Micromechanisms involved in the atypical tensile behavior observed in polyamide 11 at high temperature

Even far above the glass transition temperature, the amorphous phase in semicrystalline polymers is known to be constrained by crystals and less mobile than a pure amorphous polymer close to its equilibrium rubbery state. The aim of this paper devoted to Polyamide 11 was to investigate the existence and significance of a relaxed state in the amorphous phase of a semicrystalline polymer far above T_g. It focuses on the high temperatures, low strain‐rates, and small deformation ranges. A nonstrain‐rate dependent tensile curve (called “asymptotic curve”) was evidenced below a critical strain‐rate, consistently with reaching a fully relaxed state of the rubbery amorphous phase. Nevertheless, paradoxical mechanical features were observed at the same time (nonstrain‐rate dependent but hysteretic unloading, relaxation, and creep involving same strain‐rates as the asymptotic loading regime). Micromechanisms (orientation of primary crystals, creation of local hexagonal arrangements, orientation, and relaxation of the amorphous phase) were analyzed from DSC and X‐ray experiments. It suggested distinct amorphous and crystalline contributions depending on the loading path and therefore highlighted paradox of the mechanical behavior. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3046–3059, 2007     

The “Relaxed” state in a semicrystalline polymer: Experimental characterization and modeling

This work addresses the general issue of the mechanical behavior of the confined amorphous phase in rubbery semicrystalline polymers. Even far above the glassy transition temperature, the amorphous phase in semicrystalline polymers is known to remain constrained by crystals and is less mobile than a purely amorphous polymer close to its equilibrium rubbery state. The aim of this paper, based on Polyamide 11, is to investigate the existence and significance of a relaxed state in the amorphous phase of a semicrystalline polymer far above T _g. A strain-rate independent tensile curve (called the “asymptotic curve”) is evidenced below a critical strainrate, consistently with a fully relaxed state of the rubbery amorphous phase. Nevertheless, a contradictory mechanical phenomenology was observed at the same time (hysteretic unloading, relaxation, and creep involving the same strain-rates as the “asymptotic” loading regime), suggesting joint amorphous and crystalline processes. Modeling of this paradoxical behavior is attempted, based on the experimental results. The first one-dimensional simulations are presented. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 797–808. This article was submitted by the authors in English.     

Micromechanical modeling of the elastic properties of semicrystalline polymers: A three‐phase approach

The mechanical performance of semicrystalline polymers is strongly dependent on their underlying microstructure, consisting of crystallographic lamellae and amorphous layers. In line with that, semicrystalline polymers have previously been modeled as two and three‐phase composites, consisting of a crystalline and an amorphous phase and, in case of the three‐phase composite, a rigid‐amorphous phase between the other two, having a somewhat ordered structure and a constant thickness. In this work, the ability of two‐phase and three‐phase composite models to predict the elastic modulus of semicrystalline polymers is investigated. The three‐phase model incorporates an internal length scale through crystalline lamellar and interphase thicknesses, whereas no length scales are included in the two‐phase model. Using linear elastic behavior for the constituent phases, a closed form solution for the average stiffness of the inclusion is obtained. A hybrid inclusion interaction model has been used to compute the effective elastic properties of polyethylene. The model results are compared with experimental data to assess the capabilities of the two‐ or three‐phase composite inclusion model. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Sorption and swelling of semicrystalline polymers in supercritical CO2

The equilibrium sorption and swelling behavior of four different polymers—poly(methyl methacrylate), poly(tetrafluoroethylene), poly(vinylidene fluoride), and the random copolymer tetrafluoroethylene–perfluoromethylvinylether–in supercritical CO₂—are studied at different temperatures (from 40 to 80 °C) and pressures (up to 200 bar). Swelling is measured by visualization, and sorption through a gravimetric technique. From these data, the behavior of amorphous and semicrystalline polymers can be compared, particularly in terms of partial molar volume of CO₂ in the polymer matrix. Both poly(methyl methacrylate) and the copolymer of tetrafluoroethylene exhibit a behavior typical of rubbery systems. On the contrary, polymers with a considerable degree of crystallinity, such as poly(tetrafluoroethylene) and poly (vinylidene fluoride), show larger values of partial molar volume. These can be related to the limited mobility of the polymer chains in a semicrystalline matrix, which causes the structure to “freeze” during the sorption process into a nonequilibrium state that can differ significantly from the actual thermodynamic equilibrium. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1531–1546, 2006     

Understanding the effect of skin formation on the removal of solvents from semicrystalline polymers

