Enzymatic hydrolysis studies on novel eco-friendly aliphatic thiocopolyesters

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Abstract

Copolymers of poly(butylene succinate) (PBS) containing thiodiethylene succinate sequences with different molecular architectures (PBSPTDGS) were prepared via reactive blending in the presence of a Ti-based catalyst. In particular, a block copolymer with long sequences and a random one have been investigated for their biodegradability. The parent homopolymer PBS has been also synthesized via the usual two-stage melt polycondensation and used as reference polymer in the study.

The hydrolitic treatment carried out using lipase from Candida cylindracea ([E] = 50 U/ml, T = 30 °C and pH = 7.0), showed that copolymers biodegraded more rapidly and extensively than PBS.

ATRIR analysis and DSC measurements also performed on the resulting samples showed that the enzyme simultaneously attacks the amorphous phase and the PTDGS crystalline phase, which is characterized by lower packing density and degree of perfection with respect to the PBS one. This result suggested that the higher degradability of the copolymers under study can be correlated both to the crystallinity degree and to the nature of the crystalline phase. NMR analysis, performed to detect changes in composition of the copolymers during the degradation, demonstrated that enzyme hydrolysis preferentially targeted ester groups of TDGS sequences because of their higher hydrophilicity.

Introduction

In 2011, the worldwide plastic production was 280 million tons and more than 99% was due to polymeric materials obtained from fossil resources. Petroleum deriving plastics are extensively used (i.e., packaging of food, pharmaceuticals, cosmetics, detergents and chemicals, among others) not only due to their excellent mechanical properties, low cost, light weight and high energy effectiveness, but also for their stability, durability and chemical and biological inertness. The dramatic increase in production and the very low biodegradability of such plastics focused public attention on a potentially huge environmental accumulation and pollution problem that could persist for centuries. An effective way to mitigate this problem is to develop promising biodegradable polymers and to promote their industrial production and use. The major advantages of biodegradable plastics are (1) they can be composted with organic wastes and their carbon can be converted into organic matter for soil; (2) their use will not only reduce negative impacts on wild living organisms caused by dumping of conventional plastics but will also lessen the labor cost for the removal of plastic wastes from the environment since they are naturally degraded; (3) their decomposition will help to increase the longevity and stability of landfills by reducing the volume of garbage disposed; and (4) they could be converted to useful monomers and oligomers by microbial and/or enzymatic treatment.

There are three broad categories of biodegradable polymers: naturally occurring (like cellulose, starch, chitin etc.), modified natural polymers (like cellulose acetate, chitosan, esterified starch etc.), and those obtained by chemical polymerization of biobased building blocks or by micro-organisms. In addition to biopolymers, some synthetic chemical polymers, such as aliphatic polyesters, combine the features of biodegradability and biocompatibility with physical and chemical properties comparable with some of the most extensively used oil-based polymers, like LDPE, PP, etc. Poly(butylene succinate) (PBS), characterized by good mechanical properties and thermal stability, is one of the most representative, generally acknowledged and extensively used biodegradable aliphatic polyester [1], [2]. Therefore, several studies have been performed to investigate its mechanical properties [3], [4], [5] and biodegradability [6], [7], [8], [9], as well as its crystal structure [10], crystallization and melting behavior [11], [12], [13]. However, PBS exhibits a slow biodegradation rate due to its high degree of crystallinity [14]. One strategy of promoting PBS biodegradability is to copolymerize it with different diacids or diols. Till now, particular attention has been paid on random copolyesters, such as poly(butylene succinate-co-ethylene succinate), poly(butylene succinate-co-propylene succinate), and poly(butylene succinate-co-butylene adipate) [15], [16], [17], [18], [19], [20], [21]. Recently, PBS has been also copolymerized with poly(lactic acid) [22], resulting in improved degradation rate and introduction of different desirable properties. Biodegradable multiblock copolymers of PBS have been also investigated [23], [24], [25], [26], [27]. Generally, PBS copolymers biodegrade faster than homopolymer mainly because of their limited crystallinity.

It is worth emphasizing that the biodegradation rate of a polymer normally depends on several factors, such as the chemical structure, hydrophilic/hydrophobic ratio, molecular weight, solid-state morphology, i.e. crystallinity degree, distribution of crystal regions, crystal size and perfection [28], [29].

A simple strategy to synthesize new biodegradable polymers is the reactive blending approach. Reactive blending consists of simply mixing two or more homopolymers in the molten state in the presence of a catalyst. In this economic and solvent-free synthetic route exchange reactions among functional groups belonging to the different homopolymers lead to the formation of block copolymers. Therefore, when the starting homopolymers are selected with tailored and desired physicochemical characteristics, it is possible to produce copolymers in which the final properties can be favorably modulated, depending on the type, relative amount and distribution of the comonomeric units along the polymer chain, which in turn are controlled by reaction conditions such as initial feed and reaction time.

On the basis of the scenario described above, copolymers of PBS containing diethylene succinate sequences (PBSPDGS) with different molecular architecture have been recently prepared in our laboratories via reactive blending in the presence of Ti-based catalyst: PBS biodegradation rate was indeed enhanced and modulated, through a targeted modification of its hydrophilicity, flexibility, and capability of crystallizing [6], [30].

In the present study, block copolymers containing butylene succinate (BS) and tiodiethylene succinate (TDGS) blocks of different length, obtained by melt mixing PBS and poly(thiodiethylene succinate) (PTDGS), have been taken into consideration [31]. It has to be emphasized that PTDGS differs from PBS for the presence of a sulfur atom in the glycol sub-unit. The solid-state properties and wettability of the polymers under investigation were correlated to their enzymatic degradability. Lastly, the biodegradation rate of these copolymers has been compared with that of PBSPDGS copolymers previously studied [6] in order to evaluate the effect of the presence of sulfur atom in the polymeric chain in place of ether-oxygen one.

Section snippets

Materials

Dimethylsuccinate (DMS), 1,4-butanediol (BD), thiodiethylene glycol (TDEG), and titanium tetrabutoxide (TBT) (Sigma–Aldrich) were reagent grade products; DMS and BD were used as supplied, whereas TDEG and TBT were distilled before use. Dichloromethane (DCM), 2-chloroethanol (CE) and ethanol (EtOH) were purchased by Sigma–Aldrich and were used without any further purification. Lipase from Candida cylindracea, and pure buffer salts were purchased from Sigma–Aldrich.

Synthesis of homopolymers

Poly(butylene succinate) (PBS)

Synthesis and characterization of the copolymers

At room temperature the as-prepared polyesters were opaque and light yellow colored. The chemical structure of the copolymers is reported in the following:

PBS and PTDGS display a similar chemical structure, both containing two ester groups along a saturated aliphatic chain per repeating unit. The only difference relates to the presence of a sulfur atom in the glycol subunit of PTDGS, which is absent in PBS. Data concerning polymer molecular characterization are reported in Table 1. Both PBS and

Conclusions

The results obtained in the present study demonstrated that the introduction of sulfur atoms along poly(butylene succinate) polymer chain is a winning strategy to increase PBS biodegradability as it allows to tailor both the crystallinty degree and the hydrophilicity of the final polymer. In fact, the resulting more hydrophilic TDGS sequences, with respect to the BS ones, are preferentially hydrolyzed by lipase. The biodegradation rate can be further enhanced acting on the molecular

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