Some new operational modes and parameters of stress relaxation for the viscoelastic characterization of solid polymers. II. The “vertical‐shift” mode and intrinsic “time–strain clock”

As in part I of this study, in the same manner in the present part II as well, by the same modus operandi way, an attempt was made to introduce, by means of a given operational mode, some further practical parameters for a “by‐eye” but well‐proven experimental viscoelastic characterization of a polymeric solid. Thus, through consideration of the peculiar vertical shift behavior of the apparent modulus (?) of isotactic polypropylene (iPP) and based on the KWW model, it is shown that, in an empirical and a formalistic sense, a relevant effective or equivalent (single) characteristic relaxation time can be introduced which can give some new interpretations for the linear and nonlinear viscoelastic behavior of a polymeric material, as that of a “time–strain clock,” which, as an intrinsic function, is responsable for a functional time–strain shift of the relaxation time and, at the same time, for a shift toward to a more linear or to a more nonlinear behavior. In the above context of attempts, another functional relationship was shown, the so‐called spectral shift function and its corresponding parameter of nonlinearity strength, through which some further interpretations and characterizations connected with the existence of the permanent internal stress could be made. Finally, the introduction of the so‐called spectral isostrains and their corresponding “spectral inversion point” complete the “set” of the operative parameters proposed for the above‐mentioned purpose. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 138–148, 2003     

Application of time–stress superposition to viscoelastic behavior of polyamide 6,6 fiber and its “true” elastic modulus

The viscoelastic behavior of semi‐crystalline polyamide 6,6 fiber is exploited in viscoelastically prestressed polymeric matrix composites. To understand better the underlying prestress mechanisms, strain–time performance of the fiber material is investigated in this work, under high creep stress values (330–665 MPa). A latch‐based Weibull model enables prediction of the “true” elastic modulus through instantaneous deformation from the creep‐recovery data, giving 4.6 ± 0.4 GPa. The fiber shows approximate linear viscoelastic characteristics, so that the time–stress superposition principle (TSSP) can be implemented, with a linear relationship between the stress shift factor and applied stress. The resulting master creep curve enables creep behavior at 330 MPa to be predicted over a large timescale, thus creep at 590 MPa for 24 h would be equivalent to a 330 MPa creep stress for ～5200 years. Similarly, the TSSP is applied to the resulting recovery data, to obtain a master recovery curve. This is equivalent to load removal in the master creep curve, in which the yarns would have been subjected to 330 MPa creep stress for ～4.56 × 10⁷ h. Since our work involves high stress values, the findings may be of interest to those involved with long‐term load‐bearing applications using polyamide materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44971.     

On the nonlinear response of polymers to periodical sudden loading and unloading

In practice, it happens very often that structural parts made of plastics are submitted to a long term loading which “jumps” periodically between a constant maximal value and a constant minimal one. The knowledge of the material response to such a kind of loading demands very laborious experimental investigations. Since generally the loading takes place in the nonlinear viscoelastic range of the material, an analysis based on linear superposition cannot be applied. In the present paper a computational method is proposed to predict the response of the polymer to a low frequency “jumping” load in the nonlinear viscoelastic range, This method is based on the knowledge of the long term behavior of the material under constant loading conditions, which is approximated by finite exponential series. Both cases, stress-relaxation-type and creep-type loading histories are treated. Examples presented show a very good agreement between theoretical predictions and experimental data.     

Nonlinear viscoelastic model for the prediction of double yielding in a linear low‐density polyethylene film

Tensile testing and tensile creep experiments for linear low‐density polyethylene in a thin‐film form were examined and analyzed in terms of a nonlinear viscoelastic model. The proposed model, based on two distinct thermally activated rate processes (Eyring models), was proved to describe the double‐yield‐point tensile behavior of the material tested. The required model parameters were evaluated from the corresponding creep‐strain curves, and this revealed the relationship between the main aspects of the inelastic behavior of polymers, that is, the monotonic loading and creep response. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3519–3527, 2004     

“Binary salt” of hexane‐1,6‐diaminium adipate and “carbon nanotubate” as a synthetic precursor of carbon nanotube/Nylon‐6,6 hybrid materials

