The purpose of the present invention is to provide a, polyimide film having high transparency and improved bending resistance against multiple times of bending, and a varnish capable of providing such a polyimide film. The varnish according to the present invention contains a polymer (α) and a solvent (β). The polymer (α) is polyimide or a polyimide precursor. The varnish further contains a typical metal element having an atomic weight of 26-201 except alkali metals and alkaline earth metals, or at least one metal element among transition metal elements having an atomic weight of 26-201 or less. At least one of the metal elements contained in the varnish is present in an amount of 0.05-500 ppm relative to the polymer (α).

A method for forming foamed beads includes: saturating pellets with a blowing agent to form saturated pellets; and depressurizing the saturated pellets in a pressure vessel to form the foamed beads. The pellets include: a matrix polymer component, and from 0.01 wt% to 2 wt%, based on the weight of the pellets, of a nanostructured fluoropolymer, a nanostructured fluoropolymer encapsulated by an encapsulating polymer, or a combination thereof.

The resin composition according to one aspect is a resin composition containing a polyamic acid and a solvent, wherein the polyamic acid has a repeating unit represented by the following general formula (1) in a molecular chain, and the molecular chain has a structure represented by the following general formula (2) at one end or both ends. The proportion of the structure represented by the following general formula (2) relative to 1 mol of the repeating unit represented by the following general formula (1) is 0.001 mol or more and 0.1 mol or less. In the following general formulas, R.sup.1 is a tetravalent organic group; R.sup.2 is a divalent organic group; and R.sup.3 is an organic group having 15 or less carbon atoms.

##STR00001##

A prepreg includes composition elements [A], [B], and [C] described below, [A] a reinforcing fiber, [B] a thermosetting resin, and [C] a thermoplastic resin. [C] is present on a surface of the prepreg, [B] contains a first curing agent [b1] and a second curing agent [b2], and the reinforcing fiber of [A] that crosses over a boundary surface between a resin region containing [B] and a resin region containing [C] and that is in contact with both resin regions is present.

A manufacturing method of an SMC of the present invention comprises (i) forming chopped carbon fiber bundles by chopping a continuous carbon fiber bundle having a filament number of NK with a rotary cutter, (ii) fragmentation-processing the chopped carbon fiber bundles by using a fragmentation-processing apparatus comprising a rotating body, (iii) forming a carbon fiber mat by depositing the fragmentation-processed chopped carbon fiber bundles on a carrier film traveling below the rotary cutter, and (iv) impregnating the carbon fiber mat with a thermosetting resin composition, wherein N is 20 or more, and the fragmentation-processing apparatus comprises a first pin roller and a second pin roller which are disposed side by side, each having a rotation axis parallel to a rotation axis direction of the rotary cutter.

A polyimide copolymer according to an embodiment of the present disclosure includes a plurality of structural units. The plurality of structural units include a structural unit derived from dianhydride having an alicyclic structure and a structural unit derived from aromatic diamine including an ether group, thereby mechanical properties, thermal stability and optical characteristics of the polyimide film may be improved.

The present disclosure relates to a polyimide precursor and flexible polyimide obtained by imidization of the polyimide precursor and having high transparency, high glass transition temperature and low coefficient of thermal expansion.

In a first aspect, a polymer film includes a polyimide. The polyimide includes one or more dianhydrides and one or more diamines. Each of the dianhydrides and diamines is selected from the group consisting of crankshaft monomers, flexible monomers, rigid rotational monomers, rigid non-rotational monomers, and rotational inhibitor monomers. The polymer film has a D.sub.f of 0.005 or less, a water absorption of 2.0% or less and a water vapor transport rate of 50 (g×mil)/(m.sup.2×day) or less. In a second aspect, a metal-clad laminate includes the polymer film of first aspect and a first metal layer adhered to a first outer surface of the polymer film. In a third aspect, an electronic device includes the polymer film of the first aspect.

A polyimide film contains fluorinated substituents and cardo structures. The polyimide film exhibits excellent heat-resistance, transparency and mechanical properties. The polyimide film has a glass-transition temperature (Tg) of at least 360° C., a coefficient of thermal expansion (CTE) of 50 ppm/° C. or lower, a modulus of at least 4.0 Gpa, a b* value of 5 or lower and yellowness index of 8 or less. The polyimide film can be used as a display substrate or an optical film in a liquid crystal display (LCD), an organic light-emitting diode (OLED) and in other fields where the characteristic features are required.

Provided is a layered product that uses a high temperature-resistant transparent film having sufficient heat resistance, and that is capable of being mechanically released from an inorganic substrate after various processes are performed on the inorganic substrate since the adhesive strength between the high temperature-resistant transparent film and the inorganic substrate is appropriately weak, and that is less warped along with the inorganic substrate. In this layered product, no adhesive is used between the high temperature-resistant transparent film and the inorganic substrate, the release strength between the high temperature-resistant transparent film and the inorganic substrate is at most 0.3 N/cm, and the warpage amount of the layered product when heated at 300° C. is at most 400 .Math.m.