Aliphatic poly(ester amide)s from sebacic acid and aminoalcohols of different chain length: Synthesis, characterization and soil burial degradation

Polymer Degradation and Stability 121 (2015) 90e99

Contents lists available at ScienceDirect

Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Aliphatic poly(ester amide)s from sebacic acid and aminoalcohols of different chain length: Synthesis, characterization and soil burial degradation Paola Rizzarelli a, *, Manuela Cirica b, Gaetano Pastorelli a, Concetto Puglisi a, Graziella Valenti a a b

Istituto per i Polimeri, Compositi e Biomateriali, Consiglio Nazionale delle Ricerche, Via Paolo Gaifami 18, 95126 Catania, Italy AGRIPLAST srl, Contrada Marangio SS/11597019, Vittoria, RG, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 July 2015 Received in revised form 3 August 2015 Accepted 17 August 2015 Available online 19 August 2015

High molar mass aliphatic poly(ester amide)s were prepared from sebacic acid and aminoalcohols of different chain length by melt polycondensation and chain extension reactions. Low molecular weight aliphatic poly(ester amide)s were synthesized by melt polycondensation starting from sebacic acid and linear aminoalcohols of different chain lengths, ranging from 2 to 6 methylene groups. Then, chain extension reaction was carried out by using 1,6-hexamethylene diisocyanate (HMDI). The poly(ester amide)s obtained were characterized by viscometry, DSC, MALDI mass spectrometry, and NMR. The poly(ester amide)s with a reduced viscosity ranging from 0.5 to 1.5 dL g
1 showed a good film-ability. The degradability in soil of poly(ester amide) film samples was investigated. The weight loss of commercial poly(3-hydroxy butyrate) (PHB), poly(3-hydroxy butyrate-co-3-hydroxy valerate) 76/24 (P(HB-co-HV) 76/24), poly(caprolactone) (PCL), and two poly(butylene succinate) based samples (Bionolle), was also studied under controlled soil burial conditions. The rates of soil burial degradation of synthetic poly(ester amide)s and commercial biodegradable polyesters having different structures were compared. Interestingly, the poly(ester amide)s synthesized show higher weight loss values than polyester films after 5 and 10 days of soil burial degradation test. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Biodegradable polymers Chain extension reactions Soil burial test Poly(ester amide)s Polyesters Polymer degradation

1. Introduction The environmental pollution and the solid-waste management due to the extensive growth of plastics production have stimulated increasing interest in biodegradable polymers as green materials in the last decades. Aliphatic polyesters make up nowadays for the most important and promising family of biodegradable materials in agricultural and sanitary fields, as well as in packaging applications. Polymers of this type have already been introduced onto the market and their relevant performances are still under scrutiny to broaden their fields of application [1]. Some examples include poly(a-hydroxyacid)s such as poly(glycolic acid) (PGA) and poly(lactic acid) (PLA), poly(u-hydroxyacid)s such as polycaprolactone (PCL), aliphatic polyesters mainly constituted by 1,4-butanediol and succinic acid as the group of Bionolle by Showa High Polymer, and the naturally occurring poly(hydroxyl alkanoate)s (PHAs) such as * Corresponding author. E-mail address: [email protected] (P. Rizzarelli). http://dx.doi.org/10.1016/j.polymdegradstab.2015.08.010 0141-3910/© 2015 Elsevier Ltd. All rights reserved.

poly(3-hydroxy butyrate) (PHB) [2,3]. Unfortunately, biodegradable polyesters do not cover optimal thermal, mechanical and processing properties and this restricts, in general, their industrial applications. The incorporation of aromatic units provides acceptable melting temperatures, such as in EastarBio™ or Ecoflex™, copolyesters based on 1,4-butanediol, adipic acid and terephthalic acid. Correspondingly, the introduction of amide groups in the main chain, giving rise to strong intermolecular hydrogen-bond interactions, can significantly improve thermal and mechanical properties, combining good end-use, processing facilities and suitable biodegradability, even at relatively low molecular weight. In this way, a commercial series of poly(ester amide)s has been patented [4] and commercialized by Bayer in the past with the trade name BAK. These were statistical polymers based on adipic acid, ε-caprolactame and hexamethylenediamine as the amide components and 1,4-butanediol and ethylene glycol as the ester components [5]. Poly(ester amide)s are currently under evaluation as a new class of promising materials for biomedical applications [6].

