摘要:

AbstractFourier transform infrared (FTIR) spectroscopy in the diffuse reflectance mode was used to study polyethylene terephthalate (PET) films. The polymer film (12 μm, molecular weight M1= 18,000) was placed on a finely powdered KBr matrix, used as a reference. Infrared spectra exhibited a new band at 1684 cm−1, not usually reported in the literature. This band is assigned to the CO stretching vibration of a terminal acidic function in the presence of internal hydrogen bonds. In the carbonyl region, artifacts created by specular reflection are also discussed. The assignment of the band at 1684 cm−1is confirmed by transmission measurements on the overtone of carbonyl group (3335 cm−1), using polymer films with thicknesses greater than 200 μm and by comparison with polymer of different molecular weight. These acidic functions can be used to monitor the rate of polymerization. It is therefore possible to obtain information on the polymerization rate of PET films, using diffuse reflectance and transmission analysis, directly on the solid. © 1992 John Wiley

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301201

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe number of chain scissions per unit area that occur during the fracture of partially annealed latex films from Mn≃ 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull‐out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights fort≫ τrwheretand τrare annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley&

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301202

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThin films of poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, ultraviolet‐visible (UV‐VIS), infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in‐plane indexn∥= 1.806 and the out‐of‐plane indexn⊥= 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 μm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA‐PDA chains to align along the film plane. The frequency dispersion of the in‐plane refractive indexn∥is consistent with the results calculated by the Lorentz–Lorenz equation from the UV‐visible spectrum exhibiting several absorption bands in the 170–500 nm region. The contribution from the IR absorption in the range 7000–400 cm,−1computed by the Spitzer‐Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in‐plane refractive indexn∥. Tilt‐angle–dependent polarized IR results indicate nearly the same increase for the out‐of‐plane indexn⊥. Application of the Maxwell relation then leads to the out‐of‐plane dielectric constant ε⊥≃ 2.7 at 1.2 × 1013Hz, as compared with the measured value of ca. 3.0 at 106Hz. Assuming this small difference to remain the same for the in‐plane dielectric constants ε∥, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in‐plane dielectric constant ε∥

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301203

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have studied a 4.8 volume percent solution of a narrow distribution polystyrene with molecular weight 3.84 × 106in flows generated by a co‐rotating two‐roll mill. These flows have a stagnation point at the midpoint between the roller axes. Further, they are linear, two‐dimensional, and the magnitudes of the strain‐rates are greater than the vorticity. The overall objective of our studies is to explore the dynamics of concentrated polymer solutions which are in the highly deformed state that is generated in the two‐roll mill. Birefringence data are presented for both steady flow and start‐up of steady flow in the two‐roll mill. The steady and transient data are used to analyze thelinearandnonlinearviscoelastic regions of material behavior. In the nonlinear regime, the birefringence upon start‐up shows an initial overshoot followed by a strongundershootthat is enhanced as the ratio of elongation to rotation is increased (i.e., the flow becomes increasingly extensional in character.) We attribute this undershoot, which does not seem to appear in simple shear flows, or flows close to simple shear flow, to polymer segment stretching following an initial period of segment reorientation. Model studies are currently underway to test this notion. © 1992 Joh

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301204

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe miscibilities of ternary copolymer blends prepared from poly(styrene‐stat‐acrylonitrile), poly(styrene‐stat‐methyl methacrylate), and poly(methyl methacrylate‐stat‐acrylonitrile) were predicted by calculating the interaction parameter, χblend, for various blend combinations, from the corresponding binary segmental interaction parameters estimated from previous work. Binodal and spinodal curves were calculated using the Flory‐Huggins theory and it was observed that the most accurate estimate of the boundary between miscible and immiscible blends was given by the spinodal. It has also been demonstrated that in some of the ternary blends with fixed copolymer compositions the miscibility of the blend can be altered by changing the ratio of the three components in the mixture. Conditions for miscibility in this ternary system, and possibly a general feature of all such systems, are (a) that at least two of the binary interaction parameters χijare less than the critical value χcrit, while the third should not be too much larger, that is, one of the copolymers may act as a compatibilizer for the other two copolymers, (b) that the difference Δχ = /χ12− χ13/ is small. © 19

