Introduction with significantly different properties [39,40]. This versatility has led to the use of polyurethanes as foams, elastomers, coatings, seal- The inclusion of carbon nanotubes CNTs in polymer matrices ants, and adhesive-based products. However, the main market for has already been shown to improve their mechanical, electrical PU is in polymeric foams, which can be flexible or rigid. While rigid and thermal properties [1,2]. The reinforcement of delicate sys- PU foams are mainly used for thermal insulation, the flexible vari- tems where conventional fibres cannot be physically accommo- eties are used as cushioning materials in furnishings, transporta- dated, such as polymer films [3—11], fibres [6,12—15] and foams tion and packaging applications.
These foams offer degrees of [11,16—37], offers unique opportunities. However, most studies comfort, protection, and utility not matched by any other single on nano-filled foams have focused on nanoclays [11, cushioning material. By selecting the reactants and manufacturing 16—19,21,25—27,29,32,36,37], whereas the properties of carbon process appropriately, PU foam can be made to satisfy a wide- nanofibre CNF or CNT-reinforced polymeric foams have received range of applications see  for a full list.
These existing studies Free-rise polyurethane foams are generally obtained from the have, nevertheless, highlighted the potential benefits on both the simultaneous reaction of a polyisocyanate with a polyol matrix foaming process and mechanical behaviour of the final nanocom- and water. The reaction of the polyol with isocyanate produces posite foams. Meanwhile, the reaction of the isocyanate with Polymeric foams are important and versatile materials due to water generates CO2 which drives the foam expansion.
The poly- their outstanding strength-to-weight ratio, their resilience, and mer structure must build up rapidly to support the fragile foam, their electrical, thermal, and acoustic insulating properties, i. Polyurethane is one of the larg- ble growth. These two competing reactions are balanced by the est and most versatile families of polymers.
The control of param- addition of catalysts and surfactants. E-mail address: m. Verdejo et al. The image analysis was performed thane foam has been developed previously. In this study, we with a Matlab algorithm based on porosimetry calculations report the fabrication and properties of CNT-filled reactive PU [43—45]; the mean cell size was obtained by fitting the cell size PUR foams.
Small scale acoustic testing was performed according to ASTM 2. Materials and sample preparation frequency analyser. The acoustic absorption coefficient a is defined as the ratio of the acoustic energy absorbed by the foam Both isocyanate and polyol used in this study were supplied by Iincident Ireflected to the acoustic energy incident Iincident on the Bayer Group. The isocyanate was a modified diphenylmethane- surface and is dependent on frequency.
The catalyst used is a tertiary amine Dabco from to Hz. Due to shortage of materials, the samples LV and the surfactant is a silicone glycol Dabco DC from with 0. AirProducts and Chemicals. The samples der an inert atmosphere Ar. The diameters of as-produced CNTs were Tensile vary from 40 to 60 nm, with a length of around lm.
Oxidised CNT powder was stored in a sealed for mechanical testing. The length of the final oxCNTs is reduced, during the oxidative and shear pro- cesses, to a few microns. Results and discussion Prior to the synthesis of PU nanocomposite foams, the polyol was dehydrated under vacuum at room temperature. Both as-produced and oxidised CNTs were first mixed with the polyol at rpm for 10 min using an over- introduced initially. Subsequently, in the system, but some agglomerates were visible both during the surfactant, catalyst and distilled water Table 1 were added the mixing stage and in the final foam.
Final- dised CNTs readily yielded a better dispersion with no visible ly, the isocyanate was added and stirred for 15 s before foaming agglomerates. The different outcome can be attributed to both occurred in an open cylindrical mould. Since processing , and the increased concentration of OH and car- OxCNTs absorb moisture very rapidly, a control foaming experi- boxylic groups on the CNT surface ; these functionalities are ment was performed to investigate possible effects of chemisorbed likely to react with the isocyanate to form some direct covalent water on the reaction.
A sample with a 0. The addition of CNTs into the polyol rapidly increased the vis- 2. Sample characterisation cosity, reaching a tar-like consistency at 0. Due to this vis- cosity increase, the system no longer foamed properly at loading The samples were studied using a JEOL JSM scanning elec- fractions above 0.
The sam- controlling the stabilisation of the expanding structure . At- ples were taken around the middle of the mould.
Cross-sections tempts to increase the loading fraction by dispersing the CNTs in of the foams were fractured under liquid nitrogen both parallel ethanol or chloroform were not successful. Even the lower loadings and perpendicular to the foaming direction.
