1999 рік

A variety of techniques is used to describe the fine structure of a series of well defined linear polyethylenes, polymerized using the new generation of metallocene-based catalysts. The sample’s molecular weight (Mw) is restricted to the usual values of processability of this material ranging from 14 K to 460 K. It will be clearly shown that the overall enthalpy of the rapidly crystallized samples follows a linear decrease with the log of Mw. The relative amounts of the different phases as seen from Raman spectroscopy and small angle X-ray scattering are evaluated as a function of molecular weight. As a consequence of the overall study, the contributions of the ordered and disordered regions to the long period are clearly separated and a semi-empirical expression for this essential structural parameter is proposed.

Детальніше...  

Macromonomers based on end-functionalized poly(e -caprolactone) (EF-PCL) were prepared by different synthetic strategies using Candida antarctica lipase B as the catalyst. The first strategy: an alcohol containing the target end-functionality initiated the ring-opening polymerization of e -caprolactone (e -CL) (initiation reaction). The second strategy: acids and esters containing the target end-functionality were added to prepolymerized e -CL. Consequently acid-terminated PCL was formed (termination reaction). Using the first strategy, 9decenol-initiated PCL was formed (24 h, 99% conversion of e -CL) with an average Mw of 1980 D. From the second strategy, linoleic acidterminated PCL was formed with an average Mw of 2400 D (51 h, 99% conversion). The last strategies: initiation and termination was combined either by using a di-functionalized ester or by addition, in sequence, of initiator and terminator. By these di-functionalization strategies, 2-(4-hydroxyphenyl)-ethyl-poly(e -caprolactone)-acrylate, di-EF-PCL was synthesized with an average Mw of 1960 D (52 h, 60°C) and 1720 D (30 h, 60°C), respectively.

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Diblock copolymers composed of poly(g-benzyl L-glutamate) and poly(N-isopropylacrylamide) (PNIPAAm) (abbreviated as GN) were prepared by the ring-opening polymerization of g-benzyl L-glutamate N-carboxy anhydride (BLG-NCA) using amine-terminated PNIPAAm as a polymeric initiator. Polymeric micelles consisting of PBLG as the hydrophobic inner core and PNIPAAm as the hydrophilic outer shell were prepared by diafiltration method. Their critical micelle concentrations (cmc) were determined by fluorescent probe techniques with pyrene as a hydrophobic probe. The cmc of the polymeric micelle with the thermosensitive outer shell is less influenced by temperature than the ordinary polymeric micelle consisting of PBLG as the same innercore and poly(ethylene oxide) (PEO) as the hydrophilic outer shell (abbreviated as GE). Fluorescence results were used to estimate the thermodynamic data of micelle formation, such as the standard Gibbs energies (DG 0), the standard enthalpies (DH 0), and the standard entropies (DS 0) of micellization. For the GN/water system, DS 0 was found to be positive and so was favorable to micelle formation whereas DG 0, DH 0, and DS 0 for the GE/water system were found to be negative. The partition constant of pyrene in these micelles has a value of the order of 5 and inflection of the change of that for GN with temperature was found around the LCST of PNIPAAm.

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[2,8- 13C2] ENB was synthesized from labelled acetic acid and terpolymerized in a batch reactor to [2,8- 13C2] ENB–EPDM rubber (Mw ˆ 4.8 × 10 5, d ˆ 3, ENB content ⬃ 8 wt.%, ethene to propene ratio ⬃ 1.8). Labelled ENB, its precursors and the EPDM were characterized by NMR spectroscopy. ENB is incorporated in the polymer chain through the endo cyclic double bond. Upon high temperature exposure to air of an NMR sample, the number of double bonds decreases due to crosslinking via addition of backbone radicals to double bonds. Some oxidation of double bonds occurs, giving rise to acetyl norbornene units. No significant cleavage of double bonds takes place. The labelling/ NMR approach provides an excellent tool for studying (minor) chemical conversions in EPDM rubber.

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The thermodegradation of several ethylene– a -olefin copolymers, where a -olefin was butene-1, hexene-1, octene-1 or 4-methyl-1pentene, was investigated by the pyrolysis-gas chromatography (Py-GC) technique. The study of the various pyrograms has allowed the discrimination of these polyethylenes (PEs) produced from different catalyst systems and processes. It was shown that the pyrolysis products could be classified into two families depending upon the type of the thermal degradation mechanism. In this way, it was demonstrated that, first, the nature and the amount of the comonomer, and, second, the long chain branching are the two structural features controlling the pyrolysis process. The quantitative ratio of these two families is a promising tool to classify the various polyethylene categories. In addition, a statistical approach was applied to discriminate the different PE families.

