Shunichi Numata

Re-examination of the relationship between packing coefficient and thermal expansion coefficient for aromatic polyimides

Abstract The existence of a possible relationship between molecular packing coefficient and thermal expansion coefficient for various aromatic polyimides was investigated. Rod-like low-thermal-expansion polyimides without side groups were seen to have very high packing coefficients, pointing to free volume as a factor in lowering their thermal expansion coefficients. But the small packing coefficients for low-thermal-expansion polyimides with side groups indicated that this was not so. Also, even if the large packing coefficients tended to increase the Youngs moduli for these polyimides without side groups, the rod-like polyimides with side groups have small packing coefficients and large Youngs moduli. The polyimides with low packing coefficients were found to have very small diffusion coefficients for water vapour.

The influence of siloxane modifiers on the thermal expansion coefficient of epoxy resins

Abstract The thermal expansion behaviour of unmodified and siloxane-modified epoxy resins has been investigated to clarify the mechanism by which siloxane modifiers lower the thermal expansion coefficient of epoxy moulding compounds containing inorganic fillers. A mechanism that accounts for all the observed characteristics is proposed. The thermal expansion coefficient of epoxy resins is increased by siloxane modifiers with carboxyl end groups, and the degree of the increase is dependent on the molecular weight of the modifiers. This increase is considered to be due to two effects: an increase in the local free volume in the epoxy matrix caused by the development of thermal stresses, and the building of siloxane molecules into the network structure. For sufficiently low molecular weight siloxane modifiers the latter effect is dominant, whereas with increasing molecular weight the former effect dominates. The increased coefficient of epoxy resins due to siloxane modifiers is likely to result in thermal stress near the filler-polymer interface because of an increased difference in the thermal expansion coefficient between the materials. This is one factor causing siloxane-modified epoxy moulding compounds to have a lower thermal expansion coefficient than unmodified ones.

Thermal expansion coefficients and moduli of uniaxially stretched polyimide films with rigid and flexible molecular chains

Abstract The effects of linearity and rigidity of molecular chains and their cohesive energy density on the thermal expansion coefficient (TEC) and modulus of polyimide films were investigated, using various uniaxially stretched films. Rod-like polyimide film has a low TEC even in the case of unstretched film, and only more than 5% stretching gave a negative TEC. The larger the draw ratio, the larger the modulus in the machine direction became. By contrast, in the transverse direction, with an increase in draw ratio, the TEC became larger, and the modulus became smaller. These data suggest that this polyimide has a negative TEC and a very high modulus in the molecular chain direction, and a high TEC and a low modulus in the direction perpendicular to the molecular chain. In the cases of polyimides that had somewhat bent and rigid molecular chains, and flexible molecular chains, respectively, similar behaviour appeared. In the case of polyimide with a bent and flexible molecular chain, this behaviour hardly appeared. From these data, it was confirmed that the low TEC values of unstretched and biaxially fix-cured films were closely related not to the intermolecular cohesive energy and rigidity, but to the linearity of the molecular chain.

Low Thermal Expansion Polyimides and their Applications

Thermal expansion coefficients (TECs) for polyimides differ very much depending on their chemical structures. Polyimdes with a rod-like structure as their backbone chains have lower TEC values. This is attributed to restraining of thermal expansion by rod-like molecules within intermolecular spaces, analogous to glass fibers in FRPs. The development of new polyimides which can closely match TECs of inorganic materials, such as metal or Si, can eliminate problems produced by thermal stress including warping, cracking or delamination. A new multilevel interconnection system using multilayered dielectrics consisting of low thermal expansion polyimide and inorganic materials has been proposed as one future technology for submicron VLSIs. Consequently, adhesiveless, high quality flexible printed circuit boards have been developed using a polyimide with the same TEC as copper foil. Their most significant property is a high dimensional stability after heat treatments, such as in a soldering process. Furthermore, they have very high adhesion strength at elevated temperatures.

Studies on Thermal Cyclization of Polyamic Acids

The imidization reaction of various polyamic acids having different chemical structures has been followed by measuring the weight losses that occurred during dehydro-cyclization. From these studies it was found that when polyamic acids were heated rapidly to a given temperature, the imidization reaction proceeded during the temperature rise but slowed down very markedly after the given temperature was reached. The temperature at which the imidization reaction ended was closely related to the glass transition temperature of the resulting polyimide. Based on these observations, it is concluded that the imidization reaction slows down markedly because the glass transition temperature of the polymer rises as the reaction proceeds, and molecular motion is frozen. In other words, the free rotation of amide bonds in the main chain is frozen. As a result, suitable conformation for imidization cannot take place any more.

Relationship between structure and adhesion properties of aromatic polyimides

The adhesion properties of aromatic polymide films on SiO2 and on various polyimides without any adhesion promoter were studied. Flexibility of the polyimide chains was found to be necessary for good adhesion. Polyimides with a flexible structure gave higher adhesion than those with a rigid structure for both substrate types. On SiO2, delamination occurred within 100h on exposure to steam at 120°C. On polyimides, the adhesion depended on both the substrate and the coating, and the peel strength increased in the following order of coating/substrate combinations: rigid/rigid < flexible/rigid < rigid/flexible < flexible/flexible. For the case of a rigid polyimide substrate, the adhesive property was greatly improved when coatings were spun on partially imidized substrate. This suggests that the dominant factors affecting adhesion are the degree of freedom of the molecular chain, which will affect the interaction between polyimide and SiO2, and the diffusibility of the molecular chain at the polyimide/polyimide interface. The existence of a diffusion layer was observed at the polyimide/polyimide interface with energy-dispersive X-ray analysis/transmission electron microscopy.

A mechanism describing polyamic acid solution viscosity change on storage at high temperature

Abstract The storage of polyamic acid solution at high temperature is known to bring about a decrease in solution viscosity. This paper describes an investigation of a factor of this viscosity change at 80°C for polyamic acid solution derived from 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride and p-phenylene diamine in N-methyl-2-pyrrolidone. This reduction of viscosity was caused by a weight-average molecular weight reduction. The relationship between weight-average molecular weight measured by a light scattering method and 10 wt% solution viscosity, η, at 25°C could be described as η = 4.29 × 10−17M3.35.

Fabrications and Optical Properties of Phthalocyanine Multi-Layered Films

Abstract Phthalocyanine multi–layered films, composed of copper phthalocyanine (CuPc) and naphthalene tetracarboxylic dianhydride (NTCDA), were fabricated by an organic molecular beam deposition method. Results from several analyses, like XPS and RHEED, revealed an epitaxial layer–by–layer structure of the multilayers, pointing toward a possible candidate of molecular assembly for future photonics devices.

New heat resistant “DP resin”

The cost of electrical equipments can be lowered by reducing the size and weight of the equipment. This has led to the need for the development of inexpensive heat resistant polymers having high mechanical strength at elevated temperatures.