Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs) are a new class of infrared (IR) materials. CHIPs have the highest refractive index (RI) of any transparent synthetic polymer to date (as high as 2.2) and possess superior IR transparency in comparison to classic synthetic polymers due to a very high content of sustainable sulfur-sulfur (S-S), or sulfur-selenium (Se-S) bonds in the copolymer backbone. CHIPs can be either melt or solution processed into optical elements, such as, simple lenses, microlens arrays, phase gratings, prisms, Fresnel lenses or windows, which are directly suitable for IR-thermal imaging.

Upper-division undergraduate students are introduced to polymer processing using material extrusion fused filament fabrication 3D printing to make poly(lactic acid) (PLA) mechanical testing specimens. Computer aided design and slicing software packages are used to demonstrate the process of preparing 3D computer models for printing. Following the introduction of the relevant ASTM standards, mechanical testing specimens are created by using a commercially available desktop printer to introduce tensile, compact tension, and impact resistance testing. Using two different polymer processing parameters, the weaker 90 degrees and stronger 0 degrees raster angles, students rationalize the high degree of anisotropy observed in printed thermoplastic objects. In the 90 degrees orientation, stress is transferred across "welds" or the interfaces between tracks of deposited material, whereas in the 0 degrees orientation, stress is transferred along continuous tracks of material. Using MatLab and the results from basic rheological testing, a nonisothermal degree of healing model is coded to understand why printed objects tend to fail at the welds. Thermogravimetric analysis and differential scanning calorimetry are used to obtain the key material properties of PLA that are needed for modeling. Fracture surfaces are visualized using optical microscopy following mechanical testing to better understand the failure mechanism and to visualize void spaces formed between tracks of the material. The students are evaluated individually by using a written technical report to assess their ability to synthesize the data from many lab activities into a logical sequence to convey their results. The modularity of the laboratory makes it amenable to instructors across a wide variety of disciplines and institutions.

The mechanical performance of parts produced by fused filament fabrication (FFF) has been limited due to the presence of voids and poor interlayer welding. Recent advancements in FFF have enabled the fabrication of void-free objects with strong interlayer welding through the use of semicrystalline polymer shells such as high-density polyethylene (HDPE) along with a high viscosity core polymer like acrylonitrile-butadiene-styrene (ABS). The zero-shear viscosity (eta 0) of ABS is three orders of magnitude higher than HDPE making the ABS-HDPE core-shell configuration preferable. ABS holds the shape by preventing bulk flow and part bending while HDPE promotes full surface contact across the layers. Most polymers, however, are immiscible which causes a weak weld line along the core-shell interface. Herein, maleic anhydride (MAH) is grafted to the butadiene segment of the ABS core thereby compatibilizing the interface with HDPE, improving the interfacial adhesion. Attenuated total reflectance-Fourier transform infrared spectroscopy was employed to confirm successful grafting. Using a custom-made die affording the core-shell structure, the ABS-g-MAH is shown to improve the impact resistance by 253% and 16% compared to neat HDPE and ABS specimens, respectively. Additionally, a 10% increase compared to the unmodified ABS-HDPE core-shell configuration is observed.

Dithiophosphoric acids (DTPAs) are an intriguing class of compounds that are sourced from elemental sulfur and white phosphorus and are prepared from the reaction of phosphorus pentasulfide with alcohols. The electrophilic addition of DTPAs to alkenes and unsaturated olefinic substrates is a known reaction, but has not been applied to polymer synthesis and polymer functionalization. We report on the synthesis and application of DTPAs for the functionalization of challenging poly-enes, namely polyisoprene (PI) and polynorbornene (pNB) prepared by ring-opening metathesis polymerization (ROMP). The high heteroatom content within DTPA moieties impart intriguing bulk properties to poly-ene materials after direct electrophilic addition reactions to the polymer backbone introducing DTPAs as side chain groups. The resulting materials possess both enhanced optical and flame retardant properties vs the poly-ene starting materials. Finally, we demonstrate the ability to prepare crosslinked polydiene films with di-functional DTPAs, where the crosslinking density and thermomechanical properties can be directly tuned by DTPA feed ratios.

