Surfactant in a Polyol-CO2 Mixture: Insights from a Classical Density Functional Theory Study

Silicone-polyether (SPE) surfactants, made of a polydimethyl-siloxane (PDMS) backbone and polyether branches, are commonly used as additives in the production of polymeric foams with improved properties. A key step in the production of polymeric foams is the nucleation of gas bubbles in the polymer matrix upon supersaturation of dissolved gas. However, the role of SPE surfactants in the nucleation of gas bubbles is not well understood. In this study, we use classical density functional theory to investigate the effect of an SPE surfactant on the nucleation of CO₂ bubbles in a polyol foam formulation. We find that the addition of an SPE surfactant leads to a ∼3-fold decrease in the polyol-CO₂ interfacial tension at the surfactant's critical micelle concentration. Additionally, the surfactant is found to reduce the free energy barrier and affect the minimum free energy pathway (MFEP) associated with CO₂ bubble nucleation. In the absence of a surfactant, a CO₂-rich bubble nucleates from a homogeneous CO₂-supersaturated polyol solution by following an MFEP characterized by a single nucleation barrier. Adding a surfactant results in a two-step nucleation process with reduced free energy barriers. The first barrier corresponds to the formation of a spherical aggregate with a liquid-like CO₂ core. This spherical aggregate then grows into a CO₂-rich bubble (spherical aggregate with a vapor-like CO₂ core) of a critical size representing the second barrier. We hypothesize that the stronger affinity of CO₂ for PDMS (than polyether) stabilizes the spherical aggregate with the liquid-like CO₂ core, leading to a lower free energy barrier for CO₂ bubble nucleation. Stabilization of such an aggregate during the early stages of the nucleation may lead to foams with more, smaller bubbles, which can improve their microstrustural features and insulating abilities.

Rational Design of a Polyurethane Foam

Polyurethane (PU) foams are exceptionally versatile due to the nature of PU bond formation and the large variety of polymeric backbones and formulation components such as catalysts and surfactants. This versatility introduces a challenge, namely a near unlimited number of variables for formulating foams. In addition to this, PU foam development requires expert knowledge, not only in polyurethane chemistry but also in the art of evaluating the resulting foams. In this work, we demonstrate that a rational experimental design framework in conjunction with a design of experiments (DoE) approach reduces both the number of experiments required to understand the formulation space and reduces the need for tacit knowledge from a PU expert. We focus on an in-depth example where a catalyst and two surfactants of a known formulation are set as factors and foam physical properties are set as responses. An iterative DoE approach is used to generate a set of foams with substantially different cell morphology and hydrodynamic behaviour. We demonstrate that with 23 screening formulations and 16 final formulations, foam physical properties can be modelled from catalyst and surfactant loadings. This approach also allows for the exploration of relationships between the cell morphology of PU foam and its hydrodynamic behaviour.

A New Methodology Based on Cell-Wall Hole Analysis for the Structure-Acoustic Absorption Correlation on Polyurethane Foams

Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (S_h), the number of holes (h), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.

The Overview of Porous, Bioactive Scaffolds as Instructive Biomaterials for Tissue Regeneration and Their Clinical Translation

Porous scaffolds have been employed for decades in the biomedical field where researchers have been seeking to produce an environment which could approach one of the extracellular matrixes supporting cells in natural tissues. Such three-dimensional systems offer many degrees of freedom to modulate cell activity, ranging from the chemistry of the structure and the architectural properties such as the porosity, the pore, and interconnection size. All these features can be exploited synergistically to tailor the cell-material interactions, and further, the tissue growth within the voids of the scaffold. Herein, an overview of the materials employed to generate porous scaffolds as well as the various techniques that are used to process them is supplied. Furthermore, scaffold parameters which modulate cell behavior are identified under distinct aspects: the architecture of inert scaffolds (i.e., pore and interconnection size, porosity, mechanical properties, etc.) alone on cell functions followed by comparison with bioactive scaffolds to grasp the most relevant features driving tissue regeneration. Finally, in vivo outcomes are highlighted comparing the accordance between in vitro and in vivo results in order to tackle the future translational challenges in tissue repair and regeneration.

