Study of Laser‐Induced Photopolymerizations by Optical Pyrometry

Summary: Studies of the 355 nm laser induced free radical and cationic photopolymerization reactions of mono‐ and multifunctional monomers were conducted. These investigations were carried out with the aid of a specially constructed optical pyrometry instrument that provides rapid, reproducible temperature versus time profiles for these fast photopolymerization reactions. Using this technique, the effects of various reaction parameters and monomer structures on the rate and extent of the photopolymerization reactions were examined.

Optical pyrometry instrument for the monitoring of laser‐induced photopolymerizations.     

Liquid‐Crystalline Colloidal Particles

Summary: Here we report about the synthesis of colloidal particles of nematic and smectic liquid‐crystalline polymers. For this purpose mesogen‐containing acrylate monomers were synthesized and polymerized in a special modification of a precipitation polymerization called dispersion polymerization. By variation of the polymerization conditions colloidal particles of different size and polydispersity could be obtained including very narrowly distributed samples in optimized batches. On azobenzene‐containing colloidal particles switching experiments with polarized light were performed. It could be observed that the nematic director of the mesogens within the colloidal particles can be rotated due to the photochemical trans‐cis‐isomerization of the azobenzene chromophores.

Microscope images of a monolayer of P3‐9 .     

Temperature Dependence of the Oxygen Solubility in Acrylates and its Effect on the Induction Period in UV Photopolymerization

Summary: The influence of temperature on the kinetics of the UV photopolymerization of two diacrylates carried out under air was studied by real‐time FTIR‐ATR spectroscopy. In the temperature range up to 100 °C, the rate of polymerization obeys the Arrhenius law. The induction period was found to decrease with increasing temperature. It was shown that this decay is exclusively due to the decreasing solubility of oxygen in the acrylate. Moreover, from the induction period and the oxygen solubility, quantum yields of the α‐cleavage of some photoinitiators were estimated.

Induction period calculated from the solubility of oxygen in TPGDA in comparison with experimental data from RT‐FTIR measurements.     

Photoinduced Polymerization of Thiophene Using Iodonium Salt

Summary: Thiophene was polymerized by means of UV irradiation using diphenyliodonium hexafluorophosphate as a photoinitiator. An initiation mechanism involving electron transfer from photochemically generated onium radical cations to thiophene was proposed. N‐Ethoxy‐2‐methylpyridinium hexafluorophosphate and triphenylsulfonium hexafluorophosphate were also found to be effective in facilitating the polymerization of thiophene. Polymerization is accompanied with the film formation on the surface of the reaction tube. The photochemically obtained polymers were characterized by FT‐IR, DSC, and SEM analyses and compared with those obtained by oxidative and electrochemical means. Electrical conductivities of the samples were measured by four‐probe technique.

Photograph of the polymerization mixture containing thiophene and iodonium salt in CH₂Cl₂ before (a), after 5 min of irradiation (b), and dried film (c).     

Shape‐Memory Effect Controlled by the Crosslinking Topology in Uniaxially‐Deformed Smectic C* Elastomers

We have investigated the shape‐memory effects of uniaxially‐deformed, chiral, smectic C (SmC^*) elastomers for two different types of crosslinker, namely, a hydroquinone‐type crosslinker and a rod‐like crosslinker. Mesogens tilt with decreasing temperature from the SmA phase to the SmC^* phase in SmC^* elastomers synthesized with the hydroquinone‐type crosslinker. As for SmC^* elastomers with the rod‐like crosslinker, however, not mesogens but smectic layers are tilted in the smectic phases, because the crosslinker is sufficiently rigid to hinder mesogens from tilting. Because the shape change of such elastomers is coupled to the transformation of molecular alignment, SmC^* elastomers synthesized with the hydroquinone‐type crosslinker elongate with increasing temperature in SmC^* because of the decrease in molecular‐tilt angle, whereas those with the rod‐like crosslinker have an almost constant sample length in the temperature range of the SmC^* phase, despite the rearrangement of the layer structure. Both types of elastomer exhibit a reversible shape change that corresponds to the reversible change in molecular alignment during a heating and cooling process, within successive phase transitions between the isotropic phase and the smectic C^* phase.

