Time–temperature-dependent behavior of a substituted poly(paraphenylene): Tensile,creep, and dynamic mechanical properties in the glassy state

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1–1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T_g. Physical aging was shown to interact with long-term creep, rendering time–temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate μ. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 70: 2971–2979, 1998     

Self‐diffusion of small molecules into rubbery polymers: A lattice free‐volume theory

In the framework of the free‐volume (FV) theory, a new equation was derived for the evaluation of self‐diffusion coefficients of small molecules in polymers above the mixture glass transition temperature. The derivation of the equation turned out to be straightforward once the equivalence between the free volume and the unoccupied volume given by thermodynamic lattice theories is assumed. A parameter evaluation scheme is proposed, which is substantially simpler compared with the conventional Vrentas–Duda approach, even without losing generality. The key assumption is discussed, and its consistency is verified from a numerical viewpoint. A comparison with experimental solvent self‐diffusion coefficients for several solvent/polymer binary systems confirmed that the proposed theory presents good correlative ability over wide temperature and composition ranges. Moreover, the introduced thermodynamic foundation allows one to easily include the pressure effect too. In the frame of the proposed lattice free volume theory, the sizes of the polymer jumping units decrease with temperature and increase with pressure. Such behavior converges with theoretical expectations and opens the way for a predictive FV theory. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 529–540, 2010     

Impact of average free‐volume element size on transport in stereoisomers of polynorbornene. I. Properties at 35 °C

This article demonstrates that transport of gases through glassy polymers is significantly influenced not only by the absolute amount but also by the distribution of free volume. Two stereoisomers of polynorbornene with nearly equivalent total free volume, but markedly different average free‐volume sizes, were evaluated. The free‐volume element size was probed with positron annihilation lifetime spectroscopy, wide‐angle X‐ray scattering, gas sorption, and molecular modeling. The permeation, sorption, and diffusion of light gases were measured in each stereoisomer at 35 °C. All analytical techniques indicated that one isomer (labeled as Architecture II) had a larger average free‐volume element size but fewer elements. This isomer also had a very slightly higher bulk density (1.000 vs 0.992 g/cm³ for the other stereoisomer). Architecture II also had gas sorption and diffusion coefficients that were two to three times those of the less dense counterpart. These differences have been attributed to differences in the free‐volume element size available within the polymer matrix. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2185–2199, 2003     

New free‐volume‐related activity model for polymer solutions

A new activity model was developed with the use of the Gibbs–Helmholtz relation at constant pressure and composition. This model consists of combinatorial and residual terms. The residual term is the same as that in the UNIQUAC method, but the total area fraction of molecules is introduced on account of the hole effect. The combinatorial part takes into account the free volume (FV) effect, which plays a significant role in polymer systems. The validity of this model is demonstrated by calculating the solvent activities in 36 polymer solutions in comparison with Entropic‐FV (EFV) and UNIFAC‐FV methods. The total average absolute deviations (AAD) from the experimental observations are 8.27, 6.38, and 1.64 for EFV, UNIFAC‐FV, and the present method, respectively. It is found that the fit to these experimental data by the present model is quite good over a wide range of concentration. An estimation of the infinite dilution activity coefficients also proves the validity of the new method. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3299–3307, 2005     

Study of electrical,mechanical, and nanoscale free‐volume properties of NBR and EPDM rubber reinforced by bentonite or kaolin

The effect of Na bentonite, Ca bentonite, and kaolin fillers on the macrostructure and microstructure of acrylonitrile butadiene rubber, ethylene propylene diene rubber, and their blend (50/50) was studied through electrical and mechanical measurements, as well as with positron annihilation lifetime spectroscopy. The real part of permittivity (ε′), dielectric loss (ε″), and the crosslinking density were found to increase with increasing filler content. The increase of crosslinking density of the blend with increasing amount of fillers reflects a decrease in the equilibrium swelling up to 21.50 wt % compared with that of the unfilled blends. The mechanical investigation showed pronounced increase in the tensile strength, and in elongation at break with the addition of up to 21.50 wt % of filler. In addition, comparing between different fillers showed that the reinforcing effect of Na bentonite is more effective than Ca bentonite and kaolin but the physico‐mechanical of Ca bentonite is less than that for kaolin. The positron annihilation lifetime measurements revealed that the free‐volume properties were strongly affected by the amount and type of filler, in particular, the free‐volume fraction was dramatically decreased with increasing filler content. Furthermore, correlations were made between the free‐volume parameters and both electrical and mechanical properties. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1825–1838, 2009     

