1999 рік

Intermolecular complex formation between mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline (LC) polyimide (P11TPE) was studied by steady-state fluorescence and fluorescence lifetime measurements together with differential scanning calorimetry (d.s.c.) and wide-angle X-ray diffraction (WAXD). The P-11TPE shows various highly-ordered crystalline phases and a smectic phase during heating, according to the d.s.c. and WAXD results. By comparing the fluorescence spectra and fluorescence lifetimes between P-11TPE and its model compound (N,N 0 -diundecylter-phenyldiimide), the fluorescing complex at 438 nm of P-11TPE is confirmed to be an intermolecular ground-state complex between two terphenyldiimide moieties. The fluorescence peak wavelength of this intermolecular complex shifts slightly to shorter wavelength below the glass transition temperature and shifts abruptly to longer wavelength around crystallization, then shifts gradually to shorter wavelength again from various crystalline phases until the isotropic phase. The changes in fluorescence intensity and fluorescence lifetime around phase transition temperatures are also observed. These apparent changes in fluorescence behaviour indicate the differences in the nature and number of intermolecular complexes and radiationless deactivation processes in various phases. The characteristics of fluorescence behaviour for P-11TPE are also compared to those for various LC polyesters studied previously, and it is concluded that the red shift of fluorescnece is due to the apparent change in molecular arrangement between mesogenic moieties and the blue shift of fluorescence is due to the almost identical packing pattern with increasing thermal fluctuation during phase transition for liquid crystal polymers. By using fluorescence technique, molecular interaction between mesogenic moieties of liquid crystal polymer has been proved to show various patterns of changes during phase transition processes.

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The stress vs. strain curve with yield and the sonic modulus vs. strain curve of fibres of linear polymers below the glass transition temperature are modelled by the continuous chain model in combination with a simple yield model. It is supposed that yield of polymer fibres is due to shear deformation of the domains. The yield model is based on a critical shear yield strain. A good agreement between the experimental tensile and sonic modulus vs. strain curves and the theoretical curves has been obtained for a selection of poly(p-phenylene terephthalamide) and poly(ethylene terephthalate) fibres.

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The phase behaviour, crystallization kinetics and tensile properties of poly(b-hydroxybutyrate)–cellulose propionate (PHB–CP) blends have been investigated using differential scanning calorimetry, optical microscopy and tensile testing measurements. The results show that the blends are miscible as evidenced by the observation of a single composition-dependent glass transition temperature, a depression of the equilibrium melting temperature of PHB, and a decrease in the spherulitic growth rate of the PHB component. Finally, in the study of the tensile properties it is shown that an improvement in the ductility of PHB is brought about by blending with CP.

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The thermal transitions of a poly(ethylene terephthalate)/carbon dioxide (PET/CO2) system were investigated by using a differential scanning calorimeter accessorized with a high-pressure DSC cell. It was found that the glass transition temperature of PET decreases with an increase in the CO2 pressure due to the plasticization effect, which is quite noticeable even at rather low CO2 pressures. The sorbed CO2 enhances the mobility of the chain segments and depresses the crystallization temperature of the PET. The CO2-induced crystallization of PET at high pressure is attributed mainly to the plasticization effect, which causes a lower Tg than room temperature for PET, and hence crystallization of PET can occur at room temperature. The sorbed CO2 was also found to be able to induce the crystallization of PET at temperatures lower than the glass transition temperature of PET. The results of high-pressure DSC were supported by measurements of wideangle X-ray diffraction (WAXD).

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The phase diagram and Curie transition behavior in binary crystalline polymer blends of poly(vinylidene fluoride-trifluoethylene) P(VDF/ TrFE) and poly(1,4-butylene adipate) (PBA) were investigated using differential scanning calorimetry, light scattering, and infrared spectroscopy. In order of descending temperature, the phase diagram exhibits a lower critical solution temperature (LCST), two crystal melting transitions, a paraelectric-ferroelectric phase transition, and a single glass transition. Liquid–liquid phase separation is observed at approximately 508C higher than the melting points of P(VDF/TrFE) in the blends with PBA. The cloud point phase diagram is thermally reversible as typical for a true LCST. Increasing PBA concentration results in depression of the melting point of P(VDF/TrFE) in the blends. The analysis based on the melting point depression of P(VDF/TrFE) and PBA gives x 12 ˆ 20.592 at 1608C and 20.071 at 618C, respectively. Based on the shift of the C ˆ O absorption peak, the origin of miscibility in this blend was attributed to specific interactions between the CH2 and/or CF2 sites of P(VDF/TrFE) and the C ˆ O groups of PBA. The isothermal and non-isothermal crystallization behavior of P(VDF/TrFE) in the blends was analyzed in relation to the behavior of the ferroelectric-paraelectric phase transition, often known as the 'Curie transition’. It was found that the addition of PBA (up to 70 wt%) increases the enthalpy of the Curie transition as well as the temperature of the Curie transition. Of particular interest is the possible formation of a ferroelectric phase directly from the melt without passing through the paraelectric phase in the high PBA compositions.