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

Up to now, many routes to synthesize different types of aliphatic poly(ester amide)s by various procedures have been proposed and described in the literature [7e19]. Since the preparation of the first poly(ester amide)s from the melt mixing of the corresponding homopolymers [7], poly(amide ester)s have been synthesized as random [8], alternating, block or segmented polymers [9]. Poly(ester amide)s can be essentially classified into polydepsipeptides [10], carbohydrate derivatives [11,12], random copolymers based on monomers of commercial nylons and polyesters (i.e., 3 -caprolactam, 3 -caprolactone,1,6hexanediamine) [13,14], and amino acid derivatives [15e18]. Obviously, the properties of the materials are strongly dependent on their structure as well as their molar masses. Consequently, scientific efforts are focused on the synthesis and characterization of high molecular weights polymers to support better mechanical properties. Chain extenders, such as diisocyanates and bis(2oxazolines), as difunctional monomers, can increase the molecular weight of polymers in a fast reaction, with the addition of only low amounts of chain extender agent. The chain extenders can also introduce advantageously new functional groups, which in turn influence physical and mechanical properties as well as biodegradability of the resulting polymers [20e22]. In this paper, we describe the synthesis and the structural characterization of a set of poly(ester amide)s (PEA). Poly(ester amide)s were synthesized through a simple two-step method, based on polycondensation in the melt and chain extension reactions. High molecular weight aliphatic poly(ester amide)s derived from sebacic acid and aminoalcohols of different chain length were obtained successfully. Poly(ester amide)s were characterized and their structural and chemicalephysical properties investigated. Additionally, film samples were subjected to soil burial degradation test and the relative weight losses were evaluated. Commercial biodegradable polyesters, i.e. PHB, P(HB-co-HV) 76/24, PCL and two poly(butylene succinate) (Bionolle) based samples, were also studied for comparison. 2. Experimental section 2.1. Materials Sebacic acid, 1,2-aminoethanol, 1,3-aminopropanol, 1,4aminobutanol, 1,6-aminohexanol, tin(II) 2-ethylhexanoate (Sn(Oct)2), triphenyl phosphite (TPP), 1,6-hexamethylene diisocyanate (HMDI) and PCL were purchased from SigmaeAldrich (MI, Italy). Sebacic acid was purified by repetitive crystallizations from distilled water before use. Aminoalcohols were of the highest purity commercially available and were used without further purification. Solvents, 2-(4-hydroxyphenilazo)benzoic acid (HABA) and the fluorosulfonic acid (FSO3H) were obtained from SigmaeAldrich and used as provided. Showa Denko (Germany) supplied two commercial polyesters, having the trade name Bionolle 1001 and Bionolle 3001. Commercial poly(3-hydroxy butyrate) (PHB) was purchased from SigmaeAldrich and poly(3-hydroxy butyrate-co-3hydroxy valerate) (P(HB-co-HV)) 76/24 was obtained from Marlborough Biopolymers Ltd. 2.2. Synthesis of poly(ester amide)s Low molecular weight poly(ester amide)s (oligo-PEA), whose structures differ in the aminoalcohol moiety,

O(CH2) x NHCO(CH2)8CONH(CH2) x OCO(CH2)8CO

x=2 PEA-et x = 3 PEA-pr x = 4 PEA-bu x = 6 PEA-hex

91

were synthesized by melt polymerization starting from stoichiometric amounts of reagents, or using an excess of aminoalcohol, in the presence of tin(II) 2-ethylhexanoate, changing temperature, pressure and steps interval. High molecular weight poly(ester amide)s from sebacic acid and 1,2-aminoethanol (PEA-et 1-2) were synthesized by melt polymerization starting from stoichiometric amounts of reagents, in the presence of Sn(Oct)2 using mild conditions. In the synthesis of PEAet 1, described as an example, a three-necked flask, equipped with a stirrer, a condenser, a thermometer, and a gas inlet tube, was charged with 4.04 g (0.02 mol) of sebacic acid, 1.22 g (0.02 mol) of 1,2-aminoethanol and 0.016 g (4  10
5 mol) of Sn(Oct)2. The flask was placed in a silicone oil bath, the temperature was raised to 160  C and the mixture was kept, under nitrogen atmosphere, at this value for 30 min. Reaction was then continued, under nitrogen atmosphere, at 180  C for 3 h. In the synthesis of PEA-et 4 (similarly for PEA-pr, PEA-bu and PEA-hex), described as an example, a three-necked flask, equipped with a stirrer, a condenser, a thermometer and a gas inlet tube, was charged with 4.04 g (0.02 mol) of sebacic acid, 1.22 g (0.02 mol) of 1,2-aminoethanol and 0.005 g (2.0  10
5 mol) of Sn(Oct)2. The flask with the mixture was placed in a silicone oil bath, the temperature was raised up to 180  C and was kept at this value for 3e5 h, under nitrogen atmosphere. The poly(ester amide)s obtained were dissolved in the minimum amount of CHCl3 and precipitated into CH3CN to remove the residual catalyst and the eventual oligomers. The solid materials were filtered, washed with solvent, dried at 40  C under vacuum and characterized by viscometry, differential scanning calorimetry (DSC), size exclusion chromatography (SEC), matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry, MALDI-TOF/TOF-MS/MS [23,24], 1H NMR and 13C NMR. Afterwards, chain-linking polymerization of low molecular weight polymers (PEA-et 4, PEA-pr, PEA-bu, PEA-hex) was carried out in a three-necked flask equipped with a stirrer, a thermometer, and inlet and outlet tubes for nitrogen atmosphere, using HMDI as a chain extender. Typically, 1 g of the prepolymer powder was charged into the reactor and the temperature was raised 5e10  C above the melting temperature. After the prepolymer was molten, the chain extender was introduced into the reaction vessel (3 ÷ 6% w/w). The reactions were carried out at 180e200  C, from 30 min up to 3 h. Polymer samples were characterized by viscometry, DSC, MALDI-TOF mass spectrometry, 1H NMR and 13C NMR. 2.3. Viscometry The viscosity value was used as a parameter for molar mass estimation since poly(ester amide)s were not sufficiently soluble for SEC analysis in THF or CHCl3. The reduced viscosity, hsp/c, was measured by an Ubbelohde viscometer at a concentration of 0.5 g/ dL in trifluoroethanol or in 1,1,2,2-tetrachloroethane, at 30 ± 0.1  C. 2.4. NMR analysis The 1H NMR (200 MHz) and 13C NMR (50 MHz) spectra were recorded at room temperature in trifluoroacetic acid-d (TFA-d) or 1,1,2,2-tetrachloroethane-d2 (TCE-d2), on a Bruker AC 200 spectrometer and processed by WIN-NMR® program (Bruker). Sample concentrations of about 10 mg/0.5 mL and tetramethylsilane as internal standard were used. Additionally, for some poly(ester amide) samples NMR spectra were also recorded at room temperature in FSO3H, using dioxane as locking solvent and coaxial NMR tubes. 2D 1He1H COSY 90 spectra were obtained using the Bruker

92

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

COSY.AU program and recorded at room temperature with a digital resolution of 4.9 Hz/pt, 1024 data-points in F2, 256 increments in F1 and 16 scans per increment. The matrix was multiplied in both dimensions by a sinebell function and transformed using the WINNMR® program. 2D 1He13C HETCOR experiments were performed using the Bruker XHCORR.AU program and recorded at room temperature with a digital resolution of 3.6 Hz/pt in the 13C dimension, 2048 data-points with 100 scans per increment; with a digital resolution of 4.6 Hz/pt in the 1H dimension, with 128 increments. The matrix was multiplied in F2 by an exponential function and in F1 by a 90 shifted sinebell function, zero-filled and transformed using the WIN-NMR® program. 2.5. DSC Differential scanning calorimetry was performed using ~7 mg samples under nitrogen flow with a TA Q100 DSC thermal analysis instrument. Three scans for each sample, in the temperature range
50  C to 180  C for the poly(ester amide)s and
50  C to 290  C for Ny2,10, were performed. In the initial scan, the samples were heated at 10  C/min through fusion and left in the melt for 3 min. The cooling was performed at 50  C/min to observe crystallization from the melt. Finally, a second heating at 10  C/min was carried out to check the reproducibility of the transitions. The melting temperatures (Tm) were taken as the peak temperature of the melting endothermic. The glass transition temperatures (Tg) were calculated as the midpoint of the heat capacity change. Both temperatures and the enthalpy of fusion (DHm) were determined in the third scan.

amide) film were placed in a series of darkened vessels containing a multi-layer substrate [25]. The polymer films (2  2 cm; initial weight 45 ÷ 78 mg) were sandwiched between two layers of a mixture of milled perlite (100 g) and commercial soil (200 g), moistened with 100 mL of distilled water. The bottom and top layers were filled with 60 g of perlite moistened with 120 mL of distilled water. Perlite was added for increasing aeration to the soil and the amount of water retained. A flow of moistened air was supplied from the bottom of each vessel every 24 h for 15 min. The films were removed after intervals of 5, 10, 15, 30 and 45 days, brushed softly, washed with distilled water several times and dried under vacuum in the presence of P2O5 at room temperature, to constant weight [25]. The degree of biodegradation was evaluated as the weight loss divided by the initial sample weight. Filter paper samples were used as a positive control. 3. Results and discussion Poly(ester amide)s were synthesized by sebacic acid and aminoalcohols of different chain lengths starting from stoichiometric amounts of reagents, or using an excess of aminoalcohol, under nitrogen atmosphere and/or reduced pressure. An excess of the aminoalcohol, higher temperature than 180  C and/or longer reaction time as well as additional steps under reduced pressure, produced polymer samples with lower viscosity values. Poly(ester amide)s from sebacic acid and 1,2-aminoethanol were obtained by a polycondensation procedure (PEA-et 1, PEA-et 2), at 180  C using stoichiometric amounts of reagents, in the presence of tin(II) 2ethylhexanoate (Scheme 1a) with viscosity ranging of about

2.6. MALDI-TOF MS analysis Matrix Assisted Laser Desorption Ionization Time-Of-Flight (MALDI-TOF) mass spectra were recorded in reflector mode using a 4800 MALDI TOF/TOF™ Analyzer (Applied Biosystem, Framingham, MA, USA), equipped with a Nd:YAG laser (l ¼ 355 nm) and working in positive ion mode. This MALDI TOF instrument is equipped with a laser with wavelength of <500 ps pulse and 200 Hz repetition rate. The laser irradiance was maintained slightly above threshold. 2-(4-Hydroxyphenilazo)benzoic acid (HABA) (0.1 M in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)) was used as matrix. Appropriate volumes of polymer solution (3e5 mg/mL in HFIP) and matrix solution were mixed in order to obtain a 2:1, 1:1 and 1:2 ratios (sample/matrix v/v). 1 mL of each sample/matrix mixture was spotted on the MALDI sample holder and slowly dried to allow matrix crystallization. The resolution of the MALDI spectra reported in the text is about 12000 (FWHM), and the mass accuracy was 20e40 ppm for masses in the range of 1000e2000 Da. 2.7. Films Polymer films (thickness 0.150 ÷ 0.250 mm ± 0.01 mm) were obtained by hot pressing the polymer powder between two Teflon plates, containing a spacer, at 25e30  C above the melting temperature, under a pressure of 200e250 kg/cm2 (Carver C27 Laboratory Press) for a minute. The hot-pressed films were stored at room temperature for at least three weeks before use in order to reach equilibrium crystallinity. Films of PHB (2  2 cm; initial weight 15 ÷ 20 mg; thickness 0.04 ÷ 0.07 mm ± 0.01 mm) were prepared by solvent-casting to avoid degradation of the polymer sample. 2.8. Soil burial degradation test Tests were carried out at 30 ± 0.1  C, under moisture controlled conditions. Triplicate specimens of each polyester and poly(ester

Scheme 1. (a) Single-step polycondensation and (b) two step polymerization method, including polycondensation and chain extension reactions, to synthesize high viscosity poly(ester amide)s.

