Study of Aliphatic-Aromatic Copolyester of Aliphatic-Aromatic Copolyester Degradation in ... Biomacromolecules

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  • Study of Aliphatic-Aromatic Copolyester Degradation in SandySoil and Its Ecotoxicological Impact

    Piotr Rychter, Micha Kawalec, Micha Sobota, Piotr Kurcok,, and Marek Kowalczuk*,,

    Polish Academy of Sciences, Centre of Polymer and Carbon Materials, 34, M. Curie-Skodowska Street,41-819 Zabrze, Poland, and Jan Dugosz University, Institute of Chemistry, Environment Protection and

    Biotechnology, 13/15 Armii Krajowej Avenue, 42-200 Czestochowa, Poland

    Received July 30, 2009; Revised Manuscript Received February 16, 2010

    Degradation of poly[(1,4-butylene terephthalate)-co-(1,4-butylene adipate)] (Ecoflex, BTA) monofilaments (rods)in standardized sandy soil was investigated. Changes in the microstructure and chemical composition distributionof the degraded BTA samples were evaluated and changes in the pH and salinity of postdegradation soil, as wellas the soil phytotoxicity impact of the degradation products, are reported. A macroscopic and microscopic evaluationof the surface of BTA rod samples after specified periods of incubation in standardized soil indicated erosion ofthe surface of BTA rods starting from the fourth month of their incubation, with almost total disintegration of theincubated BTA material observed after 22 months. However, the weight loss after this period of time was about50% and only a minor change in the Mw of the investigated BTA samples was observed, along with a slightincrease in the dispersity (from an initial 2.75 up to 4.00 after 22 months of sample incubation). The multidetectorSEC and ESI-MS analysis indicated retention of aromatic chain fragments in the low molar mass fraction of theincubated sample. Phytotoxicity studies revealed no visible damage, such as necrosis and chlorosis, or otherinhibitory effects, in the following plants: radish, cres, and monocotyledonous oat, indicating that the degradationproducts of the investigated BTA copolyester are harmless to the tested plants.

    Introduction

    Biodegradable polymeric packages are meant to be compostedafter disposal. However, it is impossible to avoid improperpractices such as those at illegal and uncontrolled landfill sites.1

    Worldwide production of biodegradable polymers is about toreach more than 1 million tons per year.2 However, becauseonly about 60% of this amount is properly disposed of,approximately 400000 tons of waste is still introduced into thesoil environment annually.3

    The biodegradation process of aliphatic polyesters in varioustypes of environments was thoroughly investigated.4-14 Thelimited mechanical properties of aliphatic polyesters may beovercome by introducing aromatic units into the main chain ofthese materials. However, the higher the ratio of aromatic unitsin copolyesters, the more resistant the material is to microbes.15-17

    The sustainability challenge is to produce relatively cheappolymeric materials possessing the appropriate mechanicalproperties while still maintaining good biodegradation properties.Ecoflex, which is the aliphatic-aromatic copolyester, poly[(1,4-butylene terephthalate)-co-(1,4-butylene adipate)] (poly[(tet-ramethylene terephthalate)-co-(tetramethylene adipate)], BTA),is a very promising material to meet these criteria. Structuralstudies of BTA using NMR have focused mostly on 13C NMR,while the literature information on degradation of this materialin soil is limited and deals with gardening soil.15,18,19

    The aim of the present study was to investigate the (bio)deg-radation of BTA in standardized soil. Changes in polymermicrostructure and composition were investigated using 1HNMR, multidetector SEC, and ESI-MS techniques. The suit-ability to the ESI-MS for the molecular level structure elucida-

    tion of polyester degradation products has been demonstratedrecently.20-22 Furthermore, determination of changes in thequality of the soil as well as the effects of potential postdeg-radation products on plant growth monitored during thedegradation experiments are reported.

    Materials and Methods

    Materials. Poly[(tetramethylene terephthalate)-co-(tetramethyleneadipate)] (BTA) in the form of granules was kindly supplied by BASFLudwigshafen. A single screw laboratory extruder (ZMP-TW Gliwice;12 mm screw diameter, 20 D; four heating zones: I, 100 C; II, 135C; III, 140 C; each one 80 mm long; head, 100 C) was used for thepreparation of polymeric rods. After cooling, extruded BTA was cutinto segments and conditioned at room temperature for 1 week. Allinvestigated samples were in the form of rods weighing 0.1 g each(average diameter 2 mm and length ca. 20 mm).

    Each sample was weighed before and after degradation using aRadwag electronic balance (0.1 mg precision). Weight loss wascalculated using the following relationship:

    where Mi is the initial mass and Mf is the final mass.Poly(1,4-butylene adipate) (PBA) and poly(1,4-butylene terephtha-

    late) (PBT; both from Aldrich) were used as received.NMR Analyses. 1H and 13C NMR spectra were recorded using a

    Bruker-Avance spectrometer, operating at 600 MHz for 1H measure-ments, equipped with a BBO probe using CDCl3 as the solvent andtetramethylsilane (TMS) as the internal standard. 1H NMR spectra wereobtained with 64 scans, a 11 s pulse width, and a 2.65 s acquisitiontime, and 13C NMR spectra were obtained with 20480 scans, a 9.40 spulse width, and a 0.9088 s acquisition time.

