1 Introduction

Polymer composite membranes continue to receive tremendous attention for their potential advanced material applications [1]. Polymer composite membranes are the polymer membranes in which small amount of powder particles are dispersed. The physical, chemical, and electrostatic interactions between the polymer and filler result in improvements of electrical, optical, mechanical, thermal properties of polymer membranes [1,2,3,4]. These are needed to meet the demand in emerging technological applications for such advanced materials. More recently, the significant development occurred in the area of flexible electronic devices.

Sodium alginate (SA) is an environmentally safe, inexpensive, nontoxic, weak acidic, anionic, and linear chained natural polysaccharide material [1]. Recently, the SA acquired more significance because of its diverse properties as well as gel forming capability with metal oxides [1]. Thus, the usage of SA revealed distinct applications in controlled drug delivery, dehydration of organic solvents, removal of metal ions from aqueous solutions, textile industries, food industries etc., [1]. In addition, the SA based polymeric specimen exhibited high ε′ which in turn responsible for potential applications in actuators, artificial muscles, and charge stored capacitors [3]. The polymers containing high ε′ can also adopt high ε″. Therefore, it promotes the electrical conductivity to larger extent. Moreover, the polymer composite membranes possessing high σac value showed extensive applications in molecular electronics [2]. In order to increase the electrical conductivity and dielectric constant in polymers, one has to disperse the ceramic powder particles with high dielectric constant into the polymer solution. In this concern, many scientists used to disperse the inorganic powder particles into the polymer solution such as SA and polyaniline [4,5,6,7,8,9,10,11]. Subsequently, biomedical, electronic device and environmental cleaning applications were obtained. Hence, the polymer composite membranes can be synthesized. In the literature [2] TiO2 fillers were added to the polyaniline for further increase of dielectric constant and electrical conductivity. At this juncture, the authors got an idea to introduce the TiO2 fillers into the SA polymer solution in order to achieve the good dielectric constant and electrical conductivity. However, in the literature very limited studies were found on SA/TiO2 polymer composite membranes [12,13,14,15].

2 Experimental procedure

2.1 Materials

Sodium Alginate (viscosity > 2000 CP) (SDFC, 99.4% purity) and TiO2 (Merck, 99.2% purity) were used as the starting materials. Double distilled water was used throughout the experiment for film casting.

2.2 Membrane preparation

The polymer composite membranes of sodium alginate dispersed with TiO2 were prepared by solution casting method. At first, 4 wt% of sodium alginate solution was prepared by continuous magnetic stirring of 4 g amount of NaAlg in 90 ml of water for 48 h. The required amount of TiO2 (5, 10, 15 wt% based on weight of NaAlg) for each sample was first dispersed in 10 ml of distilled water under ultrasonication for 2 h at room temperature. The well dispersed TiO2 then added to an aqueous of 4 wt% of NaAlg solution. Furthermore, it was magnetically stirred until completely dissolved in order to obtain a homogeneous NaAlg–TiO2 viscous solution. This solution was poured into a polypropylene petri dish and left to dry at room temperature in order to obtain free standing polymer composite membranes. The thickness of NaAlg–TiO2 films for 0, 5 10, 15 wt% concentrations of TiO2 were 0.12, 0.12, 0.13 and 0.13 mm respectively. These are indicated by SA, SA1, SA2 and SA3 respectively. These films were initially dried in vacuum. Further, the films were subjected for different characterization techniques like X-ray diffractometer (Bruker X-Ray Powder Diffractometer, CuKα, λ = 0.15406 nm), Scanning Electron Microscope (Ultra 55 SEM Carl Zeiss), FT-IR spectrophotometer (IR affinity-1, Shimadzu), Differential Scanning Calorimeter (DSC 131, Setaram), Thermo gravimetric Analyzer (TGA, Linseis STA PT 1750) and LCR HiTESTER (HIOKI 3532-50, Japan) for structural, morphological, functional group, thermal and dielectric properties respectively.

3 Results and discussion

3.1 X-ray diffraction (XRD) analysis

The diffraction patterns of all membranes along with pure TiO2 were presented in the Fig. 1. There are no crystalline phases in the diffraction pattern of SA film. But nevertheless, a weak and broad diffraction peak at 2θ = 14.92° is noted for SA film. It may be generally happened owing to the instrumental error. Therefore, as a whole, it indicates the amorphous (non-crystalline) structure of sodium alginate. In the case of TiO2, the maximum intense peak at 27.68° confirmed its rutile crystalline phase. The miller indices (hkl) (110), (101), (200), (211), (220), (002), (310), and (301) are assigned to the diffraction angles 36.3°, 41.49°, 54.53°, 56.82°, 63°, 64°, 69.23° and 70.1o respectively. As 5, 10, 15 wt% of TiO2 amounts are dispersed in SA matrix, there is no significant difference in diffraction angles found for SA/TiO2 films and pure TiO2. But indeed, the decrease in intensity of peaks with decrease in concentration of TiO2 in SA films is observed. This indicates that there is no change on morphology of TiO2 which is attributed to the favorable interaction between TiO2 and SA matrix. Hence, these results suggest successful incorporation of TiO2 particles into sodium alginate matrix. In addition, the more clarity in the crystalline phases is found for pure TiO2 when compared with the dispersed materials. This confirms a fact that with increase of SA content in the titanium dioxide the phase identification becomes much clear. Thus, the conversion from non-crystalline to polycrystalline formation is obviously observed.

