Solid-state relaxations in linear low-density (1-octene comonomer), low-density,and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1633–1642, 1997     

Reversible Melting of Semi-Crystalline Polymers 2. Annealing Near to the Melting Point

Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE, HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale, to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the same value as during the cooling section. This is not the case for LDPE and LLDPE. Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle is the same. The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron radiation during a heating cycle at temperatures below the melting peak. A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect to that of folded chain lamellae. This revised version was published online in July 2006 with corrections to the Cover Date.     

结晶聚合物品相相容性的研究

通过ＤＳＣ、Ｘ－射线衍射、红外光谱及拉伸试验研究了ＨＤＰＥ与ＬＬＤＰＥ、ＨＤＰＥ与ＬＤＰＥ之间的晶相相容性．结果表明，链结构相近的ＨＤＰＥ和ＬＬＤＰＥ的晶相相容性好，能形成共晶；而链结构差异较大的ＨＤＰＥ和ＬＤＰＥ的晶相相容性较差，倾向于分别结晶，但有部分链段被对方的晶区夹持．不论是支化度大的ＬＤＰＥ链段插入以ＨＤＰＥ为主的晶区，还是支化度小的ＨＤＰＥ链段插入以ＬＤＰＥ为主的晶区，都可破坏晶区的规整性．共晶的形成使共混物的熔点、结晶度、晶粒体积等低于两组份的线性加和，而力学性能，尤其是断裂伸长率，则显著提高，呈协同效应．     

HDPE,LLDPE或LDPE高取向薄膜微结构的电子显微学研究

木工作用透射电子显微术及电子衍射技术研究3种PE(HDPE,LLDPE或LDPE)均聚物高取向薄膜的微结构。定量测定了它们的结晶尺寸。通过倾斜样品电子显微学研究确定了不同种PE纤维结构的对称性。     

Adhesive effect of linear low-density polyethylene gels on polyethylene moldings

Adhesive effect of linear low density polyethylene (LLDPE) gels in organic solvents such as decalin, tetralin, and o-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, and DSC measurements. For all of the gels the temperature at which the heated gel starts to exhibit the adhesive effect was about 70 °C, which is similar to the result of LDPE gel. In particular, when heated at 110 °C, LLDPE gel in tetralin showed such a strong bond strength that polyethylene plates of 3 mm in thickness and 20 mm in width gave rise to necking. It was found that LLDPE gel behaved as though it added LDPE gel to HDPE gel namely LDPE-like components in LLDPE resin exerted the adhesive effect at lower heating temperature, HDPE-like components exerted the strong adhesive effect at higher heating temperature.     

Post-irradiation oxidation of different polyethylenes

The radiation-induced oxidative degradation of polyethylenes (PEs) with different degrees of crystallinity was characterized after electron-beam irradiation and during storage at room temperature.UHMWPE, HDPE, LDPE, LLDPE and an ethylene-octene copolymer (Engage) were e-beam irradiated to 30 or 60 kGy in vacuum or in air and stored at room temperature in air. EPR spectroscopy was used to investigate macro-radicals produced during irradiation and their post-irradiation changes. FTIR spectroscopy was used to monitor changes in the polymer structure, induced by irradiation, and to follow post-irradiation oxidation.We found that the crystallinity and the size of the crystalline lamellae, in particular, play a major role on the post-irradiation effects. The low-crystallinity polyethylenes showed no oxidation or oxidation only to a small extent, even when irradiated and stored in air. On the contrary, development of post-irradiation oxidation was observed in HDPE and UHMWPE. We attribute these results to a different reactivity of the macro-alkyl radicals formed upon irradiation in the amorphous or in the crystalline phase. While the radicals formed in the amorphous phase decay in short time, the migration time of the radicals trapped in the crystalline phase to the amorphous one is a key factor, governing the oxidation process.     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.     

Effects of thermal history on the rigid amorphous phase in poly(phenylene sulfide)

The rigid amorphous phase of semicrystalline poly(phenylene sulfide) (PPS) has been studied as a function of thermal history using scanning calorimetry, dielectric relaxation, density, and small-angle x-ray scattering (SAXS). Based on the new heat of fusion of perfect crystalline PPS, which is 26.7±0.8 cal/gram, the weight fraction of rigid amorphous phase is shown to be nearly twice as large as previously reported [1]. The mass fraction of the rigid amorphous phase ranges from 0.24 to 0.42 and is dependent upon thermal treatment. We have taken the approach of assuming a three-phase model for the morphology of semicrystalline PPS consisting of crystalline lamellae, mobile amorphous, and rigid amorphous components. Using this three-phase model, we determine that the average density of the rigid amorphous fraction is 1.325 g/cc, which is slightly larger than the density of the mobile amorphous phase fraction and was insensitive to thermal history. From the SAXS long period, the layer thicknesses of the mobile amorphous phase, rigid amorphous phase, and crystal lamellae were estimated. Only the lamellar thickness shows a systematic variation with thermal history, increasing with melt or cold crystallization temperature, or with decreasing cooling rate.     

