Session A18: Bulk Block Copolymers I

Order-order transition among lamellae, \textit{Fddd}, and gyroid in diblock copolymer melts

We firstly found a Disorder-Gyroid-\textit{Fddd}-Lamellae transition behavior found poly(styrene-$b$-isoprene) (S-I) diblock copolymer melts in previous study. In this study, we will present the dynamics of order-order transition (OOT) among lamellae, \textit{Fddd}, and gyroid. we investigated the dynamics of OOT by using time-resolved small angle X-ray scattering with Synchrotron radiation X-ray source. We found that \textit{Fddd} structure was formed as a metastable structure during the OOT from lamellae to gyroid induced by temperature jump.   

Determination of\textit{ Fddd} phase boundary in polystyrene-\textit{block}-polyisoprene diblock copolymer

We previously reported the discovery of a novel bicontinuous microdomain structure with \textit{Fddd} symmetry in polystyrene-\textit{block}-polyisoprene (SI) diblock copolymer. In this study, we investigated the phase behavior of eight SI diblock copolymer samples having slightly different compositions (0.627$\le f_{PI} \quad \le $0.653) by SAXS and TEM to determine the phase boundary of the \textit{Fddd} structure in the phase diagram of SI. The SI having the lowest $f_{PI}$ (= 0.627) showed only disorder-lamella (L) transition but no order-order transition. The SI having the largest $f_{PI}$ (= 0.653) showed disorder-gyroid (G)-L transition with decreasing temperature, but did not show \textit{Fddd} phase. The other six SI samples having $f_{PI}$ values between these two exhibited disorder-G-\textit{Fddd}-L transition with decreasing temperature. Consequently, we could determine the compositional region where \textit{Fddd} phase is thermally stable, which is in good agreement with that predicted by SCFT.   

Stabilization of Bicontinuous Phases in Diblock Copolymer Systems

We used a coarse-grained description of the copolymer chains (i.e., dissipative particle dynamics fluid), together with continuum-space Monte Carlo and Molecular Dynamics methods, to study systems of diblock copolymers melts that have been ``filled'' with selective additives (i.e., homopolymer, and nanoparticles). Approximate phase boundaries were found via free-energy calculations. We focus on the stabilization of bi-continuous phases and the strikingly different phase behavior observed when the nature of the selective filler is changed. Our results elucidate the origins of the packing frustration that limits the viability of the gyroid, double-diamond, and plumber's nightmare phases and provide insights for overcoming it. Attention is also focused on directly determining the areas of phase diagram where macro- phase separation occurs. We compare the particle-based simulation results with the results obtained by means of self- consistent filed theory calculations.   

Structure-Properties Relationship in Proton Conductive Sulfonated Polystyrene-Polymethyl Methacrylate Block Copolymers.

We report on the dependence of proton conductivity on the morphologies of sulfonated polystyrene-poly(methyl methacrylate) (sPS-PMMA) diblock copolymers. Three diblock copolymers of varying molecular weight and block volume fraction were studied, for each one several sulfonation degrees of the PS block were considered. The investigation of the morphologies of the self-assembled sPS-PMMA diblocks was carried out by means of small angle neutron scattering and transmission electron microscopy. Depending on molecular weight and sulfonation degrees, isotropic phase (ISO), lamellar phase (LAM), cylindrical hexagonal phase (HEX) and hexagonally perforated lamellae (HPL) were observed. Proton conductivity, normalized by the volume fraction of the conductive domains (formed by PS, sPS and water), was shown to rise monotonically with the following sequence of morphologies: ISO to HEX to HPL to LAM.   

Morphology of Sulfonated Styrenic Pentablock Copolymer Solutions and Membranes

We report a systematic investigation of the morphology of sulfonated styrenic pentablock copolymer solutions and membranes obtained from Kraton Polymers LLC. The polymer studied was poly((t-butyl-styrene)-b-(ethylene-r-butylene)-b-(styrene-r-styrene sulfonate)-b-(ethylene-r-butylene)-b-(t-butyl-styrene)). Small angle x-ray scattering (SAXS) revealed that the solutions exhibited micellar morphologies. The solution SAXS data was modeled using the Kinning-Thomas model to obtain radius of the micelle core, the radius of closest approach between two micelles and the volume fraction of micelles. The membranes exhibited anisotropic morphologies with different d-spacings in-plane and through-plane. A good linear correlation was observed between the radius of closest approach between two micelles in the solutions and the d-spacings in the membranes. Efforts are underway to characterize the type of morphology in the membranes using electron microscopy and correlate them to the transport properties.   

