Eberhard Hartmann

Trialkoxysilane grafting onto nanoparticles for the preparation of clear coat polyacrylate systems with excellent scratch performance

Transparent reinforced polyacrylates were prepared using nanosized filler particles with radiation-curable acrylates. To improve embedding of the nanofillers within the acrylate matrix the filler surface was chemically modified. This modification of the silica and alumina nanoparticles was accomplished by trimethoxysilanes having methacryloxypropyl (MEMO), vinyl (VTMO), and n-propyl (PTMO) functionalities. Due to acid catalyzed condensation of organosilanes a polysiloxane shell was formed around the nanoparticles. This shell is bonded to OH groups on the filler surface. The appearance of covalent Si–O–Si–R bonds was clearly shown by multinuclear MAS NMR. MALDI-TOF mass spectroscopy revealed the formation of polysiloxane oligomers with different degrees of condensation. In the work presented, an acrylate-nanocomposite formulation contains up to 35 wt.% nanosized silica covered with polysiloxanes. For MEMO modification, surface-anchored methacryl groups can copolymerize with network acrylates. But also PTMO-modified composite materials exhibit markedly improved properties as compared to the neat acrylate polymer; e.g., an increased modulus and heat resistance, improved scratch and abrasion resistance. This makes polyacrylate nanocomposites a very promising new coating technology for many far reaching technical applications.

Preparation of scratch and abrasion resistant polymeric nanocomposites by monomer grafting onto nanoparticles, 1 FTIR and multi‐nuclear NMR spectroscopy to the characterization of methacryl grafting

Nano-sized silica and alumina particles were used as fillers for polymer reinforcement and scratch resistant coatings. For favorable embedding within the polyacrylate matrix the surface of the fillers was chemically modified by reaction with methacroyloxy (propyl)-trimethoxysilane. The formation of covalent Si(Al)-O-Si-C bonds between functional groups from silane and OH groups on silica and alumina was demonstrated by means of FTIR and MAS NMR spectroscopy. The reactivity of the various surface silanols towards the coupling agent and the yield of surface Si-O-Si bonds were estimated by 29 Si CP MAS NMR data.

Preparation of Scratch and Abrasion Resistant Polymeric Nanocomposites by Monomer Grafting onto Nanoparticles, 3. Effect of Filler Particles and Grafting Agents

After modification with different trialkoxysilanes, nano-sized silica and alumina particles were used as fillers in transparent UV/EB curable acrylates for polymer reinforcement, particularly to attain scratch and abrasion resistant coatings. The acid catalyzed condensation of the organosilanes forms a polysiloxane shell which covers the nanoparticle like a nanocapsule. CP MAS NMR spectroscopy and MALDI-TOF mass spectrometry proved to be useful for the characterization of the polysiloxane structures. Grafter oligomers with more than 20 monomeric units were observed. Nanoparticles modified by methacroyloxy(propyl)trimethoxysilane and vinyltri-methoxysilane can copolymerize with acrylates. Compared with the pure polymers, these crosslinked polyacrylate nanocomposites, containing up to 35 wt.-% silica, exhibit markedly improved surface mechanical properties. Promising scratch and abrasion resistance of radiation-cured nanocomposite materials were also obtained by propyltrimethoxysilane grafting which results in an organo-philation of pyrogenic silica. Both colloidal and pyrogenic nano-sized silica nanopowders were used as fillers in polyacrylate films. The concentration of colloidal SiO 2 in commercial acrylate formulations amounts up to 50 wt.-%, whereas pyrogenic silica, notwhithstanding their surface modification by silanes, results in a thickening effect which limits its content to about 35 wt.-%. Never-theless, a comparison showed a distinct improvement in the surface mechanical properties such as haze and diamond microscratch hardness for surface-modified pyrogenic silica.

