Thin polymer film experiments

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

The redox electrochemistry of thin polymer films is a particularly useful field of application for the quartz microbalance. As an example, we review experiments on poly(xylylviologen) films [15]. The viologen groups can be reversibly reduced in two discrete one-electron steps. 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, the overall esterification, as well as the polycondensation rate, is controlled by mass transport. Limitations of the latter arise mainly from the low solubility of TPA in EG, the diffusion of EG and water in the molten polymer and the mass transfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonstrated in experiments with thin polymer films [10]. 

The situation considered here should be applicable to experiments on spinodal decomposition in sufficiently thin polymer films with small periodic temperature modulations created by means of an optical grating technique or by local laser... 

Figure 9 Absorption spectrum (a) and electrochromism (b) of [Ru(bpy)3] dissolved in a thin polymer film at 77 K. Absorption and electrochromism spectra are taken through semitransparent conducting electrodes. Electrochromism is usually measured with the externally applied electric field parallel to the propagation direction koflight( = 0°). Experiments can also be made with the film tilted away from the normal to the propagation direction (Drawn from data in Oh and Boxer )...

Fundamental questions related to the electronic configuration of the open or colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and time-resolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans-trans-cis) isomer having a dominant quinoidal electronic configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-gel matrices and ultimately to control of degradation. 

Since the sublimation or vaporization rate of most monomers under vacuum is not insignificant, a thin polymer film was coated on top of the resist as a monomer sublimation barrier. Two water soluble polymers, poly(vinyl alcohol) (PVA) and poly(vinyl pyrroidone) (PVP), were used. It has been found that PVA solution yields more uniform films, and after e-beam exposure, can be removed easily by dipping in water for 30 seconds. There was no evidence of crosslinking in PVA after e-beam exposure. However, PVP acts as a negative resist with sensitivity (Dg ) approximately equal to 60yC cm-. PVA, therefore, was used as the monomer barrier for most experiments. 

Development of the general plasma polymerization model is restricted by an essentially non-linear contribution of different plasma components in the process. Plasma particles are able to simultaneously build and destroy thin polymer films. Nevertheless, some models are able to describe a wide variety of experiments. The plasma polymerization model developed by Osipov and Folmanis (1973) is one of those. According to this model, charged particles activate adsorbed molecules, allowing then to cross-link with the polymer film. The rate of polymer film deposition from a non-equihbrium low-pressure discharge (number of deposited molecules per unit time) can be calculated in this case as... 

In trying to answer these questions it is certainly instructive to examine previous fundamental experimental works in which such thin films have been employed. In particular, we have to mention studies on polymer interdiffusion in which thin polymer films were used for testing theoretical concepts of polymer physics [102-106]. Spin-coating was typically used to prepare the films because this process presents an easy way of obtaining smooth film of precisely controllable thickness, even in the nanometer range. This is an essential criterion for investigating, for example, polymer diffusion across an interface between two films. While these experiments successfully supported the model of chain reptation [102-106], they also indicated some deviations that hinted, for example, at the enrichment of chain ends at surfaces [104, 106]. 

We will present a few examples, which demonstrate that the balance of forces enables us to extract in a highly quantitative manner detailed information on molecular and interfacial properties of thin polymer films from relatively simple dewetting experiments. 

Now interfacial widths can be measured by depth-profiling techniques which eg use a nuclear reaction analysis to obtain the concentration profile through a thin polymer film (108,109) or by total refiection of neutrons (110) or X-rays. However, it should be noted that neither in an experiment nor in a simulation can one ever observe the intrinsic width of an interface, which is an absolutely hypothetical object (106,111). Rather the observed width always results from a convolution of the intrinsic profile with the capillary wave broadening (106,111,112). On a lateral length scale L, one obtains for the actual mean square width... 

Figure 1-3 shows a schematic diagram of a dynamic IR linear dichroism (DIRLD) experiment [20-25] which provided the foundation for the 2D IR analysis of polymers. In DIRLD spectroscopy, a small-amplitude oscillatory strain (ca. 0.1% of the sample dimension) with an acoustic-range frequency is applied to a thin polymer film. The submolecular-level response of individual chemical constituents induced by the applied dynamic strain is then monitored by using a polarized IR probe as a function of deformation frequency and other variables such as temperature. The macroscopic stress response of the system may also be measured simultaneously. In short, a DIRLD experiment may be regarded as a combination of two well-established characterization techniques already used extensively for polymers dynamic mechanical analysis (DMA) [26, 27] and infrared dichroism (IRD) spectroscopy [10, 11]. 

Dewetting experiments by Reiter [59-61] and subsequently by many others [62-66, 68-75,92-128,137-164] revealed the morphological evolution, self-organization, and pattern formation in thin polymer films. In most experiments the onset of instability is with the form of an undulation of the film surface (Figure 11.1a) [62] or the appearance of fairly equally sized but random collection of holes (Figure 11.1b) [59,60]. The mean... 

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