Interplay between domain microstructure and nematic order in liquid crystalline/isotropic block copolymers

• Published in: Colloid and polymer science Vol. 282; no. 8; pp. 810 - 824
• Main Authors: IVANOVA, Rouja, STANEVA, Rosina, GEPPERT, Steffen, HECK, Barbara, WALTER, Bernd, GRONSKI, Wolfram, STÜHN, Bernd
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01.06.2004; Springer Nature B.V
• Subjects: Applied sciences; Block copolymers; Blocking; Chains; Copolymers; Curvature; Exact sciences and technology; Mathematical analysis; Mesophase; Microstructure; Nematic; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis; Butene derivative copolymer; Phase diagram; Thermotropic state; Temperature effect; Mesomorphic state structure; Experimental study; Phase transitions; Structure composition relationship; Thermotropic liquid crystalline behavior; X-ray scattering; Side chain; Domain structure; Biphenyl derivatives; Liquid crystal polymers; Nematic state; Diblock copolymer; Liquid crystalline block copolymers; Styrene copolymer
• ISSN: 0303-402X; 1435-1536
• DOI: 10.1007/s00396-004-1128-x

The domain microstructure and the nematic LC mesophase in a series of side-chain liquid crystalline/isotropic (LC/I) diblock copolymers with systematically varied block volume fractions were studied in a broad temperature range (25-170 °C) by DSC, polarized microscopy, and wide and small angle X-ray scattering. At all temperatures the block copolymers are microphase separated. The PSLC block copolymers exhibit order at two length-scales: on one hand, a nematic LC mesophase with characteristic length-scale of 0.43 nm (intermesogen distance); on the other hand, lamellar, hexagonal or cubic domain microstructures with characteristic length-scales of 27-44 nm (lattice parameter). The LC block was either located in the matrix or confined inside the microdomains. The thermotropic behavior is characterized by the sequence g/~35 °C/n/~115 °C/i and is not affected by the domain microstructure and/or dimensions. Analysis of the lamellar dimensions showed that the LC chain is stretched. With increasing temperature, a thermal expansion of both blocks takes place followed by a retraction of the LC chain above T^sub NI^. The phase diagram is asymmetric and does not alter above T^sub NI^. No order-to-order transitions triggered by the nematic-isotropic transition are observed. It was shown that domain microstructures of low interfacial curvature (lamellar and hexagonal) are energetically favored over the geometrically expected ones of high interfacial curvature (micellar cubic) due to the presence of nematic LC mesophase in the matrix or in the microdomains. By comparison to theory a Kuhn segment length of the LC block b^sub LC^=0.86 nm was derived from the location of the lamellar/hexagonal phase boundaries.[PUBLICATION ABSTRACT]