Hydrolyzable Aromatic Copolyesters of p-Dioxanone - ACS Publications

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Biomacromolecules 2010, 11, 2512–2520

Hydrolyzable Aromatic Copolyesters of p-Dioxanone G. Giammanco, A. Martıńez de Ilarduya, A. Alla, and S. Munõz-Guerra* Departament d’Enginyeria Quı´mica, Universitat Polite`cnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain Received June 23, 2010

Entropically driven ring-opening copolymerization of mixtures of a fraction of cyclic oligo(hexamethylene terephthalate)s composed of cycle sizes from 2 to 5 and p-dioxanone was used to prepare random copolyesters covering a range of aromatic (HT) to aliphatic (DO) units ratios from 9 to 1.3. The composition and microstructure of the copolyesters were accurately determined by 1H and 13C NMR, respectively. The copolyesters showed thermal degradation and glass transition temperatures in good agreement with their comonomeric composition and microstructure, and they crystallized for contents in DO less than 30%, adopting the same crystal structure as poly(hexamethylene terephthalate). The copolyesters appeared to be sensitive to hydrolytic degradation, which was observed to take place superficially with the generation of non-water-soluble degraded fragments and with the release of water-soluble dioxanoic acid to the aqueous medium.

Introduction Semicrystalline aromatic polyesters are polymers displaying excellent mechanical and thermal properties, which combined with other specific properties such as for example high transparency or thermal stability, confer them an outstanding technological interest.1,2 Poly(ethylene terephthalate) (PET) and poly(butylene therephthalate) (PBT) are the best known members of this family, both of them being widely used in a large variety of industrial applications, either as homopolyesters or as copolyesters containing minor amounts of a diversity of second comonomers.2 However, the utilization of these materials in niches requiring either biodegradability or easy practical chemical recycling of the used polymer has been prevented by their extremely high reluctance to chemical and biological degradation. On the other hand, poly(hexamethylene terephthalate) (PHT) is a member of the terephthalate family that has not found practical use to date. This compound has considerably lower Tm and Tg than PET and PBT due to the higher chain flexibility provided by the hexamethylene segment but continues being highly resistant to hydrolysis and biodegradation. The relative low Tm of PHT facilitates its handling and processing and places it in an advantageous position to be explored for its potential in frontier biomedical applications. Very recently, copolymers of PHT containing lactic acid have been shown to be hydrolyzable under physiological conditions3 and random copolyesters of PHT with ε-caprolactone have been demonstrated to be sensitive to biodegradation by P. fluorescens.4 In this paper we wish to report on copolyesters of PHT containing p-dioxanone (DO). The polyester of p-dioxanone (PDO) is well-known in the biomedical field as a biocompatible polymer usable for the manufacture of bioreadsorbible surgical sutures and prostheses.5 It is well-known that PDO is readily hydrolyzed in aqueous medium under physiological conditions, and this is the degrading mechanism that seems to operate mainly in the in vivo degradation of the polyester.6-8 Furthermore, Nishida et al. isolated several PDO-degrading bacteria that utilize PDO as the only carbon source.9 PDO is a semicrystalline polymer melting around 105 °C and showing * To whom correspondence [email protected].

should

be

addressed.

E-mail:

glass transition in the -15 to -8 °C range. Its low stiff modulus makes it flexible enough to be used as a monofilament. Some aliphatic copolymers of DO with other aliphatic lactones have been synthesized, and their reabsorbing properties were evaluated as a function of composition.10,11 Copolymers of PHT and PDO are expected to combine beneficially the properties of the two parent homopolyesters, and exploring this approach is indeed the aim of this work. The target is to generate aromatic-aliphatic copolyesters displaying good thermal and mechanical properties but being sensitive to hydrolysis under physiological conditions. Such a pattern of behavior will be the first step toward a development of these materials for biomedical applications requiring reabsorbability. Although aromatic units have been inserted in PDO to improve the resistance to γ-radiation,11 copolyesters of PDO containing terephthalate units have not been investigated so far. Entropically driven ring-opening polymerization (ROP)12-16 will be used here for the synthesis of the copolyesters made from HT and DO. This method, which has been traditionally applied for the synthesis of certain aliphatic polyesters,17-20 including the synthesis of PDO, is achieving recently increasing importance as an alternative option for the synthesis of aromatic polyesters. We will make use of this synthetic procedure instead of traditional two-step melt polycondensation because lower temperatures are required for the reaction, which prevents DO monomer degradation and volatilization. The preparation of PET, PBT, PPT, and PHT by ROP of their corresponding cyclic oligo(alkylene phthalate)s is well described in the literature.21-24 In our preceding works on this subject, fractions of cyclic oligo(hexamethylene terephthalate)s differing in the average ring size were prepared and copolymerized with ε-caprolactone to obtain copolyesters with composition and microstructure essentially unaffected by cycle size.4,25 In this work we are using a cyclic HT fraction composed of two- to five-membered rings; the ROP of these cycles can be carried out at temperatures notably lower than those required for carrying out conventional polycondensation minimizing, therefore, the depolymerization of PDO, which is known to begin above its melting temperature.

