Crystallinity aliphatic polyesters

Polyurethane dispersions (PUD s) are usually high-performance adhesives based on crystalline, hydrophobic polyester polyols, such as hexamethylene adipate, and aliphatic diisocyanates, such as methylene bis(cyclohexyl isocyanate) (H12MDI) or isophorone diisocyanate (IPDI). These PUD s are at the more expensive end of the waterborne adhesive market but provide excellent performance. 

Ethoxylation of the carboxylic acid end groups of aliphatic polyesters significantly changes the biodegradation rate as well as the crystallinity of these materials (41). 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

These representative aliphatic polyesters are often used in copolymerized form in various combinations, for example, poly(lactide-co-glycolide) (PLGA) [66-68] and poly(lactide-co-caprolactone) [69-73], to improve degradation rates, mechanical properties, processability, and solubility by reducing crystallinity. Other monomers such as 1,4-dioxepan-5-one (DXO) [74—76], 1,4-dioxane-2-one [77], and trimethylene carbonate (TMC) [28] (Fig. 2) have also been used as comonomers to improve the hydrophobicity of the aliphatic polyesters as well as their degradability and mechanical properties. 

Hult et al. [36] have described semi-crystalline hyperbranched aliphatic polyesters where the crystallinity was induced by attachment of long alkyl chains as end groups. The crystallization was affected by several factors such as length of the end groups and the molecular weight of the hyperbranched polyester. The crystallization was proposed as being either intra- or intermolecular depending on the size of the hyperbranched polyester onto which the alkyl chains were attached. 

The polymerization of substituted lactones is an attractive strategy for extending the range of aliphatic polyesters and for tailoring important properties such as biodegradation rate, bioadherence, crystallinity, hydrophilicity, and mechanical properties [100]. Moreover, the substituent can bear a functional group, which can be very useful for the covalent attachment of drugs, probes, or control units. 

PET is a stiff crystalline polymer with a relatively high melting point. Aliphatic polyesters with more methylene groups than PET in the chain are more flexible and less crystalline than PET. 

Blending of polymers is an attractive method of producing new materials with better properties. Blends of aliphatic polyesters, especially of poly(e-CL), have been investigated extensively and have been the subject of a recent review paper [170]. Poly(e-CL) has been reported to be miscible with several polymers such as PVC, chlorinated polyethylene, SAN, bisphenol A polycarbonate, random copolymers of Vdc and VC, Vdc and AN, and Vdc/VAc, etc. A single composition-dependent Tg was obtained in the blends of each of these polymers with poly(e-CL). This is of interest as a polymeric plasticizer in these polymers. Blends of PVC and poly(e-CL) with less than 50 wt % of poly(e-CL) were homogeneous and exhibited a single Tg. These blends were soft and pliable because the inherent crystallinity of poly(e-CL) was destroyed and PVC was plasticized... 

It is believed that chain scission occurs through simple hydrolysis, but the kinetics of this hydrolysis are influenced by anions, cations, and enzymes [190]. The process is autocatalytic and the products of hydrolysis such as carboxylic groups participate in the transition state. Water preferentially enters the amorphous parts but crystalline domains are also affected [125]. The degradation of aliphatic polyesters is believed to be dominated by a hydrolytic mechanism but it is also promoted by enzymatic activities [4,7,191-193]. 

Table 10 also contains values of the unperturbed dimensions of many aliphatic polyesters. Most of these have been evaluated from the extensive results of Batzer and his collaborators (32a, 32b, 32c, 32d, 32c, 33a, 168") which combine intrinsic viscosities with osmotic molecular weights of fractions. Since almost all of these polymers have rather high crystalline melting points, and since the fractionations were generally performed at temperatures well below these melting points, we have somewhat arbitrarily chosen a value of 1.5 for the molecular weight ratio which corresponds to a figure of 1.22 for the correc-... 

The solution is a combination of aliphatic polyesters and aromatic polyesters. This involves modifying the crystalline structure of PBT by incorporating aliphatic monomer (adipic acid) in the polymer chain in such a way that the material properties of the polymer would remain acceptable (e.g., melting point of the crystalline range still around 100 °C), but the polymer would also be readily compostable/biodegradable. In this way it was possible to combine the degradability of aliphatic polyesters with the outstanding properties of aromatic polyesters. 

Aliphatic polyesters may present a crystalline a process, and as a consequence the notations (3 and y are adopted for the glass-rubber and subglass relaxations, respectively. Although fully amorphous polymers cannot be achieved by quenching, it is possible to obtain polyesters with different degrees of crytallinity by copolymerization with a noncrystallizable diol. For example, the polyester of 1,6-hexanediol condensed with adipic acid is about 60% crystalline, while the polyester of this diacid with 2,5-hexanediol is completely amorphous. By varying the l,6-hexanediol/2,5-hexanediol... 

Figure 12.2(9. Curves showing the. variation of the complex relaxation modulus with temperature, at 1. Hz, for aliphatic polyesters of different degrees of crystallinity ( ) 60%, (A).44 /o (O) 36%, (O) 30%, and (-f) 20%. (From Ref. 37.)...

Aliphatic polyesters also present a prominent y absorption caused by motions taking place in the amorphous phase. Neither the location nor the intensity of this process is sensitive to the degree of crystallinity. The variations observed in the intensity of the y relaxations in Figure 12.29 are due to changes in the chemical composition rather than to changes in the crystal-Unity. 

Figure 6.27. Predicted maximum possible crystalline fraction (xC)lllax) under isothermal quiescent crystallization, for aliphatic polyesters -[(CH2)k-COO-]n and polyamides -[(CH2)k-CON(H)-]n.

Poly(glycolic acid) (PGA) and poly(lactic acid) (PLA) are examples of poly(a-hydroxy esters.) PGA is a highly crystalline, hydrophilic, linear aliphatic polyester (Xin). As such, it has a high melting point and a relatively low solubility in most common organic solvents. PGA degrades primarily by bulk erosion. This occurs through random hydrolysis of its ester bonds. 

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