Triptycene‐containing polyetherolefins via acyclic diene metathesis polymerization

Several new triptycene‐containing polyetherolefins were synthesized via acyclic diene metathesis (ADMET) polymerization. The well‐established mechanism, high selectivity and specificity, mild reaction conditions, and well‐defined end‐groups make the ADMET polymerization a good choice for studying systematic variations in polymer structure. Two types of triptycene‐based monomer with varying connectivities were used in the synthesis of homopolymers, block copolymers, and random copolymers. In this way, the influence of the triptycene architecture and concentration in the polymer backbone on the thermal behavior of the polymers was studied. Inclusion of increasing amounts of triptycene were found to increase the glass transition temperature, from ?44 °C in polyoctenamer to 59 °C in one of the hydrogenated triptycene homopolymers ( H‐PT2 ). Varying the amounts and orientations of triptycene was found to increase the stiffness ( H‐PT1 ), toughness ( PT1₁‐b‐PO₁ ) and ductility ( PT1₁‐ran‐PO₃ ) of the polymer at room temperature. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Application of free‐volume theory to self diffusion of solvents in polymers below the glass transition temperature: A review

Theories based on free‐volume concepts have been developed to characterize the self and mutual‐diffusion coefficients of low molecular weight penetrants in rubbery and glassy polymer‐solvent systems. These theories are applicable over wide ranges of temperature and concentration. The capability of free‐volume theory to describe solvent diffusion in glassy polymers is reviewed in this article. Two alternative free‐volume based approaches used to evaluate solvent self‐diffusion coefficients in glassy polymer‐solvent systems are compared in terms of their differences and applicability. The models can correlate/predict temperature and concentration dependencies of the solvent diffusion coefficient. With the appropriate accompanying thermodynamic factors they can be used to model concentration profiles in mutual diffusion processes that are Fickian such as drying of coatings. The free‐volume methodology has been found to be consistent with molecular dynamics simulations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Evaluation of the free‐volume theory of diffusion

General characteristics of the free‐volume theory of diffusion are discussed, and a recent data‐theory comparison involving free‐volume theory is critically evaluated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 501–507, 2003     

Free‐volume change in volume phase transition of polyacrylamide gel as studied by positron annihilation: Salt dependence

Variation of free‐volume parameters—average radius size, number concentration, and size distribution—of a polyacrylamide (PAAm) gel containing 4 mol % carboxylate anions is studied during a volume phase transition (VPT) caused by a change of sodium chloride (NaCl) concentration. A positron annihilation lifetime technique is used for the determination of the free‐volume characteristics. The measurement is performed in an acetone–water 3 : 2 (v/v) [0.27 : 0.73 (mol/mol)] mixed solvent at 20°C, and the free‐volume parameters deduced from the analysis of a positron annihilation curve are utilized. An average free‐volume size of the swollen PAAm gel, ∼ 0.32 nm in radius, almost agrees with that of the mixed solvent for a corresponding salt concentration, while the size of the collapsed gel, which is ∼ 0.28 nm in radius, is smaller than that of the mixed solvent. The results for the collapsed gel indicate that the hydrogen bond plays a significant role in the nanoscopic environment. The radius of the free‐volume of the swollen PAAm gel seems to be influenced by the composition between acetone and water. An inhomogeneity of the nanoscopic structure inside the PAAm gels is discussed in terms of a dispersion of a size distribution of the free‐volume. It is concluded that a change of the nanoscopic environment of the PAAm gel during the VPT can be monitored through the free‐volume parameters obtained by the positron annihilation lifetime technique. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2634–2641, 1999     

Additive‐like amounts of HDPE prevent creep of molten LDPE: Phase‐behavior and thermo‐mechanical properties of a melt‐miscible blend

