The stimuli-responsive behavior of densely grafted polymer brushes makes them attractive for desi... more The stimuli-responsive behavior of densely grafted polymer brushes makes them attractive for designing tunable coatings and thin film sensors, provided physical properties and thermodynamics of the brush are well characterized. This work investigates the swelling of highly extended poly(methyl methacrylate) polymer brushes in solvent vapors. All brushes have identical grafting density, but differing degrees of polymerization. Brushes were equilibrated in various saturated vapors, then characterized by X-ray reflectivity to measure thickness and scattering length density. One dimensional swelling normal to the surface is observed for all brushesolvent pairs; brushes swollen in good solvents generally exhibit decreasing polymer density along the thickness direction of the brush, while poor solvents swell the brush with minimal changes in the bulk brush density profiles relative to the dry state. Comparison of changes in brush height and film density upon swelling to existing mean-field models indicate that vapor solvation thermodynamics of polymer brush thin films is not explained by existing theory.
Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, s... more Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, short-chain branched, and long-chain branched chains, can be very challenging to optimize as species with similar hydrodynamic sizes or solubility often coelute in various chromatographic methods. To better understand the effects of polymer structure on the dilute solution properties of polyolefins, a family of model linear low-density polyethylenes (LLDPEs) were synthesized by ring-opening metathesis polymerization (ROMP) of sterically hindered, alkyl-substituted cyclooctenes, followed by hydrogenation. Within this series, the alkyl branch frequency was fixed while systematically varying the short-chain branch length. These model materials were analyzed by ambient-and high-temperature size exclusion chromatography (HT-SEC) to determine their molar mass, intrinsic viscosity ([η]), and degree of short-chain branching across their respective molar mass distributions. Short-chain branching is fixed across the molar mass distribution, based on the synthetic strategy used, and measured values agree with theoretical values for longer alkyl branches, as evident by HT-SEC. Deviation from theoretical values is observed for ethyl branched LLDPEs when calibrated using either α-olefin copolymers (poly(ethylene-stat-1-octene)) or blends of polyethylene and polypropylene standards. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the entire molar mass distribution. This work demonstrates the applicability of these model materials to deconvolute structure-property relationships using chromatographic separation techniques and is a step toward determining if sequence control can minimize compositional heterogeneity and generate improved standards for determining branching content in commercial polyolefins.
The National Institute of Standards and Technology (NIST) provides science, industry, and governm... more The National Institute of Standards and Technology (NIST) provides science, industry, and government with a central source of well-characterized materials certified for chemical composition or for some chemical or physical property. These materials are designated Standard Reference Materials® (SRMs) and are used to calibrate measuring instruments, to evaluate methods and systems, or to produce scientific data that can be referred readily to a common base. In this paper, we discuss the history of polymer based SRMs, their current status, and challenges and opportunities to develop new standards to address industrial measurement challenges.
Pelagic Pacific sea turtles eat relatively large quantities of plastic (median 5 g in gut). Using... more Pelagic Pacific sea turtles eat relatively large quantities of plastic (median 5 g in gut). Using Fourier transform infrared spectroscopy, we identified the polymers ingested by 37 olive ridley, 9 green, and 4 loggerhead turtles caught as bycatch in Hawaii-and American Samoa-based longline fisheries. Unidentifiable samples were analyzed using high-temperature size exclusion chromatography with multiple detectors and/or X-ray photoelectron spectroscopy. Regardless of species differences in dive depths and foraging strategies, ingested plastics were primarily low-density, floating polymers (51% low-density polyethylene (LDPE), 26% polypropylene (PP), 10% unknown polyethylene (PE), and 5% high-density PE collectively). Albeit not statistically significant, deeper diving and deeper captured olive ridley turtles ate proportionally more plastics expected to sink (3.9%) than intermediate-diving green (1.2%) and shallow-diving loggerhead (0.3%) turtles. Spatial, but no sex, size, year, or hook depth differences were observed in polymer composition. LDPE and PP, some of the most produced and least recycled polymers worldwide, account for the largest percentage of plastic eaten by sea turtles in this region. These novel data inform managers about the threat of plastic ingestion to sea turtles and may motivate development of more environmentally friendly practices for plastic production, use, and waste management.
