Polymerization Induced Phase Separation Process Research Articles

Sol−Gel Processing of a Glycolated Cyclic Organosilane and Its Pyrolysis to Silicon Oxycarbide Monoliths with Multiscale Porosity and Large Surface Areas

This study is related to the preparation of silicon oxycarbide monoliths comprising a hierarchical network build-up from the pyrolysis of monolithic organosilica gels. A novel glycol-modified 1,3,5-trisilacyclohexane-carbosilane “[Si(OCH2CH2OH)2CH2]3” was processed via a polymerization-induced phase separation process in hydrochloric acid solution containing the Pluronic P123 block copolymer and potassium chloride. Highly porous organosilica monoliths with interconnected macropores and a large amount of uniformly sized polymer-templated mesopores within the macroscopic framework domains were obtained after supercritical fluid extraction. The monoliths were pyrolyzed in argon atmosphere at 1000 °C to yield silicon oxycarbide monoliths by maintaining the hierarchical porosity of the organosilica gel. Both the organosilica gels and the silicon oxycarbide monoliths were thoroughly investigated with standard characterization techniques. If no salt was used for the preparation of organosilica gels, no distinct ...

Functionalization of carbon nanotubes and carbon nanofibers used in epoxy/amine matrices that avoid partitioning of the monomers at the fiber interface

AbstractSingle‐wall carbon nanotubes (SWCNT), multiwall carbon nanotubes (MWCNT), and carbon nanofibers (CNF) were oxidized with a conventional H2SO4/HNO3 treatment, and the resulting COOH groups were esterified with phenyl glycidyl ether (PGE). Pristine, oxidized and esterified SWCNT, MWCNT, and CNF (1 wt%) were dispersed in diglycidyl ether of bisphenol A/polyether triamine, and cured to produce epoxy nanocomposite. The quality of the dispersion varied with surface modification in the order: COOH > unmodified > PGE, as assessed for SWCNT/epoxy composites using radial breathing modes in Raman spectra and scanning electron microscopy. Local bundling of PGE‐modified nanofillers was explained by a polymerization‐induced phase‐separation process. Dynamic mechanical analysis showed the presence of two relaxation peaks in nanocomposites prepared with unmodified and COOH‐modified SWCNT, MWCNT, and CNF, assigned to a partitioning of monomers at the carbon interface. This produced a significant decrease of the glass transition temperature. However, the esterification with PGE led to a single relaxation peak close to the one of the neat epoxy, for the three types of nanofillers without any penalty in the glassy and rubbery elastic moduli. The effect was explained by matrix exclusion from phase‐separated domains. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Memory effects of polymerized 2-hydroxyethylmethacrylate/4-cyano-[formula omitted]-pentylbiphenyl mixtures

Polymer dispersed liquid crystal (PDLC) films are heterogeneous composite materials consisting of micron-sized droplets of liquid crystal dispersed in a continuous polymer matrix. They are formed by a phase separation process of the LC component from the polymer. PDLC films can be switched from a light scattering off-state to a transparent on-state, when a sufficiently high electrical field is applied. Polymerization Induced Phase Separation (PIPS) using UV-light is one of the most common methods to prepare PDLC films and the polymerization is induced by decomposition of the photoinitiator in a mixture of LC with functional monomers.Polymer/liquid crystal (PDLC) films were prepared by a PIPS process of a mixture composed of the nematic LC material 4-cyano-4′-pentylbiphenyl (5CB), 2-hydroxyethylmethacrylate (HEMA) as monomer, and a photoinitiator (Darocur 1173). Ultraviolet (UV) radiation was employed to initiate the PIPS process. The electro-optical behaviour of the films obtained was investigated as function of applied voltage and frequency. Strong memory effects have been observed in the case of samples prepared from 60 weight.-% 5CB and 40 weight.-% HEMA.

The Effects of Conditions for Polymerization Induced Phase Separation Processes on the Electro-optic Characteristics of Polymer Dispersed Liquid Crystals

The electro-optic properties of PDLC films were investigated by process conditions such as PDLC injection temperature, UV curing temperature and UV power. The mixture of liquid crystal (TL205) and the prepolymer substituted 1,6-hexanediol diacrylate (HDDA) for trimethylolpropane triacrylate (TMPTA), and the crosslinker in PN393 was used for PDLC formulation. The contrast ratio and driving voltage of PDLC films were mainly affected by UV curing temperature, rather than by the injection temperature. The film that was prepared at a relatively low UV process temperature revealed a good contrast ratio at a low driving voltage.

