Experimental verification on UCST phase diagrams and miscibility in binary blends of isotactic, syndiotactic, and atactic polypropylenes

Issues in blends of polymers of the same chemical repeat unit but with different tacticities were addressed by investigating on the phase behavior and interaction strength of binary blends of three polypropylenes of different tacticities, i.e., isotactic polypropylene (iPP), syndiotactic polypropylene (sPP), and atactic polypropylene (aPP) using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). Although blends of polypropylenes have been widely studied in the past, there are still on-going debates on true phase behavior (miscibility vs. upper critical solution temperature (UCST) or immiscibility). Except for several earlier theoretical predictions based on the Flory-Huggins mean field theories, UCST behavior had not been experimentally proven for blends of sPP/iPP or aPP/sPP, owing to interference from PP crystallinity. In addition, interaction strength of the blends of different tactic polypropylenes is yet to be established. Using the method of equilibrium melting points, the Flory-Huggins interaction parameter of the aPP/iPP blend was shown to possess a significantly negative value (χ₁₂ = −0.21), which proves that the blend is indeed miscible in the melted amorphous as well as semicrystalline states as previously reported in the literature. However, the interaction parameters for the sPP/iPP and aPP/sPP blends were found to be nearly zero (χ₁₂ = −0.02 and −0.0071, respectively, at T = 150-180 °C), indicating that the interactions in two blends are weak and that the corresponding phase behavior for them borders on immiscibility at ambient temperature. This study also utilized novel approaches in constructing UCST phase diagrams by separating the amorphous phase domains from the crystalline spherulites, yielding data plausible for experimentally determining the UCST in iPP/sPP blend vs. aPP/sPP blend.     

Crystallization behaviors in the isotactic polypropylene/graphene composites

Crystallization behaviors and kinetics of iPP in an in-situ prepared isotactic polypropylene/graphene (iPP/G) composites were studied in this paper. In samples used in this study, the graphene fillers were well dispersed, and the interfacial adhesion exhibited enhanced features between graphene and iPP components. The thermal stability of the composites was improved by about 100 °C compared to the pristine iPP. It was found that the crystallization morphology, crystallization rate and kinetics of the iPP/G composites were significantly influenced by the presence of graphene. The nucleation and epitaxial growth of iPP on the graphene surface were observed and studied in detail. It was observed that the nucleation of iPP favored to occur at the wrinkles and edges due to the good match of the lattice parameters and the weak spatial hindrance compared to the smooth surface. Numerous nuclei epitaxially formed and the size of the crystals was very small. The schematic diagram was also proposed for the nucleation and growth process of iPP on the graphene surface in the iPP/G composites. Meanwhile, the overall crystallization kinetics and crystals growth were analyzed through Avrami equation. The obtained Avrami index n decreased with the graphene loadings and was close to 2 for the iPP/G composites, which implied that the growth of iPP in the composites was in two-dimension. And this was caused by the structure of graphene and the spatial confinement effect of graphene platelets in the iPP/G composites.     

铁系间同1,2-聚丁二烯的合成与性能

合成了一种铁系间同1,2-聚丁二烯,对其进行了分子结构表征及基本性能评价,并与日本合成橡胶公司的RB系列产品进行了比较.结果表明,该催化体系的活性较高,且产物的收率也较高,其熔点可低于130 ℃,拉伸强度大于20 MPa.     

