1992 рік

The effects of ionization and epoxy reactions on the bulk and surface properties of poly(p-phenylene terephthalamide) (PPTA), namely Kevlar 149, fibres were studied. When the ionized Kevlar 149 fibres were quenched in water, their tensile properties were significantly lowered with increasing ionization time. The lowered tensile strengths were attributed to the reactions between the residual sodium ions in the fibres and water, and possibly the subsequent hydrolytic chain scissions of the PPTA molecules. The tensile properties of the fibres were only slightly lowered with increasing Nail concentrations. With increasing ionization time, the epoxy reaction first significantly lowered and then began to compensate for the tensile properties of the fibres. The interfacial shear strength of the Kevlar 149 fibres with the epoxy was slightly higher and was associated with better wetting property of Kevlar 149 fibres and surface compositional differences between the two PPTA fibres. The ionization and epoxy reaction conditions employed for Kevlar 49 fibres did not affect the interfacial shear strengths with the epoxy resin. The interfacial shear strength for Kevlar 149 fibres was increased from 4.35 ? 10- 3 g #m- 2 for the untreated to 5.29 x t0- 3 g #m- 2 for the ionized and epoxy-reacted fibres, a 22% increase.

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Phase structure develops within composite latex particles during the polymerization process and is potentially dependent upon both the latex recipe and the polymerization process characteristics. An equilibrium thermodynamic approach is presented to predict the particle morphology as a function of the extent of conversion of a seed latex polymerization reaction. The discussion highlights the role of the monomer as it influences the phase compositions and interracial tensions throughout the polymerization. It is found that a number of different particle morphologies possess nearly the same total interfacial energy throughout a significant portion of the polymerization reaction and that it is quite likely that occluded structures will form in addition to the more fully phase-separated structures, such as core-shell and hemispheres. Detailed methods to predict the probabilities of forming a variety of different morphologies are presented.

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The dynamic viscoelastic properties of blends of poly(ethylene oxide) (PEO) with poly(vinyl acetate) (PVAc) and with poly(vinyl acetate-ran-ethylene) (PVAE) were measured, using a cone-and-plate rheometer in the oscillatory shear mode. For the study, two grades of PEO, one with weight-average molecular weight M w = 20000 (PEO20) and the other with M w = 100000 (PEO100), were used. The PVAE used was found to be amorphous, as determined by differential scanning calorimetry, and it had Mw = 12 600, as determined by gel permeation chromatography, and 73.4 wt% vinyl acetate, as determined by elemental analysis. It was found that the amorphous phase of the PEO20/PVAc blend system has one glass transition temperature Ts over the entire range of blend compositions, whereas the amorphous phase of the PEO100/PVAc blend system has one Ts for blend compositions up to 50 wt% PVAc, but two Ts values for blends with greater amounts of PVAc. Melting-point depression measurements were conducted to determine the Flory interaction parameter X to be: (a) -0.211 for PEO20/PVAc pair; (b) -0.069 for PEO100/PVAc pair; (c) -0.027 for PEO20/PVAE pair; and (d) -0.024 for PEO100/PVAE pair. Our experimental results show that plots of the logarithm of zero-shear viscosity versus blend composition at constant temperature exhibit negative deviations from linearity for the PEO20/PVAc blend system, but positive deviations from linearity for both PEO20/PVAE and PEO100/PVAE blend systems. These experimental observations are interpreted using a molecular viscoelasticity theory recently developed by Han and Kim.

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The photopolymerization kinetics of four bisphenol-A based dimethacrylate resins were studied from -40 to 160°C by isothermal differential scanning calorimetry. The limiting conversion was low at temperatures near the glass transition temperature (Ts) of the monomer but increased rapidly as the curing temperature was raised. These data were successfully fitted to a theoretical relationship between curing temperature, conversion and T~. At higher cure temperatures, the limiting conversion was topologically controlled, in agreement with the Loshaek-Fox theory. The photopolymerization rate increased rapidly to a maximum and then decreased slowly as the monomer was consumed. Using the encounter theory for reaction rates, the kinetics of the propagation and termination steps were expressed by combination of thermal (Arrhenius) and free volume (WLF) terms. These expressions were then combined with the theories of translation and reaction diffusion, allowing the prediction of the temperature dependence of the overall polymerization rate. Despite its simplicity, many of the observed kinetic features were reproduced by the model.

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Oriented liquid-crystalline solutions of the rigid heteroaromatic polymers poly (p-phenylene benzobisthiazole) and poly(p-phenylene benzobisoxazole) in polyphosphoric acid were transformed into crystalline phases by absorption of moisture. At moderate moisture levels crystal-solvate phases are formed by co-crystallization of the polymer and its solvent, whereas at high water content the crystalline polymer phase is formed by deprotonation of the polymer. Two crystal-solvate forms have been identified, as the moisture content increases up to about 10% (w/w), denoted form I and form II. Transitions between the different phases, as a function of temperature and time, were studied by X-ray diffraction using the high-intensity synchrotron source. The intensity of the crystalline reflections from the form I state decrease gradually with increasing temperature up to final melting at about 70°C. The orientation of the crystalline phase is maintained up to its melting. Recrystallization is observed upon quenching, showing the reversibility of this transition. The behaviour of the form II phase differs markedly. As the temperature is increased, a gradual crystal-crystal transition occurs in the temperature range of 250-300°C from the form II state to the crystalline polymer state. This transformation is irreversible. A novel recrystallization phenomenon is observed when the liquid-crystalline phase is heated to about 250°C. Crystalline reflections appear at spacings close to those of the crystalline polymer state. These reflections disappear upon cooling. These transformations are discussed in terms of the state of protonation of the polymer in its solution in polyphosphoric acid. Form I is considered a complex of the protonated polymer and the acid anions, which dissolves at moderate temperatures. Recrystallization at elevated temperature is suggested to result from deprotonation of the polymer. The state of protonation of the polymer in the form II state is still not understood.

