Atactic Chains Research Articles

Liquid–Liquid Equilibrium in Xylene Solubles (XS) Analysis of Polypropylene

AbstractA multicomponent Flory‐Huggins model is implemented and utilized to describe the liquid–liquid equilibrium phenomenon in mixtures of polypropylene and xylene, in the context of the well‐known xylene solubles (XS) test. The XS experiment is a common procedure in many polymer laboratories, used to determine the percentage of xylene solubles in samples of polypropylene, which provides an approximate measure of the atactic and oligomeric chains. Despite the importance of the test, the literature lacks a thermodynamic perspective regarding the description of this extraction phenomenon. In the present study, the Flory‐Huggins interaction parameter is adjusted in a multicomponent framework to ensure that equilibrium chain length distributions calculated with the proposed model best match experimental distributions. It is shown that the experimental data obtained from XS analyses can be accurately fitted by the proposed model and that the estimated Flory‐Huggins interaction parameter is more sensitive to the polymer average molar mass than to the degree of tacticity, when a particular catalyst is considered.

Conformations and Solvation of Synthetic Polymers in Water by Generalized Born Implicit-Solvent Molecular Dynamics Simulations: Stereoisomers of Poly(acrylic acid) and Poly(methacrylic acid)

We present the GB-OBC model as an approach for implicit-solvent MD simulations of a synthetic macromolecule in water. The model is tested and found to be successful in reproducing the chain dimensions and predicting the free energy of solvation of carboxylic acid vinyl polymers. The influence of stereochemistry and the hydrophobic nature of the polymer was investigated as a function of chain length (20 < N < 600) for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA). The dimensionless parameters of the GB-OBC model were parameterized to be applicable to PAA and PMA. Scaling relations for chain dimensions obtained using implicit-solvent MD simulations in this study are in good agreement with those from experiments, theory of solvated chains in good solvents, and all-atom MD simulations in explicit water. Results show that ⟨Rg2⟩/NL2 is greater for the atactic chain as compared to the isotactic chain, for PAA as well as PMA. ⟨Rg2⟩/NL2 values of chains attain constancy in water for N = 200, with the values being greater for PMA. The PMA chain is conformationally more perturbed than the PAA chain, for both isotactic and atactic stereochemistry. The solvation free energy ΔGhyd of PAA and PMA in water is negative for all chain lengths (N = 20-600) and becomes more favorable with an increase in molecular weight. The ΔGhyd values for isotactic and atactic chains are identical at lower values of N but differ slightly for N > 300. Irrespective of the hydrophobic nature of the polymer, the atactic chain is thermodynamically more soluble in water as compared to the isotactic chain. The isotactic chain is less hydrophilic as compared to the atactic chain due to the closer proximity of the COOH groups along the backbone. This implicit solvent method is an effective way to accurately simulate the configurational properties and solvation of synthetic polymers in water.

Formation and structure of iodine complex of polyacrylonitrile studied by vibrational spectroscopy

It has been reported that polyacrylonitrile (PAN) can form a complex with polyiodide ions, which can give some interesting thermal and electronic properties to PAN. However, the formation process and structure characteristics of the PAN-iodine complex interpreted from the molecular level are still unclear. The present research is devoted to clarifying the above problems by studying the PAN film immersed in the iodine aqueous solution with different concentrations and counter cations based on IR and Raman spectroscopy in detail. We have found that the PAN-iodine complex can form through the intermolecular interaction between PAN chains and I 5 − ions by immersing the PAN film into the iodine aqueous solution with an iodine concentration higher than 0.1 mol/L. The ν (CN) and δ (CH 2 ) modes in the IR spectra of PAN showed a remarkable shift through forming the complex in the 1 mol/L iodine solution. Moreover, the PAN-iodine complex formed in the iodine solutions containing the different types of counter ions (K + , Na + , Li + ) may have different structures based on different band positions of ν (CN), δ (CH 2 ), and ν (CH 2 ) mode in their IR spectra. More interestingly, we found that during the formation of the complex, the content of planer zigzag conformation along the atactic PAN chain increased obviously. The present study results may inspire to further develop PAN as a functional polymer material. • The formation process and structure characteristics of the PAN-iodine complex were investigated. • PAN-iodine complex is formed by the intermolecular interactions between PAN chains and I 5 − ions. • Content of the planer zigzag conformation along the PAN chain increases obviously through the complex formation.

