Temperature Dependence of Kinetic Friction: A Handle for Plastics Sortation?

Sortation is a crucial step in mechanical recycling of post-consumer plastics (PCR) whereby properties such as density or spectral signature are used to separate plastics. However it is difficult to sort polyolefin flakes at high throughput by these properties. We ask whether the frictional properties of plastics as a function of temperature may be used as an alternate sorting property. However, fundamental studies of friction at temperatures near their melting points are limited. Here we measure the temperature dependence of kinetic friction for three common polyolefins (high and low den- sity polyethylene and polypropylene) as well as polyethylene terephthalate (PET), focusing on the softening/melting regime. The results are augmented by differential scanning calorimetry and temperature dependence measurements of both dynamic modulus and and probe tack. For the polyolefins, we find strong increases in the coefficients of kinetic friction during temperature ramps in the melting/softening regime. For the PET, we report a notable peak in the kinetic friction which we associate with the glass transition and cold-crystallization. We discuss the enhanced friction in the context of rub- ber friction, which exhibits comparable coefficients of kinetic frictions.

Crystallization Kinetics in an Immiscible Polyolefin Blend.

Motivated by the problem of brittle mechanical behavior in recycled blends of high density polyethylene (HDPE) and isotactic polypropylene (iPP), we employ optical microscopy, rheo-Raman, and differential scanning calorimetry (DSC) to measure the composition dependence of their crystallization kinetics. Raman spectra are analyzed via multivariate curve resolution with alternating least-squares (MCR-ALS) to provide component crystallization values. We find that iPP crystallization behavior varies strongly with blend composition. Optical microscopy shows that three crystallization kinetic regimes correspond to three underlying two-phase morphologies: HDPE droplets in iPP, the inverse, and cocontinuous structures. In the HDPE droplet regime, iPP crystallization temperature decreases sharply with increasing HDPE composition. For cocontinuous morphologies, iPP crystallization is delayed, but the onset temperature changes little with the exact blend composition. In the iPP droplet regime, the two components crystallize nearly concurrently. Rheological measurements are consistent with these observations. DSC indicates that the enthalpy of crystallization of the blends is less than the weighted values of the individual components, providing a possible clue for the decreased iPP crystallization temperatures.

Percolation Implications in the Rheology of Polymer Crystallization.

The rheology of polymer crystallization is an old problem that often defies explanation due to the complex interrelationships between crystallization and flow properties. Although separate measurements of rheology and crystallinity can give some information on their relationship, it is only through simultaneous measurements that ideas on the rheology of polymer crystallization can be tested and developed. This Perspective details recent experimental developments in simultaneous crystallinity and rheology measurements as well as continuum modeling efforts for the case of quiescent and isothermal crystallization. Experimental results reveal that the rheology is dominated initially by growth of individual spherulites that evolve into spherulitic superstructures that eventually span the measurement geometry. A generalized effective medium model based on this concept of percolation can explain both the growth of the viscoelastic modulus during crystallization and the changes in the relaxation spectrum of the crystallizing polymer, including a critical gel response at percolation. The success of the combined measurement techniques and percolation concepts motivate research to extend the semicrystalline polymer materials space where these methods are applied as well as further develop novel techniques to gain additional insight into the evolution of structure and relaxation dynamics during crystallization.

Comparing polarized Raman spectroscopy and birefringence as probes of molecular scale alignment in 3D printed thermoplastics.

Polymer chain orientation is crucial to understanding the polymer dynamics at interfaces formed during thermoplastic material extrusion additive manufacturing. The flow field and rapid cooling produced during material extrusion can result in chains which are oriented and stretched, which has implications for interdiffusion and crystallization. Polarized Raman spectroscopy offers a non-destructive and surface sensitive method to quantify chain orientation. To study orientation and alignment of chains in 3D printed polycarbonate filaments, we used a combination of polarized Raman spectroscopy and birefringence (Δn) measurements. By changing the orientation of the sample with respect to polarization of incident radiation, we probe changes in the ratio between orientation-dependent vibration modes and orientation-independent modes. We used principal component analysis (PCA) and partial least squares (PLS) regression to develop correlations for birefringence and Raman measurements in samples that were pulled at different draw ratios (DRs). PCA was used to differentiate between orientation-dependent and orientation-independent modes, while PLS regression was used to calculate birefringence from Raman measurements of 3D printed samples. Birefringence measurements were compared to the polycarbonate intrinsic birefringence of 0.2, to estimate the degree of orientation. We find that measured values of birefringence underestimate orientation compared to Raman measurements.

Estimations of the effective Young's modulus of specimens prepared by fused filament fabrication.