Abstract We perform molecular dynamics (MD) simulations of a coarse-grained model of linear low-d... more Abstract We perform molecular dynamics (MD) simulations of a coarse-grained model of linear low-density polyethylenes (LLDPEs) having short-chain branching in an implicit solvent and calculate basic configurational properties (radius of gyration R g , hydrodynamic radius R h , and intrinsic viscosity [ η ] ) using the path-integration program ZENO. Solvent quality effects in our model are accounted for through the introduction of an attractive polymer-polymer interaction potential with solvent quality “strength” λ that is tuned from a “good” solvent limit, where the inter-polymer interactions are purely repulsive, to a “very poor” solvent regime, where the polymers collapse due to their self-attractive intermolecular interactions. We also identify a compensation condition or “θ point” in our model between these limits, where the polymers are “ideal” in the sense in which attractive interactions effectively nullify inter-polymer repulsions so that the chains behave akin to Gaussian polymer chains in their average conformational properties. In order to compare with experiments performed under variable solvent conditions, we define a dimensionless measure of excluded volume interaction parameter δ in terms of the mass scaling exponent ν for R g or ν h for R h , which can be determined from both experiment and simulation. We illustrate our variable solvent approach to estimate polymer solution properties in the case of polyethylenes (PEs) and evaluate the δ value that corresponds to the experimental solvent conditions. Our combined use of MD and ZENO allows for the numerical estimation of polymer solution properties for polymers having general monomer structures and solvent qualities in a computationally tractable fashion and should be useful as a general computational tool for polymer structural characterization.
We used an ultraviolet-ozone (UVO) cleaner to create substrates for atom-transfer radical polymer... more We used an ultraviolet-ozone (UVO) cleaner to create substrates for atom-transfer radical polymerization (ATRP) with varying surface initiator coverage. We collected complementary timeof-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurements to investigate the precise chemical origen of the variation in grafting density. At short exposure times, the atomic composition underwent minor changes except for the relative amount of bromine. At longer UVO exposure times, there is clear evidence of exposure-dependent surface initiator oxidation. We interpret these data as evidence of a bromine ablation process within the UVO cleaner, with additional oxidative modification of the rest of the surface. We then used these substrates to create a series of poly(methyl methacrylate) (PMMA) brushes varying in grafting density, demonstrating the utility of this tool for the control of polymer brush density. The measured brush grafting densities were correlated with the bromine concentration measured by both ToF-SIMS and XPS. XPS and brush thicknesses correlate strongly, following an exponential decay with a half-life of (18 ± 1) s.
A two-dimensional model of a solid-supported enzyme catalyst bead is fabricated on a quartz cryst... more A two-dimensional model of a solid-supported enzyme catalyst bead is fabricated on a quartz crystal microbalance with dissipation monitoring (QCM-D) sensor to measure in situ stability and mechanical properties of Candida Antarctica Lipase B (CAL B) layers under varied conditions relating to ring-opening polymerization. The model was fabricated using a dual photochemical approach, where commercially available poly(methyl methacrylate) (PMMA) thin films were crosslinked by a photoactive benzophenone monolayer and blended crosslinking agent. This process produces two dimensional, homogenous, rigid PMMA layers which mimic commercial 3D acrylic resin in a QCM-D experiment. Adsorption of CAL B to PMMA in QCM-D under varied buffer ionic strengths produces viscoelastic enzyme layers that become rigid as ionic strength increases. The rigid CAL B/PMMA interface demonstrates up to 20 % desorption of the catalyst with increasing trace water content. Increased polycaprolactone binding at the enzyme surface was also observed, indicating higher affinity of the polymer for a hydrated enzyme surface. The enzyme layer destabilized with increasing temperature, yielding near complete reversible catalyst desorption in the model.
