Novel Thermoreversible Gelation of Biodegradable PLGA‐block‐PEO‐block‐PLGA Triblock Copolymers in Aqueous Solution

The BAB‐type triblock copolymers composed of a central poly(ethylene oxide) (PEO, M̄_nPEO = 1 000) block and two poly[(D ,L ‐lactic acid)‐co‐(glycolic acid)] end blocks with molecular weights between 900 and 1 600 exhibited an interesting phase transition behavior. The copolymer aqueous solution can form micelles with PLGA loops in the core and a PEO shell and groups of micelles because of bridging between micelles caused by the PLGA blocks with raising temperature. A possible micellar gelation mechanism was suggested.     

Molecular,Nanostructural and Mechanical Characteristics of Lamellar Triblock Copolymer Blends: Effects of Molecular Weight and Constraint

While theoretical and experimental efforts have thoroughly addressed microphase‐ordered AB diblock copolymer blends with a parent homopolymer (hA or hB) or a second block copolymer, surprisingly few studies have considered comparable ABA triblock copolymers in the presence of hB or an AB diblock copolymer. In this study, we elucidate the roles of additive molecular weight and constraint by examining three matched series of miscible ABA/hB and ABA/AB blends. Self‐consistent field theory is employed to analyze molecular characteristics, e. g., segmental distributions, microdomain periods and midblock bridging fractions, as functions of blend composition. Predictions are compared to morphological characteristics discerned by transmission electron microscopy and small‐angle X‐ray scattering. The corresponding mechanical properties of these blends are measured by dynamic mechanical analysis. The results of this comprehensive work reveal that addition of hB swells the B‐lamellae of the ABA copolymer and has a generally deleterious effect on both the dynamic elastic modulus and midblock bridging fraction. In contrast, addition of a lamellar or cylindrical AB copolymer to the same ABA copolymer can promote an increase or decrease in lamellar period and bridging fraction, depending on relative block sizes.     

A Molecular Dynamics Simulation on the Self‐Assembly of ABC Triblock Copolymers, 1. Effects of Block Composition in Symmetric Triblock Copolymers

The self‐assembly of ABC triblock copolymers in the microphase‐separated state is investigated using an isothermal‐isobaric molecular dynamics simulation. For the validation of our simulation scheme, ABA triblock copolymers are also simulated. We examine the effect of the composition (f_B) of symmetric triblock copolymers on the morphology realized in these copolymers, keeping other parameters fixed. For ABA triblock copolymers, the transition from lamellar to cylindrical morphologies is observed with increasing the composition from f_B = 0.5 to f_B = 0.75, and such behavior is supported by calculation results of scattering patterns. These simulated results agree well with experimental and theoretical ones, validating our simulation method. More complex structures are predicted for ABC triblock copolymers. If midblock B is the minor component, its structures are changed from lamellar, cylindrical, to spherical morphology at the interface between A/C lamellae as f_B decreases. For ABC triblock copolymers with the midblock B as the major component, the morphology of end blocks in the matrix composed of the midblock is changed from tricontinuous to spherical structures as f_B increases.     

Glass transition temperatures of ABA poly(styrene-b-isoprene) block copolymers by differential scanning calorimetry

The glass transition behavior of two sets of ABA poly(styrene-b-isoprene) block copolymers was examined by differential scanning calorimetry. In one series, the triblock copolymers had different total molecular weights and the same (30 wt %) polyisoprene content, in the other, the molecular weight was constant (30,000 g/mol) and the elastomer content was the variable. For all triblock copolymers studied, the data show an inward shift for the glass transition temperatures T_g of the corresponding homopolymers. This shift increases for the rigid-phase T_g as the polystyrene block length decreases. Depending on their molecular characteristics, two, three, or only one T_g were found. The third T_g was interpreted in terms of the existence of an interphase. Some of these conclusions could be confirmed by transmission electron microscopy.     

Studies on the Adsorption of Asymmetrical Triblock Copolymers by Scheutjens-Fleer Theory and Monte Carlo Simulation

The adsorption of asymmetrical triblock copolymers from a non-selective solvent on solid surface has been studied by using Scheutjens-Fleer mean-field theory and Monte Carlo simulation method on lattice model. The main aim of this paper is to provide detailed computer simulation data, taking A8-kB20Ak as a key example, to study the influence of the structure of copolymer on adsorption behavior and make a comparison between MC and SF results. The simulated results show that the size distribution of various configurations and density-profile are dependent on molecular structure and adsorption energy. The molecular structure will lead to diversity of adsorption behavior. This discrepancy between different structures would be enlarged for the surface coverage and adsorption amount with increasing of the adsorption energy. The surface coverage and the adsorption amount as well as the bound fraction will become larger as symmetry of the molecular structure becomes gradually worse. The adsorption layer becomes thicker with increasing of symmetry of the molecule when adsorption energy is smaller but it becomes thinner when adsorption energy is higher. It is shown that SF theory can reproduce the adsorption behavior of asymmetrical triblock copolymers. However, systematic discrepancy between the theory and simulation still exists.The approximations inherited in the mean-filed theory such as random mixing and the allowance of direct back folding may be responsible for those deviations.     