Molecular dynamics of the interfacial properties of partially fluorinated polymer dispersed liquid crystal gratings

The interfacial properties of polymer dispersed liquid crystal (PDLC) gratings influenced by partial matrix fluorination are studied through molecular dynamics methods. The miscibility between the fluorine-substituted monomer and other materials in the prepolymer mixture is evaluated by using the solubility parameters of molecules. The interfacial effect of different fluorination levels is analysed theoretically. The results indicate that most fluorine-substituted monomers are distributed on the interface between the monomer and the LC. The interaction energy on the interface decreases with increasing fluorination, which can improve the electro-optical tunable performance of such gratings. However, it should also be noted that excessive fluorination may lead to penetration of fluorine-substituted monomers into the LC medium. Such a result will block the normal diffusion of LC during the photo-initiated polymerization-induced phase separation process. Through calculation of the radial distribution function, a reasonable alignment structure for LCs near the interface is given. The calculation of the order parameter shows that fluorination causes disorder of their orientation.

Viscoelastic phase separation in polyethersulfone modified bismaleimide resin

The polymerization-induced phase separation process of polyethersulfone (PES) modified bismaleimide resin, 4,4′-bismaleimidodiphenylmethane (BDM), was investigated by time resolved light scattering (TRLS) and scanning electronic microscopes (SEM). At the blends with 10 wt% and 12.5 wt% PES, a phase inversion structure was found by SEM. TRLS results displayed clearly the spinodal decomposition (SD) mechanism and the exponential decay procedure of scattering vector q m, which followed Maxwell-type relaxation equation. The characteristic relaxation time τ for the blends can be described by the Williams–Landel–Ferry equation. It demonstrated experimentally that the phase separation behaviors in these PES modified bismaleimide blends were affected by viscoelastic effect.

Polymerization-Induced Viscoelastic Phase Separation in Polyethersulfone-Modified Epoxy Systems

The polymerization-induced phase-separation process of polyethersulfone (PES)-modified epoxy systems was monitored in situ continuously on a single sample throughout the entire curing process by using optical microscopes, time-resolved light scattering (TRLS), scanning electronic microscopes (SEM), and a rheometry instrument. At specific PES content a viscoelastic transformation process of phase inversion morphology to bicontinuous was found with an optical microscope. The rheological behavior during phase separation corresponds well with the morphology development. Light-scattering results monitoring the phase-separation process of systems with final phase inversion morphology show a typical exponential decay procedure of scattering vector qm. The characteristic relaxation time of phase separation can be described well by the WLF equation.

Swelling and Deswelling of Poly (n-butylacrylate) Networks in Isotropic and Nematic Solvents

We report swelling/deswelling data of Poly(n-butyl acrylate) networks in isotropic and anisotropic solvents. The investigated systems are prepared via polymerization induced phase separation processes of initial mixtures composed of a reactive polymer precursor, a cross-linker, a photoinitiator and a low molecular weight solvent by ultraviolet (UV) radiation. A comparison of data obtained from isotropic organic solvents and a low molecular weight nematic liquid crystal is made in terms of temperature and cross-linking density of the network. Hexanediol-diacrylate (HDDA), a di-functional monomer is used as a cross-linker and the conditions of preparation are set in order to obtain well defined model networks. Effects of temperature and composition are evaluated by measuring the size via microscopy and the mass of dry and swollen networks. Simple modelling is used to rationalize the data.

A Static and Dynamic Study of the Mechanical Properties of Electron Beam Cured PDLC Films

This paper reports a study of static and dynamic mechanical properties of electron beam (EB) cured polymer dispersed liquid crystals. These systems consist of micron-sized droplets dispersed in a solid polymer matrix. They are prepared according to the polymerization induced phase separation process initiated by exposure to electron radiation. A difunctional monomer Tripropylene Glycol Diacrylate in short TPGDA is used with the eutectic liquid crystal mixture E7 having a nematic to isotropic transition temperature at 60°C. Strong effects are found on Young modulus and rubbery state modulus in terms of the EB dose and the liquid crystal concentration. As the concentration of small molecules increases, these mechanical parameters drop significantly together with the glass transition temperature Tg of the polymer showing a plasticizing effect. The results for Tg are confirmed by DSC thermograms in terms of liquid crystal concentration and EB dose.

