Effect of molecular structure in branched polyethylene on adhesion properties with polypropylene

Adhesion properties between branched polyethylene (PE) and isotactic polypropylene (PP) were studied by a peel test and scanning electron microscopy. In this study, two types of branched PEs were used; one is a linear low density polyethylene (LLDPE) and the other is a high pressure low density polyethylene (LDPE). The adhesive strength of the LLDPE/PP is much higher than that of LDPE/PP. Furthermore, the formation of PE influxes between PP spherulites has a small effect on the adhesion. The dynamic viscoelastic measurements for the binary blends composed of branched PE and PP were also carried out to estimate the interfacial tension by using a rheological emulsion model proposed by Palierne. The interfacial tension is 1.0 mN for LLDPE/PP and 2.1 mN for LDPE/PP, suggesting that the interfacial thickness of LLDPE/PP is about twice that of LDPE/PP. The adhesive strength between branched PE and PP will be determined by the interfacial thickness, which represents the entanglements between two polymers. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 457–463, 1998     

Rheological properties of branched low-density polyethylene

The shear flow properties of six commercially available long-chain branching low-density polyethylene resins were determined, using a cone-and-plate rheometer at low shear rates and a capillary rheometer at high shear rates. Also determined were the elongational viscosities of the resins, using an apparatus developed by Ide and White. Interpretation of the rheological measurements is given with the aid of the molecular parameters, namely, molecular weight and molecular weight distribution.     

支化PE的结构与性能

采用均相和负载α-二亚胺镍配合物催化乙烯聚合制备不同支化度的聚乙烯(PE),探讨支化度对PE密度、熔融行为、结晶度和晶型、热稳定性等的影响。随着支化度的增加,PE密度近于线性减小,熔点随之下降,结晶度呈线性下降,但热稳定性并不是单调下降。PE的差示扫描量热法升温曲线的熔融峰随着支化度增大而变低,熔融区域变宽;支化度达到48个/1 000 C时出现双峰,此时PE结晶度下降至13.0%,且衍射角为23°附近时,(200)晶面衍射峰消失,表明己转变为非结晶态的PE。     

Elution column fractionation of branched polyethylene

In a systematic study of the elution column fractionation of a well characterized branched polyethylene sample it was found that one of the most important factors contributing to the efficiency was the composition selected for the p-xylene-butyl cellosolve eluting mixture. With a mixture representing the critical solvent composition at 126°C., as determined by cloud point measurements, and deposition to an amorphous phase on a fine sand support, viscosity–distribution curves were obtained which were highly reproducible and completely free of any reversal. No further improvement was gained by using an elution temperature other than 126°C. or by the substitution of tetralin or mesitylene for p-xylene. Precipitation down a temperature gradient to produce a physical separation of species provided enhanced fractionation but is of doubtful practical interest. Fractionation was readily scaled up from 2g. to 4 or 6g., merely by increasing the mixing vessel size from 500 cc. to 1000 cc. There was no loss of efficiency, and fractions with M?_w = 8 × 10⁶, more than twentyfold higher than the sample value, were obtained despite eluted polymer concentrations exceeding 0.2%. There is evidence that many of the expected differences in the fractionation behavior of linear and branched polyethylene are erased when a solvent mixture is used which has been adjusted to the appropriate critical composition.     

Critical insights into ethylene-1-alkene copolymers and long-chain branched polyethylene microstructure on thermo-rheological and sealing behavior of multicomponent systems