The effect of glassy skin formation on the drying of semicrystalline polymers was investigated with a comprehensive mathematical model developed for multicomponent systems. Polymers with high glass‐transition temperatures can become rubbery at room temperature under the influence of solvents. As the solvents are removed from the polymer, a glassy skin can form and continue to develop. The model takes into account the effects of diffusion‐induced polymer crystallization as well as glassy–rubbery transitions on the overall solvent content and polymer crystallinity. A Vrentas–Duda free‐volume‐based diffusion scheme and crystallization kinetics were used in our model. The polymer–solvent system chosen was a poly(vinyl alcohol) (PVA)–water–methanol system. The drying kinetics of PVA films were obtained by gravimetric methods with swollen films with known water/methanol concentrations. The overall drying behaviors of the polymer system determined by our model and experimental methods were compared and found to match well. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3191–3204, 2005     

Inelastic neutron scattering from isotactic polypropylene

We used inelastic neutron scattering to probe the low‐energy excitations in semicrystalline isotactic polypropylenes with different degrees of crystallinity. The contributions from the amorphous and crystalline regions to the total scattering intensity were extracted under the assumption of a weighted linear contribution of the two regions in a simplified two‐phase system. The resulting intensity from the amorphous region showed a peak at 1.2 meV that was in good agreement with the previously determined boson peak characteristic of atactic polypropylene. The possibility of a contribution to the boson peak region by longitudinal acoustic mode modes that are characteristic of semicrystalline polymers and appear in the same low‐frequency region is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2852–2859, 2001     

Systematic coarse‐graining of semicrystalline polyethylene

In an effort to accelerate simulations exploring deformation mechanisms in semicrystalline polymers, we have created structure‐based coarse‐grained (CG) models of polyethylene and evaluated the extent to which they can simultaneously represent its amorphous and crystalline phases. Two CG models were calibrated from target data sampled from atomistic simulations of supercooled oligomer melts that differ in how accurately they represent the distribution of bond lengths between CG sites. Both models yield semicrystalline morphology when simulations are performed at ambient conditions, and both accurately predict the glass transition and melt temperatures. A thorough evaluation of the models was then conducted to assess how well they represent various properties of the amorphous and crystalline phases. We found that the model that more faithfully reproduces the target bond length distribution poorly represents the crystalline phase, which results from its inability to reproduce correlations in the structural distributions. The second model, which utilizes a harmonic bond potential and thus reproduces the target bond length distribution less accurately, represents the structure and chain mobility within the crystalline phase more realistically. Furthermore, the latter model more faithfully reproduces the vastly different relaxation timescales of the phases, a critical feature for modeling deformation mechanisms in semicrystalline polymers. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 331–342     

Viscoelastic behavior of semicrystalline polymers at elevated temperatures on the basis of a two‐process model for stress relaxation

Viscoelastic behavior at elevated temperatures of high‐density polyethylene and isotactic polypropylene was investigated by using the stress relaxation method. The results are interpreted from the view of an established two‐process model for stress relaxation in semicrystalline polymers. This model is based on the assumption that the stress relaxation can be represented as a superposition of two thermally activated processes acting in parallel. Each process is associated either with the crystal or amorphous phase of a polymer sample. It was found that the temperature dependence of viscosity coefficients and elastic moduli of these two fractions are similar in the two materials. The experimental data was correlated with literature data of α and β processes in polyethylene and polypropylene obtained from dynamic mechanical thermal analysis. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3239–3246, 2000     

Gas transport properties of substituted PEEKs

Tert-butyl and di-tert-butyl were added as pendant groups to the ether-ether phenyl ring of poly(ether ether ketone), PEEK. tert-butyl PEEK, TBPEEK, was amorphous and di-tert-butyl PEEK, DBPEEK, was semicrystalline. However, a 2 : 1 random copolymer of TBPEEK and DBPEEK, TBDBPEEK, was amorphous. Gas transport of N₂, O₂, CH₄, and CO₂ through amorphous films of PEEK, TBPEEK, TBDBPEEK, and tetramethylbiphenyl PEEK were determined at 35°C and at pressures to about 15 atm. The results support previous observations that tert-butyl and tetramethylbiphenyl groups are very effective in disrupting chain packing in the polymer. For the present polymers, these substitutions led to a 5–18-fold increase in permeability, and, in some cases, at no loss in permselectivity. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2355–2362, 1997     

Gas sorption and diffusion processes in polymer matrices at high pressures

A theoretical approach has been developed to describe the processes of gases diffusion and sorption in rubbery and glassy polymers. Various models (Flory-Huggins, dual-mode sorption, gas-polymer-matrix) used for interpreting the sorption-diffusion experiments are discussed within this approach framework. Experimental data on carbon dioxide sorption in glassy and rubbery polymers have been considered using the proposed approach. The comparison of the experimental and theoretical data has permitted to make the conclusion on the developed concepts adequacy. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1339–1348, 1997     

