Polymer Microstructure Research Articles

Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS) Quantification of Tire and Road Wear Particles (TRWP) in Environmental Matrices: Assessing the Importance of Microstructure in Instrument Calibration Protocols

Thermogravimetric methods with internal polymer standards have successfully quantified environmental tire and road wear particle (TRWP) concentrations. However, TRWP quantification in environmental matrices via pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) using butadiene rubber (BR) and styrene-butadiene rubber (SBR) marker 4-vinylcyclohexene (4-VCH; 1,4-butadiene-1,4-butadiene dimer) may be uncertain because of variable polymer compositions and BR and SBR microstructures. To determine if tire polymer microstructure is contributing to potentially over- or underestimating TRWP in the environment in py-GC-MS analyses, SBR materials (n = 8) commonly found in tire tread with varying microstructure were quantified via py-GC-MS, using 4-VCH and the deuterated internal standard d-4-VCH to provide a response ratio for each polymer. The response ratios of the dimer response to the total polymer quantity (instrument response slope) varied up to 6.8-fold for SBR, with a reduction to a 3.6-fold range when the polymer quantity was expressed as 1,4-butadiene mass rather than total polymer mass. Variability was reduced further when considering the polymerization method for emulsion-SBRs (n = 3; 1.4-fold range), but not solution-SBRs (n = 5; 2.7-fold range), which reflects the random versus structured heterosequencing of the two rubber types, respectively. Our findings suggest that py-GC-MS response should be interpreted based on empirical analysis of an appropriate number of regionally representative tire tread materials, rather than individual rubbers, because of the lack of methods available for determining unknown average microstructure in environmental samples.

A Compartmentalized Neuronal Cell-Culture Platform Compatible With Cryo-Fixation by High-Pressure Freezing for Ultrastructural Imaging.

The human brain contains a wide array of billions of neurons and interconnections, which are often simplified for analysis in vitro using compartmentalized microfluidic devices for neuronal cell culturing, to better understand neuronal development and disease. However, such devices are traditionally incompatible for high-pressure freezing and high-resolution nanoscale imaging and analysis of their sub-cellular processes by methods including electron microscopy. Here we develop a novel compartmentalized neuronal co-culture platform allowing reconstruction of neuronal networks with high variable spatial control, which is uniquely compatible for high-pressure freezing. This cryo-fixation method is well-established to enable high-fidelity preservation of the reconstructed neuronal networks and their sub-cellular processes in a near-native vitreous state without requiring chemical fixatives. To direct the outgrowth of neurites originating from two distinct groups of neurons growing in the two different compartments, polymer microstructures akin to microchannels are fabricated atop of sapphire disks. Two populations of neurons expressing either enhanced green fluorescent protein (EGFP) or mCherry were grown in either compartment, facilitating the analysis of the specific interactions between the two separate groups of cells. Neuronally differentiated PC12 cells, murine hippocampal and striatal neurons were successfully used in this context. The design of this device permits direct observation of entire neuritic processes within microchannels by optical microscopy with high spatial and temporal resolution, prior to processing for high-pressure freezing and electron microscopy. Following freeze substitution, we demonstrate that it is possible to process the neuronal networks for ultrastructural imaging by electron microscopy. Several key features of the embedded neuronal networks, including mitochondria, synaptic vesicles, axonal terminals, microtubules, with well-preserved ultrastructures were observed at high resolution using focused ion beam – scanning electron microscopy (FIB-SEM) and serial sectioning – transmission electron microscopy (TEM). These results demonstrate the compatibility of the platform with optical microscopy, high-pressure freezing and electron microscopy. The platform can be extended to neuronal models of brain disease or development in future studies, enabling the investigation of subcellular processes at the nanoscale within two distinct groups of neurons in a functional neuronal pathway, as well as pharmacological testing and drug screening.

Predicting Polymer Properties via a Coupled Kinetic, Stochastic and Rheological Modeling Approach from Reaction Conditions

AbstractA three‐step multiscale modeling approach to predict and thus control polymer properties, such as flow behavior and linear as well as non‐linear rheology, based on polymerization conditions, is developed and applied for long‐chain branched low‐density polyethylene (LDPE). The approach consists of i) a deterministic kinetic model for the description of conversion and average polymer characteristics, ii) a hybrid stochastic Monte Carlo model for the description of the polymeric microstructure, and iii) a rheology model for the evaluation of polymer melt flow properties. The modeling approach is validated via high‐pressure miniplant LDPE samples with a special focus on long‐chain branching. In the next step, the modeling approach can be successfully transferred to a tubular reactor of industrial scale. Due to its universality the approach opens up possible applications for other polymer and also copolymer systems.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization

AbstractIn this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans–cis isomerization of the azobenzene moiety. The initially targeted azobenzene‐functionalized α‐diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine–amine system bearing interrupted conjugation with the metal center, and a sandwich‐type α‐diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light‐induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light‐mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization.

