Role of surface chemistry of fibres additives on rheological behavior of ceramic particle based Shear Thickening Fluids

Author’s Accepted Manuscript Role of Surface Chemistry of Fibres additives on Rheological Behavior of Ceramic particle Based Shear Thickening Fluids Aranya Ghosh, Abhijit Majumdar, Bhupendra Singh Butola www.elsevier.com/locate/ceri

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S0272-8842(18)32289-2 https://doi.org/10.1016/j.ceramint.2018.08.213 CERI19248

To appear in: Ceramics International Received date: 3 July 2018 Revised date: 19 August 2018 Accepted date: 19 August 2018 Cite this article as: Aranya Ghosh, Abhijit Majumdar and Bhupendra Singh Butola, Role of Surface Chemistry of Fibres additives on Rheological Behavior of Ceramic particle Based Shear Thickening Fluids, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.213 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Role of Surface Chemistry of Fibres additives on Rheological Behavior of Ceramic particle Based Shear Thickening Fluids Aranya Ghosh, Abhijit Majumdar, Bhupendra Singh Butola*1 *Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas110016, New Delhi, India [email protected] Abstract

Influence of surface chemistry and size (fineness) of fibres as additives on the rheological behavior of ceramic particle (silica) based Shear Thickening Fluid (STF) has been investigated in this research work. Two different types of fibres, Kevlar microfibres (KMF) and chemically modified cellulose nanofibres (M-CNF) were used for tuning the rheological characteristics of STFs. The extraction of both the fibres, was carried out using supermass collider. The CNFs were then chemically modified with vinyl silane to impart hydrophobic character to the nano-fibers. The characterization was done by TEM-EDX and contact angle analyzer. Ultrasonication was used to obtain the fibers reinforced STFs. In both STFs, reduction in peak viscosity in comparison to virgin STFs was observed with increasing concentration of fibres in STFs. Interaction between additive fibres, silica and media (PEG) was assessed by dynamic state rheological analysis. The crossover point between storage (G′) and loss modulus (G″) for both cases (KMF & M-CNF) shifted towards lower frequency as concentration of

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fibres additives in STFs increased. It is postulated that presence of fibres with hydrophobic characters reduces the interaction between hydrophilic particles and fibres leading to decrease in peak viscosity and increase in critical shear rate. Keywords Kevlar microfibres, Cellulose nanofibres, Vinyl silane, Rheological performance, Interparticle interaction

1. Introduction Shear thickening fluids (STFs) are dense colloidal suspensions of solid particles in a carrier fluid and are characterized by a dramatic increase in viscosity with increase in shear rate [1–5]. The solid particles mostly used are ceramic materials like silica (SiO2), titania (TiO2) etc. of size 100-500 nm and spherical shape. The carrier fluids are polyethylene glycol (MW– 200) and polypropylene glycol. The remarkable shear-ratedependent behavior of STFs has attracted the interest of both the scientific and technological communities for developing smart materials like soft body armors [6–10]. Various theories have been propounded to describe shear thickening phenomena [4,11– 14], however, Order Disorder Theory (ODT) and Hydro-Cluster formation are the two most accepted mechanisms. According to the ODT theory, shear-thickening behavior arises when the applied shear rate causes a disorder in the particle arrangements in the fluid [11–13] whereas, as per hydrocluster mechanism, shear-thickening occurs when hydrodynamic shear forces overcome repulsive steric and Brownian forces between the particles [1,4,14,15]. Recently, to elucidate the mechanism of shear thickening phenomenon in dense colloidal dispersions, a new theory has been proposed which takes into consideration the contact force instead of only hydrocluster formation. The 2

basis of this new theory is contact rheology model [16–18]. According to this model, the hydrodynamic forces are dominant only in low shear stress region of an STF, where contactless interaction prevails. At higher shear stresses, however, contact forces between particles effectively contributes to thickening mechanism. This possibility increases with increasing concentration of particles in STFs, leading to higher viscosity. Since the last few years, efforts have been made to tune the rheological behavior of the STF by changing various parameters such as preparation methods, particle shape and size [5,19–21], particle volume fraction [22], surfactants [23], etc. In recent times, use of fillers and additives to tune the shear thickening phenomena of STFs has also attracted a lot of attention from researchers[16,24–30]. Wagner and Wetzel [25] were pioneers in this field. They patented the fibre reinforced STFs by incorporating short and inert chopped fibre fillers in STFs and studied the ballistic performance of those STFs. It has been claimed that fibre reinforced STFs performed better with enhanced tensile strength and load transfer properties in comparison to neat STF. It was observed that addition of different high performance fibres, such as carbon, glass and HDPE fibres, chopped in to lengths less than 1 cm, improved the stress transfer in prepared STF upon impact, due to high modulus and stiffness of the additive fibres. These STFs, are expected to exhibit significantly improved performance as ballistic, puncture and shock resistant materials. Recently, Laha et al. [26], observed that incorporation of rod shaped halloysite nanotubes (HNT) as additives improved the peak viscosity and impact energy absorption of fabrics treated with such STFs. Dimensions of the nano tubes varied from 50 to 200 nm as diameter and 200 nm to 1.3µm as length. Rheological characterization suggested that with the introduction of halloysite nano tubes in the STF, the critical shear rate reduced and peak viscosity increased significantly. It was postulated that nanotubes actually lowered the

