Influence of Composition Fluctuations on the Linear Viscoelastic Properties of Symmetric Diblock Copolymers near the Order-Disorder Transition

Molecular Engineering of Nanostructures in Disordered Block Polymers

A series of symmetric poly(methyl methacrylate-stat-styrene)-block-polylactide (P(MMA-s-S)-b-PLA) diblock terpolymers with nearly constant molar masses yet varying block interaction parameters were synthesized as a model system to probe the extent and utility of composition fluctuations in the disordered state. A combination of differential scanning calorimetry, dynamic mechanical analysis, and small-angle X-ray scattering revealed that a broad range of segregation strengths ranging from what we ascribe to essentially a mean-field disordered to a fluctuating disordered to an ordered system could be readily obtained by tuning the molar fraction of styrene in these diblocks. The P(MMA-s-S)-b-PLA diblocks were annealed above their order-disorder transition temperatures (T_ODT) and rapidly quenched to low temperatures to trap the disordered state via vitrification, as confirmed by scanning electron microscopy. Small-angle X-ray scattering and N₂ sorption analysis post-removal of PLA demonstrated that a transition from a very weakly structured, mean-field-like melt to a bicontinuous fluctuating disordered state occurred with increasing segregation strength. This work demonstrates that the extent of microphase segregation as well as the domain continuity of the disordered block polymer melt can be tuned using both synthetic design and thermal stimuli, guiding the design of disordered block polymers with targeted nanostructures that have potential technological utility.

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

We report the dynamic behavior of a sphere-forming poly(styrene)-block-poly(1,4-butadiene) (PS-PB) diblock copolymer comprising 20 vol % PB below the order-disorder transition temperature (T_ODT = 153 °C) using dynamic mechanical spectroscopy (DMS) and X-ray photon correlation spectroscopy (XPCS). A time-temperature transformation diagram was constructed by monitoring the elasticity of the sample as a function of time following rapid quenches of the disordered melt to various temperatures T < T_ODT. Isothermal frequency spectra acquired prior to nucleation of the ordered BCC phase were time-temperature superposed, and the shift factors were fit using the Williams-Landel-Ferry (WLF) equation. For comparison, XPCS measurements were used to extract relaxation times from the supercooled liquid as a function of the quench temperature. Alignment of the temperature dependence of the XPCS-based relaxation times with that of the WLF shift factors in the range T = 125-140 °C indicates that both techniques probe the fluctuating mesomorphic micelle dynamics mediated by the relaxation modes of individual chains, including interparticle chain exchange. For deeper quench temperatures, T_ODT - T ≥ 28 °C, departure of the XPCS time constant from WLF behavior is consistent with a jamming transition, analogous to that encountered in concentrated colloidal systems. We postulate that the dominant relaxation mode in the supercooled disordered liquid transitions from ergodic dynamics governed by chain exchange to a nonergodic regime dominated by local rearrangement of micellar particles at T ≈ T_erg, where T_erg denotes the ergodicity temperature.

Origins of low-symmetry phases in asymmetric diblock copolymer melts

Cooling disordered compositionally asymmetric diblock copolymers leads to the formation of nearly spherical particles, each containing hundreds of molecules, which crystallize upon cooling below the order-disorder transition temperature (TODT). Self-consistent field theory (SCFT) reveals that dispersity in the block degrees of polymerization stabilizes various Frank-Kasper phases, including the C14 and C15 Laves phases, which have been accessed experimentally in low-molar-mass poly(isoprene)-b-poly(lactide) (PI-PLA) diblock copolymers using thermal processing strategies. Heating and cooling a specimen containing 15% PLA above and below the TODT from the body-centered cubic (BCC) or C14 states regenerates the same crystalline order established at lower temperatures. This memory effect is also demonstrated with a specimen containing 20% PLA, which recrystallizes to either C15 or hexagonally ordered cylinders (HEXC) upon heating and cooling. The process-path-dependent formation of crystalline order shapes the number of particles per unit volume, n/V, which is retained in the highly structured disordered liquid as revealed by small-angle X-ray scattering (SAXS) experiments. We hypothesize that symmetry breaking during crystallization is governed by the particle number density imprinted in the liquid during ordering at lower temperature, and this metastable liquid is kinetically constrained from equilibrating due to prohibitively large free energy barriers for micelle fusion and fission. Ordering at fixed n/V is enabled by facile chain exchange, which redistributes mass as required to meet the multiple particle sizes and packing associated with specific low-symmetry Frank-Kasper phases. This discovery exposes universal concepts related to order and disorder in self-assembled soft materials.

