Soundproofing flexible polyurethane foams: Effect of chemical structure of chain extenders on micro-phase separation and acoustic damping

Abstract

In this study, a new high-performance acoustic damping flexible polyurethane foam (FPUF) was successfully designed and fabricated using synthesized linear saturated aliphatic polyester resin as polyol, methylene diphenyl diisocyanate, and ethylene glycol, monoethanolamine, and ethylenediamine as chain extenders and other reagents by one-shot bulk polymerization (isocyanate index = 100 and water content = 2.5%). The effect of the chemical structure of different chain extenders on micro-phase separation and acoustic damping properties of FPUFs were investigated using comprehensive characterization techniques such as atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), compressive strength, optical microscope, and‏ ‏impedance tube. It was indicated that the micro-phase separation degree of the FPUF matrix increased with increasing amine content in the chain extender structure due to the more bidentate hydrogen bondings formation between urea-urea groups. Also, by increasing micro-phase separation, average cell sizes decreased and compressive strength, open-cell contents%, cell walls roughness, and cell size distribution of FPUFs increased. According to the sound absorption spectra, it was found that sound absorption efficiency of FPUF containing DEA was higher than FPUF manufactured by EG by 13.23% in the range of 1500–4000 Hz due to the increase of the amine content of chain extenders. These results indicate that the acoustic properties of FPUFs can be explained with the synergistic actions of micro-phase separation including the viscoelastic behavior of hard-soft segments and increasing of airflow pathway leading to dissipating of the kinetic energy of sound waves. Finally, the results revealed that soundproofing FPUFs with an optimum condition for micro-phase separation and drainage flow can be a promising candidate for using as sound insulating materials in transportation industries such as airplanes, trains, etc.

Keywords

Polyurethane foam chain extender urea-urethane segmented copolymer micro-phase separation sound absorption coefficient microcellular polymers

Introduction

Noise pollution is one of the main environmental problems and continues to get worse due to rapid developments in transportations, modern industries, and population growth.^1,2 Engineering and domestic types of equipment and also vehicles, especially the cars are the serious sources of both structural and airborne-noise pollution.³ The majority of the structural and airborne-noise occurs in the low-frequency region (30 to 500 Hz) and in the medium to high-frequency regions (500 to 8000 Hz), respectively.⁴ The collection of sounds in these frequency ranges (30 to 8000 Hz) is common and available in urban and industrial areas. Using sound absorbing and insulating materials are effective methods for noise reduction.⁵ Microcellular polymeric foams such as polyurethane foams (PUFs), polymeric felts, glass wools, and different woven and non-woven fabrics can be installed in the required location to absorb annoying noises.

PUFs which are solid-gas composites could be classified based on the cells nature (open and closed cellular structure) and also according to their stiffness (flexible and rigid).^1,6–11 The most effective sound damping and noise insulating material up to now is flexible polyurethane foam (FPUF) with viscoelastic behavior and a high volume of interconnecting open-cells and pores.^1,7 Sound absorbers efficiency can be evaluated by the sound absorption coefficient (α) which depends on the both solid phase attributes such as ductility, thermal characteristic lengths, viscoelasticity and stiffness and also foam morphology such as porosity, tortuosity, airflow resistivity type, macroscopic density, average cell size, cell size distribution, thickness of cell walls and struts.^7,12–16

Dissipation of acoustic waves propagating through the body of FPUFs occurs by “visco-inertial and thermal damping” and “viscoelastic frame damping” mechanisms. In the former, thermal conduction in the solid phase and fluid-solid-fluid interfaces of foams and also viscous friction of sound waves on cell walls or struts causes to dampening of the sound pressure in gas phase and in later, conversion of sound energy to vibration of different parts of polymer molecule occurs which these vibrations and movements produce heat due to the friction in solid walls and struts.^11–13