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average distance a silica nanoparticles had to travel for cluster formation. On application of these HNT reinforced STFs on Kevlar fabrics, 40-50% increase in impact energy absorption was observed during drop tower test. A similar study was also carried out by Hasanzadeh et al.[27,28] with multiwall nanotube (MWNT) as nano filler in STFs. Steady and dynamic state rheological behavior of these STFs were analyzed. It was observed that addition of even small amount of MWNT to STF exhibited a disruption of shear thickening phenomena, by retarding the initiation of thickening and lowering thickening ratio. A detailed mechanism of this unusual behavior of multiphase STFs was discussed. According to authors, a network of hydrogen bonding was formed within silica-PEG-MWNT, as all the constituents possessed a propensity to form hydrogen bonding. The bonding between MWNT and PEG being stronger than that between MWNT and silica, higher shear force was required to overcome it so as to initiate the formation of hydroclusters, thus increasing the critical shear rate. Further, the puncture resistance performance of these STFs treated high modulus polypropylene (HMPP) fabrics was evaluated by quasi static puncture test. The puncture resistance performance of single phase STF impregnated HMPP fabric in terms of peak load and energy absorption increased considerably. However, impregnation of fabric with multiphase nanotube loaded STFs showed lesser improvement, due to its lesser shear thickening behavior as already confirmed by rheological analysis. Gürgen et al. [29] documented a comparative study of three different additives such as, boron carbide, silicon carbide and aluminum oxide on rheological behavior of silica based STFs. They observed that the ceramic additives actually disturbed the thickening phenomena of the reinforced STFs. At higher loading, additives actually lower the optimum concentration of silica particles, thus adversely affecting the shear thickening property. Authors proposed two possible reasons for this disruptive thickening behavior,

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firstly decrease in critical solid volume fraction of silica and secondly, disruption of network of hydroclusters by addition of micro sized ceramic particles. Hydroclusters formed at critical shear stress are chain like connected network structures and stronger hydrodynamic forces are formed with bigger hydroclusters (longer chain) which implies stronger shear thickening phenomena. Micron size additives have less interaction with silica particles which hamper the continuity of these hydrocluster structures, reducing the connectivity of these networks and resulting a weaker thickening performance. As the coarser particle is introduced in the system, interparticle distances increase and a deviation in the shear thickening is experienced. Although in case of finer additives, hydrocluster disruption is lesser than that caused by coarser ones. Additive effect on rheological behaviour with respect to temperature is also investigated elaborately. Furthermore, Gürgen and his co-researchers [31] carried out an extensive study on silicon carbide particle based STFs. Effect of different parameters, like suspension temperature, particle size of additives, and amount of additives in the STFs were investigated with three different variable levels. Silicon carbide reinforced STFs [32] were applied on Twaron fabric for evaluating the impact performance against both spike and knife impactors. Carbide particle reinforced STFs treated fabrics experienced an enhancement in impact energy absorption, although the rheological performance did not follow any direct correlation with impact performance. Authors proposed the main mechanism behind the energy absorption to be the enhancement in inter yarn friction in STF treated fabrics. This influence of inter yarn friction becomes more significant against spike impact than the knife impact where fibre cutting dominates over fibre failure mode. Ge et al. and Tan et al. [30,33] used SiC nanowires as a reinforcing additives with silica based STFs. As usual, tiny amount of SiC nanowires increased the viscosity of the