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301205

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have directly measured the free‐volume hole distributions in semicrystalline polypropylene by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi‐isotropic external pressures of 0, 4.2, and 14.7 kbar into continuous lifetime distributions. The hole radii distributions as determined from theortho‐positronium lifetime distributions are found to be between 4.0 and 0.5 Å and to have maxima at 3.0, 1.9, and 1.1 Å under the external pressures of 0, 4.2, and 14.7 kbar, respectively. © 1992 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301206

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractCloud‐point data to 180°C and 2,800 bar are presented for three poly(ethylene‐co‐methyl acrylate) copolymers [10, 31, and 41 mol % methyl acrylate (MA)] and for polyethylene in ethylene, ethane, propylene, and propane. At low concentrations of MA in the backbone of the copolymer, the saturated hydrocarbons are better solvents for the copolymer than their olefinic analogs because polarizability drives the phase behavior. For the higher MA‐content copolymers, which have more polar repeat units, the unsaturated hydrocarbons are better solvents owing to favorable quadrupolar interactions between the solvent and the polymer segments. The cloud‐point curves of the high MA‐content copolymers in the unsaturated hydrocarbons are shifted to very high temperatures to overcome strong acrylate‐acrylate interactions in the polymer. In fact, the 41 mol % MA copolymer cannot be dissolved in propane at temperatures to 180°C and pressures to 2,800 bar even though the copolymer is predominantly ethylene, while the same copolymer dissolves in propylene at 40°C and at pressures as low as 1,400 bar. Although the Sanchez‐Lacombe equation of state is used to model the cloud‐point curves, two temperature‐dependent mixture parameters are needed for a good fit of the data. © 19

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301207

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractTwo different models of order in the solid state of wholly aromatic liquid crystalline copolymers (ALCPs), namely the nonperiodic layer (NPL) model of Windle et al. and the paracrystalline lattice (PCL) model of Biswas and Blackwell, have been analyzed with respect to experimental and theoretical observations. The NPL model proposes the formation of ordered domains by matching random sequences between adjacent chains while the PCL model relies on the presence of conformational correlations between adjacent monomers, without explicit sequence matching, to form ordered domains. The presence of three‐dimensional order in the quenched state of ALCPs has been reported previously. The initial stages of crystallization are very likely to be assisted by the presence of small sequence matched domains. However, the probabilities for the formation of NPL domains having sizes comparable to those observed experimentally in the quenched state are extremely small. The PCL model, on the other hand, is not hampered by probability statistics and also provides a better fit to the experimental wide‐angle x‐ray scattering data along the fiber (Z) axis. The reported calorimetric data on ALCPs are also inconsistent with the presence of significant motion between chains, which is required for the NPL crystal to grow during the process of annealing. In contrast, the observation of a slow crystallization process, which converts the quenched hexagonal phase to the ordered orthorhombic phase, is adequately described by the PCL model as occurring via local conformational changes and small axial shifts between adjacent monomers. The observations from dielectric relaxation, nuclear magnetic resonance and dynamical mechanical spectroscopy studies also favor the PCL model over the NPL model of order in the solid state of ALCPs. © 1992 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301208

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAdvances in polymer synthesis have made possible the polymerization of “starburst dendrimer” gels which are grown from a central initiator core through controlled branching reactions with a constant multiplication of the number of monomeric blocks from one generation to the next. The structure factor for such dendrimers is calculated using Gaussian monomer‐monomer interactions. The results are somewhat lengthy but analytical forms are obtained. © 1992 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301209

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractIn a previous study, a mechanosorptive phenomenon in poly(p‐phenylene terephthalamide) fibers was reported. In this article, the mechanosorptive creep mechanism of aramid fibers and the temperature and stress influences on the mechanosorptive creep behavior of aramid fibers are addressed. Test results indicate that logarithmic creep rates and the mechanosorptive effects increase with temperature. The creep activation energies of the fibers tested are: 20 kJ/mole for the cyclic moisture condition, 4.4 kJ/mole for a high equilibrium moisture condition (RH = 95%), and 7.8 kJ/mole for a low equilibrium moisture condition (RH = 5%). Increase in stress may increase the logarithmic creep rates but may reduce the mechanosorptive effect. Aramid fibers contain hydrogen bonds between rodlike crystallites oriented at small angles relative to the fiber axis. Transient moisture conditions may cause slippage of hydrogen bonded elements and result in accelerated crystallite rotations due to breakage of hydrogen bonds, thus causing increases in logarithmic creep rate. The obtained activation energies and the reduction in fiber elastic compliance due to creep deformation support the proposed mechanisms. © 1992 John Wiley&Sons, I

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301210

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY