Koh-hei Nitta

Role of tie molecules in the yielding deformation of isotactic polypropylene

We examined the effects of the tie-molecule fraction on the yielding behavior of two isotactic polypropylenes, one having little ethylene content and the other as the homopolymer with no ethylene. The tie-molecule fraction of the samples used in this study was controlled by blending ethylene-α-olefin of an α-olefin content above 50 mol % in the blend of which the copolymers were incorporated into the amorphous regions of polypropylene (PP). An excellent linear relationship was observed between the measured yield stress and the tie-molecule fraction estimated from the Huang–Brown model, suggesting that the tie-molecule fraction and lamellar stiffness determine whether the lamellar fragmentation is easily activated or not, depending on the PP composition. Furthermore, an extended Huang–Brown model predicts a lamellar cluster connecting about five lamellae, which has a potential to account for morphological transformation of the spherulitic structure into a fibrillar one. Comparing the immiscible blends showing a phase-separated morphology with the partially miscible blends mentioned above, the yield stress was lowered by the presence of rubber phase, apparently in a similar manner; but the yielding processes were clearly discriminated between both cases when the yield stress was plotted against the tie-molecule fraction.

Dynamic mechanical properties of metallocene catalyzed linear polyethylenes

Abstract We have examined dynamic mechanical properties of metallocene catalyzed linear polyethylenes (L-PEs) with various molecular weight M w from ca. 20M to 2.6G and branched linear polyethylenes (B-PEs) having various degree of short-chain branching. The dynamic mechanical relaxation mechanisms of these metallocene catalyzed PE samples were discussed in the terms of the structural factors such as lamellar thickness, amorphous layer thickness, and crystallinity. It was found that the positions of α (crystal) relaxation and melting temperature have similar functional dependence of the inverse of the lamellar thickness 1/ L c . The β relaxation appeared around 250xa0K in the dynamic mechanical spectra for higher molecular weight PEs having more than about 200M of M w . The molecular mechanism underlying β relaxation for L-PE was found to be different from that for B-PE.

Compatibility of binary blends of polypropylene with ethylene‐α‐olefin copolymer

The compatibility for binary blends of isotactic polypropylene with rubbery ethylene-α-olefin copolymers having various α-olefin contents was investigated by means of differential scanning calorimetry, X-ray analysis, transmission electron microscopy, and dynamic mechanical analysis. It was found that “α-olefin rich” in ethylene-1-butene copolymers and in ethylene-1-hexene copolymers were miscible with amorphous polypropylene chains, when the α-olefin content is above 50 mol %. On the other hand, the blends with “ethylene rich” (above 50 mol % of the ethylene content) in ethylene-1-butene copolymers and ethylene-1-hexene copolymers showed a microheterogeneous morphology.

Tensile yield of isotactic polypropylene in terms of a lamellar-cluster model

In this study, the structural factors controlling the yield in isotactic polypropylene materials were theoretically investigated. To describe the yielding behavior of spherulitic polypropylenes, we introduced a new structural unit, lamellar clusters, which are several stacked lamellae bound by tie molecules. It was shown that tie molecules between adjacent lamellar clusters produce a concentrated load acting on the cluster surface, leading to the bending deformation of the lamellar clusters. The yielding behavior can be explained if one assumes that the disintegration of the lamellar clusters occurs when the elastic-strain energy stored by the bending deformation reaches a critical value. By applying the fracture theory of composites to a system consisting of lamellar clusters and tie molecules, we found the yield stress σy to be proportional to , in which EY is the Youngs modulus and Uy is the yield energy. The proportional coefficient between σy and depends only on the cluster size and tie-molecule density, so this proportionality is expected to be true for other spherulitic semicrystalline polymers such as polyethylenes, being independent of temperature and tensile rate.

Organic hybrid of chlorinated polyethylene and hindered phenol. I. Dynamic mechanical properties

Dynamic mechanical properties and microstructure of an organic hybrid consisting of chlorinated polyethylene (CPE) and 3,9-bis[1,1-dimethyl-2{β-(3-tert-butyl4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro[5,5]-undecane (AO-80) were investigated. The AO-80 clearly exhibited two second-order transitions at 6 and 69 °C in addition to the melting: the transition at lower temperature is assigned to the glass transition, and the transition at higher temperature is considered to be caused by the dissociation of hydrogen bond between the hydroxyl groups of AO-80. When blending with CPE, part of AO-80 molecules was dispersed into the CPE matrix, and most of them formed an AO-80-rich phase. As a result, a novel transition appeared above the glass-transition temperature of the CPE matrix. It was assigned to the dissociation of the intermolecular hydrogen bond between the α-hydrogen of CPE and the hydroxyl groups of AO-80 within the AO-80-rich phase. Dynamic mechanical properties and microstructure of CPE/AO-80 hybrid were controlled by the thermal treatment. It was found that the CPE/AO-80 hybrid is a good damping material and shows a shape memory effect.

