Synthesis and Characterization of Poly(vinyl) Alcohol-AlOOH/LiClO4 Hybrids.
Summary: PVA-AlOOH based hybrids, having varying amount of LiClO4, were synthesized by employing sol-gel process using acid as catalyst and aluminum butoxide as the precursor. At the end, thin films were obtained by solution casting technique. Thermo gravimetric Analysis (TGA), Tensile Properties, Scanning Electron Microscopy (SEM) and X-ray Diffractometry (XRD) studies were used for the characterization. The successfully prepared free standing thin films were thermally stable and with good strength. Since water was used as solvent throughout the study, our synthetic procedure is environment friendly.
Keywords: Hybrid, Poly(vinyl) Alcohol, Aluminum Butoxide, Lithium Perchlorate, Sol - gel.
Poly(vinyl) alcohol is a biodegradable synthetic polymer and is employed successfully in paper coating [1-3], textile sizing [4, 5] and flexible water soluble packaging films [6-8], etc. Films prepared from PVA possess high tensile strength and abrasion resistance and are used in binder [9, 10], blood prosthetic devices , fuel cells , double layer capacitors  etc. Inorganic fillers are commonly employed for modification of PVA based composites to improve its mechanical strength and permeability properties [14-17].
Selection of suitable filler for a PVA composite requires a thorough knowledge of the characteristics of the filler's surface which governs the interaction between the filler and the matrix. Inorganic fillers with surface silanol groups like montmorillonite , silica  and attapulgite  have been shown to interact with PVA through hydrogen bonding. In order to obtain best results, knowledge of the final state of dispersion of inorganics in polymer matrix is critical. Solution coating and casting are commonly used in the preparation of PVA composite films [18-21]. For researchers all over the world, the sol-gel procedure remained a primary choice for the synthesis of hybrid composite materials [22, 23]. The precursor compounds used are usually metal alkoxides [M (OR)n], where M represents a network-forming element (such as Si, Ti, Zr, Al, B, etc), and R is an alkyl group. The hydrolysis and polycondensation of metal alkoxides are typical sol-gel reactions.
A highly porous aerogel is produced, if the drying is carried out in supercritical conditions, otherwise, porous xerogels are formed upon drying at low temperatures (25-100 CAdeg) . Benefits of the sol-gel approach are superior control of the chemical composition of the product and low cost of the process. Materials synthesized by the sol-gel process find their application in optics, energy, electronics, biosensors, controlled drug release (medicine) and chromatography [25-29]. Study and synthesis of polymer composites with hydrogel behaviour is an active research field. Solid polymer electrolytes (SPE), which is another area attracting attention of researchers, are the alternatives to liquid electrolytes in the lithium ion batteries. SPEs acts both as separator as well as the electrolyte. Advantages of using SPEs are good compatibility with lithium metal, low self-discharge, easy processing and good electrical conductivity .
Lithium salts have been used to enhance the conductivity of PVA based SPEs to about 10-8 to 10-4 S cm-1 [31, 32].
PVA/silica system has received great interest due to the known hydrogel behaviour of PVA and is the reason a lot of literature is available on the topic [30, 31]. Work on AlOOH based composites is rare and it is very important to study these composites so that an alternative choice to silica may be available. We have reported earlier , PVA-AlOOH (PAH) based composite with desirable properties in our laboratory. Lithium perchlorate will be added to the PAH matrix to enhance conductivity of the resulting SPEs.
In the present paper, our aim is to study the effect of LiClO4 addition on PAH based hybrids. The sol-gel mediums consisted of poly (vinyl) alcohol, aluminium butoxide, and lithium perchlorate and dilute HCl (0.5N) as catalyst. Aqueous medium has been employed to avoid the use of expensive and very often toxic solvents. The PVA-AlOOH/LiClO4 (PAL) hybrid materials obtained after gelation and drying were characterized for their morphology, thermal behaviour, and mechanical properties.
Poly (vinyl) alcohol of mol. wt. 125,000 (BDH), aluminum butoxide (Fluka) and lithium perchlorate (Merck) were used without further purification. Lithium perchlorate was dried in an oven overnight at 100-110AdegC to remove moisture prior to use due to its hygroscopic nature.
A predetermined amount of PVA was added in water to achieve 8.33% solid content. The mixture was slowly heated to about 90AdegC with stirring until a homogeneous solution was obtained. More water was, then, added to compensate for any moisture loss during heating process. Required amount from LiClO4 solution and PVA solution were mixed in a beaker to get the specified composition, as mentioned in Table-1. Aluminum butoxide was, then, added drop-wise to the above mixture with continuous mixing for about 10 minutes. At the end 0.5ml of dilute HCl was added with constant mixing for about 20 minutes. Thin films of hybrids were obtained by pouring the resultant solution in clean petri dishes to allow the system to gel in vacuum at room temperature to produce film of smooth and uniform thickness.