The microstructure-property relationships in multicomponent ethylene-1-alkene copolymers with different branching in the microstructures are demonstrated. The metallocene catalyzed linear low density polyethylene (mLLDPE), was miscible with both autoclave grade low density polyethylene (LDPE-A) and/or tubular grade low density polyethylene (LDPE-T). For these multicomponent systems, the rheological response was distinctly differentiating and sensitive to the microstructure of LDPE, at higher shear regimes. The thicker lamellae of LDPE-T and/or LDPE-A might co-crystallize if there is a high density polyethylene-like fraction present in the mLLDPE. Even though the macro parameters like density and melt index (MI) of the investigated multicomponent systems are comparable, the subtle differences in the microstructure manifested by type and distribution of comonomer and/or branching affected the sealing performance. Both high comonomer content and comonomer distribution in the mLLDPE matrix affording a higher fraction of material melting below 120°C were found to be critical for the heat sealing. The fraction of material melting at lower temperatures, attributed to the tertiary branches present in the hyper-branched microstructure of LDPE-A, participate in the sealing process, and lower the sealing temperature. It was evident that mLLDPE with asymmetric distribution of lamellae is more sensitive to the microstructure of the LDPE used.     

Synthesis of long-chain branched isotactic-rich polystyrene via cationic polymerization

Cationic polymerization of styrene was conducted with 1-chloro-1-phenylethane (SCl)/AlCl₃/phenyl methyl ether (PME) initiating system in hexane/CH₂Cl₂ (60/40, v/v) at ?80 °C. The kinetics for cationic polymerization of styrene was investigated by in-situ ATR-FTIR spectroscopy. The isotactic-rich polystyrene (iPS) with m dyad of 81%, mm triad of 63% and mmmm pentad of 50% could be synthesized. Small amounts of crystalline regions in iPS formed after flow-induced crystallization and the crystallinity increased with increasing the molecular weight of iPS. Furthermore, the long-chain branched isotactic-rich polystyrene (biPS) with around 12 times higher molecular weight than that of corresponding iPS could be synthesized via cationic polymerization of styrene by introducing a small amount of isoprene (Ip) as a comonomer and branching sites as well. The possible mechanism for long-chain branching formation via intermolecular alkylation reaction by using Ip structural units along polymer chain as branching sites was proposed. The nucleation rate of biPS could be greatly enhanced with increasing the content of branching sites, leading to an obvious increase in crystallinity. The multi-melting temperatures from 140 °C to 237 °C were observed in DSC curves of these PS products. The tensile strength of commercial atactic polystyrene could be improved remarkably from 41.4 MPa to 55.7 MPa by adding 16.7% of biPS.     

支化及超支化聚乙烯研究进展

综述了支化及超支化聚乙烯的制备方法及功能化改性研究进展,重点介绍了后过渡金属催化剂催化乙烯"链行走"制备超支化聚乙烯的研究进展,同时介绍了功能性超支化聚乙烯的应用,并展望了其发展前景。     

Stress-induced crystallization of branched polyethylene terephthalate films

Polyethylene terephthalate (PET) prepolymer containing 0.0 to 0.3 mole percent pentaerythritol was polymerized to an inherent viscosity of 0.63-0.70 dl/gm by the fluidized bed technique. Rheological studies and gel permeation chromatography (GPC) examination showed the samples to be branched in character. Amorphous films were stretched at 82 and 93°C at elongation rates of 54, 161 and 267 percent-s^?1 using the T. M. Long machine. The extent of stress-induced crystallization was established by a density determination. The branched samples behaved very similarly to the linear PET material in crystallization and birefringence studies. Neither the percent crystallinity nor the birefringence appears to be a strong function of strain rate over the range 54-267 percent-s^?1. A cursory examination of the crystallization kinetics of the oriented samples suggests that extent of branching in our samples does not markedly reduce the crystallization rate at annealing temperatures of 180 and 220°C.     

