Transition from Incomplete to Complete Wetting of Solid Surface in Polymer–Solvent System: 3. The Regime of Weak Adsorption

The transition from incomplete to complete wetting occurring near the critical temperature of a two-phase polymer–solvent system at the substrate surface that weakly adsorbs macromolecules was studied using the Cahn–de Gennes model. It was shown that, depending on the force of attraction between the segments and the wall, the energy of interaction between the segments in the surface layer, as well as on the length of chain, the wetting transition can occur as the first- or second-order phase transitions, or as the tricritical wetting transition. Near the temperatures of these transitions, we determined the character of the variations in the difference of the surface concentrations that are established at the boundaries between the substrate and semidilute or dilute polymer solutions, as well as in the difference between the interfacial tensions and of the cosine of contact angles. It was shown that the temperature of each transition varies inversely to the square root of the molecular mass of polymer, and its deviation from the critical temperature is determined by the type of transition. At the first-order transitions at the SP-regime, the deviation is proportional to the energy of attraction between the chain units and the wall and is independent of polymer chain length, whereas at the critical wetting it is proportional to the squared energy of attraction between the segments and the substrate and increases with polymer chain length according to the N ^1/2law. At the considered asymptotic regime, which corresponds to the substrates that weakly attract polymer chain units, the type of the wetting transition can be regulated by varying only the length of polymer chain at the same energy characteristics of a substrate. The possibility of observing the critical wetting transitions using the solutions of high-molecular-mass compounds is discussed.     

Transitions of tethered polymer chains: a simulation study with the bond fluctuation lattice model

A polymer chain tethered to a surface may be compact or extended, adsorbed or desorbed, depending on interactions with the surface and the surrounding solvent. This leads to a rich phase diagram with a variety of transitions. To investigate these transitions we have performed Monte Carlo simulations of a bond fluctuation model with Wang-Landau and umbrella sampling algorithms in a two-dimensional state space. The simulations' density-of-states results have been evaluated for interaction parameters spanning the range from good- to poor-solvent conditions and from repulsive to strongly attractive surfaces. In this work, we describe the simulation method and present results for the overall phase behavior and for some of the transitions. For adsorption in good solvent, we compare with Metropolis Monte Carlo data for the same model and find good agreement between the results. For the collapse transition, which occurs when the solvent quality changes from good to poor, we consider two situations corresponding to three-dimensional (hard surface) and two-dimensional (very attractive surface) chain conformations, respectively. For the hard surface, we compare tethered chains with free chains and find very similar behavior for both types of chains. For the very attractive surface, we find the two-dimensional chain collapse to be a two-step transition with the same sequence of transitions that is observed for three-dimensional chains: a coil-globule transition that changes the overall chain size is followed by a local rearrangement of chain segments.     

Transition from Incomplete to Complete Wetting of Solid Surface in Polymer–Solvent System: 2. The Regime of Strong Adsorption

The transition from incomplete to complete wetting of the solid surface by a semidilute polymer solution coexisting at equilibrium with the very-dilute polymer solution was studied using the Canh–de Gennes theory under the conditions corresponding to the tricritical state of semidilute solution and strong adsorption of the chain units on a substrate. It was established that the wetting transition can occur as the first- or second-order phase transition or as the transition of tricritical wetting depending on the repulsion energy of segments that are on the substrate surface. Near the temperatures of these transitions, the character of the variations in the differences of surface concentrations that are established at the boundaries of the substrate with semidilute and dilute polymer solutions, as well as in the differences of interfacial tensions and the cosine of contact angle were determined. It was shown that the temperature of each of these phase transitions varies in proportion to the surface potential of the substrate and does not depend on the polymer molecular mass. The observed behavior differs essentially from that established near the critical point of a polymer–solvent system.     

Phase transitions of a polymer threading a membrane coupled to coil-globule transitions

We theoretically study phase transitions of a polymer threading through a pore imbedded in a membrane. We focus on the coupling between a partition of the polymer segments through the membrane and a coil-globule transition of the single polymer chain. Based on the Flory model for collapse transitions of a polymer chain, we calculate the fraction of polymer segments and the expansion factor of a polymer coil on each side of the membrane. We predict a first-order phase transition of a polymer threading a membrane; polymer segments in one side are discontinuously translocated into the other side, depending on solvent conditions and molecular weight of the polymer. We also discuss the equilibrium conformation of the polymer chain on each side of the membrane.     

