Effect of cocrystallization on kinetic parameters of high-density polyethylene/linear low-density polyethylene blend

The isothermal crystallization kinetics of a binary melt blend of high-density polyethylene (HDPE)/linear low-density polyethylene (LLDPE) is presented. An effort was made to understand the phenomenon of cocrystallization between these two constituting components of the blend with the help of kinetic parameters. The analysis based on the Avrami exponent entails that both HDPE and LLDPE undergo individual seeding of nuclei and they merge with each other in the growth process to form cocrystallites. The incorporation of the LLDPE segment in the HDPE crystallites progressively dilutes the properties of HDPE in the blend. The half-time of crystallization (t_1/2) shows variation in three distinct stages: The t_1/2 increases slowly in the region of 0–30% LLDPE content (HDPE-rich blend), remains constant in the 30–70% LLDPE-containing region (middle region of blend composition), and increases sharply thereafter. These variations of t_1/2 quite appreciably explain the change in % crystallinity, the Avrami exponent, and crystallite-size distribution. These observations were further supported by the small-angle light-scattering experiment. © 1996 John Wiley & Sons, Inc.     

Crystallization of high-density polyethylene–linear low-density polyethylene blend

The crystallization studies revealed that the high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) formed strong cocrystalline mass when they were melt blended in a single screw extruder. The progress of crystallization was observed through a small-angle light scattering instrument, scanning electron microscope, and differential scanning calorimeter. Analysis showed that these constituents followed individual nucleation and combine growth of crystallites in blends. The growth of crystallites all through the blend compositions were two-dimensional. Interestingly, the crystallites resembled each other for a particular blend composition; however, they differ widely as the composition changes. The rate of crystallization depends greatly to the number of crystallites and their interfacial boundary in contact with the amorphous phase pool. The t_1/2 and percentage of crystallinity showed a mutually exclusive trend and were seen to be varied in the following three regions of blend composition: the HDPE-rich, the LLDPE-rich, and the middle region of blend composition. The percentage of crystallinity decreases in both the HDPE-rich and LLDPE-rich blends, and it showed a plateau value in the middle region of blend composition. The t_1/2 showed opposite trend to that of % crystallinity. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2599–2607, 1998     

Interplay of crystallization and liquid-liquid phase separation in polyolefin blends: A thermal history dependence study

The kinetic interplay between crystallization and liquid-liquid phase separation (LLPS) in random copolymer blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-butene) (PEB) has been studied using optical microscopy. Morphologies of blends gone through three different thermal histories are compared: (1) single-quench (SQ), a homogeneous melt quickly cooled to isothermal crystallization temperatures (T_cry), (2) double-quench (DQ), a homogeneous melt quickly cooled to an intermediate temperature (T_lps) between binodal and equilibrium melting temperature (T_m⁰) and stored for a period of time and then cooled to T_cry, and (3) cyclic-quench (CQ), a homogeneous melt quickly cooled to T_lps and stored for a period of time, then gone through four cycles of crystallization and remelting. Comparing DQ morphologies to SQ ones, both crystal growth rate and nucleation density in the former are affected by prior LLPS. A scaling argument has been provided to partially account for the observed phenomena. In CQ, characteristic lengths of secondary features induced by crystallization depend strongly on the overall PEH composition, whereas are insensitive to temperature cycling. The contrast of large domains becomes more prominent upon cyclic crystallization and remelting. On the other hand, primary LLPS domains coarsen with CQ while loosing the contrast.     

Rheological and mechanical properties in polyethylene blends

Melt rheology and mechanical properties in linear low density polyethylene (LLDPE)/low density polyethylene (LDPE), LLDPE/high density polyethylene (HDPE), and HDPE/LDPE blends were investigated. All three blends were miscible in the melt, but the LLDPE/LDPE and HDPE/LDPE blends exibiled two crystallization and melting temperatures, indicating that those blends phase separated upon cooling from the melt. The melt strength of the blends increased with increasing molecular weight of the LDPE that was used. The mechanical properties of the LLDPE/LDPE blend were higher than claculated from a simple rule of mixtures, whiele those of the LLDPE/HDPE blend conformed to the rule of mixtures, but the properties of HDPE/LDPE were less than the rule of mixtures prediction.     

Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

Liquid-liquid phase separation and crystallization behavior of poly(ethylene terephthalate)/poly(ether imide) blend

The liquid-liquid (L-L) phase separation and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blend were investigated with optical microscopy, light scattering, and small angle X-ray scattering (SAXS). The thermal analysis showed that the concentration fluctuation between separated phases was controllable by changing the time spent for demixing before crystallization. The L-L phase-separated specimens at 130 °C for various time periods were subjected to a temperature-jump of 180 °C for the isothermal crystallization and then effects of L-L phase separation on crystallization were investigated. The crystal growth rate decreased with increasing L-L phase-separated time (t_s). The slow crystallization for a long t_s implied that the growth path of crystals was highly distorted by the rearrangement of the spinodal domains associated with coarsening. The characteristic morphological parameters at the lamellar level were determined by the correlation function analysis on the SAXS data. The blend had a larger amorphous layer thickness than the pure PET, indicating that PEI molecules in the PET-rich phase were incorporated into the interlamellar regions during crystallization.     

High-pressure solution blending of poly(?-caprolactone) with poly(methyl methacrylate) in acetone + carbon dioxide

The miscibility of blends of poly(?-caprolactone) (PCL, M_w = 14,300) with poly(methyl methacrylate) (PMMA, M_w = 15K or 540K) in acetone + CO₂ mixed solvent has been explored. The liquid-liquid phase boundaries at different temperatures have been determined for mixtures containing 10 wt% total polymer blend, 50 wt% acetone and 40 wt% CO₂. The PCL and PMMA contents of the blends were varied while holding the total polymer concentration at 10 wt%. The polymer blend solutions all displayed LCST-type behavior and required higher pressures than individual polymer components for complete miscibility. Complete miscibilities were achieved at pressures within 40 MPa. The DSC scans show that the blends are microphase-separated. The blends display the melting transition of PCL and the glass transition temperature of the PMMA phases. The presence of PMMA is found to influence the crystallization and melting behavior of PCL in the blends. The DSC results on heat of melting and the FTIR spectra, specifically the changes at 1295 cm⁻¹ band show the changes (decrease) in overall crystallinity of the blend upon addition of PMMA.     

Poly(allylammonium acrylate) as a drug-releasing matrix

A large increase in the crystallization temperature of low density polyethylene (LDPE) when blended with high density polyethylene (HDPE) is reported. Such behavior is observed for quenched LDPE rich blends when the low melting component is cooled from 119 °C under controlled conditions in the differential scanning calorimeter. It is suggested that the presence of the most linear LDPE methylene segments within the HDPE-rich crystals (cocrystallization phenomenon) facilitates the nucleation of the more branched LDPE segments on cooling. On reheating, a depression in the low melting temperature component (LDPE) is observed with increasing HDPE content in the blend. Received: 29 December 1996/Revised: 11 March 1997/Accepted: 14 March 1997     

Dynamic mechanical properties of binary blends of different polyethylenes. II. Isothermal treatment

The relaxation processes and thermal properties of a series of blends of a highly linear high-density polyethylene (HDPE) with several branched high-density, linear low-density (LLDPE), and low-density polyethylenes (LDPE) have been measured as a function of crystallization temperature, T_c, and content of branched polyethylene (BPE). The influence of composition on the dynamic mechanical spectrum of the HDPE has been rationalized taking into account the dilution with increasing content of BPE of those crystals formed during the isothermal crystallization. The influence of the type of second constituent (HDPE, LLDPE or LDPE) on the relaxation process of the HDPE has been explained in terms of segregation material data.     

