Shogo Nobukawa

Rheological properties of polymer composites with flexible fine fibers

Effect of the addition of polymeric fine fibers on the rheological properties is studied employing poly(lactic acid) PLA as a matrix. Fine fibers of poly(butylene terephthalate) PBT, whose melting point is higher than that of PLA, is prepared in a PLA matrix by melt-stretching with rapid cooling. Then, the rheological properties of the composites are evaluated at the temperature between melting points of PLA and PBT. It is found that only 1 wt. % of PBT fibers greatly enhances the elongational viscosity, although shear viscosity is hardly changed. In particular, the composite shows strain-hardening behavior, which has not been detected for polymer composites with rigid fibers. Elastic deformation of network composed of flexible PBT fibers, leading to bending of fibers and/or friction between fibers, will be responsible for the strain-hardening. The marked elasticity is also detected for a composite of silicone oil containing the PBT fine fibers.

Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.

Material design of retardation films with extraordinary wavelength dispersion of orientation birefringence: a review

Orientation birefringence and its wavelength dispersion for various types of cellulose esters are reviewed. Cellulose esters such as cellulose acetate propionate and cellulose acetate butyrate show positive orientation birefringence with extraordinary wavelength dispersion, which is determined mainly by the ester groups rather than the main chains. The acetyl group provides negative orientation birefringence with strong ordinary wavelength dispersion, whereas the propionyl and butyryl groups give positive orientation birefringence with weak wavelength dispersion. Although all groups show ordinary wavelength dispersion, the summation of their orientation birefringences gives extraordinary dispersion. Moreover, the wavelength dispersion is dependent on the stretching ratio due to the difference in the orientation relaxation of each group. On the contrary, cellulose triacetate (CTA) shows negative birefringence with ordinary wavelength dispersion because it has no positive contribution. However, doping a plasticizer having positive orientation birefringence changes the orientation birefringence of CTA from negative to positive, and the wavelength dispersion from ordinary to extraordinary. This is attributed to the cooperative orientation of plasticizer molecules to the stretching direction with CTA chains, known as nematic interaction upon a hot drawing process.

Chain packing and its anomalous effect on mechanical toughness for poly(lactic acid)

The effect of chain packing on tensile properties was studied employing amorphous poly(lactic acid) PLA. It was found that the samples cooled in the temperature range from 60 to 80 °C, that is, slightly higher than the glass transition temperature Tg, showed ductile behavior with a low brittle-ductile transition temperature. Furthermore, the samples obtained by prolonged cooling at 56 °C also showed ductile behavior, whereas a shorter cooling time at the same temperature provided a brittle product. Even for the samples quenched at 40 °C, they showed ductile behavior after the exposure to postprocessing annealing operation at 60 °C; that is, the strain at break is larger than 3. This is an anomalous phenomenon for a glassy polymer. The dynamic mechanical analysis and thermal characterization revealed that the ductile samples show slightly higher Tg than the brittle ones, presumably due to high packing density of polymer chains. Moreover, it was found from infrared spectroscopy that the ductile samples show strong absorbance at 1267 cm(-1), ascribed to high energy gauche-gauche gg conformers. Following the classic Robertsons descriptions of plastic flow, it is concluded that the increase in the gauche-gauche gg conformers, which shows the conformation change under a low stress level, reduces the critical onset stress for shear yielding. The results demonstrated that the mechanical toughness of PLA can be controlled by the cooling conditions during processing and the postprocessing annealing operation.

Selective migration of silica particles between rubbers

The migration of silica nanoparticles in the laminated sheets of poly(butadiene) (BR) and poly(styrene-co-butadiene) (SBR) rubbers is investigated. Laminated rubber sheets are subjected to various annealing conditions beyond their glass transition temperatures. After separation, the surface morphologies of separated sheets are observed by scanning electron microscopy. The transfer of particles occurs from SBR to BR during annealing, but not from BR to SBR. Since SBR exhibits a higher viscosity than BR under the experimental conditions, the transfer direction is determined by interfacial tension. Silica particles without any surface modification prefer to reside in BR. The diffusion distance is predicted from the Stokes-Einstein equation. Differential scanning calorimetric measurements show that BR crystallization is enhanced by the silica particles immigrated from the SBR sheet, because of their favorable nucleating ability. This study allows the prediction of silica particle localization in BR and SBR blends.

Morphology development of polytetrafluoroethylene in a polypropylene melt (IUPAC Technical Report)

Morphology development of polytetrafluoroethylene (PTFE) caused by applied flow history in molten isotactic polypropylene (PP) is investigated, employing a cone-and-plate rheometer and a capillary rheometer as mixing devices. Since the flow history is applied at 190 °C, PTFE is in the solid state whereas PP is in the molten state. It is found that primary PTFE particles tend to be agglomerated together by mechanical interlocking. Then they are fragmented into fibers by hydrodynamic force with reorganization process of crystalline phase. The diameter of the fragmented fibers is the same as that of the original ellipsoidal particles. Further, fine fibers whose diameter is in the range from 50 to 100 nm are also generated by yielding behavior of the particles. The prolonged shearing leads to a large number of fibers, although the diameter and length are hardly affected by the exposure time of shearing and shear stress. Moreover, the flow type (i.e., drag or pressure flow) does not affect the morphology to a great extent, although the drag flow is not efficient to reduce large agglomerated particles. The fibers form an interdigitated network structure, which is responsible for the marked melt elasticity.