Self‐Assembly of Cylinder‐Forming ABA Triblock Copolymers under Cylindrical Confinement

The self‐assembly of two types of linear ABA triblock copolymers confined in cylindrical nanopores is studied using simulated annealing. The effects of pore size and block copolymer chain architecture on morphology, chain conformations and bridging fraction are investigated. For the bulk cylinder‐forming copolymers, novel structures such as helices and stacked toroids form, which depend sensitively on the pore size. Several significant differences between the two types of copolymers are predicted and explained based on the differences in their chain conformations and chain architectures. A simple model is proposed to explain the mean square radius of gyration for the bridge and loop chains.

     

Self-assembly of linear triblock copolymers under cylindrical nanopore confinements

The self-assembly of linear ABC triblock copolymers under cylindrical confinements is investigated in two-dimensional space using the real-space self-consistent field theory. The effects of confinement degrees and preferential strengths on the triblock copolymer phase behaviors with special polymer parameters are first considered. On one hand, different confinement degrees cause different phase behaviors in nanopores with the neutral surfaces. Moreover, the strongly preferential surface fields can surpass the confinement degrees and volume fractions in determing the confined phase behaviors. On the other hand, in contrast, confined morphologies are more sensitive to the variations in the A-preferential surface field strength. Subsequently, the incompatibility degrees between different blocks are systematically varied under cylindrical nanopore confinements. Under cylindrical nanopore confinements, the morphologies are very sensitive to the variations in the incompatibility degrees. Meanwhile, nanopore confinements can affect order-disorder and order-order transition points in the bulk. The corresponding free, internal, and entropic energies as well as the order parameters are also quantificationally examined to deeply investigate the confined phase mechanisms, and a number of morphological transitions are confirmed to be of first-order. These findings may guide the design of novel nanostructures based on triblock copolymers by introducing confinements.     

Chemical composition effects on the fracture of polystyrene-block-poly(methyl methacrylate)block copolymers

The crazing and fracture behaviors of glassy–glassy block copolymers were investigated for polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers that had similar overall molecular weights but different poly(methyl methacrylate) (PMMA) molar fractions. A liquid chromatography technique was applied to separate as-synthesized PS-b-PMMA [(1) weight-average molecular weight (M_w) = 94,000 g/mol and PMMA molar fraction = 0.35 and (2) M_w = 65,000 g/mol and PMMA molar fraction = 0.28] into three fractions with different chemical compositions. With a copper-grid technique, the fracture behaviors of 0.5-μm-thick PS-b-PMMA films were studied as a function of the applied strain. For the higher M_w PS-b-PMMA samples, the median strains at crazing and fibril breakdown increased with an increase in the PMMA molar fraction from 0.24 to 0.46, corresponding to an increase in the chain entanglements in the PMMA domains. In contrast, for the lower M_w samples, the two values were not significantly changed even when the PMMA molar fraction was varied from 0.16 to 0.35. M_w of the minor component in PS-b-PMMA played a critical role in controlling the fracture behaviors of the block copolymers. Specifically, M_w/M_e of the minor component (where M_e is the molecular weight between entanglements) had to be roughly larger than 2 for the block copolymers to sustain sufficient strains before fracture. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3612–3620, 2006     

Synthesis of amphiphilic multiblock and triblock copolymers of polydimethylsiloxane and poly(N,N‐dimethylacrylamide)

The synthesis and spectroscopic characterization of a new family of amphiphilic multiblock and triblock copolymers is described. The synthetic methodology rests on the preparation of telechelic multifunctional and difunctional chain transfer agents easily available in two synthetic steps from commercially available polydimethylsiloxane‐containing starting materials. Telechelic polymers thus synthesized are used as macromolecular chain transfer agents in the reversible addition fragmentation chain transfer (RAFT) polymerization of N,N‐dimethylacrylamide (DMA) enabling the synthesis of (AB)_n‐type multiblock and ABA‐type triblock copolymers of varying compositions possessing monomodal molecular weight distribution. (AB)_n multiblock copolymers [(PDMA‐b‐PDMS)_n] were prepared with between 52 and 95 wt % poly(dimethylacrylamide) with number average molecular weights (M_n) between 14,000 and 86,000 (polydispersities of 1.20–2.30). On the other hand, ABA block copolymers with DMA led to amphiphilic block copolymers (PDMA‐b‐PDMS‐b‐PDMA) with M_n values between 9000 and 44,000 (polydispersities of 1.24–1.62). © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7033–7048, 2008     

Nucleation and crystallization of PS-b-PEO-b-PCL triblock copolymers

We have recently prepared a series of Polystyrene-b-Poly(ethylene oxide)-b-Polycaprolactone (PS-b-PEO-b-PCL or SEOCL) triblock copolymers of varying compositions and molecular weights. These ABC triblock copolymers present the peculiarity that two of the three blocks are able to crystallize upon cooling from an already phase segregated melt. When either of the crystallizable blocks or both are a minor phase, a fractionated crystallization process develops. The confinement of crystallizable blocks in the nanoscopic scale enables the clear observation in some cases of exclusive crystallization from homogeneous nuclei of two components within the triblock copolymer. The homogeneous nature of the nucleation was deduced since the supercooling attained is the maximum possible before vitrification of the material takes place. The self-nucleation domains were also found to depend on the composition and molecular weight of the copolymers. The block copolymers exhibited a marked decrease in crystalline memory and when the crystallizable blocks constitute minor phases, the self-nucleation domain disappears. The reason behind this behavior is that only at lower self-nucleation temperatures the density of self-nuclei becomes high enough to include at least one crystal fragment per confined microdomain in view of their vast numbers (e.g., 10¹⁶/cm³).     

ABA‐type liquid crystalline triblock copolymers via nitroxide‐mediated radical polymerization: Design,synthesis, and morphologies

We have designed and synthesized rod–coil–rod triblock copolymers of controlled molecular weight by two‐step nitroxide‐mediated radical polymerization, where the rod part consists of “mesogen‐jacketed liquid crystalline polymer” (MJLCP). The MJLCP segment examined in our studies is poly{2,5‐bis[(4‐methoxyphenyl)oxycarbonyl]styrene} (MPCS) while the coil part is polyisoprene (PI). Characterization of the triblock copolymers by GPC, ¹H and ¹³C NMR spectroscopies, TGA, DSC confirmed that the triblock copolymers were comprised of microphase‐separated low T_g amorphous PI and high T_g PMPCS blocks. Analysis of POM and 1D, 2D‐WAXD demonstrated that the triblock copolymers formed nematic liquid crystal phase. Morphological studies using TEM indicated the sample formed lamellar structure. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5949–5956, 2007     

Conformational structures and thermodynamic properties of compact polymer chains confined between two parallel plane boundaries

In this paper, the authors investigated the adsorption phenomenon of compact chains confined between two parallel plane boundaries using a pruned‐enriched Rosenbluth method. The authors considered three cases with different adsorption energies of ε = 0, ?1, and ?3 (in units of k_BT) for the confined compact chains of different chain lengths N, respectively. Several parameters were employed to describe the size and shape of compact chain, and some special behaviors in the conformational structures were investigated for the first time. For example, the size and shape of confined compact chains undergo distinct changes in the adsorption cases of ε = ?1 and ?3, and pass through the maximum values at the characteristic distances D_c. The authors found that this characteristic distance D_c could be scaled as D_c～ (N + 1)^ν (ν = 0.56 ± 0.01) in the case of ε = ?3. In addition, the microstructures of chains were investigated, and several significant results were obtained by analyzing the segment density distribution and the mean fractions of segment in tails, trains, bridges, and loops structures. On the other hand, the thermodynamic properties were also investigated for the confined compact chains, such as average energy per bond, Helmholtz free energy per bond, and elastic force per bond. Results show that elastic forces f have different behaviors in three cases, indicating that it is not necessary to exert an external force on the boundaries in the nonadsorption case. At the same time, the average contact energy of compact chain obviously changes when the distance between the two parallel boundaries D increases, which is similar to those of the size and shape parameters. The authors also conclude that these thermodynamic properties of compact chains depend strongly on not only the adsorption energies but also the chain lengths and the confined condition. In addition, several results of the conformational and thermodynamic parameters, such as the segment density distribution and free energy, were compared with the results from the self‐consistent field theory. These investigations may help us to deepen the knowledge about the adsorption phenomenon of confined compact chains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2888–2901, 2006     

Electroactive methacrylate‐based triblock copolymer elastomer for actuator application

A series of ABA triblock copolymers of methyl methacrylate (MMA) and dodecyl methacrylate (DMA) [poly(MMA‐b‐DMA‐b‐MMA)] (PMDM) were synthesized by Ru‐based sequential living radical polymerization. For this, DMA was first polymerized from a difunctional initiator, ethane‐1,2‐diyl bis(2‐chloro‐2‐phenylacetate) with combination of RuCl₂(PPh₃)₃ catalyst and nBu₃N additive in toluene at 80 °C. As the conversion of DMA reached over about 90%, MMA was directly added into the reaction solution to give PMDM with controlled molecular weight (M_w/M_n ≤ 1.2). These triblock copolymers showed well‐organized morphologies such as body centered cubic, hexagonal cylinder, and lamella structures both in bulk and in thin film by self‐assembly phenomenon with different poly(methyl methacrylate) (PMMA) weight fractions. Obtained PMDMs with 20–40 wt % of the PMMA segments showed excellent electroactive actuation behaviors at relatively low voltages, which was much superior compared to conventional styrene‐ethylene‐butylene‐styrene triblock copolymer systems due to its higher polarity derived from the methacrylate backbone and lower modulus. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Morphology and bridging properties of (AB)n multiblock copolymers

Motivated by recent experiments (Spontak, R. J.; Smith, S. D. J Polym Sci Part B: Polym Phys 2001, 39, 947) on morphological and mechanical properties of multiblock copolymers (AB)_n, we theoretically elucidate the links between microscopically determined properties, such as the bridging fraction of chains, and mechanical properties of these materials. We do this by applying self‐consistent mean‐field theory to determine morphological aspects such as period and interfacial width and calculate the bridging fractions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 104–111, 2003     

Monte Carlo simulation for the adsorption of symmetric triblock copolymers II. Adsorption layer information

By using Monte Carlo simulation, adsorption of both end-adsorbed and middle-adsorbed symmetric triblock copolymers from a non-selective solvent on an impenetrable surface has been studied. Influences of the adsorption energy, the bulk concentration, the chain composition and the chain length on the adsorption behavior including the surface coverage, the adsorption amount and the layer thickness are presented. It is shown that the total surface coverage for both end-adsorbed and middle-adsorbed copolymers increases monotonically as the bulk concentration increases. The higher the adsorption energy and the more the attractive segments, the higher the total surface coverage is exhibited. Surface coverage θ decreases with increasing the length of the non-attractive segments, but the product of θ and the proportion of the non-attractive segments in a triblock copolymer chain is nearly independent of the chain length. The adsorption amount increases almost monotonically with the bulk concentration. The logarithm of the adsorption amount is a linear function of the reciprocal of the reduced temperature. When the adsorption energy is large, the adsorption amount exhibits a maximum as the composition of the attractive segment increases. The adsorption isotherms of copolymers with different length of the non-attractive segments can be mapped onto a single curve under certain energy indicating that copolymers with different chain length have the same adsorption amount. The adsorption layer thickness for the end-adsorbed copolymers decreases as the energy and the number of adsorbing segments increases. The longer non-attractive segments, the larger adsorbed layer thickness is found. The tails mainly governs the adsorption layer thickness.     

Adsorption Behavior of Asymmetrical Triblock Copolymers at the Solid‐Liquid Interface by Monte Carlo Simulation

Summary: Monte Carlo simulation on a simple lattice model has been used to study the adsorption of asymmetrical triblock copolymers from a non‐selective solvent at the solid‐liquid interface. The size distributions of train, loop and tail configurations for those copolymers are obtained as well as other details of the adsorption layer microstructure. Also the influence of adsorption energy and the role of molecular symmetry are investigated. A segment‐density profile, the adsorption amount, the surface coverage, and the adsorption layer thickness have been determined. Finally, it is shown that the adsorption behavior of an asymmetrical copolymer can be predicted from the symmetrical copolymer.

Size distributions of the tail configuration for A_8−kB₂₀A_k.     

