Siu N. Leung

Numerical simulation of polymeric foaming processes using modified nucleation theory

Abstract This paper develops a modified nucleation theory and examines its application to simulate the bubble nucleating phenomena in polymeric foaming processes. Bubbles typically nucleate homogeneously in the bulk phase of the polymer/gas solution and heterogeneously at the boundaries between the bulk phase and the nucleating agents. Although past research had attempted to consider the heterogeneous nucleation to simulate the plastic foaming process, the assumption of a flat nucleating surface might not be realistic. In this context, the authors have modified the heterogeneous nucleation theory by considering random surface geometries, and have developed a computer simulation program to predict the cell nuclei density based on this heterogeneous nucleation scheme with random surface geometries. Homogeneous cell nucleation was also included in the simulation. The simulation results obtained in this study are compared with the experimentally observed data from a batch foaming process to evaluate the validity of the theory proposed herein. It is found that the modified theory does in fact account for the quantitative aspects of the experimental observations when an iterative approach is used to determine the contact angle θ c. Although the appropriateness of the values selected for θ c requires further investigation from both experimental and theoretical points of view, this study illustrated that the modified nucleation theory can represent a new access point for exploring the cell nucleating phenomena in plastic foaming processes.

Effects of Nucleating Agents’ Shapes and Interfacial Properties on Cell Nucleation

The classical nucleation theory (CNT) provides the fundamentals to understand the cell nucleation phenomena theoretically during polymeric foaming processing. Since a variety of additives and impurities are ubiquitous in commercial plastic resins and crevices are omnipresent on the internal walls of processing equipment, heterogeneous nucleation plays a significant role in plastic foaming. According to the CNT, interfacial tensions at different interfaces and surface geometries at various heterogeneous nucleating sites are important factors that govern the heterogeneous nucleation rate. Therefore, the elucidation of their roles in cell nucleation will be beneficial to the foaming industry, which can use the information to optimize its foaming technologies and develop effective nucleating agents. In this study, a series of sensitivity analyses were performed; the results indicated that the contact angle (θc ) at the gas—liquid—solid junction and the semi-conical angle (β) at the nucleating site are critical parameters that govern the free energy barrier to initiate heterogeneous nucleation (Whet ). Moreover, regardless of the surface geometries of the nucleating agents, Whet decreases as θc increases. For moderate values of θc (e.g., θc = 90°), heterogeneous nucleation is promoted in conical cavities with a smaller β. However, a smaller β is unfavorable for cell nucleation when θc is too small.

Multifunctional polymer nanocomposites with uniaxially aligned liquid crystal polymer fibrils and graphene nanoplatelets

Polymer nanocomposites have actively been studied to replace metals in different emerging applications because of their light weight, superior manufacturability, and low processing cost. For example, extensive research efforts have been made to develop advanced thermally conductive polymer nanocomposites, with good processability, for heat management applications. In this study, liquid crystal polymer (LCP)-based nanocomposites have shown to possess much higher effective thermal conductivity (keff) (i.e., as high as 2.58 W/m K) than neat polymers (i.e., ∼0.2–0.4 W/m K). The fibrillation of LCP in LCP-graphene nanoplatelet (GNP) nanocomposites also demonstrated more pronounced increase in keff than that of polyphenylene sulfide (PPS)-GNP nanocomposites. Furthermore, ultra-drawing of LCP-GNP nanocomposite led to additional increase in the nanocomposites keff because of the alignments of LCP fibrils and the embedded GNP. Experimental results also revealed that, unlike keff, the electrical conductivity (σ) o...

Fabrication and characterization of ceramic-filled thermoplastics composites with enhanced multifunctional properties

Development of novel injection moldable materials that are thermally conductive but electrically insulative are important for the continuous advancement in modern electronics. In this context, this article details the fabrication and characterization of polymer–matrix composites (PMC), which consists of linear low-density polyethylene matrix, and filled with either silicon carbide or hexagonal boron nitride. Experimental results indicated that the addition of ceramic fillers not only promoted the PMCs’ effective thermal conductivity without compromising their electrical resistivity but also resulted in the reduction of coefficient of thermal expansion and improved mechanical properties.