The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as enta... more The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as entanglement density as well as extrinsic parameters such as strain rate and test temperature. Here we use laser-induced projectile impact testing (LIPIT) to study the extreme strain rate (≈10 7 s -1 ) puncture behavior of free-standing polycarbonate (PC) thin films. We demonstrate that changes to the PC molecular mass and the degree of plasticization can lead to substantial changes in the specific puncture energy. We relate these changes to the alteration of the entanglement density of the polymer that determines the underlying failure mechanism as well as the size of the deformation zone.
Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyeste... more Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyesters over conventional heavy metal catalysts. Heterogeneous catalysis, where the enzyme is immobilized onto solid-supports, allows for easy catalyst removal and can increase the commercial feasibility of biocatalysis with a thorough understanding of the reaction kinetics and required process conditons. In this minireview, we describe our comprehensive metrology approach to fully identify key parameters to control enzymatic ringopening polymerization (ROP) of lactones: development of predictive models of reaction kinetics, experimental models of the catalyst surface micro-environment, and on-line spectroscopic analysis of reaction conversion for polyester homopolymers and copolymers. Quantitative evaluation of enzymeatic ROP illucidates advantages and limitations of current enzyme-catalyzed polymerizations and aids in the design of
Material extrusion additive manufacturing processes force molten polymer through a printer nozzle... more Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (> 100 s -1 ) wall shear rates prior to cooling and crystallization. These high shear rates can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude and importance of this effect is unknown for additive manufacturing. A significant barrier to understanding this process is the lack of in situ measurement techniques to quantify crystallinity after polymer filament extrusion. To address this issue, we use a combination of infrared thermography and Raman spectroscopy to measure the temperature and percent crystallinity of extruded polycaprolactone during additive manufacturing. We quantify crystallinity as a function of time for the nozzle temperatures and filament feed rates accessible to the apparatus. Crystallization is shown to occur faster at higher shear rates and lower nozzle temperatures, which shows that processing conditions can have a dramatic effect on crystallization kinetics in additive manufacturing.
The National Institute of Standards and Technology (NIST) provides science, industry, and governm... more The National Institute of Standards and Technology (NIST) provides science, industry, and government with a central source of well-characterized materials certified for chemical composition or for some chemical or physical property. These materials are designated Standard Reference Materials ® (SRMs) and are used to calibrate measuring instruments, to evaluate methods and systems, or to produce scientific data that can be referred readily to a common base. In this paper, we discuss the history of polymer based SRMs, their current status, and challenges and opportunities to develop new standards to address industrial measurement challenges.
Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyeste... more Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyesters over conventional heavy metal catalysts. Heterogeneous catalysis, where the enzyme is immobilized onto solid-supports, allows for easy catalyst removal and can increase the commercial feasibility of biocatalysis with a thorough understanding of the reaction kinetics and required process conditons. In this minireview, we describe our comprehensive metrology approach to fully identify key parameters to control enzymatic ringopening polymerization (ROP) of lactones: development of predictive models of reaction kinetics, experimental models of the catalyst surface micro-environment, and on-line spectroscopic analysis of reaction conversion for polyester homopolymers and copolymers. Quantitative evaluation of enzymeatic ROP illucidates advantages and limitations of current enzyme-catalyzed polymerizations and aids in the design of
The National Institute of Standards and Technology's (NIST's) approach to modernizing its synthet... more The National Institute of Standards and Technology's (NIST's) approach to modernizing its synthetic polymer reference materials is to target precise and robust synthetic methods, making homogeneous standards than can be easily tailored to include desired chemistry, molar mass, and topography. This strategy aims to meet the increasing demand for novel standards that are relevant to advanced materials and with minimial development time. Here, an alternative approach to synthetic linear and branched polyethylenes has been developed using a commercial metathesis catalyst to generate polyolefins with low dispersity and sequence control, where the degree of alkyl branching and alkyl branching distribution are known. This process is adaptable to many branch lengths and varied chemistries as it only requires replacement of the monomer feedstock with the desired chemical functionality. The resulting materials are characterized using size-exclusion chromatography with tetra detection to measure the molar mass, molar mass distribution, and degree of short chain branching. In addition to their role as potential standard reference materials (SRMs), these polymers are also being applied to study the effect of branching on polymer chain thermodynamics at surfaces, and comparing experimental results to established theoretical models.
Functional Group Depth Profiling with Resonant Soft X-ray Reflectivity DANIEL SUNDAY, EDWIN P. CH... more Functional Group Depth Profiling with Resonant Soft X-ray Reflectivity DANIEL SUNDAY, EDWIN P. CHAN, SARA V. ORSKI, CHRIS M. STAFFORD, NIST -Natl Inst of Stds Tech -The distribution and concentration of functional groups in thin films can be difficult to characterize due to the small sample volume and limited methods with depth sensitivity beyond the film surface. Soft X-rays with energies in the vicinity of an atomic absorption edge are sensitive to specific chemical functionalities. In this contribution, we demonstrate how resonant soft x-ray reflectivity can be used to quantifying the distribution of specific functional groups in polyamide thin films, which have applications as water purification membranes. Using a layer-by-layer assembly system for the synthesis of the polyamide we can control the concentration of oxygen or nitrogen based functional groups within the thin film. Three different polyamide films were prepared with different precursor materials in order to control the functional group concentration in the film. The measurement was calibrated using a series of reference films composed of random copolymers with controlled copolymer compositions. The results of this measurement can be used to determine the cross-link density and residual hydroxyl group concentration, which can be related to the performance of the membrane.
The accumulation of plastic waste in the environment has prompted the development of new chemical... more The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al 2 O 3 and Re 2 O 7 /γ-Al 2 O 3 . This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C 20 alkane, n-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an n-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.
The stimuli-responsive behavior of densely grafted polymer brushes makes them attractive for desi... more The stimuli-responsive behavior of densely grafted polymer brushes makes them attractive for designing tunable coatings and thin film sensors, provided physical properties and thermodynamics of the brush are well characterized. This work investigates the swelling of highly extended poly(methyl methacrylate) polymer brushes in solvent vapors. All brushes have identical grafting density, but differing degrees of polymerization. Brushes were equilibrated in various saturated vapors, then characterized by X-ray reflectivity to measure thickness and scattering length density. One dimensional swelling normal to the surface is observed for all brushesolvent pairs; brushes swollen in good solvents generally exhibit decreasing polymer density along the thickness direction of the brush, while poor solvents swell the brush with minimal changes in the bulk brush density profiles relative to the dry state. Comparison of changes in brush height and film density upon swelling to existing mean-field models indicate that vapor solvation thermodynamics of polymer brush thin films is not explained by existing theory.
Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, s... more Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, short-chain branched, and long-chain branched chains, can be very challenging to optimize as species with similar hydrodynamic sizes or solubility often coelute in various chromatographic methods. To better understand the effects of polymer structure on the dilute solution properties of polyolefins, a family of model linear low-density polyethylenes (LLDPEs) were synthesized by ring-opening metathesis polymerization (ROMP) of sterically hindered, alkyl-substituted cyclooctenes, followed by hydrogenation. Within this series, the alkyl branch frequency was fixed while systematically varying the short-chain branch length. These model materials were analyzed by ambient-and high-temperature size exclusion chromatography (HT-SEC) to determine their molar mass, intrinsic viscosity ([η]), and degree of short-chain branching across their respective molar mass distributions. Short-chain branching is fixed across the molar mass distribution, based on the synthetic strategy used, and measured values agree with theoretical values for longer alkyl branches, as evident by HT-SEC. Deviation from theoretical values is observed for ethyl branched LLDPEs when calibrated using either α-olefin copolymers (poly(ethylene-stat-1-octene)) or blends of polyethylene and polypropylene standards. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the entire molar mass distribution. This work demonstrates the applicability of these model materials to deconvolute structure-property relationships using chromatographic separation techniques and is a step toward determining if sequence control can minimize compositional heterogeneity and generate improved standards for determining branching content in commercial polyolefins.
