Submicron Scale Research Articles

Facile bioactive transformation of magnesium alloy surfaces for surgical implant applications.

The market for orthopedic implant alloys has seen significant growth in recent years, and efforts to reduce the carbon footprint of medical treatment (i.e., green medicine) have prompted extensive research on biodegradable magnesium-based alloys. Magnesium alloys provide the mechanical strength and biocompatibility required of medical implants; however, they are highly prone to corrosion. In this study, Mg-9Li alloy was immersed in cell culture medium to simulate degradation in the human body, while monitoring the corresponding effects of the reaction products on cells. Variations in pH revealed the generation of hydroxyl groups, which led to cell death. At day-5 of the reaction, a coating of MgCO3 (H2O)3, HA, and α -TCP appeared on sample surfaces. The coating presented three-dimensional surface structures (at nanometer to submicron scales), anti-corrosion effects, and an altered surface micro-environment conducive to the adhesion of osteoblasts. This analysis based on bio-simulation immersion has important implications for the clinical use of Mg alloys to secure regenerated periodontal tissue.

Surfaces with modified morphology and wettability arrangement: a potential medium for water harvesting in desertification areas

Abstract The cost of water transport for irrigating drought-endurable plants in desertification areas makes it important to develop other water sources. Research shows that freshwater produced by dewing could satisfy the water requirement during germination of desert plants, therefore, functional materials that promote dew formation provide solutions for reduction of plant irrigation costs in desertification regions. In this review, feasibility of utilizing trace irrigation for ensuring survival of desert plants has been demonstrated. The technologies of preparing water harvesting materials with modified microstructure and wettability arrangement have been introduced, it has been shown that surface embedded with a bump array which has counter wettability compared with its plane section is a common morphology adopted by current works. After comparing climates in desertification regions with those in dewing chambers, it has been found that currently, the expected values of dew productivities of most water harvesting materials in deserted areas will not be as efficient as they performed in experiments due to low relative humidity. However, hydrophobic surface with submicron scale hydrophilic spherical cavity array produced by nanosphere lithography has shown reliable prospects in reducing the energy barrier of heterogeneous nucleation and thus become a more potential medium in dew water production.

Aminosilane Functionalized Aligned Fiber PCL Scaffolds for Peripheral Nerve Repair.

Silane modification is a simple and cost-effective tool to modify existing biomaterials for tissue engineering applications. Aminosilane layer deposition has previously been shown to control NG108-15 neuronal cell and primary Schwann cell adhesion and differentiation by controlling deposition of ─NH2 groups at the submicron scale across the entirety of a surface by varying silane chain length. This is the first study toreport depositing 11-aminoundecyltriethoxysilane (CL11) onto aligned Polycaprolactone (PCL) scaffolds for peripheral nerve regeneration. Fibers are manufactured via electrospinning and characterized using water contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Confirmed modified fibers are investigated using in vitro cell culture of NG108-15 neuronal cells and primary Schwann cells to determine cell viability, cell differentiation, and phenotype. CL11-modified fibers significantly support NG108-15 neuronal cell and Schwann cell viability. NG108-15 neuronal cell differentiation maintains Schwann cell phenotype compared to unmodified PCL fiber scaffolds. 3D ex vivo culture of Dorsal root ganglion explants (DRGs) confirms further Schwann cell migration and longer neurite outgrowth from DRG explants cultured on CL11 fiber scaffolds compared to unmodified scaffolds. Thus, a reproducible and cost-effective tool is reported to modify biomaterials with functional amine groups that can significantly improve nerve guidance devices and enhance nerve regeneration.

Liver Cancer Vascularity Driven by Extracellular Matrix Stiffness: Implications for Imaging Research.

