Acoustic and Thermal Properties of Recycled Porous Media

This thesis is concerned with developing porous materials from tyre shred residue and polyurethane binder for acoustic absorption and thermal insulation applications. The resultant materials contains a high proportion of open, interconnected cells that are able to absorb incident sound waves through viscous friction, inertia effects and thermal energy exchanges. The materials developed are also able to insulate against heat by suppressing the convection of heat and reduced conductivity of the fluid locked in the large proportion of close-cell pores. The acoustic absorption performance of a porous media is controlled by the number of open cells and pore size distribution. Therefore, this work also investigates the use of catalysts and surfactants to modify the pore structure and studies the influence of the various components in the chemical formulations used to produce these porous materials. An optimum type and amounts of catalyst are selected to obtain a high chemical conversion and a short expanding time for the bubble growth phase. The surfactant is used to reduce the surface tension and achieve a homogenous mixing between the solid particulates tyre shred residue, the water, the catalyst and the binder. It is found that all of the components significantly affect the resultant materials structure and its morphology. The results show that the catalyst has a particularly strong effect on the pore structure and the ensuing thermal and acoustical properties. In this research, the properties of the porous materials developed are characterized using standard experimental techniques and the acoustic and thermal insulation performance underpinned using theoretical models. The important observation from this research is that a new class of recycled materials with pore stratification has been developed. It is shown that the pore stratification can have a positive effect on the acoustic absorption in a broadband frequency range. The control of reaction time in the foaming process is a key function that leads to a gradual change in the pore size distribution, porosity, flow resistivity and tortuosity which vary as a function of sample depth. It is shown that the Pade approximation is a suitable model to study the acoustic behaviour of these materials. A good agreement between the measured data and the model was attained.Ministry of Science and Technology of Thailand; Naresuan University, Phitsanulok, Thailand

The effect of continuous pore stratification on the acoustic absorption in open cell foams

This work reports new data on the acoustical properties of open cell foam with pore stratification. The pore size distribution as a function of the sample depth is determined in the laboratory using methods of optical image analysis. It is shown that the pore size distribution in this class of materials changes gradually with the depth. It is also shown that the observed pore size distribution gradient is responsible for the air flow resistivity stratification, which is measured acoustically and non-acoustically. The acoustical absorption coefficient of the developed porous sample is measured using a standard laboratory method. A suitable theoretical model for the acoustical properties of porous media with pore size distribution is adopted. The measured data for open porosity, tortuosity, and standard deviation data are used together with this model to predict the observed acoustic absorption behavior of the developed material sample. The transfer matrix approach is used in the modeling process to account for the pore size stratification. This work suggests that it is possible to design and manufacture porous media with continuous pore size stratification, which can provide an improvement to conventional porous media with uniform pore size distribution in terms of the attained acoustic absorption coefficient

Ternary blend nanofibres of poly(lactic acid), polycaprolactone and cellulose acetate butyrate for skin tissue scaffolds:influence of blend ratio and polycaprolactone molecular mass on miscibility, morphology, crystallinity and thermal properties

Ternary blends of poly(lactic acid) (PLA), polycaprolactone (PCL) and cellulose acetate butyrate (CAB) were fabricated into the form of electrospun nanofibres targeted for skin tissue scaffolds. The effects of blend ratio and molecular mass of PCL (PCL1 and PCL2) on morphology, miscibility, crystallinity, thermal properties, surface hydrophilicity and cell culture of the nanofibres were investigated. Blends with high PLA loading (80/10/10 PLA/PCL/CAB) gave fibres with a smooth surface, owing to the enhanced miscibility between the polymer chains from the presence of CAB, which acts as compatibilizer. In contrast, blends with high PCL loading were immiscible, which led to beads during the electrospinning process. The increased molecular mass of PCL2 produced smoother fibres than low-molecular-mass PCL1. The XRD patterns of blends of PLA/PCL1/CAB and PLA/PCL2/CAB were similar to one another, in which the high-crystallinity peaks of PCL seen for 20/70/10 blends were very small for 50/40/10 blends and much less prevalent for 80/10/10 blends. Better fibre formation (80/10/10>50/40/10>20/70/10) with less crystallinity occurs in well-formed fibres. Selected blends of PLA/PCL/CAB promoted growth of NIH/3T3 fibroblast cells, demonstrating that our novel biocompatible ternary blend nanofibrous scaffolds have potential in skin tissue repair applications. In addition, this work helps in the design and understanding of the factors that control the properties of nanofibrous PLA/PCL/CAB scaffolds

Multifunctional core‐shell electrospun nanofibrous fabrics of poly(vinyl alcohol)/silk sericin (core) and poly(lactide‐ co ‐glycolide) (shell)

