Oriented Polymer Films Research Articles

Design of planar waveguide based on patterning substrate and oriented polymer film

Semiconducting conjugated polymersused for light emitting devices (LEDs), lasers and amplifiers have received considerable attention due to their low cost and easy fabrication through spin-coating and photochemical processing. A promising material for LED and laser applications is poly(9, 9-dioctylfluorene-co-benzothiadiazole) (F8BT). F8BT has a low stimulated emission threshold and exhibits a large net optical gain at 570 nm. It also shows liquid crystallinity and can be readily aligned into a monodomain by using an alignment layer, polyimide (PI). Oriented film of F8BT exhibits that its charge carrier mobility is increased by more than one order of magnitude compared with isotropic film. The refractive index of the material is also greatly affected by the orientation of the polymer chain. Furthermore, it has been reported that low threshold laser can be achieved by blending P3 HT or red-F solution into F8BT via energy transfer.Here we report a planar waveguide structure obtained via patterning chain oriented area on F8BT: red-F (9 : 1) blend polymer film. The blend solution is obtained by mixing the F8BT solution with red-F solution (with the same concentration, 20 mg/ml in toluene) with a ratio of 9 : 1. The designed waveguide patterns are obtained by inkjet-printing the PI solution onto the pre-cleaned quartz substrates. Thin films (150-200~nm thick) of F8BT: Red F are deposited onto PI by spin coating (2000 rpm). The chain alignment treatment is performed by the following procedure: the films are kept in N2 at 265 ℃ for 2 min, then they are cooled down to 235 ℃ at a rate of 1 ℃/min, finally they are cooled down to room temperature sharply. The PI contacted area on the film becomes anisotropic, while the area without PI keeps isotropic. The refractive index parallel (perpendicular) to the chain direction is significantly increased (reduced) in the PI contacted area compared with outside the PI area. Therefore, the waveguide confinement could be achieved without changing the thickness of the film. Experimental investigations, including AFM images, polarized microscopy images, polarized absorption, and PL spectra of the patterned samples, clearly show the difference between the aligned area and isotropic area.The large percentage of overlap between the emission spectrum of F8BT and the absorption spectrum of red-F solution leads to an efficient energy transfer from F8BT (host) to red-F solution (guest), resulting in a red emission at a wavelength between 600-670 nm from the blend. The polarized absorption and PL spectra of the aligned F8BT: red-F film demonstrate that the absorption intensity of the polarized light parallel to the aligned chain is 5.9 times that perpendicular to the aligned chain at a wavelength of 477 nm, and their ratio is 5.5 at a wavelength of 631 nm.Our demonstration suggests that patterning chain oriented area can be a promising approach to achieving planar waveguide devices by utilizing the refraction index contrast within and beyond the chain oriented region, and the substrate of polyimide (PI) could be patterned with various widths and shapes by the use of inkjet printing technology.

Modification of a Teng-Man technique to measure both r33 and r13 electro-optic coefficients

In this paper, we present a modified Teng-Man method to measure both electro-optic coefficients in a single measurement. Using our method, we confirm a linear dependence between the applied poling field and the measured electro-optic coefficients. The ratio between the two electro-optic coefficients is close to three, which is theoretically expected from a weakly oriented polymer film. Since conductive silicon is used as substrate, no auxiliary layers of transparent oxide or metal are required on the substrate, which simplifies both the sample preparation and the evaluation of the results.

Antibacterial and barrier properties of oriented polymer films with ZnO thin films applied with atomic layer deposition at low temperatures

Concerns on food safety, and need for high quality and extended shelf-life of packaged foods have promoted the development of antibacterial barrier packaging materials. Few articles have been available dealing with the barrier or antimicrobial properties of zinc oxide thin films deposited at low temperature with atomic layer deposition (ALD) onto commercial polymer films typically used for packaging purposes. The purpose of this paper was to study the properties of ZnO thin films compared to those of aluminum oxide.It was also possible to deposit ZnO thin films onto oriented polylactic acid and polypropylene films at relatively low temperatures using ozone instead of water as an oxidizing precursor for diethylzinc. Replacing water with ozone changed both the structure and the chemical composition of films deposited on silicon wafers. ZnO films deposited with ozone contained large grains covered and separated probably by a more amorphous and uniform layer. These thin films were also assumed to contain zinc salts of carboxylic acids. The barrier properties of a 25nm ZnO thin film deposited with ozone at 100°C were quite close to those obtained earlier with ALD Al2O3 of similar apparent thickness on similar polymer films. ZnO thin films deposited at low temperature indicated migration of antibacterial agent, while direct contact between ZnO and Al2O3 thin films and bacteria promoted antibacterial activity.

A real time optical strain sensor based on a cholesteric liquid crystal network

A flexible, strain sensor for monitoring uniaxial deformations in oriented polymer films is demonstrated. The sensor consists of a crosslinked cholesteric liquid crystal layer which is spray-coated on a uniaxially oriented polyamide 6 substrate. When no deformation is applied, reflectivity measurements showed a well-defined reflection band. Upon uniaxial extension, the reflection band shifted towards lower wavelengths. A 40 nm shift was recorded for 13% of strain. This wavelength shift is attributed to a decrease in the cholesteric helix's pitch which is induced by the change in thickness of the CLC coating due to Poisson's contraction. As a result the colour of the film changed from orange to green with increasing strain. The optical response was found to follow the mechanical behaviour of the polymer substrate. Cycling loading showed a time dependent optical response of the sensor fitting with the viscoplastic mechanical response of the substrate giving real time information on the deformation.

Growth of thin Al2O3 films on biaxially oriented polymer films by atomic layer deposition

The effects of thin film nucleation and initial growth on roughness, chemistry and thermomechanical properties of polymer film surfaces were studied. Al2O3 was deposited onto commercial biaxially oriented polypropylene and polylactic acid films at 80°C by using atomic layer deposition technique. Both substrates, especially the more hydrophobic polypropylene, showed initial growth through Al2O3 clusters. There was a faster deposition of Al2O3 on polylactic acid film than on polypropylene at the early stages of the Al2O3 deposition. There were also indications of chemical interactions between polylactic acid and trimethyl aluminum used as a precursor for Al2O3. Changes in the thermo-mechanical properties of the polymer surfaces with Al2O3 also evidenced the differences between the substrate polymer films. The near surface interphase formed in polylactic acid probably contributed to the strong increase and scattering in the softening temperature during the early thin film growth.

Optically Active, Amphiphilic Poly(meta-phenylene ethynylene)s: Synthesis, Hydrogen-Bonding Enforced Helix Stability, and Direct AFM Observation of Their Helical Structures

Optically active, amphiphilic poly(meta-phenylene ethynylene)s (PPEa) bearing L- or D-alanine-derived oligo(ethylene glycol) side chains connected to the backbone via amide linkages were prepared by microwave-assisted polycondensation. PPEa's exhibited an intense Cotton effect in the π-conjugated main-chain chromophore regions in various polar and nonpolar organic solvents due to a predominantly one-handed helical conformation stabilized by an intramolecular hydrogen-bonding network between the amide groups of the pendants. The stable helical structure was retained in the bulk and led to supramolecular column formation from stacked helices in oriented polymer films as evidenced by X-ray diffraction. Atomic force microscopy was used to directly visualize the helical structures of the polymers in two-dimensional crystalline layers with molecular resolution, and, for the first time, their absolute helical senses could unambiguously be determined.

Dependence of surface free energy on molecular orientation in polymer films

Surface free energy of mechanically drawn polycarbonate films was determined using contact angle measurements and shown to increase with orientation. The increase in polymer film surface free energy was attributed to increased polymer chain packing during orientation, supported by film density measurements, which provides enhanced intermolecular interactions. Surface free energy can therefore be increased by, or used to predict, polymer orientation.

Molecular Orientation in Polymer Films for Organic Solar Cells Studied by NEXAFS

The application of new polymer materials for organic solar cells requires a detailed understanding of the orientation and the electronic properties of the system because these have a dramatic effect on the device performance and efficiency. The films were prepared by doctor blade casting, a simple preparation route from solution. We study the orientation of the low band gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly-[2,6-(4,4-bis(2-ethyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-5,8-(2,3-dioctylquinoxaline)] (PCPDTQ) as well as of the state of the art polymer poly(3-hexyl-thiophene) (P3HT) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Clear angular dependence in the NEXAFS spectra is observed for P3HT and PCPDTBT pointing to well-orientated films.

Development of an identification system for biaxially oriented polymer films based on the degree of their transverse extension

An expression has been developed that generalizes three basic geometric schemes of film deformation (axial, planar, and biaxial extension) and also all intermediate schemes. The system of quantitative identification of nonuniformly biaxially oriented films according to their transverse extension has been tested on six different film types.