Extracellular matrix stiffness represents a barrier to effective local and systemic drug delivery. Increasing stiffness disrupts newly formed vessel architecture and integrity, leading to tumor-like vasculature. The resulting vascular phenotypes are manifested through different cross-sectional imaging features. Contrast-enhanced studies can help elucidate the interplay between liver tumor stiffness and different vascular phenotypes. This study aims to correlate extracellular matrix stiffness, dynamic contrast-enhanced computed tomography, and dynamic contrast-enhancement ultrasound imaging features of 2 rat hepatocellular carcinoma tumor models. Buffalo-McA-RH7777 and Sprague Dawley (SD)-N1S1 tumor models were used to evaluate tumor stiffness by 2-dimensional shear wave elastography, along with tumor perfusion by dynamic contrast-enhanced ultrasonography and contrast-enhanced computed tomography. Atomic force microscopy was used to calculate tumor stiffness at a submicron scale. Computer-aided image analyses were performed to evaluate tumor necrosis, as well as the percentage, distribution, and thickness of CD34+ blood vessels. Distinct tissue signatures between models were observed according to the distribution of the stiffness values by 2-dimensional shear wave elastography and atomic force microscopy ( P < 0.05). Higher stiffness values were attributed to SD-N1S1 tumors, also associated with a scant microvascular network ( P ≤ 0.001). Opposite results were observed in the Buffalo-McA-RH7777 model, exhibiting lower stiffness values and richer tumor vasculature with predominantly peripheral distribution ( P = 0.03). Consistent with these findings, tumor enhancement was significantly greater in the Buffalo-McA-RH7777 tumor model than in the SD-N1S1 on both dynamic contrast-enhanced ultrasonography and contrast-enhanced computed tomography ( P < 0.005). A statistically significant positive correlation was observed between tumor perfusion on dynamic contrast-enhanced ultrasonography and contrast-enhanced computed tomography in terms of the total area under the curve and % microvessel tumor coverage ( P < 0.05). The stiffness signatures translated into different tumor vascular phenotypes. Two-dimensional shear wave elastography and dynamic contrast-enhanced ultrasonography adequately depicted different stromal patterns, which resulted in unique imaging perfusion parameters with significantly greater contrast enhancement observed in softer tumors.

Stabilizing nanograined austenitic stainless steel with grain boundary relaxation

Grain boundary (GB) relaxation can be triggered by interaction of GB with partial dislocations during plastic deformation, which is effective to stabilize nanograined structures in a number of face-centered cubic (FCC) metals and alloys. But this mechanism is interfered by deformation-induced martensitic transformation in 304 stainless steels with a metastable FCC structure. In this work, gradient FCC nanograined structures were prepared in a 304 stainless steel by using warm surface mechanical grinding treatment. GB relaxation was triggered in the samples with grains below 60 nm in size, in which partial dislocation dominates the plastic deformation. With the GB relaxation, thermal stability increases with decreasing grain sizes, contradictory to the conventional “smaller less stable” trend. The onset temperature for coarsening of nanograins of ∼30 nm in size is as high as ∼896 °C (0.7 Tm), about 250 °C above the reported value of the steel. The GB relaxation was not detected in martensitic nanograins of the same composition, of which the instability temperatures is around 650 °C (0.54 Tm), independent of grain size from 26 nm to the submicron scale, as it is controlled by the reverse martensitic transformation.

Control of mechanical and shape memory characteristics in martensitic NiTi by setting L-PBF parameters and build orientation

The current work encompasses a study of as-fabricated martensitic NiTi samples printed via Laser Powder Bed Fusion (L-PBF) in the horizontal and vertical build orientations, with systematic variations in the L-PBF process parameters. The sample densities, compositions, thermal characteristics, and mechanical responses were analysed and correlated with L-PBF parameters. Mechanical responses measured and analysed included microhardness, impact energies, and elastic modulus. The process parameters were found to have a significant effect on the resulting properties. The variation in physical properties and mechanical responses between each sample of the vertical build were less compared to those measured for the horizontal built samples. The L-PBF samples were compressed to 6% engineering strain (within elastic limit), and their shape memory strains were thermally recovered. Digital Image Correlation (DIC) based full-field strain analysis was performed during compression to measure local strain fields which revealed in detail the macroscopic effects during detwinning. The shape memory capability of the L-PBF samples was quantised based on strain recoveries attained after thermal recovery. Microstructural variations in submicron scale were also analysed in as-fabricated, post-compression and post-recovery stages, showing the sample response during different deformation mechanisms.

