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1. Mechanical Recycling of 3D-Printed Thermosets for Reuse in Vat Photopolymerization

   https://doi.org/10.1021/acsapm.4c00184  

   Maines, Erin M. ; Polley, Michaela A. ; Haugstad, Greg ; Zhao, Brenda ; Reineke, Theresa M. ; Ellison, Christopher J. ( April 2024 , ACS Applied Polymer Materials)

   Additive manufacturing, otherwise known as three-dimensional (3D) printing, is a rapidly growing technique that is increasingly used for the production of polymer products, resulting in an associated increase in plastic waste generation. Waste from a particular class of 3D-printing, known as vat photopolymerization, is of particular concern, as these materials are typically thermosets that cannot be recycled or reused. Here, we report a mechanical recycling process that uses cryomilling to generate a thermoset powder from photocured parts that can be recycled back into the neat liquid monomer resin. Mechanical recycling with three different materials is demonstrated: two commercial resins with characteristic brittle and elastic mechanical properties and a third model material formulated in-house. Studies using photocured films showed that up to 30 wt% of the model material could be recycled producing a toughness of 2.01 ± 0.55 MJ/m3, within error of neat analogues (1.65 ± 0.27 MJ/m3). Using dynamic mechanical analysis and atomic force microscopy-based infrared spectroscopy, it was determined that monomers diffuse into the recycled powder particles, creating interpenetrating networks upon ultraviolet (UV) exposure. This process mechanically adheres the particles to the matrix, preventing them from acting as failure sites under a tensile load. Finally, 3D-printing of the commercial brittle material with 10 wt% recycle content produced high quality parts that were visually similar. The maximum stress (46.7 ± 6.2 MPa) and strain at break (11.6 ± 2.3%) of 3D-printed parts with recycle content were within error the same as neat analogues (52.0 ± 1.7 MPa; 13.4 ± 1.8%). Overall, this work demonstrates mechanical recycling of photopolymerized thermosets and shows promise for the reuse of photopolymerized 3D-printing waste. 

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2. Relaxation dynamics of blends of PVDF and zwitterionic copolymer by dielectric relaxation spectroscopy

   https://doi.org/10.1002/pol.20200032  

   Clark, Andrew G. ; Salcedo Montero, Miriam ; Govinna, Nelaka D. ; Lounder, Samuel J. ; Asatekin, Ayse ; Cebe, Peggy ( March 2020 , Journal of Polymer Science)

   Relaxation dynamics of PVDF blended with a random zwitterionic copolymer (r‐ZCP) of methyl methacrylate and zwitterionic sulfobetaine‐2‐vinylpyridine (PMMA‐r‐SB2VP) were investigated using dielectric relaxation spectroscopy. FTIR spectroscopy was used to determine the PVDF crystal phase of compression molded blends. Adding 25 wt% ofr‐ZCP promoted the formation of the polarβandγcrystals over the nonpolarαphase. A structural model is proposed where ther‐ZCP biases the PVDF to form polar crystal phases. Boyd's model was used to calculate the room temperature dielectric constants and led to good agreement with our measurements. Dielectric spectra of neatr‐ZCP showed two relaxation peaks attributed to PMMA units, with no additional relaxations present from the zwitterions. Blends of PVDF withr‐ZCP were dominated by theα_crelaxation associated with the crystalline phase of PVDF, which showed an Arrhenius temperature dependence. Analysis of the conductivity spectra shows a larger DC conductivity in the blends than in eitherr‐ZCP or homopolymer PVDF. Blends show an additional peak in the loss tangent, absent in the copolymer or PVDF attributed to space‐charge polarization. Higher DC conductivity and space‐charge polarization indicate that the combination of zwitterions and unique microstructure affects charge transport properties.

