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We synthesized model polyethylene (PE) samples with nearly monodispersed molecular weight distribution and different fractions of short-chain branches to investigate the influences of the branch structure on the rheological properties. The activation energy of the flow process showed linear relations with the weight fractions of comonomer and the short-chain branch. These results suggest that the friction of the chain motion is dominated by the amount of the short-chain branch. The zero-shear viscosity showed a power law to Mw with an exponent of about 3.3 for all linear and branched PEs. The branched PEs showed large entanglement molecular weights compared with the linear PEs because the introduction of the short-chain branches results in the increase of the average molecular weight per backbone unit. Moreover, the introduction of the short-chain branches has no influence on the strain-hardening under an extensional elongation process, indicating that the short-chain branches only affect the friction.

Direct copolymerization of propylene and functional monomers using early transition metal catalysts is a prominent synthesis method to render a functionality to polypropylene maintaining high stere...

A series of di- and triblock copolymers composed of gradient norbornene (NB)/higher α-olefin (1-octene (O) or 1-dodecene (Do)) segments (NB/α-olefin-gradient segments) were synthesized with (t-BuNSiMe2Flu)TiMe2 (1) – [Ph3C][B(C6F5)4] using 2,6-bis(1,1-dimethylethyl)-4-methylphenol (BHT)-treated tri-n-octylaluminium (Oct3Al) as a scavenger. The copolymers were molded to form transparent films using a melt-pressing procedure. The strain at break behaviors of the block copolymer films were significantly improved by controlling the block length, NB mol fraction, and/or the type of α-olefin, without a corresponding loss of strength compared to the corresponding gradient copolymer films. This improvement in the mechanical properties of NB/α-olefin copolymers is expected to broaden their potential applications in optical and medical fields.

The influence of the weight-average molecular weight (Mw) on the structure–properties relationship was investigated by preparing high-density polyethylenes (HDPEs) of various Mw but constant polydispersity index (Mw/Mₙ) values. The thickness of the lamellar crystalline layer increased with increasing Mw, whereas the amorphous characteristics, such as the amorphous layer thickness and tie-molecule fraction, remained unchanged. The microscopic deformation of these HDPEs was investigated by in situ Raman spectroscopy. The orientation parameter ⟨P₂⟩ showed a power law to Mw with an exponent of −3/4 in the strain-hardening region. The Mw dependence of the molecular orientation was interpreted as the realignment of the lamellar cluster units, the size of which was determined by the end-to-end distance of the polymer chains in their melt state. It was found that higher stretching stress is applied to the crystalline chains for HDPEs of lower Mw, implying that a larger amount of taut-tie chains is formed.

The copolymerization of norbornene (NB) and various α-olefins [1-octene (O), 1-decene (De), and 1-dodecene (Do)] was achieved with high activity using (tBuNSiMe2Flu)TiMe2 (1)-[Ph3C][B(C6F5)4] and 2...

In situ Raman spectroscopy is applied for polyethylene solid under various environments to elucidate the morphological and conformational changes. The trans conformation retains up to higher temperature for high-density polyethylene, reflecting higher stability of the orthorhombic crystals composed of stacked trans chains. It is suggested that the conversion of the non-crystalline trans chains to the crystalline phase is the microscopic origin of thermal history in the crystallinity, whereas the transformation between the trans and gauche conformers is practically in thermal equilibrium. Microscopic and dynamic mechanism of deformation during uniaxial stretching is investigated for the molecular orientation and the microscopic load sharing on the crystalline and amorphous chains. Lower crystallinity results in smoother and higher orientation toward the stretching direction, as well as higher load on the amorphous chains, during tensile elongation.

The repeating cycle of freezing and thawing brings about a physical gelation for the solutions of polyacrylonitrile (PAN) in N, N-dimethylacetamide (DMA) for appropriate concentrations. It was revealed from wide-angle-X-ray diffraction that the PAN molecules were “cross-linked” by the crystallization to form a network through the freezing and thawing steps. The profiles of small-angle-X-ray scattering for the gels showed the excess scattering from the network structure formed through gelation, which could not be observed for the pure PAN solution. A reversible swelling and shrinking were observable for the PAN gel as a function of the relative content of DMA and benzene by varying the mixing ratio. The degree of swelling measured by weight, for the swollen and shrunk gels, was in a range from 0.7 to 8. A scaling relation appeared for the gel swelling between the PAN volume fraction (ϕ) and the shear moduli (G) which was measured by tensile and indentation tests, as G = Gs ×(ϕ)s; Gs= 1200kPa and s = 2.26 ± 0.23. A discussion is given on the value of the scaling exponent, s, on the basis of the conformation of flexible chain molecules in solution and the analogy in the microscopic and macroscopic properties between semidilute polymer solutions and gels.

The effects of the ultra-high-molecular-weight (UHMW) component of polypropylene (PP) on its rheological properties, crystallization behavior, and solid-state mechanical properties were investigated using various measurement techniques. The terminal relaxation time—determined by measuring the linear viscoelasticity—was increased by adding the UHMW component. The increase in the melt elasticity produced by adding the UHMW component was observed by measuring the steady-state shear flow, although the shear viscosity was not greatly affected. Owing to the long characteristic time of the Rouse relaxation of the UHMW component, PP with the UHMW component formed highly oriented structures through a shear-induced crystallization process. The addition of the UHMW component enhanced the orientation and regularity of crystalline structure for extruded films. Therefore, the Young′s modulus, yield stress, and strength were higher in the PP film containing the UHMW component than in one without the UHMW component, irrespective of the direction of tensile deformation.