Thermal transitions of benzene in copolymers and interpenetrating polymer networks based on hydrophilic and hydrophobic components: Experimental evidence of hydrophobic interaction

Thermal transitions of benzene in a hydrophobic polymer network have been explained by us in terms of the phase diagram of the polymer‐solvent system. In this work, we executed a similar study on copolymers and interpenetrating polymer networks (IPNs) with controllable hydrophilic/hydrophobic ratios. Copolymers and IPNs were swollen with different amounts of benzene and subjected to cooling and heating scans with differential scanning calorimetry (DSC). Synthesis of the IPNs was carried out in such a way that phase separation appeared, and three qualitatively different types of DSC thermograms were identified depending on the benzene content of IPN. Thermal transitions of benzene in the hydrophilic/hydrophobic copolymers can also be explained as a consequence of the phase diagram of the system, but an increase in the glass‐transition temperature of the system can be correlated with the interactions among the hydrophilic groups of the copolymer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1713–1721, 2003     

Synthesis of diblock copolymers containing both polydiene and polyalcohol blocks

A wide range of functional diblock copolymers containing both functional polymer and hydrocarbon polymer, have been prepared by the combination of anionic polymerization and borane chemistry. The distinctive hydroboration reactivity between various polydienes enables us to selectively functionalize one polydiene segment in diblock copolymers which contain two different polydiene segments. In turn, the hydroborated polymers are valuable intermediate compounds which can be converted to a whole family of functional polymers with a variety of other functionalities under mild reaction conditions. The hydroxylated diblock copolymer is used as an example to demonstrate the chemistry. The two-phase nature of these copolymers is also shown by the observation of two distinctive glass transition temperatures associated with the individual polymer segments.     

Two-dimensional self-assembly of linear poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers at the air-water interface

The interfacial properties of amphiphilic linear diblock copolymers based on poly(ethylene oxide) and poly(epsilon-caprolactone) (PEO-b-PCL) were studied at the air-water (A/W) interface by surface pressure measurements (isotherms and hysteresis experiments). The resulting Langmuir monolayers were transferred onto mica substrates and the Langmuir-Blodgett (LB) film morphologies were investigated by atomic force microscopy (AFM). All block copolymers had the same PEO segment (Mn = 2670 g/mol) and different PCL chain lengths (Mn = 1270; 2110; 3110 and 4010 g/mol). Isothermal characterization of the block copolymer samples indicated the presence of three distinct phase transitions around 6.5, 10.5, and 13.5 mN/m. The phase transitions at 6.5 and 13.5 mN/m correspond to the dissolution of the PEO segments in the water subphase and crystallization of the PCL blocks above the interface similarly as for the corresponding homopolymers, respectively. The phase transition at 10.5 mN/m was not observed for the homopolymers alone or for their blends and arises from a brush formation of the PEO segments anchored underneath the adsorbed hydrophobic PCL segments. AFM analysis confirmed the presence of PCL crystals in the LB films with unusual hairlike/needlelike architectures significantly different from those obtained for PCL homopolymers.     

Preparation and characterization of MPEG–PCL diblock copolymers with thermo‐responsive sol–gel–sol phase transition

MPEG–PCL diblock copolymers consisting of methoxy polyethylene glycol (MPEG, 750 g/mol) and poly(?‐caprolactone) (PCL) were synthesized by ring‐opening polymerization. Aqueous solutions of the synthesized diblock copolymers were prepared by dissolving the MPEG–PCL diblock copolymers at concentrations in the range of 0–20 wt %. When the PCL molecular weight was 3000 or greater, the polymer was only partially soluble in water. As the temperature was increased from room temperature, the diblock copolymer solutions showed two phase transitions: a sol‐to‐gel transition and a gel‐to‐sol transition. The sol‐to‐gel phase transition temperature decreased substantially with increasing PCL length. The sol–gel–sol transition with the increase in temperature was confirmed by monitoring the viscosity as a function of temperature. The temperature ranges of the phase transitions measured by the tilting method were in full agreement with those determined from the viscosity measurements. The maximum viscosity of the copolymer solution increased with increasing hydrophobicity of the diblock copolymer and with increasing copolymer concentration. X‐ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses revealed that the diblock copolymers exhibited crystalline domains that favored the formation of an aggregated gel because of the tight aggregation and strong packing interactions between PCL blocks. Scanning electron micrographs of the diblock copolymer solutions in the sol state showed interconnected polyhedral pore structures, whereas those of the gel state revealed a fibrillar‐like morphology. Atomic force microscope (AFM) studies of the sol and gel surfaces showed that the sol surface was covered with fine globular particles, whereas the gel surface was covered with particles in micron‐scale irregular islets. These findings are consistent with uniform mixing of the diblock copolymer and water in the sol state, and aggregation of PCL blocks in the gel state. In conclusion, we confirm that the MPEG–PCL diblock copolymer solution exhibited a sol–gel–sol transition as a function of temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5413–5423, 2006     