Micromechanical modeling of the deformation kinetics of semicrystalline polymers

The mechanical behavior of semicrystalline polymers is strongly dependent on their crystallinity level, the initial underlying microstructure, and the evolution of this structure during deformation. A previously developed micromechanical constitutive model is used to capture the elasto‐viscoplastic deformation and texture evolution in semicrystalline polymers. The model represents the material as an aggregate of two‐phase layered composite inclusions, consisting of crystalline lamellae and amorphous layers. This work focuses on adding quantitative abilities to the multiscale constitutive model, in particular for the stress‐dependence of the rate of plastic deformation, referred to as the slip kinetics. To do that, the previously used viscoplastic power law relation is replaced with an Eyring flow rule. The slip kinetics are then re‐evaluated and characterized using a hybrid numerical/experimental procedure, and the results are validated for uniaxial compression data of HDPE, at various strain rates. A double yield phenomenon is observed in the model prediction. Texture analysis shows that the double yield point in the model is due to morphological changes during deformation, that induce a change of deformation mechanism. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1297–1310, 2011     

Fast FTIR imaging: A new tool for the study of semicrystalline polymer morphology

The distribution of chemical species and the degree of orientation in semicrystalline polymer systems have been studied using fast Fourier transform infrared (FTIR) imaging. A variety of poly(ethylene glycol) systems, including pure polymer, high and low molecular weight blends, and blends with amorphous polymers, were studied. It is shown that fast FTIR imaging can be used to determine the distribution of species with different molecular weights and can be used to determine the degree of segregation of different components in blends with amorphous polymers. Additionally, by employing an infrared polarizer, the degree of orientation was determined in these systems by the generation of spatially‐resolved dichroic ratio images. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2353–2359, 1999     

Effect of diphenylsiloxane unit content on relaxation behavior of poly(dimethylsiloxane‐co‐diphenylsiloxane)

The relaxation behaviors of poly(dimethylsiloxane‐co‐diphenylsiloxane)s with different compositions were investigated using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). It is indicated that the content of Ph₂SiO unit, which is closely associated with crystallinity of polysiloxane, has a remarkable influence on its relaxation behavior. Two‐phase (crystalline and amorphous phase) structure in the semicrystalline polysiloxane of the present system can be determined for discussing relaxation behavior. An increase in relaxation strength can be reasoned to a cooperative effect of decrease in fraction of crystalline phase and increase in friction between molecular chains. And enhancements in glass transition temperature (T_g) and effective activation energy for glass transition (E_a(eff)) were ascribed more to the stiffness imposed by Ph₂SiO unit than decrease in fraction of crystalline phase. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1652–1659, 2008     

Elevation of the glass transition temperature in flexible‐chain semicrystalline polymers

In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T_g) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine T_g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T_g is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007     

On the crystallinity effect on the gas sorption in semicrystalline linear low density polyethylene (LLDPE)

This article describes the solubility of carbon dioxide, ethylene and propane in 1‐octene based polyethylene of 0.94, 0.92, 0.904, and 0.87 densities. The isotherms obtained in the gas sorption experimental device display a sorption behavior similar to that of glassy polymers. We apply the dual model to semicrystalline polymers assuming that Henry's sites are related to the amorphous phase, which decreases when the crystallinity percentage increases, whereas the surface of the crystalline phase acts as a Langmuir site with higher gas‐polymer affinity than glassy polymers. The good concordance of the calculated k_D values, using the Flory‐Huggins theory of polymer diluent mixtures, with the experimental results suggest that Henry's gas sorption fulfills this theory and, therefore, it may be a suitable way to estimate polymer‐gas enthalpic interactions. Particularly, the variation of k_D with the crystallinity fraction is exponential and the proportionality of the total sorption with the amorphous content seems only apparent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1798–1807, 2007     

Crystallinity effect on the gas transport in semicrystalline coextruded films based on linear low density polyethylene

This article describes the diffusion and permeability of oxygen, carbon dioxide, methane, ethane, ethylene, propane, and propylene in 1‐octene based polyethylene of densities 0.94, 0.92, 0.904, and 0.87. The isotherms obtained in the time‐lag experimental device display a diffusion coefficient and permeability behavior similar to that of glassy polymers. We apply the dual model to semicrystalline polymers assuming that Henry's sites are related to the amorphous phase, which decreases when the crystallinity percentage increases. Whereas the interphase of the polymeric matrix and the crystalline phase prevails and acts as Langmuir sites. Their effect is to increase both, the tortuosity of diffusion trajectories and the chain immobilization. We now explain this effect using thermodynamic considerations. In fact, the tortuosity is related to the change in activation entropy, and the chain immobilization to the cohesive energy of the polymeric matrix. In those terms, the diffusion coefficient does not follow the same crystalline percentage dependence than the solubility. According to the previous findings, the solubility changes in proportion to amorphous percentages. Instead, diffusion coefficient has exponential dependence. Furthermore, we show that the permeability changes as a consequence of the modification of diffusion and solubility, according to the product of both quantities. Comparison with previous publication has been included. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 634–642, 2010     

