Probing Liquid-Liquid Phase Separation of a Polyethylene Blend with Thermal Analysis

Summary: A series of polyethylene (PE) blends consisting of a high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with a butene-chain branch density of 77/1000 carbon was prepared at different concentrations. The LLDPE only crystallized below 50 °C, therefore, above 80 °C and below the melting temperature of HDPE, only HDPE crystallized in the PE blends. A specifically designed multi-step experimental procedure based on thermal analysis technique was utilized to monitor the liquid–liquid phase separation (LLPS) of this set of PE blends. The main step was first to quench the system from the homogeneous temperatures and isothermally anneal them at a prescribed temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from LLPS, and then cool the system at constant rate to record the non-isothermal crystallization. The crystallization peak temperature (T_p) was used to character the crystallization rate. Because LLPS results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the LLPS for the system indicated by increased T_p. The result showed that the LLPS boundary of the blend measured by this method was close to that obtained by phase contrast optical microscopy method. Therefore, we considered that the thermal analysis technique based on the non-isothermal crystallization could be effective to investigate the LLPS of PE blends.     

The influence of flow-induced crystallization on the impact toughness of high-density polyethylene

The relation between the impact toughness and flow-induced crystalline orientation of high-density polyethylene (HDPE) was investigated. Flow-induced crystalline orientation was created in the samples via injection moulding and the amount of orientation was controlled through variation of processing conditions (injection temperature) and sample thickness. The impact toughness behaviour was found to be strongly dependent on the amount of crystalline orientation, whereas the loading direction also had a strong influence, e.g. giving highest impact properties in flow direction. Subsequently, injection moulded samples of HDPE modified with calcium carbonate filler particles were tested. In this case a similar relation between crystalline orientation and loading direction was found, whereas the total amount of flow-induced crystallization was observed to be strongly influenced by the presence of the filler particles.     

Strain-induced crystallization of swollen,crosslinked polyethylene

The kinetics of strain-induced crystallization of swollen polyethylene networks have been measured using dynamometry coupled with optical birefringence. Fibers were prepared by gel-spinning ultrahigh-molecular-weight polyethylene followed by crosslinking in dicumyl peroxide and swelling in xylene. Retractive force on crystallization was monitored at various temperatures and draw ratios. Fiber visualization was achieved using optical illumination coupled with video recording and digital analysis of the in situ transformation. Avrami plots of the transformation data showed good linear fits for low draw ratios in the range α < 1.2 and moderate undercoolings (60 < T_c < 70°C). Time exponents of unity were found, indicating a one-dimensional, heterogeneous growth mechanism whose temperature dependence could be described by a formalism similar to that recently used for flow-induced growth.     

Polyethylene fiber formation in tubular flow. I. Observations of growth pattern and fiber properties

The process of seeded growth of fibrillar polyethylene crystals has been studied in a tubular flow geometry for 0.01-wt % solutions of a high-molecular-weight polyethylene in xylene. The transformation sequence has been followed visually by using polarized-light illumination in conjunction with a video camera. Data are presented to show that transformation is initiated by the formation of a concentrated, unoriented, amorphous precursor fiber within which oriented birefringent crystals subsequently grow in consequence of the stresses transmitted by the flowing solution. Time constants for the precursor formation, birefringence initiation, and completion of birefringence were measured as functions of temperature and flow rate over a range of growth conditions. Wide-angle x-ray diffraction, overall birefringence, and optical hot-stage melting data were also obtained on the grown fibers. The net result of these observations is to conclude that fibrillar crystal growth during flow is always preceded by the formation of a liquidlike phase transformation which produces the concentrated, unoriented precursor. Subsequent orientation is in consequence of stress-induced crystallization with overall fiber orientation showing an increase with solution flow rate at a fixed temperature and a decrease with temperature at a fixed flow rate. At higher temperatures and lower flow rates, birefringence develops in an oscillatory fashion, indicating a remelting process possibly due to slippage of trapped chain entanglements formed by flow. A discussion is given of the implications of these observations for the understanding of flow-induced structure development, phase transformation, and oriented crystallization; this is expanded upon in a companion paper, Part II.     