2H-NMR investigation after a polymerisation-induced phase separation process*

Polymer-dispersed liquid crystals (PDLC) are composite materials consisting of micron-sized droplets of liquid crystal dispersed in a polymer matrix. The easiest method to obtain a PDLC film is the polymerisation-induced phase separation process (PIPS). The liquid crystal is mixed with a monomer of low molecular weight and polymerisation is induced by heat or UV light. The increasing molecular weight of the polymer causes the phase separation of liquid crystal from the polymer matrix as micron-sized droplets. In this work, we have studied the structural changes induced in the polymer matrix of a PDLC after the PIPS process by deuterium nuclear magnetic resonance. Two different selectively deuterated monomers have been synthesized and investigated: isobutyl methacrylate (IBMA-d2) and methyl methacrylate (MMA-d3). The main results were the disappearance of the characteristic two-site hop in poly-IBMA, due to liquid crystal molecules, and the lack of unreacted MMA molecules in the liquid crystal droplets. In this last case, we found that it is possible to confine temporarily the unreacted MMA molecules within liquid crystal droplets.

Morphology of PDMS−PMAA IPN Membranes

Direct visualization of IPN morphology formed during polymerization-induced phase separation (PIPS) of PDMS−PMAA IPNs is reported. Laser scanning confocal microscopy (LSCM) and hydrophilic fluorescent probes of varying molecular weights were used to image hydrophilic domains in PDMS−PMAA IPNs. The images reveal complex, superimposed structures of hydrophilic domains of varying sizes and spatial distributions. This morphology is attributed to the phase-separated structures formed and partially arrested at continuously varying quench depths during the PIPS process. These observations contribute to the understanding of morphology development in IPNs.

Porous Epoxy Thermosets Obtained by a Polymerization-Induced Phase Separation Process of a Degradable Thermoplastic Polymer

A process for producing controlled porous epoxy thermosets is presented. It is based on a two-step temperature cure: a polymerization-induced phase separation at a moderate temperature (100−120 °C) of a thermoplastic, poly(vinyl methyl ether) (PVME), in an epoxy−amine matrix followed by a high-temperature treatment (above 200 °C). A porous structure is created by the oxidative thermal degradation of PVME (extraction). Different porous or semiporous structures with submicrometer-sized voids (dispersed closed cells, i.e., spheres or open cells, i.e., bicontinuous percolating canals) are generated depending on the thermoplastic content and the phase-separation mechanism. The morphologies are analyzed quantitatively from transmission electron microscopy (TEM) photographs.

Orientation control of liquid crystal droplets dispersed in a polymer matrix

The encapsulation of monomer molecules in liquid crystal droplets dispersed in a thermoplastic matrix provides a convenient method to control the orientation of liquid crystal directors. The imprint of droplet interfaces is obtained by photopolymerization of monomer molecules by means of a polymerization induced phase separation process performed in a magnetic field. In such a way, uniform orientations of liquid crystal directors can be achieved in cells without any surface treatment. The concentration of monomer molecules is an important parameter, which influences the final electro-optical properties of films.

Evolution of Electro-Optical Properties of Polymer Dispersed Liquid Crystal Materials During Processing

Polymer dispersed liquid crystal (PDLC) films are interesting materials particulary for optical devices. The initial mixture was composed of diglycidyl ether of bisphenol A (DGEBA), polypropylene oxide diamine (Jeffamine D-400), and the nematic low molecular weight liquid crystal E7. PDLC films were obtained by combined thermally and polymerization induced phase separation processes initiated by the temperature controlled polycondensation reaction of DGEBA and Jeffamine D-400. The electro-optical curves of the obtained films strongly depend on the monomer conversion. It was found that both the transmission in the off-state as well as in the on-state decrease with the extent of cure. In addition, threshold and saturation voltages increase for PDLC films characterized by higher monomer conversion values. The electro-optical curves obtained for a conversion of approximately 80% exhibit an unusual behavior. At monomer conversions approaching 100% the transmission versus voltage curves show the expected normal mode.

Effects of Perfluoration of Epoxy-Amine Polymer Matrices on the Electro-Optical Performance of Polymer Dispersed Liquid Crystal Films

Polymer dispersed liquid crystal (PDLC) films were obtained by combined thermally and polymerization induced phase separation processes initiated by the temperature controlled polycondensation reaction of monomers carrying either epoxy or amine groups. The epoxy diglycidyl ether of bisphenol A (DGEBA) and two different amines very used: an aliphatic amine and an aromatic fluorinated amine. The nematic liquid crystal mixture E7 was employed. The electro-optical performance of the obtained PDLC films was analyzed as a function of reaction cure in the absence and presence of the fluorinated amine. It was found that the addition of the fluorinated amine has a strong influence on the transmission vs voltage curves. A remarkable decrease of threshold- and saturation voltages as well as an increase of the transmission values in the on-state was observed.

