Deformation of Hierarchical Lamellar Structure Formed by a Liquid Crystalline Block Copolymer

Abstract

polystyrene-block-polybutadiene-block-polystyrene (SBS) copolymers form glassy/ rubbery type lamellae of the polystyrene (PS)/polybutadiene (PB) segments at ambient temperatures. The direction of the force with respect to the lamellar normal affects the deformation mechanism, as confirmed when using singlecrystal-like monodomain lamellar films prepared via the roll-casting method. [2,3] When force is applied perpendicular to the lamellae to increase the macroscopic strain, the lamellae tilt the normal away from the deformation direction and have sharp hinges to form chevron morphology. In contrast, when the temperature is raised to 100 °C to make the SBS form viscous/rubbery type lamellae of the PS/PB segments, the same deformation dilates the lamellae to increase the spacing without tilting them. Thus, the deformation mechanism of twophase lamellae can be affected by the state of each lamella.In this work, we investigate the deformation of viscous/ LC lamellae composed of an LC block copolymer upon being stretched in a direction parallel to the lamellar normal. The LC block copolymer, herein designated as B5-x-EMA-ϕ, consists of a main-chain smectic LC BB-5(3-Me) polyester bonded at both ends to poly(ethyl methacrylate) (PEMA) segments (Scheme 1). [5][6][7][8][9] x and ϕ are the number average molecular weight (M n ) of the BB-5(3-Me) segment and the PEMA segment volume percentage, respectively. The B5-x-EMA-ϕ copolymers at ϕ < 50 segregate these two types of segments from one another to form lamellar microdomains, within which the LC segment forms smectic layers lying parallel to the microdomain lamellae. A characteristic of B5-x-EMA-ϕ is the formation of fibers of a well-oriented "single-crystal-like" microstructure with the normal of both microdomain lamellae and smectic layers aligned along the fiber axis. Herein, we stretched the fibrous sample at a rate of 5% min −1 , and the sample stress and 2D X-ray scattering were simultaneously measured. The X-ray scattering analysis discloses the deformation mechanism of the lamellar microdomain and the smectic layers. With increasing the elongation ratio (λ) up to 1.6, the PEMA lamellae are preferentially dilated to cause the tilt of the lamellar normal with respect to the stretching direction, resulting in the lamellar undulation. The LC segment lamellae are dilated and comprise smectic layers that undulating along the lamellar boundary. With further increasing λ, the lamellae take chevron morphology that comprising straight parts connected with localized bends while recovering the spacing.The deformation of a structure of alternating liquid crystal (LC)/viscous layers upon being stretched along the normal layer is investigated using highly ordered, near-single-crystal lamellar fibers of an LC block copolymer. The block copolymer consists of poly(ethyl methacrylate) (PEMA) segments attached to a main-chain LC polyester at both ends. The LC block segregates from the PEMA block and forms a smectic LC, laying the layers parallel to the interface with PEMA block lamell...

Stress Response and Deformation of Block Copolymer Lamellae on Stretching in Normal Direction: Effects of Lateral Size of Lamellae

“…The microstructures of the copolymers were investigated using the films prepared by pulling the cast films at 180 °C and subsequent annealing at 120 °C for 6 h. For both copolymers, the LC segment segregates from the PEMA segments to form 2‐nm spaced smectic LC layers and lamellar microdomains at a spacing in the order of 10 nm. [ 15–21 ] The wide‐angle X‐ray diffraction (WAXD) patterns revealed the structure and orientation of the smectic layers. A typical WAXD pattern measured for B5‐29‐EMA‐22 ( Figure a) includes sharp reflections in the vertical direction parallel to the film length direction, indicating the smectic layers stacked up arranging their normal parallel the film length direction.…”

“…These reflections seem a line connecting a pair of two reflection spots of the smectic layer reflection at the smallest angle. [ 21 ] Combining these WAXD and SAXS patterns allows depicting the hierarchical layer structures comprising 30‐nm spaced two‐phase microdomain lamellae (Figure 2d). The two phases are smectic LC and amorphous.…”

“…The curve of B5‐29‐EMA‐22 is similar to that measured for B5‐26‐EMA‐41 in a previous study. [ 21 ] As the extension ratio ( λ ) increases, the nominal tensile stress ( σ ) increases and culminates to 0.95 MPa at λ = 1.15, then remains constant at a value of 0.75 MPa at λ ‐value of up to 3. The curve of B5‐29‐EMA‐44 almost overlaps with that of B5‐29‐EMA‐22 in the initial region at λ < 1.15; however, on further increasing λ , σ increases continuously to show strain hardening.…”

“…These LC BCPs, designated as B5‐ x ‐EMA‐ φ , comprise a main‐chain smectic LC BB‐5(3‐Me) polyester bonded at both ends to poly(ethyl methacrylate) (PEMA) segments ( Figure ). [ 15–21 ] x and φ are the number average molecular weight of the BB‐5(3‐Me) segment ( M n,LC ) in units of kDa and the PEMA segment volume percentage, respectively (see Table 1 for details). Characteristically, B5‐ x ‐EMA‐ φ forms lamellar microdomains at φ in a relatively wide range of 20–50%.…”

“…These results are similar to those obtained in our previous work using B5‐26‐EMA‐41. [ 21 ] In contrast, B5‐29‐EMA‐44 increases the stress following the yielding (strain hardening). This strain hardening involves an increase in the lamellar spacing without tilting.…”