摘要:

AbstractInterpretation of asymmetric representations of nuclear magnetic resonance (NMR) relaxation times versus reciprocal temperature requires a distribution function of correlation times that describes any relation between the slopes in the high‐ and low‐temperature regions. Although it can be shown that the Havriliak–Negami function fulfills this condition after slight modification, a more useful function (a generalization of the Fuoss—Kirkwood distribution) is introduced and discussed in detail. This new function allows for description of curves of different symmetry by an expression similar to that for a single correlation time and gives a simple rough estimation of the distribution parameters, as well as more exact computer fitting. This interpretative discussion of NMR relaxation data is purely phenomenological and has no direct connection with molecular pro

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291001

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractReactor powders of high‐ and ultrahigh‐molecular weight polyethylene have been investigated. Two different Ziegler‐Natta synthesis processes were used: polymerization in a slurry and in the gas phase. Synthesis temperature range was 30–85°C. Monoclinic crystals were identified in samples synthesized at 30°C. Investigations of thermal parameters were carried out by differential scanning calorimetry. A range of heating rates (0.4–10.0°C/min) was used to obtain information on sample reorganization on heating. The corresponding melt‐crystallized samples were scanned and their thermal parameters were compared with those obtained from the original nascent powders. Percent crystallinity and average lamellar thickness, computed by the Thompson‐Gibbs equation, were found to be controlled by conditions of synthesis. For reactor powders, the fraction of crystallinity is found to be insensitive to synthesis temperature. Crystallinity is controlled mainly by the synthesis process type: slurry or gas phase. Lamellar thickness was found to decrease as synthesis temperature was increased. This trend is the opposite of what is obtained on melt crystallization and can be interpreted on the basis of Lauritzen and Hoffman's theory of crystal growth. Nascent reactor powders give experimental support for the dependence of lamellar thickness on crystallization temperature that follows the pattern described in the theory at high undercooling. The influence of molecular weight on crystallinity and lamellar thickness of both nascent powders and melt‐crystallized samples was also studied. Catalyst and synthesis conditions were found to control crystallinity and crystallite dimensions of the reactor powders. Thus, polyethylenes suitable for a specific purpose can be obtained dir

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291002

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe lamellar thickness of polyethylene virgin reactor powder has been successfully measured using the Raman longitudinal acoustic mode (LAM). Relatively high lamellar thicknesses have been found. These values change with polymerization temperature in an unusual way, with lamellar thickness being larger for lower polymerization temperature, i.e., at higher undercooling for crystallization. Agreement within an experimental error of 10% has been obtained between values from LAM and those calculated by the Thompson—Gibbs equation from the melting point measured by differential scanning calorimetr

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291003

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA model for the effect of orientation dependence of polarizability anisotropy on birefringence is given and compared with literature data. If this dependence arises through bond angle distortion, the model shows a significant contribution to birefringence but a very small contribution to strain, accounting satisfactorily for differences in birefringence between hot‐drawn and cold‐drawn samples, for discrepancies between measurements of orientation obtained by different experimental methods and for previous observations on orientational behavior of preorientated polycarbonate at low additional str

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291004

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractFive poly (amic acid) solutions based on PMDA‐PDA, PMDA‐ODA, PMDA‐6F, ODPA‐ODA, and 6FDA‐ODA were prepared inN‐methylpyrrolidone at a polymer concentration of ca. 10 wt %. From these five poly (amic acid) solutions, six different binary blends were prepared: PMDA‐PDA/PMDA‐ODA, PMDA‐PDA/PMDA‐6F, PMDA‐ODA/6FDA‐ODA, PMDA‐ODA/ODPA‐ODA, PMDA‐PDA/ODPA‐ODA, and PMDA‐PDA/6FDA‐ODA. These blends were then characterized with respect to miscibility in the ternary state (polyamic acid‐1/polyamic acid‐2/N‐methylpyrrolidone), the condensed state (ca. 70 wt % polymer), and the fully imidized state. All blends except for PMDA‐PDA/PMDA‐6F yielded homogeneous mixtures in the ternary solution of 10 wt % polymer concentration. The PMDA‐PDA/PMDA‐6F mixture eventually became homogeneous after 10 days of mixing at room temperature. Upon solvent evaporation (condensed state) and full cure (imidized state) two blends incorporating ODPA‐ODA as one of the components exhibited apparent miscibility as evidenced by optical microscopy. The remaining blends exhibited large‐scale phase separation upon solvent evaporation with no significant differences in the overall morphology between the condensed and imidized state. However, as in the case of the PMDA‐PDA/PMDA‐6F ternary system, the morphology in the condensed and imidized state was strongly dependent on the mixing time of the precursor poly(amic acid) components; the phase‐separated domain size decreased with increasing mixing time, eventually leading to complete miscibility. These results are discussed with respect to the proposed “polymer‐monomer” reequilibration reaction as well as its implications

