The Effect of Reinforcement at Length Scale for Polyurethane Cellular

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The Effect of Reinforcement at Length Scale for Polyurethane Cellular Scaffolds by Supramolecular Assemblies Ruma Bhattacharyya, Shriram Janghela, Amit Saraiya, Debmalya Roy, Kingsuk Mukhopadhyay, and Namburi Eswara Prasad J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11978 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry

The Effect of Reinforcement at Length Scale for Polyurethane Cellular Scaffolds by Supramolecular Assemblies Ruma Bhattacharyyaa, Sriram Janghelaa, Amit Saraiyaa, Debmalya Roya*, Kingsuk Mukhopadhyaya and Namburi Eswara Prasadb a

Directorate of Nanomaterials and Technologies, DMSRDE, GT Road, Kanpur, India-208013

b

DMSRDE, GT Road, Kanpur, India-208013

Abstract: This study is aimed to represent the role of carbonaceous nanofillers to reinforce the commercially available polyurethane porous structure. The effect of dimensionality of fillers to anchor the construction of stable 3D cellular architectures has been highlighted. The cellular frameworks of commercially available thermoplastic polyurethane (TPU) have been fabricated through the thermo-reversible supramolecular self assembly route. It was established that the minimum shrinkage of TPU lattice structures was occurred when the solid state network is strengthened by the topologically engineered 3D hierarchical nanofillers where the amount of reinforcement was found to play a critical role. It has been established by series of structure-property correlations that reinforcing the cellular structure to endure the capillary stress is equally effective as supercritical drying for producing low density porous morphologies. The removal of liquid phase from gel is as important as the presence of 3D fillers in the matrix for reinforcing the cellular structures while replacing the solvent phase with air to generate the two phase solid-gas engineered morphology. The insight into the PU network structure revealed that the dimensionality, amount and distribution of fillers in the matrix are critical for reinforcing the cellular scaffolds in solid gel without any crosslinking.

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1. Introduction Thermoplastic polyurethane (TPU) is one of the extensively investigated polymers due to its unique structure-property characteristics1-2.

Polyurethanes (PU) have been used for the

development of many useful engineering applications, ranging from automobile tires, bumpers to the biomedical implants such as cardiovascular devices and tissue replacement materials. The surface and bulk properties of PU could be manoeuvred using hydrophilic and hydrophobic methods by grafting chemical functionalities onto the main chain and side groups followed by microemulsion polymerization3-4. Thermoplastic polyurethanes (PU) are made of the “soft” and “hard” segments where the glass transition temperature (Tg), tensile strength and the dimensional stability of the hard segment is higher than soft segment. It has been observed that by changing the type and functionality of the polyol and isocyanate precursors, the physical properties of PU could be varied significantly. The intramolecular rigidity is the result of chemical bonding which is determined by composition of chain structure, branching and crosslinking. The intermolecular rigidity in terms of degree of hard segments, crystallinity and hydrogen bonding is resulted from physical bonding4-6. In addition to the traditional compact PU morphology, foamed version attracts widespread attention due to its targeted properties at a very low weight. Polyurethane foams (PUFs) with the cellular structure have been used for many technological applications and dominated in the market compared to the other competing materials7-8. PUFs covered almost 29% of the total market of PU and are extensively used as insulation materials, furniture cushioning, sorbents, automotive parts and many others commercial purposes1. The porous microstructures of PUF are generated by chemical blowing reactions of isocyanates and water where the resulting carbon dioxide serves as a blowing agent. The evaporation of the physical

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blowing agents, such as chlorofluorocarbons, hydrochlorofluorocarbons, pentanes and hydrofluorocarbons were also been used to generate the cellular structures. The array of different types of foaming systems and foaming processes has been used for the synthesis of PU foams7-9. The flexible PUFs have generally open cell cellular structures where the solid phase is the constituents of the cell edges where the void space or gas phase is connected through the cell faces. The rigid PUFs are often closed cell morphology where cell faces are composed with isolated cavities filled with trapped gas or air10. Despite the outstanding properties of PUFs, they suffer from some technological disadvantages, which mainly include limited use at higher temperatures, dimensional stability, thermal aging, degradation and friability. The composite PUFs were thus developed for tailor made use for specific engineering applications where the introduction of carbon nanomaterials as filler attributed a wide range of benefits2,11. The transformation of sol to gel could be activated by the external stimuli and the thermoresponsive gels of PU were prepared by the temperature modulation in the appropriate solvent12-15. The solid network morphology of stimuli responsive liquid gel could be constructed by the removal of solvent phase under supercritical condition or controlled ambient drying where the minimum shrinkage of structure happens. The exchanges of the solvent phase with another solvent of contrasting polarity or boiling point were also attempted for the development of 3D architecture of solid gelation network from the liquid gel16-17. Although there are lots of studies on PUFs and PU gels during the synthesis of PU, however, to best of our knowledge, the cellular structures of PU in foam or gel type configuration were not attempted on the as grown PU. We first time report here that 3D solid skeletal framework of commercially available PU could be periodically developed by