Fused filament fabrication (FFF) is a 3D printing technique used to manufacture thermoplastic objects that offers users nearly unlimited design freedom compared with traditional subtractive and formative methodologies. The most significant barriers prohibiting its more widespread adoption are the reduced mechanical properties compared to injection or compression-molded counterparts and the large degree of mechanical anisotropy due to the layer-by-layer additive process, where strength between layers is developed through the reptation and entanglement of polymer chains. Herein, a highly cross-linked covalent adaptable network (CAN) based on the Diels-Alder reaction between furan and maleimide functionalities is blended with flexible and tough polycaprolactone (PCL) to afford a dynamic semi-interpenetrating polymer network (semi-IPN) that can be spooled for use on an unmodified, commercially available desktop FFF 3D printer. By leveraging the thermoreversible nature of the Diels-Alder adducts, strong covalent bonds can form across the interfaces between deposited tracks. Tensile testing is performed on specimens fabricated using 0 and 90 degrees raster angles, causing stress to be transferred either along the tracks or across the interlayer welds, respectively. The semi-IPN exhibits a degree of mechanical anisotropy of 11% without requiring specialized hardware or postprint processing steps, a marked improvement over about 60% in the typical polylactic acid used in FFF. The addition of the PCL makes the toughness of the blend 100x that of pure CAN, and the 3D-printed objects do not contain void space between tracks, making them fully dense.

In this work a fused filament fabrication (FFF) die design, capable of extruding two thermoplastics simultaneously in a core-shell configuration, is demonstrated as a means to produce composite structures in a single step. Despite the enormous advancements in 3D printing, fabrication of FFF objects with a composite structure remains a challenge due to the difficulty in finding dies to extrude such structures. We used polyethylene terephthalate glycol (PETG) and high-density polyethylene (HDPE) filaments to perform core-shell 3D printing. HDPE is one of the most commonly produced plastics but rarely used in FFF due to the severe warpage caused by volume changes upon its crystallization. Rheological and thermal analyses suggest the use of HDPE as a shell material due to its extremely short reptation time and sharp melting peak that facilitate superior surface contact and interlayer weld strength at the interface between neighboring FFF tracks. PETG is a commonly used 3D printing filament with excellent printability and sufficient zero shear viscosity to help maintain the extruded filament shape against shrinkage induced by the HDPE shell. Impact and tensile properties of core-shell objects revealed tremendous improvements in the impact resistance and toughness especially at 30 vol % HDPE shell with 1280% and 150% enhancement in impact resistance when compared to individual components: PETG and HDPE, respectively. Scanning electron microscopy was used to analyze the fracture morphology of the tested specimens to obtain an understanding of the fracture mechanism leading to the increased impact resistance. Using this die design can help open avenues of fabricating high impact resistance materials suitable for high performance applications and using HDPE in 3D printed objects with its superior solvent resistance.

Multiple relaxation times are used to capture the numerous stress relaxation modes found in bulk polymer melts. Herein, inverse vulcanization is used to synthesize high sulfur content (≥50 wt%) polymers that only need a single relaxation time to describe their stress relaxation. The S-S bonds in these organopolysulfides undergo dissociative bond exchange when exposed to elevated temperatures, making the bond exchange dominate the stress relaxation. Through the introduction of a dimeric norbornadiene crosslinker that improves thermomechanical properties, we show that it is possible for the Maxwell model of viscoelasticity to describe both dissociative covalent adaptable networks and living polymers, which is one of the few experimental realizations of a Maxwellian material. Rheological master curves utilizing time-temperature superposition were constructed using relaxation times as nonarbitrary horizontal shift factors. Despite advances in inverse vulcanization, this is the first complete characterization of the rheological properties of this class of unique polymeric material.