Local characterization of stainless steel 17-4PH produced by material extrusion additive manufacturing: Influence of the post-treatment

This study focuses on the influence of porosities present in the stainless steel 17-4 PH made by material extrusion additive manufacturing (MEAM) process. A comparison is made at the sub-micron scale between an as-fabricated sample and a H900 post-fabrication heat treatment sample. To achieve such observations, gold nanoparticles (NPs) are used as markers in order to capture the evolution of the material behavior during an in-situ mechanical tensile test under a scanning electron microscope. The obtained strain maps allow to evidence strain concentrations and their evolutions regarding the microstructure and the applied post-treatment. The heat-treated sample reveals a change in size and number of porosities. The displacements of NPs during the test highlight the quantitative evolution of the crack initiations and the porosities growth, exposing a more homogenized strain evolution for the H900 sample compared to the as-fabricated one.

Development and Characterization of Polymeric Composites Reinforced with Lignocellulosic Wastes for Packaging Applications

In this work, the effects of different fiber loadings on the mechanical properties of the composites at the sub-micron scale were studied through nanoindentation followed by physical characterization. The composites were prepared by incorporating different loadings of wheat straw, corn stalk, and rice husk in polypropylene copolymer using a melt processing method followed by thermal–hydraulic compression technique. Nanoindentation experiments in quasi-continuous stiffness mode were performed on the surfaces of produced composites to study the composites’ elastic modulus, hardness, and creep properties. The obtained results expressed the in-depth study of the micro- and macro-level structure and behavior of particle interactions. The findings demonstrated that observable shifts in composites’ hardness, elastic modulus, and creep rate had occurred. The WS-reinforced biocomposite sheet showed the highest elastic modulus of 1.09 and hardness of 0.11 GPa at 40 wt% loading in comparison to other loadings. An impact strength of 7.55 kJ/m2 was noted for the biocomposite at 40 wt% RH loading. In addition, optical microscopy, Fourier transform infrared spectroscopy, water absorption, thickness swelling, and Vicat softening point studies were conducted on biocomposite sheets to evaluate differences in physical, mechanical, and thermal properties. The outstanding mechanical performance of the newly developed composites makes them suitable for use as a biodegradable packaging material.

Movable fluid evaluation of tight sandstone reservoirs in lacustrine delta front setting: Occurrence characteristics, multiple control factors, and prediction model

The tight sandstone, deposited in a lacustrine delta front setting, constitutes substantial new petroleum reserves. Tight sandstone reservoirs' fluid mobility directly affects oil production and recovery efficiency. Consequently, it is crucial to quantitatively and synthetically characterize fluid mobility. This study presented a detailed investigation of the movable fluid of the Yanchang tight sandstone in the Ordos Basin. Multiple experiments were conducted to investigate movable fluid, including casting and cathodoluminescence thin section, nuclear magnetic resonance, X-ray diffraction, and high-pressure mercury intrusion. According to pore-throat structure investigation with petrographical observations, delta front sandstone was divided into four types of lithofacies, pore-throat structure, and three pore-throat spaces (nano-scale, submicron, and micron), which correspond to various occurrence characteristics of the movable fluid. The result shows that the fluid mobility of the micron pore-throat space is higher than the sub-micron scale, and the nano-scale is the lowest. The fluid mobility is driven by petrophysical property, pore-throat structure, diagenetic mineral, lithofacies, and sandstone strata pattern. With the deterioration of the pore-throat structure and petrophysical property, fluid mobility gradually decreases. Movable fluid content significantly correlates with quartz and feldspar content, showing that residual intergranular and feldspar dissolved pores can significantly improve fluid mobility. On the contrary, it is negatively correlated with clay and carbonate cement content. The carbonate cementation and clay mineral filling increase the proportion of nano-scale pore-throat, leading to the deterioration of the reservoir quality and pore-throat structure. Detailed researches on the spatial distribution of fluid mobility show it related to sandstone bed thickness, mudstone proximity, and mudstone/formation ratio. Typically, medium-grained sandstones forming thick beds in mudstone-poor sequences are commonly low in carbonate and clay mineral cement and highest in movable fluid content. In contrast, fine-grained sandstones occurring as thin beds or located near the mudstone-sandstone interface in mudstone-rich sequences generally exhibit high carbonate and clay cement. Hence, its movable fluid content is lowest. The frequent material exchange between sandstone and adjacent mudstone prompts the enhancement of the cementation of clay and carbonate minerals, which is a devastating factor for fluid mobility. The proposed model for predicting fluid mobility can be used for similar delta front sandstones of other lacustrine basins. These findings provide new insights into predicting the best reservoir sandstones with high fluid mobility and effectively exploiting tight sandstone reservoirs.