Morphology and mechanical properties of blown films of a low‐density polyethylene/linear low‐density polyethylene blend

The morphologies of films blown from a low‐density polyethylene (LDPE), a linear low‐density polyethylene (LLDPE), and their blend have been characterized and compared using transmission electron microscopy, small‐angle X‐ray scattering, infrared dichroism, and thermal shrinkage techniques. The blending has a significant effect on film morphology. Under similar processing conditions, the LLDPE film has a relatively random crystal orientation. The film made from the LDPE/LLDPE blend possesses the highest degree of crystal orientation. However, the LDPE film has the greatest amorphous phase orientation. A mechanism is proposed to account for this unusual phenomenon. Cocrystallization between LDPE and LLDPE occurs in the blowing process of the LDPE and LLDPE blend. The structure–property relationship is also discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 507–518, 2002; DOI 10.1002/polb.10115     

Macromolecular scission and crosslinking rate changes during polyolefin photo-oxidation

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of various thermoplastics exposed to ultraviolet irradiation (UV). Results are given for a high density polyethylene (HDPE); a low density polyethylene (LDPE); a linear low density polyethylene (LLDPE); a polypropylene homopolymer (PPHO); and a polypropylene copolymer (PPCO). As the oxidation progressed, it was observed that the scission rate for HDPE, LLDPE, PPHO and PPCO increased near to the exposed surface whereas for LDPE the rate remained almost unchanged. The crosslink rate fell near to the surface with HDPE and LDPE but increased with PPHO and PPCO. The reaction rates near to the bar centre (∼1.5 mm from the exposed surface) were low for HDPE, PPHO and PPCO; this is attributed to oxygen starvation, caused by consumption of oxygen by rapid reaction near the surface. Reaction was observed in the interior with LDPE and LLDPE, presumably because of a combination of a higher oxygen diffusion rate than for HDPE and a lower rate of consumption of oxygen near the surface than with the polypropylenes.     

Time-resolved SAXS studies of morphological changes in cold crystallized poly(ethylene terephthalate) during annealing and heating

Structural changes occurring during crystallization of quenched amorphous poly(ethylene terephthalate) (PET) and subsequent cooling/heating cycles have been studied by real-time small-angle x-ray scattering (SAXS), using synchrotron radiation. Initial crystallization is found to occur by insertion of new lamellae between the existing ones, while rapid continuous melting/recrystallization happens when the cold-crystallized PET samples are heated above the previous highest annealing temperature. Such melting/recrystalization results in irreversible increases in the lamellar long period, the crystal thickness and the density difference between the crystalline and amorphous regions; in contrast, at temperatures below the prior highest crystallization temperature, the structural changes are dominated by reversible effects such as thermal expansion. However, throughout the entire temperature range up to the melting point around 250 °C, the crystal core thickness remains quite small, less than ca. 50 Å, and the linear crystallinity of lamellar stacks remains nearly constant around 0.3. Such a low crystallinity indicates the presence of thick order-disorder interfacial layers on the lamellar surface, whose thickness increases with temperature.Dedicated to Prof. E. W. Fischer on the occasion of his 65th birthday.     

Rheooptical Fourier transform infrared spectroscopy of the deformation behavior in quenched and slow‐cooled isotactic polypropylene films

Emerging technological applications for complex polymers require insight into the dynamics of these materials from a molecular and nanostructural viewpoint. To characterize the orientational response at these length scales, we developed a versatile rheooptical Fourier transform infrared (FTIR) spectrometer by combining rheometry, polarimetry, and FTIR spectroscopy. This instrument is capable of measuring linear infrared dichroism spectra during both small‐strain dynamic deformation and large‐strain irreversible deformation over a wide temperature range. The deformation response of quenched and slow‐cooled isotactic polypropylene (iPP) is investigated. In quenched iPP, under dynamic oscillatory strain at an amplitude of ～0.1%, the dichroism from the orientation of the amorphous chains is appreciably less than that from the crystalline region. At large irreversible strains, we measured the dichroic response for 12 different peaks simultaneously and quantitatively. The dichroism from the crystalline peaks is strong as compared to amorphous peaks. In the quenched sample, the dichroism from the crystalline region saturates at 50% strain, followed by a significant increase in the amorphous region dichroism. This is consistent with the notion that the crystalline regions respond strongly before the yield point, whereas the majority of postyielding orientation occurs in the amorphous region. Our results also suggest that the 841 cm^?1 peak may be especially sensitive to the ‘smectic’ region orientation in the quenched sample. The response of the slow‐cooled sample at 70 °C is qualitatively similar but characterized by a stronger crystalline region dichroism and a weaker amorphous region dichroism, consistent with the higher crystallinity of this sample, and faster chain relaxation at 70 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2539–2551, 2002     

