Summary: Monte Carlo simulation utilizing the bond fluctuation model in conjunction with single and configurational biased Monte Carlo moves is used to study the adsorption of diblock (A‐block‐B) and alternating (A‐alt‐B) copolymers at flat, chemically heterogeneous surfaces comprising C and D domains. The main objective of this work is to address the effect of the strength of attraction between the adsorbing surface domains, D, and the copolymer adsorbing segments, B, on the copolymer's ability to recognize the chemical pattern on the surface. The results of our simulations reveal that both block and alternating copolymers have the ability to recognize the surface motif and transcribe it into the bulk material. The extent to which diblock copolymers transfer the chemical pattern from the surface to the bulk is relatively unaffected when the attractive B‐D potential is increased beyond a certain critical value. This behavior stems from the brush‐like conformation adopted by the diblock copolymer at the substrate. In contrast to the diblock copolymer, the adsorption of the alternating copolymer is influenced by the strength of the attraction between the copolymer's adsorbing segments and the adsorbing domains on the surface. Since the B segments are distributed evenly along the backbone, the alternating copolymers are more likely to adopt conformations in which the whole chain is “zipped” to the surface. The resultant entropic frustration is then alleviated through an increased formation of loops with little change to their length. Such conformational changes endow the alternating copolymer with the ability to invert the substrate pattern as the distance away from the surface is increased.
We present a statistical mechanical model, which is used to investigate the adsorption behavior of two-letter (AB) copolymers on chemically heterogeneous surfaces. The surfaces with regularly distributed stripes of two types (A and B) and periodic multiblock copolymers (Al)B(l))(x) are studied. It is assumed that A(B)-type segments selectively adsorb onto A(B)-type stripes. It is shown that the adsorption strongly depends on the copolymer sequence distribution and the arrangement of selectively adsorbing regions on the surface. The polymer-surface binding proceeds as a two-step process. At the first step, the copolymer having short blocks adsorbs onto the surface as an effective homopolymer, which does not feel chemical pattern. At the second step, when the polymer-surface attraction is sufficiently strong, the adsorbed chain adjusts its equilibrium conformation to reach the perfect bound state, thereby demonstrating ability for pattern recognition. The key element of this mechanism is the redistribution of strongly adsorbed copolymer diblocks A(l)B(l), which behave as surfactants, between multiple AB interfaces separating A and B stripes on the adsorbing surface. Such redistribution is accompanied by a well-pronounced decrease in the system entropy. We have found that marked pattern recognition is possible for copolymers with relatively short blocks at high polymer/surface affinities, beyond the adsorption threshold. 相似文献
Block copolymers have been extensively studied due to their ability to spontaneously self‐organize into a wide variety of morphologies that are valuable in energy‐, medical‐, and conservation‐related (nano)technologies. While the phase behavior of bicomponent diblock and triblock copolymers is conventionally governed by temperature and individual block masses, it is demonstrated here that their phase behavior can alternatively be controlled through the use of blocks with random monomer sequencing. Block random copolymers (BRCs), i.e., diblock copolymers wherein one or both blocks are a random copolymer comprised of A and B repeat units, have been synthesized, and their phase behavior, expressed in terms of the order–disorder transition (ODT), has been investigated. The results establish that, depending on the block composition contrast and molecular weight, BRCs can microphase‐separate. We also report that large variation in incompatibility can be generated at relatively constant molecular weight and temperature with these new soft materials. This sequence‐controlled synthetic strategy is extended to thermoplastic elastomeric triblock copolymers differing in chemistry and possessing a random‐copolymer midblock. 相似文献