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physical mixing of 3D nanofillers in PU followed by solvent exchange and replacement of solvent with air. The removal of solvent phase from the gel is extremely critical to minimize shrinkage of the gel to provide lower density. The most of the organic solvents showed high critical temperatures and pressures which lead to difficulties in handling these chemicals. The nonflammable liquid carbon dioxide has been popularly used at a much lower temperature than an organic solvent18-19. The solvent exchange is also been used to dry gel at relatively higher temperature by the judicious selection of the solvents17,20. It has been identified that the removal of liquid in the gel by evaporation produces capillary stress in the pores of the material which cause the collapse of network structures. This study is aimed to represent the role of carbonaceous nanofillers to reinforce the polyurethane porous structure to endure the capillary stress during drying. The effect of dimensionality of fillers to anchor the construction of stable 3D cellular architectures has been highlighted. We believe that our findings will have an impact on the robust, cost effective production and uses of PU foams or solid gelation network fabrication for industrial applications.

2. Experimental 2.1 Materials The two commercially available TPU of Desmopan (DP 9386) and Texin (SUN 3006) series from Bayer Material Science, AG, Germany were selected for this study which are used as it is without any modification/purification. The detailed properties of the starting polyurethanes have been listed in the supporting information (Fig S1). The other chemicals used for the experiments were purchased from sigma Aldrich and were used as received. The different low dimensional carbonaceous nanomaterials viz fullerenols (0D), acid functionalized MWCNTs (1D), hydroxyl functionalized graphenes (2D) and hierarchical structures of Page 4 of 30


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MWCNTs embedded in to graphene flakes (3D) were synthesized in our laboratory which is described elsewhere21-22. The fullerenols (0D), acid functionalized MWCNTs and edge functionalized graphenes (2D) are of the size in the range of 1-2nm diameter, 30-40 nm outer diameter/3-4µm length and 500-1000 nm flake size/10-15 graphene layers respectively. 2.2 Preparation of polyurethane nanocomposites The thermoreversible gelation of PU via the physical route without any chemical crosslinking was carried out in DMF by the reported procedure14 which is schematically shown in scheme 1. The gelation and the self assembled networks formation were carried out in nitrogen filled glove box where moisture and oxygen content were less than 10 ppm levels. It has been found that the reaction could also be carried out in atmospheric condition however at reduced reaction kinetics. The final concentration of PU is maintained at 7.5 mg/ml however the gelation temperature is raised from previously reported 80 to 900C to enhance the supramolecular assembly and phase segregation in thermoplastic PU23. The different concentrations of zero, one, two and three dimensional carbonaceous nanofillers were sonicated in DMF where the solution is repeatedly centrifuged and re-dispersed in DMF, unless a stable solution is achieved. The DMF solution with carbonaceous nanomaterials was heated to 900C and was then added to the hot solution of PU with continuous stirring. The three different weight percentages of filler concentrations viz. 0.5, 1 and 5 were used for this study. The hot solution of nanocomposite mixture is finally brought down to 300C with slowly lowering the temperature of the reaction bath when gels were formed by self assembly of PU chains in the solvent14,23. The pristine version of the nanomaterials (fullerene, MWCNT, graphene) did not produce a stable dispersed in DMF and hence the loading percentage and distribution of nanofillers in the gel could not be controlled24.

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Scheme 1: The schematic representation of the thermoreversible gelation of PU in DMF via the physical route without any crosslinking.

