Yingfeng Yu

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.

High-performance light-emitting diodes encapsulated with silica-filled epoxy materials.

Packaging materials have a great impact on the performance and reliability of light-emitting diodes (LEDs). In this study, we have prepared high performance LED devices through encapsulating LEDs by epoxy materials incorporated with filler powders. A set of evaluation methods have also been established to characterize the reliability of LED devices. No delamination or internal cracking between packaging materials and lead frames has been found for the encapsulated high performance LED devices after the package saturation with moisture and subsequent exposure to high-temperature solder reflow and thermal cycling. Four kinds of inorganic silica fillers, namely, quartz, fused silica, cristobalite, and spherical silica, and one kind of organic filler, that is, spherical silicone powder, were incorporated into the epoxy packaging materials to compare their effects on performance of LED devices. The properties of epoxy packaging materials and LED devices were studied by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), dynamic mechanical analysis (DMA), thermomechanical analyzer (TMA), ultravioletvisible spectrophotometer (UV-vis), scanning acoustic microscopy (SAM), and scanning electron microscopy (SEM). Except the spherical silicone powder filled epoxy materials, all the other filled systems showed lower equilibrium water sorption content and smaller water diffusion coefficient in both water sorption and moisture sorption tests. The coefficient of thermal expansion (CTE) values were also decreased with the addition of fillers, and the systems filled with quartz, fused, and filled with spherical silica gave the best performance, which exhibited the reduced CTE values both below and above Tg. The results of TGA essentially showed no difference between filled and unfilled systems. The glass transition temperature changed little for all the filled systems, except the one incorporated with spherical silicone. The modulus at room temperature increased with the addition of fillers. The light transmittance of filled epoxy materials varied with fillers after UV and thermal aging.

Effect of Mesoscopic Fillers on the Polymerization Induced Viscoelastic Phase Separation at Near- and Off-Critical Compositions

We have investigated the effect of mesoscopic fillers on the polymerization induced viscoelastic phase separation of thermoplastic modified thermosets at near- and off-critical concentrations using optical microscopy, time-resolved light scattering, dynamic mechanical analyses, and rheological instrument. Mesoscopic fillers including sepiolite and nanosized silica showed a significant enhancement effect in viscoelastic phase separation, and resulted in pronounced differences in the phase structures at all concentrations of polyetherimide modified epoxy resins with dynamic asymmetry. For blends near critical concentration, the introduction of fillers led to much finer phase structure with smaller characteristic length scale. At off-critical composition (i.e., blends with low concentration of slow dynamic component), the strong polymer chain entanglement resulted in enwrapped mesoscopic fillers within a slow dynamic phase. The rheological behavior of the blends clearly demonstrated the significant enhancement effect of mesoscopic fillers in the viscoelastic phase separation. The apparent activation energy of polymer chain mobility obtained from dynamic mechanical study of glass transition reflected strong wrapping behavior of polymer chains on mesoscopic fillers, which were consistent with the rheological and light scattering study.

Effect of curing conversion on the water sorption, corrosion resistance and thermo-mechanical properties of epoxy resin

The study of the relationship between curing conversion and the properties of the material is critical for optimizing the performance of epoxy materials. In this article, the change in the thermo-mechanical and corrosion resistance properties of the diglycidyl ether of a bisphenol A/diaminodiphenylsulfone epoxy system with curing conversion was studied using various instruments. The water sorption process showed the opposite trend of equilibrium water content with curing conversion at high and low temperatures, which was found to be related to different mechanisms by infrared spectroscopic study. The mechanical tests and corrosion resistance (EIS) experiments showed an optimized performance region at a curing conversion close to ca. 85%.

Enhanced viscoelastic effect of mesoscopic fillers in phase separation

Here we report the significant enhancement effect of mesoscopic fillers in viscoelastic phase separation of dynamic asymmetric polymer blends. Mesoscopic fillers with their size much larger than the dimensions of the polymer chains, from nanometres to microns, are preferentially immersed into the slow dynamic phase and phase interface due to the entanglement with polymer chains. For sufficiently high volume fraction and fine dispersion, mesoscopic fillers conduce to the pronounced slowing down of the phase separation process, and result in refined structures with sharply decreased characteristic length scales. The pinning of the phase separation is attributed to the dramatic increase of dynamic asymmetry from the entanglement of polymer chains with mesoscopic fillers. The principal difference between mesoscopic fillers in classic and viscoelastic phase separation is whether there exists a filler enforced elastic-force balance condition. This suggests a general physical scenario of entanglement selection of the polymer chains under stress.

