Homopolymer Blends Research Articles

Recent progress in synthesis of stereoblock copolymers

The synthesis of polymers with tailor-made microstructures and desired properties by controlling the regio- and stereoselectivity of olefin polymerization is an important research area in polymer science. In recent years, achievements were made to synthesis stereoblock copolymer having different tacticity of each segment such as crystalline-amorphous blocks from a single monomer. A stereoblock copolymer consisting of different stereospecific sequences in one polymer chain, shows special physical properties from the original stereoregular polymer which could be applied as thermoplastic polymer. On one hand, it leads to much wider array of polymeric materials from a limited set of commodity monomer feed stocks; on the other hand, it combines the properties of two or more homopolymers and exhibits better performance than the corresponding homopolymer blends. In this paper, research and developments about several kinds of stereoblock copolymers such as polypropylene, polydienes, polyacrylate and polylactide were reviewed and discussed, in particular, the methods to synthesize stereoblock copolymers were also summarized. The successful development of oscillating catalysts for synthesizing isotactic-atactic multi-stereoblock polypropylene opened the door of research on preparing crystalline-amorphous stereoblock polypropylene. Plenty of coordinative catalyst systems and novel polymerization methods were developed for preparing stereoblock polypropylene. The most common way is using living/controlled polymerization system and changing the selectivity simply by varying polymerization temperature, solvent polarity and pressure. Besides, two catalysts with different stereospecificity were mixed together for the propylene polymerization, by successful cross chain transfer reaction, multi-stereoblock polypropylene with thermoplastic elastomeric properties was also obtained. Stereoblock polydienes were also paid much attention for their diverse microstructures which could dramatically influence the properties of resulting materials. Much effort has been attributed to the modification of polydienes microstructure by changing the active species through incorporation of additives such as barium salt, Lewis base (PPh3) and Lewis acids in a living/controlled polymerization system. In addition, chain shuttling polymerization was also applied for diene polymerization to obtained hard-soft multiblock polydiene on the condition of appropriate catalytic systems and chain transfer agent. Progress in stereospecific living polymerization of methyl methacrylate (MMA) made it possible to synthesize stereoblock poly (methyl methacrylate) (PMMA). Stereoblock PMMA should be of interest from the viewpoint of stereocomplex formation which would occur intramolecularly as well as intermolecularly. Preparation of stereoblock PMMA with narrow molecular weight distribution was achieved by living anionic polymerization with incorporation of unique additives in the early time. Isotactic-syndiotactic stereoblock PMMA were synthesized with efficient metallocene and Lewis acid hybrid catalysts capable of switching stereospecificity of the MMA polymerization. Except for the three kinds of stereoblock polymer, other types of stereoblock polymer such as polystyrene, polylactide and polyacrylamide were also introduced. At present, although it is successful in preparing all kinds of stereoblock copolymers, several problems emerged that the feasible catalytic systems are still limited, some polymerization process demanding strict conditions is not easy to control and the obtained stereoblock polymer is often with low stereoregularity. Besides, the study on the properties of stereoblock copolymers also requires more attentions. Therefore, it is necessary for researchers to develop more practical catalytic system and novel polymerization methods for the better control of macromolecular structure, and to deeply and systematically understand the relationship between macromolecular structure and physical properties.

Homo- and Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo Diblock Copolymers of Crystalline and Amorphous Poly(lactide)s: Effects of Incorporation and Position of Amorphous Blocks.

Star-shaped four-armed stereo diblock copolymers with an amorphous poly(dl-lactide) (PDLLA) core and a crystalline poly(l-lactide) (PLLA) or poly(d-lactide) (PDLA) shell (4-DL-L and 4-DL-D) and with a crystalline PLLA or PDLA core and amorphous PDLLA shell (4-L-DL and 4-D-DL) copolymers were synthesized. The effects of incorporation and position of amorphous PDLLA blocks on homo- and stereocomplex (SC) crystallization behavior of unblended copolymers and their blends (4-DL-L/4-DL-D and 4-L-DL/4-D-DL) were investigated during isothermal crystallization from the melt and DSC heating. The incorporated amorphous PDLLA blocks disturbed the SC crystallization and orientation of SC lamellae in the copolymer blends as well as homocrystallization and orientation of homocrystalline lamellae in the unblended copolymers compared to those reported for four-armed PLLA/four-armed PDLA homopolymer blends or four-armed PLLA homopolymers, irrespective of the position of amorphous PDLLA blocks. The disturbance effect was stronger for the amorphous PDLLA shell than for the amorphous PDLLA core. The crystallizability was higher for SC crystallites in the copolymer blends than for homocrystallites in the unblended copolymers, irrespective of the position of amorphous PDLLA blocks. SC crystallization in the copolymer blends disclaimed the positional effects of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks on spherulite growth rate and crystallinity.

Phase Behavior of Diblock Copolymer–Homopolymer Ternary Blends: Congruent First-Order Lamellar–Disorder Transition

We have established the existence of a line of congruent first-order lamellar-to-disorder (LAM–DIS) transitions when appropriate amounts of poly(cyclohexylethylene) (C) and poly(ethylene) (E) homopolymers are mixed with a corresponding compositionally symmetric CE diblock copolymer. The line of congruent transitions, or the congruent isopleth, terminates at the bicontinuous microemulsion (BμE) channel, and its trajectory appears to be influenced by the critical composition of the C/E binary homopolymer blend. Blends satisfying congruency undergo a direct LAM–DIS transition without passing through a two-phase region. We present complementary optical transmission, small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and dynamic mechanical spectroscopy (DMS) results that establish the phase behavior at constant copolymer volume fraction and varying C/E homopolymer volume ratios. Adjacent to the congruent composition at constant copolymer volume fraction, the lamellar and disordered ph...

Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties.

The microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV–visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.

Linear, first and second-order, unconditionally energy stable numerical schemes for the phase field model of homopolymer blends

In this paper, we develop a series of efficient numerical schemes to solve the phase field model for homopolymer blends. The governing system is derived from the energetic variational approach of a total free energy, that consists of a nonlinear logarithmic Flory–Huggins potential, and a gradient entropy with a concentration-dependent de-Gennes type coefficient. The main challenging issue to solve this kind of models numerically is about the time marching problem, i.e., how to develop suitable temporal discretizations for the nonlinear terms in order to preserve the energy stability at the discrete level. We solve this issue in this paper, by developing the first and second order temporal approximation schemes based on the “Invariant Energy Quadratization” method, where all nonlinear terms are treated semi-explicitly. Consequently, the resulting numerical schemes lead to a symmetric positive definite linear system to be solved at each time step. The unconditional energy stabilities are further proved. Various numerical simulations of 2D and 3D are presented to demonstrate the stability and the accuracy of the proposed schemes.

Ground states of a ternary system including attractive and repulsive Coulomb-type interactions

We study a variational model where two interacting phases are embedded in a third neutral phase. The energy of the system is the sum of a local interfacial contribution and a nonlocal interaction of Coulomb type. Such models are e.g. used to describe systems of copolymer–homopolymer blends or of surfactants in water solutions. We establish existence and regularity properties of global minimizers, together with a full characterization of minimizers in the small mass regime. Furthermore, we prove uniform bounds on the potential of minimizing configurations, which in turn imply some qualitative estimates about the geometry of minimizers in the large mass regime. One key mathematical difficulty in the analysis is related to the fact that two phases have to be minimized simultaneously with both attractive and repulsive interaction present.

Quantitative study of fluctuation effects by fast lattice Monte Carlo simulations. VI. Phase behavior of incompressible symmetric binary homopolymer blends

Abstract We reported the first Monte Carlo study of incompressible symmetric binary homopolymer blends. Using fast lattice Monte Carlo simulations (Q. Wang, Soft Matter 5 (2009) 4564–4567) in a semi-grand-canonical ensemble with the cooperative motion algorithm generalized in our previous work (P. Zhang, Q. Wang, J. Phys. Chem. B 118 (2014) 12059–12067) and the finite-size scaling theory, we studied the critical properties of incompressible symmetric blends on the hexagonal lattice. We also used the replica-exchange scheme to greatly improve our sampling, and the multiple histogram reweighting technique to accurately locate the critical point. Instead of the self- and mutual-avoiding walk chain model and the nearest-neighbor interaction used in conventional lattice simulations, here we adopt the model system with multiple occupancy of lattice sites and the Kronecker δ -function interaction, which is the lattice counterpart of the standard model of incompressible bin00ary homopolymer blends used in continuum polymer field theories. This enables direct comparisons between our simulation results and mean-field (Flory-Huggins) predictions without any parameter-fitting (e.g., to determine the Flory-Huggins χ -parameter). The effects of system fluctuations/correlations on the critical point and phase diagram of incompressible symmetric blends are therefore quantitatively and unambiguously revealed in this work.

Critical Point of Symmetric Binary Homopolymer Blends

Monte Carlo field-theoretic simulations (MC-FTS) are performed on structurally symmetric binary homopolymer blends for invariant polymerization indexes of N ≥ 103. It is shown that the ultraviolet (UV) divergence that plagues MC-FTS at small N can be removed by an appropriate renormalization of the Flory–Huggins interaction parameter, χ, allowing one to extract meaningful results that are independent of the wavevector cutoff. Once the divergence is taken care of, the fluctuation corrections to mean-field theory are found to be exceptionally small. In particular, the disordered-state structure function, S(k), is virtually indistinguishable from the RPA prediction, and there is a slight shift in the critical point, (χN)c, that roughly scales as N–1/2. An implication of the small corrections is that previous experimental determinations of χ based on homopolymer blends should be relatively accurate.

Dissipative particle dynamics studies on the interfacial tension of A/B homopolymer blends and the effect of Janus nanorods

ABSTRACTJanus nanorods are used as a novel rigid compatibilizer to improve the interfacial tension of incompatible A/B homopolymer blends. Dissipative particle dynamics (DPD) methods are preformed to explore the effect of Janus nanorods on the interfacial tension. The results show that Janus nanorods are a good compatibilizer only when the appropriate length is chosen, which is different from the traditional coil compatibilizer (surfactants and block copolymers). The length of the Janus nanorods can significantly influence their orientation through the competition between the entropic and enthalpic effects. The shorter Janus nanorods preferring “standing” have a better efficiency in improving the interfacial tension than the longer ones preferring “lying.” If we can control the orientation of the longer Janus nanorods, they are still a good compatibilizer. This simulation work can widen the application of Janus nanoparticles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44098.

Computation of Free Energies of Cubic Bicontinuous Phases for Blends of Diblock Copolymer and Selective Homopolymer

Cubic bicontinuous phases like the double gyroid (G), double diamond (D), and plumber’s nightmare (P) are of great practical interest for many emerging applications requiring highly regular nanoscale networks or porous materials. Such phases can be formed from A–B diblock copolymers by the addition of A-type homopolymer over a range of compositions and relative chain lengths. Particle-based molecular simulations were used to delineate the phase diagram in a region where self-consistent field theory predicts the presence of a G–D–P triple point. Since the simulation box size must be commensurate with the morphology-specific 3D unit cell size (which is not known a priori), accurate free energy estimates are required for a range of box sizes, particularly when multiple competing phases can occur at the conditions of interest. A variant of thermodynamic integration was implemented to obtain such free energies (and hence identify the stable phases and their optimal box sizes) by tracing a reversible path conne...

Confined Pattern-Directed Assembly of Polymer-Grafted Nanoparticles in a Phase Separating Blend with a Homopolymer Matrix.

The controlled organization of nanoparticle (NP) constituents into superstructures of well-defined shape, composition and connectivity represents a continuing challenge in the development of novel hybrid materials for many technological applications. We show that the phase separation of polymer-tethered nanoparticles immersed in a chemically different polymer matrix provides an effective and scalable method for fabricating defined submicron-sized amorphous NP domains in melt polymer thin films. We investigate this phenomenon with a view towards understanding and controlling the phase separation process through directed nanoparticle assembly. In particular, we consider isothermally annealed thin films of polystyrene-grafted gold nanoparticles (AuPS) dispersed in a poly(methyl methacrylate) (PMMA) matrix. Classic binary polymer blend phase separation related morphology transitions, from discrete AuPS domains to bicontinuous to inverse domain structure with increasing nanoparticle composition is observed, yet the kinetics of the AuPS/PMMA polymer blends system exhibit unique features compared to the parent PS/PMMA homopolymer blend. We further illustrate how to pattern-align the phase-separated AuPS nanoparticle domain shape, size and location through the imposition of a simple and novel external symmetry-breaking perturbation via soft-lithography. Specifically, submicron-sized topographically patterned elastomer confinement is introduced to direct the nanoparticles into kinetically controlled long-range ordered domains, having a dense yet well-dispersed distribution of non-crystallizing nanoparticles. The simplicity, versatility and roll-to-roll adaptability of this novel method for controlled nanoparticle assembly should make it useful in creating desirable patterned nanoparticle domains for a variety of functional materials and applications.

Molar mass distributions in binary homopolymer blends by single-step two-dimensional liquid chromatography: Operation and data treatment

A single-step two-dimensional liquid chromatography technique is employed for estimating the individual molar mass distributions in blends of polystyrene and poly(methyl methacrylate). It involves an initial separation of the homopolymers by liquid chromatography under limiting condition of desorption (first column), followed by their fractionation by size exclusion chromatography (second column). The technique requires a calibration stage with narrow standards for determining (for each homopolymer): a) the molar mass calibration of the complete fractionation system; and b) the band broadening function corresponding to the first column. Then, the molar mass distributions are estimated as follows: i) the polymer blend is injected into the tandem of both fractionation columns; ii) the obtained bimodal chromatogram is pre-processed to isolate the homopolymer peaks; iii) each isolated chromatogram is corrected for the band broadening caused by the first column; and iv) the molar mass distributions are estimated from the band broadening corrected chromatograms and the corresponding molar mass calibrations. The analysis is straightforward and fast, but it requires a relatively elaborate calibration process.

Molecular Rectification in Conjugated Block Copolymer Photovoltaics

We investigate the influence that covalent linkage of electron donating and accepting blocks in high performance fully conjugated block copolymer photovoltaics has on charge generation and recombination using ultrafast mid-infrared transient absorption spectroscopy. We show that block copolymer architectures containing a conjugated bridge between the donor and acceptor groups can be used to form ordered mesoscale morphologies that lead to improved photovoltaic performance without enhancing charge recombination. Judicious placement of an electron-rich moiety in the electron accepting block of the block copolymer creates a donor–bridge–acceptor architecture that slows intramolecular charge transfer across the covalent linkage. Charge recombination in such donor–bridge–acceptor block copolymer films proceeds at the same rate as it does in their corresponding homopolymer blends for which the donor and acceptor blocks are not covalently linked, indicating that recombination is dominated by intermolecular charg...

Correction to Synthesis and Remarkable Efficacy of Model Polyethylene-graft-poly(methyl methacrylate) Copolymers as Compatibilizers in Polyethylene/Poly(methyl methacrylate) Blends

Polyethylene-graft-poly(methyl methacrylate) (PE-g-PMMA) copolymers were prepared using a combination of ring-opening metathesis polymerization (ROMP), hydrogenation, and atom transfer radical polymerization (ATRP). Approximately 20 PMMA side chains per molecule with average degrees of polymerization 6, 12, and 24 were grown via a “grafting from” approach from a Br-substituted linear PE backbone with Mw = 56 000. The resulting graft copolymers were evaluated as compatibilizing agents for binary PE/PMMA homopolymer blends. The roles of PMMA side chain length and different compatibilizer concentration in the polymer blends were investigated, and the mechanical and morphological properties of blends containing 70% PE and 30% PMMA by weight were examined. The presence of the compatibilizer reduced the average PMMA droplet size substantially, even at compatibilizer loadings as low as 1%. Furthermore, the compatibilized blends exhibited significant improvements in elastic modulus, yield strength, and scratch re...

Separation of Ethylene‐Propylene Copolymers by Crystallization and Adsorption Mechanisms. A Journey Inside the Analytical Techniques

SummaryThe separation of polyolefin copolymers in terms of composition has been typically performed by crystallization techniques. In the last years, a new liquid chromatography approach has become available that extends the range of composition analysis applicable towards higher comonomer content. Every technique has its capabilities and limitations, and the use of multiple hyphenated techniques is often required to provide unequivocal results for characterizing the microstructure of complex copolymers like high impact polypropylene. A good understanding of the individual separation mechanisms is essential to interpreting the obtained results properly, and this becomes a challenging task when dealing with ethylene‐propylene copolymers or homopolymer blends. The separation of these resins by the existing techniques is discussed. The role of solvent and the adsorbent in the separation mechanisms is investigated and a comparison of crystallization and liquid adsorption chromatography with various types of adsorbents is presented.

Neutral wetting brush layers for block copolymer thin films using homopolymer blends processed at high temperatures

Binary homopolymer blends of two hydroxyl-terminated polystyrene (PS-OH) and polymethylmethacrylate (PMMA-OH) homopolymers (Mn ∼ 16000 g mol−1) were grafted on SiO2 substrates by high-temperature (T > 150 °C), short-time (t < 600 s) thermal treatments. The resulting brush layer was tested to screen preferential interactions of the SiO2 substrate with the different symmetric and asymmetric PS-b-PMMA block copolymers deposited on top of the grafted molecules. By properly adjusting the blend composition and the processing parameters, an efficient surface neutralization path was identified, enabling the formation, in the block copolymer film, of homogeneous textures of lamellae or cylinders perpendicularly oriented with respect to the substrate. A critical interplay between the phase segregation of the homopolymer blends and their grafting process on the SiO2 was observed. In fact, the polar SiO2 is preferential for the PMMA-rich phase that forms a homogeneous layer on the substrate, while the PS-rich phase is located at the polymer-air interface. During the thermal treatment, phase segregation and grafting proceed simultaneously. Complete wetting of the PS rich phase on the PMMA rich phase leads to the formation of a PS/PMMA bilayer. In this case, the progressive diffusion of PS chains toward the polymer-SiO2 interface during the thermal treatment allows tuning of the brush layer composition.