Physical and mechanical properties of ultra-oriented high-density polyethylene fibers

Ultra-oriented high-density polyethylene fibers (HDPE) have been prepared by solid-state extrusion over 60–140°C range using capillary draw ratios up to 52 and extrusion pressures of 0.12 to 0.49 GPa. The properties of the fibers have been assessed by birefringence, thermal expansivity, differential scanning calorimetry, x-ray analysis, and mechanical testing. A maximum birefringence of 0.0637 ± 0.0015 was obtained, greater than the calculated value of 0.059 for the intrinsic birefringence of the orthorhombic crystal phase. The maximum modulus obtained was 70 GPa. The melting point, density, crystallinity, and negative thermal expansion coefficient parallel to the fiber axis all increase rapidly with draw ratio and at draw ratios of 20–30 attain limiting values comparable with those of a polyethylene single crystal. The properties of the fibers have been analyzed using the simple rule of mixtures, assuming a two-phase model of crystalline and noncrystalline microstructure. The orientation of the noncrystalline phase with draw ratio was determined by birefringence and x-ray measurements. Solid-state extrusion of HDPE near the ambient melting point produced a c-axis orientation of 0.996 and a noncrystalline orientation function of 0.36. Extrusion 50°C below the ambient melting point produced a decrease in crystallinity, c-axis orientation, melting point, and birefringence, but the noncrystalline orientation increased at low draw ratios and was responsible for the increased thermal shrinkage of the fibers.     

Effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene

The effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene (UHMW‐PE) are discussed, based on a combination of in situ X‐ray measurement and stress–strain behavior. The sample films of metallocene‐ and Ziegler‐catalyzed UHMW‐PEs with a similar viscosity average MW of ～10⁷ were prepared by compression molding at 180 °C. Stress profiles recorded at 160 °C above the melting temperature of 135 °C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt‐drawing for metallocene‐catalyzed UHMW‐PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler‐catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt‐drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt‐drawing behavior is also discussed. The obtained results indicate that the traditional temperature–strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt‐drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2455–2467, 2006     

Morphology of polyethylene crystallized by solution stirring

Dilatometric and calorimetric studies have been made of the fusion process of linear polyethylene crystallized by stirring xylene solutions at elevated temperatures. It is shown that the melting point of the crystals increases rapidly from 139.5°C to 145°C in the crystallization temperature range of 100–103°C and levels off to 146 ± 0.5°C, provided that very slow heating rates are employed. Stirrer-crystallized samples treated with fuming nitric acid show higher crystalline contents. Comparison of their enthalpies of fusion and melting points indicate that higher molecular order along the fiber axis is associated with higher crystallization temperatures. This is in general agreement with corresponding results of other modes of crystallization. The attack of fuming nitric acid on stirrer crystals is characterized by weight-loss curves similar to those of dilutesolution crystals and bulk polyethylene. The linear molecular weight dependence on time of exposure to nitric acid suggests that the oxidation proceeds mainly from the chain ends at a constant rate for samples stirred in the lower crystallization range, but an increased rate is observed for a sample stirred from xylene at 105°C. It is suggested that the lamellar overgrowths, most evident at low crystallization temperatures, are epitaxially attached to the fiber axis, whereas the smaller crossbandings observed at higher crystallization temperatures are possibly made up of elements of chains that are only partly incorporated in the highly ordered fibrous core.     

Effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black ternary composites

The effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black (iPP/HDPE/CB) composite was investigated. iPP/HDPE/CB composites were prepared by four compounding procedures (A: iPP + HDPE + CB; B: iPP/HDPE + CB; C: HDPE/CB + iPP; D: iPP/CB + HDPE). Scanning electron microscopy observation showed that CB particles are mainly distributed in HDPE in all composites, and the phase morphology of composites was obviously affected by a compounding procedure. The size of the HDPE/CB domains in the composites prepared by procedures A and D decreased with the increase in CB content, whereas that of HDPE/CB in the composites prepared by procedures B and C rarely changed with the increase in CB content. The crystallization behaviors of the composites were significantly affected by their phase morphology, which resulted from the variation of compounding procedure. The isothermal crystallization rate of iPP in the composites prepared by procedures A and D was obviously increased, which may originate from the small HDPE/CB droplets dispersed in the iPP phase. The non‐isothermal crystallization curves of composites prepared by procedure D represented two peaks because the iPP component in these composites had the fastest crystallization rate, whereas the curves of composites prepared by other compounding sequences only exhibited one peak. Moreover, the crystallinity of HDPE almost increased by one time with the incorporation of only 1 phr CB because the CB particles selectively located in the HDPE phase, and the crystallinity of HDPE decreased with the further increase of CB content because of the strong restriction of CB on the HDPE chains. Copyright © 2011 John Wiley & Sons, Ltd.     

