Elongational viscosities of random and block copolymer melts

The influence of random and block copolymerized structures on the uniaxial elongational viscosity was investigated. The investigated random copolymers were poly(ethylene-random-ethyl methacrylate) with comb-branched structure and poly(styrene-random-acrylonitrile) with linear structure. The studied block copolymers were poly(styrene-block-ethylenebutylene-block-styrene) with linear structure. The elongational viscosities of random copolymers showed strain-hardening properties. The strain-hardening property was influenced little by comonomer contents and depended on whether copolymers had linear or branched structures. In contrast, the elongational viscosities of block copolymers gave strain-softening properties. The strain-softening property was not affected by strain rates and block comonomer ratios. The causes of strain-hardening and -softening properties are discussed from relaxation spectrum and damping function based on the Bernstein–Kearsley–Zapas model. The damping functions of linear and branched random copolymers agreed with those of linear and branched homopolymers, respectively. On the other hand, linear block copolymers exhibited stronger damping than linear homopolymers. It was concluded that strain-hardening and -softening properties in the elongational viscosity of random and block copolymerized structures are correlated with their damping functions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 1765–1774, 1998     

Influence of morphology on rheological and mechanical properties of SEBS-toughened,glass-bead-filled isotactic polypropene

The influence of morphology of glass-bead-filled isotactic polypropene containing 0–20 vol% thermoplastic elastomers (TPE) on mechanical and rheological properties was investigated. Polystyrene-block-poly(ethene-co-but-1-ene)-block-polystyrene(SEBS) and the corresponding block copolymer grafted with maleic anhydrid (SEBS-g-MA) were used as thermoplastic elastomers, realizing, in the first case, a three-phase morphology with separately dispersed glass beads and SEBS particles. In the second case, SEBS-g-MA forms an elastomeric interlayer between glass beads and polypropene matrix, comprising core–shell particles. Young's modulus and tensile yield stress of the hybrid composites decrease with an increase in TPE volume fraction due to low stiffness and strength of TPE. In comparison with the three-phase morphology of hybrid composites with SEBS, SEBS-g-MA interlayers effect a reduced stiffness of the hybrid composites but improve interfacial adhesion and, thus, tensile yield stress. Rheological storage and loss moduli increase with an increase in glass bead and TPE volume fraction. Due to improved interfacial adhesion, melt elasticity and viscosity are enhanced by the SEBS-g-MA interlayer when compared with separately dispersed SEBS. Consequently, the reduced stiffening effect of the glass beads due to SEBS-g-MA interlayer decreases mechanical elasticity, whereas improved interfacial adhesion, also promoted by the SEBS-g-MA interlayer, enhances tensile yield stress and melt elasticity. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2499–2506, 1998     

Pulsed NMR study of elastomeric block copolymer under deformation

Deformation behavior of an elastomeric styrene–butadiene–styrene block copolymer (SBS) is studied by pulsed NMR techniques, and is related to lifetime distributions and the change of the microstructure in the stress relaxation process. By the measurement of spin–spin relaxation time, it is found that polybutadiene (PB) chains in the vicinity of polystyrene (PS) domains come to be in more constrained conformations with stretching than those remote from the domains mainly through the intramolecular interactions, followed by the enlargement of the constrained regions, which reflects the roles of both crosslinks and filler particles in crosslinked rubbers. In the stress relaxation process, however, the mean lifetime for SBS at the critical strain is longer than that at lower strain in contrast with the results for the crosslinked rubbers. It is estimated that the differences between the failure behaviors of SBS and those of the conventional crosslinked rubbers may be mainly caused by the characteristic change of the microstructure (the disruption of the weak interconnections between the spherical PS domains with high energy dissipation) in SBS on deformation, associated with the limited extensibility of the PB chains between the adjacent PS domains. It becomes clear that the pulsed NMR method complements the mechanical measurements with a more precise information on the heterogeneity in the rubbery polymers under deformation.     

