Microstructure‐based fatigue modeling of an acrylonitrile butadiene styrene (ABS) copolymer

In this article, we experimentally investigate the structure–property relationships of an acrylonitrile butadiene styrene (ABS) copolymer for fatigue and use a microstructure‐based multistage fatigue (MSF) model to predict material failure. The MSF model comprises three stages of fatigue damage (crack incubation, small crack growth, and long crack growth) that was originally used for metal alloys. This study shows for the first time that the MSF theory is general enough to apply to polymer systems like ABS. The experimental study included monotonic testing (compression and tension) and fully reversed uniaxial cyclic tests at two frequencies (1 Hz and 10 Hz) with a range of strain amplitudes of 0.006 to 0.04. Cyclical softening was observed in the ABS copolymer. Fractography studies of failed specimens revealed that particles were responsible for crack incubation. Although polymeric materials can be argued to be more complex in terms of failure modes and thermo‐mechano‐chemical sensitivity when compared with most metal alloys, results showed that the MSF model could be extended successfully to capture microstructural effects to polymeric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40882.     

Importance of crosslinking and disulfide bridge reduction for the mechanical properties of rigid wheat gluten bioplastics compression molded with thiol and/or disulfide functionalized additives

Thiol (SH) containing additives improve the mechanical properties of rigid, glassy gluten materials. However, the underlying molecular mechanism is still unclear. In particular, the importance of the preceding gluten‐additive mixing conditions remains to be investigated. Here, different additives containing either only SH, only disulfide or both SH and disulfide functionalities were synthesized and their impact on the gluten network using different mixing conditions prior to subsequent molding were assessed. All SH containing additives decreased the gluten molecular weight (MW) during mixing to a degree depending on the conditions. Additives with only disulfide functionality did not significantly affect protein size during mixing irrespective of the conditions used. Only when mixing induced sufficient MW reduction did the strength and failure strain of rigid gluten materials increase. This shows that factors other than the degree of cross‐linking affect the strength of rigid gluten materials. These results support our hypothesis that altered molecular conformations and improved molecular entanglements contribute to material strength. The extent to which such conformational changes occur may depend on the additive and the way of mixing. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41160.     

Synthesis of ethylene terephthalate and ethylene naphthalate (PET‐PEN) block‐co‐polyesters with defined surface qualities by tailoring segment composition

Ethylene terephthalate and ethylene naphthalate oligomers of defined degree of polymerization were synthesized via chemical recycling of the parent polymers. The oligomers were used as defined building blocks for the preparation of novel block‐co‐polyesters having tailored sequence compositions. The sequence lengths were systematically varied using Design of Experiments. The dispersive surface energy and the specific desorption energy of the co‐polymers were determined by inverse gas chromatography. The study shows that polyethylene terephthalate‐polyethylene naphthalate (PET‐PEN) block‐co‐polyesters of defined sequence lengths can be prepared. Furthermore, the specific and dispersive surface energies of the obtained block‐co‐polyesters showed a linear dependence on the oligomer molecular weight and it was possible to regulate and control their interfacial properties. In contrast, with the corresponding random‐block‐co‐polyesters no such dependence was found. The synthesized block‐co‐polyesters could be used as polymeric modifying agents for stabilizing PET‐PEN polymer blends. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40731.     

Fundamental influences on quasistatic and cyclic material behavior of short glass fiber reinforced polyamide illustrated on microscopic scale

In this work, the influences of fiber orientation and weld lines on the morphological structures and the mechanical behavior of polyamide 6.6 (PA6.6‐GF35) are investigated. In quasistatic and fatigue tests tensile and 3‐point‐bending loads are applied. Test temperatures vary between RT and 150°C. Two different specimen types are produced by using injection moulding process to create different fiber orientations as well as weld lines. Fiber orientations are determined using computer tomography. Scanning electron microscopy is used to investigate fracture surfaces of tested specimens. Results show that mechanical properties and morphological structures depend highly on fiber orientation and temperature. Transversely oriented fibers in weld lines result in brittle failure mechanisms and decreased mechanical properties. Different stress distributions in the specimens under tensile and flexural loads have influence on the material behavior as well. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40842.     