Thermorheology as a method to analyze long-chain branched polyethylenes

This paper presents correlations between polyethylenes of different compositions and branching architectures and their viscoelastic behavior in dependence on the temperature and demonstrates how effectively rheological experiments can be used for analytical purposes. Long-chain branched polyethylenes are known to be thermorheologically complex. But the thermorheological complexity of long-chain branched linear low-density metallocene polyethylenes (LCB-mLLDPE) differs from that of low-density polyethylenes (LDPE) in the way that the activation energy of LDPE becomes constant by a temperature-dependent modification of the moduli whereas a constant activation energy cannot be obtained for LCB-mLLDPE. These findings are explained by the assumption that the LCB-mLLDPE investigated consist of at least two species with distinctly different activation energies. This interpretation is supported by the thermorheological analysis of a blend of known parts of an LDPE and a linear low-density polyethylene (LLDPE). A thermorheological complexity was found similar to that of the LCB-mLLDPE which reflects the different activation energies of the two components. Results of that kind make it possible to get information on the composition of LCB-mLLDPE not available from common analytical methods.     

Relative rates of sulfonation of linear and branched long-chain alkylbenzenes

Relative rates of sulfonation of a series of 1-phenylalkanes and isomeric branched-chain alkylbenzenes by gaseous SO₃ have been determined. It has been shown that the composition of the mixture undergoing sulfonation has an important bearing on the actual relative rate constant (k _rel) values obtained. This is taken as evidence for the involvement of pyrosulfonic acids as the actual sulfonating species involved in the sulfonation reaction. Consistent k _rel values have been obtained by using an “active solvent” in 10-fold excess as the sulfonating medium. Whereas the branched-chain isomers show very little variation in their rates of sulfonation, the rates of sulfonation of the 1-phenylalkanes increase as the length of the alkyl chain decreases.     

Molecular model of partial crystallization of branched polyethylene

A model is proposed which considers various cases of arrangement of crystallizable chain elements of branched polyethylene in lamellar crystalline elements. On the bases of statistical calculations, relationships between crystallinity and long spacing of the lamellar structure are derived. Calculated relations are compared with experimental crystallinity and long spacing values determined for polyethylene samples crystallized under various cooling conditions.     

SEC-MALS method for the determination of long-chain branching and long-chain branching distribution in polyethylene

This paper systematically describes a LCB determination method that can quantify both LCB content and LCB distribution across the molecular weight distribution in polyethylene homopolymers as well as copolymers. Coupling size-exclusion chromatography with multi-angle light scattering (SEC-MALS), this method quantifies molecular weights (MW) and radii of gyration (R_g) simultaneously. The number of LCB per molecule and LCB frequency as a function of MW can be calculated by comparing R_g of a branched polymer with that of a linear control at the same MW using the Zimm-Stockmayer approach. Because the presence of short-chain branching in copolymers results in changes in R_g of the copolymers, their LCB contents cannot be obtained before the short-chain branching (SCB) effect is corrected. Using well-characterized linear PE copolymers as standards, an empirical method is successfully established in this paper to correct the SCB effect. Consequently, this method can be applied to determine LCB in PE copolymers as well. Some practical aspects, such as the selection of formalism for data processing, the LCB detection sensitivity and precision, and long-term reproducibility of this method are also discussed. Finally, examples are given to demonstrate how this method is applied to determine LCB and LCB distribution in practical PE homopolymers and copolymers.     

The nucleation effect of linear polyethylene lamellae over the crystallization of branched polyethylene

Summary The polyamides with aromatic rings in the main chain were synthesized by the solution polymerization of 4,4-diphenylmethane diisocyanate and aliphatic dicarboxylic acid in the presence of catalyst. The thermal properties and the miscibility behaviours with polyamide-6,6 of these aromatic polyamides were studied. The aromatic polyamides synthesized with one kind of dicarboxylic acid had typical thermal properties of crystalline polymers, whereas those synthesized with the mixtures of dicarboxylic acids were not easily crystallized. The observed miscibility behaviours showed some differences from those predicted by binary interaction model.     

Nuclear magnetic relaxation in linear and branched polyethylene

An assessment is made of the information provided by n.m.r. second moment, fourth moment and T₂ measurements on oriented samples of branched and linear polyethylene. Detailed theoretical comparisons are made with the previously reported experimental results of a number of workers in order to derive information about the molecular motions which underlie the α, β and γ relaxations in the two forms of the polymer. Plausible motional models are presented in each case. Particular attention has been given to the β relaxation which is explained in terms of lamellar fold motion. The relative sensitivity of the various n.m.r. measurements to molecular anisotropy and motion in the polymer is discussed.     