    * To whom correspondence should be addressed. Tel.: +48 32 2716077.Fax: +48 32 2712969. E-mail: cchpmk@poczta.ck.gliwice.pl.

    Jan Dugosz University. Polish Academy of Sciences.

    %mass loss )Mi - Mf

    Mi 100

    Biomacromolecules XXXX, xxx, 000 A

    10.1021/bm901331t XXXX American Chemical Society

  • The 1H NMR spectrum of BTA showed signals as follows: CH5 at8.10 ppm, CH62 (4.44 ppm), CH62 (m at 4.38 ppm), CH12 (m at 4.15ppm), CH12 (4.09 ppm), CH32 (2.33 ppm), CH72 (1.97 ppm), CH72 (mat 1.87 ppm), CH22 (m at 1.80 ppm), CH22 (1.69 ppm), and CH42 (1.66ppm; Figure 1).

    SEC Analyses. Molar masses and molar mass distributions ofpolymers were determined by SEC experiments conducted in CHCl3(HPLC grade stabilized with ethanol, purity min. 99.8%, POCh Gliwice)at 35 C with an eluent flow rate of 1 mL min-1, using a set of twoPLgel 5 m MIXED-C ultrahigh efficiency columns (Polymer Labo-ratories) with a mixed bed and linear range of Mw ) 200-2000000.An isocratic pump (VE 1122, Viscotek) as the solvent delivery system,differential refractive index detector stabilized to a temperature of 35C (VE3580, Viscotek), a viscometer detector (270 Dual Detector Array,viscometer only, Viscotek), and a UV-vis variable wavelength detectorat a wavelength of 260 nm (Spectra 100, Spectra-Physics) were used.A volume of 100 L of about 3% w/v sample solution in CHCl3 wasinjected. Polystyrene standards (Calibration Kit S-M-10, PolymerLaboratories) with narrow molecular weight distributions were usedto generate a universal calibration curve according to which sampleswere calculated using OmniSEC 4.6 (Viscotek) software.

    The universal calibration method for homopolymer analysis wascalibrated using the PS standard with a peak molar mass Mp ) 31420(dn/dc ) 0.165). Next, the differential refractive index increments withconcentrations (dn/dc) at 35 C were determined for PBA and PBT

    homopolymers. The refractive index increments were found to be 0.046for PBA and 0.126 for PBT. The PBA refractive index increment wasdetermined by analyzing five solutions of PBA in chloroform in theconcentration range of 2.0-0.7% w/v. In the case of PBT, 10 mg ofthe sample was solubilized in 0.5 mL of 1,1,1,3,3,3-hexafluoroisopro-panol and then diluted with CHCl3 to 10 mL. In addition, the differentialabsorbance increment with concentration (dA/dc) for PBT was deter-mined at a wavelength of 260 nm. The dA/dc was found to be equal to122.34 in relation to PS, where the dA/dc was set to 10. PBA did notshow absorbance at this wavelength (dA/dc ) 0).

    The copolymer analysis method23,24 was created from the homopoly-mer analysis method by using the determined values of dn/dc and dA/dc for poly(1,4-butylene adipate) (PBA) and poly(1,4-butylene tereph-thalate) (PBT) units.

    The concentration of counits in the analyzed sample was calculatedusing a system of two equations:

    where RIheight is the intensity of differential refractive index detectorsignal at the respective retention volume, UVheight is the intensity of

    Figure 1. 1H NMR spectrum of BTA and expanded regions: 4.60-3.90 ppm; 2.10-1.50 ppm.

    RIheight ) conc(A) dn/dc(A) + conc(B) dn/dc(B)

    UVheight ) conc(A) dA/dc(A) + conc(B) dA/dc(B)

    B Biomacromolecules, Vol. xxx, No. xx, XXXX Rychter et al.

  • UV detector signal at the respective retention volume, and conc(i) isthe concentration of unit i at the respective retention volume.

    In the case of BTA,

    where dn/dc (PBT, 35 C) ) 0.126, dn/dc (PBA, 35 C) ) 0.046, dA/dc (PBT, 260 nm) ) 122.34, and dA/dc (PBA, 260 nm) ) 0, therefore,

    The solution of this system of equations transforms the detectorsignal to analyte concentration. Molar masses were calculated accordingto real analyte concentration. P and Q factors were used with defaultvalues.

    The fractionation experiment was conducted using a SEC systemequipped with a set of two PLgel 5 m MIXED-C ultrahigh efficiencycolumns (Polymer Laboratories), a differential refractive index detectorstabilized at 35 C (VE3580, Viscotek), and a Foxy Jr. FractionCollector (Teledyne Isco, Inc.). A volume of 100 L of about 3% w/vsample solution in CHCl3 was injected and divided into five equal partsthat were collected. This procedure was repeated 15 times to collect asufficient quantity of each fraction. The fractions were then character-ized by NMR spectroscopy. For BTA samples were incubated 4 and22 months, respectively, and fractions of SEC analyte were collectedfor ESI-MS analyses at

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