Increasing Comonomer Content Research Articles

The Linear Viscoelastic Behavior of a Series of 3-Hydroxybutyrate-based Copolymers

Abstract The thermal and linear viscoelastic behavior of a series of synthetically produced 3-hydroxybutyrate-based copolymers is reported. The comonomers employed include 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), and 6-hydroxyhexanoate (6HH). The polymers investigated differed in the concentration of comonomer randomly distributed within the material and in the length of the carbon backbone of the comonomer. The glass transition temperature follows the Fox equation and decreases with increasing comonomer concentration. The molecular weight between entanglements Me was determined from the linear viscoelastic data for each copolymer, and a mixing rule was used to determine Me for homopolymers made from the comonomers studied. Depending on comonomer type and concentration, Me can be varied systematically, resulting in a wide range of potential applications depending on the chemistry employed. Using the complex viscosity data, master curves were constructed for both the zero-shear viscosity and the shear-thinning behavior depending upon the temperature and the molecular weight between entanglements. The analysis of rheological results and polymer properties based upon copolymer content provides an opportunity to process bio-based materials for a wide range of applications.

Synthesis and Characterization of Ethylene/Propylene Copolymers in the Whole Composition Range

AbstractSummary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC‐MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples.

Metallocenic Isotactic Poly(propylene) and its Copolymers with 1‐Hexene and Ethylene

AbstractA comprehensive study of the structure and properties has been performed for copolymers of propylene‐1‐hexene, CiPH, and propylene‐ethylene, CiPE, synthesized by an isotactic metallocene catalyst system. The comonomer content constitutes the most important factor affecting the structure and properties of these CiPH and CiPE copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity is observed in the two kinds of copolymers as the comonomer content increases. The structure in the CiPH copolymers evolves, however, from the typical, monoclinic crystal lattice to mesomorphic‐like, ordered entities for the highest 1‐hexene molar fraction, whereas in the CiPE copolymers the structural evolution with molar fraction goes from a monoclinic lattice to an almost amorphous material. All of these variations in crystal structure significantly influence the viscoelastic and mechanical behavior of these CiPH and CiPE copolymers. Consequently, the location and intensity of the different relaxation mechanisms, as well as the rigidity parameters (storage and Young's moduli and microhardness) and deformation mechanism are strongly dependent upon composition.magnified image

Diffusivity of n-hexane in poly(ethylene-stat-octene)s assessed by molecular dynamics simulation

Molecular dynamics simulations have been used to study diffusion of n-hexane in wholly amorphous poly(ethylene-stat-octene)s with comonomer contents ranging from 0 to 11.5mol%. The branches in the polymer increased the specific volume by affecting the packing of the chains in the rubbery state in accordance with experimental data. The diffusion of n-hexane at penetrant concentrations between 0.5 and 9.1wt% was simulated within time-scales between 0.1 and 0.2μs. The penetrant diffusivity unexpectedly decreased with increasing comonomer content. The penetrant molecule motion statistics showed that systems with high comonomer content showed a greater tendency for short distance motion (over a sampling period of 3ps) whereas the systems with lower comonomer content showed penetrant motion over longer distances. It seems that the branches retarded local chain mobility of the polymer thereby trapping the penetrant molecules. All systems studied showed a minimum in penetrant diffusivity at ca. 1wt% n-hexane and a marked increase in diffusivity at higher penetrant concentrations. The volumetric data for the different polymer–penetrant systems were consonant with additional volumes of the different components. Comparison between simulated diffusivities (for a wholly amorphous polymer) and experimentally obtained diffusivity data for semicrystalline polymers showed that constraining effect of the crystals were substantial for the highly crystalline systems and that it gradually decreased with decreasing crystallinity.

Solid-State Organization and Morphological Partitioning in Polyoxymethylene-Based Copolymers: A Solid-State NMR and WAXS Study

The solid-state organization of copolymers based on methylene oxide (MO) units and tetramethylene oxide (T) units, with T unit contents ranging between 0.9 and 10.0 mol %, is investigated by the combined use of solid-state NMR, WAXS, and DSC experiments. In these semicrystalline copolymers, the T units can be viewed as linear defects inserted along linear poly(oxymethylene) chains. As expected, the insertion of T units induces a significant decrease of both crystallinity and crystallite size: a large part of T units is located in the amorphous domains. However, some T units can also be detected within the crystalline domains and/or the interfacial regions with the amorphous phase. More precisely, the amount of T units at both sides of the noncrystalline/crystalline interfaces seems to be much higher than the T units in the interior of the crystallites. At the lowest T unit content (0.9 mol %), the composition averaged over both interfacial and crystalline phases appears to be identical to the composition of the amorphous phase. When the comonomer content increases, the amount of T units in the interfacial and crystalline zones becomes higher and higher while the partitioning coefficient of the T units in these domains tends to a limiting value of 0.40.

