Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow

Anwar, Muhammad; Berryman, Joshua T.; Schilling, Tanja

doi:10.1063/1.4896568

Cited by 65 publications

(102 citation statements)

References 80 publications

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“…When we advance from the supercooled melt towards the formation of the critical nucleus at t 0 , we observe first an increase in the global orientational order S 2 , then an increase in the radius of gyration of the segments, and finally the local density increases and the crystal structure is formed. The mechanism we observed here is the same as the one we have found in short polymer chains [30,58]. We conclude that, at 280 K the undercooling is so strong that nucleation is a local event which does not depend on the chain length.…”

Section: Nucleation Mechanismsupporting

confidence: 70%

“…We observe that nucleation is initiated by the alignment of the chain segments which participate in the formation of critical nucleus, similar to the mechanism previously observed for shorter chains [30,58]. During growth, the segments of the chains which are close to the growth front align with the long axis of the stems to attach to the cluster and then thicken by sliding/ slipping from the amorphous regions into the crystalline regions.…”

Section: Discussionsupporting

confidence: 54%

“…In order to differentiate crystallites from the melt, we used several order parameters (we have previously applied the same approach to a melt of shorter chain alkanes [30,58], thus we only briefly summarize it here. For details we refer the reader to ref.…”

Section: Model and Order Parametersmentioning

confidence: 99%

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Crystallization of polyethylene: A molecular dynamics simulation study of the nucleation and growth mechanisms

Anwar

Schilling

2015

Polymer

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a b s t r a c tWe have performed molecular dynamics simulations to study the mechanism of crystallization from an undercooled polyethylene (C500) melt. We observe that crystal nucleation is initiated by the alignment of chain segments, which is followed by straightening of the chains and densification. Growth procedes via alignment of segments, which are in the vicinity of the growth front, with the chains in the crystalline lamella. Once chains are attached, the lamella thickens by sliding of the segments along the long axis of the chain from the amorphous regions into the crystalline regions. We do not observe the formation of any folded precursors.

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Section: Nucleation Mechanismsupporting

confidence: 70%

Section: Discussionsupporting

confidence: 54%

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Crystallization of polyethylene: A molecular dynamics simulation study of the nucleation and growth mechanisms

Anwar

Schilling

2015

Polymer

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“…We have traced this deviation in part to the overly large time step of 5 fs used in that work; such a large time step was reported by Waheed et al 37 to increase the chain relaxation time by only 10%, but its effect in suppressing nucleation is apparently more dramatic, compared to the more accurate rRESPA scheme used in this work. Our estimate for nucleation rate is also higher than the rate of 105 × 10 30 m −3 s −1 reported by Anwar et al at 250 K. 43 In this case, we suspect that the discrepancy is due to the application of the MFPT analysis method proposed by Wedekind et al 14 which employs the single-exponential approximation; as shown for our system in Table I, the singleexponential approximation results in an underestimation of the true nucleation rate. From the analysis herein, we conclude that the single exponential approximation should not hold for their conditions, given that their system is larger and the supercooling is deeper, both of which are expected to give rise to a greater degree of non-exponential behavior in the FPTD.…”

Section: Case Study: N-eicosane Nucleationcontrasting

confidence: 47%

Analysis of nucleation using mean first-passage time data from molecular dynamics simulation

Nicholson

2016

The Journal of Chemical Physics

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We introduce a method for the analysis of nucleation using mean first-passage time (MFPT) statistics obtained by molecular dynamics simulation. The method is based on the Becker-Döring model for the dynamics of a nucleation-mediated phase change and rigorously accounts for the system size dependence of first-passage statistics. It is thus suitable for the analysis of systems in which the separation between time scales for nucleation and growth is small, due to either a small free energy barrier or a large system size. The method is made computationally practical by an approximation of the first-passage time distribution based on its cumulant expansion. Using this approximation, the MFPT of the model can be fit to data from molecular dynamics simulation in order to estimate valuable kinetic parameters, including the free energy barrier, critical nucleus size, and monomer attachment pre-factor, as well as the steady-state rates of nucleation and growth. The method is demonstrated using a case study on nucleation of n-eicosane crystals from the melt. For this system, we found that the observed distribution of first-passage times do not follow an exponential distribution at short times, rendering it incompatible with the assumptions made by some other methods. Using our method, the observed distribution of first-passage times was accurately described, and reasonable estimates for the kinetic parameters and steady-state rates of nucleation and growth were obtained. C 2016 AIP Publishing LLC. [http://dx

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“…At the molecular level, there have been simulation efforts using both Monte Carlo (MC) [13][14][15] and Molecular Dynamics (MD) [16][17][18][19][20][21] to study FIC; however, relatively few focused specifically on the kinetics of nucleation. 22 Given that nucleation is a transient event, a MC simulation cannot accurately capture its kinetics without an enormous amount of prior knowledge regarding the dynamic processes in the melt and their rates. For this reason, we believe that MD, which includes all of the classical physics of interacting particles, is the more attractive method for kinetic nucleation studies, where feasible, and directs readers to reviews that cover the application of MC methods to study FIC.…”

Section: Introductionmentioning

confidence: 99%

Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension

Nicholson

Rutledge

2016

The Journal of Chemical Physics

118

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Non-equilibrium molecular dynamics is used to study crystal nucleation of n-eicosane under planar shear and, for the first time, uniaxial extension. A method of analysis based on the mean first-passage time is applied to the simulation results in order to determine the effect of the applied flow field type and strain rate on the steady-state nucleation rate and a characteristic growth rate, as well as the effects on kinetic parameters associated with nucleation: the free energy barrier, critical nucleus size, and monomer attachment pre-factor. The onset of flow-enhanced nucleation (FEN) occurs at a smaller critical strain rate in extension as compared to shear. For strain rates larger than the critical rate, a rapid increase in the nucleation rate is accompanied by decreases in the free energy barrier and critical nucleus size, as well as an increase in chain extension. These observations accord with a mechanism in which FEN is caused by an increase in the driving force for crystallization due to flow-induced entropy reduction. At high applied strain rates, the free energy barrier, critical nucleus size, and degree of stretching saturate, while the monomer attachment pre-factor and degree of orientational order increase steadily. This trend is indicative of a significant diffusive contribution to the nucleation rate under intense flows that is correlated with the degree of global orientational order in a nucleating system. Both flow fields give similar results for all kinetic quantities with respect to the reduced strain rate, which we define as the ratio of the applied strain rate to the critical rate. The characteristic growth rate increases with increasing strain rate, and shows a correspondence with the nucleation rate that does not depend on the type of flow field applied. Additionally, a structural analysis of the crystalline clusters indicates that the flow field suppresses the compaction and crystalline ordering of clusters, leading to the formation of large articulated clusters under strong flow fields, and compact well-ordered clusters under weak flow fields. Published by AIP Publishing. [http://dx

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Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow

Cited by 65 publications

References 80 publications

Crystallization of polyethylene: A molecular dynamics simulation study of the nucleation and growth mechanisms

Crystallization of polyethylene: A molecular dynamics simulation study of the nucleation and growth mechanisms

Analysis of nucleation using mean first-passage time data from molecular dynamics simulation

Molecular simulation of flow-enhanced nucleation in n-eicosane melts under steady shear and uniaxial extension

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