Local Orientational Mobility Research Articles

NMR studies of excluded volume interactions in peptide dendrimers

Peptide dendrimers are good candidates for diverse biomedical applications due to their biocompatibility and low toxicity. The local orientational mobility of groups with different radial localization inside dendrimers is important characteristic for drug and gene delivery, synthesis of nanoparticles, and other specific purposes. In this paper we focus on the validation of two theoretical assumptions for dendrimers: (i) independence of NMR relaxations on excluded volume effects and (ii) similarity of mobilities of side and terminal segments of dendrimers. For this purpose we study 1H NMR spin-lattice relaxation time, T1H, of two similar peptide dendrimers of the second generation, with and without side fragments in their inner segments. Temperature dependences of 1/T1H in the temperature range from 283 to 343 K were measured for inner and terminal groups of the dendrimers dissolved in deuterated water. We have shown that the 1/T1H temperature dependences of inner groups for both dendrimers (with and without side fragments) practically coincide despite different densities of atoms inside these dendrimers. This result confirms the first theoretical assumption. The second assumption is confirmed by the 1/T1H temperature dependences of terminal groups which are similar for both dendrimers.

Local orientational mobility of polyimide-based nanocomposites

In the present study, the influence of filler concentration on the dynamic properties of nanocomposites based on the thermoplastic polyimide R-BAPO has been investigated by means of full-atomistic molecular dynamics simulations. The orientational mobility of chain fragments of different lengths has been studied before and after embedding of single-walled carbon nanotubes (SWCNT) into the polymer. Slowing down of the polymer orientational mobility in nanocomposites at temperature T = 600 K above the glass transition temperature has been observed for all considered chain fragments in comparison to unfilled systems. It was found that the relative increase of the rotation relaxation times in the diamine part of R-BAPO in the presence of SWCNT is higher for longer chain fragments. This result is in agreement with recent experimental data and with theoretical predictions for the composites based on homopolymers. On the other hand, the relative change of rotation relaxation times under filler loading in the dianhydride part of R-BAPO demonstrates non-monotonic dependence on the length of chain fragments. This result may be explained by structural inhomogeneity of the dianhydride part. Slowing down of the orientational mobility is accompanied by the increase of the glass transition temperature of the composites on ca. 5–15 K with increase of the nanofiller concentration on ca. 3–6% of mass fraction.

NMR relaxation in semiflexible Vicsek fractals

We consider the role of local orientational mobility for the NMR relaxation of hyperbranched macromolecules modeled in form of Vicsek fractals. The spectral density J(ω), that is relevant for the NMR relaxation, is studied for labeled segments of flexible and semiflexible Vicsek fractals. We show that the semiflexibility leads to strong effects. For flexible Vicsek fractals the function ωJ(ω) has a single maximum, whose position is determined by the local-scale modes and therefore independent of the location of the labeled segments. For the labeled segments of a semiflexible Vicsek fractal, we observe a broad spectral density. It is characterized by the overall relaxation of the branch that originates from the labeled segment and by the overall relaxation of the subbranches that build this branch. In this way, going from the periphery to the inner part of the macromolecule, the labeled segments display a broader spectral density. However, this effect saturates when the labeled segments get closer to the core of the hyperbranched macromolecule, so that they do not feel their remoteness from the periphery anymore.

Simulating local mobility and mechanical properties of thermostable polyimides with different dianhydride fragments

ABSTRACTThe dynamic and mechanical properties of the three thermoplastic polyimides—crystallizable polyimide BPDA‐P3 and amorphous polyimides ODPA‐P3 and aBPDA‐P3—have been simulated using the atomistic molecular dynamics technique. The three simulated polyimides differ in the chemical structure of their corresponding dianhydride fragments. Analyzing the local orientational mobility of different phenylene rings, it has been established that the increase of the glass‐transition temperature (Tg) in the ordered set is caused by the slowing down of the phthalimide rings relaxation in the corresponding dianhydride fragments. It has been observed that rather poor mechanical characteristics upon aBPDA‐P3 stretching in the strain‐hardening regime are also due to the low orientational mobility of the phthalimide rings. The correlation between the dynamic fragility and the polyimides strain‐hardening moduli has been observed; the increase of the dynamic fragility leads to the increase of the strain‐hardening modulus. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 375–382

Computer simulation of local mobility in dendrimers with asymmetric branching by brownian dynamics method

The Brownian dynamics method has been used to study the effect of the branching asymmetry on the local orientational mobility of segments and bonds in dendrimers in good solvent. “Coarse-grained” models of flexible dendrimers with different branching symmetry but with the same average segment length were considered. The frequency dependences of the rate of the spin-lattice relaxation nuclear magnetic resonance (NMR) [1/T1H(H)] for segments or bonds located at different distances from terminal monomers were calculated. After the exclusion of the contribution of the overall dendrimer rotation the position of the maxima of the frequency dependences [1/T1H(ωH)] for different segments with the same length doesn’t depend on their location inside a dendrimer both for phantom models and for models with excluded volume interactions. This effect doesn’t depend also on the branching symmetry, but the position of the maximum [1/T1H(ωH))] is determined by the segment length. For bonds inside segments the positions of the maximum [1/T1H(ωH)] coincide for all models considered. Therefore, the obtained earlier conclusion about the weak influence of the excluded volume interactions on the local dynamics in the flexible symmetric dendrimers can be generalized for dendrimers with an asymmetric branching.

