Influence of mixed soft segments on microphase separation of polyurea elastomers

Nonisocyanate Polyurethane Segmented Copolymers from Bis-Carbonylimidazolides

Bis-carbonylimidazolide (BCI) functionalization enables an efficient synthetic strategy to generate high molecular weight segmented nonisocyanate polyurethanes (NIPUs). Melt phase polymerization of ED-2003 Jeffamine, 4,4'-methylenebis(cyclohexylamine), and a BCI monomer that mimics a 1,4-butanediol chain extender enables polyether NIPUs that contain varying concentrations of hard segments ranging from 40 to 80 wt. %. Dynamic mechanical analysis and differential scanning calorimetry reveal thermal transitions for soft, hard, and mixed phases. Hard segment incorporations between 40 and 60 wt. % display up to three distinct phases pertaining to the poly(ethylene glycol) (PEG) soft segment Tg , melting transition, and hard segment Tg , while higher hard segment concentrations prohibit soft segment crystallization, presumably due to restricted molecular mobility from the hard segment. Atomic force microscopy allows for visualization and size determination of nanophase-separated regimes, revealing a nanoscale rod-like assembly of HS. Small-angle X-ray scattering confirms nanophase separation within the NIPU, characterizing both nanoscale amorphous domains and varying degrees of crystallinity. These NIPUs, which are synthesized with BCI monomers, display expected phase separation that is comparable to isocyanate-derived analogues. This work demonstrates nanophase separation in BCI-derived NIPUs and the feasibility of this nonisocyanate synthetic pathway for the preparation of segmented PU copolymers.

State-of-the-Art Polyurea Coatings: Synthesis Aspects, Structure-Properties Relationship, and Nanocomposites for Ballistic Protection Applications

This review presents polyurea (PU) synthesis, the structure-properties relationship, and characterization aspects for ballistic protection applications. The synthesis of polyurea entails step-growth polymerization through the reaction of an isocyanate monomer/prepolymer and a polyamine, each component possessing a functionality of at least two. A wide range of excellent properties such as durability and high resistance against atmospheric, chemical, and biological factors has made this polymer an outstanding option for ballistic applications. Polyureas are an extraordinary case because they contain both rigid segments, which are due to the diisocyanates used and the hydrogen points formed, and a flexible zone, which is due to the chemical structure of the polyamines. These characteristics motivate their application in ballistic protection systems. Polyurea-based coatings have also demonstrated their abilities as candidates for impulsive loading applications, affording a better response of the nanocomposite-coated metal sheet at the action of a shock wave or at the impact of a projectile, by suffering lower deformations than neat metallic plates.

Polyurethane/n-Octadecane Phase-Change Microcapsules via Emulsion Interfacial Polymerization: The Effect of Paraffin Loading on Capsule Shell Formation and Latent Heat Storage Properties

Organic phase-change materials (PCMs) hold promise in developing advanced thermoregulation and responsive energy systems owing to their high latent heat capacity and thermal reliability. However, organic PCMs are prone to leakages in the liquid state and, thus, are hardly applicable in their pristine form. Herein, we encapsulated organic PCM n-Octadecane into polyurethane capsules via polymerization of commercially available polymethylene polyphenylene isocyanate and polyethylene glycol at the interface oil-in-water emulsion and studied how various n-Octadecane feeding affected the shell formation, capsule structure, and latent heat storage properties. The successful shell polymerization and encapsulation of n-Octadecane dissolved in the oil core was verified by confocal microscopy and Fourier-transform infrared spectroscopy. The mean capsule size varied from 9.4 to 16.7 µm while the shell was found to reduce in thickness from 460 to 220 nm as the n-Octadecane feeding increased. Conversely, the latent heat storage capacity increased from 50 to 132 J/g corresponding to the growth in actual n-Octadecane content from 25% to 67% as revealed by differential scanning calorimetry. The actual n-Octadecane content increased non-linearly along with the n-Octadecane feeding and reached a plateau at 66-67% corresponded to 3.44-3.69 core-to-monomer ratio. Finally, the capsules with the reasonable combination of structural and thermal properties were evaluated as a thermoregulating additive to a commercially available paint.

