1999 рік

The assisted photochemical conversion of precursor polymer synthesized via the procedure using tetrahydrothiophenium group is studied. particularly the influence of the substrate temperature and duration of the illumination period followed by optical absorption measurements. The optimal experimental conditions are obtained for a substrate temperature T = 110°C and an illumination period of at least 30 min. The characterization of the optimized photochemically converted poly(p-phenylene vinylene) [PPV] has been investigated by means of optical absorption, infrared absorption, X-ray diffraction and resonant Raman scattering spectroscopy. We find that optical and structural properties of the optimized photoconverted are rather close to those of standard PPV.

Детальніше...  

Diffusivity and solubility data for toluene and n-heptane in semi-crystalline polyethylene were obtained by gravimetric sorption experiments conducted in two different laboratories. The effects of temperature, concentration and degree of crystallinity on the diffusion and solubility behaviour were investigated at 70°C. The diffusivity data were correlated using a modification of the Vrentas–Duda free-volume theory. All the parameters in the free-volume model were estimated from pure component data except the size of the jumping units, the tortuosity, and the free-volume characteristics of the polymers. These results indicate that both the diffusivity and penetrant solubility in the polymer amorphous phase decrease with increasing crystallinity. Further, the tortuosity or the length of the diffusion path around the crystals increases with the degree of crystallinity.

Детальніше...  

The possibility of storage of melamine formaldehyde (MF) resin solutions at elevated temperatures is examined. A series of MF resin solutions with various formulations were synthesised and storage stability was studied at room temperature and 508C. In general, the stability at 508C was higher over a much broader range of reaction times when compared with storage stability at room temperature. The storage of MF resin solutions at elevated temperatures offers many advantages like the ease of resin production on large scale and high reproducibility of storage stability. Further, it is shown, by means of 13C-NMR and light scattering analysis, that the chemical structure and molecular weight are identical at the point of instability at 508C irrespective of the initial degree of condensation i.e. reaction time.

Детальніше...  

Deuterated and protonated end-functionalized polystyrenes of low and high molecular weight were grafted onto silicon substrates by solution-spincasting, followed by annealing and removal of ungrafted chains. The remaining layers were characterised by spectroscopic ellipsometry and atomic force microscopy. The effect of a variation of the initial film thickness, the annealing temperature and time on the resulting grafted layers was investigated. It was found that only the initial film thickness and deuteration had a marked effect on the layer morphology and thickness. Further improvement in reproducibility is needed to achieve real process control.

Детальніше...  

The melting and crystallization behaviour of metallocene-catalysed commercial poly(ethylene-co-octene) was investigated by conventional and temperature-modulated differential scanning calorimetry (d.s.c. and TMd.s.c.) and by temperature-resolved wide angle X-ray scattering. The thermal behaviour of the copolymer is characterized by a very broad melting and crystallization range. Both the melting and the crystallization are complex transitions of at least two processes with different kinetics. On decreasing the temperature, crystallization starts with the formation of a separate crystalline phase and continues gradually with development of an amorphous–crystalline mesophase until the glass transition is reached. In TMd.s.c., the formation of the crystalline phase is seen as an irreversible, non-reversing process, whereas the glass transition gives a reversing heat flow signal. Similarly, on heating, the melting process is initiated after traversing the glass transition as a largely reversing process, followed by melting of the crystalline phase with non-reversing character in a narrower temperature range.

Детальніше...  

The intensities and lifetimes of positrons annihilating in low and high density poly(ethylene) (LDPE and HDPE) were measured. As expected the orthopositronium (oPs) pick-off lifetime and intensity increases with temperature; the relative values of the intensity are consistent with differential scanning calorimetry (DSC) results. Two other components with shorter lifetimes were deconvoluted from the complex exponential decay profiles. Detailed studies were carried out on quenched and annealed samples of LDPE and HDPE in an attempt to correlate the two shorter components with crystallization; both CsF and plastic scintillators were used. DSC analyses were also carried out. We conclude that the shortest lifetime component is caused by a combination of parapositronium (pPs) decay and the annihilation of free positrons in the crystalline regions and that the second longest lived component can also be associated with crystalline regions especially the crystalline/amorphous boundary.

Детальніше...  

Poly(N-vinylcarbazole) (PVK) powder has been doped by bromine at room temperature and also at 370,420 and 470 K for 24-72 h. It is shown, by electron spin resonance (e.s.r.), optical absorption, X-ray photoelectron spectroscopy (XPS), that a charge transfer complex (CT complex) is formed between PVK and Br. However, some bromine is found to react with PVK, even after doping at room temperature, by addition or substitution, since not only bromine anion, which evidences the presence of the charge transfer complex, but also neutral bromine. has been detected by CPS. After heating, our results show that PVK is transformed into a new, more insoluble polymer. This polymer is amorphous, while some traces of carbazole and NHdBr are present in the powders. The increase of the conductivity is probably related to the formation of some rr bonds in the new amorphous polymer.

Детальніше...  

The state of miscibility and possible molecular aggregation in blends of semicrystalline syndiotactic polystyrene (s-PS) with poly(vinyl methyl ether) (PVME) were investigated by using differential scanning calorimetry (DSC), optical microscopy (OM) and scanning electron microscopy (SEM). The results are then directly compared to the classical amorphous PS/PVME blend prepared using exactly the same solvent and procedures. Similar to the classical amorphous a-PS/PVME, the semicrystalline s-PS/PVME exhibit the same scales of various molecular aggregation depending on composition leading to a so-called borderline miscibility, which is thoroughly discussed using the results of T g-composition relationship and morphology evidence. The behavior of s-PS/PVME agrees mostly with that of a-PS/PVME, indicating that tacticity or crystallinity in s-PS does not affect the phase behaviour of s-PS/PVME in the amorphous domain.

Детальніше...  

The influence of two natural terpene resins on the morphology, phase structure and isothermal crystallization process of iPP was investigated by optical (OM) and electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that in dependence on temperature, composition and chemical nature of the resin, one, two or three phases can be present. The isothermal spherulite growth rate and overall crystallization rate of the blends are depressed compared to plain iPP. This depression was attributed to the increase of the energies relative to the transport of macromolecules in the melt and to the formation of nuclei of critical size, following the addition of resin. The presence of resin seems to lead to the formation of more regular folding surfaces.

Детальніше...  

X-ray microdiffraction with synchrotron radiation has been used to investigate the strain-induced crystalline modification transition in the plastic zone of a /3-phase isotactic polypropylene (/3-iPP). It was shown that the bulk /3-iPP was gradually transformed into highly oriented. conformationally disordered (condis structure) α-phase iPP as the strain increased.

Детальніше...