Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

硅烷接枝交联HDPE、LLDPE及其共混物的结构研究

利用红外光谱、差示扫描量热法等方法研究了高密度聚乙烯(HDPE)、线性低密度聚乙烯(LLDPE)及其共混物的乙烯基三乙氧基硅烷(VTEOS)接枝及交联产物的分子结构、熔融行为。结果表明,VTEOS接枝交联PE能力为:LLDPE>HDPE/LLDPE共混物>HDPE;接枝和交联使HDPE、LLDPE及其共混物的结晶度和熔点降低,晶粒变得不均匀。     

Reactive processing of LLDPEs in corotating intermeshing twin-screw extruder. I. Effect of peroxide treatment on polymer molecular structure

Commercial ethylene-octene linear low-density polyethylene (LLDPE) polymers were reactively extruded with low levels of 2,5-dimethyl-2,5 di(t-butylperoxy)hexane to modify their molecular structure and processing properties. Peroxide levels were kept low to avoid crosslinking. This article examines the effects of reactive extrusion in a corotating intermeshing extruder. Gel content analyses and examination of extruded thin tapes indicated that the products were gel-free, but line-broadening in high-resolution ¹³C-NMR spectra suggested that some crosslinking did occur. Molecular weight distributions were broadened toward higher molecular weights, as expected. SEC estimates of long-chain branching in reacted polyethylenes were consistent with the results of ¹³C-NMR analyses. Under our extrusion conditions, the products contained about one long branch per number-average molecule. This result and data on changes in carbon-carbon unsaturation indicate that the major chain extension mechanism is an end-linking reaction between terminal vinyls or allylic radicals formed at chain ends and secondary radicals. Both types are produced by hydrogen abstraction on the LLDPE. All long branches originated at tertiary branch points. Changes in thermal behavior, as measured by DSC analyses, paralleled those observed by temperature-rising elution fractionation (TREF). SEC molecular weight measurements and long-branch determinations by SEC and ¹³C-NMR can be used to quantify the effects of peroxide treatment on the molecular structure of polyethylenes. DSC and TREF techniques, however, appear to be more sensitive than are SEC or NMR. Relatively minor variations in the degree of mixing and temperature control during reactive extrusion have noticeable effects on the molecular structures of the peroxide-treated LLDPEs. © 1995 John Wiley & Sons, Inc.     

Rheology of metallocene-catalyzed monomodal and bimodal polyethylenes

The effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features different from conventional polydisperse and bimodal polyethylenes. Higher values of Newtonian viscosity (η_o) at the same values of weight average molecular weight (M_w) and stronger frequency dependence of dynamic viscosity (η′) than in conventional HDPE-s have been observed; this leads to lower values of the characteristic frequency for the onset of non-Newtonian behavior (ω_o) and higher values of the power law index (α). These features are probably due to the presence of very small amounts of long chain branching (LCB). The implications of these results in polymer processing are analyzed comparing extrusion rheometer data, which leads to the conclusion that extrusion difficulties in metallocene catalyzed polyethylenes can be overcome with bimodal molecular weight distributions.     

Slow crack growth resistance in resin blends of chromium and metallocene catalyzed ethylene‐hexene copolymers for pipe applications

A bimodal system has been developed by blending a high density and linear medium density ethylene‐hexene copolymers synthesized with chromium and metallocene catalysts, respectively. The resistance to slow crack growth examined by the Pennsylvania Edge‐Notch Tensile test (PENT test‐ASTM F1473) and the crack opening displacement was determined at 80°C and 2.4 MPa. The effects of molecular and morphological structure on the slow crack growth (SCG) resistance were evaluated, the molecular weight and lamellar thickness being the most critical parameters for this system. The great importance of the short chain branching content and distribution was determined and discussed by SEC‐FTIR. Therefore, two main factors were found determinant in the SCG resistance. On the one hand, the increase of tie molecule density and therefore the continuity in the network formed by crystals and tie molecules and, on the other hand, the short chain branching density increase as the linear medium density ethylene‐hexene copolymer fraction does so. In addition, a morphological analysis of the fracture surface was performed in order to analyze the fracture mechanisms that took place. Differences observed in the fracture surface morphology were related to the molecular and morphological structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Higher‐molecular‐weight hyperbranched polyethylenes containing crosslinking structures as lubricant viscosity‐index improvers

