Viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes in supercritical methanol

The viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes using a supercritical methanol were investigated via oscillatory dynamic shear measurements. Decrosslinked polymers at a low reaction temperature exhibited solid‐like rheological properties, as evidenced by a small slope at G′ and G″, a long relaxation time, slow stress relaxation behavior, and considerable yield stress. In contrast, decrosslinked polymers at a high temperature exhibited liquid‐like rheological properties that included a large slope in G′ and G″, a short relaxation time, fast stress relaxation behavior, and nonyield stress. The difference in the viscoelastic properties of the decrosslinked polyethylenes was attributed to the difference in the gel content with the reaction temperature. A higher gel content induced stronger solid‐like viscoelastic properties. Hence, the rheological measurements were useful for analyzing the molecular structure of decrosslinked polymers using a supercritical fluid. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1265–1270, 2010     

A novel strategy for the preparation of long chain branching polypropylene and the investigation on foamability and rheology

A novel strategy for introducing long chain branches (LCB) on commercial polypropylene (PP) was described using an extender agent, poly (hexamethylendiamine-guanidine hydrochloride) (PHGH), to couple with glycidyl methacrylate-grafted PP (PP-GMA). The presence of LCB was confirmed by GPC and melt strength measurements in two modified PP samples, MPP-1 and MPP-2. The foamability of LCB PPs and commercially linear PP (EPS) was investigated by foam extrudate. The results showed that the foamability of LCB PPs was significantly improved. To find out the difference in foamability of EPS and LCB PPs, some rheological properties were investigated. In the dynamic shear measurement, it was found that the elastic response of LCB PPs at low frequencies was distinctly enhanced in comparison with that of EPS, implying a presence of a long relaxation time mode that was not revealed in linear PP. The elongational rheology results showed that the strain hardening behaviors of LCB PPs were still intense even at higher strain rate of 1 s^?1; moreover, MPP-2 could sustain larger elongational stress and deformation, which helps to improve the foamability. In step-shear stress relaxation measurements, the fast and slow relaxation processes were observed over the entire ranges of strains, indicating the presence of two different relaxation time modes. Also, the strain dependence of slow damping function became weaker because of the introduction of LCB on linear PP. In addition, it was also found that the rheological behaviors characterizing the long relaxation time mode were further enhanced with the increase of the concentration of PHGH in the melt grafting reactions.     

Flow induced crystallization of long chain branched polypropylenes under weak shear flow

In the present work long chain branched polypropylene (LCB PP) polymers were prepared by linear polypropylene and multi-functional monomer through melt grafting reaction. A quantitative rheological method was adopted to analysis the structure parameters of LCB PP. The effects of chain branched level on the crystallization kinetics of PP were investigated by rheology, differential scanning calorimetry, polarized optical microscope and wide-angle X-ray diffraction. The dynamic viscoelastic properties of LCB PP showed that the increase in the chain branched level caused a typical deviation from the terminal behavior and a different distribution of the melt relaxation spectrum in the long relaxation time regime. It was found that the chain branched level had a significant effect on the flow induced crystallization (FIC) process of PP melts. The crystallization of LCB PP was more sensitive to shear flow than that of linear PP during induced period at low shear rates. This result also indicated that the longer relaxation time of the polymer chains played an important role in the nucleation of PP under shear flow fields. LCB PP with high chain branched level showed accelerated crystallization kinetics in comparison with that with low chain branched level.     

Poly(phenylene sulfide)/low‐melting‐point metal composites. I. Transient viscoelastic properties and crystallization kinetics

Poly(phenylene sulfide)/low‐melting‐point metal composites (PPSMs) with various loading levels were prepared by melt compounding. The nonisothermal crystallization behavior and transient viscoelastic properties were characterized by the DSC, POM, DMA, and parallel‐plate rheometer. The results reveal that the low‐melting‐point metal (LMPM) particles show nice dispersion at relative low content levels (< 30 wt %). The PPSMs composites present dual characteristics of both the filled polymer composite and polymer blend system in their transient viscoelastic behaviors, which results in occurrence of the stress overshoots with long relaxation time and nonzero residual stress especially at high shear levels. During the crystallization process, the presence of those deformable LMPM droplets facilitates the crystallization kinetics of PPS because of their flow‐promoting action. On the other hand, the LMPM has no heterogeneous nucleating effect and, only plays the role of inert filler, which results in the degradation of the crystal structure of PPS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 677–690, 2008     

