1999 рік

The photopolymerization of a diacrylate monomer with an N-aliphaticmaleimide as the photoinitiator has been investigated by photodifferential scanning calorimetry (photo-DSC), real-time infrared (RTIR) spectroscopy, and real-time UV (RTUV) spectroscopy. Photo-DSC exotherm results for relatively thick films (,150 mm) showed an increase in the polymerization reactivity (R) up to a concentration of only about 2 mol% N-aliphaticmaleimide. RTIR analysis was used to determine that the reactivity of a diacrylate thin film (,10 mm) increased continually as the concentration of AcOEMI increased from 0.1 to 30 mol%. Both RTIR and RTUV spectroscopy indicated that essentially AcOEMI was completely consumed in the polymerization process, and colorless crosslinked acrylate films with little absorbance above 300 nm were produced.

Детальніше...  

The syndiotacticity-rich (syndiotactic diad content of 57-65%) high molecular weight (number-average degree of polymerization of 4500-20 000) poly(viny1 alcohol) (PVA) microfibrillar fiber was directly produced via the saponification reaction of poly(viny1 pivalate) (PVPi) by controlling structural factors such as molecular weight and stereoregularity of PVA chains, and appetence between PVA and solvent without existing spinning procedures. By examining and observing the changes of flow birefringence, shear viscosity of reaction solution, degree of saponification, and shape of reaction product during the saponification reaction, the following novel formation mechanism of high strength microfibrillar PVA fiber is proposed. As PVPi converts into syndiotactic PVA by means of saponification using saponifying agents containing water, an oriented gel structure appears first by the interaction between water and hydroxyl groups of syndiotactic PVA formed during the saponification reaction, and then the microfibrillar structure is formed by the collapse of water-PVA interbridges and the resultant chain packing between syndiotactic PVA molecules. The larger the syndiotacticities of both PVPi and PVA, the higher the molecular orientation and the smaller the hydroxyl groups in P(VPi-VA) copolymer needed for in situ fibrillation. The well oriented microfibrillar PVA fiber prepared by such a manner had irregular cross-sections, needle-point-like ends, accumulated ultrafine microfibril structure.

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The development of global orientation and morphological features in linear polyethylene crystallizing from a sheared melt are studied using in-situ time-resolving wide angle X-ray scattering (WAXS) and ex-situ transmission electron microscopy. It is found that samples subjected to a shear rate above a critical value of ⬃ 1 s ⫺1 result in macroscopically oriented structures in the crystallized sample. This critical shear rate appears to be independent of the differences in molecular weight distribution of the samples studied although the morphologies which develop are sensitive to quite small differences in molecular weight distributions. The presence of shish kebabs in the morphology is shown to differ markedly according to variations in the upper molecular weight fraction of the molecular weight distribution, even though the resulting global orientation does not. The WAXS also reveals that areas which evidence no row nucleated structures still realize high degrees of molecular orientation. It is proposed that the formation of shish kebab or lamellar morphologies in these samples is dependent on the critical density of contiguous elongated crystallization nuclei rather than any specific global criteria.

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The polymerisations of l- and dl-lactic acid anhydrosulfite were conducted at room temperature in tetrahydrofuran with butyllithium as initiator. The polyesters obtained were analysed by gel permeation chromatography, both 1H and 13C NMR spectroscopy and also characterised by DSC. In all polymerisations, l-monomers led to highly crystalline isotactic polylactic acid, while dl-monomers produced amorphous polymers with randomly distributed l- and d- lactic acid units.

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The solid-state elimination reaction in sodium chloroacetate was studied with in-situ combined small- and wide-angle X-ray scattering (SAXS-WAXS). This reaction leads to sodium chloride cubes (d = 1 pm) finely dispersed in a matrix of partially crystalline polyglycolide, poly(hydroxyacetic acid). The wide-angle data confirm a single-step reaction without detectable intermediate. The formation of the superstructure of polyglycolide (detectable by SAXS) is delayed compared to the formation of crystalline domains (detectable by WAXS). This is indicated by the intensity of the long period diffraction peak at s = 0.104 nm-1.