Deformation behavior of styrene-block-butadiene-block-styrene triblock copolymers having different morphologies

Deformation behavior of styrene-block-butadiene-block-styrene (SBS) triblock copolymers having different morphologies was investigated. Due to the combination of different methods which provide information on different deformation levels (macroscopic, microscopic and molecular) complex deformation mechanisms for each type of SBS block copolymer (including glassy-rubber alternating lamellae, rubber cylinders in glassy matrix and hard domains in soft matrix morphology) could be revealed. In combination with tensile tests, Fourier transform infrared (FTIR) spectroscopy was successfully applied to study the change of orientation in individual phases using the absorption bands at 1493 and 966 cm⁻¹ for polystyrene (PS) and polybutadiene (PB) phases, respectively. For all the block copolymers investigated the PB phase always oriented stronger than the PS phase because of its lower Young's modulus. However, differences in orientation in both phases were influenced by an appropriate stress distribution within the specimens during deformation, which, in fact, depends on the morphology of the polymers. Additionally, atomic force microscopy revealed local morphological changes during uniaxial stretching, which, in fact, depend on the arrangement of the structural units.     

Effect of electron beam radiation on tensile and viscoelastic properties of styrenic block copolymers

The effect of electron beam (E‐beam) radiation on a series of styrenic block copolymers (SBCs) was investigated. These SBCs included newly developed poly(styrene‐block‐isoprene/butadiene‐block‐styrene) (SIBS), poly(styrene‐block‐butadiene‐block‐styrene) (SBS), and poly(styrene‐block‐isoprene‐block‐styrene) (SIS). The tensile properties, stress relaxation, molecular weight, and dynamical mechanical properties were studied. Generally, the crosslink density and tensile moduli of SBCs increased with increasing of E‐beam radiation dose. The tensile strength of SIBS and SIS was shown to first decrease at lower E‐beam radiation dose (<120 kGy) and then increase at higher radiation dose (>190 kGy). The tensile strength of SBS was significantly decreased at high E‐beam radiation dose (>190 kGy). This was attributed to the differences between entanglement before E‐beam radiation and the homogeneity of the crosslink network after exposure. POLYM. ENG. SCI., 54:2979–2988, 2014. © 2014 Society of Plastics Engineers     

Influence of the extrusion process on the morphology and micromechanical behavior of polystyrene‐block‐(polystyrene‐co‐butadiene)‐block‐polystyrene star block copolymer/homopolystyrene blends

The influence of the extrusion process on the morphology and micromechanical behavior of an asymmetric polystyrene‐block‐(polystyrene‐co‐butadiene)‐block‐polystyrene (SBS) star block copolymer and its blends with general‐purpose homopolystyrene (hPS) was studied with films prepared with a single‐screw extruder. The techniques used were transmission electron microscopy and uniaxial tensile testing. Unlike the pure SBS block copolymer possessing a gyroid‐like morphology, whose deformation was found to be insensitive to the processing conditions, the mechanical properties of the blends strongly depended on the extrusion temperature as well as the apparent shear rate. The deformation micromechanism was primarily dictated by the blend morphology. The yielding and cavitation of the nanostructures were the principal deformation mechanism for the blends having a droplet‐like microphase‐separated morphology, whereas cavitation dominated for the blends containing macrophase‐separated layers of polystyrene. The mechanical properties of the blends were further examined with respect to the influence of the temperature and shear rate on the phase behavior of the blends. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Plasticization of nano‐CaCO3 in polystyrene/nano‐CaCO3 composites

