Narrow Composition Distribution Research Articles

Ostwald ripening in immiscible polyolefin blends

AbstractThe coarsening in the quiescent melt of the phase‐segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high‐density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecular weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it formed a two‐phase system in the melt at 177°C. The blend was precipitated from xylene solution in order to obtain an initially intimately mixed system. This was the third study in a series of studies of the coarsening of phase‐segregated particles in polymer blends. This study was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system was binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet phase particles, which were mainly composed of the HPB, were observed to coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of about 0.05 μm and coarsened to about 0.2 μm after 1 h storage in the melt state. Particle dimensions were measured by scanning electron microscopy of n‐heptane‐etched and gold‐coated sections. It was found that the volume of the particles increased linearly with time and that the rate constant of coarsening was Kexp = 1.23 × 10−18 cm3/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of Kce = 0.86 × 10−18 cm3/s. In contrast the rate constant for direct particle diffusion and coalescence was Kc = 3.6 × 10−20 cm3/s. Since this was two orders of magnitude smaller than the rate constant for Ostwald ripening, it was concluded that, although the linear increase of volume with time was also consistent with the particle diffusion and coalescence mechanism, this was not a significant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism was the high melt viscosity of the matrix polymers. The application of the Ostwald ripening theory to this system could be made with a high level of confidence because it was binary. It was found that the phase concentration of the droplet phase apparently underwent a rapid increase during the first 1‐2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phase‐segregate) for about 1‐2 min. This further indicated that the long‐time coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self‐similar over the period of coarsening, as predicted by Ostwald ripening theory. © 1995 John Wiley & Sons, Inc.

The double yield in the tensile deformation of the polyethylenes

AbstractDouble yields, as well as other complex yielding behavior, that are observed during the tensile deformation of the polyethylenes have been studied in a systematic manner. The conditions under which such yielding is observed were established by studying a set of linear polyethylenes and well‐characterized ethylene copolymers of narrow molecular weight and composition distributions. A wide range in crystallinity levels were developed and the rate of deformation and the deformation temperature were varied. These experimental results make clear why such complex yielding processes are not always observed. A qualitative explanation, based on the postulate of a partial melting‐recrystallization process during deformation, is given that encompasses all of the experimental data.

Tensile Properties of Crystalline Polymers: Random Copolymers of Ethylene

Force-elongation curves of a set of random ethylene-1-alkene copolymers have been studied. The comonomers included 1-butene, 1-hexene, 1-octene, and 4-methyl-1-pentene. The copolymers all had the most probable molecular weight and narrow composition distributions. A set of hydrogenated poly(butadienes), random ethyl-branched copolymers, that have very narrow molecular weight and composition distributions were also studied as reference. Only ductile type deformations were studied. Several important generalizations resulted from this work. An important finding was that the molecular weight, copolymer composition, and chemical nature of the co-units have to be considered as independent variables in analyzing the tensile behavior. The nominal stress-strain curves of even modest molecular weights are dominated by strain-hardening. This characteristic is similar to that of very high molecular weight linear polyethylenes. However, the effect is not as large for the ethylene-butenes as compared with the other copolymers. The ultimate properties of the ethylene-butene copolymers also differ from the others. An explanation for these differences can be given by postulating that in the melt structure, and thus in the residual liquid-like region, noncrystalline portions are affected by interaction of the side groups. In contrast, the yield stress and initial modulus do not depend on the chemical nature of the co-unit but only on the crystallinity level. In the regions of overlap there is a substantial difference between the yield stress and initial modulus of linear polyethylene and the copolymers. Different portions of the force-elongation curves are governed by different structural and molecular features indicating the complexity of the problem. The wide range of experimental data that are presented, for thoroughly characterized samples, can serve as a basis for the molecular understanding of the ductile deformation of random copolymers.

The crystallization behavior of random copolymers of ethylene

The crystallization behavior of random ethylene—1-alkene copolymers is reviewed. Major attention is focused on fractions and samples that have well-defined and narrow molecular mass and composition distributions. Emphasis is given to the analysis of thermodynamic properties, the basic elements of phase structure, the purity of the crystalline phase and the supermolecular structure. The molar mass, co-unit content and chemical type are treated as independent variables in the analysis of a large amount of experimental data. As is expected from theoretical considerations, the thermodynamic properties are very sensitive to the sequence distribution of the co-units in the chain. It is found that, except for directly bonded methyl groups, the crystallite structure remains pure, irrespective of the chemical nature of the co-unit. Thermodynamic properties and the major elements of phase structure, with the exception of the interfacial structure, are independent of the nature of the side group for groups larger than methyl. The analyses of a large amount of experimental data makes it clear that the molar mass and co-unit content need to be treated as independent variables.