FPUFs are composed of reactant A (polyols, chain extenders, blowing agents, gelling and blowing catalysts, surfactants, etc.) and reactant B (isocyanates) as raw materials. Therefore, FPUFs generally consist of soft segment (SS) undertaking stability, elasticity, and mechanical strength and hard segments (HS) produced by the reaction of amine compounds such as amine chain extenders and isocyanate and contribute to the load-bearing properties of the foams.¹² By optimizing the molecular weight, chemical structure and hydroxyl values of polyols, isocyanate index and type of isocyanates, water content, composition alongside with the chain extenders, hybridization of various types of organic and/or inorganic fillers (micron or nanosize) or fibers during manufacturing and also experimental foaming processes such as one-step, two steps or pre-polymer methods, it is feasible to achieve special FPUF with high sound dampening behavior.^{1–11,17–24}

It was reported by Sung and et al. that one of the ways to increase the partial open pores which improve soundproofing in the FPUFs could be the increment of phase separation between the hard and soft domains in the PUF matrix.⁷

The aim of the present study is to investigate the effect of the chemical structure of different chain extenders on the phase separation and acoustic damping behavior of the FPUFs. FPUFs were manufactured by one-shot polymerization with different chain extenders and the other ingredients. Micro-phase separation of the hard segments in the FPUF matrix was explored by Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). The physical properties such as compressive strengths and density of the FPUFs were also measured as guidance for long-term usage. For morphological analysis, the DN-2 Microscopy Image Processing software (DN-2 MIPS) was utilized for images obtained by an optical microscope to measure average cavity and pore sizes as well as open porosity. The sound absorption efficiency was also examined by an impedance tube method in order to understand the acoustic absorbing performance of FPUFs.

Experimental

Synthesis of the linear saturated aliphatic polyester resins as polyol

Linear saturated aliphatic polyester (LSAP) resin was synthesized using 1, 4 butanediol (BD) and adipic acid (AA) monomers according to the following process. BD and AA were purchased from Merck. A 1 L 4-necked round type reactor, fitted a temperature controller, heating mantle, N₂ purge, partially refluxing condenser which was used to remove the water from esterification, was placed. BD and AA reagents were analytical grade and used without further puriﬁcations. All raw materials were charged into the reactor, the temperature was set to 220°C and esterification was carried out for 4 h. The stoichiometry of the condensation polymerizations between BD and AA was 1.5 (OH: COOH = 1.5). During the synthesis process, the acid value titration was performed using a 0.1 N KOH solution. Figure 1 illustrates the polyester synthesis set-up, schematically. To achieve high molecular polyester resin with the acid value below 5 mg KOH/g, vacuum condition was applied, so that produced water and remaining diols and diacids were filtered and removed by a vacuum pump.

Acid and hydroxyl values of LSAP resin were measured according to the ASTM D1639–90 and DIN 53240–2 (Reflux Phtalation method), respectively (Figure 1). The number average molecular weight (M_n) and weight average molecular weight (Mw) of polymer were determined by gel permeation chromatography (GPC) apparatus (waters GPC 2000, Perkin Elmer). The measurement was carried out using tetrahydrofuran (THF) as solvent, and the flow rate was 1 mL.min⁻¹. The column was calibrated using polystyrene standard samples of defined molecular weights. The density and solid content of resin were measured based on the ASTM D 1475–13 and ASTM D2369-10, respectively. The color of resins was identified according to the Gardner color scale (ASTM D 98–1544) and their viscosity was measured based on the D1545–13 ASTM. Figure 2(a) shows the molecular structures of the synthesized LSAP resin, schematically. Chemical and physical characteristics of synthesized LSAP resin are listed in Table 1.

Table. 1.

Chemical and physical characteristics of synthesized LSAP resin.

