W. Volksen

Orientational decay in poled second‐order nonlinear optical guest‐host polymers: Temperature dependence and effects of poling geometry

The orientational decay of chemically and thermally stable high‐temperature chromophores doped into thin films made from polyimides and a variety of other polymeric hosts has been investigated. The chromophores are aligned using electric field poling and second‐harmonic generation (SHG) is used to probe the decay of the electric field poling induced alignment. The decay rate of the SHG signal from films poled using both a corona discharge and side‐by‐side in‐plane electrodes was measured. When electrodes are chosen so that the effects of charge injection are minimized, little difference has been observed between the orientational decays from films poled using the two methods for either an amorphous preimidized polyimide host or a highly anisotropic film poled during imidization. The films imidized during poling showed significant orientational stability at 250 °C for over 15 h after a fast initial partial decay. In addition, the decay of the SHG signal was measured as a function of temperature below the g...

Polyimides derived from nonaromatic monomers: synthesis, characterization and potential applications

A number of mixed aromatic/cycloaliphatic as well as fully nonaromatic polyimides have been prepared. Whereas all the poly(amic acids) derived from nonaromatic diamines involved salt-formation during the initial stages of the polymerization, the majority of these eventually formed homogeneous, highly viscous polymer solutions. Only in a few select cases involving all nonaromatic monomers traditional solution polymerization was unsuccessful. The polyimide derived from hexafluoroisopropylidene diphthalic anhydride (6FDA) and trans- 1,4-diaminocyclohexane (DACH) yielded films with tough mechanical properties, a glass transition temperature of ∼360°C, good solvent resistance, and a low dielectric constant of 2.6. Thermal stability of this polyimide as determined by thermal gravimetric analysis in both air and nitrogen was quite good, exhibiting a weight loss of only 0.07 wt%/h at 350°C under isothermal conditions in nitrogen. However, mechanical properties as a function of thermal aging in both air and nitrogen indicated a maximum use temperature of only 350°C under inert conditions and less than 300°C in the presence of oxygen.

Diffusion and self-adhesion of the polyimide PMDA-ODA

Abstract The relationship between the interdiffusion of two layers of the polyimide PMDA-ODA and the adhesion between the two layers has been examined. The polyimide layers were made by successively depositing a polyamic acid layer from solution and then curing the layers at elevated temperature to the polyimide. Diffusion occurred during the curing process of the second layer and was controlled by the cure schedule. The interdiffusion was measured using forward recoil spectrometry (FRES) and the adhesion measured by peel tests. Good correlation was found between the interdiffusion distance and the adhesion. It was found that a large diffusion distance, at least 200 nm, was required to obtain a bond whose strength was equal to that of bulk material.

High-temperature polyimide nanofoams for microelectronic applications

Abstract Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term ‘nanofoam’. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block, leaving pores of the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials were investigated as well as from a number of thermally labile polymers. The foams were characterized by a variety of experiments including TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and low dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

High temperature polymer nanofoams based on amorphous, high Tg polyimides

Abstract A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology consists of a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon thermal treatment, the thermally unstable block undergoes thermolysis leaving pores of which the size and shape are dictated by the initial copolymer morphology. Triblock and graft copolymers comprising high glass transition temperature amorphous polyimide matrices with poly(propylene oxide), as the thermally decomposable coblock, were prepared. The copolymer synthesis was carried out through either the poly(amic alkyl ester) or poly(amic acid) precursor and subsequent cyclodehydration to the polyimide by either thermal or chemical means, respectively. Microphase-separated morphologies were observed for all copolymers, irrespective of the block lengths surveyed, by dynamic mechanical analysis. Upon decomposition of the thermally labile coblock, a 5–15% reduction in density was observed, consistent with the generation of a foam.

