1999 рік

Microstructural investigations below the fracture surface have revealed that the rubber particles in a number of polymer-rubber blends were deformed into remarkable S-like shapes. These shapes seem to have been largely ignored in previous microstructural studies of blends, but in fact cannot be explained from the known deformation states around a crack. We hypothesize in this paper that these shape changes develop as a consequence of macroscopic shearing of the blend as the crack front sweeps through the material. Large strain, finite element models for simple shearing of a blend are reported which demonstrate the evolution of round particles into S-shape ones for a range of material parameters, and thus support our hypothesis. The 'microscopic’ localized deformation processes are identified, and the implications for the toughening mechanism in these blends is discussed.

Детальніше...  

Mechanical properties of poly(styrene-b-n-butylmethacrylate) diblock copolymers, PS-b-PBMA, with different lengths of the polystyrene block were investigated. The copolymers display a composition range where the tensile strength of the block copolymers exceeds the values of the corresponding homopolymers. At a PS-content of 74 vol% not only the tensile strength but also the strain at break is higher than that of pure polystyrene. Furthermore, the absorbed energy shows a maximum at a polystyrene content of 29 vol%. This composition reveals a morphology of hexagonally packed PS-cylinders which turns out to be effective for the increase of toughness in PS-b-PBMA diblock copolymers. The improved properties of PS-b-PBMA diblock copolymers are discussed with respect to a possible correlation and synergism between phase behaviour, morphology, deformation mechanism, and interface formation.

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Isothermal melt crystallization in poly(butylene terephthalate) (PBT) homopolymers and glass-fiber-filled composite was studied by timeresolved simultaneous small- and wide-angle X-ray scattering (SWAXS) methods using synchrotron radiation. During primary crystallization, both average long period and lamellar thickness exhibit a significant decrease. During secondary crystallization, the long period and lamellar thickness show a smaller decrease over a longer period of time, which is approximately linear with log time. Similar to other semicrystalline polymers, the morphology of PBT formed during isothermal crystallization can be best described by a dual-lamellar stack model. In this model, primary lamellar stacks are formed first comprising thicker crystalline lamellae, whereas secondary lamellar stacks are formed later between the primary lamellar stacks (as observed by the small change of the interlayer amorphous thickness with time). In addition, no isothermal lamellar thickening is observed. From SWAXS measurements, the appearance of the SAXS peak appears to occur prior to WAXD crystal reflection peaks, suggesting that density fluctuations are probably present before melt crystallization. The lamellar morphology and crystallization rate is found to be a function of molecular weight. The glass filler in the PBT composites has no obvious effect on the lamellar structure, but it appears to act as a nucleating agent.

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The morphology of polyurethane (PU)–poly(methyl methacrylate) (PEMA) interpenetrating polymer networks (IPNs) were investigated by means of small-angle X-ray scattering (SAXS) and modulated-temperature differential scanning calorimetry (M-TDSC) techniques. Based on the analysis method employed by Tan et al. [Polymer 1997;38:4571], the interfacial thickness in the IPNs was calculated from SAXS data. The conclusion is that the interfacial thickness is zero and there are sharp domain boundaries in the PU-PEMA IPNs. M-TDSC results show that the PU-PEMA IPNs have a multi-phase morphology with a diffuse interphase region. The M-TDSC results are in agreement with DMTA and TEM findings. The M-TDSC, TEM and DMTA results, therefore, conflict with those obtained from the analysis of the SAXS data. We believe that the analysis method employed by Tan et al. is questionable for IPNs.

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The effect of casting solvents on swelling resistance, crystallinity, dynamic mechanical, and mechanical properties of hydrogenated styrene butadiene copolymer (HSBR) has been investigated. The solvents chosen are cyclohexane, carbon tetrachloride, toluene and chloroform, which cover a range of solubility parameters from 8.2 to 9.3 (cal/cm 3) 1/2. The crystallinity, swelling resistance and mechanical properties are maximum with cyclohexane cast films. The dynamic mechanical properties, tan d and E⬘, are also influenced by the nature of the solvents. Cyclohexane cast films have the highest values of dynamic modulus. For comparison, compression moulded film has also been studied. The properties of the samples have been correlated with the structure. The properties of the films are also dependent on solvent evaporation rate.