The National Institute of Standards and Technology (NIST) provides science, industry, and governm... more The National Institute of Standards and Technology (NIST) provides science, industry, and government with a central source of well-characterized materials certified for chemical composition or for some chemical or physical property. These materials are designated Standard Reference Materials® (SRMs) and are used to calibrate measuring instruments, to evaluate methods and systems, or to produce scientific data that can be referred readily to a common base. In this paper, we discuss the history of polymer based SRMs, their current status, and challenges and opportunities to develop new standards to address industrial measurement challenges.
Pelagic Pacific sea turtles eat relatively large quantities of plastic (median 5 g in gut). Using... more Pelagic Pacific sea turtles eat relatively large quantities of plastic (median 5 g in gut). Using Fourier transform infrared spectroscopy, we identified the polymers ingested by 37 olive ridley, 9 green, and 4 loggerhead turtles caught as bycatch in Hawaii-and American Samoa-based longline fisheries. Unidentifiable samples were analyzed using high-temperature size exclusion chromatography with multiple detectors and/or X-ray photoelectron spectroscopy. Regardless of species differences in dive depths and foraging strategies, ingested plastics were primarily low-density, floating polymers (51% low-density polyethylene (LDPE), 26% polypropylene (PP), 10% unknown polyethylene (PE), and 5% high-density PE collectively). Albeit not statistically significant, deeper diving and deeper captured olive ridley turtles ate proportionally more plastics expected to sink (3.9%) than intermediate-diving green (1.2%) and shallow-diving loggerhead (0.3%) turtles. Spatial, but no sex, size, year, or hook depth differences were observed in polymer composition. LDPE and PP, some of the most produced and least recycled polymers worldwide, account for the largest percentage of plastic eaten by sea turtles in this region. These novel data inform managers about the threat of plastic ingestion to sea turtles and may motivate development of more environmentally friendly practices for plastic production, use, and waste management.
Abstract We perform molecular dynamics (MD) simulations of a coarse-grained model of linear low-d... more Abstract We perform molecular dynamics (MD) simulations of a coarse-grained model of linear low-density polyethylenes (LLDPEs) having short-chain branching in an implicit solvent and calculate basic configurational properties (radius of gyration R g , hydrodynamic radius R h , and intrinsic viscosity [ η ] ) using the path-integration program ZENO. Solvent quality effects in our model are accounted for through the introduction of an attractive polymer-polymer interaction potential with solvent quality “strength” λ that is tuned from a “good” solvent limit, where the inter-polymer interactions are purely repulsive, to a “very poor” solvent regime, where the polymers collapse due to their self-attractive intermolecular interactions. We also identify a compensation condition or “θ point” in our model between these limits, where the polymers are “ideal” in the sense in which attractive interactions effectively nullify inter-polymer repulsions so that the chains behave akin to Gaussian polymer chains in their average conformational properties. In order to compare with experiments performed under variable solvent conditions, we define a dimensionless measure of excluded volume interaction parameter δ in terms of the mass scaling exponent ν for R g or ν h for R h , which can be determined from both experiment and simulation. We illustrate our variable solvent approach to estimate polymer solution properties in the case of polyethylenes (PEs) and evaluate the δ value that corresponds to the experimental solvent conditions. Our combined use of MD and ZENO allows for the numerical estimation of polymer solution properties for polymers having general monomer structures and solvent qualities in a computationally tractable fashion and should be useful as a general computational tool for polymer structural characterization.
