1999 рік

Thermoplastic hydrogels of alternating multiblock copolymers, consisting of poly(ethylene glycol) (PEG) and poly(l-lactic acid) (PLLA), were synthesized. Dicarboxylated oligomeric PLLAs were synthesized by the condensation reaction of l-lactic acid in the presence of succinic acid. Changing the feed ratio of l-lactic acid to succinic acid controlled PLLA molecular weights. Alternating multiblock copolymers with different block lengths of PEG and PLLA were obtained from the polycondensation reaction between PEG and dicarboxylated-PLLA in the presence of dicyclohexyl carbodiimide and N-dimethyl aminopyridine as catalysts. The chemical compositions of dicarboxylated PLLAs and multiblock copolymers were verified by 1H-NMR and FT-IR, and the molecular weight and distribution were measured by gel permeation chromatography. DSC thermograms showed that there existed a high degree of phase mixing, as well as microphase separation in the multiblock copolymer, demonstrated by a large Tg peak from the amorphous phase-mixing domain and a small Tm peak from crystalline microdomains of the PLLA component. The block copolymers with low molecular weights were water-soluble and clouded at a temperature which is associated with a lower critical solution temperature (LCST) phenomonon. However, the high molecular weight polymers could swell in water and their optical transparency was influenced by temperature. This observation can be attributed to an enhanced hydrophobic interaction between hydrophobic moieties in the polymer chains, caused by an increase in temperature. The weight swelling ratio (absorbed water/polymer) of the polymers was controlled by polymer composition and molecular weight. These block copolymers may offer potential for applications in drug delivery and various other biomedical projects.

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The effect of peel rate and temperature on peel toughness and delamination failure mode of coextruded microlayer sheet consisting of alternating layers of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) was studied with the T-peel test. Microlayers with thin (<1.5 mm) SAN layers always failed by interfacial delamination and consequently the interfacial toughness was independent of rate and temperature. In microlayers with thicker SAN layers, the crack alternately propagated through crazes in the SAN and along a PC–SAN interface at intermediate peel rates. The length of the crack-tip zone, the number of crazes and the amount of craze fracture increased with increasing peel rate; the peel toughness increased accordingly. Extending the tests through eight decades in peel rate revealed a transition from craze-dominated cohesive fracture at intermediate rates to predominately interfacial fracture at either very high or very low peel rates. Transitional peel behaviour resulted from the rate sensitivity of yielding and crazing in SAN as determined from tensile tests of bulk polymer.

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The structure and ferroelectric phase transition behavior were investigated for heat-treated samples of vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymers with 73 and 65% VDF molar contents. The temperature of the transition from the ferroelectric to the paraelectric phases increased when the samples were annealed in the temperature region of the above phase transition. The domain size, evaluated from the integrated width of the X-ray reflections, also increased drastically at these annealing temperatures. As the annealing temperature increased above the transition point, the resulting sample showed a reduction of the transition temperature. In this case, the domain size decreased slightly and the trans sequential length in the crystalline domain decreased, corresponding to structural disorder due to the invasion of gauche bonds into the trans zigzag chains. In the case of the 65% VDF sample, annealing in the phase transition temperature region resulted in a tilting phenomenon of chains in the crystalline region, but this phenomenon was not detected for the 73% VDF sample.

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A series of polyimide-based second-order nonlinear optical (NLO) materials were synthesized from poly(hydroxy-imide)s containing alicyclic units, followed by the Mitsunobu reaction with NLO chromophores. The resulting polymers were highly soluble in aprotic polar solvents such as DMF, DMAc, NMP, etc. The NLO polyimides exhibited the inherent viscosity range of 0.22–0.50 dL/g. Molecular structural characterization for the resulting polymers was achieved by 1H-NMR, FT-IR, and UV-visible spectroscopies. The glass transition temperature for the resulting NLO polyimides was in the range of 1618C–2338C and most of them showed high thermal stability. The polymer solutions could be spin coated on the indium–tin-oxide (ITO) glass or quartz disc substrates to form the optical quality thin films. The electro-optic coefficients (r33) at the wavelength of 1.3 mm for polymer thin films poled around the glass transition temperature were in the range of 2–6 pm/V. And, long-term thermal stability of the dipole alignment was observed up to ca. 140°C.

