Intrinsic Deformation Behavior of Semicrystalline Polymers (original) (raw)
Related papers
The Journal of Physical Chemistry B
A novel method is presented to build semi-crystalline polymer models used in molecular dynamics simulations. The method allows controlling certain aspects of the molecular morphology of the material. It relies on the generation of the polymer sections in the amorphous phase of the semi-crystalline structure according to the statistical polymer physics theory proposed by Adhikari and Muthukumar. 1 The amorphous phase is first built based on the method initially developed by Theodorou and Suter. 2 Then, the amorphous phase is stacked between crystallites, and a connection algorithm proposed by Rigby et al., 3 initially developed to build polymer thermosets, is employed to link two phases. For a given set of crystallinity degree, semi-crystalline long period, densities of the crystalline and amorphous phases and polymer molecular weight, the characteristic ratio is used to control the relative fractions of different types of polymer sections in the amorphous phase as well as the distribution of their lengths. There are three types of amorphous polymer sections: the ones that are reentering in the same crystallite called loops, those that are bonding two different crystallites called tie chains, and the chain tails ending in the amorphous region. The higher this characteristic ratio is, the higher the fraction of tie chains is. The full implementation of the theory is described and then applied to High-Density PolyEthylene (HDPE). Several samples are generated. The obtained structures are characterized. Their elastic coefficients are computed, and high uniaxial deformations are performed. It is shown that the higher the crystallinity degree, the higher the elastic coefficients. An entanglement analysis shows that the quantity of tie chains is more decisive than the entanglements in acting as stress transmitters to rigidify the structure.
Deformation of semicrystalline polymers via crystal–crystal phase transition
Journal of Polymer Science Part B: Polymer Physics, 1988
A classification is given of flexible, semicrystalline polymers based on the early stage deformation behavior in the solid state. The criteria of each category are discussed with experimental evidence from the literature on 17 polymers. The central aim of this classification is to point out the possibility of ductility induced during deformation. The understanding of the induced ductility in a given semicrystalline polymer suggests a systematic route to optimize solid-state deformation processes for achieving high draw ratios.
Structural mechanics of semicrystalline polymers prior to the yield point: a review
Journal of Materials Science, 2012
The review focuses on the current studies of the deformation response and accompanying structural transformations of thermoplastic semicrystalline polymers subjected to uniaxial tension prior to the yield point. The mechanisms of strain-induced cavitation of amorphous layers and damages of crystalline lamellae are analyzed in line with novel results on the deformation behavior of solid polymers at temperatures exceeding the glass transition point. The coupling of viscoelastic and plastic deformation mechanisms with the small-strain structural transformations is critically discussed on the basis of the advanced theoretical modeling of mechanical properties of semicrystalline polymers.
Polymer, 2017
The influence of microstructure and temperature on the initiation of yield stress and strain of highdensity polyethylene are examined using a set of linear and branched polyethylenes. The polymers were crystallized in different ways in order to get samples covering the range of crystallinity 0.5 X c 0.8 and crystallite thickness 8 nm L c 29 nm. In contrast to the conventional macroscopic yield strain and stress, the initiation strain ε yi and stress s yi were estimated from the macroscopic stress-strain curves at the onset of local plasticity as judged from in situ WAXS experiments upon tensile deformation. Phenomenological linear relation was observed between s yi crystallinity at each draw temperature T d. The dislocation model was applied to check the correlation between s yi and crystal thickness. In order to also account for the chain topology, namely the concentration of stress transmitters ST, a modified Eyring's approach was proposed. This modelling provides a good prediction the s yi dependence on L c and ST in the context of thermally activated rate processes. Finally, anelastic stress gauge, s c cr , was determined from local strain measurements in crystals at the same local strain as for s yi. This critical elastic stress at initiation of crystal plasticity displayed a good correlation with s yi at high crystallinity. However, s c cr was found to deviate from s yi with increasing T d particularly at low X c values. This finding was attributed to the activation of the crystalline mechanical relation that involves a significant drop of s yi with increasing T d in the crystalline lamellae under shear yielding whereas it does not affect the theoretical s c cr elastic stress.
Journal of Materials Science, 2000
The small strain (below yielding) tensile loading-unloading tests were carried out on the low-density polyethylene (LDPE) and polypropylene (PP) at low strain rate and room temperature. The experiments unambiguously indicate to a remarkable decrease in residual strains in comparison with those predicted by conventional viscoelastic models. These deviations cannot be explained without taking into account structural transformations of semi-crystalline polymers. As long as small deformations cannot result in significant change in content and texture of crystalline and amorphous components, it was assumed that such transformations should include disintegration of connectivity in crystallite clusters. This structural rearrangement is supposed to be caused by the strain-induced decrystallization of narrow (and thus highly stressed) "bridges" connecting domains of conjugated crystallites or inside crystallites. A simple 1D modelling of the deformation processes supports this expectation. The disconnection in polymer morphology is simulated by small portions of amorphous ligaments appearing between neighbouring crystallites in the course of deformation. In spite of simplicity of the model a precise fitting of the stress-strain diagram is obtained along with small variations in structural and material characteristics (crystallinity degree, effective rigidity and plastic ability) of the concerned polymers.
Journal of Polymer Science Part B: Polymer Physics, 2014
In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high-density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small-angle X-ray scattering (SAXS) and wideangle X-ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45 with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress-induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45 with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. V
Polymer, 2018
To establish relationships between the molecular structure of polyolefines and their physical characteristics which determine possible commercial applications, structural changes and tensile deformation response up to deformations beyond the natural draw ratio were investigated using a variety of experimental approaches. True stress-strain curves were measured at different temperatures so as to estimate the available effective network density, which will eventually define the failure mode of the material under investigation. Analysis of the deformation by means of tensile strain hardening, assuming the Haward-Thackray spring dashpot decoupling assumption by means of Edward-Vilgis' non-Gaussian rubber-elastic slip-link model, reveals the role of transient and fixed network nodes. It was established by differential scanning calorimetry and X-ray diffraction analysis that the transformation from lamellar to fibrillar morphology passes through the several pronounced stages: deformation of initial lamellae ( < 1.5); destruction of lamellar structure through the tilt; slippage of molecules in the crystallites; simultaneous formation of fibrils with structural characteristics depending on the molecular structure and on deformation conditions; deformation of the formed fibrillar structure; tiltingformation of chevrons for high molecular weight low density polyethylene or slippage of fibrils and void formation. Distinction between fixed and transient slip link network contributions reveals neatly that although there is a slight drop in the fixed link network density with increasing temperature, this contribution remains Manuscript Click here to view linked References
Journal of Polymer Science Part B: Polymer Physics, 1988
Interactions between rheology and fluid solid transformations through crystallization are demonstrated through an experimental study of crystallization of polyethylene terephthalate in uniaxial stress fields. Effect of the strength of the stress field on the kinetics of crystallization is shown to diminish greatly beyond the initiation of crystallization. Consequences of crystallization in anisotropic stress fields in the generation of mechanical properties are also described. This experimental study points clearly to the deficiencies in current theoretical formulations of kinetics of crystallization.