     

X‐Ray Diffraction Studies on the Phase‐Transformational Behavior of a Smectic Liquid‐Crystalline Elastomer Composed of Chiral Mesogens

Structural changes through successive phase transformations of a chiral smectic liquid‐crystalline elastomer are investigated by X‐ray scattering technique. In uniaxially deformed elastomers, the smectic layer seemingly tilts even in the SmA phase, in which an in‐plane chevron structure formed in the tilted smectic phase. On the basis of an analysis of the layer reflection peaks, the layer correlation length in the tilted smectic phases is shorter than that in the non‐tilted SmA phase, though smectic layers in the tilted smectic phases are better ordered than those in SmA.

Experimental arrangement for X‐ray measurements of the uniaxially deformed elastomer in the tilted smectic phase at room temperature.     

Directing the Smectic Layer Orientation by Shear Flow in Hierarchical Lamellar‐within‐lamellar Liquid Crystalline Diblock Copolymers

A self‐assembled lamellar‐within‐lamellar structure of a side chain liquid crystalline diblock copolymer was shear aligned to induce overall alignment and to direct the smectic layer orientation within the copolymer lamellae. The copolymer consisted of a polystyrene block and a poly(methyl methacrylate) block bearing cholesteryl mesogens with only short oxycarbonyloxyethyl spacers separating the mesogens from the backbone. Upon shearing, the copolymer lamellae exhibited uniaxial alignment whereas the smectic layers of the mesogens showed coexisting perpendicular and parallel orientations with respect to the copolymer lamellae. The fraction of the parallel oriented domains could be systematically increased by tuning the oscillation frequency and strain amplitude.

     

“One‐pot” Synthesis of 1‐Vinyl‐2‐pyrrolidone with Protic Functional Groups in 3‐Position

A synthetic route that allows modification of 1‐vinyl‐2‐pyrrolidone with an alcohol or thiol group in its 3‐position in a one‐pot reaction without using protecting groups is described. The strategy used to achieve this goal is the reaction of the carboxamide anion of 1‐vinyl‐2‐pyrrolidone with cyclic precursors of these functionalities. It is furthermore shown that VP‐monomers functionalized with OH groups are suitable for the preparation of the corresponding copolymers with pure VP. As a second application of the new compounds, we also describe the synthesis of the first asymmetric crosslinker based on VP.

     

Synthesis of Poly(lauryl acrylate) by Single-Electron Transfer/Degenerative Chain Transfer Living Radical Polymerization Catalyzed by Na2S2O4 in Water

Living radical polymerization of lauryl acrylate was achieved by SET/DTLRP in water catalyzed by sodium dithionite. The work describes the synthesis of a highly hydrophobic and polar monomer in aqueous medium. The plots of versus conversion and ln[M]₀/[M] versus time are linear, indicating a controlled polymerization. This method leads to α,ω-diiodopoly(lauryl acrylate)s that can be further functionalized. The MWDs were determined using a combination of three detectors: RALLS, DV, and RI. The method studied in this work represents a possible route to prepare well-tailored macromolecules made of LA in environment friendly reaction medium. The syndiotactic content is 75%.

     

Block Copolymers from Organomodified Siloxane‐Containing Macroinitiators by Atom Transfer Radical Polymerization

Alternating copolymers of 1,3‐diisopropenylbenzene and 1,1,3,3‐tetramethyldisiloxane were synthesized by hydrosilylation–polyaddition. These linear copolymers were functionalized at both ends with 2‐bromoisobutyryl or benzyl chloride moieties. Subsequently, the obtained organomodified siloxane‐containing macroinitiators were successfully used for the preparation of ABA‐type block copolymers by atom transfer radical polymerization (ATRP) of styrene and tert‐butyl acrylate. The high chain‐end functionality of the macroinitiators was confirmed by ¹H NMR analysis of the macroinitiators and GPC measurements of the obtained ABA‐type block copolymers. The macroinitiator peaks disappeared in GPC traces after ATRP, and the obtained block copolymers showed a significantly narrower molecular‐weight distribution than the macroinitiators.

Synthesis of ABA‐type block copolymers by means of ATRP using organomodified siloxane‐containing, benzyl chloride functionalized macroinitiators.     

Pyrromethene 567 Dye as Visible Light Photoinitiator for Free Radical Polymerization

In the present work we report on the photochemistry and photopolymerization activity of a new bimolecular photoinitiator system, which exhibits maximum sensitivity at 518 nm, consists of a sensitizer dye, pyrromethene 567 (PM567), and a radical generating reagent 3,3′,4,4′‐tetra(tert‐butylperoxycarbonyl)benzophenone (BTTB). The photosensitization of BTTB through the excitation of PM567 induces high polymerization rates, while different experiences have revealed that PM567 or BTTB, separately, are unable of initiating the polymerization of 2‐hydroxyethyl methacrylate (HEMA) over extensive periods of time at 40 °C, either in the dark or under irradiation. The polymerization efficiency of this photoinitiator system, PM567/BTTB, was analyzed following the bulk polymerization kinetics of monomer HEMA by differential scanning photo‐calorimetry. Photopolymerization rates and quantum yields were observed under isothermal conditions (40 °C) for continuous illumination polymerizations (at 518 nm) at different incident light intensities and different BTTB concentrations.