Quantification Characterization of Hierarchical Structure of Polyurethane by Advanced AFM and X-ray Techniques

Polyurethane (PU) with microphase separation has garnered significant attention due to its highly designable molecular structure and a wide range of adjustable properties. However, there is currently a lack of systematic approaches for quantifying PU's microphase separation. To address this research gap, we utilized an atomic force microscopy (AFM) nanomechanical mapping technique along with Gaussian fitting to recolor and quantitatively analyze the evolution of PU's microphase separation. By varying the ratios of the chain extender to cross-linking agent, we observed the changes in the hydrogen bonding between the soft and hard segments. As the ratio of the chain extender to cross-linking agent decreases, the strength of the hydrogen bonding weakens, resulting in a reduction in the quantity and phase percentage of hard segment (HS) domains. Consequently, the degree of microphase separation between the soft and hard segments decreases, leading to specific alterations in the material's mechanical properties and dynamic viscoelasticity. To further investigate the hierarchical structure of PU, we employed various techniques, such as X-ray analysis, transmission electron microscopy (TEM), and AFM-based infrared spectroscopy (AFM-IR). Our findings reveal a spherulite pattern composed of lamellae within the HS domains, with the cross-linking density gradually increasing from the center to the periphery. Overall, our comprehensive characterization of PU provides valuable insights into its hierarchical structure and establishes a quantitative framework to explore the intricate relationship between the structure and properties.

Chemical Structure and Side Reactions in Polyurea Synthesized via the Water-Diisocyanate Synthesis Pathway

Industrial polyureas are typically synthesized using diisocyanates via two possible alternative pathways: the extremely quick and highly exothermal diamine-diisocyanate pathway and the relatively slow and mild water-diisocyanate pathway. Although polyurea synthesis via the water-diisocyanate pathway is known and has been industrially applied for many decades, there is surprisingly very little analytical information in the literature in relation to the type and extent of the occurring side reactions and the resulting chemical structures following this synthesis pathway. The synthesis of polyureas exhibiting very high concentrations of carbonyl-containing groups resulted in strong and accurate diagnostic analytical signals of combined FTIR and solid-state 13C NMR analysis. Despite the strictly linear theoretical chemical structure designed, the syntheses resulted in highly nonlinear and crosslinked polymers. It was analytically found that the water-diisocyanate pathway preferentially produced highly dominant and almost equal contents of both biuret structures and tertiary oligo-uret structures, with a very small occurrence of urea groups. This is in strong contrast with the chemical structures previously obtained via the diamine-diisocyanate polyurea synthesis pathway, which almost exclusively resulted in biuret structures. The much slower reaction and crosslinking rate of the water-diisocyanate synthesis pathway enabled the further access of isocyanate groups to the already-formed secondary nitrogens, thus facilitating the formation of complex hierarchical tertiary oligo-uret structures.

Fast Li+ Transport Polyurethane-Based Single-Ion Conducting Polymer Electrolyte with Sulfonamide Side chains in the Hard Segment for Lithium Metal Batteries

Single-ion conducting polymer electrolytes (SICPEs) are considered as one of the most promising candidates for achieving lithium metal batteries (LMBs). However, the application of traditional SICPEs is hindered by their low ionic conductivity and poor mechanical stability. Herein, a self-standing and flexible polyurethane-based single-ion conductor membrane was prepared via covalent tethering of the trifluoromethanesulfonamide anion to polyurethane, which was synthesized using a facile reaction of diisocyanates with poly(ethylene oxide) and 3,5-diaminobenzoic acid (or 3,5-dihydroxybenzoic acid). The polymer electrolyte exhibited excellent ionic conductivity, mechanical properties, lithium-ion transference number, thermal stability, and a broad electrochemical window because of the bulky anions and unique two-phase structures with lithium-ion nanochannels in the hard domains. Consequently, the plasticized electrolyte membrane showed exceptional stability and reliability in a Li||Li symmetric battery. The assembled LiFePO4||Li battery exhibited an outstanding capacity (∼180 mA h g-1), Coulombic efficiency (>96%), and capacity retention. This research provides a promising polymer electrolyte for high-performance LMBs.