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

The crystallization morphologies and mechanical behaviors of iPP/sPP blends and the corresponding fibers were investigated in the present work. For all the investigated iPP/sPP blends, the starting crystallization temperature of sPP during cooling process was significantly increased with increasing iPP content. The iPP/sPP blends are strongly immiscible at the conventional melt processing temperatures, in consistence with the literature results. As isothermally crystallized at 130 °C, sPP still keeps melt state, while iPP component is able to crystallize and the spherulites become imperfect accompanied by decreasing of the crystallite size as sPP content increases. The addition of sPP decreases the crystallinity of iPP/sPP blends and fibers. The storage modulus, E′, of the iPP/sPP blends is higher than that of sPP homopolymer in the temperature range from −90 to 100 °C. The iPP/sPP fibers can be prepared favorably by melt-spinning. As sPP content exceeds 70%, the elastic recovery of the iPP/sPP fibers is approximately equal to that of sPP homopolymer fiber. The drawability of the as-spun fiber of iPP/sPP (50/50) is better than that of sPP fiber, which improves the fiber processing performance and enhances the mechanical properties of the final product. The drawn fiber of sPP presents good elastic behavior within the range of 50% deformation, whereas the elastic property of the iPP/sPP (50/50) fiber slightly decreases, but still much better than that of iPP fiber.     

Deformation-induced highly oriented and stable mesomorphic phase in quenched isotactic polypropylene

Changes of structure and morphology of quenched isotactic polypropylene (i-PP) film during tensile deformation at room temperature have been investigated by applying in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS patterns show that the isotropic diffuse scattering peak in the initial quenched i-PP film disappears after the film experiences necking and a strong streak, which accompanies obvious film stress-whitening, appears when strain reaches 600% and above. Explanation to the former is the dropping of density contrast between amorphous and mesomorphic phases due to deformation and that to the latter is the formation of microfibrils and/or microvoids in the film. WAXD patterns indicate that i-PP chains in the mesomorphic phase become highly oriented along the drawing direction with the strain value more than 5% and the three-fold helical chain conformation keeps in the deformed mesomorphic phase. The mesomorphic structure in the initial quenched i-PP film does not transform into crystal phase although high orientation appears during tensile deformation and the highly oriented mesomorphic phase is considered to be quite stable. Thermal behaviors of the initial quenched i-PP and deformed i-PP with the strain value of 650% are examined by modulated differential scanning calorimetry (MDSC). MDSC results show that the highly oriented i-PP chains in deformed mesomorphic phase are stable, which brings about restriction to chain mobility and prohibits from phase transformation of mesophase to crystal phase in the temperature range normally measurable for the initial quenched i-PP. A model about tensile deformation of the quenched i-PP is outlined to distinctly illuminate the changes of structure and morphology in both the amorphous and mesomorphic phases.     

聚丙烯／间同1，2-聚丁二烯共混物的等温结晶动力学

利用Avrami方程研究了聚丙烯（PP）及PP／间同1，2-聚T2烯（s—PB）共混物的等温结晶动力学。DSC研究表明：在相同的结晶温度下，如128℃时共混物的结晶速率为0．45min^-1，比纯PP的0．17min^-1大，s-PB起到了成核剂的作用，使共混物的结晶速率加快；共混物的Avrami指数n为2．71～323，共混物的结晶主要是以三维方式增长、异相成核。共混物的Arrhenius结晶活化能为309．35kJ·mol^-1比纯PP的338．11kJ·mol^-1低，结晶活化能的降低也证明了s-PB在共混物的结晶过程中起到了成核剂的作用并使结晶速率加快。     

Crystallization behavior of syndiotactic and atactic 1,2‐polybutadiene blends

Two 1,2‐polybutadiene samples, ie syndiotactic 1,2‐polybutadiene (sPB) and atactic 1,2‐polybutadiene (aPB), were synthesized by using the same Iron(III ) catalyst system, although with changing contents of the third catalyst component. Effects of the composition of aPB on the crystallization behavior of sPB have been characterized by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). Analysis of the results indicates that the intensity of the (010) peak of sPB is influenced much more easily than that of the (200)/(110) peak by the amorphous aPB sample, but the structure of sPB does not change with the changing composition of sPB. aPB is thermodynamically miscible with sPB in the melt, and retards the melt crystallization of sPB. At the same time, non‐isothermal crystallization kinetic studies of neat sPB and 1/2.5 blends were carried out by DSC. The Avrami equations modified by Jeziorny and Privalko, were used to fit the primary stage of the non‐isothermal crystallizations of neat sPB and the 1/2.5 blends. A larger Avrami exponent for neat sPB than for the blends was observed, and possible reasons are discussed. The activation energies (ΔE) were determined to be ?88 and ?106 kJ mol^?1 by the isoconversional Friedman method for neat sPB and sPB in the blends, respectively. Copyright © 2004 Society of Chemical Industry     

