Dielectric Relaxation in Isotropic/Liquid Crystalline Block Copolymers:  Effect of Nanoscale Confinement on the Local β and γ Dynamics

• Published in: Macromolecules Vol. 36; no. 16; pp. 6166 - 6170
• Main Authors: Zhukov, Sergei, Geppert, Steffen, Stühn, Bernd, Staneva, Rosina, Gronski, Wolfram
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 12.08.2003
• Subjects: Applied sciences; Electrical, magnetic and optical properties; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Property composition relationship; Gamma relaxation; Thermotropic state; Butadiene derivative copolymer; Experimental study; Beta relaxation; Functional polymer; Dielectric relaxation; Side chain; Biphenyl derivatives; Liquid crystal polymers; Diblock copolymer; Activation energy; Styrene copolymer
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma0301202

The effect of nanoscale confinement on the local polymer dynamics has been studied by means of dielectric spectroscopy in the frequency range from 10 mHz to 1 MHz. We have used microphase-separated PS/LC block copolymers (PS = polystyrene, LC = liquid crystalline). Depending on volume fraction, the LC blocks were confined to a layer with thickness of 21.3 and 11.2 nm (lamellae morphology) or were contained in domains of cylindrical or spherical form with diameter of 12.6 and 9.4 nm, respectively. At lower concentration of PS blocks, the LC copolymer form a continuous LC matrix with cylindrical inclusions of PS component. For all morphologies the LC block reveals two dielectric relaxations related to the local motion of mesogen (β process) and spacer (γ process). The relaxation time as well as the activation energy for the γ process was independent of spatial constraints even for the smallest characteristic length of 9.4 nm. The β relaxation, however, speeds up, and its activation energy decreases for confinement lengths less than 20 nm. The γ process has an essentially noncooperative nature, whereas the β process exhibits a positive apparent activation entropy, reflecting a partial cooperative feature. As a result, the β relaxation is affected by confinement just as the essentially cooperative dynamic glass transition (α relaxation) in polymers.