Properties Of Thin Polymer Films Research Articles

Effect of deposition temperature on the properties of thin polymer films deposited by plasma‐enhanced CVD using cyclo‐hexa‐hydrocarbons as monomers

AbstractPlasma CVD is a candidate technology for the fabrication of optical polymer waveguides. It can deposit a film on any surface geometry and any substrate material at a temperature under 200 °C in a vacuum process. It also provides good thickness controllability and uniformity of the deposition film. In the present study, the effects of the deposition temperature on film properties, specifically the refractive index, deposition rate, and molecular structure, are discussed. The refractive index decreases as the deposition temperature rises. The logarithm of the deposition rate increases with the reciprocal of the temperature and the gradient of the deposition rate depends on the relative abundance of double bonds in the monomer source. The gradient does not change when CF4 is used instead of Ar as the gas mixed into the plasma, although the deposition rate increases by a factor of about five. We speculate that the deposition rate increases due to the increase in the abundance precursors produced by the presence of CF4 in the plasma and due to an increased abundance of dangling bonds on the surface of the deposition film caused by F radicals. We further speculate that the precursors incorporated into the polymer are selected on the substrate by the density of adsorption sites and the adsorption energy. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(4): 27–35, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10171

Multiscale modeling of polymers at interfaces

Abstract A brief review of modeling and simulation methods for a study of polymers at interfaces is provided. When studying truly multiscale problems as provided by realistic polymer systems, coarse graining is practically unavoidable. In this process, degrees of freedom on smaller scales are eliminated in favor of a model suitable for efficient study of the system behavior on larger length and time scales. We emphasize the need to distinguish between dynamic and static properties regarding the model validation. A model which accurately reproduces static properties may fail completely when it comes to the dynamic behavior of the system. Furthermore, we comment on the use of the Monte Carlo method in polymer science as compared to molecular dynamics simulations. Using the latter approach, we also discuss results of recent computer simulations on the properties of polymers close to solid substrates. This includes both generic features (as also observed in the case of simpler molecular models) as well as polymer specific properties. The predictive power of computer simulations is highlighted by providing experimental evidence for these observations. Some important implications of these results for an understanding of mechanical properties of thin polymer films and coatings are also worked out.

Surface Mechanical Properties of Thin Polymer Films Investigated by AFM in Pulsed Force Mode

Atomic force microscopy in the pulsed force mode (PFM) is applied in this work to the study of thin dewetting patterns formed by drying an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM) and sodium dodecyl sulfate (SDS) on mica. This technique allows the automated acquisition of typically 4 x 10(6) force-distance curves on the sample surface together with maps showing nanodomains differentiated by their stiffness and adhesion to the tip. Topography images of dry films revealed a morphology formed by droplets distributed on the substrate. Adhesion and stiffness images with good lateral resolution show droplets containing polymer and surfactant contrasting with the substrate and also nanosized heterogeneities inside these droplets. They also revealed very small dewetted structures which could not be observed in the topography map by noncontact AFM. Adhesion interactions between the AFM tip and the polymer or the dewetted mica substrate were measured in terms of adhesion force and detachment energy, and can be used as new information to understand dewetting patterns containing silica particles, PNIPAM, and SDS. Other surface mechanical parameters such as stiffness, maximum indentation, hardness, compliance, hysteresis, and Young's modulus were obtained by sampling many points and used to characterize the PNIPAM/SDS films formed in the dewetting process.

Friction and adhesion properties of fluorocarbon polymer thin films prepared by magnetron sputtering

Fluorocarbon polymer thin films were deposited onto a SUS302 substrate with a poly(tetrafluoroethylene) (PTFE) target by three different types of r.f. magnetron sputtering systems with strong, weak and unbalanced magnetic fields. Friction and adhesion properties of these polymer thin films were evaluated. Friction coefficient of polymer thin films prepared with strong magnetic field, unbalanced magnetron and without magnetron (r.f. sputtering) was almost the same level, however, that prepared with the weak magnetic field was slightly lower than those of other thin films. Wear durability of polymer thin film increased with increase of the magnetic field. Adhesion strength between these thin films and SUS302 substrate and shear stress were measured by SAICAS. Both of the adhesion strength and shear stress of polymer thin films prepared with r.f. sputtering (without magnetron) were slightly higher than those prepared by magnetron sputtering systems.

