Molecular Characterization of Hydrogen-blown RTV Silicone Foams to Evaluate and Optimize Material Performance and Production

Researchers:

• Seth Gleiman, Polymer Materials Engineering, LANL
• Andrea Labouriau, Materials Science and Technology, LANL
• Brad Chmelka, Department of Chemical Engineering, UCSB

Objective:

Establish key functional relationships for polymeric flexible foams between molecular level properties and their microcellular structures that can be linked to macroscopic material properties by:

1. applying two-dimensional solid-state NMR techniques to quantify and correlate the local composition, structure and dynamics of polymeric flexible foams
2. developing spatially-resolved NMR techniques to elucidate polymer network structure correlated to foam cellular structures
3. investigating the interrelationships between molecular properties, microcellular structures and macroscale mechanical properties

Approach:

1. Application of two-dimensional solid-state NMR techniques to quantify and correlate the local composition, structure and dynamics of polymeric flexible foams

   Beginning with simple, stoichiometrically-balanced mixtures of di- and multifunctional polysiloxane polymers, we aim to understand the nature of the network formed during the crosslinking reaction, specifically network topology as it appears in a bulk rubber: how many crosslinks are effective, how many dangling chains exist, whether and to what extents molecular and/or macroscopic heterogeneities exist, etc., and how these parameters are affected by synthesis variables such as temperature, pressure, reaction chemistry and monomer/polymer identity. We plan to examine how and where multi-functional crosslinkers actually crosslink along their backbones and how many functional sites remain unreacted.

   We plan to use HETCOR NMR experiments (¹H{¹³C} and ¹H{²⁹Si}) to probe the local bonding, species distributions, and interfacial properties of the polymer network through dipole-dipole-coupling-mediated interactions to aid in the determination of neighboring (0.2 nm) and more distant (<2 nm) network atoms and moieties. Real-time in-situ measurements using Hadamard-encoded heteronuclear-resolved NMR diffusions and relaxations will be used to determine relative reactivity rates to compliment and compare to FTIR results.

   Development of these techniques will provide the basis for determining the molecular structures of different foams with heterogeneous structures and features, relative distributions of foam components and interfacial properties within the foams. Foams will be generated so as to maintain as much of the unfoamed network composition and chemistry as possible while generating hydrogen (blowing agent) as a crosslinking reaction by-product.

2. Developing spatially-resolved NMR techniques to elucidate polymer network structure correlated to foam cellular structures

   Spatial mapping of the network polymers within the foam is planned to elucidate molecular structure throughout the cellular morphology. We plan to determine pore sizes and interconnectivities by using NMR imaging and pulsed-fieldgradient diffusion measurements. We will also pursue ¹²⁹Xe NMR techniques that provide additional sensitivity to the local environment and can be used to determine cellular parameters. In conjunction, determination of foam cellular structure and pore size distributions will proceed through application of microscopy and tomography techniques being developed for other IMMS projects.

3. Investigating the interrelationships between molecular properties, microcellular structures and macroscale mechanical properties

   Ultimately we aim to develop new insights on complex foam composition-structure-property relationships for LANL mission-critical foams. We will incorporate mechanical property measurements such as uniaxial compression testing on our well-characterized foam materials. Mechanical property measurements will be made in conjunction with techniques developed for other IMMS projects. These results will be correlated with the molecular and interfacial structure determined via NMR.

   Development of a new NMR probehead capable of performing uniaxial compression of foam samples will allow collection of detailed foam structural information during cellular collapse and densification. Application of NMR techniques developed throughout this project, including NMR imaging, will generate never-before-seen images and provide information about interfacial interactions during densification.