Dynamic properties of star‐shaped,solution‐polymerized styrene–butadiene rubber and its cocoagulated rubber‐filled with silica/carbon black

The dynamic properties, including the dynamic mechanical properties, flex fatigue properties, dynamic compression properties, and rolling loss properties, of star‐shaped solution‐polymerized styrene–butadiene rubber (SSBR) and organically modified nanosilica powder/star‐shaped styrene–butadiene rubber cocoagulated rubber (N‐SSBR), both filled with silica/carbon black (CB), were studied. N‐SSBR was characterized by ¹H‐NMR, gel permeation chromatography, energy dispersive spectrometry, and transmission electron microscopy. The results show that the silica particles were homogeneously dispersed in the N‐SSBR matrix. In addition, the N‐SSBR/SiO₂/CB–rubber compounds' high bound rubber contents implied good filler–polymer interactions. Compared with SSBR filled with silica/CB, the N‐SSBR filled with these fillers exhibited better flex fatigue resistance and a lower Payne effect, internal friction loss, compression permanent set, compression heat buildup, and power loss. The nanocomposites with excellent flex fatigue resistance showed several characteristics of branched, thick, rough, homogeneously distributed cross‐sectional cracks, tortuous flex crack paths, few stress concentration points, and obscure interfaces with the matrix. Accordingly, N‐SSBR would be an ideal matrix for applications in the tread of green tires. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40348.     

Crystalline phase transformation of polytetrafluoroethylene in a fatigue test

In this study, we aimed to characterize the mechanical response of polytetrafluoroethylene (PTFE) laminates under a tension–tension load‐control fatigue test (frequency = 5 Hz, load ratio = 0) and provided an analysis of the failure patterns of the PTFE material with consideration of crystalline phase transformation. In the final results, the evolution of the cyclic creep strain and stress–number of cycles to failure (S–N) curves presented duplex properties accompanying the fatigue life increasing to high cycles (cycle fatigue > 10⁵). A simple phenomenological damage index was defined in this study to describe the cyclic creep process. Additionally, the scanning electronic machine investigation suggested that local fibrosis caused by crystalline phase transformation to phase I led to the initiation of fatigue crack, and the fiber formation and orientation was found to be beneficial to a higher tensile strength and better resistance to crack propagation. The aspect of cyclic‐load‐induced crystallization was observed by the microfocus hard X‐ray diffraction beamline from a new insight. The crystalline phase transformation led to a gradient distribution of crystallinity and lateral crystallite size along the crack propagation direction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41113.     

Novel kind of functional gradient poly(ε‐caprolactone) polyurethane nanocomposite: A shape‐memory effect induced in three ways

Multiwalled carbon nanotube (MWCNT) crosslinked polyurethane nanocomposites filled with iron (Fe) powders were synthesized by an in situ polymerization method. The Fe powders were deposited on one side of the nanocomposites during sample formation. Because of the gradient distribution of the Fe powders, the polymer part was affected little; this resulted in good mechanical properties of the nanocomposites. The electrical conductivities on each side of the nanocomposites were different. Because of the good magnetic properties and high electrical conductivities of the nanocomposites, the shape‐memory effect could be induced by temperature heating (temperature = 45°C), electrically resistive Joule heating (voltage (U) = 30 V), and magnetic field heating (frequency (f) = 45 kHz, intensity of magnetic field (H) = 46.5 kA/m). The shape‐memory properties were dependent on the location of the side that contained the most Fe powders (Fe side), and the nanocomposites showed better shape‐memory properties when the Fe side was located inside of the folded samples. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40220.     

Synergistic effects of carbon nanotubes and carbon black on the fracture and fatigue resistance of natural rubber composites

The quasi‐static fracture and dynamic fatigue behaviors of natural rubber composites reinforced with hybrid carbon nanotube bundles (CNTBs) and carbon black (CB) at similar hardness values were investigated on the basis of fracture mechanical methods. Mechanical measurement and J‐integral tests were carried out to characterize the quasi‐static fracture resistance. Dynamic fatigue tests were performed under cyclic constant strain conditions with single‐edged notched test pieces. The results indicate that synergistic effects between CNTBs and CB on the mechanical properties, fracture, and fatigue resistance were obtained. The composite reinforced with 3‐phr CNTBs displayed the strongest fatigue resistance. The synergistic mechanisms and dominating factors of quasi‐static and dynamic failure, such as the dispersion state of nanotubes, hybrid filler network structure, strain‐induced crystallization, tearing energy input, and viscoelastic hysteresis loss, were examined. The weakest fatigue resistance of the composite filled with 5‐phr CNTBs was ascribed to its strikingly high hysteresis, which resulted in marked heat generation under dynamic fatigue conditions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42075.     

