Method for determining resin cure kinetics with low-frequency Raman spectroscopy.

Characterizing resin extent of cure kinetics is critical to understanding the structure-property-processing relationships of polymers. The disorder band present in the low-frequency region of the Raman spectrum is directly related to conformational entropy and the modulus of amorphous materials, both of which change as the resin polymerizes. Normalizing the disorder band to its shoulder (∼85 cm-1) provides structural conversion kinetics, which we can directly correlate to chemical conversion kinetics for methacrylate and epoxy-amine based resin systems. In addition to fitting both the structural and chemical conversion data to a phenomenological kinetic rate equation, we also demonstrate a relationship between the chemical and structural kinetics which appears to relate to the softness of the material. Lastly, we use the method to investigate a methacrylate/epoxy interpenetrating polymer network resin system. We find that the structural and chemical conversions occur simultaneously during the formation of the primary (methacrylate) network, but there is a lag between the two during the formation of the secondary (epoxy-amine) network.

Itaconic Acid as a Comonomer in Betulin-Based Thermosets via Sequential and Bulk Preparation.

The inherent chemical functionalities of biobased monomers enable the production of renewably sourced polymers that further advance sustainable manufacturing. Itaconic acid (IA) is a nontoxic, commercially produced biobased monomer that can undergo both UV and thermal curing. Betulin is a biocompatible, structurally complex diol derived from birch tree bark that has been recently studied for materials with diverse applications. Here, betulin, IA, and biobased linear diacids, 1,12-dodecanedioic acid (C12) and 1,18-octadecanedioic acid (C18), were used to prepare thermosets using sequential and bulk curing methods. Thermoplastic polyester precursors were synthesized and formulated into polyester-methacrylate (PM) resins to produce sequential UV-curable thermosets. Bulk-cured polyester thermosets were prepared using a one-pot, solventless melt polycondensation using glycerol as a cross-linker. The structure-property relationships of the thermoplastic polyester precursors, sequentially prepared PM thermosets, and bulk-cured polyester thermosets were evaluated with varying IA content. Both types of thermosets exhibited higher storage moduli, Tgs, and thermal stabilities with greater IA comonomer content. These results demonstrate the viability of using IA as a comonomer to produce betulin-based thermosets each with tunable properties, expanding the scope of their applications and use in polymeric materials.

A generic scenario analysis of end-of-life plastic management: Chemical additives.

Plastic growing demand and the increment in global plastics production have raised the number of spent plastics, out of which over 90% are either landfilled or incinerated. Both methods for handling spent plastics are susceptible to releasing toxic substances, damaging air, water, soil, organisms, and public health. Improvements to the existing infrastructure for plastics management are needed to limit chemical additive release and exposure resulting from the end-of-life (EoL) stage. This article analyzes the current plastic waste management infrastructure and identifies chemical additive releases through a material flow analysis. Additionally, we performed a facility-level generic scenario analysis of the current U.S. EoL stage of plastic additives to track and estimate their potential migration, releases, and occupational exposure. Potential scenarios were analyzed through sensitivity analysis to examine the merit of increasing recycling rates, using chemical recycling, and implementing additive extraction post-recycling. Our analyses identified that the current state of plastic EoL management possesses high mass flow intensity toward incineration and landfilling. Although maximizing the plastic recycling rate is a reasonably straightforward goal for enhancing material circularity, the conventional mechanical recycling method requires improvement because major chemical additive release and contamination routes act as obstacles to achieving high-quality plastics for future reuse and should be mitigated through chemical recycling and additive extraction. The potential hazards and risks identified in this research create an opportunity to design a safer closed-loop plastic recycling infrastructure to handle additives strategically and support sustainable materials management efforts to transform the US plastic economy from linear to circular.

Molecular Weight Controls Interactions between Plastic Deformation and Fracture in Cold Spray of Glassy Polymers.

