. Pyromeliticdianhydride (PMDA) and 4’,4’ -oxydianiline (ODA) were used as monomers of polyimide. To synthesi se a water soluble polyimide precursor (polyamic acid salt), triethylamine (TEA) was added to polyamic aci d with a TEA/COOH mole ratio of 1:1. Titanate nanosheets were synthesi sed by solid -state reaction, ion -exchanged with acid, and exfoliated by TEA . Exfoliated titanate nanosheets were mixed with water soluble polyamic acid salt as reinf orcing filler. Drop c asting was deployed to synthesise polyamic acid/titanate nanosheet nanocomposite films. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to study the morphology and dispersion of nanosheets in the colloidal disper sion and the solid film composite . Modulus and hardness of nanocomposites was provided by nanoindentation. Hardness increased by 90% with ad dition of 2% TiNS while modulus increased by 103% compare d to pur e polymer. This behaviour agree s well with Halpin -Tsai theoretical prediction s up to 2 wt% filler loading ; agglomeration occurs at higher concentration s. Figure. Illustration of titanate nanosheets exfoliation and incorporation in polyamic acid salt 1. Introduction Polymer nanocomposite s consist of a hybrid organic matrix containing a dispersed nanostructure d filler. The filler can provide a significant improvement in polymer properties at low filler loading, due to the high degree of contact between filler and polymer. For example, small addition of exfoliated clay (4.7 wt%) can increase flexural modulus of nylon 6 by four times at 120oC [1]. Successful application of a clay filler to improve the mechanical properties of polymer requires further research on other nanosheet based polymer nanocomposite s containing, e.g., graphene [2], boron nitride nanoshee t [3]. Despite their increasing importance, there are few recent studies of recently titanate nanosheet s for the reinforce ment of polymer nanocomposite s [4]. Polyamic acid salt = polymer exfoliation Bulky ion Acid ion -exchange Bulky molecules ion- exchange Lepidocrocite - like titanate Smectite -like titanate Alkali ion H+ 2 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 Titanate nanosheets (TiNS) are single sh eet inorganic materials created by exfoliation of lepidocrocite - like titanium oxide. Due to their unique properties, this so -called graphene analogue attracts international research interest. It has been applied in some polymer nanocomposite s for functiona l purposes. Titanate nanosheet s act as a photo -initiator for water -soluble vinyl monomer s owing to the photocatalytic properties of titanate nanosheet s [5]. Proton -donating monomers such as N- isopropylacrylamide (IPAAm), acrylamide (AAm), and acrylic acid (AAc) can strong ly bond to the negatively charged titanate nanosheet s and polymerize with TiNS as a physical photo -crosslinker. The polymerization occurs near the TiNS and is trapped by adjacent TiNS , producing a highly sensitive optical response to therma l-stimuli. The p roton conductivity of sulfonated poly(ether ether ketone) (SPEEK) membrane can be doubled by a low loading of titanate nanosheet s (1.67 wt%) [6] and the composite is stable at high temperature (140 oC) and in a wet environment (100 % relativ e humidity) . Adding TiNS to polyethylene naphthalate (PEN) film significantly decrease helium gas permeability [7]. However, systematic study about mechanical reinforcement effects of titanate nanosheet s on the polymer nanocomposite is still limited [8]. In this work, titanate nanosheet s (TiNS) have been used to enhance the mechanical properties of polyamic acid salt (PAAS) . The m orphology of titanate nanosheet s is imag ed by scanning electron microscopy ( SEM ) and transmission electron microscopy ( TEM ). Nanoindentation has been used to examine the reduced modulus ( Er) and hardness (H) of the composites. Experimental results are co mpared with theoretical models due to Halpin -Tsai [9] and Halpin -Kardos [10]. The dispersion of titanate nanosheet s in polyamic ac id is studied by TEM to determine its effect on the mechanical properties of resultant titanate nanosheet s-polyamic acid salt composite s. 2. Experimental Methods Synthesis method of titanate nanosheet s, polyamic acid salt, and composites are considered. 