Molecular modeling of temperature dependence of solubility parameters for amorphous polymers

A molecular modeling strategy is proposed to describe the temperature (T) dependence of solubility parameter (δ) for the amorphous polymers which exhibit glass-rubber transition behavior. The commercial forcefield "COMPASS" is used to support the atomistic simulations of the polymer. The temperature dependence behavior of δ for the polymer is modeled by running molecular dynamics (MD) simulation at temperatures ranging from 250 up to 650 K. Comparing the MD predicted δ value at 298 K and the glass transition temperature (T(g)) of the polymer determined from δ-T curve with the experimental value confirm the accuracy of our method. The MD modeled relationship between δ and T agrees well with the previous theoretical works. We also observe the specific volume (v), cohesive energy (U(coh)), cohesive energy density (E(CED)) and δ shows a similar temperature dependence characteristics and a drastic change around the T(g). Meanwhile, the applications of δ and its temperature dependence property are addressed and discussed.     

Effect of temperature on elastic properties of CNT-polyethylene nanocomposite and its interface using MD simulations

This paper investigates the effect of temperature on the elastic modulus of carbon nanotube-polyethylene (CNT-PE) nanocomposite and its interface using molecular dynamics (MD) simulations, by utilizing the second-generation polymer consistent force field (PCFF). Two CNTs—armchair and zigzag—were selected as reinforcing nano-fillers, and amorphous PE was used as the polymer matrix. For atomistic modelling of the nanocomposite, the commercially available code Materials Studio 8.0 was used and all other MD simulations were subsequently performed using the open source code Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). To obtain the elastic modulus of the nanocomposite, stress-strain curves were drawn at different temperatures by performing uniaxial deformation tests on the nanocomposite material, whereas the curvatures of the interfacial interaction energy vs. strain curves were utilized to obtain Young’s modulus of the interface. In addition, the glass transition temperatures of the polymer matrix and nanocomposites were also evaluated using density-temperature curves. Based on the results, it is concluded that, irrespective of temperature condition, a nanocomposite reinforced with CNT of larger chirality (i.e., armchair) yields a higher value of Young’s modulus of the nanocomposite and its interface. It was also found that, at the phase transition (from a glassy to a rubbery state) temperature (i.e., glass transition temperature), Young’s moduli of the polymer matrix, nanocomposite, and its interface drop suddenly. The results obtained from MD simulations were verified with results obtained from continuum-based rule-of-mixtures.     

Force field comparisons of the heat capacity of carbon nanotubes

The force fields Tersoff, CHARMM, COMPASS, CVFF and PCFF are compared using molecular calculations and simulations of SWNT thermal properties. The heat capacity results from the force fields vary significantly in the low (room) temperature range. The COMPASS force field best reproduces the phonon frequencies calculated from density functional theory and is consistent with the Raman scattering results. The temperature dependent behavior of SWNT heat capacity is investigated using harmonic and quasi-harmonic dynamics theories. The impact of quasi-harmonic analysis is not significant in the low and intermediate temperature range (below 500 K). Thus, force field comparisons based on the harmonic approximation are valid in that temperature range. Above 500 K, heat capacity results based on the Tersoff force field using a quasi-harmonic approximation are further investigated.     

Study on thermomechanical properties of cross-linked epoxy resin

In this paper, molecular dynamics simulation was carried out to investigate the thermomechanical properties of cross-linked epoxy resin. The glass transition temperature, coefficients of thermal and moisture expansion, mechanical property parameters and so on are studied with the influence of temperature, water concentration and polymer conversion taken into account. The simulation results were in good agreement with existing experimental data.     

Quantifying the Effect of Polymer Blending through Molecular Modelling of Cyanurate Polymers

Modification of polymer properties by blending is a common practice in the polymer industry. We report here a study of blends of cyanurate polymers by molecular modelling that shows that the final experimentally determined properties can be predicted from first principles modelling to a good degree of accuracy. There is always a compromise between simulation length, accuracy and speed of prediction. A comparison of simulation times shows that 125ps of molecular dynamics simulation at each temperature provides the optimum compromise for models of this size with current technology. This study opens up the possibility of computer aided design of polymer blends with desired physical and mechanical properties.     

