On the crystallization of syndiotactic polypropylene (sPP)/high density polyethylene (HDPE) blends

 The crystallization kinetics of syndiotactic polypropylene (sPP)/high-density polyethylene (HDPE) blends were investigated by thermal and microscopical methods. Isothermal crystallization obtained by differential scanning calorimetry (DSC) was used to study the nucleation ability of HDPE on sPP crystallization. Morphological studies with polarized light microscopy (PLM) of solution cast films resulted in a transcrystalline-like overgrowth of the sPP on HDPE spherulites. The arrangement of the HDPE lamellar crystals in the blend were observed in transmission electron microscopy (TEM). But from these results, no straightforward conclusion about the transcrystalline nature of the sPP crystals can be given. Received: 24 November 1997 Accepted: 23 February 1998     

Effect of Carboxylic Acid Nucleating Agent on Crystallization and Mechanical Properties of PLA/PBS Blends

Poly(lactic acid) (PLA)/poly(butylene succinate) (PBS) blends were prepared using a carboxylic acid salt as nucleating agent (NA). The effect of NA on the crystallization kinetics of PLA and PLA/PBS blend was investigated using a differential scanning calorimeter, a polarized optical microscope and a wideangle X-ray diffractometer. The crystallization rate of PLA component in PLA/PBS blends is increased effectively by NA through fast nucleation and growth rate of PLA α'-form crystal, which is confirmed by isothermal crystallization behavior of PLA/PBA/NA composites. The isothermal crystallization results also show that the incorporation of NA induces heterogeneous nucleation mechanism in PLA component. The increased number of crystal nuclei hinders the increase of average grain size of PLA component in composites but contributes to a higher crystallinity of both PLA and PBS components in PLA/PBS blends. Finally, the mechanical properties and dynamic mechanical properties of PLA/PBS/NA composites are improved because of the increased crystallinity, which are superior to that of PLA/PBS blend.     

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution 1. Basic characteristics and isothermal crystallisation

Isothermal crystallisation of high density polyethylene (HDPE) blends and their parent polymers was investigated. The blends having broad bimodal molar mass distributions and various compositions were prepared by blending a high molar mass (M_w=330 kg/mol, M_w/M_n=4.8) and a low molar mass HDPE (M_w=34 kg/mol, M_w/M_n=10) in different ratios in xylene solution. The blends and their parent components were characterised by size-exclusion chromatography, dynamic rheological and density measurements. Crystallisation kinetics were studied using a polarised light microscope equipped with an in-house built hot stage and by differential scanning calorimetry. The Avrami theory was applied for crystallisation kinetics analysis. Such crystallisation kinetics parameters as nucleation rate, nucleation density, the Avrami index and cystallisation rate contant were determined for the blends and their parent polymers.According to the results obtained an increasing polydispersity of the sample had a slight increasing effect on the Avrami index, indicating gain in prevalence of the thermal nucleation over the athermal one. In all samples nucleation density increased continuously during crystallisation verifying that the presence of a certain thermal nucleation was typical for all the materials studied. Both the crystallisation rate constant and the nucleation rate decreased with increasing molar mass of the sample. The nucleation density increased proportionally to the increase in average molar mass and the values were larger at lower crystallisation temperatures.The formed supermolecular structure was found to be sensitive to the blend composition and crystallisation temperature. Irregular banded or non-banded spherulites were observed in the materials. Banding of spherulites was typical for the samples having higher average molar mass. The superstructures observed in this work were smaller and vaguer than the superstructures reported in the earlier studies of polyethylene materials having similar average molar mass but narrow molar mass distribution.     

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     

Evaluation of the fractionated crystallization of isotactic polypropylene and high density polyethylenes in their blends with Cycloolefin copolymers

Crystallization of semi-crystalline polyolefins (i-polypropylenes and HD-polyethylenes) in their blends with amorphous cycloolefin copolymers (COC) were studied. The thermal behaviour of the blends was characterized by Differential Scanning Calorimetry (DSC) whereas blend morphologies were investigated by Scanning Electronic Microscopy (SEM). In iPP/COC blends, a phenomenon of fractionated crystallization is evidenced when i-PP is finely dispersed in the COC matrix. Such a behavior is generally observed when the number of droplets is much larger than the number of heterogeneities originally present in the bulk polymer. In HDPE/COC blends, complex morphologies are observed which do not fit good correlation with DSC results. The nucleation and crystallization modes seem to be largely influenced by the characteristics of the micro-dispersed phase, largely dependent on the PE molecular weights and polydispersity indices.     