The shear rheological properties of polystyrene (PS)/nano‐CaCO₃ composites were studied to determine the plasticization of nano‐CaCO₃ to PS. The composites were prepared by melt extrusion. A poly(styrene–butadiene–styrene) triblock copolymer (SBS), a poly(styrene–isoprene–styrene) triblock copolymer (SIS), SBS‐grafted maleic anhydride (SBS–MAH), and SIS‐grafted maleic anhydride were used as modifiers or compatibilizers. Because of the weak interaction between CaCO₃ and the PS matrix, the composites with 1 and 3 phr CaCO₃ loadings exhibited apparently higher melt shear rates under the same shear stress with respect to the matrix polymer. The storage moduli for the composites increased with low CaCO₃ concentrations. The results showed that CaCO₃ had some effects on the compatibility of PS/SBS (or SBS–MAH)/CaCO₃ composites, in which SBS could effectively retard the movement of PS chain segments. The improvement of compatibility, due to the chemical interaction between CaCO₃ and the grafted maleic anhydride, had obvious effects on the rheological behavior of the composites, the melt shear rate of the composites decreased greatly, and the results showed that nano‐CaCO₃ could plasticize the PS matrix to some extent. Rheological methods provided an indirect but useful characterization of the composite structure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Influence of interfacial structure on morphology and deformation behavior of SBS block copolymers

Linear styrene-block-butadiene-block-styrene (SBS) triblock copolymers having different interfacial structures were investigated. In spite of the nearly equivalent chemical composition (about 70 vol% of styrene), these copolymers show significantly different morphologies. It was shown that the origin of the modified morphology in asymmetric block copolymers is the intermixing of short polystyrene (PS) chains or chain segments into the polybutadiene (PB) phase. It has a consequence of an increase in the glass transition temperature of the soft phase (PB phase here) and a significant decrease of the whole relaxation time of the materials. The larger the interfacial volume, the more PS molecules can mix into the PB phase. Moreover, it seems that the extent of the stress transfer in heterogeneous polymeric systems is crucially influenced by the interface. The tapered interface in an SBS block copolymer, for example, permits a more effective stress transfer compared to the sharp interface resulting in a higher degree of orientation in the individual phases of the materials.     

The morphology and mechanical properties of dynamic packing injection molded PP/PS blends

As a part of long-term project aimed at super polyolefin blends, in this work, we report the mechanical reinforcement and phase morphology of the immiscible blends of polypropylene (PP) and polystyrene (PS) achieved by dynamic packing injection molding (DPIM). The shear stress (achieved by DPIM) and interfacial interaction (obtained by using styrene-butadiene-styrene (SBS) as a compatibilizer) have a great effect on phase morphology thus mechanical properties. The shear-induced morphology with core in the center and oriented zone surrounding the core was observed in the cross-section areas of the samples. The phase inversion was also found to shift towards lower PS content under shear stress, at 70 wt% in the core and 30 wt% in the oriented zone, compared with 80 wt% for static samples (without shear). The tensile strength, tensile modules and impact strength were found largely increase by means of either shear stress or compatibilizer. The PS particle size is greatly reduced with adding of SBS, and the reduced particle size results in greater resistance to deformation, which causes the co-continuous structure at oriented zone change into droplet morphology. The morphology resulting from blending and processing was discussed based on effect of interfacial tension, shear rate, phase viscosity ratio and composition. The observed change of mechanical properties was explained based on the combined effect of phase morphology (droplet-matrix or co-continuous phase) and molecular orientation under shear stress.     

Structural relaxation and evolution of yield stress in epoxy glass aged under shear strain

Hollow cylindrical specimens of annealed epoxy glass were twisted and then aged for various periods of time under shear strain. At the end of the aging process, we twisted the specimens again to determine the stress–strain relations. For specimens aged under a shear strain of 0.005 or 0.01, the stress relaxation behavior was almost independent of the amount of strain imposed, and the value of stress at the upper yield point, regardless of aging time, was almost the same as that of the annealed specimen. On the other hand, for specimens aged under a strain of 0.02 or 0.04, the stress relaxation behavior depended on the value of the strain applied, and the value of stress at the upper yield point first decreased and subsequently increased with increasing aging time. These results led us to the following conclusions: If epoxy glass is strained largely, the originally stable structure becomes unstable. Also, when epoxy glass is aged under strain, the stability of the structure continues to decrease for a short period of time after deformation ceases, and then increases with increasing aging time. POLYM. ENG. SCI. 45:20–24, 2005. © 2004 Society of Plastics Engineers.     

