Morphology and micromechanical deformation behavior of styrene/butadiene‐block copolymers. I. Toughening mechanisms in asymmetric star block copolymers

Abstract

Lamellae-forming styrene/butadiene star block copolymers are studied to investigate the influence of morphology on micromechanical deformation mechanisms and mechanical properties by using transmission electron microscopy and tensile testing. A large homogeneous plastic deformation of polystyrene (PS) lamellae is found in styrene/butadiene star block copolymers on the basis of the new mechanism called thin-layer yielding. This mechanism depends strongly on the thickness of the PS lamellae. At a critical thickness of PS lamellae of about 20 nm, a transition from thin-layer yielding mechanism to a crazelike deformation was observed. These new deformation zones are similar to crazes with respect to their propagation perpendicular to direction of external stress and similar to shear bands with respect to an internal shear deformation component of the lamellae in the deformation zones. As a result of our investigations, the mechanical properties of star block copolymers can be understood in correlation with morphology and micromechanical deformation mechanisms.

Correlation between Molecular Architecture, Morphology, and Deformation Behaviour of Styrene/Butadiene Block Copolymers

“…The lamellar periodicity measured in the Fourier transformed image is about 44 nm. Detailed morphology of similar block copolymers has been studied by Knoll and Nießner [5] as well as by Michler et al [20] using transmission electron microscopy (TEM) with samples prepared by different methods. A TEM micrograph showing detailed morphology of this sample is given in Figure 6.…”

“…A TEM micrograph showing detailed morphology of this sample is given in Figure 6. [20] This sample shows a peculiar morphology consisting of alternating PS and PB layers with small PS domains embedded in PB lamellae. The SFM micrographs reveals both 'flat on' and 'edge on' views of alternating PS and PB layers, indicating that the morphology is close to the lamellar morphology (the 'flat on' view of lamellae is also observed in sample LN2, Figure 4).…”

“…Hence, the butadiene-rich stars may form PS domains in the butadiene lamellae together with the PS core of the styrene-rich stars. For a detailed account of the morphology of this block copolymer, readers are referred to reference [5,20]. The small PS domains in the PB phase could not be resolved satisfactorily in the SFM images, since the resolution of SFM may be limited by the tip radius [21] which might blunt due to prolonged scanning over the sample surface.…”

“…[5,[10][11][12] Styrene-based triblock copolymers with asymmetric styrene end blocks are desirable for enhanced toughness and processability. [12] There is much theoretical [13,[15][16][17][18] and experimental evidence [5,6,12,14,19,20] confirming a significant shift in the block copolymer phase diagram introduced by architectural modification. Morphological deviation (and hence a deviation in the mechanical properties and deformation mechanisms) is introduced by asymmetric architecture, tapered or broad transition, and nonlinear topologies.…”

Nanostructured Composites with Layered Morphology: Structure and Properties

“…This situation appears on coupling the glassy PS chains with polybutadiene (PB) chains to form a lamellar block copolymer. Under these circumstances, the glassy polymer lamellae can plastically flow up to the limit of theoretical value (stretching ratio of about 4) without the hindrance of liquid-like adjacent PB layers [25,26] . The deformation mechanism has been described as 'thin layer yielding' and discussed in detail elsewhere [25] .…”