Characterization of Strong and Crystalline Polyvinyl Alcohol / Montmorillonite Films Prepared by Layer-by-Layer Deposition Method

The preparation of a high-strength and highly crystalline nanocomposite with a layered structure by the use of layer-by-layer deposition (LbL) method from polyvinyl alcohol (PVOH) and montmorillonite (MMT) platelets is reported. The crystallinity and interactions between the components were studied by the use of Fourier transform infrared spectroscopy (FT-IR), wide angle Xray diffraction (WAXD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA).The nanocomposite film structure was investigated by the use of scanning electron microscope (SEM) and atomic force microscopy (AFM).The stiffness of the LbL PVOH/MMT film was significantly higher compared to pure PVOH and conventional PVOH/MMT nanocomposite. The structural and thermal studies on thin PVOH/MMT films indicated the enhanced crystallinity of the polymer.


Introduction
The layer-by-layer (LbL) assembly is a simple method of thin film deposition [1].It was successfully applied to the preparation of thin composite films combining nanoparticles and polymer.It is simple method to produce from diverseby-nature raw materials high quality coatings and films of highly layered structure with regular coplanar orientation of the components [2].Other deposition techniques that can be used for preparation of layered films are spin coating, Langmuir-Blodgett deposition, and others [3][4][5].
Nanocomposites are of scientific interest because of their improved properties due to the nanoparticle application and nanoscale interactions [6].It was demonstrated that the nanocomposites of polymers and nanoparticles could combine the best properties of the components: deformational properties of polymer matrix and high elasticity and strength of nanoparticles [7,8].It has been also reported that the thermal stability and permeability properties of composites could be greatly improved by the addition of nanoparticles [9,10].
One of the most commonly used materials for this purpose is nanoclay or montmorillonite (MMT), in which nanoparticles have platelet geometry [18].It can be used as reinforcing nanoscale filler in hydrophilic and hydrophobic polymer matrices [18].Nanoclay particles strong interaction with polymer can develop intercalated or exfoliated nanocomposites [18,19].An initial MMT is characterized by hydrophilicity, which makes it dispersible in water and compatible with hydrophilic polymers.As a result of MMT very high stiffness (modulus of elasticity  = 175 GPa), it has been widely used as reinforcing filler in polymer films produced by LbL deposition from polyvinyl alcohol (PVOH)  [20], polyurethane (PU) [21], and polyethylene oxide (PEO) [22].Such biopolymers as chitosan [23] and heparin [24] can be LbL deposited with addition of MMT.Biologically inspired and nanoscale designed LbL deposited films of MMT and hydrophilic polymer exhibit extremely high stiffness and thermal properties, as well as maintaining its high optical transparency [25][26][27].The mechanical properties were found to enhance after sequential deposition of PVOH and MMT layers due to the existence of lamina structure and strong hydrogen bonding interactions between the polymer and nanoplatelets [28].MMT also modify the crystalline structure of the PVOH [29].The polymer crystallization occurs in the ultrathin layers (only some nanometers thick).It is strictly affected by macromolecule-nanoparticle interactions.
Recently, it has also been reported that electrically conductive and cross-linked films using PVOH and nanoplatelets can be processed by LbL deposition [30].They can be used as polymer membranes for fuel cells [31].
The LbL deposition method is very simple technique to prepare polymer nanocomposites with organized structures and enhanced properties.In the present work, we are demonstrating the preparation of very strong and highly crystalline polyvinyl alcohol/montmorillonite (PVOH/MMT) thin films with regular layered structure by LbL deposition.The resulting layered films display remarkable tensile characteristics exceeding the properties of the initial polymer material and conventional polymer-nanoclay nanocomposite.We also explore, by the use of different instruments, the crystallinity degree of the polymer in the LbL film, which differs significantly from the crystalline structure of the bulk PVOH polymer.It is important because the crystalline structure strongly influences the final macromechanical properties of the polymer nanocomposite.

Experimental Details
2.1.Materials.Dellite LVF, supplied by the Laviosa Chimica Mineraria S.p.A., was used as a Na + montmorillonite (MMT).The characteristic parameters of the used MMT were shown in Table 1.Polyvinyl alcohol (PVOH) with a molecular weight of 133,000 Da, acetate group content of 0.98%, and density of 1.28 g/cm 3 was used.

