Using viscoelastic modeling and molecular dynamics based simulations to characterize polymer natural fiber composites

The potential for polymer natural fiber composites for manufacturing storage units for products with high ethanol content is explored. The influence of ethanol diffusion into the microstructure of the storage unit on its long-term mechanical (specifically creep compliance) and viscoelastic properties are measured. Burger's model for polymer viscoelasticity is used to predict durability and other fundamental properties of the composite based on the creep compliance trends. Properties such as the Maxwell moduli and Maxwell viscosities are then modeled as a function of net ethanol uptake and the concentration of natural fiber dispersed phase. Later, a combination of classical molecular dynamics (MD) and semi empirical modeling is used to predict the trends in ethanol diffusion coefficient as a function of temperature and natural fiber concentration. The most efficacious models for this purpose and the ways and means of further improving the simulation accuracy are discussed.     

Interphase‐layer effect on deformation of silicone rubber filled with nanosilica particles

The viscoelastic and statically tensile deformation properties of silicone rubber composites filled with nanosilica (300 nm in diameter) and microsilica particles (1.5 μm in diameter) were investigated on the basis of experimental results to clarify the interphase‐layer effect on these properties. The interphase layers formed around the nanoparticles without chemical coating were found to be glassy, even though the composites were in the rubbery state. The interphase layer thickness was determined to be approximately 20 nm using Guth and Gold's mixture law with the viscoelastic properties of the nanoparticle‐filled rubber in the rubbery state. The determined thickness of the interphase layer was confirmed by comparing the maximum strains at fracture for the nanoparticle‐filled rubber, which decreased for higher volume fraction of the nanoparticles. Therefore, the deformation properties were clarified to depend on the volume fraction of the apparent particles composed of the nanoparticles and interphase layers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45880.     

Relationship between structure and dynamic mechanical properties of thermoplastic polyurethane elastomer containing bi-soft segment

In this work, we investigated the microphase separation, mechanical, and dynamic mechanical properties of thermoplastic polyurethane elastomers (TPUs) with one-soft segment (polypropylene glycol, PPG, or hydroxyl-terminated polybutadiene, HTPB) or bi-soft segment (PPG and HTPB) using FTIR, XRD, SAXS and amplitude modulated-frequency modulated viscoelastic mode of AFM (AM-FM mode AFM) methods. The results showed that the microphase separation process of hard and soft segments (HS and SS) in TPU containing PPG and HTPB soft segments (PPG-HTPB-PU) was restricted by randomly alternated bi-soft segments, which results in formation of a low content of irregular-shaped hard domain (HD). In addition, the microphase separation of PPG-HTPB-PU induced a triple-phased structure of HD, HTPB rich phase and mesophase. The mesophase of PPG-HTPB-PU was formed of HS, PPG and HTPB segments which were excluded out of HD and HPTB rich domains during microphase separation process. The damping temperature range (at tan δ greater than 0.3) of PPG-HTPB-PU was from −14.6 to 32.1°C (46.5°C) which was more broad than that of TPU containing HTPB soft segment (HTPB-PU). The broad damping temperature range of PPG-HTPB-PU is mainly attributed to the enhanced energy consumption caused by the frictional motions of mixed segments of mesophase.     

Low stress creep response of PTFE sealants applied to PEM fuel cells

Viscoelastic properties of polytetrafluoroethylene (PTFE) play a crucial role in forecasting its long-term behavior in engineering applications. An attempt is made to explore the viscoelastic properties of PTFE sealants that are utilized in polymer electrolyte membrane fuel cell (PEMFC). It is to be noted that PTFE sealants are vulnerable to creep under constant loading at elevated temperatures. Moreover, the creep of sealants will lead to leakage of reactants from the cell, which affects the performance of PEMFC. PTFE is an excellent choice as a sealant material in low-temperature polymer electrolyte membrane fuel cell (LT-PEMFC), which operates in the temperature range of 60–80°C. PTFE can be prominently used as sealants in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC), as it possesses no significant change in its physical properties within the temperature range of −150 to 300°C along with the working conditions of HT-PEMFC. In LT-PEMFC, the sealants will typically be subjected to low stresses in the range of 1–5 MPa. In this article, the creep response of PTFE sealant material is extensively studied at various temperatures of 25 (room temperature), 35, 45, 55, and 65°C and at three stress levels of 2, 3, and 4 MPa. The time–temperature superposition principle is utilized to develop master curve at a reference temperature of 25°C, to forecast long-term creep characteristics of PTFE sealants. Moreover, the master curve for creep compliance is developed for 4.5 h.     