In this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans-cis isomerization of the azobenzene moiety. The initially targeted azobenzene-functionalized α-diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine-amine system bearing interrupted conjugation with the metal center, and a sandwich-type α-diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light-induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light-mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Survival of polymeric microstructures subjected to interrogatory touch.

Polymeric arrays of microrelief structures have a range of potential applications. For example, to influence wettability, to act as biologically inspired adhesives, to resist biofouling, and to play a role in the "feel" of an object during tactile interaction. Here, we investigate the damage to micropillar arrays comprising pillars of different modulus, spacing, diameter, and aspect ratio due to the sliding of a silicone cast of a human finger. The goal is to determine the effect of these parameters on the types of damage observed, including adhesive failure and ploughing of material from the finger onto the array. Our experiments point to four principal conclusions [1]. Aspect ratio is the dominant parameter in determining survivability through its effect on the bending stiffness of micropillars [2]. All else equal, micropillars with larger diameter are less susceptible to breakage and collapse [3]. The spacing of pillars in the array largely determines which type of adhesive failure occurs in non-surviving arrays [4]. Elastic modulus plays an important role in survivability. Clear evidence of elastic recovery was seen in the more flexible polymer and this recovery led to more instances of pristine survivability where the stiffer polymer tended to ablate PDMS. We developed a simple model to describe the observed bending of micropillars, based on the quasi-static mechanics of beam-columns, that indicated they experience forces ranging from 10-4-10-7 N to deflect into adhesive contact. Taken together, results obtained using our framework should inform design considerations for microstructures intended to be handled by human users.

Effective daytime radiative cooling via a template method based PDMS sponge emitter with synergistic thermo-optical activity

Radiative cooling, a passive cooling method without energy consumption, provides a sustainable alternative for existing cooling strategies. Microstructures in porous polymers have enabled further optimization of radiative cooling efficiency through selective reflection and emission. While numerous fabrication strategies have been reported, there is still a critical need for scalable method with simplicity, low cost, and designable microstructures. Herein, we report a low-cost eco-friendly template method towards scalable production of porosity-controlled sponge emitter with high radiative cooling efficiency. We demonstrate the feasibility by preparing porous polydimethylsiloxane sponge emitter (P-PDMS) with NaCl template. By modulating the template-to-PDMS mass ratio, facile porosity control is realized and the obtained sponge-like porous structure endows PDMS with higher sub-ambient cooling ability enabled by enhanced emissivity and solar reflectivity. Furthermore, the heat transfer between hot air and the object is efficiently inhibited by the micropores. Combined theoretical and experimental results demonstrate that the PDMS sponge has a remarkable reflectivity of 95% in solar spectrum and averaged emissivity of 96.5% within the 8–13 μm range, thus realizing excellent sub-ambient temperature drop of 8.0 °C. • A novel facile and eco-friendly template method for radiative cooling material fabrication was proposed. • The porosity of sponge emitter can be fine-controlled via adjusting the mass ratio between templates and polymer. • Sponge-like porous structure promotes both solar reflectance and infrared thermal emittance in daytime. • A sub-ambient cooling effect of 8.0 °C in daytime was achieved.

Nature-inspired slippery polymer thin film for ice-repellent applications

In this study, the authors investigated the anti-icing performance of a hierarchical slippery polymer thin film that is inspired by structures of the lotus leaf and Nepenthes pitcher plant. The polymer solution was mixed with the lubricant at the appropriate concentration to achieve slippery properties. The dry etching method followed by the spin-coating process was used to generate a uniform polymer microstructure on the thin film. The polymer nanostructure was then yielded by an additional plasma-etching method using carbon tetrafluoride (CF4) gas. The anti-icing efficiency was then compared with that of the non-functional samples to demonstrate the advantages of combination in all criteria. Moreover, a theoretical prediction based on the free-energy approach was used to measure the nucleation time at the interface and illustrated good agreement with real-time measurement. The results propose a new and facile approach for outdoor anti-icing applications.

