Copolymer Melts Research Articles

Development of an Antimicrobial-Coated Absorbable Monofilament Suture from a Medical-Grade Poly(l-lactide-co-ε-caprolactone) Copolymer.

In this study, a medical-grade poly(l-lactide-co-ε-caprolactone) (PLC) copolymer with a monomer ratio of l-lactide (L) to ε-caprolactone (C) of 70:30 mol % for use as an absorbable surgical suture was synthesized via ring-opening polymerization (ROP) using a novel soluble liquid tin(II) n-butoxide (Sn(OnC4H9)2) as an initiator. In fiber fabrication, the process included copolymer melt extrusion with a minimal draw followed by sequential controlled hot-drawing and fixed-annealing steps to obtain oriented semicrystalline fibers with improved mechanical strength. For healing enhancement, the fiber was dip-coated with “levofloxacin” by adding the drug into a solution mixture of acetone, poly(ε-caprolactone) (PCL), and calcium stearate (CaSt) in the ratio of acetone/PCL/CaSt = 100:1% w/v:0.1% w/v. The tensile strength of the coated fiber was found to be increased to ∼400 MPa, which is comparable with that of commercial polydioxanone (PDS II) of a similar size. Finally, the efficiency of the drug-coated fiber regarding its controlled drug release and antimicrobial activity was investigated, and the results showed that the coated fiber was able to release the drug continuously for as long as 30 days. For fiber antimicrobial activity, it was found that a concentration of 1 mg/mL was sufficient to inhibit the growth of Staphylococcus aureus (MRSA), Escherichia coli O157:H7, and Pseudomonas aeruginosa, giving a clear inhibition zone range of 20–24 mm for 90 days. Cytotoxicity testing of the drug-coated fibers showed a %viability of more than 70%, indicating that they were nontoxic.

Dissipative morphological characteristics of photo-responsive block copolymers driven by time-oscillatory irradiations: An in silico study

Dissipative or dynamic self-assembly has become one of the most important topics in the past twenty years. Under continuous energy input, the building blocks may form various dissipative structures in steady states, which will be destroyed when the energy input is stopped. Among the common forms of energy input, light is widely used for its high efficiency and adjustability. However, there is lack of study on the dissipative self-assembly of photo-responsive block copolymers under time-dependent light irradiations (especially in time-oscillatory light irradiations) as well as its design strategies. We performed non-equilibrium dissipative particle dynamics simulations of azobenzene-contained semiflexible block copolymers in melt and investigated important controlling factors that influence dissipative structures, including the volume ratio of blocks, the interaction between blocks, the oscillation period and the volume fraction of photo-responsive polymers in the system. We found that, when the oscillation period is much smaller than the diffusional timescale of block copolymer, dissipative steady structures will be formed. With the increase of oscillation period, oscillatory dissipative structures with periodic structural variation were formed based on dissipative steady structures. As oscillation period increased further, there would be no regular structure. Furthermore, the dissipative steady structures could be finely tuned by adjusting the volume fraction of photo-responsive chains in block copolymer melt. Our study provides a guidance for the design of azobenzene-contained block copolymers that can form dissipative structures in time-oscillatory irradiations and helps to predict the self-assembled morphologies.

Frank–Kasper Phases in Block Polymers

The discovery of the Frank–Kasper σ phase in diblock copolymer and tetrablock terpolymer melts catalyzed a renewed interest over the past decade in understanding particle-forming phases in block polymer systems. This Perspective provides a concise overview of the Frank–Kasper phases seen to date in block polymers (A15, σ, and the C14 and C15 Laves phases) and mechanisms known to produce them: conformational asymmetry in neat diblock copolymer melts, interfacial segregation effects in diblock copolymer blends, particle swelling in diblock copolymer/homopolymer blends, and matrix segregation effects in neat tetrablock terpolymer melts. While a qualitative understanding of the emergence of Frank–Kasper phases in block polymer systems has been achieved, a number of outstanding questions remain, in particular those arising from the low degree of polymerization used in experiments, nonequilibrium effects during thermal processing, and the large design space available in blends and multiblock systems. This Perspective discusses potential avenues for future research related to these areas as well as overarching issues underlying the connections between Frank–Kasper phase formation in block polymers to other soft matter and metals.

