Structural Studies of Three-Arm Star Block Copolymers Exposed to Extreme Stretch Suggests a Persistent Polymer Tube

Abstract

We present structural small-angle neutron scattering studies of a three-armed polystyrene star polymer with short deuterated segments at the end of each arm. We show that the form factor of the three-armed star molecules in the relaxed state agrees with that of the random phase approximation of Gaussian chains. Upon exposure to large extensional flow conditions, the star polymers change conformation resulting in a highly stretched structure that mimics a fully extended three-armed tube model. All three arms are parallel to the flow, one arm being either in positive or negative stretching direction, while the two other arms are oriented parallel, right next to each other in the direction opposite to the first arm.

Small-Angle Neutron Scattering Study of the Structural Relaxation of Elongationally Oriented, Moderately Stretched Three-Arm Star Polymers

“…Immediately after cessation of extensional flow, neutron scattering experiments showed the characteristics of a fully aligned three-armed tube. Figure 1 reviews the schematic molecules as visualized in the relaxed state (a) and in the oriented, slightly stretched state during flow (as measured immediately after cessation of flow) (b) [1]. In the relaxed state, the polymer configuration is in agreement with the structure factor calculated based on the random phase approximation, RPA.…”

“…In a recent study, prior for the present work, we studied the deformed structure of a three-armed star polymer during fast extensional flow [1]. We showed unambiguously that this system remains to have conformation characteristics of a tube.…”

“…We find that the structural relaxation falls into three surprisingly distinct time regimes with completely different characteristics, thereby supporting the conclusion from rheology. The initial configuration observed right after cessation of flow, corresponds to that of a fully extended three-armed virtual tube [1]. The initial relaxation (I) is dominated by disruption of the tube.…”

“…Architecturally complex polymers attract increased attention due to the remarkable properties. Star polymers, for example, have been the subject of several studies [1][2][3][4][5][6][7][8][9][10][11][12][13].…”

“…The Kuhn segment length of polystyrene is b ¼ 18 Å, the entanglement molar mass is M e ¼ 16.6 kg=mol and the tube diameter is a ¼ 85 Å [28]. With the molar mass M w ¼ 101.7 kg=mol for each arm, the number of entanglements is Z ≈ 6, and the equilibrium end-to-end length is R ∘ ð1 armÞ ≈ 210 Å [1]. The number of Kuhn segments in each arm is N arm ¼ ðR ∘ =bÞ 2 ¼ 135, and the total number of segments in the star polymer is N ¼ 3N arm ¼ 405.…”