Crystallization Of Isotactic Polystyrene Research Articles

Improved Polymer Crystal Phase Field Model and Numerical Simulation

The existing phase field model of polymer crystallization contains many parameters that lack actual physical meaning. Although the value of these parameters can be adjusted to obtain results consistent with the experiment, it cannot correspond to the experimental conditions. In this paper, a new phase field model is established. By adjusting the latent heat, various forms of isotactic polystyrene crystals, such as dendrites, spherulites, lamellas, etc., can be simulated. Latent heat refers to the heat absorbed or released by a substance from one phase to another and has important physical meaning during the solidification process. The finite difference method was used to solve the model, and then the data were used to visualize. The simulation results were consistent with the experiment. Numerical simulation results under pure diffusion conditions show that the newly established phase field model can qualitatively predict the polymer growth process and provide a theoretical basis for the preparation and optimization of high-performance polymers. In order to make the simulation result closer to the actual growth of the crystal, the flow velocity is added in the simulation to make the melt convection. Under forced convection, the simulated polymer crystal image is no longer symmetrical.

Formation of Stacked Three-Dimensional Polymer “Single Crystals”

Stacks of crystalline lamellae all having a uniquely oriented hexagonal shape, referred to as “3D (three-dimensional) single crystals”, were obtained via isothermal crystallization of isotactic polystyrene at a temperature close to the melting point. The height of the stacks of lamellae reached several micrometers corresponding to hundreds of superposed lamellar crystals. We propose that the mechanism of self-induced nucleation allowed propagating the orientation of the basal lamellar crystal to all other lamellae in the stack. The unique orientation of all lamellae was reflected by the preference of cracks to form along the diagonals of the hexagonal stack. Cracks were caused by a mismatch in the coefficients of thermal expansion of the substrate and “3D single crystals” during quenching from the crystallization temperature to room temperature. We believe that the presented growth mechanisms leading to “3D single crystals” can be observed for all crystallizable polymers including block copolymers with a noncrystallizable block.

28a-YC-7 アイソタクチックポリスチレンの結晶成長と膜厚

28a-YC-7 アイソタクチックポリスチレンの結晶成長と膜厚

A phase-field model for simulating various spherulite morphologies of semi-crystalline polymers

A modified phase-field model is proposed for simulating the isothermal crystallization of polymer melts. The model consists of a second-order phase-field equation and a heat conduction equation. It obtains its model parameters from the real material parameters and is easy to use with tolerable computational cost. Due to the use of a new free energy functional form, the model can reproduce various single crystal morphologies of polymer melts under quiescent conditions, including dendritic, lamellar branching, ring-banded, breakup of ring-banded, faceted hexagonal, and spherulitic structures. Simulation results of isotactic polystyrene crystals demonstrate that the present phase-field model has the ability to give qualitative predictions of polymer crystallization under isothermal and quiescent conditions.

Structure of a spin-crossover Fe(II)–1,2,4-triazole polymer complex dispersed in an isotactic polystyrene matrix

Isotactic polystyrene (i-PS) was employed as a matrix to disperse a metallo-organic polymer of [Fe(II) (4-octadecyl-1,2,4-triazole) 3(ClO 4) 2] in order to obtain novel functional materials exhibiting thermal spin-crossover transition. A detailed investigation of the structure of the metallo-organic polymer and metallo-organic polymer/iPS blends has been carried out by DSC, WAXD and SAXS techniques as a function of temperature and metallo-organic polymer/iPS proportion. The results obtained confirm on the one hand that a structural transition associated with a change in the magnetic susceptibility of the metallo-organic polymer is preserved in the presence of i-PS. This transition was found to be associated to both, an inter-conversion of lamellar structures into hexagonal structures and to an increase of inter-sheet distances within the lamellar structures in metallo-organic polymer films prepared by casting from toluene solutions. On the other hand, an increase of the degree of crystallinity of the iPS is observed in the presence of the metallo-organic polymer which suggests some nucleating effect of the metallo-organic polymer in the crystallization of isotactic polystyrene.

Oriented crystallization of isotactic polystyrene in films prepared by friction transfer

The ability to orient polymer chains by applying external forces opens up the possibility to obtain polymeric surfaces with ordered structures. Here, we employed a friction-transfer approach by moving a pin of isotactic polystyrene (i-PS) across a smooth silicon counterface at controlled velocity, pressure and temperature which led to the deposition of a molecularly thin layer of highly oriented i-PS chains. The observed morphology of the resulting film (ribbons oriented in the sliding direction) indicated that the transferred molecules were highly oriented. This was confirmed after isothermal crystallization which led to the formation of so-called “shish–kebab” crystals aligned in the sliding direction. Thus, after crystallization all polymers were preferentially oriented with their chain axis in the shearing direction. Our results strongly suggest that by extending the polymer chain conformations in the shearing direction we can introduce a significant reduction of the nucleation barrier. Accordingly, friction transfer allows to align not only the transferred polymer chains but also the subsequently forming crystalline domains within the transferred films.

Dynamic Light Scattering Studies on Crystallization of Isotactic Polystyrene from Dilute Solutions at High Supercoolings

Crystallization of isotactic polystyrene (it‐PS) from dilute solution at high supercooling has been investigated by dynamic light scattering (DLS). We successfully obtained simultaneously, in situ in solutions, the time developments of both random coils of it‐PS molecules and the growing crystals. The size of coils remains constant during growth, while the crystals pass through two stages, that is, an induction period at the early stage with very slow growth rates and a subsequent linear growth stage. It is confirmed that the temperature dependence of the linear growth rates, determined by DLS, agree well with that determined by electron microscopy. The temperature dependences of the growth rate and the inverse of induction time are dependent on the viscosity of solvent, which indicates that all dynamics are dominated by the segmental motion of polymer chains in solution at high supercoolings (low temperatures). Two possibilities are proposed for the induction period.

Crystal Growth of Isotactic Polystyrene in Ultrathin Films: Thickness and Temperature Dependence

The growth rate and morphology of isotactic polystyrene crystals grown in ultrathin films have been examined experimentally in terms of the dependences both on the film thickness and on the crystallization temperature. We have found that the thickness dependence of growth rate, G, shows a crossover change when the film thickness becomes comparable with the lamellar thickness of the polymer crystals, irrespective of the temperatures. The morphology of crystals grown in ultrathin films shows a branching typical of dendrites, the growth of which is supposed to be controlled by a diffusion field. The change in the tip width of the dendrites with crystallization temperature follows the expected dependence of the Mullins–Sekerka stability length, ℓMS ∝ (D/G)1/2, determined by the diffusion coefficient, D, and the growth rate. The results confirm that a diffusion field plays an essential role in the evolution of the structure.

Detailed interpretation of the results of two-dimensional correlation analysis of infrared spectra obtained during isothermal crystallization of isotactic polystyrene and poly(3-hydroxybutyrate)

Two sets of IR spectral data obtained during the isothermal melt-crystalization process of two different polymers are respectively analyzed by two-dimensional (2D) correlation approach. The aim of this study is to inspect this details of the obtained correlation matrices, remove the correlation artifacts and produce reliable 2D correlation results. Particular attention is thus paid to the representation of the correlation data, contour plots and correlation slices. An example of misinterpretation caused by improper thresholding of the asynchronous data is given. Correlation coefficients are also employed to confirm the simplicity of the correlation data. Finally, sample–sample correlation matrix is calculated to additionally confirm that simple two-component systems can immediately be recognized through their correlation pattern.

Phase-field modeling on morphological landscape of isotactic polystyrene single crystals

Spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization has been investigated theoretically based on the phase field model by solving temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation. In the description of the total free energy, an asymmetric double-well local free energy density has been adopted to represent the metastable melt and the stable solid crystal. Unlike the small molecule systems, polymer crystallization rarely reaches thermodynamic equilibrium; most polymer crystals are kinetically stabilized in some metastable states. To capture various metastable polymer crystals, the phase field crystal order parameter at the solidification potential has been treated to be supercooling dependent such that it can assume an intermediate value between zero (melt) and unity (perfect crystal), reflecting imperfect polycrystalline nature of polymer crystals. Two-dimensional simulations exhibit various single crystal morphologies of isotactic polystyrene crystals such as faceted hexagonal patterns transforming to nonfaceted snowflakes with increasing supercooling. Of particular interest is that heat liberation from the crystallizing front influences the curvature of the crystal-melt interface, leading to directional growth of lamellar tips and side branches. The landscape of these morphological textures has been established as a function of anisotropy of surface energy and supercooling. With increasing supercooling and decreasing anisotropy, the hexagonal single crystal transforms to the dense lamellar branching morphology in conformity with the experimental findings.