    

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3. Exciton Binding Energy in Organic Polymers: Experimental Considerations and Tuning Prospects

   https://doi.org/10.1002/aenm.202302837  

   Kashani, Somayeh ; Rech, Jeromy James ; Liu, Tuo ; Baustert, Kyle ; Ghaffari, Abbas ; Angunawela, Indunil ; Xiong, Yuan ; Dinku, Abay ; You, Wei ; Graham, Kenneth ; et al ( December 2023 , Advanced Energy Materials)

   Discrepancies in reported values of exciton binding energy (E_b) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, E_bis defined as the difference between the transport gap (E_t) and the optical gap (E_opt). Here, the E_bvalues of PBnDT‐TAZ polymer variants are determined using two commonly employed methods: a combination of ultraviolet photoemission spectroscopy and low‐energy inverse photoemission spectroscopy (UPS‐LEIPS) and solid‐state cyclic voltammetry (CV). E_bvalues obtained by UPS‐LEIPS show low dispersion and no clear correlation with the polymer structure and thedielectric properties. In contrast, CV reveals a larger dispersion (200 meV‐1 eV) and an apparent qualitative E_b‐molecular structure correlation, as the lowest E_bvalues are observed for oligo‐ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of E_b. Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of E_tis discussed. The differences in intrinsic and extrinsic factors highlight the context‐dependent nature of measurement when drawing global conclusions. Notably, the observed E_btrend derived from CV is not intrinsic to the pure materials but likely linked to electrolyte swelling and associated changes in dielectric environment, suggesting that high‐efficiency single‐material organic photovoltaics with low E_bmay be possible via high dielectric materials.

    

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4. End block dynamics in unentangled polymers by dielectric spectroscopy

   https://doi.org/10.1088/1361-648X/acdbf7  

   Wu, Mengchun ; Bichler, Karin J ; Jakobi, Bruno ; Grzesiowski, Alyssa ; Schneider, Gerald J ( June 2023 , Journal of Physics: Condensed Matter)

   Dielectric spectroscopy measures the dynamics of polymer melts over a broad frequency range. Developing a theory for the spectral shape can extend the analysis of dielectric spectra beyond determining relaxation times from the peak maxima and adds physical meaning to shape parameters determined with empirical fit functions. Toward this goal, we use the experimental results on unentangled poly(isoprene), and unentangled poly(butylene oxide), polymer melts, to test whether the concept of end blocks could be one reason for the Rouse model deviating from experimental data. These end blocks have been suggested by simulations and neutron spin echo spectroscopy and are a consequence of the monomeric friction coefficient depending on the position of the bead in the chain. The concept of an end block is an approximation which partitions the chain in a middle and two end blocks to avoid overparameterization by a continuous position dependent change of the friction parameter. Analysis of dielectric spectra shows that the deviations of the calculated from the experimental normal mode cannot be related to the end block relaxation. However, the results do not contradict an end block hiding below the segmental relaxation peak. It seems that the results are compatible with an end block being the specific part of the sub-Rouse chain interpretation close to the chain ends.

    

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5. Electrochemical performance of ionic polymer metal composite under tensile loading

   https://doi.org/10.1088/1361-665X/acecca  

   Napollion, Liya ; Kim, Kwang J. ( August 2023 , Smart Materials and Structures)

   Cracks in polymer composites can lead to premature failure, which can be disastrous for polymer-based energy storage devices. Detecting these cracks is essential to guarantee the reliability and safety of such devices. However, detecting cracks in composite polymers such as ionic polymer metal composites (IPMCs) is a challenging task, which makes it difficult to ensure their performance and safety. The overall goal of this study is to investigate the effect of cracks or damage caused by tensile loading on the mechanical properties and electrochemical characteristics of IPMC based capacitors. During tensile testing, the deformation of the IPMC strips causes changes in the ion distribution and concentration in the polymer matrix, influencing the performance of the material. The measurements were conducted utilizing electrochemical impedance spectroscopy at a room temperature (${21}^{\circ }\text{C}$) and frequency range of 10 KHz to 1 Hz. The method utilized in this study proved to be easy and quick with consistent results. The IPMC capacitor was found to increase its capacitance after major cracking in the Pt electrodes from high tensile mechanical loads. Furthermore, at lower frequency range (<100 Hz), the real (${\epsilon }^{\mathrm{\prime }}$) and imaginary (${\epsilon }^{\mathrm{\prime }\mathrm{\prime }}$) part of permittivity increase with the addition of loads. This displays that the dielectric property of the material is affected due to the increasing of the loads. It is concluded that, at frequencies above 100 Hz, the permittivity is weakly load dependent.

    

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