We used an ultraviolet-ozone (UVO) cleaner to create substrates for atom-transfer radical polymer... more We used an ultraviolet-ozone (UVO) cleaner to create substrates for atom-transfer radical polymerization (ATRP) with varying surface initiator coverage. We collected complementary timeof-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurements to investigate the precise chemical origen of the variation in grafting density. At short exposure times, the atomic composition underwent minor changes except for the relative amount of bromine. At longer UVO exposure times, there is clear evidence of exposure-dependent surface initiator oxidation. We interpret these data as evidence of a bromine ablation process within the UVO cleaner, with additional oxidative modification of the rest of the surface. We then used these substrates to create a series of poly(methyl methacrylate) (PMMA) brushes varying in grafting density, demonstrating the utility of this tool for the control of polymer brush density. The measured brush grafting densities were correlated with the bromine concentration measured by both ToF-SIMS and XPS. XPS and brush thicknesses correlate strongly, following an exponential decay with a half-life of (18 ± 1) s.
A two-dimensional model of a solid-supported enzyme catalyst bead is fabricated on a quartz cryst... more A two-dimensional model of a solid-supported enzyme catalyst bead is fabricated on a quartz crystal microbalance with dissipation monitoring (QCM-D) sensor to measure in situ stability and mechanical properties of Candida Antarctica Lipase B (CAL B) layers under varied conditions relating to ring-opening polymerization. The model was fabricated using a dual photochemical approach, where commercially available poly(methyl methacrylate) (PMMA) thin films were crosslinked by a photoactive benzophenone monolayer and blended crosslinking agent. This process produces two dimensional, homogenous, rigid PMMA layers which mimic commercial 3D acrylic resin in a QCM-D experiment. Adsorption of CAL B to PMMA in QCM-D under varied buffer ionic strengths produces viscoelastic enzyme layers that become rigid as ionic strength increases. The rigid CAL B/PMMA interface demonstrates up to 20 % desorption of the catalyst with increasing trace water content. Increased polycaprolactone binding at the enzyme surface was also observed, indicating higher affinity of the polymer for a hydrated enzyme surface. The enzyme layer destabilized with increasing temperature, yielding near complete reversible catalyst desorption in the model.
The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as enta... more The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as entanglement density as well as extrinsic parameters such as strain rate and test temperature. Here we use laser-induced projectile impact testing (LIPIT) to study the extreme strain rate (≈10 7 s -1 ) puncture behavior of free-standing polycarbonate (PC) thin films. We demonstrate that changes to the PC molecular mass and the degree of plasticization can lead to substantial changes in the specific puncture energy. We relate these changes to the alteration of the entanglement density of the polymer that determines the underlying failure mechanism as well as the size of the deformation zone.
Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyeste... more Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyesters over conventional heavy metal catalysts. Heterogeneous catalysis, where the enzyme is immobilized onto solid-supports, allows for easy catalyst removal and can increase the commercial feasibility of biocatalysis with a thorough understanding of the reaction kinetics and required process conditons. In this minireview, we describe our comprehensive metrology approach to fully identify key parameters to control enzymatic ringopening polymerization (ROP) of lactones: development of predictive models of reaction kinetics, experimental models of the catalyst surface micro-environment, and on-line spectroscopic analysis of reaction conversion for polyester homopolymers and copolymers. Quantitative evaluation of enzymeatic ROP illucidates advantages and limitations of current enzyme-catalyzed polymerizations and aids in the design of
Material extrusion additive manufacturing processes force molten polymer through a printer nozzle... more Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (> 100 s -1 ) wall shear rates prior to cooling and crystallization. These high shear rates can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude and importance of this effect is unknown for additive manufacturing. A significant barrier to understanding this process is the lack of in situ measurement techniques to quantify crystallinity after polymer filament extrusion. To address this issue, we use a combination of infrared thermography and Raman spectroscopy to measure the temperature and percent crystallinity of extruded polycaprolactone during additive manufacturing. We quantify crystallinity as a function of time for the nozzle temperatures and filament feed rates accessible to the apparatus. Crystallization is shown to occur faster at higher shear rates and lower nozzle temperatures, which shows that processing conditions can have a dramatic effect on crystallization kinetics in additive manufacturing.