Epitaxial and graphoepitaxial growth of isotactic polypropylene (iPP) from the melt on highly oriented high density polyethylene (HDPE) substrates

Although under normal conditions only the crystallization behavior of PE on oriented iPP substrates can be studied due to the higher melting point of iPP, the faster crystallization rate of a molten, oriented HDPE film compared to a nonoriented iPP layer was used to study the crystallization of iPP on the oriented HDPE film by means of transmission electron microscopy (TEM) and electron diffraction (ED). Besides the known epitaxial relationship of HDPE/iPP with their chains 50° apart, two new orientation relationships with (a) chains of both polymers parallel and (hk0)_iPP in contact with the HDPE substrate, and (b) the a‐axis of iPP crystals parallel to the chain direction of HDPE but (001)_iPP in contact with the HDPE substrate were observed. Both orientations are assumed as graphoepitaxy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1893–1898, 1999     

The effect of prior crystallinity on the flow-induced crystallization of poly(ethylene oxide)

After flowing in a dilatometer bulb for a small fraction of the duration of the transformation, a relaxed melt of poly(ethylene oxide) (M?_n = (5.9 ± 0.1) × 10³) showed marked increases in isothermal crystallization rate. The extent of increase was greater when flow was imposed at modestly later stages rather than at the earliest stage of a crystallization. Kinetic parameters for the flow-induced crystallizations were obtained via modification of the conventional mathematical treatment of the kinetics of phase change, thereby allowing the analytical resolution of the overall process into flowinduced and quiescent components. Determination of the flow-induced crystallization parameters required independent determination of the kinetic parameters for quiescent crystallizations at that temperature. The Avrami exponents n_f which characterized the flow-induced portions of the crystallizations were larger for those instances in which flow was imposed at the more advanced stages of the crystallizations, thus indicating a transition in crystallization mechanism. It is suggested that prior crystallinity present at the time of flow contributed to the crystallization by serving as a source of nucleation sites. However, in light of the experimental procedure employed, values of n_f approximating 4 that were obtained are not susceptible to mechanistic interpretations now extant.     

Differential thermal analysis of polyethylene under high pressure

The design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded-chain crystals of polyethylene and poly(ethylene–butene-1) copolymer, and melting of extended-chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius-Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene-1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded-chain and extended-chain polyethylene increases from 8.4°C at 1 bar to 25.6°C at 3000 bars.     

Heat affected zones in polymer laser marking

Laser marking is based on the laser heating of the subjected material, the heating being below the melting temperature or thermal degradation starting point. Within and nearby the mark, the material is chemically, physically and mechanically affected. This means that the main characteristics are changing in such a way that the material is ageing. Thermal and mechanical analysis can be used to determine the modification of the material, which is important and necessary to know for predicting its use lifetime. This paper investigates the physical and mechanical modification of the polymer HDPE100, when laser marking is applied. Burst stress, elongation and relaxation modulus were determined for the base material, within the heat affected zone and within the laser burned mark. Information on the crystallization rate and on the elongation viscosity is also reported. According to the results, the polyethylene has very fast crystallization and that affects the marking process if lower than appropriate maintaining during heating process is applied. It becomes stabile after 0.23 min, when it is tested at 103 °C. The elongation viscosity was analysed and values of 10⁵ Pa s were recorded for 10 s, which is a usual time of applying pressure. The performed analysis revealed about 10 % difference between the relaxation modulus of the irradiated and non-irradiated HDPE.     

Effect of orientation on the sorption of toluene by high density polyethylene

Samples of a blown high density polyethylene (HDPE) film with high orientation in machine direction were treated at 34, 74 and 114°C for different periods of time. The crystallinity and birefringence of these samples, as well as the sorption curves with toluene vapor, were determined as a function of the treatment temperature and time. The sorption curves showed that the diffusion behaviour of the oriented samples was non-Fickian. The annealing of the samples resulted in a decay of the orientation and a change of the diffusion mechanism. The latter was manifested by the decrease of the exponent n which describes the time dependence of the solvent uptake in the equation: M(t)/M∞ = k tⁿ.     

