A novel model for quantification of the moisture absorption of polymeric laminated composites

In the present study, a new model was developed that considers the amount of the environmental fluid absorption by different constituents of polymeric laminated composites including fibers, resin, fiber-matrix interphase region, ply interface region, and voids. By knowing the fluid absorption behavior of the composite constituents, the present model can predict the amount of fluid absorption of different constituents of polymeric laminated composites with an arbitrary resin volume fraction and stacking sequence. Test specimens were fabricated by glass fibers and vinyl ester resin. The environmental fluids, examined in this study, were distilled and saline water under different temperatures and salt concentrations. To investigate the absorption behavior of different constituents of polymeric composite, various tests were conducted on fibers, pure cured resin, unidirectional composite specimens, and laminated composites. Based on the results of the tests, a new theoretical model was developed to quantify and predict the amount of fluid absorption of different constituents of laminated polymeric composites. The thickness of the interphase region between the fiber and matrix was also measured using the scanning electron microscope (SEM) images and nano-indentation tests. The consistency of experimental results with the outcomes of the theoretical model indicates the accuracy of the model.     

Biobased PLA/NR-PMMA TPV with balanced stiffness-toughness: In-situ interfacial compatibilization,performance and toughening model

Using rubber to toughen polylactide (PLA) is always accompanied by the sharp reduction in stiffness. Herein, PLA/poly (methyl methacrylate) grafted natural rubber (NR-PMMA) thermoplastic vulcanizates (TPVs) with balanced stiffness-toughness were fabricated. With the addition of 40 wt % NR-PMMA, the impact strength and tensile toughness of PLA/NR-PMMA TPV significantly improved to about 102.7 kJ/m² and 66.1 MJ/m³, respectively, compared with those of 2.7 kJ/m² and 2.4 MJ/m³ for the pure PLA. Meanwhile, the yielding stress was maintained at 34.5 MPa. Fourier transform infrared spectroscopy (FTIR) confirmed the formation of in-situ interfacial compatibilization between PLA and rubber phases. Both tensile and impact toughening mechanism were studied and deduced as considerable energy dissipation provided by the continuous rubber phase. Instrumented notched impact tests demonstrated that the energy dissipating in crack propagation process contributed to the main part of impact toughness. In addition, a novel toughening model based on bicontinuous structure was incorporated, which showed good applicability in predicting the impact strength of PLA/NR-PMMA TPVs.     

Multiscale characterization and constitutive parameters identification of polyamide (PA12) processed via selective laser sintering

This study is focused on multiscale characterization and constitutive parameters identification of selectively laser sintered PA12 specimens. The process parameters and change in crystallinity due to 3D printing were identified via differential scanning calorimetry (DSC). Tension, compression, shear, flexural and fracture tests performed on samples fabricated, both in 0° and 90° directions reveal that the tensile toughness of samples printed in 0° orientation, outperform samples printed in 90° orientation by an average of 24%. Cryogenically fractured samples were analyzed via SEM and micro-computed tomography to analyze 2D/3D defects and correlate the microstructure with macroscopic properties. The constitutive parameters for a strain-rate and temperature-dependent Three Network (TN) material model were identified using the measured mechanical properties. Furthermore, mechanical response of micro-architected Kelvin lattice structure was analyzed by Finite Element Method employing the TN constitutive model and the predictions were corroborated with measurements.     

A five-parameter fractional derivative temperature spectrum model for polymeric damping materials

To capture viscoelastic behavior of polymeric damping materials based on limited dynamic mechanical analysis tests, a simple fractional temperature spectrum model representing the viscoelastic materials is proposed in this paper and experimental tests aims at stressing the validity of the model. The storage modulus, the loss modulus, and the loss factor, are established based on the five-parameter fractional derivative model and the time–temperature superposition principle. The dynamic mechanical tests of two polymeric materials are carried out to verify this temperature spectrum model. Results indicate a good agreement between the temperature spectrum model and experimental tests at various temperature conditions. Furthermore, thermodynamic coupling of the viscoelastic material is investigated by temperature rise calculation and vibration experiment test. Comparison analysis shows that the temperature rise model can simulate the temperature rise process for the shear vibration of the constrained damping, which provide references for the damping capability, thermal damage and failure of viscoelastic material.     

Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.     

Ab-initio molecular dynamics and hybrid explicit-implicit solvation model for aqueous and nonaqueous solvents: GFP chromophore in water and methanol solution as case study

Solute–solvent interactions are proxies for understanding how the electronic density of a chromophore interacts with the environment in a more exhaustive way. The subtle balance between polarization, electrostatic, and non-bonded interactions need to be accurately described to obtain good agreement between simulations and experiments. First principles approaches providing accurate configurational sampling through molecular dynamics may be a suitable choice to describe solvent effects on solute chemical–physical properties and spectroscopic features, such as optical absorption of dyes. In this context, accurate energy potentials, obtained by hybrid implicit/explicit solvation methods along with employing nonperiodic boundary conditions, are required to represent bulk solvent around a large solute–solvent cluster. In this work, a novel strategy to simulate methanol solutions is proposed combining ab initio molecular dynamics, a hybrid implicit/explicit flexible solvent model, nonperiodic boundary conditions, and time dependent density functional theory. As case study, the robustness of the proposed protocol has been gauged by investigating the microsolvation and electronic absorption of the anionic green fluorescent protein chromophore in methanol and aqueous solution. Satisfactory results are obtained, reproducing the microsolvation layout of the chromophore and, as a consequence, the experimental trends shown by the optical absorption in different solvents.     

线性响应和两种态特定方法模拟AF350电子跃迁的溶剂效应

在极化连续模型框架下比较了线性响应与两种不同态特定方法计算的溶液中Alexa Fluor 350(AF350)分子激发能和光谱移动值的差异. AF350的第一激发态S0→S1电子跃迁属于π→π^*跃迁, 主要对应于最高占据分子轨道(HOMO)到最低空轨道(LUMO)的跃迁. 该分子激发态偶极矩大于基态偶极矩, 激发态时溶质溶剂相互作用比基态时更强, 随着溶剂极性增大, 会发生光谱红移的现象. 与实验值相比, 线性响应和两种态特定方法均高估了激发能, 其中以IBSF(Improta-Barone-Scalmani-Frisch)方法得到的激发能最小, 矫正的基态反应场方法(cGSRF)得到的激发能最大. 对于光谱移动值, 3种方法与实验值相比都偏小, 线性响应方法(LR)计算出的误差最大, 而IBSF方法得到的结果与实验值最吻合, 是预测溶液中AF350分子激发能和光谱移动值最准确的方法. 对比了Marcus传统理论和基于约束平衡的非平衡溶剂化理论的结果, 发现后者得到的激发能和光谱移动值更接近于实验值.     

Automatic kinetic model generation and selection based on concentration versus time curves

The goal of the paper is to automatize the construction and parameterization of kinetic reaction mechanisms that can describe a set of experimentally measured concentration versus time curves. Using the framework and theorems of formal reaction kinetics, first, we build a set of possible mechanisms with a given number of measured and unmeasured (real or fictitious) species and reaction steps that fulfill some chemically reasonable requirements. Then we fit all the corresponding mass-action kinetic models and offer the best one to the chemist to help explain the underlying chemical phenomenon or to use it for predictions. We demonstrate the use of the method via two simple examples: on an artificial, simulated set of data and on a small real-life data set. The method can also be used to do a kind of lumping to generate a model that can reproduce the simulation results of a detailed mechanism with less species and thereby can largely accelerate spatially inhomogeneous simulations.     

应用极化水分子模型研究单分子层受限水的介电性质

受限条件下水的介电性质因测量极具挑战,其在诸多电化学过程与反应输运过程中如何扮演关键角色从未被定量地澄清.本工作利用平衡态分子动力学模拟和受限体系介电性质计算方法,系统性地探索了0.65 nm限域尺寸、5×10⁸ Pa限域压强、不同温度条件下单分子受限冰和受限水的介电性质.详细比较了恒定偶极矩SPC/E水分子模型和可极化的SWM4-NDP水分子模型在描述受限冰、水结构与介电性质上的优劣势,包括统计分析SWM4-NDP模型模拟的单分子层受限水和受限冰的瞬时分子偶极矩概率密度分布,计算每个模拟体系的静态结构因子、静态偶极空间关联函数、静态介电常数、体系偶极时间关联函数和德拜弛豫时间.首次发现了极化水分子模型描述的低维度受限水和受限冰的奇异分子极性变化,并观察到两种模型描述静态结构性质的效果相当,SWM4-NDP模型对于静态介电常数描述的优势会因受限条件的增强而被大幅削减.但在受限水介电极化弛豫动力学性质描述上SWM4-NDP模型明显优于SPC/E模型.我们推断SWM4-NDP模型在探索受限水结构相变动力学以及受限体系离子输运和溶剂化动力学等过程的模拟研究中是比SPC/E模型更好的选择.本工作将在进一步开展基于受限水系统储能、传感、输运的设计工作中提供一定的理论指导意义.