Measuring the Micro-Polarity and Hydrogen-Bond Donor/Acceptor Ability of Thermoresponsive N-Isopropylacrylamide/N-tert-Butylacrylamide Copolymer Films Using Solvatochromic Indicators

Thin polymer films are important in many areas of biomaterials research, biomedical devices, and biological sensors. The accurate in situ measurement of multiple physicochemical properties of thin polymer films is critical in understanding biocompatibility, polymer function, and performance. In this work we demonstrate a facile spectroscopic methodology for accurately measuring the micro-polarity and hydrogen-bond donor/acceptor ability for a series of relatively hydrophilic thermoresponsive copolymers. The micro-polarity of the N-isopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) co-polymers was evaluated by means of the E(T)(30), alpha, beta, and pi empirical solvatochromic polarity parameters. The data shows that increasing the NtBA fraction in the dry copolymer film reduces polarity and hydrogen-bonding ability. Within the Kamlet-Taft polarity framework, the NIPAM/NtBA copolymer films are strong hydrogen-bond acceptors, strongly dipolar/polarizable, and rather moderate hydrogen-bond donors. This characterization provides a more comprehensive physicochemical description of polymers, which aids the interpretation of film performance. Comparison of the measured E(T)(30) values with literature data for other water-soluble polymers show that dry NIPAM/NtBA copolymers are slightly more polar than poly(ethylene oxide), less polar than polyvinylalcohol, and approximately the same polarity as poly(N-vinyl-2-pyrrolidone). These findings indicate that this spectroscopic method is a facile, rapid, and nondestructive methodology for measuring polymer properties in situ, suitable for most biomaterials research laboratories.

Tuning of surface properties of thin polymer films by electron beam treatment

The aim of our work was to understand the impact of electron treatment on polymer thin films, particularly on their surface properties as well as the possibilities and limitations to tune these properties. Two different polymers, polystyrene (PS) and poly-2-vinlypyridine ( P2VP), were chosen to form thin polymer films by grafting of end-terminated linear polymer chains to a surface with sufficient grafting density, forming so called “polymer brushes”. We were able to identify the surface properties and specify ongoing physico-chemical changes after electron beam treatment by using zeta potential and contact angle measurements. By varying the absorbed dose it was possible to tune the surface properties over a wide range. The detailed knowledge about the latitude of functionalization of the tested polymers was a prerequisite for the creation of wettability gradients by electron beam treatment by adapting a special mask of known thickness and density. Hence, electron beam treatment opens an easy reproducible way to generate surface gradients in functionality.

Dewetting as an investigative tool for studying properties of thin polymer films

Employing mass conservation, time-resolved dewetting experiments of thin polymer films allow to determine in real time the dynamic contact angle and the slippage length. Moreover, based on a systematic variation of interfacial properties of a polymer brush, dewetting makes it possible to calculate the force it needs to extract a single polymer chain from its own melt. In the visco-elastic regime close to the glass transition, the temperature and molecular weight dependence of the relaxation time of residual stresses resulting from film preparation by spin-coating can be obtained from the evolution of the shape of the dewetting rim. The presented examples demonstrate that dewetting represents a powerful approach for a sensitive characterization of rheological, frictional and interfacial properties of thin polymer films.

Some issues in polymer nanocomposites: Theoretical and modeling opportunities for polymer physics

Some issues in polymer nanocomposites: Theoretical and modeling opportunities for polymer physics

Absorption cross-section control of Eu(III) complexes for increase of amplified spontaneous emission excited by third harmonic of nanosecond Nd:YAG laser

The amplified spontaneous emission (ASE) properties of polymer thin-films containing Eu(III) complexes were demonstrated by observating emissions from the edge of excited polymer thin-films. The amplification of the ASE components of Eu(III) complexes was increased in proportion to the absorption cross-sections. In particular, the amplitude percentages of the ASE component of Eu(III) complexes having naphthyl groups (absorption cross-section at 355 nm: 850 × 10 −19 cm 2) were estimated to be over 60% under 10 mJ excitation.