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When moving solvent fronts meet at the centre of a glassy polymer during sorption of solvent, making the sample entirely rubbery, sudden changes in the area, thickness and sorption rate typically occur. Examination of the sorption of water and methanol by glassy poly(2-hydroxyethyl methacrylate) showed that the magnitudes and directions of these changes can be readily modified by creating anisotropic stresses within the glassy polymer by controlled deformation of the sample prior to solvent sorption. However, the transport mechanism and sorption kinetics were not significantly altered by differing levels of initial sample anisotropy

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The new solvation equation: log L= c + rR 2 + sn~ + a~ + bfl~ + I log L 16 has been applied to the solubility of 43 gaseous probes on each of nine hydrocarbon polymers using the data of Munk et al.. In this equation, L is the gas-liquid partition coefficient of a series of probes on a given polymer, and the explanatory variables are solute properties as follows: R2 is an excess molar refraction, n~ is the probe dipolarity-polarizability, ~ and E2n are the probe hydrogen-bond acidity and basicity, and L 16 is the gas-liquid partition coefficient of the probe on hexadecane at 25°C. Each of the nine equations, one for each polymer, had correlation coefficients of around 0.999 and standard derivations of around 0.025 log units. The solubility of the gaseous probes, as log L values, as well as the polymer-probe interaction parameter X calculated by Munk, have been analysed in terms of particular polymer-probe interactions.

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The scattering behaviour of thermoplastic elastomers based on poly(ether ester) (PEE) under stress is studied. Bristles of PEE consisting of poly(butylene terephthalate) as hard segment and poly(ethylene glycol) (Mn = 1000) as soft segment in the ratio 50/50 wt% are drawn to five times their initial length and then annealed with fixed ends in order to create a standard initial structure. Samples with largely destroyed structure (by additional drawing) as well as with regenerated structure (by crystallization, solid-state reactions or chemical crosslinking) were prepared. Small-angle X-ray scattering (SAXS) measurements are carried out with single bristles subject to stress and with deformations up to 200%. An affine increase of the long period L with extension 5 up to e = 75% is observed in the samples with undestroyed structure. A second L2 appears at larger 5. Without application of stress two discrete values, L~ ?1 and L~ el, are obtained. Qualitatively, the sample with destroyed structure behaves similarly. The deformation behaviour of samples with regenerated structure depends on the method of regeneration: (i) crystallization mostly recovers the previous deformation pattern; (ii) solid-state reactions (additional condensation and exchange reactions) result in an increase of LI, L2, L~ et and L~ et due to the very high number of interfibrillar contacts; and (iii) chemical crosslinking leads to the appearance of only L 1 and L~ =1. A model is proposed suggesting the existence of two types of lamellae differing in their perfection and origin. The first and more perfect lamellae refer to the starting crystalline lamellae, while the second type of less perfect lamellae are assumed to arise during the additional stretching. The latter comprises hard segments originally dispersed in the amorphous interlamellar layers or pulled out from the neighbouring crystallites. The existence of the two types of lamellae is proved by differential scanning calorimetric measurements. By variation of the number of intra- and in particular interfibrillar contacts, the predominant role of the tie molecules in the evolution of mechanical properties of these polymer materials is demonstrated. Further, two important concepts are proposed in addition to earlier studies: (i) microfibrillar chemical healing-elimination of the interfibrillar phase boundaries as a result of solid-state reactions and (ii) deformation (by slippage) of ensembles of microfibrils in the chemically crosslinked samples, almost preserving in this way the initial L value and facilitating very high deformations (5= 200%).

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Polymer films of various gel fractions, obtained by coalescence of styrene-butadiene (SB) copolymer latices, are investigated for their viscoelastic behaviour by dynamic micromechanical spectroscopy. Isochronal temperature dependences of the storage modulus (E'), loss modulus (E") and loss tangent (tan 6) are measured in the temperature range -50 to + 130°C including the glass transition of the polymer films. Then, it is shown that the criteria of applicability for the superposition principle are fulfilled. It follows that master curves for the isothermal frequency dependence of the moduli (E', E") can be obtained over a wide range of frequency (10 -7 to 500rads-1), with shift factors obeying a law of the Williams-Landel-Ferry type. The key parameters describing the effects of degree of crosslinking and molecular interactions on the viscoelastic behaviour of the films are worked out. They are investigated at different scales (macroscopic, molecular, local) and their dependence upon chain-transfer-agent concentration, which controls the crosslinking process in the system, are determined. Moreover, it is shown that a local parameter, such as the monomeric friction coefficient, can be connected with the glass transition temperature of the SB copolymer film. Finally, the influence of the structure of the films arising from incompletely achieved coalescence of latex particles is discussed.

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The number (cq) of adsorbed molecules per phenyl group of polystyrene in solution (determined in this laboratory) was compared with the corresponding number (~) remaining after all of the excess non-adsorbed molecules were eliminated from the corresponding polystyrene-liquid (P-L) system (reported by Guenet). These correlations showed that, for P-L systems with the same polymer tacticity and the same class of liquids (based on similarity of molecular structure), ~ varies linearly with cq. The fraction of solvated polymer that formed microdomains of associated polymer (owing to the change from ct s to ~) and the average number of monomer units in the solvated segments between these microdomains, which serve as quasi-crosslinkages, were estimated on the basis of the corresponding ratio of ~ to cq. It was inferred from the results obtained that such microdomains of 'rigidized' polymer segments can evolve either via expulsion of already adsorbed solvent molecules or via incorporation of more solvent molecules to those already immobilized by adsorption

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