Co-reporter:Zhijun Nie, Wei Yu
Polymer 2017 Volume 115(Volume 115) pp:
Publication Date(Web):21 April 2017
DOI:10.1016/j.polymer.2017.03.053
•Failures of TTS and TSS were revealed from the nonlinear relaxation moduli due to mesophase separation.•A two-step relaxation mechanism was identified in olefin multiblock copolymers.•Mesophase separation of olefin multiblock copolymer could be accelerated by shear strain.The nonlinear relaxation behavior of olefin multiblock copolymers subjected to step shear strains has been investigated. Compared to homogeneous polymer, a two-step relaxation behavior was identified in olefin multiblock copolymers, with the faster one for the chain relaxation and the slower one for the domain relaxation. It is found that the domain relaxation has stronger strain dependence than the chain relaxation. Large repeated step strains can cause longer relaxation process of domains as compared to the single step strain test. The prolonged domain relaxation and nearly constant relaxation plateau modulus are ascribed to the coarsening of domain as well as the increase of the volume fraction of domains, which indicates the accelerated mesophase separation of olefin multiblock copolymer under large strains.Download high-res image (191KB)Download full-size image
Co-reporter:Wei You;Chixing Zhou
Soft Matter (2005-Present) 2017 vol. 13(Issue 22) pp:4088-4098
Publication Date(Web):2017/06/07
DOI:10.1039/C7SM00632B
Currently, it is a great challenge to characterize the dispersion quality of nanoparticles in nanocomposites through experimental techniques. In this work, we suggest a new rheological method based on the strain rate amplification effect to determine the cluster size distribution in polymer nanocomposites. The dispersion exponents of nanoparticles from this rheological method are in good agreement with the cluster analysis of transmission electron microscope (TEM) images. We also obtain a critical value of the dispersion exponent from the effective specific surface area of clusters, which separates the well-dispersed state and the phase-separated state. Our results indicate that rheology can be used as a convenient and effective structural analysis method to characterize the nanoparticle cluster size distribution in polymer nanocomposites.
Co-reporter:Kai Yang;Jun Wang
Korea-Australia Rheology Journal 2016 Volume 28( Issue 3) pp:175-180
Publication Date(Web):2016 August
DOI:10.1007/s13367-016-0017-4
Large amplitude oscillation shear (LAOS) is used to investigate the yielding and flow behavior of yield stress materials. Considering the problems in determination of the yield stress from the apparent dynamic moduli and relative harmonic intensity using Fourier Transform Rheology, we proposed a new approach based on 2D mechanical correlation spectra (2D-MCS) to quantify the yield stress. We have proved that the nonlinear synchronous self-correlation intensity as functions of stress/strain amplitude can be used to determine the yield stress unambiguously from the change of scaling exponent. The yield stresses from 2D-MCS analysis are well consistent with those from the stress ramp experiments.
Co-reporter:Jun Wang;Chixing Zhou;Ying Guo;Wim Zoetelief;Paul Steeman
Rheologica Acta 2016 Volume 55( Issue 10) pp:833-845
Publication Date(Web):2016 October
DOI:10.1007/s00397-016-0960-5
The elongational rheological behaviors of glass fiber (GF) filled poly-(ethylene-co-α-octene) POE and long chain branching POE (LCB POE) composites were investigated, with the fiber concentration varying between 0 and 50 wt.%. Three initial fiber orientations, namely planar random, parallel to the stretching flow direction and the perpendicular to it, were adopted to study the mechanism of strain hardening. The fiber orientation distributions of the samples at various Hencky strains were also determined by the optical microscopy and image analysis techniques. It is found that fibers orientate to the stretching direction during elongation and the state with fibers parallel to the flow direction has the highest elongation viscosity, which accounts for the strain hardening in fiber-filled composites. In LCB POE/GF composites, the strain hardening due to fiber orientation starts earlier than that due to chains stretching, but the latter will dominate at larger strain. Two methods were defined to evaluate the strength of strain hardening, which was found to depend on the fiber concentration, initial fiber orientation, and viscoelasticity of polymer matrix. The results were compared with two constitutive models, the Lipscomb et al.’s model and Férec et al.’s model, which could semi-quantitatively describe the effect of fiber concentration and initial fiber orientation but failed to predict the effect of polymer viscoelasticity.
Co-reporter:Jun Wang;Ying Guo;Chixing Zhou;Paul Steeman
Rheologica Acta 2016 Volume 55( Issue 1) pp:37-50
Publication Date(Web):2016 January
DOI:10.1007/s00397-015-0895-2
We investigate the linear and nonlinear viscoelasticity of a model polymer nanocomposite of fumed silica nanoparticles in poly-(ethylene-co-α-butene) (PEB). Above a critical filler fraction, a space-filling network builds up as a result of cluster agglomeration and causes the material to change from liquid-like to solid-like states. Using the Winter-Chambon criterion, the percolation threshold from the measured dynamic moduli is found to depend strongly on the matrix viscoelasticity even when the particle dispersion is identical. Such phenomenon comes from the implicit assumption in the method that the contribution from particles (or particle agglomerates) should dominate the dynamic moduli, which is usually inapplicable when polymer has a high viscosity (or elasticity). A new method is suggested to decompose the moduli of composites into the hydrodynamic part and the part from particle agglomerates by taking into consideration the different effects of particles on the strain rate and stress. The two portions are shown to depend on oscillatory frequency, which are adopted to extrapolate the dynamic moduli to lower frequency. The percolation threshold from these extrapolated data becomes independent of the matrix viscoelasticity. Furthermore, it is found that the appearance of nonlinear behavior in oscillatory shear is related to the portion of particle agglomerates in storage modulus and the strain rate is the key factor to destroy the structures.
Co-reporter:Wei Yu, Jun Wang, Wei You
Polymer 2016 Volume 98() pp:190-200
Publication Date(Web):19 August 2016
DOI:10.1016/j.polymer.2016.06.028
•A new image analysis method and a rheological method were suggested to determine the local volume fraction inside agglomerates.•The nanocomposites with aggregated particles is regarded as an emulsion.•A two-phase model was suggested to describe the linear viscoelasticity of polymer nanocomposites.The connection between the structures and the linear viscoelasticity was revealed in this work for polymer nanocomposites containing agglomerated particles. A new image analysis method using the greyscale histogram of TEM images was suggested to determine the local volume fraction of nanoparticles inside an agglomerate, which represents the degree of agglomeration. It is found that the local volume fraction of nanoparticles is strongly related to the particle-polymer interaction. Such result was justified by a new rheological analysis based on the recently suggested modulus decomposition in the linear viscoelastic properties of polymer nanocomposites, from which the combined shift factor due to the hydrodynamic effect was used to quantify the local volume fraction of nanoparticles. Moreover, a two phase model was suggested to describe the linear viscoelasticity of such polymer nanocomposites. The success of this model in different polymer nanocomposites implies the rationality to regard the isolated agglomerate as a composite droplet in the polymer matrix.