Core–shell fibres (CSFs) offer a simple route to multifunctional hybrid materials for a wide range of applications. Herein, we report the design of a core–shell electrospun nanofibrous fabric containing a hydrophilic core and hydrophobic shell. CSFs were fabricated for the first time from poly(vinyl alcohol)/silk sericin (from silk cocoons) as the core and poly(lactide-co-glycolide) as the shell. The core serves as a potential carrier for water-soluble bioactive agents, and the shell works as a barrier to prevent premature release of water-soluble agents from the core. The effect of the molecular weight of poly(lactide-co-glycolide) and the loading of silk sericin on the morphology of fibres was studied. The parameters that significantly influence the core–shell electrospinning process were studied to elucidate the most effective conditions to create our multifunctional nanofibrous fabrics with smooth fibre morphology (diameters in the range 800–1300 nm) and low bead formation. Our CSFs were shown to degrade in saline buffer solution (pH 7.4) and were readily rendered with anti-bacterial properties against Staphylococcus aureus and Escherichia coli by the post-spinning deposition of silver nanoparticles (AgNPs, 40 nm diameter) or cinnamon essential oil (CEO). The fibres were non-toxic to normal human dermal fibroblast cell lines, as the cells were shown to attach and proliferate on CSFs, CSF/AgNPs and CSF/CEO with good cell tolerance for 72 h of incubation. These smart multifunctional CSFs show great potential towards smart delivery fabrics/dressings capable of carrying water-soluble bioactive agents surrounded by a protective, but degradable, antibacterial shell to guard the cargo for more effective controlled release

Green fabrication route of robust, biodegradable silk sericin and poly(vinyl alcohol) nanofibrous scaffolds

Silk sericin (SS) has been extensively used to fabricate scaffolds for tissue engineering. However, due to its inferior mechanical properties, it has been found to be a poor choice of material when being electrospun into nanofibrous scaffolds. Here, SS has been combined with poly(vinyl alcohol) (PVA) and electrospun to create scaffolds with enhanced physical properties. Crucially, these SS/PVA nanofibrous scaffolds were created using only distilled water as a solvent with no added crosslinker in an environmentally friendly process. Temperature has been shown to have a marked effect on the formation of the SS sol–gel transition and thus influence the final formation of fibers. Heating the spinning solutions to 70 °C delivered nanofibers with enhanced morphology, water stability and mechanical properties. This is due to the transition of SS from β-sheets into random coils that enables enhanced molecular interactions between SS and PVA. The most applicable SS/PVA weight ratios for the formation of nanofibers with the desired properties were found to be 7.5/1.5 and 10.0/1.5. The fibers had diameters ranging from 60 to 500 nm, where higher PVA and SS concentrations promoted larger diameters. The crystallinity within the fibers could be controlled by manipulation of the balance between PVA and SS loadings. In vitro degradation (in phosphate buffer solution, pH 7.4 at 37 °C) was 30–50% within 42 days and fibers were shown to be nontoxic to skin fibroblast cells. This work demonstrates a new green route for incorporating SS into nanofibrous fabrics, with potential use in biomedical applications

Bio-derived and biocompatible poly(lactic acid)/silk sericin nanogels and their incorporation within poly(lactide-co-glycolide) electrospun nanofibers

Bio-derived and biocompatible nanogels based on poly(lactic acid) (PLA) and silk sericin (SS) have been synthesized for the first time. Low molecular weight PLA and SS were first modified using allyl glycidyl ether to create a PLA macromonomer and an SS multifunctional crosslinker (PLAM and SSC, respectively), as confirmed by NMR and FTIR spectroscopies. Nanogels were synthesized from PLAM/SSC and N′,N-methylene bisacrylamide (N′,N-mBAAm) as an additional bifunctional crosslinker via classical free-radical polymerization at systematically varied levels of additional crosslinking (0, 0.5, 1.0, 1.5 and 2.0 w/w% N′,N-mBAAm). Higher crosslink densities led to smaller nanogel particles with reduced accumulative drug release. Crosslinked PLAM/SSC nanogels at 0.5% N′,N-mBAAm with 400–500 nm diameter particles were shown to be non-toxic to the normal human skin fibroblast cell line (NHSF) and selected for incorporation within poly(lactide-co-glycolide) (PLGA) electrospun nanofibers. These embedded nanogel-PLGA nanofibers were non-toxic to the NHSF cell line and exhibited higher cell proliferation than pure PLGA nanofibers, due to their higher hydrophilicity induced by the PLAM/SSC nanogels. This work shows that our new crosslinked-PLAM/SSC nanogels have potential for use not only in the field of drug delivery but also for tissue regeneration by embedding them within nanofibers to create hybrid scaffolds

