共查询到20条相似文献,搜索用时 15 毫秒
1.
C. M. Roland 《Journal of Polymer Science.Polymer Physics》1988,26(4):839-856
Difference spectra of blends of cis-1,4-polyisoprene and atactic poly(vinylethylene), obtained from the measured FTIR spectra of the pure components and the blends, indicate that mixing of these polymers is not accompanied by any specific chemical interactions. Miscibility in this system arises solely due to the small combinatorial entropy of mixing. The conformation and configuration of the polymer chains in the blends are, therefore, identical to those in the pure melts. As a consequence it was found that the entanglement density of the blends varied monotonically with composition. This variation, however, was not in accordance with predictions based simply on the mechanical interaction density. The principle rheological effect of miscible blending was a large change in the monomeric friction coefficient, which arises from the strong dependence of free volume on composition. The zero shear viscosity and the terminal relaxation time of the blends reflected this change in local chain mobility. Empirical relations, which have previously been proposed for the properties of miscible polymer mixtures, were found to be without merit in describing the obtained experimental results. 相似文献
2.
Recent experiments designed to probe polymer transport in the bulk and in the vicinity of surfaces have examined the interdiffusion of multilayer sandwiches of isotopically labeled polymers. The measured time dependent concentration profiles normal to the surface are typically fit to Fick's law, with a single fitting parameter, the mutual binary diffusion coefficient (MBDC). The resulting MBDCs are found to vary over a broad range of film thicknesses and time, with the time dependence being viewed as a unique signature of the reptation mechanism of long chain motion, and the thickness dependence being attributed to the slowing down of chain dynamics near surfaces. Since the experiments are conducted at finite concentration, the MBDC, which is a product of the bare mobility and the concentration derivative of the chemical potential, could be dominated by the time and thickness dependence of this second term (which is ignored in Fick's law). To quantify this conjecture we consider the more rigorous Cahn formulation of the diffusion problem in terms of chemical potential gradients. We use square gradient theory to evaluate chemical potentials, and fit the resulting time dependent concentration profiles to the analytical solution of Fick's law. By thus mimicking the experimental analysis we find that the apparent MBDCs vary with time as t(-1/2) at short times, in good agreement with existing experiments. We show that this time dependence reflects the system's desire to minimize concentration gradients, a fact ignored in Fick's law. Since these arguments make no reference to the mechanism of chain motion, we argue that the time dependence of MBDC derived from interdiffusion experiments does not provide unequivocal support for the reptation mechanism of long chain transport. The MBDC values, which also vary with the degree of confinement, are predicted to increase with decreasing thickness for model parameters corresponding to experimental systems. In contrast, since the experimental fits yield an opposite trend, we suggest that the bare mobility of the chains decreases strongly with decreasing thickness. These findings strongly support the idea that the chains are "pinned" irreversibly to the surfaces, in good agreement with other, independent experiments. 相似文献
3.
Steady-state permeation measurements are reported for carbon dioxide (CO2) through quenched, amorphous films of a miscible blend of poly(butylene terephthalate) (PBT) and a random copolyester of bisphenol-A and iso/terephthalate acids (PAr). Permeabilities were determined at 35°C on blends with up to 60 wt % PBT and for CO2 pressures up to 300 psi (2.06 MPa). At a fixed blend composition, the permeability, P̄, decays with driving pressure, p, as described by dual-mode models for gas transport in glassy polymers. From regression fits of the data to dual-mode model predictions for P̄(p), high-and low-pressure limiting permeabilities are determined. These decrease with PBT content in a manner indicating strong, favorable energetic interactions between the PBT and PAr components in the blend. © 1996 John Wiley & Sons, Inc. 相似文献
4.
CMC and CMC-PVA were blended either with type I collagen, BSA or CS to obtain biocompatible membranes for evaluation as potential hepatocyte culture substrates. Pure and modified forms of CMC showed distinct surface, mechanical, and cell attachment properties. While the hydrophilicity decreased, the mechanical stability and the porosity of CMC membranes increased after blending. Serum proteins were adsorbed by all types of membranes. Among eight membranes tested, collagen-modified CMC was found to be a suitable membrane material for hepatocyte culture, in terms of mechanical and cell interaction properties. 相似文献
5.