Co-reporter:Peng He, Bin Chen, Wei Yu and Chixing Zhou
RSC Advances 2015 vol. 5(Issue 51) pp:40607-40619
Publication Date(Web):29 Apr 2015
DOI:10.1039/C5RA05030H
The effect of block structure on the liquid–solid transition (LST) of ethylene–octene multiblock copolymers (OBCs) during isothermal crystallization has been investigated by rheology, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). Due to the mesophase separation in OBC melts, the formation of a critical network at the LST in the OBCs with low crystallinity (7–14 wt%) was found to be different from that in homogeneous systems. The viscoelastic properties at the LST in the heterogeneous OBCs suggested a slower relaxation behavior of the critical network, and the liquid–solid transition in strongly segregated OBCs was observed to occur in the intermediate or even late stage of crystallization, demonstrated by the much higher crystallinity and large spherulites at the LST. The delayed liquid–solid transition has been discussed and can be attributed to the initial confinement of the hard-block domains in the nucleation and growth of the crystals.
Co-reporter:Zhiyong Wang, Wei Yu, Chixing Zhou
Polymer 2015 Volume 56() pp:535-544
Publication Date(Web):15 January 2015
DOI:10.1016/j.polymer.2014.11.032
•We prepare microporous membranes based on the HDPE/DIDP/DBS matrix.•The addition of HDPE-g-MAH changes the distribution of DBS.•Multiple phase separations exist in the cooling process.•Different cooling rate affects the morphology of membranes.•High permeability and good rejection rate membranes are obtained.Thermally induced phase separation (TIPS) has been widely used to prepare microporous membrane. The interaction and dynamic asymmetry between polymer and solvent sometime make the transition happen through viscoelastic phase separation (VPS) mechanism. Under such condition, cell like structure is obtained with low water permeability. In this paper, a new method is proposed to obtain high permeability membranes by TIPS. Dibenzylidene sorbitol (DBS) and HDPE-g-MAH are introduced into the polyethylene/di-iso-decyl phthalate (HDPE/DIDP) system, which exhibits an upper critical solution temperature (UCST) behavior and undergoes VPS mechanism during cooling. DBS can self-assemble and form fibril network both in HDPE and DIDP. The addition of HDPE-g-MAH makes DBS mainly distributed in HDPE-rich domains, while it mostly locates in the DIDP-rich domain in the ternary system due to their similar polarity. The self-assembly of DBS in the HDPE-rich domains can induce the secondary liquid–liquid phase separation, which significantly change the structure in walls between cells. High permeability of pure water and high retention of permeability in silica suspension can be obtained under proper control of the extent of these phase transitions.
Co-reporter:Yafang Xu, Chongwen Huang, Wei Yu, Chixing Zhou
Polymer 2015 Volume 67() pp:101-110
Publication Date(Web):12 June 2015
DOI:10.1016/j.polymer.2015.04.052
•Decoupled the amplitude and the wavelength of concentration fluctuation during phase separation by rheological approach.•A crossover frequency was determined to specify the interfacial contribution and the components' contribution.•Quantified the evolution of coexisting phase compositions during phase separation and construct phase diagram.•The crossover from the intermediate stage to the late stage was defined from the evolution of domain compositions and sizes.A systematic rheological method is suggested to study the kinetics of concentration fluctuation during phase separation. It is based on the idea that the storage modulus of a phase separating polymer blend is composed of modulus due to the growing domains (interfacial contribution) and due to the thermally induced local composition fluctuations inside the domains (components' contribution). The two part contributions to the blend modulus varied with frequency. A crossover frequency was determined, below and above which the elastic modulus is dominated by the interfacial contribution and by the components' contribution, respectively. At frequency much above the crossover frequency, the variation of storage modulus is due to the change of local compositions, which can be calculated according to the relationship between the blend composition and the dynamic modulus in miscible state. In addition, the steady state values of local compositions can be considered as the thermodynamic equilibrium compositions at the annealed temperature, which help us to construct phase diagram. Moreover, the interfacial contribution at frequency lower than the crossover frequency could be isolated by subtracting the components' contribution from the blend's modulus. The characteristic length of domains were got through YZZ model with the equilibrium interfacial tension, which is comparable to the results from transmission electron microscopy (TEM). Furthermore, from the evolution of local compositions and the growth way of the characteristic length of domains, the crossover time from the intermediate stage to the late stage of spinodal decomposition can be specified. This method is tested on poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) blend, and would be applicable in other polymer blends with viscoelastic asymmetry.
Co-reporter:Shanshan Lin, Wei Yu, Xianhong Wang, and Chixing Zhou
Industrial & Engineering Chemistry Research 2014 Volume 53(Issue 48) pp:18411-18419
Publication Date(Web):2017-2-22
DOI:10.1021/ie404049v
The degradation behavior of poly(propylene carbonate) (PPC) was investigated during melt processing to infer the mechanism and kinetics of thermal degradation. First, the degradation experiments were carried out in a miniature conical twin-screw extruder at different temperatures, rotating speeds, and processing times. Gel permeation chromatography (GPC) was applied to analyze the molecular weight and molecular weight distributions (MWDs) of melt processed PPC samples. The degradation process at various processing conditions was described by the population balance equations (PBEs) with random chain scission and chain end scission. By comparing the prediction of PBE model with the experimental evolution of molecular weight, it is proposed that random chain scission and chain end scission occur simultaneously. At temperature higher than 160 °C, random chain scission dominates with the activation energy about 120 kJ/mol. Second, a method combining the PBE model and rheology was suggested to determine the kinetics of degradation directly from the torque of mixer during melt processing without further measurements on molecular weight. Such method was applied to melt mixing of PPC in a batch mixer, from which a higher kinetic parameter of thermal degradation and similar activation energy were successfully determined as compared to those obtained from extrusion experiments.
Co-reporter:Yafang Xu, Wei Yu and Chixing Zhou
RSC Advances 2014 vol. 4(Issue 98) pp:55435-55444
Publication Date(Web):03 Oct 2014
DOI:10.1039/C4RA08985E
The miscibility in blends of polylactic acid (PLA) and poly(ethylene glycol) (PEG) was investigated by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and rheology using a specifically designed thermal procedure. The direct observation of a phase separated morphology using phase contrast optical microscopy, is difficult because of the quite close refractive indices of the two components. Liquid–liquid phase separation (LLPS) was confirmed by the appearance of two glass transitions after the samples have been annealed at low temperatures (95–125 °C). Moreover, the accelerated crystallization rate after the sample annealing at high temperatures (140–160 °C) also indicated the existence of LLPS, according to a fluctuation-assisted nucleation mechanism. An upper critical solution temperature (UCST) type of phase diagram was derived using the combination of DSC and rheological methods. Furthermore, the effects of LLPS on the kinetics of the subsequent crystallization of PLA in blends with 15 wt% PEG and 30 wt% PEG were investigated. Considering the complex effects of LLPS on the crystallization, we proposed a new model that integrates the Lauritzen–Hoffman theory and the self-concentration theory to account for the influence of miscibility on the crystallization in blends with different thermal histories. It is suggested that the effect of acceleration or deceleration of LLPS on the subsequent crystallization in a highly asymmetric system, depends on the interplay between the fluctuation-assisted nucleation and the variation of mobility during LLPS.
Co-reporter:Jinxiu You, Wei Yu, and Chixing Zhou
Industrial & Engineering Chemistry Research 2014 Volume 53(Issue 3) pp:1097-1107
Publication Date(Web):December 29, 2013
DOI:10.1021/ie402358h
The crystallization of poly(lactic acid) (PLA) is usually slow and related to the content of d-lactide. A new approach is suggested in this work to accelerate the crystallization of PLA via the synergistic effect of nanofibril nucleating agent (dibenzylidene sorbitol, DBS), plasticizer (poly(ethylene glycol), PEG) and long-chain branching in PLA. It is found that premade DBS/PEG gel can act as an active nucleating agent of PLA, which makes the crystallization peak appear during cooling. The preparation of DBS/PEG gel before mixing with PLA is important because self-assembly of DBS directly in PLA melt is difficult even in the presence of PEG. The mixing temperature is also found to be critical, which determines the amount of residual nanofibrils after melt mixing. Tuning the chain structure into long-chain branching via multifunctional monomer pentaerythritol triacrylate (PETA) will further speed up the crystallization of PLA because of the additional interaction between DBS nanofibrils and the grafted monomers. It is proven that the acceleration of crystallization is not ascribed to the change of crystal form but is due to the dominating increase in the nucleation density as well as the faster growth rate of spherulites in the presence of the plasticizer. Therefore, the problems of low melt strength and slow crystallization of PLA can be solved simultaneously via the present approach.
Co-reporter:Sijun Liu, Wei Yu, Chixing Zhou
Polymer 2014 Volume 55(Issue 8) pp:2113-2124
Publication Date(Web):10 April 2014
DOI:10.1016/j.polymer.2014.02.068
We have systematically studied phase separation behavior in ultra-high molecular weight polyethylene/liquid paraffin/dibenzylidene sorbitol (UHMWPE/LP/DBS) ternary blends. The aim of this paper is to investigate the combined effect of DBS and flow field on the structure and water permeability of UHMWPE microporous membrane. The experimental results show that DBS molecules self-assemble into fibrils firstly during cooling and the blends exhibit a gel-like state before liquid–liquid phase transition. The relaxation time of DBS fibrils is quite long, which shows a great sensitivity to flow field as compared to UHMWPE chain. UHMWPE microporous membrane was prepared via thermally induced phase separation method. DBS fibrils, as in situ formed nucleating agent, decrease the pore size and water permeability and enhance mechanical properties of membrane remarkably. Shear flow can result in alignment of DBS fibrils, which facilitates the nucleation of UHMWPE and induces the lamellae aligned perpendicular to flow direction. This feature was used to design thermal and mechanical histories and obtained oriented UHMWPE microporous membrane. In comparison to the isotropic UHMWPE microporous membrane, the oriented UHMWPE microporous membrane provides low tortuous paths across the membrane and produces high water permeability.
Co-reporter:Peng He, Wei Shen, Wei Yu, and Chixing Zhou
Macromolecules 2014 Volume 47(Issue 2) pp:807-820
Publication Date(Web):January 6, 2014
DOI:10.1021/ma402330a
Chain shuttling polymerization enables an efficient production of ethylene–octene block copolymers (OBCs) that combine different mechanical properties in a polymer chain. However, this method results in molecular weight polydispersity and multiblock chain structure. The melt-phase behavior and mesophase transition of the polydisperse OBCs with low octene content but different molecular weight and block composition were investigated by rheology, differential scanning calorimetry (DSC), atomic force microscopic (AFM), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS). Three rheological methods, namely the deviation of the scaling dependence of zero shear viscosity on molecular weight, the terminal behavior and the failure of time–temperature superposition (TTS), and two-dimensional rheological correlation spectrum, are used to reveal the mesophase separation with increasing sensitivity. The occurrence of mesophase separation transitions (MST) was observed in such low octene content and low molecular weight OBC systems, with much lower degree of segregation than the theoretical predictions in diblock copolymers. The extent of mesophase separation is further justified by its effect on subsequent crystallization behaviors.
Co-reporter:Sijun Liu, Wei Yu and Chixing Zhou
Soft Matter 2013 vol. 9(Issue 3) pp:864-874
Publication Date(Web):15 Nov 2012
DOI:10.1039/C2SM27030G
The gel behavior of dibenzylidene sorbitol (DBS) in different solvents was studied by rheology, optical microscopy and transmission electron microscopy. It was found that DBS molecules would self-assemble into fibrils with a helical structure, and the fibrillar diameter decreased with increasing solvent polarity. The gel dissolution temperature was extremely sensitive to the DBS concentration and solvent polarity, which could be explained by the Gibbs–Thomson equation through interfacial stress. The phase diagram, which classified the sol, the gel and the cluster regime in different solvents was determined by rheology and optical microscopy. The critical gel concentration was found to increase as the difference in the polar and hydrogen-bonding components of the solubility parameter (Δδph) between the gelator and solvents decreased. Meanwhile, the difference in the critical gel concentration in different solvents could explain the difference in the relaxation exponent and the gel strength at the gel point well. In the stable gel state, the plateau modulus depended on the gelator concentration according to a power-law scaling, GoN ∝ c2, which was consistent with entanglement theory and independent of the type of solvent. However, the gelator–solvent interaction and its temperature dependence were found to affect the stability of the gel substantially under large amplitude oscillatory shear. The critical strain would increase as Δδph decreased, and showed more evident temperature dependence on solvents with a smaller Δδph.
Co-reporter:Jinxiu You;Lijuan Lou;Chixing Zhou
Journal of Applied Polymer Science 2013 Volume 129( Issue 4) pp:1959-1970
Publication Date(Web):
DOI:10.1002/app.38912
Abstract
Long chain branching (LCB) of polylactic acid (PLA) was successfully prepared by melt radicals reaction with pentaerythritol triacrylate (PETA) and bis (1-methyl-1-phenylethyl) peroxide (DCP). The topological structure of the LCB was investigated by rheology and branch-on-branch (BOB) model was used to estimate the exact chain structures of the products, where comb-like LCB structures were generated due to the complex coupling between different macro-radicals. LCB structure was found to affect the crystallization of PLA products. In the temperature range of 110–130°C, the crystallization rate parameter (k) was improved sharply and the half crystallization time was decreased significantly after the grafting of PETA, which was ascribed to the enhanced hydrogen bonding in PETA-grafted long chain branching PLA. By comparing with the LCB PLA made from chain extension using multifunctional monomer, it shows that the crystallization becomes slower in a highly branched material with extremely long relaxation time if the effect of hydrogen bonding is similar. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013
Co-reporter:Chunyan Liu, Shanshan Lin, Chixing Zhou, Wei Yu
Polymer 2013 Volume 54(Issue 1) pp:310-319
Publication Date(Web):8 January 2013
DOI:10.1016/j.polymer.2012.11.047
This paper investigated the effect of catalyst on transesterification and transesterification mechanism between poly(lactic acid) (PLA) and polycarbonate (PC) under flow field. Three catalysts (zinc borate, titanium pigment and tetrabutyl titanate) were evaluated. It is found that transesterification reaction can take place without any catalyst, while three catalysts can all promote the transesterification reaction between poly(lactic acid) and polycarbonate to a greater extent. 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography and dynamic mechanical analysis revealed that structures of copolymers are not identical in the blends with and without catalyst. For pure blend, most of copolymers have relatively high molecular weight with low PC content, which implies that transesterification reaction most likely happens only once between a PLA chain and a PC chain during mixing process, and only a small amount of multiple reactions happen. However, for the catalyst systems, catalysts induce much more multiple reactions accompanying with the reducing molecular weight in copolymers and increasing PC content. Moreover, it is found that the catalysts not only affect the chain compositions of the product copolymers, but also influence the amount of polymers involved in the reaction. Tetrabutyl titanate is found to be the most effective catalyst in this study where the amount of reacted polycarbonate is more than 4 times of that in pure blend. It is found that PLA segments in copolymer are easily aligned on the interface due to its relatively high Deborah number, which increases the probability of its contact with more PC chains. Although the flow effect on the alignment of chain segment is similar in blends with and without catalysts, the acceleration of reaction due to catalyst makes it possible for multiple reactions. The match of the reaction time and contact time of chain segment of PC and PLA at interfaces is then of key importance in the interfacial transesterification reaction. The effect of flow field on the interfacial reaction is then not only from the interfacial update, but also from the change of chain conformation near the interface.
Co-reporter:Hongliang Chen, Chongwen Huang, Wei Yu, Chixing Zhou
Polymer 2013 Volume 54(Issue 6) pp:1603-1611
Publication Date(Web):8 March 2013
DOI:10.1016/j.polymer.2013.01.036
Thermally reduced graphite oxide (TrGO) was prepared by thermal exfoliation and reduction of highly oxidized graphite. The pCBT/TrGO nanocomposites were prepared by in situ ring-opening polymerization (ROP) of cyclic butylene terephthalate (CBT). The polymerization kinetics of pCBT/TrGO was monitored by dynamic time sweep in a parallel-plate rheometer. It was found that the increasing TrGO content depressed the rate and degree of CBT polymerization, which is ascribed to the reaction between the growing pCBT chains terminated with carboxyl groups and TrGO surface groups such as hydroxyl and epoxy groups at the initial polymerization stage. The grafted pCBT chains were confirmed by X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) measurements, and the grafting content was up to 53 wt%. Small amplitude oscillation shear (SAOS) was applied to investigate the rheological properties of pCBT/TrGO and the critical loading to form percolation network was determined as 0.47 vol%, which confirmed the good dispersion of TrGO in matrix. The grafting reaction was also justified from nonlinear rheology and the fractal dimension analysis.
Co-reporter:Sijun Liu, Wei Yu, and Chixing Zhou
Macromolecules 2013 Volume 46(Issue 15) pp:6309-6318
Publication Date(Web):July 24, 2013
DOI:10.1021/ma400915g
Dibenzylidene sorbitol (DBS) was chosen as an in situ forming nucleating agent to study ultrahigh molecular weight polyethylene (UHMWPE)/liquid paraffin (LP) physical gel and complex phase separation. The experimental results indicated that DBS self-assembled into fibrils first and the solution became a physical gel before liquid–liquid phase separation (LLPS) and crystallization during thermally induced phase separation (TIPS) of UHMWPE/LP/DBS solution. The temperature of DBS self-assembly and viscoelasticity of UHMWPE/LP/DBS gel show a strong dependence on DBS concentration, temperature, and time. By controlling the relative quenching depth and annealing time, the grow rate of the characteristic length showed a crossover from LLPS to crystallization, which was further justified by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). With decreasing temperature further, crystallization occurred with the aid of DBS fibrils. Those interactions were affected mutually, showed complex phase separation behavior. We proposed a new mechanism of “self-assembly assisted liquid–liquid phase separation”, which explained excellently the relationship of DBS self-assembly and LLPS. On the basis of rheometer and optical microscopy (OM and POM), we obtained the phase diagram of UHMWPE/LP/DBS ternary blends. Meanwhile, DSC indicated the overwhelming changes of crystallization kinetics after the varying LLPS in the double quenching procedure, which was consistent with the “fluctuation assisted nucleation” mechanism.
Co-reporter:Qi He, Wei Yu, Youjun Wu and Chixing Zhou
Soft Matter 2012 vol. 8(Issue 10) pp:2992-3001
Publication Date(Web):02 Feb 2012
DOI:10.1039/C2SM06963F
The rheology and morphology of 8CB (4-octyl-4-biphenylcarbonitrile) and PDMS (polydimethylsiloxane) blends with different concentrations of 8CB were investigated. The blends showed gel-like behaviour when 8CB is in the smectic state during mixing, whose morphologies resemble the foam-like structure of a highly concentrated emulsion with 8CB as the continuous phase. Once formed, such a structure is stable when 8CB is in a smectic and nematic state but unstable when 8CB is in an isotropic state. The stability of the foam-like structure is ascribed to the elasticity of 8CB films, which is justified by the concentration dependence of the interfacial contribution to the plateau modulus, i.e., the yield stress of 8CB instead of the Laplace pressure controls the deformation of the PDMS droplet. A new mechanism of phase inversion is suggested based on the direct observation of the breakup mode of an 8CB droplet. The formation of the foam-like structure was the result of the erosion breakup of the 8CB droplets and the subsequent local phase inversion, where the erosion breakup is ascribed to the shear banding inside the droplet due to the yield stress of 8CB in the smectic state.
Co-reporter:Hongliang Chen;Chixing Zhou
Polymer Engineering & Science 2012 Volume 52( Issue 1) pp:91-101
Publication Date(Web):
DOI:10.1002/pen.22050
Abstract
Cyclic butylene terephthalate oligomers (CBT) with ultra-low melt viscosity can be polymerized into poly (butylene terephthalate) (pCBT) via entropically-driven ring-opening polymerization (ED-ROP) in a short time (ranging from several seconds to 10 min) with no chemical emission and no heat generation during the polymerization process. Due to no heat generation, dynamic rheological measurements were used to monitor the polymerization of CBT from 220 to 250°C. The polymerization was accompanied by a steep increase of the melt viscosity and modulus in isothermal rheological tests, and much faster at higher temperature. With rheological results, reptation theory and Double reptation model were adopted to determine the variation of the molecular weight and concentration with time for pCBT. According to the ED-ROP mechanism of CBT, kinetics equations were also established to simulate the polymerization process. Furthermore, using the results of variation of molecular weight with time for pCBT and kinetics equations, the polymerization rate constants for initiation and propagation steps were evaluated, and the activation energy was also obtained. It was proved that rheological method is a convenient and reliable way to investigate the kinetics of ED-ROP of CBT. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers
Co-reporter:Wei Yu, Chixing Zhou
Polymer 2012 Volume 53(Issue 3) pp:881-890
Publication Date(Web):2 February 2012
DOI:10.1016/j.polymer.2011.12.025
How the viscoelastic asymmetry affects the concentration fluctuation and its manifestation in rheology of miscible polymer blends are examined in this work. The linear viscoelastic stress of polymer blends is divided into the components stress and the stress due to concentration fluctuation. The dynamic coupling between the fluctuation in concentration and the stress is represented by the two-fluid model, which is linearized and solved under small amplitude oscillatory shear to give the concentration fluctuation induced stress. A strong influence of components’ viscoelasticity on the concentration fluctuation is clearly demonstrated in the viscoelastic asymmetric system through the enhancement in the elastic modulus at low frequency. The decisive parameter is the viscoelastic length which depends mainly on the dynamic asymmetric parameter and the zero shear viscosity of blends. It is also found that the dynamic coupling effect gradually fades as it gets close to the spinodal point, where pure concentration fluctuation dominates. The dynamic moduli due to the concentration fluctuation at spinodal point exhibits the same power law dependence on oscillatory frequency, which is a characteristic of critical gel and could be used as an alternative criteria to determine the spinodal temperature.
Co-reporter:Jianping Gao, Chongwen Huang, Nan Wang, Wei Yu, Chixing Zhou
Polymer 2012 Volume 53(Issue 8) pp:1772-1782
Publication Date(Web):3 April 2012
DOI:10.1016/j.polymer.2012.02.027
The effects of silica nanoparticles on the phase separation of poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) blends are studied by the rheological method. The binodal temperatures of near-critical compositions were obtained by the gel-like behavior during spinodal decomposition, which is a character of polymer blends with co-continuous morphology. The shifted Cole–Cole plot method was introduced to determine the binodal temperatures of off-critical compositions based on the appearance of shoulder-like transition in the terminal regime of blends with droplet morphology. Such method is found also applicable in nanoparticle filled polymer blends. Moreover, a new method to determine the spinodal temperature from Fredrickson-Larson mean field theory was suggested, where the concentration fluctuation's contribution to the storage modulus is used instead of the whole dynamic moduli. This method was also successfully extended to nanoparticle filled polymer blend. The influences of the concentration and the average diameter of silica particles on the phase separation temperature were studied. It was found that the small amount of the silica nanoparticles in PMMA/SAN blends will significantly change the phase diagram, which is related to the selective location of silica in PMMA. The comparisons with thermodynamic theory of particle-filled polymer blends are also discussed.
Co-reporter:Chongwen Huang, Jianping Gao, Wei Yu, and Chixing Zhou
Macromolecules 2012 Volume 45(Issue 20) pp:8420-8429
Publication Date(Web):October 2, 2012
DOI:10.1021/ma301186b
Effects of selective location of silica nanoparticles on the phase separation of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) blends were investigated via combination of rheological method and optical microscopy. Through grafting polystyrene chain to the surface of silica nanoparticles, the silica nanoparticles were controlled to selectively locate at interfaces or in the PMMA-rich domains. Power-law analysis of the moduli and shifted Cole–Cole plots were applied to determine rheological transition temperature (apparent binodal temperature) of blend with near-critical and off-critical compositions for both neat blends and particle-filled blends. The particle location had significant influence on the rheological transition temperature but little impact on optically determined binodal temperature. This discrepancy was discussed through morphology observation via transmission electron microscopy (TEM) for blends under different phase separation conditions. It was found that nanoparticles retard coarsening of morphology during phase separation. The most striking slowdown was found in off-critical blends with nanoparticles located on the interface. On the other hand, nanoparticles preferentially locating in the minor phase could act as nucleation sites but decreased the total number of nuclei. The difference in the rheological transition temperatures is ascribed to the effect of nanoparticles on the components’ viscoelasticity and the morphology during phase separation.
Co-reporter:Ying Guo, Wei Yu, Yuanze Xu and Chixing Zhou
Soft Matter 2011 vol. 7(Issue 6) pp:2433-2443
Publication Date(Web):04 Feb 2011
DOI:10.1039/C0SM00970A
An optical visualization apparatus has been designed to measure the particle velocity of concentrated particle suspensions in a stress controlled rheometer equipped with a quartz parallel-plate. Suspensions with high and low matrix viscosity were used to investigate the nonlinear rheology and the local flow mechanism, as well as the correlations between particle velocity profiles and transition behaviors. Herein, it is shown that the sedimentation has great effects on the flow behavior of suspensions and on the particle velocity profiles. Local flow behaviors e.g. shear banding, drift in the center position of oscillatory shear and dynamic wall slip, have been studied comprehensively. By Fourier-transform rheology (FTR) analysis, the shear banding phenomenon is proved to be strongly linked with the nonlinear rheology of suspensions.
Co-reporter:Wei Yu and Chixing Zhou
Soft Matter 2011 vol. 7(Issue 13) pp:6337-6346
Publication Date(Web):06 Jun 2011
DOI:10.1039/C1SM05214D
A theoretical model for droplet dynamics and rheology of blends with viscoelastic interface and interfacial slip is suggested in this work. The interfacial constitutive equation, so called as the Boussinesq–Scriven equation, is integrated in the pertubation analysis on the flow field inside and outside the droplet, while the interfacial slip is introduced via Navier slip boundary conditions. The results of small deformation analysis is extended to larger deformation using the ellipsoidal shape tensor. The effect of Boussinesq number, the interfacial viscosity ratio and the interfacial thickness on the characteristic relaxation time of droplet, the steady deformation of droplet and the shear viscosity of blends are discussed in detail. The model is validated by certain experimental data using interfacial viscosities that are reasonably measured in experiments.
Co-reporter:Qi He;Youjun Wu;Chixing Zhou
Rheologica Acta 2011 Volume 50( Issue 7-8) pp:601-611
Publication Date(Web):2011 August
DOI:10.1007/s00397-010-0496-z
The morphology evolution of liquid crystal droplets immersed in an isotropic fluid in flow field is found to be different from flexible polymer droplets. In this paper, we investigated the retraction of a liquid crystal droplet after transient flow. It is found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. The factors that influence the rotational angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the shear rate, the flow type, and the properties of liquid crystal molecules. The rotation of liquid crystal droplet during shape recovery is ascribed to both the bulk elasticity of liquid crystal droplets and the anisotropic properties of the interface between liquid crystal and isotropic fluid.
Co-reporter:Wei Yu, Runming Li, Chixing Zhou
Polymer 2011 Volume 52(Issue 12) pp:2693-2700
Publication Date(Web):26 May 2011
DOI:10.1016/j.polymer.2011.04.024
Rheological methods have been frequently used to study the phase separation behavior of partially miscible polymer blends. Usually the binodal temperature can be determined from the failure of time–temperature superposition (TTS) principle in isothermal experiments, or the deviation of the storage modulus from the apparent extrapolation of modulus in miscible regime in non-isothermal experiments. However, these methods are shown in this work to be not widely applicable even in blends with weak dynamic asymmetry due to the thermo-rheological complexity. A rheological model which is an integration of the double reptation model and the self-concentration model is found to describe the linear viscoelasticity of miscible blends quite satisfactorily, from which it is possible to follow the contribution from the miscible blends even in the two phase regime. Then, the binodal temperature is readily defined as the deviation of experimental data from such model prediction for miscible blends. Such method is successfully applied in a model polymer blend (poly(methyl methacrylate)/poly(styrene-co-maleic anhydride), PMMA/SMA) with weak dynamic asymmetry.
Co-reporter:Sijun Liu, Chixing Zhou, Wei Yu
Journal of Membrane Science 2011 379(1–2) pp: 268-278
Publication Date(Web):
DOI:10.1016/j.memsci.2011.05.073
Co-reporter:Peng Wang;Jianye Liu;Chixing Zhou
Polymer Bulletin 2011 Volume 66( Issue 5) pp:683-701
Publication Date(Web):2011 March
DOI:10.1007/s00289-010-0382-y
The microstructure of wood plastic composite (WPC) with respect to wood particle content and maleic anhydride-grafted polypropylene (MAHPP) compatibilizer is studied by both linear and nonlinear rheological methods in this article. The complete long characteristic relaxation behavior in linear region, which is closely related to the structure of wood particle aggregates and MAHPP compatibilizing effect at the interface, is limited by observing time. Fortunately, the Fourier transform rheology (FTR) by the stress control mode is found to be an effective method for further investigating the structure with long relaxation time in WPC system. The plateau value of I31 at high stress and the range of ϕ31 are proved to be corresponding to the content of wood particle agglomerates in the WPC melts and the type of interfacial hydrodynamic interaction. The interesting outcomes suggest that MAHPP do has the effect on changing the properties of the heterogeneous interface and confirm the difference of the structure with long relaxation time in WPC can be easily captured by the high sensitive FTR indeed.
Co-reporter:Ruogu Liao, Wei Yu, Chixing Zhou
Polymer 2010 Volume 51(Issue 26) pp:6334-6345
Publication Date(Web):10 December 2010
DOI:10.1016/j.polymer.2010.11.001
The influence of rheological properties and crystallization on foam structures, such as cell diameter, cell density and cell size distribution, of semi-crystalline polymer was investigated. The rheological properties of polypropylene (PP) were controlled by long chain branching (LCB) modification with free radical reaction and its crystallinity. The foaming behavior could be well correlated with the crystal structure and the rheological properties of polymers. The results showed that the long chain branching modification changed the crystallization speed, the diameter and the number of crystal and the rheological behavior as well. The interplay between the crystallization and the rheology of polymers with different chain structures can cause different nucleation mechanism in foaming. Both the cell size of linear PP and LCB PP decrease with crystallization time, and the cell density increases with crystallization time. The crystals in PPs acted as heterogeneous nucleation cites for bubbles, but the cell density of LCB PP is much higher than that of linear PP because of it higher spherulites density. The higher viscosity of branched PP further made its cell diameter smaller than that of linear one. Therefore, the foam structure can be well controlled by tuning the chain structure and crystal structures.
Co-reporter:Ji Zhou;Jian Li;Xin Li;Chixing Zhou
Polymer Engineering & Science 2010 Volume 50( Issue 10) pp:1935-1944
Publication Date(Web):
DOI:10.1002/pen.21720
Abstract
The spinning process of noncircular fiber was investigated in this work. A combination of numerical simulations and experiments made it possible to find out the optimum processing condition for shaped fiber. First, an inverse method which combined two-dimensional/three-dimensional simulation with experiments was utilized to find out the temperature and draw ratio dependent surface tension in melt spinning of shaped fiber. The surface tension as a function of temperature and draw ratio was found to be decisive to the change of cross-section. Second, an orthogonal numerical experiment and statistics analysis were carried out to determine the optimum spinning condition for the desired noncircular fiber. The optimum spinning condition was well validated by experiments. It was then proved that the integrated method in this work, including the numerical method, the inverse method to determining the surface tension, and the orthogonal analysis, can be a good solution for the design and processing of profiled fibers. POLYM. ENG. SCI., 50:1935–1944, 2010. © 2010 Society of Plastics Engineers
Co-reporter:Ruogu Liao, Wei Yu, Chixing Zhou
Polymer 2010 Volume 51(Issue 2) pp:568-580
Publication Date(Web):21 January 2010
DOI:10.1016/j.polymer.2009.11.063
The influence of rheological properties, especially melt strength, on foam structures, such as cell size, cell density and cell size distribution, of amorphous polymer was investigated. The rheology of polystyrene (PS) was controlled by molecular modification with free radical reaction, and PS with long chain branching (LCB) level ranging from 0.15 to 1.6 branching point per 104 carbon atom was gotten. The shear and elongational rheology were found to be dependent on the LCB structure, and the strain hardening behavior of modified samples in transient elongational viscosity confirmed the existence of long branched chain. The effects of chain structure and foaming conditions such as temperature and pressure were studied by the analysis on the foam structures obtained by supercritical CO2. The experimental results revealed that increasing LCB level would decrease cell size, make cell size distribution narrower and slightly increase cell density. The effects of chain topology on the foam structures were also investigated by numerical simulation, where Pom–Pom model was used to describe the effect of backbone length and arm length. The dependence of cell size on the arm length was consistently observed in experiments and simulation. It suggested that the arm length had greater influence on the cell radius than the backbone length. Therefore, the relationship among foam structures, rheological properties and molecular structures can be established from both experiments and simulation, which can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.
Co-reporter:Jianye Liu, Lijuan Lou, Wei Yu, Ruogu Liao, Runming Li, Chixing Zhou
Polymer 2010 Volume 51(Issue 22) pp:5186-5197
Publication Date(Web):15 October 2010
DOI:10.1016/j.polymer.2010.09.002
Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of the end hydroxyl groups of PLA with pyromellitic dianhydride (PMDA) and triglycidyl isocyanurate (TGIC) together. The topological structures of the LCB generated from functional group reactions as well as free radical reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, non-linear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict exact compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the tree-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow, which improves the foaming ability substantially.
Co-reporter:Wei Yu, Wei Zhou, Chixing Zhou
Polymer 2010 Volume 51(Issue 9) pp:2091-2098
Publication Date(Web):20 April 2010
DOI:10.1016/j.polymer.2010.03.005
The co-continuous morphology of polymer blends has received much attention not only because of its potential promotion of mechanical or electrical properties of polymer blends, but also due to its importance in phase separation by spinodal decomposition. Compared to the recent advances in the characterization of co-continuous structure, the rheology of co-continuous blends has not been understood clearly. In this work, a rheological model is suggested to correlate the linear viscoelasticity and the structural information of co-continuous blends. The dynamic modulus of co-continuous blends is composed of the contribution from components and the interface. The interfacial contribution, which is most important in the rheology of blends, is calculated from a simplified co-continuous structure. This model has been compared satisfactorily with available experimental results, which proves a reasonable connection between the co-continuous structure and linear viscoelasticity of blends.
Co-reporter:Lijun Su, Lei Li, Hong Li, Junkun Tang, Yongming Zhang, Wei Yu, Chixing Zhou
Journal of Power Sources 2009 Volume 194(Issue 1) pp:220-225
Publication Date(Web):20 October 2009
DOI:10.1016/j.jpowsour.2009.04.070
Polysiloxane modified perfluorosulfonic acid (PFSA) composite membranes are prepared by using (3-mercaptopropyl) methyldimethoxysilane (MPMDMS) as a precursor of silicon alkoxide in supercritical carbon dioxide (Sc-CO2) system. In the Sc-CO2 system with the presence of water, Sc-CO2 is not only used as a solvent and swelling agent, but also functioned as an acid catalyst for the condensation polymerization of MPMDMS. Characteristics of the modified composite membranes are investigated by using attenuated total reflection-infrared spectra, scanning electron microscopy and transmission electron microscopy. The modified membrane with 13.9 wt.% poly(MPMDMS) is the best one among all the modified membranes, whose methanol permeability is extremely lower and selectivity (ratio of proton conductivity to methanol permeability) is about 5.49 times higher than that of pristine membrane and 5.88 times than that of Nafion® 117, respectively. This modified PFSA membrane still can maintain its higher selectivity value than that of Nafion® 117 in the temperature range of 25–65 °C. Therefore, the modified membranes prepared in Sc-CO2 system may be the suitable candidate electrolytes for direct methanol fuel cell applications.
Co-reporter:Lijun Su, Lei Li, Hong Li, Yongming Zhang, Wei Yu, Chixing Zhou
Journal of Membrane Science 2009 Volume 335(1–2) pp:118-125
Publication Date(Web):15 June 2009
DOI:10.1016/j.memsci.2009.03.006
Perfluorosulfonic acid (PFSA) membranes were treated by supercritical carbon dioxide (Sc-CO2) method for direct methanol fuel cell application. After the treatment in Sc-CO2 at a desired temperature, the microstructure of the PFSA membranes was changed greatly. Small-angle X-ray scattering measurement indicated that a long-range order developed and the size of the ion cluster in the fully hydrated membranes became much smaller. Wide-angle X-ray diffraction measurement showed that the relative crystallinity of the PFSA membranes increased due to Sc-CO2-induced crystallization. The macro-physical performances of the membranes were also investigated: swelling ratio of the membrane became much smaller; solubility of the membrane in ethanol/water solvent decreased dramatically; density of the wet membrane increased evidently. Differential scanning calorimetry measurement indicated that the nonfreezable water per volume of the fully hydrated membranes increased notably. As a result, the methanol permeability of the PFSA membranes was significantly reduced without the sacrifice of proton conductivity, and the mechanical properties increased evidently. All the results indicated that this novel treatment in Sc-CO2 atmosphere is a promising method to improve the properties of membrane for direct methanol fuel cell application.
Co-reporter:Lijun Su, Supeng Pei, Lei Li, Hong Li, Yongming Zhang, Wei Yu, Chixing Zhou
International Journal of Hydrogen Energy 2009 Volume 34(Issue 16) pp:6892-6901
Publication Date(Web):August 2009
DOI:10.1016/j.ijhydene.2009.05.145
In this work, polysiloxane-modified perfluorosulfonic acid (PFSA) membranes were prepared by a directed sol–gel synthesis method with (3-mercaptopropyl) methyldimethoxysilane (MPMDMS) as the precursor of silicon alkoxide in the supercritical carbon dioxide (Sc-CO2) system. Contents of polysiloxane in the modified PFSA membranes were varied according to the added amount of precursor MPMDMS. The chemical and physical properties of these modified PFSA membranes were characterized by using attenuated total reflection-infrared spectra, X-ray diffraction, thermogravimetric analysis, universal testing machine, scanning electron microscopy and transmission electron microscope. The measurement results indicated that the polysiloxane particles were not restricted in the ion clusters and well dispersed in the PFSA membrane with ordered size of about 80–100 nm. In the meanwhile, the polysiloxanes have been incorporated into the hydrophobic fluorocarbon backbone regions and interacted with C–F backbones of PFSA polymers. Dimensional stability of the modified PFSA membrane was improved after the impregnation by using Sc-CO2. The modified membranes almost can remain the same high tensile strength as the pristine membrane. Performance of these modified membranes was evaluated in terms of proton conductivity and methanol permeability. The highest selectivity value (ratio of proton conductivity to methanol permeability) of the modified membrane was about 75.6% higher than that of pristine PFSA membrane because of its higher proton conductivity and lower methanol permeability. All the results indicated that this novel synthesis method in Sc-CO2 system is a promising method to improve the properties of PFSA membrane for direct methanol fuel cell application.
Co-reporter:Ji Zhou, Wei Yu, Chixing Zhou
Polymer 2009 50(18) pp: 4397-4405
Publication Date(Web):
DOI:10.1016/j.polymer.2009.06.077
Co-reporter:Jianye Liu, Wei Yu, Wei Zhou, Chixing Zhou
Polymer 2009 50(2) pp: 547-552
Publication Date(Web):
DOI:10.1016/j.polymer.2008.11.030
Co-reporter:Wei Yu;Chixing Zhou
Journal of Polymer Science Part B: Polymer Physics 2008 Volume 46( Issue 14) pp:1505-1514
Publication Date(Web):
DOI:10.1002/polb.21486
Abstract
The effects of interfacial viscosity on the droplet dynamics in simple shear flow and planar hyperbolic flow are investigated by numerical simulation with diffuse interface model. The change of interfacial viscosity results in an apparent slip of interfacial velocity. Interfacial viscosity has been found to have different influence on droplet deformation and coalescence. Smaller interfacial viscosity can stabilize droplet shape in flow field, while larger interfacial viscosity will increase droplet deformation, or even make droplet breakup faster. Different behavior is found in droplet coalescence, where smaller interfacial viscosity speeds up film drainage and droplet coalescence, but larger interfacial viscosity postpones the film drainage process. This is due to the change of film shape from flat-like for smaller interfacial viscosity to dimple-like for larger interfacial viscosity. The film drainage time still scales as Ca0 at smaller capillary number (Ca), and Ca1.5 at higher capillary number when the interfacial viscosity changes. The interfacial viscosity only affects the transition between these limiting scaling relationships. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1505–1514, 2008
Co-reporter:Ruogu Liao;Chixing Zhou;Fengyuan Yu ;Jinghua Tian
Journal of Polymer Science Part B: Polymer Physics 2008 Volume 46( Issue 5) pp:441-451
Publication Date(Web):
DOI:10.1002/polb.21372
Abstract
The crystallization behavior of long-chain branched (LCB) polypropylene (PP) in the supercritical carbon dioxide (scCO2) atmosphere was investigated to show the influences of LCB and CO2 on the formation of γ-crystal. The crystallization experiments were performed in CO2 atmosphere with the pressure from 1.3 to 10.4 MPa and temperature between 90 and 130 °C. The effects of LCB level, CO2 pressure, and crystallization temperature on the content of γ-crystal were investigated. The results showed that the influence of LCB on the formation of γ-crystal was obvious when PP was crystallized in CO2. The content of γ-crystal increased with LCB level and reached a maximum of 88.2%. It could be explained that, as LCB increased the chainfolding energy of PP molecular chain and hindered it from folding back into crystal lamella, which made the formation of γ-crystal easier. However, CO2 was the key factor in the formation of γ-crystal, and the influence of CO2 on γ-crystal was much significant than that of LCB. It was believed that the increase of free volume after dissolving of CO2 in PP was helpful in the formation of γ-crystal. It was found that the content of γ-crystal increased almost linearly with CO2 pressure (CO2 content), and the contribution of CO2 to γ-crystal increased with pressure, while that of LCB increased with temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 441–451, 2008
Co-reporter:Quan Chen;Chixing Zhou
Journal of Polymer Science Part B: Polymer Physics 2008 Volume 46( Issue 5) pp:431-440
Publication Date(Web):
DOI:10.1002/polb.21312
Abstract
The small amplitude oscillations can be superimposed parallelly on steady shear flows. The resulting moduli provide information about time- and shear-dependent microstructure. For this purpose, model blends composed of polydimethylsiloxane and polyisobutylene with the viscosity ratio of 7.9 and 0.25 are investigated. The resulting moduli are compared with the results derived from numerical calculation as well as analytical solutions, developed here by introducing the conditions under parallel superposition flow field into MM model. Good agreement is found in the interfacial contribution of the storage moduli for blend with low volume fraction. Moreover, detailed analysis on hydrodynamic interaction between droplets is given to explain the discrepancies. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 431–440, 2008
Co-reporter:Ping Zhou;Chixing Zhou;Feng Liu;Liming Hou;Jun Wang
Journal of Applied Polymer Science 2007 Volume 103(Issue 1) pp:487-492
Publication Date(Web):23 OCT 2006
DOI:10.1002/app.25020
The morphology and electrical properties of linear low density polyethylene (LLDPE)/poly (ethylene-methyl arylate) (EMA) blends filled with carbon black (CB) are investigated in this work. Comparing to LLDPE/CB composite, the higher percolation threshold of EMA/CB composite is attributed to the good interaction between EMA and CB. However, carbon black is found to locate preferentially in the LLDPE phase of LLDPE/EMA immiscible blends from the characterization of SEM and electrical properties, which greatly decreases the percolation threshold of the composites. The viscosity of the two polymers is the key factor to determine the distribution of CB instead of interfacial energy in this system. This suggests a method to control the distribution of CB in the immiscible blends by choosing the viscosity ratio of polymer blend. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 487–492, 2007
Co-reporter:Jinghua Tian;Chixing Zhou
Journal of Applied Polymer Science 2007 Volume 104(Issue 6) pp:3592-3600
Publication Date(Web):27 MAR 2007
DOI:10.1002/app.26024
The nonisothermal crystallization kinetics of linear and long chain branched polypropylene (LCB PP) were investigated by differential scanning calorimetry (DSC) at various cooling rates. Several methods such as Avrami, Ozawa, and Jeziorny were applied to describe the crystallization process of linear PP and LCB PPs with different LCB level under nonisothermal conditions. The values of t1/2, Zc, and F(T) show that LCB has the role of heterogeneous nucleating agent and accelerates the crystallization process of PP. Moreover, the Kissinger method was used to evaluate the activation energy of linear PP and LCB PPs. The result shows that the activation energy of LCB PPs are higher than that of linear PP, indicating that the presence of LCB baffles the transfer of macromolecular segments from PP melt to the crystal growth surface. Furthermore, the crystal morphology of linear PP and LCB PPs was observed through polarized optical microscopy (POM), and fine spherulites were observed for LCB PPs. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3592–3600, 2007
Co-reporter:Wei Yu;Kai Yang;Chixing Zhou
Journal of Applied Polymer Science 2007 Volume 105(Issue 2) pp:846-852
Publication Date(Web):4 APR 2007
DOI:10.1002/app.26281
The thermal oxidation of a metallocene-catalyzed poly(ethylene octene) (POE) melt was studied by a dynamic rheological method. Furthermore, to prove the effect of oxygen on the thermal oxidation of POE, cyclic samples, which consisted of virgin POE in the outer circle and POE with antioxidant inside, were made. The results showed that a thermal-oxidation-induced crosslinking reaction occurred in the POE melt at a high temperature. The amount of the antioxidant determined the thermal oxidation of POE when the antioxidant was added, and the diffusion of oxygen controlled the thermal oxidation of POE without the antioxidant. In addition, Fourier transform infrared and gel content characterization confirmed that the crosslinking occurred and that carbonyl groups formed in the reaction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007
Co-reporter:Wei Yu;Chixing Zhou
Journal of Polymer Science Part B: Polymer Physics 2007 Volume 45(Issue 14) pp:1856-1869
Publication Date(Web):5 JUN 2007
DOI:10.1002/polb.21185
The coalescence process of two droplets in simple shear flow was modeled and simulated by the diffuse interface method. The collision between two droplets was investigated. The systems with small Peclet number, which denotes highly diffuse ability of concentration, were found to coalesce faster and easier due to the overlap of interfacial layers. The effect of matrix elasticity on droplet coalescence was studied thoroughly. The matrix elasticity was found to decrease the hydrodynamic interactions between droplets, and delay the coalescence process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1856–1869, 2007
Co-reporter:Yuanze Xu;Chixing Zhou
Journal of Polymer Science Part B: Polymer Physics 2005 Volume 43(Issue 18) pp:2534-2540
Publication Date(Web):29 JUL 2005
DOI:10.1002/polb.20542
The rheology and morphology evolution of nondilute and concentrated immiscible blends were investigated in this paper. A theoretical model was established by a Hamiltonian formalism. The interactions between droplets were integrated in the morphology-dependent drag coefficient. The phenomenological parameters in the model were determined by the comparisons with the dilute emulsion model and the Krieger–Dougherty model. The model showed better predictions in the shear viscosity and first normal stress difference than that of the dilute emulsion model. The effects of volume fraction on droplet deformation were also predicted and compared with the numerical simulations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2534–2540, 2005
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