摘要:

AbstractThe rheological behavior of semidilute galactomannan solutions in the presence of borax has been studied. The effects of polymer and borate concentrations, ionic strength, pH, and temperature are investigated and discussed. The shape of the viscoelastic functions of galactomannan–borax gels is similar to that for semidilute or concentrated polymer solutions. In 1MNaCl, electrostatic effects are screened and the characteristic moduli of the gels appear to be determined by the number of interchain crosslinks. The elastic plateau modulus varies linearly with the free borate concentration and with the 2.6 power of the galactomannan concentration. Time–temperature type superposition is applicable if a vertical shift is introduced to account for the exothermic nature of the complexation reactions. Moreover, it seems possible to control the gel reversibility and the lifetime of crosslinks by changing the complexation kinetics via the

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281301

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA new technique of thermal imidization after cold drawing of poly (amic acid) films is used to improve the tensile mechanical properties of various aromatic polyimide films. By this method PI (BDA/PPD) with linear, rigid main chains can be obtained as a high‐modulus and high‐strength material with Young's modulus of ca. 60 GPa and tensile strength of ca. 1.2 GPa. The enhancement of mechanical properties is favored by the rapid development of molecular order during imidization. The tensile properties of flexible polyimides such as PI (BDA/DDE) and PI (PMDA /DDE) are not significantly improved, because the orientation of cold‐drawn poly (amic acid) chains is easily relaxed at the temperature of imidiz

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281302

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe influence of nematiclike interactions on local order and elasticity of crosslinked polymer systems is considered. A molecular mean‐field approach is used to describe interactions between system segments. The orientation‐dependent potential is assumed to be proportional to the square of the cosine of the angle between the direction of the orienting field and the chain segment vector. The orientation factor is calculated from a self‐consistent equation, with no restrictions on the strength of intersegmental interactions. Isotropic (slightly oriented) and nematic (highly oriented) phases are observed. The influence of local order on the stress‐strain relation is derived for various values of network structural parameters (contraction coefficient, length of network chains) and the strength of segment‐segment interaction. For some values of these parameters, local order can modify considerably the elastic properties of the netwo

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281303

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA first‐order Markovian terpolymer model is used to describe the arrangement of ethylene (1), inverted propylene (2), and normal propylene (3) units in an ethylene‐propylene chain by means of chain propagation probabilities (Pset) and corresponding reactivity ratios (rset). From nuclear magnetic resonance data on the weight fractions of methylene sequences,w1:mup to and includingw5:m,P11and the methylene, ethylene, and propylene sequence length distributions can be calculated, as well as the mole fraction of ethylene (x1) and the average sequence lengthssn,sw,sz. With the usual fitting procedures, these results can be obtained for quite differentPsets andrsets. This unsatisfactory situation is characterized by the assignment of different values to the degree of inversionI=x2/(x2+x3). Insight into this indeterminacy was obtained by distinguishing the methylene sequences (s:m) ending with 2 and sequences ending with 3 (s:m, 2 ands:m, 3, respectively), the weight fractions of which appear to be independent ofIexcept forw1:m,2, andw1:m,3. In the constant sumw1:m, the ratio of the two is determined byI. The limit values ofI(IminandImax) are fixed within the model used and are reached forw1:m,2= 0 andw1:m,3= 0, respectively. The values ofP12andP13areI‐independent; by any particular choice ofI, the otherPij's and hence aPset is fixed. For everyPset there is aP″ set symmetrical with it, withI′ = 1 −I, which describes an identical chain formed in the opposit

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281304

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractFor a series of homogeneous ethylene‐propylene copolymers with mole fraction ethylene (x1) ranging between 0.40 and 0.85, the inversion (I) independent propagation probabilities (Pijs) and reactivity ratios (rijs) and the methylene, ethylene, and propylene sequence length distributions have been determined from13C nuclear magnetic resonance (NMR) data, using a first‐order Markovian terpolymer [ethylene (1), inverted propylene (2), and normal propylene (3)] model. For each sample, the limiting values ofI(Imin,Imax) are given. Calculations of the common parameters for 19 samples show that the polymerization direction characterized by therset andIis statistically more probable than the opposite direction, which is characterized by ther′ set andI′. Further,I=Iminappears to be close to the most probable value ofI. The resultingrset isr12= 119,r13= 19.7 and, forI=Imin,r21= 0,r31= 0.034,r32= 2.98. In the limited range 0.60

