Preparation of Biodegradable Gradient Polymer by Microwave

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Preparation of Biodegradable Gradient Polymer by Microwave Jiyan Liu a*, Xueqing Liu b College of Chemistry and Environmental Engineering, Jianghan University, Wuhan , China a liujiyan918@163.com; b liuxueqing2000@163.com Keywords: Biodegradable; Chitosan; polyvinyl alcohol; gradient; membrane Abstract: A biodegradable gradient polymeric membrane based on chitosan (CS) and polyvinyl alcohol (PVA) has been prepared by microwave technique. The composition and morphology variation along the thickness direction in membrane were measured by elemental analysis and scanning electron microscope (SEM). The tensile properties of CS/PVA gradient membrane were tested. Results showed that the content of either polymer shows a gradient variation along the thickness direction. SEM photographs exhibit that morphology also evolves gradually with the varying percentage ratio of two polymers. As a result of such variations in composition and structure, the mechanical properties on both sides exhibit a significant difference. Compared with conventional isotropic membrane, gradient membrane has improved mechanical properties. Introduction Chitosan (CS) is an abundant natural biopolymer. Chitosan may accelerate wound-healing, has haemostatic properties and stimulates macrophage activity, and shows a general antimicrobial effect. However, the membranes formed from solely CS show an insufficient permeability for wound exudates and are not flexible enough in a dry state to allow proper handling during the application process [1, 2]. Poly(vinyl alcohol) (PVA) is a familiar hydrophilic, biodegradable, and commercially available linear polymer with good membrane forming property, which offers good tensile strength, flexibility and barrier properties to oxygen and aroma. CS/PVA blended membrane has already been reported to have good mechanic properties for medical products and for controlled delivery of drugs [3, 4]. However, compared to pure CS membrane, some properties of blend membrane such as haemostatic and antimicrobial effect as well as accelerating wound-healing will be weakened due to the CS content reducing. A gradient material is a multi-component composite wherein the compositional gradient occurs from one component to the other [5, 6].Lots of studies [7, 8] revealed that the gradient material has special properties compared to the isotropic material of same composition. In this study, CS /PVA gradient membrane was prepared by varying CS (or PVA) content along the thickness direction, in which one side is composed of CS, and other side is PVA, between them two components change gradually. The CS face provides good antibacterial and haemostatic properties, the PVA face provides good mechanical properties, prevents oxygen permeating and harmful organic compounds invading. Layer-by-layer solution casting is one of methods for processing gradient material. However, it has disadvantage in processing CS/PVA gradient membrane. PVA and CS are water soluble, dried CS/PVA membrane is easily swollen by water, if the water doesn t evaporate from the system quickly enough, the PVA and CS molecules will penetrate into preexisting layer to form the isotropic structure. So the formation of the gradient structure depends on the equilibrium between All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,15:09:29)