Simultaneously reduced viscosity and enhanced strength of liquid silicone rubber/silica composites by silica surface modification

Silica is the most widely‐used filler to reinforce liquid silicone rubber (LSR), but the high viscosity of LSR/silica suspension significantly limits its processing flexibility. To balance the processibility and reinforcing efficiency of LSR/silica systems, two kinds of enols (propenol and 1‐undecylenyl alcohol) and a saturated alcohol (1‐undecylic alcohol) were employed to modify the silica surface. Various rheological tests were carried out to investigate the processibility as well as filler networking and crosslinking processes of the modified systems. Tensile tests were also adopted to verify the reinforcing effect. It was found that surface modification of silica by 1‐undecylenyl alcohol could significantly reduce the viscosity of its suspension with LSR. Meanwhile, the mechanical strength of LSR could be largely enhanced by six times with 10 wt % modified silica. This work will merit design and production of LSR materials with balanced processibility and mechanical performance. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45544.     

Elastic modulus of thermoplastic elastomers investigated with the instantaneous volumetric strain method

The factors influencing the elastic modulus [K(t)] of thermoplastic elastomers (TPEs) were investigated under spherical indentation mode with the instantaneous volumetric strain method. The TPE samples were prepared by the hot-pressing method, and the variation in K(t) was investigated at different thicknesses, loading forces, and spherical indenter radii (Rs). The results demonstrate that the K(t) values of the TPEs decreased nonlinearly with the depth–radius ratio. The sample thickness [H(t)] and R showed positive and negative relationships with K(t), respectively. However, the loading force did not exhibit any significant effect on K(t). In a certain depth–radius ratio range, K(t) had strong functional relationships with H(t), R, and indentation depth. These results provide a reference for the mechanical property investigation of elastoplastic materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47181.     

Mechanical properties and thermal stability of porous polyimide/hollow mesoporous silica nanoparticles composite films prepared by using polystyrene microspheres as the pore-forming template

The porous polyimide/hollow mesoporous silica nanoparticles (PI/HMSNs) composite films were fabricated via blending polymerization by using polystyrene (PS) microspheres as the pore-forming template. The morphologies, microstructures, thermal stability, thermal expansion coefficient (TEC), and mechanical performances of the porous PI/HMSNs films were characterized in detail. Results showed that the uniform dispersion of HMSNs benefits from the strong hydrogen-bonding interaction between the hydroxyl groups of HMSNs and poly(amic acid) chains. Both weight loss and TEC of the porous PI/HMSNs films are lower than those of the pure porous PI film. When 0.8 wt % HMSNs and 7.0 wt % PS were added into the PI matrix, the Young's modulus and tensile strength of composite film increased by about 32.4% and 68.1% compared with those of the pure porous PI film. Conclusively, the introduction of HMSNs in the porous PI matrix is an important strategy to enrich the diversity of porous structure, improve the thermal and mechanical properties of the porous PI material simultaneously. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48792.     

Improved processibility of silicone composites by MQ silicone resins

Processibilities of silicone composites were always a problem for their high content of SiO₂ powders. This article found that the substitution of silicone resins for linear polydimethylsiloxanes (PDMS) made processibilities easier. Three silicone resins (MQ1.0, MQ1.1, and MQ1.2) with clarified chemical structures (by FT IR, ²⁹Si NMR, and GPC) were adopted. Their shearing viscosities [η()] were greatly higher than PDMS with higher molecular weight, which could be assigned to stronger molecular interactions as surface tension and flowing activation energy ΔE indicated. On the contrary, η() of MQ‐PDMS binary blends greatly decreased to that even lower than either components (about 85% utmost decrease comparing to PDMS), for the variation of molecular interaction rather than dilution effect. Furtherly, when PDMS were partly replaced with MQ resins, process time of PDMS–SiO₂ silicone composites were greatly shortened (from >6 to 2 h), while with better SiO₂ dispersion (Mooney viscosity greatly decreased from 30.0 to 5.0 MU). Better dispersion of SiO₂ fillers in composites could be confirmed by SEM and mechanical properties. For the better dispersion, mechanical properties of composites were improved with higher elastic modulus, higher tensile strength, and higher hardness, especially with higher elongation at break (utmost increased from 190% to 277%). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46445.     