Characterization of PP/HDPE blend-based nanocomposites using different maleated polyolefins as compatibilizers

Nanocomposites based on a polypropylene (PP)/high density polyethylene (HDPE) blend were prepared using an organo-montmorillonite (15A) as a nano-filler and two maleated polyolefins (PE-MA and PP-MA) as compatibilizers. The phase morphology and typical physical properties of the prepared samples were examined. The nano-filler 15A was intercalated and/or partially exfoliated in the blend when PE-MA or PP-MA was present. The PE-MA facilitated the dispersibility of 15A to a better degree. The nano-filler 15A accelerated the crystallization of PP in the blends, whereas it hardly influenced the crystallization of HDPE. Moreover, at a slow cooling rate (i.e., 1 °C/min) the PP-MA induced a higher crystallization temperature for PP in the composite, while PE-MA impeded PP crystallization. On the other hand, the crystallization of HDPE in the composite was only slightly influenced by the presence of PE-MA or PP-MA. The thermal stability of PP/HDPE blend was enhanced after the addition of 15A regardless of the inclusion or not of PE-MA or PP-MA. The enhancement was more evident when the samples were scanned under an air environment than a N2 environment. The stiffness of PP/HDPE blend increased marginally after adding 15A and was slightly altered with the further inclusion of PP-MA. The presence of PE-MA in the composite caused a slight decline in the stiffness. The impact strength of PP/HDPE blend declined after the formation of nanocomposites, especially for the sample incorporating PP-MA.     

Quantitative Investigation on Structural Evolution of Co-continuous Phase under Shear Flow

Components of co-continuous phase can form an interpenetrating network structure, which has great potential to synergistically improve the mechanical properties of the blends, and to impart the functional blends superior electrical conductivity and permeability. In this work, the effects of shear rates (50–5000 s^?1) at different temperatures on the phase morphology, phase size and lamellar crystallites of biodegradable co-continuous polybutylene terephthalate (PBAT)/polybutylene succinate (PBS) blend are quantitatively investigated. The results show that the above features of the PBAT/PBS have a strong dependence on the shear flow and thermal field. The co-continuous phase of the blend is well maintained at 130 °C. Interestingly, this phase structure transforms into a “sea-island” structure at 160 °C, which gradually recovers to a co-continuous phase when the shear rate increases from 1000 s^?1 to 5000 s^?1. The phase size decreases with the increase of shear rate both at 130 °C and 160 °C due to the refinement and deformation of phase structures caused by strong shear stress. Unexpectedly, a unique phenomenon is observed that the shear-induced lamellar crystallites are oriented perpendicular to shear direction in the range of 500–5000 s^?1 at 130 °C, while the orientation of lamellar crystallites at 160 °C is along the shear direction within the whole range of shear rates. The degree of orientation for the PBAT/PBS blend crystals increases first and then decreases at both temperatures above. In addition, the range of shear rate has reached the level in the industrial processing. Therefore, this work has important guiding significance for the regulation of the co-continuous phase structure and the performance for the blend in the practical processing.

     

High density polyethylene/hydrogenated oligo(cyclopentadiene) blends: Tensile stress-strain behavior

The influence of hydrogenated olio(cyclopentadiene) (HOCP) on tensile mechanical properties of its blends with high density polyethylene (HDPE) has been studied at 20, 80, and 100°C. The nominal stress-strain curves performed at 20°C indicate an increase of the modulus and a conspicuous decrease of the ultimate properties increasing the HOCP content in the blend. Blends with HOCP content > 20%-wt are found to be very fragile at 20°C. These behaviors have been related to the presence of the HOCP-rich phase that has the glass transition at about 55°C. At 20°C the HOCP oligomers act as a hardener for the HDPE. The nominal stress-strain curves performed at 80 and 100°C show a strong decrease of the modulus (compared to the value at 20°C), plastic deformation for all the blends tested, and rupture of HDPE and blend samples at the beginning of the cold-drawing. Moreover, the modulus and the ultimate parameters are found to decrease with the composition. The behaviors at 80 and 100°C have been related to several factors: the higher mobility of HDPE molecules at these temperatures compared to that at 20°C; the HOCP-rich phase that is at temperatures higher than its glass transition, and so it acts as a plasticizer for the HDPE; and the decreases of crystallinity and number of entanglements when the HOCP is added to the HDPE component. ©1995 John Wiley & Sons, Inc.     

Prediction of the flow‐induced crystallization in high‐density polyethylene by a continuum model

In this work, the isothermal flow‐induced crystallization (FIC) of high‐density polyethylene (HDPE) under a simple shear flow was investigated. Two experimental modes, including steady shear and preshear treatment, were performed on the polymer melt. Based on the nonequilibrium thermodynamic theory, the FIC process of HDPE was predicted through the modification of a continuum FIC model. The theoretical predictions of the evolution of both the viscosity in steady shear flow and the complex modulus under preshear treatment were essentially related to the crystallinity of HDPE, in agreement with the experimental findings. Both experimental and predicted results showed that the applied flow field could accelerate the crystallization kinetics of HDPE significantly. However, the effect of the intensity of shear flow on the crystallization of HDPE was finite, showing a saturation phenomenon, namely, the accelerated degree of crystallization tending to level off when the shear rate was large enough. In additional, it was found that the predicted crystallinity of HDPE was very low in induction period either in steady shear flow or by preshear treatment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 531–538, 2009     