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。     

A molecular dynamics study of the effects of branching characteristics of LDPE on its miscibility with HDPE

The effects of branching characteristics of low-density polyethylene (LDPE) on its melt miscibility with high-density polyethylene (HDPE) were studied using molecular simulation. In particular, molecular dynamics (MD) was applied to compute Hildebrand solubility parameters (δ) of models of HDPE and LDPE with different branch contents at five temperatures that are well above their melting temperatures. Values computed for δ agreed very well with experiment. The Flory-Huggins interaction parameters (χ) for blends of HDPE and different LDPE models were then calculated using the computed δ values. The level of branch content for LDPE above which the blends are immiscible and segregate in the melt was found to be around 30 branches/1000 long chain carbons at the chosen simulation temperatures. This value is significantly lower than that of butene-based linear low-density polyethylene (LLDPE) (40 branches/1000 carbons) in the blends with HDPE computed by one of the authors (polymer 2000; 41:8741). The major difference between LDPE and LLDPE models is that each modeled LDPE molecule has three long chains while each modeled LLDPE molecule had only one long chain. The present results together with those of the LLDPE/HDPE blends suggest that the long chain branching may have significant influence on the miscibility of polyethylene blends at elevated temperatures.     

Positive temperature coefficient effect and mechanism of compatible LLDPE/HDPE composites doping conductive graphite powders

Linear low density polyethylene (LLDPE)/high density polyethylene (HDPE) blends doped conductive graphite powders were constructed by the traditional melt‐blending method to acquire the conductive compatible polymer composites, and corresponding positive temperature coefficient (PTC) effect of electrical resistivity was investigated. The results indicated that the room‐temperature resistivity gradually decreased and PTC effects were remarkably enhanced by regulating the graphite contents or LLDPE/HDPE ratios. Especially, with increasing graphite contents, the polymer‐fixed composites showed the notable double PTC effects, originating from the volume expansion of the co‐crystallization or their fraction. Whereas, with increasing the LLDPE/HDPE ratio, the PTC effects of the graphite‐fixed composites occurred at the lower temperature, even far below the melting points of the co‐crystallization. Therefore, the regulation of co‐crystallization morphology of compatible polymer matrices was a new idea in the improvement of PTC materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46453.     

Kinetics of pressure-induced phase separation in polystyrene + acetone solutions at high pressures

The kinetics of pressure-induced phase separation in solutions of polystyrene (M_w = 129,200; PDI = 1.02) in acetone has been studied using time- and angle-resolved light scattering. A series of controlled pressure quench experiments with different quench depths were conducted at different polymer concentrations (4.0%, 5.0%, 8.2% and 11.4% by mass) to determine the binodal and spinodal boundaries and consequently the polymer critical concentration. The results show that the solution with a polymer concentration 11.4 wt% undergoes phase separation by spinodal decomposition mechanism for both the shallow and deep quenches as characterized by a maximum in the angular distribution of the scattered light intensity profiles. Phase separation in solutions at lower polymer concentrations (4.0, 5.0 and 8.2 wt%) proceeds by nucleation and growth mechanism for shallow quenches, but by spinodal decomposition for deeper quenches. These results have been used to map-out the metastable gap and identify the critical polymer concentration where the spinodal and binodal envelops merge.The time scale of new phase formation and growth as (accessed) from the time evolution of scattered light intensities is observed to be relatively short. The late stage of phase separation is entered within seconds after a pressure quench is applied. For the systems undergoing spinodal decomposition, the characteristic wave number qm corresponding to the scattered light intensity maximum Im was analyzed by power-law scaling according to qm∼t−α and Im∼tβ. The results show β≈2α. The domain size is observed to grow from 4 μm to 10 μm within 2 s for critical quench, but about 9 s for off-critical quenches. The domain growth displays elements of self-similarity.     

