Rheological behavior during the phase separation of thermoset epoxy/thermoplastic polymer blends

Rheological behavior of thermoset/thermoplastic blends of epoxy/polyethersulphone (PES) was monitored during curing of the epoxy resin. During the isothermal curing of the mixture, a fluctuation in viscosity just before the abrupt viscosity increase was observed. This fluctuation is found to be due to the phase separation of PES from the matrix epoxy resin during the curing. The experimentally observed viscosity fluctuation is simulated with a simple two phase suspension model in terms of the increase in domain size. The viscosity profiles obtained experimentally at different isothermal curing temperatures are in good agreement with the predictions from the simple model taking into account the viscosity change due to the growth of PES domain and the network formation of the epoxy matrix.This work was supported by the Korea Science and Engineering Foundation (KOSEF) under Grant 97-0502- 0901-5. Financial aid from the Korean Ministry of Education through the Brain Korea 21 Program is also gratefully acknowledged

Dielectric Changes During the Curing of Epoxy Resin Based on the Diglycidyl Ether of Bisphenol a(DGEBA) with Diamine

The curing characteristics of diglycidyl ether of bisphenol A (DGEBA) with diaminodiphenylmethane (DDM) as a curing agent were studied using differential scanning calorimetry (DSC), rheometrics mechanical spectrometry (RMS), and dielectric analysis (DEA). The isothermal curing kinetics measured by DSC were well represented with the generalized auto-catalytic reaction model. With the temperature sweep, the inverse relationship between complex viscosity measured by RMS and ionic conductivity obtained from DEA was established indicating that the mobility of free ions represented by the ionic conductivity in DEA measurement and the chain segment motion as revealed by the complex viscosity measured from RMS are equivalent. From isothermal curing measurements at several different temperatures, the ionic conductivity contribution was shown to be dominant in the dielectric loss factor at the early stage of cure. The contribution of the dipole relaxation in dielectric loss factor became larger as the curing further proceeded. The critical degrees of cure, at which the dipolar contribution in the dielectric loss factor starts to appear, increases as isothermal curing temperature is increased. The dielectric relaxation time at the same degree of cure was shorter for a sample cured at higher curing temperature

Semiflexible Polymer Brushes: Most Probable Configuration Approach Based on Cotinuous Chain Model

The properties of semiflexible polymer brushes are studied by applying the classical limit of mean-field approach for chains with marginal chain stiffness. Using the mean-spherical Gaussian model, the most probable configuration for semiflexible chains is obtained, which reduces to the parabolic brush of Milner et al. [Mac-romolecules 1988, 21, 2610] in the flexible limit. From this configuration, equilibrium brush height as well as interactions between semiflexible brushes are estimated

Fabrication of highly ordered multilayer thin films and its applications

A new method is introduced to build up organic/organic multilayer films composed of cationic poly(allylamine hydrochloride) (PAH) and negatively charged poly(sodium 4-styrenesulfonate) (PSS) by using the spinning process. The adsorption process is governed by both the viscous force induced by fast solvent elimination and the electrostatic interaction between oppositely charged species. On the other hand, the centrifugal and air shear forces applied by the spinning process significantly enhance the desorption of weakly bound polyelectrolyte chains and also induce the planarization of the adsorbed polyelectrolyte layer. The film thickness per bilayer adsorbed by the conventional dipping process and the spinning process was found to be about 4 å and 24 å, respectively. The surface of the multilayer films prepared with the spinning process is quite homogeneous and smooth. Electroluminescence (EL) devices composed of alternating poly(p-phenylene vinylene) (PPV) and polyanions films show higher quantum efficiency when prepared by the spin self-assembly (SA) method.This work was financially supported by the National Research Laboratory Program (Grant M1-0104-00-0191) and funded in part by the Ministry of Education through the Brain Korea 21 Program at Seoul National University

Material issues for nanoporous ultra low-k dielectrics

Using the molecularly designed porogen (pore generating agent) approach, novel nanoporous low-k materials with improved mechanical properties have been achieved based on poly(methylsilsesquioxane), PMSSQ, structure. Two different methods, microphase separation system and grafted porogen system, were adopted to realize nonporous ultra low-k dielectrics with superior mechanical properties. We found that the behavior of dielectric constant as well as thin film modulus depends on the molecular structure of a porogen. Within the decomposition temperature windows of grafted porogens, a low-k material with k 6 Gpa was achieved. These results indicate that it is possible to design and fabricate nanoporous thin films with balanced low dielectric constant and robust mechanical properties, which are highly desired for microelectronic industry.This work is supported by the Collaborative Project for Excellence in Basic System IC Technology. Financial supports from the Ministry of Science and Technology (MOST) (the National Research Laboratory Fund) and the Korean Ministry of Education through the Brain Korea 21 Program are also greatly acknowledged

Effect of UV Pretreatment on the Nanopore Formation within Organosilicate Thin Films

We have investigated the low-temperature cure process to realize nanoporous organosilicate thin films at temperature below 150°C by adding a small amount of photoacid generator PAG followed by UV irradiation. The Gemini surfactant, which decomposes in the temperature range from 170 to 420°C, was used as a pore-generating material porogen for organosilicate matrix. The UV pretreatment in the presence of PAG lowers the condensation temperature of poly methyl silsesquioxane matrix and leads to the fast matrix vitrification enabling the addition of increased amount of porogens. Because the full vitrification of the matrix 150°C by UV pretreatment in the presence of PAG below the decomposition temperature of porogens 170°C prevents the pore collapse, the porosity up to 35.5% was achieved with an average pore size of 3.4 nm, as measured from X-ray reflectivity as well as ellipsometric porosimetry. It is shown that both dielectric constant and refractive index continue to decrease to 2.0 and 1.26, respectively. The present experimental system demonstrates that porogens with low degradation temperature can be successfully incorporated to realize nanoporous films without pore collapse. Consequently, this process can widen the choice of porogens to prepare nanoporous films.This work was supported by the NANO Systems Institute-National Core Research Center (NSI-NCRC) of the Korea Science and Engineering Foundation KOSEF , the Brain Korea 21 Program endorsed by the Ministry of Education of Korea, and System IC 2010 Project of Korea Ministry of Commerce, Industry and Energy

Spin-coated ultrathin multilayers and their micropatterning using microfluidic channels

A new method is introduced to build up organic/organic multilayer films composed of cationic poly(allylamine hydrochloride) (PAH) and negatively charged poly (sodium 4-styrenesulfonate) (PSS) using the spinning process. The adsorption process is governed by both the viscous force induced by fast solvent elimination and the electrostatic interaction between oppositely charged species. On the other hand, the centrifugal and air shear forces applied by the spinning process significantly enhances desorption of weakly bound polyelectrolyte chains and also induce the planarization of the adsorbed polyelectrolyte layer. The film thickness per bilayer adsorbed by the conventional dipping process and the spinning process was found to be about 4 Å and 24 Å, respectively. The surface of the multilayer films prepared with the spinning process is quite homogeneous and smooth. Also, a new approach to create multilayer ultrathin films with welldefined micropatterns in a short process time is introduced. To achieve such micropatterns with high line resolution in organic multilayer films, microfluidic channels were combined with the convective self-assembly process employing both hydrogen bonding and electrostatic intermolecular interactions. The channels were initially filled with polymer solution by capillary pressure and the residual solution was then removed by the spinning process.This work was financially supported by the National Research Laboratory Program (Grant M1-0104-00-0191) and funded in part by the Ministry of Education through the Brain Korea 21 Program at Seoul National University

Structural changes of polyaniline/montmorillonite nanocomposites and their effects on physical properties

