1999 рік

The FTIR-Raman spectra of polydiacetylenes with chiral pendent groups (–(CH2)nOCONHC *H(CH3)Ph; n ˆ 2,3,4,6 and 9; nR(S)MBU) were measured for samples as polymerised, cast from solution and precipitated on filter paper. Peaks due to the –CyC– and –CxC– backbone stretching vibrations occur above 1400 cm ⫺1. A series of peaks due to the interaction of backbone and –(CH2)n – vibrations, which reflect the structure of the alkyl chains in the pendent groups, lie between 700 and 1400 cm ⫺1. Comparison with the spectra of the related nbutoxy-carbonylmethylurethane sidegroup polymers (nBCMU) shows that, with the exception of 9R(S)MBU polymers, the alkyl chains of the nR(S)MBU polymers do not adopt a single conformation even in the as-polymerised samples. This is a consequence of the steric hindrance of the bulky chiral unit at the end of the pendent groups.

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Acrylic acid was isothermally polymerized by precipitation polymerization in toluene at various monomer and initiator concentrations over the temperature range of 408C–508C. 2,2 0 -azobis (2,4-dimethyl-valeronitrile) was employed as a chemical initiator. The rate of polymerization was found to depend on the monomer and initiator concentration to the 1.7th and 0.6th orders, respectively. The greater than first order dependence on monomer concentration signifies a secondary, monomer-enhanced, decomposition step for the initiator molecules. The approximately 0.5 order dependence of the rate of precipitation polymerization on the initiator concentration indicates that the chain termination process is predominantly bi-macromolecular. The temperature dependence on rate of precipitation polymerization obeys an Arrhenius equation with the activation energy found to be 21.49 kcal/mol. This result agrees well with the literature values of 23.5 and 22.0 kcal/mol obtained for the solution polymerization of partially neutralized acrylic acid (pH ˆ 11) and undissociated acrylic acid (pH ˆ 1), respectively. The weight average molecular weight of the polymer was found to be independent of conversion, indicating that transfer to the monomer is the chain controlling reaction.

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A new water soluble cellulose derivative 6-O-(2,3-bis(thiosulfato)propyl-oxy-2-hydroxy-propyl)-cellulose (TSHP) was synthesized by addition of tetrathionate to 6-O-(allyl-oxy-2-hydroxy-propyl)-cellulose (AHP). On gold surfaces, TSHP forms dense monolayers by chemisorption with a mean thickness of 4 ⫾ 1 nm. These monolayers were investigated by means of null-ellipsometry, AFM, FTIR-ERS, X-ray photoelectron spectroscopy and contact angle measurements. Based on the XPS results, it was proposed that the thiosulfato groups attached to the cellulose chains are homolytically cleaved and the resulting thio radicals bind covalently to the gold substrate. The formation of gold thiolates leads to a stable TSHP monolayer on the gold surface. The interaction between the TSHP films and fibrinogen and bovine serum albumin was also studied. The experimental results showed that the non-specific adsorption of these proteins was significantly reduced by the coverage of gold surfaces with TSHP.

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Melt-spun polypropylene fibres with various molecular weight distributions (MWDs) and processing parameters were subjected to stress relaxation tests at 23 and 100°C. The average level of the relaxation curves was mainly determined by processing conditions, but the MWD affected the relaxation kinetics substantially, especially at 100°C. In general, the relaxation rate was reduced by increasing the molecular orientation, increasing the degree of crystallinity, and broadening the MWD. However, the effect of the MWD was reversed for the initial relaxation at 100°C. MWD effects at 23 and 100°C were in accordance with relaxation spectra obtained by dynamic mechanical analysis. Stress plateaus at long relaxation times were observed for broad MWDs at 100°C. The presence of a crystalline structure seems to be a prerequisite for both the 'reversed’ MWD effect and the stress plateaus. The observations reported in this study can be interpreted in terms of molecular reptation, molecular constraints and structural relaxation.

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Although the kinetics of the precipitation polymerization of acrylic acid were investigated by a number of researchers, no mechanism has been proposed which considers the reaction in both continuous and dispersed phases. In this article a general mechanism is developed for the precipitation polymerization of acrylic acid in toluene. It is compared with experimental data and is found to predict rate of polymerization and molecular weight reasonably well. The mechanism consists of the initiation, propagation, transfer and termination reaction, which are common to all free-radical polymerizations. It also found that monomer-enhanced decomposition and transfer to monomer are the dominant mechanism for initiation and chain termination, respectively. The kinetic parameters were evaluated against experimental data.

