Elsevier

Polymer

Volume 50, Issue 5, 23 February 2009, Pages 1178-1186
Polymer

A novel biodegradable multiblock poly(ester urethane) containing poly(l-lactic acid) and poly(butylene succinate) blocks

https://doi.org/10.1016/j.polymer.2009.01.001Get rights and content

Abstract

A novel biodegradable multiblock poly(ester urethane) (PEU), consisting of poly(l-lactic acid) (PLLA) and poly(butylene succinate) (PBS) blocks, has been successfully synthesized via chain-extension reaction of dihydroxyl terminated PLLA (PLLA-OH) and PBS prepolymers (PBS-OH) using toluene-2,4-diisocyanate (TDI) as a chain extender. The chemical structures and molecular weights of PEUs, containing different block lengths and weight fractions of PLLA and PBS, were characterized by 1H NMR and GPC. The effects of the structures on the physical properties of PEUs were systematically studied by means of DSC, TGA, WAXD and tensile testing. The DSC results indicated that PLLA segment was compatible well with PBS segment in amorphous phase and the crystallization of PEU was predominantly caused by PBS segment, which was also confirmed by WAXD. The results of tensile testing showed that the extensibility of PLLA was largely improved by incorporating PBS segment. The PEU can be used as a potential substitute for some petroleum-based thermoplastics.

Introduction

In the past few decades, biodegradable polymers, especially aliphatic polyesters and their copolymers, have drawn growing attentions from both academic researchers and industrial workers due to their potential applications in both biomedical materials [1], [2], [3] and general environmentally friendly materials [4], [5].

Poly(lactic acid) (PLA), as a member of aliphatic polyester family, can be synthesized via either polycondensation [6], [7] of lactic acid or ring-opening polymerization [8] of cyclic lactide. The early studies of PLA mainly focus on its biomedical applications such as drug delivery [9], [10], suture [11], [12], and tissue engineering [13], [14] due to the excellent biodegradability, biocompatibility and high tensile strength. Recently, with the increased pollution and growing costs of traditional polymeric materials, PLA has attracted increasing attention in replacement of traditional petroleum-based materials. However, disadvantages of brittleness, poor thermal stability and high costs limit its applications in general plastics.

It is well known that high-molecular-weight is the prerequisite for PLA that meets the demand of mechanical properties required in practical application. However, high-molecular-weight PLA is usually synthesized via a lengthy route of ring-opening polymerization of lactide, which makes the PLA less competitive in the market. In order to reduce its cost, many approaches, such as melt/solid state polymerization [15], solution polymerization [16], and chain-extension reaction [17], have been utilized to prepare relatively higher molecular weight of PLA. Nevertheless, these methods could not improve its toughness. Thus many methods have recently been explored to toughen PLA, such as blending or copolymerizing with flexible polymers, among which a helpful option is to introduce flexible components to the backbone of PLA to form copolymers.

A common approach to confer toughness of PLA is using flexible monomer or macromolecule to copolymerize with lactide to synthesize PLA-based random or block copolymers. For random copolymer, it is difficult to form crystalline due to irregular chain structure so that the strength is usually unsatisfied. The currently reported PLA-based block copolymers include diblock, triblock and multiblock copolymers. For diblock [18] and triblock [19] copolymers, they are commonly known as amphiphilic polymers and mainly applied in drug delivery system, where the high-molecular-weight and mechanical properties are not necessary. In the case of multiblock copolymers, for examples, multiblock copolymers PLLA–PCL [20], PEG–PLLA [21], PTMC–PLLA [22], and so on, high-molecular-weights could be achieved. They usually have good mechanical properties but designed for biomedical applications. Their applications will be more extensive if their costs come down.

An economical and effective way to endow PLA with toughness is chain-extension reaction of PLA prepolymer with a flexible prepolymer in the presence of chain extender. A typical example of this method is the formation of a poly(ester urethane) (PEU). PEUs with different structures usually have various properties and can be used as versatile materials from elastomers [23] to thermoplastics [24]. Many flexible molecules, such as poly(ethylene glycol) [25], poly(ethylene oxide) [26], poly(ɛ-caprolactone) [23], [27], etc., can be used as the flexible components.

In this study, poly(butylene succinate) was used as the flexible segment. High-molecular-weight of PBS, as one of the commercially available aliphatic polyesters, has been widely researched and developed as green materials due to its biodegradability, good thermal stability, and excellent mechanical properties. Some researches [28], [29] based on blends of PLA and PBS have been reported in recent years. Harada et al. [30] investigated the blends of PLA with PBS using isocyanate as a reactive processing agent and the results indicated that the blends have better impact strength and tensile strain than those of pure PLA. A triblock copolymer PLLA-b-PBS-b-PLLA [31], with the potential application in biomaterials, was synthesized through ring-opening polymerization of l-lactide using bifunctional hydroxyl capped PBS as a macroinitiator. To our best knowledge, there is no report on such a multiblock copolymer containing PLA and PBS up to now.

In this paper, we reported a convenient synthetic route for a new and inexpensive multiblock poly(ester urethane) consisting of PLA and PBS blocks, and characterized it with 1H NMR, GPC, DSC, TGA and WAXD. Furthermore, we investigated the tensile properties and deformation recovery properties of PEU.

Section snippets

Materials

Succinic acid (AR grade) and 1,4-butanediol (AR grade) were purchased from Kelong Chemical Corporation (Chengdu, China) and were used without further purification. l-Lactic acid (LA) with 85 wt% aqueous solution was bought from Guangshui Chemical Plant (Guangshui, China), and the free water was removed by reduced pressure distillation at 80 °C for 3 h. Tetrabutyl titanate bought from Kelong Chemical Corporation was dissolved in anhydrous toluene to prepare 0.2 g/mL solution. Dihydrate tin(II)

Synthesis and characterization of PLLA-OH and PBS-OH

PLLA-OH was synthesized by direct polycondensation of l-lactic acid in the presence of 1,4-butanediol, using SnCl2·2H2O as a catalyst (Scheme 1). The chemical structure and number–average molecular weights of PLLA-OH were characterized by 1H NMR spectra (PLLA1, Fig. 1A). The signals occurring at 1.68 (δHd) and 4.11 (δHc) ppm could be reasonably assigned to the inner and outer methylene protons of –OCH2CH2CH2CH2O–, respectively. The peaks of methyl proton and methine proton at the terminus of

Conclusions

In summary, a series of poly(ester urethane)s consisting of PLLA and PBS blocks were successfully synthesized by the chain-extension reaction of PLLA-OH and PBS-OH, using toluene-1,4-diisocyanate as a chain extender. Varying the molecular weights of prepolymers and the feed mass ratio of PLLA-OH to PBS-OH, PEUs with controllable structures were prepared combining with controllable properties. Through the results of chain-extension reaction, it was confirmed that the number–average molecular

Acknowledgment

This work was supported by the National Science Fund for Distinguished Young Scholars (50525309).

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