Efficient Fully decoupled and second-order time-accurate scheme for the Navier–Stokes coupled Cahn–Hilliard Ohta–Kawaski Phase-Field model of Diblock copolymer melt

In this work, we aim to develop a highly efficient numerical scheme for the flow-coupled phase-field model of diblock copolymer melt. Formally, the model is a very complicated nonlinear system that consists of the Navier–Stokes equations and the Cahn–Hilliard type equations with the Ohta–Kawaski potential. Through a combination of a novel decoupling technique and the projection method, we develop the first full decoupling, energy stable, and second-order time-accurate numerical scheme for this model. The decoupling technique is based on the design of an auxiliary ODE, which plays a vital role in obtaining the full decoupling structure while maintaining energy stability. The high efficiency of the scheme is not only reflected by its linear and decoupled structure but also because it only needs to solve a few elliptic equations at each time step. We strictly prove that the scheme satisfies the unconditional energy stability and give a detailed implementation process. Numerical experiments further verify the convergence rate, energy stability, and effectiveness of the developed algorithm.

Stability of the Double Gyroid Phase in Bottlebrush Diblock Copolymer Melts

Stability of the Double Gyroid Phase in Bottlebrush Diblock Copolymer Melts

Identifying a critical micelle temperature in simulations of disordered asymmetric diblock copolymer melts

In melts of highly asymmetric diblock copolymers, copolymers self-assemble into spherical micelles. Indirect evidence has long suggested the existence of two distinct characteristic temperatures in such materials: A critical micelle temperature (CMT) at which a dense fluid of micelles appears over a narrow range of temperatures and an order disorder transition (ODT) temperature at which micelles crystallize. In this article, molecular dynamics simulations are used to definitively confirm the correctness of this picture in melts of high molecular weight polymers, and to show that the CMT occurs very near where self-consistent field theory predicts the ODT to occur.

Block Copolymers beneath the Surface: Measuring and Modeling Complex Morphology at the Subdomain Scale

Block copolymer (BCP) melts are a paradigm for pluripotent molecular assembly, yielding a complex and expanding array of variable domain shapes and symmetries from a fairly simple and highly expandable class of molecular designs. This Perspective addresses recent advances in the ability to model and measure features of domain morphology that go beyond the now canonical metrics of D spacing, space group, and domain topology. Such subdomain features have long been the focus of theories seeking to explain and understand mechanisms of equilibrium structure formation in block copolymer melts, from inhomogeneous curvatures of an intermaterial dividing surface to variable domain thickness. Quantitative metrics of variable subdomain geometry, or packing frustration, are central to theoretical models of complex BCP phase formation, from bicontinuous networks to complex (e.g., Frank–Kasper) crystals, and new experimental methods bring the possibility of their quantitative tests into reach. Here we not only review generic approaches to quantify local domain morphologies that both connect directly to thermodynamic models of BCP assembly but also generalize to domains of arbitrary shape and topology. We then overview experimental methods for characterizing BCP morphology, focusing on recent advances that make accessible detailed and quantitative metrics of fine features of subdomain geometry. Beyond even the critical comparison between detailed predictive models and experimental measurements of complex BCP assembly, validation of these advances lays the foundation to “mold” morphology in BCP assemblies at ever finer subdomain scale, through controlled architectures and processing pathways.

Emergence of Hexagonally Close-Packed Spheres in Linear Block Copolymer Melts.

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2-trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa fA = 0.25-0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank-Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (Đ); for example, an HCP-forming F4DF sample with fA = 0.27 has an experimentally measured Đ = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Topological Effects Near Order–Disorder Transitions in Symmetric Diblock Copolymer Melts

Topological Effects Near Order–Disorder Transitions in Symmetric Diblock Copolymer Melts

Field-Theoretic Simulations for Block Copolymer Melts Using the Partial Saddle-Point Approximation.

Field-theoretic simulations (FTS) provide an efficient technique for investigating fluctuation effects in block copolymer melts with numerous advantages over traditional particle-based simulations. For systems involving two components (i.e., A and B), the field-based Hamiltonian, , depends on a composition field, , that controls the segregation of the unlike components and a pressure field, , that enforces incompressibility. This review introduces researchers to a promising variant of FTS, in which fluctuates while tracks its mean-field value. The method is described in detail for melts of AB diblock copolymer, covering its theoretical foundation through to its numerical implementation. We then illustrate its application for neat AB diblock copolymer melts, as well as ternary blends of AB diblock copolymer with its A- and B-type parent homopolymers. The review concludes by discussing the future outlook. To help researchers adopt the method, open-source code is provided that can be run on either central processing units (CPUs) or graphics processing units (GPUs).

