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3D printed silicones, combining the unique physiochemical performances of silicones with the attractive design freedom of additive manufacturing, have aroused intensive interest from academia and industry owing to their attractive applications. Direct writing (DW) was the most used method to enable the 3D printing of silicones, but it was limited by its conflicting rheological requirements. Herein, we present novel design of an ultraviolet (UV)-thermal dual-cure ink for UV assisted DW of silicone composites. A UV curable rheological modifier was key to the ink design, which would not only increase the ink viscosity and yield stress substantially at low contents, but also further enhance its shape-retaining capability during printing by UV-assisted curing. As a result, challenging structures with good mechanical performances and isotropy were printable. Interestingly, the degree of structural collapse for the product could be controlled by tailoring the printing parameters, which proved to be a new design dimension for property manipulation of 3D-printed cellular architectures. Rheology of the inks and boundary conditions for structural collapse were discussed in detail. This work will open new opportunities for the development of 3D-printed silicones with customized architectures and tailored performances. • UV-curable rheological modifier was used for UV-DW inks for the first time. • Boundary conditions for structural collapse were resolved. • Degree of structural collapse could be tailored. • Structural collapse was used to engineer the properties of architectured foams. [ABSTRACT FROM AUTHOR]

Polypropylene (PP) was compounded with β-nucleating agent and injection-molded at 180 °C or 220 °C. The samples were subsequently treated by supercritical carbon dioxide (scCO 2 ) at different temperatures. Results show that processing temperature and scCO 2 treatment could strongly influence the tensile and impact properties of β-nucleated PP. In particular, the sample processed at 220 °C and treated at 120 °C exhibit much enhanced impact strength (4.8 times that of its untreated counterpart). FTIR, WAXD, SEM and DMA were performed to explore the effects of processing temperature and scCO 2 treatment on structure of the samples. Deformation-induced plastic flow and micro-voids were also evaluated to construct structure-property relations. It was found that the influence of processing temperature on mechanical properties is mainly associated with the β-form content and β-crystalline morphology, while the structural changes in the crystalline lamellar scale may be responsible for the toughening effect of scCO 2 treatment. [ABSTRACT FROM AUTHOR]

There are principally two mechanisms to improve the impact resistance of polymer-based composites, intrinsic toughening and extrinsic toughening. But the interplay between them is far from being well understood. Here, glass fiber was incorporated into polypropylene to promote extrinsic toughening mechanism, while addition of elastomer and annealing were adopted for intrinsic toughening. In this way, the interaction among glass fiber, elastomer and annealing could be discussed based on various characterizations, and their combined effect on mechanical properties of the composites could be determined. The results show that the intrinsic toughening mechanism of elastomer will be suppressed by glass fiber irrespective of the support of annealing, though annealing could work synergistically with glass fiber to toughen polypropylene. The possible structure-property relations are discussed. This work will provide deeper insight into the toughening behavior of polymer composites and practical guidance for the design of composites with excellent stiffness-toughness balance. [ABSTRACT FROM AUTHOR]

Abstract: β-Modification and annealing were adopted to improve the impact strength of long glass fiber reinforced (LFPP) composites. It was found that either β-modification or annealing is an efficient way to toughen LFPP composites, and largely enhanced impact toughness can be achieved via simultaneously using β-modification and annealing, particularly for the toughness at low temperatures (−40°C to −20°C). Meanwhile, the tensile strength and Young’s modulus of the composites could be maintained. Various characterizations were used to understand the microstructure changes and toughening mechanism after β-modification and annealing. The results show that the higher extent of the matrix plastic deformation could be the main reason for the enhanced toughness as induced by β-modification and annealing. The possible structural origins for the higher extent of matrix plastic deformation are also discussed. [Copyright &y& Elsevier]

Abstract: In this study, by directly melt-blending with 10wt% poly(lactic acid) (PLA) and 1.5wt% multiwalled carbon nanotubes (CNTs), yield strength of a biodegradable poly(propylene carbonate) (PPC) was increased more than one order of magnitude, and its Young’s modulus was enhanced by 8 times. Meanwhile, the high elongation at break of PPC was maintained. This overall property reinforcement could not be observed by adding either PLA or CNTs separately. At room temperature, the obtained mechanical properties of the reinforced PPC are comparable to or even better than those of some traditional petroleum-based polymers such as low density polyethylene, high density polyethylene and polypropylene (PP). Together with good biodegradability and excellent conductivity, the PPC/PLA/CNT composites prepared in this work exhibit a potential to be used as a sustainable alternative to petroleum-based polymers such as PP around room temperature. [Copyright &y& Elsevier]