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system significantly as compared to neat STFs. The dynamic response of the neat and reinforced STFs was also analyzed by the Split-Hopkinson pressure bar (SHPB). Besides adding different nanoparticles, fibres and wires,[24,31,32,30,33–37] some researchers have reported long polymer chain based macromolecules as effective additives for tuning the rheology of STFs. Recently, Liu et al. [38] used PVP K30 as a rheology modifier, and modified the STF by two different methods. In first approach, they modified the silica particles with PVP under reflux and then prepared the SiO2@PVP based STF, whereas in second approach, PVP was simply added into the STFs. The modified SiO2@PVP STF showed dramatic enhancement in viscosity as compared to the simply doped PVP based STFs. The possible explanation was that due to chemical modification in SiO2@PVP based STF, PVP molecules remained on particles surface which actually improves the interparticle interaction. In PVP doped STF, a large number of PVP molecules remain in the dispersion media and a very small amount was adsorbed by particles leading to comparatively poor interactions between the constituents. As the above discussion shows, almost all the reported works have focused on altering the rheological behavior of STF by the incorporation of various additives. However, the role of surface chemistry in influencing the particle-media or particle–particle interactions, which can alter the rheological behavior of the STFs has not been explored. In our previous study [39], influence of surface –OH group of cellulose nanofibres (CNF) as fillers on shear thickening behaviour of silica based STFs was analyzed by both steady state and dynamic state rheological analysis. Now, in order to further explore the interaction and role of surface groups of the additives on the rheological behavior of STF, the aim of the present investigation is analyzing the effect of hydrophobic Kevlar micro fibres (KMF) as well as vinyl silane modified CNFs (M6

CNF) as filler fibres on rheological behavior of silica based STFs. In this case both micro and nano sized hydrophobic filler fibres were selected to analyse the influence of size of the filler fibres also, on rheological behaviour of STFs. The modification of CNF was carried out by triethoxy vinyl silane. The KMF and M-CNF reinforced silica based STF were prepared and the interaction between all constituents of system was analyzed by steady as well as and dynamic state rheological analysis. 2. Experimental details 2.1.Materials Silica particles of diameter 500 nm were acquired from Nippon Shokubai, Japan. Polyethylene glycol or PEG (average molecular weight 200 g mol-1), triethoxy vinyl silane, acetic acid and ethanol were supplied by Merck Limited. All the chemicals and reagents used were analytical grade and used without further purification. Kevlar fibres (Kevlar 49, denier – 1420, diameter - 12 microns) used for microfibre production were collected from Kevlar yarn package. Cotton nanofibers (CNF) were produced from a cotton lap with staple fibre length of 40 mm and fineness of approximately 4-4.5µg/inch respectively. 2.2.Extraction of Kevlar microfibers (KMFs) and cotton nanofibres (CNFs) The extraction of KMFs and CNFs in the suspension form was carried out using supermasscollider (Model: MKCA6-2J, Masuko Sangyo Co. Ltd, Japan) [39–41]. Both the raw fibers were first cut in length of 3 mm length and then ground using supermass colloider for 12 h at a speed of 1500 rpm with a clearance gauge of 180 [39]. 2.3. Surface modification of CNFs The vinyl silane (VS) solution was added to the obtained CNF suspension in order to obtain a series of CNF: VS weight ratios of 1:0.5, 1:1, and 1:2. The pH of the system was maintained around 4 by addition of 0.1 M acetic acid solution. The alcoholic

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mixtures of CNF and VS were mixed under continues stirring condition for 3 h under reflux conditions [42–45]. Then the mixture was washed three times and recovered using centrifugation at 9000 rpm for 10 min. 2.4.Synthesis of STFs The neat STF of 65 % (w/w) was prepared by adding silica particles into PEG using an ultrasonicator (Elma S60 H) [39]. The overall solid content of the STF was maintained at 65%. The KMF reinforced silica based STF was prepared by adding appropriate amounts of silica nanoparticles and KMF (0.1 to 5%, w.r.t. silica content) to PEG. The ultrasonicator bath (power: 550W, frequency: 50-60 Hz) was used to completely disperse the silica and Kevlar microfibers in PEG to get a viscous homogenous suspension (supplementary figures, Fig.S1 and Fig.S2). In case of M-CNF reinforced STFs, same procedure was followed maintaining the M-CNF concentrations from 0.1 to 0.3% with respect to silica content. For all the STFs (neat and fiber reinforced) the total solid contain was maintained at 65%. 2.5.Characterization of additives and STFs The dimensions of extracted Kevlar and cotton fibres were investigated by Transmission Electron Microscopy (TEM) analysis. Modification of cotton fibres with vinyl silane was analyzed by energy-dispersive X-ray (EDX) and Fourier transform infrared spectrophotometer (FTIR, Perkin Elmer Spectrum 100). Interaction and dispersion of fibers, silica particles and PEG in STFs was investigated by scanning electron microscope (ZEISS, model: EVO 50). Hydrophobicity of the M-CNF was analyzed by KRUSS drop shape analyzer. The rheological behavior of virgin STF and KMF and M-CNF reinforced STFs was studied using Anton Parr MCR 702 TwinDrive stress controlled rheometer in cone and plate geometry (Cone diameter 25 mm and cone angle 2ο). The gap between the cone and plate was kept at 0.106 mm. For all rheological

