New explanation for chain folding in polymers

Sadler, D. M.

doi:10.1038/326174a0

Cited by 138 publications

(146 citation statements)

References 24 publications

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“…5,6 At higher temperatures, the growth front may be rough and growth is limited by the entropic cost to form a sufficiently flat surface by thermal fluctuationssthis is the situation considered by the Sadler-Gilmer model. [7][8][9] Although there exist several theoretical models for the mechanism of polymer crystallization, the role of molecular weight in polymer crystallization is still poorly understood. For instance, we have no satisfactory explanation for the phenomenon of polymer-size segregation during crystal growth.…”

Section: Ref 3)mentioning

confidence: 99%

Intramolecular Nucleation Model for Polymer Crystallization

2003

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We report a numerical study of the free energy barrier for crystallization and melting of a single homopolymer chain. The simulations show that the free energy barrier separating the crystalline and molten states at the same free energy level strongly depends on the chain length. However, at a fixed temperature the barrier for single-chain crystallization is independent of chain length. This observation is in agreement with recent experiments on multichain bulk-polymer systems and can be understood theoretically if we assume that the primary nucleation of polymer crystals is determined by intramolecular nucleation. If we further assume that the subsequent growth of polymer crystals is controlled by two-dimensional intramolecular nucleation on the growth front, we can even account for the experimentally observed molecular segregation during crystal growth as well as the chain-length independence of the free energy barrier for secondary nucleation.

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Section: Ref 3)mentioning

confidence: 99%

Intramolecular Nucleation Model for Polymer Crystallization

2003

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“…The absence of structural order in the non-crystalline region of semi-crystalline polymers imposes challenges to an extent that the issue of adjacent or non-adjacent re-entry remains unsolved even today, despite the discovery of chain folded crystals in 1957 [2,8,9]. Recently, in UHMWPE, it has been shown that the melting temperature of the as-synthesized polymer (nascent) approaches the equilibrium melting temperature because of the restricted mobility of the methylene segments in the non-crystalline region [10].…”

Section: Introductionmentioning

confidence: 99%

“…Yao et al investigated the influence of polymerization conditions on the non-crystalline region of the semi-crystalline region [7]. To recall, the authors demonstrated that the non-crystalline region in the polymer synthesized using Ziegler-Natta differed from the polymer synthesized using a single-site catalytic system [7].The absence of structural order in the non-crystalline region of semi-crystalline polymers imposes challenges to an extent that the issue of adjacent or non-adjacent re-entry remains unsolved even today, despite the discovery of chain folded crystals in 1957 [2,8,9]. Recently, in UHMWPE, it has been shown that the melting temperature of the as-synthesized polymer (nascent) approaches the equilibrium melting temperature because of the restricted mobility of the methylene segments in the non-crystalline region [10].…”

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confidence: 99%

Heterogeneous Distribution of Entanglements in a Nonequilibrium Polymer Melt of UHMWPE: Influence on Crystallization without and with Graphene Oxide

et al. 2016

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Recently, the influence of reduced graphene oxide nanoplatelets (rGON) on the rheological response of polymers has been a subject of interest. In the case of disentangled UHMWPE, it has been shown that the chain-filler interaction in the UHMWPE/rGON composite results into an everlasting non-equilibrium melt state having heterogeneous distribution in entanglement density. In this study, a thermal analysis protocol is used to follow the influence of the non-equilibrium polymer melt on the crystallization kinetics of disentangled UHMWPE with, and without, rGON. The analysis is carried out by means of differential scanning calorimetry (DSC) and the results are supported by rheology. When the disentangled UHMWPE sample, without the filler rGON, is left to crystallize under isothermal condition after melting, two endothermic peaks are observed: the high temperature peak (close to the equilibrium melting point, 141.5 °C) is related to the melting of crystals obtained on crystallization from the disentangled domains of the heterogeneous (nonequilibrium) polymer melt, whereas the low melting temperature peak is related to the melting of crystals formed from entangled domains of the melt. On increasing the annealing time in melt (160 °C), the enthalpy of the lower melting temperature peak increases at the expense of the high melting temperature peak, confirming a transformation of the nonequilibrium polymer melt to a fully entangled equilibrium melt state. However, independent of the equilibrium or non-equilibrium melt state, the recurrence of the high melting temperature peak is observed when the samples synthesized using the single-site catalytic system are left to isothermal crystallization at a specific temperature. The recurrence of the high melting temperature, close to the equilibrium melting point of the polymer, questions the differences in entanglements formed before and after polymerization in these high molar masses. The differences in the topological constraints are likely to influence the difference in melting temperatures of the isothermally crystallized samples. In the presence of rGON, the melting response of disentangled UHMWPE crystallized from its heterogeneous melt state changes; at a specific filler concentration, it is observed that the high endothermic peak remains independent of the annealing time in melt. This observation strengthens the concept that in the presence of the filler, chain dynamics is arrested to an extent that the nonequilibrium melt state having lower entanglement density is retained, facilitating the formation of crystals having high melting temperature. IntroductionThe topology of methylene segments in the non-crystalline region of the semi-crystalline polymer Ultrahigh Molecular Weight Polyethylene (UHMWPE), has a profound influence on the mechanical deformation either uniaxially or biaxially [1]. The topology can be tailored by controlling the crystallization kinetics either by dissolution and crystallization or controlled polymerization [2,3,4,5]. The influence of mo...

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“…The second approach, which was developed by Sadler and Gilmer and has been termed the entropic barrier model, is based upon the interpretation of kinetic Monte Carlo simulations 8,9 and rate-theory calculations [10][11][12] of a simplified model of polymer crystal growth. As with the surface nucleation approach, the observed thickness is suggested to result from the competition between a driving force and a free energy barrier contribution to the growth rate.…”

Section: Introductionmentioning

confidence: 99%

Kinetic Monte Carlo simulations of the growth of polymer crystals

Doye

Frenkel

1999

The Journal of Chemical Physics

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Based upon kinetic Monte Carlo simulations of crystallization in a simple polymer model we present a new picture of the mechanism by which the thickness of lamellar polymer crystals is constrained to a value close to the minimum thermodynamically stable thickness, l min . The free energetic costs of the polymer extending beyond the edges of the previous crystalline layer and of a stem being shorter than l min provide upper and lower constraints on the length of stems in a new layer. Their combined effect is to cause the crystal thickness to converge dynamically to a value close to l min where growth with constant thickness then occurs. This description contrasts with those given by the two dominant theoretical approaches. However, at small supercoolings the rounding of the crystal profile does inhibit growth as suggested in Sadler and Gilmer's entropic barrier model.

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New explanation for chain folding in polymers

Cited by 138 publications

References 24 publications

Intramolecular Nucleation Model for Polymer Crystallization

Intramolecular Nucleation Model for Polymer Crystallization

Heterogeneous Distribution of Entanglements in a Nonequilibrium Polymer Melt of UHMWPE: Influence on Crystallization without and with Graphene Oxide

Kinetic Monte Carlo simulations of the growth of polymer crystals

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