Development of Natural Active Agent-Containing Porous Hydrogel Sheets with High Water Content for Wound Dressings

This work was concerned with the fabrication of a porous hydrogel system suitable for medium to heavy-exudating wounds where traditional hydrogels cannot be used. The hydrogels were based on 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPs). In order to produce the porous structure, additional components were added (acid, blowing agent, foam stabilizer). Manuka honey (MH) was also incorporated at concentrations of 1 and 10% w/w. The hydrogel samples were characterized for morphology via scanning electron microscopy, mechanical rheology, swelling using a gravimetric method, surface absorption, and cell cytotoxicity. The results confirmed the formation of porous hydrogels (PH) with pore sizes ranging from ~50–110 µm. The swelling performance showed that the non-porous hydrogel (NPH) swelled to ~2000%, while PH weight increased ~5000%. Additionally, the use of a surface absorption technique showed that the PH absorbed 10 μL in <3000 ms, and NPH absorbed <1 μL over the same time. Incorporating MH the enhanced gel appearance and mechanical properties, including smaller pores and linear swelling. In summary, the PH produced in this study had excellent swelling performance with rapid absorption of surface liquid. Therefore, these materials have the potential to expand the applicability of hydrogels to a range of wound types, as they can both donate and absorb fluid

Water-soluble macromers based on 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (Na-AMPS) for rapid in situ hydrogel film formation

The in situ formation of hydrogels has potential for a number of biomedical applications but their generation via conventional polymerization techniques has a number of limitations, such as toxicity and reaction time. The use of macromers in hydrogel formulations can help overcome these limitations. In this work, we synthesized a new functionalized macromer formed via the copolymerization of 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) and acid-functional monomers that can undergo a ring-opening reaction with allyl glycidyl ether (AGE) to generate the desired pendant vinyl macromer functionality. These macromers were characterized by 1H nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and gel permeation chromatography (GPC) to provide evidence for successful macromer synthesis and subsequent polymerization. Using a UV-initiated crosslinking approach with poly(ethylene glycol) diacrylate (PEGDA), the hydrogels were fabricated from the macromer solution, with the gelation time being reduced from 1200 s to 10 s when compared to hydrogel formation from regular vinyl monomers. While different acidic monomers result in distinct tensile properties, hydrogels containing 2-carboxyethyl acrylate (CEA) exhibit low strength but high elongation. In contrast, those with methacrylic acid (MAA) demonstrate higher strength and lower elongation. Therefore, using a balanced combination of each is a logical approach for achieving a robust final hydrogel film. In summary, we have produced a new macromer possessing characteristics highly conducive to rapid hydrogel synthesis. This macromer approach holds potential for use in in situ hydrogel formation, where a viscous solution can be applied to the target area and subsequently hardened to its hydrogel. We envisage its application primarily in the biomedical field

In Situ Compatibilized Blends of PLA/PCL/CAB Melt-Blown Films with High Elongation: Investigation of Miscibility, Morphology, Crystallinity and Modelling

Ternary-blended, melt-blown films of polylactide (PLA), polycaprolactone (PCL) and cellulose acetate butyrate (CAB) were prepared from preliminary miscibility data using a rapid screening method and optical ternary phase diagram (presented as clear, translucent, and opaque regions) as a guide for the composition selection. The compositions that provided optically clear regions were selected for melt blending. The ternary (PLA/PCL/CAB) blends were first melt-extruded and then melt-blown to form films and characterized for their tensile properties, tensile fractured-surface morphology, miscibility, crystallinity, molecular weight and chemical structure. The results showed that the tensile elongation at the break (%elongation) of the ternary-blended, melt-blown films (85/5/10, 75/10/15, 60/15/25 of PLA/PCL/CAB) was substantially higher (>350%) than pure PLA (ca. 20%). The range of compositions in which a significant increase in %elongation was observed at 55–85% w/w PLA, 5–20% w/w PCL and 10–25% w/w CAB. Films with high %elongation all showed good interfacial interactions between the dispersed phase (PCL and CAB) and matrix (PLA) in FE-SEM and showed improvements in miscibility (higher intermolecular interaction and mixing) and a decrease in the glass transition temperature, when compared to the low %elongation films. The decrease in Mw and Mn and the formation of the new NMR peaks (1H NMR at 3.68–3.73 ppm and 13C NMR at 58.54 ppm) were observed in only the high %elongation films. These are expected to be in situ compatibilizers that are generated during the melt processing, mostly by chain scission. In addition, mathematical modelling was used to study the optimal ratio and cost-effectiveness of blends with optimised mechanical properties. These ternary-blended, melt-blown films have the potential for use in both packaging and medical devices with excellent mechanical performance as well as inherent economic and environmental capabilities