Phase Behavior of the Blends of Rod-Coil Block Copolymer and Coil Homopolymer

• Main Authors: Lai, Chia-Sheng, 賴佳昇
• Other Authors: Chen, Hsin-Lung
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/42197072822635897463

博士 === 國立清華大學 === 化學工程學系 === 100 === Rod-coil block copolymers have received increasing attention over the past decade due not only to their potential technological applications but also to their unique self-assembly behavior that clearly distinguishes from that of the conventional coil-coil block copolymers. Both experimental and theoretical studies have established that composition of the constituting blocks is also a key parameter governing the microphase-separated morphology of rod-coil block copolymers; consequently, it is of interest to inspect if the self-assembled structure of rod-coil block copolymers can be tuned systematically by blending with a coil homopolymer. This thesis undertakes systematic studies of the phase behavior of lamellae-forming Poly(2,5-di(2’-ethylhexyloxy)-1,4-phenylene vinylene)-b-Poly(methyl methcrylate) (DEH-PPV-b-PMMA) with three types of coil homopolymers, namely, PMMA, poly(ethylene oxide) (PEO) and poly(4-vinylphenol) (PVPh), with PEO and PVPh being able to form dipole-dipole interaction and hydrogen bonding with PMMA blocks in the copolymer, respectively. The blends of DEH-PPV-b-PMMA with PMMA homopolymers (h-PMMA) were found to undergo macrophase separation over the major composition window irrespective of the molecular weight of h-PMMA. The phase separation led to the formation of a copolymer-rich phase and a homopolymer-rich phase, in which microphase transition yielding a well-ordered lamellar structure and a sponge structure occurred, respectively. The phase behavior of the rod-coil diblock/coil homopolymer blends is hence fundamentally different from that of the conventional blends of coil-coil diblocks with the corresponding homopolymer in the sense that microdomain morphology transformation induced by the uniform solublization of the corresponding homopolymer into the selective microdomain was essentially inaccessible. For DEH-PPV-b-PMMA/PEO blends, it was found that the favorable interaction introduced was still unable to outweigh the strong rod-rod attraction coupled with the small again of entropy of mixing upon mixing with the rod blocks. As a result, blends still exhibited a macrophase separation, generating a diblock-rich phase and a h-PEO-rich phase. Similar to DEH-PPV-b-PMMA/h-PMMA blends, a microphase separation occurred in the diblock-rich phase, yielding a well-ordered lamellar nanostructure but the persistence length of the lamellar microdomains was smaller than that formed in the corresponding h-PMMA blend. The transformation of the rod microdomain morphology became accessible by blending with PVPh homopolymer. The DEH-PPV-b-PMMA/PVPh blends as cast from THF were found to display a lamellar morphology irrespective of the blend composition owing to the crystallization of DEH-PPV block during the solvent casting process. When the DEH-PPV crystals in the lamellar microdomains were melted upon heating, the PVPh chains were allowed to mix uniformly with the PMMA blocks to form wet-brush mixture. The large swelling of the junction point separation then destabilized the lamellar microdomains and transformed them to DEH-PPV cylinders when the overall coil volume fraction was sufficiently high (> 0.65). Therefore, a sufficiently large reduction of enthalpy of mixing between the diblock and the homopolymer is an essential condition for achieving the morphological control for rod-coil diblock copolymers via homopolymer blending.