Understanding the Role of Cohesive Interaction in Mechanical Behavior of a Glassy Polymer

Alesadi, Amirhadi; Xia, Wenjie

doi:10.1021/acs.macromol.0c00067

Cited by 30 publications

(42 citation statements)

References 87 publications

(156 reference statements)

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“…In summary, it can be seen in Figure that while the fitted T 0 is insensitive to the fitting procedure and evidently increases with increasing α, we cannot obtain a reliable trend for D . A recent paper, which focuses on the influence of cohesive interaction strength on the mechanical properties of polymers, has indicated that the fragility given by 1/ D increases with increasing cohesive interaction strength, a finding in conflict with the predictions from the GET. However, the range of τ α considered in ref is quite limited and also varies systematically for each cohesive energy parameter, so the VFT fits are expected to have the same issues, as discussed above.…”

Section: Resultsmentioning

confidence: 95%

“…Bending constraints are imposed by an angle potential having the following form proposed by Hsu and Kremer where the bond angle is given by θ = cos –1 [( b j · b j + 1 )/(| b j ∥ b j + 1 |)] in terms of the bond vector b j = r j – r j – 1 between two neighboring beads j and j – 1. Based on previous work, where we have systematically investigated the influence of the chain stiffness parameter a θ on polymer glass formation at a fixed value of b θ = 1.5, the present work selects the parameters a θ = 6ε and b θ = 1.5 to represent polymer melts with bending constraints, while polymer melts without bending constraints are modeled naturally using a θ = 0ε, i.e., without the angle potential.…”

Section: Methodsmentioning

confidence: 99%

“…As another example, cohesive interaction strength is a highly relevant variable in ionic and polar polymeric materials − despite the complexity of the intermolecular interactions involved in these materials. A number of experimental − and computational studies − aim to elucidate the role of cohesive energy and chain stiffness in polymer glass formation, and in particular, we mention the comprehensive experimental observations of Rössler and co-workers − on molecular and polymer fluids, along with a serious attempt to arrive at an improved parametric description of the functional form of the structural relaxation time τ α governing this extensive body of data. Despite progress in both experimental and modeling fronts, the development of a quantitative molecular theory capable of accurately predicting thermodynamic and dynamic properties of GF polymers in terms of basic molecular parameters and varying thermodynamic conditions remains a challenge.…”

Section: Introductionmentioning

confidence: 99%

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Role of Cohesive Energy in Glass Formation of Polymers with and without Bending Constraints

Douglas

2020

Macromolecules

121

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The influence of the monomer chemical structure and architecture on polymer glass formation has been long appreciated to be dominated by two principal molecular factors, namely, cohesive energy and chain stiffness. Here, we utilize molecular dynamics simulation, along with the analytic generalized entropy theory (GET), to compare and contrast the role of cohesive energy in determining the thermodynamic and dynamic properties of polymer melts with and without bending constraints at zero pressure. As a general finding, our simulation results support the predictions of the GET that the temperature dependence of basic thermodynamic properties, such as the reduced thermal expansion coefficient and isothermal compressibility, becomes universal when the temperature is scaled by the cohesive energy parameter regardless of the bending constraints. Our simulation results confirm that dramatic changes in the temperature dependence of the structural relaxation time and extent of cooperative motion caused by cohesive interaction strength can likewise be eliminated largely using the same scaled temperature in polymer melts without bending constraints, but this simple picture does not emerge when bending constraints are introduced, a result in general accord with the predictions of the GET. Moreover, the fine features predicted by the GET, which are associated with the nonlinear growth of characteristic temperatures and the reduction of fragility with increasing cohesive interaction strength, are also confirmed by our simulations for polymer melts with bending constraints. Going beyond the validation of the predictions of the GET, we show that our simulation results for the structural relaxation time and extent of cooperative motion conform to the string model of glass formation in both polymer melts with and without bending constraints having variable cohesive interaction strength. We then utilize the string model to understand phenomenological models of structural relaxation of recent interest in glassforming liquids. The string model also allows us to examine the dependence of the enthalpy and entropy of the high-temperature activation free energy on cohesive interaction strength. We emphasize the role of the activation entropy in improving the quantitative predictive capacity of the GET.