Acid value (mg KOH/g)	Hydroxyl value (mg KOH/g)	Hydroxyl content (OH %)	M_n	M_w	PDI	Density (g/cm³)	Color	Gardner viscosity	Solid content %
4.82	68.91	2.23	12,000	30,000	2.5	1.26	White	Solid	97

Preparation of the flexible polyurethane foams

Synthesized LSAP resin, methylene diphenyl diisocyanate (MDI), triethylenediamine (TEDA) and triethyleneamine (TEA) as blowing catalysts, dibutyltin dilaurate (DBTDL) as gelling catalyst, de-ionized (DI) water as chemical blowing agent, ethylene glycol (EG), monoethanolamine (MEA), and ethylenediamine (EDA) as chain extenders, and nonylphenol ethoxylate (NP-10 (EO)) as surfactant for the stabilization of cavities and interconnecting pores in the foams were purchased and used as received to manufacture FPUF samples. All of the raw materials except DI water and NP-10(EO) were purchased from Merck. The molecular structure of different chain extenders is shown in Figure 2(b). NCO content of MDI was measured based on ASTM D2572-97 (%NCO 33.5 ± 0.5). One-shot bulk polymerization was applied without a pre-polymer synthesis step. First, LSAP resin was heated to 50°C in order to melt completely and weighed in a 500 mL paper cup. Then gelling and blowing catalysts, DI water, chain extenders, and surfactant were added to the molten polyol and mixed for 5 min at 1500 r/min using a mechanical stirrer to obtain pre-mixture. Subsequently, pre-weighed isocyanate mixture according to the formulations mentioned in Table 2, was added to the polyol pre-mixtures and mixed further for 10 s at 1000 r/min. After the mixing process, the final mixtures were poured into an aluminum mold, and the mold was kept at 60°C for 3 h (Figure 1). FPUF were prepared by applying a hot wire cutter and the sample dimensions for each test are provided in the following characterization sections. Chemical reaction rates including gel time, cream time, rise time, and tack-free time of FPUF samples were measured and listed in Table 2.

Table 2.

Formulation and chemical reaction rate of FPUF samples.

FPUF code	F-EG	F-MEA	F-EDA
Polyester (g)	100	100	100
DBTDL(g)	0.5	0.5	0.5
NP-10 (EO) (g)	1	1	1
DI water (g)	2.5	2.5	2.5
TEDA (g)	0.25	0.25	0.25
TEA (g)	0.25	0.25	0.25
MDI (g)	64.75	64.43	64.21
EG (g)	5	–	–
MEA (g)	–	5	–
EDA (g)	–	–	5
Isocyanate index	100%	100%	100%
Cream time (s)	19	13	10
Gel time (s)	22	27	31
Rise time (s)	73	54	36
Tack-free time (s)	75	80	93

Characterizations

Morphological, physical, and mechanical characterization

Density and open-cell content of FPUF samples were measured based on ASTM D 1622–14 and ASTM D 6226–15, respectively. Cell size distribution and average cell size of FPUF samples were measured and calculated according to the ASTM D 3576–98. Based on this standard, an optical microscope was used to examine the cell structure, including cavities and interconnecting pores. The samples for optical microscope measurements were frozen in the liquid nitrogen and cut in 2 cm × 1 cm with a cutter. A thin layer of each sample was glued to the glass lamella and images were prepared by an optical microscope. Finally, images were analyzed by the DN-2 microscopy image processing system (DN-2 MIPS).

Universal mechanical test analyzer (GALDABINI sun 2500 Instruments) was used to measure the compressive strength of the FPUF sample (5 cm × 5 cm × 5 cm), according to the ASTM D 695–15. Before analysis, FPUF specimens were conditioned in an oven with humidity control at 21°C and 41% relative humidity for 24 h. Samples were then placed between two parallel plates and compressed at 5 mm/min. The compression strengths were calculated at 10% and 50% strain conditions.