Insights into the miscibility of polymethyl methacrylate (PMMA) and nile blue oxazone (NBO) flurophores: tacticity and chain length aspects

Polymethyl methacrylate (PMMA) based florescent nanoparticles are noteworthy carriers that may be explored for the delivery of numerous anti-cancer and anti-viral drugs. Particularly, the PMMA-nile blue oxazone (NBO) carriers are one of the most frequently used fluorescent probes. Despite various PMMA-based cargos being explored for drug delivery, the spatial arrangement of the methyl and ester groups on the PMMA chains with respect to the encapsulated agent remains a largely unexplored area.In the present work Molecular dynamics simulation and docking studies were carried out to explore the effect of chain length of PMMA and its tacticity on its miscibility with NBO dye molecules. Energetic behavior of the modeled chains was assessed using COMPASS force field. Total potential energy of the chains increased as their length increased. The total potential Intermolecular energy of the isotactic chains was higher as compared to their syndiotactic counterparts, for all the chain lengths considered unlike the intramolecular energy, which was higher for syndiotactic chains. Flory huggins interaction parameter (χ) was calculated for polymer chains of varying lengths and tacticities, with respect to NBO. Interaction parameter increased with increase in the chain length of the polymer chains, which indicated superior interaction with smaller chains. Syndiotactic chains showed strongest interactions with the dye molecule for all the chain lengths of the polymer, whereas isotactic chains exhibited the weakest interactions. Binding energy analysis validated the patterns obtained from flory huggins theory and also established that the polymer chains had higher propensities to self- assemble within themselves, as compared to interactions with NBO molecules. Analysis of the radius of gyration of the polymer chains in the PMMA- NBO complexes, at the end of dynamics run, exhibited that the syndiotactic chains formed smallest coiled structures while the isotactic chains formed the largest coiled structures. Flexibility of the polymer chains was calculated by determining the ratio of the radius of gyration at beginning and at the end of simulation run. Here, isotactic chains were found to have maximum flexibility, followed by atactic and syndiotactic chains. These results were more coherent when the calculations were performed on larger systems, differing in the relative number of PMMA chains and NBO molecules.

Mechanical and Thermal Properties of Mixed-Tacticity Polyhydroxybutyrates and Their Association with Iso- and Atactic Chain Segment Length Distributions

We investigated the effect of including S enantiomers on the mechanical and thermal properties of predominantly (R)-β-polyhydroxybutyrate (PHB). From tensile testing, we determined resulting ratios of meso to racemo diads, for which elastic modulus, strength, and fracture strain combine to provide maximized fracture energies. We found that these coincide with an inversion of the respective elastic moduli of the amorphous and crystalline phases. From thermocalorimetric analyses, we determined the glass-transition temperatures and enthalpic relaxations, the heat capacities of the materials and their constituent phases, the directional crystallization rates and melting points, as well as the melting enthalpies for the α-PHB phase as functions of tacticity. We present a unifying characteristic, accounting for tacticity mismatches, based on the previously determined random polymerization action of the catalyst ethylzinc β-diketiminate and 4-methoxybenzyl alcohol. This characteristic provides a qualitative indication of the transition points in nonlinear correlations encountered between the ratios of meso to racemo diads and mixed-tacticity polyhydroxybutyrates’ fracture energies, amorphous and crystalline phase elastic moduli, melting enthalpies, and lattice vibrational frequencies.

Tacticity Effect on Mesogen-Free Liquid Crystalline Self-assembly Induced by Strong Dipole–Dipole Interactions

Strong dipole–dipole interactions in mesogen-free aliphatic comblike polymers can induce unique liquid crystalline (LC) self-assembly. By comparing atactic and isotactic poly(oxypropylene)s with n-alkylsulfonyl side groups (aPOP-SO2Cn and iPOP-SO2Cn), the effect of main-chain tacticity on the ordered phase behavior was studied in this work. For aPOP-SO2Cn, LC self-assemblies were observed when n is ≥3. Two types of LC phases were formed depending on the side chain length n: (i) a three-dimensional (3D) high-order LC structure (possibly crystal B or E) for n = 3–5 and (ii) a smectic A (SmA) structure for n = 6–12. For iPOP-SO2Cn, crystallization was favored as a result of regular main-chain configuration. Only when n ≥ 10, the isotactic comblike polymers exhibited a monotropic phase behavior with the SmA phase appearing during cooling from the isotropic melt. Regardless of atactic or isotactic main chains, these ordered LC and crystalline phases featured a double-layer structure, which was dictated by the ...

Cross-linker effect on solute adsorption in swollen thermoresponsive polymer networks.

The selective solute partitioning within a polymeric network is of key importance to applications in which controlled release or uptake of solutes in a responsive hydrogel is required. In this work we investigate the impact of cross-links on solute adsorption in a swollen polymer network by means of all-atom, explicit-water molecular dynamics simulations. We focus on a representative network subunit consisting of poly(N-isopropylacrylamide) (PNIPAM) and N,N'-methylenebisacrylamide (BIS/MBA) cross-linker types. Our studied system consists of one BIS-linker with four atactic PNIPAM chains attached in a tetrahedral geometry. The adsorption of several representative solutes of different polarity in the low concentration limit at the linker region is examined. We subdivide the solute adsorption regions and distinguish between contributions stemming from polymer chains and cross-link parts. In comparison to a single polymer chain, we observe that the adsorption of the solutes to the cross-link region can significantly differ, with details depending on the specific compounds' size and polarity. In particular, for solutes that have already a relatively large affinity to PNIPAM chains the dense cross-link region (where many-body attractions are at play) amplifies the local adsorption by an order of magnitude. We also find that the cross-link region can serve as a seed for the aggregation of mutually attractive solutes at higher solute concentrations. Utilizing the microscopic adsorption coefficients in a mean-field model of an idealized macroscopic polymer network, we extrapolate these results to the global solute partitioning in a swollen hydrogel and predict that these adsorption features may lead to non-monotonic partition ratios as a function of the cross-link density.

Enhancing stereoselectivity of propylene polymerization with MgCl2-supported Ziegler-Natta catalysts by electron donor: Strong effects of titanium dispersion state

Propylene polymerization with two TiCl4/MgCl2 catalysts (Cat-1: Ti 0.1%; Cat-2: Ti 1.0%) showed different responses to (cyclo-C5H9)2Si(OCH3)2 (De) addition. Cat-1 was deactivated more than Cat-2 by De. Amount of three groups of active centers producing atactic, medium isotactic and isotactic polypropylene chains were all reduced by De addition, but those of Cat-1 were reduced more. Reactivity of the isospecific active centers (kpi) was enhanced by De, but kpi of Cat-2 was changed more sensitively than Cat-1. The lower stereoselectivity of Cat-1 than Cat-2 are attributable to weaker De adsorption on its surface Mg, and the more sparsely dispersed Ti species on Cat-1 is responsible for weak De adsorption.

Spectroscopic Analyses on Chain Structure and Thermal Stabilization Behavior of Acrylonitrile/Methyl Acrylate/Itaconic Acid-based Copolymers Synthesized by Aqueous Suspension Polymerization