S. Spall A. A. Goodwin M. D. Zipper G. P. Simon 《Journal of Polymer Science.Polymer Physics》1996,34(14):2419-2431
The dynamic mechanical and dielectric spectra of a miscible polyester and polycarbonate blend are investigated with emphasis on the latter technique. It was found that relaxation spectra for the blends from both techniques are broader than those of the constituent homopolymers. This is ascribed to greater intermolecular coupling and concentration fluctuations within the blends. The composition at which the greatest coupling occurs is dependent on the relaxation technique used and is skewed towards the component which shows the highest degree of intermolecular coupling. A number of parameters, such as relaxation time of the polymer molecules in the blend and relaxation strength, are compared as a function of reduced temperature (experimental temperature scaled by the glass transition temperature). Whereas blend behavior is generally intermediate between that of the homopolymers, it appears as though mobility of compositions with low polyester content have a greater relaxation time and possess a higher activation energy when compared to a simple, weighted average of the corresponding homopolymer values. © 1996 John Wiley & Sons, Inc. 相似文献
6.
A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-1,8-naphthyridine (2, DAN), which form an exceptionally strong quadruply hydrogen-bonding complex, are displayed at 1-10 mol % along the main backbone of PBMA and PS, respectively. (1)H NMR studies show heterocomplexation between UG and DAN exclusively. This high-fidelity, high-affinity supramolecular connection of two different polymer coils at the molecular level produces a polymer blend. Blends containing different weight ratios of the polymers and mole percent of the recognition units were characterized by AFM and DSC experiments with no isolated domains observed and a single glass-transition temperature (T(g)). The T(g) is tunable by varying the weight ratio of the polymers in the blend. In addition, viscosity measurements, size-exclusion chromatography (SEC), and dynamic light-scattering (DLS) studies demonstrate the formation of a supramolecular network structure. 相似文献
7.
This review relates the free volume properties and the morphology to the mass transport coefficients of polymer nanocomposites. Direct, utilising the method of Positron Annihilation Life-time Spectroscopy (PALS), or indirect measurements of the free volume in the nanocomposites are discussed and the influence of spherical or anisometric nanoparticles on its properties is examined. 相似文献
8.
G.C. Kapantaidakis S.P. Kaldis X.S. Dabou G.P. Sakellaropoulos 《Journal of membrane science》1996,110(2):239-247
The permeation rates of He, H2, CO2, N2 and O2, are reported for a series of miscible polysulfone-polyimide (PSF-PI) blend membranes synthesized in our laboratory. For gases which do not interact with the polymer matrix (such as He, H2, N2 and O2), gas permeabilities in the miscible blends vary monotonically between those of the pure polymers and can be described by simple mixture equations. In the case of CO2, which interacts with PI, blend permeabilities decrease somewhat, compared to pure PSF and PI. This, however, is accompanied by a two-fold improvement in the critical pressures of plasticization vs. polyimide. Permselectivities of CO2/N2 and H2/CO2 in the blends deviate from mixing theory predictions, in contrast to selectivities of gas pairs which do not interact with PI. Differential scanning calorimetry measurements of pure and PSF/PI blend membranes show one unique glass transition temperature, supporting the miscible character of the PSF/PI mixture. Optical micrographs of the blend membranes clearly indicate perfect homogenization and no phase separation. Frequency shifts and absorption intensity changes in the FTIR spectra of the blends, as compared with those of the pure polymers, indicate mixing at the molecular level. This compatibility in mixing PSF and PI, results essentially in a new blend polymer material, suitable for the preparation of gas separation membranes. Such membranes combine satisfactory gas permeation properties, reduced cost, advanced resistance to harsh chemical and temperature environments, and improved tolerance to plasticizing gases. 相似文献
9.
The purpose of this work is to describe the application of new electron microscopy techniques to the study of polymer blends with very fine dispersion of phases (miscible blends). Blends of PVC with PMMA, PCL, POM and SAN were prepared by high temperature mixing on a two roll mill, or by solvent casting. Thin sections (or cast films) were investigated in the scanning transmission electron microscope and small phases were identified in most blends. The contrast was enhanced by electronic combination of bright and dark field signals, by an irradiation and staining technique and by differential mass loss. The specimens were further characterized by measurement of mass loss, resulting from electron beam damage. The non linear changes in the mass loss rate with concentration were interpreted as being influenced by partial solubility and molecular interactions. 相似文献
10.
