1994 рік

The mechanical behaviour of sulfonated polystyrene ionomers can be altered by changing the nature of the counterion and by addition of excess neutralizing agent. Enhanced tensile strength and greater energy to fracture have been obtained by changing the counterions from monovalent Na to divalent Ca. The Ca ions create stronger ionic crosslinks that result in a more effective entanglement network. Increases in both tensile strength and toughness have also been obtained in an Na salt sulfonated polystyrene ionomer by addition of a 100% excess of neutralizing agent. The enhancement in properties is thought to arise from the presence of a small, coherent second phase that acts as a reinforcing filler and increases resistance to crack propagation.

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A series of syndiotactic polypropylene (s-PP) fractions with constant syndiotacticities and different molecular weights have been studied through differential scanning calorimetry (d.s.c.), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering experiments. The molecular weights, molecular weight distributions, syndiotacticities and sequence distributions of this series of fractions have been characterized by gel permeation chromatography, solution nuclear magnetic resonance and Fourier transform infra-red spectroscopy. The equilibrium melting temperature of sufficiently high molecular weight (above 40 000) s-PP with about 94% racemic dyads is 160___ I°C, and the heat of fusion is 8.0_+0.3 kJ mol -x. Overall crystallization rates exhibit a molecular weight dependence and a discontinuity with respect to crystallization temperature for the fractions. The temperature at which this discontinuity happens is at an undercooling of ca. 50°C. Based on nucleation theory, this discontinuity may be recognized as a regime III to regime II transition. With decreasing undercooling (increasing crystallization temperature) and molecular weight, a doubled crystal unit cell along the b axis becomes increasingly dominant during the crystallization. In this unit cell, opposite handedness of the helical chains exists along both the a and b axes (antichiral packing). Double melting peaks can be observed for all fractions in the high to middle undercooling region (AT> 50°C), while only one melting peak can be found in the relatively low undercooling region. Different heating rate experiments after isothermal crystallization in d.s.c, and WAXD indicate that the low-melting crystal may undergo reorganization and melt-recrystallization processes to form the high-melting crystal. During this transformation, doubling of the crystal unit cell along the b axis with an antichiral packing of the chain molecules is obtained

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High resolution ~H n.m.r, spectra are used to investigate the hydrogen bonds in polyester-polyurethanes based on 4,4'-diphenylmethane diisocyanate, N-methyldiethanolamine and poly(ethylene-propylene) adipate and their zwitterionomers. There are several NH peaks in the 1H n.m.r, spectra resulting from different kinds of hydrogen bonds. In concentrated dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) solutions, there are few hydrogen bonds between the polyurethanes and the solvent or water. The solvent effect can be ignored for hydrogen bonds. The relative amounts and the half-widths of each NH peak remain the same in the temperature range 23-82°C. The chemical shifts of the NH peaks move to lower frequency at higher temperature. The strong hydrogen bonds between groups of the hard segments make the hard segments align in a very orderly fashion. The percentage of NH groups forming hydrogen bonds with C~O groups of the hard segment is large (> 45%). Neither the solvent nor H20 can destroy the strong hydrogen bonds of the hard segment groups in concentrated solution. We deduce the conformation of the hard segments from two-dimensional nuclear Overhauser effect spectroscopy. There is no exchange between NH groups. The conformation of the polyester-polyurethanes in concentrated solution is similar to that in the solid state. Thus we can study the hydrogen-bonding interactions in concentrated solution and deduce those interactions in the solid state.

