1,720,981 research outputs found
Highly Stereoregular Polymerization of 1,3-Cyclohexadiene in the presence of Cp2Ni - MAO catalyst.
No full textCocrystallization in Blends of random tetrafluoroethylene fluorinated copolymers: the effect of chain structure and crystallization conditions
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Influence of regio- and stereoregularity of propene insertion on crystallization behavior and elasticity of ethene-propene copolymers.
No full textThermal Fractionation of Ethylene/1-Octene Multiblock Copolymers from Chain Shuttling Polymerization
No full textEthylene/1-octene statistical multiblock copolymers (OMBCs) consisting of the alternation of crystalline, hard blocks and amorphous, soft blocks, with ≈0.5 and 20 mol % of 1-octene units, respectively, are subjected to thermal fractionation resorting to successive self-nucleation and annealing (SSA). The study is extended to random copolymers (RCs) of high (44 kDa) and low (3.4 kDa) number average molecular mass Mn, mimicking in 1-octene content the crystalline hard blocks and to the OMBC fractions extracted in boiling n-hexane and cyclohexane through a suitable solution fractionation protocol. For all the samples, the melting endotherms are well resolved in a multiplicity of peaks corresponding to the melting of crystals of different thicknesses generated in the SSA protocol that reflect the distribution of the methylene sequence length (MSL) in between consecutive interruptions along the chains. It is shown that, regardless of molecular mass, the MSL distributions of the RC samples are shifted toward greater values than those of the OMBC samples, and that also the shapes of the distributions are different. Since the MSL distribution depends on the frequency and distribution of the defects along the chains, and the defects act as interruption points, the higher fraction of long crystallizable sequences in the RC samples suggests that whereas for the RC samples the interruptions are merely represented by the 1-octene units that are rejected outside the crystals, for the OMBCs, the interruption of the regular methylene sequences belonging to the crystalline hard blocks due to the amorphous soft blocks linked to them should also play a role. Indeed, due to the partial miscibility of the hard and soft blocks, the hard blocks of major length tend to crystallize in a confined environment. This prevents the formation of thicker crystals and induces decrease of the MSL values as well as changes in the shape of the MSL distributions (topological confinement). On the other hand, the hard blocks of shorter length tend to crystallize, crossing the hard block rich regions and causing melting point depression and consequent shift of the MSLs toward lower values with respect to RCs (diluent effect). It is shown that differences in the MSL distribution of OMBCs are the result of the interplay between topological confinement and diluent effect, which in turn reflects differences in the OMBC chain microstructure, that is, differences in the distribution of block length at the inter- and intra-chain level
Polymorphism and form II – form I transformation in Ziegler-Natta isotactic 1-butene-ethylene copolymers having a multiblock molecular structure
No full textA study of the crystallization behavior and kinetics of form II-form I transition in isotactic butene-ethylene copolymers (iPBEt) prepared with Ziegler-Natta catalyst with ethylene concentration in the range 9–44 mol% is presented. The samples have been separated with boiling solvents in different fractions containing chains with different ethylene concentration. Crystallization of form II from the melt is observed in each fraction even at high ethylene concentration, up to nearly 16 mol%. For higher ethylene content the samples do not crystallize from the melt but still crystallize during aging at room temperature in form I′. The comparison with the crystallization behavior of iPBEt copolymers prepared with homogeneous metallocene catalysts, characterized by a random distribution of ethylene units, has shown that the form II-form I transformation in Ziegler-Natta copolymers is much slower than in metallocene copolymers having the same ethylene concentrations and for the metallocene copolymers a low concentration of ethylene units of nearly 6 mol% is sufficient to prevent crystallization from the melt of form II. This indicates that in the chains of Ziegler-Natta iPBEt copolymers the regular butene sequences interrupted by the ethylene units are much longer than those of the chains of copolymers prepared with metallocene catalysts of the same ethylene concentration, giving further evidence of the multiblock structure of Ziegler-Natta iPBEt copolymers
Propylene-Butene Copolymers: Tailoring Mechanical Properties from Isotactic Polypropylene to Polybutene