Polyaniline/montmorillonite (MMT) nanocomposites containing different PANI contents were prepared by the intercalation of aniline monomer into pristine MMT followed by the subsequent oxidative polymerization of the aniline in the interlayer spacings. The polyaniline/MMT nanocomposite structure intercalated with polyaniline (PANI) was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). From the full-width at half-maximum (FWHM) of the (001) reflection peaks in the XRD patterns, the PANI/MMT nanocomposite containing 12.3 wt% PANI (PMN12) was found to be in the most disordered state. The physical interaction between the intercalated PANI and the basal surfaces of MMT was monitored by FT-IR. The room-temperature conductivity (RT) varied from 9.1 × 10–9 to 1.5 × 100 S cm–1 depending on the PANI content in the nanocomposites. The temperature dependence of dc conductivity (dc(T)) of all the samples follows the quasi-1D variable range hopping (quasi-1D VRH) model (i.e., dc(T) exp [–(T0/T)1/2]). The charge transport behavior of this system was interpreted from the slopes (T0) of the dc curves and the highest T0 value was found for the PANI/MMT nanocomposite with 12.3 wt% PANI (PMN12). The FT-IR, dc(T) and RT results for the nanocomposites with varying content of PANI are consistently related to the structure of the PANI/MMT nanocomposites discussed in the XRD analysis. The structural argument was further supported by scanning electron microscopy (SEM) of all the samples. Thermogravimetric analysis (TGA) showed improved thermal stability for the intercalated nanocomposites in comparison with the pure PANI and a simple PANI/MMT mixture.We are very grateful for the financial support from the National Research Laboratory Fund from the Ministry of Science and Technology (MOST), the Korean Ministry of Education through the Brain Korea 21 Program, and the Research Institute of Advanced Materials (RIAM) at Seoul National University. We also thank Mrs Mee Jeong Kang for her kind SEM and TEM measurements

Origin of the Thermoreversible fcc-bcc Transition in Block Copolymer Solutions

The thermoreversible fcc-bcc transition in concentrated block copolymer micellar solutions is shown to be driven by decreases in the aggregation number as the solvent penetrates the core, leading to a softer intermicelle potential. Small-angle neutron scattering measurements in a dilute solution are used to quantify the temperature-dependent micellar characteristics. The observed phase boundary is in excellent agreement with recent simulations of highly branched star polymers.This work was supported primarily by the MRSEC Program of the National Science Foundation under Grant No. DMR-0212302, and also in part by the Brain Korea 21 Program endorsed by the Korean Ministry of Education. We acknowledge helpful discussions with C. N. Likos

Temperature-dependent micellar structures in poly(styrene-b-isoprene) diblock copolymer solutions near the critical micelle temperature

The temperature dependence of the micelle structures formed by poly(styrene-b-isoprene) (SI) diblock copolymers in the selective solvents diethyl phthalate (DEP) and tetradecane (C14), which are selective for the PS and PI blocks, respectively, have been investigated by small angle neutron scattering (SANS). Two nearly symmetric SI diblock copolymers, one with a perdeuterated PS block and the other with a perdeuterated PI block, were examined in both DEP and C14. The SANS scattering length density of the solvent was matched closely to either the core or the corona block. The resulting core and corona contrast data were fitted with a detailed model developed by Pedersen and co-workers. The fits provide quantitative information on micellar characteristics such as aggregation number, core size, overall size, solvent fraction in the core, and corona thickness. As temperature increases, the solvent selectivity decreases, leading to substantial solvent swelling of the core and a decrease in the aggregation number and core size. Both core and corona chains are able to relax their conformations near the critical micelle temperature due to a decrease in the interfacial tension, even though the corona chains are always under good solvent conditions.This work was supported primarily by the National Science Foundation, through the University of Minnesota MRSEC ~Grant No. DMR-0212302!, and also in part by the Brain Korea 21 Program endorsed by the Korean Ministry of Education. Further support for this research was provided by the National Institute of Standards and Technology, U.S. Department of Commerce, through the neutron research facilities