Enhancement of Mechanical Properties Upon Hydration in Copolymers of PEG and Iodinated Tyrosine-derived Poycarbonates
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Enhancement of Mechanical Properties Upon Hydration in Copolymers of PEG and Iodinated Tyrosine-derived Poycarbonates F. Bedoui,1 L. K. Widjaja,2 A. Luk,1 D. Bolikal,1 N. S. Murthy,1 and J. Kohn1 1
2
New Jersey Center for Biomaterials, Rutgers University, The State University of New Jersey, Piscataway, NJ
Nanyang Technological University, School of Materials Science and Engineering, Singapore
ABSTRACT Increase in modulus upon hydration in copolymers of desaminotyrosyl-tyrosine ethyl ester (DTE) and poly(ethylene glycol) (PEG) with iodinated tyrosines, poly(I2DTE-co-PEG carbonate)s, was investigated by varying the fraction and the molecular weight of the hydrophilic PEG component. Water, as expected, acts as plasticizer in polymer with PEG content < 15 wt% and > 30 wt%. But, water has the opposite effect in iodinated polymers with moderate PEG contents, between 15 to 20 wt%: it enhances the Young’s modulus. The strength and modulus of hydrated poly(I2DTE-co-15%PEG2K carbonate)s increased by as much as fifteen fold upon hydration. While the decrease in the mechanical properties in most polymeric materials with diluents such water is due to the solvent-induced swelling, the increase in strength and modulus that is observed is most likely due to the reinforcing effect of the increased cross-linking efficiency of the hydrated PEG domains in the iodinated polymer. INTRODUCTION In this paper we report the observation that some polymers, which are elastomeric in the hydrated state, are also stronger than in the non-hydrated state. Such behavior is sometimes referred to in the literature as antiplasticization [1,2]. The polymers we studied belong to a family of tyrosine derived polycarbonates, copolymers of desaminotyrosyl-tyrosine ethyl ester (DTE) and poly(ethylene glycol) (PEG), poly(DTE-co-PEG carbonate)s and their iodinated derivatives. Poly(I2 DTE-co-PEG carbonate)s in which desaminotyrosine rings are iodinated, have been found to be useful because medical devices such as stents made from these polymers are visible in x-ray fluoroscopy [3]. Here we explore the effect of hydration on mechanical properties, and report the antiplasticization behavior in some iodinated derivatives over a limited range of hydrations. The observations will be explained in light of the structural changes occurring at nano length scales. MATERIALS AND METHODS Copolymers of tyrosine-derived polycarbonates and PEG of molecular weights of 1000 and 2000 Da at different mole fractions (10, 15, 20 and 30 %) were synthesized by methods described in our previous publications [4-6]. The polymers used in this study are listed in Table I, and will be referred to as “polymer” in this paper. Poly(I2DTE -co- x% PEGnk carbonate)s and poly(DTE -co x% PEGnk carbonate)s, where x is the mole% of PEG and n is its molecular weight in 1000’s, will be referred to as iodinated and non-iodinated polymers, respectively, with w %
2
PEGnk where w is the wt% of PEG; this is because wt% rather than mole % of PEG is relevant for this study. Control samp
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