Polymer cold spray has gained considerable attention as a novel manufacturing process. A promising aspect of this technology involves the ability to deposit uniform polymer coatings without the requirements of solvent and/or high-temperature conditions. The present study investigates the interplay between shear instability, often considered to be the primary mechanism for bond formation, and fracture, as a secondary energy dissipation mechanism, collectively governing the deposition of glassy thermoplastics on similar and dissimilar substrates. A hybrid experimental-computational approach is utilized to explore the simultaneous effects of several interconnected phenomena, namely the particle-substrate relative deformability, molecular weights, and the resultant yielding versus fracture of polystyrene particles, examined herein as a model material system. The computational investigations are based on constitutive plasticity and damage equations determined and calibrated based on a statistical data mining approach applied to a wide collection of previously reported stress-strain and failure data. Results obtained herein demonstrate that the underlying adhesion mechanisms depend strongly on the molecular weight of the sprayed particles. It is also shown that although the plastic deformation and shear instability are still the primary bond formation mechanisms, the molecular-weight-dependent fracture of the sprayed glassy polymers is also a considerable phenomenon capable of significantly affecting the deposition process, especially in cases involving the cold spray of soft thermoplastics on hard substrates. The strong interplay between molecular-weight-dependent plastic yielding and fracture in the examined system emphasizes the importance of molecular weight as a critical variable in the cold spray of glassy polymers, also highlighting the possibility of process optimization by proper feedstock selection.

Molecular-Weight-Dependent Interplay of Brittle-to-Ductile Transition in High-Strain-Rate Cold Spray Deposition of Glassy Polymers.

Based on the cold spray technique, the solvent-free and solid-state deposition of glassy polymers is envisioned. Adiabatic inelastic deformation mechanisms in the cold spray technique are studied through high-velocity collisions (<1000 m/s) of polystyrene microparticles against stationary target substrates of polystyrene and silicon. During extreme collisions, a brittle-to-ductile transition occurs, leading to either fracture- or shear-dominant inelastic deformation of the colliding microparticles. Due to the nonlinear interplay between the adiabatic shearing and the thermal softening of polystyrene, the plastic shear flow becomes the dominant deformation channel over brittle fragmentation when increasing the rigidity of the target substrate. High molecular weights (>20 kDa) are essential to hinder the evolution of brittle fracture and promote shear-induced heating beyond the glass transition temperature of polystyrene. However, an excessively high molecular weight (∼100 kDa) reduces the adhesion of the microparticles to the substrate due to insufficient wetting of the softened polystyrene. Due to the two competing viscoelastic effects, proper selection of molecular weight becomes critical for the cold spray technique of glassy polymers.

Synthesis and Characterization of a Low-Molecular-Weight Novolac Epoxy Derived from Lignin-Inspired Phenolics.

The need for renewable polymers capable of replacing their petrochemical counterparts continues to grow as sustainability concerns rise. Bisguaiacol (BG), a bioinspired alternative to bisphenol-A (BPA), has been synthesized using vanillyl alcohol and guaiacol via an electrophilic aromatic condensation. Purification provides both BG and an oligomeric coproduct with a consistent number average molecular weight and dispersity of ∼650 Da and ∼1.00, respectively. This coproduct has been well characterized as a low-molecular-weight novolac averaging five hydroxyls per molecule and was transformed into an epoxy resin suitable for use in thermosetting resins. The bioinspired thermoset produced in this work, consisting of the epoxidized coproduct and an amine curing agent (Epikure W), exhibited a glass transition temperature over 100 °C and glassy storage modulus value of ∼3 GPa at 25 °C. When compared to a commercial cresol novolac epoxy, the cured epoxidized coproduct resin shows comparable thermal and thermomechanical properties. When compared to a commercial BPA-based resin, the cured epoxidized coproduct resin shows improved mode 1 fracture values of 1.34 J m1/2 (K 1C) and 448.16 J/m2 (G 1C). By utilizing the coproduct strategically, the overall production of BG has the potential to become more economically feasible.

Hydrophilic modification of SLA 3D printed droplet generators by photochemical grafting.

Few droplet generators manufactured using desktop stereolithography 3D printers have been reported in the literature. Moreover, 3D printed microfluidic chips are typically hydrophobic, limiting their application to water in oil droplets. Herein, we present designs for concentric and planar 3D printed microfluidic devices suitable for making polymeric microparticles using an off-the-shelf commercial stereolithography printer and resin. The devices consist of a microscope slide, binder clips, and printed components. Channels were modified by an ultraviolet grafting of methacrylic acid to the surface of chips, yielding a hydrophilic coating without modification to the bulk polymer. The water contact angle decreased from 97.0° to 25.4° after grafting. The presence of the coating was confirmed by microscopy and spectroscopy techniques. Polystyrene microparticles in the <100 µm size range were generated with varying molecular weights using the described microfluidic chips. Our work provides a facile method to construct droplet generators from commercial stereolithography printers and resins, and a rapid surface modification technique that has been under-utilized in 3D printed microfluidics. A wide range of microfluidic devices for other applications can be engineered using the methods described.

