Study of PVDF composite electret for energy harvesting MEMS device

PVDF films with various additives were fabricated by the spin coating and solution casting method. The additives include Cloisite 20A, acid-treated MWCNTS and carbon black. The films were subsequently characterized by Fourier transform infrared (FTIR), polarized light microscopy and differential scanning calorimetry (DSC). PVDF film samples with very high content of -phase was achieved with the use of additives. The samples were subjected to cyclic corona charging in order to investigate their suitability as electret materials. The B-phase content of the PVDF films has no appreciable influence upon the charge retention ability of the material. Neat PVDF however, has been demonstrated to be a feasible electret material for energy harvesting MEMS device applications.Bachelor of Engineering (Materials Engineering

Acid degradation of thermoset materials

Thermoset polymers are a class of materials known for its insolubility in solvent due to its crosslinked network, which consequently impart upon it a high degree of chemical resistance. However, the degradation of thermoset materials upon exposure to strongly acidic conditions are not well understood by the scientific community due to a surprising lack of established research work in this topic. Often, it is presumed that acid degradation of polymer is chemical in nature, and thus the physical diffusion aspect tends to be overlooked. This research work intends to study and understand both the chemical and physical degradation behaviour of thermoset materials under strong acids, thus facilitating the identification of acid resistant or susceptible moieties, which may then either enable the design of an acid resistant thermoset material or allow for the development of a process to recycle thermosets. A new methodology based on dielectric spectroscopy to quantify acid degradation of thermoset polymers was developed, in order to relate the observed degradation phenomena with quantitative data and thus substantiate the proposed degradation mechanism with evidence. Amine-cured epoxy swollen with HCl acid was observed to possess very interesting dielectric properties, which warranted further investigation in this Chapter. The unique dielectric features including negative permittivity and resonant tan δ peaks indicative of specific binding interactions were observed, which was thoroughly investigated by dielectric spectroscopy. Oscillatory Bode plots and spiral Nyquist plots from impedance analysis of HCl-swollen TGAP-NBDA provided insights into the charge transfer mechanism within such systems. The mechanism of acid degradation in thermoset composite materials and various contribution from composite morphology and filler content was studied in an attempt to quantify the influence of the interface. the mass uptake behaviour of epoxy/silica composites with different filler morphology, particle size and filler content in HCl acid immersion was thoroughly investigated to study the role of the interface/interphase region in the acid degradation mechanism. Dielectric analysis was performed to elucidate insights into the state of the polymer network between the inert silica fillers. The role of the interface was determined to the most dominant factor governing the acid degradation of epoxy/silica composites, regardless of the filler shape morphology, size or content level. The relevance of this research topic and impact justification was demonstrated through the utilisation of the fundamental principles of acid degradation uncovered in this thesis to develop an innovative approach for carbon fibre composite recycling. The acid delamination process has the ability to transit polymer thermal degradation in air from an anaerobic mode to oxidation dominated decomposition simply by increasing interfacial exposure to air, which could significantly improve the economics of the carbon fibre recovery process. This two-step acid and oxidation method can recover carbon fibre from CFRP waste without the need for significant size reduction pre-treatment, hence enabling the recovery of entire carbon fibre textile and long fibres, which is more suitable for more applications than the short fibres recovered by current techniques.Doctor of Philosoph

Unexpected H₂ solubility of polyimide/polyphthalonitrile H₂-selective membranes with tailorable microstructure and performance

Membrane separation technologies are emerging as energy-efficient alternatives to traditional distillation processes. The growing demand for clean energy on “hydrogen economy” has highlighted the need for high-performance H2-selective gas separation membranes. Polyimides (PIs) show promise in these applications due to their excellent thermal stability, good hydrogen permeability, and processability. However, PIs often have insufficient selectivity, mainly because they have a poor affinity H2 over other gases. Despite the rigid molecular structure of polyimides provides excellent size-sieving property for hydrogen, the low H2 solubility of PIs limits the H2 selectivity, which is crucial for hydrogen purification. To address this issue, this study demonstrated a simple blending method to fabricate polyimide/polyphthalonitrile blend membranes with adjustable microstructure. The study found that the blend membranes exhibited remarkable H2 separation performance that the H2/CO2 and H2/N2 selectivity over 60 and 1600, respectively, exceeding the Roberson's upper bound (2008). The improved gas separation performance was attributed to enhanced H2 solubility, resulting in superior H2 solubility selectivity. The addition of polyphthalonitrile benefits the blend membranes toward a more narrowed distribution of fractional free volume, promoting the sorption of H2 over 8 times higher than both polyimide or polyphthalonitirle. Additionally, gas separation performance of the blend membranes can be further adjust through thermal crosslinking of the blend membranes into a semi-interpenetrating network (semi-IPN). Overall, this study presents a novel approach to tailoring the polymer matrix of polyimide-based membranes, opening up possibilities for the development of advanced gas separation membranes