Fig. 1
figure 1

XRD pattern of SA/TiO2 films

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3.2 SEM analysis

SEM photographs of the surface morphology for the NaAlg/TiO2 are shown in the Fig. 2. Depending on the amount of TiO2 present in the polymer matrix, the morphology of NaAlg/TiO2 composite membranes will vary and greatly influence its properties. These images show many aggregates or chunks distributed on the top surface indicating the TiO2 particles tended to form aggregates and dispersed into NaAlg polymer matrix. From the images of pure NaAlg and NaAlg titanium dioxide composites, it was observed that the apparent roughness and whiteness (due to small particle size) increases with increase of the content of TiO2 and its dispersion in polymer matrix. In case of the distribution of grains, the homogeneous grains were seen. But these were accommodated like cluster of grains at different spots of the polymer films. Particularly, the shape of grains was almost seemed to be like stones.

Fig. 2
figure 2

SEM micrographs of SA/TiO2 films

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3.3 FTIR analysis

FTIR analysis (Fig. 3) is used to analyze characteristic bonds. The FTIR spectrum of TiO2 has broad absorption bands at 3371, 1596 and 1388 cm−1 that are attributed to –OH, C=O vibrations respectively. In case of polymer composite membranes, in FTIR spectrum of SA membrane, the peak at 3409, and 2815–2731 cm−1 are assigned to stretching vibrations of O–H [16,17,18] and C–H bonds [2] respectively. The strong absorption bands at 1596 and 1388 cm−1 are attributed to the asymmetric stretching vibration of (C=O) carboxylate group. The peaks at 763, 702, and 609 cm−1 attributed to Na–O bond in SA [2]. After the incorporation of TiO2 into these membranes, the FTIR spectra show the systemic increase in OH absorption with increase in concentration of TiO2. This in turn leads to increase in intensity which is determined from the interaction between TiO2 and NaAlg.

Fig. 3
figure 3

FTIR spectra of SA/TiO2 films

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3.4 DSC analysis

The Fig. 4 shows DSC curves of the mixed matrix membranes. It is evident that the heat flow is first decreasing with increase of temperature up to approximately 100 °C and beyond that the heat flow started to increase till 185.76 °C. The DSC curve of SA showed sharp endothermic peak at 185.76 °C and is associated to melting temperature. This peak was shifted to higher temperature region of 218.71 °C, 218.9 °C, and 219.22 °C for SA1, SA2 and SA3 respectively when sodium alginate is incorporated with titanium dioxide. This confirmed a fact that the incorporation of inorganic filler TiO2 into sodium alginate matrix attributes to increase in the crystallization from pure SA to SA3. For further increase of temperature, the heat flow is diminished beyond 260 °C which is a usual behavior.

Fig. 4
figure 4

Variation of heat flow versus temperature

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3.5 TGA analysis

In general, TGA is used to find the thermal stability of present polymer film. With increase of temperature, the thermal degradation of polymer film is observed. It is obvious from the Fig. 5 that the weight loss was decreased with increase of temperature from room temperature to 600 °C. The weight loss is found at two different temperatures. That is, the TGA curves (Fig. 5) of pure sodium alginate and sodium alginate-TiO2 membranes show the two-stage degradation behavior. The first stage corresponds to 10–15% weight loss from room temperature to 180 °C due to expulsion of moisture and residual solvent in sodium alginate. The major weight loss occurred above 180 °C. As concentration of TiO2 increases, a decrease in weight loss is noted with increase in temperature. At high concentration of TiO2, the high thermal stability is noticed. This establishes a fact that the SA1, SA2 and SA3 samples expressed almost constant trend of weight loss with increase of temperature. Therefore, it indicates the thermal stability between 250 and 600 °C.

Fig. 5
figure 5

Variation of weight loss versus temperature

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3.6 Dielectric properties

The dielectric behaviour of polymer composite materials is a powerful technique for studying relaxation and conduction mechanisms in polymer material. Dielectric permittivity is complex quantity given by: ε* = ε′ − iε″, where ε′ and ε″ are the real and imaginary parts permittivities. The real part generally is referred as dielectric constant while the imaginary part is associated to the dielectric loss. The dielectric constant is calculated by using the relation: ε′ = Cd/Aεo, where ‘C’ is the capacitance, ‘A’ is area of cross section of material, ‘d’ is the thickness and permittivity of free space [19]. The dielectric loss (ε″ = ε′ tanδ) is computed using the product of dielectric constant and loss tangent [20]. Both ε′ and ε″ parameters of SA, SA1, SA2, and SA3 mainly depend on two factors such as applied temperature and frequency. These are described as follows.

