Crystallization and Structure Formation of Block Copolymers Containing a Rubbery Amorphous Component

Abstract

Changes of higher-order structure of poly(ethylene glycol)−poly(butadiene) (PEG−PBd) di- and triblock copolymers in isothermal crystallization from microphase-separated melts were studied using time-resolved synchrotron small-angle X-ray scattering (SR−SAXS) techniques. Lamellar microphase structure of PEG−PBd with weight fractions f PEG = 0.57 and 0.51 was destroyed on crystallization and changed to a lamellar structure distinct from that in the melt. A diblock copolymer with f PEG = 0.34 crystallized, keeping the cylindrical structure preexisting in the melt. On the other hand, a f PEG = 0.7 diblock copolymer whose melt structure was cylindrical showed two kinds of structure changes: at low crystallization temperatures T c the SAXS peak positions changed discontinuously but the structure remained cylindrical, while the structure changed from cylindrical to lamellar at high T c. Triblock copolymers PBd−PEG−PBd with f PEG = 0.67 and 0.42 showed a structure change similar to the f PEG = 0.57 and 0.51 diblock copolymers. Behavior of structure changes in the melting process was the reverse of that in the crystallization process for all the samples. Crystallization behavior, in particular crystallization kinetics, was also investigated by DSC and polarized optical microscopy (POM) with a 530 nm retardation plate. Avrami exponents evaluated from DSC results for the block copolymers were almost the same as those for PEG homopolymers even in crystallization from the cylindrical melt, which was quite different from the result in crystallization from a frozen microphase-separated melt reported previously. For the f PEG = 0.34 diblock and f PEG = 0.42 triblock copolymers with a small-sized microdomain in the melt, overall crystallization rates were suppressed and apparent activation energies were high. No spherulite with a clear Maltese cross was observed by POM, but the crystal region was propagated with irregularly distributed blue- and yellow-colored portions for all the block copolymers including the f PEG = 0.34 diblock copolymers. This behavior of the propagation and the above result of Avrami exponents for the f PEG = 0.34 copolymer suggest that crystallization can traverse from one cylinder domain to another with preservation of the melt structure. From the kinetic results obtained by both DSC and POM, it was suggested that nucleation was easy for the middle block in the triblock copolymers.

Structure Formation and Crystallization Behavior of Ethylene-Isoprene Block Copolymers and Their Blends with Corresponding Homopolymers

“…In our previous papers, it was found that the Avrami indices were reduced for block copolymers having a glassy amorphous component, because crystallization occurs within confined rigid microdomains 19,20) . On the other hand, for PEG-PBD, in which the amorphous component is rubbery in crystallization, we reported that the Avrami index is not reduced even when crystalline PEG is in cylindrical microdomains 26) . In the system studied here, the reduction of n was not found even in the blends with the small amount of the crystalline component (HEI/LPIp-17 and HEI/HPIp-17).…”

“…On the other hand, when crystallized at the intermediate crystallization temperatures (60ՅT c Ͻ94°C), the peak from the microphase separation structure in the melt disappears completely, indicating that the microphase separation structure was disrupted in crystallization of the E block. For crystalline-rubbery amorphous block copolymers, we reported the crystallization of PEG-PBD diblock copolymers, in which PBd is rubbery in crystallization of PEG 26) . In the crystallization of PEG-PBD, reorganization of the phase structure depended on the crystallization temperature as well as the microphase separation structure, i.e., the melt structure tends to be preserved at lower T c , while it is reorganized to the alternating structure of crystalline and amorphous layers at higher T c .…”

“…Nojima et al 25) also investigated a crystallization process of crystalline-rubbery amorphous block copolymers for poly(e-caprolactone)-polybutadiene (weakly segregated system) through a time-resolved smallangle X-ray scattering (SAXS) technique using synchrotron radiation as a light source, and observed that a preexisting SAXS peak from the microphase separation disappeared with development of a new peak originating from the long period of crystal lamellae. Furthermore, we have reported that a high molecular weight poly(ethylene glycol)-poly(butadiene) block copolymer (PEG-PBD), which is relatively strongly segregated system, with f PEG ϭ0.58 crystallizes within the preexisting lamellar microdomain, but for a low molecular weight PEG-PBD with f PEG ϭ0.58, the lamellar microdomain is destroyed in crystallization 26) .…”

“…In addition to segregation strength of melt, we have elucidated that the structure change in crystallization from a microphase-separated melt also depends on some other factors 26) . For the PEG-PBD block copolymers, the microphase separation structure as well as the molecular weight (i.e., segregation strength) can affect the structure change.…”

Crystallization of graft copolymers 1. Graft chains miscible with main chains

“…[1][2][3][4][5][6][7][8][9][10][11][12] In immiscible polymer blends and block copolymers, competition or combination between crystallization and phase separation affects both crystallization and the resulting structure. [5][6][7] Especially in the crystallization of block copolymers from microphase-separated melts, the restriction of crystallization by microdomains is also important. [7][8][9][10] By contrast, for miscible polymer blends, the kinetics of crystallization is affected by diluent effects and by the composition-dependent glass transition temperature (see Mandelkern, 2 Chapter 11).…”