1999 рік

By using fluorescence spectroscopy together with d.s.c. and WAXD, structure analysis for main-chain thermotropic LC polyesters (PB-n), containing n ¼ 7–18 methylene units, was carried out. The strong excitation-wavelength dependence of fluorescence and red shift of fluorescence peak wavelengths excited at various wavelengths during heating for all PB-n were found to depend on different degrees of overlap between mesogenic biphenyl moieties. The red shifts for fluorescence excited at 320 nm from room temperature to crystalline–S H transition temperature for PB-16 (26 nm) and PB-18 (36 nm) are larger than those for PB-8 (14 nm) and PB-10 (16 nm), owing to the mobile packing structure in crystalline phase for PB-16 and PB-18 with flexible and longer spacer lengths. According to the much longer average fluorescence peak wavelengths for PB-16 (399 nm) and PB-18 (401 nm) in S H phase than those for PB-8 (381 nm) and PB-10 (385 nm), the degree of overlap between two mesogenic biphenyl moieties in S H phase for PB-n was proved to decrease and the interaction between a carbonyl group and a phenyl ring increased with its increasing methylene units. Fluorescence spectroscopy was demonstrated to be effective in proving the difference in molecular interaction between mesogenic moieties for LCP with various spacer lengths.

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Electron-beam (EB) curing of two epoxy resins, one acrylated and one methacrylated, has been investigated. The change in thermomechanical properties, such as Tg, and the change of residual unsaturation have been studied as a function of dose. These results, in combination with in situ measurements of the temperature during cure, have shown the importance of sample geometry for the final properties of the thermoset. The thermal history of the sample during cure greatly affects the properties of the cured resin.

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The effect of the polystyrene chain as a bulky long ester group of methacrylate monomer on the chain tacticity was investigated in the radical chain polymerization of w-methacryloyloxyethyl polystyrene macromonomers of different molecular weights. The macromonomers and 3-phenylbutyl methacrylate (SPBMA), which corresponds to the unimer model of the macromonomer, were synthesized and polymerized with azobisisobutylonitrile in benzene at 60°C. The polymers obtained were converted to poly(methy1 methacrylate)s by acid hydrolysis reaction followed by methyl esterification with diazomethane to estimate the chain tacticity of the original polymers by 'H- and '“C-n.m.r. It was found that the backbone chains of poly(macromonomer)s as well as the poly(3PBMA) chain had a syndiotactic triad in the range of X4-62%, but the chains still had a similar tactic structure comparable with those of poly(methacrylate)s having a short alkyl chain and the fractions of tactic triads obeyed the Bemoullian statistics. This fact indicates that the specific multibranched structure around the propagating radicals as well as the ester polymer chain of the macromonomer does not have much effect on the chain tacticity, although the molecular conformation of the central backbone chain of the formed poly(macromonomer)s is strongly affected by the polystyrene branch chains of high branch density, as described previously.

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The possibility of using various types of non-covalent cross-linking, to promote the structure of interpenetrating networks in blends based on side-chain liquid crystalline polymers, was investigated. The polymers used included: (1) random copolymers of liquid crystalline polyacrylates containing benzoic acid units for hydrogen-bond cross-linking through the formation of intermolecular dimers; (2) liquid crystalline polyacrylate-ionomers bearing sodium salt groups for cross-linking through ionic aggregates; and (3) a styrene–butadiene– styrene triblock copolymer having a network structure with polystyrene microdomains as cross-links. The preparation method consists of making a dilute solution of two polymers, then evaporating the solvent and drying the mixture. No evidence of network interpenetrating was observed. The results suggest, however, that blends composed of two components having a network structure, for both H-bond/ionicaggregate and H-bond/triblock-copolymer combinations, have a greater miscibility, compared with blends containing only one component capable of forming a network structure through non-covalent cross-linking. The improved miscibility is characterized by reduced domain sizes and, for blends of two liquid crystalline polymers, changes in the phase transition behavior.

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Recent investigations on elastic behaviour, using uniaxial compression measurements of the swelling degree of poly(dimethylsiloxane) rubber (PDMS) networks lead to the assumption that a transition from the affine to the free-fluctuating phantom network occurs. To study the swelling process in different solvents, PDMS networks with different crosslink densities, molecular weight distributions and memory terms were synthesized. Additionally, another fast method for network characterization is given by the analysis of the transversal nmr relaxation. For elastomer networks, well above the glass transition temperature Tg, the presence of topological constraints and permanent crosslinks leads to a non-zero average of homo- and hetero-nuclear interactions, which results in a solid-like relaxation behaviour. The shape of the decay of proton transversal magnetization, using the Hahn echo sequence in the dry PDMS network, is related to the crosslink density. The results of the nmr experiments are compared with compression measurements on dry and swollen networks.

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Various electron donors and electron acceptors have been tested in order to examine the possibility of the application of the Marcus equation to the description of the kinetics of free radical polymerization photoinitiated via photoinduced electron transfer (PET). Variations of the driving force of the electron transfer, for selected dyes (xanthene dyes, camphorquinone), were introduced by using: a series of tertiary aromatic amines (TAAs) and N-phenylglycine derivatives; a series of electron acceptors and only one type of electron donor (cyanine cations with tetraorganylborate anion); a mixed system with a series of benzophenones as electron acceptors and TAAs as electron donors. Several important conclusions follow from the experimental data. (1) For the case with the rate of PET much lower than the rate of diffusioncontrolled processes, the Marcus theory can be used for analysing or predicting the ability of organic redox systems for light-induced free radical polymerization. (2) For a process controlled by diffusion, the reactivity of free radicals formed as a result of PET limits the rate of initiation of polymerization. It is shown that this relationship can be also presented as a function of thermodynamic driving forces of the photoredox reaction ( - Go). For this type of process, the relationship between the rate of polymerization and -AG” is linear, indicating the 'inverted-region-like’ kinetic behaviour. Additionally, it is shown that the bleaching process of the dyes competes with the polymerization photoinitiation in a way that suggests that, after free radical formation, two parallel reactions occur and that the rate of bleaching can be expressed as a fraction of the total rate of electron transfer.

