Degradation Kinetics of Some Coordination Biopolymers of Transition Metal Complexes of Alginates: Influence of Geometrical Structure and Strength of Chelation on the Thermal Stability

1 Citation: Hassan RM (2019). Degradation Kinetics of Some Coordination Biopolymers of Transition Metal Complexes of Alginates: Influence of Geometrical Structure and Strength of Chelation on the Thermal Stability. Material Science 1(1): 3. Degradation Kinetics of Some Coordination Biopolymers of Transition Metal Complexes of Alginates: Influence of Geometrical Structure and Strength of Chelation on the Thermal Stability Refat M Hassan1*, Hideo D Takagi2

The crosslinks have been existed through formation of partially ionic and partially coordinate bonds between the chelated metal ion and both the carboxylate and hydroxyl groups, respectively, of the alginate macromolecular chains of the guluronic blocks in order to form its corresponding coordination biopolymer metal-alginate gel complexes [9,10].
Although, the recognized high importance of those polymeric biomaterials in biotechnological industry through numerous studies of its physicochemical properties [11][12][13][14][15][16][17][18], a little attention was paid to studies of degradation kinetics from the thermal stability points of view.
In terms of the arguments mentioned above and our interest on the thermal decomposition studies of such coordination biopolymer complexes metal-alginates [19][20][21], we have prompted to undertake the cited work in an effort to gain some information on the influence of the thermal stability of these complexes on the performance efficiency in terms of the model structure, coordination geometry and strength of chelation points of view. Again, the results obtained will be DOI: https://doi.org/10.35702/msci.10003 compared with that reported previously for other coordination biopolymers complexes of divalent-metal alginates [19][20][21].

MATERIALS
All materials used were of Analar quality (BDH). Samples of granule nature of such metal alginate complexes were prepared by replacement of the sodium counter ions in alginate (Na +) (Cica Reagent Chemical Co.) by Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ metal ions. The process takes place by stepwise addition of alginate powder to solutions of metal ion electrolytes placed in glass conical flasks (ca. 0.5-1.0 M) whilst stirring the solutions vigorously to avoid any gelatinous precipitate formation, which swells with difficulty. After completion of between the two exchangeable ions (2-3 hrs), the formed granules of metal alginates were washed with deionized water several times, and then with doubly distilled water until the resultant washings became free from the surrounding metal ions [20,21]. The sample complexes were dried at 105 o C under vacuum over anhydrous CaCl 2 or P 2 O 5 [20,21].

TECHNIQUES
Thermogravimetry (TG) and differential thermogravimetry (DTG) of metal alginate complexes were carried out using a Mettler TA 3000 thermal analyzer in static air. Three degrees 5, 10 and 20 K/min of heating rates were used.
A Pye Unicam SP 3100 spectrophotometer using the KBr disc technique (4000~200 cm -1 ) was used to record the IR spectra.
A Philips 1710 diffractometer, the patterns run with copper as a target and nickel as a filter (λ = 1.54187 Å) at 40kV and 30 mA was applied to record the X-ray diffraction patterns. The scanning speed was 3.6 K/min in the range 2θ = 2-60.

RESULTS AND DISCUSSION
Generally, the divalent metal ions from the electrolytes were replaced the Na + counter ions of added alginate powder through ion exchange process. This ion exchange is inherently a stoichiometric process [11][12][13], even the two counter ions may different in either valences or mobilities [6]. The exchange processes can be represented by the following stoichiometric equation (Na-Alg) n + n M 2+ = (M-Alg 2 ) n + n ( 2Na + ) (1)

Solid Electrolyte Solid complex Electrolyte
Where M denotes Mn, Co, Ni, Cu or Zn metals.
A kind of chelation occurs between the interdiffused divalent metal ions with both the carboxylate and hydroxyl functional groups of the alginate macromolecular chains. This chelation is not just simple, but partially ionic and partially coordinate bonds are formed between the metal ion and the carboxylate and hydroxyl groups, respectively, through sort of bridges.
Thus in turn, the chelation will lead to the formation of the corresponding coordination biopolymer complexes in an eggcarton like structure [4,5]. Hence, two configurations of the geometrical structures can be suggested in such chelation [22]. In the first geometry, the functional groups involved of -OCO-group (ν s ) were shifted from 1400 cm -1 for alginate to higher frequencies, respectively. This result indicates that the functional groups are participating into the coordination process. Again, the enhancement of the ν OH band to lower frequencies is larger and broader than that of alginate for all metal alginate complexes which may indicate that the OH groups take part in chelation. Again, these complexes were found to be amorphous in nature. found to be nearly similar for all complexes. This means that the decomposition behavior of these complexes proceeds through similar manner [19,20].    [24] and Satava [25] have discussed how to evaluation of the reaction mechanism by non-isothermal methods. This method is based on the assumption that non-isothermal reactions proceed in infinitesimal time intervals and, hence, the rates can be expressed by the Arrhenius type equation where Z is the frequency factor, t is the time, α is the decomposition fraction, and F(α) depends on the mechanism of the process.
A theoretical method for the reduced time was suggested by Sharp and co-workers [26] in order to distinguish between the different methods where the time scales in the kinetic equation is (6) where g(α) is the integrated form of F(α) which altered so that (7) Here t 0.5 is the time for 50% decomposition and A is a constant depending on the form of g(α).
The determined t 0.5 which corresponds to α = 0.5 (using the experimental data) was used to convert the data to curves of the form α -(t/t 0.5 ) plots. Linear regression analysis was applied to analyze the non-isothermal data according to various kinetic model functions [24]. Table 3 summarized the best fitting models of the hydration processes for the investigated  samples.
Samples of 10 mg were used at the three heating rates 5,10 and 20 K/min in order to obtain reliable kinetic parameters.
However, different methods may be used to calculate the kinetic parameters. The following equation was applied in the present study where φ is the heating rate, E is the activation energy, T is the absolute temperature, T p is the peak temperature of decomposition and R is the gas constant. Equation (8) was based on a proposed reaction mechanism given by Satava together with Coot's-Redfern equation [27]. The g(α) assumes different forms to describe the rate process depending on the model applied. Plots of log g(α)/T 2 against 1/T should be linear as was observed experimentally from which slopes and intercepts, the activation energy and frequency factor can be determined. These values were calculated by using the method of least-squares and are summarized in Table 3.
Again, the entropy of activation can be evaluated from the following relationship [27] (8)    [20].
The magnitude of the activation energies and the maximum temperatures of decomposition shown in Table 2 & 3 may be indicative to the stability of these complexes. The stability was found to increase in the order Zn < Mn ≤ Co < Ni < Cualginates, in good agreement with their magnitudes of M-O bond energies [28] and with the same order of magnitude reported elsewhere [16,18,[29][30][31]. The high stability of crosslinked copper-alginate complex may be attributed to its distorted octahedral structure and high degree of orientation [17].
The influence of heating rates on the thermal decomposition