Copolymer Phase Research Articles

Preparation of high toughness and high transparency polylactide blends resin based on multiarmed polycaprolactone-block-poly(l-lactide)

Modification of poly(l-lactide) (PLLA) via blending with two or more polymers is one of the effective approaches used to overcome the brittleness of PLLA, which often requires the addition of a compatibilizer and generate opaque materials. To solve this problem, multiarmed polycaprolactone-block-PLLA (PCL-b-PLLA) was synthesized by consecutive ring-opening polymerization of e-caprolactone and l-lactide using multihydroxyl alcohols as initiator. The structure and composition was confirmed by proton nuclear magnetic resonance and gel permeation chromatography. PLLA/multiarmed PCL-b-PLLA blends with various blend ratios were prepared via melt mixing. The presence of multiarmed PCL-b-PLLA (30%) in the PLLA matrix exhibited more than 80 times improvement in the elongation at break, as compared to unmodified PLLA. The addition of multiarmed PCL-b-PLLA in the PLLA contributed to the enhancement of the storage modulus in the low frequency, which was related to the entanglement of the PLLA and multiarmed PCL-b-PLLA. The blend interface had no obvious phase separation, and showed good adhesion between dispersed block copolymer phases within the continuous PLLA phase. The compatibilization mechanism and toughing mechanism were proposed. The resulting PLLA blends also showed good transparency. The current research opened a new route available to prepare transparent PLA-based resin with enhanced properties. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Effect of Geometrical Asymmetry on the Phase Behavior of Rod-Coil Diblock Copolymers.

The effect of geometrical asymmetry β (described by the length-diameter ratio of rods) on the rod-coil diblock copolymer phase behavior is studied by implementation of self-consistent field theory (SCFT) in three-dimensional (3D) position space while considering the rod orientation on the spherical surface. The phase diagrams at different geometrical asymmetry show that the aspect ratio of rods β influences not only the order-disorder transition (ODT) but also the order-order transition (OOT). By exploring the phase diagram with interactions between rods and coils plotted against β, the β effect on the phase diagram is similar to the copolymer composition f. This suggests that non-lamellae structures can be obtained by tuning β, besides f. When the rods are slim compared with the isotropic shape of the coil segment (β is relatively large), the phase behavior is quite different from that of coil-coil diblock copolymers. In this case, only hexagonal cylinders with the coil at the convex side of the interface and lamella phases are stable even in the absence of orientational interaction between rods. The phase diagram is no longer symmetrical about the symmetric copolymer composition and cylinder phases occupy the large area of the phase diagram. The ODT is much lower than that of the coil-coil diblock copolymer system and the triple point at which disordered, cylinder and lamella phases coexist in equilibrium is located at rod composition fR = 0.66. In contrast, when the rods are short and stumpy (β is smaller), the stretching entropy cost of coils can be alleviated and the phase behavior is similar to coil-coil diblocks. Therefore, the hexagonal cylinder phase formed by coils is also found beside the former two structures. Moreover, the ODT may even become a little higher than that of the coil-coil diblock copolymers due to the large interfacial area per chain provided by the stumpy rods, thus compensating the stretching entropy loss of the coils.

Synthesis and Self-Assembly of Discrete Dimethylsiloxane-Lactic Acid Diblock Co-oligomers: The Dononacontamer and Its Shorter Homologues.