We have investigated, in terms of the Cohen-Turnbull theory, a relationship for polycarbonate (PC) glasses between average stress relaxation times, <to, and average free volume sizes, 〈vf〉, obtained from positron annihilation lifetime spectroscopy. This examination suggests that the minimum free volume required for stress relaxation, v*, decreases with decreasing temperature and that, near the glass transition temperature, only a subset of extremely large free volume elements contributes to the stress relaxation of PC glasses. This suggestion is consistent with the idea that near the glass transition temperature, the viscoelastic response is dominated by large-scale, main-chain motion, whereas at lower temperature it is controlled by local motion. Moreover, comparison with the v* value estimated from gas diffusivity through various PC species at room temperature shows that the required free volume size for stress relaxation in the glass transition region is much larger than that for gas diffusion. Previously we showed that the Doolittle equation fails to correlate viscoelastic relaxation times of polymer glasses with changing temperature; determining the free volume fraction, h, from theoretical analysis of volume recovery data and theory, the Doolittle equation is shown to be valid in PC above 135°C (Tg - 14°C) irrespective of temperature and physical aging times. This result supports the idea suggested in the previous article that, as glassy polymers approach the transition region, viscoelastic properties increasingly tend to be controlled by free volume. © 1996 John Wiley & Sons, Inc. 相似文献
11.
The concept of crystallization dynamics method evaluating the miscibility of binary blend system including crystalline component
was proposed. Three characteristic rates, nucleation, crystal growth rates (N*, G*) and growth rate of conformation (G
c*) were used to evaluate the miscibility of PVDF/at-PMMA and PVDF/iso-PMMA by the simultaneous DSC-FTIR. N*, G* and G
c* depended remarkably on both temperature and blend fraction (ϕPMMA) for PVDF/at-PMMA system, which indicated the miscible system. PVDF/iso-PMMA showed small ϕPMMA dependency of N*, G* and G
c*, was estimated the immiscible system. The ΔT/T
m0 values, corresponding to Gibbs energy required to attend the constant G* and G
c*, evaluated from G* and G
c* showed the good linear relationships with different slope. The experimental results suggested that the concentration fluctuation
existed in PVDF/iso-PMMA system. 相似文献
12.
The complex thermorheological behaviour observed in miscible polymer blends is modelled by combining two existing theoretical approaches: The Thermal Concentration Fluctuation model (A. Zetsche, E.W. Fischer, Acta Polymer. 1994 , 45, 168) and the Effective Concentration model (T.P. Lodge, T.C.B. McLeish, Macromolecules 2000 , 33, 5278), giving rise to a simple model with only one adjustable parameter. This model is then tested in the case of two model blends allowing to show its abilities and limitations to describe how the respective segmental dynamics of lowest or highest Tg component are affected by blending. 相似文献
13.
Hiromu Saito Mamoru Takahashi Takashi Inoue 《Journal of Polymer Science.Polymer Physics》1988,26(8):1761-1768
Orientation relaxation of dissimilar chains in the molten miscible blends, poly(methyl methacrylate)/poly(vinylidene fluoride) and poly(methyl methacrylate)/poly(vinylidene fluoride-co-trifluoroethylene), were investigated by measuring (1) the change of infrared dichroic ratio with time after the uniaxial stretching of film specimens, (2) the shear stress relaxation spectrum, and (3) birefringence relaxation in shear. The dissimilar polymers showed an identical time variation of the normalized Hermans orientation function. The blend showed a relaxation spectrum with a single characteristic relaxation time τc, depending on the blend composition. The birefringence relaxed monotonically, remaining positive. These results suggest that the dissimilar polymers do not relax independently but cooperatively. This behavior may be induced by a constraint due to the specific interactions between the dissimilar polymers, e.g., weak hydrogen bonding. For the cooperative chain relaxation, a third power relationship was found; τc/τe vprop; (M/Me),3 where τe and Me are the relaxation time and molecular weight of entanglement strand, respectively, and M is the number average molecular weight in the blend. 相似文献
14.
15.
