1992 рік

The temperature dependence of the elastic modulus E 1 of crystalline regions in the direction parallel to the chain axis has been measured by X-ray diffraction for poly (vinyl alcohol ) ( PVA ) with different tacticities. The E l value for PVA with 55.4% syndiotactic diads, sPVA(55.4%), decreased above 120°C, which was in accordance with that of atactic PVA. The lattice spacing for the (0 2 0) plane (the fibre axis is b) also decreased with increasing temperature. The thermal expansion coefficient ~c changed discontinuously around the temperature range where E l began to decrease. These phenomena could be explained using the kinked-chain model, where a small amount of a contracted portion, such as a kink, was incorporated into the chain molecule as in the case of atactic PVA. On the other hand, the E~ value of sPVA(63.0%) remained unchanged till 150°C. The value of ~o did not change with increasing temperature in this case. Consequently, the molecular motion that is responsible for the decrement of E~ is considered to be restricted for sPVA (63.0%) because of strong intermolecular hydrogen bonds. Further, increasing the syndiotacticity is considered to be one of the most effective ways to introduce high heat resistance into PVA

Детальніше...  

Linear polydiene chains selectively end-capped by an ionic group have been investigated by dielectric spectroscopy in relation to both polydispersity and the glass transition temperature (Tz) of the polymer and nature of the dipolar end groups. An s-relaxation associated with the micro-Brownian motion of chain segments is systematically observed. When the chain length distribution is broad the structure of the or-relaxation is complex and reflects an incomplete segregation of ionic and non-ionic components. The thermal dissociation of the ionic multiplets allows unpaired carboxylate groups to be released (E phenomenon); it is observed as a large relaxation process when it occurs at a temperature much higher than Tg (e.g. 100°C) and when carboxylate end groups are associated with alkaline cations (excluding alkaline earth cations). When the glass transition of the matrix and the thermal dissociation of the multiplets interfere, the situation is more complex : the or-relaxation is observed at a temperature lower than expected ; and the Y phenomenon is no longer detected (in the usually investigated temperature range) although an unexplained low amplitude relaxation is noted beyond the or-relaxation.

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From X-ray diffraction experiments, it is observed that on-substrate cured polyimide films are so oriented that the imide chains are preferentially aligned in the plane direction of the films. For films with rigid-rod polyimide, film orientation is especially prominent and decreases significantly as the film thickness increases. The effect of coating thickness on the orientation and ordering of polyimide films is most pronounced in pure pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA) films, slightly less in films with 50% pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) and 50% PMDA-PDA, and relatively insignificant in those with 100% PMDA-ODA. According to the C=O and C-N stretching absorption bands from Fourier-transform infra-red-attenuated total reflectance spectroscopic experiments, imide molecules located near the centre of the film exhibit poorer structure ordering than those near the surface. This reveals why the average film orientation decreases and the corresponding thermal expansion coefficient increases with increase of film thickness.

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Raman spectroscopy has been used to measure the molecular stress distribution in high-modulus, high-strength polyethylene fibres (Allied Spectra 1000 and Spectra 900) at low temperature and as a function of time during creep at room temperature. Increase in band width or change of band shape is interpreted as due to a distribution of stress or strain in molecules in the all-trans configuration. Fibres loaded at 77 K show two peaks in the stress distribution, as was found by van Eijk et al. at 240 K. About one-sixth of the all-trans molecules in a Spectra 1000 fibre as produced carry little load. The rest carry from half to twice the mean load. Spectra 1000 fibres loaded at over 1 GPa for a few minutes at room temperature show two well defined peaks in each of the C-C stretching Raman bands at 1064 and 1130 cm- 1 and thus a sharp bimodal distribution of stress. At long creep times, the higher stress peak increases in width while keeping the same area. Eventually, the stress distribution has a single peak with a broad tail on the high-stress side. This result unifies the previous two observations, one of which found only a single asymmetric peak and the other only two peaks in the stress distribution. As the tail broadens, the number of very highly strained molecules increases, and a significant number reach strains of 10%. It is not surprising that this leads to fracture of the fibre. After loading for less than a minute at room temperature or for a few minutes at 273 K, there is only a single peak in the stress distribution. The fibre structure is thus undergoing complex changes during creep, and the stress distribution seen at room temperature is not simply related to the original structure.