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Comparative studies on the dynamic and static vulcanization of blends of polyethylene (PE) and ethylene-propylene-diene monomer rubber (EPDM) are reported. The studies were made using a PE/EPDM blend ratio range of 40/60 to 20/80, over which the occurrence of phase inversion was indicated from torque rheometric studies. The state and rate of sulfur cure were varied by use of different appropriate doses of (a) tetramethylthiuram disulfide-mercaptobenzthiazyl disulfide-sulfur (TMTD-MBTS-S) combination as a conventional curative and (b) TMTD-Si69-S combination as a silane curative system. Of the two curative systems, the conventional system imparted measurably higher cure rates. For a particular blend ratio and for a given crosslink density level established in each case, the tensile strength and elongation at break measured at 298 K were higher for vulcanizates obtained statically compared to those obtained dynamically, while the corresponding modulus values followed the opposite trend. High-temperature tensile properties (at 403 K) and hot elongation and hot set values (at 523 K) for the vulcanizates were also evaluated. The property differences for vulcanizates from static and dynamic curing have been explained in the light of differences in the morphology developed

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The processing, mechanical and chemical properties of poly(ether ester)s, prepared from pivalolactone (PVL), 1,4-butanediol (4G) and dimethyl terephthalate (DMT), were studied. The poly(ether ester)s could easily be processed by injection moulding, owing to their favourable rheological and thermal properties. The tensile response of a poly(ether ester) with a butylene terephthalate (4GT) content of 72 mol%, which exhibited the phenomena of necking and strain-hardening, was related to the morphology of these copolymers. The influence of the short 4G-PVL segments was reflected in a high Young's modulus and yield stress, and resulted in a tough behaviour for the poly(ether ester), with an ultimate elongation of 500%. The poly(ether ester)s were stable towards treatment at room temperature with water or weakly acidic or alkaline solutions. Conditioning at 90°C in water for 264 h resulted in a water uptake of 1 wt%, whereas the rate of hydrolysis was 0.0003 (expressed in At/re~ h-t) for the poly(ether ester) with a 4GT content of 72 mol%. Although a decay in the mechanical properties for the PVL-based poly(ether ester) after exposure to water at 90°C was observed, these materials were assumed to have a higher hydrolytical stability than other poly(ether ester)s.

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The spherulitic growth rates of e-phase poly(pivalolactone) (PPVL) in blends with poly(vinylidene fluoride) (PVF2) were measured by polarized optical microscopy as a function of blend composition and isothermal crystallization temperature Tx between 160 and 215°C. The PPVL weight fraction in the blends ranges from 100 to 10 wt%, which constitutes the largest compositional range investigated in any such study. Using the Lauritzen-Hoffman kinetic theory of crystallization, the composition dependent equilibrium melting temperatures Trn , the nucleation constants Kg(ii) and Kg(lll ) and the surface free energy product acr? were determined directly from the temperature dependence of the spherulitic growth rate data for each blend. The equilibrium melting temperature, the nucleation constants and the product of the fold and lateral surface free energies of PPVL e-phase crystals are observed to decrease with increasing PVF/content. The observed depression in equilibrium melting temperature was successfully analysed following the treatment proposed by Nishi and Wang and based on Scott's expression for chemical potentials in a binary polymer mixture to yield a negative interaction parameter (X=-0.13+0.05). The magnitude of this interaction parameter is consistent with that found in earlier studies of poly(vinylidene fluoride)/poly(methyl methacrylate) blends. Finally, the observed decrease in crystal/melt surface free energy product is discussed in the context of a recent model correlating the lateral crystal/melt interracial free energy with the characteristic ratio of the crystallizable polymer chain. Our analysis suggests that the lateral crystal/melt interface thickness should increase with PVF2 concentration in the blend in order to minimize the demixing of a crystallizable chain as it diffuses into the melt/crystal interface to become physically adsorbed onto the crystal growth front.