Development of phase-separated morphologies has been studied in polymer blend solutions and forced mixtures of highly immiscible polymers by optical microscopy, DSC and light scattering. Unmixing was induced by different courses of solvent evaporation and by thermal agitation, respectively. Blends comprising different kinds of flexible and rigid chain molecules display various phase patterns in the late stage of phase decomposition which can be characterized roughly as being either regularly bicontinuous or irregularly shaped. Effects of polymer-polymer interaction and the blending ratio are discussed. Generally, evolution of morphologies depends on both internal parameters of the system and externally imposed unmixing conditions. Limits of percolating morphologies in the course of phase separation can be established qualitatively. 相似文献
16.
M. D. Zipper G. P. Simon P. Cherry A. J. Hill 《Journal of Polymer Science.Polymer Physics》1994,32(7):1237-1247
Homopolymers and blends of polycarbonate/ThermX have been investigated by differential scanning calorimetry, dynamic mechanical analysis, density measurements, and positron annihilation lifetime spectroscopy. The study focuses on the chain mobility and free volume in the amorphous miscible regions of the blends and how this mobility and free volume are affected by increasing crystallinity. It is proposed that judicious annealing, which results in increasing crystallinity (while avoiding ageing or transesterification), induces a constraint on the amorphous region leading to an increase in Tg and coalescence of free volume sites. © 1994 John Wiley & Sons, Inc. 相似文献
17.
The dynamic viscosity for miscible blends of 1,4-polyisoprene and poly(vinyl ethylene) (PVE) at the extremes of composition (1% tracer in 99% matrix) has been measured as a function of temperature. Time-temperature superposition failure is observed in these tracer blends. This observation indicates that intramolecular contributions are crucial in defining the local environment experienced by a polymer segment, as anticipated by self-concentration models. The temperature dependence of the dynamics of the PVE tracer has been extracted, and compared with the model of Lodge and McLeish. The self-concentration effect was found to be even stronger than predicted. 相似文献
18.
Gerhard Zifferer Thomas Petrik Brigitte Neubauer Oskar Friedrich Olaj 《Macromolecular Symposia》2002,181(1):331-340
Polymers molecules in solution or melt are more or less flexible and continuously change their shape and size. Thus, characteristic properties of the system fluctuate around statistical mean values which are dependent on the concentration of the solution, on the quality of the solvent used, and on the specific structure of the molecules, e.g. linear or star-branched. The most direct approach to these quantities on a molecular level are computer simulations. Due to restrictions of computer power fully atomistic simulations of macromolecules are presently still at the beginning but several arguments justify the use of simplified models. The most efficient way dealing with polymer systems are Monte Carlo simulations based on lattice chains, at least as long as static properties are of interest only. In the present paper a short introduction to the field is given and selected examples are presented in order to demonstrate the usefulness of these methods. 相似文献
19.
Poly(methylmethacrylate) (PMMA)/poly(vinylidene fluoride) (PVdF) blend based electrolyte films containing different lithium salt concentrations are prepared using solvent casting technique. The complexation has been confirmed using XRD and FTIR spectral studies. Ionic conductivity and thermal behaviour of PMMA/PVdF complexes were studied with various salt concentrations, temperature and plasticizer content. The network structure of the polymer complexes are also investigated using SEM. The maximum value of conductivity 4.2×10−3 S/cm is obtained for the PMMA(7.5)-PVdF(17.5)-LiClO4(8)-DMP(67) polymer complex at 303 K. 相似文献
20.
Kruse J Rätzke K Faupel F Sterescu DM Stamatialis DF Wessling M 《The journal of physical chemistry. B》2007,111(50):13914-13918
PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) is a well-known membrane material showing good gas separation properties. The incorporation of nanoparticles can enhance or deteriorate the performance of composite membranes, sometimes depending only on the way of the composite preparation. We have modified the PPO polymer with C60 fullerenes up to a content of 2 wt %. Previous investigations showed a strong dependence of permeability on whether the C60 is simply dispersed in the polymer or chemically bonded to the polymer chains. Free volume effects were suggested as an explanation but not experimentally confirmed. Here, we present free volume studies by positron annihilation lifetime spectroscopy. An additional long positron lifetime shows the increased free volume of composite samples, while the high electron affinity of C60 helps to indicate the homogeneity of the samples. Combining the presented results with permeability measurements refines the understanding of this promising membrane material. 相似文献