10.1021/bm1007025  2010 American Chemical Society Published on Web 08/04/2010

Aromatic Copolyesters of p-Dioxanone

Experimental Section Dimethyl terephthalate (DMT; 99%), 1,6-hexanediol (HD; 99%), dibutyltin oxide (DBTO; 98%), and 1,2-dichlorobenzene (DCB; 99%) were purchased from Sigma-Aldrich Co. and used without further purification. Tetrabutyl titanate (TBT; Merck-Schuchardt) was reagent grade and used as received. 1,4-Dioxan-2-one (p-dioxanone, DO; 99.5%) was purchased from Leap Labchem Co. Ltd., and it was distilled and recrystallized from ethyl acetate (99.5%, Sharlau) before use. Solvents for purification and characterization purposes, such as chloroform (99.5%, Panreac), diethyl ether (99.7, Panreac), methanol (99.5%, Panreac), dichoromethane (DCM, 99.9%, Panreac), tetrahydrofuran (THF, 99.5%, Panreac), and dichloroacetic acid (DCA, 98%, Panreac) were used as received. Measurements. 1H and 13C NMR spectra were recorded on a Bruker AMX-300 spectrometer at 25.0 °C operating at 300.1 and 75.5 MHz, respectively. Both polymers and cyclic compounds were dissolved in CDCl3 (99.8% D, Euriso-top) and spectra were internally referenced to tetramethylsilane. About 10 and 50 mg of sample in 1 mL of solvent were used for 1H and 13C NMR, respectively. A total of 64 scans were recorded for 1H NMR and between 1000 and 10000 scans for 13C NMR, with 32 and 64 K data points and relaxation delays of 1 and 2 s, respectively. High performance liquid chromatography (HPLC) analysis was carried out at 25.0 °C in a Waters apparatus equipped with a UV detector of Applied Biosystems operating at 254 nm wavelength and a Scharlau Science column (Si60, 5 µm; 250 × 4.6 mm). Cyclic oligomers (1 mg) were dissolved in chloroform (1 mL) and eluted with the hexane/ 1,4-dioxane mixture (70/30 v/v) at a flow rate of 1.0 mL · min-1. Gel permeation chromatography (GPC) for determination of polymer molecular weights and their distributions was performed in a Waters equipment provided with RI and UV detectors using 1,1,1,3,3,3hexafluoroisopropanol (HFIP, 99% Apollo Scientific Lim.) containing sodium trifluoroacetate (98%, Sigma-Aldrich Co.; 6.8 g · L-1) as mobile phase. A total of 100 µL of 0.1% (w/v) sample solution were injected and chromatographed with a flow of 0.5 mL · min-1. HR5E Waters linear Styragel columns (7.8 × 300 mm, pore size 103-104 Å) packed with cross-linked polystyrene and protected with a precolumn (VanGuard, 1.8 µm, 2.1 × 5 mm) were used. Molecular weight averages and distributions were evaluated against poly(methyl methacrylate) standards. Intrinsic viscosities were measured from diluted polymer solutions in DCA using an Ubbelohde viscometer thermostatted at 25.0 ((0.1 °C), and calculations were performed using the Mark-Houwink parameters reported for poly(ethylene terephthalate): K ) 6.7 × 10-3, a ) 0.47.26 The thermal behavior of cyclic oligomers and polymers was examined by differential scanning calorimetry (DSC) using a Perkin-Elmer Pyris 1 apparatus. Thermograms were obtained from 4-6 mg samples at heating and cooling rates of 10 °C · min-1 under a nitrogen flow of 20 mL · min-1. Indium and zinc were used as standards for temperature and enthalpy calibration, respectively. Glass transition temperature (Tg) was taken as the inflection point of the heating DSC traces recorded at 20 °C · min-1 from melt-quenched samples, and the melting temperature (Tm) was taken as the maximum of the endothermic peak appearing on heating traces. The thermal stability was determined under a nitrogen atmosphere in a Perkin-Elmer TGA-6 thermogravimetric analyzer at a heating rate of 10 °C · min-1 in the range of 30-600 °C. X-ray diffraction patterns were recorded on an INEL CPS-120 diffractometer with a Debye-Scherrer configuration using the Cu KR radiation of wavelength 0.1542 nm. Samples were prepared by pulverizing the pristine polymer while immersed in liquid N2. After drying under vacuum overnight, they were placed in a radiationtransparent glass capillary with an internal diameter of 1 mm for examination. Polymerization and Cyclo-Depolymerization Reactions. Synthesis of Cyclic Oligo(hexamethylene terephthalate)s c(HT)2-5. PHT was synthesized by polycondensation of DMT with HD using TBT as catalyst according to the method previously reported by us.4 The

