Fracture toughness and toughening mechanisms of poly(butylene terephthalate)/polycarbonate (PBT/PC) blends

Poly(buty1ene terephthalate)/polycarbonate (PBT/PC) system was selected in the present study on the fracture toughness and toughening mechanisms of rigid-rigid polymer blends. A comprehensive review on the toughening mechanisms for traditional rubber toughened and rigid polymer toughened polymer blends was given. Techniques for fracture toughness characterisation and toughening mechanism analysis were introduced and compared. The equivalence of the critical J—integral and the specific essential work of fracture was proven experimentally. There is no geometry dependence of the specific essential work of fracture as verified with three different specimens, namely, Single-Edge-Notched Three-Point-Bend (SEN—3PB), Compact Tension (CT) and Double-Edge-Notched-Tension (DENT). The specific essential work of fracture concept has been successfully extended to impact testing and the kinetic energy effect is discussed. A relatively thorough investigation on the morphology, mechanical properties, fracture toughness and toughening mechanisms was first conducted on a commercial grade PBT/PC/impact modifier (IM) blend under different testing conditions. Toughening mechanisms and sequence of toughening events were studied by SEM and TEM. It is suggested that the expansion of IM particles and matrix crazing under triaxial stress takes place first, which is followed by the cavitation inside of the IM particles and/or at the boundary between IM particle and matrix. Further expansion of the cavities reduces the thickness of the ligaments between them and eventually relieves the plane—strain constraint allowing massive shear deformation in the matrix to occur. It is believed that this matrix shear deformation provides the major contribution in toughening this commercial blend. To be able to modify the microstructures and hence to study the structure-property relationship a series of PBT/PC blends with and without IM particles were subsequently fabricated using a twin screw extruder with different processing control successfully. It was found that the morphology of the blends changed gradually with the percentage of PET from PC-matrix/PBT-particle to PC-particle/PBT-matrix. Bi-continuous structure was observed with the PBT/PC (40/60) and (50/50) blends. A relatively strong boundary between PET and PC domains was found in the PC—rich blends which could be tentatively attributed to the PBT/PC copolymer generated by transesterification during processing. In the toughened PBT/PC blends without IM, new toughening mechanisms are first observed. These include: (1) crazes formation in matrix under triaxial stress state, (2) crazes stabilisation by PC domains which prevent the crazes from developing into harmful cracks, (3) debonding—cavitation at the interface between PET and PC when the triaxial stress reaches its debonding strength, (4) relief of plane—strain constraint via debonding-cavitation which promotes massive shear yielding in matrix and (5) crack bridging by PC domains which is suggested as a possible major toughening mechanism. It is significant that in these blends the PC domains not only stabilise the growing crazes but also bridge the crack surfaces, enlarge the plastic zone size and result in a high fracture toughness. To enable these toughening mechanisms an appropriate bond strength between PET and PC is critical. It should be strong enough to provide the crack-bridging effect but not too strong to prevent debonding—cavitation to occur. The effects of processing condition, testing temperature, strain rate and IM addition on the fracture behaviour of the laboratory prepared PBT/PC blends were also studied. Mechanisms responsible in toughening PBT/PC blends at impact loading rate were proposed; and possible correlations between fracture toughness and molecular relaxation processes were discussed. The optimum volume fraction of IM particle in this PBT/PC/IM system was found and the role of the IM in toughening was investigated using OM and SEM. Based on the results of the present study, some suggestions on future work in this area and on the design of tough rigid—rigid polymer blends are given

Winner's Curse Free Robust Mendelian Randomization with Summary Data

In the past decade, the increased availability of genome-wide association studies summary data has popularized Mendelian Randomization (MR) for conducting causal inference. MR analyses, incorporating genetic variants as instrumental variables, are known for their robustness against reverse causation bias and unmeasured confounders. Nevertheless, classical MR analyses utilizing summary data may still produce biased causal effect estimates due to the winner's curse and pleiotropic issues. To address these two issues and establish valid causal conclusions, we propose a unified robust Mendelian Randomization framework with summary data, which systematically removes the winner's curse and screens out invalid genetic instruments with pleiotropic effects. Different from existing robust MR literature, our framework delivers valid statistical inference on the causal effect neither requiring the genetic pleiotropy effects to follow any parametric distribution nor relying on perfect instrument screening property. Under appropriate conditions, we show that our proposed estimator converges to a normal distribution and its variance can be well estimated. We demonstrate the performance of our proposed estimator through Monte Carlo simulations and two case studies. The codes implementing the procedures are available at https://github.com/ChongWuLab/CARE/

Fracture toughness and fracture mechanisms of PBT/PC/IM blends PART IV Impact toughness and failure mechanisms of PBT/PC blends without impact modifier