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

0.5 dL/g (Table 1). In the same conditions, poly(ester amide)s from aminoalcohols with longer chain lengths showed very low viscosity values. The polymerization method, including polycondensation and chain extension reactions was successful to obtain high molecular weight PEA (PEA-et 4 HMDI1, PEA-et 4 HMDI2, PEA-et 4 HMDI3, PEA-pr HMDI, PEA-bu HMDI, and PEA-hex HMDI) (Scheme 1b). The low molecular weight prepolymers were obtained by stoichiometric amounts of reagents. The chain extender was added into the reaction vessel when the prepolymer was molten. Reduced viscosities of the chain-extended poly(ester amide)s ranged from 0.7 to 1.5 dL  g
1 (Table 1). Chain-extension was also carried out using low molecular weight PEA samples synthetized from an excess of aminoalcohol, constituted by macromolecular chains terminated essentially by amino or hydroxyl-groups at both chain ends. In such cases, cross-linking and gelification occurred since the chain extender was introduced into the reaction vessel. Polymer films from samples with viscosity values lower than 0.5 dL/g were brittle and could not be exposed to soil burial test.

Table 1 Properties of the polymer samples synthesized. % HMDI % Urethane hsp (dl/g) Tg ( C) Tm ( C) DHm (J/g) bonds PEA-et 1a PEA-et 2a PEA-et 4 PEA-et 4 HMDI1 PEA-et 4 HMDI2 PEA-et 4 HMDI3 PEA-pr PEA-pr HMDI PEA-bu PEA-bu HMDI PEA-hex PEA-hex HMDI a

e e e 3% 3% 3% e 4% e 6% e 5%

e e e 2.8 1.8 2.7 e 2.8 e 1.8 e 1.8

0.62 0.53 0.18 1.46 0.70 1.16 0.36 0.70 0.16 0.90 0.14 0.80

40 44 49 51 58 / 38 47 53 60 52 57

104 106 104 108 106 109 92 93 103 106 97 104

70 74 72 65 69 67 67 62 69 51 71 41

Sample synthesized by the single step method.

NH-CH2-CH2-O-CO-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CO b

a

d

i

h

g

93

g

f

e

f, g, h

n

c

(a)

NHCH2CH2OH at

d

a b c

i e at

3,85 ppm

(b)

NH(CH2)2OCONHCH2(CH2)4CH2NHCOO(CH2)2NH u

u

u 3,33 ppm

4.8

4.4

4.0

3.6

3.2

2.8

2.4

2.0

1.6

ppm Fig. 1. 1H NMR spectra of (a) PEA-et 4 and (b) PEA-et 4 HMDI2 in TFA-d.

1.2

0.8

0.4

94

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

Poly(ester amide)s with hsp higher than 0.5 dL/g showed good filmability and were tested for degradation in soil. 3.1. Structural and thermal characterization In Table 1 are listed the properties of the poly(ester amide)s synthesized. Reduced viscosities of the synthesized poly(ester amide)s, chain-linked and not, ranged from 0.5 to 1.5 dL  g
1. Properties of the commercial biodegradable polyesters (Bionolle 1001 and 3001, a poly(butylene succinate) homo-polymer and a poly(butylene succinate-co-butylene adipate) 80/20 co-polymer, respectively); PHB and P(HB-co-HV) 76/24 were reported previously [25]. MALDI-TOF/TOF-MS/MS gave structural information about the ester and amide bonds sequences in PEA, showing that during the synthesis ester/amide, ester/ester and amide/amide units are produced in a random sequence [23,24]. The chemical structures of chain linked poly(ester amide)s were studied and the polymerization influence of carboxyl-, amino- and hydroxyl-reactive end groups of prepolymers with the chain extender was investigated with the employ of NMR and MALDI/ TOF. The assignments of NMR spectra were performed with the additional information obtained by COSY and HETCOR spectroscopy (Figs. 1S and 2S e Supporting Information). Fig. 1 shows the 1H NMR spectra of (a) PEA-et 4 and (b) PEA-et 4 HMDI2 with the corresponding assignments. 1H NMR signals at 3.85 ppm (Fig. 1a), belonging to methylene groups linked to alcoholic terminal groups, are not detected after the chain extension reaction and new signals at 3.33 ppm (Fig. 1b) reveal the presence of urethane bonds in the polymer chains. All of the other poly(ester amide)s showed

essentially similar features by NMR spectra, with only minor changes corresponding to the different number of methylene carbons inside of the chain. A complete assignment of peaks in 13C NMR spectra has be done with the help of two-dimensional techniques, which revealed that, contrary to the expectations, the resonance attributed to the CH2 in b to the amide groups in the sebacic moiety appear downfield from the CH2 in b to the ester groups. End group structures of the oligo-PEAs were checked by MALDI MS as a well-established method in the characterization of biodegradable polymers [26]. Predictably, in MALDI mass spectra of oligo-PEA synthesized using stoichiometric amounts of reagents, ions of linear chains terminated by all the possible combinations of the diverse end groups appeared, i.e. carboxyl, hydroxyl or amine. Ions of linear chains bearing eOH or eNH2 end groups cannot be differentiated from the mass values and are present reasonably with the same proportion in the MALDI spectra. Ions of linear chains bearing hydroxyl or amine end groups at both chain ends were mainly detected in the MALDI spectra of polymer samples obtained using an excess of the aminoalcohol. In such case, the intensity of the peaks due to linear chains bearing sebacic acid at both chain ends was comparable to the background noise (Fig. 3S e Supporting Information). Fig. 2 shows the MALDI mass spectra of (a) PEA-bu and (b) PEA-bu HMDI samples. The most abundant ions in MALDI spectra of PEA-bu are due to sodiated cyclic (m/z 1298.9 þ 255.2) and linear chains bearing sebacic acid (m/z 1245.8 þ 255.2) or aminoalcohol (m/z 1387.9 þ 255.2) at both chain ends, otherwise terminated with carboxyl at one end and with aminoalcohol groups at the other end (m/z 1316.9 þ 255.2).

Fig. 2. MALDI mass spectra of (a) PEA-bu and (b) PEA-bu HMDI. (U ¼ eO(CH2)4NHCO(CH2)8COe^eNH(CH2)4OCO(CH2)8COe).

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

95

Additionally, ions due to linear species originated from thermal degradation reactions at m/z 1369.9 þ 255.2, m/z 1351.9 þ 255.2 and m/z 1298.9 þ 255.2 (isobar with cyclic chains) are detected in the spectrum. These end groups are reasonably generated during the synthesis by a b-H transfer mechanism, well known in thermal degradation of polyesters [27]. Potassiated ions of all the species are present in the mass spectrum (þ16 Da respect to the sodiated ones). Finally, the corresponding sodium salts of the species terminated with sebacic acid at one chain end (m/z 1338.9 þ 255.2 and m/z 1320.9 þ 255.2) or at both chain ends (m/z 1267.8 þ 255.2 and m/z 1289.8 þ 255.2) are detected as well. In the mass spectrum of the PEA-bu HMDI sample (Fig. 2b), the abundances of ions due to linear chains bearing aminoalcohol end groups significantly decrease respect to PEA-bu spectrum, reasonably as a consequence of the reaction of hydroxyl and amino end groups with HMDI. The most abundant ions are due to sodiated linear chains bearing sebacic acid (m/z 1245.8 þ 255.2) at both chain end, to cyclic species (m/z 1298.9 þ 255.2) and to ions of linear oligomers terminated with carboxyl at one end and olefin groups at the other end. Species terminated with eCOOH groups react more slowly with isocyanate in the reaction conditions. The concurrent presence of isobaric cyclic and linear chains bearing sebacic acid and olefin at m/z 1298.9 þ 255.2 is confirmed by the detection of signals at m/z 1320.9 þ 255.2, derived from the corresponding sodium salt of the linear species terminated with sebacic acid at one chain end. Unlikely, in the low mass range, peaks of chain extended chains are not detected. Nevertheless, ions at m/z 1442.9 þ 255.2 were assigned to linear chains with the following structure:

CH2 CH(CH2)4NHCO

U

5

OH

originated from a b-H transfer mechanism in the urethane moiety of chain-extended macromolecular chains. The thermal behaviour of poly(ester amide)s was examined by DSC (Table 1). The melting temperatures of poly(ester amide)s range from 92 to 104  C. The values are similar to that of Bionolle samples [25] and higher than the corresponding poly(alkylene sebacate)s [28,29]. In Fig. 3 aec the trends of the melting temperature (Tm), glass transition temperature (Tg) and enthalpy of fusion (DHm) as a function of the number of methylene groups in the aminoalcohol moiety for PEAs, chain extended and not, are reported. These plots reveal higher Tm and Tg values for the samples with an even number of methylene groups compared to those of the sample PEA-pr with an odd number of methylene groups (Fig. 3a and b). Such behaviour, the so-called “odd-even effect”, has been reported previously for similar polymer samples [30]. Chainextension did not influenced significantly the melting temperatures (Fig. 3a) that are only a little bit higher for PEA samples chain extended. In agreement with the introduction of urethane bonds along the polymer backbone, the chain extended PEAs showed Tg values higher than that of the oligo-PEAs (Fig. 3b) and lower crystallinity and consequently DHm values, which decrease with the increase of the number of CH2 groups in the aminoalcohol moiety (Fig. 3c). 3.2. Soil burial degradation Biodegradation tests were carried out at 30 ± 0.1  C, under moisture controlled conditions. Triplicate specimens of each poly(ester amide) and polyester film were placed in a series of darkened vessels containing a multi-layer substrate [25]. The polymer films were sandwiched between two layers of a mixture of

Fig. 3. Trend of the melting temperature (Tm), glass transition temperature (Tg) and enthalpy of fusion (DHm) as a function of the number of CH2 groups in the aminoalcohol moiety.

milled perlite and of commercial soil, moistened with distilled water. The bottom and top layers were filled with perlite moistened with distilled water. Perlite was added for increasing the amount of water retained and aeration to the soil. A flow of moistened air was supplied from the bottom of each vessel every 24 h for 15 min Table 2 shows some representative photos of the film samples after 30 days of soil burial degradation test. Yellowing and embrittlement was observed for all the samples. Fig. 4a shows the kinetic of degradation of PEA-et 1 and PEA-et 2 samples obtained by the single step procedure from 1,2aminoethanol and sebacic acid and PEA-et 4 HMDI1. Fig. 4b report the weight loss (%) vs. degradation time for the chain