Novel split-tip proximal probe for fabrication of nanometer-textured, in-plane oriented polymer films

Novel fabrication schemes are required to deposit nanoscale materials that contain molecules oriented in the plane of the surface. The breakage of in-plane symmetry allows devices to be fabricated in this plane, enabling molecular electronics to follow this successful paradigm of semiconductor devices. The authors discuss here the fabrication of a unique split-tip optical nanoprobe that can be used to both orient molecules on a surface with a strong, localized electric field and deposit them with nanoscale resolution. Ultraviolet light injected through the probe into the region of aligned molecules causes the deposition. The production of the split-tip probe is significantly different than that of the related near-field scanning optical microscope (NSOM) probe, since the stresses in the metal layer must be held by the metal film–silica interface rather than within the film as it encircles the silica of a NSOM probe. Mounting of the probe to ensure reliable electrical contacts is also described.

Aggregation and excitation trapping of 3,3′-diethyl-9-methylthiacarbocyanine iodide in disordered and uniaxially oriented polymer films

Photophysical properties of 3,3′-diethyl-9-methylthiacarbocyanine iodide (MDTCI) in disordered and uniaxially stretched poly(vinyl alcohol) films (PVA) are reported at high dye concentration. Ground-state formation of fluorescent MDTCI aggregates at high concentration was observed in unstretched and stretched PVA films using steady-state and time-resolved spectroscopic methods. It was found that fluorescence of MDTCI aggregates is almost totally depolarized in disordered matrix, but it remains partly polarized in the stretched PVA film despite energy transfer from monomers. The differences between spectral properties of aggregates in both matrices may result from partial orientation of aggregate transition moment and its immobilization in polymer cage.

Modeling the mechanical properties of highly oriented polymer films: A fiber/gel composite theory approach

AbstractControlling the extent of orientation is of great interest in polymer processing and is effected by the choice of polymer, the fabrication technique and the processing conditions. Understanding the crystalline transitions that form highly oriented fibrils is necessary for modeling the changes in physical properties, relative to degree of orientation. A model is proposed to describe the mechanical properties of drawn semicrystalline polymer films based on structural transitions. With a minimal amount of experimental data (requiring testing on only two drawn films samples), this model can be used to predict film properties. These properties include the critical and maximum draw ratios, the moduli at the maximum draw ratio, the moduli of the fiber, the modulus of the nonfibrous gel relative to draw ratio, the volume fraction of fibers, and the rate of fibrillation. Where high degrees of uniaxial orientation are required, the polymer is typically drawn in the solid state, meaning the polymer is stretched in a single direction at temperatures below the melting point. During this process, pre‐existing crystallites are transformed into fiber‐like structures with large aspect ratios. The presence of these rigid asymmetric structures significantly enhances the moduli and break strength of the polymer. This work presents a model that describes the formation of fiber‐like structures. The volume fraction of fibers is predicted to be linear in draw ratio. The derived relationship between volume fraction of fibers and draw ratio can then be used for the prediction of the various properties of the oriented film. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 607–618, 2008

Orientation of triplet and singlet transition dipole moments in polyfluorene, studied by polarised spectroscopies

The orientation of the transition dipole moments for four commonly studied optical transitions of the conjugated polymer polyfluorene have been deduced using polarised spectroscopies, including fluorescence, phosphorescence, and photoinduced absorption in oriented polymer films. The data show that all the transitions except from the phosphorescence are highly polarised parallel to the chain orientation. The phosphorescence in this polyfluorene homopolymer however, has its dominant component orthogonal to the chain orientation. The data are interpreted by regarding shape and electron distribution of the excited states. It has been deduced that the singlet states and higher triplet states are strongly delocalised along the chain whereas the T 1 excited state is more localised and has its electron distribution oriented orthogonal to the chain.

All‐in‐one Layer: Anisotropic Emission due to Light‐Induced Orientation of a Multifunctional Polymer

AbstractAn efficient procedure to create oriented polymer films with strongly anisotropic properties is described. It is based on a two‐step process combining a photochemical and a thermal aligning step. The orientation of the polymer parallel or perpendicular to linearly polarised incident light can be adjusted. The method presented allows for simple fabrication of films with anisotropic absorption and emission characteristics with a dichroism of 0.7 and a fluorescence anisotropy of about 8.magnified image

Multistep energy migration between 3,3′-diethyl-9-methylthiacarbocyanine iodide monomers in uniaxially oriented polymer films

Multistep energy migration was studied for 3,3′-diethyl-9-methylthiacarbocyanine iodide (MDTCI) in uniaxially stretched and unstretched poly(vinyl alcohol) films. At low and intermediate concentrations fluorescence anisotropy decay measurements yield completely different results for disordered and ordered systems due to strong redistribution and angular correlation of fluorophores transition moments in ordered polymer matrix. These results and other selected properties of energy transport in uniaxially stretched polymer films were analyzed using the technique of Monte–Carlo simulation.