2.3 Preparation of porous polyurethane 3D cellular structures The swollen PU nanocomposite gels in DMF were soaked at 300C into the different organic solvents like ethanol, THF-methanol (4:1) and toluene to diffuse out the impurities and to exchange DMF. The soaking solvents were replaced with fresh solvents in regular intervals of 6 hrs for a day to enhance the kinetics of exchange process. The gels were then left to slowly evaporate out the solvents at 300C for a day in case of low boiling solvents and three days for high boiling solvent in a vacuum oven where the vacuum is optimized at one mill bar. 2.4 Characterization The scanning electron microscopic (SEM) images of the different graphite morphologies were recorded on a SUPRA 40 VP, Gemini, Carl Zeiss scanning electron microscope. The TEM images were recorded on a PHILIPS CM200 model TEM, operating at a 200 kV voltage. The samples for TEM studies were prepared by microtome. The topographic AFM Page 6 of 30


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images were recorded on A100 SPM A.P.E. Research, Italy system. The solid cellular samples were cut and fixed on the freshly cleaved HOPG substrate using double sided carbon tapes. The powder diffraction XRD patterns of the PU nanocomposite films were recorded on a Bruker AXS, Germany (D8 Advanced) diffractometer. The X-rays were produced using a sealed tube and the wavelength of X-ray was 0.154 nm (Cu K-alpha). X-Ray pole figures were recorded by relatively large area scan (2 mmx2 mm) for the texture analysis to maintain the reasonable statistics. The subsequent texture analyses were conducted using a commercial program LaboTex. Thermal properties of the PU and nanocomposites were measured on a TA Instrument Hi-Res TGA 2950 thermogravimetric analyzer (TGA) attached to a Thermal Analyst 2100 (Du Pont Instruments) thermal analyzer at a 100C/min heating rate. The crystallization and melting features at different temperatures were carried out in an inert atmosphere using a differential scanning calorimeter DSC Q200 V24.4 Build 116 at a 050C/min heating rate. In dynamic mechanical analysis (DMA) experiment, the temperature scans were conducted in a TA Instruments model Q800 in tension mode with an oscillatory frequency of 1 Hz and a heating rate of 050C/min. The dimensions of the samples were 30x10x2 mm3 were tested in a temperature range of 20 to 2000C. The

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helium (99.99% pure) pycnometer was used to determine the volume of the samples by measuring the pressure change of helium in a calibrated volume. Density is calculated by the formula ρ = m/V and percentage porosity by ([1- ρ/ ρ0] x100) where ρ0 is 1.2 for Texin PU. Tensile testing of the neat PU and nanocomposites were conducted on a universal testing machine of S. C. Dey and Co. in accordance to the ASTM D882. The specimen dimension of 2″x0.5″ at the gauge length and were cut from the pristine and nanocomposite TPU samples where the 50 mm/min crosshead rate was used. The five specimens of each sample were selected from the material drawn in the tensile machine and the average data were reported. Page 7 of 30



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The microtomographic images have been recorded on the GE microtomography instrument. The acquired images were recorded in the datos|x 2.3 software using 0.8-1.1µm voxel size where the samples dimensions were 3x3.5x2 mm3.

3. Results and Discussion It has been reported that commercially available PU donot form gel in organic solvent14 and we also found that Desmopan DP 9386 did not form gel in DMF at 900C even after 24hrs at controlled atmosphere. The incorporation of small molecular weight diols as chain extender is associated with ancillary reactions with diisocyanates to generate hard segments in TPU by physical crosslinking. The role of chain extender is thus critical for the gelation kinetics by extending the hydrogen bonding and overall length of segmented zones14,23. The Desmopan DP 9386 is composed of polytetramethylene glycol, diphenyl methane diisocynate and 1, 4 butanediol (BD) as chain extender25 which could not able to provide enough rigidity by the extended crosslinking using BD which corroborates the earlier findings14. The Texin SUN PU of 7.5 mg/ml concentration however showed good gelation behaviour in DMF at 900C in as low as 06hrs time. The supramolecular self assembly in commercially available PU in solvent could be attributed to the higher degree of coupling between the dicyclohexylmethane diisocyanate with the combination of 1,4-butanediol and 1,6-hexanediol (HD) in the Texin grade. The higher crosslinking density of aliphatic diisocynate units with the blends of BD and HD in Texin SUN 3006 compared to aromatic Desmopan DP 9386 was further confirmed by the TGA experiment (supporting information, S2). The typical two stage degradation was found in Texin due to the higher contrast between the hard and soft segments compared to Desmopan and subsequently form gel in DMF14,26.