Preparation of scratch and abrasion resistant polymeric nanocomposites by monomer grafting onto nanoparticles, 2 Characterization of radiation‐cured polymeric nanocomposites

By in situ grafting methacroyloxy functionalized-silanes on commercial nanoglobular silica polymerization-active silico-organic nanoparticles were prepared. In radiation (UV, EB) induced polymerization reactions these modified nanoparticles form covalent crosslinks to acrylate substrates, thus efficiently modifying their viscoelastic properties. The transparent nanopowder composites can be used as scratch resistants coatings. The composite films show excellent adhesion on PC and PVC and good abrasion resistance after the Taber abraser test with haze values of about 6% after 500 cycles.

Study of the tetragonal-to-cubic phase transition in PbTiO3 nanopowders

The CPP (combined polymerization and pyrolysis) preparation route, in its enhanced liquid-precursor-based version, was combined with consecutive soft milling. For studies of temperature- and size-dependent structural changes occurring in ferroelectric lead titanate, this combined route yields a nanopowder series covering the relevant particle-size region at target quality. This material basis enables consistent SEM, TEM, XRD, Raman, EPR and dielectric measurements, which furnish a comprehensive picture of the cooperation between temperature rise and size reduction to eliminate tetragonality and concomitant ferroelectricity. Our previous original EPR studies on nanosized barium titanate are now extended to the lead titanate case. Furthermore, as compared to the pertinent literature standard, the materials basis is extended to powder samples of smaller mean particle sizes, comprising the critical size at which a PbTiO3 particle undergoes a transition into cubic paraelectric phase. Thus, the size-driven phase transition can be observed in a direct way (at 7 nm, which compares to 40 nm for BaTiO3), and the EPR data suggest a much less spacious gradient shell at the particle surfaces (thickness ≈ 2 nm) than in previous analogous investigations on BaTiO3 (15 nm).

Size Effects in BaTiO3 Nanopowders Studied by EPR and NMR

Abstract To study the effect of the particle size on the material properties, Mn 2 + EPR and 137 Ba NMR measurements were performed on BaTiO 3 particles with average diameters between 15 nm and 255 nm. Typical changes in the fine structure tensor and the quadrupole coupling tensor, respectively, are observed which allow us to make conclusions about the structure model for the nanograins in the ferroelectric tetragonal phase.

Polarization distribution in ceramic-polymer nanocomposites

We report pyroelectric responsivity versus depth profiles in nanocomposites of powdered barium titanate and tripropylene glycol diacrylate obtained by using the thermal pulse (TP) method. The ceramic nanopowder (mean particle size 270 nm) was prepared by combined solid-state polymerization and pyrolysis of a metallo-organic precursor. Films of powder/monomer dispersion (thickness ∼30 μm) were cured by irradiation with an electron beam (energy 140 keV, dose 80 kGy). The observation of a TP response after electron beam curing (EBC) indicates the appearance of a poling electric field as an effect of the EBC. The polarization depth profiles were studied for different filler contents. The composites show potential promise for piezo and pyroelectric sensors applications and for charge storage in microelectronics.

Dielectric investigations and theoretical calculations of size effect in lead titanate nanocrystals

The dielectric properties of nanograin ferroelectric lead titanate crystals are presented. The PbTiO3 samples were prepared by pressing nanopowders into plates and were studied experimentally by dielectric permittivity measurements in a wide frequency and temperature range. The TC dependence obtained showed a critical change of behavior with increasing mean nanoparticle size in the 9-nm region. The theoretical calculations based on Monte Carlo simulation were performed to describe the behavior of this material. It was shown that the distribution of nanoparticle sizes in the sample taken into account with the Monte Carlo method describes the dielectric properties of PbTiO3 nanocrystals quite well

Dielectric Investigations and Theoretical Calculations of Size Effect in Lead Titanate Nanocrystals

In this paper the dielectric properties of nanosize ferroelectric lead titanate crystals are presented. The PbTiO3 samples were prepared by pressing nanopowder into the plates and have been studied experimentally by dielectric permittivity measurements in a wide frequency and temperature range. The obtained TC dependence shows a critical change of behavior with increasing of mean nanoparticle size in a 9 nm region. The theoretical calculations based on Monte Carlo simulation were performed to describe such behavior of this material. It was shown that taking into account distribution of nanoparticle sizes in sample is possible to describe dielectric properties of PbTiO3 nanocrystals rather well.