In this contribution, we describe the syntheses, characterization, and properties of ABA, CBA and CBC triblock copolymers with glassy (A), elastomeric (B), and crystalline (C) blocks. These three hard-soft-hard triblock copolymers were prepared via living ring-opening metathesis copolymerization by use of Grubbs third generation catalyst through one-pot sequential monomer addition and subsequent hydrogenation. These hard-soft-hard triblock copolymers based thermoplastic elastomers have been analyzed by proton NMR, differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), IR, and transmission electron microscopy (TEM). The mechanical properties of these triblock copolymers were also measured by monotonic and step cyclic tensile test. Compared with the ABA triblock copolymer, the CBC triblock copolymer containing highly crystalline hard end-blocks shows enhanced tensile strength and the best elastic recovery of 90.8%. The hybrid CBA triblock copolymer displays much improved elongation, which is almost twice as long as the other two TPEs, and excellent elastic recovery of 87.0%. 相似文献
Summary: The effects of copolymer sequence distribution on the dynamics of a copolymer in a homopolymer matrix are studied using computer simulations within the framework of the bond‐fluctuation model on blends containing low concentrations (10%) of copolymers dispersed in a homopolymer matrix. The sequence distribution of the two copolymer components was changed while maintaining the overall copolymer composition at 50/50. Our results indicate that copolymers with disordered sequence distributions exhibit dynamics that are faster than that of a homopolymer melt, while those with ordered sequence distributions exhibit a tendency to form aggregates that lead to slower dynamics as well as phase separation. Analysis of the structure suggests that copolymers with an alternating sequence distribution form large aggregates that are short‐lived, while diblocks form permanent micelle‐like structures. Analysis of the local composition around a copolymer molecule indicates that aggregation between copolymer chains has a direct impact on the local composition. This in turn has a significant impact on system dynamics. Our results indicate that the dynamics of random, random‐blocky, and alternating copolymers are nearly identical and are faster than that of a homopolymer melt. However, alternating copolymers form aggregates and hence are not uniformly distributed throughout the matrix phase. Thus, alternating copolymers are at a disadvantage in their ability to be effective compatibilizers. From a dynamic perspective, copolymers with random and random‐blocky copolymers seem to be ideal compatibilizers since they are distributed uniformly throughout the system and move rapidly through the matrix phase.
Snapshots of aggregates of alternating copolymer chains. Dark and bright spheres represent A and B monomers, respectively. 相似文献
Adsorption of copolymers on patterned surfaces is studied using lattice modeling and multiple Markov chain Monte Carlo methods. The copolymer is composed of alternating blocks of A and B monomers, and the adsorbing surface is composed of alternating square blocks containing C and D sites. Effects of interaction specificity on the adsorbed pattern of the copolymer and the sharpness of the adsorption transition are investigated by comparing three different models of copolymer-surface interactions. Analyses of the underlying energy distribution indicate that adsorption transitions in our models are not two-state-like. We show how the corresponding experimental question may be addressed by calorimetric measurements as have been applied to protein folding. Although the adsorption transitions are not "first order" or two-state-like, the sharpness of the transition increases when interaction specificity is enhanced by either including more attractive interaction types or by introducing repulsive interactions. Uniformity of the pattern of the adsorbed copolymer is also sensitive to the interaction scheme. Ramifications of the results from the present minimalist models of pattern recognition on the energetic and statistical mechanical origins of undesirable nonspecific adsorption of synthetic biopolymers in cellular environments are discussed. 相似文献
