1999 рік

Two analogous series of thermotropic main-chain polyesters, poly(alkyl-4,4’-diphenoxy terephtbalate)s and poly(alky1 terephthaloyl-bis4-oxybenzoate)s, have been analyzed by dielectric techniques. Analysis of the dielectric properties as a function of temperature and frequency show three main processes, termed (α, /3 and /3*. The cx transition has been associated with the glass transition of the amorphous phase. The low-temperature processes /3 and /3* have been related to the local motion of internal and external polar groups in the mesogenic unit. The activation energy and dielectric strength values associated with these relaxations have been determined, and a very important effect of the crystalline order on these parameters has been observed. The effect of the thermal history on the dielectric behaviour has also been studied

Детальніше...  

The applicability of the thermally induced phase separation (TIPS) process to the production of anisotropic membranes was investigated. To induce an anisotropic structure, diluent was evaporated from one side of the polymer–diluent melt-blended, thereby creating a concentration gradient in the nascent membrane prior to inducing phase separation. The system used to prepare these membranes was isotactic polypropylene (iPP) in diphenyl ether. The resulting membrane structures showed that this evaporation process was useful in producing anisotropic structures. The effects of evaporation time and initial polymer concentration on the anisotropic membrane structure were investigated. The evaporation process was analysed by solving appropriate mass transfer and heat transfer equations. The agreement between the calculated results and the experimental data on the membrane weight loss and the membrane thickness was satisfactory. The membrane structures are discussed in detail based on the calculated polymer volume fraction profiles in the membranes.

Детальніше...  

The nature of secondary crystallization in poly(ethylene terephthalate) (PET) was examined during isothermal crystallization and subsequent melting by time-resolved synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) techniques. In one experiment, the process of isothermal crystallization was sustained over 72 h to induce a relatively large crystallinity (46%, by weight). The purpose of this experiment was to resolve the issue of controversial assignment for the crystal lamellar thickness (lc) by the correlation function analysis of the SAXS data. Results suggest that a two-stage decrease mechanism exists in both long period (L) and lc during isothermal crystallization: (1) a significant decrease in the initial stage (primary crystallization dominant), and (2) a much slower decrease in the later stage (secondary crystallization dominant) that is nearly linear with log time. We attribute this behavior to the formation of thinner separate stacks of lamellae between the primary stacks by secondary crystallization. Both secondary and primary stacks can undergo a great deal of crystal perfection and rearrangement with time. From DSC measurements, a triplemelting behavior was observed in the samples crystallized at 205 and 215°C for 1 h, and a double-melting behavior at higher temperatures of 225 and 231°C for 2 h. Temperature scanning SAXS and MDSC directly characterize aspects of crystal perfection and melting. Consistent with some of the literature, we confirm that for short annealing ( ⬃ hour) at 200–220°C, the first (low) endotherm is related to melting of secondary crystals, the middle endotherm is due to melting of primary crystals, and the third endotherm is due to melting of crystals reorganized during heating. With prolonged crystallization at 231°C for 24 and 72 h, a single higher melting endotherm was observed even though SAXS experiments indicate a slight decrease in average lamellar thickness. In PET, ester exchange reactions contribute to unusual high mobility, allowing chains to avoid topological constraints such as entanglements and tie chains. The results suggest that the change in population of tie molecules in the non-crystalline phase reduces the entropy of melting causing an increase in Tm, and that this overwhelms the contribution of the decrease in lc.

Детальніше...  

Employing 13C CP/MAS NMR spectroscopy, the existence of immobile regions in natural rubber (cis-1,4-polyisoprene) corresponding to a few percent of the monomer units has been detected at ambient temperature. For synthetic rubbers no immobile regions have been detected at all. Applying different physical and chemical treatments to natural rubber it is shown that mastication, g-irradiation, and increasing the temperature, slightly above the ambient, reduce the amount of immobile regions.

Детальніше...  

This theoretical work evaluates the errors in average molecular weights due to instrumental broadening when a size exclusion chromatograph is fitted with ideal on-line Mn orMw, sensors, and a correction method is proposed to compensate for such errors. The basic assumptions are that linear homopolymers are analyzed, and that the instrumental broadening is uniform. It was verified that an ideal molar mass detector systematically underestimates the polydispersity and that such bias may be simply obtained from the spreading function polydispersity. The correction method uses an estimate of the instantaneous polydispersity. Such function can be directly obtained from the spreading function alone, since it is proven to be little dependent on the shape of the analyzed MWD

Детальніше...  