Dynamic rheological behaviors of metallocene-based ethylene-butene copolymers and their blends with low-density polyethylene

The dynamic rheological properties of three metallocene-based ethylene-butene copolymers were examined and were compared with those of some conventional ethylene-butene copolymers and of a low density polyethylene (LDPE). Compared with the conventional ethylene copolymers, the metallocene-based copolymers exhibit the following dynamic rheological features: (1) lower viscoelastic moduli and viscosity at small frequencies, but larger viscoelastic moduli and viscosity at large frequencies, thus a small shear thinning effect; (2) larger values of flow activation energy; (3) a relatively fast relaxation rate. These features are the results of simultaneous absence of high molecular weight tails and low molecular weight tails in the metallocene-based copolymers. The dynamic rheological properties of blends of various ethylene-butene copolymers with LDPE were also investigated. It is found that the addition of LDPE can raise the viscosity at low frequencies but lower the viscosity and elasticity at higher frequencies, and retard the relaxation rate of the metallocene-based ethylene copolymers. However, the improvement in rheological properties by LDPE varies with the polymer samples and there is no improvement for the conventional copolymer G.     

In situ fibrillation of polyamide 6 in isotactic polypropylene occurring in the laminating‐multiplying die

The polyamide 6 (PA6)/isotactic polypropylene (iPP) in situ fibrillation composites are prepared by a novel extrusion die with an assembly of laminating‐multiplying elements (LMEs). The scanning electron micrographs illustrate that the dividing‐multiplying processes in LMEs elongate, break, and stabilize the dispersed PA6 phase in the iPP matrix along the flowing direction (FD). The morphology development of PA6 with different LME numbers greatly affects the rheological properties, crystalline behaviors, and mechanical properties. The dynamic rheological test performed at 195°C shows that the increased spatial restriction of the high‐aspect‐ratio PA6 particles increases the viscoelastic moduli, complex viscosity, and relaxation time. The crystalline analysis reveals that the heterogeneous nucleation becomes predominant and the transcrystalline morphology is observed in those samples produced by more LMEs. The tensile tests indicate that both, breaking strength and elongation, enhanced simultaneously because of the fibrillation of dispersed phase and the improvement in interfacial adhesion between the fibers and the matrix. Copyright © 2009 John Wiley & Sons, Ltd.     

Kinetics study on melt compounding of carbon nanotube/polypropylene nanocomposites

The multiwalled carbon nanotubes/polypropylene nanocomposites (PP/CNTs) were prepared by melt mixing using maleic anhydride grafted polypropylene (mPP) as the compatibilizer. The effect of mPP on dispersion of CNTs was then studied using the tool of rheology, aiming at relating the viscoelastic behaviors to the mesoscopic structure of CNTs. To further explore the kinetics of hybrid formation, a multilayered sample with alternatively superposed neat mPP and binary PP/CNTs microcomposites (without addition of mPP) sheets was prepared and experienced dynamic annealing in the small amplitude oscillatory shear flow. The results show that melt blending CNTs with PP can only yield the composites with microscale dispersion of CNTs, while adding mPP promotes nanoscale dispersion of CNTs as smaller bundles or even as individual nanotubes, reducing percolation threshold as a result. However, the values of apparent diffusivities of the composites are in same order with that of self‐diffusion coefficients of the neat PP, indicating that the presence of detached CNTs nearly does not inhibit PP chain motion. Hence, the activation energy of hybrid formation is close to the self‐diffusion of PP. This also indicates that although addition of mPP can improve the compatibility between CNTs and PP thermodynamically, those dynamic factors, such as shear flow, however, may be the dominant role on hybrid formation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 608–618, 2009     

Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

Pressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Shear‐induced change of exfoliation and orientation in polypropylene/montmorillonite nanocomposites

For the improved dispersion of montmorillonite (MMT) in a polypropylene (PP) matrix, PP/MMT nanocomposites prepared via direct melt intercalation were further subjected to oscillating stress achieved by dynamic packing injection molding. The shear‐induced morphological changes were investigated with an Instron machine, wide‐angle X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. The original nanocomposites possessed a partly intercalated and partly exfoliated morphology. A transformation of the intercalated structure into an exfoliated structure occurred after shearing, and a more homogeneous dispersion of MMT in the PP matrix was obtained. However, the increase of the exfoliated structure was accompanied by the scarifying of the orientation of MMT layers along the shear direction. Some bended or curved MMT layers were found for the first time by TEM after shearing. However, the orientation of PP chains in the PP/MMT nanocomposites became very difficult under an external shear force; this indicated that the molecular motion of PP chains intercalated between MMT layers was highly confined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1–10, 2003     