In-situ polymer-derived SiOC strengthened aluminum sheets: Investigations on its manufacturing and structure-property correlations

Ceramic nanoparticles are used to strengthen metals/alloys but their agglomeration within the matrix, weak particle-matrix interface, and high cost are concerning issues. Polymer-derived ceramics could overcome these problems. Pyrolyzed SiOC was obtained from poly(methylhydrosiloxane) (PMHS) in-situ within an Al matrix. Before pyrolysis, PMHS was fragmented and distributed in the nano and sub-micron scale by accumulative roll bonding (ARB). In the mid-section/central region of the composite along the thickness, a multimodal grain size distribution with a combination of large elongated and fine equiaxed grains (recrystallized grains) was observed. However, near the surface, only the latter was present. The PDC-containing specimen exhibited an improvement in strength by ∼ 8.5 times compared to the parent material. Whereas, the improvement compared to a similarly processed specimen but without the polymer-derived SiOC particles was by ∼ 6 times. Fractography revealed ductile fracture with sheet debonding in the composite.

Assessing the Scalability of the High-Pressure High-Temperature Quartz Crystal Microbalance with Dissipation (HPHT QCM-D) for Milk Fouling Studies

The thermal processing of milk is negatively impacted by fouling, which decreases the rate of heat transfer and increases the frequency of cleaning. Small-scale tools that can predict fouling on larger scales are crucial to understanding and mitigating fouling. In this work, milk fouling is compared between the submicron scale, using a high-pressure high-temperature quartz crystal microbalance with dissipation (HPHT QCM-D), and the pilot scale. Fouling rates are monitored in situ and foulants are characterized ex situ using Raman spectroscopy and atomic force microscopy (AFM). The results reveal that a proteinaceous foulant is formed on both scales. Also, a gradient in the magnitude of fouling is observed at both scales as a function of the local residence time and temperature with the greatest amount of fouling observed for the smallest local residence time and at the highest temperature (132 °C). The results suggest a scale-up factor of approximately 70–120 times when converting between QCM-D and pilot-scale fouling thicknesses. Similarities in the foulant properties and fouling rates support QCM-D as a viable technology for scale-up studies of milk fouling.

Submicrometer analyses of the polymer flow modifier effects on metal–polymer direct joining

Polymer modification is a promising approach to improve the joining performance of injection molded direct joining (IMDJ) technology. However, the factors contributing to the improved joining strength of submicron scale surface pattern joints have not been fully elucidated. In this study, we investigated the mechanism for improving the joining performance of polyamide 6 (PA6)/aluminum alloy A5052 joints with Blast + hot water treated metal structure using a flow modifier (OSGOL MF-11). Enhanced mechanical polymer infiltration at the submicron scale was confirmed, and the hydrogen bond generation at the joint interface was characterized. These findings elucidate the reason for the joining performance improvement caused by the flow modifier in the submicron scale.

37‐2: Distinguished Paper: Efficiency Enhancement of Submicron‐size Light‐Emitting Diodes by Triple Dielectric Layers

We demonstrate highly efficient submicron scale nanorod LEDs (nLEDs) passivated by the triple dielectric layers. The diameter, length, and wavelength at a peak EQE of nLEDs are 580nm, 5¼m, and 460nm, respectively. We report a peak EQE of 22.15 ± 0.34% with HfO2 based triple dielectric layer. We also explore the relation of indium‐fluctuation in multi quantum wells (MQWs) to sidewall effect of micro LEDs. We show that higher indium content in MQWs successfully reduce non‐radiative recombination on sidewall of InGaN blue nLEDs.