Abstract: Controlling the hierarchical structure of melt-processed polymers is vital to “structuring” processing and tailoring properties of the product. In this work, polypropylene (PP)/octene-ethylene copolymer (POE) blends were injection-molded using so-called dynamic packing injection technique, which imposed oscillatory shear on the gradually cooled melt during the packing solidification stage. In this way, samples with highly oriented PP matrix and elongated POE particles were obtained. Most interestingly, it was found for the first time that the elongated POE particles could not improve any impact toughness of oriented PP, which is completely different from that for the isotropic ones. Polarized optical microscope, scanning electron microscope, micro-Fourier transform infrared spectroscopy and differential scanning calorimetry were used to characterize the microstructures along sample thickness. The crack-initiation term, impact fractured surface and cross-section of the impact surface were inspected to understand the difference in impact behavior between the oriented PP/POE blends and their isotropic counterparts. The results show that massive crazing or plastic flow of the matrix could not be effectively initiated in the oriented blends. Our work provides a good example for better understanding structure–property relationship of polymers via well controlling their internal hierarchical structure. [Copyright &y& Elsevier]

Cellular silicone foams are renowned for their exceptional flexibility, ultra-elasticity, and durability, which have gained significant attention in diverse applications. However, manufacturing porous silicone foams featuring substantial bending curvatures and customized mechanical properties remains a challenge. Herein, this work presents a new strategy for manufacturing curved porous silicone foams with both 4D printed bending curvature and tailored meta-mechanical properties. The foam is achieved by direct ink writing of composite silicone inks embedded with thermal expandable microspheres as foaming agents. The work studied in detail the thermal expansion of microspheres, foaming of composite silicone inks, as well as properties of the expanded foam. Utilizing the strain mismatch under thermal stimuli, the printed bilayer structures achieved shape-shifting. Additionally, silicones composed of stacked bilayer filaments exhibited negative stiffness properties under compression, leading to enhanced energy absorption capacity, which can be fine-tuned through different printing structural designs, demonstrating its potential in fields such as energy dissipation and shock absorption while protecting objects with curved shapes. This work presents a novel 4D printing strategy for manufacturing thin-shelled cellular silicone foams with both substantial bending curvature and tailored meta-mechanical properties. The foam is achieved by direct ink writing of composite silicone inks embedded with thermal expandable microspheres as foaming agents. This study investigated the thermal expansion of microspheres and composite silicone inks, as well as changes in the size, density, and modulus of the expanded silicone foam. By printing structures of stacked bilayer filaments with varying thermal expansion ratios, 4D printing of various geometrical structures due to strain mismatch, as well as negative stiffness effect due to snap- through buckling of stacked filaments under compression, were achieved. The combination of those characteristics demonstrated enhanced energy absorption capacity and potentials of the 4D printed cellular silicones in protecting objects with curved shapes from impact. [Display omitted] [ABSTRACT FROM AUTHOR]

3D printed hierarchical structured cellular silicones with high mechanical performance, shape morphing properties, multifunctional piezoresistive and temperature sensing properties were reported. The hierarchical structured PDMS/TEM cellular silicones demonstrated 3D printed lattice structure and micro-scaled porous filaments which was achieved upon thermal expansion of microspheres. The optimal 3D printed hierarchical structured silicones showed extreme load-bearing capacity (load is more than 165000 times its weight), ultra-high elasticity (negligible stress and strain loss under 80% compression), and high cycle durability (less than 4% strain loss under 1000 compression cycles). Besides, shape morphing property was endowed to the silicones to manufacture complex structures facilely. Furthermore, piezoresistive effect and varying temperature-sensing properties were endowed to the silicones by adding conductive fillers as MWCNTs, which demonstrate excellent versatility of this method in potential applications such as soft and wearable sensors. [Display omitted] Multifunctional lightweight cellular silicone with adjustable properties has aroused great interests in many fields. However, it remains a challenge to facilely prepare multifunctional lightweight porous silicones with high load-bearing capacity. Herein, this work developed a 3D printing technique to prepare lightweight hierarchical structured cellular silicones with macroscale lattice structure and microscale intra-strand close-cell porosities, which was achieved by the expansion of thermally expandable microspheres (TEM) with plastic shells dispersed in formulated silicones. The obtained silicone foam with hierarchical porosity distributions shows excellent mechanical properties, including extreme load-bearing capacity (load is more than 165000 times its weight), high elasticity (negligible stress and strain loss under 80% compression), and high cycle durability (less than 4% strain loss under 1000 compression cycles). Besides, the incorporation of conductive fillers of MWCNTs endowed the foam with multifunctional piezoresistive and temperature-sensing properties. Furthermore, by printing multiple mixture inks of varying expansion ratios, shape morphing ability was endowed to the printed foam, to achieve complex curvature geometry facilely, demonstrating excellent versatility and potential applications in manufacturing flexible and conformal electronics of this method. [ABSTRACT FROM AUTHOR]