Molecular and structural analysis of a triepoxide‐modified poly(ethylene terephthalate) from rheological data

The effects of reactive melt modification of poly(ethylene terephthalate) (PET) with a multifunctional epoxide—triglycidyl isocyanurate—that lead to chain extension/branching and formation of gel‐like structures were rheologically characterized. The storage and loss moduli and the complex viscosity of the modified PET were larger than those of the unmodified PET. The elastic or solidlike behavior of PET was enhanced after reactive modification as a result of chain extension/branching. Modified Cole–Cole plots revealed that the modified resins show higher elasticity than the unmodified one. Reactive modification characterized by the presence of long‐chain branching resulted in a wider molecular weight distribution. The effects of mixing temperature and the concentration of modifier corresponding to different stoichiometries were investigated. Higher amounts of modifier resulted in a polymeric structure near the sol–gel transition point whose linear viscoelastic properties obey scaling law. The relaxation spectrum was calculated from the oscillatory shear data by Tschoegl equations. Certain modified resins have rheological properties suitable for process operations such as extrusion foaming, blow molding, and thermoforming. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 958–969, 2003     

Viscoelastic properties of silica‐grafted poly(styrene–acrylonitrile) nanocomposites

The melt state viscoelastic properties of styrene–acrylonitrile random copolymers grafted to silica nanoparticles and prepared by in situ atom transfer radical polymerization are examined. Linear dynamic oscillatory viscoelastic measurements indicate that nanocomposites with silica contents as low as 1.2 vol % exhibit solid‐like response. Steady shear results in considerable alteration to the mesoscale structure and results in the failure of the Cox‐Merz rule. Flow reversal studies confirm the elastic nature of the mesoscale structure and demonstrate that the relaxation of the flow altered structure is slow, non‐Brownian, and possibly cooperative. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2014–2023, 2006     

Orientation and relaxation of polymer–clay solutions studied by rheology and small-angle neutron scattering

The influence of shear on viscoelastic solutions of poly(ethylene oxide) (PEO) and clay [montmorillonite, i.e., Cloisite NA+ (CNA)] was investigated with rheology and small-angle neutron scattering (SANS). The steady-state viscosity and SANS were used to measure the shear-induced orientation and relaxation of the polymer and clay platelets. Anisotropic scattering patterns developed at much lower shear rates than in pure clay solutions. The scattering anisotropy saturated at low shear rates, and the CNA clay platelets aligned with the flow, with the surface normal parallel to the gradient direction. The cessation of shear led to partial and slow randomization of the CNA platelets, whereas extremely fast relaxation was observed for laponite (LRD) platelets. These PEO–CNA networklike solutions were compared with previously reported PEO–LRD networks, and the differences and similarities, with respect to the shear orientation, relaxation, and polymer–clay interactions, were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3102–3112, 2004     

Viscoelastic interfacial properties of compatibilized poly(ε‐caprolactone)/polylactide blend

Poly(ε‐caprolactone)/polylactide blend (PCL/PLA) is an interesting biomaterial because the two component polymers show good complementarity in their physical properties. However, PCL and PLA are incompatible thermodynamically and hence the interfacial properties act as the important roles controlling the final properties of their blends. Thus, in this work, the PCL/PLA blends were prepared by melt mixing using the block copolymers as compatibilizer for the studies of interfacial properties. Several rheological methods and viscoelastic models were used to establish the relations between improved phase morphologies and interfacial properties. The results show that the interfacial behaviors of the PCL/PLA blends highly depend on the interface‐located copolymers. The presence of copolymers reduces the interfacial tension and emulsified the phase interface, leading to stabilization of the interface and retarding both the shape relaxation and the elastic interface relaxation. As a result, besides the relaxation of matrices (τ_m) and the shape relaxation of the dispersed PLA phase (τ_F), a new relaxation behavior (τ_β), which is attribute to the relaxation of Marangoni stresses tangential to the interface between dispersed PLA phase and matrix PCL, is observed on the compatibilized blends. In contrast to that of the diblock copolymers, the triblock copolymers show higher emulsifying level. However, both can improve the overall interfacial properties and enhance the mechanical strength of the PCL/PLA blends as a result. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 756–765, 2010     

Poly(ether ether ketone)/poly(aryl ether sulfone) blends: Melt rheological behavior

Dynamic rheological measurements were carried out on blends of poly(ether ether ketone) (PEEK)/poly(aryl ether sulfone) (PES) in the melt state in the oscillatory shear mode. The data were analyzed for the fundamental rheological behavior to yield insight into the microstructure of PEEK/PES blends. A variation of complex viscosity with composition exhibited positive–negative deviations from the log‐additivity rule and was typical for a continuous‐discrete type of morphology with weak interaction among droplets. The point of transition showed that phase inversion takes place at composition with a 0.6 weight fraction of PEEK, which agreed with the actual morphology of these blends observed by scanning electron microscopy. Activation energy for flow, for blend compositions followed additive behavior, which indicated that PEEK/PES blends may have had some compatibility in the melt. Variation of the elastic modulus (G′) with composition showed a trend similar to that observed for complex viscosity. A three‐zone model used for understanding the dynamic moduli behavior of polymers demonstrated that PEEK follows plateau‐zone behavior, whereas PES exhibits only terminal‐zone behavior in the frequency range studied. The blends of these two polymers showed an intermediate behavior, and the crossover frequency shifted to the low‐frequency region as the PEEK content in PES increased. This revealed the shift of terminal‐zone behavior to low frequency with an increased PEEK percentage in the blend. Variation of relaxation time with composition suggested that slow relaxation of PEEK retards the relaxation process of PES as the PEEK concentration in the blend is increased because of the partial miscibility of the blend, which affects the constraint release process of pure components in the blend. A temperature‐independent correlation observed in the log–log plots of G′ versus loss modulus (G″) for different blend systems fulfilled the necessary condition for their rheological simplicity. Further, the composition‐dependent correlations of PEEK/PES blends observed in a log–log plot of G′ versus G″ showed that the blends are either partially miscible or immiscible and form a discrete‐continuous phase morphology. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1548–1563, 2004     

Exploring rheological properties of aqueous polyaniline‐PVP dispersion

Stable aqueous dispersion of polyaniline (PAn) stabilized by a hydrophilic polymer poly(vinyl pyrrolidone) (PVP) exhibits interesting rheological properties different from its components. Shear thinning observed for both PVP and PAn–PVP colloid (PP) indicates partially entangled nature of the later. Linear viscoelastic response of PVP solution exhibit strong frequency dependence of elastic (G′) and viscous (G″) modulus over the whole frequency range (0.1–100 ras/s) where G′ never exceeds G″ indicating the applicability of the Rouse‐Zimm model to this system. On the other hand, there is a crossover of G′ and G″ in the rheological profile of PP dispersion so that a single relaxation time model can be applicable. Therefore, PVP presents an entangled polymeric system and supposed to have a spectrum of relaxation times, whereas PP resembles to a physically crosslinked system with a single relaxation time. Increasing the extent of hydrogen bonding within the system (by raising the fraction of PAn or by leaving the solution undisturbed for long) relaxation time also becomes longer. The large difference in values of steady and complex shear viscosity (η and η*) within LVE regime reflects that original Cox‐Merz rule is obviously inapplicable to these systems. But at larger strain amplitude, η and η* are satisfactorily coincident that indicates a broader applicability of the modified Cox‐Merz rule. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2443–2455, 2008     

Rheological properties and foam processability for blends of linear and crosslinked polyethylenes

The effect of blending crosslinked linear low‐density polyethylene (cLLDPE) on the rheological properties and foam processability of linear low‐density polyethylene was studied. A small addition of cLLDPE, which had a low density of crosslink points, enhanced strain‐hardening behavior in the elongational viscosity to a great degree, although it had little effect on the steady‐state shear viscosity. The enhanced strain hardening reduced heterogeneous deformation during foaming. As a result, a foam with a uniform cell size distribution was obtained. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2159–2167, 2001     

Apparent relaxation‐time spectrum cutoff in dilute polymer solutions: An effect of solvent dynamics