LbL Preparation of Layered Thin Films.
For LbL preparation of thin film, room temperature distilled water was used to prepare a suspension of MMT at concentration of 0,5 wt.%.The suspension was stirred for 12 h and sonicated with Hielscher sonicator for 2 h.The PVOH powder was mixed with distilled water and then it was heated to about 90 ∘ C to dissolve the polymer.
Thin film of PVOH/MMT was synthesized by LbL deposition method on a glass substrate [1].Before the deposition of PVOH/MMT films, the microscope glass slides were immersed into "piranha" solution for 1 hour to remove all possible organic contaminants."Piranha" solution was the mixture H 2 SO 4 /H 2 O 2 with the ratio 3 : 1.It was followed by continuous washing with deionized water and drying.
The PVOH/MMT film preparation was the following: glass slide was immersed in 0.5 wt.% solution of PVOH for 30 s, washed with distilled water for 1 min and dried with nitrogen for 1 min, and then immersed in 0.5 wt.% MMT water dispersion for 30 s. Accordingly, one double layer was developed.The procedure continued till 130 double layers were received.A free-standing thin film of PVOH/MMT was detached from the substrate for further examinations.
Two additional samples were prepared to compare the results: pure PVOH film and PVOH/MMT * traditional nanocomposite.Film of pure PVOH was prepared by polymer solution casting in a 100 mm diameter polystyrene Petri dish.It was dried in oven at 60 ∘ C for 48 h.PVOH/MMT * nanocomposite with MMT loading of 3 wt.%was prepared by a solution-intercalation casting method.PVOH was dissolved in the MMT water dispersion, and solution was sonicated for 60 min.The prepared films were kept in closed dish over the dried KBr powder until testing.

Investigation Methods. Atomic force microscopy (AFM)
images of PVOH/MMT thin film surface were obtained using a CP II Scanning Probe Microscope from Veeco Instruments.The instrument was operated in tapping mode with SiN 3 cantilever tips (Veeco Nanoprobes) at a scan rate of 1 Hz.
The surface and the cross section of the PVOH/MMT film were examined with a scanning electron microscope (SEM) Tescan, Mira//LMU Schottky, at different magnifications and a voltage of 25 kV.The surfaces were covered with gold (Emitech, K550X).
The wide angle X-ray diffraction (WAXD) measurements were carried out to evaluate the layered structure of the film.They were obtained on a Panalitical X'Pert PRO diffractometer at a temperature of 20 ∘ C. The monochromatic irradiation of CuK  with a wavelength of  = 0.154 nm in the range of diffraction angles 2theta from 2 to 40 deg was used.
The voltage was 40 kV and the scanning rate 0.125 deg/s.Diffraction patterns were analyzed by the use of software FITYK-093.Crystallinity  WAXD was calculated by [32,33] where   is experimental integrated intensity of amorphous phase and   is experimental integrated intensity of crystalline phase.FT-IR spectra were collected at a resolution of 4 cm −1 on a Bruker VERTEX 80 FT-IR spectrometer in the region of 500-4000 cm −1 .The FT-IR absorption bands were used to study the interactions of the components and to calculate the where   is experimental intensity of the crystallinity band at 1142 cm −1 ;   is experimental intensity of the crystallinity band at 1432 cm −1 ;  and  are constants which were calculated using values of the degrees of crystallinity.The calorimetric tests were carried out on a Mettler DSC-1 instrument.Specimens about 10 mg in weight were scanned in heating mode, by using nitrogen as a purge gas in the temperature range from 25 to 300 ∘ C. The DSC ran at a rate of 10 ∘ C/min.The melting enthalpy and temperature   were calculated from the experimental heating curves.Degree of crystallinity was calculated from [32,38] where Δ exp is experimental melting enthalpy of crystalline phase, J/g; Δ 100% is theoretical melting enthalpy of PVOH crystalline phase, 138.7 J/g [39].
The PVOH and PVOH/MMT thermal stability and the content of MMT were evaluated from the weight-loss heating curves from within a thermogravimetric analysis (TGA).The TGA tests were performed on a Mettler TG50 instrument.Specimens of about 10 mg in weight were heated in air up to 800 ∘ C. The degradation temperature   and weight loss were calculated, according to ASTM D3850, by using the Mettler original software.
The mechanical properties were analyzed by the use of universal tester Zwick BDO.The uniaxial tension was carried out under ambient conditions according to EN ISO 527.The specimens of rectangular strip shape, length 30 mm and width 3 mm, were stressed at constant crosshead rate of 1 mm/min, which is equal to the start deformation speed of 3%/min.More than five parallel samples were tested.The error of the measurements was about 20%.The elastic modulus  was calculated from the stress-strain curves tangent at the beginning of the coordinate axis.