Thermodynamic Behavior of Poly(styrene-b-styrene sulfonate) Block Copolymers With Varying Counterions

A series of poly(styrene-b-styrene sulfonate) (PS-b-PSS) block copolymers have been prepared by RAFT polymerization. The counterions in the PSS block have been varied by neutralizing the sulfonate groups with alkyl amines or quaternary ammonium ions. The choice of counterion has a strong effect on the lipophilicity of the PSS block. This presentation will focus on the resulting morphology and bulk thermodynamic behavior of these polymers as a function of the PSS counterion. The use of these materials for preparing ion-exchange membranes will be discussed.   

Salt Doping in PEO Containing Block Copolymers: Counterion and Concentration Effects

Salt-doped poly(ethylene oxide)-based block copolymers are promising candidates for lithium battery polymer electrolytes, which require high ionic conductivities and adequate mechanical integrity. We studied the phase behavior of poly(styrene-b-ethylene oxide) block copolymers doped with various lithium salts over a range of [EO]:[Li] ratios. Small-angle X-ray scattering, transmission electron microscopy, and differential scanning calorimetry experiments were used to characterize the phase behavior of our samples. Specimens were prepared in an argon atmosphere and rigorously dried to reduce the effects of moisture uptake on phase behavior. We found that we can tune the copolymer microstructure by varying the lithium counterion as well as the salt doping ratio. Using strong segregation theory, we estimated an effective interaction parameter for the salt-doped copolymers, which varies linearly with salt concentration, where the slope is influenced by the nature of the counterion.   

Ion transport through block copolymer electrolytes

Poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) is a candidate material for electrolytes for rechargeable lithium metal batteries. The PS phase suppresses lithium dendrite growth on the anode during recharge, and the PEO phase solvates lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt to form conducting pathways. Complete electrochemical characterization of PEO/LiTFSI mixtures requires measurement of conductivity, salt diffusion coefficient, and lithium ion transference number. The present study covers SEO copolymers that exhibit lamellar and cylindrical morphologies in the absence of salt. The addition of salt affects morphology but the relationships between morphology and electrochemical characteristics have not yet been clarified. Some aspects of these relationships will be presented.   

Morphology of Novel Semicrystalline Ethylene-$\alpha$-Olefin Block Copolymers

In semicrystalline block copolymers, the solid-state structure can be set either by block incompatibility or by crystallization of one or more blocks. Depending on the block interaction strength, a wide array of solid-state morphologies may be observed, ranging from spherulitic to confined crystallization within preexisting microphase-separated domains. Dow Chemical has recently developed a novel chain shuttling polymerization process to produce olefin block copolymers with alternating amorphous and semicrystalline chain segments, where each block exhibits the most-probable distribution. We examined the melt and solid-state morphologies of these novel olefin block copolymers, having a high octene content in the amorphous block, using two- dimensional synchrotron small-angle and wide-angle x-ray scattering on specimens oriented by channel die compression. Multiblock and diblock copolymers with near-symmetric compositions showed well-ordered lamellar structures at room temperature with long periods exceeding 100 nm, with little dependence on thermal history, indicating the presence of a mesophase-separated melt which templates crystallization.   

Self-assembly of crystalline bioinspired block copolymers

Polypeptoids are sequence-specific biologically inspired polymers based on N-substituted glycines for which monodisperse, polymeric molecular weights can be achieved. Sequence control allows for a degree of tunability in both the self-assembly and thermal properties not available in classical polymer systems. We demonstrate that a series of homopolypeptoids are thermally stable to 300C and are crystalline with melting transitions ranging from 150C to 250C. The introduction of defects at precise locations in the polymer sequence (as a side chain substitution) allows crystallization and hence the melting temperature to be suppressed. Symmetric block copolymers with two crystalline polypeptoid blocks exhibit co-crystallization of the two blocks but distinct melting behaviors, indicating a disordered melt. If samples are carefully prepared to allow for microphase separation, block copolymer lamellae with long range order are formed with an order-disorder transition temperature well below the melting transition temperature of the polymer.   

Crystallization, Crystal Orientation and Morphology of Poly(ethylene oxide) under 1D Defect-Free Nanoscale Confinement

One-dimensional (1-D) defect-free nanoscale confinement is created by growing single crystals of PS-b-PEO block copolymers in dilute solution. Those defect-free, 1-D confined lamellae having different PEO layer thicknesses in PS-b-PEO lamellar single crystals (or crystal mats) were used to study the polymer recrystallization and crystal orientation evolution as a function of recrystallization temperature (T$_{rx})$ because the T$_{g}^{PS}$ is larger than T$_{m}^{PEO}$ in the PS-b-PEO single crystal. The results are summarized as follows. First, by the combination of electron diffraction and known PEO crystallography, the crystallization of PEO only takes place at T$_{rx}<$-5$^{o}$C. Meanwhile a unique tilted PEO orientation is formed at T$_{rx }>$-5$^{o}$C after self-seeding. The origin of the formation of tilted chains in the PEO crystal will be addressed. Second, from the analysis of 2D WAXD patterns of crystal mats, it is shown that the change in PEO c-axis orientation from homogeneous at low T$_{rx}$ to homeotropic at higher T$_{rx}$ transitions sharply, within 1$^{o}$C. The mechanism inducing this dramatic change in crystal orientation will be investigated in detail.   