We have synthesized a series of copolymers with different compositions of acrylonitrile (AN, 80–100 wt%), methyl acrylate (MA, 4–20 wt%) and itaconic acid (IA, 0–3 wt%) by using an efficient aqueous suspension polymerization, and have investigated the molecular structure and thermal stabilization behavior of PAN homopolymer, AN/MA-based bipolymers, and AN/MA/IA-based terpolymers by adopting 1H/13C-NMR and thermal FT-IR analyses. The viscosity-average molecular weight of the synthesized polymers were measured to be ~263,000 g/mol. The reactivity ratios of AN and MA for all the copolymers were evaluated to be 0.99 and 1.05, respectively. Accordingly, the output compositions of the synthesized copolymers were quite consistent with the input monomer compositions. The 13C NMR analysis revealed that all the synthesized polymers have an atactic chain configuration, regardless of the feed composition. The structural evolutions during the thermal stabilization process of the copolymers in air environment were characterized by monitoring the temperature-dependent changes of characteristic absorbance bands at 2240 cm-1 (C≡N), 1595 cm-1 (C=N) and 1660 cm-1 (C=O) with aid of thermal FT-IR spectroscopy. It was found that the IA unit in the terpolymers accelerated the oxidation and cyclization reactions, unlike the retarding effect of MA unit, and that the onset temperatures of the oxidation reaction of the copolymers with IA unit was lower than that of the cyclization reaction.

Structure, Dynamics, and Apparent Glass Transition of Stereoregular Poly(methyl methacrylate)/Graphene Interfaces through Atomistic Simulations

We provide a detailed investigation of the structure and dynamics of stereoregular poly(methyl methacrylate) (PMMA) chains confined between graphene layers via atomistic molecular dynamics simulations. The density, conformation, and interfacial segmental dynamics of low molecular weight isotactic, syndiotactic, and atactic PMMA chains are examined at various temperatures, ranging from 490 K up to 580 K. For all stereoisomers, a tendency of chains to adsorb on graphene via ester-methyl groups is observed. Compared to other stereoisomers, isotactic chains are stretched and form better organized layers at the interface. Concerning dynamical properties, various dynamical modes of PMMA are studied as a function of distance from graphene as well as over the entire confined system. The interfacial backbone and ester side group motions are restricted, whereas the fast motion of methyl groups remains unaffected. In the temperature range studied here, the interphase thickness remains almost constant; however, the r...

Mechanism of internal and external electron donor effects on propylene polymerization with MgCl2‐supported Ziegler–Natta catalyst: New evidences based on active center counting

ABSTRACTTwo TiCl4/Di/MgCl2 type supported Ziegler–Natta catalysts were prepared by loading dibutylphthalate or dicyclopentyldimethoxysilane (DCPDMS) (internal donor, Di) and TiCl4 on activated δ‐MgCl2 in sequence, and a blank catalyst was prepared by loading TiCl4 on the same δ‐MgCl2 without adding Di. These catalysts have similar specific surface area and pore size distribution, thus form a suitable base for comparative studies. Propylene polymerization with the catalysts was conducted in n‐heptane slurry using triethylaluminum (TEA) as cocatalyst, and the effects of Di as well as De (external donor, in this work it was DCPDMS) on the number of active centers, the distribution of active centers among three polypropylene (PP) fractions (isotactic, medium isotactic, and atactic PP chains), and chain propagation rate constants of the PP fractions were studied by counting the number of active centers in the PP fractions using a method based on selective quench‐labeling of the propagation chains by 2‐thiophenecarbonyl chloride. When De was not added in the polymerization, introducing a phthalate type Di in the catalyst evidently changed the active center distribution by enhancing the proportion of active centers producing isotactic PP (iPP) ( ), but scarcely changed reactivities of the three groups of active centers forming the three fractions. When the De was added in the polymerization system with TiCl4/phthalate/MgCl2 catalyst, further shifting of active center distribution in favor of took place, meanwhile reactivities of the three groups of active centers also remarkably changed in favor of . Mutual effects of these changes led to overwhelming dominance of iPP production in the TiCl4/Di/MgCl2–TEA/De system (Di = phthalate, De = alkoxysilane). In contrast, though using alkoxysilane as Di also caused shifting of active center distribution in favor of when De was not added, addition of alkoxysilane De caused reverse shifting of active center distribution in favor of those producing PP of lower stereoregularity. This unfavorable change largely counteracted the reactivity changes in favor of caused by the De, rendering the catalytic system rather poor isospecificity. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46605.

