Mechanical characterization of polymeric films using depth-sensing instrument: Correlation between viscoelastic-plastic properties and scratch resistance

The scratch behavior of polymer films deposited on PMMA substrate by three different coating techniques is investigated by scratch tests using a depth-sensing instrument. Using an improved measurement technique, we develop an advanced methodology based on a more appropriated model that includes an estimation of realistic stress and strain states during scratch tests. The scratch resistance is evaluated by comparing the average contact pressure for which the coating cracks, by taking into account the elastic–plastic behavior of the layer. The proposed model allows the determination of the true contact depth and the elastic recovery at the rear side of the elastic–plastic contact, and thus the true projected contact area between the moving tip and the polymeric surface. This determination depends first on a rheological factor estimated from standard load–displacement curves obtained by nanoindentation and then on the tip geometry. The viscoplacticity index and the activation volume of each type of coating are then determined by nanoscratch. The viscoplasticity index determined during an elastic–plastic contact and the activation volume related to the ductile–brittle transition are discussed as reliable criteria for determining which bilayer system (polymeric film on PMMA substrate) will truly exhibit better wear and scratch resistances in service.     

Viscoelastic behavior of anhydride‐cured epoxy system initiated by thermal latent catalyst

The thermal latency and viscoelastic behavior during the cure of a new catalytic (N‐benzylpyrazinium hexafluoroantimonate) anhydride‐cured epoxy system were studied with differential scanning calorimetry and a rheometer under isothermal conditions. The gelation time was obtained from the evaluation of the storage modulus, loss modulus, and damping factor. The temperature dependence on the reaction time was described by the Arrhenius expression, and the crosslinking activation energy was determined from the Arrhenius plot based on the gelation time and reaction temperature. The gelation time and crosslinking activation energy characterized from rheological behaviors increased with increasing anhydride composition and showed a maximum value with a mixing ratio of 0.65, which was due to the compact crosslinking network without a side reaction in the ratio. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 646–653, 2001     

Effect of temperature on the compressive stress-strain properties of polystyrene

The compressive stress-strain behavior of a commercial polystyrene has been studied and the effect of deformation temperature on modulus, yield stress, percent yield strain and yield energy was determined. Yield energy is the only one of these parameters that is linear with temperature in the ductile region. A change in the mode of failure from ductile to brittle occurs between 5–30°C at a strain rate of O.1/in./in./min. At all temperatures studied, the yield or fracture stress varied linearly with the rate of deformation for strain rates ranging from 0.1 to 1.0 in./in./min. The yield data as a function of temperature were analyzed via a rate expression modified to incorporate the Coulomb-Navier yield criterion, Activation energy was found to be a function of deformation temperature with a change in slope occurring near the β transition. Activation volume increased linearly with deformation temperature, for the range studied. Agreement of dynamic mechanical and yield activation energies imply that the type of motion and the height of the energy barrier are similar for both. However, an increase in activation volume for stressed vs unstressed conditions suggests that a greater number of chain segments move as a result of stress biasing. Also the increase of both activation volume and activation energy with temperature implies that the correlated length of chain movement increases as temperature is increased. Similar to activation energy, yield stress exhibits a change in temperature dependence near the β transition. Data on other glassy polymers suggest that the highest temperature sub-T_g, transition is related to the change in the temperature dependence of yield stress.     

Dynamic mechanical properties of sisal fiber reinforced polyester composites fabricated by resin transfer molding

The dynamic mechanical properties of sisal fiber reinforced polyester composites fabricated by resin transfer molding (RTM) were investigated as a function of fiber content, frequency, and temperature. Investigation proved that at all temperature range the storage modulus (E′) value is maximum for the composites having fiber loading of 40 vol%. The loss modulus (E″) and damping peaks (tan δ) were lowered with increasing fiber content. The height of the damping peaks depends upon the fiber content and the fiber/matrix adhesion. The extent of the reinforcement was estimated from the experimental storage modulus, and it has been found that the effect of reinforcement is maximum at 40 vol% fiber content. As the fiber content increases the T_g from tan δ curve showed a positive shift. The loss modulus, storage modulus, and damping peaks were evaluated as a function of frequency. The activation energy for the glass transition increases upon the fiber content. Cole–Cole analysis was made to understand the phase behavior of the fiber reinforced composites. Finally, attempts were made to correlate the experimental dynamic properties with theoretical predictions. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effect of aging on the compressive yielding of PAN-based carbon fiber/polycarbonate composites