Properties of thin films of isotactic polypropylene blended with polyisobutylene and ethylene-propylene-diene terpolymer rubbers

An experimental study was carried out to investigate the kinetic, morphological and thermodynamic properties of thin films of isotactic polypropylene (iPP) blended with several elastomers such as ethylene-propylene-diene terpolymer (EPDM) and three samples of polyisobutylene (PIB) with different molecular masses. The addition of the rubber to iPP causes drastic modifications in the morphology, nucleation density, spherulite growth rate and thermal behaviour of iPP. Such modifications depend strongly on the chemical and molecular mass of the added elastomer and on the composition of the blend. All the elastomers studied seem to act as nucleating agents for the iPP spherulites. The addition of PIB to iPP results in a reduction of the spherulite growth rate G, whereas the addition of EPDM does not seem to have a great influence. For the iPP/PIB_HM iPP/PIB_MM and iPP/EPDM blends a depression of the equilibrium melting temperature T_m, with respect to that of pure iPP, is observed. This depression is increased for the blend containing 20% rubber. This effect is probably related to phenomena of partial miscibility in the melt and to the coexistence of processes such as molecular fractionation and preferential dissolution of the more defective molecules.     

Crystallization behaviour of fractions of isotactic polypropylene with different degrees of stereoregularity

The influence of the degree of stereoregularity on the overall rate of crystallization and on the melting behaviour of fractions of isotactic polypropylene has been investigated. Fractions with different stereoregularity were obtained by extraction methods. The stereochemical pentad population was determined by ¹³C n.m.r. analysis. The results show that at constant T_c the overall rate constant of crystallization decreases with the increase of configurational chain defects whereas at a given ΔT the contrary is observed. An evident phenomenon of secondary crystallization is observed only in fractions of IPP with a higher concentration of defects. Such observation may explain the double peaked endotherm of fusion of more defective IPP fractions. The free energy of formation of a nucleus of critical dimensions has been calculated for each fraction and its value correlated with the degree of stereoregularity.     

s-PB/TPI釜内原位聚合共混物的非等温结晶动力学

采用2种负载型催化体系在反应釜内原位合成了间同聚丁二烯（s-PB）及s-PB/反式-1,4-聚异戊二烯（TPI）（80/20）共混物。用DSC方法研究了s-PB及共混物的非等温结晶行为,结果表明：Jeziorny修正的Avrami法和Mo法对s-PB及s-PB/TPI（80/20）共混物的非等温结晶行为有较好的适用性,Ozawa法则存在一定的缺陷。s-PB及其共混物的结晶活化能分别为-253 kJ/mol和-310 kJ/mol。TPI的加入降低了s-PB的结晶速率,使得共混物的结晶活化能绝对值增大。     

Relation between PVT measurements and linear viscosity in isotactic and syndiotactic polypropylenes

Pressure-Volume-Temperature (PVT) data of an isotactic and a syndiotactic PP are fitted to a modification of the Simha-Somcynsky equation of state (S-S), to calculate characteristic parameters, such as V^∗, T^∗, P^∗ and the solubility parameter δ. The hole fraction of the S-S model and the free volume are deduced from these parameters. The application of a modified Doolittle equation and a modified Berry-Fox equation to estimate viscosity, leads to extract novel conclusions on the differences between both types of PPs. An equation which accounts for the effect of temperature on the characteristic ratio of syndiotactic PP is presented.     