Micro-phase separation properties

Fourier transform infrared spectroscopy (FT-IR) analyses were performed to investigate the hydrogen bondings formed between urea-urea and urethane-urethane hard segments in FPUF samples. The FTIR measurements were carried out using an FTIR BOMEM MB-Serie spectrometer from Hartmann & Braun Michelson. KBr pellets were used for foam specimens. Spectra in the range of 4000 to 400 cm⁻¹ were recorded in the transmission mode acquired from four scans at a resolution of 4 cm⁻¹.

Atomic force microscopy (AFM) was used to investigate the micro-phase separation and measuring the surface roughness of cell walls and struts. AFM (Ambios Tech., USA) was operated at a resonance frequency of 150 kHz and spring constant 38 N/m. All the samples were scanned with silicon cantilever in the non-contact mode at a scan rate of 2 Hz. The images and data analyses were performed by Q port software. In order to prepare AFM samples, FPUF samples were frozen in liquid nitrogen and cut in 1 cm ×1 cm with a cryo-microtome. Finally, the cryo-microtomed samples were glued to silicon substrates and dried in a refrigerator at 25°C for 24 h. Also, the surface roughness of each sample was calculated by Q port software.

Acoustic performance

Sound absorption coefficient (α) values were measured using an impedance tube device (Type 4002 Standing Wave Impedance Tube, B&K Co. Ltd) following the ISO10534-2 based on the transfer function mode. Three measurements were performed for each sample in the frequency range from 200 Hz to 5000 Hz with two 1/4 inch microphones. α is defined as the difference between the energy of the incident and reflected acoustic waves. The thickness of all the samples was 10 mm and the diameters for high (500–6300 Hz) and low frequencies (200–500 Hz) were 29 and 99 mm, respectively. The schematic illustration of an impedance tube system including FPUF sample is shown in Figure 3.

Result and discussion

Micro-phase separation studies

Cream time, gel time, tack-free time, and rise time of each sample were measured and mentioned in Table 2. F-EG, F-MEA, and F-EDA consist of EG, MEA, and EDA as chain extenders, respectively. The results showed that F-EDA had the lowest amount of cream time and rise time due to the higher amine content in EDA compared to the MEA and EDA. The reaction rate of the amine with isocyanate group shown by cream and rise time is higher than the reaction rate of the hydroxyl groups or water with isocyanate groups, so the reaction of water with isocyanate groups which produces CO₂ was postponed.²⁵ Also, it was observed that by increasing the hydroxyl content of chain extenders, gel and tack-free time decreased.

Generally, by increasing the temperature and rate of mixing of the liquid foam mixture and also foam curing temperature, average cell size, and open cell content increase.^14,19,25,26

Figure 4 shows the FT-IR spectra of FPUF samples manufactured by different chain extenders through baseline correction. Not only urethane and urea functional groups but also hydrogen bonding content in the urethane and urea linkages has a strong influence on the absorbance peaks intensity. The monodentate and bidentate hydrogen bonding absorbance peaks of the urea functional groups appear at 1660 cm⁻¹ and 1640 cm⁻¹, respectively. Free urethane and urea groups generally show absorbance peak at 1730 cm⁻¹ and 1710 cm⁻¹, respectively but they are shifted to the lower wavenumber location due to the existence of monodentate and bidentate hydrogen bonding conditions.^25,27–30 The absorbance peak intensity of monodentate urea is clearly different in FPUF samples and these peak intensity increases as follows: F-MEA > F-EDA >F-EG.

Also, the absorbance peak intensity of bidentate urea increases as follows: F-EDA> F-MEA > F-EG.

