Vito Speranza** *Special Issue Editors*
