Anne K. St. Clair

Fibers from a low dielectric constant fluorinated polyimide: Solution spinning and morphology control

Control of the internal morphology of wet-spun fibers from a fluorinated polyimide has been achieved by varying the rate of polymer coagulation through adjustments in nonsolvent/solvent miscibility and precipitation strength of the coagulation bath. Filament internal morphologies ranged from very porous or sponge-like to fully solid. Intermediate structures included fibers containing a spongy core with a nonporous skin, sponge-like fibers containing large voids, and a relatively solid material containing randomly spaced small voids. The cross-sectional shape of the fiber is dependent upon the coagulation process as well as the volume contraction of the initial extrudate. Drawn fibers (3×) retained the original asspun cross-sectional shape and also lost porosity. Mechanical properties of poly(6FDA-4BDAF) fibers have an inverse relationship to filament porosity. Maximum modulus and break strength for drawn fibers is approximately 6 CPa and 200 MPa, respectively. Asspun mechanical properties were dependent upon the processing conditions and have moduli between 0.4–3.0 Gpa and break strengths of 10–160 MPa. A dielectric constant of 2.50 for nonporous films was measured over a frequency range between 1.0 MHz to 1.8 GHz, showing little dispersion.

Dimensionally stable polyimide copolymers for microelectronics applications

Polyimides are finding increased applications in microelectronics due to their high thermal stability, good chemical resistance, good adhesion, low moisture absorption, good mechanical properties, and low coefficient of thermal expansion (CTE). Four series of non-fluorinated copolyimides were synthesized and characterized for potential application as encapsulants, interlayer dielectrics, and stress relief layers. Several candidates exhibited good combinations of physical and mechanical properties, with inherent viscosities from 1.21-1.42 dL/g, Tgs ranging from 251-277{degrees}C, 10% weight loss temperatures occurring between 503-527{degrees}C, and CTEs ranging from 33-39 ppm/{degrees}C. Mechanical properties at RT for the best candidates included tensile strengths of 17.8-21.3 ksi, moduli between 388-506 ksi, and elongations of 11-43%. Moisture uptake for these copolyimides ranged between 0.85-1.38 weight percent.

Poly(aryl ether ketones) bearing alkylated side chains

Poly(aryl ether ketones) containing pendant alkyl side chains were prepared by the nucleophilic displacement reaction of 2,2-bis(4-hydroxphenyl)tridecane, 2-2,bis(4hydroxyphenyl)nonadecane and bisphenol AF with 2,2-bis-[4-(4-fluorobenzoyl)phenyl]propane and 2,2-bis[4-(4-fluorobenzoyl)phenyl]hexa-fluoropropane in the presence of potassium carbonate in N,N-dimethylacet-amide. The polymers had inherent viscosities up to 1.04 dl g−1 in chloroform, formed creaseable films and were soluble in common organic solvents. Their dielectric constants were below 2.60 (at 10 GHz). The inclusion of the alkyl side chains lowered the T g, thermal stability and char yield of fluorinated poly(ether ketone) as expected.

Molecularly Oriented Films for Space Applications

Because of their inherent toughness and flexibility, low density, thermal stability, radiation resistance and mechanical strength, aromatic polyimides have excellent potential for use as advanced materials on large space structures. Polyimides were uniaxially and biaxially oriented to further enhance their dimensional stability, stiffness, elongation and strength. Both unoriented and oriented polymeric thin films were also cryogenically treated to temperatures below -184 °C to show their stability in cold environments. Successful stretching depended on film quality, film thickness, soak time, grip pressure, stretching rate and stretching temperature. In-plane birefringence increased with increased stretch ratios as expected. Coefficients of thermal expansion (CTEs) decreased with increased stretch ratio by as much as 31%. Tensile strengths increased by up to 85% with increasing stretch ratio. Elongations increased by as much as 95% with stretching. Significant improvements in elongation were obtained with only a 2× uniaxial orientation. Moduli did not change significantly with stretching. Mechanical properties increased more with uniaxial stretching than biaxial stretching. Cryogenic treatment increased the tensile strength of some stretched samples by a maximum of 10–20%. Properties such as modulus and CTE did not change significantly after the films had been slow cooled to cryogenic temperatures.