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The compatibilization effect of poly(styrene-b-2-ethyl-2-oxazoline) diblock copolymer, P(S-b-EOx), on immiscible blends of poly(2,6dimethyl-1,4-phenylene oxide) (PPO) and poly(ethylene-co-acrylic acid) (EAA) is examined in terms of phase structure and thermal, rheological and mechanical properties, and its compatibilizing mechanism is investigated by Fourier-transform infrared spectroscopy. The block copolymer, synthesized by a mechanism transformation copolymerization, is used in solution blending of PPO/EAA. Scanning electron micrographs show that the blends exhibit a more regular and finer dispersion on addition of a small amount of P(S-b-EOx). Thermal analysis indicates that the glass transition of PPO and the lower endothermic peak of EAA components become closer on adding P(S-b-EOx), and the added diblock copolymer is mainly located at the interface between the PPO and EAA phases. The interfacial tension estimated by rheological measurement is significantly reduced on addition of a small amount of P(S-b-EOx). The tensile strength and elongation at break increase with the addition of the diblock copolymer for PPO-rich blends, whereas the tensile strength increases but the elongation at break decreases for EAA-rich blends. This effect is interpreted in terms of interfacial activity and the reinforcing effect of the diblock copolymer, and it is concluded that the diblock copolymer plays a role as an effective compatibilizer for PPO/EAA blends. The specific interaction between EAA and polar parts of P(S-b-EOx) is mainly hydrogen bonding.

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The relationship between the structure and the viscoelastic properties of resol resins was studied. Six phenolic resins (resol) were synthesized with different molar ratios of formaldehyde to phenol. These resols were cured by means of temperature and without catalyst. The characterization of the resol was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of fully cured resins, characteristic properties such as: storage modulus (E 0 ), tan d , width of the tan d (DT) and damping peak (tan d ) were obtained. The compression modulus and the void content were also determined. A maximum in the methylene bridge and the result of the viscoelastic properties allow us to say that the resol with F/Ph between 1.3 and 1.4 has the highest crosslinking density.

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Silver nanoparticles were chemically synthesized by means of microemulsions techniques. Their self-organization behavior, including detailed description of the mechanism, the process and the resultant structure changes, was investigated by examining the electrically conductive characteristics of silver nanoparticles/solvent suspension, silver nanoparticles/epoxy resin mixture and its cured version, respectively. For silver nanoparticles suspension, higher particle concentration was found to be unfavorable to the formation of conducting paths owing to particles aggregation. For silver/epoxy mixture, transition-like changes in conductivity as a function of temperature were observed, manifesting two-stage self-organization of the nanoparticles: one related to the formation of some texture and the other related to melting and growth of the particles. As a result, conductive networks can be established by freezing the nanoparticles aggregation at the first stage under a particle loading even lower than the percolation threshold estimated conventionally. Accordingly, curing reaction of epoxy resin should be so designed as to adequately match the self-organization process.

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The dynamic mechanical behaviour of a series of hydrogenated syndiotactic polystyrenes (HsPS) with various degrees of hydrogenation has been investigated. In the case of crystallized HsPS samples, the primary dispersion is much broader due to the overlapping of two glass relaxation processes arising from one purely amorphous component and the other amorphous component under restraint by crystallites, as seen in semicrystalline polymers. There exist two secondary dispersions ascribed to chair–chair flip motion and oscillatory rotation of the cyclohexyl group in the range from 110 to 250 K. As compared with glassy complete HsPS, the strength of loss tangent (tan d) peaks ascribed to the molecular motions of the cyclohexyl group in the crystalline polymer was lowered, suggesting that the molecular motions of this group in the crystalline part are sterically hindered.

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This study deals with the effect of temperature and frequency on the electrical properties of highly cross-linked epoxy resin/bisphenol Abased polycarbonate. Impedance measurements were carried out on neat resin and on blend containing 10 and 15 wt% of polycarbonate in the frequency range 10–10 6 Hz at 25°C and 100°C. It was found that the impedance, dielectric constant are stable and nearly constant in the measured frequency and temperature range. The plot of the imaginary component of the impedence Z vs frequency at 100°C shows a relaxation peak in the blend which was interpreted as a local mode motion of the hydroxyl group formed during the high temperature dissolution process.

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The steady-state fluorescence and dilatometric techniques were used to study the free-radical crosslinking copolymerization of styrene (S) and commercial divinylbenzene (DVB) in bulk. Pyrene (Py) was used as a fluorescence probe for the in situ polymerization experiments. The time required for a sudden increase in the fluorescence intensity of Py was recorded for various DVB content and temperature. The monomer conversions and the gel points were recorded by dilatometry. To interpret the experimental data, a kinetic model was developed for the SDVB copolymerization system. It was shown that both the pendant vinyl groups and Py molecules are affected by the reaction medium in a similar way, their mobility decreases as the DVB concentration increases or the temperature decreases. The reaction time at which the Py intensity in the fluorescence spectra exhibits a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. The results show that the fluorescence technique can be used to follow the onset of the gel effect in SDVB copolymerization.

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