Comparison of Crystallization and Melting Characteristics of Quiescent and Melt-Spun Poly(ethylene-co-octene) Copolymers

An experimental study of crystallization and melting of polyethylene and its octene copolymers under quiescent conditions and their melt-spun fibers is presented. This includes copolymers produced by both Dow's Insite® Technology homogeneous catalyst and by a Ziegler heterogeneous catalyst. It is found that with increasing comonomer content, both isothermal and non-isothermal crystallization rate decrease. The melting peaks become broader and the melting temperatures decrease for polymers with more comonomer. Multiple melting peaks are observed for copolymers. Except the unchanged peak with the highest melting temperature, the other peaks move to higher temperature after being annealed under higher temperature. Attempts were made to correlate crystallization kinetics and melting behavior under quiescent conditions with melting temperatures and crystallinities determined for their melt-spun fibers. The results show that besides cooling effects, the melt temperatures and the crystallinities of the melt spun fibers are also affected by the degree of molecular orientation, which is mainly decided by the spin-line stress. These two competitive factors determine the final melt temperatures and crystallinities of the melt-spun fibers.

Characterization of Some New Olefinic Block Copolymers

Exciting new developments in polyolefin synthesis give rise to olefinic block copolymers with properties typical of thermoplastic elastomers. The blocky copolymers synthesized by chain shuttling technology consist of crystallizable ethylene−octene blocks with low comonomer content and high melting temperature (hard blocks), alternating with amorphous ethylene−octene blocks with high comonomer content and low glass transition temperature (soft blocks). This paper describes the materials science of these unique polymers as characterized by thermal analysis, X-ray diffraction, microscopy, and tensile deformation. The crystallizable nature of the hard block and the crystalline morphologies are consistent with an average hard block length that is well in excess of 200 carbon atoms. The crystallizable blocks are long enough to form well-organized lamellar crystals with the orthorhombic unit cell and high melting temperature. The lamellae are organized into space-filling spherulites in all compositions even in copolymers with only 18 wt % hard block. The morphology is consistent with crystallization from a miscible melt. Crystallization of the hard blocks forces segregation of the noncrystallizable soft blocks into the interlamellar regions. Good separation of hard and soft blocks in the solid state is confirmed by distinct and separate β- and α-relaxations in all the blocky copolymers. Compared to statistical ethylene−octene copolymers, the blocky architecture imparts a substantially higher crystallization temperature, a higher melting temperature and a better organized crystalline morphology, while maintaining a lower glass transition temperature. The differences between blocky and statistical copolymers become progressively more apparent as the total comonomer content increases.

Synergistic Activity of Hydrophilic Modification in Antibiotic Polymers

Quaternized poly(vinylpyridine) is known to kill up to 99% of drug-resistant gram-positive and -negative bacteria but shows minimal biocompatibility. We report enhanced bactericidal activity of vinylpyridine through copolymerization with hydroxyethyl methacrylate and poly(ethylene gycol) methyl ether methacrylate. Copolymers with increasing comonomer content were synthesized by radical polymerization and quaternized with hexylbromide. We assessed the effects of the changes in polymer composition on the bactericidal activity of the surface activity using a bioluminescent pathogenic strain of Escherichia coli (O157:H7). By recording the photoluminescence emitted by these bacteria in contact with the copolymers, it was shown that several of the copolymers possess better antibacterial efficiency than quaternized poly(vinylpyridine). Results indicate that several of the copolymers synthesized possess antibacterial activity approximately 20 times greater than the pure quaternized poly(vinylpyridine) homopolymer, while only containing 1 wt % hexylated pyridinium. This behavior is explained by the increased surface wettability of the copolymers containing lesser amounts of poly(vinylpyridine), as bactericidal behavior correlates to the hydrophilicity of the system as measured by contact angles. A hydrophilicity based design-paradigm can significantly improve both the efficacy and the biocompatibility of antibacterial materials.