Monitoring elasticity and orientation in syndiotactic polypropylene

The effect of temperature on the elasticity and structure of syndiotactic polypropylene (sPP) is investigated using a combination of WAXD and rheo-optical FTIR spectroscopy. sPP has a rich crystal structure, which leads to unique mechanical behavior. Beyond yield point, it exhibits elastic response associated with deformation-induced structure-structure transformation. The structure and orientation were measured both during and after uniaxial tensile stretching of films (up to 200%) as a function of temperature (25-70 °C). Our WAXD and rheo-FTIR results suggest that as the temperature increases, there is a reduction in the extent of helical to trans-planar conformational change upon stretching. When the strain is released, there is partial transformation of trans-planar conformation back to helical. The presence and orientation of the trans-planar conformation plays a key role on the elastic behavior of sPP beyond the yield point. Rheo-optical FTIR dichroism studies provide further insights into the elasticity in syndiotactic polypropylene. Different proportions of helical and trans-planar conformations orient to different extents. The helical conformation does not orient appreciably at higher temperature though they are present beyond the yield point. In contrast, the trans-planar chains show a significant increase in dichroism beyond the yield point, suggesting that there is a difference in mobility (orientation) of the helical and trans-planar chains. This further supports the importance of trans-planar chains on the elastic behavior.     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

Unusual crystallization behavior of isotactic polypropylene and propene/1-alkene copolymers at large undercoolings

During the investigation of the crystallization of metallocene isotactic polypropylene and copolymers with low amount of 1-butene and 1-hexene at large undercoolings, an unexpected behavior has been found. Random copolymers crystallize faster than the homopolymer between 80 and 40 °C, while at high temperatures the overall crystallization rates follow the expected trend. On the basis of structural and morphological evidences we suggest that the overall structuring kinetics of the homopolymer is slowed down by the concomitant formation of mesophase and monoclinic structures. This effect is absent in the copolymers because the branched counits retard the development of mesophase.     

Correlation between mechanical properties and orientation of the crystalline and mesomorphic phases in isotactic polypropylene fibers

The microstructure and mechanical properties of melt-spun isotactic polypropylene (iPP) fibers were studied for different take-up velocities and throughputs with a combination of small- and wide-angle X-ray scattering, calorimetry and tensile tests. With the increase of take-up velocity the mesomorphic fraction of the fibers steadily decreases while the crystalline fraction increases. The addition of an alpha-nucleating agent led to an increase of crystallinity and a slight decrease of the orientation factor. By contrast, a beta-nucleator resulted in a complete suppression of the fiber crystallinity. The orientation factor of the mesophase and/or crystalline phase is found to have a one-to-one correlation with the fiber tenacity and deformation at break. At the same time, the absolute values of crystallinity and/or mesomorphicity are fully uncorrelated with these mechanical characteristics. The observed correlations can be used for example to design and control the fiber mechanical properties by tuning the processing conditions such as take-up velocity, throughput and addition of nucleating agents.     

Crystallization behaviour of isotactic polypropylene/linear low density polyethylene blends

Crystallization behaviour of isotactic polypropylene/linear low density polyethylene (iPP/LLDPE) blends has been investigated by optical microscopy and DSC. Crystallization of iPP depends upon blend composition and thermal history. When blended with LLDPE, the crystallization temperature of iPP, T_c, decreased slightly. Crystallinity did not change in the range 0-80wt% LLDPE; there were only slight changes in the crystalline structure, but LLDPE seemed to resist forming the β type of spherulites. Below 80 wt% of LLDPE, iPP was a continuous phase. The iPP spherulite growth rate was almost constant; however, overall crystallization decreased due to decreasing primary nuclei density.     