Physical and Mechanical Properties of Iron Powder Filled Polystyrene Composites

This article reports on an experimental study of the physical and mechanical properties of Polystyrene (PS) and Fe-PS polymer composites containing 5, 10, and 15 vol.% of Fe powder. After mixing Fe powder and PS in a twin-screw extruder, an injection-molding machine was used to prepare unfilled PS and Fe-PS polymer composite samples. After that, the material properties were experimentally determined for each sample. The investigated material properties included the modulus of elasticity, yield and tensile strength, % elongation, Izod impact strength (notched), hardness (Shore D), melt flow index (MFI), heat deflection temperature (HDT), Vicat softening point, and glass transition temperature (T _g ). The results indicated that, compared to the unfilled PS, an addition of Fe into PS decreases the yield and tensile strength, % elongation, and Izod impact strength. Furthermore, it was determined that the Fe particles increase the modulus of elasticity, hardness, MFI, Vicat softening point, and HDT values.     

Morphology and rheological behavior of maltene–polymer blends. I. Effect of partial hydrogenation of poly(styrene‐block‐butadiene‐block‐styrene‐block)‐type copolymers

Because of the importance of the maltene–polymer interaction for the better performance of polymer‐modified asphalts, this article reports the effects of the molecular characteristics of two commercial poly(styrene‐block‐butadiene‐block‐styrene‐block) (SBS) polymers and their partially hydrogenated derivatives [poly{styrene‐block[(butadiene)_1?x–(ethylene‐co‐butylene)_x]‐block‐styrene‐block} (SBEBS)] on the morphology and rheological behavior of maltene–polymer blends (MPBs) with polymer concentrations of 3 and 10% (w/w). Each SBEBS and its parent SBS had the same molecular weight and polystyrene block size, but they differed from each other in the composition of the elastomeric block, which exhibited the semicrystalline characteristics of SBEBS. Maltenes were obtained from Ac‐20 asphalt (Pemex, Salamanca, Mexico), and the blends were prepared by a hot‐mixing procedure. Fluorescence microscopy images indicated that all the blends were heterogeneous, with polymer‐rich and maltene‐rich phases. The rheological behavior of the blends was determined from oscillatory shear flow data. An analysis of the storage modulus, loss modulus, complex modulus, and phase angle as a function of the oscillatory frequency at various temperatures allowed us to conclude that the maltenes behaved as pseudohomogeneous viscoelastic materials that could dissipate stress without presenting structural changes; moreover, all the MPBs were more viscoelastic than the neat maltenes, and this depended on both the characteristics and amount of the polymer. The MPBs prepared with SBEBS were more viscoelastic and possessed higher elasticity than those prepared with SBS. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

An experimental study of the influence of carbon black on the rheological properties of a polystyrene melt

A broad range of experiments on carbon black filled polystyrene melts shows the reinforcing effect of the filler. This study represents one of the most extensive investigations of a series of highly filled polymer melts. Stress relaxation and dynamic experiments characterize the small strain behavior while the steady state shear viscosity, normal stresses, and elongational flow experiments describe the large strain deformation rate response. Extrudate swell and unconstrained shrinkage of extrudates are also measured. Highly filled systems exhibit yield values. This is seen in the dynamic experiments and in the shear and elongational viscosities. Viscosity does not level off at finite values with decreasing deformation rate but continues to increase in an approximately inverse manner. This corresponds to yield values of order 5 × 10⁵ dynes/cm². The storage modulus also does not tend to zero at low frequencies. The small strain dynamic properties and stress relaxation results suggest high memories for small strain experiments. Txtrudate swell values are however small and the systems exhibit minimal delayed recovery. The implications of this are considered. Generally it is argued that at volume loadings between 10 and 20 percent, the system takes on the characteristics of a gel and the response is similar to that of a Schwedoff body.     