Viability of Low Molecular Weight Lignin in Developing Thiol-Ene Polymer Electrolytes with Balanced Thermomechanical and Conductive Properties.

Polymer electrolytes with high aromatic content are prepared through thiol-ene polymerization with functionalized, low molecular weight fractions of softwood pine Kraft lignin, and wheat straw/Sarkanda grass soda lignin. Differing solubility, functionality, and aromatic content of the lignin fractions vary the glass transition temperatures of the resulting polymers and the suitability for electrolyte applications. The softwood pine Kraft lignin is used as a precursor for a gel polymer electrolyte (GPE) with room temperature conductivity of 72 × 10-7 S cm-1 , while the wheat straw/Sarkanda grass soda lignin is utilized in solid polymer electrolytes (SPEs) with room temperature conductivity values in the range of 5 × 10-5 - 7 × 10-5 S cm-1 . The lignin-based GPE displays similar conductivity but improved thermal stability to a comparable, recently reported GPE containing an allylated, monophenolic, lignin-derived, vanillin-derived monomer. The lignin-based SPEs exhibit excellent cationic transport with ion transference values up to 0.90. The promising conductivity and ion transference results reveal the potential for use of functionalized, low molecular weight wheat straw/Sarkanda grass soda lignin in SPE applications as a way to improve thermal stability, electrochemical performance, and incorporate an abundant, sustainable resource in a high performance application.

Efficient UV Filter Solubilizers Prevent Recrystallization Favoring Accurate and Safe Sun Protection.

Sun protection is a global concern, and maximizing sunscreen stability and efficacy depends partially on the prevention of UV filters recrystallization. We aimed to study the efficacy of hydrophobic solubilizers in preventing the recrystallization of solid hydrophobic UV filters in predissolutions, sunscreen formulations, and during simulated human use. Recrystallization of UV filters induced by ultrasonication, temperature variation, or simulated human application was analyzed by different methods. Maximum solubility of UV filters in solubilizers was determined. Surprisingly, the best solubilizer was not necessarily the best solvent to prevent recrystallization, suggesting there are different forces controlling these phenomena. Hydrophobic solubilizers tend to perform better than ethanol in predissolutions, but the presence of other components in final products may change their performance. Results suggest that some UV filters tend to form liquid clusters, which may behave as crystals and affect the desired even distribution of UV filters on the skin. UV filters were also found to respond differently to Hansen Solubility Parameters. Scanning electron microscopy supports the fact that recrystallization upon sunscreen application is an issue to be tested during development. A timesaving method to predict recrystallization of UV filters in clear systems was developed and is presented as a tool to enhance the efficacy of sunscreens.

Towards Sustainable High-Performance Thermoplastics: Synthesis, Characterization, and Enzymatic Hydrolysis of Bisguaiacol-Based Polyesters.

The utilization of wood-derived building blocks (xylochemicals) to replace fossil-based precursors is an attractive research subject of modern polymer science. Here, we demonstrate that bisguaiacol (BG), a lignin-derived bisphenol analogue, can be used to prepare biobased polyesters with remarkable thermal properties. BG was treated with different activated diacids to investigate the effect of co-monomer structures on the physical properties of the products. Namely, derivatives of adipic acid, succinic acid, and 2,5-furandicarboxylic acid were used. Moreover, a terephthalic acid derivative was used for comparison purposes. The products were characterized by 1 H NMR spectroscopy, attenuated total reflectance FTIR spectroscopy, gel-permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry to assess their structural and thermal properties in detail. The polymers showed glass-transition temperatures ranging up to 160 °C and thermal stabilities in excess of 300 °C. Furthermore, the susceptibility of the polyester to enzymatic hydrolysis was investigated to assess the potential for further surface functionalization and/or recycling and biodegradation. Indeed, hydrolysis with two different enzymes from the bacteria Thermobifida cellulosilytica led to the release of monomers, as quantified by HPLC. The results of this study indicate that our new polyesters represent promising renewable and biodegradable alternatives to petroleum-based polyesters currently employed in the plastics industry, specifically for applications in which high-temperature stability is essential to ensure overall system integrity.

Incorporating allylated lignin-derivatives in thiol-ene gel-polymer electrolytes.