Polysaccharides in desert reclamation: Compositions of exocellular proteoglycan complexes produced by filamentous blue-green and unicellular green edaphic algae

The filamentous blue-green alga Nostoc calcicola Geitler, strain 79WA01, showed a peak production of ∼70% of its biomass as a mixture of exocellular proteoglycan complexes, containing 3–30% of a polypeptide moiety and >70% of a complex glycuronoglycan. The former contained high proportions of Asp, Glu, Arg, and amido-NH 3, in addition to ∼35% of “hydrophobic” amino-acids. The latter varied in composition in different fractions: GalA (2.5–10.3%), GlcA (4.7–11.5%), Glc (11.7–39.0%), Xyl (5.7–17.9%), Man (2.7–9.5%), Gal (5.7–9.5%), Fuc (1.5–11.1%), Ara (1.9–4.3%), and Rha (1.4–4.4%). None of the fractions showed a stoichiometric ratio of sugar residues. Palmelloid cells of three unicellular green soil-algae of the genus Chlamydomonas yielded ∼70% of their dry weight as capsular mucilage. About 50% of the sodium salt of this material was soluble in water, and contained 3–12% of polypeptide and 88–97% of glycuronoglycan (GlcA:Glc:Xyl = 1:1:3 for C. humicola Lucksch, and GlcA:Gal = 1:2 for C. peterfii Gerloff and C. sajao Lewin). These categorical differences in sugar composition, together with narrow composition distributions, suggested regular structures for the glycuronoglycans. The remainder of the mucilages contained essentially the same glycuronoglycan chains, but a higher proportion of polypeptide. These materials did not form true solutions in water, but dispersed as microgel particles.

Separation and characterization of low‐conversion poly(styrene‐co‐methyl methacrylate) by liquid adsorption chromatography

AbstractPoly(styrene‐co‐methyl methacrylate) copolymers of narrow chemical composition distributions were separated according to composition by linear gradient elution at column temperatures between 10 and 80°C using silica gel as an adsorbent. The initial mobile phase was a mixture of chloroform and ethanol (99.0 : 1.0, v/v) and the final one (95.5 : 4.5). The relationship of retention volume and the composition of the copolymers was obtained. Higher column temperature (and/or less ethanol content in the mobile phase) is preferable for the separation of copolymers having less MMA and vice versa. Molecular weight dependence on retention volume was not observed. The chromatographed copolymers were still chemical heterogeneous: the compositional difference of the front and rear halves of an adsorption chromatogram of a copolymer was 2.1 %. The front half had a higher styrene content and lower molecular weight averages than those for the rear half of the peak. This separation method was reproducible and the relative standard deviation was about 0.6%. Recovery of fractions was 98.2% by isocratic elution and 96.4% by gradient elution.

The structure of copolymer crystals formed from dilute solution and in bulk

AbstractThe thermodynamic and structural properties of a series of hydrogenated polybutadienes, crystalized in the bulk and from dilute solution, have been investigated. These polymers are ethyl‐branched ethylene copolymers with narrow molecular weight and composition distributions. Despite the fact that for both modes of crystallization these random‐sequence copolymers display a lamellar crystalline habit, all of the independent physical chemical measurements are quantitatively consistent in indicating a relatively thin crystallite with a large amorphous disordered overlayer. This is seen to be a very general phenomenon for copolymer crystallization. The core thicknesses, determined from Raman LAM and from small‐angle x‐ray scattering, are in good agreement. Quantitatively consistent values of the degree of crystallinity are obtained from the density, enthalpy of fusion, Raman internal modes, Raman LAM, and small‐angle x‐ray scattering. It is significant that independent thermodynamic and structural methods give the same result.

The compositional inhomogeneity of ethylene-propylene copolymers formed by polymerization with the (C 2H 5) 2AlClVOCl 3, [formula omitted] and (iso-C 4H 9) 2AlClVOCl 3 catalyst systems

It is shown that the compositional inhomogeneity of ethylene-propylene copolymers obtained with the given systems is explained by the presence in the latter of both soluble and insoluble complexes, with different activities in copolymerization of ethylene and propylene. Copolymers with a narrow compositional distribution are formed from the soluble catalyst, and copolymers with a broad distribution from the insoluble catalyst. Further inhomogeneity arises as a result of precipitation from solution of the part of the copolymer richer in ethylene and subsequent chain propagation within this copolymer in bulk. When the 1 2 ( C 2 H 5) 3 Al 2 Cl 3 VOCl 3 complex system is prepared beforehand in the absence of monomers, at Al:V ratios of 9:1 and 18:1, copolymers of homogeneous composition are formed, because almost all of the catalyst is soluble in the hydrocarbons.