Simple Ways to Characterize Non-isothermal Crystallization of Homogeneous Poly(ethylene-1-co-octene) Copolymer

Non-isothermal crystallization of homogeneous poly(ethylene-co-1-octene) copolymers with different octene contents was studied using power-compensating differential scanning calorimetry as functions of cooling rate and comonomer content. With increasing cooling rate, the crystallization temperature decreases for all polymers. A single crystallization peak was observed for polyethylene, while multiple crystallization peaks were observed for copolymers. The non-isothermal crystallization rate of these copolymers was estimated by continuous cooling transformation (CCT) curves, crystallization rate coefficient (CRC), and crystallization rate parameter (CRP). With increasing comonomer content, the CRC value decreases and CCT curves shift to longer time under the same degree of supercooling, indicating the crystallization rate decreases. However, the CRP value can't successfully estimate the crystallization rate of these copolymers.

Chemical composition distributions and microstructures of ethylene–hexene copolymers produced by a rac-Et(Ind) 2ZrCl 2/TiCl 4/MAO/SMB catalyst

A silica-magnesium bisupport (SMB) was prepared by a sol–gel method for use as a support for a metallocene/Ziegler–Natta hybrid catalyst. The SMB was treated with methylaluminoxane (MAO) prior to the immobilization of TiCl 4 and rac-Et(Ind) 2ZrCl 2. The prepared rac-Et(Ind) 2ZrCl 2/TiCl 4/MAO/SMB catalyst was applied to the ethylene–hexene copolymerization with variable amounts of 1-hexene used. The catalytic activity of rac-Et(Ind) 2ZrCl 2/TiCl 4/MAO/SMB showed a volcano feature with respect to the amount of comonomer used. The melting point attributed to the Ziegler–Natta catalyst was not changed significantly, while that attributed to the metallocene catalyst was broadened and gradually shifted to the lower temperature with increasing 1-hexene content. The number of chemical composition distribution (CCD) peaks was increased and the short chain branches were distributed over the lower temperature region with increasing 1-hexene content. The majorities of lamella distributions whose content was more than 50 wt.% shifted to the smaller lamella size with increasing 1-hexene content. The average sequence length of ethylene was steadily decreased with increasing comonomer content, while that of 1-hexene was not changed significantly. Both blocky sequence ([EHH]) and non-blocky sequence ([EHE]) were increased with increasing comonomer content. However, the increasing rate of blocky sequence formation was much higher than that of non-blocky sequence.

Comparison of propylene/ethylene copolymers prepared with different catalysts

AbstractThis study compared a series of experimental propylene/ethylene copolymers synthesized by a transition metal‐based, postmetallocene catalyst (xP/E) with homogeneous propylene/ethylene copolymers synthesized by conventional metallocene catalysts (mP/E). The properties varied from thermoplastic to elastomeric over the broad composition range examined. Copolymers with up to 30 mol % ethylene were characterized by thermal analysis, density, atomic force microscopy, and stress–strain behavior. The xP/Es exhibited noticeably lower crystallinity than mP/Es for the same comonomer content. Correspondingly, an xP/E exhibited a lower melting point, lower glass transition temperature, lower modulus, and lower yield stress than an mP/E of the same comonomer content. The difference was magnified as the comonomer content increased. Homogeneous mP/Es exhibited space‐filling spherulites with sharp boundaries and uniform lamellar texture. Increasing comonomer content served to decrease spherulite size until spherulitic entities were no longer discernable. In contrast, axialites, rather than spherulites, described the irregular morphological entities observed in xP/Es. The lamellar texture was heterogeneous in terms of lamellar density and organization. At higher comonomer content, embryonic axialites were dispersed among individual randomly arrayed lamellae. These features were characteristic of a copolymer with heterogeneous chain composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1651–1658, 2006

Syndiotactic polypropylene and its copolymers with alpha-olefins. Effect of composition and length of comonomer

A set of copolymers of propene and alpha olefins (1-hexene, 1-octene and 1-octadecene) and the corresponding homopolymer (sPP) have been synthesized using a syndiotactic metallocene catalyst. The effect of either the incorporation or length of these conomomeric units on the structure and final properties exhibited has been analyzed. As expected, there is a considerable decrease in crystallinity with the increase of comonomer content. Thus, a completely amorphous copolymer is obtained if the molar fraction is high enough. The structural variations drastically influence the viscoelastic and mechanical behaviors of these copolymers. Three or four relaxation processes can be observed depending on composition and length of comonomer. At temperatures above the process associated with the glass transition (β relaxation), a deep drop (in one or two steps) in E′ and a shoulder in E″, overlapped with that β mechanism, are observed. The existence of a relaxation involving crystalline regions is postulated because of its variation with crystal characteristics. Moreover, a relaxation related to internal motions within the comonomeric units is seen in the range of very low temperatures. This process, primarily ascribed to movement of methylenic segments within the comonomer, is strongly depending on composition and length of the pendant aliphatic chains on incorporated units. On the other hand, a decrease in stiffness and microhardness as well as the brittle–ductile transition are observed by simply varying composition when deformation takes place at room temperature.