Tear anisotropy in films blown from polyethylenes of different macromolecular architectures

Blown films of different types of polyethylenes, such as branched low‐density polyethylene (LDPE) and linear high‐density polyethylene (HDPE), are well known to tear easily along particular directions: along the film bubble's transverse direction for LDPE and along the machine direction (MD) for HDPE. Depending on the resin characteristics and processing conditions, different structures can form within the film; it is therefore difficult to separate the effects of the crystal structure and orientation on the film tear behavior from the effects of the macromolecular architecture, such as the molecular weight distribution and long‐chain branching. Here we examine LDPE, HDPE, and linear low‐density polyethylene (LLDPE) blown films with similar crystal orientations, as verified by through‐film X‐ray scattering measurements. With these common orientations, LDPE and HDPE films still follow the usual preferred tear directions, whereas LLDPE tears isotropically despite an oriented crystal structure. These differences are attributed to the number densities of the tie molecules, especially along MD, which are considerably greater for linear‐architecture polymers with a substantial fraction of long chains, capable of significant extension in flow. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 413–420, 2005     

Polyethylene blends: A correlation study between morphology and environmental resistance

HDPE is widely used in outdoor applications in which dielectric, mechanical and thermal behaviors combined with high environmental resistance are a major concern. Unfortunately, the required processing conditions of the material often result in residual stresses that in connection with environmental exposure are responsible for premature failure. In an attempt to combine good durability with more facile processing conditions for HDPE based components, blends of this polymer with LDPE and LLDPE were studied, as a function of the blend composition ratio. Measurements of crystallinity and dynamical mechanical studies were conducted which established a quantitative relationship between morphological features and composition. The optimal results of performance were obtained with HDPE/LLDPE blends which are discussed here in terms of their morphology and mobility of the amorphous phase.     

Mechanical properties and characterization of slowly cooled isotactic polypropylene/high‐density polyethylene blends

In previous studies, we found that Young's moduli of quenched isotactic polypropylene/high‐density polyethylene (iPP/HDPE) exceeded the upper bound, calculated from the Voigt model, with the moduli of the quenched homopolymers as those of the two components. We suggested that this might be due to crystallization, as the components crystallized at higher temperatures in the blend than on their own. We repeated the same set of measurements, this time on iPP/HDPE blends that were cooled slowly. We also examined crystallization at various rates of cooling with differential scanning calorimetry. At slow cooling rates, the HDPE and iPP components in the blends crystallize at lower temperatures than in the pure homopolymers, suggesting that the presence of one component inhibits rather than promotes the crystallization of the other. Electron microscopy of slowly cooled blends revealed very different interfacial morphologies depending on whether the HDPE or the iPP crystallizes first. Young's moduli of most of the blends lie on the upper bound; however, some blends with co‐continuous morphologies fall well below the lower bound. The mechanical properties are discussed in terms of the interfacial morphology, the crystallization behavior, and the large‐scale phase separation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1384–1392, 2003     

Fuels obtained by thermal cracking of individual and mixed polymers

Utilization of oils/waxes obtained from thermal cracking of individual LDPE (low density polyethylene), HDPE (high density polyethylene), LLDPE (linear low density polyethylene), PP (polypropylene), or cracking of mixed polymers PP/LDPE (1: 1 mass ratio), HDPE/LDPE/PP (1: 1: 1 mass ratio), HDPE/LDPE/LLDPE/PP (1: 1: 1: 1 mass ratio) for the production of automotive gasolines and diesel fuels is overviewed. Thermal cracking was carried out in a batch reactor at 450°C in the presence of nitrogen. The principal process products, gaseous and liquid hydrocarbon fractions, are similar to the refinery cracking products. Liquid cracking products are unstable due to the olefins content and their chemical composition and their properties strongly depend on the feed composition. Naphtha and diesel fractions were hydrogenated over a Pd/C catalyst. Bromine numbers of hydrogenated fractions decreased to values from 0.02 g to 6.9 g of Br₂ per 100 g of the sample. Research octane numbers (RON) before the hydrogenation of naphtha fractions were in the range from 80.5 to 93.4. After the hydrogenation of naphtha fractions, RON decreased to values from 61.0 to 93.6. Diesel indexes (DI) for diesel fractions were in the range from 73.7 to 75.6. After the hydrogenation of diesel fractions, DI increased up to 104.9.     