2.1. Synthesis of t itanate nanosheet s A synthesis method for single -layer titanate nanosheets adapted from Sasaki et.al. [11] Sasaki’s single -layer titanate nanosheet s are based on two starting material s as alkali precursor s (K, Li, Rb, Cs) and TiO 2 powder. For syn thesis of titanate nanosheets by the caesium route, TiO 2 (Degussa P25) from Aeroxide and Cs 2CO 3 from Sigma -Aldrich were used without further purification. There are three steps to synthesis single -layer titanate nanosheets. First ly, solid -state reaction of TiO 2 and Cs 2CO 3 was conducted ba sed on the work of Grey et.al. [12] 2.6 g of Cs 2CO 3 and 2 g of TiO 2 (molar ratio 1: 5.3) was mixed, ground and put in a platinum crucible , an inert crucible being crucial to prevent reaction o f alkali precursor and crucible , for one hour heating at 800 oC. The mixture was allowed to cool 3 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 spontaneously. I t was ground then heated at 800 oC for 20 h . The material was ground then the heat treatment cycle was repeated . The resulting product was a white powder of lepidocrocite -like caesium titanate (Cs 0.7Ti1.8250.175O4) with representing a vacancy [13,14]. Second ly, acid ion -exchange was used to create smectite -like acid titanate (H 0.7Ti1.8250.175O4·H2O) [13,14 ]. 2.4 g of Cs0.7Ti1.8250.175O4 was stirred with 96 mL of 1 M HCl for 4 days. The acid solution was replaced with a fresh acid solution every day to maintain the amount of H+ ions for acid leaching of Cs+ ions. Then, the solution was decanted with deionized (DI) water three times followed by vacuum filtration using 0.2 μm nylon membrane and washed with d istilled water unti l the conductivity around 10 μS cm-1. Third ly, bulky molecules ion -exchange was demon strated by exfoliating 0.08 g of H0.7Ti1.825□0.175O4·H2O with 20 mL of distilled water and 400 µL triethylamine (TEA) with bath ultrasonicator for 9 h. The solution was kept for 4 days to separate non -exfoliated sheets at the bottom. Finally, the upper, exfoliated nanosheet s were directly used to make composites. Figure 1. Schematic representation of t itanate nanosheet synthesis 2.2. Synthesis of polyam ic acid salt The work of Lee, et.al. [15] has been used as a guide to synthesise polyamic acid salt. An aromatic polyamic acid salt were prepared from pyromelliticdianhydride (PMDA) and 4,4’ - oxydiani line (ODA) as monomers and N,N -dimethylacetamide (DMAc) as a solvent. All the reactions were shielded by nitrogen gas. As a typical example, a mixture of 1.201 g of ODA 4 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 (1 mol ) and 22.59 g of DMAc was stirred in a 250 m L round bottom flask at room temperature (22 oC). After ODA had dissolved, 1.309 g of PMDA (1 m ol) was added to the solution followed by stirring for 1 day, yielding a 10 wt% DMAc solution of polyamic acid. 1.656 mL of triethylamine (TEA; 2 mol of monomer) was added. Local wh ite precipitation was visible upon addition of TEA to the solution as state d by Kreuz, et.al. [16] and disappear ed after approximately 30 min. The resulting solution was poured into acetone and filtered with a 0.2 µm pore size nylon membrane, followed by washing twice with acetone . After drying at 22 oC for 24 h, light yellow PAAS powder was collected and stored at 22 oC and 40 ±5% relative humidity. 2.3. Synthesis of polyamic acid salt -titanate nanosheet composites All composites in this work were made by solution mixing. A known concentration (determined by UV-vis spectrometry) of tit anate nanosheet colloidal suspension was mixed with PAAS powder to produc e 5 wt% of PAAS -TiNS aqueous solution with various concentration of TiNS ( 0.5, 1, 2, 3, and 5 wt% of solid). For pure polymer, 0.215 g of PAAS was stirred in 4 m L distilled water and 80 µL TEA to assist dissolution of PAAS , yielding 5 wt% of PAAS aqueous solution . The solutions were stirred for 3 days and drop cast on glass. The drop cast solutions were kept at 22±2 oC and 40±5% relative humidity for 4 days to dry and achieve an equili brium moisture content in the films. 