Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard-Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic-nematic transition to higher densities but does not significantly change the position of nematic-smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase.The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.     

Adsorption of Homopolymer Chains on a Strip-Patterned Surface: A Monte Carlo Study

In this study we investigated the process of chemisorption of polymers on solid surfaces. The formation of a strongly adsorbed polymer film was studied by Monte Carlo simulations. The adsorbing planar surface was patterned with strip-like repulsive sites. The polymer chains were represented by a sequence of schematically constructed objects (united atoms) and we considered star-branched macromolecules with f = 3 arms of equal length. The chains were studied at good solvent conditions and thus the excluded volume was the only potential of interaction between the polymers. A Metropolis-like sampling algorithm was employed in order to calculate the properties of the adsorbed chains. The influence of the chain length, the system density and the type of the pattern on the adsorbing surface on size of chains and the structure of the polymer film were determined and discussed. We found that the roughness of the polymer film surface depends non-monotonics on the number of polymer beads in the system. The shape of this surface reflects the pattern imposed on the substrate.     

Properties of branched polymer chains adsorbed on a patterned surface

The aim of this study was to investigate the properties of polymer chains strongly adsorbed on a planar surface. Model macromolecules were constructed of identical segments, the positions of which were restricted to nodes of a simple cubic lattice. The chains were in good solvent conditions, thus, the excluded volume was the only interaction between the polymer segments. The polymer model chain interacted via a simple contact potential with an impenetrable flat surface with two kinds of points: attractive and repulsive (the latter being arranged into narrow strips). The properties of the macromolecular system were determined by means of Monte Carlo simulations with a sampling algorithm based on the local conformational changes of the chain. The structure of adsorbed chains was found to be strongly dependent on the distance between the repulsive strips, whenever this distance was very short. The mobility of the chains was also studied and it was found that diffusion across repulsive strips was suppressed for large distances between the strips.     

Role of entanglements and bond scission in high strain-rate deformation of polymer gels

A key factor that limits the practical implementation of polymer gels is low gel toughness. Here, we present coarse-grained molecular dynamics simulations of the effects of solvent molecular weight on the toughness of entangled and non-entangled polymer gels in the ballistic impact regime. Our results demonstrate that higher molecular weight solvents enhance gel toughness, and that mechanical properties including strength and toughness can be influenced by bond scission. Further, we find a remarkable two-step gel fracture mechanism on the molecular level: network chains undergo scission first (and well before fracture), followed by scission of solvent chains. For strain rates greater than inverse relaxation time of the solvent, long, highly entangled solvent chains provide fracture resistance even after the network chains break by effectively increasing the number of chains that must be broken as a crack propagates.     

Generalized concentration dependence of makromolecule selfdiffusion coefficients in polyethyleneoxide solutions

Summary The polyethyleneoxide (PEO) macroraolecule translational mobility in dioxane and benzene solutions was studied by pulsed field gradient nmr. The generalized dependence describing the macromolecule translational mobility in solutions — invariant with respect to the polymer molecular mass, solvent,as well as to temperature — was obtained.     

Synthesis and properties of solvent absorptive methyl methacrylate‐divinylbenzene copolymer beads

Methyl methacrylate‐divinylbenzene copolymer beads were synthesized by radical suspension polymerization. The effects of the divinylbenzene concentration and the composition of the toluene/heptane diluent were studied with regard to the polymer bead formation, surface morphology, solvent swelling ratio, and absorption kinetics. The crosslinking density and diluent composition were responsible for solvent swelling. The interaction between the polymer and the diluents is attributed to phase separation, which controls the formation of a network‐type or pore‐type polymer, or a combination. For the optimum bead swelling in toluene, a combined morphology of more flexible polymer networks and a small amount of pores is essential for the desired absorption–desorption behavior. Dynamic swelling behavior of the polymer beads was elucidated. The mechanism of toluene transport into the beads became more a relaxation control. POLYM. ENG. SCI., 47:447–459, 2007. © 2007 Society of Plastics Engineers.     