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analyses, temperature was maintained at 25±0.1°C. The steady state rheological behavior was analyzed in a wide range of shear rates, from 1 to 1000 s-1. Strain sweep experiment was done before the frequency sweep analysis for determination of linear viscoelastic region (LVR) for all the STFs. Strain rate was varied from 0.01% to 1000% at a fixed frequency of 10 Hz. The frequency sweep test was carried out over the frequency range of 0.01 to 1000 Hz within the linear viscoelastic region as determined from strain sweep experiment. 3. Results and Discussions 3.1.Morphological and chemical analysis It was observed that the diameters of KMF and CNF were in the range of 70 to 600 and 50 to 110 nm respectively. The size of KMF and M-CNF are shown in Figure 1. Chemical structure of Kevlar fibre is Poly paraphenylene terepthalamide. Its internal atomic structure follows a “pi stacking” interaction towards neighboring polymer chains (Scheme 1), which results in very high tensile strength. Kevlar chains are rigid in nature and generally form planner sheets. Kevlar is a synthetic multifilament fibres made by solution spinning technique. It is anisotropic in nature, so the Kevlar fibres are stronger in the axial direction but not in the transverse direction. So during grinding process, due to weak lateral forces, fibrillation of filaments occurred and the effective diameter of the individual filaments was reduced to micro form. However, in case of cotton fibre mainly cellulose microfibrills (as shown in scheme 2) were separated during frictional grinding process in supermass collider. Repeated grinding process reduced the diameter of the fibrils to nano size, though length of the fibrils remained in micron range, resulting in a good aspect ratio, one of the good potential criteria for a fibrous filler. The FTIR study (Figure 2) was carried out to investigate the chemical modification of CNF by silane. Three different weight fractions of triethoxy vinyl silane (VS) were used 9

for modification process. The bands at 1320 cm-1 and 1429 cm-1 are ascribed to symmetric bending of –CH2 and bending vibrations of the C-H and C-O bonds in polysaccharide ring of cellulose repeat units [39] and are present in both unmodified and modified cellulosic cotton samples. The modified cellulose shows peaks at 1600 cm-1 and 1408 cm-1 which are characteristics of C=C stretching present in vinyl silane and -CH2 and =CH scissors vibrations [46,47]. Also, the peak intensity increased as the weight percentage of vinyl silane increased. This can be attributed to the blocking of more –OH group of the CNF which makes it hydrophobic in nature as compared to unmodified CNF. The vibration at 1276 cm-1 is representation of silicone based linkage Si-C [46]. The broad absorption band around 1000-1200 cm-1 is attributed Si-O-Si and Si-O-C stretching vibrations. The modification of CNF was further analyzed by TEMEDX (Figure 3). The presence of Si peak in EDX confirms the modification of CNF with VS. Based on FTIR and EDX results, CNF: VS (1:2) weight ratio of modified CNF was used for further study as it shows higher intensity of vinyl stretching i.e. more blocking of –OH groups present on the CNF surface. In order to support and confirm the blocking of surface hydroxyl groups of CNF with VS, modified CNF dispersion was spin coated on a glass slide and the contact angle was measured for both cellulose nanofibres and modified cellulose nanofibres coatings. In case of cellulose nanofibres, the drop of water was instantly absorbed. It was difficult even to capture the image as the absorption process was very fast. Whereas, M-CNF coating held the drop for longer duration and a contact angle of 47.55 degree was measured from the sample (Figure 4). The reaction mecahnism of CNF with VS has also been demonstrated in Scheme 3. CNF:VS (1:2) weight ratio of modified CNF was used for further study as it was

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expected to influence the rheological behavior of STF to a greater extent as compared to unmodified samples (reported in our previous publication). Figure 5 shows SEM images representing the interaction between fibers and silica nano particles in different STFs. Figure 5(a) signifies the particles without any additives at lower and higher magnifications. As can be seen, particles are well dispersed in PEG, however few agglomerated particles can also be seen due to higher surface area of silica nanoparticles. Figure 5(b) and 5(c) show SEM images of KMF and M-CNF reinforced STFs respectively. In both the cases, particles are more crowded over fibres (clearer at higher magnification). 3.2. Steady state and dynamic rheological analysis of fiber reinforced silica based STFs The effect of both KMF and M-CNF on rheological behaviour of STFs were investigated using steady state analysis. In this analysis, the response of viscosity over a range of shear rates is analysed. As the shear rate is gradually increased, the shear stress which is proportionate with shear rate also increased. As a result of this, particlemedium-additive interactions change. 3.2.1