Bidentate and monodentate hydrogen bonding can promote the aggregation of hard segments (urea-urea linkage) and aggregated hard domain leads to micro-phase separation.^26,31–33

AFM investigations are capable to clarify variations of micro-phase separation; so, phase images of different samples were used to study micro-phase separation and hard-soft phase distributions. Figure 5 illustrates the phase images of FPUF samples obtained by AFM test. It is clear in Figure 5 that the surface of the images is segmented into different colors based on the maximum and minimum amount of hard modulus with an arbitrary unit (au). In general, micro-domains with higher modulus are shown as lighter points compared to the points with lower modulus in AFM phase images. The degree of micro-phase separation can be shown by the maximum amount of sample modulus and hard-soft micro-domains distribution.^33,34 Maximum modulus of F-EG, F-MEA, and F-EDA are 0.2982 kau, 5.466 kau, and 14.73 kau, respectively. According to the maximum modulus and hard-soft micro-domains distribution, it can be concluded expressly that foam based on the EDA (F-EDA) displayed more micro-phase separation morphologies while foam based on the EG (F-EG) was featureless relatively.

It is clear in Figure 4 that F-EG has the least monodentate and bidentate absorbance peak, its free urethane and urea absorbance peak have no shifts, and also phase image related to the F-EG has the lowest hardness modulus and micro-phase separation distribution. Figure 4 also shows that F-EDA has a strong bidentate hydrogen bonding between urea-urea groups and absorbance peak shift of its free urethane and urea to the lower wavenumbers is obvious in FT-IR spectra; therefore its maximum micro-phase separation is justifiable. Therefore, by increasing the bidentate urea content, urea aggregation increases and micro-phase separation becomes sharper. It was reported that the degree of micro-phase separation and the size of hard domains depend on the chemical structure, polarity, crystallization ability, and solubility parameters of soft segment and hard segment contents.³⁴ Due to the higher amine content in EDA, more urea bonds are generated in F-EDA compared to F-EG and F-MEA during the reaction between MDI with EDA; hence, the intensity of the absorbance peak of bidentate hydrogen bonding is higher in F-EDA. Consequently, AFM findings are in good agreement with the FT-IR results.

The stress-strain curves of manufactured FPUFs are shown in Figure 6. The compression strength of the FPUFs was measured at 50% strain conditions. Curves of all samples show the typical behavior of plastic foams distinguished by a pseudo-solid-like or initial linear portion, related to the elastic deformation of cells; a macroscopic yield with an almost horizontal plateau (rubbery like), and self-reinforcing which is the result of entropy reduction at high strains. The slope of the linear region represents the apparent compressive elastic modulus of the FPUFs.³⁵ However, after the removal of the gas phases, the samples show a high rubbery like strain behavior as a result of easy deformation of the foam structure. At high strain amplitude, the dynamic of polymer chains is limited by the applied stress, and cells are almost collapsed completely.

It is obvious that by increasing the separated hard domains in the samples, compressive strength increase. Each hard domain in the FPUF can be supposed as a physical crosslinking site and collaborates with the foam’s stiffness and cohesive force.^34–36 Subsequently, the increase in the cohesive forces is related to a high level of micro-phase separation in FPUFs and Young modulus due to the higher monodentate and bidentate hydrogen bondings. In other words, increasing hydrogen bonding can produce a higher resistance to deformation under compression circumstances.

It was observed that all samples, regardless of the chain extender type, show an almost entire recovery to the first size, which declares the suitable elastic behavior of the FPUFs.

Based on the model explained by Onck et al, at a constant sample dimension the compressive strength of foams is proportional to their cell size reversely. Smaller cells with higher micro-phase separation degree could have more supporting struts and walls, so compression strength could be promoted. When pore and cell sizes decrease, the size of strut increase which can act as a sturdy bearing wall.³⁷

Table 3 reveals the Young elastic modulus (slope of the linear region), toughness (area under the stress-strain curve) and compressive stress at 50% strain measured according to the stress-strain curves. The results manifest that toughness of FPUF samples increased by an increment of the micro-phase separation degree, implying higher stored elastic energy which makes the samples to resile to their initial volume after omitting the stress.

Table 3.

Compressive stress at 50% strain, toughness and Young elastic modulus of FPUFs.

