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 C 21 H 44 where the NCCT mass fraction is expected to be large and then apply it to the early stage crystallization kinetics of PE. We find that the NCCT conformations appear in conjunction with orthorhombic crystallinity, indicating that the growing early stage clusters are rich in NCCT conformations. ■ INTRODUCTIONThe structure and physical properties of semicrystalline polymers are determined during crystallization, in which the material transforms from an entangled melt into a semicrystalline, multi-length-scale structure. 1 While precise control of crystallization is key to the final properties, it is mainly empirical. Among the broad array of crystallizing polymers, PE holds a special place for economic, scientific, and life-cycle reasons. Despite its maturity, industrial producers continue to develop new strategies to improve properties and processing. Scientifically, PE is important because it is the simplest polymer that forms a crystalline structure, and it serves as a model system for testing our understanding of polymer crystallization in general. A large literature has thus developed for PE devoted to understanding the molten phase, the lamellar semicrystalline state, and the kinetics of the transformation of the former into the latter. Despite much research on PE, there remains vigorous debate on the crystallization pathway. In particular, the events that occur during the early stages of crystallization, known as the precrystalline or induction period, are little understood.Here we demonstrate that Raman spectroscopy is a powerful tool to probe the kinetics of isothermal crystallization of PE due to its ability to simultaneously measure the evolution of local conformational states and orthorhombic crystallinity. It can be used to measure the mass fraction of conformations that are locally straight but are not part of the orthorhombic crystalline structure, which we call the noncrystalline consecutive trans (NCCT) conformation. We are particularly interested in the early time behavior where early stage structures are predicted to be rich in trans conformational states that are noncrystalline.In this work, we ...

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.

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 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 contribution can be a function of the dilation rate and the radius of the bubble. Thus, the radius of curvature can be used as a control parameter with which to separate surface tension and interfacial rheology effects, aiding in validation of interfacial constitutive models. We examine the limits of validity of the small amplitude assumption and provide guidelines for determining the operating limits of a capillary pressure tensiometer. Finally, we compare several existing devices, including a microtensiometer we developed previously that oscillates the pressure inside small (R ∼ 10 μm) droplets.

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.

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.

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.

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