ABA‐ and BAB‐type triblock copolymers possessing pendant, self‐assembling motifs in the A and B blocks were synthesized, with 2‐ureidopyrimidinone (UPy) and benzene‐1,3,5‐tricarboxamide (BTA) for the A and B block, respectively. They were investigated to assess if and how the polymer’s microstructure influences the self‐assembly behavior of the supramolecular motifs and, as a result, the single‐chain folding process. BAB‐type triblock copolymers were synthesized via atom transfer radical polymerization (ATRP) with molecular weights ranging from 30 to 120 kg mol?1; the BTA and UPy motifs were attached using a post‐functionalization approach. The ABA‐type triblock copolymers were available from previous work. In highly dilute solutions, both types of triblock copolymers fold into single‐chain polymeric nanoparticles (SCPNs) via thermally induced BTA self‐assembly and photo‐triggered UPy dimerization. Chain collapse induced by intramolecular UPy dimerization was evaluated using size‐exclusion chromatography (SEC). The BTA self‐assembly was monitored by circular dichroism (CD) spectroscopy. The microstructures of SCPNs were visualized by atomic force microscopy (AFM). SEC analysis indicated a more loose packing for the BAB‐type folded nanoparticles than for the ABA‐type ones, which implies that topological differences in the polymer architecture do affect the folding behavior, although only slightly. The facile synthetic protocol developed here provides topologically different triblock architectures and opens up the area for single‐chain folding technology that is applicable in artificial enzymatic systems with compartmentalized domains. 相似文献
An (AB)n-type multiblock copolymer containing alternating poly(l-lactide) (PLLA) and poly(dimethyl siloxane) (PDMS) segments was synthesized by chain extension of hydroxyltelechelic PLLA-PDMS-PLLA triblock copolymers, which were prepared by the ring-opening polymerization of l-lactide initiated by α,ω-functionalized hydroxyl poly(dimethyl siloxane), using 1,6-hexamethylene diisocyanate as a chain extender. The triblock and the multiblock copolymers were characterized by FT-IR, 1H NMR and GPC. From the results of thermal analysis, two glass transition temperatures which were measured by DSC showed the occurrence of phase separation phenomena in the triblock and multiblock copolymers because of the difference of solubility parameters between PLLA and PDMS segments. The effect of the chemical composition of the triblock copolymers, including the Mw and the constitutive segment chain length of the macrodiol, on the development of the Mw of the multiblock was discussed based on diffusion effect. Furthermore, the consumption of the isocyanate groups was determined by FT-IR to investigate the dependence of the reaction kinetics of the urethane formation on the chemical composition of the triblock copolymer. The results reveal that the order of the chain extension reaction depended on the Mw of the triblock copolymer: a second order reaction was transformed into a third reaction as the Mw of the triblock copolymer increased from 7000 to 25,000 (g/mol) perhaps because of the inhibition of the formation of an active complex involved in the catalyzed-urethane reaction by the polymer chain aggregation. Finally, the mechanical properties of the multiblock copolymers demonstrated that the introduction of the extremely flexible PDMS segment substantially improved the elongation at breakage, and the tensile strength and the tensile modulus declined due to the intrinsic elasticity of such segments. 相似文献
The objective of this project was to utilize the alternating copolymerizability of electrondonor monomers with electron-acceptor monomers to selectively introduce nucleic acid bases into copolymers in a controlled sequence distribution. To this end, maleimide monomers containing the adenine, thymine, cytosine, and 6-chloropurine moieties were converted to their hompolymers. The homopolymer of 1-(vinyloxyethoxy)thymine was also prepared. Alternating copolymers of the adenine maleimide monomer and the corresponding 6-chloropurine maleimide monomer with 1-(vinyloxyethoxy)thymine were prepared. The latter copolymer was converted to the alternating adenine–thymine copolymer by reaction with ammonia. Characterization of the polymers and copolymers via spectroscopic methods and physical measurements confirm their proposed structures. Monomer syntheses and characterization, as well as studies designed to establish the extent and nature of adenine–thymine interactions in the copolymers, are reported in accompanying papers. 相似文献
Methyl acrylate and styrene have been copolymerized in the presence of zinc chloride either by photoinitiation or spontaneously. The copolymerization mechanism is investigated by analyses of copolymers composition and monomer sequence distribution. The resulting copolymers are not always alternating, their composition being dependent especially on the monomer feed ratio. Appreciable deviation to higher methyl acrylate unit content from an equimolar composition occurs at monomer feed fractions of methyl acrylate over 0.7. The larger deviation is induced by higher temperature, by photoirradiation, and by greater dilution of the reaction mixture with toluene. The 13C-NMR spectrum of the alternating copolymer shows a sharp