PAA Films Research Articles

Luminescence studies of polymer matrices. III. Characterization and evaluation of acrylic acid-based polymers as hosts for a phosphorescent coding system

The phosphorescence characteristics of naphthyl labelled poly(acrylic acid) film samples have been studied as a function of temperature, with a view to investigating the effect of matrix control on the level of triplet emission observed. Two relaxations, which serve to deactivate the excited triplet states have been detected from phosphorescence lifetime measurements: the α (or glass) and the βtransition (which can be associated with the onset of rotation of the carboxylic acid group). Investigation of the emission from 2-benzoyl naphthalene dispersed within both an acrylic acid-methyl methacrylate copolymer and a PAA film, respectively, has revealed that the more intense, longer-lived phosphorescence results from the modified polymer. This is considered to reflect the existence of (i) hydrogen-bonding interactions induced by the presence of carboxylic acid groups which serve to form a rigid matrix and (ii) intramolecular aggregates of methyl methacrylate units which create hydrophobic-rich pockets, capable of sustaining intense phosphorescence at room temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2127–2136, 1999

Thermal and mechanical properties of the precursor polymers: Comparison of their properties with poly(amic acid)

The DSC thermograms of P-PHA show a large endothermic peak at 450-550°C. As the annealing temperature increases from 250°C to 400°C, the endothermic peaks become smaller and then disappear for samples annealed above 450°C. As observed for P-PHA, the endothermic enthalpy of PHA and PAA became smaller with an increasing annealing temperature. The cyclization onset temperature (T 1 ) of the three precursors increases linearly with an increasing annealing temperature at a constant annealing time (30 min). Otherwise, the initial decomposition onset temperature (T 2 ) was shown to be constant. T 2 of P-PHA, PHA, and PAA were observed in the temperature ranges of 601-603°C, 576-577°C, and 532-534°C, respectively. These TGA results confirm that all of the samples are thermally stable. Increasing the annealing temperature of the three precursor polymers significantly increases the tensile properties of the films. The tensile properties of all annealed precursors were much higher than those of the unannealed films. In contrast, the initial modulus of PAA is improved only slightly when compared with the other two polymers regardless of the heat treatment. The biaxial stresses in the PHA and PAA films were investigated by holographic interferometry. The stresses in the films were 6.85-7.61 MPa for PHA and 27.01-27.70 MPa for PAA.

Chemically Grafted Polymeric Filters for Chemical Sensors: Hyperbranched Poly(acrylic acid) Films Incorporating β-Cyclodextrin Receptors and Amine-Functionalized Filter Layers

We report a new “molecular-filter” approach for enhancing the selectivity of chemical sensors. Specifically, we describe electrochemical sensors prepared from Au electrodes coated with β-cyclodextrin-functionalized, hyperbranched poly(acrylic acid) (PAA) films capped with a chemically grafted, ultrathin polyamine layer. The hyperbranched PAA film is a highly functionalized framework for covalently binding the β-cyclodextrin molecular receptors. The thin, grafted polyamine overlayer acts as a pH-sensitive “molecular filter” that selectively passes suitably charged analytes. Poly(amidoamine) dendrimers or poly-d-lysine is used as 10−15-nm-thick filter layers. The results show that at low pH, when the polyamines are fully protonated, positively charged redox probe molecules, such as benzyl viologen (BV), do not permeate the filter layer. However, at high pH, when the filter layer is uncharged, BV penetrates the filter layer and is reduced at the electrode. The opposite pH dependence is observed for negativel...

Development of an optical model for steady state porous anodic films on aluminium formed in phosphoric acid

Porous anodic oxide films on aluminium formed in phosphoric acid (PAA) have been characterized nondestructively by spectroscopic ellipsometry. Compared to previous studies on porous films formed in sulfuric acid, the optical behaviour of PAA films reveals new features which have been attributed to film-substrate interface roughness and optical anisotropy effects. On one hand relatively large interface roughness has been simulated by a graded index model. On the other hand, the implementation of uniaxial anisotropy in the optical model of the PAA film enables to interpret spectroscopic ellipsometry data acquired at multiple angles of incidence in terms of the morphology of the films. More specifically, accurate and physically realistic values are found for the porosity and porous film thickness. Although more difficult to interpret from the optical findings, the thickness of the barrier part of the porous film can also be estimated. The ellipsometry characterizations are confirmed by complementary TEM analysis of various films. Finally, the anisotropy exhibited by the PAA films is in line with recent theoretical predictions of the optical behaviour of arrays of parallel cylindrical capillaries in an isotropic medium proposed by other authors.