Acceleration or retardation to crystallization if liquid-liquid phase separation occurs: Studies on a polyolefin blend by SAXS/WAXD, DSC and TEM

Blends of statistical copolymers containing ethylene/hexene (PEH) and ethylene/butene (PEB) exhibited the behavior of upper critical solution temperature (UCST). The interplay between the early and intermediate stage liquid-liquid phase separation (LLPS) and crystallization of the PEH/PEB 50/50 blend was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Samples were treated by two different quench procedures: in single quench, the sample was directly quenched from 160 °C to isothermal crystallization temperature of 114 °C; while in double quench, the sample was firstly quenched to 130 °C for 20 min annealing, where LLPS occurred, and then to 114 °C. It was found that in the early stage of crystallization, the integrated values of Iq² and crystallinity, X_c, in the double quench procedure were consistently higher than those in the single quench procedure, which could be attributed to accelerated nucleation induced by enhanced concentration fluctuations and interfacial tension. In the late stage of crystallization, some morphological parameters were found to crossover and then reverse, which could be explained by retardation of lamellar growth due to phase separation formed during the double quench procedure. This phenomenon was also confirmed by DSC measurements in blends of different compositions at varying isothermal crystallization temperatures. The crystal lamellar thickness determined by SAXS showed a good agreement with TEM observation. Results indicated that the early stage LLPS in the PEH/PEB blend prior to crystallization indeed dictated the resulting lamellar structures, including the average size of lamellar stack and the stack distribution. There seemed to be little variation of lamellar thickness and long period between the two quenching procedures (i.e., single quench versus double quench).     

Sub-micron dispersed-phase particle size in polymer blends: overcoming the Taylor limit via solid-state shear pulverization

A comparison was made of the fineness of dispersion in immiscible polymer blends achieved by a continuous mechanical alloying technique, solid-state shear pulverization, relative to that achieved by melt mixing. Two polymer blend systems were investigated. A polystyrene (PS)/polyethylene (PE) wax blend was studied because, based on a classic analysis by G.I. Taylor, melt mixing was expected to yield a number-average dispersed-phase domain size, D_n, well above 1 μm. A PS/high density polyethylene (HDPE) blend was also studied because it was known to produce a sub-micron number-average dispersed-phase particle size when mixed by twin-screw extrusion. In the case of the PS/PE wax blend at compositions ranging from 1 to 15 wt% polyethylene wax, pulverization resulted in nearly identical D_n values (typical value of 0.7 μm) independent of minor-phase content; these D_n values were an order of magnitude smaller than the anticipated Taylor limit for melt-mixed blends. In contrast, PS/PE wax blends made by batch, intensive melt mixing yielded D_n values between ∼3 μm at both 1 and 5 wt% minor-phase content and 17.5 μm at 15 wt% minor-phase content. The increase in D_n with increasing dispersed-phase content in the melt-mixed blend is a consequence of coalescence present during melt processing; such effects are disallowed in the pulverization process occurring in the solid state. Scanning electron microscopy of a 95/5 wt% PS/HDPE blend provided D_n values of 500 and 270 nm in the twin-screw extruded and pulverized samples, respectively. Fractionated crystallization studies further corroborated the ability of pulverization to result in a finer, nanoscopic dispersion of the minor phase as compared to extrusion.     

Study of the effect of branch content of octene‐based linear low‐density polyethylene on its melt miscibility with high‐density polyethylene by inverse gas chromatography

Inverse gas chromatography was used to measure Flory–Huggins interaction parameters (χ₂₃) for five binary blends consisting of high‐density polyethylene (HDPE) and octene‐based linear low‐density polyethylene (LLDPE) with different compositions at four elevated temperatures. The branch content of the LLDPE used in each pair of the blends ranged from 2 to 87 branches per 1000 backbone carbons. To obtain solvent‐independent χ₂₃, the data analysis approach recently proposed by Zhao and Choi (Polymer 2001, 42, 1075) was used. The results indicate that the higher the branch content of LLDPE, the higher the measured χ₂₃, signifying that HDPE/LLDPE blends with low branch content LLDPEs are relatively more miscible than those with high branch contents. In particular, when the branch content of LLDPE is higher than 50 branches per 1000 backbone carbons, phase separation may occur. This result is in good agreement with other researchers' results obtained from different techniques. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1927–1931, 2004     