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Fig 1: The SEM images of Texin Sun 3600 PU solid gel skeletal morphology soaked with toluene and dried at 300C under 1 mbar for 3 days without any filler (A) and in presence of zero (B), one (C) and two (D) dimensional 1 weight percentage carbonaceous nanofillers. The scale bars of SEM images are 2µm.

One of the characteristic features of the thermoreversible polymeric gel is to show temperature dependant physical properties due to the supramolecular self assembly of polymeric chains to construct phase segregated structures in the solvent. The change of solubility of PU in DMF with the change in temperature results insolubility at lower temperature which leads to supramolecular self assembly to physically produce gels without any crosslinking or functionalization27. The replacement of solvent by air/gas for producing solid-gas cellular network is generally prepared by supercritical drying of solvent to avoid shrinkage of polymeric microstructure28. The fabrication of skeletal framework of solid gel Page 9 of 30



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without supercritical drying was also reported by using the fluid in the pores of the gel with lesser surface tension29. The swollen Texin Sun PU gels in DMF were replaced by the three different organic solvents of dissimilar surface tension like ethanol, THF-methanol (4:1) and toluene. The SEM image, pycnometric data and AFM micrographs (Fig 1A, 2A and 2C respectively) do not reveal the cellular framework in Texin PU however the density of dried gel is reduced by 5%. The spherulites type phase segregated morphologies were found to be evolved in the PU matrix in presence of one weight percentage filler of zero (Fig 1B) and two dimensional (Fig 1D) carbonaceous nanomaterials. The polycrystalline polymeric spherulites are generally composed of the irregularly grown microfibrillar crystals where lots of reports were available in varieties of semicrystalline polymeric spherulites29-30.

Fig 2: Helium pycnometric studies are summarized in graphical format (A) where the density (standard derivations by error bars) and porosity were displayed versus different PU Page 10 of 30


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nanocomposites. PU-S, PU, 0-PU, 1-PU, 2-PU and 3-PU represents starting PU and solidified gels of PU with 0, 1, 2 and 3 dimensional carbonaceous nanofillers respectively. The SEM (B) and AFM (C, D) images of the frameworks of solid PU gels were illustrated where the images of B and D contain 3D nanofillers and image C is without any filler. The scale bar of SEM image is 2µm. The nucleating agent like low-dimensional nanofiller is required to initiate the phase segregation self assembly of polymer with the low degree of crystallinity like TPU31-32. Nanomaterials surface provides the template for surface induced conformational ordering of polymeric chains by promoting inter chain interactions. The fullerenols (0D) and edge functionalized graphenes (2D) are of the size in the range of 1-2 and 500-1000 nm respectively. We report here the typical seed mediated embryo of TPU chains which resulted into thermoreversible gel during hard and soft phase segregation. The higher accessibility of surface curvature and active area in case of fullerenol and functionalized graphene by the PU gel lead to the controlled orientation of polymeric chains which leads to spherulites. On the other hand, the functionalized carbon nanotube (1D) is 3-4 micron in length and were found to catalyzed the complex and multifaceted structural hierarchies in the solidified PU gel (Fig 1C). The detailed studies were carried out earlier on the epitaxial crystallization in 1D polymer nanocomposite by statistical modelling and experimentations. It was found that the nanohybrid shish-kebab architecture was form in place of spherulites type crystal structure by carbon nanotube32-34. The bulk densities and porosities of the solidified gel of pristine and nanocomposite Texin PUs were determined from the fall of helium gas pressure in the measuring chamber using the standard pycnometer. It has been observed that the CNT based solidified PU gels has the lowest density compared to the 0 and 2 dimensional fillers with spherulites morphologies. The highly folded phase segregated structures of PU gel in Page 11 of 30



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presence of 1-D filler are apparently random (Fig 1C) and repeatedly branched to form space filling structure which is found to be more porous than the aggregate like spherulites (Fig 1B & 1D) [35].

Fig 3: The TEM images of Texin PU solid gel cross section by microtome with 1% fullerenols (A), acid functionalized MWCNTs (B), functionalized graphene (C) and MWCNTs immobilized into graphitic flakes (D) respectively. The scale bars of figures A-D are 200,100,100 and 50 respectively.