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number-average molecular weight (Mn) and polydispersity index (PDI) measured by GPC were 10400 g · mol-1 and 2.1, respectively. Cyclodepolymerization of PHT was carried out by refluxing a suspension of finely powdered polymer (3.7 g previously dried overnight in a vacuum oven at 60 °C) in DCB (200 mL) containing 112 mg of DBTO. The mixture was vigorously stirred and the reaction was allowed to proceed for 4 days at 180 °C, cooled to room temperature and filtered. The filtrate was rapidly evaporated to dryness at 50 °C under vacuum, and the recovered solid (3.5 g, 85% w/w) was dissolved in chloroform and precipitated with cold diethyl ether. Once the solid was discarded, the clean solution was evaporated to dryness to afford an oligomer fraction enriched in cycle sizes ranging from 2 to 5 (1.75 g, 47% w/w). This fraction c(HT)2-5 was dried at 50 °C under vacuum for 48 h before characterization. 1H NMR (δ, CDCl3, 300 MHz): 1.54, 1.63 (m, 4H), 1.83 (q, 4H), 4.36 (m, 4H), 8.08, 7.87 (s, 4H). 13C NMR (δ, CDCl3, 75.5 MHz): 25.7, 26.1, 28.3, 28.6, 65.3, 129.5, 134.1, 165.8. Synthesis of PHT by Ring-Opening Polymerization of c(HT)2-5. A total of 1.0 g of the cyclic oligomer fraction c(HT)2-5 and 0.5 mg of DBTO were placed inside a previously silanized glass reactor. The mixture was then dissolved in DCM (1 mL), the solvent was slowly evaporated under vigorous stirring, and the residue was dried overnight at 50 °C under reduced pressure. The glass reactor was then purged with nitrogen, sealed, and immersed into a bath containing an eutectic mixture of molten sodium nitrite, sodium nitrate, and potassium nitrate at 180 °C. Polymerization was carried out at this temperature for 2 h. After cooling to room temperature, the solidified reaction product was dissolved in chloroform and precipitated with methanol (0.92 g, 92% w/w). 1H NMR (δ, CDCl3, 300 MHz): 1.54 (m, 4H), 1.83 (qt, 4H), 4.36 (t, 4H), 8.08 (s, 4H). 13C NMR (δ, CDCl3, 75.5 MHz): 25.7, 28.6, 65.3, 129.5, 134.1, 165.8. Synthesis of Poly(hexamethylene terephthalate-co-p-dioxanone) Copolyesters (coPHTxDOy) by Ring-Opening Polymerization. A mixture of cyclic oligomers c(HT)2-5, DO with the selected composition, and DBTO (0.5 mol %) was prepared as described in the previous paragraph and introduced in a silanized glass reactor. The system was purged with nitrogen and sealed to avoid contact with oxygen and moisture and then immersed into a salt bath containing an eutectic mixture of molten sodium nitrite and sodium and potassium nitrates at 180 °C for 4 h. The solid formed after cooling to room temperature was dissolved in chloroform and precipitated with methanol. Hydrolytic Degradation Essays. Amorphous 0.2 mm thick films of polyesters were prepared by melt compression followed by quenching into ice-cold water. Films were cut into 1 × 1 cm squares with a weight of 10-12 mg. Hydrolytic degradation was performed by incubation in both 0.1 M saline phosphate, pH 7.1, and citric acid, pH 2.3, buffers. For this, film pieces were placed into glass vials containing 30 mL of buffered solutions, and vials were sealed and stored in a heat chamber at 37 °C. At the end of the scheduled incubation period, the film pieces were withdrawn from the buffer solution, washed carefully with distilled water, and dried under vacuum. All samples were then subjected to weighting and GPC analysis.

Results and Discussion The ring-opening copolymerization reaction between c(HT)2-5 and DO catalyzed by DBTO is depicted in Scheme 1. These reactions were conducted in bulk at 180 °C with the cycles being in the molten state. Convenient c(HT)2-5 to DO ratios were chosen for the feed to provide a series of copolyesters covering a range of contents in DO between approximately 10 and 40%. A first evaluation of the chemical structure and composition of the resulting polymers was provided by FTIR spectroscopy, where changes in composition become noticeably reflected in the absorptions associated to the ester carbonyl groups (information provided as Supporting Information). Copolymerization results regarding product yields, polymer molecular weights and polydispersities, and copolymer com-

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Scheme 1. Synthesis of coPHTxDOy Copolyesters

anone) centered triad sequences are feasible for the coPHTxDOy chain when the allowed reaction combinations between the c(HT) and DO cycles and subsequent transesterification reactions are taken into account. Furthermore, the two possible orientations for the DO unit along the polyester chain must also be considered. In Table 2, the chemical structures of the 10 triads that are feasible for the combination of the H, D, and T moieties are represented with every NMR distinguishable proton and carbon atom properly labeled. A simple account of this labeling indicates that up to 25 signals could be present in the NMR spectra of these copolyesters making their analysis notably complex. Both the 1H and 13C NMR spectra of coPHT57DO43 with partial assignation of the signals are shown in Figure 1 for illustration. The compositions of coPHTxDOy were determined by the integration method using the phenylene and R-CH2 signals observed at 4.3 and 4.5 ppm in the 1H NMR spectra. The area ratio for these signals afforded accurately the compositions given in Table 1, where they are compared with the compositions used in their respective copolymerization feeds.

positions are summarized in Table 1. Fair yields and molecular weights were attained but decreasing in parallel to the increase in content of DO, while polydispersity remained essentially unaffected. NMR Analysis: Composition and Microstructure of Copolyesters. A detailed 1H and 13C NMR analysis supported on data provided by both unidimensional and bidimensional spectra has been carried out to determine the copolyester composition and to elucidate their chain microstructure. A number of H (hexamethylene), T (terephthalate), and D (diox-

As it is seen, the content in DO units of the resulting copolyesters is diminished respect to that used in their respective feeds in an amount that increased from 6 to 12% as the HT to DO ratio in the feed decreased from 85/15 to 45/55. The observed differences are attributable to the experimental conditions used for copolymerization. In fact, the reaction temperature

Table 1. Synthesis Results of Ring-Opening Copolymerization of c(HT)2-5 with DO composition (mol %) molecular size

polyesterd

feed

polyester

yield (%)

Mna (g · mol-1)

Mwa (g · mol-1)

PD

[η]b (dL · g-1)

Mvc (g · mol-1)

HT

DO

HT

DO

PHT coPHT91DO9 coPHT84DO16 coPHT69DO31 coPHT57DO43 PDO

89 80 84 76 63 65

18000 13600 11450 10650 5650 8550e

41800 16500 21550 26800 10850

2.3 1.8 1.9 2.5 1.9

0.58 0.51 0.49 0.40 0.34

13400 10000 9400 6000 4100

100 85 75 60 45 0

0 15 25 40 55 100

100 91.1 83.9 68.9 57.0 0

0 8.9 16.1 31.1 43.0 100

a Determined by GPC. b Intrinsic viscosity determined in DCA. c Viscometric molecular weight determined using the Mark-Houwink parameters K ) 6.7 × 10-3 and a ) 0.47, reported for PET.26 d Molar composition determined by integration of the 1H NMR spectra. e Determined by 1H NMR end-group analysis.