The effect of annealing on the time-dependent behavior of isotactic polypropylene at finite strains

Four series of tensile relaxation tests are performed on isotactic polypropylene at elongations up to the necking point. In the first series of experiments, injection-molded samples are used without thermal pre-treatment. In the other series, the specimens are annealed for 24 h prior to testing at 110, 120 and 130 °C, respectively. Results of mechanical experiments are compared with DSC measurements.A constitutive model is derived for the time-dependent response of semicrystalline polymers at finite strains. A polymer is treated as an equivalent temporary network of macromolecules bridged by junctions (physical cross-links, entanglements and crystalline lamellae). At random times chains separate from their junctions and merge with new ones (the viscoelastic response), whereas junctions slip with respect to their positions in the bulk material (the viscoplastic behavior). The network is thought of as an ensemble of active meso-regions with various potential energies for detachment of chains from temporary nodes and passive meso-domains, where separation of chains is prevented by surrounding radial and tangential lamellae.Experimental data demonstrate that the content of active meso-domains increases with elongation ratio driven by the release of constrained amorphous phase induced by fragmentation of lamellae. In the sub-critical region of deformation (relatively small strains), the growth of the concentration of active meso-regions is associated with breakage of subsidiary (thin) lamellae developed at annealing. In the post-critical region (large strains), an increase in the fraction of active amorphous domains is attributed to disintegration of primary (thick) lamellae.     

Infrared spectroscopy study on crystalline and conformational changes in solution-cast syndiotactic polystyrene

Fourier transform infrared (FT-IR) analysis was used to investigate the conformation and the effects on crystals in syndiotactic polystyrene (sPS) subjected to different solvent and/or thermal treatments. It was found that the original helical conformation in sPS cast from a solution of 1,1,2,2-tetrachloroethane could transform to an all-traps planar conformation as the sPS was heated to temperatures above 200 °C. The spectra of sPS showed that the helical conformation chain varied with different casting solvents and the band positions and relative intensities also differed slightly owing to different interactions between the solvent molecule and the δ-type sPS crystalline phase. The solution-casting temperature also had a significant effect on the crystal forms in sPS. Lower temperatures (casting below ∼100 °C) produced helical crystals while the higher temperatures (casting above 120 °C) yielded all-trans crystals. In addition, spectra evidence of different crystals (α-and β-crystals in all-trans conformation) in sPS are discussed and identified in this study.     

The effect of strain rate on the viscoplastic behavior of isotactic polypropylene at finite strains

Two series of uniaxial tensile tests are performed on isotactic polypropylene with the strain rates ranging from 5 to 200 mm/min. In the first series, injection-molded specimens are used without thermal pre-treatment. In the other series of experiments, the samples are annealed for 51 h at 160 °C prior to testing.A constitutive model is developed for the viscoplastic behavior of isotactic polypropylene at finite strains. A semicrystalline polymer is treated as equivalent heterogeneous network of chains bridged by permanent junctions (physical cross-links and entanglements). The network is thought of as an ensemble of meso-regions connected with each other by links (lamellar blocks). In the sub-yield region of deformations, junctions between chains in meso-domains slide with respect to their reference positions (which reflects sliding of nodes in the amorphous phase and fine slip of lamellar blocks). Above the yield point, the sliding process is accompanied by displacements of meso-domains in the ensemble with respect to each other (which reflects coarse slip and fragmentation of lamellar blocks). To account for alignment of disintegrated lamellar blocks along the direction of maximal stresses (which is observed as strain-hardening of specimens in the post-yield regions of deformations) elastic moduli are assumed to depend on the principal invariants of the right Cauchy-Green tensor for the viscoplastic flow.Stress-strain relations for a semicrystalline polymer are derived by using the laws of thermodynamics. The constitutive equations are determined by five adjustable parameters that are found by matching observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. A noticeable difference is revealed between the mechanical responses of non-annealed and annealed specimens: (i) necking of samples not subjected to thermal treatment precedes coarse slip and fragmentation of lamellar blocks, whereas cold-drawing of annealed specimens up to a longitudinal strain of 80% does not induce spatial heterogeneity of their deformation; (ii) the elastic modulus increases with the strain rate for non-annealed specimens and decreases for annealed samples.