Residual stress behaviour of isomeric PMDA-ODA polyimides

Poly(3,4′-oxydiphenylene pyromellitimide) (PMDA-3,4′-ODA), an isomer of poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-4,4′-ODA), was synthesized from pyromellitic dianhydride (PMDA) and 3,4′-oxydiphenylene diamine (3,4′-ODA). For these two polyimide isomers and their poly(amic acid) precursors in the condensed state on Si wafers, residual stress behaviour over the range 25–400°C was investigated by the dynamic measurement of wafer bending. During thermal imidization both isomers did not show any difference in stress versus temperature behaviour. Once imidized, however, one isomer exhibited a quite different stress behaviour from that of the other during cool-down: the stress of PMDA-3,4′-ODA increased rapidly from zero at 400°C to ≈45 MPa at 40°C, whereas that of PMDA-4,4′-ODA rose gradually from zero to ≈27 MPa. For both cured isomers, stress-temperature profile on heating was the same as that on cooling, with some deviation due to moisture uptake over the temperature range 25–150°C, indicating that their stresses were insensitive to thermal cycling or thermal annealing. From independently measured properties (thermal expansion coefficient, modulus, Poissons ratio of 0.34) of both polyimides, the thermal stresses were calculated and compared with the measured overall stresses. It is concluded that for both polyimides the overall residual stress results primarily from the thermal stress. In comparison with PMDA-4,4′-ODA, PMDA-3,4′-ODA showed a much higher stress despite its slightly lower thermal expansion coefficient. This leads to the conclusion that the large difference between the stresses of the isomers results from the large difference in their moduli (5.0 GPa for PMDA-3,4′-ODA and 3.0 GPa for PMDA-4,4′-ODA). This behaviour is further supported by the difference in morphological structures of these two isomers as determined by wide-angle X-ray diffraction: PMDA-3,4′-ODA showed a well-developed crystalline structure, whereas the PMDA-4,4′-ODA did not. In addition, the interfacial adhesion between polyimide film and Si wafer primed with A1100 was investigated.

Correlation between polymer architecture and sub-glass-transition-temperature light-induced molecular movement in azo-polyimide polymers: influence on linear and second- and third-order nonlinear optical processes

We report on light-induced linear and second- and third-order nonlinear optical effects in high-glass-transition-temperature (Tg) photosensitive nonlinear optical azo polyimides. We present evidence of light-induced orientation of azo chromophores at room temperature in very high-Tg polyimides (Tg up to 350 °C) even though the chromophore is firmly embedded into the polymer backbone. We show that the isomerization reaction and the light-induced polar and nonpolar orientation depend on the molecular structure of the unit building blocks of the polymer. The mechanism of the photoassisted poling process is clarified, and it is shown how the linear and second- and third-order nonlinear optical effects can be controlled by light.

Polyimide Nanofoams For Low Dielectric Applications

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term “nanofoam”. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block leaving pores the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials, were investigated as well as a number of thermally labile polymers. The foams were characterized by a variety of experiments including, TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

Low-stress polyimide block copolymers

Abstract In order to produce polyimide films in situ on silicon substrates, while minimizing the thermally induced residual stresses on cooling from the cure temperature, we have prepared various polyimide-polydimethylsiloxane based block copolymers derived from pyromellitic dianhydride (PMDA) and p,p′ -oxydianiline (ODA) via the poly(amic-alkyl ester) precursor route. This approach allows more synthetic flexibility than the poly(amic acid) route because it permits the use of a wider range of solvents and solvent mixtures, allowing copolymers with considerable variety in the polyimide backbone, as well as in the coblock, block length and composition. PMDA/ODA containing 20 wt% polydimethylsiloxane blocks of molecular weight 5400 g mol −1 was found to show no residual thermal stress after curing at 350°C, although its Youngs modulus and yield stress remained high (about half those of the PMDA/ODA homopolymer, which showed residual stresses of about 40 MPa for comparable film thicknesses). The rapid stress relaxation is attributed to the relatively good phase separation and minimal perturbation of the molecular ordering and orientation of the PMDA/ODA matrix in this copolymer.

Interdiffusion at polyimide interfaces

Abstract Interdiffusion at interfaces between deuterated polyamic acid (d-PAA) and polyimide (PI) films was investigated with forward recoil spectrometry, an ion beam analysis technique. The PI films were prepared by spin coating a solution of PAA on a silicon substrate, followed by an anneal at a temperature T i , which produced partial or complete conversion of the PAA to PI. An overlayer of d-PAA was added, in one set of samples by spin coating from solution, and in another set by transferring (in the absence of solvents) a dry d-PAA film onto the PI surface. The bilayer samples were then either annealed at a temperature T d or allowed to stand at room temperature. No interdiffusion occurred in the solventless transfer samples for any combination of temperatures T i or T d from room temperature up to 400°C. Bilayers prepared by spin coating d-PAA from solution directly on partially cured PI films did, however, show significant interdiffusion distances w , which decreased with increasing values of T i to immeasurable levels by T i = 200°C. The decrease in w with increasing T i is thought to be caused by a positive Flory parameter between PAA and PI which increases with the imide fraction in the PI film; the result is an increasing immiscibility between the swollen polymer layers. Subsequent annealing of these spin-coated bilayers at a temperature T d up to 400°C was ineffective in producing any additional interdiffusion. The absence of any thermally activated interdiffusion (even for initially unimidized samples), with either the solventless-transfer or spin-coated preparation methods may be attributed to the rapid increase of the glass transition temperature of the polymer with imidization.