Plastics are an extremely important class of materials that are prevalent in all facets of society; however, their widespread use over time, combined with limited end-of-life strategies, has led to increasing levels of waste accumulation. Although currently considered a burden, plastics waste is potentially an untapped feedstock for numerous chemical and manufacturing processes. In this review, we discuss the state of the art of approaches for valorization of plastics waste from a materials research perspective, including previous efforts to utilize plastics waste and recent innovations that have opportunities to add significant value. Although additional progress is necessary, we present several diverse capabilities and strategies for valorization that, when brought together, address end-of-life challenges for plastics at every stage of design and product consumption. In short, a materials research–based framework offers a unique perspective to address the urgent issues posed by plastics, unlocking the potential of polymers and plastics waste.

Plastic accumulation is a growing sustainability challenge. Most plastics are thermally inert at room temperature, requiring high temperatures for depolymerization. In the quest for developing sustainable plastic upcycling, we investigate silicon carbide (SiC) as microwave absorbing susceptors for reaching depolymerization temperatures. We utilize three SiC topologies - particles, hot-pressed plastic films containing SiC particles, and monoliths. SiC particles of >0.5 mm in diameter can provide heating rates approaching -200 degrees C/min at modest powers. Still, physically mixed plastic-susceptor beds suffer from density differences, causing non-uniform contact during plastic softening and melting. Hot-pressed plastic films containing SiC particles allow uniform plastic-susceptor contact but suffer from slow heating as only a single layer of susceptors exists between polymer layers. SiC monoliths mitigate these challenges by preferentially dissipating the microwaves and rapidly heating the plastics independently of polymeric properties.

Mobile-amorphous-free crystals of a semiconducting polymer, poly(3-hexyl thiophene) (P3HT), were made via a flow induced crystallization technique to yield crystals that were microns long while the other two dimensions were 6 and 21 nm in size. Thermal analysis revealed a melting point depression that is described by the Gibbs-Thomson (GT) equation resulting in an interpretation that is physically sound. On the contrary, contemporary analysis of the melting enthalpy, which also shows a depression below that for an infinitely sized crystal, yields results that are nonphysical and do not agree with the GT analysis. A simple argument is made to correct this discrepancy, using the first law of thermodynamics, to include the sensible heat of the molten crystal mass required for an extrapolation to the melting enthalpy of an infinitely sized crystal. A satisfactory comparison to the results from the GT equation is now found to reconcile older, literature data to good effect. However, the comparison is not as good for the P3HT crystals made here and believed to be because of the finite size of the crystals in two dimensions coupled with the rather large size of the polymer molecules prohibiting a continuum analysis.

The morphology of poly (3-hexylthiophene)(P3HT) in its liquidphase and its manipulation via flow-induced solution crystallizationand its crystallization kinetics was studied to determine its mechanism.Shear flow-induced ordering of semiconducting P3HT, which generatesmore perfect crystal structures than quiescent methods, is elucidatedusing in situ rheo-SANS and rheo-SALS measurements, and an Avramianalysis is performed. Characteristic lengths of P3HT crystals weremeasured as a function of time, and 3-D networks of percolated P3HTfibril crystals were determined by measuring the apparent fractal,∼2.6, by fitting the rheo-SANS data with a power law function.Additionally, UV–vis and DSC results revealed a process ofP3HT crystal perfection determined by following the evolution of absorptionpeak characteristics of pi–pi stacking at 600 nm and the meltingpeaks as they shifted and narrowed with respect to increasing sheartime. The Avrami exponent, m, reached a maximum value of 2 indicatinghomogeneous nucleation of P3HT macromolecules that allowed one-dimensionalfibril crystal growth and was limited by contact time between theP3HT molecules rather than the diffusion of P3HT chains and this isattributed to the highly directional pi–pi stacking attractionsof electron pi in the thiophene rings.

The critical role of nanoparticle dispersion on Faraday rotator activity was studied, revealing new routes for fabricating “plastic garnets” as low cost alternatives to existing inorganic materials for optical isolation and magnetic sensing.