Poly(l-lactic acid)(PLLA) was blended with poly(ethylene-co-vinyl acetate)(EVA) and the miscibility of the blend was investigated by DSC, polarizing optical microscopy and rheometry. The blend of PLLA with EVA70, that contained 70 wt% of vinyl acetate, was immiscible because the T g and the spherulitic growth rate of the blend were nearly constant regardless of the change in the blend composition. On the other hand, the T g, equilibrium melting temperature, the spherulitic growth rate during the isothermal crystallization of the PLLA–EVA85 blend were decreased with the increase in the EVA content. Moreover, the Flory–Huggins interaction parameter of the blend was found to be negative, which clearly means the PLLA–EVA85 blend was miscible. The tensile strength and modulus of the PLLA–EVA85 blend were dropped rapidly, followed by a more gradual decrease with the increase in the EVA85 content. Strain-at-break, however, was increased rather slowly up to 70 wt% of EVA85 and then increased quite rapidly around 90 wt% of EVA85.

Детальніше...  

Temperature dependence of the elastic modulus El of the crystalline regions of poly(p-phenylene benzobisthiazole) (PBZT) in the direction parallel to the chain axis was investigated by X-ray diffraction over a temperature range from ⫺ 180 to 400°C. The El value of PBZT was 372 GPa at room temperature, which was the highest El value ever investigated. This value coincided with that reported by Lenhert and Adams (395 GPa) [Lenhert PG, Adams WW, Mat. Res. Soc. Symp. Proc. 1989; 134: 329] within experimental error. The axial chain thermal expansion coefficient was constant at ⫺ 2.9 × 10 ⫺6 K ⫺1 from 20 to 445°C, and correspondingly the El value remained unchanged up to 400°C. Moreover, the El value at ⫺ 185°C also coincided with that at room temperature. It is considered that the deformation mechanism remains unchanged over a wide temperature range and the skeletal conformation of PBZT in the crystalline regions is stable against the heat.

Детальніше...  

Polyelectrolyte complexes of calf-thymus DNA and polycations, such as PDADMAC, IONENE, and P4VP were formed and analysed with respect to their thermodynamical stability. The UV-VIS spectrographic melting curves yield two denaturation temperatures, Tm;1 and Tm;2 . This is explained as follows: DNA molecules contain two kinds of repeat unit sequences. There are sequences that are not complexed, they denature at Tm;1 ; the other DNA repeat units are complexed with polycation repeat units, they denature at Tm;2 . Thereby, Tm;2 is much larger than Tm;1 . That is, polycation binding stabilises calf-thymus DNA. Surprisingly, the absolute value of Tm;1 depends neither on the degree of complexation nor on the polycation type. Tm;1 agrees quite well with the denaturation temperature of the pure, uncomplexed DNA. Also Tm;2 does depend not on the degree of polycation complexation, but on the polycation type. One observes Tm;2 PDADMAC† . Tm;2 P4VP† . Tm;2 IONENE†: The probable reason for this series is the polycation molar mass. Tm;2 increases slightly as Mw is increased. While Tm;1 depends on the concentration of the added NaCl-salt, Tm;2 does not. This is explained using the fact that the complexed DNA repeat units are electrically neutral, so that there are no salt/charge interactions. Measurements in aqueous/organic solvent mixtures show that both Tm;1 as well Tm;2 decrease continually as the content of the organic solvent is increased. Very marked is this effect for water/N-methylformamide. At wNMF < 40%, Tm;1 and Tm;2 are nearly half as large as in pure water.

Детальніше...  

A new 2,2’-bis(furyl)benzidine (FurylBZ) with a well-defined conjugation length was synthesized, and its polyimide was prepared through thermal imidization of the poly(amide acid) obtained from the polymerization with pyromellitic dianhydride. FurylBZ monomer and its polyimide film were characterized to exhibit intense blue light emission with a relatively narrow peak width in the photoluminescence. The relative fluorescence quantum yield was measured at room temperature to be 0.52 for FurylBZ in 1,Cdioxane and 0.074 for the polyimide in films. The polyimide was thermally stable up to 400°C. The polyimide is amorphous, due to the bulky fury1 side groups. However, the polyimide chains were favorably aligned in the film plane due to the rod-like chain characteristic

Детальніше...  

The conformation of substituted poly(p-phenylene terephthalate)s has been studied with molecular mechanics and Metropolis Monte Carlo methods. It was shown that substitution lowers the trans–gauche energy difference of the most important dihedral angle of the backbone more when the substituent is attached to the hydroquinone residue (HQ) than when it is attached to the terephthalic acid residue (TA). The substitution also lowers the trans–gauche energy barriers for both HQ and TA, but the gauche states are more stable in HQ. The trans region is wider for HQ compared with the unsubstituted monomer and TA. The mean squared radius of gyration, the mean squared end-to-end distance and the persistence length were calculated with the Metropolis Monte Carlo method for polymers with octyloxy side chains, when the side chain was attached either to the hydroquinone unit (PHQ8) or to the terephthalic acid unit (PTA8). In PHQ8 the side chains are oriented mainly parallel to the backbone; in PTA8 they are oriented more randomly. The main chain of PTA8 is more extended than that of PHQ8, owing to the wider trans minimum in PHQ8 than in PTA8.

Детальніше...