Behaviour of mesogenic side group polyacrylates in dilute and semidilute regime

The solution behaviour of a new mesogenic side group polyacrylate in tetrahydrofuran and toluene has been investigated by static and dynamic light scattering. In the dilute regime the polymer behaves as typical polydisperse linear chains in good solvent and the dynamics is dominated by a single fast mode. Cluster formation was detected starting at a concentration around 50 g l⁻¹. It seems to be independent of the solvent as well as of polymer molecular weight. In the semidilute regime, the behaviour of the reduced osmotic modulus leads to the conclusion that repulsion between the chains is stronger than in linear macromolecules. The appearance of larger clusters was revealed above a characteristic concentration and is slightly dependent on the polyacrylate molecular weight. The dynamics was generally characterised by a fast mode related to the cooperative diffusion and by a slow mode associated with large clusters. The existence of a network of multiconnected clusters is envisaged with increasing solution concentration.     

Dynamics of rigid and flexible polymer chains in confined geometries III. Wall-bead hydrodynamic interaction

As a continuation of the previous paper, dynamics of rigid and flexible polymer chains in viscous medium was reexamined in order to include the interaction of boundary and polymer chains. As models for rigid and flexible chains, rigid and linear elastic dumbbells were considered, which are essentially composed of two beads of frictional sources. The orientation distribution function was obtained by including anisotropic diffusivities of the bead due to the presence of the walls, and Theological properties were predicted to give us the dependence on an additional parameter, ε_go,h relative size of bead to the gap width, as expected. Dynamics of flexible polymer chains showed a similar dependence on the relative size of bead, but exhibited no dependence on the shear rate as the case of no boundaries.     

Effects of monomer structure on the morphology of polymer network and the electro‐optical property of reverse‐mode polymer‐stabilized cholesteric texture

The morphologies of the polymer networks in the polymer network/LC composite of reverse‐mode polymer‐stabilized cholesteric texture (PSCT) films was observed. The polymer network/LC composite was prepared from photopolymerization of the acrylate monomers, which had rod‐like rigid cores in monomer/LC mixture. The effects of the structure of the acrylate monomers on the morphology of polymer network were studied. The acrylate monomer without flexible pacers between the acrylate functional groups and the rigid core formed rice‐grain‐like polymer network with poor orientation. The acrylate monomer with flexible pacers formed fiber‐like polymer network with better orientation. Meanwhile, the effects of morphology of polymer network on the electro‐optical property of reverse‐mode PSCT films were also investigated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

粒子在溶液中吸附聚电解质行为及粒子间的相互作用(Ⅱ)数值分析和结果讨论

用Newdon -Raphson迭代法对链节密度分布模型进行数值分析 ,从理论上探讨了聚合物分子链在粒子表面的构象状况 ,聚合物分子量、分子链电荷密度、聚合物和溶剂的相互作用参数及聚合物和粒子表面相互作用参数等因素对分子链在粒子表面的吸附行为和吸附有聚合物的粒子间的相互作用的影响 ,并通过聚丙烯酰胺(PAM)和阳离子聚丙烯酰胺 (CPAM)在炭黑表面的吸附验证了所建立的模型     

Fracture toughness of highly ordered liquid crystalline epoxy thermosets achieved by optimized composition of curing agents

Liquid crystalline (LC) epoxy resin was prepared by using different compositions of aromatic amine as curing agents, in order to control curing rates and chemical compositions. The progress of the curing reaction was investigated based on the gel fraction and epoxy groups of conversion determined by Fourier-transform infrared spectroscopy. The ordered networked polymer structure was analyzed by polarized optical microscopy and X-ray diffraction. Highly oriented network chains in the obtained epoxy thermosets were promoted by the incorporation of flexible chains in the network and the provision of sufficient time for vitrification. Furthermore, it was clarified that a curing temperature higher than T_g is required to promote the transition to the smectic LC phase in order to prepare highly ordered epoxy thermosets. The increase in the formed smectic LC phase in the network chains resulted in significant higher fracture toughness and achieved up to 2.7 times higher value.     

Simulation of diffusion and relaxations of non-dilute star chains

Star polymer chains with 12 arms in a good solvent have been investigated using a dynamic Monte Carlo algorithm with the bond fluctuation model in the dilute, semidilute and concentrated regimes. We have characterized the self-diffusion coefficient and different relaxation modes of individual chains. We discuss the variation of the results with the arm length (or number of beads) and concentration. The ratio of diffusion coefficient of a star to that of a homologous linear chain is shown to increase progressively with concentration for overlapped non-entangled chains. It is also observed that diffusion and elastic relaxation of individual arms are more sensitive to the chains overlapping than global rotation or disentanglement between arms.     