Viscoelastic and thermal decomposition behaviors of polytetrahydrofuran binder prepared using glycerin as a crosslinking modifier

Polytetrahydrofuran (PTHF) is an effective binder ingredient used for improving the performance of propellants. PTHF becomes sufficiently rubbery for use as a binder with the addition of an adequate crosslinking modifier. This study investigated the viscoelastic and thermal decomposition behaviors of the PTHF binder prepared using glycerin as a crosslinking modifier, as well as the influence of the molecular weight of PTHF on the characteristics of the PTHF binder. The curing behavior of the PTHF binder was suitable for the manufacture of propellants, and the superior tensile properties of the PTHF binder made it suitable for use as a propellant binder. The degree of crosslinking of the samples decreased as the molecular weight of the PTHF increased. The PTHF binder has unique dynamic mechanical properties owing to its melting and chemical structure, and these properties were dependent on the molecular weight of PTHF. The glass transition temperature (T_g) and the loss tangent at T_g decreased as the molecular weight of the PTHF increased. The temperature and frequency dependence of the PTHF binder were influenced by the melting point of PTHF. The viscoelastic properties of the binder prepared using PTHF with a molecular weight of 650 followed the time–temperature superposition principle. The activation energy for the relaxation of this binder varied remarkably at the melting point of PTHF. The thermal decomposition behavior indicated that at low temperatures, the consumption rate of the binder with low‐molecular‐weight PTHF was slightly larger than that of the binder with high‐molecular‐weight PTHF. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Platinum-terpyridine complexes in polymers: A novel approach for the synthesis of self-healing metallopolymers

The synthesis of novel platinum-based metallopolymers for self-healing applications is presented. For this purpose, terpyridine-platinum complexes were studied using isothermal titration calorimetry regarding their complexation behavior with pyridine. The obtained knowledge was utilized for the preparation of metallopolymers using the reversible addition fragmentation chain-transfer -polymerization technique resulting in well-defined polymers. Crosslinking with a tetravalent pyridine-crosslinker enabled the synthesis of a metallopolymer network featuring self-healing properties. From these experiments, more information about the molecular preconditions for the design of healable metal-containing polymers could be drawn enabling a further optimization of these systems in the future. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47064.     

Ternary nanocomposites based on epoxy,modified silica,and tetrabutyl titanate: Morphology,characteristics, and kinetics of the curing process

In this study, the effects of unmodified nanosilica and nanosilica modified by an isopropyl tri[di(octyl) phosphate] titanate coupling agent (KR-12; m-nanosilica) on the structure, morphology, thermomechanical properties, and kinetics of the curing process of epoxy–tetrabutyl titanate (TBuT) nanocomposites were investigated. The viscosity, tensile strength, and flexural strength of the cured epoxy and cured epoxy–m-silica–TBuT nanocomposites were determined with a Brookfield viscometer and an Instron 5582-100KN universal machine. The morphology and gel fraction content of the nanocomposites were analyzed with transmission electron microscopy and scanning electron microscopy methods and Soxhlet extraction. The viscosity, mechanical properties, gel fraction content, and morphology results of the cured epoxy–m-silica–TBuT nanocomposites confirm that 5 wt % m-nanosilica was the most suitable for improving the dispersion of m-nanosilica in the epoxy matrix and the properties of these materials. The thermal behavior of the nanocomposites was determined by thermogravimetric analysis and differential scanning calorimetry (DSC) methods. On the basis of DSC data, the average value of the activation energy of the cured epoxy–TBuT system, calculated according to Flynn–Wall–Ozawa and Kissinger equations, was 67.893 kJ/mol. The calculation according to the Crane equation showed that the first-order kinetics complied with the curing reaction for the neat epoxy. When we introduced the unmodified nanosilica and modified nanosilica into the epoxy matrix, the order kinetics of the curing reaction for the nanocomposites also followed first-order kinetics, but the activation energy of their curing reaction decreased significantly. Some other properties were also investigated with dynamic mechanical analysis and Fourier transform infrared analysis and are discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47412.     

Highly toughened poly(lactic acid) blends prepared by reactive blending with a renewable poly(ether-block-amide) elastomer

Renewable poly(ether-block-amide) (PEBA) elastomer was grafted with glycidyl methacrylate (GMA) to prepare PEBA-GMA, then it was melting blended with poly (lactic acid) (PLA) in an effort to achieve fully bio-based super-toughened PLA materials. The notched Izod impact strength of PLA/PEBA-GMA blend was significantly enhanced when the content of PEBA-GMA was higher than 20 wt%, and the tensile toughness was also improved. It was found a new copolymer was formed at the interface due to the reaction of the end groups ( OH,  COOH) of PLA with the epoxide group of PEBA-GMA. This greatly improves the interfacial adhesion between PLA and PEBA-GMA component, leading to finer dispersed particles of PEBA-GMA which were better wetted by the PLA matrix. Therefore, the highly enhanced notched impact strength was ascribed to the effective reactive compatibilization promoted by the interfacial reaction. This provides a new idea for preparing super tough PLA materials with bio-based elastomer, which will widely extend the application of PLA.     

Insights into the synergistic mechanism of reactive aliphatic soft chains and nano-silica on toughening epoxy resins with improved mechanical properties and low viscosity

Toughening epoxy resin (EP) without sacrificing strength, modulus, and processing performance is always a harsh task. Here, a series of epoxy systems containing soft butyl glycidyl ether (BGE) and rigid nano-silica (nano-SiO₂) were prepared. Micro-phase separation structures derived from the self-assembly effect of BGE can be observed in atomic force microscopy images by controlling the total amount of BGE and nano-SiO₂ at 2 wt% for the EP_C:Si-m:n (m + n = 4) systems. Due to the synergistic effect of self-assembly effect of BGE and the rigid effect of well dispersed nano-SiO₂, EP_C:Si-2:2 system exhibited improvement of tensile strength of 59.3% (92.63 MPa), tensile modulus of 24.8% (3.52 GPa), elongation at break of 78.6% (4.84%), and glass transition temperature of 2.4% (138.4°C) compared with Pure EP system. Besides, due to the low loading of nano-SiO₂ (≤2 wt%) and the dilution effect of BGE, the viscosity of all the toughening systems is lower than 600 mPa·s, which can provide this toughening system with superior processing performance for large production of composites by automotive manufacturing methods such as vacuum assistant resin infusion technology.     

Plasticizing effect of biodegradable dipropylene glycol bibenzoate and epoxidized linseed oil on diglycidyl ether of bisphenol A based epoxy resin

Major limitation for use of epoxy thermosets in engineering applications is its sudden brittle failure. In the present study dipropylene glycol dibenzoate (DPGDB) based plasticizer is used to modify diglycidyl ether of bisphenol A (DEGEBA) based epoxy resin system via simple blending technique. Bio-based epoxidized linseed oil was also used to modify epoxy resin system and compared with DPGDB modified resin. For DPGDB modified resin storage modulus and loss modulus of the epoxy system modified with 10% plasticizer increased by 7.54% and 12.24%, respectively. The primary mechanism responsible for such behavior is improved crosslinking density. With 5% plasticizer loading, flexural strength increased by 21%. There was an improvement of 312.74% in strain at failure for 10% plasticizer loading, while preserving its mechanical strength. It was found that DPGDB based modification was better than epoxidized linseed oil modification.     

Impact of ultrasonication on carbon nanotube demixing and damage in polymer nanocomposites

While ultrasonication is universally employed for dispersion and distribution of carbon nanotubes (CNTs) in a solvent or polymer solution, the current work focuses on the underlying mechanisms of CNT demixing and CNT damage that can occur during processing. Here, multi-walled CNTs were dispersed in a polycaprolactone polymer matrix using an established solution processing technique. Electrical, rheological, and mechanical characterization results suggest that once nanocomposite property enhancements reach an optimal level, further sonication leads to a decrease in the corresponding properties due to a combination of CNT damage and demixing mechanisms. Evidence of CNT damage from transmission electron microscopy, poor CNT distribution from optical image analysis and shear-induced crystallization results, and reagglomeration observed from ultraviolet–visible results, taken together, suggest that mechanisms of demixing and damage of the CNTs coexist for excessive sonication times.     

Interface engineering for the improvement of mechanical and thermal properties of covalent functionalized graphene/epoxy composites

Functionalized reduced graphene oxide (GO)/epoxy composites are fabricated through solution mixing. GO is functionalized using 3‐amino‐1,2,4‐triazole (TZ) in presence of potassium hydroxide (KOH). KOH is expected to serve dual role as catalyst for nucleophilic addition reaction between GO and TZ, and also as reducing agent. The grafting of TZ moiety on GO is confirmed by Fourier transform infrared spectroscopy, X‐ray diffraction, and thermogravimetric analysis. The prepared composites show remarkable improvement in mechanical and thermal stability. The fracture toughness of the composites (critical stress intensity factor, K_IC) achieved from single edge notched bending testing is improved by ～111% against pure epoxy at 0.1 wt % loading of TZ functionalized GO. Further, the tensile strength and Young's modulus are improved by ～30.5% and 35%, respectively. Thermal stability of the composites as investigated by thermogravimetric analysis showed 29 °C rise in onset degradation temperature for 0.1 wt % TZ functionalized GO incorporated composite. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46124.     

