摘要:

AbstractThe structure and properties of presumed block copolymers of polypropylene (PP) with ethylene‐propylene random copolymers (EPR), i.e., PP‐EPR and PP‐EPR‐PP, have been investigated by viscometry, transmission electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, gel permeation chromatography, wide‐angle x‐ray diffraction, and other techniques testing various mechanical properties. PP‐EPR and PP‐EPR‐PP were synthesized using δ‐TiCl3‐Et2‐AlCl as a catalyst system. The results indicate that the intrinisic viscosity of these polymers increases with each block‐building step, whereas the intrinsic viscosity of those prepared by chain transfer reaction (strong chain‐transfer reagent hydrogen was introduced between block‐building steps during polymerization) hardly changes with the reaction time. Compared with PP/EPR blends, PP‐EPR‐PP block copolymers have lower PP and polyethylene crystallinity, and lower melting and crystallization temperatures of crystalline EPR. Two relaxation peaks of PP and EPR appear in the dynamic spectra of blends. They merge into a very broad relaxation peak with block sequence products of the same composition, indicating good compatibility between PP and EPR in the presence of block copolymers. Varying the PP and EPR content affects the crystallinity, density, and morphological structure of the products, which in turn affects the tensile strength and elongation at break. Because of their superior mechanical properties, sequential polymerization products containing PP‐EPR and PP‐EPR‐PP block copolymers may have potential as compatibilizing agents for isotactic polypropylene and polyethylene blends or as potential

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291201

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractMethanol‐induced opacity in poly (methyl methacrylate) (PMMA) is investigated subject to two cooling processes; furnace cooling and air cooling. The glass transition temperature of PMMA decreases with increasing time of exposure to methanol at 40–60°C and then increases during cooling, due to progressive desorption. Voids form during cooling as long as specimen temperature remains above its glass transition temperature. Since furnace cooling affords enough time for holes to expand larger than the light wavelengths, the transmittance of furnace‐cooled PMMA is independent of wavelength. The transmittance of PMMA subjected to rapid cooling in the air is wavelength dependent due to scattering by holes smaller than light wavelengths. The transmittance of PMMA bearing a given weight gain of methanol (measured at absorption temperature) prior to cooling for furance cooling is lower than that for the same material subjected to air cooling. A sharp front between outer and inner regions is found in specimens removed quickly from the thermostated water bath to air at ambient tempe

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291202

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractLow‐Frequency Raman spectra of odd α,ω‐dibromo‐ and α,ω‐dihydroxy‐n‐alkanes were recorded. The longitudinal acoustic mode (LAM‐1) frequencies were assigned by references to the published results forn‐alkanes and even α,ω‐disubstitutedn‐alkanes and also by taking account of the effects of end intermolecular forces and end‐group masses by use of the chai

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291203

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractPoly (n‐butylisocyanate)‐benzene solutions prepared by solubilization at 45°C, followed by aging at room temperature were found to be metastable for months, although, eventually, they separated into a birefringent polymer‐rich phase and an isotropic solution. These metastable solutions, as well as isothermally phase‐separated biphasic samples, flowed and exhibited dynamic moduli indicative of low polymer connectivity. By contrast samples prepared by a freeze‐thaw cycle were uniformly and highly birefringent and showed network (gel) behavior at room temperature. The mechanism of gel formation is most likely the exclusion of the polymer from the benzene crystal during crystallization, forcing the polymer to align and exist at grain boundaries. Films formed from solutions have different moduli than those formed from gels, and are consistent with the proposed

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291204

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe β relaxation process was shown to occur at different temperatures and exhibit a different activation energy for dry and wet samples prepared by reaction injection molding (RIM) of Nylon‐6. By employing a low operating frequency and controlled low moisture levels, it became possible to distinguish the two β relaxation processes, β1and β2, in one sample by dynamic mechanical spectroscopy. It was shown that while β1was associated with moisture in the sample, β2was related to a structural mechanism independent of moisture. The effect of moisture content on the chain‐chain association (α relaxation process) and chain‐water association (β1, relaxation process) in nylon‐6 was assessed. The annealing mechanism of RIM Nylon‐6 at 85°C was probed by both dynamic mechanical spectroscopy and

