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1. Acousto‐Photolithography for Programmable Shape Deformation of Composite Hydrogel Sheets

   https://doi.org/10.1002/smll.202204288  

   Li, Minghao ; Mei, Jiyang ; Friend, James ; Bae, Jinhye ( October 2022 , Small)

   Stimuli‐responsive hydrogels with programmable shapes produced by defined patterns of particles are of great interest for the fabrication of small‐scale soft actuators and robots. Patterning the particles in the hydrogels during fabrication generally requires external magnetic or electric fields, thus limiting the material choice for the particles. Acoustically driven particle manipulation, however, solely depends on the acoustic impedance difference between the particles and the surrounding fluid, making it a more versatile method to spatially control particles. Here, an approach is reported by combining direct acoustic force to align photothermal particles and photolithography to spatially immobilize these alignments within a temperature‐responsive poly(N‐isopropylacrylamide) hydrogel to trigger shape deformation under temperature change and light exposure. The spatial distribution of particles can be tuned by the power and frequency of the acoustic waves. Specifically, changing the spacing between the particle patterns and position alters the bending curvature and direction of this composite hydrogel sheet, respectively. Moreover, the orientation (i.e., relative angle) of the particle alignments with respect to the long axis of laser‐cut hydrogel strips governs the bending behaviors and the subsequent shape deformation by external stimuli. This acousto‐photolithography provides a means of spatiotemporal programming of the internal heterogeneity of composite polymeric systems.
2. Fabricating polymer-matrix composite materials with aligned microfibers using ultrasound directed self-assembly and stereolithography

   Niendorf, Karl ; Raeymaekers, Bart ( April 2022 , Proc. of the American Chemical Society Spring Meeting, San Diego, CA (USA), 20-24 March 2022)

   Fabricating polymer-matrix composite materials with microfibers aligned along a user-specified direction is important to obtain specific material properties, such as anisotropic electrical and thermal conductivity and improved mechanical strength. We quantify macro- and microscale alignment of microfibers embedded in photopolymer resin, 3D-printed using ultrasound directed self-assembly (DSA) and stereolithography, as a function of three dimensionless input parameters: microfiber weight fraction, dimensionless ultrasound transducer input power, and dimensionless ultrasound transducer separation distance. We use regression analysis to determine microfiber alignment as a function of the fabrication process parameters. Microscale alignment is primarily determined by microfiber weight fraction, whereas macroscale alignment is a function of microfiber weight fraction, dimensionless ultrasound transducer separation distance, and dimensionless ultrasound transducer input power. Relating microfiber alignment to the fabrication process parameters is a crucial step towards 3D-printing composite materials with specific anisotropic material properties.
3. Nanoscale investigation of two-photon polymerized microstructures with tip-enhanced Raman spectroscopy

   https://doi.org/10.1088/2515-7647/abdcba  

   Kazantseva, Anastasiya V. ; Chernykh, Elena A. ; Crook, Cameron ; Garcia, Evan P. ; Fishman, Dmitry A. ; Potma, Eric O. ; Valdevit, Lorenzo ; Kharintsev, Sergey S. ; Baldacchini, Tommaso ( February 2021 , Journal of Physics: Photonics)

   We demonstrate the use of tip-enhanced Raman spectroscopy (TERS) on polymeric microstructures fabricated by two-photon polymerization direct laser writing (TPP-DLW). Compared to the signal intensity obtained in confocal Raman microscopy, a linear enhancement of almost two times is measured when using TERS. Because the probing volume is much smaller in TERS than in confocal Raman microscopy, the effective signal enhancement is estimated to be ca. 10⁴. We obtain chemical maps of TPP microstructures using TERS with relatively short acquisition times and with high spatial resolution as defined by the metallic tip apex radius of curvature. We take advantage of this high resolution to study the homogeneity of the polymer network in TPP microstructures printed in an acrylic-based resin. We find that the polymer degree of conversion varies by about 30% within a distance of only 100 nm. The combination of high resolution topographical and chemical data delivered by TERS provides an effective analytical tool for studying TPP-DLW materials in a non-destructive way.
4. Natural cellulose fiber‐reinforced polyamide 6 thermoplastic composites prepared via in situ anionic ring‐opening polymerization

   https://doi.org/10.1002/pc.24808  

   Kashani Rahimi, Shahab ; Otaigbe, Joshua U. ( February 2018 , Polymer Composites)

   Thermoplastic polyamide 6 composites reinforced with flax fiber fabric and kraft pulp cellulose mat were prepared by anionicin situring‐opening polymerization (ROP) using a special vacuum‐assisted resin infusion process. The effects of the polymerization initiator, fiber alkali pre‐treatment, fiber type, polymerization temperature, and surface modification of fibers with organosilane coupling agents on the ROP reaction, polymer conversion and the physical and mechanical properties of the composites were investigated. The results showed that a combination of alkali pre‐treatment and the use of relatively less reactive magnesium bromide based anionic initiator gave a successful ROP reaction and high polymer conversion. Analysis of the physical and mechanical properties of the composite panels showed that optimal mechanical properties could be achieved by using a polymerization temperature of 150°C while polymerization at higher temperatures lowered both the flexural and tensile properties due to generation of more internal microvoids, as well as, decreased crystallinity of the matrix. In addition, it was found that optimal mechanical properties of the composites could be obtained using 2 wt% aminopropyltriethoxysilane (APS) coupling agent while higher APS concentrations negatively impacted the interfacial adhesion, resulting in lower mechanical properties. POLYM. COMPOS., 40:1104–1116, 2019. © 2018 Society of Plastics Engineers
5. A Computational Framework for Predicting Properties From Multifield Processing Conditions in Polymer Matrix Composites

   https://doi.org/10.1115/SMASIS2020-2390  

   Widdowson, Denise ; von Lockette, Paris ; Erol, Anil ; Rodriguez, Manuel A. ( September 2020 , ASME 2020 Conference on Smart Materials, Adaptive Structures and Intelligent Systems)

   Composites can be tailored to specific applications by adjusting process variables. These variables include those related to composition, such as volume fraction of the constituents and those associated with processing methods, methods that can affect composite topology. In the case of particle matrix composites, orientation of the inclusions affects the resulting composite properties, particularly so in instances where the particles can be oriented and arranged into structures. In this work, we study the effects of coupled electric and magnetic field processing with externally applied fields on those structures, and consequently on the resulting material properties that arise. The ability to vary these processing conditions with the goal of generating microstructures that yield target material properties adds an additional level of control to the design of composite material properties. Moreover, while analytical models allow for the prediction of resulting composite properties from constituents and composite topology, these models do not build upward from process variables to make these predictions.

   This work couples simulation of the formation of microscale architectures, which result from coupled electric and magnetic field processing of particulate filled polymer matrix composites, with finite element analysis of those structures to provide a direct and explicit linkages between process,more »structure, and properties. This work demonstrates the utility of these method as a tool for determining composite properties from constituent and processing parameters. Initial particle dynamics simulation incorporating electromagnetic responses between particles and between the particles and the applied fields, including dielectrophoresis, are used to stochastically generate representative volume elements for a given set of process variables. Next, these RVEs are analyzed as periodic structures using FEA yielding bulk material properties. The results are shown to converge for simulation size and discretization, validating the RVE as an appropriate representation of the composite volume. Calculated material properties are compared to traditional effective medium theory models. Simulations allow for mapping of composite properties with respect to not only composition, but also fundamentally from processing simulations that yield varying particle configurations, a step not present in traditional or more modern effective medium theories such as the Halpin Tsai or double-inclusion theories.

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