Polyanhydride mechanical properties

Table I. Selected mechanical properties of bone and the crosslinked polyanhydrides [32,33]...

Polyanhydrides have been modified by incorporating amino acids into im-ide bonds. The imide with the terminal carboxylic acids is activated with acetic anhydride and copolymerized with sebacic acid or CCP. Poly(anhydride-imides) increase the mechanical properties of the polyanhydrides. Degradation of poly(anhydride-imide)s is similar to that of polyanhydrides (i.e., surface erosion). Two different cleavable bonds (anhydride and ester) in the polymer chains have been included in polyanhydrides. Carboxylic acid-terminated e-caprolactone oligomers or carboxylic acid-terminated monomers (e.g., salicylic acid) have been polymerized with activated monomers (e.g., SA). 

The development of unsaturated polyanhydrides responded to the necessity of improving the mechanical properties of the polymers in applications such as the temporary replacement of bone. " Unsaturated polyanhydrides, prepared by melt or solution polymerization, include homopolymers of fumaric acid (FA), acetylene-dicarboxylic acid (ACDA), and 4,4 -stilbenzenedi-carboxylic acid (STDA). The chemical structures of poly(FA) and poly(ACDA) are shown in Table 1. These polymers are highly crystalline and insoluble in common organic solvents. The double bonds of these monomers make them suitable for further crosslinking to improve mechanical properties of polyanhydrides. When copolymerized with aliphatic diacids, less crystalline polymers with enhanced solubility in chlorinated solvents result. 

The physical and mechanical properties of polyanhydrides can be altered by modification of the polymer structure with a minor change in the polymer composition. Several such modifications include the formation of polymer blends, branched and crosslinked polymers, partial hydrogenation and reaction with epoxides. 

Biodegradable polymer blends of polyanhydrides and polyesters have been used as drug carriers [59], Polyflactic acid) (PLA), polyfhydroxybutyrate) (PHB), and poly(caprolactone) (PCL), of 2000 and 50000 molecular weights were mixed with poly(sebacic anhydride) (PSA), and the properties of these mixtures were studied. Mixtures of PHB and low molecular weight PLA or PCL formed uniform blends with various amounts of PSA. These blends possess different physical and mechanical properties compared to the parent polymers. The release rate of drugs from these polymeric blends increases with the increase in the content of the rapidly degrading component, PSA. 

In the past decade, extensive research has been carried out on the characterization of polyanhydrides. This section will describe the methods used for the characterization of polyanhydrides and data obtained about their chemical composition and structure, crystallinity and thermal properties, mechanical properties, and thermodynamic and hydrolytic stability. 

A comprehensive evaluation of their toxicity showed that, in general, the polyanhydrides possess excellent in vivo biocompatibility. Their employment in load-bearing orthopedic applications, however, is restricted owing to inadequate mechanical properties. Polyanhydrides-based devices for bone repair are limited to drug-release systems, as in the ease of poly(sebaeie anhydride) (PSA) and poly-D,L-lactide (PLA), for the treatment of ehronie osteomyelitis and in the prophylaxis of bone infeetion." ... 

Another strategy employed to enhance the mechanical strength, lies in the integration of aromatic imide bonds into the backbone of the polyanhydrides. This particular choice aims at combining the good mechanical properties of polyimides with the degradative properties of polyanhydrides. 

Unsaturated homopolymers are crystalline and insoluble in common organic solvents, whereas copolymers with aliphatic diacids are less crystalline and soluble in chlorinated hydrocarbons. The unsaturated polyanhydrides also provide a means for cross-linking through the double bonds that remain intact during the polymerisation process, and thus their mechanical properties can be improved (Domb et al., 1991). 

The physical and mechanical properties of polyanhydrides can be altered by minor modifications. Biodegradable polymer blends of polyanhydrides and polyesters have been investigated as dmg carriers (Leong et al., 1984). A polymeric blend of poly (trimethylene carbonate) (PTMC) with poly(adipic anhydride) (PAA) and the matrix of PTMC—PAA blend was found to be biocompatible in vitro and in vivo experiments as well as a promising candidate for controlled drug delivery erosion with tuneable erosion rate achieved by varying the proportion of PTMC and PAA (Edlund and... 

The most extensively investigated polyanhydride is poly [(carboxyphenoxy) propane-sebacic acid] p(CPP-SA). Ghadel is based on p(CPP-SA), which is used as a dehvery matrix for controlled dehvery of the chemotherapeutic carmustine to treat brain cancer. Septacin, a copolymer based on 1 1 S A and EAD, is used for the delivery of gentamicin in the treatment of osteomyehtis. Poly(anhydrides-co-imides) have been developed to combine the mechanical properties of polyimides with surface-eroding properties of polyanhydrides, and they are being explored as scaffolds for tissue engineering. 

The effect of y-irradiation on polyanhydrides for sterilization purposes has been studied (Mader et al, 1996). Aliphatic and aromatic homo- and copolymers were y-irradiated at a 2.5 Mrad dose under dry ice and the properties of the polymer before and after radiation were monitored. All polymers did not change in physical or mechanical properties and their iH-NMR and IR spectra remain the same. A slight increase in molecular weight was found for aliphatic polyanhydrides. 

Polyanhydrides have been characterized with regard to their chemical composition, structure, crystallinity and thermal properties, mechanical properties, thermodynamic properties, and hydrols c stability A representative set of analysis data has been summarized for the polyanhydrides poly(EAD-SA) and poly(CPP-SA) (Table 4). H NMR spectroscopy indicates the degree of randomness that suggests whether the polyanhydrides is either a random or a block copolymer the average length of sequence (Ln) and the frequency of occurrence of specific comonomers sequences (58). The anhydride bond presents characteristic peaks in... 

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