Most of the theoretical and computational descriptions of the phase behavior of block copolymers describe the chain ensembles of perfect and uniform polymers. In contrast, experimental studies on block copolymers always employ materials with disperse molecular makeup. Although most polymers are so-called monodisperse, they still have a molecular weight dispersity. Here, we describe the synthesis and properties of a series of discrete length diblock co-oligomers, based on oligo-dimethylsiloxane (oDMS) and oligo-lactic acid (oLA), diblock co-oligomers with highly noncompatible blocks. By utilizing an iterative synthetic protocol, co-oligomers with molar masses up to 6901 Da, ultralow molar mass dispersities (Đ ≤ 1.00002), and unique control over the co-oligomer composition are synthesized and characterized. This specific block co-oligomer required the development of a new divergent strategy for the oDMS structures by which both bis- and monosubstituted oDMS derivatives up to 59 Si-atoms became available. The incompatibility of the two blocks makes the final coupling more demanding the longer the blocks become. These optimized synthetic procedures granted access to multigram quantities of most of the block co-oligomers, useful to study the lower limits of block copolymer phase segregation in detail. Cylindrical, gyroid, and lamellar nanostructures, as revealed by DSC, SAXS, and AFM, were generated. The small oligomeric size of the block co-oligomers resulted in exceptionally small feature sizes (down to 3.4 nm) and long-range organization.

AFM study of excimer laser patterning of block-copolymer: Creation of ordered hierarchical, hybrid, or recessed structures

Abstract We report fabrication of the varied range of hierarchical structures by combining bottom-up self-assembly of block copolymer poly(styrene-block-vinylpyridine) (PS-b-P4VP) with top-down excimer laser patterning method. Different procedures were tested, where laser treatment was applied before phase separation and after phase separation or phase separation and surface reconstruction. Laser treatment was performed using either polarized laser light with the aim to create periodical pattern on polymer surface or non-polarized light for preferential removing of polystyrene (PS) part from PS-b-P4VP. Additionally, dye was introduced into one part of block copolymer (P4VP) with the aim to modify its response to laser light. Resulting structures were analyzed by XPS, UV–vis and AFM techniques. Application of polarized laser light leads to creation of structures with hierarchical, recessed or hybrid geometries. Non-polarized laser beam allows pronouncing the block copolymer phase separated structure. Tuning the order of steps or individual step conditions enables the efficient reorientation of block-copolymer domain at large scale, fabrication of hierarchical, hybrid or recessed structures. The obtained structures can find potential applications in nanotechnology, photonics, plasmonics, information storage, optical devices, sensors and smart surfaces.

Silver Nanoparticle Arrays Prepared by In Situ Automatic Reduction of Silver Ions in Mussel‐Inspired Block Copolymer Films

Block copolymer phase separation has been employed to form templates for the fabrication of nanoscale patterns of metals due to their characteristic size scales that range from several nanometers to tens of nanometers. In the present work, poly(vinyl catechol‐block‐styrene) PVCa‐b‐PSt synthesized with various compositions forms unique microphase‐separated structures. Silver nanoparticles are spontaneously formed inside of the PVCa phases due to the reductive properties of the catechol moieties. The alignment and formation of silver nanoparticle arrays is successfully achieved without the need for high‐temperature thermal treatment or chemical reduction processes. The present system can be applied to the formation of nanoparticle arrays with a wide variety of metals by control of their arrangement. image

Sequential graft-interpenetrating polymer networks based on polyurethane and acrylic/ester copolymers

Highly transparent and tough graft-interpenetrating polymer networks (graft-IPNs) were synthesized using an elastomeric polyurethane phase (PU) and a highly stiff acrylate-base copolymer phase. The grafting points between the two networks were generated with the purpose of minimizing the phase separation process of the polymeric systems. In order to generate the grafting between the networks, an acrylic resin capable of undergoing both free radical and poly-addition poly- merization was employed. The thermo-mechanical properties, fracture toughness properties as well as network and surface phase morphology of the graft-IPNs synthesized were evaluated in this work. Data obtained suggested that the minimiza- tion of the phase separation was achieved by the generation of crosslinking points between both networks. High trans- parency was obtained in all samples as an indication of the high level of interpenetration achieved. The relative high values obtained for the fracture toughness tests suggest that generating chemical crosslinks between networks is a good approach for increasing the fracture toughness of polymeric materials.