Phase-Change Microcapsules with a Stable Polyurethane Shell through the Direct Crosslinking of Cellulose Nanocrystals with Polyisocyanate at the Oil/Water Interface of Pickering Emulsion

Phase-change materials (PCMs) attract much attention with regard to their capability of mitigating fossil fuel-based heating in in-building applications, due to the responsive accumulation and release of thermal energy as a latent heat of reversible phase transitions. Organic PCMs possess high latent heat storage capacity and thermal reliability. However, bare PCMs suffer from leakages in the liquid form. Here, we demonstrate a reliable approach to improve the shape stability of organic PCM n-octadecane by encapsulation via interfacial polymerization at an oil/water interface of Pickering emulsion. Cellulose nanocrystals are employed as emulsion stabilizers and branched oligo-polyol with high functionality to crosslink the polyurethane shell in reaction with polyisocyanate dissolved in the oil core. This gives rise to a rigid polyurethane structure with a high density of urethane groups. The formation of a polyurethane shell and successful encapsulation of n-octadecane is confirmed by FTIR spectroscopy, XRD analysis, and fluorescent confocal microscopy. Electron microscopy reveals the formation of non-aggregated capsules with an average size of 18.6 µm and a smooth uniform shell with the thickness of 450 nm. The capsules demonstrate a latent heat storage capacity of 79 J/g, while the encapsulation of n-octadecane greatly improves its shape and thermal stability compared with bulk paraffin.

Effects of Fiber Volume Fraction and Length on the Mechanical Properties of Milled Glass Fiber/Polyurea Composites

Composites of polyurea (PU) reinforced with milled glass fiber (MG_f) were fabricated. The volume fraction and length of the milled glass fiber were varied to study their effects on the morphological and mechanical properties of the MG_f/PU composites. The morphological attributes were characterized with scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The SEM investigations revealed a uniform distribution and arbitrary orientation of milled glass fiber in the polyurea matrix. Moreover, it seems that the composites with longer fiber exhibit better interfacial bonding. It was found from the FTIR studies that the incorporation of milled glass fiber into polyurea leads to more phase mixing and decreases the hydrogen bonding of the polyurea matrix, while having a negligible effect on the H-bond strength. The compression tests at different strain rates (0.001, 0.01, 0.1, 1, 2000 and 3000 s⁻¹) and dynamic mechanical properties over the temperature range from −30 to 100 °C at 1 Hz were performed. Experimental results show that the compressive behavior of MG_f/PU composites is nonlinear and strain-rate-dependent. Both elastic modulus and flow stress at any given strain increased with strain rate. The composites with higher fiber volume fraction and longer fiber length are more sensitive to strain rate. Furthermore, the elastic modulus, stress at 65% strain and energy absorption capability were studied, taking into account both the effect of fiber volume fraction and mean fiber length. It is noted that an increase in fiber volume fraction and fiber length leads to an increase in elastic modulus, stress at 65% strain and absorbed energy up to ~103%, 83.0% and 137.5%, respectively. The storage and loss moduli of the composites also increase with fiber volume fraction and fiber length. It can be concluded that the addition of milled glass fiber into polyurea not only improves the stiffness of the composites but also increases their energy dissipative capability.

Engineering bio-inspired peptide-polyurea hybrids with thermo-responsive shape memory behaviour

Inspired by Nature's tunability driven by the modulation of structural organization, we utilize peptide motifs as an approach to tailor not only hierarchical structure, but also thermo-responsive shape memory properties of conventional polymeric materials. Specifically, poly(β-benzyl-L-aspartate)-b-poly(dimethylsiloxane)-b-poly(β-benzyl-L-aspartate) was incorporated as the soft segment in peptide-polyurea hybrids to manipulate hierarchical ordering through peptide secondary structure and a balance of inter- and intra-molecular hydrogen bonding. Employing these bioinspired peptidic polyureas, we investigated the influence of secondary structure on microphase-separated morphology, and shape fixity and recovery via attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). The β-sheet motifs promoted phase mixing through extensive inter-molecular hydrogen bonding between the hard block and peptide segments and provided an increased chain elasticity, resulting in decreased shape fixity compared to a non-peptidic control. In contrast, intra-molecular hydrogen bonding driven by the α-helical arrangements yielded a microphase-separated and hierarchically ordered morphology, leading to an increase in the shape fixing ratio. These results indicate that peptide secondary structure provides a convenient handle for tuning shape memory properties by regulating hydrogen bonding with the surrounding polyurea hard segment, wherein extent of hydrogen bonding and phase mixing between the peptidic block and hard segment dictate the resulting shape memory behaviour. Furthermore, the ability to shift secondary structure as a function of temperature was also demonstrated as a pathway to influence shape memory response. This research highlights that peptide secondary conformation influences the hierarchical ordering and modulates the shape memory response of peptide-polymer hybrids. We anticipate that these findings will enable the design of smart bio-inspired materials with responsive and tailored function via a balance of hydrogen bonding character, structural organization, and mechanics.

