A study of polymer tertiary structure in Langmuir monolayer films of poly(dimethyl siloxane) by time-of-flight secondary ion mass spectrometry, reflection-absorption Fourier transform infrared spectroscopy and high-resolution electron energy loss spectroscopy
Piwowar, Alan M.
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The primary focus of this thesis is development of vibrational spectroscopy and the use of time of flight secondary ion mass spectrometry (ToF-SIMS) to identify surface tertiary structures of macromolecules. For this approach a series of model polymer systems of poly (dimethyl siloxane) (PDMS) were made using the Langmuir-Blodgett (LB) technique of film compression. This written work is divided into three parts: the first part examines the tertiary conformations of PDMS in various regions of a surface pressure-area isotherm using reflection-absorption Fourier transform infrared spectroscopy (RAFTIR), part two explores the fragmentation mechanism and ion formation probabilities for structurally different films of PDMS using ToF-SIMS and the third part utilizes high resolution electron energy loss spectroscopy (HREELS) to investigate (i) the tertiary structure of random and ordered films of PDMS and (ii) the quantitative ability of the technique. Reflection-absorption Fourier Transform Infrared Spectroscopy (RAFTIR) has been utilized to investigate the tertiary structures of a series of Poly (dimethyl siloxane) (PDMS) films transferred to silver substrates using the Langmuir-Blodgett technique. A surface pressure-area isotherm of PDMS of molecular weight of 2400 Da with low polydispersity (PDI of 1.09) was measured on a water subphase at 20°C. Films were transferred onto silver substrates at various surface pressures associated with regions on the isotherm where structural changes are indicated. The infrared data from the films indicated the structural changes identified by analysis of the isotherm could be related to polymeric backbone orientational adjustments. This was evidenced by relative intensity changes of the asymmetric and symmetric Si-O-Si stretches located at 1050 cm -1 and 1110 cm -1 respectively. Infrared data from the films also showed increases in absorption consistent with varying film thickness. Ratios of the peak intensity of 1265 cm -1 for the various films were compared with theoretical and experimentally observed film thicknesses reported by Fox, Taylor and Zisman. There was a positive correlation between the peak intensity ratios and structural thickness ratios reported by these authors which supports the proposed structures of caterpillar, zig-zag and helical coil. Furthermore we report new infrared data supporting the model of structural change in the isotherm proposed by Lenk and Koberstein based on the formation of helical coils with varying number of monomeric units per turn which form based on the surface pressure. A series of model systems of Poly (dimethyl siloxane) (PDMS) of molecular weight 2400 Da and low polydispersity (1.09) were prepared using the Langmuir-Blodgett technique to investigate the effects of tertiary structure on the ion formation probability in time of flight secondary ion mass spectrometry (TOF-SIMS). Using data from the measured surface pressure-area isotherm for PDMS ordered monolayer films were transferred to silver mirror substrates from the various regions in the isotherm where structural changes are observed. Two particular conformations of the polymer are examined here: a linear caterpillar-like structure and a coiled helical structure. Reflection absorption Fourier transform infrared (RA-FTIR) spectroscopic analysis indicates structural changes for the films related to adjustments to the orientation of the polymeric backbone consistent with the formation of two distinct structures. The polymeric changes translate into differences in ion formation probability of fragments in both the low (<250 Da) and high (>1000 Da) mass range. Data is also presented to analyze how tertiary structure may affect the apparent polydispersity index calculated from the ToF-SIMS spectra. High resolution electron energy loss spectroscopy was used to investigate a series of polymer films for both tertiary structure information and to investigate the quantitative abilities of the technique. A series of films of PDMS-2400 were manufactured using the Langmuir-Blodgett technique and by a watercasting technique which produced ordered and disordered structures respectively. Characteristic peaks corresponding to Si-O-Si and methyl vibrations were ratioed in the spectra and higher ratios for the Langmuir-Blodgett films for the majority of peaks was observed. Additionally, the data was compared to RA-FTIR data collected on the same system. The major difference between the two techniques was the HREELS detection of a CH 3 rocking vibration which was unresolved in the RA-FTIR data. This is due to HREELS increased sensitivity to the topmost surface molecular vibrations. For the quantitative study, a series of surface modified films of poly (tetrafluoroethylene- co -hexafluoropropylene) (FEP) with measurable densities of hydroxide, amine and carboxylic acid functionalities were investigated with attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR) and their surface concentrations were calculated. HREELS data for hydroxide modified FEP (FEP-OH) was collected and compared with both ATR-FTIR and RAFTIR data for the same system.