engleski

Influence of Polymerization Conditions on the Stable Structure of APST

Aminopropylsilanetriol (APST) NH2CH2CH2CH2Si(OH)3 belongs to a new family of organosilane compounds with increasing industrial application due to the simple and good contacting properties of this substance with various optical fibers, semiconducting substrates and nanoparticles ; all of different chemical composition [1]. The polymerization process of this coupling agent is simple for use but far from being completely understood. In spite of many efforts over the last decade the diversity of behavior is not satisfactorily explained even for pure APST polymer as well as for APST-metal, APST-glass and APST-plastic contacts. Until now trans and gauche conformers, ladder, cube and cage structures (Fig.1) were offered and studied in order to improve a knowledge of this polymerization process [2]. Most authors agree only in the claim that mechanical properties of pure matter as well the contacts with other materials are significantly affected by temperature/humidity conditions of the environment. Our previous work was more focused on light, temperature and substrate dependence of the polymerization process [3, 4]. Samples were prepared from commercial 25 % water solution of APST (ABCR Company) under different conditions and on various substrates. The best samples were obtained by slow water evaporation at room temperature in the dark, on the polyethylene substrate. Under these conditions quite sturdy, thin, clear plates (thickness about 1 mm) of excellent transparency were obtained. It was established that polymerization process can be accelerated by exposing the samples to daily or even to ultraviolet light or/and by improved water evaporation. However, in all of these cases deteriorated mechanical properties were found sometimes together with yellowish coloring of the sample. Also, using light microscopy many different, complicated patterns were observed on the surface or even inside of the bulk, but their successful correlation with conditions of the polymerization process was not found. Main methods in our previous studies were Raman spectroscopy and vibrational bands assignment based on ab initio force field calculation. Owing to that the observed spectra of various samples were assigned and conclusions on present structures were derived. E.g. for the best samples (optical and mechanical properties) only the bands corresponding to trans conformer were detected, and the Si-O-Si stretching band was observed at 1145 cm-1 with the highest intensity, strongly corroborating ladder structure hypothesis. We have studied very carefully low-frequency Raman spectrum dependence on temperature because of Boson peak observation. The observed spectra were successfully fitted according to the model of Martin and Brening, supporting the idea of medium range order in disordered polyAPST. However, some polarization properties would suggest ladder-like layered structure oriented parallel to the plane of growth. We emphasize that presented fitting procedure does not exclude the possibility of the nanosize particles formed arround hypothetical cube or cage structures. In order to improve the understanding of observed structural/mechanical instabilities we paid here the particular attention to the high frequency part of the Raman spectrum of APST that was solidified as a bulk matter in a PE cube. Raman difference spectrum obtained for the same sample before and after the drying at 60 0C for 60 h exhibits very complicated spectral feature (Fig.2). Part of that can be understood easily as a result of the slight change of O-H and N-H bond abundance. However, there are some weak, broad features in the 3040– 3280 and 3450– 3700 cm-1 interval that we interpret as a change of water content. To our opinion the lower frequency interval corresponds to bulk water, the upper interval to free monomer and dimer water. This time a more reliable confirmation was found using scanning electron microscopy in spite of its weakness against water containing samples. Examined samples were prepared by evaporation at 60, 22 and 8 0C, than evacuated for 2 days and finally left in the atmosphere for next 4 months. Direct application of SEM to the intact samples was not possible since they belong to the category of insulators. Due to that and because of strong possibility of the water evaporation inside of the microscope chamber we deposed rather thick Au layer using standard magnetron sputtering technique. Again, for different samples we observed various surface patterns, most of them were static (Fig.3). For the sample prepared at 60 0C some growing and breathing blobs were observed, but not from the very beginning. This observation, together with simple but precious weight measurements, corroborated the previous indication of the important role of water molecule in the stabilization of APST structure that was firstly derived from Raman difference spectra.

Aminopropylsilanetriol (APST); Scaning electron microscopy; Raman spectroscopy

nije evidentirano