Role of organometallic complexes in olefin polymerization: a review report

• A detailed discussion on various organometallic complexes used for olefin polymerization, has been done. • Regulation on composition and microstructure of the polymer products can be done by altering the metal center and the auxiliary ligand of an organometallic complex. • The organometallic catalysts of group 3 metals for the polymerization of olefins are still less discovered and very uncommon and group 4 metal-based catalysts holding diverse auxiliary ligands have been widely researched and investigated. • Increase in performance of homogeneous catalysts by introducing them on metalasiloxanes. The chemistry of catalytic polymerization of ethylene and propylene employing titanium complexes and aluminum-alkyls as co-catalysts is well known since last 50 years. Thorough studies show that invention of homogeneous organometallic complexes as catalysts such as metallocenes and other transition metal complexes with methylaluminoxane (MAO) as co-catalyst for the polymerization of olefins has achieved a lot of interest in this area as compared to the well-known and conventional heterogeneous catalysts i.e. Ziegler–Natta and Phillips catalyst. The possibilities of controlling the polymer composition, microstructure, tacticity, stereoregularity and special properties with high precision are found to be high in homogeneous catalysts. Availability of single active accessible site on metallocene catalysts in contrast to their classical equivalents is responsible for improved catalytic polymerization. Furthermore, these single-site catalysts, such as half titanocenes/MAO, zirconocenes, and other single-site catalysts, can control important parameters like co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, polymer branching etc . The purity in products found less when Ziegler–Natta catalysts were used. This review summarises and compares the recent advances in the polymerization of various olefins (especially ethylene or α-olefins) using various organometallic catalysts consist of rare-earth elements and lanthanides, group 4 metals, and other transition metals (V,Cr and Ni). At the end the review discusses modulation of catalytic activity to control polymerization reactions and concludes the study with future challenges in the area.

Bowls, vases and goblets\u2014the microcrockery of polymer and nanocomposite morphology revealed by two-photon optical tomography

On the >1 µm scale the morphology of semicrystalline plastics like polyethylene or Nylon features spherulites, “shish-kebabs”, cylinddrites and other crystalline aggregates which strongly affect mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects. Here we show how they can be visualized in 3D using fluorescent labels and confocal microscopy. As a result, we see spherulites in 3D, both in neat polymers and their nanocomposites, and observe how unevenly nanoparticles and other additives are distributed in the material. Images of i-polypropylene and biodegradable poly(lactic acid) reveal previously unsuspected morphologies such as “vases” and “goblets”, nonspherical “spherulites” and, unexpectedly, “shish-kebabs” grown from quiescent melt. Also surprisingly, in nanocomposite sheets spherulite nucleation is seen to be copied from one surface to another, mediated by crystallization-induced pressure drop and local melt-flow. These first results reveal unfamiliar modes of self-assembly in familiar plastics and open fresh perspectives on polymer microstructure.

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Chemical orthogonality, defined as 100% selectivity in chemical reactions, has proven to be a powerful concept making significant impacts in the fields of synthetic organic chemistry, chemical biology, and materials science. The review by James F. Reuther and coworkers on page 1748 details the recent advancements towards the use of chemically orthogonal reactions in synthetic polymer chemistry, allowing the synthesis of complex, structurally diverse polymer microstructures and topologies. Utilization of orthogonal “click” chemistry post-polymerization yielded complex materials that are not possible using traditional polymerization techniques. The advent of stimuli-triggered orthogonal polymerization methods that differ in mechanism has provided streamlined, one-pot routes to unique block and bottle-brush copolymer structures simplifying procedures for synthesis and purification. (DOI: 10.1002/pol.20210345)

Spectral Universality of Elastoinertial Turbulence

Dissolving small amounts of polymer into a Newtonian fluid can dramatically change the dynamics of transitional and turbulent flows. We investigate the spatiotemporal dynamics of a submerged jet of dilute polymer solution entering a quiescent bath of Newtonian fluid. High-speed digital Schlieren imaging is used to quantify the evolution of Lagrangian features in the jet revealing a rich sequence of transitional and turbulent states. At high levels of viscoelasticity, we identify a new distinct transitional pathway to elastoinertial turbulence (EIT) that does not feature the conventional turbulent bursts and instead proceeds via a shear-layer instability that produces elongated filaments of polymer due to the nonlinear effects of viscoelasticity. Even though the pathways to the EIT state can be different, and within EIT the spatial details of the turbulent structures vary systematically with polymer microstructure and concentration, there is a universality in the power-law spectral decay of EIT with frequency, f^{-3}, independent of fluid rheology and flow parameters.