Table 2. Chemical Structure of Possible Sequences in the Copolyester Chain


Figure 1. 1H (top) and


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C NMR (bottom) spectra of coPHT57DO43.

was chosen as the minimum required for ring-opening of the c(HT) cycles at an acceptable rate in spite of being higher than advisible for the polymerization of DO. It has been recently reported that the ROP of DO at temperatures as low as 80 °C involves a cyclo-polymerization-cyclo-depolymerization equilibrium with formation of considerable amounts of monomer and cyclic oligomers due to the occurrence of a backbiting mechanism that becomes enhanced with increasing temperatures.27 To achieve a complete assignment of the signals appearing in the NMR spectra of these copolyesters, cosy and hetcor 2D spectra, which give information on the 1H-1H and 1H-13C spin-spin coupling, respectively, were registered from coPHT57DO43 (Supporting Information). The analysis of the sequences at the triad level was carried out using the signals appearing in the 13C NMR spectra at 134.1, 68.4, and 25.7 ppm, which arise from f-f′′′, c-c′′′, and i-i′′′ carbons contained in the H, D, and T moieties, respectively. These three signals were found to be sensitive to sequence distribution and they appear in the spectra resolved as much as to be reliably used for area quantification after deconvolution (Figure 2). The probability of occurrence of the triad sequences centered in T are given by the following expressions

PHTH ) XT · XH2; PDTD ) XT · 1/4XD2; PHTD ) PDTH ) XT · XH · 1/2XD where X refers to the molar fraction of the corresponding moieties H, D, or T and where the asymmetry of the DO unit has been duly taken into account for the estimation of P. Similar expressions were used for the determination of the probability of H and D triad centered sequences. All the experimental and

theoretical values obtained for the three types of triads for the set of coPHTxDOy copolyesters studied in this work are given in Table 3. The microstructure analysis of these copolyesters becomes particularly complicated by the directional nature of the DO unit which may be inserted in the chain with two opposite orientations, both needing to be considered in the determination of the randomness degree B. Additionally, as a consequence of transesterification reactions new DHD or DTD triad sequences have to be taken into account. A practical methodology has been recently put forward by Fradet and colleagues for evaluating B in copolyesters obtained by polycondensation of monomers of A-A, B-B, and A-B types.28 We have made use of the algorithm reported by these authors for determining B in coPHTxDOy copolyesters. The expressions for the present case and referred to H units are

a ) FH(HT + TH) ) FH(THT) +

FH(THD + DHT) 2 B ) (1 - a)

2t + d d

where FH is the molar fraction of H-centered triads and t and d are the molar fractions of T and D moieties in the copolyester, which were estimated by 1H NMR, as described above. Similar expressions were used for evaluating BT and BD. The B value averaged from the BH, BT, and BD values obtained by applying such expressions are given in Table 3 for every copolyester. As it is seen, all B values are very close to unity, indicating a fair homogeneous distribution of the H, T and D units along the copolymer chain. This result is contrary to expectations because the reactivity of c(HT) is much lower than that of DO,

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Figure 2.

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C NMR signals used for the microstructural analysis with indication of the triads to which they are assigned.

Table 3. Chain Microstructure of coPHTxDOy Copolyesters Determined by NMRa T-centered triads

H-centered triads

D-centered triads

copolyester

HTH

HTD + DTH

DTD

THT

THD + DHT

DHD

TDH

TDD + DDH

DDD

Bb

coPHT91DO9 coPHT84DO16 coPHT69DO31 coPHT57DO43

90.48 (90.78) 83.46 (83.50) 69.46 (66.00) 55.76 (52.70)

9.52 (9.00) 15.48 (15.80) 27.31 (29.80) 38.17 (39.80)

(0.00) (0.22) 1.06 (0.75) 3.22 (3.40) 6.07 (7.50)

90.03 (90.78) 83.01 (83.50) 65.36 (66.00) 53.88 (52.70)

8.87 (9.00) 16.47 (15.80) 29.94 (29.80) 38.10 (39.80)

1.10 (0.22) 0.52 (0.75) 4.70 (3.40) 8.03 (7.50)

90.82 (90.80) 82.7 (83.02) 65.7 (66.68) 54.7 (53.90)

9.18 (8.98) 16.5 (16.20) 28.9 (29.95) 37.3 (39.03)

(0.00) (0.22) 0.80 (0.79) 5.50 (3.36) 8.00 (7.06)

1.13 1.00 1.02 0.97

a Sequence distribution of T-, H-, and D-centered triads calculated on the basis of the intensity ratio of the peaks contained in the signals appearing at 134, 25.6, and 68.4 ppm, respectively, in the 13C NMR spectra. Values are calculated for a theoretical distribution according to Bernouillian statistics and taking into account the respective compositions are given in parentheses. b Average randomness degree calculated from sequence distributions for T-, H-, and D-centered triads.