Mechanical properties derived from phase separation in co-polymer hydrogels

Hydrogels can be synthesized with most of the properties needed for biomaterials applications. Soft, wettable, and highly permeable gels with a practically unlimited breadth of chemical functionalities are routinely made in the laboratory. However, the ability to make highly elastic and durable hydrogels remains limited. Here we describe an approach to generate stretchy, durable hydrogels, employing a high polymer-to-crosslink ratio for extensibility, combined with an aggregating copolymer phase to provide stability against swelling. We find that the addition of aggregating co-polymer can produce a highly extensible gel that fails at 1000% strain, recovers from large strains within a few minutes, maintains its elasticity over repeated cycles of large amplitude strain, and exhibits significantly reduced swelling. We find that the gel׳s enhanced mechanical performance comes from a kinetically arrested structure that arises from a competition between the disparate polymerization rates of the two components and the aggregation rate of the unstable phase. These results represent an alternative strategy to generating the type of stretchy elastomer-like hydrogels needed for biomedical technologies.

Enhanced Block Copolymer Phase Separation Using Click Chemistry and Ionic Junctions.

In addition to the traditional parameters of chi (χ) and degree of polymerization (N), we demonstrate that the segregation strength of a diblock copolymer can be increased by introduction of an ionic unit at the junction of the two blocks. Compared to neutral linking groups, the electrostatic interactions between counterions of adjacent domain junctions leads to increased enthalpy, segregation strength, and phase separation. As a result, the order disorder transition temperatures of block copolymers with a 1,2,3-triazolium ionic junction were observed to be significantly higher than the corresponding neutral block copolymers. To demonstrate the potential of block copolymers with ionic junctions for nanopatterning, block copolymers were prepared by click coupling of homopolymers and then used to fabricate well-defined sub-10 nm line features. We believe that the concept of improved thin-film assembly through the introduction of ionic junctions is a powerful tool for block copolymer lithography and complements chi (χ) and degree of polymerization (N) in the design of macromolecular systems with enhanced phase separation.

Single Nanoparticle Localization in the Perforated Lamellar Phase of Self-Assembled Block Copolymer Driven by Entropy Minimization

Although precisely controlled microdomains of block copolymers (BCP) provide an excellent guiding matrix for multiple nanoparticles (NPs) to be controllably segregated into a desired polymer block, localization and positioning of individual NPs have not been demonstrated. Here, we report a unique one-to-one positioning phenomenon of guest Au NPs in the host BCP microdomains; each of polystyrene-functionalized Au NPs is embedded within the perforation domain of hexagonally perforated lamellar (HPL) morphology of poly(dimethylsiloxane-b-styrene) BCP. The local minimization of free energy achieved by the placement of Au NPs into the center of the perforation domain is theoretically supported by the self-consistent field theory (SCFT) simulation. We propose a novel design principle for more precisely controllable nanocomposites by developing a new route of NP arrangement within a polymer matrix.

Conjugated polymer/fullerene nanostructures through cooperative non-covalent interactions for organic solar cells

We have previously reported the formation of polymer/fullerene core–shell composite nanofibers (NFs) through self-assembly of functionalized conjugated block copolymers and fullerene derivatives, by cooperation of orthogonal non-covalent interactions including block copolymer phase separation, fullerene aggregation, poly(3-hexylthiophene) (P3HT) crystallization and complementary hydrogen bonding. These NFs displayed improved solar cell performances and the core–shell type structures are generally insensitive to polymer/fullerene weight ratios. In order to better understand the self-assembly mechanisms of such complex systems, we have prepared a new block copolymer (P3) having a different block length ratio and studied its solution and thin film morphologies in the presence of fullerene derivatives at different weight ratios. Worm-like micelles are observed for P3 in mixed-solvent solutions, distinctly different from the well-defined NFs observed previously. Upon fullerene additions, the solution morphologies gradually change into hierarchically complex nanostructures with varying fullerene contents. Thin film morphologies are studied by atomic force microscopy (AFM) and the blends are applied in organic solar cell (OSC) devices. Our present studies provide detailed information on the self-assembly behaviors of conjugated block copolymers and fullerenes under cooperative orthogonal non-covalent interactions.