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281305

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractStress‐induced vibrational frequency shifts in the infrared and Raman spectra of polymer chains are interpreted theoretically on the basis of the quasiharmonic lattice dynamical treatment. The calculated frequency shifts are in good agreement with experiment for planar‐zigzag polyethylene and helical polyoxymethylene. The anharmonic terms of the force constants are estimated numerically in order to obtain the parameters of anharmonic potential functions such as the Morse potential. The close relationship between stress‐induced frequency shifts and molecular deformation mechanisms is discussed in terms of the force field and the geometry of the polymer

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281306

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe partitioning of linear polymers between small pores and bulk solution is examined theoretically for the case of long‐chain molecules which are attracted by the walls of cylindrical or slitlike pores. The polymer is considered to be a freely jointed chain ofNmass points connected by rectilinear segments of lengthl. The interaction potential is represented as a square well with width comparable tol. It is assumed thatNis very large and thatlis much smaller than the pore radius, allowing the calculations to be performed by solving a “diffusion equation” with a suitable source term. The results presented cover a wide range of interaction potentials and relative polymer and pore sizes. A critical interaction potential is identified for which entropic and enthalpic effects exactly cancel, resulting in equal concentrations in the pore and bulk solution. As the critical potential is approached, the solution to the diffusion equation rapidly changes character. The critical potential may be interpreted as defining a transition from “free” to weakly adsorbe

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281307

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAs‐spun poly(p‐phenylene terephthalamide) (PPTA) fibers have been annealed by three different methods: (1) the relaxation mode; (2) the constant temperature mode; and (3) the constant tension mode. Wide‐angle x‐ray diffraction (WAXD) was used with all three modes to characterize the changes in three crystal structure parameters: the degree of crystal orientation, the apparent crystal size, and the lattice strain along the c‐axis of the crystal unit cell. It was found that internal stresses in the fibers, which are frozen‐in during either the fiber spinning process or in later annealing, is the key factor causing these structure parameter changes. Our thermal shrinkage stress measurements may reasonably represent the fiber's internal stresses. The thermal shrinkage modulus and the tensile modulus of the PPTA fibers under these annealing modes have been determined. Based on a serial model, quantitative structure–property relationships can be established. The effects of annealing temperature and applied tension on both the microscopic crystal structures and the macroscopic properties are a

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281308

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA comparison between the entirely statistical and the combined kinetic and statistical model for amine‐epoxy networks, both studied previously in the literature is made. The chemistry studied involves the generation of a secondary site which reacts in the network through chainwise addition rather than the stepwise addition mode of the primary reaction. It is shown that the combined model is exact and the naïve statistical models are approximate. The assumptions underlying the two models are delineated. It is shown, that under certain circumstances, the predictions of the two models can differ by several hundred percent. Further, the combined model is extended to predict the postgel properties of the netwo

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281309

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe technique of forced Rayleigh scattering has been used to examine the tracer diffusion of probe photochromic dyes, azobenzene and methyl red, in poly (vinyl acetate)/ toluene solutions, as a function of polymer concentration (0–96% by weight) and temperature (5–45°C). It is demonstrated that the dye diffusivity can be quantitatively scaled to the solvent diffusivity, thus permitting direct application of predictions for two‐component systems. The results have been interpreted using the free volume theories of Fujita and Vrentas and Duda. While both approaches are able to describe the concentration dependence extremely well, the Vrentas‐Duda expression has the virtue of being employed without any freely adjustable parameters. However, neither theory is particularly successful in predicting the observed temperature de

ISSN:0887-6266    

DOI:10.1002/polb.1990.090281310

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY