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  • 1
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The morphology, the crystallization and thermal behaviour of isotactic polypropylene (IPP) in its blends with two different samples of low density polyethylene (LDPE) was investigated at temperatures high enough to prevent any solidification of LDPE. It is found that pre-existing liquid LDPE domains are incorporated in intra-spherulitic regions during the isothermal crystallization of iPP. The radial growth rate of spherulites is almost unaffected by the LDPE content. The overall rate of crystallization of iPP, on the contrary, is strongly depressed by the addtion of LDPE. A depression of the equilibrium melting temperature of iPP, due to kinetic and morphological effects, is also observed. The depression of the overal kinetic rate constant is accounted for by the negative effect (decrease in the number of nuclei) that the addition of LDPE has on the primary nucleation process of iPP.
    Additional Material: 15 Ill.
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  • 2
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The isothermal crystallization kinetics, the morphology and melting behaviour of blends of poly(tetramethylene terephthalate) {poly(butylene terephthalate), (PBT)} with a thermotropic liquid-crystalline polyester, poly(decamethylene 4,4′-terephthaloyldioxydibenzoate) (HTH10), were studied in the composition range 0 - 50 wt.-% of HTH10 by optical microscopy and differential scanning calorimetry. The spherulite growth rate and the overall crystallization rate of PBT from melt blends are markedly depressed by the presence of the mesomorphic component. Changes of superstructure and of primary nucleation density of PBT spherulites in the blends were observed. The analysis of the melting behaviour of these samples indicates that the stability of PBT crystals and their reorganization processes on heating are dependent on the blend composition. A decrease of the equilibrium melting temperature of PBT is found with increasing the amount of HTH10. According to previously reported investigations on the same system, the results are discussed in respect of the onset of interactions between the two polymers in the liquid state.
    Additional Material: 13 Ill.
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  • 3
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In order to elucidate the relations between morphological habits, thermal behaviour and chemical structure of polymers, aliphatic polyesters with variable amount of trans-double bonds along the backbone chain were crystallized from dilute solutions in 1-propanol and 1-butanol. The crystals were studied with electron microscopy, differential scanning calorimetry, and wide angle X-ray diffraction. The morphology of solution grown single crystals and some thermodynamic quantities such as the equilibrium dissolution and melting temperature and the enthalpy and entropy of fusion are influenced by the percentage of unsaturated comonomer. The thermal behaviour and the X-ray diffraction of these crystals suggest that saturated and unsaturated repeating units are able to cocrystallize.
    Additional Material: 10 Ill.
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  • 4
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The influence of composition on the overall rate of isothermal crystallization of isotactic random propylene/1-butene copolymers is investigated. At a constant crystallization temperature a small fraction of 1-butene co-units inserted along a polypropylene chain causes a drastic reduction in the overall rate of crystallization. Further it is found that for a given rate of crystallization it is possible to crystallize the isotactic polypropylene at lower crystallization temperature by adding along the chain a small amount of 1-butene as comonomer. The equilibrium melting temperature, the surface free energy of folding, the enthalpy and the entropy of fusion decrease with increasing the contents of 1-butene in the copolymer samples.
    Additional Material: 9 Ill.
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  • 5
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The crystallization behavior of high density polyethylene/isotactic polypropylene blends (HDPE/PP) was investigated. The overall kinetics of crystallization of HDPE may be strongly influenced by the addition of a small fraction of PP. At 398 K the half crystallization time of the blend containing 10% by weight of PP is around three times larger than for pure polyethylene. This effect may be accounted for by an increase in the melt viscosity of liquid HDPE caused by PP molecules. For crystallization temperatures high enough to prevent HDPE crystallization (Tc 〉 400 K), the presence in the melted blends of liquid HDPE influences the crystallization of PP. In fact, the half time of crystallization of PP increases with increasing the fraction of HDPE in the blend. The kinetics show a minimum at a well defined composition (around 60% in PP) before phase inversion. The same trend is observed when the surface free energy of folding and the lamellar thickness of crystals are reported as function of composition.
    Additional Material: 9 Ill.
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  • 6
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Blends of poly(tetramethylene terephthalate) [poly(butylene terephthalate) (PBT)] with a liquid crystalline polyester, poly(decamethylene 4,4′-terephthaloyldioxydibenzoate) (HTH 10), obtained by solution precipitation, were characterized by optical microscopy, X-ray diffraction and differential calorimetry. The morphology and phase behaviour of the blends are sensibly influenced by the composition. X-ray data support the existence of separated crystal phase of the two polymers only for HTH10 contents higher than 30 wt.-%. An increase of the clearing temperature of the original blends is found with decreasing concentration of the mesomorphic polymer in the range 100 to 30 wt.-%. On cooling from the isotropic melt, a single crystallization process is observed, whose temperature decreases with increasing amount of HTH10. A large depression of the melting temperature of PBT is found in the composition range 0 to 50 wt.-% of HTH10. Blends quenched from the isotropic state exhibit one single glass transition temperature, intermediate between those of the starting polymers. The results suggest the occurrence of miscibility phenomena of PBT and HTH10 in the amorphouse phase.