Numerical simulation is used to understand the melting and pressurization mechanism in fused filament fabrication (FFF). The results show the incoming fiber melts axisymmetrically, forming a cone of unmelted material in the center surrounded by melted polymer. Details of the simulation reveal that a recirculating vortex of melted polymer is formed at the fiber entrance to the hot end. The large viscosity within this vortex acts to effectively seal the system against back-pressures of order 1000 psi (10 MPa), which are typical under standard printing conditions. The Generalized Newtonian Fluid (GNF) model was appropriate for simulation within the region that melts the fiber, however, a viscoelastic model, the Phan-Thien-Tanner (PTT) model, was required to capture flow within the nozzle. This is due to the presence of an elongational flow as molten material transitions from the melting region (diameter of 3 mm) to the nozzle at the exit (diameter of 0.5 mm). Remarkably, almost half the pressure drop occurs over the short capillary (0.5 mm in length) attached to the end of the converging flow region. Increased manufacturing rates are limited by high pressures, necessitating more consideration in the nozzle design of future FFF printers.

Extensive efforts have been employed to improve the power conversion efficiency of organic solar cells. One of the most successful approaches is the morphological control of the solar cells' active layer, the bulk heterojunction of the donor and acceptor. However, many morphological control techniques have faced challenges transferring from the lab to industrial scale due to lack of process scalability, stringent environmental requirements, and high temperature requirements during thermal annealing. In this report, we develop a novel strategy to manipulate the organic solar cells' morphology using a simple stirring technique. By stirring of poly(3-hexylthiophene) (P3HT) and mixtures of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) solutions in a nonhalogenated and less toxic solvent system, more perfect P3HT crystals can be made. Networks of P3HT crystal fibrils, percolated P3HT crystal domains, and PCBM phase separated domains are critical factors for optimal solar cell performance. We demonstrate a simple fabricating technique utilizing brush painting to easily deposit the precrystallized components of the devices' active layer. As a result, the painted solar cells (without thermal annealing) achieve similar performance to a control group prepared via the commonly used standard process of spin-coating from a mixture of P3HT and PCBM dissolved in dichlorobenzene, followed by thermal annealing at an elevated temperature. Moreover, this report reveals an increase in power conversion efficiency of 60% to 90% from the painted devices made from precrystallized components (after stir solutions) in comparisons to the devices made from the pristine components (before stir solutions). We have demonstrated an easy thin-film processing technique that achieves high degrees of morphological control, showing promise not only for applications in other semiconducting polymers, but also demonstrating a technique that is scalable for mass production.

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

Stresses result when polymer feed stock is extruded through the nozzle of a three-dimensional (3D) printer, causing undesirable surface roughness called "sharkskin," which hinders effective bonding to the substrate. A promising method to remove the sharkskin is to reheat the polymer after extrusion. However, questions remain about the appropriate design parameters to guarantee success. A mathematical model is presented for this system, and both amorphous and crystalline polymers are examined. The former is a heat transfer problem; the latter a Stefan problem. Several effectiveness conditions are considered, including exit temperature and a duration condition related to the polymer relaxation time. Our results provide guidance on designing effective postextrusion heaters.

New amine functional sulfur prepolymers were synthesized from inexpensive poly(sulfur-random-styrene) and 1,3-meta-phenylenediamine (PDA) via a proposed electrophilic aromatic substitution (SEAr) reaction. These chalcogenide hybrid inorganic/organic polymer resins show improved solubility in organic solvents. The aromatic amine functional groups were utilized to react with epoxides on polyhedral oligomeric silsesquioxanes through post-polymerization modification which resulted in crosslinked sulfur polymers.

Fused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material. Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts.

AbstractOptical technologies in the long‐wave infrared (LWIR) spectrum (7–14 μm) offer important advantages for high‐resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR‐transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so‐called IR‐fingerprint region. We report on a new molecular‐design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co‐monomers.

Many applications of 3D printing are enhanced by increased printing speed. In the hot end of a 3D printer, the polymer feed stock flows in a heated cylinder at a set temperature. Since the polymer must be hot enough to reach a pliant state before extrusion, this establishes a maximum velocity beyond which the polymer is too rigid to be extruded. A mathematical model is presented for this system, and both amorphous and crystalline polymer systems are examined. The former is a heat transfer problem; the latter is a Stefan problem. Several different conditions for establishing the maximum velocity are considered; using the average polymer temperature in the hot end matches well with experimental data.