Combined effects of clay modifications and compatibilizers on the formation and physical properties of melt‐mixed polypropylene/clay nanocomposites

The melt mixing technique was used to prepare various polypropylene (PP)‐based (nano)composites. Two commercial organoclays (denoted 20A and 30B) served as the fillers for the PP matrix, and two different maleated (so‐called) compatibilizers (denoted PP‐MA and SMA) were employed as the third component. The results from X‐ray diffraction (XRD) and transmission electron microscope (TEM) experiments revealed that 190 °C was an adequate temperature for preparing the nanocomposites. Nanocomposites were achieved only if specific pairs of organoclay and compatibilizer were simultaneously incorporated in the PP matrix. For example, PP/20A(5 wt %)/PP‐MA(10 wt %) and PP/30B(5 wt %)/SMA(5 wt %) composites exhibited nanoscaled dispersion of 20A or 30B in the PP matrix. Differential scanning calorimetry (DSC) results indicated that the organoclays served as nucleation agents for the PP matrix. Generally, their nucleation effectiveness increased with the addition of compatibilizers. The thermal stability enhancement of PP after adding 20A was confirmed with thermogravimetric analysis (TGA). The enhancement became more evident as a suitable compatibilizer was further added. However, for the 30B‐included composites, thermal stability enhancement was not evident. The dynamic mechanical properties (i.e., storage modulus and loss modulus) of PP increased as the nanocomposites were formed; the properties increment corresponded to the organoclay dispersion status in the matrix. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4139–4150, 2004     

Solvent‐ and interface‐induced surface segregation in blends of isotactic polypropylene with poly(ethylene‐co‐propylene) rubber

The surface compositions and morphologies of melt‐quenched blends of isotactic polypropylene (iPP) with aspecific poly(ethylene‐co‐propylene) rubber (aEPR) were characterized by atomic force microscopy, optical microscopy, and X‐ray photoelectron spectroscopy. The surface morphologies and compositions formed in the melt are frozen‐in by crystallization of the iPP component and, depending on the processing conditions, are enriched in iPP or aEPR or contain a phase‐separated mix of iPP and aEPR. Enrichment of iPP is observed for blends melted in open air, in agreement with earlier work showing the high surface activity of atactic polypropylene at open interfaces. Surface segregation of iPP is suppressed at confined interfaces. Blends melt‐pressed between hydrophilic and hydrophobic substrates have phase‐separated iPP and aEPR domains present at the surface, which grow in size as the melt time increases. Surface enrichment of aEPR is observed after exposing melt‐pressed blends to n‐hexane vapor, which preferentially solvates aEPR and draws it to the surface. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 421–432, 2004     

Structure,morphology, and crystallization process of isotactic polypropylene/hydrogenated hydrocarbon resin blends

The structure, morphology, and isothermal and nonisothermal crystallization of isotactic polypropylene/low‐molecular‐mass hydrocarbon resin blends (iPP/HR) (up to 20% in weight of HR) have been studied, using optical and electron microscopy, wide‐ and small‐angle X‐ray and differential scanning calorimetry. New structures and morphologies can be activated, using appropriate preparation and crystallization conditions and blend composition. For every composition and crystallization condition, iPP crystallizes in α‐form, with a spherulitic morphology. The size of iPP spherulites increases with resin content, whereas the long period decreases. In the range of crystallization temperatures investigated, HR modifies the birefringence of iPP spherulites, favoring the formation of radial lamellae and changing the ratio between tangential and radial lamellae. Spherulitic radial growth rates, overall crystallization rates, and melting temperatures are strongly affected by resin, monotonically decreasing with resin content. This confirms miscibility in the melt between the two components of the blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3368–3379, 2004     

Thermal stability of shear-induced precursor structures in isotactic polypropylene by rheo-X-ray techniques with couette flow geometry

Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s⁻¹ and duration t_s = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s⁻¹ and duration, t_s = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006     

Metallocene copolymers of propene and 1‐hexene: The influence of the comonomer content and thermal history on the structure and mechanical properties

The relationships between the structure and properties have been established for copolymers of propylene and 1‐hexene synthesized with an isotactic metallocene catalyst system. The most important factor affecting the structure and properties of these copolymers is the comonomer content. The thermal treatment, that is, the rate of cooling from the melt, is also important. These factors affect the thermal properties, the degree of crystallinity, and therefore the structural parameters and the viscoelastic behavior. A slow cooling from the melt favors the formation of the γ phase instead of the α modification. Regarding the viscoelastic behavior, the β relaxation, associated with the glass‐transition temperature, is shifted to lower temperatures and its intensity is increased as the 1‐hexene content raises. The microhardness values are correlated with those of the storage modulus deduced from dynamic mechanical thermal analysis curves, and good linear relations have been obtained between these parameters. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1253–1267, 2006     

Morphologies of microporous polyethylene and polypropylene crystallized from solution in supercritical propane

The morphologies of solvent-free, microporous, mechanically self-supporting cylinders of linear polyethylene and isotactic polypropylene, crystallized from solution in supercritical propane, were examined by an SEM technique. The morphology of gels (or foams), obtained with little or no shrinkage from 2% to 35% solutions by weight of polyethylene (99% to 85% porosities), is shown in some detail. Lamellae with very little or considerable mutual organization occur, often in the form of stacks with straight or coiled axes (axialites). Further growth of these can lead to particles with a roughly spherical overall shape and a predominantly radial orientation of the lamellae at the particle surface. Subcooled isotactic polypropylene, on the other hand, crystallizes in the form of perfectly shaped birefringent microspheres of very uniform size. ©1995 John Wiley & Sons, Inc.     

聚丙烯基纳米碳纤维复合材料结晶行为研究

采用示差扫描量热法(DSC)研究了纳米碳纤维对PP／CNF复合材料中PP结晶行为的影响。结果表明：纳米碳纤雏可提高PP的结晶温度，但略降低其结晶速率和结晶度。提高纳米碳纤雏含量或减小纳米碳纤雏直径，PP结晶行为的上述变化更为明显，但结晶速率随纳米碳纤维含量出现先减小后增大的趋势。     

反相液体色谱中饱和醇保留行为的研究 Ⅰ.低碳醇保留行为与溶质分子结构的关系

本文从吸附-疏溶剂理论出发,根据热力学平衡观点,导出了一定固定相和流动相时,描述溶质保留行为与其分子结构关系的理论模型,并用之预测了饱和醇的保留值,获满意结果。从而表明本模型不但能用于同系物,而且亦可用于同分异构体及立体异构体保留行为的预测。     

Conformations of fold part in isotactic polypropylene lamella with diamond lattice model

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