Effect of Aging on the Mechanical Properties of Polymeric Materials

Abstract

Aging can modify polymer structure at the molecular, macromolecular, and/or the morphological level and thus induce changes in the mechanical properties. Stiffness is generally not modified for nonrubbery materials, except for mass transfer (solvent plasticization or plasticizer loss) in amorphous polymers or phase transfer (crystallization or crystal destruction) in semicrystalline polymers. The most significant modulus changes occur in the radiochemical aging of semicrystalline polymers whose amorphous phase is in the rubbery state. Yield properties generally vary in the same way as stiffness. Physical aging at T < T _g can lead to a significant increase in the yield stress. Very general features can be observed for rupture properties, for instance: 1) Only ultimate elongation ε is a pertinent variable in kinetic studies of aging involving tensile testing and related methods, 2) the amplitude of ε variation for a given degradation conversion is considerably higher for initially ductile materials than for brittle ones, and 3) the rupture envelope σ = f(ε), i.e., the ultimate stress, is often very close to the initial tensile curve except for rubbery materials undergoing predominant crosslinking. The mechanisms of ultimate property changes are reviewed. A kinetic approach is proposed for the very important case of heterogeneous, diffusion-controlled aging.     

Molecular dynamics simulation of water diffusion inside an amorphous polyacrylate latex film

Molecular dynamics simulations were applied to investigate the diffusion behaviors of water molecules at temperatures ranging from 323 to 443 K inside amorphous polyacrylate. The results showed that the simulated diffusion coefficients and activation energies were similar to those of experiments. Moreover, the activation energy of water molecules at high temperatures was higher than that at low temperatures by 3.16 kcal mol^?1, which was close to the hydrogen‐bonding energy between water and polyacrylate. An analysis of the experimental desorption curves of water molecules and their activation energies has confirmed that there are two forms of water molecules inside rubbery polyacrylate, namely, free water and bound water. In addition, it has been concluded that bound water molecules move from one polar group of polyacrylate to another, and this is followed by occasional jumps. Simulated information is very helpful in designing new polyacrylate latex systems and optimizing existent polyacrylate systems. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 884–891, 2007     

The influence of cavitation phenomenon on selected properties and mechanisms activated during tensile deformation of polypropylene

The cavitation phenomenon accompanies the tensile deformation of most semicrystalline polymers when negative pressure inside the amorphous phase is generated. Over the years, this phenomenon has been marginalized, due to the common belief that it does not have any significant influence on the properties or micromechanisms activated during plastic deformation of such materials. In this article, for the first time, the influence of the cavitation phenomenon on the value of yield stress/strain, the intensity of the lamellae fragmentation process, the reorientation dynamics of the crystalline and amorphous component, the degree of crystals orientation at selected stages of deformation, and the amount of heat generated as a result of activating characteristic micromechanisms of plastic deformation were systematically analyzed. The research has been conducted for cavitating/non‐cavitating polypropylene model systems with an identical structure of crystalline component during their tensile deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1853–1868     

A morphological study of membranes obtained from the systems polylactide-dioxane-methanol,polylactide-dioxane-water and polylactide-N-methyl pyrrolidone-water

The influence of liquid–liquid demixing, solid–liquid demixing, and vitrification on the membrane morphologies obtained from several polylactide-solvent-nonsolvent systems has been investigated. The polymers investigated were the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA). The solvent-nonsolvent systems used were dioxane-water, N-methyl pyrrolidone-water and dioxane-methanol. For each of these systems it was attempted to relate the membrane morphology to the ternary phase diagram at 25°C. It was demonstrated that for the amorphous poly-DL-lactide the intersection of a glass transition and a liquid–liquid miscibility gap in the phase diagram was a prerequisite for the formation of stable membrane structures. For the semicrystalline PLLA a wide variety of morphologies could be obtained ranging from cellular to spherulitical structures. For membrane-forming combinations that show delayed demixing, trends expected on the basis of phase diagrams were in reasonable agreement with the observed membrane morphologies. Only for the rapidly precipitating system PLLA-N-methyl pyrrolidone-water were structures due to liquid–liquid demixing obtained when structures due to solid–liquid demixing were expected. Probably, rapid precipitation conditions promote solid–liquid demixing over liquid–liquid demixing, because the activation energy necessary for liquid–liquid demixing is lower than that for crystallization. © 1996 John Wiley & Sons, Inc.