Local orientational mobility in regular hyperbranched polymers.

We study the dynamics of local bond orientation in regular hyperbranched polymers modeled by Vicsek fractals. The local dynamics is investigated through the temporal autocorrelation functions of single bonds and the corresponding relaxation forms of the complex dielectric susceptibility. We show that the dynamic behavior of single segments depends on their remoteness from the periphery rather than on the size of the whole macromolecule. Remarkably, the dynamics of the core segments (which are most remote from the periphery) shows a scaling behavior that differs from the dynamics obtained after structural average. We analyze the most relevant processes of single segment motion and provide an analytic approximation for the corresponding relaxation times. Furthermore, we describe an iterative method to calculate the orientational dynamics in the case of very large macromolecular sizes.

Local Chain Dynamics and Dynamic Heterogeneity in Cross-Linked Epoxy in the Vicinity of Glass Transition

We studied the local chain dynamics as well as the dynamic heterogeneity in highly cross-linked epoxy near glass transition by molecular dynamics simulation. In previous work (Lin and Khare in Macromolecules 2009, 42, 4319 ), we had reported creation of fully relaxed atomistic structures of cross-linked epoxy; the glass transition temperature of these structures was also determined from volume−temperature behavior. The local chain dynamics in these structures is characterized by using molecular dynamics simulation in this work. Local translational mobility of cross-linked epoxy as measured by mean-squared displacement of atoms exhibited two subdiffusive regimes within the simulation time of 200 ns. Local orientational mobility was determined by monitoring the autocorrelation function (ACF) of a vector associated with the epoxy monomer. Time dependence of an order parameter based on this ACF showed that there was almost no orientational relaxation when the temperature was near or below glass transition tem...

Simulated glass transition in free‐standing thin polystyrene films

AbstractIn this article, we investigate the glass transition in polystyrene melts and free‐standing ultra‐thin films by means of large‐scale computer simulations. The transition temperatures are obtained from static (density) and dynamic (diffusion and orientational relaxation) measurements. As it turns out, the glass transition temperature of a 3 nm thin film is ∼60 °K lower than that of the bulk. Local orientational mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second‐order P2(t). The α and β relaxation times are obtained from the spectral density of P2(t). Our simulations reveal that interfaces affect α and β‐relaxation processes differently. The β relaxation rate is faster in the center of the film than near a free surface; for the α relaxation rate, an opposite trend is observed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1160–1167, 2010

Local orientational mobility in dendrimers. Theory and computer-aided simulation

The orientational mobility of segments in dendrimers are studied by the method of Brownian dynamics, and the results are analyzed in terms of an earlier proposed analytical theory. The orientational autocorrelation function for the cosine of an angle of segmental rotation in dendrimers of a given generation P 1(t) is controlled by three relaxation processes with the corresponding relaxation times. Characteristic times and the contribution from the above processes to P 1(t) are calculated. The first process refers to the local mobility of a selected segment; the second process, to the rotations of a dendrimer branch, which originates from the selected segment of a given generation; and the third process, to the rotation of a dendrimer macro-molecule as a whole. The proposed approach makes it possible to estimate the relaxation spectrum of a dendrimer by studying the orientational mobility of segments in different generations. The relaxation times can be used to describe various relaxation processes observed by different experimental methods, such as dielectric relaxation, NMR, dynamic birefringence, and polarized luminescence.

Atomistic Simulation of Bulk Mechanics and Local Dynamics of Amorphous Polymers

AbstractSummary: In order to have better insight into the polymer specifics of the dynamic glass transition molecular dynamics (MD) computer simulations of three glass‐formers have been carried out: low‐molecular‐weight isopropylbenzene (iPB), brittle atactic polystyrene (PS) and tough bisphenol A polycarbonate (PC). Simulation of the uniaxial deformation of these mechanically different types of amorphous polymers shows that the mechanical experimental data could be realistically reproduced. Now the objective is to study the local orientational mobility in the non‐deformed isotropic state and to find the possible connection of the segmental dynamics with the different bulk mechanical properties. Local orientational mobility has been studied via Legendre polynomials of the second order and CONTIN analysis. Insight into local orientational dynamics on a range of length‐ and time scales is acquired. The fast transient ballistic process describing the very initial part of the relaxation has been observed for all temperatures. For all three simulated materials the slowing down of cage escape (α‐relaxation) follows mode‐coupling theory above Tg, with non‐universal, material‐specific exponents. Below Tg universal activated segmental motion has been found. At high temperature the α process is merged with the β process. The β process which corresponds to the motions within cage continues below Tg and can be described by an activation law.