A novel chemical approach was established to produce carbon nanotube/Nylon‐6,6 hybrid materials from readily available substrates, that is, Nylon‐6,6 salt and oxidized multiwall carbon nanotubes (O‐MWCNTs). The key synthetic precursor hexane‐1,6‐diaminium adipate and “carbon nanotubate”—“Binary nanotube salt”—was obtained and isolated as stable and easy‐to‐handle solid in over 80% yield and with no nanotube losses. The final hybrid materials of various nanotube loadings were synthesized at 270°C and were easily purified from the homopolymer. Purified hybrids were comprehensively analyzed (yields and grafting ratios, SEM, TEM, FT‐IR) revealing a two‐phase characteristics—individually grafted nanotubes and cross‐linked nanotube material. Isothermal TGA kinetic studies showed that in the “binary salts” diamine and diacid molecules were anchored to the nanotube outer shells and then held electrostatically enabling growth of polymer immobilized on O‐MWCNTs (“grafting‐from” mechanism). Depending on the density and type of nanotube functionalities and filler concentration in the “binary salt,” the O‐MWCNT/Nylon‐6,6 hybrids can be treated as hybrid material of a proportion of aliphatic polyamide and polyaramide properties. POLYM. COMPOS., 35:523–529, 2014. © 2013 Society of Plastics Engineers     

Chromatographic performance of monodisperse–macroporous particles produced by “modified seeded polymerization.” I: Effect of monomer/seed latex ratio

In this study, the monodisperse–macroporous particles produced by a relatively new polymerization protocol, the so‐called, “modified seeded polymerization,” were used as column‐packing material in the reversed phase chromatography (RPC) of proteins. The particles were synthesized in the form of styrene‐divinylbenzene copolymer approximately 7.5 μm in size. In the first stage of the synthesis, the monodisperse polystyrene particles 4.4 μm in size were obtained by dispersion polymerization and used as the “seed latex.” The seed particles were swollen by a low‐molecular‐weight organic agent and then by a monomer mixture. The monodisperse–macroporous particles were obtained by the polymerization of monomer mixture in the seed particles. In the proposed polymerization protocol, the number of successive swelling stages was reduced with respect to the present techniques by the use of sufficiently large particles with an appropriate average molecular weight as the seed latex. A series of particles with different porosity properties was obtained by varying the monomer/seed latex ratio. The separation behavior of HPLC columns including the produced particles as packing material was investigated in the RPC mode using a protein mixture including albumin, lysozyme, cytochrome c, and ribonuclease A. The chromatograms were obtained with different flow rates under an acetonitrile–water gradient. The theoretical plate number increased and chromatograms with higher resolutions were obtained with the particles produced by using a lower monomer/seed latex ratio. The separation ability of the column could be protected over a wide range of flow rates (i.e., 0.5–3 mL/min) with most of the materials tested. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 607–618, 2004     

Tensile creep of a long‐fibre glass mat thermoplastic (GMT) composite. II. Viscoelastic‐viscoplastic constitutive modeling

In Part I of this article, the short‐term tensile creep of a 3‐mm‐thick continuous long‐fibre glass mat thermoplastic composite was characterized and found to be linear viscoelastic up to 20 MPa. Subsequently, a nonlinear viscoelastic model has been developed for stresses up to 60 MPa for relatively short creep durations. The creep response was also compared with the same composite material having twice the thickness for a lower stress range. Here in Part II, the work has been extended to characterize and model longer term creep and recovery in the 3‐mm composite for stresses up to near failure. Long‐term creep tests consisting of 1‐day loading followed by recovery were carried out in the nonlinear viscoelastic stress range of the material, i.e., 20–80 MPa in increments of 10 MPa. The material exhibited tertiary creep at 80 MPa and hence data up‐to 70 MPa has been used for model development. It was found that viscoplastic strains of about 10% of the instantaneous strains were developed under load. Hence, a non‐linear viscoelastic–viscoplastic constitutive model has been developed to represent the considerable plastic strains for the long‐term tests. Findley's model which is the reduced form of the Schapery non‐linear viscoelastic model was found to be sufficient to model the viscoelastic behavior. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the nonlinear model has been developed. The model predictions were found to be in good agreement with the average experimental curves. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Linear‐nonlinear dichotomy of the rheological response of particle‐filled polymers