The elastic response of homogeneous isotropic materials is most commonly represented by their Young's modulus (E), but geometric variability associated with additive manufacturing results in materials that are neither homogeneous nor isotropic. Here we investigated methods to estimate the effective elastic modulus (Eeff) of samples fabricated by fused filament fabrication. We conducted finite element analysis (FEA) on printed samples based on material properties and CT-scanned geometries. The analysis revealed how the layer structure of a specimen altered the internal stress distribution and the resulting Eeff. We also investigated different empirical methods to estimate Eeff as guides. We envision the findings from our study can provide guidelines for modulus estimation of as-printed specimens, with the potential of applying to other extrusion-based additive manufacturing technologies.

Upper bound of feed rates in thermoplastic material extrusion additive manufacturing☆.

In this work, we develop a simple model to determine the upper bound of feed rates that do not cause jamming in material extrusion additive manufacturing, also known as fused deposition modeling (FDM)™ or fused-filament fabrication (FFF). We first derive a relation between the tube temperature and Péclet number for the solid portion of polymer filaments. We focus on the boundary between the solid and molten polymer in the heated portion of the tube. We find the Péclet number that corresponds to the point at which this boundary makes contact with the nozzle, and identify this as the upper bound of the feed rate. We compare our predictions to experimental results. We find good agreement for tube temperatures sufficiently above the glass-transition temperature, which is the temperature region of typical additive manufacturing.

AMB2018-03: Benchmark Physical Property Measurements for Material Extrusion Additive Manufacturing of Polycarbonate.

Material extrusion (MatEx) is finding increasing applications in additive manufacturing of thermoplastics due to the ease of use and the ability to process disparate polymers. Since part strength is anisotropic and frequently deviates negatively with respect to parts produced by injection molding, an urgent challenge is to predict final properties of parts made through this method. A nascent effort is underway to develop theoretical and computational models of MatEx part properties, but these efforts require comprehensive experimental data for guidance and validation. As part of the AM-Bench framework, we provide here a thorough set of measurements on a model system: polycarbonate printed in a simple rectangular shape. For the precursor material (as-received filament), we perform rheology, gel permeation chromatography, and dynamical mechanical analysis, to ascertain critical material parameters such as molar mass distribution, glass transition, and shear thinning. Following processing, we conduct X-ray computed tomography, scanning electron microscopy, depth sensing indentation, and atomic force microscopy modulus mapping. These measurements provide information related to pores, method of failure, and local modulus variations. Finally, we conduct tensile testing to assess strength and degree of anisotropy of mechanical properties. We find several effects that lead to degradation of tensile properties including the presence of pore networks, poor interfacial bonding, variations in interfacial mechanical behavior between rasters, and variable interaction of the neighboring builds within the melt state. The results provide insight into the processing-structure-property relationships and should serve as benchmarks for the development of mechanical models.

Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers.

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities.

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

AMB2018-04: Benchmark Physical Property Measurements for Powder Bed Fusion Additive Manufacturing of Polyamide 12.

Laser sintering (LS) of polyamide 12 (PA12) is increasingly being adopted for industrial production of end-use parts, yet the complexity of this process coupled with the lack of organized, rigorous, publicly available process-structure-physical property datasets exposes manufacturers and customers to risks of unacceptably poor part quality and high costs. Although an extensive scientific literature has been developed to address some of these concerns, results are distributed among numerous reports based on different machines, materials, process parameters, and users. In this study, a single commercially important LS PA12 feedstock has been processed along four build dimensions of a modern production LS machine, characterized by a wide range of physical techniques, and compared to the same material formed by conventional melt processing. Results are discussed in the context of the literature, offering novel insights including distributions of particle size and shape, localization of semicrystalline phase changes due to LS processing, effect of chemical aging on melt viscosity, porosity orientation relative to LS build axes, and microstructural effects on tensile properties and failure mechanisms. The resulting datasets will be made publicly available to modelers and practitioners for the purpose of improving certifiability and repeatability of end-use parts manufactured by LS.

Evaluating models for polycaprolactone crystallization via simultaneous rheology and Raman spectroscopy.

The crystallization of a polymer melt is characterized by dramatic structural and mechanical changes that significantly impact the processing conditions used to generate industrially-relevant products. Relationships between crystallinity and rheology are necessary to simulate and monitor the effect of processing conditions on the properties of the final product. However, separate measurements of crystallinity and rheology are difficult to correlate due to differences in sample history, geometry, and temperature. Recently, we have developed a rheo-Raman microscope for simultaneous rheology, Raman spectroscopy, and polarized reflection-mode optical measurements of soft materials, which allows for quantitative crystallinity measurements through features in the Raman spectrum. In this work, we apply this technique to monitor the isothermal crystallization of polycaprolactone to probe the relationship between structure, crystallinity, and rheology. Both crystallinity and the shear modulus vary over comparable timescales, but the birefringence increases much earlier in the crystallization process. We directly plot rheological parameters as a function of crystallinity to probe a range of suspension-based and empirical models relating the complex modulus to crystallinity, and we find that the previously developed models cannot describe the crystallinity-modulus relationship over the crystallization process. By developing a suspension-based model we can fit the complex modulus over the crystallization range. The crystallization process is characterized by a critical percolation fraction and a single scaling exponent.