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Section: Resultsmentioning

confidence: 95%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Role of Cohesive Energy in Glass Formation of Polymers with and without Bending Constraints

Douglas

2020

Macromolecules

121

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“…We adopt b θ = 1.5 based on our previous work. 85 Nonbonded pair interactions are described by a truncated-and-shifted LJ potential,…”

Section: Molecular Dynamics Simulationmentioning

confidence: 99%

Equation of State and Entropy Theory Approach to Thermodynamic Scaling in Polymeric Glass-Forming Liquids

Douglas,

2021

Preprint

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Numerous experimental and computational studies have established that most liquids seem to exhibit a remarkable, yet poorly understood, property termed 'thermodynamic scaling' in which the structural relaxation time τ α , and many other dynamic properties, can be expressed in terms of a 'universal' reduced variable, T V γ , where T is the temperature, V is the material volume, and γ is a scaling exponent describing how T and V are linked to each other when either quantity is varied. Here, we show that this scaling relation can be derived by combining the Murnaghan equation of state (EOS) with the generalized entropy theory (GET) of glass formation. In our theory, thermodynamic scaling arises in the non-Arrhenius relaxation regime as a scaling property of the fluid configurational entropy density s c , normalized by its value s * c at the onset temperature T A of glass formation, s c /s * c , so that a constant value of T V γ corresponds to a reduced isoentropic fluid condition. Molecular dynamics simulations on a coarse-grained polymer melt are utilized to confirm that the predicted thermodynamic scaling of τ α by the GET holds both above and below T A and to test whether the extent L of stringlike collective motion, normalized its value L A at T A , also obeys thermodynamic scaling, as required for consistency with thermodynamic scaling. While the predicted thermodynamic scaling of both τ α and L/L A is confirmed by simulation, we find that the isothermal compressibility κ T and the long wavelength limit S(0) of the static structure factor do not exhibit thermodynamic scaling, an observation that would appear to eliminate some proposed models of glass formation emphasizing fluid 'structure' over configurational entropy. It is found, however, that by defining a low temperature hyperuniform reference state, we may define a compressibility relative to this condition, δκ T , a transformed dimensionless variable that exhibits thermodynamic scaling and which can be directly related to s c /s * c . Further, the Murnaghan EOS allows us to interpret γ as a measure of intrinsic anharmonicity of intermolecular interactions that may be directly determined from the pressure derivative of the material bulk modulus. Finally, we show that many phenomenological relationships related to the glass formation and melting of materials can be understood based on the Murnaghan EOS

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“…Nanosized fillers are commonly added to polymer matrices to impart desirable properties such as conductivity and mechanical strength. − However, the enhancement of such properties depends on a complex interplay of the nanoparticle–polymer and nanoparticle–nanoparticle interactions and the total surface area of nanoparticles exposed to the polymer matrix. − The exposed surface area, in turn, critically depends on the degree of dispersion of the nanofillers in the polymer host. As nanoparticle–polymer attractive interactions generally suppress chain relaxation dynamics and repulsive or nonattractive interactions enhance them; increasing the nanoparticle–polymer attraction would be expected to reduce bulk free volume, V f , and increase the glass-transition temperature, T g .…”

Section: Introductionmentioning

confidence: 99%

Surface Functionalization-Induced Effects on Nanoparticle Dispersion and Associated Changes in the Thermophysical Properties of Polymer Nanocomposites

2021

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The degree of nanoparticle dispersion in a polymer matrix is challenging to predict a priori from the structure and composition of polymer nanocomposites. The consequences of changing the nanoparticle surface’s chemical nature on its tendency to disperse remain an open question. In the present work, we adopted a fully atomistic molecular dynamics simulation to systematically compare the dispersion tendency of hydroxylated fullerene (C60) nanoparticles to bare fullerene nanoparticles in a poly(methyl methacrylate) (PMMA) matrix. We demonstrate that hydroxylated nanoparticles show better dispersion than bare nanoparticles at an equivalent loading. We also uncover that the enhanced dispersion is primarily an outcome of increased inter-nanoparticle repulsion between hydroxylated nanoparticles. Comparing polymer chain dynamics around individual nanoparticle surfaces reveals that the interfacial boundary is more dynamic for hydroxylated nanoparticles with numerous chains traversing the interfacial boundary. Finally, to ascertain the repercussions of disparate nanoparticle dispersion states on the observable macroscopic properties, we have calculated the glass-transition temperature T g at multiple nanoparticle loadings for both bare and hydroxylated nanoparticles. We observe that T g shows a similar trend with increasing nanoparticle loading for both cases. However, at any given nanoparticle loading, T g is lower for systems containing hydroxylated nanoparticles. We attribute this to a combination of nanoparticle–polymer energetic interactions and an increase of bulk free volume for nanocomposites containing hydroxylated nanoparticles. Thus, our study also provides crucial insights into the importance of accounting for nanoparticle dispersion effects in molecular simulations of polymer nanocomposites and proves that not accounting for such effects may ultimately lead to inaccurate conclusions.

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Understanding the Role of Cohesive Interaction in Mechanical Behavior of a Glassy Polymer

Cited by 30 publications

References 87 publications

Role of Cohesive Energy in Glass Formation of Polymers with and without Bending Constraints

Role of Cohesive Energy in Glass Formation of Polymers with and without Bending Constraints

Equation of State and Entropy Theory Approach to Thermodynamic Scaling in Polymeric Glass-Forming Liquids

Surface Functionalization-Induced Effects on Nanoparticle Dispersion and Associated Changes in the Thermophysical Properties of Polymer Nanocomposites

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