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291205

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA coupling model that has been shown in the past to be capable of relating macroscopically measured relaxation parameters to molecular ones has been presented. In this article the coupling model is applied to the analysis of stress relaxation data collected by Cama and Sternstein on PVC and plasticized PVC. The Kohlrausch‐Williams‐Watts form, exp — (t/τ*)1−n, usingn= 0.77 is found to be capable of describing the stress relaxation master curve at temperatures below the glass transition,Tg. From the temperature‐independent apparent activation energy found by Cama and Sternstein, the primitive activation energy of the α‐relaxation was calculated to be 7.5 kcal/mol, which is a reasonable value for the energy barrier to internal rotational isomerism in PVC. Support for this value is found from the data on two plasticized PVCs with differentTgs and apparent activation energies. By applying the coupling model in a similar manner, the primitive activation energies were found to be 8.5 kcal/mol for PVC plasticized with 6 pph dioctylphthalate and 7.7 kcal/mol for PVC plasticized with 6 pph tricresyl phosphate. Within experimental uncertainties, the three primitive activation energies can be considered to be the same. This finding is consistent with the physical basis for primitive activation energy and its identification with the internal rotation barrier, which should be independent of the type and amount of plasticizer in the system. Analysis of Cama and Sternstein's data on the effect of repeated stress aging on stress relaxation of quenched samples of PVC and plasticized PVC show that the coupling constantnincreases systematically with each successive stress‐aging cycle until it approaches the value for slow‐cooled samples. These results are consistent with the notion that stress‐aging changes the structural state of the glass in ways simila

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291206

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractGlass transition temperatures (Tg) of nonstoichiometric epoxy‐amine networks based on the diglycidylether of bisphenol A (DGEBA), are analyzed in terms of the network structure. In most cases reasonable predictions ofTgcan be made using an empirical equation reported by L. E. Nielsen together with the experimentalTgvalue of the stoichiometric network and statistical calculations of the concentration of elastic chains. It is stated that in these rigid networks the concentration of elastic chains is the main structural factor associated to the variations ofTgwith stoichiometry. For flexible networks based on the diglycidylether of butanediol (DGEBD), the effect of elastic chains on theTgvalue is much less significan

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291207

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe morphology of segmented polyurethane (PU) elastomers was studied by means of ESR at various temperatures ranging from 100K‐450K. The PU's were based on 4,4'‐diphenylmethane diisocyanate (MDI), poly (oxytetramethylene) glycols (PTMO) and hydroxyl‐terminated random compolymers of tetrahydrofuran and 3‐methyl‐tetrahydrofuran (THF/Me‐THF). Purified 1,4‐butanediol (BD) was used as a chain extender. The nitroxide probe, 4‐hydroxy‐2‐2',6‐6'‐tetramethylpiperidine‐1‐oxyl (TEMPOL), was used to label the polyurethanes by reaction with an NCO group of MDI. Analysis of the electron spin resonance (ESR) spectra suggested that the label situates at heterogeneous sites, which means that a two‐phase domain is present. The morphologies at 300K and 400K were compared. It was found from ESR spectra that polyurethanes made from soft segments having higher MW exhibited greater phase separation at 300K than their corresponding elastomers made from lower MW soft segments. However, at 400K the more isotropic ESR spectra for the PUs containing higher MW soft segments suggested that the PUs were more mobile than their analogs having lower MW soft segments. The introduction of 3‐methyl side groups decreased the phase mixing

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291208

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe current technique for the measurement of linear viscoelastic dynamic moduli is reconsidered. The transfer function analysis reveals the existence of nonlinear components in the response of the sample in a wide range of excitation frequencies around the reasonances of the measuring system. This analysis permits the disclosure of the components of the response which behave in accordance to linear viscoelastic theory and separates the undesirable nonlinear components. This results in partial corrections of the anomalous behavior of the calculated dynamic moduli through the standard technique.

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291209

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe absorption and subsequent desorption of benzene, toluene, carbon disulfide, and chloroform in amorphous and 27% crystalline poly (aryl‐ether‐ether‐ketone) (PEEK) were determined. At 35°C, the equilibrium weight gain (solubility) of benzene, toluene, chloroform, and CS2are 23.5, 19.8, 51.2, and 21.2 wt%, respectively. The initial weight gain is linear with root‐time and pseudodiffusion constants for absorption into amorphous PEEK ranging from 0.35 to 9.85 x 10‐12m2/s were calculated. The desorption processes are two‐step and are controlled by theTgof the penetrant‐resin mixture. The rate of diffussion into the crystalline material is extremely slow; crystalline PEEK reaches saturation (12.5 wt%) after immersion in CS2(35°C) for several hundred hours but, even after 1300 h immersion, the other fluids do not r

ISSN:0887-6266    

DOI:10.1002/polb.1991.090291210

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY