Influence of compositional gradient on the phase behavior of ternary symmetric homopolymer-copolymer blends: A Monte Carlo study

The formation of bicontinuous microemulsions (BUEs) in ternary symmetric blends including two immiscible homopolymers A and B as well as a linear gradient copolymer G is verified by Monte Carlo simulations. Four phase diagrams as a function of segregation strength and blend composition are constructed to show the effects of compositional gradient. Compared with the blends of A/B/diblock copolymers, the BUE phase occupies a broader region in the phase diagram of the A/B/G blends and is replaced by the three-phase coexistence phases of A + B + L or A + B + D at a much lower chain length ratio. Moreover, the lamellar phase is formed at a higher value of segregation strength, has a shorter periodic length and lower orientational order, and spans a narrower range of the phase diagram. A “counteraction effect” is proposed to explain those discrepancies caused by gradient copolymers.     

Differences in enthalpy recovery of gradient and random copolymers of similar overall composition: styrene/4-methylstyrene copolymers made by nitroxide-mediated controlled radical polymerization

Styrene/4-methylstyrene (S/MS) random and gradient copolymers were synthesized by nitroxide-mediated controlled radical polymerization (NM-CRP) and compared to random copolymers made by conventional free radical polymerization (ConvFRP). The gradient copolymers have molecular weight (MW) values approaching 85,000 g/mol, making these some of the higher MW gradient copolymers reported to date. Due to the proximity of the glass transition temperatures (T_g) of polystyrene (PS) and poly(4-methylstyrene) (PMS), there is no significant difference in T_g between the gradient and random copolymers, with both copolymer types yielding single T_gs that typically increase slightly with increasing MS content. While enthalpy relaxation studies demonstrate similarity in random copolymers made by NM-CRP and ConvFRP, they reveal significant differences between random and gradient copolymers. Gradient copolymers exhibit broad enthalpy recovery peaks, whereas random copolymers exhibit narrower enthalpy recovery peaks. The maxima in the enthalpy recovery peaks are at substantially lower temperature, as much as 17 °C, in the gradient copolymers as compared to random copolymers of equal overall composition. While random and gradient copolymers of a given overall composition exhibit similar enthalpy recovery values at a common physical aging time and quench depth relative to T_g, the major differences in the enthalpy recovery peaks indicate that differences in sequence distribution along the chain length can lead to unusual behavior in gradient copolymers relative to random copolymers.     

Molecular weight distribution in living polymerization

An easily obtained parameter for characterizing polymers is the molecular weight distribution (MWD). Polymers with different MWD have different properties. The MWD can be easily influenced with controlled polymerization technologies. Examples of how these methods influence the properties of block and gradient copolymers in wetting and dispersing additives are discussed. These wetting and dispersing additives are used to make solvent-borne paints and aqueous pigment concentrates. An advantage of modulation of the MWD is the ability to fine tune-specific properties, such as the compatibility or the wetting activity of the polymer.     

Achievement of quasi-nanostructured polymer blends by solid-state shear pulverization and compatibilization by gradient copolymer addition

Nanoblends, in which dispersed-phase domains exhibit length scales of order 100 nm or less, are made using a continuous, industrially scalable, mechanical process called solid-state shear pulverization (SSSP). An 80/20 wt% polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend processed by SSSP and consolidated by platen pressing, without melt processing, exhibits a quasi-nanostructured morphology with many irregular, minor-phase domain sizes of ∼100 nm or less. After short-residence-time single-screw extrusion, the pulverized blend exhibits spherical dispersed-phase domains with a number-average diameter of 155 nm. Thus, SSSP followed by certain melt-processing operations can yield nanoblends. However, the pulverized blend exhibits significant coarsening of the dispersed-phase domains during long-term, high-temperature static annealing, indicating that SSSP followed by other melt processes may yield microstructured blends. In order to suppress coarsening, a styrene (S)/methyl methacrylate (MMA) gradient copolymer is synthesized by controlled radical polymerization. When 5 wt% S/MMA gradient copolymer is added to the PS/PMMA blend during SSSP, the resulting blend exhibits a nanostructure nearly identical to that of the blend without gradient copolymer, and coarsening is nearly totally suppressed during long-term, high-temperature static annealing. Thus, SSSP with gradient copolymer addition can yield compatibilized nanoblends. Morphologies obtained in the pulverized PS/PMMA nanoblend are compared with those in blends of PS/poly(n-butyl methacrylate) and PS/high-density polyethylene made using identical SSSP conditions, providing for commentary on the ability of SSSP to produce nanostructured blends as a function of blend components.     

Influence of the longest ethylene and isotactic propylene sequences on crystallization elution fractionation of ethylene/propylene copolymers

Crystallization elution fractionation (CEF) is the newest crystallization-based technique for estimating the chemical composition distribution of ethylene/1-olefin copolymers. Understanding the separation mechanism of CEF for ethylene/propylene copolymers over their full compositional range is challenging because the crystallizabilities of the copolymer chains depend on the longest ethylene sequence and on longest isotactic propylene sequence. We developed a mathematical model to describe the CEF mechanism for ethylene/propylene copolymers over the entire compositional range using population balances for the crystallization and dissolution stages. The joint distribution of longest ethylene and isotactic propylene sequences determines how the copolymer populations crystallize and dissolve. The model was validated with experimental CEF profiles of ethylene/propylene copolymers varying from pure ethylene to propylene homopolymers.     

Block, blocky gradient and random copolymers of 2-ethylhexyl acrylate and acrylic acid by atom transfer radical polymerization

The controlled synthesis of low-T_g poly(2-ethylhexyl acrylate) (P2EHA) and derived random, block and blocky gradient copolymers via atom transfer radical polymerization (ATRP) is described. After optimizing the reaction conditions for the homopolymerization of 2EHA via ATRP, the synthesis of a variety of copolymers with poly(t-butyl acrylate) (PtBuA) was investigated. First, AB-block copolymers were targeted, starting from P2EHA and PtBuA as macroinitiators. Second, random copolymers of tBuA and 2EHA with different monomer ratios were synthesized. Finally, the synthesis of “blocky” gradient copolymers via a one-pot procedure was investigated, starting with the homopolymerization of tBuA, followed by the addition of 2EHA. The hydrolysis of the PtBuA-segments to poly(acrylic acid) (PAA), which was carried out with methanesulfonic acid, resulted in block, blocky gradient and random copolymers consisting of PAA and P2EHA. Solubility testing of the copolymers in slightly basic water (pH ∼ 9) demonstrated that the gradient structure significantly enhances solubility compared to the block copolymer structures with equal composition. The polymers have been characterized by MALDI-TOF MS, GPC and ¹H NMR.     

Synthesis and application of styrene/4-hydroxystyrene gradient copolymers made by controlled radical polymerization: Compatibilization of immiscible polymer blends via hydrogen-bonding effects

Styrene (S)/4-hydroxystyrene (HS) copolymers are synthesized by hydrolysis of S/4-acetoxystyrene copolymer precursors; two gradient copolymer precursors are made by semi-batch, nitroxide-mediated controlled radical polymerization, and a random copolymer precursor is prepared by conventional free radical polymerization. Conventional heat curves from differential scanning calorimetry indicate two glass transition temperatures (T_gs) and a broad T_g in well-annealed 59/41 mol% and 25/75 mol% S/HS gradient copolymers, respectively, both of which contain short S end-blocks. In contrast, a narrow T_g is observed in a 57/43 mol% random copolymer. Each S/HS copolymer is added at 5 wt% by solution mixing to an 80/20 wt% polystyrene (PS)/polycaprolactone (PCL) blend and tested for its ability to compatibilize the blend during melt processing; the hydroxyl groups on the HS units can form hydrogen bonds with the PCL ester groups. The S/HS random copolymer fails as a compatibilizer while both gradient copolymers are good compatibilizers. Relative to the blend without copolymer, the blend with 59/41 mol% S/HS gradient copolymer also exhibits a major reduction in initial dispersed-phase domain size and irregularly shaped domains, which are indicators of a sharply reduced interfacial tension. In contrast, the blend with 25/75 mol% S/HS gradient copolymer has an average PCL domain size comparable to the blend without copolymer and a broad domain size distribution. The presence of S/HS copolymers in the blend leads to reduced PCL crystallization and melting temperatures as well as reduced enthalpies of crystallization and melting, consistent with some solubilization of copolymer in the PCL domain interiors.     

梯度共聚物的可控制备及其性能

梯度共聚物是近年来伴随着活性聚合方法而发展起来的一种新型共聚物,其特点在于单体单元组成沿着分子链方向逐渐变化,链结构界于常见的无规共聚物和嵌段共聚物之间。本文从梯度共聚物的结构特点入手,总结了其可控制备方法、表征手段、物化性质以及应用前景。基于共聚动力学模型控制单体加料速率的半连续活性/可控自由基聚合可实现梯度共聚物的结构定制,基于多步单体进料方式的RAFT乳液聚合则由于其简单和高效将成为梯度共聚物可控制备的重要方法。梯度共聚物的自组装行为和微观聚集态不同于嵌段共聚物,表现出独特的界面活性、热学特性和力学性能,组成梯度结构有望成为调控高分子材料性能的新参数,梯度共聚物有望在乳化剂、相相容剂、阻尼材料、多形状记忆材料等领域得到应用。     

Thermo-responsive brush copolymers with structure-tunable LCST and switchable surface wettability

Thermo-responsive brush copolymers poly(methyl methacrylate (MMA)-co-2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM)-graft-(N-isopropyl-acrylamide) (NIPAAm)) were synthesized using Cu-mediated “living” radical polymerization (LRP) approach. Varied grafting densities of the brushes were obtained through adjusting backbone structure as random, gradient and block respectively. The effect of grafting densities on their thermo-responsive phase transition behaviors in aqueous solution and on surface were investigated in detail. The lower critical solution temperature (LCST) of brush copolymers in solution was adjusted as 35, 37 and 38 °C through random, gradient and block backbone structure respectively. Their structure tunable thermo-responsive phase transition in solution were further confirmed by the different micelle aggregation behaviors above LCST which monitored by transmission electron microscopy (TEM) images and dynamic light scattering (DLS). In addition, surfaces modified by the resulted brush copolymers have a temperature tunable wettability based on thermo-responsive phase transition in solid, the similar WCA variation range of three brush copolymers implies that the composition of backbone does not much affect the switchable wettability of surfaces.     

Gelatin/Poly(ethylene oxide) Blend Films with Compositional Gradient: Fabrication and Characterization

In this study, novel protein/biodegradable polymer blend biomaterials‐gelatin/poly(ethylene oxide) blend films with compositional gradients were successfully fabricated by a dissolution/diffusion method. Two kinds of compositional gradient films, which were different in gradient structure, were prepared. The compositional gradient structure in the films was characterized by polarized optic microscopy, ATR‐FTIR, and trans‐FT‐IR mapping measurement. In addition, the mechanical properties of the gradient films were characterized with reference to their gradient structure. It is found that the compositional gradient films have better mechanical properties than the pure gelatin film. These protein/biodegradable polymer compositional gradient films have a potential for many biomedical applications.

     

Characterisation of block copolymer self‐assemblies by thermal field‐flow fractionation

The ability of block copolymers to self‐assemble into various nanostructures (such as micelles) has attracted significant attention over the years as these block copolymers provide a versatile platform that can readily be modified for a wide range of applications. As such, the solution behaviour of block copolymers has been at the forefront of the modern nanotechnology revolution. However, these novel block copolymer‐based self‐assemblies lack suitable characterisation techniques to determine size, molar mass and compositional distributions simultaneously. This mini‐review gives a short background on the current techniques used to determine important micelle properties as well as their limitations for characterising complex samples. Current column‐based fractionation techniques used for determining property distributions are addressed. As a result of the limitations of column‐based fractionations, a multidetector thermal field‐flow fractionation (ThFFF) approach is put forward as a powerful alternative for determining not only size and molar mass distributions, but also other micelle properties as a function of temperature. More importantly, ThFFF is highlighted as the only current characterisation platform capable of addressing the analytical challenges associated with compositional distributions of polymer self‐assemblies. © 2017 Society of Chemical Industry     

Comparison of thermomechanical properties of statistical, gradient and block copolymers of isobornyl acrylate and n-butyl acrylate with various acrylate homopolymers

Well-defined statistical, gradient and block copolymers consisting of isobornyl acrylate (IBA) and n-butyl acrylate (nBA) were synthesized via atom transfer radical polymerization (ATRP). To investigate structure-property correlation, copolymers were prepared with systematically varied molecular weights and compositions. Thermomechanical properties of synthesized materials were analyzed via differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA) and small-angle X-ray scattering (SAXS). Glass transition temperature (T_g) of the resulting statistical poly(isobornyl acrylate-co-n-butyl acrylate) (P(IBA-co-nBA)) copolymers was tuned by changing the monomer feed. This way, it was possible to generate materials which can mimic thermal behavior of several homopolymers, such as poly(t-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA) and poly(n-propyl acrylate) (PPA). Although statistical copolymers had the same thermal properties as their homopolymer equivalents, DMA measurements revealed that they are much softer materials. While statistical copolymers showed a single T_g, block copolymers showed two T_gs and DSC thermogram for the gradient copolymer indicated a single, but very broad, glass transition. The mechanical properties of block and gradient copolymers were compared to the statistical copolymers with the same IBA/nBA composition.     

Compositional and configurational sequence determination of methacrylonitrile–vinylidene chloride copolymers by nuclear magnetic resonance spectroscopy

Methacrylonitrile–vinylidene chloride (M/V) copolymers of different composition were prepared by bulk polymerization using benzoyl peroxide as an initiator under nitrogen atmosphere in a sealed tube. The copolymer composition was determined from quantitative ¹³C[¹H] NMR spectra. The reactivity ratios for M/V copolymers obtained from a linear Kelen–Tudos method and nonlinear error‐in‐variables method are r_M = 2.47 ± 0.14, r_V = 0.40 ± 0.02, and r_M = 2.43, r_V = 0.39, respectively. The complete spectral assignment in term of compositional and conformational sequences of these copolymers were done with the help of distortionless enhancement by polarization transfer, two‐dimensional heteronuclear single‐quantum coherence spectroscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1865–1874, 2005     

Thermal and X-ray investigations of ethylene–α-olefin copolymers obtained with highly active supported Ti–Mg and V–Mg catalysts

Copolymers of ethylene with propylene, butene-1 and hexene-1 were prepared using a titanium–magnesium (TMC) or a vanadium–magnesium catalyst (VMC). The copolymers were examined for thermal stability by TGA, melting and crystallization behaviour by DSC and crystallinity by XRD. Fractionated samples of ethylene–hexene-1 copolymers were also similarly characterized. Results indicate that VMC produces copolymers with a higher degree of crystallinity and greater compositional homogeneity than TMC.     

Separation of propene/1-alkene and ethylene/1-alkene copolymers by high-temperature adsorption liquid chromatography

A high performance liquid chromatography column (HPLC) Hypercarb^® packed with porous graphite has proven to discriminate polyolefin molecules due to differences in their adsorption and desorption behaviour. While linear polyethylene (PE) and syndiotactic polypropylene (sPP) are adsorbed on the graphite packing, isotactic polypropylene (iPP) is not adsorbed. The column operates at 160 °C with 1-decanol as sample solvent and mobile phase. We have now tested this HPLC system for separations of random propene/1-alkene and ethylene/1-hexene copolymers: While copolymers of propene with 1-butene, 1-hexene and 1-octene copolymers eluted in size exclusion mode without adsorption, propene/1-octadecene and ethylene/1-hexene copolymers are strongly retained and eluted only after application of a linear gradient starting from 1-decanol and ending with pure 1,2,4-trichlorobenzene. The retention of propene/1-alkene (>11 carbons in the side chain) copolymers increases with the concentration of comonomer, making this HPLC system suitable to separate these copolymers according to their chemical composition. In contrast, the retention of ethylene/1-hexene samples decreases with increasing 1-hexene content. Branching in this case shortens the length of continuous methylene sequences of the polymer backbone, which are expected to adsorb in a planar conformation to the graphite layers. This is the first report on the separation of short chain branched polyolefins by high-temperature adsorption liquid chromatography.     

Compositional heterogeneity,thermostable, and shape memory properties of ethylene oxide-ethylene terephthalate segmented copolymer with long soft segment

Ethylene oxide-ethylene terephthalate segmented copolymers (EOET) with long PEO segment or high PET content have showed an obvious compositional heterogeneity. The EOET copolymers with compositional heterogeneity could be separated into soluble and insoluble fractions by extraction with chloroform. ¹H-NMR measurements showed that the former contains much lower PET content than the average content value, and the latter is in reverse. DSC results revealed that PET segments in the latter would crystallize more easily, but in the former PEO segments exhibits more intensive melting peak. The thermogravimetric behaviors of EOET copolymers were between PEO and PET homopolymers. The EOET copolymers with serious compositional heterogeneity showed two stages of weight loss. TGA was sensitive to indicate the compositional heterogeneity in EOET copolymers. The compositional heterogeneity could impart a great influence on the shape memory behavior of EOET copolymers. The recovery curve of EOET copolymers with serious compositional heterogeneity also can exhibit two stages of deformation recovery. Generally, the component with worse memory behavior in EOET copolymer is an unfavorable factor, and the addition of EOET copolymer with better memory behavior into the blend is a favorable factor for the blend system. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 947–955, 1998     

SEC-Gradients, an alternative approach to polymer gradient chromatography: 1. Proof of the concept

A new approach to gradient chromatography of polymers is presented, in which the sample is introduced at the end of the gradient and elutes within the elution volume range typical for size exclusion chromatography (SEC). Due to the gradient the samples are retarded and elute nearly independent of molar mass at the adsorption threshold. The concept was proven for a series of narrowly distributed poly(methy methacrylate)s (PMMA) in a chloroform-tetrahydrofuran (THF) SEC-gradient. The application of the SEC-gradient to a blend of PMMA and polystyrene standards of similar molar masses, which could not be separated by SEC due to their similar hydrodynamic sizes, resulted in a clear separation according to chemical composition. Since SEC-gradients allow dissolving the sample in strong eluents, which might result in breakthrough peaks in conventional gradients, the new approach is a valuable alternative to conventional gradient chromatography.     

Comparison of methods for characterizing comonomer composition in ethylene 1-olefin copolymers: 3D-TREF vs. SEC-FTIR

In this paper, two methodologies for determining comonomer composition in ethylene 1-olefin copolymers, namely three detectors coupled to a temperature rising elution fractionation unit (3D-TREF) and size exclusion chromatography coupled to a Fourier transfer infrared detector (SEC-FTIR), are examined and compared. Because the two methods are based on different separation mechanisms, insight into the resin's molecular architecture is gained from two entirely different, yet complementary perspectives. The choice of which method to use will be determined by the specific structure vs. property issue under study. Comparative results from the analysis of copolymers produced by Ziegler-Natta, chromium and metallocene catalysts show that both the methods are useful for characterizing LLDPE resins. However, the 3D-TREF method may offer more insight into the heterogeneity of resin blends, particularly when the blend components have similar molecular weights. Although some MW-dependency information of the temperature fractions can be ascertained via viscometer and light scattering detectors, SEC-FTIR is the more appropriate method to detect compositional heterogeneity in resin blends that are composed of two or more resins with the same copolymer compositions, but with different molecular weights.     

Synthesis and characterization of functional gradient copolymers of glycidyl methacrylate and butyl acrylate

Epoxy-functional spontaneous gradient copolymers of glycidyl methacrylate (G) and n-butyl acrylate (B) were synthesized via atom transfer radical polymerization (ATRP). The copolymerization reactions were carried out in toluene solution at 70 °C, using methyl 2-bromopropionate (MBrP) as initiator and copper chloride with N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as the catalyst system. The kinetic behaviour of the statistical copolymerizations was studied in a wide composition interval with molar fractions of G ranging from 0.10 to 0.75. The synthesized copolymers were characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. ¹H NMR was employed to determine the copolymer composition, demonstrating the gradient character of the copolymers along the main chain in the whole monomer conversion interval. Apart from this, the sequence distribution and stereoregularity were analyzed. These microstructural experimental data agreed well with those calculated from Mayo-Lewis terminal model (MLTM) and a Bernoullian statistic with an isotacticity parameter of σ_G = 0.28 and a coisotacticity parameter of σ = 0.30.     

梯度共聚物的最新进展

阐述了梯度共聚物的概念及梯度共聚物的研究意义,介绍了梯度共聚物分类及特点,以及与这些特点相对应的各种用途。重点综述了近年来各种梯度共聚物的制备方法如常规乳液聚合、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移聚合(RAFP)和氮氧调控自由基聚合(NMCRP)等,以及梯度共聚物的各种表征方法,如红外光谱(FT-IR)、核磁共振(NMR)、薄层色谱(TLC)以及X射线光电子能谱(XPS),并进一步分析了梯度共聚物的研究方向和研究重点。