The compressive yield behavior and the effect of aging in boiling water on the mechanical properties of polycarbonate composites reinforced with PAN-based carbon fibers were studied at different filler contents and over a range of strain rates. The Young's modulus, yield stress, and yield strain are reported as a function of aging time. Other mechanical parameters such as the activation energy and volume of the yielding process were determined through the Eyring theory. The increase of both Young's modulus and yield stress with aging in boiling water is explained by structural changes. The mechanical properties of the composite were correlated with the morphology and its glass transition temperature.     

Tensile yield in poly(4-methyl pentene-1)

Uniaxial tension tests to the yield point were performed on a crystalline polymer, poly(4-methyl pentene-1) (PMP) as a function of temperature from 21° to 200°C at a strain rate of 2 min^?1. After testing, the specimens showed considerable stress whitening as a result of microvoid formation. Yield energy was found to be a linear function of temperature extrapolating to zero at the melting point (240°C). Thus, the behavior of this crystalline polymer is similar to that of glassy polymers, but with the melting temperature, rather than the glass transition temperature, as the reference point. The ratio of thermal to mechanical energy input to produce yielding is an order of magnitude smaller for PMP than it is for glassy polymers. The ratio of yield stress to Young's modulus is about 0.02, which is typical for polymers. Yield stress is a linear function of log strain rate, which implies that yielding can be described as a segmental flow rate process in which the applied stress biases the activation energy. The activation volume is on the order of 20 monomer unit volumes and increases as the temperature increases. The activation energy is 19 kcal/mol.     

Viscoelasticity of porous composite materials: Experimentals and theory

The viscoelasticity of latex/PS beads composites, having the same volume content of latex and PS beads, but various porosity ratios and specific surfaces of particles has been investigated in the glass temperature range of the latex matrix. In order to give evidence for the main factors governing the viscoelastic properties, data were compared to numerical simulations based on the Christensen and Lo model and Skorokhod's approach. Such modelings were performed by considering either the latex matrix of the PS beads as the continuous phase. It was concluded that the glassy modulus is mainly governed by the volume fraction of porosity, while the rubbery modulus is controlled by both the interactions between phases and the void volume content. The location of the mechanical relaxation seems to be governed by the mechanical coupling between phases, which implies some local phase inversion. The magnitude of the mechanical relaxation depends not only on the chemical coupling induced by the interactions between phases, but also on the mechanical coupling.     

Effect of layering pattern on dynamic mechanical properties of randomly oriented short banana/sisal hybrid fiber–reinforced polyester composites

Dynamic mechanical test methods have been widely employed for investigating the structures and viscoelastic behavior of polymeric materials to determine their relevant stiffness and damping characteristics for various applications. Randomly oriented short banana/sisal hybrid fiber–reinforced polyester composites were prepared by keeping the volume ratio of banana and sisal 1 : 1 and the total fiber loading 0.40 volume fraction. Bilayer (banana/sisal), trilayer (banana/sisal/banana and sisal/banana/sisal), and intimate mix composites were prepared. The effect of layering pattern on storage modulus (E′), damping behavior (tan δ), and loss modulus (E″) was studied as a function of temperature and frequency. Bilayer composite showed high damping property while intimately mixed and banana/sisal/banana composites showed increased stiffness compared to the other pattern. The Arrhenius relationship has been used to calculate the activation energy of the glass transition of the composites. The activation energy of the intimately mixed composite was found to be the highest. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2168–2174, 2005     