Steady state rheological analysis of KMF reinforced based STFs

At first the effect of KMF on rheological behaviour of STFs was explored. The neat STF (without any additives), generally follows three basic distinct zones, namely initial shear thinning followed by an abrupt upturn in viscosity or shear thickening and finally again shear thinning at higher shear rates. As discussed earlier, as the shear force (or eventually the shear rate) increases, the particles align themselves in different layers along the flow which reduces the viscosity and therefore the initial shear thinning zone occurs. After that at a particular shear rate, the shear force dominates over the repulsive force between the particles. At this particular point, hydrodynamic lubrication force 11

come to act between the particles. Therefore, this results in transient flocculation or clustering of particles, causing the viscosity of the system to increase abruptly. This particular shear rate is defined as critical shear rate of the system. Besides this, maximum viscosity of the STF system is defined as peak viscosity. At first, very less amount of KMFs (0.1%, 0.2%, and 0.3% w.r.t. total silica content) were added to the silica based STF and their steady state rheological analysis was carried out. From Figure 6, it is evident that neat STF shows the best shear thickening behavior in comparison to all KMF reinforced STFs. Neat STF shows the highest peak viscosity of 1126.4 Pa.s and the critical shear rate of 2.01 s-1. However, when KMF (0.1%) was introduced in the STF system, a decrease in peak viscosity, i.e. 310 Pa.s and a shift in critical shear rate towards a higher value of 16.6 s-1 is observed. However, when concentration of KMF was further increased to 0.2 and 0.3%, peak viscosity value increased up to 350 Pa.s and 442 Pa.s, respectively, and critical shear rate shifted to lower values of 4.07 and 4.06 s-1 respectively. So the experimental plan was designed for higher KMF concentrations (i.e. 0.3, 2 and 5% KMF) to study its influence on shear thickening behavior (Figure. 7). Interestingly, in case of all the KMF reinforced STFs (KMF concentrations - 0.3, 2 and 5%), the peak viscosity is observed to be less than neat STF, and the percentage drops in peak viscosity are 61, 78 and 70% respectively. All the samples show a negative correlation with peak viscosity on addition of KMF as additive. However, the critical shear rate of the system shifts to higher values, i.e. 4.58 and 7.32 for 0.3 and 2% KMF respectively. As the total solid content of all STFs were maintained at a constant value of 65%, with increase in KMF concentration, silica particle concentration get reduced in the system, which may hamper the main shear thickening property, so KMF add-on was not increased beyond 5%.

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The decrease in peak viscosity with addition of KMFs can be attributed to two factors, firstly due to micro size and secondly, to hydrophobic nature of the additive fibres. In the light of contact rheology model, it can be inferred that due to micro size thickness of KMFs and their inert surface, the contacts point between silica particles are reduced. As shown in scheme 4, the micro size Kevlar microfibres actually disturb the continuity network of silica particles based hydroclusters, resulting in decrease in peak viscosity. On the other hand, the critical shear rate of the system gradually shifts towards higher shear rate with the addition of KMF. The reason of this behavior can be attributed to increase in inter-particle distance among the silica particles due to introduction of micro-sized Kevlar fibres. As the particle to particle distance increases, more shear force (i.e. higher shear rate) is necessary to reach the point where hydrodynamic lubrication forces dominate the inter particle repulsive forces and thus the critical shear rate shifts to higher shear rate with the addition of 2% KMF. However, in case of 5% KMF, the critical shear rate shifted to a little lower value i.e. 5.79 s-1, which is slightly higher than the virgin STF. 3.2.2. Dynamic state rheological analysis of Kevlar microfibres based STFs For detailed analysis of the interaction between KMF, silica and PEG in the STF, the oscillatory dynamic viscoelastic properties were analyzed (Figure 8). At first, the linear viscoelastic region (LVR) of the STFs was determined from strain sweep analysis (Supplementary figure Fig.S3). For all KMF based STFs, the LVR region was found to dominate below 1% critical strain which means that up to 1% strain rate, the structural integrity of the dispersion would be maintained. Hence the frequency sweep analysis was carried out by fixing the strain rate at 1%. Frequency sweep data can be used for analyzing or characterizing the STF. It can be clearly seen from Figure 8 that the storage (G′) and loss moduli (G″) are greatly 13