Entry pressure and elastic behavior of a branched polyethylene

Die entry pressures and elastic parameters derived from these have been determined for one low-density polyethylene (LDPE) at four temperatures using three dies having different entry angles. The elongational viscosity shows a somewhat different behavior with respect to temperature than the shear viscosity. Mostly the elastic parameters show a temperature-independent relation to the shear stress but only if the data at the lowest temperature used (130°C) are ignored. The latter data show that the material is much more elastic at this temperature than at higher ones; this is so even when the temperature is increased by only 20°C, after which any further increase gives negligible difference in elasticity. This behavior is probably due to a structural difference which may be a result of increased crosslink sites created by flow-induced crystallization. © 1995 John Wiley & Sons, Inc.     

On the kinetics of isothermal crystallization of branched polyethylene

Isothermal crystallization of branched polyethylene was studied by time-dependent small-angle X-ray scattering experiments and dilatometry. A strict proportionality between scattering intensities and density changes was found for all times. This result indicates complete absence of crystal thickening and perfectioning processes. Isotherms directly reflect the increase in specific inner surface resulting from nucleation and lateral growth of lamellae. The shape of isotherms suggests a time-dependent non-uniform internal structure of spherulites with a decrease in specific inner surface with increasing distance from centre.     

Steady flow and dynamic viscoelastic properties of branched polyethylene

The presentation of viscoelastic properties of molten high polymers in a complex plane makes three characteristic rheologic parameters appear. These are examined for a series of commercial samples of low density polyethylene. For instance, it enables products to be recognized which are very similar as far as the melt index is concerned but have different molecular weight distribution, different long chain branching index and consequently different processing properties.     

Characterization of branched polyethylene fractions from the elution column

Fractions from several elution column runs on samples of up to 6 g. of a well-characterized high-pressure polyethylene were analyzed by absolute molecular weight methods and several other techniques. The M_n and M_w integral distribution curves are free from any reversal, as was the viscosity distribution curve. Fractions with M_w as high as 8 × 10⁶ were recovered, more than 20 times higher than the original sample's M_w. The polydispersity of the fractions increases from M_w/M_n = 1.5 or less in the low molecular weight fractions to a nearly constant value of 4.5–5.0 in fractions above 60% cumulative sample weight. Nonetheless, refractionation on the elution column shows that the fractions are narrowly distributed in terms of solubility, while GPC analysis reveals that the fractions have an extremely narrow size distribution. It is concluded from the combined results that long-chain branching plays an important role in determining the equilibrium solubility and, further, that long-chain branching increases the polymer solubility. Sample calculations are provided, which illustrate the effect of fraction polydispersity on calculated original sample molecular weights and the fit of the fractionation results to several model distribution functions.     

Effect of dimethyl formamide in the synthesis of linear low density polyethylene on branched and molecular structure

In this research work, effect of dimethyl formamide (DMF) as an external electron donor in gas phase ethylene/1‐butene copolymerization process in the presence of Ziegler–Natta catalyst has been investigated. Different experiments were performed using TiCl₄/MgCl₂ as catalyst, triethyl aluminum as cocatalyst and in the presence of various amounts of DMF as an external electron donor. Then, effect of DMF on various parameters such as catalyst activity, molecular weight, and short chain branching content of the samples has been studied. The results showed that the more the ratio of DMF/Ti, the lower the catalyst activity. Moreover, the curves relating molecular weight and short chain branching content of the samples to the ratio of DMF/Ti passed through an extremum point at about DMF/Ti = 0.25. A justification for the occurrence of the extremum point has been proposed in this research work, and some analysis methods are adopted to confirm the suggested justification. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1267–1272, 2012