Foam	F-EG	F-MEA	F-EDA
Compressive Stress at 50% strain (kPa)	93.9	145.21	174.25
Standard deviation	0.95	0.82	0.89
Toughness (J.m⁻³)	9887.105	10558.325	15055.945
Standard deviation	2.3	1.9	2,98
Young’s Modulus (kPa)	7.76	13.32	19.88
Standard deviation	0.58	0.77	0.87

It was concluded that Young elastic modulus, toughness, and compressive strength of foams increased by increasing the hard segments content and micro-phase separation; so, F-EDA has the most compressive strength and toughness. This can be justified by the resistance of monodentate and bidentate hydrogen bonding to deformation before the yield stress point of FPUFs.²⁵

Sound absorption properties

Generally, sound waves can be reflected, transmitted, and absorbed (dissipated) when they strike a cell surface in FPUF samples.³⁰ Gayathri et al.³⁸ reported that dissipation of sound waves to heat occurs due to their collision to the FPUF matrix including of main polymer backbone, side chains, soft segment crystals or hard domains as well as the friction between sound wave with air molecules in and/or out of the cells and with cell walls due to the sound pressure.¹⁷ The cell morphology and the modulus of cells structure including of walls and struts have intense effects on the α values of the FPUFs generally.^8,15,33,34 α values for the 1 cm thick of the manufactured FPUF samples at frequencies ranging from 100 to 5000 Hz were measured and plotted in Figure 7. It is obvious in Figure 7 that α values of the FPUFs have two distinct regions: low (100–500 Hz) and high (500–5000 Hz) frequency ranges. Generally, FPUFs have maximum α in the second region which can be related to the hard segments. It is shown in Figure 7 that α values begin to increase at a frequency of about 1000 Hz to an optimum value (3100 Hz), and decrease slowly for all of the FPUF samples. It can be deduced that at high frequencies, viscous motions happen by the FPUF matrix, leading to the more damping of the wave energy. In general, pore size, cell morphology, and density of FPUFs seem to play a major role in low frequencies. In turn, the viscoelastic behavior of the urea-urethane block copolymer network seems to be the determinative parameter at higher frequencies. However, at high frequencies (higher than maximum value), the time-dependent viscous motions of FPUF’s segments occurs slightly, which leads to the decrease of the wave energy damping and hence reduction of α values.¹⁷

Average α values for frequency ranges between 1500 and 4000 Hz and area under the sound absorption curves of FPUF samples are summarized in Table 4. Average α values of F-EG, F-MEA, and F-EDA are 0.65, 0.69, and 0.74, respectively. Also, the area under the sound absorption curves of F-EG, F-MEA, and F-EDA are 2843, 3026.451, and 3219.075, respectively. It is clear that F-EDA has higher α values which are nearly 0.95 at 2100 Hz.

Table 4.

Physical characteristics of FPUF samples and the area under the absorption coefficient-frequency curve.

FPUF code	F-EG	F-MEA	F-EDA
Area under absorption curve (Hz)	2843	3026.45	3219.075
Standard deviation	2.44	3.5	2.98
Average sound absorption coefficient value (α) (1500–4000 Hz)	0.65	0.69	0.74
Standard deviation	2.9	4	3.6
Roughness (nm)	154.73	199.54	328.49
Standard deviation	4.56	5.88	3.56
Average cell size (µm)	243.42	131.1	111.8
Standard deviation	21.98	16.8	11.87
Open-cell content %	0.8	11.2	24
Standard deviation	0.06	1.9	1.43
Density (kg/m³)	52.83	98.4	103.76
Standard deviation	0.89	0.73	0.95

Some of the studies revealed that the foams with smaller cell size have better sound absorption efficiency due to the suitable tortuosity in the foams.^32,37,39 Also, Ghaffari Mosanenzadeh et al.³⁹ reported that the average cell sizes between 250 and 500 μm of the polylactic acid open-cell foams showed the best sound absorption performance. Cell sizes of FPUF samples were measured by optical microscope images shown in Figure 8.