Table-1: Composition of the PAL Hybrids
Hybrid Code###ABO (Wt %)###LiClO4 (Wt. %)
Apparatus and Procedure
TGA Analysis. Thermal analysis was carried out on a Diamond Series TG/DTA System, made by Perkin-Elmer, USA, at nitrogen atmosphere with a heating rate of 5K min-1 from 298K to 1298K.
Mechanical Testing. Tensile strength, elongation at break and young's modulus of hybrid films were measured using Universal Testing Machine (UTM) 100-500KN M350/500 by Testometric Inc.(UK) at room temperature (27AdegC), using a crosshead speed of 15mm per minute. The sample dimension was approximately 60 x 25 x 0.3mm (l x b x h) and the length between the clamps was 25mm. Procedure employed for measuring tensile properties was ASTM D3039.
SEM Analysis. Scanning electron micrographs were obtained using Scanning Electron Microscope (JEOL, Germany). Samples were gold coated employing sputter coater machine.
XRD Analysis. X-ray diffractometer (Rigaku, Japan) was used for X-ray diffraction (XRD) patterns of the samples to identify the phases represented in the samples and calculation of crystal sizes. During the test CuK[alpha] radiation of wavelength 0.15406 nm, 35kV acceleration voltage and 20mA current was used. The range of diffraction angle was 10.00 to 70.00 - 2I, while scan rate was 0.04Adeg/s.
Results and Discussion
Thermogravimetric Analysis (TGA)
Thermogram of PA is shown in Fig. 1. Mishra and Rao  reported that PVA is stable till 265AdegC but it decomposes in two steps, later on. The weight loss around 350AdegC is due to the decomposition of side chain while the weight loss near 450AdegC is attributed to the degradation of backbone of the PVA [34, 35]. In the present study, the TG curve shows that the PA hybrid is stable till 204AdegC with a 28.0% weight loss due to the structural water and moisture from the surface. There followed a gradual weight loss of 27.0% up to 434AdegC, which is attributed to the degradation of side chain of PVA. Degradation of the main backbone of PVA and transformation of c-AlOOH to c-Al2O3 occurs , with a net weight loss of 41.0%, from 435AdegC till the end of the curve. Thermogram of PAL4 (Fig. 2), resembles that of PA in shape and follows similar steps of weight losses. Degradation temperature of PAL4 is 329.5AdegC, which is higher than PA (319.5AdegC), indicating that addition of salt has imparted stability to the hybrid.
TGA curves of the PAL hybrids with increasing amount of LiClO4 show nearly similar trend (Fig. 2). TGA curve of PAL6 is slightly different, around 400AdegC, from the rest. The reason for this behaviour is oozing out of salt from the matrix of composite. The effect of LiClO4 contents on the degradation temperature of PAL series is shown by Fig. 3. Initially, degradation temperature rises from 319.5AdegC, with the concentration of LiClO4, until it reaches a maximum value of 330.5AdegC at 0.6% LiClO4 concentration (PAL6), after which degradation temperature decreases to a value of 324.5AdegC (PAL10), suggesting a chemical interaction by lithium perchlorate with the PAH matrix.
Stress-strain curve of PA (Fig. 4) showed ductile deformation and uniform yielding over a wide range, with a noticeable yielding point. Humayun et. al.  studied the stress - strain curve of neat PVA, which showed a ductile deformation and undergone a uniform yielding over a wide range. Moreover, stress-strain curve of poly(vinyl) alcohol composites with alumina displayed ductile to brittle transition with the increase of alumina contents. Similar results are reported by Nakane and Bandyopadhyay [16, 36], while studying PVA-Silica composites. Lim and coworkers  also reported that neat polymer went through transition from a ductile to brittle behaviour upon addition of filler.
In Fig. 4, stress - strain curves of PAL hybrids of various compositions are shown. From Fig. 4, important tensile properties like, tensile strength, young's modulus and elongation at break, were calculated and presented in Table-2.
Table-2: Variation of Tensile Strength, Young's Modulus and Elongation at Break with %LiClO4 loading
PA###0.0###8.68 + 0.23###37.08 + 1.43###89.43 + 3.21
PAL2###0.2###8.82 + 0.35###33.71 + 3.01###137.53 + 7.22
PAL4###0.4###13.04 + 0.84###29.50 + 2.35###174.59 + 9.61
PAL6###0.6###15.02 + 1.02###10.28 + 2.83###159.42 + 8.28
PAL10###1.0###14.84 + 0.73###7.62 + 1.04###133.81 + 6.47
Fig. 5 shows variation of tensile strength with the loading of LiClO4 in PAL hybrids. Tensile strength showed an initial sharp increase with the increase of LiClO4 concentration which nearly levelled off at around 0.6% LiClO4 concentration in the final hybrid. This may be due to the interaction of Li+ with the hydroxyl group of PVA and AlOOH in the matrix and thus providing reinforcement to the resulting hybrid .
