Phase Of Semicrystalline Polymers Research Articles

Retrospecting the relationship between interlamellar amorphous structure and intrinsic modulus of semicrystalline polymers via in-situ positron annihilation lifetime spectroscopy and in-situ X-ray scattering

The structure-property relationship of the interlamellar amorphous phase in semicrystalline polymers has been a fundamental yet controversial topic in polymer physics for several decades. The intricate hierarchical structure and limited availability of experimental techniques pose significant challenges in quantitatively characterizing the evolution of the amorphous structure and establishing its connection to the mechanical behavior. In this work, the unannealed and annealed isotactic polypropylene hard-elastic films composed of idealized series-arranged lamellar stacks are studied to simplify the mechanical coupling between the two phases. By utilizing the synchrotron-based in-situ small and wide-angle X-ray scattering (SAXS/WAXS) techniques, the apparent modulus and overall volumetric strain of the amorphous phase are estimated. Moreover, with our significant progress in the counting rate of positron annihilation lifetime spectroscopy (PALS), the minute-scale structure evolution of amorphous phase (free-volume pores) during the tensile process is now feasible to be captured. Combing in-situ X-ray scattering and in-situ PALS results, in the elastic regime, it is found that triaxial stress in the amorphous phase leads to the expansion of free-volume pores (about 23% volume increase for the annealed sample, 35% volumetric strain for the unannealed one) and density decrement within occupied volume by overcoming Van Der Waals force. Our quantitative analysis also reveals that the annealing process not only enhances the crystallinity and aspect ratio of lamellae but also weakens the intrinsic modulus and chain packing density of amorphous phase.

Thermal and structural properties of polyzwitterions: Effects of monomer chemistry and salt addition

Thermal and structural properties of polyzwitterions (PZI)s comprising the sulfobetaine moiety were studied as a function of LiCl addition and variations in monomer chemistry. The polyzwitterions chosen for this study are: poly(sulfobetaine methacrylate), PSBMA; poly(sulfobetaine acrylate), PSBA; and, poly(ethyl sulfobetaine methacrylate), PESBMA. When LiCl salt is added, the Li+and Cl− ions disrupt the dipole-dipole electrostatic interaction of the PZI side groups. Fourier transform infrared spectroscopy shows that the vibrational frequencies of the symmetric and asymmetric S=O bonds increase as LiCl content increases. Structural studies by wide angle X-ray scattering show that all the PZIs are amorphous, and the average interatomic spacing, d, increases in the order: dPSBA(= 0.456±0.006nm) < dPSBMA (= 0.474±0.015nm) < dPESBMA (= 0.493±0.011nm). Upon the addition of LiCl, d-spacing for a given PZI decreases as LiCl content increases. Thermogravimetry showed that PZIs had degradation onset temperatures Td ∼ 300°C. As LiCl content increases, the onset temperature of degradation decreases suggesting that the disruption of dipole-dipole crosslinks destabilizes the polymer. All the PZIs are hydrophilic and hygroscopic, and a drying procedure was developed so that well-defined glass-to-liquid transitions could be observed in the dry solid state. PZIs without LiCl have glass transition temperatures, Tg, increasing in the order: Tg,PSBA(= 200±1°C) < Tg,PESBMA (= 204±1°C) < Tg,PSBMA (= 237±1°C). As the LiCl content increases, Tg decreases, and the liquid state heat capacity increases, in all PZIs. The specific heat capacity increment at Tg was studied for PSBA as a function of LiCl content. In PSBA, the heat capacity increment reached a maximum for molar ZI:LiCl content at and above 1:1, and its value agreed well with predictions based on an empirical bead model of polymers. Thermal data on the dry state of PZI-LiCl complexes suggest an analogy between electrostatically crosslinked, fully amorphous PZIs and semicrystalline polymers. The dipolar crosslinks confine the molecular motion of PZIs just as crystal domains confine the molecular motion of the amorphous phase in semicrystalline polymers.

A modified non-equilibrium lattice fluid model based on corrected fractional free volume of polymers for gas solubility prediction

We propose a model based on non-equilibrium lattice fluid (NELF) theory and corrected fractional free volume of polymers to effectively and accurately predict the solubility of gases in different polymers. The method to achieve this purpose is based on the utilization of NELF model infinite dilution solubility coefficient (S0) as the base of predictive calculations. To account for the isolated pore in the polymer matrix in density estimation, a fractional free volume correction factor (β) was introduced in NELF model. The modified NELF model was successfully applied for prediction of solubility of C3H8 and CO2 in polyethylene oxide (PEO) and CO2 in polyethylene terephthalate (PET), isotactic polypropylene (i-PP), polyetherimide (PEI), polymethyl methacrylate (PMMA) and polyethyl methacrylate (PEMA) with adjustments in β value and depth of diffusion of gases in polymer matrix (ζ) at different pressures and temperatures. This work involves multi-objective optimization using genetic algorithm of MATLAB toolbox with adjusted settings. It applies to find the optimum temperature at which the minimum standard deviation of β for different gas-polymer systems is obtained. β showed the same trend of change with temperature as the constrained pressure imposed on the amorphous phase in semi-crystalline polymers. A cubic correlation for standard deviation for β versus temperature was obtained which was able to anticipate the changing trend of β at different temperatures. The chi-square test results verified that compared with original NELF model, a more accurate model for prediction of gas solubilities in polymers has been proposed.