Predict the glass transition temperature of glycerol-water binary cryoprotectant by molecular dynamic simulation

Vitrification is proposed to be the best way for the cryopreservation of organs. The glass transition temperature (T_g) of vitrification solutions is a critical parameter of fundamental importance for cryopreservation by vitrification. The instruments that can detect the thermodynamic, mechanical and dielectric changes of a substance may be used to determine the glass transition temperature. T_g is usually measured by using differential scanning calorimetry (DSC). In this study, the T_g of the glycerol-aqueous solution (60%, wt/%) was determined by isothermal-isobaric molecular dynamic simulation (NPT-MD). The software package Discover in Material Studio with the Polymer Consortium Force Field (PCFF) was used for the simulation. The state parameters of heat capacity at constant pressure (C_p), density (ρ), amorphous cell volume (V_cell) and specific volume (V_specific) and radial distribution function (rdf) were obtained by NPT-MD in the temperature range of 90–270 K. These parameters showed a discontinuity at a specific temperature in the plot of state parameter versus temperature. The temperature at the discontinuity is taken as the simulated T_g value for glycerol–water binary solution. The T_g values determined by simulation method were compared with the values in the literatures. The simulation values of T_g (160.06–167.51 K) agree well with the DSC results (163.60–167.10 K) and the DMA results (159.00 K). We drew the conclusion that molecular dynamic simulation (MDS) is a potential method for investigating the glass transition temperature (T_g) of glycerol–water binary cryoprotectants and may be used for other vitrification solutions.     

Computation of densities,bulk moduli and glass transition temperatures of vinylic polymers from atomistic simulation

The aim of molecular modeling is to mimic reality by considering approximations appropriate to the scale at which the simulation is carried out. At the atomic level, forcefields that represent average atomic interactions are used. However, the phase space has to be adequately explored in order to compare successfully computed and experimental properties. The procedure exposed in this article considers an initial selection of relevant configurations on which a simulated annealing process is applied using the first generation forcefield OPLS, followed by a uniform hydrostatic compression using the second generation forcefield COMPASS©. The resulting data are fitted by an equation of state, from which density and bulk modulus are determined. The glass transition is then simulated and T _gs are computed. Our approach is tested using a series of vinylic polymers, which differ from each other by small variations in atomic interaction combinations. The excellent agreement with experimental data shows the validity of the procedure exposed. Moreover, a clear linear relationship between simulated and experimental T _gs is revealed.     

Bone-tissue-engineering material poly(propylene fumarate): correlation between molecular weight, chain dimensions, and physical properties

Poly(propylene fumarate) (PPF) is an important biodegradable and cross-linkable polymer designed for bone-tissue-engineering applications. For the first time we report the extensive characterization of this biomaterial including molecular weight dependences of physical properties such as glass transition temperature Tg, thermal degradation temperature Td, density rho, melt viscosity eta0, hydrodynamic radius RH, and intrinsic viscosity [eta]. The temperature dependence of eta0 changes progressively with molecular weight, whereas it can be unified when the temperature is normalized to Tg. The plateau modulus and entanglement molecular weight Me have been obtained from the rheological master curves. A variety of chain microstructure parameters such as the Mark-Houwink-Sakurada constants K and alpha, characteristic ratio Cinfinity, unperturbed chain dimension r0(2)/M, packing lengthp, Kuhn length b, and tube diameter a have been deduced. Further correlation between the microstructure and macroscopic physical properties has been discussed in light of recent progress in polymer dynamics to supply a better understanding about this unsaturated polyester to advance its biomedical uses. The molecular weight dependence of Tg for six polymer species including PPF has been summarized to support that Me is irrelevant for the finite length effect on the glass transition, whereas surprisingly these polymers can be divided into two groups when their normalized Tg is plotted simply against Mw to indicate the deciding roles of inherent chain properties such as chain fragility, intermolecular cooperativity, and chain end mobility.     