极低密度聚乙烯与其它聚乙烯的共混

从结构角度，用ＤＳＣ，ＷＡＸＤ，ＳＡＸＳ研究了聚乙烯（ＰＥ）家族中极低密度聚乙烯（ＶＬＤＰＥ）与其它ＰＥ的互容性．ＨＤＰＥ／ＶＬＤＰＥ是共晶互容的，以其大量无规部分“溶解”了ＨＤＰＥ的结晶缺陷部分，提高了ＨＤＰＥ的Ｔｃ，Ｔｍ，Ｘｃ，结晶峰半高宽变窄，晶胞参数随组成而有最低值．ＶＬＤＰＥ与ＬＬＤＰＥ结构极为相似，ＤＳＣ及ＷＡＸＤ证明其共混物是共晶相容体系．ＬＤＰＥ／ＶＬＤＰＥ的结晶度符合按组成的计算值，但晶胞参数ａ，ｂ以及晶粒尺寸增大，ＤＳＣ上有分别相应于两组份的两个Ｔｍ；ＶＬＤＰＥ的Ｔｃ，Ｔｍ峰高之和高于按组份的计算值，ＬＤＰＥ的Ｔｍ，Ｔｃ则低于计算值．认为是正如ＬＬＤＰＥ／ＬＤＰＥ，ＬＤＰＥ向充满整个体积的ＶＬＤＰＥ中不断填入，以ＶＬＤＰＥ为晶核而结晶，形成相分离的不相容体系．     

Characterization of linear low-density polyethylene: Cross-fractionation according to copolymer composition and molecular weight

The structure of ethylene copolymers modified by α-olefins has become an area of intense investigation since the successful commercialization of so-called linear low-density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene-1. The resins were fractionated according to short-chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x-ray diffraction, ¹³C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short-chain branching, long-chain branching, and the molecular-weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short-chain branching distribution. No significant long-chain branching was found in the LLDPE resins. The short-chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid-state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α-olefin in these commercial LLDPE resins.     

Morphological explanation of the extraordinary fracture toughness of linear low density polyethylenes

The fracture toughness of commercial linear low-density polyethylenes (LLDPE) has been found to be extraordinarily high relative to commercial low-density (LDPE) and high-density (HDPE) polyethylenes in previously reported investigations. The present investigation shows that this extraordinary fracture toughness cannot be explained by differences in molecular structure variables, such as molecular weight, long-chain and short-chain branching, fractional crystallinity, and comonomer content. Instead, the presence of a second soft phase, which was extractable with a weak solvent, in a hard semicrystalline matrix was discovered by morphological investigations of LLDPE resins. This second phase arises from the extreme compositional heterogeneity of the copolymers which comprise these LLDPE resins. No evidence for a similar morphological entity was found in LDPE and HDPE resins. This finding provides persuasive evidence that this very-low-crystallinity second phase performs a function similar to that of the rubberlike second phase in other high impact resins and, thus, leads to the observed extraordinary fracture toughness of LLDPE resins. Evidence for the nature and existence of this second phase is given from temperature-rising elution fractionation and scanning electron microscopy investigations.     

高密度聚乙烯／低密度聚乙烯／乙烯－醋酸乙烯共聚物共混体系的结晶行为

通过ＤＳＣ和ＷＡＸＤ研究了高密度聚乙烯／低密度聚乙烯／乙烯－醋酸乙烯共聚物（ＨＤＰＥ／ＬＤＰＥ／ＥＶＡ）三元共混体系的热行为和结晶性能。发现当ＨＤＰＥ含量小于４０％时，ＥＶＡ对ＬＤＰＥ起稀释剂作用，促进ＨＤＰＥ、ＬＤＰＥ的晶相分离，使ＨＤＰＥ、ＬＤＰＥ单独结晶．当ＨＤＰＥ含量高于４０％时，ＬＤＰＥ片晶进入ＨＤＰＥ晶相。形成与ＬＤＰＥ在片晶水平上的共晶。