Moonhor Ree

Influences of chain rigidity, in‐plane orientation, and thickness on residual stress of polymer films

Films of several polyimides having various chain rigidities were prepared on silicon substrates from the poly(amic acid) precursors by a conventional spin‐coat/dry/thermal imidization process: poly(p‐phenylene pyromellitimide) (PMDA‐PDA), poly(4,4’‐oxydiphenylene pyromellitimide) (PMDA‐ODA), poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA), and poly(4,4’‐oxydiphenylene benzophenonetetracarboximide) (BTDA‐ODA). The film properties, such as residual stress, refractive indices, and birefringence were investigated using wafer bending and prism coupling techniques. The stress was relatively low for both fully rodlike PMDA‐PDA and semirigid BPDA‐PDA, intermediate for semiflexible PMDA‐ODA, and high for flexible BTDA‐ODA. However, even for the rigid polyimides such low stress is achievable only in thin films (≤10 μm thick for PMDA‐PDA and ≤20 μm thick for BPDA‐PDA). Otherwise, they are no longer low stress materials. Overall, the film stress was very sensitive to the film thickness in the rigid polyimides but less sensitive to it in the flexible polyimides. The lower stress polyimide film exhibited the higher anisotropy of refractive indices (i.e., higher birefringence, which is a measure of molecular in‐plane orientation). The molecular in‐plane orientation, which is a critical factor to the film stress, becomes higher as the polymer chain rigidity is greater and the film is thinner. In the polyimide films prepared from the precursors in N‐methyl‐2‐pyrrolidone, which has a high boiling point and low vapor pressure, the molecular in‐plane orientation is mainly induced by the surface of substrates rather than other film process factors including spinning in spin coating.

Anisotropic properties of high‐temperature polyimide thin films: Dielectric and thermal‐expansion behaviors

Multilayer poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA) polyimide films of 172 μm total thickness (11.4 μm per layer) were prepared from the poly(amic acid) precursor solution through repetition of a spin‐coat/softbake/cure process. Wide‐angle x‐ray diffraction results indicate that the polyimide molecules in the multilayer films are highly ordered along the chain axes as well as in the lateral direction and furthermore are highly oriented in the film plane as observed in a single‐layer film of 11.4 μm thickness. The multilayer films showed the same dynamic mechanical properties and glass transition behavior (Tg = 330u2009°C) as a single‐layer film. For the multilayer films both the in‐plane dielectric constant (e’XY) and out‐of‐plane thermal‐expansion coefficient (αZ) were measured using time‐domain spectroscopy and conventional thermal mechanical analysis, respectively. The e’XY at room temperature was 3.69 (±0.08) over a frequency range of 0.35–2.50 THz. A similar e’XY is predicted at frequencies ...

Effect of precursor history on residual stress and relaxation behaviour of high temperature polyimides

Abstract Using a wafer bending technique, residual stress and its relaxation were investigated for four different kinds of high temperature polyimides: rodlike PMDA-PDA, semi-rigid BPDA-PDA, semi-flexible PMDA-ODA, and flexible BTDA-ODA. Residual stress was measured in situ on silicon wafers during thermal imidization of the polyimide precursors and subsequent cooling of the resulting polyimides as a function of temperature over the range of 25–400°C. The stress of the cured films at room temperature was significantly relaxed by the moisture uptake of the films rather than their creep behaviour. Both residual stress and moisture-induced stress relaxation in the cured polyimide films were strongly dependent upon the molecular nature (that is, molecular chain rigidity, degree of molecular orientation and packing), as well as the precursor origin. In addition, the diffusion coefficients of water in the cured polyimide films were estimated by best-fitting stress relaxation against time profiles.