Wetting characteristics of polymer adhesives with different chain bending stiffness

Polymer adhesives are widely used in daily applications and in industry owing to their flexibility and overall non-toxicity, particularly in interfacial adhesion. The spreading of polymer adhesives on adherend is one of the essential considerations for the interfacial adhesion of polymer adhesives, which is strongly related to their wetting behaviors. While relationships between polymer microstructure and adhesion have been investigated in previous studies, it remains challenging to unveil the effect of polymer microstructure on wettability. To address this issue, here we utilize coarse-grained molecular dynamics (CGMD) simulations to systematically elucidate how the wettability of a polymer adhesive droplet on a surface depends on bending stiffness. The wetting dynamics and the contact angle are studied to show the evolution of morphology of droplets during the wetting process. The results indicate the wettability is weakened by the increase of bending stiffness of polymer chain. Detailed thermodynamic property analysis is further conducted, revealing that the adhesion between the polymer droplet and substrate deteriorates due to the decline of wettability. Interestingly, we observe such deterioration becomes more significant by both increasing the temperature and decreasing the bending stiffness. Our study sheds light on the dependence of chain bending stiffness and temperature on the wetting behavior of polymer adhesive droplets, and offers insights, which, upon experimental validation can then be used for the design of adhesives or hydrogels.

Performance Evaluation of the Multifunctional Variable-Viscosity Slick Water for Fracturing in Unconventional Reservoirs.

The traditional guar gum fracturing fluid system has the drawbacks of the fracturing process of unconventional oil and gas deposits, such as high drag resistance and large residuum harm, which is gradually replaced by the system of the slick water fracturing fluid. The conventional slick water system, however, still has the features of low sand-carrying capability. Therefore, high-viscosity slick water is often used in fracturing operations, but most of the high-viscosity slick water is difficult to prepare, dissolve, and break gels, which needs to be improved. Based on the abovementioned problems, a new type of multifunctional variable-viscosity slick water is proposed in this paper. The self-made loop drag test unit, a dynamic crack sand-carrying model, a multifunctional core flow device, and other equipment were used for testing, and a set of systematic evaluation methods for the performance of multifunctional variable-viscosity slick water are established. In addition, the mechanism of improving sand-carrying capacity and increasing viscosity and solubilization was explained through the macroevaluation experiment of polymer properties and the analysis of the polymer microstructure. The experimental results show that compared with high-viscosity slick water, the multifunctional variable-viscosity slick water has good drag-reducing performance, the drag-reducing rate can reach more than 75%; the intersection value of viscoelastic modulus is about 0.01 Hz, the sand carrying capacity is higher; the gel-breaking time is faster, the residue content is lower, 38.5 ppm; it has the characteristics of low harm, the harm rate to the core is 18.30%; and it also has the performance of enhancing oil recovery.

Self‐Adhesive Silicone Microstructures for the Treatment of Tympanic Membrane Perforations

Inspired by the gecko foot, polymeric microstructures have demonstrated reliable dry adhesion to both stiff objects and sensitive surfaces such as skin. Microstructured silicone patches are proposed, herein, for the treatment of tympanic membrane perforations with the aim of serving as an alternative for current surgical procedures that require anesthesia and ear canal packing. Sylgard 184 PDMS micropillars of 20 μm in diameter and 60 μm in length are topped by a Soft Skin Adhesive (SSA) MG7‐1010 terminal layer, of about 25 μm thickness. The adhesion is evaluated by specially designed tack tests against explanted murine eardrums and, for comparison, against a rigid substrate. Functional effects are evaluated using auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAE). The adhesion strength of the microstructure and unstructured controls to explanted murine tympanic membranes is comparable (typically 12 kPa), but the microstructured patches are easier to handle by the surgeon. For the first time, partial recovery of hearing performance is measured immediately after patch application. The novel patches adhere without the need for further fixation, removing the need for ear canal packing. The proposed material design holds great promise for improving clinical treatments of tympanic membrane perforations.

Quantitative analysis of the representative volume element of polymer grouting materials based on geometric homogenization

In recent years, polymer grouting material has been applied to engineering combined with the development of grouting technology. Two-component foaming polyurethane is one type of water insensitive polymer grouting material, in which the cell structure plays a key role on its physical properties. To explore the impact of the microstructure of the polyurethane polymer on its macroscopic performance, it is necessary to study the representative volume element (RVE) quantitatively. In this study, the definition of the geometric RVE for this kind of polymer material is given. Taking the cell content, average cell diameter, and standard deviation as microscopic statistical characteristics of polymer meso-structure, the method to determine quantitatively the RVE of polymer is proposed. Scanning electron microscopy (SEM) images of two-component foaming polyurethane specimens with different densities were observed, and the mesoscopic characteristics were calculated. Random distributed cells were selected to establish the 2D Voronoi polygon models for low-density polymers and the 2D circular models for medium- and high-density polymers, respectively. Considering the random position of the overall numerical sample and the increasing sized sample, the size of the geometric RVE for polymers with different densities were calculated based on each microscopic statistical characteristic, and the influence of the density and microscopic statistical characteristics on the size of the polymer geometric RVE was analyzed. The results show that the RVE size of the polymers with different densities is approximately 15–21 times of the average cell diameter and the main factor of RVE size is cell content.

