Blockiness and Sequence Polydispersity Effects on the Phase Behavior and Interfacial Properties of Gradient Copolymers

Stimuli-Responsive Control over Self-Assembled Nanostructures in Sequence-Specific Functional Block Copolymers

The precise sequence of a protein's primary structure is essential in determining its folding pathways. To emulate the complexity of these biomolecules, functional block copolymers consisting of segmented triblocks with distinct functionalities positioned in a sequence-specific manner are designed to control the polymer chain compaction. Triblock polymers P- b -C- b -F and P- b -F- b -C and random diblock copolymer P- b -C- r -F consist of a hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic block with coumarin (C) and ferrocene (F) moieties that are grafted in a sequence-specific or random manner onto the hydrophilic block. External stimuli such as UVB light, redox, and chemical cues influence the functional hydrophobic block to alter the packing parameters that are monitored with spectroscopic and scattering techniques. Interestingly, the positioning of the stimuli-responsive moiety within the hydrophobic block of P- b -C- b -F, P- b -F- b -C, and P- b -C- r -F affects the extent of the hydrophobic-hydrophilic balance in block copolymers that renders orthogonal control in stimuli-responsive transformation of self-assembled vesicles to micelles.

Influence of Microstructure on the Elution Behavior of Gradient Copolymers in Different Modes of Liquid Interaction Chromatography

We studied the influence of microstructure on the chromatographic behavior of gradient copolymers with different gradient strengths and block copolymer with completely segregated blocks by using gradient liquid adsorption chromatography (gLAC) and liquid chromatography at critical conditions (LCCC) for one of the copolymer constituents. The copolymers consist of repeating units of poly(propylene oxide) and poly(propylene phthalate) and have comparable average chemical composition and molar mass, and a narrow molar mass distribution to avoid as much as possible the influence of these parameters on the elution behavior of the copolymers. On both reversed stationary phases, the elution volume of gradient copolymers increases with the increasing strength of the gradient. The results indicate that for both modes of liquid interaction chromatography, it is important to consider the effect of microstructure on the elution behavior of the gradient copolymers in addition to the copolymer chemical composition and molar mass in the case of gLAC and the length of the chromatographically visible copolymer constituent in the case of LCCC.

Where Biology and Traditional Polymers Meet: The Potential of Associating Sequence-Defined Polymers for Materials Science

Polymers with precisely defined monomeric sequences present an exquisite tool for controlling material properties by harnessing both the robustness of synthetic polymers and the ability to tailor the inter- and intramolecular interactions so crucial to many biological materials. While polymer scientists traditionally synthesized and studied the physics of long molecules best described by their statistical nature, many biological polymers derive their highly tailored functions from precisely controlled sequences. Therefore, significant effort has been applied toward developing new methods of synthesizing, characterizing, and understanding the physics of non-natural sequence-defined polymers. This perspective considers the synergistic advantages that can be achieved via tailoring both precise sequence control and attributes of traditional polymers in a single system. Here, we focus on the potential of sequence-defined polymers in highly associating systems, with a focus on the unique properties, such as enhanced proton conductivity, that can be attained by incorporating sequence. In particular, we examine these materials as key model systems for studying previously unresolvable questions in polymer physics including the role of chain shape near interfaces and how to tailor compatibilization between dissimilar polymer blocks. Finally, we discuss the critical challenges-in particular, truly scalable synthetic approaches, characterization and modeling tools, and robust control and understanding of assembly pathways-that must be overcome for sequence-defined polymers to attain their potential and achieve ubiquity.

Polymerization-Induced Microphase Separation with Long-Range Order in Melts of Gradient Copolymers

In this work, we studied the question of whether it is possible to develop a one-step approach for the creation of microphase-separated materials with long-range order with the help of spontaneous gradient copolymers, i.e., formed during controlled copolymerization solely due to the large difference in the reactivity ratios. To that end, we studied the polymerization-induced microphase separation in bulk on the example of a monomer pair with realistic parameters based on styrene (S) and vinylpirrolydone (VP) by means of computer simulation. We showed that for experimentally reasonable chain lengths, the structures with long-range order start to appear at the conversion degree as low as 76%; a full phase diagram in coordinates (fraction of VP-conversion degree) was constructed. Rather rich phase behavior was obtained; moreover, at some VP fractions, order-order transitions were observed. Finally, we studied how the conversion degree at which the order-disorder transition occurs changes upon varying the maximum average chain length in the system.