Effect of freeze‐thaw pretreatment on thermal drying process and physicochemical properties of chitosan

The effect of freeze‐thaw pretreatment on the thermal drying process and physicochemical properties of chitosan was investigated in this study. Results showed that the freeze‐thaw treatment changed the form of chitosan paste and reduced 75.6–77.7% of the water content. The freeze‐thaw treatment decreased the drying time of chitosan from 16–19 h to 2.75–4 h and the dried product was loosely packed powder. After freeze‐thaw treatment, the molecular weight of chitosan was unchanged during the thermal drying. The heat‐induced browning effect of chitosan during drying was greatly alleviated by the pretreatment. Furthermore, the pretreatment increased the 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical‐scavenging activity of dried product by 40.4–59.8%. The molecular weight, color, and DPPH radical‐scavenging activity of the pretreated dried chitosan product were close to those of freeze‐dried product. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41017.     

Investigations of the electrical conduction mechanisms of polyaniline‐DBSA/poly(acrylonitrile‐butadiene styrene) blends

The electrical properties of Al/PANI‐DBSA/ABS/Au blend with PANI (5%) w/w have been investigated by using of current‐voltage (I‐V) measurements, in a temperature range of 100–313 K. The analysis of I‐V characteristics in the forward direction was based on thermionic emission mechanism for applied electrical field till ～3 × 10² V/cm. The thickness dependence of the current‐voltage relationship, clearly demonstrates that the electrical current for larger fields is space charge limited current (SCLC). Temperature dependences of the ideality factor, barrier height, and series resistance have been calculated. The mobility of carriers which is temperature dependent was calculated using the trap free SCLC as 1.53 × 10^?4 cm² V^?1 s^?1 at room temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40688.     

Effect of geometry and loading fractions on energy absorbing properties of fiberglass polymer composite under quasi‐static and low‐velocity impact loadings

The focus of this study is to experimentally investigate the mechanical properties of fiberglass reinforced composite with various aspect ratios and loading fractions in the quasi‐static and low‐velocity impact loading conditions. In this study, short fiberglass reinforced polycarbonate composite materials were fabricated via a solution mixing method and characterized for their tensile properties by varying both fiberglass loading fraction and aspect ratio. The tensile properties including tensile toughness of the fiberglass reinforced composites were characterized and compared. It was observed in this study that the toughness of the composite was dramatically improved whereas the tensile strength and Young's modulus were moderately enhanced over the neat polymer, which were measured to be only up to 15% and 70% increase, respectively. The low‐velocity impact behaviors of the fiberglass composites were also investigated and compared to the tensile toughness of the corresponding composites. Besides, the effect of thickness on their low‐velocity impact properties was investigated. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40821.     

Capillary study on geometrical dependence of shear viscosity of polymer melts

Geometrical dependence of viscosity of polymethylmethacrylate (PMMA) and high density polyethylene (HDPE) are studied by means of a twin‐bore capillary rheometer based on power‐law model. Contrary geometrical dependences of shear viscosity are observed for PMMA between 210 and 255°C, but similar geometrical dependences are revealed for HDPE between 190 and 260°C. The fact that wall slip can not successfully explain the irregular geometrical dependence of PMMA viscosity is found in this work. Then, pressure effect and dependence of fraction of free volume (FFV) on both pressure and temperature are proposed to be responsible for the geometrical dependence of capillary viscosity of polymers. The dependence of shear viscosity on applied pressure is first investigated based on the Barus equation. By introducing a shift factor, shear viscosity curves of PMMA measured under different pressures can be shifted onto a set of parallel plots by correcting the pressure effect and the less shear‐thinning then disappears, especially at high pressure. Meanwhile, the FFV and combining strength among molecular chains are evaluated for both samples based on molecular dynamics simulation, which implies that the irregular geometrical dependence of PMMA viscosity can not be attributed to the wall slip behavior. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39982.     