Novel nanoparticles, polymer‐particle coupling agents, and functionalized polymers are being developed to enhance the performance of particle‐reinforced polymer systems such as advanced rubber compounds for automobile tires. Understanding the complex rheological behavior of rubber is critical to providing insights into both processability and end‐use properties. One unique aspect of the rheology of filled elastomers is that the incorporation of particles introduces a hysteretic softening (Payne effect) at small dynamic strains. This study demonstrates that this nonlinear viscoelastic behavior needs to be considered when attempting to correlate steady shear response (Mooney viscosity) to oscillatory shear measurements from test equipment such as the Rubber Process Analyzer (RPA). While a wide array of unfilled gum elastomers show good correlation between Mooney viscosity and dynamic torque from the RPA at all of the strain amplitudes used, rubber compounds containing silica and carbon black particles only exhibit good agreement between the two measures of processability when the oscillatory strain amplitude is high enough to sufficiently break up the filler network. Other features of the filler network and its influence on nonlinear rheology are considered in this investigation, including the effects of polymer–filler interactions on filler flocculation and the use of Fourier transform rheometry to illustrate the “linear‐nonlinear dichotomy” of the Payne effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40818.     

High‐Temperature Creep Behavior of SiOC Glass‐Ceramics: Influence of Network Carbon Versus Segregated Carbon

Three silicon oxycarbide samples with different carbon contents are analyzed in the present study with respect to their high‐temperature creep behavior. The tests were performed in compression at 1100°C, 1200°C, and 1300°C; in this temperature range the mechanism of creep relies on viscoelastic flow within the samples and has been modeled with the Jeffreys viscoelastic model. After the release of the applied mechanical stress, a viscoelastic recovery behavior was observed in all samples. The creep behavior of the investigated samples indicates two rheological contributions in SiOC: (i) a high viscous answer, coming from the silica‐rich network, and (ii) an elastic response from the segregated carbon phase within the samples. Furthermore, two distinct effects of the carbon phase on the HT creep behavior of SiOC were identified and are discussed in the present paper: the effect of the carbon presence within the SiOC network (the “carbidic” carbon), which induces a significant increase in the viscosity and a strong decrease in the activation energy for creep, as compared to vitreous silica; and the influence of the segregated carbon phase (the “free” carbon), which has been shown to affect the viscosity and the activation energy of creep and dominates the creep behavior in phase‐separated silicon oxycarbides.     

Synthesis and characterization of side‐chain liquid‐crystalline polyacrylates containing azobenzene moieties

Syntheses of novel liquid‐crystalline polymers containing azobenzene moieties were performed by a convenient route with an acrylate backbone. The azobenzenes were key intermediates of the monomers, and side‐chain liquid‐crystalline polymers were prepared, that is, poly[α‐{4‐[(4‐acetylphenyl)azo]phenoxy}alkyloxy]acrylates, for which the spacer length was 3 or 11 methylene units. In addition, poly[3‐{4‐[(3,5‐dimethylphenyl)azo]phenoxy}propyloxy]acrylate was prepared with a spacer length of 3 methylene units. The structures of the precursors, monomers, and polymers were characterized with Fourier transform infrared, ¹H‐NMR, and ¹³C‐NMR techniques. The polymers were obtained by conventional free‐radical polymerization with 2,2′‐azobisisobutyronitrile as an initiator. The phase‐transition temperatures of the polymers were studied with differential scanning calorimetry, and the phase structures were evaluated with a polarizing optical microscopy technique. The results showed that two of the monomers and their corresponding polymers exhibited nematic liquid‐crystalline behavior, and one of the monomers and its corresponding polymer showed smectic liquid‐crystalline behavior. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2653–2661, 2002     

Prediction of viscoelastic behavior of interphase in polypropylene‐chopped rice husk composites for β‐relaxation domain

The effect of chopped rice husk (CRH) content on viscoelastic properties and crystallinity of polypropylene (PP) composites was investigated. Composites containing 0, 20, and 40 part per hundred plastics (php) of CRH into PP were prepared by twin‐screw extruder, with maleic anhydride‐grafted PP as the coupling agent. The viscoelastic behavior and the crystallinity of these composites have been studied by dynamic mechanical analysis as well as differential scanning calorimetry, respectively. By the incorporation of CRH into PP, the storage modulus (E′) was found to be increased progressively, whereas the mechanical loss factor (tan δ) decreased in a nonlinear manner. A self‐consistent analysis was proposed for the prediction of viscoelastic response of the interphase between PP matrix and CRH particles. A three‐phase model was applied in a reverse mode, and the viscoelastic behavior of the interphase was extracted and compared with the unfilled matrix. Differential scanning calorimetry results indicated that CRH influences crystallization temperature as well as the degree of crystallinity of the composites. An entrapped polymer within CRH filler and PP matrix was detected by scanning electron microscope, which can be attributed to the interfacial layer with a good adhesion between the main components. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Some new operational modes and parameters of stress relaxation for the viscoelastic characterization of solid polymers. I. The “virtual modulus” mode