Arbitrary lattice symmetries via block copolymer nanomeshes.

Self-assembly of block copolymers is a powerful motif for spontaneously forming well-defined nanostructures over macroscopic areas. Yet, the inherent energy minimization criteria of self-assembly give rise to a limited library of structures; diblock copolymers naturally form spheres on a cubic lattice, hexagonally packed cylinders and alternating lamellae. Here, we demonstrate multicomponent nanomeshes with any desired lattice symmetry. We exploit photothermal annealing to rapidly order and align block copolymer phases over macroscopic areas, combined with conversion of the self-assembled organic phase into inorganic replicas. Repeated photothermal processing independently aligns successive layers, providing full control of the size, symmetry and composition of the nanoscale unit cell. We construct a variety of symmetries, most of which are not natively formed by block copolymers, including squares, rhombuses, rectangles and triangles. In fact, we demonstrate all possible two-dimensional Bravais lattices. Finally, we elucidate the influence of nanostructure on the electrical and optical properties of nanomeshes.

Structural Diversity of Arthropod Biophotonic Nanostructures Spans Amphiphilic Phase-Space.

Many organisms, especially arthropods, produce vivid interference colors using diverse mesoscopic (100-350 nm) integumentary biophotonic nanostructures that are increasingly being investigated for technological applications. Despite a century of interest, precise structural knowledge of many biophotonic nanostructures and the mechanisms controlling their development remain tentative, when such knowledge can open novel biomimetic routes to facilely self-assemble tunable, multifunctional materials. Here, we use synchrotron small-angle X-ray scattering and electron microscopy to characterize the photonic nanostructure of 140 integumentary scales and setae from ∼127 species of terrestrial arthropods in 85 genera from 5 orders. We report a rich nanostructural diversity, including triply periodic bicontinuous networks, close-packed spheres, inverse columnar, perforated lamellar, and disordered spongelike morphologies, commonly observed as stable phases of amphiphilic surfactants, block copolymer, and lyotropic lipid-water systems. Diverse arthropod lineages appear to have independently evolved to utilize the self-assembly of infolding lipid-bilayer membranes to develop biophotonic nanostructures that span the phase-space of amphiphilic morphologies, but at optical length scales.

Widely Tunable Morphologies in Block Copolymer Thin Films Through Solvent Vapor Annealing Using Mixtures of Selective Solvents.

Thin films of block copolymers are extremely attractive for nanofabrication because of their ability to form uniform and periodic nanoscale structures by microphase separation. One shortcoming of this approach is that to date the design of a desired equilibrium structure requires synthesis of a block copolymer de novo within the corresponding volume ratio of the blocks. In this work, we investigated solvent vapor annealing in supported thin films of poly(2-hydroxyethyl methacrylate)-block-poly(methyl methacrylate) [PHEMA-b-PMMA] by means of grazing incidence small angle X-ray scattering (GISAXS). A spin-coated thin film of lamellar block copolymer was solvent vapor annealed to induce microphase separation and improve the long-range order of the self-assembled pattern. Annealing in a mixture of solvent vapors using a controlled volume ratio of solvents (methanol, MeOH, and tetrahydrofuran, THF), which are chosen to be preferential for each block, enabled selective formation of ordered lamellae, gyroid, hexagonal or spherical morphologies from a single block copolymer with a fixed volume fraction. The selected microstructure was then kinetically trapped in the dry film by rapid drying. To our knowledge, this paper describes the first reported case where in-situ methods are used to study the transition of block copolymer films from one initial disordered morphology to four different ordered morphologies, covering much of the theoretical diblock copolymer phase diagram.

Regulating block copolymer phases via selective homopolymers.

