Abstract3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.
ABSTRACT:The aim of this work was to study the effects of incorporation of low molar mass additives on the molecular mobility and water vapor transport properties of the polysulfone (PSF). The additives used in this work were N-phenyl-2-naphthylamine (PNA) at 10, 18, and 30 wt % concentration and 2,6-di-terc-butyl p-cresol (BHT) at 5, 10, 15, and 20 wt % concentration. The additive incorporation resulted in changes on molecular mobility and thermal properties of the polysulfone glassy matrix associated with antiplasticization phenomenon. The effects observed on the polysulfone were reduction in glass transition temperature, reduction in the magnitude of secondary loss transition peak, changes in secondary loss transition peak for higher temperatures, and increase in elastic modulus EЈ as compared with those of the unmodified polymer. Changes in molecular mobility were correlated to reductions in PSF water vapor permeability. In PSF-PNA mixtures, the water vapor permeability was reduced up to 95% for 30 wt % additive incorporation and 81% for mixtures PSF-BHT with 20 wt % incorporation.
This work dealt with the effect of using an acrylic acid modified polypropylene (PP-g-AA) as a compatibilizing agent for the intercalation/exfoliation of an organically modified montmorillonite (o-MMT) in a polypropylene matrix (PP). Two PP-g-AA containing the same AA content (6 wt %) and having different molar masses were used. The o-MMT content was 0, 1, or 5 wt % of total mass and the PP-g-AA/o-MMT mass ratio was 0/1, 1/1, 2/1, or 5/1. Results of wide angle X-ray scattering (WAXS) and transmission electronic microscopy (TEM) showed that without the PPg-AA, the o-MMT was dispersed in the PP/o-MMT in a micrometer scale, similar to a conventional microcomposite. With the PP-g-AA, the o-MMT was much better dispersed and its interlayers were intercalated and partly exfoliated by the polymer chains. Compared with the neat PP, some PP/PP-g-AA/o-MMT systems exhibited higher G 0 values and a yield stress at low frequencies, indicating that the PP-g-AA promoted the intercalation/exfoliation of the o-MMT. The compatibilizing efficiency of those two PP-g-AA was very similar. Generally speaking, the higher the PP-g-AA/o-MMT mass ratio, the better the state of dispersion and the degree of intercalation/ exfoliation.
Poly(ethylene terephthalate)/organically modified montmorillonite (PET/o-Mt) nanocomposites were prepared via melt intercalation in a twin-screw extruder using a polyester ionomer (PETi) as compatibilizer. The o-Mt content used was 0, 1, 3 or 5 wt% and the compatibilizer/o-Mt mass ratio was 0/1, 1/1 or 3/1. The main objective was to study the effects of the addition of o-Mt and compatibilizer on the barrier properties of PET/o-Mt nanocomposites. The nanocomposites showed a significant reduction in CO 2 permeability of up to 50% when compared to the neat PET, without significant change in the CO 2 solubility revealing the importance of the diffusional path imputed by the organoclay on the overall permeation process. Water vapor permeability was reduced for all nanocomposites, achieving up to 30% reduction for the nanocomposite containing a compatibilizer/o-Mt mass ratio of 1/1. Overall, the nanocomposite containing 5 wt% of organoclay and compatibilizer/o-Mt mass ratio of 1/1 showed the best barrier properties.
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