Growing environmental and economic concerns as well as the uncertainty that accompanies finite petrochemical resources contributes to the increase in research and development of bio-based, renewable polymers. Concurrently, industrial and consumer demand for smaller, safer, and more flexible technologies motivates a global research effort to improve electrolytic polymer separators in lithium-ion batteries. To incorporate the aromatic structural advantages of lignin, a highly abundant and renewable resource, into gel-polymer electrolytes, lignin-derived molecules, vanillyl alcohol and gastrodigenin are functionalized and UV-polymerized with multi-functional thiol monomers. The resulting thin, flexible, polymer films possess glass transition temperatures ranging from -42.1°C to 0.3°C and storage moduli at 25°C ranging from 1.90MPa to 10.08MPa. The crosslinked polymer films swollen with electrolyte solution impart conductivities in the range of 7.04×10-7 to 102.73×10-7Scm-1. Thiol molecular weight has the most impact on the thermo-mechanical properties of the resulting films while polymer crosslink density has the largest effect on conductivity. The conducting abilities of the bio-based gel-polymer electrolytes in this study prove the viability of lignin-derived feedstock for use in lithium-ion battery applications and reveal structurally and thermally desirable traits for future work.

Experimental Data Extraction and in Silico Prediction of the Estrogenic Activity of Renewable Replacements for Bisphenol A.

Bisphenol A (BPA) is a ubiquitous compound used in polymer manufacturing for a wide array of applications; however, increasing evidence has shown that BPA causes significant endocrine disruption and this has raised public concerns over safety and exposure limits. The use of renewable materials as polymer feedstocks provides an opportunity to develop replacement compounds for BPA that are sustainable and exhibit unique properties due to their diverse structures. As new bio-based materials are developed and tested, it is important to consider the impacts of both monomers and polymers on human health. Molecular docking simulations using the Estrogenic Activity Database in conjunction with the decision forest were performed as part of a two-tier in silico model to predict the activity of 29 bio-based platform chemicals in the estrogen receptor-α (ERα). Fifteen of the candidates were predicted as ER binders and fifteen as non-binders. Gaining insight into the estrogenic activity of the bio-based BPA replacements aids in the sustainable development of new polymeric materials.

Synthesis and characterization of ionic polymer networks in a room-temperature ionic liquid.

Ionic liquid gels (ILGs) for potential use in ion transport and separation applications were generated via a free radical copolymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and N,N'-methylene(bis)acrylamide (MBA) using 1-ethyl-3-methylimidazolium ethylsulfate (IL) as a room temperature ionic liquid solvent medium. The AMPS and MBA monomer solubility window in the IL in the temperature range of 25 to 65 °C was determined. In situ ATR-FTIR showed near complete conversion of monomers to a cross-linked polymer network. ILGs with glass transition temperatures (T(g)s) near -50 °C were generated with T(g) decreasing with increasing IL content. The elastic moduli in compression (200 to 6600 kPa) decreased with increasing IL content and increasing AMPS content while the conductivities (0.35 to 2.14 mS cm⁻¹) increased with increasing IL content and decreasing MBA content. The polymer-IL interaction parameter (χ) (0.48 to 0.55) was determined via a modified version of the Bray and Merrill equation.

Lignin model compounds as bio-based reactive diluents for liquid molding resins.

Lignin is a copious paper and pulping waste product that has the potential to yield valuable, low molecular weight, single aromatic chemicals when strategically depolymerized. The single aromatic lignin model compounds, vanillin, guaiacol, and eugenol, were methacrylated by esterification with methacrylic anhydride and a catalytic amount of 4-dimethylaminopyridine. Methacrylated guaiacol (MG) and methacrylated eugenol (ME) exhibited low viscosities at room temperature (MG: 17 cP and ME: 28 cP). When used as reactive diluents in vinyl ester resins, they produced resin viscosities higher than that of vinyl ester-styrene blends. The relative volatilities of MG (1.05 wt% loss in 18 h) and ME (0.96 wt% loss in 18 h) measured by means of thermogravimetric analysis (TGA) were considerably lower than that of styrene (93.7 wt% loss in 3 h) indicating the potential of these chemicals to be environmentally friendly reactive diluents. Bulk polymerization of MG and ME generated homopolymers with glass transition temperatures (T(g)s) of 92 and 103 °C, respectively. Blends of a standard vinyl ester resin with MG and ME (50 wt % reactive diluent) produced thermosets with T(g)s of 127 and 153 °C, respectively, which are comparable to vinyl ester-styrene resins, thus demonstrating the ability of MG and ME to completely replace styrene as reactive diluents in liquid molding resins without sacrificing cured-resin thermal performance.