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The effects of annealing a glassy polymer at temperatures well below Tg on the dynamic linear viscoelastic properties were investigated. The polymer was an epoxy thermoset with a Tg of 182°C. Specimens were annealed from 2 h to up to 600 h at temperatures from −100°C (Tg −282°C) up to Tg. At annealing temperatures just below Tg, there was no effect of annealing, and annealing at −100°C (Tg −282°C) showed no difference between 2 and 6 h annealing, although annealing effects might be seen at longer times. At temperatures between −50°C (Tg −232°C) and 170°C (Tg −12°C) the storage isotherms increased with annealing while the loss isotherms decreased. In addition, the heat capacity as measured via DSC after annealing at 50°C (Tg −132°C) and 100°C (Tg −82°C) exhibited endothermic peaks approximately 40°C above the annealing temperature. This study clearly shows that a material deep in the glassy state is not frozen, but undergoes continuing evolution. The nature of deep glass aging is different than physical aging in the Tg region, where deep in the glass the relaxation spectrum changes magnitude and shape but does not shift along the log frequency axis in contrast to physical aging in the Tg region. [ABSTRACT FROM AUTHOR]

In this article, a novel constitutive model using fractional calculus is developed to capture the stress relaxation behavior of glassy polymers, where a variable‐order differential operator based on Marchaud fractional derivative is adopted. To assess the validity of the proposed model, a series of stress relaxation tests of the representative glassy polymer, poly(ethylene glycol‐co‐1,4‐cyclohexanedimethanol terephthalate), are conducted, and the stress responses of the specimens under different ambient temperatures are obtained. In view of the basic theory of fractional viscoelasticity, the varying order is assumed to bea function of time. Through the comparison of the fitting effect and reasonability of four kinds of order function based on the experimental data, it is concluded that the linear order function of time is suitable for our model which shows high accuracy. Moreover, the physical significance of order function is analytically derived, and the rising order is found to have a direct connection with the continuous softening of polymers during relaxation. Deeper investigations manifest that high ambient temperature accelerates the material softening, and the change of microstructures inside polymers could be reflected by the varying order, which is possible to provide a new perspective on manufacturing proper polymers with excellent resistance to stress relaxation. [ABSTRACT FROM AUTHOR]

The diffusion of small molecules through amorphous polymers proceeds by the thermally activated jumps whose rate changes from one region to another due to polymer heterogeneity. Little is known at present about the origin of diffusion heterogeneity. The oxidation of triplet nitrene and quenching of phenanthrene phosphorescence have been used to study the movement of molecular oxygen on a nanometer length scale in glassy poly(ethyl methacrylate) and poly(n‐butyl methacrylate) far below the glass transition temperature. It has been found that, as temperature increases, the jump rates in all regions increase by the same factor. This finding points to the equality of the activation energies of oxygen jumps in different regions of the polymers. We conclude that the correlated elastic displacements of polymer chains inside regions about several nanometers in size provide molecules jumps. Due to the large size of these regions, the activation energy of jump does not depend on local polymer structure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1097–1104 [ABSTRACT FROM AUTHOR]

In this study, the rate‐ and temperature‐dependent strain hardening and the Bauschinger effect is studied for three glassy polymers. It appeared that for all materials, an equal distribution of elastic and viscous hardening was necessary to accurately predict the Bauschinger effect, as well as the rate‐ and temperature‐dependent strain hardening response. As for the elastic contribution, the viscous contribution appears to increase with an increase in entanglement network density. Investigating the effect of temperature on the Bauschinger effect revealed that at elevated temperatures the model predictions are not accurately enough. It is shown that this is caused by the magnitude of the elastic hardening contribution; to improve the predictions, a temperature‐dependent elastic contribution is necessary. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1001–1013 [ABSTRACT FROM AUTHOR]