The effect of catalytic system on the composition of ethylene-propylene copolymers

Copolymers with a relatively narrow composition distribution (less than 5% deviation from the average) can be produced in the ethylene copolymerization with propylene over catalytic complexes based on diethyl-, sesquiethyl- and di-isobutyl-aluminium chlorides with VOCl 3 or VCl 4, prepared in the presence of the monomers. Copolymers with an irregular composition will be obtained when the alkyl aluminium chlorides are replaced by tri-isobutyl aluminium under the same conditions. The method of preparation of the catalytic complex (with or without the monomers present) i.e from 1 2 ( C 2 H 5) Al 2Cl 3&.zsbnd;VOCl 3 , does not affect the distribution range of composition in the copolymers, while other systems, such as (C 2H 5) 2) AlClVOCl 3, or iso-(C 4H 9) 2AlCl 3VOCl 3, will give a much greater composition irregularity when the complex is prepared without the monomers.

Infrared Determination of Composition of Ethylene-Propylene Copolymers

Abstract Carbon-14 labeled ethylene and propylene were used to synthesize a series of copolymers of known composition to serve as standards for copolymer analyses. Polymers with broad and narrow compositional distributions and differing sequence distributions were produced by varying the catalyst system. Solution and combustion counting techniques were used to determine sample composition and then infrared calibration curves were determined on pressed polymer films utilizing several different infrared peaks. 1. Within the ranges systematically varied neither compositional nor sequence distributions affect the peak ratios studied. 2. No isotope effects exist in the polymerizations. 3. NMR analysis yields the same results as 14C analysis. 4. Use of the 1378 cm−1 methyl band as calibrated with atactic polypropylene yields agreement with the 14C data if the average of polypropylene and copolymer results is used. 5. We amend the results previously published by our laboratory.

Molecular weight distributions of block copolymers of poly(isoprene:styrene) by gel permeation chromatography

Gel permeation chromatograms of block copolymers of cis 1:4 polyisoprene and polystyrene with narrow molecular weight and composition distributions have been examined by the normal - probability plot method. The parameters M̄ w, M̄ n and standard deviation σ have been evaluated and the results compared with M̄ N obtained by high speed osmometry. Fractionation appears to be achieved according to chain length for a particular composition.

Transition metal catalysts. IX. Random ethylene‐propylene copolymers with a low pressure polymerization catalyst

AbstractEthylene–propylene copolymers were prepared at atmospheric pressure with the VCl4–(C6H5)4Sn–AlBr3, hydrocarbon‐soluble, polymerization catalyst. A controlled rate of initiation and thus a constant comonomer composition were obtained by the introduction of a constant amount of oxygen into the monomer stream. Four copolymers, containing 2.6, 3.8, 10, and 27 mole‐% propylene, were found to have uniform comonomer compositions when fractionated by successive extraction with boiling solvents, in contrast to copolymers prepared with other reported Zieglertype catalysts. An elution fractionation of the 10 mole‐% propylene copolymer was conducted with cellosolve‐xylene mixtures above the copolymer melting point. The molecular weight distribution was shown to approximate the “most probable” distribution. This is in agreement with a previously proposed polymerization mechanism. The propylene contents of each of the fractions from the 10% copolymer were within ±1% propylene of the value of the whole copolymer. This narrow composition distribution is in accord with the statistical distribution predicated theoretically for random copolymers and is in accord with the previous finding that r1r2 ≅ 1. These data suggest that Ziegler‐type catalysts containing a single active species are amendable to the same statistical and kinetic treatment that has been used for the classic polymerization catalysts.1Phillips, G. W., and W. L. Carrick, J. Am. Chem. Soc., 84, 920 (1962).

Transition Metal Catalysts. IX. Random Ethylene-Propylene Copolymers with a Low Pressure Polymerization Catalyst

Ethylene–propylene copolymers were prepared at atmospheric pressure with the VCl4–(C6H5)4Sn–AlBr3, hydrocarbon-soluble, polymerization catalyst. A controlled rate of initiation and thus a constant comonomer composition were obtained by the introduction of a constant amount of oxygen into the monomer stream. Four copolymers, containing 2.6, 3.8, 10, and 27 mole-% propylene, were found to have uniform comonomer compositions when fractionated by successive extraction with boiling solvents, in contrast to copolymers prepared with other reported Zieglertype catalysts. An elution fractionation of the 10 mole-% propylene copolymer was conducted with cellosolve-xylene mixtures above the copolymer melting point. The molecular weight distribution was shown to approximate the “most probable” distribution. This is in agreement with a previously proposed polymerization mechanism. The propylene contents of each of the fractions from the 10% copolymer were within ±1% propylene of the value of the whole copolymer. This narrow composition distribution is in accord with the statistical distribution predicated theoretically for random copolymers and is in accord with the previous finding that r1r2 ≅ 1. These data suggest that Ziegler-type catalysts containing a single active species are amendable to the same statistical and kinetic treatment that has been used for the classic polymerization catalysts.11