influenced by angular frequency (ω). Generally, a solid like behavior is characterized by the elastic nature of the system i.e., G′>G″. On the contrary, a viscous behavior is characterized by the dominance of loss modulus i.e., G″> G′. A crossover point indicates the transition from elastic to viscous states, where both the moduli become equal i.e., G″=G′. For all these STFs, at lower frequency, the elastic behavior is predominant where the storage modulus is higher than the loss modulus. For neat STF (0% KMF), this nature prevails due to higher silica particle to particle interactions. However, as the STF is compounded with KMF, the elastic nature of the system is enhanced which can be attributed to the reinforcing effect of Kevlar fibres with high aspect ratio. As the angular frequency increases, the time for relaxation decreases and thus the loss modulus or viscous nature of the system becomes prominent. With increase in angular frequency, at a critical angular frequency, a crossover point between storage and loss moduli is obtained. Interestingly, dynamic rheological measurements revealed that the crossover point between G′ and G″ shifts to lower angular frequency as the KMF concentration in STF increases. The results are summarized in Table 1. It was observed that crossover point shifted from 48.28 to 22.87 rad/sec with increase in KMF concentration from 0 to 5%. Therefore, it can be inferred that with increase in KMF concentration in STF, due to its inert nature and micro size, interaction between silica, PEG and KMF actually decreases in STF which hinders the interparticle and mediaparticle interactions and also the stability of the formed hydrocluster is disturbed. In view of above results and discussion, and to confirm that the reduced interactions among particles, medium and additives were responsible for the change in rheological behavior of STF, a separate study with chemically modified cellulose nanofibres (MCNF) was undertaken. In this study, CNFs were modified by vinyl silane to block the hydrophilic –OH group over the surface of the fibres and to render them partially

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hydrophobic. It has been reported in our previous study [39] that unmodified hydrophilic CNFs as additive favor the shear thickening phenomenon in silica based STF due to enhanced interaction between particles and nanofibers. This is attributed to the presence of a large number of –OH groups on the surface of nanofibers, which favors hydrocluster formation at higher share stresses, resulting in increase in peak viscosity and shifting of critical shear rate to lower values (Scheme 5). 3.2.3. Steady state rheological analysis of M-CNF reinforced silica based STFs Figure 9 represents shear rate vs. viscosity analysis of virgin and M-CNF based STFs. Addition of even very small amount of M-CNF in the STFs actually modifies the peak viscosity as compared to neat STF. The neat STF exhibited peak viscosity of 325.29 Pa.s and the critical shear rate attained was 14.71 s-1. However, addition of M-CNF (0.1, 0.2 and 0.3%) reduced the peak viscosity to 298.43, 243.75 and 146.76 Pa.s respectively. However, very little shift in critical shear rate from 13.1 to 11.7s-1 was observed for the STFs reinforced with 0.1 and 0.3% M-CNF respectively. As already discussed, in case of Kevlar micro-fibre addition, the inert or hydrophobic nature of the additives is one of the important factors in preventing the hydrocluster formation. The continuity hydrocluster networks gets affected by lower interaction of filler fibres and silica particles. After the modification of CNF with VS, most of the surface –OH groups (which favor hydrocluster formation) are blocked. Thus M-CNF acquire a chemical nature similar to KMF, albeit with finer thickness. It is already reported [16] that according to contact rheology model, the presence of nano additives has less effect on the continuation of hydroclusters of silica. As the particles and nanofibers both have similar dimensions, the nano sized M-CNF additives do not cause the discontinuity of contact force in the STFs. Therefore, surface chemistry of additives plays a major role in altering the interaction between silica particles by increasing the mismatch between 15

hydrophobic nature of M-CNF additive and hydrophilic silica particles. These M-CNFs hinder hydrocluster formation as particle-fibres interactions get reduced due to hydrophobic nature of these nano-fibres. As a result, the peak viscosity of M-CNF-STF system which is related to the ease of hydrocluster formation, decreases in comparison with the neat STF. While the peak viscosity drops on addition of M-CNF, the critical shear rate for all the STFs remains almost equal. These results correlate well with the thickness of the additive fibres, i.e., micro vs nano. The thickness of the reinforcing fibres determines interparticle distances. In case of M-CNF, there is very little change in the interparticle distances due to its nano size but its hydrophobic surface does not allow much interaction between particles and nanofibers. Hence the critical shear remains almost equal for all M-CNF reinforced STFs (Scheme 6). 3.2.4. Dynamic state rheological analysis M-CNF silica based STFs For better understanding of the of above results, dynamic state rheological analysis of M-CNF based STFs was also carried out. At first the liner viscoelastic region (LVR) of the STFs was determined from strain sweep analysis (Supplementary figure Fig.S4). For all the M-CNF based STFs, the LVR region dominates under 1% critical strain. The trend of storage and loss modulus (Figure 10) for M-CNF based STFs also followed the trend shown by KMF based STFs. At lower frequency, the elastic behavior is predominant where the storage modulus is higher than the loss modulus and at higher frequency, loss modulus dominates over the storage one. In case of 0.3% M-CNF, both the modulus values increased a little with respect to 0, 0.1 and 0.2% M-CNF based STFs. Similar to KMF case, with addition of M-CNF, crossover point shifted towards lower angular frequency from 74.38 to 44.38 rad/sec for 0 and 0.3% M-CNF based STFs (Table 2) respectively.