Results and Discussion
TGA was performed to evaluate the thermal properties and the weight loss of the specimens.Figure 1 showed the TGA curves of PVOH and PVOH/MMT films.The degradation temperature   for PVOH was 315 ∘ C. It decomposed very fast starting at 250 ∘ C. PVOH/MMT characterized the decrease of   till 279 ∘ C (see Table 2).It was established from TGA weight loss curves that heating the PVOH/MMT to 600 ∘ C decreases its weight by about 50%.The observed initial decrease in the weight of the nanocomposite could be connected to the some residual water vaporization and in the range of 300-600 ∘ C was also caused by decomposition of the polymer [18,40,41].The layered structure of the film was testified by the use of WAXD. Figure 2 showed the WAXD pattern of pure PVOH, MMT, and PVOH/MMT film with 30, 80, and 130 double layers.The diffractogram curve of initial MMT contained a clear characteristic diffraction peak at 2Theta = 7.0 deg, which was caused by the MMT layers periodicity and corresponded to the basic or interlayer distance  001 = 1.26 nm (Table 1).Diffraction peak of layered structure of the film PVOH/MMT appeared at smaller diffraction angles of 2Theta.It was assumed that, with increased number of the double layers in the film, the characteristic diffraction intensity of layered structure also increased correspondingly.The interlayer distance  001 was about 3. intensity and the shape of the peaks revealed the modification of polymer crystal structure.The macromolecular chain mobility could be restricted due to the absorption on MMT layers and development of strong bond interactions [29].So the PVOH crystallization occurred in the limited space, that is, confined environment [42,43].The degree of crystallinity  WAXD of PVOH was calculated using the method of area under the diffraction curve [32,33].The calculated values were given in Table 2.The diffraction pattern intensity of the PVOH/MMT crystalline phase was more intensive in comparison to the PVOH, which corresponded also to higher crystallinity values. WAXD for PVOH was about 52%;  WAXD for PVOH/MMT was increased till 77%.MMT layers possibly acted as nucleating agent changing the overall polymer crystalline structure and amount of crystalline phase [42,43].FT-IR spectra were received to characterize the developed interactions between the components.FTIR spectra, measured in the range 500-4000 cm −1 for PVOH, MMT, and PVOH/MMT samples, were shown in Figure 3.For PVA, the characteristic absorption peaks were at 3330-3340 cm −1 (O-H stretching vibration), 2942 cm −1 (C-H stretching vibration), 1430 cm −1 (CH 2 bending vibration), 1141 cm −1 (C-C and C-O-C stretching vibration), 1096 cm −1 (C-O stretching vibration), 916 cm −1 (CH 2 rocking vibration), and 850 cm −1 (C-C stretching vibration) [35].For initial MMT, the characteristic bands were observed at 920 cm −1 (Al-O-H), 796 cm −1 (Al (Mg)-O-H), 1040-1120 cm −1 (Si-O), and 525 cm −1 and 470 cm −1 (Al (Mg)-O-Si) [44].The spectrum of PVOH/MMT film showed combined peaks in the bands region of the initial components.A broad band in the region 3300-3700 cm −1 (O-H vibration) was observed, which corresponded to the presence of O-H groups involved in formation of hydrogen bonding between the components.
The results of DSC were presented in Figure 4 as thermograms for the pure PVOH and PVOH/MMT thin film.The heating curves showed broad endotherms at interval 200-300 ∘ C associated with melting of PVOH polymer.Melting temperature   and degree of crystallinity  DSC were calculated from the endotherms and summarized in Table 2.
The PVOH/MMT endotherm shifted to the higher temperature values.The transition started at lower temperatures and ended at higher temperatures.It can be concluded that both values of   and  DSC were very significantly increased for PVOH/MMT films.As it can be seen,   was 273.1 ∘ C; but  DSC was 85.3% compared to 226.5 ∘ C and 44.9% for initial PVOH correspondingly.Observed behavior also could be attributed to the nucleation effect of MMT layers and changes of quality and quantity in the polymer crystalline structure, and FTIR showed strong interaction in the interphase between PVOH and MMT [42,43].Crystalline phase content for PVOH/MMT is two times higher than for initial PVOH.However, more perfect crystalline structure was also formed during PVOH and MMT LbL deposition, which corresponded well to the observed increase of   .