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In this paper, a study of the structural changes due to annealing of injection-moulded polyamide-6,6 has been carried out. The spectroscopic behaviour of polyamide-6,6 is quite similar to that of poly(ethylene terephthalate) (PET). Using the band at 1650cm-1 as an internal reference band, the intensity changes of the bands situated at 1146 and 936cm -1 were followed. The former decreases when the annealing temperature exceeds 180°C whereas the latter increases. Furthermore, the bandwidth of the band at 936 cm- 1 decreases from 23 to 20cm -~ for annealing above 180°C. These spectroscopic changes were related to gauche/trans isomerism induced by the thermal treatment. Moreover, polyamide-6,6 verifies a two-phase conformational model, similarly to PET. As far as the thermal behaviour is concerned, two endothermic peaks at low (LM peak) and high (HM peak) temperature were found in thermograms of samples annealed above 150°C, and these displayed similar behaviour to those found in PET. The LM peak can be attributed to melting of crystals with increasing perfection and fold-surface smoothing of the crystalline layers due to the annealing treatment, and the HM peak to melting of the recrystallized crystalline structure during heating in differential scanning calorimetry (d.s.c.). On the other hand, the influence of the fabrication process in polyamide-6,6 seems not to be as important as in PET. The correlation between Fourier-transform infra-red spectroscopy with photoacoustic detection (p.a.-FTi.r.) and d.s.c, measurements show that the amide group works as an important constraint that limits the mobility of the crystalline molecules, and most of the conformational changes occur in the amorphous phase for annealing temperatures above 180°C. In the ordered phase, a slight crystallinity increase beyond this temperature can be related to crystalline perfection and fold-surface smoothing.

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Solvent polymeric ion-selective electrode membranes are composed primarily of 33% polymer and 66% plasticizer. Puncture tests were conducted on membranes that incorporated blends of poly(vinyl chloride)s (PVCs), fractionated and whole, with different molecular-weight distributions. The extent to which the minor membrane component determines eight modified mechanical properties, i.e. four elastic parameters (elastic limit, tangent stiffness, resilience, flexibility) and four failure parameters (strength, secant stiffness, toughness, ductility), was investigated. The mechanical properties of fractionated and whole blends were not substantially different. Membrane ductility was always independent of thickness. Elastic parameters showed no effect of polymer number-average molecular weight, M,. Two of the failure parameters, however, showed a linear blending relationship based on the blend M.. From that relationship, the per mer contributions of PVC to the membrane strength/thickness and toughness/thickness equalled 0.96 mg/~m- 1 and 0.78 mg mm ttm- 1, respectively.

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A new method for poly(ether ether ketone) (PEEK) molecular-mass characterization by room-temperature (r.t.) size (steric) exclusion chromatography (s.e.c.) based on a derivatization procedure that improves the polymer solubility is described. Upon dissolution in 99.5% sulfuric acid at room temperature, PEEK is chemically modified by sulfonation of phenyl rings without polymer degradation. As complete sulfonation is achieved, the reproducibility of the chemical modification is ensured as well as the increase of molecular mass. Sulfonated PEEK is soluble at room temperature in different solvents suitable for s.e.c. N-Methyl-2-pyrrolidine (NMP) is selected as PEEK s.e.c, solvent. Owing to the polyelectrolyte nature of sulfonated PEEK, a salt (0.1 M LiBr) is added to NMP. Various broad-dispersity PEEK standards are used to establish specific and universal calibrations. The viscosity laws of sulfonated PEEK in NMP + LiBr (0.1 M), in methanesulfonic acid (MSA) and in 99.5% H2SO4 are determined. As sulfonated PEEK is a hygroscopic polymer, some spectroscopic methods are developed to determine the absolute concentration of the standards in order to perform accurate viscometric experiments giving the Mark-Houwink-Sakurada parameters in NMP-salt and in MSA. The s.e.c, universal calibration curve is constructed with poly(methyl methacrylate) narrow-dispersity standards.

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Folded chain lamellar crystals of polyethylene were examined by atomic force microscopy. Particular care was taken to ensure that the areas examined at high magnification were on the fold-containing surface of the crystals. High spots observed on this surface formed a 2D array. The spacings and angles of the array were consistent with the positions expected for the folds when viewed from a direction perpendicular to the fold-containing surface and were not the same as expected if viewed along the chain axis. Spacings appropriate for the usual orthorhombic unit cell and for the monoclinic unit cell that can be produced by mechanical deformation were observed. Local variations in the shape and position of the spots indicate that folds of different orientation and structure were present. In many observations of similar fold surfaces at high magnification, only images of lower quality which did not show the periodic arrays of spots were observed. Reasons for the images without periodic order are proposed.

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