A novel method to predict the glass transition of 70% glycerol aqueous solution

Vitrification has been used to successfully cryopreserve cells and tissues for over 60 years. Glass transition temperature (T _g) of the vitrification is a critical parameter, which has been investigated experimentally. In this study, an isothermal–isobaric molecular simulation (NPT-MD) is proposed to investigate the glass transition and T _g of such vitrification solution. The cohesive energy density, solubility parameter (δ) and bulk modulus of the solution during the process of the glass transition are investigated as well. The results indicate that these properties as functions of temperature can give a definite inflexion; thus, these properties can be used to predict T _g more accurately than the heat capacity (C _p ), density (ρ), volume (V) and radial distribution function (rdf). At the same time, the predicted values of T _g agree well with the experimental results. Therefore, molecular dynamics simulation is a potential method for investigating the glass transition and T _g of the vitrification solutions.     

Prediction of Selected Physical and Mechanical Properties of a Telechelic Polybenzoxazine by Molecular Simulation

Molecular simulation is becoming an important tool for both understanding polymeric structures and predicting their physical and mechanical properties. In this study, temperature ramped molecular dynamics simulations are used to predict two physical properties (i.e., glass transition temperature and thermal degradation temperature) of a previously synthesised and published telechelic benzoxazine. Plots of simulated density versus temperature show decreases in density within the same temperature range as experimental values for the thermal degradation. The predicted value for the thermal degradation temperature for the cured polybenzoxazine based on the telechelic polyetherketone (PEK) monomer was ca. 400°C, in line with the experimental thermal degradation temperature range of 450°C to 500°C. Mechanical Properties of both the unmodified PEK and the telechelic benzoxazines are simulated and compared to experimental values (where available). The introduction of the benoxazine moieties are predicted to increase the elastic moduli in line with the increase of crosslinking in the system.     

Molecular dynamics simulation of cross-linked urea-formaldehyde polymers for self-healing nanocomposites: prediction of mechanical properties and glass transition temperature

Urea-formaldehyde polymers, which are utilized in the adhesives industry, have recently been shown to be suitable materials for synthesizing micro/nanocapsules for use in self-healing (nano)composites. In this study, molecular dynamics was employed to simulate the process in which urea and formaldehyde are cross-linked via methylene and ether cross linkers, and to study the structure and mechanical/thermal properties of simulated poly(urea-formaldehyde)s (PUFs). The elastic stiffness constants of the simulated materials were calculated using the constant-strain (static) method. A temperature cycle was applied to the cross-linked PUFs, and the glass transition behavior of each material was investigated through the mean squared displacement (MSD) and temperature evolution of the energy and the specific volume of the polymer. The simulation results confirmed that there was considerable improvement in the properties of the poly(UF) materials upon cross linking. The radial distribution function was also used to study the local structures of the polymers, and this revealed that increasing the temperature and cross linking density results in a significant drop in hydrogen bonding intensity in the cross-linked PUF systems.     

Separation of the variables of time and temperature in the mechanical properties of high sugar/polysaccharide mixtures

Experimental results from previous studies were analyzed in order to separate the dynamic mechanical properties of high sugar/polysaccharide mixtures into a basic function of temperature alone and a basic function of time alone. In doing so, the energy of vitrification as derived from the Williams, Landel, and Ferry equation, and the distribution function of relaxation times were used. It was found that the temperature course of vitrification depends on the nature of the polymer and the composition of the mixture. Thus, at the same level of cosolute, the glass transition temperature of the mixture is determined by the structural behavior of the macromolecule and, it appears, that cation-mediated associations--for example, of kappa-carrageenan--are more efficient "vitrifiers" than the neutral associations of agarose. Regardless of the glass transition temperature, vitrification requires five times the activation energy of elementary flow in the melt or of the viscoelastic relaxation in the rubbery state. In the region of long time scales of measurement, the time function is determined by the molecular weight distribution and the ability of the polysaccharide to form a three-dimensional network. In the area of short times, free volume effects leading to vitrification are similar for all materials.     