and therefore, B values lower than 1 corresponding to a block microstructure should be found. It is very likely that extensive transesterification reactions leading to the homogenization of the microstructure of the initially formed copolyester have taken place at high temperatures and long residence times used in these copolymerizations. There is solid evidence demonstrating the occurrence of transesterification reactions: (a) HT sequence lengths longer than observed, and lower degrees of randomness would result in the absence of transesterification reactions since a dimer-trimer cycle is polymerizing, and (b) the occurrence of DHD and DTD sequences is only feasible by transesterification; in the absence of such reactions, all dyads centered in T or H should contain the repeating unit TH or HT. Thermal Properties of Copolyesters. The TGA traces of coPHTxDOy copolyesters registered under an inert atmosphere are comparatively depicted in Figure 3 along with those of polyesters PHT and PDO. The much greater sensitivity to heating exhibited by PDO compared to PHT is obviously the reason for the decreasing thermal stability observed for coPHTxDOy with increasing contents in DO units. At difference with the parent homopolyesters, decomposition of the copolyesters takes place in two stages, presumably corresponding to independent DO and HT decompositions. According to antecedents, DO sequences are expected to decompose, generating pdioxanone29 by a clean backbiting mechanism, whereas HT segments will decompose at higher temperatures through a β-elimination mechanism, generating double bonds and carboxylic groups.30 It is very worthy to note that the amount of sample released in the first degradation step is very close to the amount of DO contained in the copolyester, which strongly supports this interpretation. Nevertheless, decomposition of the copolyesters did not start to be noticeable until well over 200

°C, clearly above the onset decomposition temperature of PDO. The decomposition onset and maximum rate decomposition temperatures, as well as the residual weights left after every decomposition stage, are given in Table 4 for PHT, PDO, and the copolyesters. The DSC analysis of samples quenched from the melt revealed the occurrence of a unique Tg for these copolyesters with a value steadily decreasing with the content in DO, and confined between those of the parent homopolyesters, which are 10 °C and -15 °C. The heating traces of powdered pristine samples of PHT, PDO, and coPHTxDOy showing the inflection characteristic of glass transition are compared in Figure 4a and b, and the variation of Tg and Tm with composition is plotted in Figure 4c. The pattern of behavior observed is fully consistent with which should be expected for a copolyester series varying gradually in composition and having a homogeneous distribution of the two comonomers along the polymer chain. On the other side, DSC of the “as synthesized” copolyesters gave heating traces with melting endotherms within the temperature range of 80-140 °C, indicating that all of them are semicrystalline. DSC traces registered at first heating compared for the whole set of polyesters are depicted in Figure 4b, and the trend followed by Tm as a function of the copolyester composition is shown in Figure 4c. In a similar way as observed for Tg, the melting temperature is observed to decrease as the content in DO increases: in this case, more rapidly and falling down to values even below the PDO’s Tm. All copolyesters except coPHT57DO43 were able to crystallize upon cooling from the melt, showing sharp crystallization peaks at temperatures gradually decreasing with the content in DO units. Both melting and crystallization enthalpies displayed also the same trend as temperatures, that is, decreasing as the content in DO units



Figure 3. Compared TGA traces of copolyesters coPHTxDOy and polyesters PHT and PDO registered under an inert atmosphere.

increases. These results clearly indicate that the presence of DO in the PHT chain not only hinders the crystallization process from the melt but also depress the crystallinity of the polyester. The fact that the copolyester with 43% of DO was unable to crystallize instead of showing two melting temperatures is fully consistent with the random distribution that DO and HT units have along the polymer chain.

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Figure 5. Compared WAXS profiles from PHT, PDO, and coPHTxDOy copolyesters registered at 20 °C from powder samples coming directly from synthesis.

The ability of coPHTxDOy to crystallize is worthy of attention given the random distribution that the comonomeric units have in these copolyesters. The WAXS profiles of pristine samples of PHT, PDO, and the two copolyesters coPHT91DO9 and coPHT69DO31 chosen as representative of the series are compared in Figure 5. It is known that PHT may adopt three crystal forms depending on the history of the sample.31-33 Such polymorphs have slightly different melting points and are also

Table 4. Thermal Properties of coPHTxDOy Copolyesters DSC

TGA

polyester

Tma (°C)

∆Hma (J · g-1)

Tcb (°C)

∆Hcb (J · g-1)

Tgc (°C)

