Session D26: Physics Concepts in Polymer Engineering

Polymer crystallization at high melt-supercooling

Melt-processing of polymers typically involves rapid cooling and solidification of the melt at high supercooling. At such condition, crystallization may follow a homogeneous-nucleation path, and, as a consequence, semicrystalline morphologies and properties often are qualitatively different compared to crystallization at low melt-supercooling. This paper provides a comprehensive review of semicrystalline structures of both commodity and engineering polymers forming at different supercooling, and of our experimental approaches to gain such information. Specifically, fast scanning chip calorimetry is introduced as an invaluable tool to obtain information about the kinetics of nucleation and crystallization as well as, when coupled to microscopy or scattering techniques, about the morphology of semicrystalline polymers formed at any thermal condition. With the possibility of suppression of crystallization and vitrifying the entire melt, also glass-relaxation and -crystallization is discussed, important for understanding physical aging and long-term change of properties.   

Semicrystalline polyethylene as a network in the gelation regime

We propose an equivalence between the mass average tie molecule probability in semicrystalline polymers and the gel fraction from the theory of networks in the gelation regime. Not only does this equivalence provide a formal framework for analysis of semicrystalline polymer structure (e.g., chain and network topology) and properties (e.g., deformation resistance)—and thus set the foundation for a quantitative structure-performance relationship for semicrystalline polyethylene—but also it establishes how to calculate tie molecule probabilities for semicrystalline polymer systems with molar mass dispersity. We use the framework to predict that a polyethylene blend consisting of specific low molar mass and high molar mass components will exhibit deformation response equivalent to a neat polymer with an intermediate molar mass, and we measure stress strain response over a range of temperatures to confirm our prediction.   

Porous Melt Blown PBT Fibers with High Ductility and High Temperature Structure Stability

Melt blowing is an efficient and highly scalable method of producing nonwoven microfibers commonly used in high performance filtration and coalescence media. Development of new melt blowing resins in the form of immiscible polymer blends can produce nonwoven mats with tailorable structures and diverse properties, enabling application to fields like water remediation and tissue scaffolding. Specifically, cocontinuous blends can contain a sacrificial phase which templates structure during melt blowing and can then be extracted to produce porous melt blown fibers. We applied this templating method to produce porous poly(butylene terephthalate) (PBT) fibers which display remarkable ductility and retain fine structural features at temperatures over 150 °C. Single fiber tensile testing, detailed electron microscopy, and microstructural characterization demonstrate the role of the characteristic rapid quenching and extension of the melt blowing process in defining fiber morphology and properties. The highly oriented pore structures and thermal stability are ideal for traditional melt blown fiber applications, while the decreasing radial pore size distribution and demonstrated utility of the ester functionalization suggest applicability in other high value applications.   

Processing-dependent microstructure and gas permeability of polyethylene blends for improved oxygen barrier

    This work demonstrates a greater than expected enhancement of oxygen barrier properties in linear low-density polyethylene (LLDPE)-based materials by blending LLDPE with high-density polyethylene (HDPE). The films made by melt pressing the LLDPE/HDPE blends had a greater reduction in oxygen permeability coefficients (P_O2) than predicted using common permeability reduction models, i.e., the harmonic average model and zero-permeability nanofiller model. The reduction of P_O2 was attributed to the presence of spherulite crystal structures as revealed by atomic force microscopy combined with infrared spectroscopy (AFM-IR). The LLDPE matrix exhibited significant spherulite formation even at a relatively low addition of HDPEs, which likely formed tortuous pathways for diffusing oxygen molecules. Transport results from melt pressed films contrast with the results from films with similar compositions prepared by film blowing, which did not show barrier enhancement beyond expectation. AFM-IR revealed that the blown films lacked spherulite crystals likely due to stretching in the machine direction followed by rapid cooling. These findings demonstrate the role of processing in controlling microstructures and thus the oxygen barrier performance. This work offers the possibility of achieving easily recyclable LLDPE-based packaging materials by simple blending of polyethylenes with different crystalline content.   