Temperature dependence of polyolefin melt rheology

The rheology of polymer melts depends strongly on temperature. Quantifying this temperature dependence is very important for fundamental, as well as practical, reasons. The purpose of this paper is to present a unified framework for handling the temperature dependence of rheological data. We considered the case (by far the most common in polymer melts) where all relaxation times (in the context of linear viscoelasticity) have the same temperature dependence (characterized by a “horizontal shift activation energy”) and all relaxation moduli have the same temperature dependence (characterized by a “vertical shift activation energy”). The horizontal and vertical activation energies were extracted from loss tangent vs. frequency and loss tangent vs. complex modulus data, respectively. This is the recommended method of calculation, as it allows independent estimation of the two activation energies (statistically uncorrelated). It was shown theoretically, and demonstrated experimentally, that neglect of the vertical shift leads to a stress (or modulus) dependent activation energy and necessitates different activation energies for the superposition of loss and storage modulus data. The long standing problem of a stress-dependent activation energy in long chain branched LDPE was identified as originating from the neglect of the vertical shift. The theory was applied successfully to many polyolefin melts, including HDPE, LLDPE, PP, EVOH, LDPE, and EVA. Linear polymers (HDPE, LLDPE, PP) and EVOH do not require a vertical shift, but long chain branched polymers do (LDPE, EVA). Steady-shear viscosity data can be superimposed using activation energies extracted from dynamic data.     

Some analogies in the mechanical behavior of filled polymers

The temperature and frequency dependences of complex shear modulus and mechanical losses were studied for expoxy resin composition in the presence of different amounts of quartz and polystyrene fillers. The data obtained were analyzed by the use of the Williams-Landell-Ferry method. It was shown that in the mechanical behavior of filled polymers, except for the well-known temperature-time analogy there exist some lows connected with the presence of filler. The change in filler concentration leads to the same change in the real part of complex modulus as change in frequency (concentration–time analogy), and change in temperature is equivalent to concentration change (temperature–concentration analogy). The existence of these analogies is explained by a change in deformation conditions for polymeric matrix in the presence of different amount of filler, by the existence of surface layers of polymer at the interface with solid filler, and by peculiarities of the mechanical behavior of filler. It is also established that the thickness of surface layer which was determined from experimental data depends on temperature and has an extremum in the temperature region of the α-transition.     

Influences of Grafted Side Chains on the Viscoelastic Behavior of Ternary Graft Copolymers

The novel ternary graft copolymers with long grafted side chains were prepared by solid phase grafting maleic anhydride (MAH), methyl methacrylate (MMA), and butyl acrylate (BA) onto polypropylene (PP). GPC was employed to show the length of the grafted side chains. The viscoelastic behavior was investigated by DMA. The results showed that the storage modulus and the loss modulus decreased with increase of the length of the grafted side chains but the α relaxation characterized by tanδ has no distinct change after various grafting. The distinct change of the internal damping of the graft copolymers was characterized by Cole-Cole curves. The storage modulus was successfully superposed according to the WLF and VFTH equations, which are suitable to describe the TTS principle for these ternary graft copolymers. When the weight percentage of MAH/MMA/BA was 6/6/2, the PP matrix had been overwhelmed by the grafted side chains, resulting in the change of the matrix properties and the dramatic increase of activation energy which was calculated by the Arrhenius equation. Meanwhile, the length of the grafted side chains was long enough to be discovered by TGA and DTG.     