ABSTRACT Although gas sorption in glassy polymers is a well-studied phenomenon, no general microscopical model is developed which is able to describe the gas sorption in a wide temperature range using only characteristics of polymer and gas molecule. In this work, sorption isotherms and desorption kinetics of O2, Ar, and N2 for glassy poly(ethyl methacrylate) have been measured in the temperature range from 160 to 308 K. To describe both the phenomena, the model is developed which postulates that, in the frozen structure of glassy polymer, any cavities between macromolecules are the sorption sites for small molecules. The cavities of small size can expand elastically to accommodate a gas molecule. The sorption sites are considered to be the potential wells and their depths are distributed according to Gaussian law. The concentration of sorption sites, their mean depth and depths dispersion, and the frequency of molecules oscillations in the sorption sites are the only parameters which determine both the gas transport and sorption. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 288-296 [ABSTRACT FROM AUTHOR]

ABSTRACT Infinite dilution gas solubility coefficients in glassy polymers of diverse chemical structure are predicted on the basis of a simple computational model of short polymer chain conformations. In contrast to existing models that describe the infinite dilution solubility coefficient S of various gases in certain polymers or S for the particular gas in different polymers, our approach allows to deal with the broad variety of gases and glassy polymers of different structure. With the van der Waals volume, accessible surface area, and some derivative geometrical descriptors being the predicting variables, the multivariate linear regression on the testing set gives correlation 0.89 with the logarithm of the solubility coefficient for the considered classes of relatively low-permeable polymers (the mean relative error of solubility coefficient S prediction is 68%). Regression accuracy can be improved up to correlation 0.95 and the mean relative error 39% when a separate regression is constructed for each penetrant gas. Implications are discussed to the analysis of solubility selectivity. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 228-244 [ABSTRACT FROM AUTHOR]

ABSTRACT Sorption of vapors of four organic compounds in two glassy polymers, cellulose triacetate (CTA) and poly[(trimethylsilyl)propyne] (PTMSP), has been reported and analyzed in terms of Guggenheim-Anderson-De Boer (GAB) model. These two structurally and physicochemically different glassy polymers both independently showed that one sorption site was formed by about three monomeric units. This finding held true for vapors of all characterized compounds; that is, for methanol, for its derivatives dimethyl carbonate and methyl acetate, and for acetone. The 'rule of three' might thus also be applicable to other sorbates and glassy polymers. Further, an original modification of the GAB model for the sorption of alcohols in PTMSP was derived and successfully tested. Overall, the analyses of the sorption isotherms, heats of sorption and diffusion coefficients supported the view that the sorption of vapors in glassy polymers has adsorptive nature. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 561-569 [ABSTRACT FROM AUTHOR]

ABSTRACT The linear rheological responses of a series of specially designed wedge-type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, Tg. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at Tg −60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 899-906 [ABSTRACT FROM AUTHOR]

ABSTRACT The suitability of the Guggenheim-Anderson-De Boer (GAB) model for the parameterization of gas sorption isotherms and their dependences on temperature is explored. The GAB model implies that molecules adsorb on inner surfaces of the polymer in multilayers, which contrasts with the assumptions of the classical Dual Mode Sorption (DMS) model which implies the simultaneous occurrence of Henry-like dissolution and Langmuir's case I adsorption. The GAB model shows similar efficacy of the parameterization of the gas sorption isotherms in polymers as the DMS model. The isosteric heat of adsorption shows clear dependence on relative surface coverage for carbon dioxide sorption in cellulose acetate, polyethylene terephthalate, and the first polymer of intrinsic microporosity (PIM-1), thus allowing for the occurrence of adsorption multilayers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1490-1495 [ABSTRACT FROM AUTHOR]

ABSTRACT The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well-known Williams-Landel-Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile-to-strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 907-915 [ABSTRACT FROM AUTHOR]