Ultra-High-Q Racetrack Resonators on Thick SOI Platform Through Hydrogen Annealing Smoothing

We implemented a hydrogen annealing based post-processing technique as a tool to improve the sidewall roughness of 3 μm thick silicon-on-insulator (SOI) waveguides and demonstrated ultra-high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factors on racetrack resonators leveraging on the propagation loss reduction achieved through the smoothing process. The designed racetracks are based on a combination of rib waveguides and strip-waveguide-based Euler bends. We measured intrinsic quality factors of 14×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for a racetrack with a footprint of ∼5.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 10.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for a smaller racetrack with footprint of ∼1.48 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The estimated propagation loss for the rib waveguides was ∼2.7 dB/m, representing a ×3 reduction respect to the previously measured losses of 3 μm thick SOI rib waveguides treated with thermal oxidation smoothing. Overall, the post-processing technique allowed to significantly reduce the sidewall roughness without altering the geometry of the waveguides, unlike in sub-micron scale SOI platforms, making it an attractive solution for applications demanding ultra-low losses.

Electrochemical micromachining based on sinusoidal signals with variable resistance

ABSTRACT Reducing pulse duration is the sole effective strategy to improve the processing accuracy in ultra-short pulse electrochemical microfabrication. High cost and unsuitability in practical production also restrain its development. This study aims to propose a method based on sinusoidal signals with variable resistance. The bilayer capacitor cannot be fully charged to achieve a transient machining state. An equivalent circuit differential equation is established, and the resistance becomes an effective parameter to control the machining accuracy. The experimental result shows that the machining accuracy increases significantly with the increase in resistance. The approach is applied to machine microstructures and achieves sub-micron scale accuracy on a pure nickel sheet. The same superior machining accuracy can be obtained by machining microstructures on hard-to-cut superalloy plates.

Heterogeneity of microstructures in a Cu–Zr based amorphous alloy composite reinforced by crystalline phases

Amorphous alloy composites (AACs) comprising reinforcing crystalline phases exhibit far better mechanical properties than their base amorphous alloy counterparts, and Cu–Zr based alloys consisting of B2 phase particles embedded in an amorphous matrix are well-researched AACs in this respect. Traditionally, the B2 particles in Cu–Zr based AACs may appear to be homogeneously crystalline (without any amorphous phase inside), the glassy matrix homogeneously amorphous, and their interfaces atomically sharp. Yet, such a picture would pose difficulties in understanding how crystallization can happen in the glassy matrix, since the crystallized product is denser than but iso-stoichiometric with the matrix. The present work provides insights into this puzzle, by using electron microscopy, nanoindentation and molecular dynamics (MD) simulations to investigate the structures and mechanical properties of the crystalline and amorphous phases, and their interfaces in a Cu62Zr34.5Al3Nb0.5 AAC. Surprisingly, in this AAC, the superficially crystalline particles with tens to hundreds of microns large are found to be heterogeneous at the sub-micron scale, comprising nano-crystallites embedded in a glassy matrix, and the amorphous phase outside also exhibits a gradient in free volume content towards the interface. Smaller particles containing only B2 nano-crystallites are found to be softer than the amorphous matrix, while larger particles are harder than the amorphous phase due to martensitic transformation into the B19’ phase. MD simulations indicate that the B2 phase is marginally more stable than the glassy state, and on crystallization the densification leads to free-volume generation in the surrounding glassy matrix as observed experimentally. Mechanics analysis indicates that the strain energy in the glassy matrix associated with free-volume generation scales with the volume of the crystallization, as would be the crystallization energy itself. Hence, crystallization in the AAC leading to a two-phase structure is a spontaneous but kinetically limiting process arising from the net energy difference between the crystallization energy and free-volume energy produced in the glassy matrix. This work provides a detailed understanding of the heterogeneity in the microstructures in Cu–Zr based AACs, to shed light on the crystallization of the iso-stoichiometric B2 phase from the amorphous phase.

Improved Therapeutic Delivery Targeting Clinically Relevant Orthotopic Human Pancreatic Tumors Engrafted in Immunocompromised Pigs Using Ultrasound-Induced Cavitation: A Pilot Study.