A convenient preparation of photoactive monolayers for liquid crystal photoalignment by the surface adsorption of an aminoalkylated azobenzene on poly(acrylic acid) films

Spin-cast poly(acrylic acid) (pAA) films on a substrate plate are immersed in a hexane solution of 4-butyl-4′-(10-aminodecyloxy)azobenzene, resulting in surface-selective adsorption and leading to the formation of photoactive monolayers. The surface adsorption behaviour is critically affected by the concentration of the azobenzene adsorbate solutions, and a 2.5×10–5 mol dm–3 solution gives an azobenzene monolayer with a surface density of 2 molecules nm–2 without any modification of the surface morphology of the pAA film. The surface density of the azobenzene is controlled by a binary system consisting of the azobenzene and n-octadecylamine. Photoirradiation of the surface-modified film with linearly polarised UV light for E-to-Z photoisomerisation results in the orientational transformation of the nematic liquid crystal layer from homeotropic to homogeneous alignment. It has been found that the minimum exposure energy of polarised UV light required for the liquid crystal alignment photocontrol is determined by the surface density of the azobenzene. There is a critical density; no photoresponse is observed for average densities of the azobenzene smaller than ca. 0.7 molecules nm–2 . With densities larger than this value, the smaller the density of the chromophore on pAA surface is, the faster the photoreorientation of the liquid crystal effectively takes place.

Real-Time Poling Vapor Co-deposition of Dye-Doped Second-Order Nonlinear Optical Polymer Thin Films

Dye-doped second-order nonlinear optical (NLO) poly(amic acid) (PAA) and polyimide (PI) thin films were prepared via a newly developed real-time poling vapor co-deposition process. This method would plausibly enable many polymeric materials, including high glass transition ones, to be matrix films and many NLO-active chromophores to be desired dopants for making numerous NLO thin films. The PAA film, after curing at 150 °C, showed a considerable improvement in thermal stability. Fourier transform infrared characterization also revealed that a 23 wt % disperse red 1 doped PI thin film exhibited reasonable second harmonic generation (SHG) characteristics even at relatively low poling fields; that is, the SHG coefficient had increased from 0.8 to 5.8 pm/V for a poling field increasing from 0.01 to 0.1 MV/cm.

Inhibition of Electrochemical Reactions at Gold Surfaces by Grafted, Highly Fluorinated, Hyperbranched Polymer Films

We report the synthesis and passivation properties of surface-grafted, highly fluorinated, hyperbranched poly(acrylic acid) (PAA) films that contain up to 50 atom % F. These films are very hydrophobic (water contact angle of up to 114°) and block electrochemical reactions on gold electrodes. Cyclic voltammetry in basic solution shows that while an electrode covered with a three-layer PAA (3-PAA) film mimics an assembly of microelectrodes, a fluorinated three-layer PAA (3-PAA/F) film blocks nearly all electron transfer to the gold surface. The charge transfer resistance of electrodes covered with a 3-PAA/F film is about 40 times greater than that for an electrode covered with a nonfluorinated 3-PAA film. 3-PAA/F films are initially 10 times more blocking than C16SH SAMs. Additionally, while the SAM is desorbed by electrochemical cycling, 3-PAA/F films are still highly blocking after ten potential sweeps over a 1.5 V range. Thus fluorinated PAA films are promising candidates for corrosion-inhibiting coatings.

Synthesis and Characterization of Surface-Grafted, Hyperbranched Polymer Films Containing Fluorescent, Hydrophobic, Ion-Binding, Biocompatible, and Electroactive Groups

We report the derivatization of hyperbranched poly(acrylic acid) (PAA) films with a wide range of amino- or alcohol-terminated molecules. These molecules can include moieties such as pyrene, ferrocene, poly(ethylene glycol), 15-crown-5, and a dye. To derivatize PAA films, we activate their carboxylic acid groups with isobutyl chloroformate and allow them to react with amine- or alcohol-containing molecules. Infrared spectroscopy demonstrates the formation of amide and ester bonds upon coupling as well as the presence of the derivative functional groups. Excimer fluorescence from pyrene-containing films implies a high density of pyrene groups. However, we can control the amount of pyrene in the film (and obtain monomer fluorescence) by varying the concentration of Py(CH2)3CONH(CH2)2NH2 in the derivatization solution. Cyclic voltammetry of ferrocene-containing films shows an electrochemically addressable ferrocenyl surface coverage of (6 ± 3) × 10-9 mol/cm2 in three-layer PAA films. PAA films and their deri...