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The nucleophilic substitution reaction on poly(vinyl chloride) (PVC) with sodium 2-mercaptobenzothiazolate has been carried out in solution in cyclohexanone at different temperatures. The kinetic parameters indicate the occurrence of one process having an activation energy of 63 KJ.mol ¹1. The evolution of unreacted iso, hetero and syndiotactic triad contents with degree of substitution has been followed by 13C n.m.r. spectroscopy. By comparing quantitatively the microstructural changes with degree of substitution and taking into account that the reaction is of S N2 type only the mechanisms of substitution through the mm triad of mmr tetrad and the rm of rrmr pentads are proved to react. On the contrary, owing to steric hindrance the reaction by the mr triad in the mmr tetrad is quite unlikely, thus enhancing the stereoselective nature of the reaction. This conclusion was confirmed on the basis of the FTi.r. results and is more feasible than using other nucleophiles to study the effect of the aforementioned structures on the polymer properties and therefore provides new approaches to better comprehension at the molecular level of the structure–property relationships. These novel contributions are an original approach to the prominent role of the microstructure-dependent local conformation in the mechanisms of analogous reactions of PVC.

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A series of liquid-crystalline copolymers poly{6-[4-(4-hexyloxy benzoyloxy)phenoxy]hexyl methacrylate}-co-{6-[4-(S-2-methyl-1-butyloxycarbonylphenylazo)phenoxy]hexyl methacrylate} with an azobenzene moiety as photoreactive mesogenic unit were prepared. Their liquid crystalline properties and photoinduced anisotropy were investigated. The results showed that these polymers exhibited layered smectic phases and showed good photoinduced anisotropy both in the glass states and in the mesophase states.

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The rheological behavior of the room temperature vulcanized polydimethylsiloxane (precursor) with tetraethylorthosilicate (crosslinker) was studied. Data collected from a constant stress rheometer showed that, during the vulcanization, the phase angle increased until reaching the gel point where the elasticity predominated over the viscosity causing a drastic decrease. At a higher ratio of crosslinker to precursor, the vulcanization accelerated and the gel point was reached sooner, thus resulting in a shorter gel time. However, data collected from our extraction experiments indicated that the degree of completion for vulcanization was actually lowered because of an increased amount of sol species, thus necessitating a longer cycle time for a complete cure. The validity of this observation was corroborated by comparing the data collected from the extraction method against those collected from two independent thermal analytical methods, namely the heat capacity method and the cold crystallization method. The degree of completion for vulcanization at each reaction instant determined using these three methods were in excellent agreement with each other. In addition, the presence of residue sol species in the vulcanized products caused a slight decrease in the thermal stability.

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Eight amorphous, thermoplastic aromatic poly(ether)s and poly(ether ketone)s, some containing fluorine, were investigated by dielectric relaxation spectroscopy over the frequency range 20–10 5 Hz and the temperature range ¹130–300°C. For the polymers containing perfluorophenylene units, three relation processes were observed; one corresponding to the glass transition (a-relaxation), and a further two subT g secondary process (b- and g-processes). For polymers without perfluorophenylene units, a b-process was not detected. The sub-T g transitions followed Arrhenius behaviour and were sensitive to polymer structure and chain flexibility. The sub-ambient relaxation exhibited a strong dependence on absorbed moisture. The polymers containing perfluorophenylene units showed a significant decrease in dielectric permittivity at 100 kHz and this was attributed mainly to a reduced electronic polarisability. The a-relaxation strength of the perfluorinated polymers, which arises from dipolar motions, was also reduced.

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Multiblock copoly(oxybenzoate-p-ethylene terephthalate) (POB-PET) was synthesized by a direct reaction of poly(hydroxybenzoic acid) (PHBA), acetoxybenzoic acid and poly(ethylene terephthalate). The synthesized multiblock POB-PET becomes more blocky and less soluble in the mixed solvent of trifluoroacetic acid and chloroform when the degree of polymerization of PHBA used in the synthesis is higher as evidenced in the solid-state and liquid-state nuclear magnetic resonance study. The thermal curves displayed by the synthesized multiblock 20/80 and 40/60 POB-PET contained distinctive contributions from both PET blocks and POB blocks in the copolymers when the degree of polymerization of PHBA used in the synthesis is seven. Only 20 mol% of POB is needed to form liquid crystalline phase in these multiblock POB-PET as opposed to a minimum of 40 mol% of POB is required to form liquid crystalline phases for random POB-PET.

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