Unraveling the mechanistic origins of epoxy degradation in acids

Water diffusion into polymers like thermosetting epoxies is well-studied; however, comparably little has been reported thus far on the related but very different mechanism of acid diffusion and the corresponding influence on material degradation. The diffusion of hydrochloric acid into an amine-cured epoxy system was studied in this work using gravimetric analysis and dielectric monitoring concurrently, and the mass uptake behavior was observed to differ significantly compared with water diffusion, faster by an order of magnitude. A unique 3-stage diffusion of acid into epoxy was observed due to the influence of Coulombic interactions between oppositely charged ionic species diffusing at different rates. Material characterization studies have revealed that the dominant degradation mechanism is physical in nature, with the formation of surface cracks driven by the swelling stresses due to the core-shell swelling behavior in highly concentrated hydrochloric acid, leading to an erosion-type degradation phenomenon. The insights gained from understanding acid electrolyte diffusion could serve to design a more effective and efficient process to enable thermoset recycling by facilitating rapid material breakdown or the design of acid-resistant materials for various applications in chemical storage tanks, batteries, and protective coatings in a corrosive environment.Published versio

Design rationale for stimuli-responsive, semi-interpenetrating polymer network hydrogels–a quantitative approach

Stimuli‐responsive semi‐interpenetrating polymer network (semi‐IPN) hydrogels form an important class of polymers for their tunable properties via molecular design. They are widely investigated for a diverse range of applications including drug delivery, sensors, actuators, and osmotic agents. However, in‐depth studies on some of the critical design principles affecting diffusion/leaching of linear polymer from semi‐IPN hydrogels are lacking. Herein, for the first time, by preparing a series of model semi‐IPN hydrogels based on thermally responsive poly (N‐isopropyl acrylamide) (PNIPAM) network and linear poly(sodium acrylate) (PSA), a systematic and quantitative study concerning linear polymer chain retention and swelling/deswelling kinetics is reported. The study shows that PSA retention is significantly affected not only by PSA molecular weight and concentration, but also by polymerization temperature, which could be linked to homogeneity and internal morphology of the hydrogel. Surprisingly, there is no obvious influence of crosslinking density of PNIPAM network toward PSA retention, while faster swelling and deswelling at higher crosslinking density are observed in terms of swelling rate constant and deswelling activation energy. These findings offer new insights on the factors affecting structural and physicochemical properties of such semi‐IPN hydrogels, which should in turn serve as a general guideline for materials design.Accepted versionThe author (Gupta) would like to thank Nanyang Technological University for the research scholarship via the Interdisciplinary Graduate School (IGS)

Phthalonitrile prepolymer and PAN blends : new strategy for precursor stabilization and pyrolytic char yield enhancement

New polymer blends of polyacrylonitrile (PAN) and resorcinol-based phthalonitrile prepolymer (pPN) are studied as superior carbon precursors. pPN has a hyperbranch-like structure with multiple terminal nitrile groups available for chemical interactions. The addition of pPN into PAN significantly lowers the cyclization temperature by more than 15 °C during the oxidative stabilization stage which is unprecedented and highly desirable giving rise to a multitude of advantages during carbonization. The presence of pPN also leads to large synergy in char yield by due to the specific interaction between the nitrile terminal groups in the hyperbranch-like pPN and PAN chains. The char yield at 600 °C increased from 57.7% to a remarkable 69.0% when 10 wt% of pPN is added into PAN even though under the same condition the char yield of neat pPN itself is only 48.8%. Additional advantages of this new approach, i.e., large shrinkage reduction and property enhancement, are also observed in the carbonaceous materials obtained from the pPN/PAN blends. Raman spectra reveal that the carbon structure is retained when 10 wt% or less pPN is used.Nanyang Technological UniversityThis paper was supported by Temasek Laboratories@ NTU and School of Materials Science and Engineering, Nanyang Technological University, Singapore

Large toughening effect in biomimetic geopolymer composites via interface engineered 3D skeleton

Green and eco-friendly geopolymers with high thermal/acid resistance represent potential candidates for the replacement of traditional Portland cement in construction, as well as many other applications; however, the intrinsic brittleness and low toughness typical of ceramic hinders widespread adoption of this material in various applications. In this work, we fabricated a new type of geopolymer composites by impregnated with interface engineered 3D skeleton resembling the lotus root structure. Highly porous melamine foam was selected as the 3D skeleton and its interior surface was coated with elastomeric polydimethylsiloxane–polyurea block copolymer. Under loading, the interfacial elastomer could deform and absorb large amount of energy concurrently with crack deflection of melamine foam and delamination of interfaces, thus the toughness was substantially improved as results indicated a transition of fracture behavior from brittle failure mode to a more ductile one. With as low as 2.5 wt % elastomer, the fracture toughness and work of fracture were increased by 258% and 654%, respectively. Owing to the three-dimensional reinforcement preform, the issue with dispersion of reinforcing fillers is circumvented. The obtained geopolymer composites with enhanced toughness allow for applications requiring high load capacity. This strategy of manufacturing composites through 3D skeleton opens new pathway to improving mechanical performance of various brittle materials and material processing techniques.Nanyang Technological UniversityAccepted versionXuelong Chen acknowledges the scholarship from Nanyang Technological University. Liying Zhang acknowledges the support by the initial research funds for Young Teachers of Donghua University. The authors also acknowledge the funding supported by Nanyang Technological University(NTU) with grant number M4061124 and support from School of Materials Science and Engineering at NTU for this work. The authors thank the Facility for Analysis, Characterization, Testing and Simulation (FACTS) lab where SEM and XRD were performed

