Relaxation processes in a lower disorder order transition diblock copolymer

Abstract

The dynamics of lower disorder-order temperature diblock copolymer leading to phase separation has been observed by X ray photon correlation spectroscopy. Two different modes have been characterized. A non-diffusive mode appears at temperatures below the disorder to order transition, which can be associated to compositional fluctuations, that becomes slower as the interaction parameter increases, in a similar way to the one observed for diblock copolymers exhibiting phase separation upon cooling. At temperatures above the disorder to order transition TODT, the dynamics becomes diffusive, indicating that after phase separation in Lower Disorder-Order Transition (LDOT) diblock copolymers, the diffusion of chain segments across the interface is the governing dynamics. As the segregation is stronger, the diffusive process becomes slower. Both observed modes have been predicted by the theory describing upper order-disorder transition systems, assuming incompressibility. However, the present results indicate that the existence of these two modes is more universal as they are present also in compressible diblock copolymers exhibiting a lower disorder-order transition. No such a theory describing the dynamics in LDOT block copolymers is available, and these experimental results may offer some hints to understanding the dynamics in these systems. The dynamics has also been studied in the ordered state, and for the present system, the non-diffusive mode disappears and only a diffusive mode is observed. This mode is related to the transport of segment in the interphase, due to the weak segregation on this system.

Grain Growth and Coarsening Dynamics in a Compositionally Asymmetric Block Copolymer Revealed by X-ray Photon Correlation Spectroscopy

“…The average relaxation times remain nearly constant at approximately τ̅ ≈ 1000 s for all quench temperatures, suggesting that the dominant relaxation mode measured by XPCS is independent of temperature. Consistent with prior XPCS studies of ordering in block polymers, , the “on grain” structural relaxations are much longer than those that we previously reported for the supercooled fluctuating disordered state of this system . The order of magnitude increase in the structural relaxation times upon ordering implies a change in mechanism from one dominated by diblock polymer chain exchange between micelles in the disordered state (prior to ordering after supercooling from 170 °C to 130 < T < 153 °C) to another mode of relaxation likely tied to the ordered domain dynamics.…”

“…However, we anticipate that the concentration of such intrinsic defects is relatively low due to the constraint of filling space at constant density, and therefore this effect should not contribute significantly to the fluctuating scattered intensity. Substantial fluctuations in the (110) interplanar distance may also be caused by fluctuations in micelle size resulting from individual chain exchange events, by analogy to the lamellar diblock polymers studied by Sanz et al However, we previously measured considerably faster relaxation times in the fluctuating disordered state of this same system prior to BCC grain nucleation that are associated with block polymer chain exchange events . The order of magnitude longer relaxation times in the ordered state likely imply that the physical mechanism of structural relaxation is caused by longer-ranged, collective motion.…”

“…In block polymers, XPCS has been used to probe dynamics in the fluctuating disordered melt state, ,, with more recent work extending to ordered, microphase-separated structures. ,, Patel et al used XPCS to probe the effect of ordering on “micelle relaxation times” during thermal quenches of a compositionally asymmetric diblock polymer into a hexagonally packed cylinders (HEX) phase . Sanz et al observed a diffusive relaxation mode for ordered lamellar diblocks that they attributed to interfacial chain diffusion . These results differ from recent reports by Hallinan and co-workers, who reported a hyperdiffusive relaxation mode in strongly segregated lithium salt-doped lamellar diblocks, which they ascribed to grain rotation .…”

“…X-ray photon correlation spectroscopy (XPCS) is a noninvasive technique that probes dynamic phenomena at nanometer length scales in disordered and ordered soft materials − and is thus well-suited for studies of GBs. In block polymers, XPCS has been used to probe dynamics in the fluctuating disordered melt state, ,, with more recent work extending to ordered, microphase-separated structures. ,, Patel et al used XPCS to probe the effect of ordering on “micelle relaxation times” during thermal quenches of a compositionally asymmetric diblock polymer into a hexagonally packed cylinders (HEX) phase . Sanz et al observed a diffusive relaxation mode for ordered lamellar diblocks that they attributed to interfacial chain diffusion .…”

Dynamics of a Supercooled Disordered Sphere-Forming Diblock Copolymer as Determined by X-ray Photon Correlation and Dynamic Mechanical Spectroscopies

“…Traditional techniques for investigating block polymer dynamics include dynamic mechanical spectroscopy (DMS), − forced Rayleigh scattering, , forward recoil spectroscopy, pulsed-field gradient nuclear magnetic resonance spectroscopy, depolarized light scattering, , and various photon correlation spectroscopies. , Recently, X-ray photon correlation spectroscopy (XPCS) has emerged as a useful tool for characterizing the relaxation mechanisms in soft materials. − However, few connections that bridge the dynamics probed by XPCS and those measured by more established techniques such as DMS have been established. Several studies have used XPCS to study diblock copolymer dynamics in both the ordered and disordered states, ,− yet the dynamics of compositionally asymmetric, particle-forming diblock copolymers below the order–disorder transition (ODT) temperature have yet to be interrogated with this technique. This regime is a prime target for XPCS experiments due to recent discoveries of complex, low symmetry particle packings of diblock copolymer micelles that strongly depend on the system dynamics and sample processing history. − …”