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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.

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 ( $$\Delta n$$ ) 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.

In high-resolution stereolithography, printed parts deviate significantly from the projected photomask because of the reaction complexity on the voxel scale. To better understand the reaction proce...

© 2020 The Society of Rheology. Polymer crystallization occurs in many plastic manufacturing processes, from injection molding to film blowing. Linear low-density polyethylene (LLDPE) is one of the most commonly processed polymers, wherein the type and extent of short-chain branching (SCB) may be varied to influence crystallization. In this work, we report simultaneous measurements of the rheology and Raman spectra, using a Rheo-Raman microscope, for two industrial-grade LLDPEs undergoing crystallization. These polymers are characterized by broad polydispersity, SCB, and the presence of polymer chain entanglements. The rheological behavior of these entangled LLDPE melts is modeled as a function of crystallinity using a slip-link model. The partially crystallized melt is represented by a blend of linear chains with either free or cross-linked ends, wherein the cross-links represent attachment to growing crystallites, and a modulus shift factor that increases with the degree of crystallinity. In contrast to our previous application of the slip-link model to isotactic polypropylene, in which the introduction of only bridging segments with cross-links at both ends was sufficient to describe the available data, for these LLDPEs, we find it necessary to introduce dangling segments, with cross-links at only one end. The model captures quantitatively the evolution of viscosity and elasticity with crystallization over the whole range of frequencies in the linear regime for the two LLDPE grades.

The ability to introduce isotopic labeling in a controlled fashion into polymers has long been an area of keen interest, as it allows for analysis by methods such as small-angle neutron scattering. In this work, polyhomologation (also known as “C1 polymerization”) of a deuterated ylide was performed, yielding linear polyethylene of high deuterium content. Copolymerization with the protonated analog allowed for the preparation of statistical copolymers, in which linear polyethylene with a uniform and tunable amount of deuterium is obtained. The amount of deuterium present, as well as its uniform distribution, is supported by vibrational spectroscopy, size-exclusion chromatography with infrared detection, and 2H nuclear magnetic resonance spectroscopy. This technique was extended to prepare a precision triblock copolymer in which the chain-ends are selectively deuterated. All polymers reported are of narrow dispersity, and will find use in myriad measurements sensitive to deuterium labeling, most notably sm...

Raman spectroscopy is a powerful non-destructive technique for the identification and characterization of plastics, but a major shortcoming of this technique is that environmental weathering, dyes, and additives in the material can generate a strong fluorescence background that overwhelms the Raman scattering. Here, we demonstrate that time-gated Raman spectroscopy can be used to successfully reduce the fluorescence signal and measure Raman spectra of recovered plastics. Time-gating removes a significant amount of background signal from the Raman spectra such that the polymers and color additives can be identified using similar measurement times compared to continuous-wave Raman spectroscopy. Examples of this are shown for a small subset of samples recovered from Hawaiian marine environments and a nonweathered commercial plastic. Time-gated Raman spectroscopy can also be used to characterize samples that are black in color due to carbon-based additives like graphite, which can be challenging to characterize via other common vibrational spectroscopic techniques.

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.

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.

Multiple melting peaks of high-density polyethylene produced by complex thermal pretreatment are investigated with variable temperature Raman spectroscopy and differential scanning calorimetry (DSC). We examine the origin of the observed multiple melting peaks with three possible model mechanisms: melting–recrystallization, premelting via mesophases, and melting of multithickness lamellae. We quantitatively analyze Raman spectral change associated with each melting peak with respect to temperature. The Raman spectral change is represented by a least squares moving-window (LSMW) method, which is effective in suppression of high-frequency derivative noise. The conformational similarity among multiple melting peaks strongly suggests that dominant melting occurs from the crystalline phase directly to the amorphous phase at all transitions. The observed multiple melting transitions are consistent with one-step melting of multithickness lamellae as the dominant origin. However, two-dimensional Raman correlation...

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.

The rheology of crystallizing polymers is critical to polymer processing, but our current understanding of how crystallinity affects rheology during crystallization is lacking. The challenge is twofold: first, we must measure rheology simultaneously with crystallinity, and then we must develop models that can describe those measurements as well as prior phenomenological observations. Here, we further develop a generalized effective medium model to describe the frequency-dependent shear modulus of a crystallizing polymer. Through a simple model system, we show that the percolation transition in the effective medium model recovers the relaxation dynamics of a critical gel with a relaxation spectrum that can be approximated as the power mean of the initial melt and final semicrystalline material. We demonstrate the success of this model on the isothermal crystallization of polycaprolactones. From the generalized effective medium model, we can calculate the percolation fraction and power-law relaxation exponent at the critical point, even when the measurement frequency range is dominated by shear thinning of the melt phase.

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.