Validation of Milner's visco-elastic theory of sintering for the generation of porous polymers with finely tuned morphology

Sacrificial sphere templating has become a method of choice to generate macro-porous materials with well-defined, interconnected pores. For this purpose, the interstices of a sphere packing are filled with a solidifying matrix, from which the spheres are subsequently removed to obtain interconnected voids. In order to control the size of the interconnections, viscous sintering of the initial sphere template has proven a reliable approach. To predict how the interconnections evolve with different sintering parameters, such as time or temperature, Frenkel's model has been used with reasonable success over the last 70 years. However, numerous investigations have shown that the often complex flow behaviour of the spheres needs to be taken into account. To this end, S. Milner [arXiv:1907.05862] developed recently a theoretical model which improves on some key assumptions made in Frenkel's model, leading to a slightly different scaling. He also extended this new model to take into account the visco-elastic response of the spheres. Using an in-depth investigation of templates of paraffin spheres, we provide here the first systematic comparison with Milner's theory. Firstly, we show that his new scaling describes the experimental data slightly better than Frenkel's scaling. We then show that the visco-elastic version of his model provides a significantly improved description of the data over a wide parameter range. We finally use the obtained sphere templates to produce macro-porous polyurethanes with finely controlled pore and interconnection sizes. The general applicability of Milner's theory makes it transferable to a wide range of formulations, provided the flow properties of the sphere material can be quantified. It therefore provides a powerful tool to guide the creation of sphere packings and porous materials with finely controlled morphologies.

Shape memory polyurethane-urea foams with improved toughness

Current vascular aneurysm treatments often require either highly invasive strategy to surgically occlude an aneurysm or endovascular occlusion via metal coils. While endovascular coils are safer, they have limited efficacy. Endovascular coils that are integrated with shape memory polymer (SMP) foams have the potential to improve occlusion and reduce coil risks; however, the mechanical performance and limited homogeneity of SMP foams can hinder their effective use. To address this issue, SMP foams are synthesized using the monomer diethanolamine (DEA) in place of triethanolamine (TEA) to provide improved mechanical properties for medical device applications. Mechanical testing and micro-fracture analysis were performed on DEA and TEA foams. DEA foams show improved toughness and reduced micro-fractures compared to the control. This work presents the utility of DEA in SMP synthesis to enable the potential production of safer aneurysm treatment.

Effects of Isosorbide Incorporation into Flexible Polyurethane Foams: Reversible Urethane Linkages and Antioxidant Activity

Isosorbide (ISB), a nontoxic bio-based bicyclic diol composed from two fuzed furans, was incorporated into the preparation of flexible polyurethane foams (FPUFs) for use as a cell opener and to impart antioxidant properties to the resulting foam. A novel method for cell opening was designed based on the anticipated reversibility of the urethane linkages formed by ISB with isocyanate. FPUFs containing various amounts of ISB (up to 5 wt%) were successfully prepared without any noticeable deterioration in the appearance and physical properties of the resulting foams. The air permeability of these resulting FPUFs was increased and this could be further improved by thermal treatment at 160 °C. The urethane units based on ISB enabled cell window opening, as anticipated, through the reversible urethane linkage. The ISB-containing FPUFs also demonstrated better antioxidant activity by impeding discoloration. Thus, ISB, a nontoxic, bio-based diol, can be a valuable raw material (or additive) for eco-friendly FPUFs without seriously compromising the physical properties of these FPUFs.