No full textIsotactic propylene-butene copolymers [i(P-co-B)] with precise and controlled molecular structures were synthesized with various organometallic catalysts having different stereoselectivities. Stereoregular and stereodefective samples of i(P-co-B) with 1-butene (B) content variable in the whole range of composition were synthesized. All samples crystallize regardless of composition, indicating cocrystallization of propene and 1-butene units, which are incorporated in the unit cells of polymorphic forms of isotactic poly(propylene) (iPP) and isotactic poly(1-butene) (iPB). The copolymers show a continuum change of crystal morphology with the composition, transforming from big spherulites to bundle-like and needle-like crystals, to granular crystals. The cocrystallization allows maintaining high crystallinity of copolymers for any composition and provides an opportunity to develop outstanding mechanical properties that can be tailored by changing the isotacticity and composition. This allows, ideally, combining in the same material the different properties of stiffness of iPP and flexibility of iPB. These copolymers show, indeed, mechanical properties intermediate between iPP and iPB, ranging from stiffness/brittleness and ductility/flexibility depending on the composition and isotacticity, with high strength and Young's modulus that may be regulated by the stereoregularity of the iPP and iPB sequences, which is, in turn, dictated by the catalyst structure
Pseudohexagonal crystallinity and thermal and tensile properties of ethene-propene copolymers
No full textMechanical properties of isotactic 1-butene-ethylene copolymers from Ziegler-Natta catalyst
No full textThe mechanical properties and elastic behavior of Ziegler-Natta isotactic 1-butene-ethylene (BuEt-ZN) copolymers, having a broad composition from 1.7 to 45.5 mol% of ethylene units, are studied. The multi-site nature of the catalytic system determines a non-uniform distribution of the comonomers along the polymer chains; the copolymers present a blocky microstructure and composed of fractions of chains made of long and short butene sequences, which can be separated by solvent fractionation procedures. Incorporation of ethylene induces improving of drawability and decrease of rigidity with respect to homopolymer, associated with the lower crystallinity, and outstanding elastomeric behavior when the ethylene amount exceeds 5.5 mol%. In these copolymers the presence of irregular fractions of chains with short butene sequences, showing crystallization of the defective and low-melting temperature form I′ upon aging of the amorphous at room temperature, is responsible for the good elasticity of the materials
Reinforcing mechanisms of natural fibers in green composites: Role of fibers morphology in a PLA/hemp model system
No full textGreen composites, i.e. biodegradable polymers reinforced with natural fibers, are attracting interest as potential substitutes for conventional composites based on petroleum derived plastics. The role of the inherently complex morphology of natural fibers in their reinforcing mechanisms is not completely understood and this is the topic of the present study. The selected system was poly-(lactic acid) filled with 3 and 6 wt% of short hemp fibers. Such a low fiber amount was chosen to help visualization of the fiber – matrix interface at the scanning electron microscope. Remarkable differences in the mechanical behavior were found between composites containing fibers that were alkali treated with respect to untreated fiber filled materials, but unexpectedly it was found that the quality of the fiber – matrix interface was only marginally influenced by the alkaline treatment. Interface properties were thus not exhaustive in explaining the observed differences. On the other hand, the main difference between treated and untreated fibers was the presence, in the untreated fibers population, of a volumetrically relevant sub-population of thick fiber bundles. It was further argued that this fraction did not carry the loads transferred across the fiber-matrix interface uniformly in its cross section, thus determining a reduction in the effective fiber volume fraction. In contrast, the combined action of alkalization and the mechanical stresses during melt mixing resulted in a narrow distribution of isolated elementary fibers, which were more effective in providing higher mechanical properties, in agreement with theoretical predictions. The key message for the scientific community interested in maximizing the mechanical performances of green composites is that, besides trying to improve the quality of the fiber-matrix interface, one should also aim at minimizing the amount of fiber bundles
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