Macromolecules with specific constitution and supramolecular structures

Segmented triblock and multiblock copolymers consisting of highly flexible polydisperse segments and molecularly uniform segments of well defined architecture capable of either the formation of a mesophase, chainfolded crystallites or double-helical metal complexes have been synthesized; the bulk superstructure as well as the physical and material properties correlated with the primary structure: block copolymers with liquid crystalline segments formed microphase separated systems, and the phase behaviour depended on the constitutional composition. Multiblock copolymers with symmetrically chain-folded polyurethane hard segments and associative groups in the chain fold led to specially shaped lamellar superstructures. Block copolymers with bipyridine containing segments gave tetracoordinated and double-helical copper(I) complexes which segregated resulting in supramolecular polymer systems.     

无规共聚物熔体结晶的计算机模拟

1956年Flory基于Onsager的思想提出了高分子有序化的"堆积原理",即由于链的非柔顺性导致链构象的空间各向异性,只有有序化排列才能在有限的空间中放入大量的各向异性分子而不必改变其构象,由此发展的平均场格子理论被向列型有序的MonteCarlo模拟所证明。     

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.     

Shear-induced structure in polymer-clay nanocomposite solutions

The equilibrium structure and shear response of model polymer-clay nanocomposite gels are measured using X-ray scattering, light scattering, optical microscopy, and rheometry. The suspensions form physical gels via the "bridging" of neighboring colloidal clay platelets by the polymer, with reversible adsorption of polymer segments onto the clay surface providing a short-range attractive force. As the flow disrupts this transient network, coupling between composition and stress leads to the formation of a macroscopic domain pattern, while the clay platelets orient with their surface normal parallel to the direction of vorticity. We discuss the shear-induced structure, steady-shear rheology, and oscillatory-shear response of these dynamic networks, and we offer a physical explanation for the mesoscale shear response. In contrast to flow-induced "banding" transitions, no stress plateau is observed in the region where macroscopic phase separation occurs. The observed platelet orientation is different from that reported for polymer-melt clay nanocomposites, which we attribute to effects associated with macroscopic phase separation under shear flow.     

Supramolecular structure and properties of specially designed two-phase macromolecular systems

Multiphase polymer systems with special phase structures and properties can be achieved by the proper design of the polymer constitution and the appropriate sample treatment. Multiblock copolymers consisting of molecularly uniform polyurethane segments of various molecularly engineered structures and connected by highly flexible polyether segments have been synthesized. The segregation of the hard segments and the size, shape and perfection of the hard domains can be controlled by the hard segment's architecture in that either extended chain crystallization or chain folding occurs. The same superstructure and properties as of the above multiblock copolymers can be mimicked by specially designed graft copolymers with polyether backbone and polyurethane branches, again uniform and molecularly engineered. This study has shown that the morphology and superstructure of macromolecular systems can be precisely controlled by the molecularly designed chain architecture and the sample history, and is correlated with special material properties.     

Monitoring surface thermal transitions of ABA triblock copolymers with crystalline segments using phase contrast tapping mode atomic force microscopy

Variable-temperature tapping mode atomic force microscopy was used to follow thermal transitions in nanoscale phase separated triblock copolymers containing partially crystalline poly(octadecyl methacrylate) or poly(docosyl methacrylate) and glassy (poly(tert-butyl acrylate)) segments. Melting/crystallization and devitrification/vitrification transitions in phase separated domains were followed with the aid of "phase shift thermograms" constructed from cantilever phase shift maps acquired at different temperatures. This type of analysis turned out to be particularly useful in following melting/crystallization and devitrification/vitrification transitions occurring in the same temperature range and thus difficult or impossible to resolve using differential scanning calorimetry.     

接枝共聚法制备聚乙二醇(PEG)/聚乙烯醇(PVA)高分子固-固相变材料性能研究

用接枝共聚法将具有相变特征的聚乙二醇(PEG)接枝到具有较高熔点的聚乙烯醇(PVA)主链上,得到了系列性能稳定的PEG/PVA高分子固-固相转变材料,用DSC,WAXD和POM对其相变行为及形态结构进行了研究.结果表明,该材料呈现出可逆的固-固相转变特性;其结晶峰值温度和相变焓比纯PEG低,接枝率对相变温度和归一化相变焓影响不大;接枝率只影响结晶与熔融行为,不影响结晶结构.     