Copolymer Informatics with Multitask Deep Neural Networks

Polymer informatics tools have been recently gaining ground to efficiently and effectively develop, design, and discover new polymers that meet specific application needs. So far, however, these data-driven efforts have largely focused on homopolymers. Here, we address the property prediction challenge for copolymers, extending the polymer informatics framework beyond homopolymers. Advanced polymer fingerprinting and deep-learning schemes that incorporate multi-task learning and meta-learning are proposed. A large data set containing over 18,000 data points of glass transition, melting, and degradation temperature of homopolymers and copolymers of up to two monomers is used to demonstrate the copolymer prediction efficacy. The developed models are accurate, fast, flexible, and scalable to more copolymer properties when suitable data become available.

Rheological Properties of Lamellae‐Forming Diblock Copolymers

AbstractThe present study investigates rheological properties of microphase‐separated lamellar structures formed by unentangled and weakly entangled diblock copolymer melts under a finite amplitude oscillatory shear flow and a steady shear flow. To simulate such a system, dissipative particle dynamics (DPD) simulation is employed to reproduce the lamellar structures, where a multi‐chain slip‐spring model is combined with DPD simulation to mimic entanglement effects. By imposing an oscillatory shear flow, this model enables us to measure storage and loss moduli for three different (parallel, perpendicular, and transverse) lamellar orientations with respect to the flow direction. These orientations display distinctive rheological behaviors, and especially the transverse orientation exhibits a peculiar behavior owing to a finite expansion of the lamellar interfaces, which differs from a previous study. In a steady shear flow, steady shear viscosities of parallel and perpendicular orientations are evaluated. In this case, the orientations of lamellar domains and the local orientation and stretching of bonds near the lamellar interfaces affect the steady shear viscosities. A spatial distribution of entanglements also affects the steady shear viscosities differently from another previous study.

Melt elasticity and flow properties of polypropylene impact copolymer/polyolefin elastomer blends subjected to varied capillary extrusion conditions

In present study, rheological properties of polypropylene impact copolymer (PPcp) and polyolefin elastomer (POE) blend melts were evaluated on a capillary rheometer under shear and elongational flows. The flow and melt elastic properties (die swell and first normal stress difference) studied at varied extrusion conditions were correlated with blend morphology and elastomer content by means of image analysis and theoretical models. Dispersed particle break-up and coalescence were observed to be influenced by POE content and the viscosities of the constituent polymers which were in turn affected by the capillary extrusion conditions (shear rate in particular). The blend melts demonstrated typical pseudoplastic behavior obeying Cross model under shear flow. The elongational flow also corroborated well with the shear flow behavior. All the blends illustrated prominent dependence of melt elastic properties on POE content and the capillary extrusion conditions. The melt elastic properties were also found to critically rely on the inter-particle distance of the POE phase.

Theory of Microphase Segregation in ABA Triblock Comb-Shaped Copolymers: Lamellar Mesophase

A theory describing lamellar mesophases formed in the melt of ABA triblock copolymers with strongly incompatible comblike blocks is developed using a combination of a strong-stretching self-consistent field (SS-SCF) analytical approach and numerical self-consistent field calculations based on the Scheutjens-Fleer (SF-SCF) methods. The structural and thermodynamic properties of the lamellae are analyzed as a function of their architectural parameters, i.e., grafting density and polymerization degree of the main and side chains in the comb-shaped blocks. In particular, we distinguish between the loops and bridges formed by the middle comb-shaped block and demonstrate how a fraction of the bridges connecting the neighboring AB interfaces of the lamellar layers and shear modulus of the mesophase are mediated by the architectural parameters of the comblike blocks. In particular, we predict an increase in the shear modulus upon replacement of the linear ABA triblock copolymers by comblike ones with the same composition. The asymptotic analytical predictions of the theory are complemented by the results of numerical modeling.

Parallel Hybrid Simulations of Block Copolymer Nanocomposites using Coarray Fortran

AbstractComputer simulations of experimentally comparable system sizes in soft matter often require considerable elapsed times. The use of many cores can reduce the needed time, ideally proportionally to the number of processors. In this paper a parallel computational method using coarray Fortran is implemented and tested for large systems of purely block copolymer melts, as well as block copolymer nanocomposites. A satisfactory strong scaling is shown up to 512 cores while a weak scaling with a drop in performance is achieved up to 4096 cores. The scaling of the parallel cell dynamic simulations scheme displays no drawbacks over MPI and provides an example of the simplicity of the coarray approach. The code has been tested on several architectures and compilers. The hybrid block copolymer/nanoparticle algorithm can achieve previously unavailable system sizes.

Fluctuation correction for the order-disorder transition of diblock copolymer melts.