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The surface structure and bulk compositions of random copolymer systems based on poly (N-vinylpyridine) (N =2,4) and poly(N-vinylimidazole) copolymerized with polystyrene were examined by X-ray photoelectron spectroscopy (X.p.s.) or electron spectroscopy for chemical analysis, Fourier transform infra-red spectroscopy (FTi.r.) and static secondary ion mass spectrometry (s.i.m.s.). The spectroscopic results were correlated with morphology determined from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The X.p.s. results from solution-cast thick films and films cast at the air/water interface from poly(N-vinylpyridine-co-styrene) copolymers indicate equivalent surface and bulk compositions for all angle- and energy-dependent measurements, which are further supported by FTi.r. and EM analyses. On the other hand, surface enrichment of polystyrene was observed in angle-dependent X.p.s. analysis of poly(N-vinylimidazole-co-styrene) preparations. Attenuated total reflectance and transmission i,r. peak intensity ratios are equivalent, which suggests that segregation of polystyrene in the poly(N-vinylimidazole-co-styrene) system is localized to a region less than 30 A from the air/polymer interface. In addition, the s.i.m.s, technique was shown to be sensitive to structural differences arising from pH changes as well as to the detection of dimer ions in these systems.

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The relationship between structure and mechanical properties has been investigated for high-modulus fibres of poly(2,5(6)-benzoxazole) (ABPBO). It has been found that the modulus of the fibres increases from 91 to 133 GPa upon heat treatment at 525°C. The strain to failure decreases from about 4.3% to 2.0%, whereas the fibre strength remains virtually unchanged at about 2.1 GPa. The improvement in fibre modulus is accompanied by the development of a three-dimensional crystal order in the fibres. Molecular deformation in the fibres has been monitored using Raman microscopy and it has been shown that the position of the Raman bands in the fibres is sensitive to the application of mechanical stress. This has enabled a detailed study of molecular deformation in the fibres to be undertaken for both tensile and compressive deformation. It has been found that failure takes place at relatively low stresses and strains in compression through the formation of kink bands. It has also been shown that the fibres tend to have a lower modulus in compression than for tensile deformation. The molecular processes involved during fibre deformation have been discussed in detail.

Детальніше...  

Elongational flow techniques are applied to the examination of flow-induced chain scission of macromolecules in solution. An opposed-jets apparatus is used both to produce mechanical scission and to monitor the molecular-weight distribution of the scission products. Gel permeation chromatography is also used to complement the latter. In dilute solutions of almost monodisperse high-molecular-weight atactic polystyrene (aPS), degradation occurs as closely central scission of the molecules beyond a critical strain rate. As the concentration is increased to the semidilute regime, this preferential breaking near the centre of the molecule disappears (for 1.5% aPS/decalin solutions, Mp = 5.8 x 10 6 ). Therefore, according to expectation, the scission is randomized as the number of entanglements per chain is greatly increased. The results also indicate that, once entanglements are present, the parameter that seems to be most relevant to the process of molecular fracture is the applied stress.

Детальніше...  

The morphology of heterogeneous polystyrene/poly(methyl methacrylate) (PS/PMMA) blends has been modified by the addition of diblock copolymers of PS and PMMA and investigated as a function of block copolymer composition, molecular weight and concentration by optical and electron microscopies. A sharp decrease in dispersed phase dimension is observed with the addition of a few per cent of block copolymers having equal segmental mass (50/50 PS/PMMA), followed by a levelling off as the copolymer content is increased above the critical micelle concentration. For concentrations below the critical value, the particle size reduction is linear with copolymer volume fraction and agrees well with the predictions of Noolandi and Hong. The compatibilizing efficiency of the copolymer is strongly dependent on its composition and molecular weight. The effects of processing conditions, such as casting solvents and compression moulding, on the phase morphology have also been studied. Finally, the conformation of the copolymer at the interface is discussed with the help of models : calculations based on the experimental results indicate that the compatibilizer is neither located entirely at the interface, nor penetrates completely into the corresponding homopolymer phases.

Детальніше...  

Computer-generated atomic models are employed to evaluate the effect of disruptor units containing an m-phenylene ring on the modulus of aromatic polyester chains. The incorporation of isophthaloyl units results in quasiplanar chains which affect the chain modulus in a complex way. At a given concentration of disruptor units the chain modulus is strongly influenced by chain regularity and length. Chains containing m-phenylenedioxy and m-oxybenzoyl units cannot be represented by quasiplanar models and this implies a stronger detrimental effect of such units on chain modulus. The intuitive notion, employed in the design of thermotropic polymers, that the fewer disruptor units the better, is clearly a misconception.

Детальніше...  

The conformational analysis of some liquid crystalline polyesters with bulky side groups has been carried out by means of computer modelling. The possible packings of the macromolecular fragments with: overlapped tilted side groups are proposed on the basis of analysis of the conformational states and the: corresponding X-ray data. Some trends in the formation of ordered structures in the liquid crystalline state of the studied polyesters are discussed

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