Mechanical properties of borate crosslinked poly(vinyl alcohol) gels

Boric acid does not introduce crosslinks in poly(vinyl alcohol) solutions, but gelation does occur in the presence of cations. In this experimental study, the dynamic mechanical properties of these gels were determined using test-tube torsion pendulums and an air-bearing torsion pendulum. The modulus at a fixed concentration of polymer and boric acid increases with increasing sodium ion concentration up to the point where the atom ratio of sodium to boron reaches 1. Higher sodium concentrations do not increase the modulus. The log decrement, on the other hand, decreases with increasing sodium concentration continuously without reaching a plateau at the equal atom ratio. Log decrements as low as 0.02 can be measured. The storage modulus depends on the logarithm of borate concentration and on the 4.7 power of poly(vinyl alcohol) concentration. Only a very small portion of the borates in solution take part in effective crosslinks. The activation energy for breaking individual bonds in a function of temperature and the cation to boron ratios. At a fixed cation concentration, this activation energy is more negative with increasing amount of boron ions due to a transformation of monomeric crosslinks into polymeric crosslinks, so that the storage modulus which measures crosslink density decreases as a temperature rises.     

Antiplasticization behavior of polycaprolactone/polycarbonate-modified epoxies

Antiplasticization behavior was found in the polycaprolactone (PCL)/polycarbonate (PC)-modified epoxy system, cured with an aromatic amine. The initial modulus increased and the fracture toughness and the elongation at break decreased with the addition of the PCL/PC modifier. The glass transition temperature (T_g) decreased slightly. In this system, the antiplasticization phenomenon can be explained well by the formation of hydrogen bonding between the carbonyl groups in the PCL/PC and the hydroxyl groups in the epoxy. The hydrogen bonding proportion, as analyzed from Fourier transform infrared spectra, increased with the addition of PCL/PC up to 15 parts. This is consistent with the trend observed in the mechanical properties. It was thought that for antiplasticization to occur, a strong molecular interaction is necessary for a restriction of molecular motion, in turn decreasing the free volume of the matrix and thereby causing an increase in the modulus of the material. The dynamic mechanical thermal behavior of these systems was also studied. It was found that there was no change in the molecular weights between crosslinks, which excluded the effect of crosslink density on the changes of the properties. Instead, the activation energy was increased and the peak area was decreased for the β relaxation because of the restriction of the molecular motion by hydrogen bonding. The results indirectly support the hypothesis that the motion of the 2-hydroxypropyl ether is responsible for the β relaxation process.     

RDX/PEG悬浮液的流变性能

为获取RDX对固体火箭推进剂药浆流变性能的影响规律,采用稳态和动态流变学方法研究了RDX在硝酸酯增塑的聚乙二醇黏合剂体系中的流变特性、黏弹性及温度对悬浮液流变性能的影响.结果表明,随RDX含量的增加,颗粒间相互作用增强,从而导致悬浮液体系假塑性程度升高和损耗模量频率谱偏离线性程度增大.在RDX(I)悬浮液中RDX存在临界体积分数,且在硝酸酯增塑的聚乙二醇黏合剂体系中的表观黏度随温度的升高呈增加趋势,这与RDX的粒度和固-液界面相关.     

基于分子动力学模拟的CH4溶解对原油分子间作用的影响机制研究

为了研究甲烷（CH₄）溶解对原油分子间作用的影响机制,利用分子模拟方法,分别构建正庚烷与正庚烷、蜡分子、胶质、沥青质的最低能量构型,分析CH₄氛围对原油分子间相互作用的影响。以CH₄/原油分子体系模型为研究对象进行分子动力学模拟,考察CH₄溶解对原油分子体系黏度的影响,根据原油分子间相互作用、径向分布函数、体积应变、自扩散系数以及体系内聚能密度变化规律揭示CH₄溶解对原油分子间作用的影响机制,发现CH₄的溶解增大了原油分子间的间距,削弱了体系内原油分子间的范德华作用;体积膨胀为原油分子的热运动提供了更多的空间,原油分子热运动加剧,使得油品的流动能力增强;不同原油分子体系的黏度、径向分布函数、体积应变、自扩散系数以及内聚能密度的变化趋势类似,原油分子体系中蜡、胶质、沥青质的存在并未改变CH₄对原油分子间作用的影响机制。     

Effect of nano clay on the constrained polymer volume of chlorobutyl rubber nanocomposites