Phase Behavior of Gradient Copolymer Melts with Different Gradient Strengths Revealed by Mesoscale Simulations

Design and preparation of functional nanomaterials with specific properties requires precise control over their microscopic structure. A prototypical example is the self-assembly of diblock copolymers, which generate highly ordered structures controlled by three parameters: the chemical incompatibility between blocks, block size ratio and chain length. Recent advances in polymer synthesis have allowed for the preparation of gradient copolymers with controlled sequence chemistry, thus providing additional parameters to tailor their assembly. These are polydisperse monomer sequence, block size distribution and gradient strength. Here, we employ dissipative particle dynamics to describe the self-assembly of gradient copolymer melts with strong, intermediate, and weak gradient strength and compare their phase behavior to that of corresponding diblock copolymers. Gradient melts behave similarly when copolymers with a strong gradient are considered. Decreasing the gradient strength leads to the widening of the gyroid phase window, at the expense of cylindrical domains, and a remarkable extension of the lamellar phase. Finally, we show that weak gradient strength enhances chain packing in gyroid structures much more than in lamellar and cylindrical morphologies. Importantly, this work also provides a link between gradient copolymers morphology and parameters such as chemical incompatibility, chain length and monomer sequence as support for the rational design of these nanomaterials.

Effect of Poly(vinyl butyral) Comonomer Sequence on Adhesion to Amorphous Silica: A Coarse-Grained Molecular Dynamics Study

Modulating a comonomer sequence, in addition to the overall chemical composition, is the key to unlocking the true potential of many existing commercial copolymers. We employ coarse-grained molecular dynamics (MD) simulations to study the behavior of random-blocky poly(vinyl butyral-co-vinyl alcohol) (PVB) melts in contact with an amorphous silica surface, representing the interface found in laminated safety glass. Our two-pronged coarse-graining approach utilizes both macroscopic thermophysical data and all-atom MD simulation data. Polymer-polymer nonbonded interactions are described by the fused-sphere SAFT-γ Mie equation of state, while bonded interactions are derived using Boltzmann inversion to match the bond and angle distributions from all-atom PVB chains. Spatially dependent polymer-surface interactions are mapped from a hydroxylated all-atom amorphous silica slab model and all-atom monomers to an external potential acting on the coarse-grained sites. We discovered an unexpected complex relationship between the blockiness parameter and the adhesion energy. The adhesion strength between PVB copolymers with intermediate VA content and silica was found to be maximal for random-blocky copolymers with a moderately high degree of blockiness rather than for diblock copolymers. We attribute this to two main factors: (1) changes in morphology, which dramatically alter the number of VA beads interacting with the surface and (2) a non-negligible contribution of vinyl butyral (VB) monomers to adhesion energy because of their preference to adsorb to zones with low hydroxyl density on the silica surface.

Effect of sequence dispersity on morphology of tapered diblock copolymers from molecular dynamics simulations

Tapered diblock copolymers are similar to typical AB diblock copolymers but have an added transition region between the two blocks which changes gradually in composition from pure A to pure B. This tapered region can be varied from 0% (true diblock) to 100% (gradient copolymer) of the polymer length, and this allows some control over the microphase separated domain spacing and other material properties. We perform molecular dynamics simulations of linearly tapered block copolymers with tapers of various lengths, initialized from fluids density functional theory predictions. To investigate the effect of sequence dispersity, we compare systems composed of identical polymers, whose taper has a fixed sequence that most closely approximates a linear gradient, with sequentially disperse polymers, whose sequences are created statistically to yield the appropriate ensemble average linear gradient. Especially at high segregation strength, we find clear differences in polymer conformations and microstructures between these systems. Importantly, the statistical polymers are able to find more favorable conformations given their sequence, for instance, a statistical polymer with a larger fraction of A than the median will tend towards the A lamellae. The conformations of the statistically different polymers can thus be less stretched, and these systems have higher overall density. Consequently, the lamellae formed by statistical polymers have smaller domain spacing with sharper interfaces.

Diffusion of Selective Penetrants in Interfacially Modified Block Copolymers from Molecular Dynamics Simulations

To show the influence of the interface on structure and dynamics of microphase separated polymer systems, we study interfacially modified AB block copolymers with small molecule penetrants. The polymers have a random midblock or tapered midblock whose composition varies from pure A to pure B (or from pure B to pure A for an inverse taper) between two pure blocks of A and B. We perform simple coarse-grained molecular dynamics simulations of symmetric polymers that form lamellae. With normal tapering, both polymer and penetrant diffusion parallel to the lamellae increases as taper length increases. Inverse tapered polymers exist in different conformational states (e.g., stretched vs folded back and forth across the interface) with different dynamic behavior, leading to nonmonotonic trends in their diffusion. However, the local mixing of monomers (rather than polymer conformation) appears to be the most important factor in determining penetrant diffusion.