Scanning electron microscopy was used to test uniformity of surface and cross section of the prepared LbL film.The first deposited layer on the glass substrate was PVOH polymer.The surface that contacted the glass was shown in Figures 5(a Atomic force microscopy (AFM) examinations of tapping mode at different resolutions of the deposited films surface also verified dense coverage of the MMT layer (see Figure 6).Micrographs also demonstrated dense and high homogeneity structure of deposited MMT layer.
SEM micrographs of PVOH/MMT film cross section revealed layered structure with high lamina orientation of PVOH and MMT layers (Figure 7).MMT elementary platelets had coplanar spatial orientation.The film could be characterized by dense structure and uniform thickness.The observed thickness of the film cross sections was 3.0±0.5 m.
The measured thickness of double layer was about 35-50 nm.
To understand the difference between the mechanisms of the reinforcement, the mechanical properties of LbL film of PVOH/MMT were compared with data of the nanocomposite of PVOH/MMT * , which was prepared by conventional solution casting (Table 2).It is well known that the solution preparation of nanoclay-polymer nanocomposites acquires the possibility of macromolecules to penetrate into the platelet interlayer and promotes development of intercalated (stacked) or delaminated (exfoliated) structure of the nanocomposite [18,19].Such nanocomposites can demonstrate good tensile property improvements.The conventional nanocomposite of PVOH/MMT * is characterized only by relatively low (about 1.5 times) improvements in elasticity and strength ( = 80.2 MPa;  = 3.2 GPa), because reinforcing efficiency (aspect ratio) decreases due to possible stacking of the nanoparticles.The total interactions intensity at the polymer-nanoparticle interface also decreases with agglomeration of the nanoparticles in the polymer matrix [18,19].
In comparison, the mechanical properties for PVOH/ MMT LbL film showed two times higher strength  = 150.2MPa and ten times higher modulus of elasticity  = 11.2GPa when compared with pure PVOH polymer ( = 85.1 MPa;  = 1.9 GPa).SEM micrographs (Figure 7) testified that PVOH/MMT film had a structure of the laminate,  which consisted of coplanar layers of MMT and PVOH.MMT layers acted as reinforcement with very large aspect ratio and the modulus of elasticity of about 175 GPa [45].PVOH and MMT were bound through formation of strong hydrogen bonds.It was testified by FTIR tests.As a result, very dense network of bonds at the components interface can be developed.The polymer consisted mainly (about 69-85%) of the strong and dense crystalline phase [29].It also contributed to the total increase of the stiffness of the PVOH/MMT LbL deposited film.

Conclusions
PVOH/MMT film was prepared by LbL deposition method.PVOH/MMT film consisted of polymer and MMT coplanar layers.The resulting PVOH/MMT film was found to be strong and highly crystalline.Mechanical properties of PVOH/MMT were superior to the initial polymer and traditional nanocomposite.Formation of high crystalline polymer and hydrogen bonding interactions between the MMT and PVOH also enhanced the strength and stiffness properties of the obtained PVOH/MMT nanocomposite.Concluding, it should be noted that the degree of crystallinity was verified by the use of three different approaches: WAXD, DSC, and FT-IR.The received crystallinity values were in the range of 69-85%.
) and 5(b).The surface looked very smooth and flat in different magnifications.In the comparison, Figures5(c) and 5(d) showed the upper surface with the absorbed MMT layers in which homogeny and dense covered the surface.

Figure 7 :
Figure 7: SEM micrographs of cross section of a 130-double-layer PVOH/MMT film at different magnifications (a, b).