Gradient Architecture as Means of Phase Diagram Manipulation in Copolymers: Accessing Both LCOT and UCOT in High Molecular Weight Styrene/n-Butyl Acrylate Systems

Traditionally, phase transitions of block copolymers could only be tuned through molecular weight and relative block length. Here, we introduce comonomer sequence design through gradient compositions as a means of further manipulating phase diagram boundaries. In such gradient copolymers, the reduced repulsion between chain segments allows access to phase transitions even at high molecular weights (MW). Rheological and x-ray scattering studies were performed to study the impact of comonomer sequence on phase behavior in styrene/n-butyl acrylate (S/nBA) systems. In S/nBA block copolymers, only upper critical ordering behavior was observed. In contrast, by using a gradient architecture of higher MW we observed both upper and lower ordering transitions similar to those seen in very weakly segregating S/n-butyl methacrylate block copolymers, where such dual ordering transitions were first detected by Russell et al. This is the first study to access a miscibility gap in gradient copolymers. Access to such behavior is very rare in blends and block copolymers, limited to low MW and/or very weakly segregating systems.   

Effects of polydispersity on the order-disorder transition of diblock copolymer melts

The effect of polydispersity on an AB diblock copolymer melt is investigated using lattice based Monte Carlo simulations with parallel tempering (PT) techniques. We consider melts where the B blocks are monodisperse and the A blocks are polydisperse with a Schultz-Zimm distribution. Expanding our previous work on polydisperse melts of symmetric composition, we now construct a polydisperse phase diagram, investigating the size of the domains and locations of the order-disorder (ODT) and order-order (OOT) transitions. The PT method has yielded a number of benefits over single-processor temperature scans, including: simulating a number of temperatures simultaneously, annealing out defects in the configurations more readily and capturing the distinctive spike in the heat capacity that occurs at the ODT, allowing the location of the transition to be determined more accurately than in previous studies. The results are compared to those of experiment and to the predictions of self-consistent field theory (SCFT).   

Dynamics of Disordered PI-PtBS Diblock Copolymer

Viscoelastic ($G^*$) and dielectric ($\varepsilon''$) data were examined for a LCST-type diblock copolymer composed of polyisoprene (PI; M = 53K) and poly(\textit{p}-\textit{tert}- butyl styrene) (PtBS; M = 42K) blocks disordered at $T \quad \le 120 \textrm{C}^{\circ}$. Only PI had the type-A dipole parallel along the chain backbone. Thus, the $\varepsilon''$ data reflected the global motion of the PI block, while the $G^*$ data detected the motion of the copolymer chain as a whole. Comparison of these data indicated that the PI block relaxed much faster than the PtBS block at low $T$ and the dynamic heterogeneity due to PtBS was effectively quenched to give a frictional nonuniformity for the PI block relaxation. The $\varepsilon''$ data were thermo-rheologically complex at low $T$, partly due to this nonuniformity. However, the block connectivity could have also led to the complexity. For testing this effect, the $\varepsilon''$ data were reduced at the iso- frictional state defined with respect to bulk PI. In this state, the $\varepsilon''$ data of the copolymer at low and high $T$, respectively, were close to the data for the star-branched and linear bulk PI. Thus, the PI block appeared to be effectively tethered in space at low $T$ thereby behaving similarly to the star arm while the PI block tended to move cooperatively with the PtBS block at high $T$ to behave similarly to the linear PI, which led to the complexity of the $\varepsilon''$ data. The PtBS block also exhibited the complexity (noted from the $G^*$ data), which was well correlated with the complexity of the PI block.   

Morphology of Renewable Polylactide / Soybean Oil Blends Compatibilized by Block Copolymers

Renewable composites derived from polylactide and soybean oil (soy) were prepared by melt blending. The blend morphology was tuned with the addition of poly(isoprene-b-lactide) block copolymers. Due to the extreme differences in the viscosities of soy and polylactide, a critical block copolymer block ratio was found to induce a phase inversion in which the morphology changed from soy droplets in a polylactide matrix to polylactide droplets in a soy matrix, even though soy was the minority component. This transition was not only due to the thermodynamic interactions between the block copolymer and the two immiscible phases, but also was a result of shear forces acting on the mixture during melt blending. The droplet size of the soy droplets in the polylactide matrix was also highly dependent on the block copolymer composition. In binary polylactide/soy blends there was a limiting concentration of soy which could be incorporated into the polylactide matrix (5 percent of the total blend weight) due to the mismatch in viscosities resulting in the loss of soy during mixing. The addition of block copolymer with an appropriate block ratio allowed full incorporation of soy up to 20 percent of the total blend weight.