金沢大学理工研究域フロンティア工学系, The influences of the polydispersity index (Mw/Mn) on the morphology and deformation behavior of high-density polyethylene solids under uniaxial stretching were investigated using in situ Raman spectroscopy. An increase in Mw/Mn led to a larger fraction of tie molecules connecting several lamellar crystals, whereas no obvious influence on the crystalline structures was observed. Although the stress-strain curves essentially remained unchanged up to the neck propagation region irrespective of Mw/Mn, the strain-hardening modulus was higher for higher Mw/Mn samples. These results indicate that the mechanical response of the network structure appears in the strain-hardening region because of the tie molecules connecting several lamellar crystals fragmented at the yield point. In addition, the molecular orientation of the crystalline chains into the stretching direction and the formation of long consecutive trans chains were enhanced for higher Mw/Mn samples. Copyright © 2019 American Chemical Society., Embargo Period 12 months

An appropriate processing protocol was proposed to improve the rubber elasticity of a thermoplastic polyester elastomer (TPEE). Rheology measurements at the processing temperature and thermal measurements determined the critical temperature at which small numbers of crystals exist. It was found from dynamic mechanical analyses, wide- and small-angle X-ray measurements, and electron microscope observations that cooling from the critical temperature markedly promoted the crystallization of hard segments. Moreover, tensile tests revealed that the modulus and yield stress increased. Most importantly, compared with films obtained by cooling from high temperatures, the residual strain at a fixed stress was low—i.e., the rubber elasticity was improved owing to its well-developed crystalline structure. These effects were accelerated by a more optimized process—i.e., subsequent heating at slightly below the critical temperature. The technique is noteworthy because a TPEE with high modulus generally shows poor elasticity.

金沢大学理工研究域フロンティア工学系, Raman spectroscopy has been used to reveal the microscopic structural changes of low-density polyethylene under ultraviolet irradiation. The fraction of non-crystalline consecutive trans chains (the molecular chains in the trans conformation separate from the orthorhombic crystalline phase) drastically decreases at ∼600 h, together with a decrement in the molecular weight and an increment in the crystallinity owing to chemicrystallization. This suggests that short trans chains are prolonged to form consecutive trans chains before chemicrystallization. Moreover, gradual increases of the stretching and compression stresses are observed on the trans and amorphous chains, respectively. These conformational changes are detected by Raman spectroscopy even in the early stage of photodegradation. Chemicrystallization at ∼600 h is accompanied by structural changes, such as shortening of the interchain distance of the lamellar crystals and thinning of the amorphous layer, which induce formation of surface cracks on the macroscopic specimen., Embargo Period 24 months

The effects of magnesium salts with various anion species on the structure and properties of a poly(vinyl alcohol) (PVA) film were studied. The glass transition temperature of the PVA film increased following the addition of a magnesium salt. Furthermore, the salt greatly enhanced the modulus and yield stress and reduced the crystallinity of the film. These effects were attributed to the strong ion–dipole interactions between the magnesium salts and the PVA chains. The strength of interaction, i.e., the reduction of segmental motions, depended on the anion species in the following order: Mg(ClO4)2, MgBr2, MgCl2, Mg(CH3COO)2, and MgSO4. The order corresponded to the Hofmeister series, which predicts the ability to break the structure of water.

We prepared model polyethylene (PE) samples that contain the controlled-amounts of the high-molecular-weight component by blending a monodispersed PE into a polydispersed PE, to elucidate the influence of the high-molecular-weight component of a polymer on its mechanical properties. The strength and strain-hardening modulus were significantly enhanced by the addition of a monodispersed PE of sufficiently high molecular weight, and the critical value of Mw was determined to be ~3.0 × 105. The improved strength and strain-hardening modulus are attributed to higher amounts of inter-cluster links that bridge adjacent lamellar cluster units because these links act as stress transmitters between these units.

Triblock copolymers, poly(L-lactide)-b-poly(2-methyl-1,3-propanediyl succinate)-b-poly(L-lactide) (PLLA-b-PMPS-b-PLLA), were synthesized as new biodegradable thermoplastic elastomers by two-step polymerization. In the first step, hydroxy-telechelic poly(2-methyl-1,3-propanediyl succinate) (PMPS) was synthesized from 2-methyl-1,3-propanediol (MP, small excess) and succinic acid (SA) using two-stage reaction of esterification and simple polycondensation processes. In the second step, plant derived L-lactide (LLA) was polymerized using the bifunctional PMPS as a macroinitiator to synthesize PLLA-b-PMPS-b-PLLA triblock copolymers. The structures of the triblock copolymers as well as the PMPS macroinitiator were characterized by NMR and GPC analysis. PMPS is an amorphous polymer with Tg at lower than −24 °C. The triblock copolymers also exhibited low Tg for the PMPS segment at lower than −20 °C as well as Tm at around 150 °C for the PLLA segment. The tensile tests of the triblock copolymers demonstrated their low modulus and high elongation. The biodegradation tests of these polymers were also performed using enzymes and seawater to reveal their biodegradability.

The molecular orientation behavior of isotactic polypropylene (iPP) is investigated by using in situ Raman spec- troscopy under tensile tests. A versatile method of the tilt-angle correction for the orientation parameters is newly developed, where the molecular orientation in highly oriented specimens is assumed to be entropically favorable. The real-time changes of orientation parameters and orientation distribution functions are determined for the molecular chain axis of iPP during uniaxial stretching. The molecular orientation remains random in the elastic region, and increases after the first yield point. In the yielding region, a broad distribution of orientation toward an intermediate angle of 30-70° from the stretching direction is observed. This is interpreted as reorientation of the crystalline chains being hindered by rigid, bulky lamellar cluster units. After the yielding region, orientation toward the stretching direction proceeds rapidly, approaching highly oriented states.