According to the results mentioned in Table 4, foam based on the EDA with high micro-phase separation has the smaller cell size. As expected, decreasing the cell size from 243.42 to 111.8 µm leads to increasing the sound absorption efficiency. So, by decreasing cell size, average α (1500–4000 Hz) and area under sound absorption curves increased from 0.65 to 0.74 Hz and 2843 to 3219.075 Hz, respectively. Cell size distribution of FPUF samples was illustrated in Figure 9. Also, topology images based on the AFM investigations are shown in Figure 10. The results of the open-cell content % and roughness measurements of cell walls were mentioned in Table 4. It is clear that higher cell size distribution, open-cell content %, and roughness increased the sound absorption efficiency; this is due to the better damping of the sound wave energy as a result of the higher collision of sound waves with cell walls and air molecules (increasing the pathway) and higher reflection and scattering. Also, it can be seen in Figure 7 that F-EDA has a little higher α values at the low-frequency region compared to the F-EG and F-MEA. Based on the results, it can be concluded that by increasing micro-phase separation of hard domains, sound absorption efficiency improves generally. Sung et al.⁷ and Kim et al.⁴⁰ reported similar results. It seems that the micro-phase separation influences on the two main sound absorption parameters: (1) increasing open pores and cell content % and (2) viscoelastic behavior of FPUFs.^41,42 During cell formation, cell rupturing occurs due to the extensional thinning and stress concentration points which can be created by the high amount of hard micro-domains aggregation in the FPUF cell. Cell rupturing increases the open-cell content % which can increase the collisions of sound waves with gas molecules, struts and walls.^32,41,42

Also, hard segments have viscoelastic behavior due to the hydrogen bonding between urea-urea groups; hence, it seems that hard segments can contribute to sound energy dissipation mechanism. By vibrating hard segments, the other attached soft segments are forced to vibrate; so chain length and glass transition temperature (Tg) of soft segments are also important parameters in the viscous loss.^34,41,42 Therefore, the extent of sound energy dissipation of FPUFs is influenced by the dynamic of the polymer chain segments and consequently ease of dipoles polarization which is governed by micro-phase separation.

Conclusion

In this study, FPUF samples were synthesized with various chain extenders including of EG, MEA, and EDA to assess the effects of chain extenders chemical structure on the micro-phase separation, cell morphology (density, average cell size, cell size distribution, and open-cell s content of FPUFs), and sound absorption behavior. AFM and FT-IR investigations showed that FPUF prepared by EG did not show any features indicating the presence of micro-phase separation compared to the FPUFs containing EDA and MEA. On the other hands, the degree of micro-phase separation in the FPUF matrix increased with increasing amine content in chain extender structure due to the forming more bidentate urea hydrogen bondings. It was observed that the physical parameters of the foam, including average cell size, cell size distribution, roughness of the cell wall surface, open-cell content %, etc. are affected by micro-phase separation. Therefore, by increasing micro-phase separation, the average cell size of FPUFs decreased and compressive strength, open-cell contents %, cell walls roughness, and cell size distribution of FPUF samples increased. Also, it was observed that sound absorption efficiency of FPUF containing DEA increased by 13.23% compared to FPUF manufactured by EG (area under the sound absorption curves and average α increased from 2843 to 3219.075 and 0.65 to 0.74 in the range 1500–4000 Hz, respectively). Therefore, the acoustic properties of FPUFs can be explained by the synergistic actions of micro-phase separation including the viscoelastic behavior of hard segments and increasing of airflow pathway leading to dissipating of the kinetic energy of sound waves. So, different factors such as hard and soft segment content, micro-phase separation, and soft segment ability to vibrate can have intensive effects on the sound absorption efficiency. Therefore, an optimum amount of micro-phase separation is necessary to achieve good acoustic behavior in the FPUFs.

Footnotes

Acknowledgements

The authors would like to acknowledge Color & Polymer Research Center (CPRC) for the kind support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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