Abstract
Structural and morphological interplay between hard and soft phases determine the bulk properties of thermoplastic polyurethanes. Commonly employed techniques rely on different physical or chemical phenomena for characterizing the organization of domains, but detailed structural information can be difficult to derive. Here, total scattering pair distribution function (PDF) analysis is used to determine atomic-scale insights into the connectivity and molecular ordering and compared to the domain size and morphological characteristics measured by AFM, TEM, SAXS, WAXS, and solid-state NMR 1H–1H spin-diffusion. In particular, density distribution functions are highlighted as a means to bridging the gap from the domain morphology to intradomain structural ordering. High real-space resolution PDFs are shown to provide a sensitive fingerprint for indexing aromatic, aliphatic, and polymerization-induced bonding characteristics, as well as the hard phase structure, and indicate that hard phases coexist in both ordered and disordered states.
Highlights
How do the atomic arrangements within thermoplastic polyurethanes (TPUs) affect their bulk properties? This is a complicated question to answer because of the small size and weak ordering of the domains, microphase segregation and intermixing, complex morphologies, and strongly diverging chemistry- and processing-driven structuring pathways
The amount, size, and morphology of hard and soft phases can be studied by methods such as small-angle X-ray scattering (SAXS),[1−3] transmission electron microscopy (TEM),[4−6] atomic force microscopy (AFM),[7−9] and various solid-state nuclear magnetic resonance (NMR) techniques.[10−15] the methods are sensitive to different sample properties, for example, variations in electron density, mechanical response, or molecular mobility and proton density
The similarity between the pair distribution functions (PDFs) here and for previous TPU model hard phases, and the disproportionately low intensities of the long-range ordered content, show that there is a large concentration of disordered hard domains coexisting with larger, long-range ordered domains, despite all three showing different structures
Summary
How do the atomic arrangements within thermoplastic polyurethanes (TPUs) affect their bulk properties? This is a complicated question to answer because of the small size and weak ordering of the domains, microphase segregation and intermixing, complex morphologies, and strongly diverging chemistry- and processing-driven structuring pathways. The amount, size, and morphology of hard and soft phases can be studied by methods such as small-angle X-ray scattering (SAXS),[1−3] transmission electron microscopy (TEM),[4−6] atomic force microscopy (AFM),[7−9] and various solid-state nuclear magnetic resonance (NMR) techniques.[10−15] the methods are sensitive to different sample properties, for example, variations in electron density, mechanical response, or molecular mobility and proton density. The idea of density distributions can be generalized for all scattering regimes: the isotropic atomic-pair density correlations obtained from different regions of X-ray scattering momentum transfer imbue useful differences in structural length-scale resolution
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