PEG-POSS Star Molecules Blended in Polyurethane with Flexible Hard Segments: Morphology and Dynamics

A star polymer with a polyhedral oligomeric silsesquioxanne (POSS) core and poly(ethylene glycol) (PEG) vertex groups is incorporated in a polyurethane with flexible hard segments in-situ during the polymerization process. The blends are studied in terms of morphology, molecular dynamics, and charge mobility. The methods utilized for this purpose are scanning electron and atomic force microscopies (SEM, AFM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and to a larger extent dielectric relaxation spectroscopy (DRS). It is found that POSS reduces the degree of crystallinity of the hard segments. Contrary to what was observed in a similar system with POSS pendent along the main chain, soft phase calorimetric glass transition temperature drops as a result of plasticization, and homogenization of the soft phase by the star molecules. The dynamic glass transition though, remains practically unaffected, and a hypothesis is formed to resolve the discrepancy, based on the assumption of different thermal and dielectric responses of slow and fast modes of the system. A relaxation α′, slower than the bulky segmental α and common in polyurethanes, appears here too. A detailed analysis of dielectric spectra provides some evidence that this relaxation has cooperative character. An additional relaxation g, which is not commonly observed, accompanies the Maxwell Wagner Sillars interfacial polarization process, and has dynamics similar to it. POSS is found to introduce conductivity and possibly alter its mechanism. The study points out that different architectures of incorporation of POSS in polyurethane affect its physical properties by different mechanisms.

Fluorescent linear polyurea based on toluene diisocyanate: Easy preparation, broad emission and potential applications

In contrast to conventional fluorescent polymers featured by large conjugation structures, a new class of fluorescent polymers without above conjugations are gaining constant interest owing to their significant academic importance and promising applications in diverse fields. These unconventional fluorescent polymers are in general composed of heteroatoms (e.g. N, O, P, and S) under different forms. Here we report our recent study on polyurea, prepared by a very simple one step precipitation polymerization of toluene diisocyanate in a binary solvent of water-acetone. This polyurea, basically consisting of phenyl ring and urea group, shows fluorescent emission in a broad concentration range, from very low (10-5 mg/mL) to its solubility limit (50 mg/mL), and in a wide range of emission wavelength from UV to visible regions of up to 500 nm under varied excitation wavelength. The emission behaviors were fully studied under different concentrations and excitations. It was concluded that the emission in UV region was intrinsic due to the conjugation between the phenyl and the adjacent urea unit; while the emission in visible region, strongly excitation dependent, was caused by the cluster formation of the molecular chains, in accordance with the cluster-triggered-emission (CTE) mechanism. The formation of the cluster was tested through dynamic light scattering, FTIR and UV absorbance. Tested in presence of different metal ions, Fe3+ demonstrated a quenching effect with high selectivity. Based on this study, different paper-based sensors were designed to detect Fe3+, H2O2 in bioanalysis and for data encryption. This work provides a simple way to prepare a polyurea, a novel type of unconventional fluorescent polymer, with high emission performance distinct from its known analogues.

Preparation and properties of semi-interpenetrating networks combined by thermoplastic polyurethane and a thermosetting elastomer

A novel energetic macromolecule of semi-interpenetrating materials was prepared via a sequential-IPN process, which can be used in the areas of aerospace industry and missile technology.

Effect of thermal processing temperature on the microphase separation and mechanical properties of BAMO/THF polyurethane

This paper describes the influence of thermal processing temperature on the microphase separation, hydrogen bonding, phase transitions and mechanical properties of 3,3-bis(azidomethyl)oxetane (BAMO)/tetrahydrofuran (THF) polyurethane binder, which is used for propellant. Fourier transform infrared (FTIR) spectroscopy confirmed that the intended polyurethane was synthesized and was used to determine the state of the local hydrogen bonding in these polyurethanes. The results showed that the thermal processing clearly imparts significant changes to the H-bonded environment and this was confirmed in a quantitative fashion using small-angle X-ray scattering (SAXS). The dynamic mechanical analysis (DMA) revealed rather significant changes in dynamic segmental relaxations and storage moduli for this series of BAMO/THF polyurethanes, which are in keeping with the findings from other experiments.