Relaxation In Glassy Polymers Research Articles

Structural relaxation in glassy polymers predicted by soft modes: a quantitative analysis.

We present a quantitative analysis of the correlation between quasi-localized, low energy vibrational modes and structural relaxation events in computer simulations of a quiescent, thermal polymer glass. Our results extend previous studies on glass forming binary mixtures in 2D, and show that the soft modes identify regions that undergo irreversible rearrangements with up to 7 times the average probability. We study systems in the supercooled- and aging-regimes and discuss temperature- as well as age-dependence of the correlation. In addition to the location of rearrangements, we find that soft modes also predict their direction on the molecular level. The soft regions are long lived structural features, and the observed correlations vanish only after >50% of the system has undergone rearrangements.

Prediction of the relationship between the rate of deformation and the rate of stress relaxation in glassy polymers

Kim, et al. (Polymer, 54(15), 3949, 2013) recently reported on the unexpected relaxation behavior of an amorphous polymer in the Tg-region, where the rate of stress relaxation increased with deformation at a strain rate of 1.5 × 10−4 s−1 but decreased at a strain rate of 1.2 × 10−5 s−1. This inversion in the ordering with strain rate challenges the underlying structure of the existing nonlinear viscoelastic and viscoplastic constitutive models, where the key nonlinearity is a deformation dependent material clock. The nonlinear stress relaxation predictions of a recently developed stochastic constitutive model, SCM, (Medvedev, et al., J. Rheology, 57(3), 949, 2013) that acknowledge dynamic heterogeneity of the glass have been investigated. The SCM predicts the inversion in the ordering of the mobility with the loading strain rate as reported by the stress relaxation response. The change in perspective on the nonlinear viscoelastic behavior of glassy polymers engendered by the SCM is discussed.

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T ( g ) + 8 K to T ( g ) - 83 K and times as long as 10(5) minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20 nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T ( g ) value. The surface relaxation times are only weakly temperature dependent and near T ( g ), the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a "surface glass transition" and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.

Stress effects on mass transport in polymers: a model for volume relaxation

A model is introduced here for volume relaxation in glassy polymers, induced by sorption of low molecular weight species. The model relies on description of thermodynamic properties of glassy mixtures obtained through NET-GP model and it considers simple linear viscoelastic laws to interpret the stress–strain relation for the polymeric material. Results for the case of sorption kinetics in constant fugacity processes, obtained from the model, are compared with experimental values for vapour sorption in glassy polymeric microspheres. Satisfactory representations were obtained both for instant solute content measured in experimental run and for mass uptake registered in ‘relaxation stage’ of sorption processes. Viscosity data obtained from fitting of sorption experimental results are finally discussed in terms of temperature dependence and compared with values obtained form purely mechanical volume relaxation experiments.

Enthalpy relaxation in glassy polymers

Constitutive equations are derived for enthalpy recovery in amorphous glassy polymers. The model is based on the concept of cooperative relaxation which treats a polymer as an ensemble of flow units rearranging at random times. The rate of rearrangement is determined by the Eyring theory of thermally activated processes. Fair agreement is demonstrated between results of numerical simulation and experimental data for polycarbonate, poly(ether imide), poly(methyl methacrylate), polystyrene and a glycerol glass.

A model of traps for structural relaxation in glassy polymers

Constitutive equations are derived for enthalpy and volume recovery in amorphous glassy polymers after thermal treatment. The model is based on the concept of cooperative relaxation, where flow units are assumed to change their traps at random times. Fair agreement is demonstrated between numerical predictions and observations in calorimetric and dilatometric tests for polycarbonate, polystyrene and poly(vinyl acetate).

Volume and Enthalpy Relaxation in Glassy Polymers

Volume and Enthalpy Relaxation in Glassy Polymers

Modelling an anomalous stress relaxation in glassy polymers (the Kitagawa effect)

Constitutive relations are derived for the nonlinear viscoelastic response of glassy polymers. The model is based on the concept of adaptive links (a version of the theory of transient networks), where the rates of annihilation of active links are determined by the Eyring formula. Adjustable parameters in the constitutive equations are found by using experimental data for a polyester resin in standard relaxation tests. The stress-strain relations are applied to describe an anomalous stress relaxation in solid polymers observed in tensile test after strain reversal (the Kitagawa effect). Qualitative agreement is demonstrated between results of numerical simulation and observations.