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This behavior again implies lower interaction between particles and fibres in M-CNF based STFs as was also the case with KMF based STFs. The chemical nature or surface chemistry of the additive fibres plays an important role in influencing the interactions between fibres, particles and medium, which in turn dictates STF properties. Therefore when tuning the STF properties with fibrous additives, the chemical properties and the size of the fibres should also be taken into consideration. 4. Summary and Conclusions This research demonstrates the role of surface chemistry of fibrous additives on rheological behavior of STFs. Both Kevlar micro-fibres (KMF) and modified cellulose nanofibres (M-CNF) based STFs were prepared and their effect on shear thickening property of STFs was investigated by both steady and dynamic state rheological analysis. Kevlar micro-fibres, possess both inert surface chemistry and micron range thickness. Both the factors influence the peak viscosity and critical shear rate parameters of STF system in significant way. The micro size of filler fibres increases the interparticle distance, which delays the initiation of hydrocluster formation which means higher shear force is required to overcome the interparticle repulsive force, shifting the critical shear rate to higher values. In addition, the inert nature of Kevlar fibre reduces interaction between the particles. This reduces the ease of hydrocluster formation. Moreover the Kevlar micro fibres break the continuity of the formed hydrocluster network. As a consequence, the peak viscosity of the system decreases. Dynamic rheological results support the above mentioned findings, by showing a shift in the crossover point of the multiphase STFs to lower angular frequencies in KMFSTFs. These results show that Kevlar micro fibres actually do not create any reinforcement effect in the STF system, due to weak interactions among the constituents. In case of M-CNF reinforced STFs, M-CNF possess inert nature along 17

with nano-size thickness. In this case, the peak viscosity decreased due to weak interaction between all the constituents, whereas the critical shear rate was hardly affected by the M-CNFs. As the nano-fibres possess nano size, interparticle distances are hardly affected. Again the Dynamic analysis followed the same trend as was witnessed in case of KMF reinforced STFs. So this study establishes that the chemical nature of the filler fibres is a key factor for tuning the shear thickening behavior of STFs. Apart from the surface chemistry, size or thickness of the filler fibres (micro or nano) also plays a significant role for determining the inter particle distance which influences the hydrocluster formation mechanism. Acknowledgement The research was financially supported by CSIR (Project No. 22/664/14/EMR-II) and DRDO (Project No. ST-13/TBR-1298). References [1] N.J. Wagner, J.F. Brady, Shear thickening in colloidal dispersions, Phys. Today. 62 (2009) 27–32. doi:10.1063/1.3248476. [2] N. Wagner, J.E. KirkWood,, R.G. Egres JR., Shear thickening fluid containment in polymer composites, US 20060234572A1. [3] Y.S. Lee, N.J. Wagner, Rheological Properties and Small-Angle Neutron Scattering of a Shear Thickening, Nanoparticle Dispersion at High Shear Rates, Ind. Eng. Chem. Res. 45 (2006) 7015–7024. doi:10.1021/ie0512690. [4] J.W. Bender, N.J. Wagner, Optical Measurement of the Contributions of Colloidal Forces to the Rheology of Concentrated Suspensions, J. Colloid Interface Sci. 172 (1995) 171–184. doi:10.1006/jcis.1995.1240.

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Scheme 1: Chemical structure of Kevlar fibres Scheme 2: Chemical Structures of Cellulose Nano fibres Scheme 3: Reaction mechanism between cellulose and tri ethoxy vinyl silane Scheme 4: Interaction of Kevlar microfibers in STF hydrocluster system under shear force

24

Scheme 5: Schematic representation of hydrocluster formation for CNF reinforced silica based STFs. Scheme 6: Schematic representation of hydrocluster formation for M-CNF reinforced silica based STFs.