2 1950 Application of Chemical Engineering water evaporating and PVA or CS permeating in system. Only when the water evaporates faster than the PVA or CS permeates, the gradient structure may be hold. Microwave is an efficient energy source for polymer processing and substance drying, especially when the system contains polar molecule such as water, it favors the absorption of microwaves and accelerates water evaporation and benefits holding of gradient structure. In addition, the microwave absorption of the material depends on the dielectric constant itself; when the first layer is casted and cured, the dielectric constant of the sample is reduced. The microwave for irradiating subsequent layers has little effect on it, so every layer in membrane has equal microwave absorption. In this paper, microwave was used to process CS/PVA gradient membrane. The structure and mechanical properties of membrane were investigated. Experimental Materials: Chitosan (Commercial grade) with MW of 70,000 (85% N-deacetylation degree) was purchased from Golden-shell Biochemical Co. (Zhejiang, China). PVA (chemical grade, DP= , alcoholysis degree is 99%), glutaraldehyde (solution 50 wt% in water) and acetic acid (glacial, analytical grade) were supplied by Kemiou Co. (Tianjing, China). The CS/PVA gradient membrane preparation:the CS/PVA membranes were prepared by the solution casting method. 5 wt % of PVA solutions were prepared by dissolving the polymer in deionized water at 90 C with stirring. 2.5 wt % CS solutions were prepared by dissolving CS in 2 wt % aqueous acetic acid solution at room temperature with stirring. Both polymer solutions were degassed in a vacuum chamber and then carefully mixed at given mass ratios. Then, the glutaraldehyde was slowly added under constant stirring. The final concentration of glutaraldehyde in the solution was 2 wt %. The resulted homogeneous solution was cast onto a Teflon mold, irradiating at 600 W of microwave power for 2 min to evaporate the great mass of water in system and subsequently at 100W for 30 min. The membrane was submitted for the next casting. Except for the bottom and top layer, the amount of solution of every layer was same. Characterization: The morphology of the membrane was measured by using a scanning electron microscopy (SEM, Hitachi, S-570). The tensile properties of the system were measured by SANS tensile tester (SANS, TAS-10). The test was carried out at room temperature at a strain rate of 5 mm/min; tensile modulus (E) was reported at 0.1% strain. All samples dried in vacuum at 60 C for 10 h before testing. Five measurements were taken for each specimen.gradient membrane of about 0.5 mm in thickness was cut five layers evenly, each of 0.1 mm thickness. The chemical composition of cut layers was measured with Element analyzer (VarioEL III). The swelling ratio (SR) of the membrane was obtained from the difference between the weight of the dried membrane s (W d ) and the weight of the membranes after immersion in distilled water for 24 h at 25 C and subsequent removal of the surface water by blotting with tissue paper (Ws). SR is given by Eq(1): Ws W SR = W d d 100% (1) Results and discussion The microwave power for removing the water in solution is very important. Although higher microwave power will accelerate water evaporating from system, the membrane contracts so fast cause lots of rumples on the surface when microwave power is above 600 W. In this study, CS/PVA

3 Advanced Materials Research Vols solution was irradiated with 600W to remove a large amount of water within 2 minutes and subsequently using 100W microwave to remove residue water. The effect of microwave irradiation time on the tensile properties of CS/PVA membrane is shown in Fig.1. Fig.1. Variation of tensile strength and elongation at break of CS/PVA membrane with Microwave irradiation time.(cs/pva=5:5,w/w) It can be found that tensile strength and elongation at break of CS/PVA membrane increase with time and reach the maximum at 25 min, prolonging irradiation time does not influence the properties. The CS/PVA isotropic membrane with different composition prepared by casting the mixed solution of PVA, CS and glutaraldehyde onto the Teflon mould, irradiating at 600W for 2 min followed by 100W for 30 min. Tensile strength, modulus and elongation at break of CS/PVA isotropic membrane are shown in Fig.2 All above properties change with CS/PVA mass ratio. Up to 8/2 by CS/PVA mass ratio, the tensile strength increases with PVA content. However, it shows a decreased value over that content. With the PVA content increasing, the tensile modulus decrease and elongation increase. Tensile properties show that the flexibility of the membranes increases with PVA content increasing. Fig.2. Variation of tensile properties and SR of CS/PVA isotropic membranes with composition. The CS/PVA gradient membrane was formed by lay-by-layer solution casting, the mass ratio of CS/PVA for casting changes stepwisely in the following order: CS /PVA =10:0,9:1,8:2,7:3,6:4, 5:5,4:6,3:7,2:8,1:9,0:10. The resulting gradient membrane is about 0.5 mm in thickness, it was cut five layers evenly. Fig.3 shows the nitrogen content and corresponding CS content in each layer. It can be seen that in the gradient membrane, the nitrogen content varies from 5.59 wt % for the first layer to 1.2 wt % for the fifth layer and the CS content changes from 81 wt % for the first layer to17 wt % for the fifth.