Establishing structure-processing-property relationships in 3D printed thermoplastic : pi-conjugated blends

Polymer processing displays at its core structure-processing-property relationships which are key for understanding the materials’ behavior and properties. As useful and promising additive manufacturing is shown to be, there is still a gap in our understanding of how the process will impact the printed material at the molecular level. To contribute tackling this challenge, our research group is currently developing strategies to investigate the structure-processing-property relationships at play upon the additive manufacturing of polymeric materials, more specifically, pi-conjugated polymers, which are well known for their optoelectronic properties. Herein, as a proof-of-concept, blends prepared form polycaprolactone (PCL) and poly(3-hexylthiophene-2,5-diyl) (P3HT) are evaluated for their printing capabilities in the context of hot-melt extrusion additive manufacturing. Our work consisting in the establishment of systematic correlations between the printability, print fidelity, and optoelectronic properties of these polymer blends via rheology, metrology and spectroscopy will be presented. Our conclusions highlight the challenges and opportunities involved in the additive manufacturing of functional polymer materials to enable the preparation of freeform materials with built-in functionalities.   

The Role of Shear and Extensional Flows in 3DP-Induced Alignment of Block Copolymer Nanostructures

Biological systems frequently achieve remarkable functional material properties through self-assembly of complex, hierarchically ordered structures. We have previously demonstrated controlled alignment of block copolymer (SEBS) nanostructures by leveraging the flows intrinsic to melt-extrusion 3D printing. These processing flows align the stiff cylindrical polystyrene domains within the rubbery matrix to achieve nearly two orders of magnitude of mechanical anisotropy parallel versus perpendicular to the print path. Here, we investigated the relationship between the local shear and extensional flow history and the resulting material nanostructure and extent of alignment. To accomplish this, we employed high spatial resolution imaging techniques to capture the spatial heterogeneity in structure induced by the non-uniform flow profile of 3D printing. Our outcomes help inform the rational design of 3DP nozzles and flow conditions to improve control over material anisotropy in 3D printed nanostructured polymers.   

Residual orientation mapping in material extrusion processes

Material extrusion is a common additive manufacturing process that subjects molten polymers to non-steady deformation and thermal processing to build customizable parts. Often, the complex processing conditions result in localized residual orientation, which is associated with variations in semicrystalline microstructure<span style="font-size:10.8333px"> and decreased weld strength between layers<span style="font-size:10.8333px"> in the printed part. There is a critical need to identify and characterize the spatial variation in polymer chain orientation due to these melt flow processes. Here, we use polarization imaging to characterize the spatial variation in residual stress and residual chain orientation in a glassy polylactide. By imaging the sample in a refractive index-matched fluid using a four-quadrant polarization camera we can measure the spatial variation in retardance in printed parts as a function of the Weissenberg number (Wi) in the printer nozzle. When combined with micro-computed tomography, we identify birefringence variations across the printed part that can be decomposed into residual orientation effects from the extrusion-deposition process and residual stress from spatial variations in temperature during cooling. The resulting analysis shows that residual orientation exhibits an approximate power-law dependence on Wi, which can be related back to the melt flow and relaxation that occurs in the extrusion-deposition process. We show that the residual orientation can be scaled as a function of processing and material parameters to generate a master plot of residual orientation. Our results show that a controlled amount of residual orientation can be templated into materials by controlling nozzle temperature and print speed.   

Evaluation of Fiber Orientation Measurement Techniques and Anisotropic Tensile Analysis of Additively Manufactured Carbon Fiber Poly-Lactic Acid

Additively manufactured parts are prone to anisotropic mechanical performance due to the time-based deposition of material, where decreased layer adhesion from the manufacturing process acts as a limiting condition for the part’s mechanical strength and applications. To increase the mechanical performance, chopped fibers can be added to the polymer matrix to increase feedstock strength while limiting process changes required. In additive parts containing short fibers, the direction of deposition is mechanically strongest due to the fibers orienting in line with material flow as a function of process parameters. The present work evaluated the relationships between fiber orientation and tensile strength for varying print parameters. Fiber orientation was measured using X-Ray Microscopy and image analysis, which was validated via a Monte Carlo simulation. A correlation between extrusion rate and fiber orientation was observed where the highest fiber orientation corresponded to the lowest extrusion rate with R² values of up to 0.90. Samples from the prints were tensile tested to determine correlation between fiber orientation and isotropy of tensile behavior. For all extrusion rates tested, as the extrusion multiplier increased, the ultimate tensile strength anisotropy decreased, however, the tensile strength of an estimated isotropic additive part was 236.78 ±  43.37 psi, approximately 25 % below that of the injection-molded samples. The use of image analysis for determination of fiber orientation shows micro scale analysis of additive parts is accessible in a research setting. The isotropy analysis of tensile strength in additive parts highlights the need to adjust print and process settings to specific boundary conditions and loading directions.   