PHT coPHT91DO9 coPHT84DO16 coPHT69DO31 coPHT57DO43 PDO

146 138 129 106 82 100

45 35 28 25 19 79

117 107 96 65

–51 –39 –38 –26

26

–32

10 6 1 –6 –11 –15

o

Tdd (°C) 373 357 317 265 240 197

Tdd (°C)

RW1e (%)

RW2f (%)

411 411 409 408 406 270

95.3(96.4) 92.8(93.8) 85.8(88.7) 78.6(76.3)

7.0 2.6 1.4 2.4 2.4 0.1

m

Melting temperature and enthalpy measured at the first heating DSC scan recorded at 10 °C · min-1. b Crystallization temperature and enthalpy measured on the cooling DSC scan recorded at 10 °C · min-1. c Glass transition temperature measured at the second heating DSC scan recorded at 20 °C · min-1 from samples quenched from the melt. d Onset decomposition temperature corresponding to 5% of the initial weight loss and maximum rate decomposition determined by TGA at a heating rate of 10 °C · min-1. e Percentage (w/w) of the initial weight after the first decomposition step. In parentheses, the percentage (w/w) of HT in the original sample. f Percentage (w/w) of initial weight remaining after heating at 600 °C. a

Figure 4. Compared DSC traces from PHT, PDO, and coPHTxDOy copolyesters. (a) Melting peak observed on traces recorded at heating at 10 °C · min-1 from pristine samples. (b) Scans registered at heating at 20 °C · min-1 from samples quenched from the melt showing Tg observed as trace inflections. (c) Tm and Tg values as a function of the content in DO units.

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Figure 6. Evolution of the hydrolytic degradation of PHT, PDO, and coPHT69DO31 upon incubation at 37 °C in aqueous solution at pH 2.3 and pH 7.0. (a) Remaining weight and (b) number-average molecular weight of the residual polymer as a function of incubation time.

Figure 7. 1H NMR spectra of coPHT69DO31 after 15 weeks of incubation in buffer at pH 7.2: (a) original sample, (b) residue, and (c) products released to the aqueous medium; *water peak; **signal of CH2 p-xylyleneglycol used as reference.

discernible by slight differences in their WAXS profiles. Two forms β and γ, both of them consisting of a triclinic lattice are reported to exist in PHT samples obtained by precipitation from solution, and as expected, the profile of PHT included in Figure 5 is consistent with the presence of these two crystal forms. The profiles recorded from the copolyesters are similar and essentially identical to that of PHT, indicating that they must adopt the same crystal structure. Because the diffraction

scattering produced by crystalline PDO is perfectly distinguishable from that of PHT, the presence of PDO crystal phase should be detected even when present in small amounts. No sign of diffraction characteristic of PDO is however observed in the copolyesters traces for whichever composition proving therefore that DO units are unable to crystallize. This interpretation is well supported by the absence of melting peak assignable to PDO in the copolyesters DSC traces. It can be concluded therefore that crystallites present in the crystallized copolyesters must be made exclusively of HT units, provided that the insertion of aliphatic DO units in the PHT lattice should be discarded. Accordingly, crystallites would be made from HT sequences having the minimum length needed to render a thermodynamically stable lamellar thickness. This makes really striking the observation of crystallinity in DO enriched copolyesters such as coPHT69DO31 and coPHT57DO43. According to NMR determinations, the number-average of HT sequences in these copolyesters is approximately 3.2 and 2.3 repeating units, respectively, which will produce exceptionally low lamellar thickness. This behavior is, however, not inedited because extremely thin crystallites comprised of two or three repeating units have been reported to exist in other aromatic copolyesters.34 Hydrolytic Degradation. To evaluate the influence of copolymerization on the sensitivity to hydrolytic degradation, a comparative study of the two homopolyesters and the copolyester coPHT69PDO31 was made. Degradation experiments were carried out at 37 °C in aqueous buffers at pH 7.0 and 2.3. In Figure 6a the weight loss is plotted against time along an incubation period of 15 weeks. As it could be anticipated PHT remained practically unaffected over the whole period whereas PDO almost disappeared taking more or less time depending on pH. The higher degradability displayed by PDO at lower pH is consistent with the mechanism reported for the hydrolysis of this polyester, which is known to be autocatalyzed by the carboxylic end groups that are generated in the process.6 Conversely, the copolyester showed a very slow weight loss that hardly achieved 10% at the end of the period; although the response is moderate, it is of interest because it is indicative anyway of the influence exerted by the presence of DO units in the PHT chain. Although the weight change is even smaller, it can be appreciated that, different from PDO, the copolyesters hydrolyzed somewhat faster at neutral pH. This unexpected