Synthesis, Thermal Properties, and Morphologies of Amphiphilic Brush Block Copolymers with Tacticity-Controlled Polyether Main Chain

A series of brush block copolymers (BBCPs) consisting of poly(decyl glycidyl ether) (PDGE) and poly(10-hydroxyldecyl glycidyl ether) (PHDGE) blocks, having four different types of chain tacticities, i.e., [at-PDGE]-b-[at-PDEGE], [at-PDGE]-b-[it-PDEGE], [it-PDGE]-b-[at-PDEGE], and [it-PDGE]-b-[it-PDEGE], where the it and at represent the isotactic and atactic chains, respectively, were prepared by t-Bu-P4-catalyzed sequential anionic ring-opening polymerization of glycidyl ethers followed by side-chain modification. The corresponding homopolymers, i.e., at-PDGE, it-PDGE, at-PHDGE, and it-PHDGE, were also prepared for comparison with the BBCPs. The PDGE homopolymers were significantly promoted in the phase transitions and morphological structure formation by the isotacticity formation. In particular, it-PDGE was found to form only a horizontal multibilayer structure with a monoclinic lattice in thin films, which was driven by the bristles’ self-assembling ability and enhanced by the isotacticity. However, t...

Effect of tacticity on the structure and glass transition temperature of polystyrene adsorbed onto solid surfaces.

We have carried out atomistic (all-atom) molecular dynamics simulations to investigate the effect of tacticity on the structure and glass transition temperature (Tg) of polystyrene (PS) thin films adsorbed on two distinct types of solid substrates. The systems consist of thin films made of atactic, isotactic, and syndiotactic PS chains supported by graphite or hydroxylated α-quartz substrates, which are known to be atomically flat but chemically and structurally different. We have observed a marked dependence of the film structure on substrate type as well as on tacticity. For instance, rings' orientation near substrate surfaces depends on substrate type for atactic PS and isotactic PS films, while no such dependence is observed for syndiotactic PS films whose interfacial structure seems to result from their propensity to adopt the trans conformation rather than their specific interaction with the substrates. Moreover, our results indicate that glass transition temperatures of substrate supported polystyrene films are higher compared to those of the corresponding free-standing films. More specifically, PS films on graphite exhibit larger Tg values than those on α-quartz, and we have noticed that syndiotactic PS has the largest Tg irrespective of the substrate type. Furthermore, the local Tg in the region of the film in contact with the substrates shows a strong tacticity and substrate dependence, whereas no dependencies were found for the local Tg in the middle of the film. Substrate-film interaction energy and chains' dynamics near substrate-film interfaces were subsequently investigated in order to substantiate the obtained Tgs, and it was found that films with higher Tgs are strongly adsorbed on the substrates and/or exhibit smaller interfacial chains' dynamics essentially due to steric hindrance.

Coil-Globule Collapse of Polystyrene Chains in Tetrahydrofuran-Water Mixtures.

We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20-150 monomers in tetrahydrofuran-water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, XW, is roughly below 0.25, whereas the chains collapse into globules when XW ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, Rg, in these regimes, i.e., Rg ∝ N3/5 and Rg ∝ N1/3 for good and poor solvent conditions, respectively. For poor solvent conditions with XW = 0.75, we find a sizable fraction of residual tetrahydrofuran trapped inside the collapsed polymer chains with an excess amount located at the globule surface, acting as a protective layer between the hydrophobic polystyrene and the surrounding water-rich mixture. These findings have important implications for nanoparticle fabrication techniques where solvent exchange is exploited to drive polymer aggregation, since residual solvent can significantly influence the physical properties of the precipitated nanoparticles.