Field-theoretic simulations of random copolymers with structural rigidity

Copolymers play an important role in a range of soft-materials applications and biological phenomena. Prevalent works on block copolymer phase behavior use flexible chain models and incorporate interactions using a mean-field approximation. However, when phase separation takes place on length scales comparable to a few monomers, the structural rigidity of the monomers becomes important. In addition, concentration fluctuations become significant at short length scales, rendering the mean-field approximation invalid. In this work, we use simulation to address the role of finite monomer rigidity and concentration fluctuations in microphase segregation of random copolymers. Using a field-theoretic Monte-Carlo simulation of semiflexible polymers with random chemical sequences, we generate phase diagrams for random copolymers. We find that the melt morphology of random copolymers strongly depends on chain flexibility and chemical sequence correlation. Chemically anti-correlated copolymers undergo first-order phase transitions to local lamellar structures. With increasing degree of chemical correlation, this first-order phase transition is softened, and melts form microphases with irregular shaped domains. Our simulations in the homogeneous phase exhibit agreement with the density-density correlation from mean-field theory. However, conditions near a phase transition result in deviations between simulation and mean-field theory for the density-density correlation and the critical wavemode. Chain rigidity and sequence randomness lead to frustration in the segregated phase, introducing heterogeneity in the resulting morphologies.

Multiblock copolymers prepared by patterned modification: Analytical theory and computer simulations

We describe a special type of multiblock copolymers which are synthesized by a hypothetic procedure of the modification of monomer units in a polymer melt according to a certain geometrical criterion. In particular, we explore the case of lamellar-like structures: the sequence statistics of the resulting multiblock copolymers is described and their ability to self-assemble is studied. It is found that the block-size distribution P(k) for such random copolymers contains a large fraction of short blocks with the asymptotic dependence ∼k(-3/2), where k is the block size. A characteristic feature of such multiblock copolymers is their extremely high block-size polydispersity with the polydispersity index being proportional to the space period of the modification. The morphological behavior of such copolymers is simulated by means of dissipative particle dynamics. A stable self-assembled lamellar structure is observed, but the domain size appears to be sufficiently larger than the initial pattern period.

Biodegradable polymeric nanostructures in therapeutic applications: opportunities and challenges

Biodegradable polymeric nanostructures (BPNs) have shown great promise in different therapeutic applications such as diagnosis, imaging, drug delivery, cosmetics, organ implants, and tissue engineering.

Gradient and block side-chain liquid crystalline polyethers

Synthesis of gradient liquid crystalline copolymers is reported for the first time, phase structures of which on multiple length scales with composition and temperature are investigated and compared with the corresponding diblock copolymers.

Phase Behavior of Tapered Diblock Copolymers from Self-Consistent Field Theory

Tapered diblock copolymers are similar to AB diblock copolymers, but the sharp junction between the A and B blocks is replaced with a gradient region in which composition varies from mostly A to mostly B along its length. The A side of the taper can be attached to the A block (normal) or the B block (inverse). We demonstrate how taper length and direction affect the phase diagrams and density profiles using self-consistent field theory. Adding tapers shifts the order-disorder transition to lower temperature versus the diblock, and this effect is larger for longer tapers and for inverse tapers. However, tapered systems' phase diagrams and interfacial profiles do not simply match those of diblocks at a shifted effective temperature. For instance, we find that normal tapering widens the bicontinuous gyroid region of the phase diagram, while inverse tapering narrows this region, apparently due to differences in polymer organization at the interfaces.

Precise Compositional Control and Systematic Preparation of Multimonomeric Statistical Copolymers

A comprehensive approach to target exact molecular weights and chemical compositions for multimonomeric statistical copolymers using a new controlled statistics method with reversible addition-fragmentation chain transfer free-radical (RAFT) polymerization is presented. The system chosen to illustrate this procedure is an acrylic quarterpolymer consisting of methyl acrylate, 2-carboxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-propylacetyl acrylate, modeling a well-known macromolecule utilized to deliver poorly water-soluble drugs (hydroxypropyl methylcellulose acetate succinate, HPMCAS). The relative reactivities at 70 °C between monomer pairs were measured and employed to predict the feed ratio necessary for synthesizing well-defined compositions based on the Walling-Briggs model. Application of Skeist's equations addressed compositional drift and anticipated the general monomer incorporation distribution as a function of conversion, which was verified experimentally. This new and simple paradigm combining both predictive models provides complementary synthetic and predictive tools for designing macromolecular chemical architectures with hierarchical control over spatially dependent structure-property relationships for complex applications such as oral drug delivery.