Low‐density polyethylene (LDPE) is the preferred type of polyolefin for many medical and electrical applications because of its superior purity and cleanliness. However, the inferior thermo‐mechanical properties as compared to, for example, high‐density polyethylene (HDPE), which arise because of the lower melting temperature of LDPE, constitute a significant drawback. Here, we demonstrate that the addition of minute amounts of HDPE to a LDPE resin considerably improves the mechanical integrity above the melting temperature of LDPE. A combination of dynamic mechanical analysis and creep experiments reveals that the addition of as little as 1 to 2 wt% HDPE leads to complete form stability above the melting temperature of LDPE. The investigated LDPE/HDPE blend is found to be miscible in the melt, which facilitates the formation of a solid‐state microstructure that features a fine distribution of HDPE‐rich lamellae. The absence of creep above the melting temperature of LDPE is rationalized with the presence of tie chains and trapped entanglements that connect the few remaining crystallites. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 146–156     

Wide‐angle X‐ray scattering study of the lamellar/fibrillar transition for a semi‐crystalline polymer deformed in tension in relation with the evolution of volume strain

This work is devoted to the study of the deformation mechanisms of a high‐density polyethylene deformed in tension. Specific treatments were applied to synchrotron wide‐angle X‐ray scattering patterns obtained in situ with the aim of quantifying: (i) the evolution of the apparent crystal sizes during the deformation process, (ii) the reorientation dynamics of the fragmented crystals while aligning their chains along the drawing axis during the establishment of the fibrillar morphology, and (iii) the reorientation dynamics of the amorphous chains. In addition, the volume strain evolution was measured using 3D digital image correlation. The cavitation phenomenon was found to mainly occur during the lamellae fragmentation phase. At the end of the deformation process, when the lamellar structure is destroyed, the fragmented crystals have new degrees of freedom and become free to rotate to align their chains along the drawing axis. Crystal fragmentation is then no longer needed to allow material deformation, and there is no further volume strain increase. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1470–1480     

Viscoelastic characterization of polymers using instrumented indentation. I. Quasi‐static testing*

The use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time‐dependent and slight nonlinear responses in both rheometry and indentation measurements. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1794–1811, 2005     

A study on free‐volume properties of styrene‐acrylic ester type copolymers

The positron annihilation lifetime measurements have been performed on a number of amorphous styrene–methyl acrylate copolymers and styrene–butyl methacrylate copolymers. The densities of copolymers were obtained with immersion method by using a capillary pycnometer and the average molecular weights were determined by gel chromatography. The lifetime τ₃ of ortho‐positronium (o‐Ps) pick‐off annihilation have been found to correlate with side group volume and polarity of macromolecular chains in the copolymers, and relative intensity I₃ is attributed mainly to the electron‐attracting groups trapping the spur electrons and positrons. The experimental results have been discussed on the basis of the structural variation of macromolecular chains. In addition, the PALS measurement as a function of time for polystyrene and several styrene–methyl acrylate copolymers has also been performed. The result shows that an electric field is built in polymers during extended positron annihilation spectroscopy measurement, and the field effect is a main factor which causes the decrease in I₃ with time. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2476–2485, 1999     

TGSA: A molecular superposition program based on topo‐geometrical considerations

In this article, a new molecular alignment procedure to provide general‐purpose, fast, automatic, and user‐intuitive three‐dimensional molecular alignments is presented. This procedure, called Topo‐Geometrical Superposition Approach (TGSA), is only based on comparisons of atom types and interatomic distances; hence, the procedure can handle large molecular sets within affordable computational costs. The method is able to accurately align 3D structures using the common molecular substructures, as inferred by the bonding pattern (atom correspondences), where present. The algorithm has been implemented into a program named TGSA99, and it has been tested over eight different molecular sets: flavilium salts, amino acids, indole derivatives, AZT, steroids, anilide derivatives, poly‐aromatic‐hydrocarbons, and inhibitors of thrombine. The TGSA algorithm performance is evaluated by means of computational time, number of superposed atoms, and index of fit between the compared structures. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 255–263, 2001     

Free‐volume change during the volume phase transition of polyacrylamide gel studied by positron annihilation: Solvent‐composition dependence