4 1952 Application of Chemical Engineering The membrane is very thin, it is difficult to test accurately the mechanical properties and water absorption of sliced layer. They were obtained by estimating from the date in Fig.2 with interpolating method. The results are illustrated in Fig.4 and Fig.5 It is found that in gradient membrane, from first layer to the fifth layer, the tensile strength changes from 65.4 MPa to 33.2 MPa,elongation at break changes from 28.6% to 81.1%,the tensile modulus changes from 4.75 GPa to1.17 GPa, and the SR varies from 53.6% to 253%. Fig.3. Nitrogen and CS content in layers of CS/PVA gradient membrane. Fig.4. Tensile strength and elongation at break in layers of CS/PVA gradient membrane. Fig.6 is stress-strain curves for CS/PVA isotropic and gradient membrane, it can be seen that the membrane with gradient structure has higher tensile strength and elongation at break compared to the isotropic membrane with same composition. This result agrees to report by G.C. Martin, et al [9],they proved that creating a gradient structure in a polymeric material can markedly improve its mechanical properties. They explained this phenomenon that all layers are stretched to one and the same degree when the gradient material is strain, and the stress in every layer corresponds to its modulus. Such a stress distribution promotes plastic deformation rather than brittle fracture and thereby increases breaking elongation and fracture energy. Fig.7a is the whole morphology of membrane. Figs.7b-8f is SEM photographs of fracture surfaces for the gradient membrane along the thickness direction. Fig.7b and Fig.7c are for the CS-rich layers, Fig.7d and Fig.7e are for the CS-PVA co-exhibiting layers and Fig.7f is for PVA-rich layer. Except for the CS-rich layer (Fig.7b), all layers exhibit phase-separation structure. In the CS-PVA co-exhibiting layers, many thin ribbon slices with thin white border can be observed. The white border becomes heavier when the PVA increases. Formation of ribbon slices is induced by areas with aggregated PVA causing local centers of high stresses. The white borders coming out from the fracture surface, which is believed to be the crack-propagation direction. At higher PVA

5 Advanced Materials Research Vols content (Figs.7d-7f), the domain of ribbon slices becomes small and short, PVA becomes the continuous phase and CS dispersed in PVA. From SEM results, it can be presumed that in the gradient membrane, the high elastomeric PVA-rich phase can prevent craze and crack initiation and improve its elongation. Fig.7. SEM of the fracture surface for CS/PVA gradient membrane. Conclusions CS/PVA gradient membrane was successfully processed by microwave technique. Its gradient structure is proved by element analysis and SEM. Along the thickness direction, CS content changes from 81 wt % to 17 wt %, tensile strength changes from 65.4 MPa to 33.2 MPa,elongation at break changes from 28.6% to 81.1%, tensile modulus changes from 4.75GPa to1.17gpa, and the water absorption varies from 53.6% to 253%. Compared to CS/PVA isotropic membrane, CS/PVA gradient membrane has higher tensile strength and elongation at break. In gradient membrane, CS-rich side provides good antibacterial, haemostatic properties; PVA- rich side provides the good mechanical properties and prevents oxygen permeating and harmful organic compounds invading. It is expected to be used as wound dressing material. Acknowledgment This work was supported by the Wuhan Science and Technology Fund ( ). References [1] Z. P. Liang, Y. Q. Feng,S.X. Meng: Chin. Chem. Lett. 16 :135. (2005) [2] S. Tripathi, G. K. Mehrotra, P. K. Dutta : Carbohydr. Polym. 79, 711. (2010) [3] P. C. Srinivasaa, M. N. Rameshb, K.R. Kumarc: Carbohydr. Polym. 53, 431. (2003) [4] F. Peng, L. Lu, C. Hu, H. Wu, Z.Y. Jiang : J. Membr. Sci. 259, 65. (2005) [5] B.Y. Wen,G. Wu,J. Yu:Polymer 45,3359. (2004) [6] J. Stabik, A. Dybowska : J. Achi. Mater. Manu. Eng. 25, 67. (2007) [7] P. Tsotra, K. Friedrich:Composites Part A :34,75. (2003) [8] C. L. Qin, W. M. Cai, J. Cai:Mater. Chem. Phys. 85, 402. (2004) [9] G. C. Martin, E. Enssani, and M. Shen:J. Appl. Polym. Sci. 26, (1981)