As a new grade of polyethylene materials with unique chain architectures, hyperbranched polyethylenes synthesized by chain walking ethylene polymerization have great potential for industrial application as novel viscosity index (VI) improver in lubricant formulation. Although high‐molecular‐weight hyperbranched polyethylenes (weight‐average molecular weight of about 10⁵ g/mol) possess high shear stability, their viscosity thickening properties are compromised due to their compact chain architectures. In this work, we aim at improving their viscosity thickening property by increasing polymer molecular weight. A range of hyperbranched polymers of various enhanced molecular weights were synthesized by chain walking ethylene polymerization in the presence of small amounts of 1,4‐butanediol diacrylate as a difunctional crosslinker. The molecular weight dependences of viscosity thickening power and shear stability of these polymers containing crosslinking structures were evaluated. It is found that, with the increase of molecular weight via crosslinking, these polymers showed consistently enhanced viscosity thickening power, but with the reduced shear stability. However, their shear stability was still significantly better compared to linear polymers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Effect of molecular structure and topology on network formation in peroxide crosslinked polyethylene

The present study compared the crosslinking performance of single site linear low density polyethylenes (LLDPE) with high pressure, free radical polymerised, low density polyethylenes (LDPE). The difference in crosslinking performance is not fully explained by different structural parameters such as molar mass distribution (MMD), M_n, MFR₂ value and vinyl groups but is related more to the phenomenon of a long chain branched LDPE macromolecule being smaller in size in the molten state than a macromolecule of a linear LLDPE sample of the same molar mass. The result of the difference in size is that the LDPE will contain a larger number of intramolecular crosslinking points than the LLDPE, which, on the other hand, will contain a larger fraction of intermolecular crosslinking points. The crosslinking points mentioned are of either a physical or chemical nature. From the perspective of the network build-up, the intermolecular crosslinking points are the most efficient. To compensate for the larger fraction of intramolecular crosslinking points in LDPE, more peroxide can be added or vinyl groups can be introduced.     

From high molecular weight precursor polyrotaxanes to supramolecular sliding networks. The ‘sliding gels’

This paper presents the synthesis and characterization of an original class of supramolecular networks, the ‘sliding’ gels. In this new class of network materials the crosslink points are not fixed but sliding. The molecular structure is based on intermolecularly crosslinked α-cyclodextrins/poly(ethylene-glycol) precursor polyrotaxanes. A synthetic method was developed to obtain various high molecular weight precursor polyrotaxanes with various amounts of α-cyclodextrins (α-CD) per template chain (the so-called degree of complexation, N). The crosslinking reaction of the precursor polyrotaxanes was carried out with the divinyl-sulfone allowing an efficient and good control of the crosslinking density. This control permitted to obtain various network materials with unusual physical/mechanical properties depending on the crosslinker amount and on N. The unusual properties originate from the sliding character of the crosslink points, in these new network materials, which induces a variable and eventually controllable mean mass distribution of the entanglement length. The mechanical spectroscopy and rheological experiments showed two viscoelastic regimes evidencing the molecular mechanism of the change of the mean mass distribution between the crosslink points due to the sliding of α-CD rings on the template PEG chain.     

Some aspects of structural electrophysics of irradiated polyethylenes

In the case of the insulation polymeric materials, such as polyethylenes, it is of essential interest to understand correlations between structural changes and (di)electric properties. The dielectric behavior of different polyethylenes, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE), irradiated to different absorbed doses of gamma radiation, was studied through dielectric loss (tan δ) analysis. Dielectric relaxation behavior is related to the changes in the initial structure of different polyethylenes and to the radiation-induced processes of oxidative degradation and crosslinking. Differential scanning calorimetry (DSC), IC spectroscopy and gel measurements were used to determine the changes in the crystal fraction, oxidative degradation and degree of network formation, respectively.     

Primitive chain network study on uncrosslinked and crosslinked cis-polyisoprene polymers

In this paper, the polymer chain packing and primitive path (PP) network of uncrosslinked and crosslinked cis-polyisoprene (PI) polymer are analyzed upon employing coarse-grained molecular dynamics simulation. The crosslinking effect is found to enhance intra-chain packing of PI polymers, while weakening their inter-chain packing. Surprisingly, these effects cancel each other in the global packing behavior of this polymeric system. We systematically study the effects of molecular weight (MW) and crosslink density on the PP. Both the PP contour length and number of entanglements per chain, 〈Z〉, are found to increase linearly with MW for uncrosslinked cis-PI. The corresponding entanglement molecular length N_e of cis-PI is estimated to be 76 ± 1, in good agreement with experimental results. The polymer end-to-end distance, the PP contour length as well as 〈Z〉 of crosslinked PI are reduced by higher intra-chain packing density, compared with uncrosslinked PI, if the crosslinkers are ignored in the PP analysis. At the same time, the tube diameter of crosslinked PI is enlarged by the sparse inter-chain packing. By dividing the crosslinked cis-PI chain network into subchains through crosslinked or crosslinker beads, the PP networks of these partial systems are treated as well. We obtain scaling laws between MW/crosslinking density and 〈Z〉 for crosslinked PI polymers. The simulation results indicate that the random walk assumption, often encountered during the analysis of PPs, can only be applied to the entanglement-dominated (low crosslink density) polymers. For crosslink-dominated (high crosslink density) polymers, whose subchains have a molecular length below 100, this assumption would imply a greatly overestimated entanglement density; we thus avoid the assumption in our analysis. To our best knowledge, this is the first work to uncover the PP of crosslinked polymers.     

高相对分子质量双峰HDPE及其应用

综述了高相对分子质量高密度聚乙烯树脂的性能、生产技术、应用领域及发展趋势．指出双峰聚乙烯中高相对分子质量段赋予良好的强度、低相对分子质量段使树脂具有良好的加工性．是日前用途广泛、市场极有前景的一种通用树脂。同时,介绍了台湾塑胶工业股份有限公司在薄膜级及管材级双峰聚乙烯、中空吹塑级宽峰聚乙烯产品上的生产工艺及生产现状。     

Smooth Muscle Cell Responses to Poly(ε-Caprolactone) Triacrylate Networks with Different Crosslinking Time

Poly(ε-caprolactone) triacrylate (PCLTA) is attractive in tissue engineering because of its good biocompatibility and processability. The crosslinking time strongly influences PCLTAs cellular behaviors. To investigate these influences, PCLTAs with different molecular weights were crosslinked under UV light for times ranging from 1 to 20 min. The crosslinking efficiency of PCLTA increased with decreasing the molecular weight and increasing crosslinking time which could increase the gel fraction and network stiffness and decrease the swelling ratio. Then, the PCLTA networks crosslinked for different time were used as substrates for culturing rat aortic smooth muscle cells (SMCs). SMC attachment and proliferation all increased when the PCLTA molecular weight increased from 8k to 10k and then to 20k at the same crosslinking time. For the same PCLTA, SMC attachment, proliferation, and focal adhesions increased with increasing the crosslinking time, in particular, between the substrates crosslinked for less than 3 min and longer than 5 min. This work will provide a good experimental basis for the application of PCLTA.     

Thermal and rheological studies on the molecular composition and structure of metallocene‐ and Ziegler–Natta‐catalyzed ethylene–α‐olefin copolymers

The relationship between the molecular structure and the thermal and rheological behaviors of metallocene‐ and Ziegler–Natta (ZN)‐catalyzed ethylene copolymers and high‐density polyethylenes was studied. Of special interest in this work were the differences and similarities of the metallocene‐catalyzed (homogeneous) polymers with conventional coordination‐catalyzed (heterogeneous) polyethylenes and low‐density polyethylenes. The short‐chain branching distribution was analyzed with stepwise crystallization by differential scanning calorimetry and by dynamic mechanical analysis. The metallocene copolymers exhibited much more effective comonomer incorporation in the chain than the ZN copolymers; they also exhibited narrower lamellar thickness distributions. Homogeneous, vanadium‐catalyzed ZN copolymers displayed a very similar comonomer incorporation to metallocene copolymers at the same density level. The small amplitude rheological measurements revealed the expected trend of increasing viscosity with weight‐average molecular weight and shear‐thinning tendency with polydispersity for the heterogeneous linear low‐density polyethylene and very‐low‐density polyethylene resins. The high activation energy values (34–53 kJ/mol) and elevated elasticity found for some of our experimental metallocene polymers suggest the presence of long‐chain branching in these polymers. This was also supported by the comparison of the relationship between low shear rate viscosity and molecular weight. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1140–1156, 2002     

Radiation-induced crosslinking of UHMWPE in the presence of co-agents: chemical and mechanical characterisation

Radiation-induced crosslinking has been shown to have a beneficial effect on the wear resistance of ultra high molecular weight polyethylene (UHMWPE). Since we postulate that crosslinking takes place through reactions involving terminal double bonds, unsaturated additives were added to UHMWPE in this study to enhance crosslinking. UHMWPE specimens soaked in 1,7-octadiene, methylacetylene and ethylene, respectively, were irradiated with electron beam to different doses in single or multiple passages. FTIR spectroscopy was used for the chemical characterisation of the crosslinked polymer. Tensile tests were performed with all samples in order to monitor changes in the mechanical properties.Gel fraction measurements proved that crosslinking took place in all the irradiated samples, but 1,7-octadiene turned out to be the most effective additive for the present purpose, exhibiting a good efficiency in enhancing crosslinking. The FTIR analyses showed that consumption of vinyl double bonds is proportional to the irradiation dose, with an efficiency which increases with increasing their initial concentration, confirming the hypothesis of the involvement of these groups in the crosslinking process. In the case of UHMWPE with 1,7-octadiene irradiated to high doses in multiple passages, the results of the tensile tests indicated a significant decrease in both elongation at break (up to 65%) and ultimate stress (up to 25%). A possible explanation of the diminished strength of the crosslinked material has also been proposed.     

Semi‐interpenetrating polymer network hydrogels based on aspen hemicellulose and chitosan: Effect of crosslinking sequence on hydrogel properties

Semi‐interpenetrating network hydrogel films were prepared using hemicellulose and chemically crosslinked chitosan. Hemicellulose was extracted from aspen by using a novel alkaline treatment and characterized by HPSEC, and consisted of a mixture of high and low molecular weight polymeric fractions. HPLC analysis of the acid hydrolysate of the hemicellulose showed that its major constituent sugar was xylose. X‐ray analysis showed that the relative crystallinity of hydrogels increased with increasing hemicellulose content up to 31.3%. Strong intermolecular interactions between chitosan and hemicellulose were evidenced by FT‐IR analysis. Quantitative analysis of free amino groups showed that hemicellulose could interrupt the chemical crosslinking of chitosan macromolecules. Mechanical testing and swelling experiments were used to define the effective network crosslink density and average molecular weight between crosslinks. Swelling ratios increased with increasing hemicellulose content and mainly consisted of H‐bonded bound water. Results revealed that by altering the hydrogel preparation steps and hemicellulose content, crosslink density and swelling behavior of semi‐IPN hydrogels could be controlled without deteriorating their mechanical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of crosslink density and length on the number of intramolecular crosslinks (defects) introduced into a rubbery network

Estimates are presented for the effects of the crosslink density, or the molecular weight of polymer chains between crosslinks, and the length of crosslinks, or crosslinking agents, upon the expected ratio of internal or intramolecular (possibly elastically ineffective) to external or intermolecular (elastically effective) crosslinks introduced in rubber networks crosslinked in both the dry and dissolved states. Model calculations are performed on cis-1,4-polyisoprene with the following results: (i) in rubber networks formed by crosslinking in the dry state, the number of possibly inactive, intramolecular crosslinks introduced is negligibly small; and (ii) the relative number of intramolecular crosslinks introduced may become appreciable for those networks formed by lightly crosslinking low molecular weight rubber molecules in solutions, where the volume fraction of rubber present is small, using initiators that produce short crosslinks.     

Proposed solution for the Flory-Charlesby equation for crosslinked polymers and application for 1,2-polybutadiene crosslinked with AIBN and aryl diazide

The derivation and properties of a tractable function that approximates well the solution of the Flory-Charlesby equation for crosslinked polymers with logarithmic normal primary molecular size distribution is presented. This function gives the possibility to determine the crosslinking density, ρ, from measurements of insoluble fraction, what is the only quantitative method that can be applied for postsynthesis crosslinked polymers. Applications to 1,2-polybutadiene crosslinked with AIBN and 2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone by heating allowed to obtain information about the gel point as well as about the crosslinking yield and mechanism. In addition, it was found that the temperature where the damping factor in DMA curves is maximum increases with ρ following a second order dependence.     

Synthesis and crosslinking of partially disulfonated poly(arylene ether sulfone) random copolymers as candidates for chlorine resistant reverse osmosis membranes

Biphenol-based, partially disulfonated poly(arylene ether sulfone)s synthesized by direct copolymerization show promise as potential reverse osmosis membranes. They have excellent chlorine resistance over a wide range of pHs and good anti-protein and anti-oily water fouling behavior. Crosslinking of these copolymers that have high degrees of disulfonation may improve salt rejection of the membranes for reverse osmosis performance. A series of controlled molecular weight, phenoxide-endcapped, 50% disulfonated poly(arylene ether sulfone)s were synthesized. The copolymers were reacted with a multifunctional epoxy resin and crosslinked thermally. The effects on network properties of various factors such as crosslinking time, copolymer molecular weight and epoxy concentration were investigated. The crosslinked membranes were characterized in terms of gel fraction, water uptake, swelling and self-diffusion coefficients of water. The salt rejection of the cured membranes was significantly higher than that for the uncrosslinked copolymer precursors.     

Structure–property relationships of lightly chemical crosslinked poly(vinyl chloride) thermoplastic elastomer

Chemical crosslinked poly(vinyl chloride) (C‐PVC) was synthesized by vinyl chloride suspension polymerization in the presence of diallyl phthalate (DAP) and plasticized to prepare poly(vinyl chloride) (PVC) thermoplastic elastomer (TPE) materials. The chemical crosslinking and physical crosslinking structure in chemical crosslinked PVC‐TPE were investigated. It showed that the gel fraction and the crosslinking density of gel increased as the feed concentration of DAP increased. C‐PVC prepared by VC/DAP copolymerization was lightly crosslinked as compared with irradiation crosslinked PVC. Physical entanglements would greatly influence the crosslinking density of gel when the gel fraction was high. Chemical crosslinking had little influence on the recrystallization behavior of PVC. A structure model of chemical crosslinked PVC‐TPE was proposed, in which chemical networks acted with physical networks cooperatively. It also showed that chemical crosslinking and physical crosslinking influenced the processability and mechanical properties of chemical crosslinked PVC‐TPE cooperatively. Although the processability of PVC‐TPE deteriorated with chemical crosslinking, the dimension stability and elasticity of PVC‐TPE were improved as the permanent chemical networks were introduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 868–874, 2000     

茂金属聚乙烯材料结构与加工性能研究

研究了茂金属聚乙烯结构,发现茂金属聚乙烯(mPE)具有比传统聚乙烯更规整的结构、更窄的分子量分布;指出:通过改进mPE的合成技术,使用复配催化剂生产双峰或宽峰聚乙烯,改进树脂加工设备等手段可以制得性能优良的茂金属乙烯制品。