The cellular structure of CNTs affected the rising step of the foaming process; although the final volume of the foam was restricted, the rising time was ex- tended compared to that of the pure system. This lower foaming rate affects the cellular structure by enabling the diffusion of gas Table 1 from small cells to larger cells driven by the free energy of the sys- Percentage by weight of the reactants tem .
It is likely that increased viscosity, MDI Optical examination Fig. Photographs showing the texture and increasing darkening of the PU foam samples. Low magnification SEM images show an anisotropic structure initially increases on the addition of CNTs, but subsequently with cells elongated parallel to the foaming direction Fig. Increased cell sizes can, in general, be favoured by which is consistent with the foaming process. Meanwhile, the increased gas diffusion ; one hypothesis is that diffusion is cellular structure perpendicular to the foaming direction Fig.
Close inspection of the polymeric matrix reveals and increased free volume in the polymer. However, since the a good dispersion of CNTs throughout the sample, in both the nanotube dispersion is good and the chemistry suggests direct walls, and particularly the struts of the cellular structure Fig. Image analysis Fig. Given the Fig. This behav- for many fluids [38,48—50], including polymers, and the known iour is characteristic of open-cell foams.
On the addition of CNTs, influence of CNTs on viscosity , a depression of the pore nucle- the acoustic absorption of the system improves over the entire fre- ation rate seems likely.
The decrease in cell size at higher CNT load- quency range. The lowest CNT loadings have relatively little effect, ing can also be attributed to the higher viscosity, which will limit but even 0.
This improvement cannot be attributed ought to lead to smaller cell sizes, even at low loading fractions. However, the observed shift of The densities of the nanocomposite foam samples Fig.
Given the low concen- to the increase in density. The acoustic activity Fig. The vacuum-dried sample 0. Sound waves are characterisitc to the standard control, implying that the effect of absorbed by two main mechanisms : conversion to mechanical any adsorbed water is minimal. The presence of the CNTs within polymers is known to tion coefficient of the samples, as a function of frequency. The sam- have a strong influence on mechanical energy dissipation, with sig- ples show a clear absorption peak over the range of — Hz, nificant increases in loss modulus observed above the glass transi- where human sensitivity to noise is high .
At higher frequen- tion temperature in dynamic mechanical experiments . The damping effects are attributed to the large surface area at the poly- mer-CNT interface, where energy can be dissipated by interfacial sliding and stick-slip behaviour , even in the presence of some covalent bonds.
The compressive response of the foams did not show an improvement with CNT content. The apparent negative impact on the compressive mechanical properties could be due to the CNTs or the changes in foam microstructure.
The result was an opaque film 1 mm thick with no cellular structure. In mechan- ical tests under tension, these samples showed a monotonic improvement in strength and stiffness on adding CNTs, consistent with the expected favourable interactions between oxidised CNTs and isocyanate. Thus, the fluctuating compression properties ob- served in the foamed samples can be attributed to the varying den- Fig.
PU foam density and standard error as a function of CNT content. Mechanical reinforcement of polymers using carbon nanotubes. Adv Mater ;18 6 — Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol ;61 13 — The present work demonstrates a straightforward route to the  Cadek M et al.
Reinforcement of polymers with carbon nanotubes: the role of production of CNT-filled flexible PU foams through a direct reac- nanotube surface area. Nano Lett ;4 2 —6. This system has potentially wide relevance as poly-  Coleman JN et al. Improving the mechanical properties of single-walled carbon urethanes have amongst the top six sale volumes of all polymers nanotube sheets by intercalation of polymeric adhesives.
Appl Phys Lett ;82 11 —4. Here, an initial study has shown that even very low  Lahiff E et al. Selective positioning and density control of nanotubes within a CNT loading fractions can deliver a significant increase in the polymer thin film.
Nano Lett ;3 10 —7. Effect of orientation on the modulus of SWNT films and fibers. Nano Lett ;3 5 — Layer-by-layer assembled composites from multiwall Further research is required to minimise the effect of the CNTs carbon nanotubes with different morphologies.
Nano Lett ;4 10 : on the foaming rate and hence the cellular structure. Possible strat- —9. Synthesis and characterization of thickness-aligned carbon egies could include modifying the mixing procedure, reducing the nanotube-polymer composite films. Chem Mater ;17 5 — Multiwalled carbon nanotube polymer tubes, or increasing the surfactant content used to stabilise the composites: synthesis and characterization of thin films.