Temperature induced phase transition of interpenetrating polymer networks composed of poly(vinyl alcohol) and copolymers of N-isopropylacrylamide with acrylamide or 2-acrylamido-2-methylpropyl-sulfonic acid

Interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly( N-isopropylacrylamide- co-X) (PNIPAAm- co-X) were prepared by using a two-step method. Hydrophilicity of PNIPAAm chains was increased by adding acrylamide (AAm; hydrophilic neutral monomer) or 2-acrylamido-2-methylpropylsulfonic acid (AMPS; anionic charged monomer). The effect of the incorporation of chemically crosslinked PVA into the temperature induced phase transition as well as swelling behavior of the responsive hydrogels were studied. The volume phase transition (VPT) of IPNs was investigated by cloud point measurement and differential scanning calorimetry. Significant differences in volume phase transition enthalpies of PNIPAAm (Δ H VPT) and volume phase transition temperatures ( T VPT) were found. The Δ H VPT decreases with increasing co-monomer concentration. In several cases the phase transition was not followed by macroscopic shrinking of IPNs. Thus this property is advantageous for several technical applications.

Micromechanical interpretation of the modulus of ethylene–(meth)acrylic acid copolymers

Statistical incorporation of a comonomer into polyethylene is well known to reduce its crystallinity, and consequently the small-strain modulus. However, we have found that when the comonomer is methacrylic or acrylic acid, the modulus of homogeneous copolymers initially decreases and then increases with increasing comonomer content. The modulus increase is traced to an elevation of the amorphous phase glass transition temperature (Tg) with comonomer content; Tg passes through room temperature at comonomer contents sufficiently low that substantial crystallinity remains. When the modulus is measured at temperatures above Tg, such that the amorphous phase is fully relaxed, a monotonic increase in modulus with crystallinity is both anticipated and observed. Several two-phase composite models are investigated to describe the modulus vs. crystallinity data above Tg; the Davies model provides a good quantitative description for the copolymers examined. The Davies model may also be applied at temperatures where the amorphous phase is incompletely relaxed, to extract its contribution to the semicrystalline polymer's modulus.

Metallocenic Copolymers of Isotactic Propylene and 1‐Octadecene: Crystalline Structure and Mechanical Behavior

AbstractSummary: A set of copolymers of isotactic propylene and 1‐octadecene (CiPOD) and the corresponding isotactic poly(propylene) homopolymer (iPP) have been synthesized using a metallocene catalyst. The influence of the incorporation of this long chain comonomer on the structure and final properties exhibited by iPP has been examined. Wide‐angle X ray diffraction and optical microscopy have been used to analyze the effect of introducing 1‐octadecene comonomer within the iPP backbone. The initial monoclinic crystal lattice with a well‐developed lamellar morphology is transformed into a mesomorphic modification showing very small entities. All these structural changes significantly influence the mechanical behavior of these copolymers. Thus, stiffness and microhardness are diminished and the brittle‐ductile transition can be observed by simply varying composition when deformation takes place at room temperature. On the other hand, the intensity of the process associated with cooperative movements within the amorphous phase (β relaxation) significantly goes up with the incorporation of 1‐octadecene and its location is shifted to lower temperature because of the decrease in the crystallinity content.X‐ray diffraction patterns at room temperature, with increasing comonomer content.magnified imageX‐ray diffraction patterns at room temperature, with increasing comonomer content.

A quantitative differentiation method for acrylic fibers by infrared spectroscopy

Absorbance peak areas of nitrile (2240 cm −1), carbonyl (1730 cm −1) and C H (1370 cm −1) groups were obtained for 48 colorless acrylic fibers by infrared (IR) microspectroscopy. The carbonyl signal, related to the comonomers most commonly used in acrylic fibers, was ratioed against the nitrile and C H bands, pertaining to the backbone of the polymer chains. The ratios A 1730/ A 2240 and A 1730/ A 1370, a relative measure of the comonomer content in the fiber, were used to differentiate the samples. A decrease in the crystallinity of fibers has been noted with increasing comonomer content. Relative standard deviation (R.S.D.) of the ratios were 1 and 3% for repetitive analyses on the same location and along the length of the same single fiber, respectively. When different fibers of the same sample were examined, results were reproducible within 6%. This simple method can greatly enhance the evidential value of colorless acrylic fibers, being able to discriminate them and thus helping the Court to better assess their significance.