Sequence Distribution-Glass Transition Effects. II. Alkyl Methacrylate/Vinyl Chloride Copolymers

The glass transition temperatures of cyclized and un-cyciized alkyl methacrylate/vinyl chloride copolymers have been measured and found to differ from values predicted using conventional glass transition prediction relationships. An additive relationship was developed that relates the sequence distribution of the copolymer to the polymer glass transition temperature. Using this relationship, good agreement between experimental and sequence distribution predicted glass transitions was found.     

Bulk and surface properties of blends with semifluorinated polymers and block copolymers

The goal of the investigation presented here is the development of extremely hydrophobic materials based on polysulfone that can be applied, for instance, as fouling-resistant membrane materials. The concept used is the addition of semifluorinated polymers to polysulfone in suitable blend compositions. The influence of molecular parameters like chain structure of the semifluorinated polymer (segmented block copolymers, random copolymers) and segment molecular weight on the state of phase separation in the bulk and its influence on the surface properties have been systematically examined. It could be shown that segmented block copolymers with semifluorinated polyester segments with intermediate segment molecular weight are more suitable in blends with polysulfone than random polysulfone copolymers having semifluorinated side chains with respect to form homogeneous thin films (coatings) with highly non-wetting properties.     

A novel thermogelling matrix for microchannel DNA sequencing based on poly-N-alkoxyalkylacrylamide copolymers

We have developed a novel class of thermogelling polymer networks based on poly-N-alkoxyalkylacrylamides, and demonstrated their use as DNA sequencing matrices for high-throughput microchannel electrophoresis in capillary arrays. Polymers and copolymers of N-ethoxyethylacrylamide (NEEA) and N-methoxyethylacrylamide (NMEA) were synthesized by aqueous-phase free-radical polymerization and characterized by tandem gel permeation chromatography-multi-angle laser light scattering. These copolymer matrices exhibit "re-entrant"-type volume phase transitions, forming entangled networks with high shear viscosity at low (< 20 degrees C) and high (> 35 degrees C) temperatures, and undergoing a "coil-to-globular", lower critical solution temperature (LCST)-like phase transition over an intermediate temperature range (20-35 degrees C). Hence, matrix viscosity is relatively low at room temperature (25 degrees C), and increases rapidly above 35 degrees C. The material properties and phase behavior of these thermogelling polymer networks were studied by steady-shear rheometry. These matrices are easily loaded into capillary arrays at room temperature while existing as viscous fluids, but thermogel above 35 degrees C to form transparent hydrogels via a thermo-associative phase transition. The extent of the intermediate viscosity drop and the final viscosity increase depends on the composition of the copolymers. DNA sequencing by capillary array electrophoresis with four-color laser-induced fluorescence (LIF) detection shows that these thermogelling networks provide enhanced resolution of both small and large DNA sequencing fragments and longer sequencing read lengths, in comparison to appropriate control (closely related, nonthermogelling) polymer networks. In particular, a copolymer comprised of 90% w/w NMEA and 10% w/w NEEA, with a molecular mass of approximately 2 MDa, delivers around 600 bases at 98.5% base-calling accuracy in 100 min of electrophoresis.     

Phase behaviors of cyclic diblock copolymers

A spectral method of self-consistent field theory has been applied to AB cyclic block copolymers. Phase behaviors of cyclic diblock copolymers, such as order-disorder transition, order-order transition, and domain spacing size, have been studied, showing good consistency with previous experimental and theoretical results. Compared to linear diblocks, cyclic diblocks are harder to phase separate due to the topological constraint of the ring structure. A direct disorder-to-cylinder transition window is observed in the phase diagram, which is significantly different from the mean field phase diagram of linear diblock copolymers. The domain spacing size ratio between cyclic and linear diblock copolymers is typically close to 0.707, indicating in segregation that the cyclic polymer can be considered to be made up of linear diblocks with half of the original chain length.     

Polymer surface science

Molecular level studies of the structure and mechanical properties of polymer surfaces have been carried out by sum frequency generation (SFG) surface vibrational spectroscopy and atomic force microscopy (AFM). The surfaces of different grades of polyethylene and polypropylene have been characterized-including during the glass transition and when mechanically stretched. Copolymers that have hard and soft segments with different glass transition temperatures show phase separation, an effect of hydrogen bonding between the hard and soft segments, that influences their adhesive and friction properties. AFM and SFG show that low surface energy additives migrate to the surface and alter the surface mechanical properties. Polymers, where the chemical nature of the end groups is different from the backbone, show surface segregation of the hydrophobic part of the chain in air and the hydrophilic part in water. Likewise, in miscible polymer blends, surface segregation of the more hydrophobic component in air and the more hydrophilic component in water is observed. This area of surface science requires increased attention because of the predominance of polymers as structural materials and as biomaterials.