Changes in the free‐volume parameters of polyacrylamide (PAAm) gels during the volume phase transition (VPT) were studied with the positron annihilation lifetime technique. The VPT was induced through the variation of the solvent composition in a mixture of acetone and water. The PAAm gels containing 0 and 4 mol % carboxyl groups in their polymer chains were adapted to compare the effect of the presence of ionic groups on the microscopic environment. The change of the free‐volume property is discussed on a nanoscopic scale, with attention paid to the interactions between the polymer chains and the solvent molecules. It is proven that the variations of the free‐volume parameters correlate significantly with the VPT phenomenon. The results of the free volume for both gels are well‐explained when an interaction parameter, ε_g, is assumed. The interpretation suggests that the state of the interactions among the components (the polymer chain, acetone, and water molecules) plays an important role in the change of the free volume of PAAm gels during the VPT. An increase of the dispersion of the free‐volume size near the VPT point was observed for the ionized PAAm gel. The broadened size distribution of the free volume of the ionized PAAm gel around the VPT point lay between those of pure water and the corresponding mixed solvent, suggesting that a local minimum of the average free‐volume size at the VPT point is caused by the increase of a specific interaction, hydrogen bonding. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 922–933, 2000     

A solvation‐free‐energy functional: A reference‐modified density functional formulation

The three‐dimensional reference interaction site model (3D‐RISM) theory, which is one of the most applicable integral equation theories for molecular liquids, overestimates the absolute values of solvation‐free‐energy (SFE) for large solute molecules in water. To improve the free‐energy density functional for the SFE of solute molecules, we propose a reference‐modified density functional theory (RMDFT) that is a general theoretical approach to construct the free‐energy density functional systematically. In the RMDFT formulation, hard‐sphere (HS) fluids are introduced as the reference system instead of an ideal polyatomic molecular gas, which has been regarded as the appropriate reference system of the interaction‐site‐model density functional theory for polyatomic molecular fluids. We show that using RMDFT with a reference HS system can significantly improve the absolute values of the SFE for a set of neutral amino acid side‐chain analogues as well as for 504 small organic molecules. © 2015 Wiley Periodicals, Inc.     

Free volume and water vapor transport properties of temperature‐sensitive polyurethanes

Segmented polyurethanes (PU) with crystalline soft segments were prepared with different crystalline polyols as soft segments. Morphology and microstructure of the PUs were investigated using Differential Scanning Calorimetry (DSC), Wide‐angle X‐ray Diffraction (WAXD), and Positron Annihilation Lifetime Spectra (PALS). Water vapor transport properties of the PU membranes were measured in the temperature range of crystal melting of their soft segments. Dependence of free volume of the PUs on temperature and the relationship between the free volume and water vapor permeability of the PU membranes were investigated. The results show that the mean free volume size and fractional free volume increase more rapidly in the temperature range of crystal melting than in other temperature intervals. In the specified temperature range, water vapor permeability of the polyester based PU membranes increases significantly, caused by the steep increase in free volume, due to crystal melting of the soft segments. Water vapor permeability of the polyester based PUs exhibits approximately direct correlation with the fractional free volume within the temperature range of crystal melting. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1865–1872, 2005     

Effects of temperature and stress on creep properties of ethylene tetrafluoroethylene (ETFE) foils for transparent buildings

Creep properties of ethylene tetrafluoroethylene (ETFE) foils are indispensable for evaluating serviceability limit state, especially under high temperature and high stress. This paper concerned temperature and stress effects on creep properties of ETFE foils with experimental and theoretical studies. Experimental results showed that dimensionless stress effect on creep properties could be higher than that of temperature effect. A unified equation incorporating temperature, stress and time based on experimental results was determined and could be utilized to calculate the stress limits and long-term creep strains. The stress limits in response to creep strain of 10% were less than 5 MPa, 4 MPa and 3 MPa for temperature ranges of 40–50 °C, 50–60 °C and 70–80 °C, respectively. The long-term creep strain of ETFE foils under 40 °C was 5.96% concerning 50-year working time.Master curves of ETFE foils were evaluated considering time-temperature superposition (TTSP) and time-stress superposition (TSSP). Long-term creep strains with these master curves were identified and compared with experimental creep strains. It is found that TTSP could be a little underestimation of creep strains while TSSP could overestimate creep strains to some extent. Moreover, the maximum creep strain difference was only 0.48%, which justified the feasibility and suitability of using the unified equation to predict creep strains of ETFE foils.