Structure and Properties of Homogeneous Copolymers of Propylene and 1-Hexene

The solid state structure and properties of homogeneous copolymers of propylene and 1-hexene were studied by examining melting behavior, dynamic mechanical response, and morphology primarily with atomic force microscopy, wide- (WAXS) and small-angle X-ray scattering, and tensile deformation. Chain microstructure was analyzed by 13C NMR. The results indicate that copolymers used in this study have an essentially random distribution of comonomer. For copolymers with less than 10 mol% hexane, crystallinity decreases with increasing comonomer content, as expected for exclusion of comonomer from the polypropylene crystal. The peak melting and crystallization temperatures also decrease with increasing hexene content. Copolymers with more than 10 mol% hexane crystallize with a new crystal structure that permits incorporation of hexene units. This is inferred from a higher level of crystallinity than would be expected if comonomer were excluded from the crystal and better development of the crystals as the hexene content increases. Copolymers with the new crystal structure crystallize slowly. After an incubation period, long fibrous lamellae form sheaf-like arrays that develop into small spherulites. The corresponding enthalpy change as a function of time assumes an S-shape characteristic of a phase transition described by the Avrami process. The new crystallographic form has not been reported for either polypropylene or for poly(1-hexene). It follows from WAXS studies of highly oriented films that the crystallographic unit cell has orthorhombic symmetry with a = 1.9860 nm, b = 1.7176 nm, and c = 0.6458 nm. The most intense diffracting planes are identified as the (210) plane reflecting at 2θ = 10.30°, the (230) plane reflecting at 2θ = 17.65°, the (040) plane reflecting at 2θ = 20.60°, the (031) plane reflecting at 2θ = 20.73°, and the (112) plane reflecting at 2θ = 28.52° for Cu Kα radiation. On the basis of pole figures, it is evident that the easiest slip during plastic deformation of the new crystal form occurs along (0k0) planes.

Synthesis and properties coming from the copolymerization of propene with α-olefins using different metallocene catalysts

Copolymerization of propene with two α-olefins (1-hexene and 1-octadecene) using iso- and syndioselective metallocene catalysts (EtInd2ZrCl2, Et(2-MeInd)2ZrCl2, Me2SiInd2ZrCl2, Ph2CFluCpZrCl2 and Me2CFluCpZrCl2) activated with methylaluminoxane (MAO) is reported. The so-called comonomer effect was seen in the catalytic activity of the Me2SiInd2ZrCl2/MAO system. Incorporation of syndiotactic copolymers was greater than that of isotactic copolymers. The molecular weight of the isotactic copolymers was not affected significantly by the presence of the comonomer, but the molecular weights of the copolymers obtained with the syndioselective catalysts decreased with increasing comonomer concentration in the medium. Tensile properties were studied. Syndiotactic copolymers with incorporation of the order of 6mol% of 1-octadecene presented elastomeric properties.

Isothermal crystallization of metallocene-based propylene/?-olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene-1 copolymers and two propylene/octene-1 copolymers prepared with metallocene catalyst were investigated. It is found that γ-modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (Tc) the crystallization rate constant changes more rapidly with Tc and the crystallization half-time substantially increases. Double melting peaks were also observed at high Tc, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005

Nonisothermal crystallization of metallocene propylene–decene‐1 copolymers

AbstractThe nonisothermal crystallization behavior of one metallocene‐based isotactic polypropylene and three propylene–decene‐1 copolymers was studied. The effects of comonomer content and cooling rate were investigated. It was found that comonomer units enchained systematically reduce the crystallization temperature (Tc), melting temperature (Tm), fusion enthalpy (ΔHf), and crystallinity (Xc). Such an effect becomes more evident at a faster cooling rate. With increasing comonomer content, the supercooling required for crystallization increases and the overall crystallization rate is reduced. The Avrami equation is applicable to describe the nonisothermal crystallization kinetics of propylene–decene‐1 copolymer. It was shown that, although the reduced crystallization rate constant Zc increases with comonomer content, the Avrami exponent decreases with comonomer content and cooling rate, leading to the smaller overall crystallization rate and larger crystallization half‐time of the copolymer with higher comonomer content. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1724–1730, 2004