Computer Simulation of the Dynamics of Neutral and Charged Dendrimers

Dynamic properties of dilute solutions of neutral and charged dendrimers with explicit excluded-volume, electrostatic, and hydrodynamic interactions have been investigated by Brownian dynamics simulation. Three different types of motions in dendrimers up to g = 5 generations have been considered: the motion of a dendrimer as a whole; the size and shape fluctuations (pulsations); the local reorientations of the individual monomers. The influence of the excluded-volume, electrostatic, and hydrodynamic interactions on these motions has been studied. The characteristic relaxation times have been compared with the theoretical predictions of the Rouse and Zimm models. The self-diffusion of a dendrimer can be described with the help of the preaveraged Zimm approach, and a dendrimer may be considered as an impenetrable sphere with the hydrodynamic radius Rh. For both neutral and charged dendrimers the hydrodynamic radius is smaller than the gyration radius Rg. The dynamics of the size fluctuations for a dendrimer with rigid spacers differs significantly from the theoretical predictions for a dendrimer with flexible spacers. The relaxation of these fluctuations is weakly sensitive to the presence of the hydrodynamic interactions, and the behavior of a dendrimer is close to that of an elastic body in a viscous medium. The local orientational mobility of individual monomers is significantly influenced by the ionization of the terminal groups.

Computer simulation study of bulk atactic polystyrene in the vicinity of the glass transition

AbstractMolecular dynamics (MD) simulations of bulk atactic polystyrene have been performed in a temperature range from 100 K to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long‐time asymptotic slope of these dependencies is 0.54 at T>Tg, showing Rouse behavior. Cross‐over from motion in the cage to Rouse like dynamics has been studied at T>Tg with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode‐coupling theory (MCT). Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs, (Legendre polynomials of the first and second, order) of both the main‐chain and side‐group bonds. The relaxation times of the orientational α‐relaxation follow the same power law (γ∼2.9) as the characteristic translational diffusion time. Below T>Tg both types of dynamics are described by the same activated law. The ACFs time‐distribution functions reveal the existence of activated local rearrangements already above T>Tg.

Correlated Segmental Dynamics in Amorphous Atactic Polystyrene: A Molecular Dynamics Simulation Study

Molecular dynamics simulations of bulk atactic polystyrene have been performed in a temperature range from 100 to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T > Tg, showing Rouse behavior. Crossover from motion in the cage to Rouse-like dynamics has been studied at T > Tg with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). We studied the coherent intermediate scattering function and show the agreement with scaling predictions of MCT. Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs (Legendre polynomials of the first and second order), of both the main-chain and side-group bonds. The ACF relaxation times analysis shows strong coupling between the phenyl groups and the backbone relaxation. The relaxation times ...

Large-scale computer simulation of local segmental dynamics in amorphous atactic polystyrene

Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The MD-determined values of the glass transition temperature T g are in a good agreement with experimental PVT data at different values of applied pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > T g , showing diffusive behaviour. The cage effect when local translational motions are essentially frozen in the glassy state has been studied. Local orientational mobility is studied with the help of Legendre polynomials of the first, P 1 ( t ), and second, P 2 ( t ), order for the bonds in the main chain and along the phenyl side groups. Temperature dependencies of the orientational relaxation times at different values of pressure are well described by the Vogel–Fulcher law.

Collapse of diblock copolymer in poor solvent. Molecular dynamics study

AbstractThe modification of unsaturated polyesters (UPE) by poly (ethylene glycol) (PEG) end groups [1] and the computer simulation of the collapse of a single molecule of a diblock copolymer AB consisting of PEG (A) and UPE (B) blocks [2] were reported recently by us. In the present paper the static and dynamic characteristics of the final collapsed structure were investigated by a molecular dynamics simulation. The structure and the local dynamic of collapsed homopolymers (AA and BB) were studied also. The initial conformations in all cases were taken near completely extended ones. The final structures of the diblocks are similar to the structure of a core (UPE) and shell (PEG) model. The differences in torsion angle distributions between diblock and homopolymer are maximal for the CCOC dihedral angles (both in UPE and PEG blocks). The orientational order of the COC vectors (‘chords’ along backbone of polymer chain) inside both blocks is larger in the diblock than in the homopolymer. Local orientational mobility of the COC vector and the translational mobility of the monomers is larger in the diblock than in the homopolymers for both blocks.