Effects of ABS rubber particles on rheology,melt failure,and thermoforming

The rubber particles included in rubber modified polymeric materials such as acrylonitrile‐butadiene‐styrene (ABS) polymer and impact modified polymers play an important role in determining their rheological properties, processing behavior, and mechanical properties. In this study both small strain oscillatory shear viscosity in the frequency range from 10^?2 to 10² s^?1 and uniaxial elongational viscosity behavior at two elongation rates ( = 0.1 and 1.0 s^?1) over the range of temperatures from 140°C to 200°C were measured for commercial ABS polymers with different contents and deformability of rubber particles. The influences of rubber content and deformability on rheological properties such as melt elasticity, elongational viscosity, strain hardening and/or softening, the onset of nonuniform deformation, and thermoforming performance were investigated. The Wagner two‐parameter nonlinear viscoelastic constitutive model was used to describe strain hardening behavior, while the Considère criterion was used to determine the onset point of nonuniform deformation. The part thickness distribution obtained through use of a vacuum snap‐back forming process was simulated to investigate the effects of rheological changes associated with different rubber particles on the thermoforming performance. It was found that ABS polymers with larger contents of hard rubber particles exhibited more melt elasticity, stronger strain hardening, a maximum of biaxial elongational viscosity, onset of nonuniform deformation at later time, and better thermoforming performance. Strain hardening and the Considère criterion provide simple, reliable indicators of the thermoforming performance of ABS polymers.     

Linear and nonlinear melt‐state viscoelastic properties of polypropylene/organoclay nanocomposites

Rheological behavior of polypropylene (PP)/organoclay nanocomposites varying in compatibilizer (PP‐g‐MA) and organoclay concentration was investigated. The samples were prepared by melt intercalation method in an internal mixer. The wide angle X‐ray diffraction patterns and results of rheological measurements showed that the compatibilizer had strong influence in increasing the interlayer spacing. The observed low frequency liquid‐like to solid‐like transition and apparent yield stress in simple shear flows, along with convergence of transient shear stress to nonzero values in stress relaxation after the cessation of flow experiments, were found to be consistent with formation of a physical network in quiescent conditions which could be easily ruptured with applying low shear rates. The values of stress overshoot strain in flow reversal experiments were independent of shear rate, organoclay, and compatibilizer content. From the results of frequency sweep experiments in different nonlinear strain amplitudes it was shown that extended Cox‐Merz analogy was valid in nonlinear dynamic deformations while the shear viscosity showed positive deviation from this analogy with higher deviations at lower shear rates. Results of storage modulus recovery and flow reversal experiments at different shear rates suggested that network structure is reformed with a much slower rate compared to the rotational relaxation of organoclay platelets. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Flow-induced structure and rheology of a triblock copolymer

Linear viscoelastic properties are found to be a sensitive measure of flow-induced structural changes in a block copolymer. Styrene-butadiene-styrene block copolymer (SBS) with 26% polystyrene (PS) forms a macrostructure in the quiescent state with grains of the order of 1–10 μm. Within each grain, phase separation gives rise to a regular two-phase microstructure with cylindrical PS domains with radius of the order of 200 Å. Large-amplitude oscillatory shear (γ = 4.5) at temperatures between 139 and 181°C was applied to after the grain structure with the objectives of removing the discontinuities at the grain boundaries and of aligning the domains into a continuous ultrastructure. The SBS behaved like a solid (tan δ < 1 at low ω) before and like a liquid (tan δ > 1) after shear modification. This change expressed itself in the removal of the long relaxation times from the linear viscoelastic spectrum; the intermediate and low relaxation times were not affected by the shear modification. The viscoelastic spectrum slowly recovered during annealing with recovery times of the order of the longest relaxation time of the quiescent structure. Birefringence studies showed that the SBS did not recover into its original grain structure but into a highly oriented domain structure. The discontinuities at the grain boundaries could not be removed completely.     

