Mechanisms of polymer ca2+ interaction and their effects on the characteristics of alginate microspheres and films 3

Part III. Influence of incorporating other polymers on the properties of the alginate matrix A. Light transmittance, thickness and percent weight loss of films and FTIR spectroscopy The copolymers chosen in this study are natural gums that have a long history of applications in the food industry. These gums are safe, biocompatible and biodegradable. Their influence on the properties of the alginate matrix was investigated. Light transmittance measurements are often used as an empirical method for the determination of phase mixing in polymer composite materials (Krause, 1972). Composite materials which are not homogeneous i.e. the individual polymers are not completely miscible, will reduce light transmittance because of the quality of scattered and reflected light. This will result in lower transmittance values of the composite materials than those obtained from individual polymers (Zhou and Zhang, 2001). Light transmittance through a film may also be affected by its thickness. Preliminary studies using SA films showed insignificant difference in the percent transmittance values obtained if the film thickness varied within 0.0389 to 0.0578 mm (Table 18). Hence, this film thickness range was used for all test films in the subsequent measurement of light transmittance in all studies. Pectin and ι-carrageenan showed good miscibility with sodium alginate as shown by the high light transmittance values of uncross-linked AP and AI films (Table 19). Yoshida et al. (2001) found that the miscibility of polymers depended strongly on polymer-polymer interactions, such that a slight difference in the conformation of the polymer chain might affect the miscibility of the polymers with 130 Table 18. The influence of thickness of SA films on light transmittance. Film thickness (mm) Transmittance (%)* 0.0380 0.0396 0.0488 0.0526 0.0578 87.0 86.7 85.5 84.8 84.0 SA films were composed of sodium alginate (Manucol® DH, uncross-linked). *Only determination of light transmittance was carried out for the films of different thickness. 131 one another. This was especially evident with the addition of κ-carrageenan to alginate. κ-carrageenan is structurally similar to ι-carrageenan with the exception of having only sulphate group (Figure 3) but the transmittance of uncross-linked AK was significantly lower than that of uncross-linked AI. The addition of κ-carrageenan and gellan gum to sodium alginate gave rise to lower light transmittance values indicating that the polymers were not completely miscible (Table 19). Thickness is an important film property as it affects the tensile strength and permeability of films. The degree of cross-linking might also be reflected to a certain extent by the percent change in film thickness after cross-linking. The changes in film thickness after cross-linking are shown in Figure 22a. Although the amount of film former (sodium alginate with or without copolymer) used was constant, significant differences in film thickness were observed (p0.05) probably led to the minute changes in film permeability of the cross-linked AP films (p>0.05). The alginate matrix became more permeable with the addition of carrageenans (Table 22). The cross-linked composite AI and AK films had greater permeability coefficients than cross-linked SA films with the exception of AK 50/50. This higher film permeability was attributed to the greater extent of hydration of the composite films as carrageenan is highly hydrophilic. The cross-linked KC and IC films were hydrated to a considerable extent and disintegrated when placed in aqueous media. This allowed easy drug diffusion across the cross-linked composite films. Work by 165 Sipahigil and Dortunc (2001) also suggested greater matrix drug permeation for κcarrageenan than cross-linked alginate. Higher drug encapsulation efficiency was reported for alginate beads than κ-carrageenan beads produced by the extrusion of polymer-drug solution into cross-linking solution (Sipahigil and Dortunc, 2001). It appeared that more drug was able to diffuse out of these beads than cross-linked alginate beads during bead production. The trends in film permeability brought about by the addition of 10 %, 30 % and 50 % of copolymer are as follows: AI > AK = AP = AG, AI > AK > AP = AG and AI > AG > AP = AK. It can be clearly seen that cross-linked AI films were most permeable to the drug molecules, compared to the other cross-linked composite films, for all proportions of copolymer used (p[...]