1994 рік

A more complete description of the morphology of poly(p-phenylene terephthalamide) (PPTA) in both as-polymerized polymer and various fibre forms is developed by the utilization of a number of methods (dark-field transmission electron microscopy, X-ray diffraction, solid state n.m.r, spectroscopy, and partial isotopic exchange of accessible amide moieties). This description focuses primarily on the effect of lateral crystallite dimensions on the microscopic dynamic structure and the concomitant macroscopic accessibility properties. The observation of a heterogeneous dynamic structure reflects the imperfect structural consolidation during the processing of PPTA in both as-polymerized polymer and fibre forms. The imperfect microcrystalline structure formed during processing is shown to be the origin of macroscopic accessibility for small molecules and also provides a plausible structural basis for biological accessibility.

Детальніше...  

The reactivity parameters of the copolymerization of N,N-bis(2-methacryloyloxyethyl)-p-toluidine (BMAT) (Mr) with methyl methacrylate (MMA) (M2) in 1,4-dioxane, i.e. rl = 1.07+0.20, and r2=0.11 +0.19, and with bisphenol-A bis(2-hydroxypropyl methacrylate)(Bis-GMA)(M2) in benzene, i.e. rl =0.58+0.04, and r2 = 0.03 + 0.03, were determined at 60°C at low concentration and low conversion (~ 5%) of the monomers. Strongly delayed gelation occurred during the copolymerization of BMAT and MMA at monomer conversions of 20-30%, relatively independently of the composition of the monomer mixture. The efficiency of BMAT and N-acryloyl-N'-phenylpiperazine as activators in the redox initiated copolymerization of Bis-GMA in dental composites at ambient temperatures was found to be comparatively low. In the case where BMAT was used in equimolar proportions to benzoyl peroxide the former component was not detectable in the extract of the cured composite.

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The crystalline content of a range of celluloses has been evaluated by X-ray diffraction and infrared spectroscopy. Chemical derivatization of these materials with trifluoroacetic anhydride in conjunction with X-ray photoelectron spectroscopy has been used to measure hydroxyl accessibility. A linear relationship is found between cellulose crystallinity and the rate of hydroxyl labelling.

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An imaging plate was used to quantitatively measure the electron diffraction intensities of graphite and polyethylene (PE) crystals. After the signs of the structure factors were assigned by using a direct phasing procedure, potential maps were made in order to determine atomic positions. Peaks corresponding to the carbon atoms in graphite were seen at (0,0), (1/3,2/3) and (2/3,1/3) in the map of the c-axis projection. For PE, hydrogen atoms were resolved in addition to carbon atoms due to the higher scattering powers of hydrogen for electron beam than for X-ray. The setting angle of the molecular chain in the c-axis projection of the unit cell was determined to be 46.0 °, which is consistent with the results of X-ray experiments. R-factors were found to be 0.228 and 0.197 for graphite and PE, respectively. It is shown that the high sensitivity, the wide dynamic range, the good linear response and the digital output data of the imaging plate are useful for structure analysis using electron diffraction

Детальніше...  

The miscibility behaviour of poly(3-chloropropyl methacrylate) (PCPMA) and poly(2-iodoethyl methacrylate) (PIEMA) with a series of aliphatic polyesters was investigated by differential scanning calorimetry. Binary blends of PCPMA and PIEMA with poly(butylene adipate), poly(2,2-dimethyl-l,3-propylene adipate), poly(e-caprolactone) (PCL) and poly(hexamethylene sebacate) (PHS) exhibit single, composition-dependent glass transition temperatures, characteristic of miscible systems. However, PCPMA and PIEMA are immiscible with poly(ethylene succinate), poly(ethylene adipate) and poly(2,2-dimethyl-l,3-propylene succinate). Blends of PCPMA and PIEMA with poly(2,2-dimethyl-l,3-propylene sebacate) exhibit upper critical solution temperature behaviour. Interaction parameters for PCPMA/PHS, PIEMA/PHS, PCPMA/PCL and PIEMA/PCL blends were evaluated from melting-point depression analysis.

