Molecular Simulations of the Interlamellar Phase in Polymers:  Effect of Chain Tilt

Gautam, Siddharth; Balijepalli, Sudhakar; Rutledge, Gregory C.

doi:10.1021/ma0012503

Cited by 127 publications

(142 citation statements)

References 26 publications

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“…23 The decay in density in the interfacial region for the branched systems was sharp compared to that for the unbranched system. This density decay depends on the local packing of the chains near the crystal surface and also on the "chain flux" through the interface, which in turn is governed by the populations and butyl-branched (green) systems were modeled using the TRaPPE-UA force field.…”

Section: Resultsmentioning

confidence: 93%

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Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

2017

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We use molecular simulations with a united atom force field to examine the effect of short chain branching (SCB) on the noncrystalline, interlamellar structure typical of linear low density polyethylene (LLDPE). The model is predicated on a metastable thermodynamic equilibrium within the interlamellar space of the crystal stack and accounts explicitly for the various chain topologies (loops, tails, and bridges) therein. We examine three branched systems containing methyl, ethyl, and butyl side branches, and compare our results to high density polyethylene (HDPE), without branches. We also compare results for two united atom force fields, PYS and TraPPE-UA, within the context of these simulations. In contrast to conventional wisdom, our simulations indicate that the thicknesses of the interfacial regions in systems with SCB are smaller than those observed for a linear polyethylene without branches, and that branches are uniformly distributed throughout the interlamellar region. We find a prevalence of gauche states along the backbone due to the presence of branches, and an abrupt decrease in the orientational order in the region immediately adjacent to the crystallite.3 Introduction:

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Section: Resultsmentioning

confidence: 93%

“…[22][23][24] The method has been applied to linear HDPE 25,26 and to isotactic polypropylene, with good results. 27 More recently, representative structures generated by this method have been used as the starting point for large strain deformations of HDPE using nonequilibrium molecular dynamics.…”

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confidence: 99%

Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

2017

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“…As noted previously, 12 cyclic chains (or cycles) containing 2 or more crystal stems can be formed as illustrated in Figure 2 short segments, longer loops and tails were present, as expected according to topological equilibration. 18 These segments are expected to play an important role in the mechanical properties of semicrystalline PE through their entanglements.…”

Section: Interphase Monte Carlo Equilibrationmentioning

confidence: 99%

Mechanical and Structural Characterization of Semicrystalline Polyethylene under Tensile Deformation by Molecular Dynamics Simulations

2015

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ABSTRACT. We have studied tensile deformations of semicrystalline polyethylene (PE) with molecular dynamics simulations at two different strain rates and temperatures. Compared to earlier studies, the modeled systems were approximately five times larger, which allowed significantly larger strains up to about 120% to be examined. Two different modes of structural transformation of semicrystalline PE were observed at the higher temperature of 350 K, depending on the strain rate. At the faster strain rate of 5×10 7 s -1 , cavitation in the non-crystalline region dominated, with little change in the crystalline region, resulting in monotonically declining stress with increasing strain after the yield point. However, in a small number of cases, 2 significant deviations from the average stress-strain profile were observed that correlated with topological constraints, such as bridges and bridging entanglements connecting crystalline regions separated by the non-crystalline region, and destabilization of the crystalline region. At the slower strain rate of 5×10 6 s -1 , we observed repeated melting/recrystallization events and significant oscillations in stress associated with variations of density in crystalline and noncrystalline regions and the displacement of polymer chains from crystalline to non-crystalline regions. When averaged over an ensemble of starting configurations for semicrystalline PE, the oscillations were found to be less coherent from microstate to microstate and offset one another.The post-yield stress became notably smoother, and began to resemble the plastic flow observed macroscopically, followed by stress hardening at the later stage of deformation. At the lower temperature of 250 K, cavity formation was the only mechanism observed, for both strain rates.The interplay between the thermodynamic stability of the crystalline region and the topological constraints imposed by bridges and entanglements in the non-crystalline region is crucial to understanding structural transformations of semicrystalline PE during tensile deformations.

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“…The modeling of such defects is at present still best done with force field based approaches. 3,[31][32][33] The calculations show that even if synthesis of pure crystalline phases of nylon6 were possible, the defects are likely to be induced at room temperature, because vacancy formation energies in both phases (0.66 and 0.64 kcal/mol per monomer in α and γ phases, respectively) are of the order of kT at room temperature. This study does not however consider the kinetics of defect formation.…”

Section: Discussionmentioning

confidence: 99%

Defects in alpha and gamma crystalline nylon6: A computational study

Arabnejad

Manzhos

2015

AIP Advances

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We present a comparative Density Functional Tight Binding study of structures, energetics, and vibrational properties of α and γ crystalline phases of nylon6 with different types of defects: single and double chain vacancies and interstitials. The defect formation energies are: for a single vacancy 0.66 and 0.64 kcal/mol per monomer, and for an interstitial strand 1.35 and 2.45 kcal/mol per monomer in the α and γ phases, respectively. The presence of defects does not materially influence the relative stability of the two phases, within the accuracy of the method. The inclusion of phononic contributions has a negligible effect. The calculations show that even if it were possible to synthesize the pure phases of nylon6, the defects will be easily induced at room temperature, because vacancy formation energies in both phases are of the order of kT at room temperature. The formation of interstitial defects, on the contrary, requires the energy equivalent to multiple kT values and is much less likely; it is also much less probable in the γ phase than in α. The vibration spectra do not show significant sensitivity to the presence of these defects.

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Molecular Simulations of the Interlamellar Phase in Polymers: Effect of Chain Tilt

Cited by 127 publications

References 26 publications

Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

Mechanical and Structural Characterization of Semicrystalline Polyethylene under Tensile Deformation by Molecular Dynamics Simulations

Defects in alpha and gamma crystalline nylon6: A computational study

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