X-ray scattering studies of thin films of photosensitive polyimides

Abstract Using the techniques of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), the morphology of polyimide thin films thermally imidized from several photosensitive polyimide (PSPI) precursors has been investigated and compared with that of the thin films prepared from the corresponding poly(amic acid) precursors: poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), poly( p -phenylene biphenyltetracarboximide) (BPDA-PDA) and poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). The WAXD results indicate that regardless of the precursor origin, the BTDA-ODA polyimide is amorphous, whereas the other polyimides exhibit a molecular order. For both the PMDA-ODA and the BPDA-PDA, the molecular order is relatively higher in the films prepared from the PSPI precursors than in those from the corresponding poly(amic acid)s, indicating that during thermal imidization, the photosensitive groups play an important role to improve the mobility of the polymer chains, which may be critically needed to make better molecular packing, in spite of their bulkiness having the potential to hinder the molecular packing. In Lorentz-corrected SAXS analyses, a long period (130–156 A mean periodicity) was observed for all the polyimides except the PMDA-ODA. In particular, the microstructure in the BPDA-PDA could be described by an extended chain-based two-phase (ordered and less ordered phase) model with diffuse boundaries because of its high chain rigidity. In addition, the Guinier SAXS analyses indicate the presence of voids in all the polyimide films, regardless of the precursor origin. The size of voids was 251–349 A in radius, depending upon the type of polyimide.

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.

Chain Conformations Of Aromatic Polvimides And Their Ordering In Thin Films.

We have investigated the conformation – order relationships of four aromatic polyimides prepared by thermal imidization of the precursor poly(amic acids) in thin films. They are: (i) PMDA-4,4′-ODA polyimide from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) monomers; (ii) PMDA-PDA polyimide from PMDA and p -phenylene diamine (PDA); (iii) PMDA-Benzidine polyimide from PMDA and benzidine and (iv) BPDA-PDA polyirnide from biphenylene tetracarboxylicdianhydride (BPDA) and PDA. X-ray diffraction results and their analyses by molecular modeling show that all the four polyimides exhibit extended chain conformations in various smectic-type ordered structures that form monomer repeat layers but differ in the details of interchain packing. Furthermore, the polyimide chains are highly aligned along the film plane.

Structure And Properties Of Binary Rodlike/Flexible Polyimide mixtures.

Binary mixtures of a rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA) and a flexible 6F-BDAF polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride and 2,2-bis(4-aminophenoxy-p-phenylene) hexafluoropropane were prepared by solution-blending of the meta-PMDA-PDA poly(amic ethyl ester) and 6F-BDAF poly(amic acid) precursors, followed by solvent evaporation and thermal imidization. The size scale of the phase separation, as measured by light scattering, is ca. I μm or smaller in most cases. Dynamical mechanical thermal analysis measurements indicate that the glass transition temperature of 6F-BDAF is unaffected in all of the mixtures studied, indicating complete demixing of rodlike and flexible polyimides in agreement with theory. X-ray photoelectron spectroscopy results show a strong surface segregation of 6F-BDAF in mixtures containing as low as 10% by weight of the 6F-BDAF component in the bulk. The mixtures with PMDA-PDA as the major matrix component therefore maintain excellent bulk properties of rodlike polymers, i.e., high modulus to 500°C, and low coefficients of thermal expansion (

Structure And Properties Of BPDA-PDA Polyimide From Its Poly(Amic Acid) Precursor Complexed With An Aminoalkyl Methacrylate

BPDA-PDA poly(amic acid) precursor was functionalized through its carboxylic acid groups being linked with a crosslinkable aminoalkyl methacrylate, 2-(dimethylamino)ethyl methacrylate (DMAEM), by acid/base complexation. BPDA-PDA polyimide films, which were thermally imidized from the precursors complexed with various amounts of DMAEM, were characterized by means of wide angle x-ray diffraction, stress-strain analysis, and residual stress analysis. The structure and properties of the BPDA-PDA polyimide film were dependent upon the history of the precursor, that is, the complexation of the poly(amic acid) precursor with DMAEM. The molecular packing order was enhanced with the history of DMAEM loading while the molecular order along the chain axis was disrupted. Overall, physical properties, such as mechanical properties and residual stress, were degraded with DMAEM loading. The moisture induced stress relaxation behavior was sensitive to the history of DMAEM loading, whereas the creep induced stress relaxation was varied little due to its high T g . These properties are understood in terms of structure/property relationships, as well as microvoids, which were possibly generated by outgassing the bulky DMAEM pendent groups during thermal imidization.