Thermal Decomposition of Generation 4 Polyamidoamine Dendrimer Films



 My research

    The purpose of my research is to monitor the decomposition of the dendrimer particle as it is heated to different temperatures. Infrared Spectroscopy, Thermal Gravimetric Analysis, Temperature Programmed Desorption, and X-Ray Photoelectron Spectroscopy are the  main ways to monitor this decomposition. It is hypothesized that the data obtained from the different methods will support each other.

What is a Dendrimer?

    Dendrimers are hyperbranched polymers that have a well defined structure characterized by three distinct features: a central core, repetitive branching units, and terminal groups. Dendrimer particles are usually spherical in shape and contain interior void spaces. The dendrimer is grown outward from the central core to the terminal functional groups. As the generation of the dendrimer increases, the number of terminal groups increases. G4-OH, the main dendrimer used in this research, has 64 terminal groups while the G2-OH has 16. Another distinct feature of the dendrimer is the appearance of the amide I, II peaks (with wave numbers of 1640 cm-1 and 1550 cm-1 respectively) in the unique spectrums obtained from different vibrational spectroscopies such as Infrared spectroscopy.


What happens as the dendrimer particle decomposes?

    Thermal decomposition refers to a chemical reaction where a single compound breaks up into two or more simpler compounds or elements when heated. As the dendrimer particles are heated to higher temperatures, their bonds, specifically their signature amide peaks, are broken.

Why use dendrimers? 

The most important goal when synthesizing catalysts is to be able to control the size, dispersion, and composition of the metal nanoparticles. 
Traditional catalyst preparation methods do not provide the level of control required for the design of new heterogeneous catalysts with specific properties. Dendrimer stabilized nanoparticles (DSN) provide a new approach to controlled synthesis of supported metal catalysts. Dendrimers are hyperbranched polymers formed by attaching monomers to a core. The monomer branching creates interior voids that can be used to create metal nanoparticles. Once the catalyst is formed, the dendrimer must be removed before the catalyst can be used. Thermal decomposition is one method of dendrimer removal.

Methods of monitoring the dendrimer particle decomposition

 Thermal Gravimetric Analysis (TGA)

    TGA examines the process of weight changes as a function of time, temperature, and other conditions that may be created within the apparatus. For this research project, TGA will be used to monitor the weight loss of polymeric dendrimer particles as they are thermally decomposed.

Infrared Spectroscopy

    Infrared spectroscopy involves the absorption of infrared light causing chemical bonds to bend and stretch. Each stretch corresponds to a particular functional group and frequency. For example, the stretching of a carbon-hydrogen bond occurs at a frequency of around 3000 cm-1. Thus, Infrared Spectroscopy can be used to identify functional groups and types of bonds in a given molecule.

    The dendrimer particles have characteristic amide I and amide II peaks that occur at 1640 cm-1 and 1550 cm-1 respectively. As the dendrimer decomposes, the intensity of these peaks slowly decreases in the Infrared spectrum. Once the peaks have completely vanished, the dendrimer bonds have been fully broken.

Temperature Programmed Desorption (TPD)

   TPD involves heating a sample while contained in a vacuum and simultaneously detecting the residual gas in the vacuum by means of a mass analyser. As the temperature rises, certain absorbed species will have enough energy to escape and will be detected as a rise in pressure for a certain mass

    The temperature of the sample is slowly increased (between 15 seconds to several minutes). As the temperature rises and a particular species is able to desorb from the surface, the pressure will rise. As the temperature rises still further the amount of the species on the surface will reduce causing the pressure to drop again. This results in a peak in the pressure versus time plot. The temperature of the peak maximum provides information on the binding energy of the bound species.

X-Ray Photoelectron Spectroscopy (XPS)

  In XPS the photon is absorbed by an atom in a molecule or solid, leading to ionization and the emission of a core (inner shell) electron. The kinetic energy distribution of the emitted photoelectrons (i.e. the number of emitted photoelectrons as a function of their kinetic energy) can be measured using any appropriate electron energy analyzer and a photoelectron spectrum can thus be recorded. Since the electron's energy is present solely as kinetic energy (KE), the binding energy can be determined from the following equation: KE = hn - BE. Since the binding energy each different species is unique, XPS can determine which species are present  in a sample. XPS can also indicate if these species are bound to others or free (unbound).


Results

IRAS

IRAS studies show that with the Pt particles present, most amide bonds are broken at 225°C while amide bonds for the G4OH remain up to 275° C. This is because the Pt particles hinder the formation of the oxidation products that form during G4OH decomposition


TGA

Figure (a) shows TGA studies done in air. The procedure reproduces the IRAS studies. These results also show that the G4OH decomposes at higher temperatures due to the formation of oxidation products. Figure (b) shows TGA studies done in Argon. These studies were done in order to bridge the project from studies in air to studies in vacuum. These results show that without the presence of oxygen, the decomposition of the G4OH occurs more rapidly since it does not for the oxidation products.

TPD

TPD experiments show desorption of large dendrimer fragments from G4OH films, whereas decomposition to H2 gas appears to be the dominant pathway for the Pt-G4OH films. This indicates that Pt-G4OH is more fully decomposed.

XPS

                            (a) G4-OH                                                                                            (b) Pt G4-OH

The XPS data shows that the Pt particles catalyze decomposition. This is evident in the comparison of figures (a) G4-OH and (b) Pt G4-OH. At room temperature, Pt-G4OH has a third oxygen species present in addition to the hydroxyl oxygen and carbonyl oxygen. This species indicates that decomposition of the dendrimer has already begun. Also, the figure shows that Pt-G4OH has a greater shift towards elemental oxygen and is more fully decomposed.