Figure 1 TEM images of (a) extracted KMF and (b) M-CNF (a′) and (b′) shows the high magnification images of the respective fibre Figure 2 Full range FTIR spectra of CNF, modified by varying the concentration of vinyl silane with respect to CNF. Figure 3 TEM and EDX analysis of vinyl silane (1:2) modified CNF. Figure 4 Contact angle measurement of vinyl silane (1:2) modified cellulose nanofibers Figure 5 SEM images of (a) silica-PEG based STF (b) KMF reinforced STF and (c) MCNF reinforced STF at different magnifications Figure 6 Viscosity-shear rate profile of silica-PEG and KMF reinforced silica-PEG STFs Figure 7 Steady state rheological behavior of silica-PEG and KMF reinforced silicaPEG STFs Figure 8 Response of G′ and G″ against frequency for silica-PEG and KMF reinforced silica-PEG STFs Figure 9 Steady state rheological behavior of neat and M-CNF reinforced silica based STFs

25

Figure 10 Response of G′ and G″ against angular frequency for silica-PEG and all MCNF reinforced silica-PEG STFs Table 1: Changes in cross-over points by addition of different concentration of KMF STFs

Angular Frequency (rad/sec)

0% KMF

48.28

0.3% KMF 42.32 2% KMF

34.53

5% KMF

22.87

Table 2: Changes in the G’ and G’’ cross-over points for different M-CNF based STFs STFs

Angular Frequency (rad/sec)

0% M-CNF

74.38

0.1% M-CNF 49.29 0.2% M-CNF 37.33 0.3% M-CNF 44.38

26

Figure 1

(a)

(a′)

1 µm

100 nm

(b′)

(b)

1µm

600nm

Figure 2

80

40 90

20

1000-1200

3400-3300

60

Transmittance

Transmittance (%)

60

0

30

0 1800

-20 4000

1600

1408 1276

1600

1400

1200

1000

800

Wave number

3500

3000

2500

2000 -1

Wavenumber (cm )

27

1500

CNF:VS 1:0 CNF:VS 1:0.5 CNF:VS 1:1 CNF:VS 1:2

1000

500

Figure 3

Figure 4

28

Figure 5

Figure 6

29

1000

0% KMF 0.1% KMF 0.2% KMF 0.3% KMF

Viscosity [Pa·s]

100

10

1 1

10

Shear Rate [1/s] 100

1000

Figure 7

Viscosity [Pa·s]

1000

100

0% KMF 0.3% KMF 2% KMF 5% KMF

10

1

1

10

Shear Rate [1/s]

Figure 8

30

100

Figure 9

Viscosity [Pa·s]

100

10

0 MCNF 0.1 MCNF 0.2 MCNF 0.3 MCNF 1

10

100

Shear Rate [1/s]

31

100

80

10

70 60

Storage Modulus & Loss Modulus [Pa]

Storage Modulus [Pa] and Loss Modulus [Pa]

Figure 10

/

0% M-CNF G // 0% M-CNF G / 0.1% M-CNF G // 0.1% M-CNF G / 0.2% M-CNF G // 0.2% M-CNF G / 0.3% M-CNF G // 0.3% M-CNF G

1

0.1

0.1

50

40

30

20

20

1

40 Angular Frequency

60

10

80

100

100

Angular Frequency (rad/s)

Scheme 1 O C H N

O

C

O

C

O

C

N

H

H O

N

Kevlar Chemical Structure

C

C O

O

C

H

N

N

H C O

Kevlar filament Micro Fibres

Kevlar Multifilament

32

Scheme 2 H

H2COH

O

H H

CNF Chemical Structure

OH

O

H2COH

H H

H H O

H H

H H

OH

H2COH

O OH H

OH

O

O O

OH

H H

H

OH

Crystalline Region

Amorphous Region Cellulose Microfibrils

Cotton Fibre

Cellulose Nano Fibres (CNF)

Scheme 3 CH2 H5C2O

Si

1. Dissolution in ethanol/water in acidic pH

OC2H5

OC2H5

CH2

Silane coupling agent

O Hydrolyzed silane coupling agent

3. Thermal treatment 3h at reflux condition

OH

Si

HO

Hydrogen bonding

H

H O

CH2

CH2 HO

2. Hydrogen bond formation with CNF

covalent bonding

Si O -H2O

O

Si

O

OH Vinyl silane modified cellulose (M-CNF)

33

Scheme 4 Continuation of network of hydrocluster get disturbed by micro size of Kevlar fibres

Under Shear Force

Bigger distance to form hydrogen bonding

O H O H O H OH

H O O H O

H O

H

Inter particles distance increased

Inert nature Kevlar fibre provides less number of surface functional groups

Scheme 5

34

Under Shear Force

Cellulose nano fibres increase the number of hydrocluster

H OH O H O O O H OHHO H O HO O O H H HO H O O O H

H

Strong hydrogen bonding between CNF and Particles

Inter particles distance

CNF having more surface active groups help in interaction

Scheme 6

Under Shear Force

hydrocluster get disturbed due to hydrophobic character of M-CNF

H

Less interaction due surface modification of CNF

C2 H 5 O H 5 C2 H O O HO H C2 H 5 H 5 C2 O O H H H C2HH5 O OH5C2 M-CNF having more less surface active groups doses not help in Hydrogen boding interaction

35

Inter particles distance