The National Institute of Standards and Technology (NIST) provides science, industry, and governm... more The National Institute of Standards and Technology (NIST) provides science, industry, and government with a central source of well-characterized materials certified for chemical composition or for some chemical or physical property. These materials are designated Standard Reference Materials ® (SRMs) and are used to calibrate measuring instruments, to evaluate methods and systems, or to produce scientific data that can be referred readily to a common base. In this paper, we discuss the history of polymer based SRMs, their current status, and challenges and opportunities to develop new standards to address industrial measurement challenges.
Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyeste... more Enzyme-catalyzed polymerization provides a green alternative to synthesize biodegradable polyesters over conventional heavy metal catalysts. Heterogeneous catalysis, where the enzyme is immobilized onto solid-supports, allows for easy catalyst removal and can increase the commercial feasibility of biocatalysis with a thorough understanding of the reaction kinetics and required process conditons. In this minireview, we describe our comprehensive metrology approach to fully identify key parameters to control enzymatic ringopening polymerization (ROP) of lactones: development of predictive models of reaction kinetics, experimental models of the catalyst surface micro-environment, and on-line spectroscopic analysis of reaction conversion for polyester homopolymers and copolymers. Quantitative evaluation of enzymeatic ROP illucidates advantages and limitations of current enzyme-catalyzed polymerizations and aids in the design of
The National Institute of Standards and Technology's (NIST's) approach to modernizing its synthet... more The National Institute of Standards and Technology's (NIST's) approach to modernizing its synthetic polymer reference materials is to target precise and robust synthetic methods, making homogeneous standards than can be easily tailored to include desired chemistry, molar mass, and topography. This strategy aims to meet the increasing demand for novel standards that are relevant to advanced materials and with minimial development time. Here, an alternative approach to synthetic linear and branched polyethylenes has been developed using a commercial metathesis catalyst to generate polyolefins with low dispersity and sequence control, where the degree of alkyl branching and alkyl branching distribution are known. This process is adaptable to many branch lengths and varied chemistries as it only requires replacement of the monomer feedstock with the desired chemical functionality. The resulting materials are characterized using size-exclusion chromatography with tetra detection to measure the molar mass, molar mass distribution, and degree of short chain branching. In addition to their role as potential standard reference materials (SRMs), these polymers are also being applied to study the effect of branching on polymer chain thermodynamics at surfaces, and comparing experimental results to established theoretical models.
Functional Group Depth Profiling with Resonant Soft X-ray Reflectivity DANIEL SUNDAY, EDWIN P. CH... more Functional Group Depth Profiling with Resonant Soft X-ray Reflectivity DANIEL SUNDAY, EDWIN P. CHAN, SARA V. ORSKI, CHRIS M. STAFFORD, NIST -Natl Inst of Stds Tech -The distribution and concentration of functional groups in thin films can be difficult to characterize due to the small sample volume and limited methods with depth sensitivity beyond the film surface. Soft X-rays with energies in the vicinity of an atomic absorption edge are sensitive to specific chemical functionalities. In this contribution, we demonstrate how resonant soft x-ray reflectivity can be used to quantifying the distribution of specific functional groups in polyamide thin films, which have applications as water purification membranes. Using a layer-by-layer assembly system for the synthesis of the polyamide we can control the concentration of oxygen or nitrogen based functional groups within the thin film. Three different polyamide films were prepared with different precursor materials in order to control the functional group concentration in the film. The measurement was calibrated using a series of reference films composed of random copolymers with controlled copolymer compositions. The results of this measurement can be used to determine the cross-link density and residual hydroxyl group concentration, which can be related to the performance of the membrane.
The accumulation of plastic waste in the environment has prompted the development of new chemical... more The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al 2 O 3 and Re 2 O 7 /γ-Al 2 O 3 . This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C 20 alkane, n-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an n-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.
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Papers by Sara Orski