A new operational (functional) parameter, the so‐called virtual modulus, ē(t), is introduced. By this, an attempt was made for the approximation of the function of the real modulus E(t), which, as known, is valid only for instantaneous loading, namely, for zero loading times. Thus, through a simple theoretical modeling and an algorithmic approach, the determination of E(t), from ē(t), sets sail, at the end, to the solution of a Volterra integral equation of the second type, which, in turn, sets sail to the solution of a differential equation. By the aid of numerical integration and also of some experimental evidence, it seems that this solution is valid only for loading times approximately above 0.2 s, thus obtaining, in fact, a “pseudomodulus” of relaxation. To assess the validity of this pseudomodulus, the well‐known Kohlrausch–Williams–Watt (KWW) and the power‐law models were used as some crude “criteria.” By means of best fit, it appeared, at the first instance, that the so‐calculated pseudomodulus better obeys the power‐law model than it does the KWW model. This is a certain contradiction with the so‐called apparent modulus, which was obtained from experiment with a finite loading time superior to 1 s. Two other criteria that were used have shown a satisfactory proof of the validity of this modeling. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 121–137, 2003     

Unique block molecules based on glycerol skeleton as C‐3 building blocks for liquid crystals formation by self‐assembly and their future potential for the “nano‐chemistry”

Studies on design of liquid crystalline block molecules with non‐conventional mesophase morphoplogies have been proposed by Tschierske and co‐workers. These block molecules have a rigid core, a lateral substituent, and an alkyl chain segmenting blocks from each other. Related to this, diglyceryl alkyl ethers found by us have glycerol as a rigid core, a lateral substituent, and an alkyl chain similar to the Tschierske design. These two cases are compared with each other with regard to the relationship between the molecular structure and the liquid crystalline morphologies and other properties. Recently, new soft materials suitable for liquid crystals exhibiting self‐assembly of phase‐segregated structures have been designed. Typical examples of such “block” molecules containing glycerol having a C‐3 building block include: (i) undecyl‐glycerylether‐ modified siloxane derivatives with a siloxane segment as the rigid core and alkyl chains with 2, 3‐dihydroxypropoxy group as a hydrophilic group at a lateral or terminal position of the siloxane segment; (ii) novel hyper branched dendrimers forming the basis of polyglycerol nanocapsules with a core‐shell molecular architecture; (iii) carbon nanotubes based on cyclodextrins (CDs); (iv) polymerizable amphiphilic diacetylene‐containing phospholipids suitable for construction of functional nanocomposites. This is done by self‐assembly and polymerization of diacetylene creating a “block molecular structure” with a polyacetylene chain as a rigid core segment, the lipid headgroups as the hydrophilic segment, and terminal flexible alkyl chains. On the basis of these results, future potential of block molecules as a soft building material for liquid crystalline structures was discussed.     

Interrogation of “surface,” “skin,” and “core” orientation in thermotropic liquid‐crystalline copolyester moldings by near‐edge X‐ray absorption fine structure and wide‐angle X‐ray scattering

Injection molding thermotropic liquid‐crystalline polymers (TLCPs) usually results in the fabrication of molded articles that possess complex states of orientation that vary greatly as a function of thickness. “Skin‐core” morphologies are often observed in TLCP moldings. Given that both “core” and “skin” orientation states may often differ both in magnitude and direction, deconvolution of these complex orientation states requires a method to separately characterize molecular orientation in the surface region. A combination of two‐dimensional wide‐angle X‐ray scattering (WAXS) in transmission and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy is used to probe the molecular orientation in injection molded plaques fabricated from a 4,4′‐dihydroxy‐α‐methylstilbene (DHαMS)‐based thermotropic liquid crystalline copolyester. Partial electron yield (PEY) mode NEXAFS is a noninvasive ex situ characterization tool with exquisite surface sensitivity that samples to a depth of 2 nm. The effects of plaque geometry and injection molding processing conditions on surface orientation in the regions on‐ and off‐ axis to the centerline of injection molded plaques are presented and discussed. Quantitative comparisons are made between orientation parameters obtained by NEXAFS and those from 2D WAXS in transmission, which are dominated by the microstructure in the skin and core regions. Some qualitative comparisons are also made with 2D WAXS results from the literature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Molecular Dynamics Simulations of Polymer‐Bonded Explosives (PBXs): Modeling,Mechanical Properties and their Dependence on Temperatures and Concentrations of Binders