Value of young's modulus showed a decrease, in Fig. 6, with the increase in concentration of LiClO4. This can be readily explained by taking into the account chemical interaction of Li+ with the hydroxyl group of PVA and AlOOH in the matrix of hybrid. Similar trend was observed by Rahmat  when studying PVC-PEO/LiClO4 composites.
Another conclusion which one can easily draw is that addition of LiClO4 in the PAH system causes a decrease in crystallinity, which is proportional to the value of Young's modulus. Work of Tambelli  and Rahmat  are in compliance with our findings.
Elongation at Break
Fig. 7 exhibits variation in elongation at break with the concentration of LiClO4. Elongation at break initially increases, reaches a maxima and then decreases. This behaviour may be due to the extensive co-ordination between PAH matrix and the lithium ion of the salt. Thus fluidity of the hybrid improves till it reaches maxima. The 0.4% LiClO4 concentration is acting as threshold limit beyond which the behaviour is changing. Further addition beyond this point deteriorates the fluidity of the hybrid. Gulfam  reported similar results while studying PVA/CNT composites.
Scanning Electron Microscopy (SEM)
Humayun  studied the SEM image of virgin PVA and reported that surface of PVA is approximately smooth with some of its own bulging. SEM image of PA thin film is shown at a high resolution in Fig. 8. The AlOOH particles in the SEM image are clearly crystalline. The particle size of AlOOH particles in the PA are about 2-3 um. It is important to note that these particles are almost uniform in size. Since ABO (precursor) had three hydrolysable butoxy groups, each group was able to form three dimensional networks. Protected relations of AlOOH particles in the hybrid were responsible for agglomerations observed in the SEM image . A low resolution image, Fig. 9 of the PA shows an excellent dispersion of the AlOOH particles which is evident from the smooth surface of the PA hybrid.
Fig. 10 shows a representative SEM image of PAL4 hybrid. The surface morphology of the hybrid is smooth having no holes or roughness. This is clear evidence that both AlOOH and LiClO4 were well dispersed in the PVA matrix. A closer look at the Fig. 10, also shows certain agglomerated part of probably AlOOH and some grains of the components of polymer hybrid.
Fig. 11 is the XRD pattern of LiClO4, which shows that LiClO4 is crystalline. Important peaks in the XRD pattern are at 2I, values of 21.1, 23.1, 26.2, 31.4, 32.9, 35.6, 39.4, 49.3 and 52.2.
PA's XRD pattern (Fig. 12) showed a peak at 2I, value of 20.9, which resembles peak at similar point of PVA's XRD pattern . Other important peak of PA is at 2I, value of 29.0. A quick glance at Fig. 12 shows that crystallinity of PA has increased, which is suggested by more and sharp peaks in its XRD pattern as compared to PVA. Thus it can be concluded that addition of AlOOH has positively contributed to the crystallinity of the resulting hybrid. Reason of improved crystallinity of PA is due to a hopping among coordinate sites, confined structural lessening and motions of segments of the polymer. Studies conducted by Humayun  are in compliance with our work. XRD diffraction pattern of various PAL hybrids are shown in Fig. 13. XRD patterns of these hybrids have similarities in shape with the patterns of PA hybrid, as is noted by the peaks at 2I, values of 20.9 and 29.0. Peaks due to LiClO4 were absent, indicating absence of free LiClO4 in the hybrid.
Moreover, it can also be concluded that LiClO4 is incorporated in to the PAL hybrid. Another feature of XRD patterns of PAL series is a continuous decrease in peak at 29.0 with the increase of concentration of LiClO4. This decrease in crystallinity can be attributed to the presence of LiClO4 which tends to complex with PVA . During the complexation, LiClO4 distorts the predominant crystalline nature of PVA. The decrease in crystallinity with the addition of LiClO4 indicates a strong interaction of PVA and LiClO4. These results are in compliance with the work of Gulfam on PVA-CNT composites , Rahmat on PVC-NaClO4 system  and Jayathilaka on PEO based composites .The AlOOH is also playing its part in decreasing the crystallinity as it is more or less amorphous in nature.
PAL hybrids have been successfully prepared employing sol-gel process using acid as catalyst and aluminum butoxide as the precursor. At the end thin films were obtained employing solution casting technique.
Addition of salt has imparted thermal stability to the resulting hybrid as is shown by the TGA studies. Loading of LiClO4 has positive effect on tensile strength and elongation at break while Young's modulus decreased with the increasing concentration of LiClO4. Possible explanation of these observations is the bonding of Li+ with the oxygen of PVA-AlOOH matrix. These results also suggest that addition of salt has decreased crystallinity of the hybrid and the same is confirmed from XRD studies of the hybrids.
Financial support by Higher Education Commission of Pakistan (HEC) is greatly acknowledged.
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|Author:||Khan, Mohammad Saleem; Rab, Abdur; Muhammad, Haneef; Zia, Abid; Humayun, Muhammad|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Aug 31, 2017|
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