Polypropylene-block-poly(ethylene-co-propylene) addition to polypropylene/poly(ethylene-co-propylene) blends: morphology and mechanical properties

Abstract The additive effects of a diblock copolymer of polypropylene and ethylene–propylene rubber PP- b -EPR(50–50) on the morphology and mechanical properties of PP/EPR(50/50) blends were investigated. It was found from an electron microscopy study that the block copolymer is compatible with the PP phase but incompatible with the EPR phase. Dynamic mechanical spectra showed that the PP- b -EPR block copolymers are incorporated into the PP phase, and consequently, the EPR portions of the block are trapped into the interlamellae of PP.

Organic hybrid of chlorinated polyethylene and hindered phenol. II. Influence of the chemical structure of small molecules on viscoelastic properties

The viscoelastic properties and stabilities of those properties of organic hybrids consisting of chlorinated polyethylene (CPE) and tetrakis[methylene-3-(3-5-di-tert-butyl-4-hydroxy phenyl)propionyloxy]methane (AO-60) and triethylene glycol bis[3-(3-tert-butyl-4-hydroxy-5-methyl phenyl)propionyloxy] (AO-70) were investigated. The CPE/AO-70 hybrids show only one transition, whereas for the CPE/AO-60 hybrids, one novel relaxation appears above the glass-transition temperature of CPE. This relaxation on the higher temperature side in the mechanical spectrum for CPE/AO-60 is associated with the appearance of the AO-60-rich phase. Furthermore, the stabilities of the viscoelastic properties and microstructures of the organic hybrids consisting of CPE and multifunctional hindered phenols are dominated by the strength of the intermolecular interaction between CPE and phenols and the conformations of the middle skeletal parts of hindered phenols.

Mechanical properties of binary blends of polypropylene with ethylene-α-olefin copolymer

The compatibility of isotactic polypropylene and ethylene-l-butene copolymers changes with the content of 1-butene in the copolymer so that the morphology of their blends changes drastically. The relationship between morphology and mechanical properties for blends of isotactic polypropylene and ethylene-1-butene copolymer are investigated using a rheo-optical technique; the infrared absorbance spectroscopy and the stress are measured simultaneously under a constant rate of elongation. It is found that the compatible blends affinely deformed accompanied by little plastic deformation such as microvoids and craze, whereas the incompatible blends occurring a phase separation cause a segregation at interface between the two phases in the low strain region.

Rheological properties for binary blends of i-PP and ethylene-1-hexene copolymer

The dynamic viscoelastic properties for binary blends consisting of an isotactic polypropylene (i-PP) and an ethylene-1-hexene copolymer (EHR) were investigated in both solid and molten states to reveal the relation between miscibility in the molten state and the morphology in the solid state. In this study, two types of EHRs were employed: “ethylene-rich” EHR and “1-hexene-rich” EHR. The blend of i-PP and EHR of 30 mol % 1-hexene content shows a very long time relaxation due to the phase separation in the molten state. The blend film shows two separate glass-relaxation processes associated with those of the pure components. These indicate that the blend shows phase-separated morphology in the solid state as well as in the molten state. On the other hand, dynamic moduli in the molten state of the blends of i-PP and EHR of 57 mol % 1-hexene were found to be intermediate between those of individual pure components. Furthermore, the apparent activation energy of the blends is constant and is identical with those of i-PP and the EHR. The blend films show a single glass-relaxation process at the temperatures between those of the pure components, indicating that the EHR molecules are incorporated in the amorphous region of i-PP in the solid state. Accordingly, it was found that the polymer miscibility in the molten state for the i-PP/EHR blends directly affects the morphology in the solid state.

Novel Proposal of Lamellar Clustering Process for Elucidation of Tensile Yield Behavior of Linear Polyethylenes

We propose a new solidification process in crystallizable polymers in which lamellar clustering takes place in the crystallization process under high supercoolings. Clustering of lamellae is a two-step process: the first step is the exclusion process of chain ends from the space of a single Gaussian chain accompanied by the cooperative alignment of stiffened segments. This leads to anisotropic cylindrical units including several intertwined chains and is followed by the two-dimensional development of the partially solidified units into the final lamellar cluster. Using a series of polyethylenes with different molecular weights we show that the use of the lamellar cluster model makes it possible to provide a consistent understanding of both the solidification process and tensile yielding behavior in semicrystalline solids. The present model well explains the intercrystalline links found by Keith et al. (J. Polym. Sci. A2 1966, 4, 267–282) and the results which show that the calculation of the strength of the links agree with that of ultra-high-modulus fibers.