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A detailed characterization of hydroxylated poly(vinyl chloride) (PVC), obtained by modification of PVC with p-mercaptobenzyl alcohol and its crosslinked homologues, is described. The selective character of the modification with respect to the thiol group and the presence of free hydroxyl groups in the polymer is shown using 1H-NMR. The stereoselectivity of the reaction is provided from 13C-NMR. The presence of physical interactions in the system by hydrogen bonding is deduced from light scattering measurements and analysis of the thermal and mechanical properties. The critical concentration of hydroxyl groups required in the chains in order to form an efficient physical network is determined to be about 4%. Chemical networks are obtained by reaction of hexamethylene diisocyanate with the hydroxyl groups present in the polymer, and the process is followed by IR spectroscopy.

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Photo-oxidative degradation of poly(ethylene oxide) with small amount (1 and 3 wt.%) of cobalt (II) chloride was studied in acetonitrile solutions and in solid state. On the basis of FTIR, UV-Vis spectroscopy, GPC and viscometry measurements it was found that cobalt salt accelerates photodecomposition of PEO. DSC was applied for study of crystallinity changes in PEO and PEO 1 CoCl2 during UVirradiation. The structure of PEO 1 CoCl2, mechanism of photoreactions and the influence of physical factors are discussed.

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The effect of physical ageing on the d.s.c. heat flow and yielding behaviour has been investigated for two series of epoxy resins in which the crosslink density is varied without significant compositional variation. At low degrees of undercooling, the extent of enthalpic ageing approached a thermodynamic limit with increased ageing time, but at larger degrees of undercooling, full densification was not achieved in the time-scale of the experiments due to kinetic control of the ageing process. At constant degree of undercooling, the relaxation enthalpy decreased as the crosslink density was raised, which is attributed to topological restrictions on the process of segmental re-organization leading to densification. However, neither crosslink density nor amine nature appeared to significantly affect the ageing kinetics when measured at the same degree of undercooling. The influence of ageing on the room temperature yield stress was also investigated. Even though the yield stress increased with crosslink density, the influence of ageing on the yield stress was reduced. The time-scales for enthalpic ageing and yield stress enhancement did not appear comparable.

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The crosslinking of poly(vinyl chloride) (PVC) modified with 4-mercaptobenzyl alcohol is studied as a function of the number of hydroxyl groups present in the polymer and the amount of crosslinking agent added. Comparison of crosslinking reaction of a hydroxylated PVC polymer synthesized in our laboratory and a commercial system shows a higher reaction rate and degree of conversion for the former. Swelling experiments of partially crosslinked polymers reveal the existence of two kinds of interpenetrated networks, chemical and physical. The appearance of the physical network is due to hydrogen bonding interactions between the remaining hydroxyl groups in the polymer chains after crosslinking. Thermogravimetric analysis and traction tests show good thermal stability of the networks obtained, and a significant improvement in the mechanical properties with respect to linear PVC. Mc values obtained from swelling experiments, mechanical tests and dynamomechanical tests are in agreement with the theoretical values.

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A generalized Flory–Huggins parameter x is utilized to study immiscibility-loop phase behaviour of binary polymer blends. Special emphasis is placed on the temperature dependence of the x parameter. It turns out that specific interactions, that gradually become weaker with ascending temperature, lead to miscibility at low temperatures and, eventually, to phase separation at sufficiently high temperatures (lower critical solution temperature behaviour). In addition, if a favourable noncombinatorial entropy contribution to parameter x becomes more and more dominant with increasing temperature then it leads again to a homogeneous system (upper lower critical solution temperature behaviour). From a molecular point of view, this stabilization of the mixture at high temperatures might be attributed to nonrandom packing effects caused by structural disparities of the constituents. It is also demonstrated that immiscibility-loop phase behaviour can only be observed in blends of constituents having sufficient asymmetry in the polymerization indices.

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