Competitive Effects of Hydrogen Bonds and Molecular Weights on the Phase and Crystallization Behaviors of Binary Block Copolymers

Abstract

We investigated the competitive effects of the hydrogen-bonding interaction and molecular weight on the phase and crystallization behaviors of polystyrene-block-poly(ethylene oxide) (PS-b-PEO)/polystyrene-block-poly(acrylic acid) (PS-b-PAA) blends. The hydrogen-bonding interaction between PEO and PAA chains improved their miscibility, enabling the distinct block copolymers to co-organize into common PS and PEO/PAA microdomains. When the molecular weight ratio of PS-b-PEO to PS-b-PAA exceeded 4.5, the entropy penalty (caused by long PEO chains organizing into the PEO/PAA microdomains) suppressed the hydrogen-bonding interaction, inducing macrophase separation. Localization of large PS-b-PEO chains and small PS-b-PAA chains at the common interface caused the PEO/PAA microdomains to be composed of coexisting PEO/PAA and PEO regions (Regions I and II, respectively). Hydrogen bonds hampered PEO crystallization in Region I, whereas the PEO in Region II was crystallizable. When the molecular weight ratio of PEO to PAA was decreased, PEO crystallization in Region II became strongly confined, reducing the crystallization temperature and kinetics. Increasing the PS-b-PAA content expanded the separation distance between PEO chains in Region I. Consequently, PEO chains in Region II became more stretched to maintain the microdomain incompressibility, which inhibited PEO crystallization in Region II.

Hydrogen Bonding-Induced Crystal Orientation Changes in Confined Microdomains Constructed by Block Copolymer Blends

“…We only used PS- b -PEO whose molecular weight was comparable to or slightly higher than the molecular weight of PS- b -PAA. When the molecular weight of PS- b -PEO was considerably higher than that of PS- b -PAA, the organization of long PEO chains into coexisting PEO/PAA microdomains caused entropy penalty, resulting in macrophase separation . Similar to the SAXS patterns of PS 96 - b -PEO 261 /PS- b -PAA blends, those of PS 207 - b -PEO 455 and its blend with 20 wt % PS- b -PAA exhibited a preferentially oriented microdomain morphology, which was verified by the arc feature along the z⃗ direction when the patterns were recorded using the X-ray beam through the y⃗ direction (Figure a,b).…”

“…However, the hydrogen bonds hampered PEO crystallization, causing substantial decreases in the crystallization kinetics and crystallinity of PEO. Although previous studies have demonstrated the crystallization behavior of binary block copolymers, ,, crystal orientation in binary block copolymers remains unclear. In the present study, we varied the molecular weight of PS- b -PEO but not that of PS- b -PAA to investigate the crystal orientation of PEO in the resulting microdomain morphologies.…”

“…Previously, we investigated the crystallization behavior of binary block copolymers composed of PS- b -PEO and polystyrene- block -poly­(acrylic acid) (PS- b -PAA). , In terms of molecules, only PEO chains were crystallizable. The hydrogen-bonding interaction between PEO and PAA molecules improved the miscibility of the copolymers, thus inducing the co-ordering of two PS block chains segregated into common PS microdomains and PEO and PAA block chains assembled into common PEO/PAA microdomains.…”

Compositional asymmetry in a crystalline–amorphous block copolymer influences the phase and crystallization behaviors of its blend with an amorphous block copolymer

“…A study investigated the crystallization in symmetric block copolymer mixtures comprising polystyrene- block -poly(ethylene oxide) (PS–PEO)/polystyrene- block -poly(acrylic acid) (PS–PAA) blends. 17 In these blends, only PEO molecules were crystallizable and PAA hydrogen bonded with PEO, hindering PEO crystallization. Because PEO and PAA were co-organized into coexisting PEO/PAA microdomains, PEO crystallization was strongly affected by the molecular weight ratio of PEO to PAA.…”

“…13 In contrast to chemically identical block copolymer mixtures, chemically unidentical binary block copolymer mixtures demonstrate microphase separation when the blends are endowed with specific molecular interaction. 9, [14][15][16][17] For instance, in a mixture of (A-B) a and (A-C) b block copolymers in the presence of a hydrogen-bonding interaction between the B and C block chains, the hydrogen-bonding interaction facilitates coordering of the B and C block chains in a coexisting microdomain, which then results in the formation of a microphaseseparated morphology. 17 Furthermore, hydrogen bonds reverse the phase behavior of each block copolymer in the blend that exhibits an upper-critical-ordering-transition-type interaction parameter to a lower-critical-ordering-transition-type phase diagram.…”