Linear low-density polyethylene/poly(ethylene-ran-butene) elastomer blends: Miscibility and crystallization behavior

The phase and crystallization behavior of the blends consisting of LLDPE (0.7 mol% hexene copolymer) and PEB (26 mol% butene copolymer) have been investigated using optical microscopy (OM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The blends exhibited an upper critical solution temperature of 162°C. The solubility parameter analysis showed that the solubility parameter of LLDPE decreased more rapidly than that of PEB with temperature. However, due to the slow kinetics of phase separation, at lower crystallization temperatures, the crystallization and melting behavior of LLDPE mainly reflected the miscibility between LLDPE and PEB. Crystallization from the two-phase state could present two crystallization peaks. PEB didnt change the crystal cell unit and crystallinity of LLDPE, but changed its distribution of lamellar thickness or crystal perfection. The dilute effect of PEB also changed the overall nature of the nucleation and growth process of LLDPE. The equilibrium melting temperature in this blend could be obtained by the Hoffman-Weeks method, and comparing with that of the pure LLDPE, it was reduced and kept relatively constant in the bi-phase state. The phase diagram made up of the LLPS boundary, equilibrium melting temperatures and melting temperatures observed may be better to indicate the phase and crystallization behavior of LLDPE/PEB blends.     

Liquid phase behavior and its effect on crystalline morphology in the extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) blend

Morphology in an extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) was investigated using time-resolved light scattering, optical microscope and small-angle X-ray scattering. During annealing at 280 °C, the domain structure via spinodal decomposition preceded, the transesterification followed, and then the transesterification between the two polyesters induced the dissolution of the liquid-liquid (L-L) phase separation, i.e. the homogenization. The annealed specimen for various time periods (t_s) at 280 °C was subjected to a temperature-drop to 120 °C for the isothermal crystallization and then the effects of liquid phase morphology on crystallization was investigated. With t_s, the H_ν (cross-polarization) light scattering patterns exhibited the dramatic change from a four-leaf clover pattern with maximum intensity at azimuthal angle 45° (×-type scattering pattern) to a diffuse pattern of circular symmetry and then a four-leaf clover pattern with maximum intensity at azimuthal angles 0 and 90° (+-type scattering pattern). This suggests that the crystalline structure depends on the level of the block and/or random copolymer produced by the transesterification during annealing. The H_ν scattering patterns reflected differences in the principle polarizability of the crystalline lamellae with respect to the spherulitic radius. On the other hand, the long period L_B, an average distance between two adjacent crystalline lamellae, increased with t_s at 280 °C. The dependence of L_B on t_s was explained by the change in the crystallization rate G.     

Crystallization behavior of poly(l-lactic acid) affected by the addition of a small amount of poly(3-hydroxybutyrate)

The miscibility, crystallization and subsequent melting behavior in binary biodegradable polymer blends of poly(l-lactic acid) (PLLA) and low molecular weight poly(3-hydroxybutyrate) (PHB) have been investigated by differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). DSC analysis results indicted that PLLA showed no miscibility with high molecular weight PHB (M_w = 650,000 g mol⁻¹) in the 80/20, 60/40, 40/60, 20/80 composition range of the PHB/PLLA blends. On the other hand, it showed some limited miscibility with low molecular weight PHB (M_w = 5000 g mol⁻¹) when the PHB content was below 25%, as evidenced by small changes in the glass transition temperature of PLLA. The partial miscibility was further supported by changes of cold-crystallization behavior of PLLA in the blends. During the nonisothermal crystallization, it was found that the addition of a small amount of PHB up to 30% made the cold-crystallization of PLLA occur in the lower temperature. Meanwhile, the crystallization of PHB and PLLA was observed in the heating process by monitoring characteristic IR bands of each component for the low molecular weight PHB/PLLA 20/80 and 30/70 blends. The temperature-dependent IR and WAXD results also revealed that for PLLA component crystallization, the disorder (α′) phase of PLLA was produced, and that the α′ phase changed to the order (α) phase just prior to the melting point.     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.