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To understand the phase dissociation behaviour of PU gel during solidification in presence of nanomaterials, we have introduced a 3D hierarchical structure of graphene-CNT hybrid into the Texin PU. We have observed the lowest density of the PU matrix when we placed these 3D nanofillers where MWCNTs were embedded into the graphitic flakes21. The graphene flakes are expected to play a key role for producing epitaxial lattice matched crystals whereas CNTs contribute to the geometrically confined topological structures36. We have, in fact, found in SEM and AFM studies (Fig 2B&2D) that 3D nanofillers produced the spherulites in all over the solid PU gel [supporting information S3]. The further insight into the spherulites formed into the solid gel matrix with different low dimensional nanofillers disclosed that there are distinct differences in the morphologies. The zero dimensional filler produced typical particulate growth in TPU gels whereas 2D filler produced lamellar type nanoscale structures, however both these nanometre building blocks were assemble into micron sized spherulites during drying of gel (Fig 1& Fig 3). It has been reported earlier that the flat surface of graphene produced the better interaction with the matrix which resulted the rigid geometry of nanocomposite. In contrast, CNTs embedded into polymer matrix is flexible and originates point type contact with the matrix due to the small junction areas35. The flexibility of 1D nanofillers in the thick TPU gel restricts the spherulites growth (Fig 1B), however CNTs immobilized into graphene flakes produced spherulites (Fig 1D & Fig 3D). The CNTs bridged between graphene flakes infused additional rigidity in the TPU gel which in turn catalyzed the formation of globular morphology. The TPU spherulites constituted from 2D and 3D nanofillers have different topological structures, graphene produced flake like lamellar nanoscale geometry (Fig 1D) whereas the typical microfibrilar assemblies were developed by CNT-graphene hybrid hierarchical nanofillers (Fig 2B).

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Fig 4: 3D imaging of 2.5x2.5 mm samples of Texin PU solid gel morphologies with 1% fullerenols (A), acid functionalized MWCNTs (B), functionalized graphene (C) and MWCNTs immobilized into graphitic flakes (D) using X-Ray tomography.

X-ray computed tomography (CT) is in much demand among the non-destructive assessment techniques to study the three dimensional morphologies37. The heterogeneity in composite materials often necessitates qualification inspection to defects densities and structural integrity. We have used X-ray CT to demonstrate 3D porous cellular skeletal of TPU gel (Fig 4D) by the 3D nanomaterials reinforcement which carries the representative signatures of both 1D (Fig 4B) and 2D (Fig 4C) nanofillers. The 3D imaging permits us to map the overall distribution of nanofillers in the gel network and the absence of contrasting images in case of 3D fillers (Fig 4D) ensure no micron size aggregated cluster formation in the matrix. The orientation of nanofillers in the semicrystalline TPU matrix was further evaluated by X-ray pole figures (Fig 5). The pole figures are the most common source for texture information Page 14 of 30


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and have been used for morphological characterization in semicrystalline polymer matrix38. In the present work, the reference plane for the pole figures was the plane of PU peak at 200 2θ. The fairly uniform pole densities clustered around the PU crystalline plane indicates the uniform distribution of nanofillers in the matrix (Fig 5). The evenly distributed pole (Fig 5D) in a plane very close to the TD/ND plane corroborate the observation of CT that 3D filler distribution is found to be uniform in the matrix.

Fig 5: The pole figures at the characteristic polyurethane peak around 2θ=200 for the Texin PU nanocomposite with 1% fullerenols (A), acid functionalized MWCNTs (B), functionalized graphene (C) and MWCNTs immobilized into graphitic flakes (D) respectively. Page 15 of 30



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The studies on structural geometries and density measurements suggest that the functional carbonaceous nanomaterials could efficiently reinforce the cellular structures of rigid gelation networks. It has been established that supercritical drying or low temperature drying are required to retain the porous morphology16-17 and our study indicates that the strengthening the porous network is equally effective to avoid the collapse of cellular frameworks. The enhancement of dimensional stability of the rigid gelation network by reinforcement with carbonaceous nanofillers was further supported using the dynamic mechanical analysis experiments (Fig 6A). As per the datasheet, Texin Sun TPU has the glass transition temperature of -400C, however the thermo reversible gelation route has increased the rigidity of the segmented phases of TPU and hence the Tg value is increased to 400C (Fig 6A). The similar elevation of Tg values were also reported in the literature for polyurethane foam and porous structures by reacting polyols with isocyanates in presence of chemical blowing agents39. The introduction of low dimensional nanomaterials into the pristine TPU gel bestowed additional support to the network and hence the glass transition temperatures were increase even higher. The significant increment in the storage modulus values (Fig 6A) in the nanocomposite and particularly in the rubbery region indicates the strong interfacial interaction between nanofillers and TPU matrix. As reported earlier, fullerenols could not effectively restrict the polymeric chain movement due to its small size and hence Tg and modulus values were not increased compared to the TPU matrix40. The highest Tg value, increment in storage modulus and highest reduction of tanδ peak furnished further evidences that 3D hierarchical carbonaceous nanofillers (supporting information S4) highly impedes the polymer chain motion by enhanced interfacial interactions supporting information S5).