Nanoporous Thermosets with Percolating Pores from Block Polymers Chemically Fixed above the Order-Disorder Transition

A lamellar diblock polymer combining a cross-linkable segment with a chemically etchable segment was cross-linked above its order-disorder temperature (TODT) to kinetically trap the morphology associated with the fluctuating disordered state. After removal of the etchable block, evaluation of the resulting porous thermoset allows for an unprecedented experimental characterization of the trapped disordered phase. Through a combination of small-angle X-ray scattering, nitrogen sorption, scanning electron microscopy, and electron tomography experiments we demonstrate that the nanoporous structure exhibits a narrow pore size distribution and a high surface to volume ratio and is bicontinuous over a large sample area. Together with the processability of the polymeric starting material, the proposed system combines attractive attributes for many advanced applications. In particular, it was used to design new composite membranes for the ultrafiltration of water.

Unusual phase separation and rheological behavior of poly(ethylene oxide)/ionic liquid mixtures with specific interactions

The phase separation behavior of poly(ethylene oxide) (PEO) in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was investigated by rheological, optical microscopy, FT-IR and DSC measurements. It is demonstrated that specific interactions, particularly the hydrogen bonding between PEO and the ionic liquids as evidenced by FT-IR, in which a subtle but apparent absorption peak shift near the phase transition appears, account for the unusual low critical solution temperature (LCST) phase separation. Unlike the typical trend in which the storage modulus G' simply increases with temperature near the phase boundary for polymer blends without specific interaction, in our study, a novel "V-shaped" rheological response is observed, namely a dip in G' followed by an upturn, especially at low PEO concentration (<50 wt%). The magnitude of the "V" dip has heating rate and frequency dependences, while Tr (the phase transition temperature) is almost unchanged with heating rate and frequency. Upon increasing the alkyl chain length on the imidazolium ring from an ethyl to a butyl, the "V-shape" becomes more prominent and shifts to higher temperature, which is consistent with the results of FT-IR and DSC, evidently due to the stronger hydrogen bonding interaction between PEO and [BMIM][BF4] than [EMIM][BF4]. This unusual "V" dip might be tentatively ascribed to the coupling effects of the breaking of the "hydrogen bonding cage" formed between PEO chains and IL molecules and dissolution of the heterogeneous clusters as verified by FT-IR and TEM, respectively, and the following upturn is dominated by the interface formation upon phase separation.

Tailoring the Static and Dynamic Mechanical Properties of Tri-Block Copolymers through Molecular Dynamics Simulation

Although the research of the self-assembly of tri-block copolymers has been carried out widely, little attention has been paid to study the mechanical properties and to establish its structure-property relation, which is of utmost significance for its practical applications. Here, we adopt molecular dynamics simulation to study the static and dynamic mechanical properties of the ABA tri-block copolymer, by systematically varying the morphology, the interaction strength between A-A blocks, the temperature, the dynamic shear amplitude and frequency. In our simulation, we set the self-assembled structure formed by A-blocks to be in the glassy state, with the B-blocks in the rubbery state. With the increase of the content of A-blocks, the spherical, cylindrical and lamellar domains are formed, respectively, exhibiting a gradual increase of the stress-strain behavior. During the self-assembly process, the stress-strain curve is as well enhanced. The increase of the interaction strength between A-A blocks improves the stress-strain behavior and reduces the dynamic hysteresis loss. Since the cylindrical domains are randomly dispersed, the stress-strain behavior exhibits the isotropic mechanical property; while for the lamellar domains, the mechanical property seems to be better along the direction perpendicular to than parallel to the lamellar direction. In addition, we observe that with the increase of the dynamic shear amplitude and frequency, the self-assembled domains become broken up, resulting in the decrease of the storage modulus and the increase of the hysteresis loss, which holds the same conclusion for the increase of the temperature. Our work provides some valuable guidance to tune the static and dynamic mechanical properties of ABA tri-block copolymer in the field of various applications.

Structure, viscoelasticity, and interfacial dynamics of a model polymeric bicontinuous microemulsion

We have systematically studied the equilibrium structure and dynamics of a polymeric bicontinuous microemulsion (BμE) composed of poly(cyclohexylethylene) (PCHE), poly(ethylene) (PE), and a volumetrically symmetric PCHE-PE diblock copolymer, using dynamic mechanical spectroscopy, small angle X-ray and neutron scattering, and transmission electron microscopy. The BμE was investigated over an 80 °C temperature range, revealing a structural evolution and a rheological response not previously recognized in such systems. As the temperature is reduced below the point associated with the lamellar-disorder transition at compositions adjacent to the microemulsion channel, the interfacial area per chain of the BμE approaches that of the neat (undiluted) lamellar diblock copolymer. With increasing temperature, the diblock-rich interface swells through homopolymer infiltration. Time-temperature-superposed linear dynamic data obtained as a function of frequency show that the viscoelastic response of the BμE is strikingly similar to that of the fluctuating pure diblock copolymer in the disordered state, which we associate with membrane undulations and the breaking and reforming of interfaces. This work provides new insights into the structure and dynamics that characterize thermodynamically stable BμEs in the limits of relatively weak and strong segregation.