Diffusion of linear macromolecules and spherical particles in semidilute polymer solutions and polymer networks

The dynamics of fluorescently labeled linear macromolecules and spherical particles that are enclosed in semidilute polymer matrixes was studied by fluorescence recovery after photobleaching. The experiments were designed such that the transition from a semidilute solution to a permanent network could be covered. This was achieved by employing a matrix polymer, polyacrylamide, carrying pendant dimethylmaleimide groups. Stepwise irradiation of such samples in the presence of a triplet sensitizer causes successive dimerization of the maleimides leading to progressive crosslinking.Studies were performed with varying concentrations of matrix polymer (20-80 g L⁻¹) as well as different molar masses (200,000-1,300,000 g mol⁻¹) and particle radii (17 and 36 nm) of enclosed labeled probes. Results show notable differences between the behavior of linear and spherical tracers: while the mobility of flexible linear chains remains nearly unaffected by the transition from a semidilute polymer solution into a chemically crosslinked network, spherical tracers get completely immobilized when the degree of crosslinking exceeds a certain threshold.     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Insight into the liquid crystalline behavior of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}: a typical thermotropic mesogen-jacketed liquid crystalline polymer

Based on the thermodynamic and kinetic analyses, the thermotropic hexagonal columnar liquid crystalline (LC) phase of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PBPCS) at high temperature is self-assembled by driving forces of the conformation entropy and macromolecular interaction, respectively. Because of the strong ??coupling effect?? between flexible backbone and rigid side chain, the molecular structure of PBPCS can be regarded as ??dual chains?? model that a flexible backbone and a rigid side chain are in parallel, the backbone and side chain are corresponded to different molecular thermal motion characteristics. When the orientation requirements of the side mesogenic chain are dominant, LC phase appears; when the random motions of flexible backbone are dominant, LC phase disappears. PBPCS demonstrates various types of metastable phases in the LC phase transition. Above LC phase transition temperature, molecular motion of rigid side chain is dominant; therefore, LC phase is formed. In the temperature range from glass transition to LC phase transition, molecular motion of flexible backbone is dominant; therefore, metastable LC phase may be relaxed partly and even disappear. The stability of LC phase can be determined by the kinetic factors, it is related to the residence time for LC phase formation and the interaction among the aggregated columnar macromolecular chains. Metastable LC phase disappeared in the subsequent cooling procedure, however, the stable LC phase was not.     

The role of clay network on macromolecular chain mobility and relaxation in isotactic polypropylene/organoclay nanocomposites

It is well known that a so-called “three-dimensional filler network structure” will be constructed in the polymer/layered silicate nanocomposites when the content of layered clay reaches a threshold value, at which the silicate sheets are incapable of freely rotating, due to physical jamming and connecting of the nanodispersed layered silicate. In this article, the effect of such clay network on the mobility and relaxation of macromolecular chains in isotactic polypropylene(iPP)/organoclay nanocomposites was investigated in detail with a combination of DMTA, DSC, TGA, TEM, rheometry and melt flow index measurements. The main aim is to establish a relationship between the mesoscopic filler network structure and the macroscopic properties of the polymer nanocomposites, particularly to explore the role of the clay network on the mobility and relaxation of macromolecular chains. It was found that the nanodispersed clay tactoids and layers play less important or dominant roles on the mobility of iPP chains depending on the formation of percolating filler network. The turning point of macroscopic properties appeared at 1 wt% organoclay content. Before this point, the effect of organoclay can be negligible, and the increase of chain mobility was ascribed to the decrease of molecular weight of polymer chains, as commonly occurs during dynamic melt processing; after this point, however, a reduced mobility of chains and a retarded chain relaxation were observed and attributed to the formation of a mesoscopic filler network. The essential features of such a mesoscopic organoclay network were estimated and discussed on the basis of stress relaxation and structural reversion measurements. A schematic model was proposed to describe the different relaxation and motion behaviors of macromolecular chains in the unfilled polymer and the filled hybrids with partial and percolated organoclay networks, respectively.     

