Читать книгу Poly(lactic acid) - Группа авторов - Страница 67

The amount of diisocyanate determines to a large extent how the linking reaction proceeds. For a hydroxyl‐terminated prepolymer and an equimolar amount of diisocyanate, the chain‐extension reaction proceeds rapidly and the molecular weight reaches its maximum in a few minutes, after which the molecular weight starts to decrease due to thermal degradation [55]. Side reactions become more evident if the amount of diisocyanate is increased, which can be seen from an increase in weight‐average molecular weight but not in the number‐average molecular weight. This is caused by the formation of isocyanate‐terminated prepolymers in excess of chain extender and the further reaction with urethane bonds yielding allophanates, branching, and eventually cross‐linking. The terminal groups of the prepolymers are also important for the progress of the chain extension. Carboxylic acid end groups can react with isocyanates and yield amides, carboxylic anhydrides, or ureas even if the reactivity with hydroxyl groups is significantly higher than with carboxylic acid groups. The presence of carboxylic acid end groups has furthermore been suggested to retard the reaction between hydroxyls and isocyanates [56]. The acid number of the prepolymers is consequently an important parameter and useful as a tool for controlling the chain‐extension reaction [55, 57]. Molecular weight, molecular weight distribution, and long‐chain branching can be varied by changing reaction conditions and acid number of the prepolymer. This gives a possibility for tailoring the viscoelastic properties of the polymer and tailoring LA‐based polymers for specific processing equipment and conditions [57]. The third important parameter in the linking process is the catalyst, which will affect not only the reaction rate, but also the racemization. The racemization, that is, degree of lost tacticity, determines the ability of the poly(ester‐urethane) to crystallize, which in turn will affect the mechanical and thermomechanical properties of the polymer. It can be concluded that the preparation of poly(ester‐urethane)s can be best controlled by the following: (i) the presence of more than one type of end groups in the prepolymer being as low as possible; (ii) the semicrystalline nature being retained during both the prepolymer preparation and the linking steps; (iii) keeping the molecular weight of the prepolymer below a certain level to perform successful linking; and (d) using a catalyst that preferably is also nontoxic, with retained activity in both reaction steps [58].