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The upper critical solution temperature (UCST) and lower critical solution temperature (LCST) have been predicted for 45 polymer/ solvent systems based on Patterson’s theory. The configurational energy and configurational heat capacity used in the theory are determined by the following thermodynamic relations ¹ Uconf: ¼ gV TV CP, conf: ¼ aP TgV V where the thermal expansion coefficient and thermal pressure coefficient are obtained from our empirical equations   T ¹T ln(aP T) ¹ 1 ¼ 1:1820 þ 0:8425 ln C T ln(gV =gVC ) ¼ 0:8452 þ 1:1324 ln(V=VC ) þ 2:8940[ln(V=VC )]2 where T C, V C and gVC are the critical temperature, critical volume and thermal pressure coefficient at the critical state. The deviations between predicted and experimental UCST and LCST are generally less than 20°C for 28 polymer/solvent systems. The molecular weight dependence of UCST and LCST is predicted for polystyrene (PS)/cyclohexane, PS/methyl cyclohexane and PS/toluene systems, and good agreement with experimental results is obtained.

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Plasma polymer coatings deposited from hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDSA) were monitored by XPS, FTIR and contact angle (CA) measurements as they aged in air after fabrication. Of particular interest was the influence of the monomer structure on the long term properties of the plasma deposited materials: in conventionally synthesized organosilicon materials, the siloxane unit provides very good long-term stability whereas the silazane structure is prone to hydrolytic attack. During plasma deposition of both coatings, abstraction of methyl groups was the major activation mechanism and the monomer structure was retained to a substantial extent. In the case of plasma polymerised (pp) HMDSA, however, other reactions such as Si–N bond cleavage resulted in considerably more structural diversity. During storage in air, the ppHMDSO film underwent minor chemical changes such as incorporation of additional siloxane crosslinks and a small extent of loss of methyl groups. The chemical structure of both the freshly deposited material and the aged coating were unusually homogeneous, compared with the broad range of chemical structures typically found in most other plasma polymers. The structure of the ppHMDSA coating, in contrast, changed dramatically on ageing: almost all silazane moieties were lost after one year and substantial amounts of oxygen incorporated, mainly in the form of siloxane links. In spite of the initial chemical differences, the two materials became more similar over time, with the final structures of the aged materials based on a cross-linked siloxane backbone. The wettability data reflected the structural differences between the two materials. However, correlations between structures and surface properties were not predictable. The overall wettability of these surfaces was determined by a complex balance of several factors such as chemical structure, topography and mobility.

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Blends of poly(vinylidene fluoride), PVDF, and poly(o-methoxyaniline), POMA doped with toluene sulfonic acid, TSA, were prepared by casting at various compositions and studied by scanning electron microscopy, X-ray diffraction and differential scanning calorimetry. The blend composition has a great influence on the morphology obtained. As the concentration of POMA-TSA is increased in the blend an interconnecting fibrillar-like morphology is formed and the spherulites characteristic of pure PVDF are destroyed. The variation of blend morphology is further discussed based on X-ray diffraction and differential scanning calorimetry analysis.

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The computational method described in this paper allows the calculation of the dielectric relaxation strength of an amorphous polymer based solely upon its chemical structure. The 4,4' oxydiphthalic anhydride (ODPA) dianhydride and bis-aminophenoxybenzene (APB) diamine based polyimides, (b-CN) APB–ODPA and APB–ODPA were studied. Amorphous cells were constructed and then poled using molecular dynamics. Dielectric relaxation strengths of De ¼ 17.8 for (b-CN) APB–ODPA and De ¼ 7.7 for APB–ODPA were predicted. These values are in excellent agreement with the experimental values. It was found that both the pendant nitrile dipole and the backbone anhydride residue dipole make significant contributions to the polyimides’ dielectric response. Specifically, it was shown that the difference in the magnitude of the dielectric relaxations is directly attributable to the nitrile dipole. The size of the relaxations indicate an absence of cooperative dipolar motions. The model was used to explain these results in terms of the average orientation of the nitrile and anhydride dipoles to within 51° and 63°, respectively, of the applied electric field.

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The change in mechanical properties induced by irradiation of electron (2 MeV), proton (10 and 20 MeV), He 21 (20 and 50 MeV), and 220 MeV C 51 ions were studied for polymer films of polyethylene (PE), bis-phenol A based polysulphone (PSF) and polyethersulphone (PES). There is no difference between the irradiations of proton and electron in the mechanical properties for PE compared by absorbed dose. On the contrary, in aromatic polymers as PES and PSF, the decrement in the tensile strength and elongation with dose is extremely depressed in the irradiation of ion. It was found from the measurements of gel fraction and glass transition temperature that the probability of crosslinking and/or chain scission depends on linear energy transfer (LET).

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