Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

High Efficiency and Low Migration Hyperbranched Silicone Contain Macrophotoinitiators for UV-Cured Transparent Coatings

A kind of hyperbranched silicone containing macrophotoinitiators (HBSMIs) were synthesized from 2-hydroxy-2-methyl-1-phenyl propanone (HMPP) and the UV-curing behaviors of HBSMIs were investigated in UV-cured transparent polyurethane-acrylate (PUA) coatings. HBSMIs show higher UV-initiating efficiency than HMPP. The migration of HBSMIs from the UV-cured coatings can be as low as 1.7–6.0 wt%, which is obviously lower than the migration of HMPP. There is a remarkable improvement of the tensile strength of the UV-cured materials initiated by HBSMI in comparison to that of the materials prepared with the same PUA initiated by HMPP. Especially for the UV-cured materials prepared from PUA with 20 wt% 1,1,1-tris(hydroxymethyl)propane (TMP), the tensile strength and the strain at break increased from 6.81 MPa to 12.14 MPa and from 43.0% to 71.9%, respectively. The fraction of improvement for the tensile strength and the strain at break is as high as 78.9% and 67.2%, respectively. The coatings prepared with HBSMI also have better UV resistance ability than those coatings prepared with HMPP because they turn slightly yellow when they are aged by UV for about 15 min while the coating prepared with 4 wt% of HMPP will turn yellow only aged by UV for 2 min. These results suggest that preparation hyperbranched silicone containing macrophotoinitiators will be one of the good strategies to improve the curing efficiency of the UV-curing system, reduce the migration of UV initiator from cured material, improve the mechanical and UV resistance performance of UV-cured materials.

Onium salts improve the kinetics of photopolymerization of acrylate activated with visible light

The aim was study the influence of onium salts on the kinetics of photopolymerization in the visible light region. Trimethylolpropane triacrylate TMPTA was selected as a monomer, and activated by 1,3-bis(phenylamino)squaraine (SQ) used as a photosensitizer in addition to tetramethylammonium n-butyltriphenylborate (B2). The iodonium salt [A–I–B]⁺X⁻ functioned as a second radical initiator, bearing a different substitution pattern for the cation. The ternary system was formulated with different concentrations of both borate and diphenyliodonium salts. Differential scanning calorimetry was used to investigate the polymerization reaction over the photoactivation time carried out at 300 nm < λ < 500 nm irradiation. When the squaraine dye/borate salt was used as photoinitiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a third component (onium salt) increased the polymerization rate and conversion. Ternary photoinitiator systems showed improvement in the polymerization rate of triacrylate leading to high conversion in a short photoactivation time. The photoinitiating ability of bi- and tri-component photoinitiators acting in the UV-Vis region for initiation polymerization of triacrylate was compared with those of some commercially used photoinitiating systems. It was also found, that, the parallel electron transfer from an excited state of the sensitizer to [A–I–B]⁺X⁻, and an electron transfer from a ground state of R(Ph)₃B⁻N(CH₃)₄⁺ to an excited state of the sensitizer results in two types of initiating radical.

The chemical mechanisms were investigated by steady state photolysis and nanosecond laser flash photolysis experiments. A mechanism for initiating polymerization using both onium salts is proposed here.

Tuning Surface Microstructure and Gradient Property of Polymer by Photopolymerizable Polysiloxane-modified Nanogels

This paper reports a series of photopolymerizable polysiloxane-modified nanogels for regulating surface microstructure and gradient property of polymers, which were synthesized by solution polymerization under different feed ratios of a methacrylate-modified polysiloxane, urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of a thiol chain transfer agent. The nanogel structure and composition were characterized by proton nuclear magnetic resonance (1H-NMR), Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscope (TEM), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The dispersion of these nanogels in triethylene glycol dimethacrylate (TEGDMA) can reduce the onset and magnitude of shrinkage stress during polymerization without compromise to mechanical properties of the resulting polymers. Most importantly, as demonstrated by elemental analysis and X-ray photoelectron spectroscopy (XPS), the nanogels exhibit good self-floating ability in the monomer/polymer matrix and the increase of polysiloxane content in the nanogel can enhance the self-floating capability due to the lower surface tension and energy associated with the polysiloxane component. As a result, the polysiloxane-modified nanogels can spontaneously form a concentration gradient that can be locked in upon photopolymerization leading to a well-controlled heterogeneous polymer that presents a gradient change in thermal stability. With the increase of polysiloxane content, the thermal stability of the polymer was improved significantly. Furthermore, the enrichment of the nanogel on the surface resulting from the good self-floating ability can reduce the dispersion surface energy of gradient polymer film and generate a more hydrophobic surface with altered surface microstructure. These photopolymerizable polysiloxane-modified nanogels are demonstrated to have potential broad application in the preparation of gradient polymer with controlled surface properties.