Modelling of effective thermal conductivity of polymer matrix composite foams with biaxially aligned filler networks

Recent research revealed potentials to develop polymer matrix composite foams filled with thermally conductive filler network as light-weight thermal management materials. Since polymeric foams are commonly used for thermal insulation, the concept of thermally conductive polymer matrix composite foams seems to be counter-intuitive, and the underlying factors that govern polymer matrix composite foam’s effective thermal conductivity (keff) were not clear. In this context, this paper develops new models to predict polymer matrix composite foams’ keff and to elucidate the dependence of keff on their cellular morphology. Linear low density polyethylene–hexagonal boron nitride composite foams were used as case examples to verify the model. The model demonstrated that the composite foam’s keff would be promoted when the volume expansion was over a threshold percentage. At low hexagonal boron nitride loadings (e.g. 10 vol.%) and fixed cell size, linear low density polyethylene–hexagonal boron nitride foams’ keff increased with volume expansion percent through an increase in cell population density. Constrained foaming with preferential expansion in the heat flow direction also enhanced their keff.

Development of novel multifunctional biobased polymer composites with tailored conductive network of micro-and-nano-fillers

Biobased/green polymers and nanotechnology warrant a multidisciplinary approach to promote the development of the next generation of materials, products, and processes that are environmentally sustainable. The scientific challenge is to find the suitable applications, and thereby to create the demand for large scale production of biobased/green polymers that would foster sustainable development of these eco-friendly materials in contrast to their petroleum/fossil fuel derived counterparts. In this context, this research aims to investigate the synergistic effect of green materials and nanotechnology to develop a new family of multifunctional biobased polymer composites with promoted thermal conductivity. For instance, such composite can be used as a heat management material in the electronics industry. A series of parametric studies were conducted to elucidate the science behind materials behavior and their structure-toproperty relationships. Using biobased polymers (e.g., polylactic acid (PLA)) as the matrix, heat transfer networks were developed and structured by embedding hexagonal boron nitride (hBN) and graphene nanoplatelets (GNP) in the PLA matrix. The use of hybrid filler system, with optimized material formulation, was found to promote the composite’s effective thermal conductivity by 10-folded over neat PLA. This was achieved by promoting the development of an interconnected thermally conductive network through structuring hybrid fillers. The thermally conductive composite is expected to afford unique opportunities to injection mold three-dimensional, net-shape, lightweight, and eco-friendly microelectronic enclosures with superior heat dissipation performance.

Fabrication of electroactive poly(vinylidene fluoride) through non-isothermal crystallization and supercritical CO2 processing

Poly(vinylidene fluoride) (PVDF) with enhanced β phase content has conventionally been manufactured by mechanical stretching and electric poling of PVDF films. More recently, scientific research has been conducted to investigate the use of electrospinning to promote the piezoelectric properties of PVDF. In this work, we developed a novel processing strategy using a combination of non-isothermal crystallization and supercritical carbon dioxide (ScCO2) processing to promote the electroactive (i.e., β and γ) phase content of PVDF. Differential scanning calorimetric, X-ray diffraction, and infrared spectroscopy results revealed that the preferential formation of β and γ crystal phases were induced by the ScCO2 foaming step and the non-isothermal crystallization step, respectively. The sequential crystallization of these electroactive phases were coupled by their relationships with the ScCO2-induced foam expansion. Furthermore, both ScCO2s plasticization effect and annealing helped the promotion of PVDFs degree of crystallinity. Overall, this work offers a new industrially viable methodology to tailor PVDFs crystalline structures. This represents a useful technique to fabricate piezoelectric PVDF film for sensors, actuators, and energy harvesting applications.

Preparation and characterization of 100% bio-based polylactic acid/palmitic acid microcapsules for thermal energy storage

Phase change materials (PCM) have gained extensive attention in thermal energy storage applications. In this work, microencapsulation of vegetable-derived palmitic acid (PA) in bio-based polylactic acid (PLA) shell by solvent evaporation and oil-in-water emulsification was investigated. Fourier transform infrared spectroscopy and scanning electron microscopy were conducted to confirm the successful encapsulation of PA in PLA shells. Differential scanning calorimetry was performed to evaluate the thermal properties, thermal reliability, and core content of the fabricated PCM microcapsules (microPCM). Through a series of parametric studies, the effects of PCM and solvent content, oil phase-to-aqueous phase ratio, as well as surfactant type and content on the morphology, particle size, and thermal properties of the PCM microcapsules were investigated. Experimental results showed that PVA was a superior emulsifier to SDS in the emulsion systems being studied. There also existed an optimal PVA concentration to reduce the average size of microPCM. When the PVA concentration was above this optimal level, the emulsifier molecules tend to form micelles among themselves. This led to the adhesion of tiny microspheres on the surface of microPCM as well as larger microPCM. In short, this work has demonstrated the possibility of using the solvent evaporation method to fabricate 100% bio-based PCM-polymer microcapsules for thermal energy storage applications.

Applications of multifunctional polymer-matrix composites in hybrid heat sinks

Designers of electronic devices and telecommunications equipment have used three-dimensional chip architecture, comprised of a vertically integrated stack of chips, to increase the number of transistors on integrated circuits. These latest chips generate excessive amount of heat, and thus can reach unacceptably high temperatures. In this context, this research aims to develop thermally conductive liquid crystal polymer (LCP)/hexagonal boron nitride (hBN) composite films to replace the traditionally-used Kapton films that satisfy the electrical insulation requirements for the attachment of heat sinks to the chips without compromising the heat dissipation performance. Parametric study was conducted to elucidate the effects of hBN contents on the heat dissipation ability of the composite. The performance of the hybrid heat sinks were experimentally simulated by measuring the temperature distribution of the hybrid heat sinks attached to a 10 W square-faced (i.e., 10 cm by 10 cm) heater. Experimental simulation show that the maximum temperature of the heater mounted with a hybrid heat sink reduced with increased hBN content. It is believed the fibrillation of LCP matrix leads to highly ordered structure, promoting heat dissipation ability of the electrically insulating pad of the hybrid heat sink.

Multi-stage crystallization mechanism of electroactive phase polyvinylidene fluoride induced by thermal and supercritical carbon dioxide processing

Polyvinylidene fluoride (PVDF) is garnering interest in many applications ranging from sensors to high-frequency transducers to energy harvesters due to its piezoelectric properties. It is also an inexpensive, conformable, and more environmentally friendly alternative to polycrystalline ferroelectric ceramic lead zirconate titanate. This paper decouples the thermal and supercritical carbon dioxide (ScCO2) processing of PVDF proposed in our previous study into individual processing steps to elucidate the multi-stage crystallization mechanism of electroactive phase PVDF. Differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy were employed to analyze the processed PVDF samples. As a result, the processing-to-structure relationships with respect to each crystal polymorph (i.e., α, β and γ phases) in different phases during the multi-stage mechanism were successfully identified. The γ phase content in PVDF was predominantly governed by isothermal and non-isothermal crystallization before the ScCO2 injection as well as ScCO2-assisted isothermal crystallization before foam expansion. In contrast, the β phase content in PVDF was governed by the foaming behaviours of PVDF during the ScCO2 foaming phase. It was also observed that foam morphologies, including both cell population density and average cell size, governed the amount of local strain in PVDF matrices and thereby the α to β phase transformation during the foam expansion stage. The highest fraction of the PVDF electroactive phase achieved by the thermal and ScCO2 processing (i.e., 72.2%) was shown to be slightly better than that yielded by mechanically stretching the PVDF film (i.e., 68%).