... (Figure 22a) .The Ca2+ enhanced gelation of κ-carrageenan by stabilization of the helical conformation of the polymer, through shielding of the charge of sulphate groups by Ca2+ (Watase and Nishinari, 1986) This phenomenon enabled tight binding and enhanced aggregation of the helices, which accounted for the reduction in thickness of KC and AK films after cross-linking The increased aggregation of the helices... properties reveal the actual extent of interaction or association between constituent molecules The effect of polymer selfassociation or hetero-association on the mechanical properties of composite matrices depends on the proportion and type of polymer/ copolymer involved The tensile strength and elastic modulus of SA films increased after crosslinking with Ca2+ The cross-linking action pulled polymer chains... proportion of alginate to copolymer and the moisture sorption ability of the individual polymer components Drug release across the alginate matrix was affected by varying the type and amount of copolymer used Generally, the cross-linked matrices maintained their integrity in water The carrageenans increased the hydrophilicity of the alginate matrix and would most likely promote diffusion of drug across the. .. chains do not interact with Ca2+ Their presence could hinder the formation of the ‘egg box’ structure with alginate Therefore interactions between alginate and pectin are likely to occur along some segments of the polymer chains, resulting in the composite film having a more loosely packed structure than the SA and PT films (Figure 26) The interaction between the polymers and Ca2+ were evident from FTIR... proportion of copolymer in AP films increase, the films became stiffer after cross-linking It could be deduced from the results that the tensile strength and elastic modulus of the matrix were influenced to a greater extent by the structural arrangement of the polymer chains than the extent of interaction of the polymer with Ca2+ Thus, by adjusting the proportions of alginate and copolymers, composite matrices... deionised water ( 1.2 ( ) ) and USP buffer pH 158 Table 21 Hydration indices of films Film code Hydration index (%) SA 286.91 ± 3. 55 PT 33 6.95 ± 9 .30 KC - IC - GG 692.97 ± 18 .37 AP90/10 288.99 ± 6.75 AP70 /30 30 1.11 ± 30 .25 AP50/50 271.15 ± 36 .28 AK90/10 36 1.14 ± 2 .32 AK70 /30 36 5.58 ± 8.26 AK50/50 406.42 ± 5.89 AI90/10 33 1. 83 ± 4.89 AI70 /30 34 6.09 ± 6.01 AI50/50 458 .38 ± 4.04 AG90/10 278.19 ± 4 .31 AG70 /30 ... in the films and produced films, as indicated by the lower tensile strength In AP films, the presence of methoxy groups scattered along the pectin chains tend to hinder the formation of the ‘egg box’ structure with alginates and this caused lower tensile strength for the AP films (Figure 30 ) Since the methoxy and amide groups are unable to cross-link with the carboxyl groups in the presence of Ca2+ and. .. sequestered by phosphate ions which led to the disruption of cross-linkages and consequently the disintegration of films (Østberg et al., 1994) Thus, addition of the copolymers to alginate enhanced the stability of the matrix in an acidic medium These composite matrices are potentially useful for enteric applications as they exhibit reasonable integrity under gastric conditions and disintegrate at intestinal... aggregation of the helices (Michel et al., 1997) This aggregation of the helices contributed to increased tensile strength and elastic modulus in AI and AK films and this was especially prominent in cross-linked AI 70 /30 and AK 70 /30 films Michel et al (1997) also suggested that 155 addition of Ca2+ extended the crystalline regions of carrageenan gels, hence increasing their elastic moduli Cross-linked AI and. .. blocks of the alginate (Sartori et al., 1997) which mainly involved interactions with Ca2+ Thus, the above observations from the FTIR spectra indicated a change in polymer ring and linkage conformation before and after cross-linking in both polymers, suggesting interactions between the two types of polymer chains in the cross-linking process The higher light transmittance of cross-linked AP films as . Ca 2+ . Their presence could hinder the formation of the ‘egg box’ structure with alginate. Therefore interactions between alginate and pectin are likely to occur along some segments of the polymer. the ease of the constituent polymer to dissolve in the cross-linking medium and the extent of interaction between the cross-linker and polymer chains, bringing the chains closer together. The. influenced the film thickness (Figure 22a) .The Ca 2+ enhanced gelation of κ-carrageenan by stabilization of the helical conformation of the polymer, through shielding of the charge of sulphate