How to fold back grafted chains in dipolar brushes

Birshtein, T.M.; Polotsky, Alexey A.; Glova, Artyom D.; Amoskov, Victor M.; Mercurieva, Anna A.; Nazarychev, Victor M.; Lyulin, Sergey V.

doi:10.1016/j.polymer.2018.05.076

Cited by 13 publications

(31 citation statements)

References 27 publications

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“…Only a fraction of the grafts was stretched from the surface, and, in contrast to the previous results, the other grafts were backfolded to the surface and adopted a loop-like conformation [10,11,16]. The Scheutjens-Fleer self-consistent field (SF-SCF) numerical calculations, the theoretical arguments based on a simple two-state model, and the atomistic MD simulations allowed us to confirm that the key factor leading to the backfolding of the lactide chains grafted via similar ends is related to dipole-dipole interactions between the longitudinal dipoles of the grafts [20]. While the performed MD simulations have provided valuable insights into the structure of the grafted lactide chains, a missing factor was that they were restricted to consideration of grafts with a fixed length.…”

Section: Introductionsupporting

confidence: 69%

“…Both the backfolding and the structural ordering may be viewed as the initial stage of the crystallization of the layer of grafted lactide chains. In contrast to the known behavior for conventional polymer brushes in the "dry brush" regime, the structure of the grafted lactide chains can be either amorphous or ordered, depending on the graft length N and the grafting density σ when their product Nσ is fixed.of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in 96 comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains 97 N was varied.…”

mentioning

confidence: 77%

“…Such a modification corresponded to the grafting density σ = 1.76 nm −2 , at which the "dry brush" regime was observed [19]. Moreover, the segregation of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains N was varied.…”

Section: Model and Simulation Methodsmentioning

confidence: 99%

“…Following our previous simulations [18][19][20][21], we performed atomistic molecular dynamics simulations of polylactide-based nanocomposites filled with surface-modified cellulose nanocrystals (CNC). The simulated systems comprised 20 free lactide chains that surrounded the filler, with the length of the free chains equal to 150.…”

Section: Model and Simulation Methodsmentioning

confidence: 99%

“…of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in 96 comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains 97 N was varied.…”

mentioning

confidence: 95%

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Grafting-Induced Structural Ordering of Lactide Chains

et al. 2019

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The structure of a grafted layer of lactide chains in the "dry brush" regime immersed in a melt of chemically similar polymer was examined while varying graft lengths. To this end, microsecond atomistic molecular dynamics simulations were performed. Almost no influence of graft length on the fraction of the grafted chains backfolded to the grafting surface was found. However, a structural ordering was unexpectedly observed in the system when the length of the grafted lactide chains was close to approximately 10 Kuhn segments. This ordering of the grafts is characterized by the formation of helical fragments whose structure is in good agreement with the experimental data for the α crystal of the lactide chains. Both the backfolding and the structural ordering may be viewed as the initial stage of the crystallization of the layer of grafted lactide chains. In contrast to the known behavior for conventional polymer brushes in the "dry brush" regime, the structure of the grafted lactide chains can be either amorphous or ordered, depending on the graft length N and the grafting density σ when their product Nσ is fixed.of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in 96 comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains 97 N was varied. Six lengths of the grafted chains, N = 13, 17, 22, 30, 40 and 50, were examined. Snapshots 98 of the systems in the case of N = 13 and 50 are presented in Figure 1. Additionally, a grafted layer in 99 the "dry brush" regime with the grafting density σ = 0.88 nm −2 and the graft length N = 60 was 100 simulated. This system was compared with the one where the grafting density σ = 1.76 nm −2 and the 101 graft length N = 30 in order to examine difference in the structure between the two grafted layers at 102 a fixed product of Nσ. According to our previous estimate of the characteristic ratio [22], the Kuhn 103 segment length A for the lactide chains is equal to approximately 1.6 nm. The contour length L of the 104 grafts under consideration could be calculated as  L aN , where a = 0.38 nm was the contour length 105 of the monomer in the graft. These calculations gave contour lengths L in the range 5-19 nm for the 106 considered length of the grafts at σ = 1.76 nm −2 . Thus, the number of Kuhn segments in the grafts at 107 this σ varied from approximately 3 to 12 units, lying in different orders of magnitude. For the grafts 108 where σ = 0.88 nm −2 , the contour length and the number of Kuhn segments were equal to about 23 nm 109 and 14 units, respectively. The chosen graft lengths were limited by the computational demands 110 required to perform simulations of the corresponding composites. To the best of our knowledge, the 111 considered grafts were among the longest ever studied by using atomistic molecular dynamics 112 simulations. In the present study, the total number of atoms in the system with the grafted layer at 113 σ = 1.76 nm −2 and N = 13 was about 53,000, while the composite c...