The phase behavior of strongly segregated AB diblock copolymer and selective C homopolymer blends is examined theoretically using a combination of strong stretching theory (SST) and self-consistent field theory (SCFT). The C-homopolymer is immiscible with the B-blocks but strongly attractive with the A-blocks. The effect of homopolymer content on the order-order phase transitions is analyzed. It is observed that, for AB diblock copolymers with majority A-blocks, the addition of the C-homopolymers results in lamellar to cylindrical to spherical phase transitions because of the A/C complexation. For diblock copolymers with minor A-blocks, adding C-homopolymers leads to transitions from spherical or cylindrical morphology with A-rich core to lamellae to inverted cylindrical and spherical morphologies with B-rich core. The results from analytical SST and numerical SCFT are in good agreement within most regions of the phase diagram. But the deviation becomes more obvious when the composition of A-blocks is too small and the content of added C-homopolymers is large enough, where the SCFT predicts a narrow co-existence region between different ordered phases. Furthermore, it is found that the phase behavior of the system is insensitive to the molecular weight of C-homopolymer.

The Use of Solid‐State NMR to Investigate the Development of Segmental Mobility in Commercial Heterophasic Ethylene Propylene Copolymers (HEPCs)

The effect increasing ethylene incorporation on the development of commercial heterophasic ethylene propylene copolymers (HEPCs) was previously evaluated in terms of particle morphology, chemical composition, crystallinity, and microstructure. These polymers were obtained from a commercial gas‐phase process and enabled the visualization of the early development of the copolymer phase while including the effects of large‐scale production processes on the resulting polymer properties. Based on the ethylene‐dependent changes observed for the HEPCs as well as some semi‐crystalline TREF fractions, further variable temperature solid‐state 13C NMR experiments were done to observe temperature‐dependent shifts in localised mobility within the bulk samples during a heating/melting profile. From these experiments, the development of amorphous polypropylene signals was observed with increasing temperature, which could indirectly be related to the introduction of ethylene defects. It was also observed that polypropylene components with high rigidity remained at high temperatures for the HEPCs but not for the homopolymer. T1ρ experiments were used to differentiate between changes in crystalline and non‐crystalline phases based on ethylene incorporation as well as during melting. In this publication, the observations from the perspective of the solid‐state analyses are outlined and placed in the context of the evolution of microscopic, chemical, microstructural, and mechanical properties.

Mechanical Properties of Long Glass Fiber-Reinforced Polyolefin Composites

Mechanical properties and fracture morphology for long glass fiber-reinforced-polyolefin composites and their blends were investigated. The properties like tensile, flexural, and impact strength of the composites at low temperature increased considerably and elongation at break reduced visibly with increase of glass fiber contents. Impact strength at room temperature was an exception, and impact strength of the composites was higher than that of impact polypropylene copolymer. The results are caused by interaction of rubber phase and amorphous parts in impact polypropylene copolymer. Extraction or fracture for glass fiber in the composites consumed more impact energy, which means they have higher impact strength.

Dynamic orientation transition of the lyotropic lamellar phase at high shear rates.

The dynamic orientation behavior of the lamellar phase of a triblock copolymer is studied in a wide range of shear rates as a function of solvent composition. We find that various phases can be induced by increasing the shear rate. At low shear rates, the onion phase forms from planar lamellae with many defects. A further increase of the shear rate caused the onion structure to break down, and the lamellar phase recovers with fewer defects. Finally, the transition of the orientation from parallel to perpendicular is observed at high shear rates. In the orientation transition at high shear rates, a stable intermediate structure, to our knowledge, is found for the first time. We also find that the critical shear stress of the rupture of the onion phase coincides with the orientation transition. The consistency of the critical shear stress suggests that all orientation transitions at a high shear rate are dominated by a mechanical balance between the applied viscous stress and the internal relaxation mode of the lamellae.

Volume shrinkage and rheological studies of epoxidised and unepoxidised poly(styrene-block-butadiene-block-styrene) triblock copolymer modified epoxy resin-diamino diphenyl methane nanostructured blend systems.