Effect of cellulose nanocrystals on crystallization kinetics of polycaprolactone as probed by Rheo-Raman.

The development of biocompatible polymer nano-composites that enhance mechanical properties while maintaining thermoplastic processability is a longstanding goal in sustainable materials. When the matrix is semi-crystalline, the nanoparticles may induce significant changes to crystallization kinetics and morphology due to their ability to act as nucleating agents. To fully model this behavior in a process line, an understanding of the relationship between crystallinity and modulus is required. Here, we introduce a scalable model system consisting of surface-compatibilized cellulose nanocrystals (CNC) dispersed into poly(ε-caprolactone) (PCL) and study the effects of nanoparticle concentration on isothermal crystallization kinetics. The dispersion is accomplished by exchange of the Na+ of sulfated cellulose nanocrystals by tetra-butyl ammonium cations (Bu4N+) followed by melt mixing via twin-screw extrusion. Crystallization kinetics are measured through the recently developed rheo-Raman instrument which extracts the relationship between the growth of the transient mechanical modulus and that of crystallinity. With extrusion and increasing CNC content, we find the expected enhancement of crystallization rate, but we moreover find a significant change in the relative kinetics of increase in modulus versus crystallinity. We analyze this via generalized effective medium theory which allows computation of a critical percolation threshold ξ c and discuss the results in terms of a change in nucleation density and a change in the anisotropy of crystallization.

Effect of processing conditions on crystallization kinetics during materials extrusion additive manufacturing.

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.

Determining conformational order and crystallinity in polycaprolactone via Raman spectroscopy.

Raman spectroscopy is a popular method for non-invasive analysis of biomaterials containing polycaprolactone in applications such as tissue engineering and drug delivery. However there remain fundamental challenges in interpretation of such spectra in the context of existing dielectric spectroscopy and differential scanning calorimetry results in both the melt and semi-crystalline states. In this work, we develop a thermodynamically informed analysis method which utilizes basis spectra - ideal spectra of the polymer chain conformers comprising the measured Raman spectrum. In polycaprolactone we identify three basis spectra in the carbonyl region; measurement of their temperature dependence shows that one is linearly proportional to crystallinity, a second correlates with dipole-dipole interactions that are observed in dielectric spectroscopy and a third which correlates with amorphous chain behavior. For other spectral regions, e.g. C-COO stretch, a comparison of the basis spectra to those from density functional theory calculations in the all-trans configuration allows us to indicate whether sharp spectral peaks can be attributed to single chain modes in the all-trans state or to crystalline order. Our analysis method is general and should provide important insights to other polymeric materials.

Weld formation during material extrusion additive manufacturing.

Material extrusion (ME) is a layer-by-layer additive manufacturing process that is now used in personal and commercial production where prototyping and customization are required. However, parts produced from ME frequently exhibit poor mechanical performance relative to those from traditional means; moreover, fundamental knowledge of the factors leading to development of inter-layer strength in this highly non-isothermal process is limited. In this work, we seek to understand the development of inter-layer weld strength from the perspective of polymer interdiffusion under conditions of rapidly changing mobility. Our framework centers around three interrelated components: in situ thermal measurements (via infrared imaging), temperature dependent molecular processes (via rheology), and mechanical testing (via mode III fracture). We develop the concept of an equivalent isothermal weld time and test its relationship to fracture energy. For the printing conditions studied the equivalent isothermal weld time for Tref = 230 °C ranged from 0.1 ms to 100 ms. The results of these analysis provide a basis for optimizing inter-layer strength, the limitations of the ME process, and guide development of new materials.

The rheo-Raman microscope: Simultaneous chemical, conformational, mechanical, and microstructural measures of soft materials.

The design and performance of an instrument capable of simultaneous Raman spectroscopy, rheology, and optical microscopy are described. The instrument couples a Raman spectrometer and optical microscope to a rotational rheometer through an optically transparent base, and the resulting simultaneous measurements are particularly advantageous in situations where flow properties vary due to either chemical or conformational changes in molecular structure, such as in crystallization, melting, gelation, or curing processes. Instrument performance is demonstrated on two material systems that show thermal transitions. First, we perform steady state rotational tests, Raman spectroscopy, and polarized reflection microscopy during a melting transition in a cosmetic emulsion. Second, we perform small amplitude oscillatory shear measurements along with Raman spectroscopy and polarized reflection microscopy during crystallization of a high density polyethylene. The instrument can be applied to study structure-property relationships in a variety of soft materials including thermoset resins, liquid crystalline materials, colloidal suspensions undergoing sol-gel processes, and biomacromolecules. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

Raman analysis of bond conformations in the rotator state and premelting of normal alkanes.