Details of the intermolecular interactions in poly(vinyl alcohol)-iodine complexes as studied by quantum chemical calculations

Abstract The electronic state of poly(vinyl alcohol) (PVA)-I 3 − ion complex has been successfully described for the first time on the basis of the density functional theoretical (DFT) calculations. (1) As the first trial, complex structure models between a short PVA chain (5-mer) and I 3 − ion were constructed, which were geometrically optimized and the energy change during the complexation was analyzed. Mainly the interaction between OH groups of PVA and iodine has been found to determine the stability of the complex. The interaction strength depends on the stereoregularity of the chain: the syndiotactic PVA sequence gave the strongest interactions between I 3 − and four or five OH sequence along the chain axis, while the atactic and isotactic chain models are less stable than the former case. The calculated interaction energy between PVA and I 3 − species was 36.0, 31.2 and 25.6 kcal/mol for syndiotactic , atactic and isotactic PVA, respectively. The calculation of orbital interactions revealed that the highest occupied molecular orbital (HOMO) of I 3 − is relatively close to the lowest unoccupied MO (LUMO) of PVA, causing an electron transfer from I 3 − to PVA. (2) In the second-stage calculation, the crystal structure model of PVA-iodine complex including counter ions (K + ) was built up by referring to the X-ray analyzed structure. The K + ions were found to be localized in the highly-negatively-charged space surrounded by OH groups and iodine atoms, corresponding well to the electrostatic potential minimum created by the PVA chains and iodine ions. The local molecular orbitals (LMO) calculated for these two models [(1) and (2)] revealed the existence of H-bonding interaction between the OH group and the iodine ion.

Computing thermomechanical properties of dry homopolymers used as raw materials for formulation of biomedical hydrogels.

Different static properties have been calculated with COMPASS force field for polyacrylamide, poly(2-hydroxyethylacrylate) (HEA), poly(2-hydroxyethylmethacrylate) (HEMA), poly(glycidylmethacrylate) (GMA), polyethylene glycol (PEG), and poly(2,2,2-trifluoroethylmethacrylate) (TFEM). For each polymers, the calculated values were averaged on five equilibrated configurations of amorphous cell composed of one atactic chain containing 100 repeat units. The ranking obtained from the densities calculated at 300K is TFEM > HEA ≈ xpolycrylamide > HEMA ≈ GMA > PEG. Concerning the glass transition temperature we have obtained polyacrylamide > HEMA ≈ GMA ≈ HEA > PEG, and polyacrylamide > HEMA ≈ HEA > GMA ≈ PEG > TFEM for the bulk modulus. The calculated results, when available, have been compared with experimental data coming from literature.

Radical Polymerization of Pendant (Meth)acrylates on a Rigid Helical Polyisocyanate Backbone Producing Poly(meth)acrylates with “Ideally” Atactic Main Chain Configuration

This study includes a topic that poly(methyl methacrylate) (PMMA) with an “ideally” atactic main chain configuration, approximately triad tacticities of mm/mr/rr = 1/2/1, has been produced in solution by the “one-dimensionally regulated” radical polymerization using rigid helical polyisocyanate as a macromolecular template. Three types of isocyanate monomers with pendant (meth)acryloxy groups, such as 2-(methacryloxy)ethyl isocyanate (MOI), 2-[2-(methacryloxy)ethoxy]ethyl isocyanate (MOI-EO), and 2-(acryloxy)ethyl isocyanate (AOI), were polymerized using a titanium alkoxide complex to produce the corresponding polyisocyanates, MOI-n, MOI-EO-n, and AOI-n, respectively, in which n represents the number-averaged degree of polymerization. From the SAXS measurements of MOI-26 in hexafluoro-2-propanol and MOI-EO-38 in tetrahydrofuran at 25 °C, the polyisocyanates were found to be considered as rigid rod molecules. The radical polymerization of the pendant (meth)acryloxy groups was performed, and the polyisocyanate template was removed by alkali hydrolysis followed by methyl esterification, eventually producing the poly[methyl (meth)acrylate]s. The diad, triad, and pentad tacicities of the products were determined by the 1H and 13C NMR spectra, and these probabilities followed those calculated based on the Bernoulli statistics. The influences of the polymerization conditions, such as polymerization temperature and monomer concentration, on the tacticity of the final polymers were clarified. The effect of the flexible spacer between the polymerizable group and polyisocynate template was also evaluated. The radical polymerizations in which the polymerizable groups are molecularly regulated on a rod-like helical polyisocyanate backbone produced vinyl polymers with inimitable tacticities even in the solution state.

