1999 рік

Physical properties of thin films of soluble and insoluble aromatic polyimides were compared by d.s.c., u.v.-visible and fluorescence spectroscopy, and prism coupler technique. D.s.c. results showed that the thermal properties of insoluble polyimides are superior to those of soluble ones, owing to the specific molecular interactions of insoluble polyimides, revealed by fluorescence spectroscopy. However, the specific molecular interactions sacrifice the transparency in their thin films, shown by u.v.-visible transmission spectra. Negative birefringence of thin films, not only for soluble polimides but also for polyamic acids of insoluble polyimides, was confirmed by prism coupler, while thin films of insoluble polyimides thermally imidized lack high levels of negative birefringence. It is regarded that thin films of insoluble polyimides thermally imidized are likely to be isotropic because of molecular relaxation above their glass transition temperatures (Z’,) in the course of thermal imidization, according to Plot-y’s principle. On the contrary, thin films of insoluble poyimide chemically imidized keep the same levels of negative birefringence as those of the relevant polyamic acid thin films. Judging from the opposite cases, it is recognized that molecular relaxation above Tg weakens the optical anisotropy of polyimide thin films.

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Poly(styrene-b-isobutylene-b-styrene) block copolymers made through controlled–living cationic polymerization using the t Bu-m-DCC– TiCl 4 initiating system in 60/40 (v/v) hexane–methyl chloride cosolvents were analyzed using curve-resolution software in conjunction with high-resolution g.p.c. Fractional precipitation and selective solvent extraction were applied to a representative sample in order to confirm the identity of contaminating species. The block copolymers were found to consist of 70–75 wt% of the target molecule, and about 20 wt% of coupled (and higher linked) block copolymers formed by intermolecular electrophilic aromatic substitution linking reactions occurring during the styrene polymerization. Minor contaminants were identified as diblock copolymer and low molecular weight polystyrene homopolymer.

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The influence of cross-linking on the phase behaviour of a series of side-chain liquid crystalline elastomers has been studied. For samples cross-linked in the temperature range corresponding to the nematic phase, the phase transition was shifted compared to that observed when an identical sample was cross-linked in the isotropic phase. This shift represented a stabilisation of the nematic phase in the former case, in line with theoretical expectations. By utilising a novel, slow cross-linking method, which allows the polymer backbone to take up an equilibrium conformation prior to network formation, it proved possible to monitor the shifts in phase transition temperature as a function of the length of the methylene chain coupling the mesogenic units to the polymer backbone. The results obtained are related to the backbone anisotropy and indicate that the level of orientational order of the polymer in the nematic phase backbone increases with a reduction in the length of the coupling chain.

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Poly(ether ether ketone) with high content of sodium sulfonate groups prepared directly from 5,5’-carbonylbis(2-fluorobenzene sulfonate), 4,4’-difluorobenzophenone and phenolphthalein via aromatic nucleophilic substitution reaction. The polymer showed excellent thermal stability, high permeability of water vapour, high permselectivity of water vapour over nitrogen, and high permselectivity of anions.

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This paper reports on the synthesis of aliphatic polyester-grafted starch by in situ ring-opening polymerization (ROP) of e-caprolactone in the presence of starch-like polysaccharides that may be granular or plasticized, or a mixture of these, wherein the in situ ROP is conducted in the bulk, i.e. in the absence of solvents. The polymerization is initiated from the starch phase with the amylose/amylopectin hydroxyl functions in the presence of a catalyst such as Sn(Oct) 2, or by adequate modification/activation of the surface hydroxyls into titanium or aluminium alkoxides, thereby promoting grafting and good interfacial adhesion between the two starch/polycaprolactone (PCL) phases. Evidence for coupling of the active initiator species onto the starch hydroxyls, aluminium alkoxides for instance, is shown by an aluminium peak in the X-ray photoelectron spectroscopy scan of a starch surface reacted with triethylaluminium. Good interfacial adhesion is confirmed by scanning electron microscopy images of surface fractures and by selective extraction experiments in toluene, which is a good solvent for solubilizing the non-grafted PCL chains.