Electro-Optical, Spectroscopical, and Calorimetric Investigations of Electron Beamand Ultraviolet- Cured Polymer Dispersed Liquid Crystal Materials

The must convenient method used for the preparation of polymer dispersed liquid crystal (PDLC) films is the polymerization induced phase separation (PIPS) of mixtures composed of polymer precursors and low molecular weight liquid crystals (LMWLC). Ultraviolct (UV) and electron beam (EB) radiation was employed to initiate the PIPS process. A mixture of the nematic LMWLC material E7, an aromatic polyester acrylatc, and additional monomeric acrylates was exposed to the EB radiation. Lucirin TPO as a photoinitiator has been included in the initial mixture in the case of UV exposure. The clectro-optical properties of the obtained PDLC films strongly depend on the curing conditions, and arc highly reproducible. In general it was found that the EB-cured PDLC films show a better electro-optical performance as compared to the UV-cured samples. The extent of cure has been determined by infrared spectroscopy measurements of the carbon-carbon double bond of the acrylate groups. Differential scanning calorimetry was employed to obtain the glass transition temperature of the polymer and the LMWLC solubility limit in the polymer matrix.

Polymer Dispersed Liquid Crystal Films Prepared by Electron Beam Curing: Influence of the Nature of the Polymer Precursor

Polymer dispersed liquid crystal (PDLC) films were prepared by a polymerization induced phase separation (PIPS) process of mixtures composed of monomers and the nematic liquid crystal (LC) E7. Electron beam (EB) radiation was employed to initiate the PIPS process. Mixtures of the LC E7 and selected well characterized di- and tri-functional acrylate monomers as polymer precursors have been exposed to a defined amount of dose of EB radiation. Tripropylene-glycoldiacrylate (TPGDA), propoxylated glyceroltriacrylate (GPTA), and a 1:1 mixture of these materials were used as monomers. The concentration of E7 in the initial mixture was kept constant at 70 weight-%. It was shown that the electro-optical properties strongly depend on the nature and the functionality of the monomers.

Theory and simulation of polymerization-induced phase separation in polymeric media

A rigorous model of polymerization-induced phase separation (PIPS), based on the non-linear Cahn-Hilliard (C-H) and Flory-Huggins (F-H) theories combined with a second-order polymerization reaction equation, has been formulated and its solutions characterized. The model describes phase separation in system consisting of a non-reactive polymer and a monomer that undergoes condensation polymerization. The model consists of a balance equation for the low molecular weight polymerization regime and another balance equation for the high molecular weight entangled regime. The model equations are solved, and the solutions are characterized to identify the dynamical and morphological phenomena of the PIPS process. The extent of phase separation increases significantly with time during the early stage of phase separation, and slows down in the intermediate stage. The various types of phase-separated morphologies are fully characterized using a novel morphological characterization techniques, known as the intensity and scale of segregation. Both the dynamical and morphological features of the PIPS method are sensitive to the magnitudes of the dimensionless diffusion coefficient D* and the dimensionless reaction rate constant K*. The scale of segregation and the droplet size decreases as D* and K* increase. On the other hand, the intensity of segregation increases with K*, but decreases with D*. The present results extend the present knowledge of the PIPS process by taking into account the effects arising from the presence of a non-reactive polymer.

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system: an X-ray diffraction study

A Polymer Dispersed Liquid Crystal composite based upon a smectic liquid crystal and reactive crosslinkable epoxide-amine monomers was studied. The composite material is obtained by a polymerization induced phase separation process. The polymerization and the phase separation kinetics are both monitored. The polymerization induced phase separation is detected and quantitatively monitored by X-ray diffraction. The normalized X-ray intensity is used to calculate a phase separation rate and to define the phase separation kinetics which is compared to the polymerization rate and to the polymerization kinetics. The rate difference accounts for changes in morphology.

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system: an X-ray diffraction study

A Polymer Dispersed Liqued Crystal composite based upon a smective liquid crystal and reactive crosslinkable epoxide-amine monomers was studies. The composite material is obtained by a polymerization induced phase separation process. The polymerization and the phase separation kinetics are with monitored. The polymerization induced phase separation is detected and quantiatively monitored by X-ray diffraction. The normalized X-ray intensity is used to calculate a phase separation rate and to define the phase separation kinetics which is comppared to the polymerization rate and to the polymerization kinetics. The rate difference accounts for changes in morphology.