Scheme of the photosensitization process of the bimolecular photoinitiator system of PM567/BTTB.     

UV Laser Deposition of SiS/Poly(thiacarbosilane) Composites and their Conversion to SiO/Poly(thiacarbosiloxane) Composites

ArF laser irradiation of gaseous mixtures of carbon disulfide and silane allows efficient deposition of Si? S bond‐containing poly(thiacarbosilanes) incorporating SiS bodies. These SiS/poly(thiacarbosilane) composites are the first example of silicon sulfide/polymer composites. Composite formation is analyzed by GC/MS analysis of volatile products and the structure of the composite as determined by electron microscopy and FT‐IR spectra. The composites undergo reaction with air moisture and methanol vapor, evolve H₂S and evolve to nano‐sized poly(thiacarbosiloxane)s and poly(methoxythiacarbosiloxane)s.

     

Synthesis and Orientation of Fluorene Containing Reactive Mesogens

Summary: The synthesis of new fluorene containing photocrosslinkable reactive mesogens is described. Both monodisperse trimers or pentamers and oligomeric mixtures containing two photocrosslinkable acrylate end groups were obtained by Suzuki‐cross‐coupling reactions. The pentamer 12a shows an ideal phase behavior for orientation experiments with a broad nematic phase between the glass transition at ?10 °C and 123 °C. In the oligomeric mixtures 14a – g the transition temperature from the nematic to the isotropic phase can be tailored from 100 to 310 °C by adjusting the molecular weight of the oligomers by end‐capping. This process can be easily characterized by MALDI‐TOF spectroscopy. The pentamer 12a and the oligomeric mixture 14c were oriented on rubbed polyimide layers and orientation ratios of 15:1 in photoluminescence were obtained. Experiments with different film thicknesses show that the orientation is not homogeneous throughout the film but decreases with increasing distance from the orientation layer.

     

Preparation of Positively and Negatively Charged Organic Colloids from a Single Precursor

A synthetic route is presented for the preparation of electrically charged colloidal particles with either sign from the same precursor. In the first step, methyl methacrylate was copolymerized with ω‐bromoalkyl acrylate and a cross‐linker, leading to monodisperse, bromo‐functionalized colloids. Substitution of the bromo‐functions by either trimethylamine or Na₂SO₃ transformed the neutral precursors into positively or negatively charged particles, respectively. The charge density could be varied in two ways: (i) by varying the fraction of bromo‐functionalized monomers; (ii) by increasing the degree of substitution of the bromo‐functions. Derivation of the charged particles from the same precursor colloid made it easier to explain various interesting physical properties of the charged colloidal particles. Higher charge densities were achieved for the cationic colloids. If a threshold value for the charge density of 55 μC · cm^?2 was surpassed, the particles showed a significant swelling in aqueous solution.

     

Preparation of Fluorescent Carboxyl and Amino Functionalized Polystyrene Particles by Miniemulsion Polymerization as Markers for Cells

Summary: A series of fluorescent polystyrene latex particles with carboxyl and amino functionalities on their surface were synthesized by the miniemulsion technique. The fluorescent dye N‐(2,6‐diisopropylphenyl)perylene‐3,4‐dicarboximide (PMI) was incorporated into the copolymer nanoparticles formulated from styrene and acrylic acid or styrene and aminoethyl methacrylate hydrochloride. The resulting latexes were stable and showed a monodisperse size distribution. The particle size depended on the amount and nature of the functional comonomer and was in the range 100–175 nm. All latexes were characterized by transmission electron microscopy (TEM), dynamic light scattering, UV‐Vis spectroscopy and zeta potential measurements. The amount of surface functional groups was determined by electrolyte titration. Furthermore, the functionalized fluorescent particles were utilized as markers for HeLa cells and cell uptake was visualized using fluorescence microscopy. The correlation of the uptake of nanoparticles with the surface charge was determined by FACS measurements.

Confocal fluorescent microscopy of HeLa cells after the uptake of amino functionalized particles (green).     