Deducing Multiple Interfacial Dynamics during Polymeric Foaming

Several interfacial phenomena are active during polymeric foaming, the dynamics of which significantly influence terminal stability, cell structure, and in turn the thermomechanical properties of temporally evolved foam. Understanding these dynamics is important in achieving desired foam properties. Here, we introduce a method to simultaneously portray the time evolution of bubble growth, lamella thinning, and plateau border drainage, occurring during reactive polymeric foaming. In this method, we initially conduct bulk and surface shear rheology under polymerizing and nonfoaming conditions. In a subsequent step, foaming experiments were conducted in a rheometer. The microscopic structural dimensions pertaining to the terminal values of the dynamics of each interfacial phenomena are then measured using a combination of scanning electron microscopy, optical microscopy, and imaging ellipsometry, after the foaming is over. The measured surface and bulk rheological parameters are incorporated in time evolution equations that are derived from mass and momentum transport occurring when a model viscoelastic fluid is foamed by gas dispersion. Analytical and numerical solutions to these equations portray the dynamics. We demonstrate this method for a series of reactive polyurethane foams generated from different chemical sources. The effectiveness of our method is in simultaneously obtaining these dynamics that are difficult to directly monitor because of short active durations over multiple length scales.

Liquid foam templating - A route to tailor-made polymer foams

Solid foams with pore sizes between a few micrometres and a few millimetres are heavily exploited in a wide range of established and emerging applications. While the optimisation of foam applications requires a fine control over their structural properties (pore size distribution, pore opening, foam density, …), the great complexity of most foaming processes still defies a sound scientific understanding and therefore explicit control and prediction of these parameters. We therefore need to improve our understanding of existing processes and also develop new fabrication routes which we understand and which we can exploit to tailor-make new porous materials. One of these new routes is liquid templating in general and liquid foam templating in particular, to which this review article is dedicated. While all solid foams are generated from an initially liquid(-like) state, the particular notion of liquid foam templating implies the specific condition that the liquid foam has time to find its "equilibrium structure" before it is solidified. In other words, the characteristic time scales of the liquid foam's stability and its solidification are well separated, allowing to build on the vast know-how on liquid foams established over the last 20 years. The dispersed phase of the liquid foam determines the final pore size and pore size distribution, while the continuous phase contains the precursors of the desired porous scaffold. We review here the three key challenges which need to be addressed by this approach: (1) the control of the structure of the liquid template, (2) the matching of the time scales between the stability of the liquid template and solidification, and (3) the preservation of the structure of the template throughout the process. Focusing on the field of polymer foams, this review gives an overview of recent research on the properties of liquid foam templates and summarises a key set of studies in the emerging field of liquid foam templating. It finishes with an outlook on future developments. Occasional references to non-polymeric foams are given if the analogy provides specific insight into a physical phenomenon.

Renewable polyols for advanced polyurethane foams from diverse biomass resources

This review highlights recent advances in the synthesis of renewable polyols, used for making polyurethane foams, from biomass.

Skin Development in Free Rise, Flexible Polyurethane Foam

The behavior of top skin layer in reacting flexible polyurethane foam was studied to interpret its role in determining the properties of the foam. Temperature profile near the top skin rises much slower than that at the center of the foam bun. Parallel plate rheometer and infrared spectroscopy showed that the material near the top skin is still liquid after the onset of cell opening at the center of the bun, due to the slow temperature increase. The cells near the top skin are supposed to still be closed at visual blow-off.

The visual blow-off time changed with different silicone surfactant structure used in the formulations, although the onset of cell opening time did not change. This indicates that surfactants are playing some role to prevent CO2 gas from coming out by breaking the top skin and cell layers under it. Cell opening was shown to be continuously occurring between onset of cell opening and visual blow-off time. Delaying the visual blow-off was efficient to get higher foam bun height and could affect air flow rate. However, the visual blow-off time alone cannot account for the difference in the air flow of final foam products.

A New Cell Opening Mechanism in Flexible Polyurethane Foam

Literature on the polyurethane foam cell opening mechanism is reviewed and discussed. A new cell opening mechanism is proposed. Cell opening is observed to be triggered by urea precipitation which changes the rheology of the polyurethane dramatically. The foam matrix shows extensional thinning behavior after urea phase separation. The extensional thinning behavior will lead to localized cell window thinning.

The cell window thickness distribution will lead to different film rupture times for individual cell windows and will result in cell windows in different states. Some windows are totally missing and others leave torn-off films. Windows with only pin holes or totally closed were also observed. The cell opening event is shown to be a continuous process rather than a sudden event.

Influence of Cell Opener in High Resilience Molded Polyurethane Foam

In a chemical method for cell opening of high resilience (HR) molded polyurethane (PU) foam, two types of cell openers, the poly(ester-ether) type, PESE, having ester linkage and hydrophilic polyether segment in the molecule and the conventional polyether type, poly(propylene oxide-ethylene oxide), PPEO, were employed. The influence of structure and concentration of cell opener on the kinetics, stability, open cell content and mechanical properties of molded foam were investigated.

The PESE-based formulation exhibited delayed reaction times and good stability compared to the PPEO-based formulation. The open cell content increased with cell opener amount and was independent of the cell opener structure. The mechanical properties clearly depend on the structure of cell opener. The PESE-based formulation had improved mechanical properties such as indentation hardness, tensile strength and tear strength. The increase of PPEO concentration significantly deteriorated the molded foam’s mechanical properties, while the foam with PESE gave less deterioration of mechanical properties up to a specific concentration of the PESE. With the above observations, PESE was found to be a useful cell opener for preparation of HR molded PU foam which has acceptable mechanical properties.

Some Aspects of Matrix Formation of Flexible Polyurethane Foams

The reaction of liquid raw materials like isocyanates, polyols, and water, to build a structural polymer network is a very complex sequence of different chemical reactions.

We can describe some of these essential processes by the use of a dynamic measurement of the viscosity during the course of the foaming reaction, on-line FTIR-spectrometry measurements and the determination of the curing behaviour.

We employed a laboratory FT-IR system, that was interfaced to an optical fibre loop. The formation of the soluble and associated urea species, the decay of isocyanate groups and the development of urethane species during the polymerization reaction were studied by FT-IR. The onset of the formation of associated urea species is influenced by the chemical composition of the reacting mixture.

For the viscosity measurements we used a rotational viscosimeter. The network development was shown by the determination of the viscosity profile.

By extrapolation of the course of the indentation force over time, estimated after the demould of the flexible foam, we determined the green strength of the moulding and the vitrification of the foam at a frozen morphology.

We also illustrated the dependence of the mechanical properties of flexible PU foams on the chemical structures formed during the reaction.

Effects of chain extender in biodegradable polyurethane foams

Molded flexible polyurethane (PU) foams were synthesized from a starch/petroleum based polyol, 2,4/2,6-toluene diisocyanate (TDI-80), using a one shot method with water as the blowing agent. The effects of chain extender type [starch (S series), diethanolamine (DEA, D series), glycerol (G series)] and content (0, 2, 5, 10 pphr) were extensively studied. The rate of foam formation, density, compression strength, glass transition temperature (Tg) and rubbery modulus of the foam increased with the addition and increasing content of extender. At the same extender content, DEA showed the highest of these properties, while starch showed the lowest. The rate of biodegradation in a buffer solution decreased with the addition of DEA and glycerol, due to the increased crosslinking density and hard segment content, but increased with starch, owing to its biodegradability.