Effect of pH, Fluorination, and Number of Layers on the Inhibition of Electrochemical Reactions by Grafted, Hyperbranched Poly(acrylic acid) Films

AbstractWe report the electrode‐passivation properties of both fluorinated and unmodified hyperbranched poly(acrylic acid) (PAA) films as a function of pH and the number of PAA layers. Both cyclic voltammetry and ac‐impedance spectroscopy show that the extent of blocking increases with the number of layers, regardless of the solution pH. However, passivation resulting from unfluorinated PAA films decreases with increasing pH, while passivation due to fluorinated films increases with increasing pH. Three‐layer fluorinated films can increase the charge transfer resistance of the electrode by up to a factor of 6 × 104. Although Randles' equivalent circuit can be used to model the electrochemistry of electrodes covered with unfluorinated PAA, fluorinated PAA‐coated electrodes often require additional circuit elements due to the high resistance of these hydrophobic films.

Spontaneous Molecular Orientation of Polyimides Induced by Thermal Imidization. 2. In-Plane Orientation

The degree of in-plane molecular orientation of various polyimides (PIs) and their precursors, poly(amic acid)s (PAAs), were estimated by measuring the visible dichroic absorption at an incidence angle for a rod-like dye (perylenediimide, PEDI) dispersed in the matrices. The effects of PI chain structure, film thickness, heating rate, and residual solvent on a spontaneous in-plane orientation phenomenon were examined to fully understand the mechanism. All PAA films cast on a substrate showed the low degrees of in-plane orientation of the chain axis, nearly independent of the chain structure. Upon thermal imidization of the PAA films adhered on a substrate, a striking spontaneous in-plane orientation behavior was observed for some PI systems with rigid chains; in contrast to that, some flexible PI systems showed no spontaneous behavior. Cure of the PAA films adhered on a substrate induced the spontaneous orientation even if the films were considerably thick (∼50 μm); in contrast to that, the cure of the fr...

Solvent Effects in Thermal Curing of Poly(4,4'-oxybis(phenylenepyromellitamic acid))

Thermal curing of poly(4,4'-oxybis(phenylenepyromellitamic acid)) (PAA) films prepared from PAA solutions using a novel solvent system composed of tetrahydrofuran (THF) and methanol (MeOH) has been investigated. In the drying process of casted PAA films, MeOH was entirely evaporated, while a significant amount of THF remained in the dried film. The progress of thermal imidization was observed during the process of drying at 80 °C. NMR measurements for the dried film revealed that the thermal imidization of trans-amidized PAA predominated over that of cis-amidized PAA. Dynamic mechanical thermal analyses (DMTA) pointed out that the dried PAA film containing THF showed no significant plasticization during the thermal cure. This indicates the mobility of polymer chain would be low during the thermal cure so that the resulting polyimide film has a higher in-plane orientation. The coefficient of thermal expansion (CTE) of the polyimide film was found to be low compared with a film obtained from a PAA solution using NMP.

Spontaneous molecular orientation of polyimides induced by thermal imidization. I. Uniaxial stretching of polyamic acid film

The uniaxial molecular orientation of poly(amic acid)s (PAA) and polyimides (PI) [N(CO) 2 Ph-Ph(CO) 2 N-Ar] n (Ar= Ph, PhOPh) as a function of draw ratio of PAA film. The dichroic spectra showed that for semirigid PI (Ar= Ph), spontaneous molecular orientation occurs during thermal imidization of slightly drawn PAA film. High Young's modulus of the oriented PI (Ar= Ph) film (∼ 60 GPa) was rationalized in term of the high orientation factor

Evidence of mucoadhesion by chain interpenetration at a poly (acrylic acid)/mucin interface using ATR-FTIR spectroscopy

Poly (acrylic acid) is considered to be an important mucoadhesive for controlled release applications. Yet, the specific mechanisms responsible for its bioadhesive behavior are not well understood. Attenuated total reflection infrared spectroscopy (ATR-FTIR) was developed for investigation of chain interpenetration at a poly (acrylic acid) (PAA) and mucin interface. A thin film of acrylic acid, polymerized below the gelation point, was cross-linked directly on an ATR crystal. The cross-linked PAA film was contacted with a buffered mucin solution and the ATR-FTIR spectrum was collected in situ as a function of time. The experimental results show evidence in support of chain interpenetration at the PAA/mucin interface. The experimental results indicate that as PAA and mucin are compatible in the 5–7 pH range, an important mechanism of mucoadhesion is the swelling of PAA by mucin and adhesion is limited by the extent of chain interpenetration across the biointerface.

Hydrodynamic flow and surface tension temperature dependence during the nematic-isotropic phase transition

A radial hydrodynamic flow in cylindrical freely suspended films of HOAB, OOBA, DOBA, PAA and BOBT around the nematic-isotropic phase transition is observed. The flow is explained by the existence of a nonlinear temperature dependence of the surface tension. The infusion of a warmer isotropic liquid into the nematic layer due to the flow and vice versa generates bubble domains around the N-I interface in HOAB, OOBA and DOBA.