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Fig 6: The DMA (A), XRD (B) results of pristine (black thin line) and one weight percentage cellular solid TPU nanocomposite where blue dashed, magenta dotted, green dash dot and red thick line represent fullerenols, functionalized MWCNTs, functionalized graphenes and 3D carbonaceous hierarchical nanostructures based nanocomposite respectively.

The XRD spectra of pristine TPU and its nanocomposite illustrated a clear reconstruction of TPU microstructures (Fig 6B). The 2θ peaks at 200 and 22.50 have been assigned for the amorphous halo due to the presence of short range regularly ordered hard, soft domains and crystallization of the hard segments respectively41-42. It is clear that the presence of low dimensional functional carbonaceous nanofillers significantly influenced the crystalline structure of the hard segments (Fig 6B). The reduced intensity of the shoulder at 22.50 by the addition of nanofillers implied that the functional nanomaterials considerably shortened the hard segments by penetrate into the microstructural phases. The increase in the width of amorphous halo in case of 3D hierarchical nanostructure indicates that the dynamics of both soft and hard segments of TPU matrix was markedly disordered due to the presence of interpenetrating network of 3D nanofillers in TPU matrix. The analogues type of reduction in

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the intensity of the hard segments was also reported earlier in waterborne polyurethane nanocomposite with functionalized graphene sheets43.

Fig 7: Tensile measurement of the elastic region below (A) and above (B) glass transition temperature of the pristine (black thin line) and one weight percentage cellular solid TPU nanocomposite where blue dashes, magenta dots, green dash dots and red thick line represent fullerenols, functionalized MWCNTs, functionalized graphenes and 3D carbonaceous hierarchical nanostructures respectively.

For further insight into the phase segregated microstructural details of the reinforcement of cellular morphology by low dimensional carbonaceous nanofillers, the mechanical measurement of elongation were studied using the applied load. The elastic deformation of the cellular structures happens in the initial stage and after the yield point, the large deformation arises due to buckling of cells of the foam which produces stress plateau44. The deformation resistance increase beyond the post-buckling deformation of cellular structures where strain softening and deformation localisation were observed44. The typical stress-stain curves before and after glass transition temperature (Fig 7) of the TPU and its nanocomposite Page 18 of 30


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revealed the similar information as we received from DMA experiments (Fig 6A). The fullerenols could not able to efficiently support the solid state structure of TPU and hence the reduction in elasticity is modest (Fig 7A). The differences in percentage elongation values in nanocomposite are significant in the glassy region (Fig 7A), however above the glass transition temperature, the tensile properties are reduced and averaged out (Fig 7B). The normalization of stain at a given applied stress in the elevated temperature could be attributed to the high segmental dynamic of polymeric chains beyond the glass transition temperature45. The tensile properties indicate that low dimensional carbonaceous nanofillers could able to successfully reinforce the cellular networks of TPU in solid state which assists the formation of solid state network scaffolds without supercritical drying of the solvent phase. Now, to check the effect of the percentage of loading of this 3D hierarchical structure in the TPU matrix, the filler loading is varied from 0.5 to 5 weight percentage. It has been found that no well ordered network structure of TPU could be assembled in solid state at lower and higher loading the 3D nanofillers (Fig 8C and F). The similar phenomenon also observed in 0 and 2D nanofillers where no regular phase segregated spherulites were produced at lower and higher loading of nanofillers in the TPU matrix (Fig 8A, B, D and E). The formation of porous cellular lattice at an optimum weight percentage loading of nanofillers into the TPU matrix suggests the critical structure-property relation. The differential scanning calorimetric measurement indicates the loss of crystallinity of TPU matrix at lower and higher loading of 3D fillers (supporting information, S6). The lower weight percentage of filler failed to mechanically strengthen the cellular morphology by coherent interfacial interactions during the evaporation of liquid phase. At higher loading, the dispersions of nanofillers into the thick gels are inconsistent and therefore irregularly generate interpenetrating networks which also reported in the other studies46-47. Page 19 of 30



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Fig 8: The SEM images of solid state structure Texin Sun PU soaked with toluene and dried at 300C under 1 mbar for 3 days with 0.1 and 5 weight percentage fullerenols (A and D), functionalized graphene (B and E), and MWCNTs immobilized into graphitic flakes (C and F) respectively where the scale bar is 1µm.