Morphology re‐entry in asymmetric PS‐PI‐PS' triblock copolymer and PS homopolymer blends

ABSTRACTHere, we report the morphology variation in a series of PS‐b‐PI‐b‐PS' asymmetric triblock copolymer and PS homopolymer (hPS) blends, where PS' and PS are polystyrene blocks with a molecular weight ratio of approximately 0.11 and PI is poly(isoprene). We find that adding a small amount of hPS results in significant order–order transition (OOT) boundary deflection toward higher PS volume fractions fPS, which is accompanied by morphology re‐entry. For example, the neat triblock copolymer with a PS + PS' volume fraction of fPS = 0.38 exhibits a lamellar microphase; adding a small amount of hPS reverts the morphology into a hexagonal phase with PS cylinders, while further increasing the hPS fraction leads to normal OOTs from PS cylinders to lamellae, to PI cylinders and finally to spheres. The morphology variation reported here is significantly different from that reported in binary blends of diblock or symmetric triblock copolymer with homopolymer. While the domain features of the LAM structure can be correctly reproduced by self‐consistent field theory (SCFT), the observed morphology re‐entry is absent in the theoretical SCFT phase diagram. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 169–179

Ordered-bicontinuous-double-diamond structure in block copolymer/homopolymer blends

The morphologies of the microdomain structures in polystyrene-polyisoprene (PS-PI) diblock copolymer and polyisoprene (PI) homopolymer blends were investigated by small-angle X-ray scattering. The PS-PI investigated in the present study has number-average molecular weight and volume fraction of PI , and PI has . The weight fraction of added homopolymer was 0.17. We demonstrated that the existence of ordered-bicontinuous-double-diamond (OBDD) structure was explicitly identified in the PS-PI/PI blend by the scattering method. It was confirmed that OBDD microdomain changed to gyroid (Gyr) when the sample was heated, and the order-order transition between OBDD and Gyr thermo-reversibly occurred. The kinetics of the transformation between OBDD and Gyr was also investigated. In this study, the intermediate structure during the transition between OBDD and Gyr was not observed. A comparison between the rates of transformation revealed that the rate of the OBDD to Gyr transformation is faster than that of the Gyr to OBDD.

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.

A new periodic pattern with five‐neighbored domain packing from ABC triblock terpolymer/B homopolymer blend

ABSTRACTSelf‐assembled structures from poly(isoprene‐b‐styrene‐b‐2‐vinylpyridine)(ISP)/styrene homopolymer blend were investigated. Five terpolymers whose total molecular weight, M, is in the narrow range of 121k &lt; M &lt; 214k, and volume fractions of the center block polystyrenes, ϕS, are similar at around 0.55, were prepared as parent block terpolymers. Their ϕP/ϕI ratios, used as an indicator of asymmetry, are varied in the range 0.32 &lt; ϕP/ϕI &lt; 2.46. Three low‐molecular weight styrene homopolymers with molecular weights of 3k, 9k, and 12k, respectively, were mixed with those block terpolymers to produce blends with almost constant styrene content within the range 0.65 &lt; ϕS &lt; 0.68. Both ISP/S(3k) and ISP/S(12k) blend series show a morphological transition from tetragonally packed cylinders (TPC) to double hexagonal structure (DHS) with hexagonally arrayed polyisoprene cylinders, each surrounded by six thin cylinders as satellites. If one focuses on ISP‐III(150k)/S blends whose ϕP/ϕI is 0.88, TPC for ISP‐III/S(3k) was transformed to DHS for ISP‐III/S(12k), evidently due to the molecular weight effect of the added homopolymer. Finally a new periodic pattern, having P cylinders surrounded by five I cylinders each, has been discovered from ISP‐III/S(9k) at overall composition of ϕI/ϕS/ϕP = 0.17/0.68/0.15 and polystyrene block/styrene homopolymer ratio of wS(b)/wS(h) = 1.4. This structure was confirmed to possess hexagonal symmetry with larger unit cell than regular patterns ever known by X‐ray diffraction experiment. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 907–911