J Appl Polym Sci ;84 14 —9. This observation supports the significance of a positive line tension and thus confirms that its contribution to the free energy of cell nucleation must be included in the models describing foaming. These results further underline the importance of obtaining an enhanced understanding of the interactions between highly curved particles with viscoelastic polymers when particle sizes are at the nanometer length scale.
Related knowledge would allow one to fully exploit the potential of nanoparticles as highly efficient nucleation agents in nanocellular foaming, as well as line tension effects in numerous other applications, such as in electronics, 42 sensors, 43 , 44 adhesives, 45 and templated porous materials.
Louis, MO. These particles were dispersed in aqueous solution and have surface-exposed silanol groups on the surface as received.
Ethanol absolute for analysis was obtained from Merck Darmstadt, Germany. Unless otherwise mentioned, all other chemicals were used as received. After 1.
Subsequently, the collected SiO2 was redispersed in ethanol and centrifuged again. This washing step was repeated 2 more times, followed by vacuum-drying the collected SiO2 NPs at room temperature for 12 h. The reaction was conducted for 1. The collecting, washing, and drying steps of these NPs were the same as those described for NPs of 80 nm.
Subsequently, hydrochloric acid was added to the dispersion while stirring at rpm until the pH of the solution reached a value of approximately 1. After 4 h, the dispersion was centrifuged at 10 rpm for 30 min.
The collected NPs were redispersed in Milli-Q water and centrifuged again. This washing step was repeated 2 more times, followed by drying the silanol functional NPs SiO2-OH in vacuum at room temperature for 12 h. The dispersion was left to stir at rpm at room temperature for 17 h. This washing step was repeated 2 more times, followed by drying the collected SiO2-NH2 NPs in vacuum at room temperature for 12 h.
The amino-functionalized NPs SiO2-NH2 with diameters of 12 and 20 nm were collected by the addition of 5 mL of calcium chloride 1 mol L—1 that induces reversible aggregation of the NPs, followed by centrifugation at 10 rpm for 30 min. The reversible NP aggregation aids in their sedimentation during centrifugation.
The particles were subsequently redispersed in ethanol. Following cooling to room temperature, the reaction mixture was washed with THF and centrifuged at 10 rpm for 30 min. Nanocomposite Film Preparation Nanocomposites were prepared by dispersing an amount of functional silica NPs 2. The number density of NPs was kept constant at the value mentioned throughout this study.
Subsequently, the PMMA nanocomposite was collected and left to cool to room temperature. The samples were left to dry in air for at least 12 h prior to further analysis. For a scheme of the custom-built foaming setup we used, see Figure S1. The spectra were collected in the range of — cm—1 spectral solution of 4 cm—1, scans. Background spectra were recorded against air. The applied air flow was 20 mL min—1. Images were obtained in the bright field mode with a kV acceleration voltage.
The typically used electron acceleration voltage was 5 keV. Prior to analysis, the nanocomposite foams were freeze-fractured after cooling in liquid nitrogen for 10 min. Together with the magnification factor of the micrograph M , Nv can be calculated according to eq 1.
Conductive carbon nanofiber-polymer foam structures. Subsequently, the collected SiO2 was redispersed in ethanol and centrifuged again. We report here that the less efficient nucleation for the smaller particles is ascribed to positive line tension values acting at the three-phase contact line among the nanoparticle, CO2 nucleus, and CO2 swollen polymer. The number density of NPs was kept constant at the value mentioned throughout this study. It is likely that increased viscosity, MDI Since processing , and the increased concentration of OH and car- OxCNTs absorb moisture very rapidly, a control foaming experi- boxylic groups on the CNT surface ; these functionalities are ment was performed to investigate possible effects of chemisorbed likely to react with the isocyanate to form some direct covalent water on the reaction.
Introduction with significantly different properties [39,40]. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. Effects of carbon nanotubes on the crystallization behavior of tions in nanotube diameter surface area , and interfacial chemis- polypropylene. Nano Lett ;3 5 — PU foam density and standard error as a function of CNT content. Chem Commun ;—5.
The difference in the foam morphologies of the foam produced by samples with the diameters discussed above is found to be insignificant. Polyurethane is one of the larg- ble growth.
This lower foaming rate affects the cellular structure by enabling the diffusion of gas Table 1 from small cells to larger cells driven by the free energy of the sys- Percentage by weight of the reactants tem . This washing step was repeated 2 more times, followed by vacuum-drying the collected SiO2 NPs at room temperature for 12 h. The applied air flow was 20 mL min—1. These particles were dispersed in aqueous solution and have surface-exposed silanol groups on the surface as received.
Chem Commun ;—5. Radial decomposition of discs and spheres.