ガラス状高分子の非晶構造と緩和 誘電緩和でみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 誘電緩和でみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 低分子輸送とガラス状高分子の非晶構造

ガラス状高分子の非晶構造と緩和 低分子輸送とガラス状高分子の非晶構造

ガラス状高分子の非晶構造と緩和 オルトポジトロニウムでみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 オルトポジトロニウムでみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 ガラス状高分子の非晶構造と破壊

ガラス状高分子の非晶構造と緩和 ガラス状高分子の非晶構造と破壊

ガラス状高分子の非晶構造と緩和 電子用エポキシ樹脂の微細構造と物性

ガラス状高分子の非晶構造と緩和 電子用エポキシ樹脂の微細構造と物性

ガラス状高分子の非晶構造と緩和 ガラス形成高分子の緩和過程と不均一性

ガラス状高分子の非晶構造と緩和 ガラス形成高分子の緩和過程と不均一性

ガラス状高分子の非晶構造と緩和 熱分析でみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 熱分析でみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 固体ＮＭＲでみるガラス状高分子の非晶構造と緩和

ガラス状高分子の非晶構造と緩和 固体ＮＭＲでみるガラス状高分子の非晶構造と緩和

Orientation relaxation in glassy polymers. II. Dipole-size spectroscopy and short-time kinetics

The orientational diffusion of a rodlike particle embedded in a glassy polymeric matrix is considered; the underlying kinetics is that of local rearrangements. A defining parameter of the theory is the length of the particle. The timing of steps of the random walk in orientation space is determined by rearrangements. We discuss the physical properties of the glass state in connection with the rearrangement kinetics. The orientational diffusion is influenced by the local disorder; this influence is different for dipoles of different length. For a short dipole, the resulting diffusion is of generalized Debye type. Nonexponential relaxation of physical quantities may then be caused by the distribution of rearrangement barriers. For longer dipoles and if the orientation is uniquely determined by the configuration of the embedding cluster, the motion is a random walk on a given random map on a sphere. An ensemble of random mappings is considered. For even longer dipoles, hierarchical (multiscale) relaxation is expected. We discuss the relation of the theory to the short time depoling kinetics in a system of dipoles having different length, such as are found in relaxation of electrically poled polymer materials.

Principal features of structural relaxation in glassy polymers. A Review

AbstractThe principal features of models of structural relaxation in the glassy state were reviewed. The concepts of fictive temperature as the structural parameter and nonexponentiality and nonlinearity of structural relaxations were described. Various expressions for the structural relaxation time were presented and compared. One particular model was selected and utilized in a parametric optimization study of structural relaxation. Model predictions were checked against experimental results obtained by volumetric and calorimetric measurements.

Proposed correlation between enthalpic and viscoelastic measurements of structural relaxation in glassy polymers

An investigation was carried out seeking to correlate the results of enthalpic and viscoelastic measurements of the progress of structural relaxation in glassy polymers. Experimentally, changes in enthalpy were followed by calorimetry, and changes in the viscoelastic response by stress relaxation. The Narayanaswamy/Moynihan model was used in the simulation of enthalpy relaxation. An optimized set of model parameters was determined, for which the calculated enthalpy relaxation times were comparable to the experimental stress relaxation times, thus providing an empirical link between two different manifestations of the same process.

Structure as a function of relaxation in glassy polymers

AbstractPreviously, amorphous glasses were simulated by carrying out energy minimization on initial conformations generated by growing polymer chains in a periodic cube. It was not known what degree of relaxation these simulated glasses possessed. The degree relaxation is determined by the thermal history of the bulk polymer and in turn determines numerous important properties. Constant stress molecular dynamics (CSMD) followed by energy minimization was used to simulate different thermal histories of an isotactic poly(propylene) glass. This simulation approach produced glasses in which the degree of energetic relaxation was a function of the thermal history. Based on the simulated cohesive energy density, the simulation using minimization alone produced a glass that was energetically annealed. However, the local geometry and x‐ray structure factor indicate that it has a different structure than those obtained using CSMD followed by energy minimization and may not be structurally relaxed.