ABSTRACT Crosslinking can fundamentally change the mechanical properties of a linear glassy polymer. It has been experimentally observed that when lightly crosslinked, poly(methyl-methacrylate) (PMMA) has a characteristically more ductile response to mechanical loading than does linear PMMA despite having a higher glass transition temperature. Here, molecular dynamics (MD) simulations are used to investigate conformational and energetic differences between linear PMMA and lightly crosslinked PMMA under shear deformation. As consistent with experiments, crosslinked PMMA is found to have a reduced yield stress relative to linear PMMA. Using the probing capabilities of our explicit atom MD approach, it is observed that while the crosslinks have a minimal direct energy contribution to the total system, they can alter how the main chains conform to macroscopic loading. In crosslinked PMMA, the backbone aligns more with the direction of external loading, thereby reducing the force applied to (and associated deformation of) the polymer bonds. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 444-449 [ABSTRACT FROM AUTHOR]

Recently, we have reported that azobenzene-functionalized polyimide materials exhibit a substantial increase in photogenerated stress in comparison to azobenzene-functionalized acrylate-based liquid crystal polymer networks. Here, we show that pre-straining the azo-CP2-20 material further increases the magnitude of the photomechanical response visualized as bending in the cantilever geometry as well as in direct measurements of photogenerated stress. Additionally, we also report on the ability to optically fix both optically and mechanically generated shapes in azo-CP2-20. The optically fixable shape memory of azo-CP2-20 is enabled by the introduction of excess free volume during thermal processing as the material was pre-strained. [ABSTRACT FROM AUTHOR]

Deformation is known to enhance the atomic mobility in disordered systems such as polymer materials. To elucidate the origin of this phenomenon, we carry out two types of simulations: molecular dynamics (MD) simulations, which determine the atomic trajectories at finite temperature, and quasi-static simulations, which determine the atomic trajectories in the limit of zero temperature (and in the limit of zero shear rate). The quasi-static simulations show discontinuous changes in properties, such as system energy and atomic mobility. We use a normal mode analysis to show that these discontinuous changes arise from fold catastrophes of the potential energy landscape, in which energy minima flatten out and the heights of energy barriers decrease to zero; this was demonstrated by normal mode frequencies following a power law with an exponent of 0.5 as the discontinuous change is approached. After the fold catastrophe, the system relaxes to a different energy minimum, giving rise to atomic displacements. These fold catastrophes are the only mechanism for diffusive atomic displacements in the quasi-static simulations, where there is no thermal energy. We compared the mean-squared displacements as a function of strain from the quasi-static simulations to those from MD simulations (which do include thermal effects)-the similarity of the values of the mean-squared displacements in these two types of simulations demonstrates that the fold catastrophes underlie the enhanced dynamics in strained polymer systems even at finite temperature. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

The modeling and quantification of a viscous contribution to strain hardening is discussed. Traditional strain hardening models, based on rubber-elasticity, show serious deviations from the experimentally observed large-strain response of glassy polymers. Therefore, to capture both the strain rate and temperature dependence of strain hardening correctly, a part of the rubber-elastic strain hardening is replaced with a viscous contribution. This is realized by introducing a deformation-dependent viscosity in the Eindhoven Glassy Polymer model. Verification of the accuracy of the model proposed is done using a set of uniaxial compression tests on polycarbonate at different strain rates and temperatures. An accurate description of all experimental data results and, moreover, the model quantitatively captures the Bauschinger effect observed in oriented polycarbonate. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Considering the current view that physical aging of glasses results in an increase of activation barriers to plastic deformation, it is surprising that, until now, no influence of physical aging was observed experimentally on the temperature dependence of plastic deformation in polymeric glasses. This study evaluates why such an influence has not been found, and it is shown that detailed analysis of a set of uniaxial compression data on polycarbonate (PC) at different strain rates and temperatures leads to the conclusion that a significant influence indeed exists. As a consequence, the Eyring activation energy depends on the aging history of the material. These experimental observations are rationalized in terms of a simple physical interpretation of the aging phenomenon. The article also contains a discussion of the apparent deaging of amorphous polymers induced by large-strain plastic deformation. This discussion is of key importance here, because this study compares yield stresses of aged PC with those of mechanically deaged, or rejuvenated, PC. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