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Crystallization behaviors of crystallizable poly(ethylene terephthalate co ethylene isophthalate) with different molar ratios of polyethylene terephthalate (PET) to polyethylene isophthalate (PEI) were studied both non-isothermally and isothermally by differential scanning calorimetry (DSC). Avrami and Ozawa plots were used to describe the non-isothermal crystallizations. However, the Avrami treatment cannot predict the mechanisms of non-isothermal crystallizations for the above copolyesters, while the Ozawa explanation can be taken to reveal that the non-isothermal crystallization is dominated by heterogeneous nucleation. The activation energies of crystallization were calculated from the Kissinger equation. The copolyester 90/10 shows synergistic effects on non-isothermal crystallization, in that it has the lowest activation energy of crystallization. Isothermal crystallizations were explained by Avrami plots. From lower degrees of supercooling to higher ones, the crystals of the copolyesters grow from low dimension to multidimensions, but the crystallization rates become lower. The percentage of spherulite growth of the copolyester 100/0 (PET homopolymer) is higher than that of the other copolyesters under the same degree of supercooling. Equilibrium crystallization temperatures (Tc0 ) and melting temperatures (Tm0 ) were postulated respectively. Tc0 and Tm0 decrease with the increasing composition of PEI, indicating that the equilibrium crystals of the copolyesters become imperfect. With involvement of PEI, the copolyester is not likely crystallizable perfectly and the crystal becomes less perfect.

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A series of poly(a-n-alkyl P-L-aspartate)s (where IZis the number of carbon atoms in the linear alkyl side chain, with n = 12, 18 and 22) was studied by differential scanning calorimetry. The effect of the length of alkyl side chain on the non-isothermal crystallization kinetics on cooling was investigated. It was found that the AH, and crystallization rate of PAALA samples on cooling increase with the alkyl side chain length. A similar trend is observed for the activation energy of the crystallization process. It was found that the Johnson-Mehl-Avrami model cannot be applied for a quantitative description of non-isothermal d.s.c. data corresponding to this crystallization process.

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The molecular weight distributions of the free-radical polymerization of acrylamide in solution and in inverse emulsion are examined as a function of concentrations of initiator and of amount of added chain transfer agent (sodium formate). When these distributions are displayed as ln(number molecular weight distribution), readily obtained from the GPC trace (essentially by dividing by the square of molecular weight), the results can be semi-quantitatively interpreted in terms of the amounts of chain stoppage by transfer and by termination.

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The dielectric relaxation behavior of a series of amorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3-HB-co-3-HV) having the hydroxyvalerate content: 0, 7, 14, 20 and 24 mol%, was investigated at the temperature and frequency ranges of 08–1008C and 10 3 –10 5 Hz, respectively. The dielectric spectra showed that the crystallization process starts during the temperature scan measurements at t 308C regardless of the 3-HV-content. All samples of P(3-HB-co-3-HV) exhibited very similar relaxation spectra indicating the preservation of the main features of P(3-HB). However, some differences were observed in the relaxation spectra including the peak height, the onset crystallization temperature and the activation behavior. It is suggested that these differences observed in the various samples of the copolyester containing different 3-HV content are mainly controlled by: (1) the rate of crystallization, (2) the ultimate percentage of crystallinity, and; (3) the heating rate of measurements. Comparison of the onset crystallization temperature, Tonset, of different samples showed that the higher the 3-HV contents, the higher is onset temperature. However, measurements of Tonest, at different heating rates showed the lower the rate, the weaker is the composition dependence of the onset temperature, so that the zero-rate onset temperature could be considered to be independent of the 3-HV content and equals that of pure P(3-HB). In addition, the study of the kinetics of cold crystallization showed that while the onset crystallization time is shortened from 14 to 4 min. by increasing the 3-HV-content from 0 to 24% HV, the offset crystallisation time is extended from 65 to 150 min. Moreover, by fitting the experimental results using Avrami equation to calculate the mechanistic exponent “n” the results showed surprisingly that “n” changes from 3.8 to t 2, t 2 and t 1 for samples containing 0, 7, 14, 20 and 24% HV, respectively. This change indicated the existence of mechanistic changes in the cold crystallization process. The results obtained are discussed and correlated to the literature data.

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Polymerization of acrylamide was specifically performed under conditions wherein no polymerization could occur in the absence of ultrasound and initiator. Polymerizations were carried out in the presence of peroxodisulfate and ultrasound. The rate of polymerization and the rate of peroxodisulfate disappearance were followed under various conditions to arrive at the rate dependencies. The observed results were adequately explained in terms of the proposed mechanism. The rate parameters were evaluated and provided as evidence for the polymerization which can occur in the presence of ultrasound.

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