The Diffusion Mechanism of Water Transport in Amine-Cured Epoxy Networks

In the present work, time-resolved attenuated total reflection Fourier transform infrared spectroscopy (ATR-IR), near-infrared (NIR) spectroscopy, and generalized two-dimensional (2D) correlation analysis were used to investigate water diffusion processes and the state of water molecules in six different epoxy resins. Positron annihilation lifetime spectroscopy (PALS) experimental results and IR results suggested that water diffusion is controlled by local chain reorientation and bond dissociation of water molecules from epoxy networks. Dynamic mechanical analysis (DMA) results of glass transition temperatures of epoxy resins after immersion in hot water correlated well with the PALS and IR results. In addition, four types of water molecules, termed nonbonded (S0), single bonded (S1), loosely double hydrogen bonded (S2L), and tightly double bonded (S2T), were detected. It was likewise found, as verified by rough estimation, that water molecules with double hydrogen bonds mostly accomplished diffusion.

Monitoring extent of curing and thermal–mechanical property study of printed circuit board substrates

Abstract Precise control of curing conversion for epoxy-based printed circuit board (PCB) substrates and clarification of curing–property relationship are critical for the performance and reliability assessment, and for the design optimization of electronic systems. In this article, various epoxy composites for PCB substrates were analyzed by infrared spectroscopy (IR), differential scanning calorimetry (DSC), rheometry, dynamic mechanical analysis (DMA), and scanning electron microscope (SEM). Compared with mid-IR and DSC, near-IR (NIR) is found to be a reliable method for the characterization of curing conversion process by detecting the consumption of epoxy groups. And DMA is a powerful method for measuring the conversion of PCB materials by testing glass transition temperatures ( T g ) and viscoelastic properties. The curing behaviors of a variety of epoxy composites show distinct differences in both curing rate and activation energy, and the growth tendency of T g with curing conversion also changed depending on the material compositions. Correlation of curing conversion versus thermal properties shows that the activation energy of curing at different stage by DSC resembles the tendency of T g transitions tested by DMA. Mechanical properties of the composites show close relationship with the curing conversions. Peel strength, the indicator of adhesion strength between copper foil and epoxy composites, was tested on all the specimens of different curing conversions, and the results showed a maximum value at curing conversion between ca. 90 and 95%.

Effect of pi–pi stacking on the self-assembly of azomethine-type rod–coil liquid crystals

The role of pi–pi stacking in the molecular self-assembly behaviour, the phase transitions and the fluorescent properties was investigated for coil–rod–coil liquid crystals with two ethylene oxide coil units and aromatic azomethine rod units. Three kinds of rod–coil liquid crystals with different rod units were synthesised and characterised by differential scanning calorimetry (DSC), polarised optical microscopy (POM), X-ray diffraction (XRD), transmission electronic microscopy (TEM), absorption and photoluminescence. With strong pi–pi stacking interaction between coplanar molecules, fibril structure in solution is formed while the fluorescent emission shows a rapid increase of the intensity of short wavelength with the increase of solution concentration. Spherical aggregations in solution are formed with the increase of the twisting of the rod segment plane, which decreases the pi–pi stacking interaction and thus enlarges the intermolecular spacing. Simulation work carried out in Material Studio corresponds well with the experiment results of aggregations and phase transitions.

Effect of size and content of mesoscopic fillers on the polymerization induced viscoelastic phase separation

Fillers have great impact on the phase separation process and material properties of multi-component heterogeneous polymer composites. In this article, we have investigated the effect of surface affinity, size and content of mesoscopic fillers on the polymerization induced viscoelastic phase separation of thermoplastic modified thermosets by using optical microscopy, time-resolved light scattering, and dynamic mechanical analyses. Nanosized mesoscopic silica type fillers, both surface treated and untreated, showed significant enhancement effect on the viscoelastic phase separation of polyethersulfone modified epoxy resins with dynamic asymmetry, which resulted in a clear decrease of characteristic length scale of phase structure and an increase of relaxation time of phase separation. With increase of filler size from nanoscale to micron-size, the “enhanced viscoelastic effect” of fillers at the same content diminished quickly as both the phase evolution process and the size of phase structure shifted to those of blends without fillers. While increase of the content of fillers showed an enlarged enhancement effect on viscoelastic phase separation. Glass transition temperatures of filled blends with various filler contents and sizes obtained from dynamic mechanical study were consistent with the phase separation study.

Polyoxometalate cured epoxy resins with photochromic properties

Novel photochromic thermosetting materials were facilely prepared by polyoxometalate, Keggin-type H3PW12O40, cured epoxy networks with ethylene oxide blocks. The dual functions of polyoxometalate as both hardener and photochromophore were studied by differential scanning calorimetry, infrared spectroscopy, ultraviolet–visible spectroscopy (UV–vis), and generalized two-dimensional correlation analysis. Polyoxometalate initiates the cationic polymerization of epoxy resin through dissolving in either polyethylene oxide epoxy or organic solvents. When subjected to UV irradiation, the transparent thermosetting materials with ethylene oxide blocks change from colorless to blue, and could be bleached in air at various temperatures to recover its initial state. From the UV–vis measurements, all the resultant thermosetting materials demonstrated similar photochromic behavior after ultraviolet irradiation showing characteristic d–d transition band and intervalence charge transfer band. The 2D correlation analysis of the photochromic spectra clearly revealed the sequence of electron movements in the framework of PW12 anion.