Estane: Poly(ester urethane)

Segmented poly(ester urethanes) are an important class of structural polymers that derive their desirable mechanical properties from the nanophase separation of immiscible crystalline (hard) and rubbery (soft) segments that make up the polymer. It is well known that the thermoplastic character of polyurethanes is a result of changes in structure that occur on heating and cooling. Phase sensitive atomic force microscopy imaging is sensitive to the differences in mechanical stiffness of the hard segment (HS) and soft segment (SS) domains formed by the segregation of the polyurethane components, as shown in previous work in our laboratory [1]. We have also studied time dependent evolution of estane with various hard segment contents after heating. Full details of the experiments and results are given in the papers on our publications page. General features are presented below.

The objective of this study is to determine the time-dependent evolution of estane samples over several weeks after heating at 100°C. In addition, we studied changes in 23% HS samples were studied as a function of cooling rate. Scanning Probe Microscope (SPM) scan rates are fast on time scales relative to the rate of expected structural changes; the method provides near surface information on the time dependent component segregation.

Tapping Mode Phase Images: Hand Segment Content


Figure 1, above, shows phase mode AFM images from samples with various amounts of hard segments. The 19% and 23% samples show obvious segregation of the hard segments into rod or fiber like domains with an orientation that may be related to the pressing direction during sample formation. The 30% hard segment sample does not obviously show these features, however it may be that the background of rod like shapes are too small to resolve.

Tapping Mode Phase Images of 23% Sample: Thermal History

Figure 2, above, shows AFM data depicting the changes in the 23% HS sample after a heating cycle has been applied. The change during heating is rapid but the recovery to domains of hard segment regions is very slow.

Tapping Mode: Phase Imaging of 23% Sample: Quench Rate

Figure 3, above, shows the results of different quenching speeds on the 23% hard segment sample. Quenching leads to the formation of a network of a very hard rod-like microphase. More rapid quenching resulted in a more extensively developed structure, consisting of ~90 degree smaller branches emanating from the hard rod-like network. Slowly cooled low hard segment samples initially show very little nanophase segregation. However, over an extended period, strand-like nandomains, seen prior to heating, redeveloped, coalescing into dense mats. Coincident with the formation of these strands is the formation of a background structure with a length scale similar to that observed in small-angle scattering experiments. It consists of ~ 10 nm interpenetrating soft and hard domains.

References
[1] M. E. Hawley, E. B. Orler, D. A. Wrobleski, R. P. Hjelm, and G. W. Brown, "Comprehensive Structural Study of Pre- and Post-Heat Treated Compression Molded Polyurethane Samples of Varying Composition Studied by Scanning Probe Techniques," eds. A. I. Nakatani, R. P. Hjelm, M. Gerspacher, and R. Krishnamoorti, Mat. Res. Soc. Symp. KK Proceed. 661, KK4.7.1 (2001).