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Section: Introductionsupporting

confidence: 69%

mentioning

confidence: 77%

Section: Model and Simulation Methodsmentioning

confidence: 99%

Section: Model and Simulation Methodsmentioning

confidence: 99%

mentioning

confidence: 95%

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Grafting-Induced Structural Ordering of Lactide Chains

et al. 2019

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Conformational Behavior of a Single Dipolar Chain under Stretching Force

Merzliakova,

Gordievskaya,

Kramarenko

2023

Macromolecules

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The stretching of a single dipolar chain by a tensile force acting on the chain end is studied using molecular dynamics simulations and theoretical considerations. In both theory and simulations, the dipoles are modeled as two oppositely charged beads; this approach makes it possible to account for the excluded volume of the charged beads and electrostatic interactions on the many-body level. In simulations, the main attention is paid to the conformational and mechanical responses of the polymer globule formed due to the charge fluctuation correlation under strong electrostatic coupling. When a force is applied, the globule undergoes a transition to an open coil in several stages. The most striking one is realized at rather strong electrostatic interactions when the stretching of the chain favors the "head-to-tail" arrangement of dipoles, with the formation of linear ionic aggregates oriented along the force vector. This state of the stretched chain appears as a quasi-plateau on the stress−strain curve. A diagram of the states of a dipolar chain under the action of a tensile force is constructed via analysis of the stress−strain dependences, as well as the microstructure of the chain, the orientational correlations of the dipoles at different magnitudes of the tensile force, and the strength of the electrostatic interactions. The developed theory allowed for the description the regimes of continuous and jump-like unfolding of the dipolar chain at f > 0.5 observed in simulations and defined by a fine interplay of conformational entropy, excluded volume, and electrostatic interactions. The results obtained provide deeper insight into the stretching behavior of ion-containing polymers in low-polar media, which is important for the design of stimuli-responsive systems.

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Grafted Dipolar Chains: Dipoles and Restricted Freedom Lead to Unexpected Hairpins

et al. 2020

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We use atomistic molecular dynamics simulations to investigate the structure of oligohydroxybutyrate (OHB) and oligolactide (OLA) chains grafted onto cellulose nanocrystals (CNCs) in melts of polyhydroxybutyrate and polylactide chains, respectively. We observed that, contrary to the established theory for neutral chains, the more flexible OHB grafts extend further from the CNC surface, whereas the stiffer OLA ones tend to fold back toward the surface by forming hairpin-like structures. Both OHB and OLA grafts have dipoles but are charge-neutral. To resolve this apparent contradiction, interactions between the grafts' monomers as well as their flexibilities were evaluated. The interaction analysis, however, showed insignificant differences. We examined this further by systematically switching off the dipolar interactions and dihedrals from the OLA grafts. The observed counterintuitive behavior is due to a subtle combination of the dipole−dipole interactions and the additional methylene group in OHB that is critical in allowing the OHB grafts to sample more conformations than the more restricted OLA chains: the steric restrictions are critical and lead to hairpin-like structures and enable dipole−dipole interactions to effectively form a zipper that maintains the hairpin-like conformations.

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How to fold back grafted chains in dipolar brushes

Cited by 13 publications

References 27 publications

Grafting-Induced Structural Ordering of Lactide Chains

Grafting-Induced Structural Ordering of Lactide Chains

Conformational Behavior of a Single Dipolar Chain under Stretching Force

Grafted Dipolar Chains: Dipoles and Restricted Freedom Lead to Unexpected Hairpins

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