The reinforcement of cellular structure of nanocomposite TPU for enduring the capillary force during exchange of solvent phase by air is further studied using the 3D deformations of the solid gel in volumetric digital image correlation48-49. The isotropic distributions of nanofillers in the gel matrix produced the 3D microstructures which were further subdivided into smaller specimen for volumetric correlations. The compactions of the cellular pristine and nanocomposite TPU samples were achieved by applying the compressive load in the axial direction with the help of a solid brass cylinder. The digital volume correlation enables the calculation of 3D stains at the voxel level using the Chebyshev spline functions as Page 20 of 30


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proposed by Sjodahl50. The global 3D stain mapping (Fig 9B) successfully demonstrates the constant deformation due to translation, shear and rotation of the reinforced porous structures under the applied load in the nanocomposite TPU. The pristine TPU is almost remained uncompacted (Fig 9A) under compression stress as all the cellular structures were already collapsed in pristine TPU during evaporation of solvent phase. We have thus showed that the cellular structures of a gel could be strengthened by choosing the optimized amount of correct reinforcing nanofillers. The polymeric foams have been extensively used for a range of applications and we believe that our process for fabrication of low density porous structures of commercially available TPU will help to redefine the usages of TPU nanocomposites.

Fig 9: The 3D stain mapping by digital volume correlation of the solid state structure Texin TPU soaked with toluene and dried at 300C under 1 mbar for 3 days with 0 (A) and 1 (B) weight percentage 3D nanofillers using X-ray 3D computed micro tomography.

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4. Conclusions The solid regular cellular frameworks of commercially available TPU have been fabricated through the thermo-reversible supramolecular self assembly route where low dimensional carbonaceous nanomaterials were used as reinforcing fillers. It was established that the minimum shrinkage of TPU lattice structures was occurred when the solid state network is strengthened by the topologically engineered 3D hierarchical nanofillers where the amount of reinforcement was found to play a critical role. The internal free spaces in the 3D fillers were demonstrated to be responsible for the generation of highly folded phase segregated porous structures in the TPU matrix. It has been established by series of structure-property correlations that reinforcing the cellular structure is equally effective as supercritical drying for producing low density porous morphologies. Our findings may have an impact on the robust, cost effective production and uses of PU foams or aerogels fabrication for industrial applications.

Author Information Corresponding Author * Tel/Fax: +91 512 2451759-78. E-mail address: [email protected]/[email protected] (Dr. Debmalya Roy).

Supporting Information The data sheet of Texin SUN 3006 provided by Bayer MaterialScience (S1), TGA thermograph of Desmopan DP 9386 and Texin SUN 3006 PU (S2), SEM images of Texin PU nanocomposite solid gel (S3), 3D hierarchical structures comprising of carbon nanotubes embedded into graphitic flakes (S4), 3D hierarchical carbon nanomaterials assemblies are mostly undisturbed in matrix after making composite (S5) and DSC thermograph of pristine and nanocomposite PU of 0.1, 1 and 5 weight percentage 3D hierarchical nanofillers (S6). Page 22 of 30


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Acknowledgements The authors acknowledge help from Dr. Kavita Agrawal, DMSRDE for the SEM and XRD studies. We are thankful to Mr. Mahesh Choudhuri, DMSRDE for the DSC and DMA measurements. Authors thank Prof. Bikramjit Basu of IISc, Bangalore for pycnometric density measurements. We are thankful to Mr. Ranajit Guha, technical director, Synthetic Moulders Ltd., Kolkata for tensile measurements. Authors acknowledge the help and support of the research scholars and the staff members of the directorate of nanomaterials and technologies, DMSRDE for the experimentations and characterizations. Authors thank Dr. D. K. Setua and Mr. Subhash Mandal, DMSRDE for fruitful discussions and suggestions. Authors are grateful to the Director, DMSRDE, Kanpur for help, financial support in terms of a task project and to permit us to publish our experimental findings.

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The Effect of Reinforcement at Length Scale for Polyurethane Cellular

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