Simulation study of biobutanol production in a polymer-loaded two-phase partitioning bioreactor (PL-TPPB): Model development

The conventional two-phase partitioning bioreactor (TPPB) containing an organic solvent as a second phase was found to be hardly efficient for biobutanol production because of the relatively low partitioning coefficient of butanol between the organic solvent and aqueous solution. Polymer bead was alternatively employed as the second phase in the TPPB, and Dowex Optipore L-493, a copolymer of styrene and divinyl benzene, was chosen as the optimum polymer because it shows the highest partitioning coefficients of butanol, acetone, ethanol and butyric acid against the aqueous phase among candidate polymers. The mass transfer coefficients of compounds from the aqueous phase into polymer beads were experimentally determined with respect to agitation speed. The mass transfer coefficient related to the stripping of volatile compounds by nitrogen gas was also determined, and the influence of gas flow rate turned out to be greater than that of the agitation speed, though both influences were remarkable. A mathematical model for the TPPB containing the polymer beads was suggested and as many as 40 parameters were cited from other publications or determined in this study. This mathematical model will be subsequently used for the detailed simulation study.     

Polymer molecular weight and chain transfer during the photopolymerization of an aliphatic monoacrylate in a smectic liquid crystal

Polymer/liquid crystal (LC) composites offer a unique opportunity to study polymerizations in ordered media, specifically the potential effect mesophase order can have on polymer properties including molecular weight. To develop successful polymer/LC composites for display applications, it is important to understand the effect of mesophase order on polymer molecular weight in order to optimize the electro-optic (EO) properties of the polymer/LC composite. Polymer molecular weight may be influenced in a LC by changes in polymerization rate as LC order is modulated and by chain transfer. This work focuses on the photopolymerization of an aliphatic monoacrylate monomer, decyl acrylate (DA), both in the ordered LC phases of 8CB as well as in isotropic solutions with LC and co-solvent.When DA is polymerized using the LC as the solvent, enhanced polymerization rates and polymer molecular weights are observed in the highly ordered smectic phase compared to the less ordered nematic and isotropic phases. When conducted strictly in an isotropic environment using a co-solvent with increasing 8CB percentages, a dramatic decrease in the polymerization rate and a significant reduction of the polymer molecular weight is observed, implying degradative chain transfer to the LC. NMR results show that this chain transfer is a result of hydrogen abstraction from the liquid crystals, which leads to the reduction in the polymerization rate with increasing 8CB concentration. The most likely site of hydrogen abstraction is from the benzyl hydrogens of the alkyl chain of 8CB. This chain transfer also plays a role for polymerizations performed in the ordered phases of the LC. Chain transfer appears to be less significant when polymerizations are conducted in the smectic phase due to the anti-parallel association of the LC molecules. When polymerizations occur in the less ordered phases, chain transfer dominates leading to a large reduction in polymer molecular weight and polymerization rate.     

Characterization of compositional heterogeneity in copolymers and coatings systems by GPC/FTIR

One of the long-sought ‘holy grails' of polymer characterization has been the simultaneous determination of polymer composition as a function of molecular weight distribution. The recent commercialization of a solvent evaporative interface between a Gel Permeation Chromatograph (GPC) and a Fourier Transform Infrared (FTIR) Spectrometer has provided one useful solution to the problem of determining polymer composition as a function of molecular weight for a range of polymers. This study will focus on the use of a solvent evaporative interface in conjunction with a GPC-viscometer chromatograph and a FTIR spectrometer in order to provide functional group information as a function of molecular weight. The solvent evaporative interface will be described and its use with a GPC-viscometer/FTIR system will be discussed in terms of operational variable and data analysis considerations. Application of the GPC-viscometer/solvent evaporative interface/FTIR system to a variety of polymer and coatings systems as a tool for product problem solving and elucidation will be presented. In addition, examples of the use of the solvent evaporative interface to elucidate compositional heterogeneity of copolymers will be illustrated. The potential use of the solvent evaporative interface in GPC/LC cross fractionation studies for very fine elucidation of polymer compositional heterogeneity will be discussed.