    Additional Material: 10 Ill.
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  • 7
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Blends of polyphenylene sulfide (PPS) with a commercial, wholly aromatic, liquid crystal copolyesteramide (Vectra-B950) have been prepared by meltblending. The crystallization behavior of neat and blended PPS has been studied by differential scanning calorimetry (DSC), under both non-isothermal and isothermal conditions. It has been found that blending PPS with Vectra-B leads to an increase of the temperature of non-isothermal crystallization and to a pronounced acceleration of the isothermal crystallization, without any reduction of the degree of crystallinity. All these effects have been found to occur independent of the Vectra-B concentration, within the investigated range (2 to 20%, w/w). The results have been interpreted in terms of an incrased nucleation density of the blends, probably due to heterogeneous substances, initially present in the Vectra-B bulk, which dissolve to saturation in the PPS phase, during melt-blending.
    Additional Material: 8 Ill.
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  • 8
    ISSN: 0098-1273
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solution-grown crystals of fractions of isotactic polypropylene (IPP) with different degrees of stereoregularity have been obtained by isothermal crystallization from α-chloronaphthalene, using a self-seeding technique. Electron micrographs of samples, crystallized under the same undercooling, show that, with decreasing fraction of isotactic pentads, the perfect rectangular shape of the single crystal is lost and the presence of more complex morphologies is increasingly observed. The equilibrium dissolution temperature Td of IPP fractions, from polymers prepared with a titanium based catalyst, decreases linearly with decreasing percentage of isotactic pentads. An extrapolated value of 171°C is obtained for the equilibrium dissolution temperature of a crystal of IPP with 100% isotactic pentads, i.e., an IPP crystal free of configurational defects. The melting temperature T′m and the apparent enthalpy of fusion ΔH*f of crystallized and annealed crystal aggregates have been determined by differential calorimetry. The equilibrium melting temperature Tm also depends greatly upon the isotactic pentad concentration. For 100% concentration the extrapolated value of Tm is 181°C. Tm decreases about 1°C per 1% decrease in the isotactic pentad population. The observed equilibrium melting and dissolution temperature depression does not follow the predictions of the Flory equation for copolymer crystallization. In fact, the effect of decreasing probability of isotactic sequence propagation is to depress Td and Tm much more rapidly. The apparent enthalpy of fusion of both solution-grown crystals and melt-recrystallized samples decreases with an increase in the number of configurational impurities along the chain. For the most stereoregular fraction the average length of isotactic stereoblocks has been compared with the lamellar thickness of solution-grown lath-shaped single crystals.
    Additional Material: 14 Ill.
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  • 9
    ISSN: 0959-8103
    Keywords: ethylene/1-butene copolymers ; Ziegler-Natta catalysts ; sequence distribution ; thermal properties ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The crystallization and melting behaviour of two sets of ethylene/1-butene copolymers have been analysed by DSC. The samples, with comonomer content in the range from 0 to 21.5 mol%, were obtained by industrial processes using both Mg/Ti-based catalyst systems. The composition dependences of melting and crystallization temperatures were found to be strictly affected by the catalyst type. Moreover, logarithmic plots of the melting and crystallization enthalpy as a function of the ethylene content (mol%) in the copolymers fitted linear relationships whose slopes have been related to the critical sequence length of crystallizable ethylene units, depending on the catalytic system. These results are compared with those reported in the literature for ethylene/1-butene copolymers synthesized by other catalysts and are accounted for by a different distribution of the comonomer units in the macromolecules of the two sets of samples.
    Additional Material: 6 Ill.
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  • 10
    ISSN: 0959-8103
    Keywords: fourier transform-infrared microspectroscopy (FTIR-M) ; DSC ; poly(vinylidene fluoride) ; polymorphism ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Poly(vinylidene fluoride) (PVDF) samples, obtained by casting from tetrahydrofuran solutions and submitted to various thermal treatments, have been examined by Fourier transform-infrared microspectroscopy (FTIR-M) and differential scanning calorimetry (DSC). This kind of analysis allowed us to examine microdomains of samples with different morphological characteristics and to obtain an indication of the polymorphism of PVDF. In some cases the simultaneous presence of two or three forms has been evidenced thanks to the comparison of FTIR-M spectra and DSC traces. Vibrational spectra of single crystalline forms can be recorded by FTIR-M on phase homogeneous microdomains.
    Additional Material: 12 Ill.
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