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The influence of glycerol on the retrogradation kinetics for waxy maize starch-water systems was monitored by Fourier transform infra-red spectroscopy. The spectra showed the C-C and C-O stretching region (1300-800 cm-1) to be sensitive to the retrogradation process. A multistage kinetic process in terms of structural changes on a molecular level was observed during the retrogradation of waxy maize in a 10% (w/w) gel. For the first two stages (formation of helices and induction time for helix aggregation) no significant kinetic differences were observed in the gels with various glycerol contents. Stage three is described as the primary aggregation and crystallization step. In the final stage, syneresis of water occurs. The calculated rate constants clearly show decreasing retrogradation of waxy maize starch with increasing glycerol content. The effects are explained in terms of starch-glycerol interactions and stabilization of water resulting in a decreased mobility of the starch chains.

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Light incident on a total internal reflection surface will tunnel through a submicrometre gap in the presence of a dielectric surface. This tunnelling phenomenon is used in the photon tunnelling microscope to image polyethylene single crystals, providing a topographical map of the single-crystal surface. Tunnelling increases exponentially with sample height and is quantified using video photometry of the grey-scale tunnelling image.

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Non-extractable poly(methyl methacrylate) (PMMA)-montmorillonite adducts were prepared by two distinct processes: (a) In the first method, free-radical copolymerization of MMA with clay previously treated with 2-(N-methyl-N,N-diethylammonium iodide) ethyl acrylate (QD1) or 2-(N-butyl-N,N-diethylammonium bromide) ethyl acrylate (QD4) was performed. (b) The second method was based on the direct interaction of montmorillonite with various copolymers of MMA with QD1 or QD4. The structure of the adducts as determined by Fourier-transform infra-red spectroscopy, thermogravimetric analysis and X-ray diffraction was shown to consist of the insertion of MMA macromolecules between lamellar layers whose separation was consequently higher than in the polymer-free clay. The polymer was strongly fixed to the inorganic surfaces, probably due to cooperative formation of electrostatic bonding. The thermal stability of the organic polymers was substantially enhanced in the adducts

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Gel-spun ultrahigh molecular weight polyethylene (UHMWPE) was examined by advanced solid-state '3C n.m.r, techniques to extract detailed information on the morphology and nature of the crystalline phases. Two partially overlapping resonances separated by 1.5 ppm in the 13 C magic angle spinning n.m.r, spectrum have been assigned to monoclinic and orthorhombic crystalline phases. To analyse better the small amount of monoclinic phase, a 13C chemical shift filter was employed to suppress partially the dominant orthorhombic signal. The monoclinic phase exhibits a slightly smaller chemical shielding anisotropy of 27 ppm with chemical shift principal tensor values of 50, 36.5 and 16ppm as compared to a shielding anisotropy of 30ppm and principal tensor values of 50, 35.5 and 13ppm for the orthorhombic phase. Additionally, a minor fraction of interface was detected arising from heterogeneous molecular packing. Both crystalline phases reveal an extremely high degree of order, with (P2) around 0.94. Whereas the conformations of the chains in the non-crystalline regions of both gel-spun and melt-spun polyethylene are alike, the mobility of the former is largely reduced. The morphology of UHMWPE is compared with that of melt-spun polyethylene.

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Polyaniline-dodecylbenzenesulfonate( PANI-DBSA) fibrils prepared directly during emulsion polymerization or by precipitation from solution have been studied by transmission electron microscopy. High-aspect-ratio fibrils are obtained with typical dimensions of order 1 mm by 1/~m. The small crystallites within the fibrils are oriented with c axis parallel to the fibril axis. The crystal structure of the PANI-DBSA crystallites has been determined as orthorhombic with two PANI chain repeat units and two DBSA molecules in the unit cell; the lattice parameters are a= 1.178 nm, b= 1.791 nm and c=0.716 nm. Based on this structure, the calculated density is 1.12 g cm- 3. The systematic absences in the PANI-DBSA diffraction data are consistent with those expected for the Pmnm space group.

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