Glass Transition Temperature and β Relaxation Temperature around Chain End of Polystyrene Determined by Site Specific Spin Labeling

A glass transition temperature, T(g), and a β relaxation temperature, T(β), of spin-labeled polystyrene (PS) having a number average molecular weight (M(n)) of ca. 25 kDa were determined by the microwave power saturation (MPS) method of electron spin resonance (ESR). Spin labeling was selectively carried out at chain ends or midchain segments. This method allowed us to determine the local T(g) and the local T(β) around the spin-labeled sites, selectively. The T(g) determined by the ESR, T(g,ESR), was in good agreement with that determined by differential scanning calorimetry, T(g,DSC); the T(g,ESR) decreased with decreasing M(n) with blending oligomers as well as the T(g,DSC). The T(g,ESR) for the end-labeled PS (PS-E) was equal to that for the midchain-labeled PS (PS-M) irrespective of the M(n). However, we previously reported that the PS-E showed distinctly higher segmental mobility than the PS-M in the temperature range 423-463 K (above T(g)). Therefore, we conclude that the chain ends intrinsically have higher segmental mobility than midchain segments due to the discontinuity of repeat units; however, the mobilities of chain ends and midchain segments are averaged out in the vicinity of T(g) due to the cooperativities with neighboring numerous chain segments. Concerning the β relaxation, the T(β) determined by the MPS was in good agreement with those determined by dielectric and dynamic mechanical spectroscopies and dilatometry. The T(β) of the PS-E was the same with that of the PS-M within experimental uncertainties; the T(β) was insensitive to the M(n) in contrast to the T(g). Therefore, we conclude that the effect of chain end is little on the β relaxation of PS due to its local character. In addition, the effect of annealing at 353 K was found to be the same for the T(β)s of the PS-E and PS-M.

Development of microporous structure and its application to optical film for cellulose triacetate containing diisodecyl adipate

Phase separation in plasticized cellulose triacetate (CTA) films is investigated to produce a microporous film that can be used in optical devices. Hot-stretched CTA films containing diisodecyl adipate (DIDA) show negative orientation birefringence similar to the hot-stretched pure CTA. After extracting DIDA from the stretched films by immersion into an organic solvent, however, the films exhibit positive birefringence. Moreover, the magnitude of the birefringence increases with the wavelength, known as extraordinary dispersion, which is an essential property in the preparation of an ideal quarter-wave plate. Numerous ellipsoidal pores with micro-scale were detected in the film after the immersion, indicating that DIDA were segregated and formed ellipsoidal domains in the CTA matrix during annealing and stretching. These results indicate that extraordinary wavelength dispersion is given by the combinations of orientation birefringence from CTA and form birefringence from micropores. Furthermore, it was found that annealing time and stretching condition affect the phase separation as well as the shape and size of pores.

Effect of thermal modification on rheological properties of polyethylene blends

We examined the effects of thermal modification under flow field on the rheological properties of linear low-density polyethylene (LLDPE) with high molecular weight, low-density polyethylene (LDPE), and their blends, without thermal stabilizer. Although structural changes during processing are not detected by size extrusion chromatography or nuclear magnetic resonance spectroscopy, linear viscoelastic properties changed greatly, especially for the LLDPE. A cross-linking reaction took place, leading to, presumably, star-shaped long-chain branches. Consequently, the modified LLDPE, having high zero-shear viscosity, became a thermorheologically complex melt. Moreover, it should be noted that the drawdown force, defined as the uniaxial elongational force at a constant draw ratio, was significantly enhanced for the blends. Enhancement of elongational viscosity was also detected. The drawdown force and elongational viscosity are marked for the thermally modified blend as compared with those for the blend of thermally modified pure components. Intermolecular cross-linking reactions between LDPE and LLDPE, yielding polymers with more than two branch points per chain, result in marked strain-hardening in the elongational viscosity behavior even at small strain. The recovery curve of the oscillatory modulus after the shear modification is further evidence of a branched structure.

Melting Point Elevation of Isotactic Polypropylene

The melting point of a conventional isotactic polypropylene (PP) was enhanced by a rapid annealing procedure of an extruded sheet composed of β trigonal form crystals having thick lamellae, which was prepared by T-die processing with a specific β-nucleating agent, N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide. Although the melting point of PP with α monoclinic form, prepared by a conventional processing method, is known to be located around at 165°C, the sample obtained by the present technique showed a higher melting point, 170°C. The phase transformation from β-to α-form crystals, retaining the lamellar thickness, was responsible for the melting point elevation.