Mikael Trollsås

Transport properties of hyperbranched and dendrimer-like star polymers

Moisture transport properties were assessed by sorption and desorption measurements on hydroxyl-functional hyperbranched polyesters based on 2,2-bis(methylol) propionic acid (bis-MPA) as AB(2)-mono ...

Highly branched radial block copolymers via dendritic initiation of aliphatic polyesters

Living ring opening polymerization of e-caprolactone initiated from the numerous chain-end hydroxymethyl groups of the analogous dendrimeric and hyperbranched polyesters derived from 2,2-bis(hydroxymethyl) propionic acid is described. By controlling the size of the dendritic macromolecule and the molar ratio of e-caprolactone, a variety of highly branched radial block copolymers are obtained. Comparison of the results obtained for the dendrimeric and hyperbranched initiators demonstrates that the reactivity of the chain-end hydroxymethyl groups in the dendrimer are significantly greater than in the isomeric hyperbranched case.

Templating Nanopores Into Poly(MethylSilsesquioxane): New Lowdielectric Coatings Suitable for MicroElectronic Applications

Thin films of nanoporous poly(methylsilsesquioxane) have been prepared from organic/inorganic polymer hybrids. Low molecular weight 6-arm star poly(caprolactone)s are incorporated into partially pre-condensed methylsilsesquioxane resin. Upon heating, the glass resin cross-links around the organic polymer templates. Thermal decomposition of the orgainc polymer produces nanoporous thin films which show tunable dielectric constants as low as 1.9 and exhibit properties suitable for use in interconnect devices. Experimental data including porosity, morphology, and dielectric constants are presented.

A pyroelectric liquid crystal polymer (PLCP) for second-harmonic generation

Abstract In order for a material to show a non-vanishing second order non-linear susceptibility, the material has to have a polar axis. This is hard to achieve in a liquid crystal system because of the strong quadrupolar order along the director. Electrostatic poling of polymers and polymer liquid crystals at high temperature and subsequent quenching to lower temperature only gives a small value of the polar order parameter and, in addition, results in materials which are not in thermodynamic equilibrium and therefore not stable over long times. Starting with a ferroelectric liquid crystal (which has polar order perpendicular to the director) we have succeeded in making a material with true polar order. It is not ferroelectric, although the ferroelectric properties of the starting and intermediate materials are basic for the procedure and for the final product which can be used for frequency doubling, of importance, for instance, in a wide area of optic communication applications.

Starlike dendrimers in solutions: Structural properties and internal dynamics

We measured the shape and the internal dynamics of starlike dendrimers under good solvent conditions with small-angle neutron scattering and neutron spin-echo (NSE) spectroscopy, respectively. Architectural parameters such as the spacer length and generation were varied in a systematic manner. Structural changes occurring in the dendrimers as a function of these parameters are discussed, i.e., in terms of the fractal dimension and deviations of the radius of gyration from the Gaussian value. A first cumulant evaluation of the NSE spectra for each scattering vector q separately yields the length scale dependent relaxation rates. We observe a local minimum in the normalized relaxation rates Omega(q)q(3) on length scales corresponding to the overall dendrimer dimension. The dynamics is discussed within a Rouse-Zimm approach generalized to the case of starlike dendrimers of arbitrary geometry. The model allows an identification of the modes contributing to the relaxation of the dendrimer in the q and time range of the NSE experiment. The local minimum is due to collective breathing motions of (parts of) the dendrons relative to each other. Shape fluctuations are not observed.

Dielectric relaxation of liquid crystalline polymers used in non-linear optical materials

Abstract Dielectric permittivity and loss have been measured over the frequency range 0.1–1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of smectic monomers. The polyacrylates were based on mixtures of chiral monofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and nonchiral bifunctional monomers, either with an ester group in the mesogen or with an ester group in the mesogen and a lateral nitro group, creating networks with different crosslink densities. Poled copolymers based on these monomers exhibit second-order nonlinear optical properties. Polymers based only on monomers with the chiral and nitro groups exhibited a pronounced γ process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temperature subglass process (β) associated with reorientation of the mesogen. The polymers with the bifunctional monomer without the nitro group exhibited clearly both γ and β processes. The dielectric data indicate that the ester groups in the bifunctional units undergo conventional reorientation according to the β mechanism and that the β process is suppressed in the pendant chains with the lateral nitro groups. The β process involves a coordinated torsion about two bonds in the mesogen and the swept-out volume involved in the motion becomes extensive when a lateral nitro group is present.

Influence of molecular tilt angle on the SHG response of pyroelectric liquid crystal polymers

Abstract Second harmonic generation in novel pyroelectric liquid crystal polymers (PLCP) made from a series binary mixtures, was studied using 1100 nm as the fundamental wavelength. The PLCPs were prepared by photo-polymerization of binary mixtures of two monomers which exhibit a smectic C ∗ phase, A2c (4″-( R )-(−)-2-[(10-acrylo-yloxy)decyl]oxy-3-nitrophenyl 4-{4′-[(11-acryloyloxy)-undecyloxy]phenyl}benzoate) and Alb (4″-(( R )-(+)-2-octyloxy)-3″-nitrophenyl 4-(4′-(11-acryloyloxy)undecyloxy)-phenyl)-benzoate). The highest d 16 and d 23 coefficients were found to be in the range 0.65–0.8 pm/V, and differed depending on the detailed preparation of the sample. All cases of polymers formed from the chiral smectic C∗ phase showed an SHG-signal with no external field present, indicating that polar order became fixed. The SHG-signal was found to increase with the tilt angle of the FLC molecules.

Preparation and Characterization of Crosslinked Liquid Crystalline Polymer Films as Nonlinear Optical Materials

Liquid crystalline monomers have been synthesized and characterized in purpose to make polymeric second-order nonlinear optical (NLO) materials. Crosslinked polymers have been made to thermally stabilize the orientation created in aligned samples. The orientation has been measured by means of x-ray scattering and infrared dichroism. Oriented, crosslinked poly(vinyl ether)s were made by cationic polymerization whereas crosslinked, nonlinear optical poly(acrylate)s were made by radical polymerization. Polymerization was initiated both thermally and photochemically. The NLO activity of the poly(acrylate)s was measured as the Pockels effect and the second harmonic generation signal, giving values of 0.15 pm/V and 0.40 pm/V, respectively.

The influence of chiral strength on the spontaneous polarization and the second-order nonlinear optical susceptibility in ferroelectric liquid crystals

Abstract The influence on the spontaneous polarization and the second-order nonlinear optical susceptibility of the mole fraction of the chiral component in binary mixtures of ferroelectric liquid crystals was studied theoretically. The model is based on the different molecular sizes which the molecules occupy in the mixture and on a model of the rotational distribution of the lateral dipole moment and the hyperpolarizability. The model is found to give a nonlinear relationship between the spontaneous polarization or the NLO-susceptibility and the mole fraction. A comparison is made with spontaneous polarization measurements performed on a series of systematically synthesized chiral and non-chiral liquid crystal structures, which where mixed in different proportions and combinations. The agreement with experiment is very good, and some model parameters are evaluated. The usefulness of the model is discussed.