Ep16 Osmotic pressure, light scattering, MALDI-TOF-MS - NANO 134 Darren Lipomi

The script introduces the concepts of Gel Permeation Chromatography (GPC) and Size Exclusion Chromatography (SEC), which are methods used to measure molecular weight rapidly. It discusses the convenience of these techniques, especially with the use of an autosampler, and the challenge of determining the absolute molecular weight of standards. The lecture promises to cover various experimental methods to ascertain molecular weights, including end group assay, osmotic pressure, light scattering, and MALDI-TOF MS, each in a brief 10-minute segment, acknowledging the complexity of the subject matter within the constraints of a 10-week polymer class.

This paragraph delves into the end group assay method for determining the number of polymer chains by counting end groups, which is practical for low molecular weight polymers. It mentions the limitations due to the dilute concentration of end groups in high molecular weight polymers and the use of titration or NMR for quantitative analysis. The paragraph then transitions to osmotic pressure, explaining it as the pressure exerted across a semipermeable membrane due to a difference in chemical potential, and how it can be used to calculate molecular weight through the measurement of the osmotic pressure at varying concentrations.

The script describes the practical execution of an osmotic pressure experiment, where the pressure exerted by a polymer solution against a pure solvent is measured. It explains how data is collected by plotting osmotic pressure over concentration and using the relationship between osmotic pressure, concentration, and molecular weight. The example of PMMA in acetone is given, and the process of deriving the number average molecular weight from the y-intercept of the plot is outlined, including a specific calculation that results in a molecular weight of 380 kilodaltons.

The paragraph discusses light scattering as a technique for determining the weight average molecular weight of a polymer solution. It explains the phenomenon of scattering as the polarization and depolarization of electron clouds and how it is influenced by the size of the particles in solution. The script touches on the complexity of interpreting light scattering data due to the broad peaks resulting from various magnetic and electronic environments in polymers, and introduces the concept of polarizability, which affects the strength of the induced dipole in a molecule.

This section provides an overview of the experimental setup for light scattering, including the sample cell, detector, incident beam, and transmitted beam. It explains the concept of isotropic scattering for small particles and how the intensity of scattered light is measured. The script presents the equation relating the scattered intensity to molecular weight and describes the process of plotting data to obtain a straight line, from which the weight average molecular weight can be derived, using a specific example with calculated slope and y-intercept values.

The script introduces Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) as a technique for directly determining molecular weights. It describes the process of placing a polymer sample on an organic matrix, ablation by a laser, ionization of the fragments, and their acceleration towards a detector based on their mass-to-charge ratio. The paragraph explains how the mass spectrum is obtained and how it can be used to determine the molecular weight distribution of a polymer, with a focus on the ideal scenario of obtaining a clear peak representing the molecular weight.

The final paragraph wraps up the lecture by emphasizing that molecular weights can be determined through direct methods, not just indirectly through GPC and SEC. It acknowledges the limitations of creating polymers with exact molecular weights by simply adding monomers and the challenges of controlling the polymerization process for high molecular weight materials. The script mentions the use of protecting groups for more controlled polymer synthesis and the importance of combining various techniques for accurate molecular weight determination. It concludes with a note on the upcoming exam and an invitation for students to bring questions for a discussion session.