A new gas/vapor mixture permeation system is described to investigate the effect of organic molecules on oxygen (O2) and carbon dioxide (CO2) transport in barrier materials. Methanol vapor was considered as a flavor simulant mainly because of its conveniently high diffusion coefficient, which makes the experimental time accessible. A highly accurate syringe pump was used to introduce a desired activity level of vapor into gas feed stream. Adsorption of methanol on high energy surfaces is carefully characterized to prevent underestimation of methanol permeability. A special permeation cell was also developed to study the effect of interacting vapors on O2 and CO2 transport in barrier materials. Systematic permeation measurements were conducted for binary and ternary gas/vapor permeation measurements (e.g., MeOH/O2 and O2/CO2/MeOH) to verify the feasibility of our new vapor/gas permeation system. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Wrinkles with two distinct wavelengths formed sequentially on the same surface are investigated. A series of aligned wrinkles are formed through local strain application on a partially crosslinked elastomer. After the formation of these primary wrinkles, the elastomer is fully crosslinked, and a mechanical compressive strain is applied to the sample orthogonal to the primary wrinkles. This mechanical strain results in smaller secondary wrinkles superimposed on the larger primary aligned wrinkles. Resulting biaxial morphologies suggest that the primary pattern directs the formation of the smaller wrinkles. The modulus mismatch of the substrate on primary and secondary wrinkle formation dictates the ratio between the two resulting wavelengths, as well as the specific biaxial morphologies, ranging from zigzag ridges to ellipsoidal bumps or corn-on-the-cob structures to the classic herringbone. The sequential strain wrinkling process has the potential to be used on an industrial scale for the facile formation of surface topography with two discrete, tunable lateral dimensions over large surface areas. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Light can wirelessly direct functional responses in photoresponsive polymeric materials and composites. The intensity, phase, and polarization of light can be spatially modulated into complex patterns with holography (intensity or polarization) or masking (intensity or phase). Deriving from these foundational properties of light, photoresponsive macromolecular systems exhibit exceeding potential to yield rapid and highly engineered macroscopic as well as spatially selectable mechanically adaptive responses useful as soft actuators or topographical surfaces in aerospace, automotive, and biomedical applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Summary: Thick bulk films of linear amorphous polymers with different chain architecture and molecular weight were brought into contact with themselves in a lap-shear joint geometry at bonding temperatures ( T) below the glass transition temperatures of their bulk ( T [ABSTRACT FROM AUTHOR]

The bulk mechanical properties of linear triblock and pentablock copolymers that self-assemble into hexagonally packed cylinders with glassy, unentangled matrices of poly(cyclohexylethylene) (PCHE for a homopolymer, C for a block copolymer) with rubbery poly(ethylene- alt-propylene) (P) and semicrystalline polyethylene (E) minority components are examined. The tensile properties of high C content CEC triblock copolymer could not be quantified; however, CPC can plastically deform under uniaxial strain, unlike brittle PCHE. Both CECEC and CPCPC pentablock copolymers exhibited ductile tensile behavior, but the tensile properties of blends of these two pentablock copolymers show that the addition of crystallinity in the minority phase prevents strain softening after yielding and necking, which indicates that these samples deform only via crazing. On the other hand, the white gage region of CPCPC and the ability of CPCPC to neck indicate that high C content materials deform via shear yielding and crazing when the minority component is a rubbery material. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Back Cover: A large increase in the magnitude of the photogenerated stress is observed in a crosslinked azobenzene‐functionalized polyimide. Subjecting the material to pre‐strain aligns the azobenzene units in the polyimide, which serves to enhance the cooperative motion of the photo chemical mechanism to yield a photogenerated stress of 1.25 MPa. This value is more than two times larger than the stress generated in the “as‐prepared” polyimide. Further details can be found in the article by K. M. Lee, D. H. Wang, H. Koerner, R. A. Vaia, L.‐S. Tan,* and T. J. White* on page 1189. [ABSTRACT FROM AUTHOR]