Understanding Nonlinear Fluid Dynamics for All-aqueous Printing of a Viscoelastic Droplet in Yield-Stress Fluids

Analogous of pixels to two-dimensional (2D) pictures, voxels – in the form of either small cubes or spherical particles – are the basic building blocks of three-dimensional (3D) object. Voxelated bioprinting may provide a standardized approach for tissue engineering, transforming basic and translational biomedicine. Recently, we proved the concept of a voxelated bioprinting technology that relies on all-aqueous printing of viscoelastic droplets in yield-stress fluids. Despite the practical success, the fundamental science of such a printing process is largely unknown. Here, we study the process of printing an aqueous viscoelastic droplet in an aqueous yield-stress fluid without the help of large interfacial tension. We develop a printing platform that allows for real-time quantification of the printing process. Further, we design hybrid biomaterials to independently control viscosity and shear-thinning properties and identify the parameter space for printing droplets of good fidelity in three-dimensional (3D) space. We discover that the fidelity of droplet printing, defined as droplet roundness and roughness, is determined by the viscosity and shear-thinning properties of bio-inks. Finally, we demonstrate printing 3D structures made of interconnected yet distinguishable droplets made of multi-materials. Our results provide the knowledge and tools for 3D printing of highly viscoelastic droplets, paving the way for engineering highly functional tissues.   

Modeling Electrosprayed Particle Assembly on Geometrically Controlled Sessile Droplet Surfaces through Brownian Dynamics Simulations

Interfacial assembly of colloidal particles holds significant potential for advancing the additive manufacture of thin film materials. Nevertheless, many fundamental questions remain regarding the dynamics of particles and the evolution of monolayer structures confined at the interface. In this study, we utilize electrospray atomization as a nonintrusive method to deliver colloidal particles to geometrically controlled sessile droplets, enabling the creation of colloidal monolayers at the droplet surface. The distinctive feature of electrospray is its ability to impart a substantial electric charge to both the sprayed particles and the receiving substrate. Consequently, the dynamics and assembly of particles can be controlled by electrostatic interactions among the particles themselves and between particles and the substrate. To simulate the impact of these electrostatic interactions on the assembly structure, we combine a mesh-constrained Brownian dynamics algorithm with ANSYS electric field simulations to model particle movement on nonparameterizable surfaces. Consistent with previous experimental observations, our simulations predict the presence of a particle-depletion region near the droplet contact line, arising from long-range electrostatic repulsion between particles and the substrate. Our investigation systematically quantifies the influence of particle and substrate surface charge on assembly structures and the development of the depletion region. Additionally, we also probed the formation of rounded assembly boundaries observed in experiments. These results provide valuable insights into the interplay among various electrostatic interactions during the interfacial assembly of electrosprayed particles.   

Modelling the interplay between printing conditions, rheology, and crystallization in MatEx additive manufacturing

Material extrusion (MatEx) of thermoplastics continues to be one of the most common and economical additive manufacturing techniques. The printing process is predominantly shear-flow through a nozzle followed by rapid cooling and solidification of cylindrical filaments, which are built up layer-by-layer. Uniform strength and structural integrity rely on achieving sufficient inter-diffusion and isotropy of the polymers before the onset of either the glass transition or crystallization. Here I discuss a modelling framework to capture the interplay between MatEx conditions, rheology, and crystallization for several different polymers. The aim is to understand the microstructural properties, such as molecular alignment and spherulitic crystallinity, within a single layer and how this may affect the mechanical properties of a layered structure.   

Effects of Cooperative Association on Polymer Conformational Transition in Solutions

Numerous endeavors have been dedicated to unraveling the physical intricacies underlying polymer conformational transitions, which acts as an important role in polymer engineering. One well-known example of it is the coil-globule transition accompanied with the polymer dehydration, that is lower critical solution temperature (LCST) transition. The abrupt conformation change at the critical temperature as well as the hysteresis observed upon heating and cooling signifies a first-order transition nature. One characteristic of the first-order transition is the coexistence of the collapsed and swollen state at the transition point. However, our coarse-grained model investigation demonstrates that two-state behavior cannot be observed without the explicit inclusion of association correlation among neighbored segments, indicating the insufficiency of models focusing solely on hydrophobic and hydrophilic interactions in studying LCST transitions. Consistently, previous theoretical, computational, and experimental findings all suggest the pivotal role of cooperative association during the transition. To delve deeper, we integrate the cooperative association directly into coarse-grained model to run Monte Carlo simulation and employ several techniques to improve sampling efficiency. Consequently, we not only observe the coexistence of two-states within chain conformation distributions but also confirm it through free energy calculations.