Synthesis and characterization of new fluorinated polyacrylates: 2

A series of new fluorinated acrylate polymers and copolymers (with methyl acrylate and methyl methacrylate) has been prepared starting from 1,3-bis(hexafluoro-2-hydroxy-2-propyl)benzene (1,3-HFAB). 1,3-HFAB was first reacted with dimethylsulfate in the presence of aqueous sodium hydroxide to yield 1-(hexafluoro-2-hydroxy-2-propyl)-3-(hexafluoro-2-methoxy-2-propyl)benzene which was reacted with acryloyl or methacryloyl chloride to give the corresponding fluorinated acrylate or methacrylate monomers. The polymers were obtained by bulk or solution free radical polymerization techniques with α,α′-azobisisobutyronitrile as initiator. The polymers had viscosities ranging from 0.21–0.41 dl g−1, and the polyacrylates formed brittle films by solution casting. Thermogravimetric analysis showed that they had thermal stability up to 391°C in nitrogen and glass transition temperatures from 50 to 104°C.

Synthesis and characterization of elastomer-modified polyimide films

Abstract A study has been conducted to determine the effects of added elastomers on the film properties of a linear aromatic polyimide. Polyamic acids have been prepared via reaction of BTDA (3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride) with ODA (4,4′-oxydianiline) and various ATBN (aromatic amineterminated butadiene acrylonitrile) elastomers. Films were prepared and thermally converted to the corresponding polyimides. Optimum cure conditions for BTDA + ODA/ATBN films were evaluated. The effect of elastomer concentration on BTDA + ODA thermal properties, density, tensile properties, morphology and tear resistance were evaluated. Variation of the acrylonitrile content of the ATBN elastomer in BTDA + ODA was performed and properties evaluated. Incorporation of ATBN into BTDA + ODA caused a decrease in the glass transition temperature, thermal stability, tensile strength and modulus of films. Increases of film tear resistance were observed for several formulations of ATBN in the BTDA + ODA polyimide.

N-Methylated copoly(imide amide)s containing hexafluoroisopropylidene

Abstract A synthetic method was developed to produce N,N′ -dimethylated diamines which were then used to prepare poly(imide amide)s. Thus, N,N′ -dimethyl-4,4′-bis(4-aminophenoxy)biphenyl and N,N′ -dimethyl-2,2-bis(4-aminophenyl)hexafluoropropane were condensed at 0°C in dimethylacetamide with 2,2-bis[ N -(4-chlorocarboxyphenyl)phthalimidyl]hexafluoropropane. The yields were 76–92%, viscosities 0.28–2.26 dl g −1 , water absorptions 0.16–0.37 wt% and thermal stabilities around 460°C by thermogravimetric analysis. The addition of the methyl group caused lower molecular weights but superior thermal stability (isothermally) and decreased colour.

Polyimide Adhesives: Modified with ATBN and Silicone Elastomers

A series of studies evaluating the effects of added elastomers on the properties of a high-temperature addition polyimide adhesive is reviewed. First, thermoset polyimides containing various butadiene/acrylonitrile and silicone elastomers were synthesized either as physical polyblends or by chemically reacting the elastomers directly onto the polymer backbone. The modified adhesive resins were characterized for thermomechanical properties, fracture toughness and adhesive strength. A second series of elastomer-containing polyimides was also prepared in order to study the effects of the elastomer chain length on polymer properties. Aromatic amine-terminated silicone rubbers with repeat units varying from n=10 to 105 were reacted onto the polyimide backbone, and the resulting polymers were characterized for their adhesive properties.

N-Propargyl-substituted aromatic polyamides: preparation and thermal crosslinking

Abstract A series of high molecular weight aromatic polyamide copolymers derived from 4,4′-bis(methylamino)-diphenylmethane, 4,4′-bis(propargylamino)diphenylmethane and isophthaloyl dichloride was prepared as potential candidates for use as matrix resins in Kevlar(R) fibre composites. These polyamides, which contain pendant propargyl groups, underwent facile crosslinking at 280°C as evidenced by dramatic increases in their glass transition temperatures (Tg) and accompanying loss of solubility. Other attempts to effect crosslinking by exposure to ultraviolet light, electron beam or gamma radiation were unsuccessful.

Free volume model for dielectric constant of polymer films

A slow positron flux generator reported in another paper at this conference was used to measure positron lifetime in a series of especially developed fluorine containing thin polyimide films. The positron lifetime spectra was analyzed into 2‐components using a standard least square routine. No evidence for positronium formation was observed in any of test films studied. The trapped positron lifetimes were used to calculate the radii of the shallow trap sites. Equating the total volume occupied by the traps with the saturation of the shallow trap sites. Equating the total volume occupied by the traps with the saturation moisture content of Kapton (reference) films, free volume fractions (f) were calculated in all the samples. These free volume fractions affect the dielectric constants (e) of the test films as follows: 1/e= (1−f)/eR+f(1−d)/eAir+fd/eWater Where, eR is the dielectric constant of the trap‐free medium, eAir is the dielectric constant of air, eWater is the dielectric constant of water, and d...