Molecular simulation of realistic membrane models of alkylated PEEK membranes

Atomistic molecular modelling has proven to be a useful tool for the investigation of transport properties of small gas molecules in polymer membrane matrices. The quality of the predictions of these properties based on molecular simulation depends principally on the quality of the membrane model. The predicted gas transport properties of small gas molecules in the same glassy polymer membrane show often a large scatter in gas diffusion and solubility simulated values. In order to reduce the scatter in predicted gas transport properties in glassy polymer membranes, numerical analysis of structural features of the membrane model is used for pre-selecting only the realistic ones for further simulations using transition-state theory (TST) approach. Simulation results of gas solubility and diffusion in alkylated poly-ether–ether–ketone (PEEK) membranes will illustrate the approach.     

Constant normal pressure, constant surface tension, and constant temperature molecular dynamics simulation of hydrated 1,2-dilignoceroylphosphatidylcholine monolayer

A constant normal pressure, constant surface tension, and constant temperature (NP(N)gammaT) molecular dynamics (MD) simulation of the liquid condensed phase of a 1,2-dilignoceroylphosphatidylcholine (DLGPC) monolayer has been performed at 293.15 K. A DLGPC molecule has two saturated 24-carbon acyl chains, giving the hydrocarbon core thickness of the monolayer approximately 28 A, which is close to the hydrocarbon core thickness of a membrane of a living system. NP(N)gammaT ensemble was used to reproduce the experimental observations, such as area/lipid, because surface tension is an essential factor in determining the monolayer structure. Data analysis on DLGPC/water monolayer shows that various liquid condensed-phase properties of the monolayer have been well reproduced from the simulation, indicating that surface tension 22.9 mN/M used in the simulation is an appropriate condition for the condensed-phase NP(N)gammaT simulation. The simulation results suggest that this long-chain phospholipid monolayer shares many structural characteristics with typical short-chain 1,2-diacylphosphatidylcholine systems, such as DPPC/water monolayer in the condensed phase and DPPC/water bilayer in the gel phase. Furthermore, it was found that DLGPC/water monolayer has almost completely rotationally disordered acyl chains, which have not been observed so far in short-chain 1,2-diacylphosphatidylcholine/water bilayers. This study indicates the good biological relevance of the DLGPC/water monolayer which might be useful in protein/lipid studies to reveal protein structure and protein/lipid interactions in a membrane environment.     

COMPASS II: extended coverage for polymer and drug-like molecule databases

The COMPASS II force field has been developed by extending the coverage of the COMPASS force field (J Phys Chem B 102(38):7338–7364, 1998) to polymer and drug-like molecules found in popular databases. Using a fragmentation method to systematically construct small molecules that exhibit key functional groups found in these databases, parameters applicable to database compounds were efficiently obtained. Based on the same parameterization paradigm as used in the development of the COMPASS force field, new parameters were derived by a combination of fits to quantum mechanical data for valence parameters and experimental liquid and crystal data for nonbond parameters. To preserve the quality of the original COMPASS parameters, a quality assurance suite was used and updated to ensure that additional atom-types and parameters do not interfere with the existing ones. Validation against molecular properties, liquid and crystal densities, and enthalpies, demonstrates that the quality of COMPASS is preserved and the same quality of prediction is achieved for the additional coverage.     

Thermodynamic aspects of vitrification

Vitrification is a process in which a liquid begins to behave as a solid during cooling without any substantial change in molecular arrangement or thermodynamic state variables. The physical phenomenon of vitrification is relevant to both cryopreservation by freezing, in which cells survive in glass between ice crystals, and cryopreservation by vitrification in which a whole sample is vitrified. The change from liquid to solid behavior is called the glass transition. It is coincident with liquid viscosity reaching 10¹³ Poise during cooling, which corresponds to a shear stress relaxation time of several minutes. The glass transition can be understood on a molecular level as a loss of rotational and translational degrees of freedom over a particular measurement timescale, leaving only bond vibration within a fixed molecular structure. Reduced freedom of molecular movement results in decreased heat capacity and thermal expansivity in glass relative to the liquid state. In cryoprotectant solutions, the change from liquid to solid properties happens over a ∼10 °C temperature interval centered on a glass transition temperature, typically near −120 °C (±10 °C) for solutions used for vitrification. Loss of freedom to quickly rearrange molecular position causes liquids to depart from thermodynamic equilibrium as they turn into a glass during vitrification. Residual molecular mobility below the glass transition temperature allows glass to very slowly contract, release heat, and decrease entropy during relaxation toward equilibrium. Although diffusion is practically non-existent below the glass transition temperature, small local movements of molecules related to relaxation have consequences for cryobiology. In particular, ice nucleation in supercooled vitrification solutions occurs at remarkable speed until at least 15 °C below the glass transition temperature.     