Three-Dimensional Photoluminescent Crypto-Images Doped with (CdSe)ZnS Quantum Dots by One-Photon and Two-Photon Polymerization

Three-dimensional (3D) photoluminescent crypto-images can be created by nanopatterning quantum dots (QDs) within a polymeric micro-object. A UV-sensitive photoresist, doped with (CdSe)ZnS core–shell QDs, is employed to fabricate fluorescent devices, by two-photon polymerization direct laser writing (TPP-DLW). In this work, we report on the expulsion of the majority of QDs from the irradiated area, caused by the cross-linking of the monomers, determining their accumulation on the photopolymer surface. At the same time, QDs get excited and trigger a surrounding thermal polymerization, leading to a variation of the spatial resolution of the fabricated structures. By opportunely combining these physical phenomena, it is possible to control the local density of QDs with nanometric resolution and, as a consequence, to tune the local photoemission of the fabricated polymeric structure. A photopolymer doped with five different concentrations of (CdSe)ZnS core–shell QDs is analyzed and tested. We define the best conditions to create nanostructures containing only few QDs to be integrated in optical devices, as well as to create 3D photoluminescent micro crypto-images. By TPP-DLW and one-photon lithography, a 3D fluorescent design can be concealed inside a polymeric microstructure that is swiftly fabricated using a photoresist. An example of a 3D crypto-image is shown to illustrate the technique.

Regiochemistry-Driven Organic Electrochemical Transistor Performance Enhancement in Ethylene Glycol-Functionalized Polythiophenes.

Novel p-type semiconducting polymers that can facilitate ion penetration, and operate in accumulation mode are much desired in bioelectronics. Glycol side chains have proven to be an efficient method to increase bulk electrochemical doping and optimize aqueous swelling. One early polymer which exemplifies these design approaches was p(g2T-TT), employing a bithiophene-co-thienothiophene backbone with glycol side chains in the 3,3' positions of the bithiophene repeat unit. In this paper, the analogous regioisomeric polymer, namely pgBTTT, was synthesized by relocating the glycol side chains position on the bithiophene unit of p(g2T-TT) from the 3,3' to the 4,4' positions and compared with the original p(g2T-TT). By changing the regio-positioning of the side chains, the planarizing effects of the S-O interactions were redistributed along the backbone, and the influence on the polymer's microstructure organization was investigated using grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The newly designed pgBTTT exhibited lower backbone disorder, closer π-stacking, and higher scattering intensity in both the in-plane and out-of-plane GIWAXS measurements. The effect of the improved planarity of pgBTTT manifested as higher hole mobility (μ) of 3.44 ± 0.13 cm2 V-1 s-1. Scanning tunneling microscopy (STM) was in agreement with the GIWAXS measurements and demonstrated, for the first time, that glycol side chains can also facilitate intermolecular interdigitation analogous to that of pBTTT. Electrochemical quartz crystal microbalance with dissipation of energy (eQCM-D) measurements revealed that pgBTTT maintains a more rigid structure than p(g2T-TT) during doping, minimizing molecular packing disruption and maintaining higher hole mobility in operation mode.

Microstructural evidence of the toughening mechanisms of polyurethane reinforced with halloysite nanotubes under high strain-rate tensile loading

In this study, we have investigated the relationship between the spherulitic morphology and the dynamic tensile response of polyurethane reinforced with Halloysite nanotubes (HNTs). The polyurethane prepolymer is partially silane end-capped and filled with only 0.8 wt.% of acid-treated Halloysite nanotubes. The resultant nanocomposite material presents a 35% higher spall strength compared to the neat polyurethane and 21% higher fracture toughness. We show evidence that the HNTs are not the toughening phase in the nanocomposite, but rather it is their influence on the resultant spherulitic structures which alters the polymer microstructure and leads to a tougher dynamic response. Microstructural characterization is performed via Scanning Electron Microscopy, Atomic Force Microscopy and Field Emission Scanning Electron Microscopy, and crystallinity examination via X-ray diffraction. The spherulitic structures present a brittle fracture character, while the interspherulitic regions are more ductile and show large deformation. The nanocomposite presents a finer and more rigid spherulitic structure, and a more energy dissipative fracture mechanism characterized by a rougher fracture surface with highly deformed interspherulitic regions.

Randomness of Polymer Microstructure in the Resist Film as Shot Noise

Randomness of Polymer Microstructure in the Resist Film as Shot Noise