Effect of anionic surfactants on grafting density of gelatin modified with PDMS-E
Graphical abstract
Introduction
Gelatin, a water-soluble protein produced from hydrolysis of animal collagen with poor mechanical properties especially when exposed to wet and humid conditions [1], [2], [3], [4], [5], [6]. The chemical properties of gelatin are switched from purely organic biopolymer to inorganic–organic grafted polymers by introducing increased levels of PDMS [7], [8], [9]. The compatibility between gelatin and PDMS plays a crucial role in determining the degree of grafting in heterogeneous system. Surfactant receives considerable attention to promote compatibility of immiscible polymer in heterogeneous system because of their ability to impart significant changes to the interfacial and physico-chemical properties of polymers [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Theoretical modeling of gelatin/anionic surfactant system suggests that the gelatin segments bind in the surfactant micelle palisade layer and shield a fraction of the hydrophobic core from contacting with water, resulting in a decrease in the interfacial free energy of gelatin solutions [22]. Commonly, free energy of micellization of the surfactant (ΔG°M) is determined by the value of the critical micellization concentration (cmc) of a surfactant in aqueous solution [23], [24], [25]. However, the interfacial behavior of gelatin chains induced by anionic surfactant which is particularly relevant in determining the compatibility for the two immiscible polymers has not yet been systematically studied.
The electrostatic and hydrophobic interactions between gelatin and anionic surfactants in a nondilute solution can cause dispersed particles to accumulate preferentially at a fluid interface [26]. The self-assembly of the gelatin/anionic surfactants in a phase-separating system has the potential to arrest the process of demixing arising from spinodal decomposition (or nucleation and growth) by the formation of a viscoelastic particle layer at the liquid–liquid interface [27], [28], [29], [30], [31], [32], [33]. By following the distribution and movement of particles within the evolving system, there is the opportunity to gain quantitative information about the local microstructure of different microphase regions [34], [35].
The purpose of this paper is to study the effect of anionic surfactants on the aggregation behavior of gelatin chains at a fluid interface. A particular aim will be to shed light on the mechanism by which anionic surfactants control the grafting density (=a decrease in number of NH2 groups per unit mass) of PDMS-E grafted gelatin (PGG) polymers.
Section snippets
Material
Gelatin (type A obtained from pigskin, having an approximate Mw of 50 000 and isoelectric point at pH 8.4–8.8 determined by fluorescence measurements) was obtained from China National Medicine Corporation without further purification or treatment. The alkyl sulfates and sulfonates were purchased from Alfa Aesar and recrystallized from ethanol before use, including sodium heptyl sulfate (SHepS), sodium octyl sulfate (SOS), sodium decyl sulfate (SDecS), sodium undecyl sulfate (SUS), sodium dodecyl
Chemical modification of gelatin by PDMS-E
The chemical modification of gelatin through grafting reactions between free NH2 groups in gelatin and PDMS-E was performed in an aqueous medium. XPS results show that the peak N1s shifts from 401.5 eV to 399.87 eV, which identifies the formation of CN bond. N/O changed from 0.7516 to 0.1594, which indicates that the free amino groups were consumed. The relative concentration of Si at the surface of gelatin film reached to 18.64%.
The concentration of the gelatin solution was 5% (w/w) and the
Conclusions
The mechanisms by which anion surfactants control grafting density of PGG polymers are revealed. Based on the above results of viscosity, surface tension and HR-TEM, the effect of anionic surfactant on the aggregation behavior of gelatin chains at a fluid interface was investigated for explaining the change of grafting density at different surfactant system. Hydrophilic segments of gelatin bound in the sulfate surfactant micelle palisade layer and prevented a fraction of the hydrophobic core
Acknowledgments
This work is supported by the National Natural Science Funds of China (Nos. 21376125, 21276149), National Natural Science Funds of Shandong (No. 2013ZRB01572) and supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.
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