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 because of thermal broadening near the melting temperature. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Semicrystalline polymer melts are commonly used in material extrusion (MatEx) for 3D printing. Although flows have a profound effect on polymer crystallization, the relationship between typical MatEx deformation rates and printed-part crystal morphology is yet to be understood. Here, MatEx is used to print a wall of polylactic acid filaments. The linear rheology and quiescent crystallization kinetics are characterized, infrared imaging is used to measure temperature variations during the MatEx process, and optical microscopy is employed to determine the resulting crystal morphology before and after a postprinting thermal annealing process. Our flow-enhanced crystallization model demonstrates that MatEx-induced polymer stretch leads to a higher nucleation density and greater space filling in the weld regions between deposited filaments. Consequently, after annealing, the weld regions feature smaller spherulites than the filament center, as shown by optical microscopy. Finally, flow-induced crystallization is proposed as a method to improve weld toughness.

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.

The dilatational properties of insoluble monolayers are important for understanding the mechanics of biological systems and consumer products, but isolating the dilatational response of an interface is challenging due to the difficulties in separating dilatation from shear and other deformation modes. Oscillations of a microscale bubble radius are useful for generating purely dilatational flows, but the current deposition methods for insoluble layers onto fluid interfaces are not easily scaled down. In this paper, we describe a miscible solvent exchange procedure for generating insoluble layers at an air-water interface pinned at the tip of a capillary tens of micrometers in diameter. We show that the amount of surfactant adsorbed at the interface can be controlled by the initial concentration dissolved in isopropanol (the starting solvent) and the volumetric flow rate of solvent exchange. Surface pressure-area isotherms and dilatational moduli are measured concurrently for three insoluble surfactants: palmitic acid (PA), dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). The isotherms measured on the microscale interface compare well with previous experiments performed on a Langmuir trough. However, the magnitudes of the dilatational moduli differ from those measured on either Langmuir trough or pendant drop apparatuses. Several possible reasons for the observed differences are discussed. A comparison of the dilatational modulus with the Gibbs elasticity is used to determine the presence of dilatational extra stresses at the interface. The isotherm and dilatational modulus of the insoluble component of the industrial surfactant Tween 80 are measured using this approach. The methods developed here also open the possibility for future study of the important role of finite size effects on microstructure formation and the resulting interfacial mechanics.

The kinetic pathway by which molten polymers transform into multi-length-scale, semicrystalline structures upon cooling is critical to their processing, properties, and ultimate performance. However, for the case of polyethylene (PE), critical questions remain concerning early stage kinetics of isothermal crystallization. Here we utilize Raman spectroscopy, in conjunction with turbidity and depolarization measurements, to probe crystallization kinetics in PE because of its unique ability to simultaneously measure intrachain consecutive trans conformations (locally straight) and interchain orthorhombic crystallinity. We analyze the spectra within the context of a three-state model to extract the mass fraction of conformations that are locally straight but not part of the orthorhombic crystal, which we term noncrystalline consecutive trans (NCCT). We first validate this methodology on the n-alkane C21H44 where the NCCT mass fraction is expected to be large and then apply it to the early stage crystallizatio...

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(e-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.

The dilatational rheology of complex fluid-fluid interfaces is linked to the stability and bulk rheology of emulsions and foams. Dilatational rheology can be measured by pinning a bubble or droplet at the tip of a capillary, subjecting the interface shape to small amplitude oscillations, and recording the resulting pressure jump across the interface. The complex dilatational modulus is obtained by differentiating the interfacial stress with respect to the area change of the interface. In this paper, we perform a regular asymptotic expansion to analyze the interface response in pressure-controlled capillary pressure tensiometers to determine the dilatational modulus as a function of the measured radius of curvature. We show that small amplitude oscillatory dilation of a spherical bubble is neither stress nor strain rate controlled. The resulting dilatational modulus contains contributions from both surface tension effects as well as extra stresses. Depending on the specifics of the interface, each contribu...

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.

The Rheo-Raman Microscope combines rheology, Raman spectroscopy and polarization light microscopy and provides comprehensive insight into a material’s bulk as well as micro structural properties under well defined and reproducible conditions such as temperature or shear profiles. The simultaneous acquisition with three independent analytical methods is advantageous for investigation of structural changes occurring for example in gelation, melting or crystallization. Details of this hyphenated instrumentation as well as selected results including temperature induced melting of a polymer emulsion and crystallization of a polymer melt are presented in this contribution.

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.

The adsorption of particles to fluid–fluid interfaces is a key step in the generation of colloidosomes and particle-stabilized emulsions. Microfluidic channels are a promising tool for generating particle-stabilized drops and bubbles with independent control over the bubble size and the concentration of particles adsorbed at the fluid interface. In this paper, we present experimental observations of the adsorption of a nanoparticle-surfactant suspension to confined bubbles translating along a microchannel. Long bubbles exhibit a unique two-lobed shape that is linked to the adsorption of surface-active particles to the interface at a timescale comparable to the residence time in the channel. An accompanying decrease in the bubble velocity results from the added viscous drag at the bubble interface. We develop a transport model to describe the rate of particle adsorption to the interface and find good agreement between the model estimates of bubble shape changes and experimental observation. The formation of the two-lobed shape is due to a difference in the velocity of the front and rear of the bubble, which can promote bubble break-up.