2.4. Characterisation The morphology of titanate nanosheets was examined by transmission electron m icroscopy (TEM) (JEOL -3010) and field e mission scanning electron m icroscopy (FE -SEM) (JEOL JSM -6500 ). To prepare TEM sample s, a diluted colloidal suspension was dropped on a copper grid containing perforated carbon film and dried for 1 day in 22 oC. A d iluted colloidal suspension of titanate nanosheet s was dropped on top of a silicon wafer for SEM characterisation. The c oncentr ation of titanate nanosheet suspension was determined with UV -vis spectroscopy. TEM (JEOL -3010) was employed to study dispersion of titanate nanosheets in polyamic acid salt. Samples were dropped on a copper grid with perforated carbon film and spin coated at 4300 rpm for 1 min to achieve a very thin film for TEM. The o ptical properties of polymer composites were evaluated with a 20.1 megapixel camera. The reduced modulus and hardness were obtained with a nanoindenter (Nanotest Platform 3, Micro Materials L td, UK). Reduced moduli were converted to Young’s moduli using the Oliver -Pharr equation [17]. To prepare the samples, a sliver of the film (around 0.5 cm x 0.5 cm) was glued by epoxy glue to the soda -lime glass with the smoother surface (cast side) on top to be measured. Then, 5 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 the glass was glued with acrylic glue to cylinder holder. The cylinder holder with glass and samples on top was put inside the nanoindenter to be measured. A Berkovich (3 -side pyramidal) diamond tip was employed for nanoindentation . A constant loading and unloading rate of 20 mN s-1 were applied with 30 seconds holding time after reaching the maximum depth. 20 indentations were perform ed on each sample at a 0.5 mN applied load. 3. Results and D iscussion 3.1. Morphology of t itanate nanosheet s The structure of the nanosheets is shown in Figure 2. Figure 2. Morphology of t itanate nanosheet s: (a) SEM Images; (b) TEM I mages Here we report the use of tri ethylamine to exfoliate titanate nanosheet s. The morphology of titanate nanosheets is shown in Figure 2a. The nanosheets tend to restack into a large platelet when dried , creating a tactoid phase or ‘skewed’ agglomerates [18]. TEM is needed to identify the size of the nanosheet s. TEM images of TiNS can be seen in Figure 2b. The extremely thin nanos heets result in a very fe int contrast, making it hard to distinguish nanostructured material from the background substrate19. Instead of single layer s, some nanosheet s stack horizontally on the substrate and become more prominent than single layer nanosheet s. The colloidal suspension consists mainl y of small , irregular ly shape d nanosheet particles with a diameter of ca. 100 nm. However, there are some large sheets with a diameter of ca. 300 nm together with very small nanotubes ( ca. 50 nm in length ). Accordi ng to our previous work [20] titanate nanosheets tend to scroll into nanotube s in an alkali ne environment . Such scrolling may also occur with very small nanoshee ts in triethylamine solution . 3.2. Experimental and theoretical analysis of the mechanical p ropertie s of polyamic acid salt -titanate nanosheet c omposites 6 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 Nano indentation and transmission electron microscopy were used to study the behaviour of titanate nanosheets embedded in polymer at controlled concentration s (0.5, 1, 2, 3, 5 wt%). TEM images of PAAS/T iNS are provided in Figure 3. At low concentration (1 wt%, Figure 3a), titanate nanosheets tend to retain their exfoliated form inside the polyamic acid salt. Very thin and small nanosheets ( ca. 100 nm long) were mostly well dispersed, with some trace s of few layer nanosheet s. This shows that the polymer may interact well with the tit anate nanosheets preventing agglomeration . However, some nanosheets lay down on the substrate in a very thin layer which were difficult to observe . At 3 wt% of titanate nanoshe ets, the distance between adjacent nanosheets became closer and van der W aal’s force s between nanosheets became predominant, resulting in agglomerates (Figure 3b) . Such agglomerates are likely to form elongated , ‘skewed’ assemblies of densely packed nanosh eets. Figure 3. TEM images of titanate nanosheets incorporated into the polyamic acid salt matrix at (A) 1 w t% and (B) 3 wt% of TiNS. To study and analyse the reinforcement effect of titanate nanosheet s, nanoindentation data of the polyamic acid salt -titanate nanosheet (PAAS/TiNS) were compared with theoretical predictions due to Halpin -Tsai [9] and Halpin -Kardos [10]. Halpin -Tsai is a well -known model to estimate reinforcement of unidirectional oriented short fibre s and has been successfully used to predict the reinforcement of carbon nanotube s at low filler content (>1 wt%) [21]. It can also be adapted to predict the modulus of polymer nanocomposite with nanosheet s (e.g., clay and graphene) as filler [22,23]. The modified Halpin -Tsai equation is: (1) 7 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 Where Ec and Em are the Young’s modulus of composite (PAAS/TiNS) and matrix (PAAS), respectively. The Young’s modulus of matrix (PAAS) is adapted from nanoindentation measurements of pure PAAS. Af is the filler aspect ratio ( l/h), which in th is case ( l) is the nanosheet length (100 nm) and (h) is the nanosheet thickness (0.75 nm, assuming all the nanosheets are completely exfoliated to single layer s). Φf is the volume fraction of filler which obtained by converting the weight filler fraction , knowing the density of TiO 2 (4.23 g mL-1) and polyamic acid (1.04 g mL-1). µ is a geometric factor, which is (2) Where Ef is modulus of TiO 2 (282.76 GPa, by assuming TiNS as TiO 2) as taken from the CRC [24]. The Halpin -Tsai equation is also able to predict hardness of micro and nanocomposite by simply changing the modulus with hardness [25]. The h ardness of the matrix ( Hm) is determined by nanoindentation of pure PAAS while the hardness of TiO 2 (10.99 GPa) is taken from literature [24]. The Halpin -Kardos equation can also be used for comparison. Originally, this equation wa s applied to randomly oriented short fibre s with a quasi -isotropic laminate assumption involving the [0/+45/90/ - 45] n configuration [10]. However, the expression can be modif ied and has been successfully used to predict the reinforcement of clay up to 2 wt% in a polymer blend [23]. The authors argue d that the tactoid phase of clay may act in a similar fashion to short fibre s. The modified Halpin -Kardos equation is: (3) Where ηL and ηT can be determined with the following equations (4) (5) 8 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 To obtain the h ardness, this equation was modified by substitut ing modulus for hardness . Hardness and modulus of polyamic acid salt -titanate nano sheet (PAAS/TiNS) nanocomposites is shown in Figure 4. Figure 4. The effect of titanate nano sheets on the mechanical p roperties o f polyamic acid s alt compared to the predictions of theoretical models: (A) hardness; (B) m odulus. The addition of titanat e nanosheet s significantly enhanced the hardness and modulus of water -soluble polyam ic acid salt. Hardness increased 90% with addition of 2% TiNS , while the modulus increased 103% compare d to the pure polymer. The improvement i n hardness follows the Halpin -Tsai theory up to 2 wt% while the modulus follows up to 1 wt%. This might happen due to the nanosheets at low filler concentration being fully exfoliate d in Figure 3a) maintaining the same aspect ratio and polymer - filler interaction. Moreover, t itanate nanosheet s have an anisotropic reinforcement which is highest when the loading is perpendicular to the nanosheet [5]. Figure 3a ) shows that most of the nanosheets lay flat on the substrate, rendering them