微流道中聚合物熔体的弹性特性研究

采用双机筒毛细管流变仪,以直径1.5 mm的口模作为宏观流道、直径0.5 mm的口模作为微观流道、直径1.0 mm的口模作为临界参考,通过毛细管入口压力降和聚合物熔体应力松弛时间研究了微尺度下聚甲基丙烯酸甲脂（PMMA）、聚苯乙烯（PS）、高密度聚乙烯（PE-HD）及聚丙烯（PP）4种聚合物熔体的弹性特性,并讨论了在口模直径为0.5 mm条件下温度对入口压力降和应力松弛时间的影响。结果表明,微流道中4种聚合物熔体的入口压力降与应力松弛时间与宏观流道相比均有增大的趋势,且流道尺寸越小,增大趋势越明显;随着剪切速率的增大,4种聚合物熔体的入口压力降增大,但在高剪切速率下,增大趋势变缓;微流道中,随着温度的提高,4种聚合物熔体的入口压力降降低,应力松弛时间变短。     

Tensile stress measurements on linear and branches low-density polyethylene melts. Interpretation of results with a generalized Maxwell model

Dynamic shear experiments in the linear range of deformation and extensional tests at constant strain rate have been carried out on a linear low-density polyethylene (LLDPE) melt and on two branched low-density polyethylene (LDPE) melts with different amounts of long-chain branching. Both the dynamic shear moduli and the tensile stress obey the time–temperature superposition principle. A simple model based on a nonaffine generalized Maxwell model with two relaxation times is proposed to describe the rheological behavior in elongation of these melts. Close agreement between the model and the experimental data can be obtained by adjusting the two relaxation times and the “slip parameter” of entanglements. The variations of these parameters with strain rate and their relationship with molecular structure are discussed.     

Melt Rheology and Morphology of Nitrile Rubber and Ethylene-Vinyl Acetate Copolymer Blends

The melt Theological behavior of nitrile rubber (NBR)/ethylene-vinyl acetate (EVA) copolymer blends was studied with special reference to the effect of the blend ratio, cross-linking systems, and shear rate using a capillary rheometer. At a given shear stress at 90°C, the viscosities of the blends vary slightly with composition. The effect of cross-linking systems [viz., sulfur (S), peroxide (DCP) and mixed (S+DCP) systems] on the viscosity of NBR/EVA blends is negligible. The melt viscosity of the blends decreases with increasing shear rate, showing pseudoplastic behavior. The flow behavior index values also support the pseudoplastic nature of these blends. Various theoretical models were used to predict the melt viscosity of the blends. Parameters such as die swell, principal normal stress difference, recoverable shear strain, and shear modulus were calculated to characterize the melt elasticity of these blends. The melt elasticity of the system was increased by the addition of NBR to EVA. The extrudate deformation at different shear rates was also studied. It was observed that as the shear rate increases, the extrudate surface exhibits a higher degree of deformation. The morphology of the extrudates of the blends at different shear rates has been examined by a scanning electron microscope. The morphology was found to be dependent on the blend ratio and shear rate.     

Transient uniaxial orientation of flexible polymer chains in a wide range of elongation rates

Affine evolution of chain end-to-end vectors distribution function is derived analytically for non-linear polymer liquids subjected to uniaxial elongational flow, controlled by time-evolution of chain deformation coefficients. Peterlin approximation for non-Gaussian chain elasticity is applied, with Padè approximation for the inverse Langevin function. The approach enables calculations of transient molecular deformation coefficients in entire range of elongation rates and times.Equations controlling time evolution of the molecular deformation coefficients in elongational flow are solved analytically with an assumption of dominating elongational component. The approach allows to decouple evolution equations and obtain an approximate closed form analytical formula describing time evolution of the molecular deformation with high accuracy, in particular at higher elongation rates, above the Gaussian limit.Predictions of the analytical formula are compared with numerical computations to evaluate the approximation and ranges of its validity.The analytical formula enables predicting evolution of average functions in non-linear systems, such as free energy, tensile stress, molecular orientation, etc. The formula is used to discuss molecular vs. macroscopic deformation in wide range of elongation rates and times, as well as evolution of stress, axial orientation factor, apparent elongational viscosity.