Polyanhydrides crystallinity

B. Characterization of Thermal Properties, Crystallinity, and Phase Behavior of Polyanhydrides... 

It is important to characterize the thermal properties of polyanhydrides that are proposed for drug delivery applications, as changes in crystallinity... 

Most of the commonly used polyanhydrides, including the copolymers, are semicrystalline. Crystallinity is characterized by a variety of techniques... 

Fig. 4. Crystallinity of several polyanhydride copolymers as a function of composition. From Mathiowitz et al. (1990b). Reprinted with permission.

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. 

It has been shown that the crystallinity of polymers affects erosion and drug release rates, because crystalline regions erode slower than amorphous ones. Moreover, highly crystalline polyanhydrides affect the device morphology as it creates irregular external... 

Mathiowitz, E. Ron, E. Mathiowitz, G. Amato, C. Langer, R. Morphological characterization of bioerodible polymers. 1. Crystallinity of polyanhydride copolymers. Macromolecules 1990, 23, 3212-3218. 

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. 

Aliphatic polyanhydrides synthesized lirom saturated diacid monomers are crystalline, melt at tanperatuies below 100 °C, and are soluble in chlorinated hydrocarbons. They are degraded and eliminated from the body within weeks [39],... 

These are the copolymers of aromatic and aliphatic diacid monomers. Polyanhydrides of diacid monomers containing poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB) are synthesized by either melt or solution with molecular weights of up to 44,600 [17,18]. They were found to be less crystalline in nature and possessed slow degradation due to their aromatic content. 

N. Yoda, Syntheses of polyanhydrides. XII. Crystalline and high melting polyamidepolyanhydride of methylenebis (p-carboxyphenyl) amide, J. Polym. Sci. Part A Gen. Pap. 1 (1963) 1323-1338. 

Aliphatic polyanhydrides presenting saturated bonds along the chain and a crystalline structure. 

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). 

---