3D printed silicones have demonstrated great potential in diverse areas such as soft robots, tissue engineering, flexible sensors and energy dissipation. However, most silicone inks face inevitable curing issue once it is prepared. During the materials extrusion based printing process, the curing-related viscosity change in silicone inks hamper the ink stability and printing controllability, leading to complex relationship among ink properties and printing parameters, as well as printing structures and material properties of final products. Herein, this work presents active mixing of two component silicone ink to realize stable printing, achieving exact and continuous match of ink properties and printing parameters. Using a specially formulated ink containing separate starting components with reactive groups, catalyst and thixotropic agents, this work studies the active mixing process thoroughly for the first time from molecular level to materials performances under various conditions regarding inlet flow rates, impeller rotation speed and component ratios. Besides, cellular silicones with controlled distribution of component ratios and tailored gradient structures are printed, leading to integrated manufacturing of compression performance and shape morphing property. This work will pave way for active mixing and printing of two-component silicones, and provide guidance for other reactive multi-material 3D printing systems. Active mixing of two-component viscoelastic silicone inks is studied, to solve the inevitable curing issue of mixture silicone ink, and to improve printing stability for continuous 3D printing of cellular silicones. Under optimal parameters, good mixing effects are achieved, from molecular distribution to mechanical performances. Furthermore, printed cellular silicones with tunable gradient compositions show adjustable compression and shape-morphing properties. [Display omitted] • Active mixing of two-component viscoelastic silicone inks is studied to improve printing stability for DIW printing of cellular silicones. • Under optimal parameters, good mixing effects are achieved, from molecular distribution to mechanical performances. • Printed cellular silicones with tunable gradient compositions show adjustable compression and shape-morphing properties. [ABSTRACT FROM AUTHOR]

Abstract: In this study, cellulose nanocrystals (CNCs) were incorporated into poly(propylene carbonate) (PPC), which is a biodegradable alternative copolymer of propylene oxide and carbon dioxide, CNCs were prepared by sulfuric treatment of cellulose filter. An effective approach to prepare PPC environment-friendly composites is shown. The obtained PPC/CNCs nanocomposites display orders of magnitude increase in mechanical properties by adding a small amount of CNCs, in particular, with only 0.1wt% of CNCs incorporated, a 10-fold increase in tensile strength and 7-fold increase in Young’s modulus were achieved. The thermal stability of the nanocomposites is significantly improved as measured by dynamic mechanical analysis and differential scanning calorimetry. This is due to a combination of CNCs reinforcement in the soft matrix and increased effective molecular weight of the nanocomposties. The present nanocomposites outperform PPC composites reinforced with other nano-sized stiff and anisotropic fillers. [Copyright &y& Elsevier]

The relaxation behaviors and impact damping effects of phenyl silicone rubber were investigated using dynamic mechanical analysis (DMA) and a home-designed impact damping test. The effects of rubber phenyl content, sample thickness and compression on the damping performance were evaluated. DMA results over a broad temperature and frequency range indicate that silicone rubber with higher phenyl content would overall exhibit better damping capacity owing to the increase in friction between molecular chains, and the impact damping performances agree well with the relaxation results. Besides, sample thickness and compression will also strongly influence the damping performances as directly evidenced by the impact damping tests. It is demonstrated that a combination of DMA and impact damping test is of critical use for designing rubber damping devices according to the frequency range of interest. This work will provide deeper insight into damping performances of rubbers and practical guidance for their engineering applications. • The "true" impact damping effects of silicone rubbers under work conditions were evaluated using impact damping test. • The impact damping results were correlated with DMA relaxation results. • The loss peak area of silicone rubber will increase almost linearly with phenyl content. • Sample thickness will strongly influence the damping effect at a certain frequency. • Compression will lower the damping effect at high frequencies, but enhance the damping effect at lower frequencies. [ABSTRACT FROM AUTHOR]