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Fully rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA) exhibits a high modulus but is considered a useless polymer because of its high brittleness. In this study, the high brittleness was successfully healed with only a minor effect on the high modulus by incorporating short side groups, such as methyl, methoxy and trifluoromethyl, into the PDA unit of the polymer backbone. Furthermore, refractive indices, dielectric constants, and their anisotropies were all reduced by the incorporation of side groups. However, both in-plane thermal expansivity and thermal stability were relatively degraded by the incorporation of side groups. The polyimides exhibited relatively low packing coefficients because of the limited bulkiness of the side groups. They exhibited glass transition temperatures which are not expected for PMDA-PDA polyimide, indicating that the chain mobility was improved by incorporating side groups. With the improved chain mobility, molecular ordering was slightly improved, showing a bundle-like ordering without regular packing.

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Compatibilization of the immiscible polycarbonate (PC)/polyvinylidenefluoride (PVDF) pair by a third homopolymer, i.e. polymethylmethacrylate (PMMA), was studied in relation to phase morphology and mechanical properties of the polyblends. Scanning electron microscopy showed a more regular and finer phase dispersion when the original PMMA content in PVDF exceeded 20 wt.%. The premixing of PVDF with ca. 40 wt.% PMMA also had a beneficial effect on mechanical properties, such as ultimate tensile strength, elongation at break, and notched impact strength. All these experimental results are consistent with the interfacial activity of PMMA in the PC/PVDF blends.

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A dual platen hot-tool welding machine, in which the temperatures of the two hot-tool surfaces can be independently controlled, was used to study the weldability of bisphenol-A polycarbonate. In these experiments, the outflow in the melting phase was controlled by means of stops, the thickness of the molten film was controlled by the heating time, and the outflow during the final joining phase was also controlled by displacement stops. Strength data for butt welds are reported for a series of tests — both on dried and undried specimens — in which the hot-tool surface temperatures, the heating times, and the displacement stop positions were varied, but the pressure was not. Weld strength data are reported for three specimen thicknesses. It is shown that very high weld strengths can be achieved, even in the undried material.

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The compressional relaxation performance of poly(tetrafluoro ethylene) (PTFE) in the a-transition region has previously been analysed and systematized using a two-component (TC) model. To further validate the consistency of the model and of the properties of its parameters, tand is calculated for extended ranges of temperature (from ¹ 50 to 250°C) and frequency (from 10 ¹6 to 10 8 Hz) and compared to dynamic mechanical as well as dielectric data. Modifying the original temperature dependence of one of the parameters of the model, namely of the characteristic relaxation time, leads to good qualitative and quantitative agreement between experiments and predictions for the whole experimentally accessed frequency ( ⬇ 1 to 10 7 Hz) and temperature range (120–200°C) of the a-transition.

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Studies of the thermal properties and lamellar morphology of a highly structurally regular fraction of a Ziegler–Natta type isotactic poly(propylene) have been carried out. This fraction has an isotacticity content of mmmm ˆ 0:995 and a molar fraction of defects of 0.001. It is thus, among the most structurally regular isotactic poly(propylene) samples whose properties are reported. Differential scanning calorimetry as well as electron and optical microscopy were used to characterize the specimens. The fraction was crystallized from the melt over a very wide range of crystallization temperatures (135°C ⱕ Tc ⱕ 167°C). Monoclinic, a type crystals were formed over the whole crystallization range. The formation of cross-hatching, or lamellae branching, was also observed over the complete interval of crystallization temperatures. The formation of the tangential 'daughter’ lamellae at temperatures greater than 160°C can be attributed to the high structural regularity of the fraction studied. Relatively low crystallization temperatures (130°C to 150°C) show extended regions of woven lamellae having similar thicknesses with occasional groups of parallel long radiating lamellae. A morphology of rather thick long radiating lamellae and thin, transverse lamellae is formed at temperatures ⱖ 160°C. The angle between the daughter and mother lamellae of approximately 100° is in agreement with crystallographic predictions. The two endotherms that are observed by differential scanning calorimetry can be identified with the melting of the two distinct lamellae populations. It is consistent with the optical microscopy observations where a change in the sign of the birefringence is observed on the melting of the daughter lamellae. When formed at relatively high temperatures (Tc > 160°C) the mother lamellae subsequently melt at temperatures > 180°C.

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