Metallocene polyolefins and their blends#

Commercial copolymers of 1‐octene and ethylene: metallocene catalyzed (mLLDPE) and Ziegler‐Natta catalyzed (znLLDPE), a low density polyethylene (LDPE), and high density polyethylene (HDPE), were characterized with respect to branching, crystallization behaviour and dynamic‐mechanical properties. It was found that the crystallinity of the polymers is more influenced by the homogeneity of the short‐chain branching than by its content. The study of blends of mLLDPE and znLLDPE with LDPE and HDPE showed that the interaction between mLLDPE and LDPE is stronger than between znLLDPE and LDPE. Blends containing mLLDPE showed a composition depending improvement of the storage modulus G' which was not observed in znLLDPE/LDPE blends. The HPDE blends followed a linear mixing rule. Co‐crystallization was found mLLDPE/LDPE and partially in znLLDPE/LDPE and znLLDPE/HDPE blends, respectively.     

Crystallization behavior of disproportionately high and low molecular weight PEO blends

The crystallization behavior of PEOs with molecular weight of 1 Ok and 200k as well as their blends was studied in details. The results show that the lower molecular weight PEO crystallizes with faster crystallization rate as judged from a shorter time for completing the crystallization. On the other hand, the higher molecular weight PEO crystallizes at relatively higher temperature, indicating an early start of crystallization compared with the lower molecular weight one. The blends of these two PEOs with different blend ratios always cocrystallize during the cooling processes. It is confirmed that mixing of the 10k PEO with the 200k one is in favor of the crystallization of the system. This is not only demonstrated by the early start of the crystallization at higher crystallization temperature, and also a faster crystal growth of the blend with respect to the 200k PEO. The crystallization of the blends at higher temperature is caused by an early start of nucleation and an increment of nucleus density. This may originate from the density fluctuation of the blend and a reduction in energy barrier for nucleation. Moreover, it is found that the crystallinity of the 1 Ok PEO rich blends increases with increasing concentration of the 10k PEO. This is caused by the solvent effect of the 10k PEO toward the 200k PEO. On the other hand, the crystallinity of the 30/70 （10k/200k） PEO blend is decreased a little bit. This may be a balanced result of the improved crystallization of the 200k PEO at the expense of the high crystallization ability of the 1 Ok PEO.     

The effect of various compatibilizers on thermal,mechanical, and morphological properties of polystyrene/polypropylene blends

The effects of various compatibilizers on thermal, mechanical and morphological properties of 50/50 polypropylene/polystyrene blends were investigated. Various compatibilizers, polystyrene-(ethylene/butylenes/ styrene) (SEBS), ethylene vinyl acetate (EVA), polystyrene-butylene rubber (SBR) and blend of compatibilizers SEBS/PP-g-MAH, EVA/PP-g-MAH, and SBR/PP-g-MAH were used. Differential scanning calorimetry, thermogravimetric analysis, wide-angle X-ray scattering, scanning electron microscopy, microhardness, and Izod impact strength were adopted. It was found that the influence of various compatibilizers was appeared on all the properties studied. The properties of the blends compatibilized with SEBS, EVA, and SBR are very distinct from those of blends compatibilized with blend of compatibilizers. Results show that compatibilized blends with the blend of compatibilizers EVA/PP-g-MAH, SBR/PP-g-MAH, and SEBS/PP-g-MAH or SBR were relatively more stable than the uncompatibilized blend and blend compatibilized with SEBS or EVA. The compatibilizer does not only reduce the interfacial tension or increase the phase interfacial adhesion between the immiscible polymers, but greatly affects the degree of crystallinity of blends.

     

Crystallinity and rheology of HDPE/PA12 blends compatibilized with HDPE-alt-MAH

This work studies the crystallinity and rheology of HDPE/PA12 blends compatibilized with 2 wt% of HDPE-alt-MAH. Specimens of HDPE/PA12 blends were extruded and injected into a mold with 75/25, 50/50, and 25/75 HDPE/PA ratios. The Fourier-transform infrared spectroscopy (FTIR) analysis showed that no oxidation reaction occurred in the high-temperature processing and that stronger interactions between the components of the blends occurred in the polyamide's functional groups. The x-ray diffraction (XRD) analysis showed that the crystallinity degree of the blends and the mean crystallite sizes decreased with the addition of PA12 for both blends. The HDPE's lattice parameters were consistent with the values in the literature, whereas for the PA12, it was not possible to fit its lattice parameters. The rheology analysis evaluated the relationship between the shear stress and viscosity and found that the HDPE/PA 75/25 blend was the most pseudoplastic, presenting the best processability under high shear rates.     