Structure Changes during the Induction Period of Cold Crystallization of Isotactic Polystyrene Investigated by Infrared and Two-Dimensional Infrared Correlation Spectroscopy

The structural changes during the induction period of isotactic polystyrene (iPS) cold crystallization were investigated by using infrared (IR) spectroscopy together with 2D correlation analysis. It was found that the ordered orientation of the phenyl ring, local conformation change, and structural formation of 31 helix chain of iPS already appear during the induction period. Moreover, careful interpretation of the asynchronous 2D IR spectra generated from the time-dependent spectra enabled us to delineate the structural evolution during the induction period of iPS cold crystallization: Prior to the crystallization of iPS, the amorphous phase changes first, and then the ordering of the phenyl rings of iPS takes place. After that, the polymer chains adjust their local conformations to form the short 31 helix structure. The present study has also demonstrated that the generalized 2D IR correlation spectroscopy is powerful in studying small spectral changes in the induction period of polymer crystallization.

Undulation of Lamellar Crystals of Polymers by Surface Stresses

The periodical striped contrast is observed in the single crystals of isotactic polystyrene and poly(oxymethylene) by transmission electron microscopy. The contrast is attributed to a periodical tilting of the chain stem around the axis normal to the crystal growth plane; the tilting accompanies the surface undulation, which is observed by atomic force microscopy. The wavelength of the undulation increases with the lamellar thickness. In situ observation of the growing crystal by transmission electron microscopy shows that this undulation is introduced during the growth and not through the cooling process of the crystal. It is proposed that the stress of fold surfaces makes a flat lamellar crystal elastically unstable to cause the buckling deformation during the crystal growth.

Transmission electron microscopy observations on fine structures of shish-kebab crystals of isotactic polystyrene by partial melting

The composite morphology and melting behavior of isothermally cold-crystallized isotactic polystyrene (iPS) shish-kebab crystals have been studied by transmission electron microscopy (TEM). The fine structures of iPS shish-kebabs are reflected at lamellar resolution by partial melting treatment using bright- and dark-field imaging modes. TEM observations indicate that the entire shish-kebab entity consists of several components: the central extended-chain micro-shish crystals, the partially extended-chain macro-shish crystals which are intrinsically connected with the micro-shish cores, the overgrown micro-kebabs by primary crystallization, and the macro-kebabs formed by secondary crystallization on macro-kebab nuclei. The reversible morphological transformation between the micro- and macro-kebabs has been visualized on nanoscale via melting and recrystallization processes, due to the difference in their thermal stability.

Helix Inversions in Polypropylene and Polystyrene

A conformational analysis is done in order to investigate on the mobility of joints separating d helical sequences from l ones along molecules of polystyrene and polypropylene both isotactic and syndiotactic in solution or in the liquid state. Results indicate that, in the case of isotactic polystyrene, severe interactions between side phenyl rings separated by three and four monomer units play a relevant role in hindering the molecular motions necessary to move the joint. Only a coordinate torsional motion of a number of chain bonds (four) and the overcoming of a rather high energy barrier allows for the motion of the joint along the chain. Syndiotactic polystyrene and iso- and syndiotactic polypropylene show a much easier conformational situation. This may be related to the extreme slowness affecting the crystallization of isotactic polystyrene as compared to the crystallization rate of the other polymers studied. Finally a connection is proposed between results of the present analysis and a possible interpretation of the X-ray diffraction pattern recorded from the oriented (fringed micelle) crystallites contained in crystalline gels of i-PS.

Growth shape of isotactic polystyrene crystals in thin films

The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11nm thick films. The it-PS crystals grown in the thin films show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a diffusion field appears: dendrites and compact seaweed. The branching morphology is formed through a morphological instability caused by the gradient of film thickness around a crystal; the thicker the film thickness, the larger is the lateral growth rate of crystals. Regardless of the morphological change, the growth rate as well as the lamellar thickness depends on the crystallization temperature as predicted by the surface kinetics.

Gelation Crystallization of Isotactic Polystyrene in Solvents of Varying Molecular Size

Gelation Crystallization of Isotactic Polystyrene in Solvents of Varying Molecular Size

Structure of single-molecule single crystals of isotactic polystyrene and their radiation resistance

Based on the one-band tight-binding model, we systematically investigate the interlayer exchange coupling and the angular dependence of the coupling energy in a magnetic sandwich covered on both sides by nonmagnetic films. Our results show that (i) the thickness of magnetic and outer nonmagnetic films influence significantly the oscillatory behavior of exchange coupling, (ii) the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers, (iii) the nonoscillatory component of the coupling varies generally with the thickness of magnetic or outer nonmagnetic films, and (iv) the results in the case where the thickness of both magnetic or both outer nonmagnetic films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic or outer nonmagnetic films is fixed while the other is varied. These results are qualitatively in agreement with the experimental measurements. (C) 1998 American Institute of Physics. [S0021-8979(98)02302-0].

Interfacial Reactions of Pd and Pd/Si on GaAs

Single-chain single crystals of isotactic polystyrene and poly(ethylene oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single-chain single crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single-chain particles on copper grids, followed by isothermal crystallization. A statistical analysis of the sizes of single-chain crystals was found to match with the known molecular weight distribution of original sample, indicating the particles to be composed of single chain. Observation of the morphology and electron diffraction gave evidence of the single crystal nature. Regular-shaped single-chain crystals were obtained after isothermal crystallization for a longer lime. By close observation, several types of morphologies were found for single-chain crystals of isotactic polystyrene and poly(ethylene oxide); in addition to the conventional morphologies observed for multi-chain crystals, new morphologies were observed in both cases. The morphologies of poly(ethylene oxide) were explained according to the crystal structure and twin modes. Tent-like single-chain crystals were often observed. Because of the small size of the crystals, they can avoid collapse on the substrate. The crystalline c-axis of single-chain crystals were found to orient preferably in the direction normal to the substrate. The investigation of electron diffraction and high resolution electron microscopy revealed that the structure of the single-chain crystals of isotactic polystyrene is the same as far multi-chain crystals. A reasonable explanation is given for the unusual resistance to electron irradiation and the missing of lower-index reflections. Regular periodic stripes were found on the top surface of single-chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition, a statistical thermodynamics theory was developed for single-chain crystal. It is found that the equilibrium dimensions are related to molecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.

Single‐chain single crystals

AbstractSingle‐chain single crystals of isotactic polystyrene and poly(ethylene oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single‐chain single crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single‐chain particles on copper grids, followed by isothermal crystallization. A statistical analysis of the sizes of single‐chain crystals was found to match with the known molecular weight distribution of original sample, indicating the particles to be composed of single chain. Observation of the morphology and electron diffraction gave evidence of the single crystal nature. Regular‐shaped single‐chain crystals were obtained after isothermal crystallization for a longer time. By close observation, several types of morphologies were found for single‐chain crystals of isotactic polystyrene and poly(ethylene oxide); in addition to the conventional morphologies observed for multi‐chain crystals, new morphologies were observed in both cases. The morphologies of poly(ethylene oxide) were explained according to the crystal structure and twin modes. Tent‐like single‐chain crystals were often observed. Because of the small size of the crystals, they can avoid collapse on the substrate. The crystalline c‐axis of single‐chain crystals were found to orient preferably in the direction normal to the substrate. The investigation of electron diffraction and high resolution electron microscopy revealed that the structure of the single‐chain crystals of isotactic polystyrene is the same as for multi‐chain crystals. A reasonable explanation is given for the unusual resistance to electron irradiation and the missing of lower‐index reflections. Regular periodic stripes were found on the top surface of single‐chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition, a statistical thermodynamics theory was developed for single‐chain crystal. It is found that the equilibrium dimensions are related to molecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.

The morphology of isotactic polystyrene crystals grown in thin films: the effect of substrate material

Thin isotactic polystyrene films (∼50 nm thick) have been crystallized from the melt on various substrate materials (carbon, glass, mica, polyimide sheet and silicon). The morphology of the crystals has been examined using atomic force microscopy, and was found to be dependent on the nature of the substrate, with two basic types of crystal forming. Crystals either develop around giant screw dislocations, or around small bundles of lamellae growing perpendicular to the substrate surface. It has further been observed that the number of screw dislocations generated in the lamellae is also dependent on the substrate, as is the growth rate of the spiral terraces. These effects are interpreted in terms of interactions between the molecules in the melt and the substrate surface.