3.7 Temperature dependence of ε′ and ε″

The variation of dielectric constant and dielectric loss with temperature at 1 MHz frequency is shown in the Fig. 6. The dielectric constant remained a constant value for all the samples up to 90 °C. Beyond this temperature, ε′ is increased with increase of temperature. On the other hand, the dielectric loss exhibited a slow increasing trend with temperature till 80 °C. Above this temperature, a drastic increasing manner is seen. It can be attributed to the fast response of charge carriers in the membranes [21, 22]. However, it is observed that dielectric constant and dielectric loss performed the relaxation behaviour with temperature and undergone transition. This type of relaxation behavior is attributed to the accumulation of charge carriers at the grain boundary interface. Thus, the resonance takes place and further it leads to the peaking value of both dielectric constant and loss values. The dielectric behavior of the pure polymer and membranes SA2, SA3 is same (except SA1). Among all these membranes, SA1 composite membrane shows very interesting behavior in respect of dielectric properties. The highest values of dielectric constant and dielectric loss with temperature are 0.36 and 0.06 respectively for SA1.

Fig. 6
figure 6

Temperature dependence of ε′ and ε″ of SA/TiO2 films

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3.8 Frequency dependence of ε′ and ε″

Figure 7 shows the variations of the dielectric constant and dielectric loss with frequency for different concentrations of TiO2 at 311 K. The dielectric constant and dielectric loss are decreased with increase in frequency. High value of dielectric constant at low frequencies is due to space charge polarization (interfacial polarization) [23]. This value decreases which could be due to dipoles unable to follow the field variation at high frequencies. As frequency goes on increasing, the pure SA, SA2, SA3 shows similar behavior i.e. they exhibit increasing of ε′ and ε″ values at particular frequency instead of usual dielectric behavior [24,25,26]. In general, the polycrystalline materials are composed of two segments such as grains and grain boundaries. The high value of dielectric constant and loss can be attributed to the grain boundary effect. Since, the grain boundaries are more active at lower frequencies. Further at high frequencies, the grains become more active and subsequently, the ε′ and ε″ parameters will be reduced. This behavior is different for SA1 between 2 and 3 MHz. That is, ε′ and ε″ started to increase. For the further increase of frequency again the ε′ and ε″ are decreased. It can be attributed to noticeable enhancement of polarization of composites. The highest values of dielectric constant and dielectric loss with frequency are 0.56 and 0.2 respectively for SA1 at 10 kHz.

Fig. 7
figure 7

Frequency dependence of ε′ and ε″ of SA/TiO2 films

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3.9 Temperature and frequency dependence of AC-conductivity

The Fig. 8 shows the variation of ac-conductivity as a function of temperature and frequency. As the temperature goes on increasing, the ac-conductivity gradually increases. This kind of manner may be acquired due to increase of hopping of charge carriers. That is, for the present samples, the ac-conductivity is almost constant up to 70 °C. This can be achieved as a result of the less response of charge carries for the provided temperature. In general, these polymer films will have less number of charges. For further increase of temperature, the ac-conductivity starts gradually increasing for all the materials. It normally implies that the dielectric relaxation becomes thermally activated and hence this can be governed by Arrhenius equation: σac = σo exp (−Ea/KBT), σo is pre-exponential factor, KB is the Boltzmann’s constant, Ea is the activation energy and T is the absolute temperature [23,24,25,26,27,28]. The high ac-conductivity achieved in this study is 3.3 × 10−8 S/cm at 369 K for SA1. The dielectric constant and dielectric loss are high at particular frequency. This variation in turn allows having high ac-conductivity at that particular frequency. The high ac-conductivity achieved in this study is 2.3 × 10−7 SA3 at 4 MHz. Therefore, this kind of high conducting polymer composite membranes may be expected for molecular electronics [2].

Fig. 8
figure 8

Temperature and frequency dependence of ac-conductivity of SA/TiO2 films

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4 Conclusions

  • The SA/TiO2 polymer composite membranes were prepared by solution casting method.

  • The diffraction pattern confirmed the change of sodium alginate polymer matrix structure from amorphous to polycrystalline on dispersing the TiO2 powder particles into sodium alginate solution.

  • The SEM micrographs evidenced the non-crystalline and polycrystalline nature of formed films.

  • The FTIR spectra showed the existence of functional groups like –OH, C=O, O–H, C–H and Na–O.

  • The DSC curve of SA showed sharp endothermic peak at 185.76° and is associated to melting temperature. This peak was shifted to higher temperature region of 218.71°, 218.9°, 219.22° for SA1, SA2 and SA3 respectively.

  • The TGA analysis showed the major weight loss above 180 °C due moisture and residual solvent in sodium alginate.

  • The high dielectric constant and loss observed at 5 wt% of TiO2 dispersed in the sodium alginate solution.

  • The high σac ~ 2.3 × 10−7 noticed for SA3 at 4 MHz may be well suited for molecular electronics applications.