96

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

Table 2 Representative photos of the film samples recovered after soil burial degradation test. Sample

Time: 30 days

Sample

PEA-et 1

PEA-et 4 HMDI2

PEA-et 2

PEA-bu HMDI

PEA-et 4 HMDI1a

PEA-hex HMDI

PHB

Bionolle 3001

a

Time: 30 days

Time: 45 days.

extended poly(ester amide)s with aminoalcohol of diverse chain length. Films of PEA-et 1, PEA-et 2, PEA-hex HMDI and PEA-bu HMDI were removed after 30 days as fragments and the test was not prolonged. Surprisingly, PEA-et 1 sample showed the highest degradation rate even though it should have the same chemical structure of PEA-et 2 (Fig. 4a). This could be due to a little bit lower DHm (Table 1), related to the degree of crystallinity that affects degradation in soil of polymers [25]. The weight loss (%) of the corresponding chain-extended PEA (PEA-et 4 HMDI1) shows a similar trend to that of PEA-et 2, up to 30 days. Comparably to other biodegradable polymers such as polylactide [1], it could be assumed that an induction period is needed in order to reduce the molecular weight and promote the degradation process. In the first step, hydrolysis of ester and amide bonds occur, likely encouraged by the presence of microorganisms in the soil, and the films become more and more brittle. After the time needed to decrease the molecular weight, the oligomeric products are eroded from the surface and the weight loss increased. The degradation trend of PEA HDMI samples (Fig. 4b) is reasonably influenced by diverse factors, i.e. chemical structure, molecular weight, Tm, Tg, and DHm. Undoubtedly, chain extended poly(ester amide)s with aminoalcohol containing an even number of methylene groups display a higher degradation rate during the first 30 days (Fig. 4b). In Fig. 5 the weight loss percentage is plotted as a function of degradation time for all the commercial polyesters and compared with that of PEA-et 1. After five days, the polyesters have similar weight loss (%) and ten-fifteen days occur to observe a speedy rise, except for Bionolle 1001. Among the commercial polyesters (Fig. 5), PHB and Bionolle 3001 degraded faster than the other film samples. It

80 (a)

Weight loss (%)

70

PEA-et 1

60 50 40

PEA-et 4 HMDI1

30 20 10 0

PEA-et 2 5

10

15

20

25

30

35

40

45

50

40 (b)

PEA-pr HMDI

Weight Loss (%)

30

PEA-hex HMDI PEA-et 4 HMDI1

20

PEA-bu HMDI 10

0 5

10

15

20

25

30

Degradation time (days)

35

40

45

50

Fig. 4. Weight loss percentage vs. degradation time of (a) polymer samples from 1,2-aminoethanol and sebacic acid, chain extended (PEA-et 4 HMDI1) and not (PEA-et 1 and PEA-et 2), and (b) chain extended poly(ester amide)s with aminoalcohol of diverse chain length.

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

97

80

Bionolle 3001 70

PHB PHB 76/24

60

PEA-et 1

Weight loss (%)

50

40

PCL 30

20

Bionolle 1001 10

0 5

10

15

20

25

30

35

40

45

50

Degradation time (days) Fig. 5. Weight loss percentage vs. degradation time of the commercial polyesters and PEA et 1.

Fig. 6. Comparison of weight losses (%) after (a) 5 and (b) 30 days of soil burial degradation tests of high molecular weight poly(ester amide)s synthesized and commercial polyesters. (Filter paper was not recovered after 10 days of degradation in soil burial test).

98

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99

could be suggested that an induction period is needed in order to reduce the molecular weight and promote the degradation through the erosion process. In the first step, hydrolysis of ester bonds should occur and therefore molecular weight decrease. Subsequently, the low molecular weight products are lost from the surface and the weight loss of the films detected. Interestingly, PEA-et 1 showed a very similar trend of weight loss (%) vs. degradation time. Additionally, PEA-et 1 displayed the highest weight loss after five and ten days. In such case, the viscosity is lower than that of the commercial polyesters [25] and an induction period is not observed. In Fig. 6 the weight loss after (a) 5 and (b) 30 days, obtained in the controlled soil burial degradation tests, of all the poly(ester amide)s and polyesters investigated are compared. PEA-et 1 sample displays the highest weight loss among the poly(ester amide)s and a higher value than P(HB-co-HV) 76/24 copolymer and PCL, after five days of soil burial test (Fig. 6a). Surprisingly, in the initial step, PHB, PCL and Bionolle 3001 show lower degradation than poly(ester amide)s but at 30 days, PHB and Bionolle 3001 exhibit the highest values of weight loss percentage. Among chain-extended poly(ester amide)s, PEA-et 4 HDMI2 is the most degradable with a weight loss of 35% (Fig. 6b). 4. Conclusions High molar mass aliphatic poly(ester amide)s derived from sebacic acid and aminoalcohols were obtained successfully with high yields (60 ÷ 80%). Direct polycondensation and a two-step polymerization method, including polycondensation and chain extension reactions, allowed to prepare aliphatic poly(ester amide)s with a good filmability. Cross-linking reactions and gelification occurred when HMDI was used to chain extend oligo(ester amide)s synthesized by using an excess of aminoalcohol. The direct polycondensation method was successful only to obtain PEA from sebacic acid and 1,2-aminoethanol with a good filmability. Film samples were subjected to soil burial degradation test and the relative weight loss were compared. Reasonably, water as well as enzyme assisted hydrolysis play both an important role in the degradation process. As expected PHB shows excellent soil burial degradation, comparable to Bionolle 3001. The results point out that the poly(ester amide)s show satisfactory degradation levels even if the chain extended poly(ester amide)s revealed a lower degradation rate. Remarkably, diverse induction periods are observed for the different classes of polymer and they appear longer for most of the chain-extended poly(ester amide) samples. On the contrary, the poly(ester amide)s synthesized without chain extender do not display an induction period and show higher weight loss values than polyester films after 5 and 10 days of soil burial degradation test. Undoubtedly, chemical structure as well as molar masses affect degradation in soil. In fact, not chain extend poly(ester amide)s display higher weight loss (%) values than the chain extended samples with urethane bonds along the backbone. Polymer films of commercial polyester samples, as a consequence of higher viscosity values than poly(ester amide)s, require more time to reduce their initial weight. Crystallinity, Tg and Tm appear to be less important in such experiments. Acknowledgements This work was financially supported by the project “SHELF-LIFE e Integrated use of innovative technological approaches to improve the shelf-life and preserve the nutritional properties of food products” carried out by the Cluster Sicily Agrobio and Fishing Industry and funded by the Research Fund PON R&C 2007e2013, DD 713/Ric. (PON02_00451_3361909). The authors wish to thank Dr. Giuseppe Impallomeni (CNR IPCB UoS of Catania, Italy) for NMR 2D