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Novel lactamimide-containing polymers were first synthesized by the bisimide-lactamimide ring contraction of 1,8:4,5-tetracarboxylic bisimide-containing polymers. The conversion of bisimide moieties was in the range 60–100% depending on polymer chain flexibility. The yields of lactamimide units were between 75 and 100% with respect to bisimide group conversion owing to the insolubility of polymers during lactam ring closure. No appreciable hydrolytic degradation of polymers was observed as followed from GPC data. Both u.v. longwave absorption and emission maxims in polylactamimides were 20–50 nm red shifted compared with those of polyimides due to intramolecular charge transfer. It was found that the thermostability of lactamimide units was close to that of bisimide as proved by TGA.

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The thermal behavior of a series of polybenzoxazines based on aniline and various phenols is examined under both inert and oxidative environments. Under an inert environment, the various phenols substantially affect the char yield of polybenzoxazines without altering the mechanism of Mannich base cleavage. Under an oxidative environment, the various phenols have a significant influence on the degradation behavior of polybenzoxazines below 600°C. The mechanism of the Mannich base cleavage under oxidative degradation has been proposed.

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An experimental study of the strain rate and temperature dependencies of the stress–strain behavior of PET under large strain deformation was conducted over a range in strain rates (⫺0.005 to ⫺2.0 s ⫺1) and temperatures (90°C–105°C) in both uniaxial and plane strain compression. The increase in crystallinity content with varying strain rate and temperature and for the two different states of strain that developed as a result of these deformation conditions was investigated using Differential Scanning Calorimetry (DSC). The nature and evolution of the crystallographic texture was studied using Wide Angle X-ray Diffraction (WAXD) measurements. The stress–strain behavior is found to exhibit four characteristic features: (1) a relatively high initial stiffness, (2) followed by a rollover in the stress–strain curve to flow behavior, (3) followed by an increase in stress with continuing strain, (4) followed by a dramatic increase in stress with strain at very large strains. These four features strongly depend on strain rate, temperature and state of strain. DSC and WAXD measurements on deformed specimens indicated an increase in crystallinity with increasing strain rate and decreasing deformation temperature under both uniaxial and plane strain conditions. The WAXD measurements also revealed that the preferential crystallographic texture that develops is consistent with the molecular orientation which develops with the different states of strain.

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The influence of hard domains on the mechanical and thermomechanical properties of polyurethane and unsaturated polyester hybrid networks has been investigated. The hybrid networks consist of a polyurethane linkage formed by reacting unsaturated polyester polyol with polymeric 4,4⬘-diphenylmethane diisocyanate (MDI) and free-radical crosslinking through styrene monomer and vinylene groups in the unsaturated polyester. Hard segments were formed by condensing two different types of chain extender, ethylene glycol (EG) and 1,6hexanediol (HD), with MDI. Incorporation of chain extenders in the hybrid networks varied from 0% to 12% by weight based on the weight of unsaturated polyester polyol. The thermomechanical properties of the polyurethane and unsaturated polyester hybrid networks were characterized by heat distortion analysis and by dynamic mechanical analysis. Flexural three-point bend tests and unnotched Izod impact analysis were used to investigate mechanical properties at ambient temperature. Hybrid networks with hard segments formed by MDI and EG showed an increase in the glass transition temperature. A second glass transition was found with incorporation of more than 6 wt% EG due to the formation of phase-separated hard domains. The rubber plateau of the hybrid networks decreased owing to the lower crosslinking density when chain extenders were incorporated. Phase-separated hard domains enhanced the rubber plateau by acting as physical crosslinks in the hybrid network until the glass transition was reached. The hybrid network had improved mechanical properties when more hard segments were added into it without creating the phase-separated hard domains. A dramatic drop in mechanical properties was observed for the sample with a two-phase structure.

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3,3 0 -Bis(4-chloro-1,8-naphthalimido)diphenylsulfone (1a) and 4,4 0 -bis(4-chloro-1,8-naphthalimido)diphenylsulfone (1b) and model imides have been synthesized and characterized by n.m.r. and i.r. spectroscopies and by d.s.c. The results of the structural investigations indicate that 1a and 1b are pure naphthalimides, without traces of isoimides. 1a and 1b were used to generate three series of poly(thioetherimide-sulfones). The polymer synthesis involved a nucleophilic displacement polymerization between 1a or 1b and Li2S or dithiol reactants such as 1,3-benzenedithiol and 2,5-dimercapto-1,3,4-thiadiazole under basic conditions in NMP or DMAc. The copolymers were characterized by i.r., n.m.r. and thermal analysis. Model compounds were synthesized for the correct assignment of 13C n.m.r. chemical shifts. All the copolymers showed a good thermal stability, exhibiting 10% mass loss in the range 400–500°C. The copolymers containing parasubstituted phenylsulfone units and/or the ones containing thiadiazole units were soluble in common polar solvents such as DMSO, DMF, NMP and DMAc.

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