Polyurethane molded foams with high content of hyperbranched polyols from soybean oil

This paper examines the feasibility of using polyols from vegetable oils as base polyols (i.e. with 50% or more in a blend with petrochemical polyols) for flexible molded polyurethane foams. A series of hyperbranched (HB) polyols were synthesized by transesterification of hydroxy fatty acid methyl esters and different modifiers to control viscosity, hydrophilicity, molecular weight, and functionality. All HB polyols had hydroxyl numbers around 85 mg KOH/g, with the exception of one which was 105 mg KOH/g. When mixed with petrochemical polyols with OH numbers 35 and 28 mg KOH/g, the HB polyols acted primarily as high molecular weight crosslinkers that increased the stiffness of the polymeric network and the load-bearing properties but decreased the tensile strength, elongation, and tear strength. However, most of the foams met the targeted tensile and tear strength values while some of the foam formulations provided satisfactory elongation. The best mechanical properties were obtained from foams with phthalic anhydride-modified HB polyols. It was demonstrated that flexible molded foams with satisfactory properties can be obtained with 50% and 65% of HB soy polyols in a blend with PPO polyols.

Probing nanodispersions of clays for reactive foaming

Nanodispersions of clays in polyurethane components have been prepared. Nanoclays (both natural and organically modified) of various aspect ratios are used. The fillers are dispersed separately in polyurethane components, viz., polyol and polyisocyanate. The nanodispersions are characterized by the combined use of solution rheology, X-ray scattering, cryo-electron microscopy, and IR spectroscopy. Reactive foaming of these nanodispersions is carried out to make polyurethane nanocomposite foams. The status of the dispersion of fillers in components and in foams has been compared to investigate the effect of the foaming process in exfoliation. Interpretation of the results from different characterization techniques describes the state of the dispersion of fillers in components and in foam. The rheological and physicochemical behaviors of nanodispersions are shown to have a significant influence on the properties of nanocomposite foams.

A Simple Transient Method for Measurement of Thermal Conductivity of Rigid Polyurethane Foams

Rigid polyurethane foams (PU) are widely used as thermal insulators in various applications. The thermal conductivity of the foam is the key parameter that governs the efficiency of thermal insulation provided by the foam. The usual technique employed to measure thermal conductivity is based on the rate of steady state heat transfer across a known thickness, induced by two different known temperatures at two opposite surfaces of the foam. We introduce a technique based on the transient measurement of heat transfer measured by an embedded needle probe. This technique is not only rapid but the instrumentation required for such a measurement is simple and the cost is only a fraction of the steady state counterpart. The values of thermal conductivity obtained by both methods are compared and found to agree within 4% over the range of 0.02—0.03 W/mK, which is the usual range of thermal conductivity for commercial rigid PU foams. The sensitivity of the needle probe technique is demonstrated by measuring the thermal conductivity values of foams made with various concentrations of chemical blowing agent (water). The present technique is also shown to be effective for measuring the thermal conductivity of small samples, especially, free rise cup foams for which the steady state technique can not be used.

Role of Silicone Surfactant in Flexible Polyurethane Foam

Grafted copolymers which consist of a polydimethylsiloxane backbone and polyethylene oxide-co-propylene oxide pendant groups are used as surfactants to stabilize the foam cells in the flexible polyurethane foaming process. The mechanical properties of the cured polyurethane foam such as air permeability and foam cell size are affected significantly by the structure of the silicone surfactant used in the formulation. It is shown that silicone surfactant has an important impact on both the bubble generation and the cell window stabilization stage. A series of silicone surfactants with different structures was tested. Surfactants with higher silicone content will provide lower surface tension and thus help increase the number of air bubbles introduced during mixing. These air bubbles serve as the starting point for foam cell growth. As a result, the cured polyurethane foam made with higher silicone content surfactant has a smaller bubble size. It is also shown that silicone surfactant can reduce the cell window drainage rate due to the surface tension gradient along the cell window. The Gibbs film elasticity, the dynamic film elasticity, and the film drainage rate were measured for the first time versus surfactant composition. Surfactants with longer siloxane backbones are shown to give higher film elasticity. Using the vertical film drainage and foam column tests, it is shown that surfactants with higher film elasticity will yield slower drainage rate and better foam cell stability. Copyright 1999 Academic Press.