Mechanical investigation of confined amorphous phase in semicrystalline polymers: Case of PET and PLA

The effect of confinement onto the mechanical properties of the amorphous phase of Polyethylene terephthalate (PET) and poly(lactic acid) (PLA) was investigated. These polymers have the advantage of being in bulk amorphous or in semicrystalline state allowing mechanical and physical investigation of the amorphous phase on bulk and confined configuration. Based on small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) experiments, the micro-structural arrangement of the amorphous and crystalline phase, the rigid amorphous fraction, and the visco-elastic mechanical properties of the different semicrystalline samples were investigated. DSC results help quantifying the rigid amorphous fraction dependence on the crystallinity. DMA measurements lead to quantify the viscoelastic properties of the free and confined amorphous phases for PET and PLA polymers. Indeed, based on the DMA tests, where the maximum of tan(δ) peak is usually related to the glass transition temperature, shifts upon crystallization, the mechanical properties of the restricted and mobile amorphous phase were determined. This result was correlated along with the amorphous phase thickness distribution determined by SAXS results. This observation was bolstered based on literature results about geometrical confinement configurations and their effect on the glass transition temperature of polymeric materials. POLYM. ENG. SCI., 55:397–405, 2015. © 2014 Society of Plastics Engineers

Micro-mechanical analysis of the confined amorphous phase in semi-crystalline polymers

The mechanical modelling of semi-crystalline polymers is highly complex due to their different micro-structural scales and to some lacks regarding the understanding of the respective contributions of crystalline and amorphous phases on the macroscopic behaviour. In particular, the important role of the confined amorphous phase has to be investigated. To this aim, this work proposes to combine full-field simulations with kinematics field measurements at the same scale. The material is the linear low density polyethylene (LLDPE), chosen for its “simple” microstructure. Displacement fields at the micro-scale are measured by using the digital image correlation technique in the SEM. It is scheduled to apply the experimental strain as periodic boundary conditions to a unit cell whose morphological content will be built from the observed real microstructure and to analyse both the resulting macroscopic average response and local fields. To this aim, specific numerical tools are developed notably for statistical field analysis. The results on a simple stack model under various loading cases/paths are presented as a preliminary stage before the complete analysis for the LLDPE under tensile tests. The long term aim is to better understand the strain mechanisms in the confined amorphous phase, to determine its mechanical behaviour and conclude on its influence on the macroscopic behaviour.

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.

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

AbstractEven 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 Tg. 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

Effects of fiber aggregates on the transverse mechanical behavior of commingled PP–glass fiber composites

AbstractIn previous articles, mechanical models were proposed to predict the reinforcement effect of polymers by particulates as well as unidirectional fibers over wide ranges of volume fraction of fillers and temperatures. On the basis of image analyses and the definition of representative morphological motifs, these models are able to predict the viscoelastic properties of quasi‐isotropic and unidirectional composites or to extract the behavior of a phase, such as the interphase in filled rubbers or the transcrystalline phase in semicrystalline polymers. In this work, based on a 2D image processing, this approach is extended to predict the viscoelastic properties of commingled PP–glass fiber composites. It is shown that fiber aggregates, composed of fibers and surrounding polymer, might be considered as the reinforcing phase. In addition, the different failure modes of these composites are separated as a function of the volume fraction of fillers or temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3466–3476, 2006

Glass Transition Temperature and the Nature of the Amorphous Phase in Semicrystalline Polymers: Effects of Drawing, Annealing and Hydration in Polyamide 6

The influence of the organization of the amorphous chains segments on the glass transition temperature (Tg) in semicrystalline polymers is analysed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystaline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the Tg. Tg increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on Tg and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of Tg can be understood in terms of a two Tg model in which the oriented amorphous component has a higher Tg than the unoriented component.

Pre‐ordering state in the crystallization process of polymer blends detected by pulsed NMR

AbstractReal time measurement by pulsed NMR revealed that poly(ϵ‐caprolactone)/polystyrene (PCL/PS) blends and PCL homopolymer form a pre‐ordering state at the onset of crystal formation. The spin‐spin relaxation signals were decomposed into three components. When the intermediate phase appeared, its fractional amount was larger than that of the interfacial phase in semicrystalline polymers and the spin‐spin relaxation time was longer than that of the interface. The fractional amount decreased in the first stage of crystallization and then increased as the process proceeded. This behavior was more significant for blends with higher PS content or those crystallized at elevated temperatures. This state is assigned to a pre‐ordering state of the crystalline polymer that has to exclude the amorphous polymer on the formation of lamellae.