Preparation and properties of dynamically cured poly(vinylidene fluoride)/silicone rubber blends

Blends of poly(vinylidene fluoride) (PVDF) and silicone rubber (SR) were prepared through dynamic vulcanization. The effects of SR content on crystallization behavior, rheology, dynamic mechanical properties and morphology of the blends were investigated. Morphology characterization shows that the crosslinked spherical SR particles with an average diameter of 2-4 μm form a “network” in the PVDF continuous phase. The dynamic mechanical properties indicate the interface adhesion between PVDF and rubber phase is improved by the dynamic vulcanization. The rheology study shows that with the increase of rubber content the blends pseudoplastic nature is retained, while the viscosity increases, and hence the processability is less good. The incorporation of SR phase promotes the nucleation process of PVDF, leading to increased polymer crystallization rate and crystallization temperature. However, a higher content of SR seems to show a negative effect on the crystallinity of the PVDF component.     

原位聚合聚乙烯/蒙脱土纳米复合材料的结晶行为

用DSC研究了HDPE与MMT负载的催化剂熔融共混和原位聚合得到的两种纳米复合材料的熔融、 结晶行为和等温结晶动力学.  结果表明, HDPE与熔融共混样品的结晶度、 平衡熔点、 球晶生长速率和结晶能力大体相同; 原位聚合得到的HDPE/MMT纳米复合材料的结晶度和平衡熔点高于纯HDPE; 在相同过冷度条件下熔融结晶速率和结晶能力低于纯HDPE, 而在相同结晶温度Tc下, 熔融结晶速率和结晶能力则高于纯HDPE.  纯HDPE的晶体生长侧向单位面积表面自由能最小, 其次是熔融共混样品, 原位聚合样品最大, 且随ＭＭＴ含量的增加逐渐升高.     

Impact of plasticizers and tackifiers on the crystallization of isotactic poly(1-butene)

Crystallization of a semi-crystalline polyolefin in the presence of low molecular weight modifiers was quantified by differential scanning calorimetry and optical microscopy. The polyolefin was a commercial grade of isotactic poly(1-butene) (iPB). Two modifiers were used: an oligomeric plasticizer, designated HOAO, which decreased the glass transition temperature (T_g) of the system, and an oligomeric tackifier, designated HOCP, which increased T_g. Binary iPB/modifier blends containing 10% or 20% by weight of HOAO or HOCP were examined to determine how their addition affects T_g, while ternary iPB/HOAO/HOCP blends containing 10% or 20% by weight of total modifier were examined to determine the effects of dilution by using a ratio of HOAO to HOCP that matched the T_g of iPB. The addition of modifier decreased the nucleation rate, spherulitic crystal growth rate, and final crystallinity of each blend. However, only the nucleation rate showed a dependence on the type of modifier, with nucleation retarded more by HOCP than by HOAO. A Hoffman-Weeks analysis of the melting point as a function of crystallization temperature confirmed that the driving force for nucleation was reduced, and that the effect was larger for HOCP. An Avrami analysis of the bulk crystallization kinetics was consistent with these observations, as the Avrami exponents were in the range of 3-4.     

Thermal and shrinkage behaviour of stretched peroxide-crosslinked high-density polyethylene

The thermal shrinkage of stretched crosslinked high-density polyethylene (HDPE) was investigated with the aim to produce heat shrinkable materials. The heat shrinkable property was achieved by a process of heating-stretching-cooling by aid of tensile machine on crosslinked HDPE obtained by compounding with various amount of peroxide. Effect of stretching ratio and stretching temperature on thermal and shrinkage behaviour at varying peroxide contents was investigated. The results showed that crosslinking hindered the crystallization process by decreasing the melting and crystallization temperatures as well as the total degree of crystallinity. The stretching ratio had no significant effect on shrink temperature but rather on ultimate shrinkage. The stretching temperature had relatively significant influence on the shrink temperature. Crosslinked HDPE stretched at above melting point (140 °C) had higher shrink temperature as compared to those stretched at lower temperature (90 °C). These effects could be reasonably explained by Hoffman theory and changes in crystallites size and total amount of crystallinity.     

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.     

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.     

Dynamic mechanical behavior of acrylonitrile butadiene rubber/poly(ethylene‐co‐vinyl acetate) blends

The dynamic mechanical behavior of uncrosslinked (thermoplastic) and crosslinked (thermosetting) acrylonitrile butadiene rubber/poly(ethylene‐co‐vinyl acetate) (NBR/EVA) blends was studied with reference to the effect of blend ratio, crosslinking systems, frequency, and temperature. Different crosslinked systems were prepared using peroxide (DCP), sulfur, and mixed crosslink systems. The glass‐transition behavior of the blends was affected by the blend ratio, the nature of crosslinking, and frequency. sThe damping properties of the blends increased with NBR content. The variations in tan δ_max were in accordance with morphology changes in the blends. From tan δ values of peroxide‐cured NBR, EVA, and blends the crosslinking effect of DCP was more predominant in NBR. The morphology of the uncrosslinked blends was examined using scanning electron and optical microscopes. Cocontinuous morphology was observed between 40 and 60 wt % of NBR. The particle size distribution curve of the blends was also drawn. The Arrhenius relationship was used to calculate the activation energy for the glass transition of the blends, and it decreased with an increase in the NBR content. Various theoretical models were used to predict the modulus of the blends. From wide‐angle X‐ray scattering studies, the degree of crystallinity of the blends decreased with an increasing NBR content. The thermal behavior of the uncrosslinked and crosslinked systems of NBR/EVA blends was analyzed using a differential scanning calorimeter. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1556–1570, 2002     