Two models, i.e. “covering” and “cutting” models, for the polymer‐bonded explosives (PBXs) were proposed for different researching aspects. Used for choosing polymeric binders, the “covering” models are mainly applied to find the relations of temperatures and concentrations respectively with elastic properties of the PBXs. The “cutting” model is especially used to describe the highly anisotropic behavior of 1,3,5‐triamino‐2,4,6‐trinitrobenzene crystals (TATB). These models were realized by using molecular dynamics methods. It is found that the ductility of crystalline TATB can be effectively improved by blending fluorine‐containing polymers in small amounts. The moduli for the PBXs decrease with increase in temperature and concentration of binders. Different crystalline surfaces interacting with the same polymer binder have different modulus‐decreasing effects due to the highly anisotropic behavior of TATB. The modulus‐decreasing effect for different crystalline surfaces ranking order is (010)≈(100)>(001).     

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

The inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Evaluation of the characteristics of a viscoelastic material from creep analysis using the universal viscoelastic model. I. Isolation of the elastic component designated as the “Projected Elastic Limit”

In a preceding publication this author introduced a new universal viscoelastic model to describe a definitive relationship between constant strain rate, creep, and stress relaxation analysis for viscoelastic polymeric compounds. One extremely important characteristic of this new model is that it also characterizes secondary creep very well. Because secondary creep is the linear portion of creep after the completion of primary creep, then a straight line with a slope and an intercept can describe secondary creep. To effectively define a straight line in the secondary creep region it was found necessary to obtain averages of the instantaneous slope and the instantaneous intercept strain by averaging over a series of equally spaced data points in the secondary slope region. Most importantly, this average intercept strain was found to be independent of creep stress and creep time. This means that all the secondary creep straight lines must pass through the same intercept creep strain for all creep stresses. The results presented in this study strongly indicate that this secondary creep intercept strain is independent of creep stress and creep time, and appears to increase as the value of the efficiency of yield energy dissipation decreases. Because a decrease in the efficiency of yield energy dissipation, n, appears to correlate with an increase in the elastic solid like character of a material, then it appears that this secondary creep intercept strain should be a direct measure of the strain that the material can survive to retain its full elastic character. Therefore, this secondary creep intercept strain has been designated as the “Projected Elastic Limit” of a given viscoelastic material. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2923–2936, 2003     

“Dark” Singlet Oxygenation of Hydrophobic Substrates in Environmentally Friendly Microemulsions

The molybdate‐catalyzed “dark” singlet oxygenation of hydrophobic compounds with hydrogen peroxide proceeds efficiently with low catalyst loadings (10 ^–3 mol %) in chlorine‐free w/o microemulsions. These micro‐heterogeneous systems are composed of sodium dodecyl sulfate (SDS)/n‐butanol/water/organic phase, the latter being either a ”green” solvent such as ethyl acetate or a liquid substrate, such as α‐terpinene or β‐citronellol. Very high reactor yields with improved product/SDS ratio can be obtained for the ”dark” singlet oxygenation of such liquid substrates.     

A novel water‐soluble polythiophene derivatives based fluorescence “turn‐on” method for protein determination

A lable‐free, simple, and sensitive fluorescence “turn‐on” approach is designed to rapidly detect protein using a conjugated polythiophene derivative (PDPMT‐Cl). The fluorescence of PDPMT‐Cl solution can be efficiently quenched by PtCl42? ions. Upon adding trypsin to the (bovine serum albumin, BSA) PDPMT‐Cl–PtCl42? solution, the BSA is cleaved into amino acid or peptide fragments, which are stronger PtCl42? ions chelators to form more stable complexes with PtCl42? ions. Thus, the PtCl42? ion is displaced from PDPMT‐Cl and the fluorescence of PDPMT‐Cl is recovered. By triggering the “turn‐on” signal of PDPMT‐Cl, it is successful to detect the protein in real time. “Turn‐on” response as readout signal is able to effectively reduce background noise and increase detection sensitivity. This method offers good selectivity for detecting protein in the presence of other common amino acids and metal ions. Under optimized conditions, the concentration of BSA in the range of 0.0004–1.75 mg/mL exhibits a linear relationship with the relative fluorescence intensity, and the correlation coefficient is 0.9997. The limit of detection is 4.47 × 10?4 mg/mL. The system is successfully applied for detecting protein in milk and egg. Due to the simplicity, sensitivity, and rapid response, this assay shows great potential for protein detection in the future. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 939‐943, 2013