3.2 shows the changes in the material properties that have been achieved by varying the prepolymer composition [50, 62].
TABLE 3.2 Thermal and Mechanical Properties of Poly(ester‐urethane)s
| Composition | Ratio | T g (°C) | Tensile Strength (MPa) | Strain (%) |
|---|---|---|---|---|
| LA : 1,4‐butanediol | 98 : 2 | 53 | 47 ± 2 | 3.7/0.3 |
| LA : CL | 93 : 7 | 35 | 23 ± 3 | 420/20 |
| LA : CL | 63.7 : 36.3 | −5 | 1.6 ± 30.1 | 900/50 |
| LA : DL‐mandelic acid : butanediol | 89.1 : 8.9 : 2 | 58 | 34 ± 38 | 1.8/0.4 |
| LA : DL‐mandelic acid : butanediol | 78.9 : 19.1 : 2 | 60 | 49 ± 31 | 3.1/0.1 |
The softening point of poly(ester‐urethane)s based on CL–LA prepolymers can be varied to a large extent by changing the ɛ‐caprolactone (CL) content. The properties of thermoplastic poly(L‐lactic acid‐co‐ɛ‐caprolactone‐urethane)s changed according to the molar ratio of the monomers in the copolymer. Small amounts of CL increased the strain of the poly(ester‐urethane)s, while at higher CL content the poly(ester‐urethane)s exhibited lower strength but higher elongation [50, 62]. By utilizing well‐defined four‐armed CL–LA precursors that were cross‐linked with diisocyanates, a variety of mechanical properties were attained [63].
The low heat deflection temperature of PLA limits its use for several application fields, such as in packaging materials and electronic components. The introduction of rigid building blocks [64] or cross‐links [65] is known, for instance, to increase the glass transition temperature and/or heat resistance of LA‐based polymers. The effect of different amounts of comonomers in the prepolymers on the T g and mechanical properties of poly(ester‐urethane)s is demonstrated in Table 3.2. The heat resistance of poly(ester‐urethane)s can be improved by the copolymerization of LA with D,L‐mandelic acid. This broadening of the operating temperature range is of clear practical importance. The incorporation of other comonomers that impede rotation and make polymer chains less mobile also causes an increase in T g, even if the same comonomers can depress the rate of polycondensation [50].
The hydrolysis behavior of amorphous LA‐based poly(ester‐urethane)s is similar to that of regular PLA, with a typical water absorption and a decrease in molecular weight followed by weight loss at a later stage [66]. The biodegradation of poly(ester‐urethane)s has been evaluated in several studies [67]. It has been found that increasing the amount of diisocyanate used as a linking agent increases the biodegradation rate to some extent, which has been explained by an activating effect of a degradation product attributed to the linking agent. All the poly(ester‐urethane)s in this study did biodegrade; that is, 90% of the theoretical CO2 was produced during six months, as stipulated by the European Committee for Standardization (CEN) for biodegradability of packaging materials [68]. In a further part of the study, the Flash test, which is based on the kinetic measurement of bioluminescence of Vibrio fischeri, was applied to evaluate the formation of potentially toxic metabolites in the compost matrix during the biodegradation. The poly(ester‐urethane) based on 1,6‐hexamethylene diisocyanate produced a toxic response in the test. The poly(ester‐urethane) prepared by using 1,4‐butane diisocyanate, on the other hand, did not show any toxic effects [67].
3.3.2 Chain Extension with Bis‐2‐Oxazoline
Bis‐2‐oxazolines were described in the 1960s as useful in the preparation of poly(ester‐amide)s and manufacturing processes were later developed, for instance, for chain extension of aromatic polyesters [69, 70]. Bis‐2‐oxazolines have also been applied in the linking of LA‐based prepolymers. Prepolymers with predominantly carboxylic acid termination were linked using 2,2′‐bis(2‐oxazoline) as chain extender [71]. 2‐Oxazolines are inert toward aliphatic alcohols [72] and accordingly react selectively with the carboxyl end group of the prepolyester through ring‐opening between positions 1 and 5 of the oxazoline, yielding compounds possessing both amide and ester bonds (Figure 3.5).
3.3.2.1 Chain‐Extension Reaction Parameters
The molecular weight of the poly(ester‐amide) strongly depends on the polymerization temperature and the molar ratio of oxazoline and carboxylic acid end groups [71]. High‐molecular‐weight polymers can be produced only within a narrow range of polymerization parameters. The amount of 2‐oxazoline must be optimized because too high an excess of oxazoline results in a dominant blocking reaction and the hydroxyl end group concentration becomes too high, leading to faster degradation than polymerization. An optimal polymerization was achieved when a molar ratio of end groups of 1.2 : 1.0 (Ox/COOH) at 200°C was used. At lower temperature, the linking reaction is insufficient and at higher temperatures, significant thermal degradation takes place. At optimal conditions, the linking process can be completed in a few minutes [71].
FIGURE 3.5 Chain‐extension reactions of LA‐based prepolymers using bis‐2‐oxazolines.
3.3.2.2 Properties of Poly(Ester‐Amide)s
Poly(ester‐amide)s are, like poly(ester‐urethane)s, amorphous polymers, but provide an interesting alternative to other biodegradable polyesters due to the incorporated oxamide linkage in the polyester backbone. This feature equips the polymer with different mechanical properties as well as stability when compared to poly(ester‐urethane)s. A slightly higher mechanical strength and lower elongation have their origin in the rigid configuration of the linking agent. The presence of the oxamide linkage affects both the hydrolytic and the thermal stability. The blocking of the terminal groups reduces the melt degradation, and the increased hydrophilicity speeds up the hydrolytic degradation [55, 71, 73]. Poly(ester‐amide)s undergo biodegradation well in the time framework stipulated in the norms. The ecotoxicity issue observed for the poly(ester‐urethane)s prepared by 1,6‐hexamethylenediisocyanate can also be avoided if 2,2′‐bis(2‐oxazoline) is used as the chain extender [67].
3.3.3 Dual Linking Processes
The selective reactivity of oxazolines provides a possibility of performing dual linking processes with both diisocyanates and oxazolines. It has been shown that the order of addition of the linking agent affects both the reaction and the structure of the polymers, for example, the degree of branching. Simultaneous addition of 2,2′‐bis(2‐oxazoline) and 1,6‐hexamethylene diisocyanate results in a slower increase of the molecular weight than does sequential addition. This approach