Styrene-block-butadiene-block-styrene (SBS) copolymers epoxidised at different epoxidation degrees were used as modifiers for diglycidyl ether of the bisphenol A-diamino diphenyl methane (DGEBA-DDM) system. Epoxy systems containing modified epoxidised styrene-block-butadiene-block-styrene (eSBS) triblock copolymer with compositions ranging from 0 to 30 wt% were prepared and the curing reaction was monitored in situ using rheometry and pressure-volume-temperature (PVT) analysis. By controlling the mole percent of epoxidation, we could generate vesicles, worm-like micelles and core-shell nanodomains. At the highest mole percent of epoxidation, the fraction of the epoxy miscible component in the triblock copolymer (epoxidised polybutadiene (PB)) was maximum. This gave rise to core-shell nanodomains having a size of 10-15 nm, in which the incompatible polystyrene (PS) becomes the core, the unepoxidised PB becomes the shell and the epoxidised PB interpenetrates with the epoxy phase. On the other hand, the low level of epoxidation gave rise to bigger domains having a size of ∼1 μm and the intermediate epoxidation level resulted in a worm-like structure. This investigation specifically focused on the importance of cure rheology on nanostructure formation, using rheometry. The reaction induced phase separation of the PS phase in the epoxy matrix was carefully explored through rheological measurements. PVT measurements during curing were carried out to understand the volume shrinkage of the blend, confirming that shrinkage behaviour is related to the block copolymer phase separation process during curing. The volume shrinkage was found to be maximum in the case of blends with unmodified SBS, where a heterogeneous morphology was observed, while a decrease in the shrinkage was evidenced in the case of SBS epoxidation. It could be explained by two effects: (1) solubility of the epoxidised block copolymer in the DGEBA leads to the formation of nanoscopic domains upon reaction induced phase separation and (2) the plasticisation effect of the epoxidised block copolymer in the epoxy resin.

The Tricontinuous 3ths(5) Phase: A New Morphology in Copolymer Melts

Self-assembly remains the most efficient route to the formation of ordered nanostructures, including the double gyroid network phase in diblock copolymers based on two intergrown network domains. Here we use self-consistent field theory to show that a tricontinuous structure with monoclinic symmetry, called 3ths(5), based on the intergrowth of three distorted ths nets, is an equilibrium phase of triblock star-copolymer melts when an extended molecular core is introduced. The introduction of the core enhances the role of chain stretching by enforcing larger structural length scales, thus destabilizing the hexagonal columnar phase in favor of morphologies with less packing frustration. This study further demonstrates that the introduction of molecular cores is a general concept for tuning the relative importance of entropic and enthalpic free energy contributions, hence providing a tool to stabilize an extended repertoire of self-assembled nanostructured materials.

Thermoresponsive anionic copolymer brushes containing strong acid moieties for effective separation of basic biomolecules and proteins.

A thermoresponsive copolymer brush possessing the sulfonic acid group, poly(N-isopropylacrylamide (IPAAm)-co-2-acrylamido-2-methylpropanesulfonic acid (AMPS)-co-tert-butylacrylamide (tBAAm)), was grafted onto the surface of silica beads through surface-initiated atom transfer radical polymerization. Prepared copolymer and copolymer brushes on silica beads were characterized by observing the phase transition profile, CHNS elemental analysis, X-ray photoelectron spectroscopy, and gel permeation chromatography. The phase transition profile indicated that an appropriate AMPS composition for enabling thermally modulated property changes is 5 mol %, while excessive amounts of sulfonic acid groups prevented copolymer phase transition. Chromatographic elutions of catecholamine derivatives and basic proteins were observed, using the prepared copolymer brush-modified beads as chromatographic matrices, and the results suggest that the beads interact with these analytes through relatively strong electrostatic interactions. Thus, poly(IPAAm-co-AMPS-co-tBAAm) brush-modified beads will be useful for effective thermoresponsive chromatography matrices that separate basic biomolecules through strong electrostatic interactions.