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Three measurement protocols were performed to investigate the temperature dependence of positron lifetime spectra for polystyrene over the temperature range ¹ 70 to 180°C. The polystyrene samples were first subjected to a heating schedule in sequential 10°C increments. This was followed by a cooling schedule, again in 10°C increments. The third schedule involved rejuvenation of the specimen above T g between each measurement. The measured lifetime spectra were resolved into three components. We observed a minimum in the temperature dependence of the o-Ps intensity I 3 at ⬃20°C for polystyrene specimens subjected to the heating and cooling schedules, but not for the rejuvenated samples. The influence of the duration of e þ-irradiation on the lifetime and intensity of o-Ps annihilation was investigated under isothermal conditions. A decrease in o-Ps intensity I 3 with duration of exposure to e þ-irradiation was found whose rate was maximal at 23°C. The results suggest that the time dependence of the o-Ps intensity in the glass can be interpreted within the framework of the spur-model. Specifically, the variation of I 3 is ascribed to the formation at low temperature of free radicals with small electron affinities. After correcting for the e þ-irradiation effect, no significant aging effect in the glassy state was detected in the o-Ps intensity and lifetime.

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Crystalline PVDF was precipitated, respectively, from water/DMF and 1-octanol/DMF solutions to produce membranes with asymmetric and uniform morphologies. The formation mechanisms of these specific structures were described both in the aspects of thermodynamics (equilibrium phase behavior) and the kinetics (diffusion trajectory). The phase diagrams of the investigated systems indicated the possibility of liquid–liquid demixing or crystallization or both during the immersion-precipitation process. The sequence of these phase separation events, which determined the ultimate membrane structure, was attributed to the kinetic factors. Into this context, a quantitative model describing the immersion-precipitation process was considered. The calculated diffusion trajectory and concentration distribution in the nascent membrane were found to be consistent with the experimental membrane morphologies. Moreover, the precipitation rate of the PVDF solution in water and 1-octanol were examined by the light transmission experiments. The latter results further confirmed the validity of the mass transfer calculations.

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Copolyesters (V,-V, and VI,-VZ,J derived from 2-(6-oxido-6H-dibenz(c,exl,2)oxaphosphorin-6-yl)-l,4-bis(2-hydroxyethoxy) phenylene (II) and bis(Z-hydroxyethyl) naphthalate (ZZZ)or bis(4-hydroxybutyl) naphthalate (Iv) were synthesized and characterized. Copolyesters V,-V,, VI.-VII had been characterized by inherent viscosity, solubilities, t.g.a., d.s.c., X-ray diffraction, 'H n.m.r., 31P n.m.r., UL-94 and d.m.a. D.s.c. and X-ray diffraction show that the VI series of copolyesters were crystalline polymers and the crystallinity decreased with the increase in ZZcontent, whereas the V series copolyesters were amorphous polymers. The inherent viscosity of these copolyesters ranged from 0.53 to 0.68 dl g-'. The T, values of the V series copolyesters ranged from 118 to 126°C and those of the VI series copolyesters were in the range 68-77°C; these were higher than the Ts values of polyethylene naphthalate (PEN, Ta: 116°C) and polybutylene naphthalate (PBN, Tg: 67°C). The Td values (10%) for the V series of copolyesters in nitrogen ranged from 450 to 470°C and those for the VI series copolyesters were in the range 410-415°C; these values were also higher than that of PEN (Td: 449°C) and PBN (Td: 410°C). UL-94 V-O grade could be achieved with a phosphorus content as low as 0.48 wt% for the V series and 0.97 wt% for the VI series copolyesters. T.g.a. kinetics also indicated that the V and VI series copolyesters had higher activation energies of degradation than did PEN and PBN. When these copolyphosphates were treated with fire for 10 s, intumescence is observed on the surface of copolyphosphates which retards further flaming of esters have good mechanical properties (G’: lO8-lo9 Pa) before glass transition the polymers. DMA showed that all copolyphosphate temperatures.

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The interactions between hexamethylenetetramine (HMTA) and models for both poly(N-(hydroxyphenyl) maleimides) and phenol-formaldehyde resins have been examined. There is direct interaction between HMTA and the N-(hydroxyphenyl) succinimide and xylenols used as model compounds. When the model compounds both contain a free ortho position, four different benzoxazine derivatives are formed, that is, the succinimide–succinimide and xylenol–xylenol benzoxazines and the two 'mixed’ systems. The rate of reaction of HMTA with the mixed system was measured via 13C nuclear magnetic resonance spectroscopy (n.m.r.). The relative rates of formation of the benzoxazine derivatives are dependent on the substitution pattern of the phenolic rings. Control over the interactions between HMTA and the models was achieved via selective masking of the phenolic groups using the thermally labile tetrahydropyranyl (THP) group.

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