Synthesis,helical conformation,and infrared emissivity property study of optically active substituted polyacetylenes derived from serine

Serine‐based monosubstituted acetylene monomers were synthesized and polymerized with rhodium zwitterion catalyst in THF to afford optically active polyacetylene derivatives (LPA and DPA) and corresponding racemic polyacetylenes (RPA) with moderate molecular weights in good yields. All of the substituted polyacetylenes (SPA) were characterized by FT‐IR, NMR, GPC, UV‐Vis spectroscopy, circular dichroism (CD) spectroscopy, and TGA. LPA and DPA were soluble in common organic solvents and possessed single‐handed helical conformation according to their intense Cotton effect and large specific rotations, while RPA presented random coiled polymer chain. The characterization results showed that helical structure of these SPAs was stabilized by intra‐ and intermolecular hydrogen bonding between the substituents which played a significant role in creating and maintaining the helix. Further, the infrared emissivity properties of them at wavelength of 8 to 14 μm were investigated at room temperature. Consequently, the LPA and DPA exhibited lower infrared emissivity values than RPA, which came down to 0.632 and 0.616. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41210.     

Acidochromicity of a low‐molecular‐weight pyrimidine‐based copolymer

A new type of low‐molecular‐weight polypyrimidine in a π‐conjugated main chain was prepared by a Grignard reaction between 2‐amino‐4,6‐dichloropyrimidine and 1,4‐dibromo‐2,5‐didodecyloxybenzene in the presence of [1,2‐Bis(diphenylphosphino) ethane]dichloronickel(II). The structure of the copolymer was fully elucidated by Fourier transform infrared spectroscopy, ¹H‐NMR, and elemental analysis. The copolymer had good solubility in common organic solvents. The copolymer displayed a bathochromic shift when protonated with an organic or inorganic acid in chloroform or tetrahydrofuran. The copolymer depicted facile p‐doping and good electron‐transporting electrochemical properties in a 1M H₂SO₄ aqueous solution. The copolymer showed a narrow polydispersity of 1.04. Thermogravimetric analysis showed that the copolymer had a certain thermal stability with no decomposition at a temperature of 250°C under N₂. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41174.     

Viscoelastic and thermal decomposition behaviors of polytetrahydrofuran binder prepared using glycerin as a crosslinking modifier

Polytetrahydrofuran (PTHF) is an effective binder ingredient used for improving the performance of propellants. PTHF becomes sufficiently rubbery for use as a binder with the addition of an adequate crosslinking modifier. This study investigated the viscoelastic and thermal decomposition behaviors of the PTHF binder prepared using glycerin as a crosslinking modifier, as well as the influence of the molecular weight of PTHF on the characteristics of the PTHF binder. The curing behavior of the PTHF binder was suitable for the manufacture of propellants, and the superior tensile properties of the PTHF binder made it suitable for use as a propellant binder. The degree of crosslinking of the samples decreased as the molecular weight of the PTHF increased. The PTHF binder has unique dynamic mechanical properties owing to its melting and chemical structure, and these properties were dependent on the molecular weight of PTHF. The glass transition temperature (T_g) and the loss tangent at T_g decreased as the molecular weight of the PTHF increased. The temperature and frequency dependence of the PTHF binder were influenced by the melting point of PTHF. The viscoelastic properties of the binder prepared using PTHF with a molecular weight of 650 followed the time–temperature superposition principle. The activation energy for the relaxation of this binder varied remarkably at the melting point of PTHF. The thermal decomposition behavior indicated that at low temperatures, the consumption rate of the binder with low‐molecular‐weight PTHF was slightly larger than that of the binder with high‐molecular‐weight PTHF. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Tailoring the molecular and thermo–mechanical properties of kraft lignin by ultrafiltration

This study has shown that ultrafiltration allows the selective extraction from industrial black liquors of lignin fraction with specific thermo‐mechanical properties, which can be matched to the intended end uses. Ultrafiltration resulted in the efficient fractionation of kraft lignin according to its molecular weight, with an accumulation of sulfur‐containing compounds in the low‐molecular weight fractions. The obtained lignin samples had a varying quantities of functional groups, which correlated with their molecular weight with decreased molecular size, the lignin fractions had a higher amount of phenolic hydroxyl groups and fewer aliphatic hydroxyl groups. Depending on the molecular weight, glass‐transition temperatures (T_g) between 70 and 170°C were obtained for lignin samples isolated from the same batch of black liquor, a tendency confirmed by two independent methods, DSC, and dynamic rheology (DMA). The Fox–Flory equation adequately described the relationship between the number average molecular masses (M_n) and T_g's‐irrespective of the method applied. DMA showed that low‐molecular‐weight lignin exhibits a good flow behavior as well as high‐temperature crosslinking capability. Unfractionated and high molecular weight lignin (M_w >5 kDa), on the other hand, do not soften sufficiently and may require additional modifications for use in thermal processings where melt‐flow is required as the first step. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40799.     

Preparation of polysiloxane oligomers bearing benzoxazine side groups and tunable properties of their thermosets

A new pathway for the preparation of polysiloxane oligomers bearing benzoxazine side groups were reported via the hydrolysis and co‐polycondensation of benzoxazinyl siloxane (SBZ) with dimethyldiethoxysilane (DEDMS). The structures of SBZ and oligomers were characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The average molecular weights of the obtained oligomers were estimated from size exclusion chromatography and ¹H‐NMR to be in the range of 2000–4000. The oligomers gave transparent films by casting in THF solution. The films were further thermally treated to produce crosslinked films via the ring opening polymerization of benzoxazine side group. The effects of siloxane content on polymerization behavior, glass transition temperature, and mechanical properties of the polybenzoxazine thermosets were investigated. Tensile test of the films revealed that the elongation at break increased with increasing siloxane content. The elongation at break of poly(I‐50) was up to 12.1%. Dynamic mechanical analysis of the thermosets showed that the T_gs were in the range of 119–165°C. Thermogravimetic analysis also revealed a better thermal stability as evidenced by the 5% weight loss temperatures in the range of 363–390°C. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40960.     

Synthesis,characterization, and properties of biodegradable poly(butylene succinate) modified with imide dihydric alcohol

A series of high molecular weight poly (butylene succinate) and its copolyester containing rigid imide units were synthesized in this article. The chemical structure and composition of the copolyesters were determined by ¹H NMR spectroscopy and Fourier transform infrared spectroscope (FT‐IR). The thermal properties, crystallization behavior and mechanical properties of polymers were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide‐angle X‐ray diffraction (WAXD) and mechanical testing. The enzymatic degradation was investigated using pancreatic lipase solution. The results showed that the melting temperature (T_m) of the copolyester decreased with the increment in pyromellitic imide unit content. However, the thermal degradation temperature (5% decomposition temperature) changed little. Meanwhile, the enzymatic degradation rate of poly (butylene succinate) was enhanced. The mechanical properties showed that the tensile strength had a trend of decrease, but the elongation at break was improved with the increment in imide units. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40807.     

Curing characteristics,morphology, thermal stability,mechanical properties,and irradiation resistance of methylethylsilicone/methylphenylsilicone rubber blends

Methylethylsilicone rubber (MESR)/methylphenylsilicone rubber (MPSR) blends were cured with 2,5‐dimethyl‐2,5‐di(tert‐butylperoxy)hexane. The curing characteristics, morphology, thermal behaviors, mechanical properties at different temperatures, radiation resistance, and thermal aging resistance of the MESR/MPSR blends were investigated. The results show that a high MPSR content could decrease the optimum curing time and improve the scorch safety. Dynamic mechanical analysis revealed that the glass‐transition temperature of the blends increased slightly with the addition of MPSR. Scanning electron microscopy showed that MESR and MPSR had good compatibility in the blends. Thermogravimetric analysis indicated that the thermal stability of the blends increased with increasing quantity of MPSR. The blends had excellent mechanical properties at low temperatures. However, these properties were significantly reduced when the temperature was increased. Moreover, changes in the mechanical properties decreased with increasing MPSR content at high temperatures, especially at temperatures higher than 100°C. In addition, the radiation resistance and thermal aging resistance of the blends increased with increasing MPSR content. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40529.     

Dependence on mean stress and stress amplitude of fatigue life of EPDM elastomers

Abstract

This paper describes the dependence on test parameters of the fatigue resistance of EPDM. Fatigue was investigated using dumbbell specimens under load control at 1 Hz until failure. Tests were made in order to create a common Wöhler (S–N) curve while increasing the stress amplitude and also to show the influence of increasing minimum stress at constant stress amplitude on fatigue properties. The results of these tests confirmed the well known amplitude dependence of fatigue life in filled rubbers. An additional significant influence on fatigue life is seen to be the minimum stress applied during each cycle to these materials. Fatigue life is not dependent on strain crystallisation in EPDM as it is for natural rubber. The results of this research give component designers the opportunity to increase the fatigue lives of components made from this material.