Understanding hydrogen solubility and free volume characteristics in charge-transfer phthalonitrile prepolymer and polyimide blend membranes

The increasing demand for hydrogen production has necessitated the development of H2-selective membranes. Polyimides are excellent membrane materials for gas separation; however, commercial polyimides generally lack sufficient H2 selectivity due to their low H2 affinity. Understanding the relationship between gas transport properties and free volume microstructure is critical to advancing H2-selective membrane design. Herein, we report a facile material strategy to adjust the free volume characteristics and H2 separation performance via blending Matrimid (PI) and crosslinkable resorcinol-based phthalonitrile prepolymer (RPN) with electron donor/acceptor properties. The novel RPN30/PI70 membrane exhibits H2/N2 and H2/CO2 permselectivity of 1637 and 66.4, respectively, with H2 permeability of 2.7 Barrer in pure gas test, surpassing Robeson upper bounds (2008). The increased H2 permselectivity of RPN/PI membranes was attributed to the narrowed free volume size and distribution, giving rise to the considerably improved H2 solubility and selectivity of the blends. Moreover, the H2 permeability of crosslinked RPN30/PI70 membranes can be further improved via thermal treatment. The H2/CO2 mixed-gas test reveals that the H2 gas separation performance of the RPN30/PI70 membrane is influenced by plasticization effect and competitive sorption. This study demonstrates a new versatile strategy for designing high-performance hydrogen-selective polymeric membranes.Nanyang Technological UniversityThe authors would like to thank Nanyang Technological University, Singapore, for the research scholarship via the Interdisciplinary Graduate Programme

Epoxy-assisted ball milling of boron nitride towards thermally conductive impregnable composites

It remains a major challenge to develop insulating polymeric composites with high thermal conductivity and low viscosity for impregnation applications. Hexagonal boron nitride (hBN) has proven a competitive thermal conductivity enhancer for polymers. However, when introduced into a polymer matrix, raw BN usually undergoes severe sedimentation and thus uneven distribution throughout the matrix, which undermines the uniformity in thermal conductivity and other physical properties. In this work, BN was ball milled with part of the host epoxy resin in slurry state and then directly mixed with the rest in the same pot to obtain a composite resin. Without the tedious processes of filler separation, purification and re-dispersion, the epoxy-assisted slurry-state ball milling greatly improved the suspension stability of BN platelets in epoxy resin, leading to a uniform filler distribution after curing. Consequently, the thermal conductivity of epoxy resin was boosted from 0.19 W·m−1·K−1 by 132% to 0.44 W·m−1·K−1 with the addition of 9.1 wt% loading of BN. Moreover, the composite resin demonstrated high impregnation performance on litz wire. The material preparation method developed in this work is facile, time-effective and scalable, especially suitable for fabricating thermally conductive yet low-viscosity composite resins for impregnation applications. The method may also be extended to develop other polymeric composites incorporated with layer-structured filler materials.Nanyang Technological UniversityNational Research Foundation (NRF)The authors would like to thank Nanyang Technological University, Rolls Royce and National Research Foundation of Singapore for the financial support provided through the Corp Lab@University scheme

High-precision 3D printing of high-strength polymer-derived ceramics: impact of precursor's molecular structure

Additive manufacturing (3D printing) offers new opportunities to create complex structures for many applications. The development of suitable precursors for high-resolution 3D printing of ceramics is increasingly essential to meet evolving material requirements. Herein, a new hybrid preceramic formulation based on thiol-ene click chemistry for precision printing of polymer-derived ceramic has been developed to enable the fabrication of complex 3D objects using high-resolution projection microstereolithography (PμSL). Two low-odor thiol compounds with either three (trithiol) or four thiols (tetrathiol) moieties have been examined, to investigate the influence of thiol structure on the mechanical properties of converted ceramics. Pyrolysis of the printed green bodies leads to the formation of silicon oxycarbide (SiOC) with high fidelity after polymer-to-ceramic transformation. The SiOC printed specimen converted from the tetrathiol formulation (4T) demonstrates excellent mechanical strength surpassing that of the trithiol-based formulation (3T) and previously reported SiOC preceramic polymers. The ceramic honeycomb fabricated using the tetrathiol compound shows remarkable improvement in compressive strength, which is two times higher than that of the trithiol-derived ceramic. This work proposes a simple and effective way to formulate 3D printable preceramic polymers through molecular design. The achieved 3D printed SiOC can fulfill the requirement for high-strength ceramic materials with complex shapes.Nanyang Technological UniversityThe authors acknowledge support from the Nanyang Technological University (NTU) with grant number of 04IDS000677N040