Studies on preparation of HDPE/CB composites including a novel oriented structure by the microwave heating and their characterization

Microwave heating has several advantages over traditional methods of heating, including rapid and uniform heating, greater penetration depth of heat into material, lower power costs and selective heating within the material and so on. In this paper, effects of microwave heating on the properties of high‐density polyethylene/carbon black (HDPE/CB) composites were studied. The results show that the HDPE/CB composites can be heated via microwave irradiation, and composites with different CB concentration exhibit different microwave heatability. The 20 wt% CB composites have the most rapid heating rate, and its temperature reaches 78°C after 10 sec, and 159°C after 150 sec, respectively. Meanwhile, microwave heating improves the mechanical properties of HDPE/CB composites. Scanning Electron Microscopy (SEM) analysis shows a better combination between CB particles and HDPE after microwave irradiation. Furthermore, selective heating of microwave was used to prepare a novel oriented structure, which the core layer has preferential orientation and the surface layer has little orientation. Characterization of the novel oriented structure was also studied. Wide angle X‐ray diffraction (WAXD) analysis of 25 wt% CB composites with the novel oriented structure shows that the diffraction peaks of the surface layer are obviously weaker than those of the core layer, which indicates that orientation in the core layer is more intensive than that in the surface layer. The novel oriented structure is different to the traditional skin‐core structure, in which the surface layer has preferential orientation and the core layer has little orientation. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and characterization of side‐chain liquid‐crystalline polymers and study of their role in the crystallization of high‐density polyethylene

Side‐chain liquid‐crystalline polymers (SCLCPs) as nucleating agents for high‐density polyethylene (HDPE) were investigated. For this purpose, the molecular architectures of four different vinyl monomers with liquid‐crystalline properties were designed and prepared with 1‐butanol, 1‐pentanol, 4‐hydroxybenzoic acid, hydroquinone, and acryloyl chloride as the starting materials through alkylation and acylation reactions. The corresponding polymers were synthesized by homopolymerization in 1,4‐dioxane with benzoyl peroxide as the initiator at 60 °C. Both the monomers and the synthesized polymers were characterized with elemental analysis, Fourier transform infrared, and ¹H NMR measurements. Differential scanning calorimetry, thermogravimetric analysis, and hot stage polarized optical microscopy were employed to study the phase‐transition temperature, mesophase texture, and thermal stability of the liquid‐crystalline polymers. The results showed that all the polymers had thermotropic liquid‐crystalline features. Being used as nucleating agents, SCLCPs effectively increased both the crystallization temperature and rate and, at the same time, raised the crystallinity for HDPE. In comparison with common small‐molecule nucleating agents, such as 1,3:2,4‐dibenzylidenesorbitol, SCLCPs are more efficient and are indeed excellent nucleating agents for HDPE. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3067–3078, 2005     

Kinetics of the amorphous—anatase phase transformation in copper doped titanium oxide

Samples of TiO₂ doped with 2 and 5 mol% of Cu²⁺ were prepared by the sol-gel process. Titanium(IV) isopropoxide and copper(II) nitrate were used as precursors. The samples were prepared as monolithic shapes, dried at 80°C for 72 h and heat treated at various temperatures in the range 200–900°C for 2 h. The structural transformation and texture of the samples were investigated by X-ray powder diffraction (XRD) and nitrogen adsorption. Significant changes were observed during the crystallization process; on the one hand, the crystallization profiles show that crystallization occurs uniformly and is practically insensitive to the dopant concentration, but when the transformation at a given temperature is followed as a function of time, the rate of the amorphous-anatase transformation is larger for the sample containing 2 mol% Cu²⁺. Electron spin resonance (ESR) results show that in this sample there is no segregation of Cu²⁺ ions. The sample containing 2 mol% of Cu²⁺ was selected for the kinetic studies and the temperatures selected were 300, 325, 350, 375 and 400°C, which were taken from the amorphous to anatase crystallization profile. An activation energy of 137 ± 4 kJ/mol for the crystallization process was estimated from the kinetic data. These results showed that the effect of the open structure present in the TiO₂ amorphous phase provides the atomic mobility required for the crystallization. On the other hand, the differences in the crystallization rate due to the amount of Cu²⁺ were explained by the segregation of copper ions to the surface of the samples.