Rheology - the new science of deformation and flow for a material showing complicated mechanical responses - is necessary in order to develop advanced polymeric materials. Our laboratory is carrying out material design of functional and high-performance polymers based on the rheological approach to create novel displays, next-generation automobile parts, eco-friendly materials including biomass-based plastics, and so on. Moreover, innovation of polymer processing is also studied with our industrial partners, considering trouble-shooting of actual processing operations Our research activities are outlined below.
Dr. Professor
School of Materials Science
Materials Chemistry Frontiers Research Area
[E-mail]
m_yama at jaist.ac.jp
[Phone]
+81-761-51-1621
[Facsimile]
+81-761-51-1149
[Degrees]
B.E. (1987), M.E. (1989), Doctor degree (1999) from Kyoto University
[Business career]
TOSOH Corporation (1989-2005)
* New Jersey Institute of Technology / Polymer Processing Institute (2000-2002)
Research projects
[1] Research and Development of high-performance polymers and functional polymers
Polymer materials are used in various applications such as fibers, plastics, rubbers, adhesives, paints, and cosmetics. For most of the applications, additives and fillers are added to improve their performance and/or to provide an attractive function. In our laboratory, specific organic compounds and nanoparticles are employed to modify polymeric materials with a novel idea. This simple technique is quite conventional and available in industry. Some of the research projects are exemplified as follows.
Functional optical films using orientation correlation
A low-molecular-compound (LMC) dissolved in a polymer matrix shows cooperative orientation with matrix polymer chains as illustrated in Figure 1. The oriented LMC provides optical anisotropy, which can be used to develop an optical retardation film needed for various display applications. Up to now, a novel material design to produce a retardation film with extraordinary wavelength dispersion (Figure 2) was proposed (Original paper 82).
Figure 1 Orientation correlation of LMC molecules dispersed in polymer chains.Figure 2 Material design of optical retardation film with extraordinary wavelength dispersion.
When the long axis of an LMC is perpendicular to the polarizability anisotropy, the refractive index in the normal direction of a film can be enhanced as shown in Figure 3, which is strongly required in industry (original paper 140). Furthermore, we proposed an idea to employ orientation correlations considering various combinations of the shape of an LMC and the deformation mode (original paper 104). This technique will be applicable to improve the ultraviolet (UV) resistance because most LMCs showing UV absorption are disk-shaped molecules.
Figure 3 Refractive index enhancement in the normal direction of a film
High-performance plastics with nucleating agent
In general, a nucleating agent is added to semicrystalline polymer to reduce the cycle time for processing. Of course, the plastics with a nucleating agent show high modulus and improved heat distortion temperature due to the enhanced crystallinity. We are proposing various techniques to improve the performance of crystalline plastics using various nucleating agents. High-density polyethylene (HDPE) is known to show rapid crystallization, and thus, a nucleating agent is barely added. However, we found that the addition of a small amount of carbon nanotubes enhances the crystallinity greatly only in the flow field, because CNTs act as shish in the shish-kebab structure. As a result, the modulus of HDPE with 1% of CNTs is twice as high as that of HDPE without CNTs as shown in Figure 4 (original paper 162).
Figure 4 Tensile modulus of HDPE containing CNT with an SEM image of HDPE crystals on the CNT surface
We further proposed the technique to improve the transparency of a product by addition of a nucleating agent for polypropylene and poly(lactic acid) (original papers 44 and 169) as shown in Figure 5. This is an anomalous phenomenon because the crystallinity, leading to light scattering, is always enhanced by a nucleating agent. Moreover, a novel method to obtain the plywood structure is proposed using a conventional injection molding with a unique nucleating agent (Figure 6, original paper 94). The product shows excellent impact strength without sharp-edge of broken species, which must be required for automobile parts and so on. This nucleating agent is also employed to enhance the melting point of polypropylene by using phase-transformation (original paper 97) and provide a porous film suitable for a battery separator (original paper 88).
Figure 5 Poly(lactic acid) film with high transparency and high heat distortion temperature. Figure 6 Injection-molded polypropylene plate with plywood structure and the plates after impact tests.
Modification by specific salts
We found that the addition of a specific salt enhances the glass transition temperature for various polar polymers such as poly(methyl methacrylate), poly(lactic acid), and polyamide 6 (original papers 126, 152, and 153). Furthermore, the salt addition provides the transparency for polyamide 6, which can be used for plastic glass (Figure 7).
Figure 7 Glass transition temperature of polyamide 6 with various amounts of lithium bromide.
[2] Material design using advanced mixing techniques with precise structure control
Polymer alloys, blends, and composites are still strong tools to develop advanced materials. Recently, such efforts are performed for biomass-based plastics and biodegradable plastics. Moreover, various intelligent materials are developed using the mixing technologies, such as shape-memory plastics and thermochromic films. We are trying to establish a novel concept available for the material design in this field.