Structural Reasons for Restricted Backbone Motion in Poly(n‐alkyl methacrylates): Degree of Polymerization,Tacticity and Side‐Chain Length

Summary: The complex dynamics of poly(n‐alkyl methacrylates) is studied by advanced ¹³C NMR spectroscopy as well as mechanical and dielectric relaxation. Extended backbone conformations are identified as the molecular units involved in structural relaxation. From the variation in the degree of polymerization and a comparison with the presence of stereoregular sequences in the sample, the length of the extended units is determined to involve about five, at most ten monomeric units. Syndiotactic and isotactic sequences behave similarly. These findings are indicative of locally structured polymer melts.

     

Synthesis of Magnetic Nanocomposite Based on Amphiphilic Polyurethane Network Films

Magnetic nanocomposites, showing extremely small coercivity and hysteresis loop, were synthesized using amphiphilic polyurethane networks based on amphiphilic urethane acrylate nonionomer (UAN) precursor chains. Three kinds of UAN gel films were synthesized through UV‐curing of UAN chains in the absence of a solvent, or crosslinking polymerization of UAN/water and UAN/DMAc mixtures. Even though these gel films were synthesized with the same UAN, these UAN gel films had very different microstructures depending on the preparation conditions; as a consequence, magnetic composites prepared with these crosslinked films exhibited very different size and morphology of magnetic nanoparticles. Atomic force microscopy and mechanical property measurements showed that UAN gel film (UANH) synthesized with UAN/water mixture had the highly microphase‐separated structures whereas the gel films synthesized via UV‐curing (UANV) or polymerization of UAN/DMAc mixture (UAND) had relatively homogeneous structures. The largest ellipsoidal particles were formed in UANH gel films and the smallest ellipsoidal particles and rod‐shaped particles were made up in UANV gel and UAND gel film, which was confirmed by transmission electron microscopy.

Transmission electron micrograph of magnetic composite film prepared with UANV.     

Double‐Gyroid Morphology of a Polystyrene‐block‐Poly(ferrocenylethylmethylsilane) Diblock Copolymer: A Route to Ordered Bicontinuous Nanoscale Architectures

A polystyrene‐block‐poly(ferrocenylethylmethylsilane) diblock copolymer, displaying a double‐gyroid morphology when self‐assembled in the solid state, has been prepared with a PFEMS volume fraction ?_PFEMS = 0.39 and a total molecular weight of 64 000 Da by sequential living anionic polymerisation. A block copolymer with a metal‐containing block with iron and silicon in the main chain was selected due to its plasma etch resistance compared to the organic block. Self‐assembly of the diblock copolymer in the bulk showed a stable, double‐gyroid morphology as characterised by TEM. SAXS confirmed that the structure belonged to the Ia d space group.

     

Synthesis and Characterization of New Acrylate and Methacrylate Monomers with Pendant Pyridine Groups

To overcome some drawbacks of polyvinylpyridines, new monomers of acrylate and methacrylate type with pendant pyridine groups i.e., 4‐(3‐methacryloylpropyl)pyridine 1a and 4‐(3‐acryloylpropyl)pyridine 1b were successfully prepared, although it turned out to be challenging work to synthesize the acrylate monomer 1b . First polymerization studies showed that the new monomers could be polymerized easily by atom transfer radical polymerization (ATRP). The new polymers show excellent characteristics, such as very good solubility, low glass‐transition temperature, and easy quaternization.

Design and structure of new monomers 1a and 1b .     

Pyrene Containing Polymers for the Non‐Covalent Functionalization of Carbon Nanotubes

Pyrene containing diblock copolymers based on poly(methyl methacrylate) were synthesized and investigated regarding their adsorption on carbon nanotubes (CNT). The pyrene units were introduced using a reactive ester monomer for the build up of the second block which later on was reacted polymer‐analogously with amine functionalized pyrene derivatives. As we started from the same reactive ester intermediate, full block length identity is given. We varied the length of the anchor block to find an optimal block length and used pyren‐1‐yl‐methylamine as well as 4‐pyren‐1‐yl‐butylamine as anchor units. For both anchor units a maximal adsorption was found for 13 and 20 anchor units, respectively. The absolute adsorption was best for the 4‐pyren‐1‐yl‐butylamine anchor units as the longer spacer enhances the mobility of the anchor unit. The dispersion diagram of CNTs and diblock copolymer in terms of dispersion stability was investigated and a stable dispersion of 2.5 mg · ml^?1 CNTs in THF was found.