spectra, helpful discussions and suggestions. Many thanks are due to Mr. Roberto Rapisardi for his continuous and skillful technical assistance. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymdegradstab.2015.08.010. References [1] S. Ebnesajjad, Handbook of Biopolymers and Biodegradable Plastics, Elsevier, Oxford, UK, 2013. [2] A. Lendlein, A. Sisson, Handbook of Biodegradable Polymers e Synthesis, Characterization and Applications, Wiley-VCH Verlag GmbH & Co., Weinheim, DE, 2011, http://dx.doi.org/10.002/9783527635818. [3] S. Pilla, Handbook of Bioplastics and Biocomposites Engineering Applications, Wiley-VCH Verlag GmbH & Co., Weinheim, DE, 2011. [4] R. Timmermman, R. Dujardin, R. Koch, United States Patent 5,646,020, 1997. [5] E. Grigat, R. Koch, R. Timmermann, BAR 1095 and BAK 2195: completely biodegradable synthetic thermoplastics, Polym. Degrad. Stab. 59 (1e3) (1998) 223e226, http://dx.doi.org/10.1016/S0141.3910 (97)00174-2. ~es, Biodegradable poly (ester amide)s e a [6] A.C. Fonseca, M.H. Gil, P.N. Simo remarkable opportunity for the biomedical area: review on the synthesis, characterization and applications, Prog. Polym. Sci. 39 (7) (2014) 1291e1311, http://dx.doi.org/10.1016/j.progpolymsci.2013.11.007. [7] Y. Tokiwa, T. Suzuki, T. Ando, Synthesis of copolyamide-esters and some aspect involved in their hydrolysis by lipase, J. Appl. Polym. Sci. 24 (7) (1979) 1701e1711, http://dx.doi.org/10.1002/app1979.070240710. [8] A.M. Abdoulkader, S. Slim, A. Souhir, E.G. Rachid, F. Alain, Random and quasi-alternating polyesteramides deriving from 3-caprolactone and b-alanine, Eur. Polym. J. 53 (2014) 160e170, http://dx.doi.org/10.1016/ j.eurpolymj.2014.01.023. [9] E. Sorta, G. Della Fortuna, Poly(ester amide)-polyether block copolymers: preparation and some physcochemical properties, Polymer 21 (7) (1980) 728e732, http://dx.doi.org/10.1016/0032-3861 (80)90287-6. [10] M. Yoshida, M. Asano, M. Kumakura, R. Katakai, T. Mashimo, Sequential polydepsipeptides containing tripeptide sequencer and a-hydroxy acids as biodegradable carrier, Eur. Polym. J. 27 (3) (1991) 325e329, http://dx.doi.org/ 10.1016/0014-3057(91)90113-3. rez, [11] M. Bueno-Martìnez, I. Molina-Pinilla, F. Zamora-Mata, J.A. Galbis-Pe Hydrolytic degradation of poly(ester amide) derived from carbohydrates, Macromolecules 30 (11) (1997) 3197e3203, http://dx.doi.org/10.1021/ ma961476z. [12] A. Alla, A. Rodrìguez-Galan, A.M. de Ilarduya, S. Munoz-Guerra, Degradable poly(ester amide)s based on L-tartaric acid, Polymer 38 (19) (1997) 4935e4944, http://dx.doi.org/10.1016/S0032-3861 (96)01092-0. [13] K.E. Gonsalves, X. Chen, J.A. Cameron, Degradation of nonalternating poly(ester amides), Macromolecules 25 (12) (1992) 3309e3312, http://dx.doi.org/ 10.1021/ma00038a047. [14] I. Arvanitoyannis, A. Nakayama, N. Kawasaki, N. Yamamoto, Synthesis and study of novel biodegradable oligo(ester amide)s based on sebacic acid, octadecanedioic acid, 1,6-hexanediamine and ε-caprolactone: 2, Polymer 36 (4) (1995) 857e866, http://dx.doi.org/10.1016/0032-3861(95)93118-6. n, J. Puiggalı, C. Peraire, Studies on the biodeg[15] N. Paredes, A. Rodrìguez-Gala radation and biocompatibility of a new poly(ester amide) derived from Lalanine, J. Appl. Polym. Sci. 69 (8) (1998) 1537e1549, http://dx.doi.org/ 10.1002/(SICI)1097-4628(19980822)69:8. n, M. Pelfort, J.E. Aceituno, J. Puiggalı, Comparative studies [16] A. Rodrìguez-Gala on the degradability of poly(ester amide)s derived from L- and L, D-alanine, J. Appl. Polym. Sci. 74 (9) (1999) 2312e2320, http://dx.doi.org/10.1002/(SICI) 1097-4628(19991128)74:9. n, L. Fuentes, J. Puiggalı, Studies on the degradability of a [17] A. Rodrìguez-Gala poly(ester amide) derived from L-alanine, 1, 12-dodecanediol and 1, 12dodecanedioic acid, Polymer 41 (15) (2000) 5967e5970, http://dx.doi.org/ 10.1016/S0032-3861(99)00756-9. n, L. Franco, J. Puiggali, Biodegradable Polyurethanes and [18] A. Rodrìguez-Gala poly(ester amide)s, in: Handbook of Biodegradable Polymers: Synthesis, Characterization and Applications, 2011, pp. 133e154, http://dx.doi.org/ 10.1002/9783527635818.ch6 chapter 6. [19] A. Borriello, L. Nicolais, S.J. Huang, Poly(amide-ester)s derived from dicarboxylic acid and aminoalcohol, J. Appl. Polym. Sci. 95 (2) (2005) 362e368, http://dx.doi.org/10.1002/app.21270. [20] J. Kylm€ a, J. Tuominen, A. Helminen, J. Sepp€ al€ a, Chain extending of lactic acid oligomers. Effect of 2, 2-bis (2-oxazoline) on 1, 6-hexamethylene diisocyanate linking reaction, Polymer 42 (8) (2001) 3333e3343, http://dx.doi.org/ 10.1016/S0032-3861(00)00751-5. €, J. Sepp€ [21] J. Tuominen, J. Kylma al€ a, Chain extending of lactic acid oligomers. 2. Increase of molecular weight with 1, 6-hexamethylene diisocyanate and 2, 2bis (2-oxazoline), Polymer 43 (1) (2002) 3e10, http://dx.doi.org/10.1016/ S0032-3861(01)00606-1.