Adsorption Behavior of Polydisperse Polymeric Systems

Adsorption behavior of polydisperse polymers at interfaces is studied by the Monte Carlo simulation method based on a lattice model. Effects of temperature and adsorption energies on the polymer density profile, cluster distributions and adsorption layer thickness are evaluated in two different polydisperse systems. It is found that adsorption properties are greatly different in these two systems. In normal distribution polydisperse systems, polymers are more sensitive to the excluded volume effects while in average distribution polymers are more inclined to adsorb on adsorbing interfaces. As higher temperature and lower attraction constrain the polymer adsorption, more clusters are found under these conditions. When temperature approaches the critical temperature of monodisperse systems, stable and large clusters exist in these two polydisperse systems. These results suggest that micro phase transition may exist in polydisperse systems. Polydisperse systems take little change of phase transition temperatures but altered the clusters morphology to some extent. The adsorption layer thickness changes are more sensitive to both temperature and polymer–interface interactions in average distribution systems when the whole polymer concentration increases slightly. This work also suggests significant differences between the polydisperse and the monodisperse systems in adsorption behavior. Therefore, quantitative system errors may exist when the monodisperse system models are used in simulation to evaluate polymer adsorption properties.     

The rubber‐to‐glass transition in high sugar agarose systems

The small and large deformation properties of agarose in the presence of high levels of sugar were investigated. Mixtures can be described as lightly cross‐linked rubbers, which undergo vitrification upon cooling. The combined Williams–Landel–Ferry (WLF)/free volume framework was used to derive the glass transition temperature, the fractional free volume, and the thermal expansion coefficient of the glass. Sucrose‐rich cosolute crystallizes, but addition of the polymer encourages intermolecular interactions, which transform the mixture into a high viscosity glass. The mechanical properties of glucose syrup, a noncrystalline sugar, follow WLF behavior in the glass transition region and revert to an Arrhenius‐type prediction in the glassy state. Measurements on sugar samples and agarose–sugar mixtures were resolved into a basic function of temperature alone and a basic function of frequency (time) alone. The former traces the energetic cost of vitrification, which increases sharply with decreasing temperature. The latter, at long time scales, is governed by the infinite molecular weight of the agarose network. In the region of short times, the effect of free volume is active regardless of the sample composition. © 1999 John Wiley & Sons, Inc. Biopoly 49: 267–275, 1999     

The effect of alumina nanoparticles on the thermal properties of PMMA: a molecular dynamics simulation

Metal oxides, as one of the most promising flame retardant additives, improve the fire retardant and the thermal stability properties of polymers. In the present study, molecular dynamics (MD) simulations based on the united atom model were applied to study the effect of alumina nanoparticles on the density, thermal conductivity, heat capacity, and thermal diffusivity of isotactic poly(methyl methacrylate) (is-PMMA). Thermal diffusivity of PMMA and PMMA/alumina nanocomposite were investigated through calculating thermal conductivity, density and heat capacity in the range of 300–700?K. Heat capacity can be calculated using fluctuations properties of energy. Thermal conductivity was calculated through the nonequilibrium molecular dynamics (NEMD) simulation by Fourier’s law approach. Our results show that the addition of alumina nanoparticles decreases the heat capacity and increases the glass transition temperature (T_g), thermal conductivity and thermal diffusivity of the PMMA. Therefore, the addition of alumina nanoparticles to PMMA improves the fire retardancy of the polymer. In addition, we illustrate the links between the intermolecular and bulk properties of PMMA in the presence of the alumina nanoparticles.