Experimental investigation and statistical analysis of additively manufactured onyx-carbon fiber reinforced composites

Availability of additive manufacturing has influenced the scientific community to improve on production and versatility of the components created with several associated technologies. Adding multiple substances through superimposing levels is considered as a part of three-dimensional (3D) printing innovations to produce required products. These technologies are experiencing an increase in development nowadays. It requires frequently adding substance and has capacity to fabricate extremely complex geometrical shapes. However, the fundamental issues with this advancement include alteration of capacity to create special products with usefulness and properties at an economically viable price. In this study, significant procedural parameters: layer designs/ patterns (hexagonal, rectangular and triangular) and infill densities (30%, 40%, and 50%) were considered to investigate into their effects on mechanical behaviors off fused deposition modeling or 3D-printed onyx-carbon fiber reinforced composite specimens, using a high-end 3D printing machine. Mechanical (tensile and impact) properties of the printed specimens were conclusively analyzed. From the results obtained, it was observed that better qualities were achieved with an increased infill density, and rectangular-shaped design exhibited an optimum or maximum tensile strength and energy absorption rate, when compared with other counterparts. The measurable relapse conditions were viably evolved to anticipate the real mechanical qualities with an accuracy of 96.4%. In comparison with other patterns, this was more closely predicted in the rectangular design, using regression models. The modeled linear regression helps to define the association of two dependent variables linked with properties of the dissimilar composite material natures. The models can further predict response of the quantities before and also guide practical applications.     

Microlayer and nanolayer tubing and piping via layer multiplication coextrusion. I. Validation

Layered annular structures produced with layer multiplication coextrusion, utilizing both a standard in-line “spider” die, and a custom annular die, are compared in structures up to 129 layers. One multilayered system, of a Dow LDPE 5004I was utilized in generating experimental results to validate the custom die design performance. It was found that the custom design demonstrates successful extrusion of high layer number annular structures with substantial benefits over the standard spider die. Moreover, a design method incorporating angular rotation was implemented within the custom die to eliminate weld lines and attain concentric layer structures to further enhance commercial viability and mechanical integrity. Results indicate angular rotation may be utilized to generate idealized annular products with concentric layer structures. Additionally, exploration of flow through the annular die land was conducted with ANSYS Polyflow in under several conditions of angular rotation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48683.     

Microlayer and nanolayer tubing and piping via layer multiplication coextrusion. II. Rheologically mismatched systems

An advanced technology, presented in Part 1, was utilized with a custom annular die to produce structures having 9–129 layers. Two material systems were chosen to produce layer structures of viscosity and elasticity stratified type both with/without the application of a rotating boundary wall within the annular land. A maximum rotation window for nine-layer structures was determined to maintain the layer structure while minimizing the appearance of the weld lines and achieving concentric layers. ANSYS Polyflow was utilized in conjunction to study trilayered systems of generic material properties in stratified forms, along with replication of the experimental systems. Combination of experimental and simulation approaches herein are allowing for the achievement of high layer number, low layer thickness, systems in annular form for the first time. The continued development of this technology can lead to application areas of tubes/pipes with advanced properties such as optical filtering, pressure resistance, and permeation reduction. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48684.     

Preparation of epoxidized acrylonitrile butadiene rubber and its compatibilization effect on water-swellable rubber composites

In this research, water-swellable rubbers (WSRs) were prepared by the blending of acrylonitrile butadiene rubber (NBR) with epoxidized acrylonitrile butadiene rubber (ENBR), precipitated silica, vulcanizing agents, and polyacrylic acid sodium (PAAS) as superabsorbent polymers (SAPs). ENBR was prepared with a molybdenum trioxide catalyst and a tert-butyl hydroperoxide oxidant through a free-radical reaction under specific conditions and was used as a compatibilizer to improve the interfacial adhesion of the NBR matrix and hydrophilic components of the WSRs. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to analyze the structure and properties of ENBR. The dispersion of silica and SAPs in the rubber matrix was investigated by transmission electron microscopy. The presence of ENBR enhanced the water-absorption properties of the WSRs. The NBR–ENBR composites exhibited a higher mechanical performance after water absorption than that without ENBR. Both the swelling ratio and the absorption rate increased with the ENBR content. When the weight ratio of NBR–ENBR was 25/75, an equilibrium swelling ratio of 172% (13% higher than that of the sample without ENBR) was obtained. Scanning electron microscopy analysis revealed that the addition of ENBR improved the compatibility between the SAPs and the NBR matrix. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47694.