Читать книгу Poly(lactic acid) - Группа авторов - Страница 68

The thermal and mechanical properties of poly(ester‐urethane)s are similar to those of polylactide prepared by ring‐opening polymerization, but most of the poly(ester‐urethane)s described in the literature are amorphous, with a few exceptions [58]. This means that some of the properties need to be improved to make useful end products. For many applications, the brittleness is an issue and for others the low heat resistance. Different approaches have been suggested for reducing the brittleness of PLA, for example, by copolymerization [59], blending [60], or adding plasticizing compounds [61].

The copolymerization approach has successfully been applied for poly(ester‐urethane)s by equipping the prepolymers with elastomeric properties by copolymerization. CL–LA copolymers have been reported to result in a significant increase in the strain and the flexibility [62]. Table 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].