Role of Diblock Copolymers toward Controlling the Glass Transition of Thin Polymer Films

The physical properties of thin polymer films are often thickness, h, dependent, influenced by confinement and by interfacial interactions between the chains and the external interfaces. We show that the magnitude and film thickness dependence of the average glass transition temperature, Tg, of the polystyrene−silicon oxide (PS/SiOx/Si) system are influenced appreciably with the addition of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. The Tg can be “tailored” to increase, or decrease, with decreasing h or to remain independent of h. Tg shifts of as much of 35 °C are obtained for films of h ≈ 20 nm. Additionally, we report that the critical micelle concentration, φcmc, of the copolymer in thin films is considerably larger than for the bulk; specifically, micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains in the system. The h dependence of Tg is not influenced by the ϕcmc or by the number of micelles in this system.

Models for transport properties of polymer thin films

AbstractWe have observed that thin films of different undoped non‐conjugated polymers exhibit high conductivity in metal–polymer–metal (M‐P‐M) structures. Below the critical temperature, Tc of the superconductivity of the contacts, a supercurrent in superconductor–polymer–superconductor (S‐P‐S) structures was obtained. Investigations of the current‐voltage characteristics as a function of contact diameter at different locations on the polymer showed that only in some small channels does a high conductivity appear, whereas at other places a reversible on/off switching was obtained. We postulate that the conductivity effect is connected with the phenomenon of electrification. The electrons or holes are supplied by the electrodes which are in contact with the polymer. These injected charge carriers accumulate in localised states on different locations in the polymer matrix. As a result, a large inner electrical field appears in the polymer giving rise to local changes between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the high field region the forbidden gap can vanish, resulting in small channels with a high “metallic‐like” conductivity of the polymer. At present, there exists no theory explaining all the peculiarities of the electrical transport observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dimensionality of electronic excitations in organic semiconductors: A dielectric function approach

We present a detailed investigation on the effective dimensionality (associated with the degree of delocalization) of electronic excitations in thin organic films using the dielectric function as obtained from ellipsometry. To this end, we study first the best analytical representation of the optical dielectric function of these materials and compare different approaches found in the literature: (i) the harmonic oscillator approximation, (ii) the standard critical-point model (SCP), (iii) the model dielectric function (MDF), and (iv) the Forouhi-Bloomer model. We use these models to analyze variable angle spectroscopic ellipsometry raw data for a thin poly(9,9-dioctylfluorene) (PFO) film deposited on quartz (taken as an archetypal sample). The superiority of the SCP model for PFO films and a wide range of other spin-coated conjugated polymers (and guest-molecules in polymers) is demonstrated. Moreover, we show how the SCP model can be used to gain physical information on the microscopic structure. As an example, we show that the delocalization of excitons decreases for nonconjugated polymers, such as polymethylmethacrylate and polyimide, while the conjugation length and exciton delocalization are, respectively, enhanced in cases where a planar conformation (e.g., $\ensuremath{\beta}$ phase of PFO) or a high degree of crystallinity [e.g., poly(3-hexylthiophene)] is achieved. As an additional example, we employ the SCP excitonic model to investigate the temperature dependence of the dielectric function of crystalline and glassy PFO films. We propose that the SCP excitonic model should be adopted as the standard choice to model the optical properties of polymer thin films from ellipsometry data.

Measurement of radiative thermal properties of thin polymer films by FTIR

Although a number of test methods have been reported for the characterization of radiative thermal properties of semi-transparent materials and textile fabrics, the need for specialized equipment limited the application of these test methods. This paper reports on the use of a conventional instrument, viz. Fourier transform infrared (FTIR) spectroscopy, for the characterization of radiative thermal properties of thin polymer films for application within an intermediate temperature range. Experimental measurements on spectral transmission with FTIR were performed on four types of thin films with high mass fraction, viz. one made of polyethylene (PE), two made of polyamide (NB and NW) and one made of aluminium foil (AF). From the measurements, extinction coefficients were calculated by using Beer's Law, and then the Rosseland mean extinction coefficients and radiative thermal conductivities were determined. The results showed that the thin AF film was the best at blocking radiation, whereas the thin PE film was the weakest. The two polyamide films (NB and NW), made of the same polymer but different in colour, had similar intermediate capability in blocking radiation, indicating colour has little effect on radiative thermal properties of polymeric materials.

Nanomechanical properties of polymer thin films measured by force–distance curves

Force–displacement curves have been acquired with a commercial atomic force microscope on thin films of poly( n-butyl methacrylate) on glass substrates. Different film thicknesses, from 10 up to 430 nm, were chosen to examine in detail the so called “mechanical double-layer” topic, i.e., the influence of the substrate on the determination of the mechanical properties of thin films. Taking advantage of the Hertz theory we calculated for all films the contact radius between tip and sample as a function of the applied load. Further Young's modulus of the samples was derived from the experimental data as a function of the applied load and, alternatively, of the deformation. The results of this analysis for 10 different film thicknesses were fitted with several half empirical equations proposed by several researchers. The focus of this work is to evaluate such existing half empirical theories for mechanical double-layers and to show the need for an alternative consistent approach.

Fabrication and nonlinear optical properties of polymeric thin films doped with a novel tricyanofuran chromophore

A novel second-order nonlinear optical chromophore (DCDHF-2-V) containing a tricyanofuran acceptor, was synthesized by a two-step reaction and characterized by elemental analysis, 1H NMR, FT-IR and thermal analysis, respectively. Spin-coated films of poly methyl methacrylate (PMMA) doped with DCDHF-2-V were fabricated and their poling behaviors were studied by UV–visible absorption spectra and atom force microscopy (AFM). The second-order nonlinear coefficient d 33 was taken from the second harmonic generation (SHG) experimental setup and thereby the calculated value was 15.2 pm/V at the wavelength of 1064 nm. The relation between the second-order nonlinear coefficients d 33 and d 13 for the poled polymer film was also discussed in detail.

Organic‐Inorganic Nanocomposites: Optical and Electrophysical Properties

A short review on achievements of conducting and luminescent nanocomposites as well as spectroscopic and electrophysical properties of thin polymer films containing metal and semiconductor nanoparticles and the data of studies of thin films and ITO/nanocomposite/Al or metal/nanocomposite/metal structures mainly based on poly‐N‐epoxypropylcarbazole (PEPK) with Ag or Ag/Au and CdS nanoparticles are presented. It is shown that these nanocomposites are very promising materials for creation of printed electronics circuitry and scintillation screen of field emission displays.

Robustness of wetting and morphology of maleimide copolymer films to choice of solvent and annealing

We report the surface properties of thin polymer films prepared by spin coating from different solvents and subsequent annealing. Surface morphology is studied using optical and scanning force microscopy (SFM). Low-rate dynamic advancing contact angles of water are measured by axisymmetric drop shape analysis (ADSA). Optical, SFM and contact angle measurements give complementary information on the surface properties. We find that annealing at moderate temperatures and choice of suitable solvent have little or no effect on surface morphology and wettability of poly(propene- alt- N-( n-dodecyl)maleimide) films. Thus, contact angle measurements reflect material properties of these surfaces, and are unchanged by normal environmental effects.

Morphology and photophysical properties of polymer thin films dispersed with dye nanoparticle

We prepared nanoparticles of an organic dye, acridine orange (AO), dispersed in poly(methacrylic acid) (PMA) films by spin-coating the solution of the two components. The surface of the AO/PMA films became bumpier with increasing AO concentration ( c AO). The absorption and fluorescence spectra of AO/PMA films exhibited a marked dependence on c AO at low c AO ( c AO < 2.1 × 10 − 1 mol kg − 1 ), and were independent of c AO at high c AO ( c AO > 2.1 × 10 − 1 mol kg − 1 ). The peak shift of fluorescence spectra with changing c AO was as large as ∼ 100 nm.

Mechanical properties of polymer thin film measured by the bulge test

The bulge test was used to measure the mechanical properties of polymer thin films with thickness in the range of 77 nm to 352 nm. The mechanical properties of polymeric thin films were extracted by comparing differences between curves of load vs. bulge height obtained from composite film configurations with and without the polymer layer. Both square and long rectangular windows were used to obtain the Poisson ratio and Young's Modulus. Composite film with 230 nm silicon nitride layer and 30 nm Al layer has a composite Poisson ratio of 0.29 and a Young's Modulus of 234 ± 0.8 GPa. The Poisson ratio extracted for a 352 nm Poly(methyl methacrylate)-based thermoplastic polymeric thin film was 0.39. The Young's Modulus extracted for the 77 nm thick polymeric film is 4.9 ± 0.8 GPa and for the 352 nm thick film is 5.8 ± 0.2 GPa. In the thickness range investigated, no clear thickness dependence of the Young's modulus was observed using the Bulge test.

Reversibly cross-linked surface-grafted polymer brushes.

Reversibly cross-linked surface-grafted polymer brushes.