Energy transfer between unlike fluorophores in uniaxially oriented polymer films monitored by time-resolved and steady-state emission anisotropy

Excitation energy transfer is studied for 3,3′-diethylthiacyanine iodide (DTTHCI, donor) and rhodamine 6G (R6G, acceptor) (system I) and 3,3′-diethyloxacarbocyanine iodide (DTCI, acceptor) (system II) in uniaxially stretched and unstretched polyvinyl alcohol films (PVA). Both systems differ strongly in the orientation degree of acceptor transition moments in the stretched polymeric matrix. As a result of weak orientation of R6G in PVA stretched matrix, strong acceptor depolarization of fluorescence upon donor excitation was observed for system I despite ordering the PVA matrix. However, for system II the transition moments of both DTTHCI and DTCI are correlated and orientate strongly along the direction of stretching resulting in the preservation of acceptor emission anisotropy upon steady-state and pulse excitation.

Mechanism of polarization and piezoelectric behavior in crystallizable ferroelectric polymers from the standpoint of propagation of soliton waves

Published experimental data concerning the process of poling and the piezoelectric response in poly(vinylidene fluoride)-based ferroelectric polymers are discussed in terms of the soliton mechanism of propagation of excitation energy throughout the polymer. For the poling processes, the earlier proposed model of the twist conformational defect, which can travel with a subsonic velocity in a crystal, is considered. To have increased values of remanent polarization in uniaxially drawn ferroelectric polymers, a structure that satisfies certain conditions should be formed. The thickness of amorphous interphases between the faces of microfibrillar crystals must be as low as possible. It is desirable to have a microstructure of interlamellar regions with a high packing density due to an increased amount of all-trans tie chains. Such topology and microstructure of interlamellar phases ensures low scattering of dynamic conformational defects described by soliton waves. Experimental data show that the generation of a conformational defect in a crystal (emergence of a soliton wave) is limited by the mobility in the interfibrillar amorphous phase in tie chains close to crystals. Higher values of remanent polarization are observed in textured films, in which the aforementioned micro-Brownian dynamics is characterized by reduced values of activation parameters. Such conditions, for example, are created when isotropic films with certain initial morphology are textured by uniaxial cold drawing. Meeting the above requirements for the structure of an oriented film also creates conditions for field-induced postcrystallization, which also facilitates the formation of high remanent polarization. In the case of the lateral piezoelectric effect, the role of the dynamic behavior of vacancies that are formed in a polymer crystal by elastic loading of an oriented polymer film along the draw axis is considered. In such a model, the piezoelectric effect must be the strongest for structures that provide the lowest damping of soliton waves that describe the motion of vacancies. It was found that these favorable structures could also be produced by means of cold drawing used to form the above morphology.

Control of Crystal Size and Orientation in Polymer Films by Host−Guest Interactions

The unprecedented occurrence of two different kinds of uniplanar orientation by solution casting, observed for syndiotactic polystyrene (s-PS) films, has been rationalized in terms of host−guest interactions into molecular complex phases. Correlation lengths along different crystallographic directions (Dhkl), of molecular complex crystalline domains of s-PS with several different guest molecules, have been investigated by wide-angle X-ray diffraction. The chemical nature of the guest has a strong influence on the correlation length perpendicular to the ac plane (i.e., perpendicular to the layers of close-packed enantiomorphous helices) and a weak influence on the correlation lengths along a and c (chain) axes. Less and more favorable host−guest interactions lead to molecular complex domains with low and high degree of order perpendicular to the ac layers (D010 < 6 nm and D010 > 7 nm) and generate cast films with uniplanar (010) and (002) orientations, respectively.

Scratch resistance anisotropy in biaxially oriented polypropylene and poly(ethylene terephthalate) films

Using a diamond-tipped stylus, scratch tests were conducted on biaxially oriented polypropylene and poly(ethylene terephthalate) films in the two draw directions, i.e., the machine-direction (MD) and the transverse-direction (TD) along which the draw ratios are different. Atomic force microscopy study of those scratches revealed a significant anisotropy in the scratch resistance between the MD and TD for both of the polymer films. We confirmed that the scratch resistance of polymer strands is closely related to the draw ratios, which determine the mechanical strength and optical clarity of biaxially oriented polymer films.

Laser writing on oriented polymer films

Polymer films are suitable substrates for optical data storage due to their mechanical and optical properties. Furthermore, oriented polymer films show an additional effect to laser writing. In this review the effects of laser writing on oriented polymer films are investigated by atomic force microscopy. A change in surface topography of the film is observed. Different polymer types and different draw ratios are used. The dependence of topography effects on the draw ratio is shown. A model of the writing effect is framed.