Modification of sustainable plastics
Sustainable plastics such as polylactide, poly(vinyl alcohol), isosorbide polycarbonate, and cellulose derivatives need various modifications to replace from conventional plastics. We propose various ideas to improve various properties of biomass-based plastics and biodegradable plastics (original papers 149, 158, 169, 172, 175 and so on).
Figure 8 Improvement of mechanical toughness for poly(lactic acid) with high transparency.Figure 9 Shape-memory plastics comprising of biomass-based plastic, PLA.
Polyolefin blends
Flory-Huggins interaction parameter and interfacial tension between polyolefins are determined by the chain rigidity. Therefore, we can predict them only by the chemical structure. We are developing new polyolefin blends with precise morphology control.
Figure 10 Morphology of blends composed of polypropylene and ethylene-butene copolymers.
Segregation behavior of miscible polymer blends
Concentration of one component in a miscible polymer blend is homogeneous in general. However, we found that velocity gradient and temperature gradient induce the segregation behaviors for some miscible blends. This phenomenon can be employed to develop functional polymer blends having concentration gradient, such as optical fiber, injection-molded plate with anti-scratch property, and self-healing plastics (original papers 112 and 139).
Figure 11 Development of a material with concentration gradient.Figure 12 Scratch property of PC (left) and modified PC (right).
Immiscible polymer blends with a third component
Commercially available polymer blends often contain a third component such as additive, filler, and plasticizer.
We are developing the advanced material, in which a third component shows interfacial transfer by a stimulus.
All season's tire, that shows low modulus in winter and high modulus in summer, can be developed by this technique.
Figure 13 Material design of all season's tire.
Localization of functional nanofillers
Localization of a third component in an immiscible polymer blend is sometimes preferred.
In such cases, a third component having appropriate interfacial tensions with individual polymers is selected.
In our study, a third component is localized by another advanced method. For example, as shown in Figure 14, carbon nanotubes are localized either in polypropylene or ethylene-propylene elastomer, which is controlled by the processing condition (original paper 134). Furthermore, a third component such as conductive fillers should be localized between phases in a co-continuous polymer blend. We are developing a new method to localize CNTs considering the diffusion constant of CNTs in a polymer melt (original paper 166).
Figure 14 CNT dispersion in PP/EPR blend and their tensile properties.
Design of thermochromic material using refractive index difference
Rubbery materials are often added to rigid plastics to improve the mechanical toughness. The mixture usually loses the transparency because of the light scattering ascribed to the refractive index difference. In other words, a mixture composed of polymers having the same refractive index shows transparency even though the blend shows phase separation. We developed the immiscible polymer blend which changes the light transmittance according to the ambient temperature. It can be applicable as a smart curtain and so on.
[3] Study on Polymer Processing
Rheology provides fundamental information on polymer processing. Based on the experiences and basic information on polymer sciences, extensive study on various processing operations such as foaming, injection-molding and extrusion are carried out considering properties of final products. The obtained results provide new ideas for material design of high-performance or functional polymers.
Modification of rheological responses under elongational flow
Strain-hardening behavior in transient elongational viscosity is important for processing, such as reduction of neck-in, heat-sagging, and draw resonance. Furthermore, it is responsible for uniform wall thickness at blow-molding and thermoforming, and stable bubble at tubular blown film. We poposed various ideas to provide the strain-hardening, such as flexible nanofiber method (Figure 15) and critical gel method (Figure 16) . Moreover, a simple addition of long-chain branched polymer is found to be effective recently (original papers 157, 167, 168, and 174).
Figure 15 Growth curves of elongational viscosity (left) poly(lactic acid) and (right) poly(lactic acid) containing 1wt% of poly(butylene terephthalate) fibers.Figure 16 Growth curves of elongational viscosity and its foam for (left) linear low-density polyethylene and (right) linear low-density polyethylene containing 1 wt% of a weak gel of polyethylene.
Improved flowability
The addition of plasticizer is known to be effective to reduce the shear viscosity, which is pronounced at low shear rate region. We found a specific blend system to show low viscosity in the high shear rate region as shown in Figure 17, which is a great benefit at injection-molding (original papers 159 and 171).
Figure 17 Flow curves of polycarbonate (PC), PC with PC oligomer, and PC with low MW polystyrene.
Troubleshooting
Long-chain branched polypropylene is often added to linear polypropylene to improve the processability. However, an appropriate mixing method is required to obtain good processability, at which shear modification has to be considered. This is only one example of the problems at polymer processing. We consider the origin of each problem happened at polymer processing.