Gas Transport Properties of Various Imidized Polyamic Acid Films

The sorption and permeation of CO2 in and through various imidized polyamic acid [PAA] films were investigated. Dual-mode sorption and partial immobilization models were applied to their phenomena and their behavior and structure are discussed. The amount of CO2 sorption increased with imidization in PAA film up to the imide content of ca. 40%, and then decreased. This behavior can be attributed mainly to the variation of the Langmuir sorption capacity term (CH′). In other words, it can be regarded that an unrelaxed volume and/or a microvoid fraction [(Vg−Vl)/Vg] vary with imidization, which are functions of Tg and difference in the expansion coefficient between liquid and glassy states. CH′ was described by the product (Tg−25) and ΔCp, difference of heat capacity between liquid and glassy states used instead of the expansion coefficient. On the other hand, the diffusion coefficient of Henry’s law mode (DD) is enhanced by the decrease of carboxyl group in PAA reducing the diffusion of penetrant with imidization up to ca. 40%, and then is depressed by restricted thermal mobility caused by considerably increased rigidity in the polymer chain with imidization. Variation in the diffusion coefficient of Langmuir mode behaved parallelly to DD.

Solvent and Curing Effects on Diffusion at Polyimide Interfaces

AbstractInterdiffusion 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 Ti, 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 Td or allowed to stand at room temperature. Bilayers prepared by spin – coating d-PAA from solution directly on partially cured PI films had interdiffusion distances w that decreased with increasing values of Ti to immeasurable levels by Ti = 200 °C. The decrease in w with increasing Ti 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. No interdiffusion occurred in the solventless – transfer samples for any combination of temperatures Ti or Td from room temperature up to 400 °C. Post – annealing of spin – coated bilayers at a temperature Td up to 400 °C was ineffective in producing any additional interdiffusion. Both of these results indicate that thermally activated interdiffusion (even for initially unimidized samples) does not exist in the absence of solvents, an effect attributed to the rapid increase of the glass transition temperature of the polymer with imidization.

Molecular aggregation and fluorescence spectra of aromatic polyimides

Effects of molecular aggregation on emission behaviour have been studied, together with the u.v.-vis absorption spectra and DSC measurements, for several polyimides (PIs) prepared by the combination of biphenyltetracarboxylic dianhydride (BDA), pyromellitic dianhydride or benzophenonetetracarboxylic dianhydride with p-phenylene diamine (PPD) or 4,4′-diaminodiphenyl ether. The intensities of the fluorescence, previously attributed to the intermolecular charge-transfer (CT) interaction [1], increase sharply with annealing or with increasing imidization temperature for all the PIs studied. For PI(BDA/PPD), annealing induces not only increase of the intensity of the emission at 525 nm (CT 1) but also the appearance of a new emission at 540 nm (CT 2) which is considered to be due to a ground state intermolecular CT complex. The efficiency in energy transfer, measured by a depolarization technique, also increases on annealing. The CT emission spectra are also sensitive to change of molecular orientation. With increasing draw ratio of the films of polyamic acid PAA (BDA/PPD) which is a precursor of PI, the emission intensities of the resulting PI(BDA/PPD) increases for CT 1 but decreases for CT 2, showing the completely different natures of the two CT complexes.

PAA film application and the amalgam-PAA-glass ionomer bond

The influence of application thickness of the PAA film and the vigor with which it is brush-applied on the bond shear strength of the amalgam-PAA-glass ionomer bond was investigated. Six different PAA film thicknesses (0.020″, 0.015″, 0.010″, 0.005″, 0.002″, and “thin”) and 2 techniques of brush application (vigorous brushing for 10 s or minimal brushing) were employed. Samples were shear-tested after one week storage at 37°C and 100% relative humidity. ANOVA revealed that only for the minimally brushed groups did PAA film application thickness have a statistically significant effect on bond shear strength, while vigorous brush application improved bond shear strength only at the thicker film thickness applications. The results are explained in terms of differences in the composition of the adhesive film.

Complexes of acid amides with aprotic polar solvents IV. Complexes of poly(acid amides) with aprotic solvents

The process of solid-phase cyclodehydration of films cast from solutions of poly(acid amides) (PAA) in dimethylformamide, dimethylacetamide and N-methyl-2-pyrrolidone has been investigated by mass-spectrometric thermal analysis (MTA). It is shown that MTA is a structure-sensitive method making it possible to follow the appearance and changes in the character of ordered structures formed in PAA films. The authors relate these structures to the process of crystallization of complexes of PAA with aprotic polar solvents.