In this part of the series, the impact behaviour of the PBT and PC blends without impact modifier was studied. Failure mechanism of the blends under various conditions was discussed. It was found that the key toughening process, i.e. interfacial debonding-cavitation, was disabled when the blends were subjected to impact loading. Hence, the fracture of the thick PBT/PC specimens with strong interface occurred under plane-strain condition. Their impact toughness obeys the rule of mixtures and synergistic toughening could not be achieved. When thinner specimens were tested, the fracture took place under non-plane-strain condition. But, the toughness of the blends was much lower than the value predicted by the rule of mixtures. Negative blending effect was obtained. Study on the strain rate effect suggests that under impact loading, the PC domains in the blends are subjected to an additional plastic constraint imposed by the neighboring PBT matrix, which is more rigid at a higher strain rate. Since fracture of the PC is highly sensitive to the plastic constraint at the crack-tip, the PBT imposed high plastic constraint promotes brittle fracture of the PC, leading to a deteriorated impact resistance. Evidences from TEM, SEM and OM studies support the mechanism proposed. Based on this mechanism, some suggestions on the selection of polymer components and design of microstructure for rigid-rigid polymer blends are also given.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44763/1/10853_2004_Article_254468.pd

Fracture toughness and fracture mechanisms of polybutylene-terephthalate/polycarbonate/ impact-modifier blends

A series of polybutylene-terephthalate/polycarbonate (PBT/PC) blends with different compositions were prepared using a twin-screw extruder. The morphologies of the blends were revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that a 50/50 PBT/PC blend possessed a bicontinuous structure and the other blends had a dispersed phase of either PBT or PC depending on which was the minor component. A relatively strong interface was observed in the blends with 20%, 40% and 50% PBT; but poor interfacial adhesion was found in the blends with 60% and 80% PBT. The strength of the interfacial boundary was believed to depend on the composition and blending conditions of the individual blend. Fracture experiments showed that the sharp-notch fracture toughness of PC could be significantly increased by mixing with up to 50% PBT without losing its modulus and yield stress. The toughening mechanisms involved in the fracture processes of the blends were studied using both SEM and TEM together with single-edge-double-notched-bend (SEDNB) specimens. It was found that in the toughened blends the growing crazes initiated by the triaxial stress in front of the crack tip were stabilized by the PC domains. The debonding-cavitation mechanism occurred at the PBT/PC interface, which relieved the plane-strain constraint and promoted shear deformation in both PBT and PC. This plastic deformation absorbed a tremendous amount of energy. Crack-interface bridging by the PC domains was clearly verified by the TEM study. Thus, the PC domains not only stabilized the growing crazes they also bridged crack surfaces after the crack has passed by. This effect definitely caused a large plastic-damage zone and hence a high crack resistance. Poor crack resistances of the blends rich in PBT was caused by the poor interfacial adhesion between PBT and PC. In these polymer blends, the growing crazes easily developed into cracks, which subsequently passed through the weak interface of PBT/PC and finally produced fast unstable fracture.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44731/1/10853_2004_Article_BF00376274.pd

Research on Temperature Gradient Effect to Solder Joint Reliability

In order to evaluate the solder joint reliability under different thermal treatment condition, the thermal cycling test was usually used as a benchmark method to evaluate the quality of different electronic products. However, in real case, the product would suffer power cycling more than the thermal cycling, particularly personal portable devices, like laptop computer, mobile phone, and tablet PC etc., which would often be on and off. For the size limitation, a cooling system will be used to dissipate the heat from chip by cooling the PCB in these kinds of products. It means a large temperature gradient will be set at two sides of solder joint. The temperature of die side would be hot, while the substrate side would be cool. The temperature gradient condition would affect the metal diffusion inside solder, change the intermetallic compound grain growth, and finally influence the solder joint reliability. A new test method was introduced in this paper. A temperature gradient would be applied on the two sides of BGA chip - heat at die side and cool at substrate side. Three solder materials - eutectic solder (EU), lead-free solder (LF) and high-lead solder (HL) were introduced and compared. And three thermal profiles are applied in this research. FEM simulation was also applied to compare the temperature gradient difference effect and the stress distribution of whole package. The cross-section figures of solder bump before and after the experiments are compared and observed by SEM and EDX. It found that the metal migration phenomenon in different solder materials and the cycling thermal loading would be the two key facts to make the crack occur easily

A comprehensive study on intercalation and exfoliation of epoxy/clay nanocomposites

In this work, a series of epoxy/montmorillonite (MMT) nanocomposites were synthesized. As a general procedure, organo-modified clay of 1, 3 or 5 wt% loadings was thoroughly mixed with epoxy resin diglycidylether ofbisphenol A (DGEBA), at 80°C for Ih. After that, a stoichiometric amount of curing agent was added and the mixture was then degassed, molded and cured at 80°C for 2h and 160°C for another 2h. The nanocomposites samples were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM), in order to characterize the nanoscale dispersion and intergallery spacing ofnanoclay. Following the above general procedure, the intercalation and partial exfoliation of epoxylMMT nanocomposites were easily achieved, with the intergallery spacing of32-41A in average. In order to realize exfoliation, various processing conditions were tried in laboratory, e.g., extending the mixing time to 24h, elevating the mixing temperature to 200°C, or shearing the mixture with strong agitation, etc. However, these efforts seemed useless in expanding clay intergallery spacing. Higher initial curing temperature was also tried. For those samples with 1 and 3 wt% clay loadings cured at 160°C and 5 wt% at 200°C, the [001] diffraction peak of the clay disappeared from the XRD patterns, which indicated that the exfoliation was achieved. Other characterization methods have been applied to the above nanocomposites. Further investigation is currently continuing in progress