The dynamic mechanical properties of chlorobutyl rubber nanocomposites containing different varieties of clay have been investigated. The clay moieties have been chosen so that they vary in their organic modification, modifier concentration, and d spacing. The viscoelastic properties such as storage modulus, damping behavior, and loss modulus of polymer composites depends on matrix filler interaction, crystallinity, and extent of crosslinking. The prepared composites were characterized by X Ray Diffraction, and the extend of exfoliation/intercalation was studied. It has been observed that the storage modulus of the composites increased with the addition of filler due to the enhancement in stiffness of the material. The damping behavior was found to decrease with the addition of filler and this was attributed to the restricted movement of the polymer segments. The higher surface area to volume ratio of the layered silicate resulted in the better interaction between the polymer matrix and filler. The variation of loss as well as storage modulus of the nanocomposites were evaluated as a function of filler loading, and a comparison of the properties of the rubber nanocomposites containing different organic clay was also carried out. Finally, a calculation of constrained volume of polymer chains was done in the nanocomposites. POLYM. COMPOS., 36:2135–2139, 2015. © 2014 Society of Plastics Engineer     

Preparation of poly(p-phenylene terephthalamide)/poly(vinyl chloride) molecular composite and its thermal and mechanical properties

Poly(p-phenylene terephthalamide) (PPTA) was blended with poly(vinyl chloride) (PVC) by solution-blending method. PPTA was metalated for dissolving in dimethyl sulfoxide. Dimethyl sulfoxide was used as a common solvent. In PPTA/PVC composite, PPTA accelerated the thermal degradation of PVC. PPTA molecules are aggregated as microfibrillar form in PVC matrix. Such microfibrils are dispersed homogeneously in PVC matrix, according to polarizing microscopic observation. The average diameter of the microfibrils becomes smaller in the composite with lower content of PPTA. In the surface region of PPTA microfibrils the intermolecular hydrogen bonds between C? Cl of PVC and N? H of PPTA are formed. Young's modulus and the yield stress at room temperature were higher in the composites than those in PVC. The modulus of the composites was higher, especially at the high temperatures above their glass transition temperatures, than that in PVC. The temperature dependence of modulus can be calculated by using the mechanical model equivalent to the quasi-3-dimensional microfibrillar model which will be approximately applied to the composite structure. It becomes apparent that the modulus of the PPTA microfibrils evaluated by using the mechanical model is higher in the higher molecular weight PPTA.     

Effects of temperature and strain rate on the tensile behavior of unfilled and talc‐filled polypropylene. Part I: Experiments

The tensile behavior of unfilled and 40 w% talc‐filled polypropylene has been determined at four different temperatures (21.5, 50, 75 and 100°C) and three different strain rates (0.05, 0.5 and 5 min^?1). Experimental results showed that both unfilled and talc‐filled polypropylenes were sensitive to strain rate and temperature. Stressstrain curves of both materials were nonlinear even at relatively low strains. The addition of talc to polypropylene increased the elastic modulus, but the yield strength and yield strain were reduced. The temperature and strain rate sensitivities of these materials were also different. An energy‐activated, rate sensitive Eyring equation was used to predict the yield strength of both materials. It is shown that both activation volume and activation of energy increased with the addition of talc in polypropylene.     

Short Nylon Fibre Reinforced PP: Melt Rheology

Short fiber reinforced thermoplastics have generated much interest these days since fibrous materials tend to increase both mechanical and thermal properties, such as tensile strength, flexural strength, flexural modulus, heat deflection temperature, creep resistance, and some times impact strength of thermoplastics. If the matrix and reinforcement are both based on polymers the composite are recyclable. The rheological behavior of recyclable composites based on nylon fiber reinforced polypropylene (PP) is reported in this paper. The rheological behavior was evaluated both using a capillary rheometer and a torque rheometer. The study showed that the composite became pseudoplastic with fiber content and hence fiber addition did not affect processing adversely at higher shear rates. The torque rheometer data resembled that obtained from the capillary rheometer. The energy of mixing and activation energy of mixing also did not show much variation from that of PP alone.