singlet at the carbonyl region, whereas the spectra of random copolymers prepared by benzoyl peroxide initiation at 60°C show a triplet splitting at the carbonyl carbon region, irrespective of copolymer composition. The relative intensities of the triplet peaks for the random copolymers are in good correspondence to the contents of triad sequences calculated by means of conventional radical copolymerization theory. These results clearly indicate that the carbonyl splitting is caused predominantly by variation of the monomer sequence and not by variation of the stereosequence. The monomer sequence distribution in the copolymers is thus directly and quantitatively measured from the split carbonyl resonance. Although the same triplet splitting appears in the spectra of methyl acrylate–rich copolymers prepared in the presence of zinc chloride at high feed ratios (>0.7) of methyl acrylate, the relative intensities of the split peaks do not fit the sequence distributions of random copolymers calculated by means of the Lewis–Mayo equation. The copolymerization yielding these peculiar sequences and the alternating sequence in the presence of zinc chloride is fully comprehended by a copolymerization mechanism proceeding between two active coordinated monomers, i.e., the ternary molecular complex composed of zinc chloride, methyl methacrylate, and styrene, and the binary molecular complex composed of zinc chloride and methyl methacrylate. 相似文献
The complex microstructures of amphiphilic ABC linear triblock copolymers in which one of the end blocks is relatively short and hydrophilic, and the other two blocks B and C are hydrophobic in a dilute solution, have been investigated by the real-space implementation of self-consistent field theory (SCFT) in two dimensions (2D). In contrast to diblock copolymers in solution, the aggregation of triblock copolymers are more complicated due to the presence of the second hydrophobic blocks and, hence, big ranges of parameter space controlling the morphology. By tailoring the hydrophobic degree and its difference between the blocks B and C, the various shapes of vesicles, circlelike and linelike micelles possibly corresponding to spherelike, and rodlike micelles in 3D, and especially, peanutlike micelles not found in diblock copolymers are observed. The transition from vesicles to circlelike micelles occurs with increasing the hydrophobicity of the blocks B and C, while the transition from circlelike micelles to linelike micelles or from the mixture of micelles and vesicles to the long linelike micelles takes place when the repulsive interaction of the end hydrophobic block C is stronger than that of the middle hydrophobic block B. Furthermore, it is favorable for dispersion of the block copolymer in the solvent into aggregates when the repulsion of the solvent to the end hydrophobic block is larger than that of the solvent to the middle hydrophobic block. Especially when the bulk block copolymers are in a weak segregation regime, the competition between the microphase separation and macrophase separation exists and the large compound micelle-like aggregates are found due to the macrophase separation with increasing the hydrophobic degree of blocks B and C, which is absent in diblock copolymer solution. The simulation results successfully reproduce the existing experimental ones. 相似文献
We use Monte Carlo simulations to model the formation of random copolymers with tunable monomer sequence distributions. Our scheme is based on the original idea proposed a few years ago by Khokhlov and Khalatur [Physica A 249, 253 (1998); Phys. Rev. Lett. 82, 3456 (1999)], who showed that the distribution of species B in A-B random copolymers can be regulated by (a) adjusting the coil size of a homopolymer A and (b) chemically modifying ("coloring") monomers that reside at (or close to) the periphery of the coil with species B. In contrast to Khokhlov and Khalatur's work, who modeled the polymer modification by performing the coloring instantaneously, we let the chemical coloring reaction progress over time using computer simulations. We show that similar to Khokhlov and Khalatur's work, the blockiness (i.e., number of consecutive monomers) of the B species along the A-B copolymer increases with increasing degree of collapse of the parent homopolymer A. A simple analysis of the A-B monomer sequences in the copolymers reveals that monomer sequence distributions in homopolymers "colored" under collapsed conformations possess certain degrees of self-similarity, while there is no correlation found among the monomer sequence distributions formed by coloring homopolymers with expanded conformations. 相似文献