P. Rizzarelli et al. / Polymer Degradation and Stability 121 (2015) 90e99 [22] Y. Wang, C. Ruan, J. Sun, M. Zhang, Y. Wu, K. Peng, Degradation studies on segmented polyurethanes prepared with poly (D, L-lactic acid) diol, hexamethylene diisocyanate and different chain extenders, Polym. Degrad. Stab. 96 (9) (2011) 1687e1694, http://dx.doi.org/10.1016/ j.polymdegradstab.2011.06.015. [23] P. Rizzarelli, C. Puglisi, G. Montaudo, Sequence determination in aliphatic poly(ester amide)s by matrix-assisted laser desorption/ionization time-offlight and time-of-flight/time-of-flight tandem mass spectrometry, Rapid Commun. Mass Spectrom. 19 (17) (2005) 2407e2418, http://dx.doi.org/ 10.1002/rcm.2075. [24] P. Rizzarelli, C. Puglisi, Structural characterization of synthetic poly(ester amide) from sebacic acid and 4-amino-1-butanol by matrix assisted laser desorption ionization time-of-flight/time-of-flight tandem mass spectrometry, Rapid Commun. Mass Spectrom. 22 (6) (2008) 739e754, http:// dx.doi.org/10.1002/rcm.3417. [25] P. Rizzarelli, C. Puglisi, G. Montaudo, Soil burial and enzymatic degradation in solution of aliphatic co-polyesters, Polym. Degrad. Stab. 85 (2) (2004) 855e863, http://dx.doi.org/10.1016/j.polymdegradstab.2004.03.022.

99

[26] P. Rizzarelli, S. Carroccio, Modern mass spectrometry in the characterization and degradation of biodegradable polymers, Anal. Chim. Acta 808 (2014) 18e43, http://dx.doi.org/10.1016/j.aca.2013.11.001. [27] P. Rizzarelli, S. Carroccio, Thermo-oxidative processes in biodegradable poly(butylene succinate), Polym. Degrad. Stab. 94 (10) (2009) 1825e1838, http:// dx.doi.org/10.1016/j.polymdegradstab.2009.06.007. [28] J.J. O'Malley, T.J. Pacansky, W.J. Stauffer, Synthesis and characterization of poly(hexamethy1ene sebacate)-poly(dimethylsiloxane) block copolymers, Macromolecules 10 (6) (1977) 1197e1199, http://dx.doi.org/10.1021/ ma60060a007. [29] G.Z. Papageorgiou, D.N. Bikiaris, D.S. Achilias, S. Nanaki, N. Karagiannidis, Synthesis and comparative study of biodegradable poly(alkylene sebacate)s, J. Polym. Sci. Part B Polym. Phys. 48 (6) (2010) 672e686, http://dx.doi.org/ 10.1002/polb.21937. [30] H. Tetsuka, Y. Doi, H. Abe, Synthesis and thermal properties of novel periodic poly(ester-amide)s derived from adipate, butane-1,4-diamine, and linear aliphatic diols, Macromolecules 39 (8) (2006) 2875e2885, http://dx.doi.org/ 10.1021/ma052566j.