Untersuchungen zur Polymerdynamik an laserdeponierten Poly(alkyl methacrylat)-Filmen

Abstract

Pulsed Laser Deposition is an extremely versatile method for preparation of thin films of a wide range of material classes. The associated ablation mechanisms are very complex - especially for organic materials - and strongly depend on the applied deposition parameters which lead to a variety of different film properties. In the past these properties have been investigated in detail for the model system poly(methyl methacrylate) (PMMA).In this contribution the set of investigated laser deposited polymers is expanded by further homologues of the poly(alkyl methacrylate)s (PAMAs). Using different analysis techniques as fouriertransformation infrared spectroscopy, size exclusion chromatography, scanning electron microscopy, and x-ray scattering the universal character of the ablation and deposition mechanisms for these materials could be demonstrated. At a laser fluence of some J/cm2 photochemical processes lead to an initial modification of the target material (incubation) before the modified material is ablated.From the point of view of polymer dynamics the studied PAMAs are an ideal model system for the investigation of microscopic relaxation processes. The gradual elongation of the alkyl side group from PMMA to poly(butyl methacrylate) implies an internal plastification leading to an increased mobility of chain segments and thus to a successive decrease of the glass transition temperature. The underlying mechanisms can suitably be explored applying spectroscopic techniques. Here mechanical spectroscopy is in the center of interest. With the Plasma Plume eXcited Reed (PPXR) a flexible method to obtain the temperature dependent complex elastic modulus of thin films at fixed resonance frequency is employed. Introducing a new experimental setup using the Double Paddle Oscillator (DPO) as substrate a variety of modes of vibration is available. This allows the in-situ determination of the temperature and frequency dependent complex shear and elastic moduli of the thin films with high accuracy.The as-prepared polymer thin films are very sensible on thermal treatment. An irreversible increase of both loss- and storage modulus is obtained upon annealing the films to about 35 K above their glass transition temperature. A cross linking of macromolecules, which is benefited by small fragments with radical character that are embedded in the film material due to the deposition process, as well as a structural relaxation of the polymer can be made responsible for this thermally induced relaxation. Relaxed samples that were annealed well above their glass transition temperature exhibit a bulk-like simple glass transition identified as α-relaxation. The influence of the thermally induced relaxation is confirmed by dielectric spectroscopy measurements. During the very first annealing the films show irreversible changes in dielectric properties, while in a relaxed state they show bulk-like behavior. The corresponding dielectric loss maxima can explicitly be assigned to a rotation of the alkyl side groups (β-relaxation).