The apparent short time cutoff of the relaxation‐time spectrum at surprisingly long times for polymers in solution is a well known but not yet understood observation. To elucidate its origins we revisit viscoelastic and oscillatory flow birefringence data for solutions and melts of two linear polymers (polystyrene and polyisoprene) and present new measurements of oscillatory flow birefringence of the latter. Previous measurements have suggested that the “flexibility” of both polymers in solution is smaller than in the melt on the basis of the breadth of the relaxation‐time spectrum of the solution as compared with that of the melt. Our new measurements have explored a higher effective frequency range than was previously possible. This has allowed us to observe the effect of the rotational relaxation time of the solvent on the dynamics of the solution at high frequencies. To obtain the polymer global motion contribution, one now needs to subtract from the solution properties a frequency‐dependent complex solvating environment contribution. We show that the decrease in apparent “flexibility” for solutions arises from the presence of a solvent that exhibits a rotational relaxation time and thus simple viscoelastic behavior somewhat near the frequency window of the experiment. Although recent predictions of a model for a chain in a solvent with a single relaxation time are in qualitative agreement with our results, our data suggest that the solution results may reflect the influence of solvent on the development of the “entropic spring” forces at short times. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2860–2873, 2001     

Relaxation dynamics of stretched polymer chains traced by shear induced Shish‐Kebab structure

We have developed a morphologic method to investigate the relaxation processing of the stretched polymer chains in melts, in which an atomic force microscope probe was used to shear the surface of an isotactic polypropylene melt to obtain the isolated shish‐kebab structure. We present the results of the time dependence of length of the isolated shish‐kebab structure and the stress dependence of the kebab density along the direction of shish in this paper. Our results demonstrate that the shear‐oriented polymer melts show the relaxation dynamics of worm‐like chain where the length deficit of the isolated shish‐kebab structure is scaled with the relaxation time as a power of 1/3. The melting behavior of shish‐kebab structure was also investigated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 907–914     

The influence of variant PE‐b‐PEO segments on physical properties of LLDPE graft copolymers

A method was adopted to fix a series of polymers of PE‐b‐PEO with different PEO/PE segments on the chains of LLDPE. Maleic anhydride (MA) reacting with hydroxyl group of PE‐b‐PEO (mPE‐b‐PEO) was used as the intermediate. The structures of intermediates and graft copolymers were approved by ¹H NMR and FTIR. XPS analysis revealed a great amount of oxygen on the surface of grafted copolymers although the end group of PEO was fixed on the LLDPE chains through MA. Thermal properties of the graft copolymers as determined by differential scanning calorimetry (DSC) showed that PE segments in the grafted monomers could promote the heterogeneous nucleation of the polymer, increase T_c, and crystal growth rate. While the amorphous PEO segments which attached to the crystalline PE segments in LLDPE, impaired their ability to fit the crystal lattice, and depressed the crystallization of LLDPE backbones. In this study, it was also verified through the dynamic rheological data that increasing M_n of grafted monomers significantly increased the complex viscosity and enhanced the shear‐thinning behavior. Long‐branched chains formed by grafted monomers enhanced the complex moduli (G′ and G″) value and retarded relaxation rate. However, there were little influence on the rheological properties when increasing the amounts of PEO segments (or decreasing PE segments) of grafted monomers with similar molecular weight. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 506–515, 2008     

Linear viscoelastic characteristics of in situ compatiblized binary polymer blends with viscoelastic properties of components variable

By Friedel‐Crafts alkylation reaction, catalyzed by a Lewis acid of anhydrous aluminum chloride (AlCl₃), binary polymer blends of polypropylene (PP)/polystyrene (PS) with volume proportion of 80/20 were in situ compatiblized and prepared in an XSS‐30 melt mixer at 210 °C. The linear viscoelastic characteristics of the blends were investigated by checking the variations of storage modulus, loss modulus, complex modulus, and complex viscosity of the in situ compatiblized blends, which were dependent on AlCl₃ content. In addition, Han plots of the in situ compatiblized blends with different AlCl₃ content were also used to characterize the linear viscoelastic properties of the blends. The results showed that both the dynamic rheological parameters and the Han plots were obviously influenced by the rheological properties of the matrix and slightly influenced by the rheological properties of the dispersed phase. Further investigations revealed that phase geometry contributions to the dynamic rheological parameters of the blends could be ignored in comparison with the contributions of the components and the interfacial modification, which were defined and obtained according to log‐linear‐additivity rule. The linear viscoelastic characteristics of the blends were mainly controlled by the combination of the effects of interfacial modification between phases and the rheological properties of the matrix. Storage modulus is the most sensitive dynamic rheological parameter to characterize the interfacial compatiblization effects in the in situ compatiblized binary polymer blends with rheological properties of components variable. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1349–1362, 2010