Novel triblock siloxane copolymers: Synthesis,characterization, and their use as surface modifying additives

Synthesis of novel triblock, polycaprolactone-b-polydimethylsiloxane (PDMS) and poly(2-ethyl-oxazoline)-b-PDMS copolymers were demonstrated. These materials were obtained via the ring-opening polymerization of ?-caprolactone or 2-ethyl-2-oxazoline monomers by using organofunctionally terminated PDMS oligomers as initiators and comonomers. Segment molecular weights in these copolymers were varied over a wide range between 1000 and 2000 g/mol and the formation of copolymers with desired backbone compositions were monitored by ¹H-NMR spectroscopy and GPC. DSC and TMA studies showed the formation of two phase morphologies with PDMS (T_g, ?120°C) and polycaprolactone (T_m, 50–60°C) or poly(2-ethyl-2-oxazoline) (T_g, 40-60°C) transitions respectively. The use of polycaprolactone-b-PDMS copolymers as surface modifying additives in polymer blends were also investigated. When these copolymers were blended at low levels (0.25–10.0% by weight) with various commercial resins such as, polyurethanes, PVC, PMMA, and PET, the resulting systems displayed silicone-like, hydrophobic surface properties, as determined by critical surface tension measurements or water contact angles. The effect of siloxane content, block length, base polymer type and morphology on the resulting surfaces are discussed.     

Morphology and microphase separation of star copolymers

Star copolymers have attracted significant interest due to their different characteristics compared with diblock copolymers, including higher critical micelle concentration, lower viscosity, unique spatial shape, or morphologies. Development of synthetic skills such as anionic polymerization and controlled radical polymerization have made it possible to make diverse architectures of polymers. Depending on the molecular architecture of the copolymer, numerous morphologies are possible, for instance, Archimedean tiling patterns and cylindrical microdomains at symmetric volume fraction for miktoarm star copolymers as well as asymmetric lamellar microdomains for star‐shaped copolymers, which have not been reported for linear block copolymers. In this review, we focus on morphologies and microphase separations of miktoarm (A_mB_n and ABC miktoarm) star copolymers and star‐shaped [(A‐b‐B)_n] copolymers with nonlinear architecture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1–21     

三嵌段共聚物PS-PHB-PS的原子转移自由基聚合

为了克服聚β-羟基丁酸酯(PHB)的弱点, 得到性能良好的新材料, 本文利用原子转移自由基聚合方法, 以Br-PHB-Br为大分子引发剂, 苯乙烯为单体, 在CuBr/N,N,N′,N″,N″-五甲基–二乙基三胺(PMDETA)催化体系作用下合成了一种新的三嵌段共聚物聚苯乙烯-聚β-羟基丁酸酯-聚苯乙烯(PS-PHB-PS). 共聚物的链结构利用¹H NMR和¹³C NMR进行了表征, 分子量特性和链段组成利用凝胶渗透色谱(SEC)方法进行了测定. 聚合物的分子量随单体转化率的增加而线性增加, 分子量分布指数相对较窄. 这些特征都满足原子转移自由基活性聚合的理想要求. 所得到的共聚物PS-PHB-PS具有较好的生物相容性, 与PHB相比具有良好的耐热性.     

Stimuli‐responsive ABC triblock copolymers by sequential living cationic copolymerization: Multistage self‐assemblies through micellization to open association

Stimuli‐responsive ABC triblock copolymers with three segments with different phase‐separation temperatures were synthesized via sequential living cationic copolymerization. The triblock copolymers exhibited sensitive thermally induced physical gelation (open association) through the formation of micelles. For example, an aqueous solution of EOVE₂₀₀‐b‐MOVE₂₀₀‐b‐EOEOVE₂₀₀ [where EOVE is 2‐ethoxyethyl vinyl ether, MOVE is 2‐methoxethyl vinyl ether and EOEOVE is 2‐(2‐ethoxy)ethoxyethyl vinyl ether; the order of the phase‐separation temperatures was poly(EOVE) (20 °C) < poly(EOEOVE) (41 °C) < poly(MOVE) (70 °C)] underwent multiple reversible transitions from sol (<20 °C) to micellization (20–41 °C) to physical gelation (physical crosslinking, 41–64 °C) and, finally, to precipitation (>64 °C). At 41–64 °C, the physical gel became stiffer than similar diblock or ABA triblock copolymers of the same molecular weight. Furthermore, the ABC triblock copolymers exhibited Weissenberg effects in semidilute aqueous solutions. In sharp contrast, another ABC triblock copolymer with a different arrangement, EOVE₂₀₀‐b‐EOEOVE₂₀₀‐b‐MOVE₂₀₀, scarcely exhibited any increase in viscosity above 41 °C. The temperatures of micelle formation and physical gelation corresponded to the phase‐separation temperatures of the segment types in the ABC triblock copolymer. No second‐stage association was observed for AB and ABA block copolymers with the same thermosensitive segments found in their ABC counterparts. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2601–2611, 2004