perpendicular to the direction of nanoindentation loa ding. This is consistent with the Halpin -Tsai theory which assumes that the reinforcement direction is unidirectional to the loading. At higher loading of filler, ‘skewed’ platelet s or tactoid phase s were 9 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 created (Figure 3b). Such elongated agglomerates ma y act as short fibre s with random orientation, in agreement with the Halpin -Kardos prediction. Similar effects occur with clay filler s [23]. However, experimental results do not follo w set theories at higher filler concentration (5 wt%) , possibly due to severe agglomeration s acting as stress concentration sites [23]. Addition of nanostructures may induce brittleness in polymers [26]. Nanoindentation is also able to give an indication of brittleness via the plasticity index. This index is the ratio of plasti c work done on the material to total work (plastic work and elastic work) applied [27]. The index ranged from 0 to 1; brittle materials , such as glass , tend to have low value (0.38) while ductile steel has a higher value (0.72). Table 1 shows that the addit ion of ti tanate nanosheet s up to 5 wt% did not significantly change the brittleness of polyamic acid salt. Table 1. Plasticity index of the pure p olymer (PAAS) and the c omposite s (PAAS/TiNS) for various loading of TiNS Sample name wt% of TiNS plasticity i ndex Pure PAAS 0 0.63208 PAAS /TiNS /0.5 0.5 0.691766 PAAS /TiNS /1 1 0.660274 PAAS /TiNS /2 2 0.636735 PAAS /TiNS /3 3 0.622159 PAAS /TiNS /5 5 0.69291 Figure 5 shows that the transparency of polyamic acid salt is reduced when the concentration of titanate nanosheet s exceeds 1 wt%. The agglomeration may start to form at a higher titanate loading in Figure 3b) causing scattering of light inside the polyamic acid salt. Figure 5. A photograph showing t he effect of t itanate nanosheets on the optical propert ies of polyamic acid s alt (titanate content from left to r ight: 0, 0.5, 1, 2, 3, 5 wt%) 10 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054 4. Conclusions Polyamic acid salt -titanate nanosheet composites have been successfully prepared by solution mixing. Triethylamine (TEA) can be used as a bulky molecule , instead of the more common tetrabutyl ammonium hydroxide , to exfoliate titanate nanosheets. Ultrasonication can assist intercalation of TEA into the nan osheets and their delamination into thin nanosheet s. Addition of titanate nanosheet to water soluble polya mic acid significantly enhanced hardness and modulus of the polymer. The hardness increase d by 90% on addition of 2% TiNS while the modulus increased by 103% compare d to pure polymer. A h igh aspect ratio ( l/d = 133.33) and uniform distribution of titanate nanosheets provide s significant reinforcem ent of polymer. This improved behaviour is consistent with Halpin - Tsai theory at low filler content, assuming that the hardness and modulus of titanate nanosheet are the same as TiO 2. At high filler loading (>2 wt% ), the mechanical properties of composites do not fit established theories due to agglomerat ion of titanate nanosheets. These sheets were stacked on each other , creating several layers of ‘skewed’ agglomerates or tactoid phase s. This causes poor distributi on of nanosheets and might disturb stress distribution between nanosheet and polymer. The incorporation of titanate nanosheet s has no significant impact on the plasticity of polymer , as indicated by the plasticity index of composite ; this assure s flexibili ty of the composites . However, the inclusion of the nanosheets slightly reduces the transparency of polymer , particula rly when the titanate nanosheet weight percentage exceeds 1 wt% , due to agglomeration . Acknowledgements The author s are grateful to the Indonesian Endowment Fund for Education (LPDP) for funding . 11 1234567890Innovation in Polymer Science and Technology 2016 (IPST 2016) IOP Publishing IOP Conf. Series: Materials Science and Engineering 223 (2017) 012054 doi:10.1088/1757-899X/223/1/012054