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291005

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe dielectric permittivity and loss of LiClO4solutions in poly (propylene glycol (PPG)), molecular weight 2000, have been measured over a concentration range up to a ratio of Li+to oxygen atoms in PPG of 33.3:100, between 77 and 350 K. The data have been analyzed in both the permittivity and electrical modulus formalisms. Addition of LiClO4to poly (propylene glycol) first increases the height of the β‐relaxation peak, and ultimately a second sub‐Tgrelaxation peak at a higher temperature emerges. This is in addition to the β‐relaxation peak due to the reorientation of PPG dipoles, whose strength decreases from that in pure PPG‐2000. For a fixed temperature, the dc conductivity initially decreases with increasing Li+concentration up to 20 Li+per 100 O atoms and thereafter increases. This concentration corresponds to that at which theTgof the solution reaches its limiting value of ca. 310 K. It is concluded that the formation of ion pairs causes a second and slower sub‐Tgrelaxation process and that the increase in the efficiency of chain packing reduces the strength of the β‐relaxation

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291006

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractIn order to clarify the structural changes that occur in the thermochromic phase transition of poly (3‐dodecylthiophene) [P3DT] and poly (3‐hexylthiophene) [P3HT], the temperature dependence of x‐ray diffraction and Fourier transform infrared spectra was measured. (1) Orthogonal unit‐cell parameters were determined at room temperature: a=25.83 Å, b=7.75 Å, c (fiber axis)=7.77 Å for P3DT and a=16.63 Å, b=7.75 Å, and c=7.77 Å for P3HT. A large variation of the a‐axis length between P3DT and P3HT indicates the extended trans conformation for the alkyl side chains which are oriented along the lateral a‐axis direction. (2) The interplanar spacing, intensity, and integral width of the x‐ray (h00) and (00l) reflections were found to change drastically in the transition region. (3) Polarized infrared measurements at high temperature revealed a marked increase of the gauche band intensity for the alkyl side group modes followed by a decrease in the band intensity of the thiophene ring modes. (4) The layer reflections of the x‐ray fiber diagram become diffuse at high temperatures, indicating that the transition occurs in a liquidcrystalline manner with the orientation of the main chain axes preserved but with almost no axial correlation between the neighboring main chains. These results provide experimental support for the structural model proposed earlier: as the temperature increases, the trans‐type side chains begin to disorder by introduction of gauche bonds. This disordering disrupts the regularity of the main chain conformation and decreases the effective length of the polythio

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291007

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractTheoretical x‐ray scattering curves have been developed for the lamellar structure in semicrystalline polymers in which there are present distributions of lamellar thickness and crystallinity. The models have been tested against samples of linear low‐density, low‐density and crosslinked polyethylenes. When variation in crystallinity is present in a material, a major effect is an increase in the magnitude of near‐zero angle scattering. The Bragg maximum can appear as an ill‐defined hump on an apparently high level of background scattering. The shape of the Bragg peak is influenced more by crystallinity distribution than by lamellar thickness distribution. Of the polymers we have studied so far only linear low density polyethylene shows significant crystallinity distribution effects. A “rule‐of‐thumb” method for rapid estimation of crystallinity distribution effects has been developed, obviating the need for l

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291008

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe trend of the room‐temperature transitions of native polytetrafluoroethylene versus annealing temperature has been studied. A method of deconvolution for the calorimetric curvesCp(T), based on Pearson VII type functions and the Marquardt's nonlinear leastsquares method, allowed the evaluation of the temperatures and of the heat of each transition. Annealing at 330°C determines the loss of the crystalline fraction responsible of the lowest room‐temperature transition. An interpretation of the lowest room‐temperature transition is sug

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291009

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractAn analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition‐dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation‐period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor‐phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 se

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291010

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY