No abstract

Unmodified polyvinyl chloride (PVC) has low thermal stability and high hardness. Therefore, using plasticizers as well as thermal stabilizers is inevitable, while it causes serious environmental and health issues. In this work, for the first time, pure food-grade PVC with potential biomedical applications is processed and 3D printed. Samples are successfully 3D printed using different printing parameters, including velocity, raster angle, nozzle diameter, and layer thickness, and their mechanical properties are investigated in compression, bending, and tension modes. Scanning electron microscopy is also used to evaluate the bonding and microstructure of the printed layers. Among the mentioned printing parameters, raster angle and printing velocity influence the mechanical properties significantly, whereas the layer thickness and nozzle diameter has a little effect. Images from scanning electron microscopy also reveal that printing velocity greatly affects the final part's quality regarding defective voids and rasters' bonding. The maximum tensile strength of 88.55 MPa is achieved, which implies the superiority of 3D-printed PVC mechanical properties compared to other commercial filaments. This study opens an avenue to additively manufacture PVC that is the second most-consumed polymer with cost-effective and high-strength features.

The main novelty of this paper is the use of poly-ethylene terephthalate glycol (PETG) as a new shape memory polymer with excellent shape memory effect (SME) and printability. In addition, for the first time, the effect of programming temperature on PETG 4D printed samples has been studied. The amorphous nature of the PETG necessitates that molecular entanglements function as net points, which makes the role of programming temperature critical. SME comprehensively was conducted under compression loading for three programming conditions as well as various pre-strains. Significant results were obtained that summarized the gross differences exhibiting that the hot, cold, and warm programmed samples had the highest shape fixity, shape recovery, and stress recovery, respectively. The recovery and fixity ratios fell and rose, respectively, as the programming temperature increased. This effect intensified in hot programmed samples as the applied strain (loading time) expanded. So, the recovery ratio dropped from 68% to 50% by raising the pre-strain from 20% to 80%. The maximum stress recovery was 16 MPa, suggesting the fantastic benefit of warm programming conditions in PETG 4D printed parts. The locking mechanism (recovery force storage) for cold and hot programming is quite different. The dominant mechanism in cold programming is increasing internal energy by potential energy level enhancement. Contrary to this, in hot programming, the entropy reduction applies to the majority of the molecular segments, playing this role. By cooling, the state of the material changes from rubbery to glassy, and with this phase change, the oriented conformation of the deformed polymer chains is maintained under deformation. The results of this research can be used for various applications that require high shape fixity, recovery, or stress recovery.

There are a few shape memory polymers (SMPs) like polylactic acid (PLA) and polyurethane (PU) that are 4D printable, and other SMPs must be synthesized with a complicated chemical lab effort. Herein, considering dual‐material extrusion printing and microscopic mechanism behind shape memory effect (SME), bilayer‐encapsulated polycaprolactone (PCL)–thermoplastic polyurethane (TPU) shape memory composite structures are 4D printed for the first time. The SME performance is investigated by assessing fixity, shape recovery, stress recovery, and stress relaxation under bending and compression loading modes. PCL, TPU, and melting temperature of PCL play the role of switching phase, net point, and transition temperature, respectively. Due to the destruction and dripping of molten PCL in contact with water, PCL is encapsulated by TPU. Encapsulation successfully solves the challenge of bonding/interface between printed layers, and the results show that the SME performance of the encapsulated structures is higher than bilayer PCL–TPU one's. Experiments reveal that maximum stress recovery in 4D‐printed composites remains constant over time. This is a great achievement compared to the previous extrusion‐based SMP structures that have great weakness in stress relaxation due to weak and low crystalline fractions and the unraveling of molecular entanglements in semicrystalline and amorphous thermoplastic SMPs, respectively.

No abstract