Equivalence of chain conformations in the surface region of a polymer melt and a single Gaussian chain under critical conditions

In the melt polymer conformations are nearly ideal according to Flory's ideality hypothesis. Silberberg generalized this statement for chains in the interfacial region. We check the Silberberg argument by analyzing the conformations of a probe chain end-grafted at a solid surface in a sea of floating free chains of concentration φ by the self-consistent field (SCF) method. Apart from the grafting, probe chain and floating chains are identical. Most of the results were obtained for a standard SCF model with freely jointed chains on a six-choice lattice, where immediate step reversals are allowed. A few data were generated for a five-choice lattice, where such step reversals are forbidden. These coarse-grained models describe the equilibrium properties of flexible atactic polymer chains at the scale of the segment length. The concentration was varied over the whole range from φ = 0 (single grafted chain) to φ = 1 (probe chain in the melt). The number of contacts with the surface, average height of the free end and its dispersion, average loop and train length, tail size distribution, end-point and overall segment distributions were calculated for a grafted probe chain as a function of φ, for several chain lengths and substrate∕polymer interactions, which were varied from strong repulsion to strong adsorption. The computations show that the conformations of the probe chain in the melt do not depend on substrate∕polymer interactions and are very similar to the conformations of a single end-grafted chain under critical conditions, and can thus be described analytically. When the substrate∕polymer interaction is fixed at the value corresponding to critical conditions, all equilibrium properties of a probe chain are independent of φ, over the whole range from a dilute solution to the melt. We believe that the conformations of all flexible chains in the surface region of the melt are close to those of an appropriate single chain in critical conditions, provided that one end of the single chain is fixed at the same point as a chain in the melt.

Hierarchical Multiscale Modeling of Polymer–Solid Interfaces: Atomistic to Coarse-Grained Description and Structural and Conformational Properties of Polystyrene–Gold Systems

A hierarchical simulation approach was developed in order to study polystyrene films sandwiched between two parallel Au(111) surfaces. The coarse-grained potentials describing the interaction of polystyrene with the gold surface were developed systematically using constrained all-atom molecular simulations of a styrene trimer on the Au(111) surface. The model was validated by studying a 5 nm film of short (10-mer) atactic polystyrene chains using all-atom and coarse-grained molecular dynamics simulations. The density, structure, and conformational properties of coarse-grained films were found to be in excellent agreement with all-atom ones. The coarse-grained model was then used to study the structural and conformational properties of roughly 10 and 20 nm thick films with 10-, 50-, 100-, and 200-mer chains. The width of the interphase region of the polymer films is property specific. The density profiles reached the bulk value around 1.5 nm from the interface, for all chain lengths. An estimate of the width of the interphase region based on the conformation tensor profile indicates that the interphase width is around 2–3 times the radius of gyration, which is proportional to the square root of the chain length (number of monomers), and for 200-mer chains is approximately 6–10 nm.

Self-Templated Free-Radical Polymerization To Form Tactic Chains in Confined Environment

It is known that the free-radical propagation chain possesses no stereospecificity as the single electron in the p-orbital of radical is equally accessible to the reactants from the bilateral sides, leading to the formation of atactic polymer chains. This sterically uncontrolled radical chain propagation could be rectified through repressing the degrees of freedom of the reactive vinyl monomer molecules by compression as predicted by our molecular simulation. The simulation unveils that molecules undergo ordered alignment upon being compressed to a certain extent to counteract the increase in chemical potential. We validated this concept by polymerizing N-isopropylacrylamide, a solid monomer, in a highly confined environment, from which a block copolymers consisting of 68% isotactic blocks was achieved. The additional account was attained from polymerizing liquid styrene under compression, which led to 43.1% isotactic polystyrene blocks. It is thus conceived that an ordered array of vinyl monomers would regulate their steric orientation until they join the propagating polymer chains. In conclusion, specifically, molecular confinement results in self-templated effect and hence offers a unique pathway for stereoselective chain growth.