Pancreatic tumors can be resistant to drug penetration due to high interstitial fluid pressure, dense stroma, and disarrayed vasculature. Ultrasound-induced cavitation is an emerging technology that may overcome many of these limitations. Low-intensity ultrasound, coupled with co-administered cavitation nuclei consisting of gas-stabilizing sub-micron scale SonoTran Particles, is effective at increasing therapeutic antibody delivery to xenograft flank tumors in mouse models. Here, we sought to evaluate the effectiveness of this approach in situ using a large animal model that mimics human pancreatic cancer patients. Immunocompromised pigs were surgically engrafted with human Panc-1 pancreatic ductal adenocarcinoma (PDAC) tumors in targeted regions of the pancreas. These tumors were found to recapitulate many features of human PDAC tumors. Animals were intravenously injected with the common cancer therapeutics Cetuximab, gemcitabine, and paclitaxel, followed by infusion with SonoTran Particles. Select tumors in each animal were targeted with focused ultrasound to induce cavitation. Cavitation increased the intra-tumor concentrations of Cetuximab, gemcitabine, and paclitaxel by 477%, 148%, and 193%, respectively, compared to tumors that were not targeted with ultrasound in the same animals. Together, these data show that ultrasound-mediated cavitation, when delivered in combination with gas-entrapping particles, improves therapeutic delivery in pancreatic tumors under clinically relevant conditions.

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (gm), with the measured peak gm up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 μm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of gm; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.

Automated trackspinning of aligned lignin fibers as precursors of green carbon nanofibers

At present, most carbon fibers are made from non-renewable polyacrylonitrile. Substantial efforts have been made to replace petroleum-based precursors for carbon fiber production. Interestingly, lignin is a carbon fiber precursor material that is cheap, highly available and sustainable. Submicron-scale lignin-based carbon nanofibers are used in numerous areas, such as electronic devices, batteries, supercapacitors and low-cost, high-performance structural composite materials. Trackspinning (TS) technology offers a way to scale up the versatile, but inefficient contact drawing technique to produce small-diameter lignin fibers from environmentally friendly aqueous solutions. In this study, the effects of TS based on probe drawing of low-concentration lignin nanofibers blended with poly(ethylene oxide) and glycerol in sodium hydroxide (NaOH) solution were investigated. The TS lignin fibers were well aligned and reached diameters as small as 500–1000 nm as the drawing length was increased. Lignin fiber macromolecular alignment was isotropic at low levels of draw, and the dichroic ratio increased from 1 to 2.25 with doubling of the drawing length. The most highly drawn trackspun lignin fibers had a mechanical strength of 3.92 MPa and a Young’s modulus of 2.15 GPa, which were similar to reported values for solvent-electrospun lignin nanofibers. These findings support the potential to utilize TS to produce small-diameter lignin fibers using a simple aqueous solvent approach.

Interplay of photopolymerization and phase separation kinetics and the resulting structure-property relationship of photocurable resins

In this work we develop thermoset materials with heterogeneous microstructures on sub-micron scales by photopolymerization-induced phase separation (photo-PIPS). To this end, we designed a photo-curable resin based on pentaerythritol tetraacrylate (PETA), an acrylate monomer, combined with 2-ethylhexyl methacrylate (2-EHMA), a methacrylate diluent. Polypropylene glycol (PPG) was used as a phase separation agent. Phase separation was monitored by reactive light transmittance using a custom-built light transmission apparatus and through dynamic mechanical analysis (DMA). The photopolymerization kinetics and the microstructure morphology were characterized using real-time Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), respectively. We observe double phase separation: PETA and 2-EHMA separate due to different reactivities, while PPG phase separates as the network develops due to immiscibility. A synergy is observed between the two processes: PPG phase separation leads to an enhanced separation of 2-EHMA. Phase separation leads to reduced transmittance due to scattering, which is primarily associated with the separation of 2-EHMA. Phase separation also causes the reduction of stiffness due to the formation of PPG subdomains. The kinetics is enhanced by increasing the PPG molecular weight and increasing the irradiation intensity.