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Figure 8. Surface SEM micrographs of a film of coPHT69PDO31 before (a) and after hydrolytic degradation at 37 °C and pH 7.0 for 100 days (b) (Inset in (b): edge-view of the incubated film fractured after quenching).

behavior suggests that the factors determining the hydrolytic rate of the copolyester must be different to those operating in PDO. The changes in the number-average molecular weight of the polyesters taking place upon incubation were followed by GPC and the results expressed as percentages of the initial value are plotted in Figure 6b. In accordance to changes observed in sample weight, the Mn of incubated PHT remained invariable corroborating that this polymer is fully resistant to the aqueous attack under the experimental conditions used in this study. On the contrary, the decrease in Mn of PDO became apparent from the beginning and kept on with a rate that follows the same dependence on pH as observed for the loss of sample weight. Also, coPHT69DO31 showed a significant reduction in Mn that reached almost 50% of the initial value at the end of the incubation period for both pH 7.0 and 2.3. This result confirms the occurrence of hydrolytic degradation of the copolyester in non-water-soluble fragments that remain attached to the sample so changes in weight are hardly appreciated. The opposite influence of pH on degradation of the copolyesters compared to PDO becomes understandable if releasing of degraded fragments to the medium is assumed to be the main factor determining the hydrolysis rate in the case of the aromatic-based copolyesters; at acidic pH, releasing is even more repressed, and the shell effect on water of enriched HT fractions is more effective. The NMR analysis of the degraded products provided evidence indicative of the degradation mechanism. In Figure 7, the 1H NMR spectra of the products released to the aqueous medium and the residual material resulting after 15 weeks of incubation of coPHT69DO31 at pH 7.2 are compared with the spectrum of the original copolyester. It is revealed that the only product released to the medium is dioxanoic acid, and quantification of the signals indicated that the copolyester has lost approximately 20% of dioxanone at the end of the incubation period. It can be concluded therefore that hydrolysis of the copolyester takes place essentially by breaking of the carboxylate groups associated to the DO units. The analysis by SEM (Figure 8) added valuable information in support of such degradation mechanism. Micrographs of the surface of the coPHT69PDO31 film before and after the whole incubation period at pH 7.0 are compared in Figure 8a,b, bringing into view the erosion undergone by the copolyesters due to aqueous degradation. The edge view of a fresh fracture of the incubated film is shown in the inset of Figure 8b, where it is clearly seen that degradation is strictly confined to the

surface of the film reaching a depth of only about 5% of the film thickness.

Conclusions Ring-opening copolymerization of mixtures of a fraction of cyclic oligo(hexamethylene terephthalate)s embracing cycle sizes from 2 to 5 repeating units and p-dioxanone was used to prepare coPHTxDOy copolyesters within a range of aromatic (HT) to aliphatic (DO) units ratios going from 9 to 1.3. Copolyester compositions deviated significantly from those used in the feed as a consequence of the back p-dioxanone depolymerization that took place at the temperatures used for reaction. The random microstructure of these copolyesters was elucidated by combining 1H and 13C NMR data, which could be interpreted on the basis of the signal assignment attained by making use of bidimensional analysis. The copolyesters showed decomposition and glass transition temperatures in agreement with their composition and microstructure. They were able to crystallize from the melt up to contents in DO of 31% adopting the same crystal structure as the homopolyester PHT with DO units being excluded from the crystalline domains. The incorporation of DO units in the PHT chain enhanced moderately the hydrolysis of the polyester, which was found to occur through a surface erosion process due to the non-water-solubility of the HT containing fragments originated by degradation. Hydrolysis took place almost exclusively on DO units with releasing of the dioxanoic acid to the medium. Acknowledgment. Financial support for this work was received from the Spanish Ministerio de Ciencia e Innovacioń (MICINN) with Project Grant MAT2006-13209-CO2-02 and MAT 2009-14053-CO2-01. Our thanks to the Venezuelan government (CONICYT) for the Master Thesis grant awarded to one of the authors (G.G.). Supporting Information Available. FTIR and 2D NMR (COSY and HETCOR) spectra of coPHTxDOy copolyesters. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Hydrolyzable Aromatic Copolyesters of p-Dioxanone - ACS Publications

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