We perform Raman spectroscopic measurements on normal alkanes (CnH2n+2) to quantify the n dependence of the conformational disorder that occurs below the melt temperature. We employ a three-state spectral analysis method originally developed for semi-crystalline polyethylene that posits crystalline, amorphous, and non-crystalline consecutive trans (NCCT) conformations to extract their respective mass fractions. For the alkanes studied that melt via a rotator phase (21 ≤n≤ 37), we find that conformational disorder can be quantified by the loss of NCCT mass fraction, which systematically decreases with increasing chain length. For those that melt directly via the crystal phase (n≥ 40), we observe NCCT conformational mass fractions that are independent of chain length but whose disordered mass fraction increases with length. These complement prior IR measurements which measure disorder via gauche conformations, but have not been able to measure the mass fraction of this disorder as a function of n. An interesting feature of the three-state analysis when applied to alkanes is that the measured fraction of disordered chain conformations in the rotator phase of (10 to 30)% greatly exceeds the mass fraction of gauche bonds (1 to 7)% as measured from IR; we reconcile this difference through DFT calculations.

Lightweight, Flexible, High-Performance Carbon Nanotube Cables Made by Scalable Flow Coating.

Coaxial cables for data transmission are ubiquitous in telecommunications, aerospace, automotive, and robotics industries. Yet, the metals used to make commercial cables are unsuitably heavy and stiff. These undesirable traits are particularly problematic in aerospace applications, where weight is at a premium and flexibility is necessary to conform with the distributed layout of electronic components in satellites and aircraft. The cable outer conductor (OC) is usually the heaviest component of modern data cables; therefore, exchanging the conventional metallic OC for lower weight materials with comparable transmission characteristics is highly desirable. Carbon nanotubes (CNTs) have recently been proposed to replace the metal components in coaxial cables; however, signal attenuation was too high in prototypes produced so far. Here, we fabricate the OC of coaxial data cables by directly coating a solution of CNTs in chlorosulfonic acid (CSA) onto the cable inner dielectric. This coating has an electrical conductivity that is approximately 2 orders of magnitude greater than the best CNT OC reported in the literature to date. This high conductivity makes CNT coaxial cables an attractive alternative to commercial cables with a metal (tin-coated copper) OC, providing comparable cable attenuation and mechanical durability with a 97% lower component mass.

Phase-Specific Raman Analysis of n-Alkane Melting by Moving-Window Two-Dimensional Correlation Spectroscopy.

We use moving-window two-dimensional correlation spectroscopy (MW-2DCOS) for phase-specific Raman analysis of the n-alkane (C21H44) during melting from the crystalline solid phase to the intermediate rotator phase and to the amorphous molten phase. In MW-2DCOS, individual peak-to-peak correlation analysis within a small subset of spectra provides both temperature-resolved and spectrally disentangled Raman assignments conducive to understanding phase-specific molecular interactions and chain configurations. We demonstrate that autocorrelation MW-2DCOS can determine the phase transition temperatures with a higher resolving power than commonly-used analysis methods including individual peak intensity analysis or principal component analysis. Besides the enhanced temperature resolving power, we demonstrate that asynchronous 2DCOS near the orthorhombic-to-rotator transition temperature can spectrally resolve the two overlapping peaks embedded in the Raman CH2 twisting band in the orthorhombic phase, which had been only predicted but not observed due to thermal broadening near the melting temperature.

Infrared thermography of welding zones produced by polymer extrusion additive manufacturing.

In common thermoplastic additive manufacturing (AM) processes, a solid polymer filament is melted, extruded though a rastering nozzle, welded onto neighboring layers and solidified. The temperature of the polymer at each of these stages is the key parameter governing these non-equilibrium processes, but due to its strong spatial and temporal variations, it is difficult to measure accurately. Here we utilize infrared (IR) imaging - in conjunction with necessary reflection corrections and calibration procedures - to measure these temperature profiles of a model polymer during 3D printing. From the temperature profiles of the printed layer (road) and sublayers, the temporal profile of the crucially important weld temperatures can be obtained. Under typical printing conditions, the weld temperature decreases at a rate of approximately 100 °C/s and remains above the glass transition temperature for approximately 1 s. These measurement methods are a first step in the development of strategies to control and model the printing processes and in the ability to develop models that correlate critical part strength with material and processing parameters.