The order-disorder transition (ODT) of diblock copolymer melts is evaluated for an invariant polymerization index of N¯=104, using field-theoretic simulations (FTS) supplemented by a partial saddle-point approximation for incompressibility. For computational efficiency, the FTS are performed using the discrete Gaussian-chain model, and results are then mapped onto the continuous model using a linear approximation for the Flory-Huggins χ parameter. Particular attention is paid to the complex phase window. Results are found to be consistent with the well-established understanding that the gyroid phase extends down to the ODT. Furthermore, our simulations are the first to predict that the Fddd phase survives fluctuation effects, consistent with experiments.

Photo-induced bond breaking during phase separation kinetics of block copolymer melts: a dissipative particle dynamics study.

Using a dissipative particle dynamics (DPD) simulation method, we study the phase separation dynamics in block copolymer (BCP) melts in d = 3, subjected to external stimuli such as light. An initial homogeneous BCP melt is rapidly quenched to a temperature T < Tc, where Tc is the critical temperature. We then allow the system to undergo alternate light "on" and "off" cycles. An on-cycle breaks the stimuli-sensitive bonds connecting both the blocks A and B in the BCP melt, and during the off-cycle, the broken bonds recombine. By simulating the effect of light, we isolate scenarios where phase separation begins with the light off (set 1); the cooperative interactions within the system allow it to undergo microphase separation. When the phase separation starts with the light on (set 2), the system undergoes macrophase separation due to bond breaking. Here, we report the role of alternate cycles on domain morphology by varying the bond-breaking probability for both set 1 and set 2, respectively. We observe that the scaling functions depend upon the conditions mentioned above that change the time scale of the evolving morphologies in various cycles. However, in all the cases, the average domain size respects the power-law growth: R(t) ∼tφ at late times, where φ is the dynamic growth exponent. After a short-lived diffusive growth (φ∼ 1/3) at early times, φ illustrates a crossover from the viscous hydrodynamic (φ∼ 1) to the inertial hydrodynamic (φ∼ 2/3) regimes at late times.

Microphase separation in the melts of diblock copolymers with amphiphilic blocks.

Self-assembly of graft diblock copolymers is an actual topic in the development of materials with desirable properties. In the paper, microphase separation in a melt of the diblock copolymer with amphiphilic and non-amphiphilic blocks is investigated using the analytical theory in the strong segregation approximation. Non-amphiphilic blocks are strongly immiscible with the backbone chains of amphiphilic ones but miscible with their side chains. In the theory, the amphiphilic units are considered as dimers, which can easily orient at interfaces. In the case of weakly amphiphilic dimers, the interfacial tension at a flat interface is calculated using density-functional theory. The amphiphilicity effect leads to a decrease in the surface tension and, hence, to weakening of the block stretching and decrease of the spatial period of the structure. In the case of strongly amphiphilic dimers, the phase diagrams are calculated taking into account basic morphological types (spheres and inverse spheres of amphiphilic blocks, cylinders and inverse cylinders, and lamellae). If the amphiphilicity effects dominate, the characteristic size of the amphiphilic block domain is equal to the side chain length, spherical and cylindrical micelles are formed only at very low fractions of the amphiphilic blocks, the lamellae are formed at slightly larger factions, and the micelles from non-amphiphilic blocks are separated by thin interconnected layers from amphiphilic blocks in the broad range of compositions.

Distinct Viscoelasticity of Hierarchical Nanostructures Self-Assembled from Multiblock Copolymers

We employed a nonequilibrium dissipative particle dynamics method to study viscoelastic properties of hierarchical lamellar structures self-assembled from heptablock copolymers. Three ways of shearing with respect to lamellar planes, including parallel, vertical, and transverse shearing, were imposed to study the viscoelasticity of ordered systems. In parallel shear, with the morphology transformation from disorder to general lamellae to parallel lamellae-in-lamella, both the storage and loss moduli of multiblock copolymer melt show a remarkable improvement. In addition to the parallel lamellae-in-lamella, the multiblock copolymers can form perpendicular lamellae-in-lamella with different viscoelastic behaviors. In vertical shear, parallel lamellae-in-lamella shows terminal behavior, while a low-frequency plateau in the storage modulus exists for the perpendicular lamellae-in-lamella. In transverse shear, the storage moduli for both perpendicular and parallel lamellae-in-lamellae with strong separation of small-length-scale structure exhibit a solid-like plateau at low frequency. The physical origin underlying the distinct viscoelasticity of various hierarchical lamellae was revealed by monitoring the motions of polymer blocks at different length scales. The present work could provide information for preparing advanced materials based on packed lamellae.

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.