Morphology dependence of crystallization of natural rubber in blends

Crystallization of natural rubber (NR) was investigated in different morphology for NR/styrene butadiene rubber (SBR) blend and NR/polystyrene-(b)-polyisoprene (SI)/polystyrene (PS) blend. A purified NR (PC-TE) was prepared from pale crape via transesterification. In the blends, PC-TE formed various morphologies; that is, matrix phase, island phase and continuous phase with a nano-scale, respectively, in dependence upon the ratio of the rubbers. The crystallization rate of the blends was also significantly associated with the morphology of the rubbers.     

Effects of partial replacement of silicone rubber with flurorubber on properties of dynamically cured poly(vinylidene fluoride)/silicone rubber/flurorubber ternary blends

Blends of poly(vinylidene fluoride) (PVDF), silicone rubber (SR) and flurorubber (FKM) were prepared via peroxide dynamic vulcanization. The effect of FKM loading on the morphology, mechanical properties, crystallization behavior, rheology and dynamic mechanical properties of the PVDF/SR/FKM ternary blends was investigated. A “network” was observed in the PVDF/SR binary blends, which disappeared in the ternary blends, but a core-shell-like structure was formed. The mechanical properties were significantly improved. The Izod impact strength of PVDF/SR/FKM blend with 19 wt% FKM was 18.3 kJ/m², which was 3–4 times higher than the PVDF/SR binary blend. The complex viscosity and storage modulus of the PVDF/SR/FKM blends decreased with increasing FKM content, hence the processability was improved. The increase of FKM content seemed to show a favorable effect on the crystallization of the PVDF component. It promoted the nucleation process of PVDF, leading to increased polymer crystallization rate and higher crystallization temperature. The glass-rubber transition temperature of the PVDF phase moved to a lower temperature.     

Phase morphology, crystallisation behaviour and mechanical properties of isotactic polypropylene/high density polyethylene blends

The phase morphology, crystallisation behaviour and mechanical properties of isotactic polypropylene (iPP)/high density polyethylene (HDPE) blends were investigated. It was found that the properties are intimately related to each other. The morphology of the blends showed a two phase structure in which the minor phase was dispersed as domains in the major continuous matrix phase. The domain size of the dispersed phase increased with increasing concentration of that phase due to coalescence. It was also found that the domain size of the dispersed phase depends on the viscosity difference between the two phases. For a given HDPE/iPP blend, where HDPE is the matrix and iPP is the dispersed phase, the iPP domains were smaller than HDPE domains of the corresponding iPP/HDPE blend where iPP is the matrix and HDPE is the dispersed phase. A co-continuous morphology was observed at 50/50 PP/HDPE composition. Crystallinity studies revealed that blending has not much effect on the crystalline melting point of polypropylene and high density polyethylene. The crystallisation enthalpy and heat of fusion values of HDPE and PP in the blend were decreased as the amount of the other component increased. The variation in percent crystallinity of HDPE and PP in the blend was found to depend on the morphology of the blend. All the mechanical properties except Young's modulus and hardness showed negative deviation from the additivity line. This is due to the incompatibility of these blends.     

Thermogravimetric and wide angle X-ray diffraction analysis of thermoplastic elastomers from nylon copolymer and EPDM rubber

Nylon copolymer (PA6, 66) and ethylene propylene diene (EPDM) blends with and without compatibilizer were prepared by melt mixing using Brabender Plasticorder. The thermal stability of nylon copolymer (PA6, 66)/ethylene propylene diene rubber (EPDM) blends was studied using thermogravimetric analysis (TGA). The morphology of the blends was investigated using scanning electron microscopy (SEM). In this work, the effects of blend ratio and compatibilisation on thermal stability and crystallinity were investigated. The incorporation of EPDM rubber was found to improve the thermal stability of nylon copolymer. The kinetic parameters of the degradation process were also studied. A good correlation was observed between the thermal properties and phase morphology of the blends. By applying Coats and Redfern method, the activation energies of various blends were derived from the Thermogravimetric curves. The compatibilization of the blends using EPM-g-MA has increased the degradation temperature and decreased the weight loss. EPM-g-MA is an effective compatibilizer as it increases the decomposition temperature and thermal stability of the blends. Crystallinity of various systems has been studied using wide angle X-ray scattering (WAXS). The addition of EPDM decreases the crystallinity of the blend systems.