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Role of 2-hydroxyethyl end group on the thermal degradation of polyethylene terephthalate and reactive melt mixing of poly(ethylene terephthalate)/poly(ethylene naphthalate) blends.

INTRODUCTION

Thermal stability of thermoplastic engineering polyesters such as poly(ethylene terephthalate) (PET) is of great technology importance. PET and its blends with other polyesters (i.e., poly(ethylene naphthalate), referred as PEN) have subjected to processing operations at about 300[degrees]C, and therefore the understanding of their behavior at this temperature is of crucial importance in the end applications. The thermal degradation processes that occur in PET have received continuing attention in the literature (1-18). The overall evidence presented there indicated that the pyrolysis of neat PET proceeds through the primary formation (at about 300[degrees]C) of cyclic oligomers that decompose further (at about 400[degrees]C) via [beta]-hydrogen atom abstraction to generate acetaldehyde ([CH.sub.3]CH0), water, carbon dioxide, and low-molecular-weight oligomers terminated with carboxyl groups (1), (2). PET oligomers containing anhydride units were also observed (1). Formation of acetaldehyde in PET samples processed at various temperatures was detected by [.sub.1]H-NMR (1). Several studies report that at about 280-300[degrees]C the [CH.sub.3C]HO, one of the major products of the thermal degradation of PET, would be generated by a mechanism involving the hydroxyethyl end groups (--[CH.sub.2]--[CH.sub.2]--OH) (1), (14), (18). However, some questions are unanswered, to date, on the formation of acetaldehyde by a mechanism involving the vinyl end groups generated by [beta]-chain scission.

PET/PEN blends are extensively studied and several works report that exchange reactions between the two polymers occur during their processing in the molten state at about 280-300[degrees]C (19-30). Three types of interchange reactions can occur in polyesters having chains terminated by hydroxyl or carboxyl groups. These reactions include

alcoholysis, acidolysis, and ester exchange that are referred here as transesterification. The first exchange process leading to the formation of block copolymers becomes more random as the exchange reactions proceed. The formed block copolymer acts as a compatibilizer of the initial immiscible blends. PET/PEN blends offer advantages over PET in their mechanical, thermal, and barrier properties.

The role of the hydroxyl end groups on the thermal stability of PET at the processing temperature (300[degrees]C) and also in the transesterification reactions occurring during the melt mixing of PET/PEN blends at 300[degrees]C is here investigated using two PET samples: one with unmodified end groups the other with the hydroxyl end groups capped by reaction with benzoyl chloride. All homopolymers were heated for different times in the same processing conditions to investigate their thermal stability, and were are then characterized by means of matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) (31), (32) and [.sup.1]H-NMR to obtain detailed information on their end groups. The studied samples were also analyzed by means of thermogravimetry (TG), in inert ([N.sub.2]) and oxidant (air) atmosphere, and by differential scanning calorimetry (DSC) tools.

EXPERIMENTAL

Materials

All the polymer and chemicals were purchased from Aldrich. PET ([M.sub.v] = 18000) and PEN ([M.sub.v] = 24000) were dried at 70[degrees]C for 1 week before use. Tetrachloroethane (TCE) was freshly distilled and stored under nitrogen before use. All the other chemicals were used without further purification. PET was treated with [CH.sub.2][C1.sub.2] at reflux to extract the cyclic oligomers.

End-Capping of PET

The end-capping of purified PET was performed by reacting benzoyl chloride with the commercial PET sample in TCE, at 100[degrees]C, using N, N-dimethylaniline as proton acceptor as shown in Scheme 1. The end-capping reaction was carried out as follows: in a three-neck round bottom flask equipped with a nitrogen inlet, containing 150 mL of a vigorously stirred TCE solution of PET (5 g) and 1 mL of N, N-dimethylaniline, 50 mL was dropwise added in a TCE solution of benzoyl chloride (0.02 mol). The reaction mixture was allowed to stir for 24 h at 100[degrees]C. The solvent was removed by vacuum distillation, and the solid residue was washed several times with boiling acetone and dried at 70[degrees]C for 2 days. The so-obtained polymer will be referred thereafter as Bz-PET.

[.sup.1]H-NMR ([d.sub.2]-TCE): d = 8.50 (d, [H.sup.[alpha]]), 7.7 (dd, [H.sup.[beta]]), 7.5 (dd, [H.sup.[gamma]]), 8.09 (large s, [H.sup.c, b]), 4.09 (large s, [H.sup.a]).

Preparation of PET/PEN Physical Blends

The PET/PEN and the Bz-PET/PEN blends, with a PEN content of 20% (w/w), were prepared by dissolving the polymers in a mixture of phenol/TCE 60/40% (w/w). The solutions were then precipitated in acetone. After several washings with boiling acetone, the product was dried under vacuum at 120C to constant weight.

Transesterification Processes

The reaction was carried out in a laboratory static mixer, under nitrogen flow, in the presence of Ti(isoprop-oxide)4 as catalyst. Typically, 4 g of the PET/PEN and Bz-PET/PEN blends were melt mixed at 285[degrees]C for 10 min. The catalyst 48 mg (0.12% w/w) was then added and this was considered at time zero. The blends were allowed to react at 300[degrees]C for 30, 60, and 120 min.

[.sup.1]H-NMR Measurements

All samples were analyzed by [.sup.1]H-NMR using a 500-MHz Varian Inova spectrometer. Deuterated sym-1,1,2,2-TCE solvent was used for PET samples, whereas PEN and PET/PEN blends were analyzed in a 70/30 (v/v) mixture of deuterated chloroform/deuterated trifluoroacetic acid, at a polymer concentration of 20 mg/mL.

MALDI Analysis

MALDI-TOF mass spectra of PEN and PET samples were recorded in linear mode, using a Voyager-DE STR mass spectrometer (Perceptive Biosystem), equipped with a nitrogen laser emitting at 337 nm, with a 3 ns pulse width, and working in positive ion mode. The accelerating voltage was 20 kV; the grid voltage and delay time (delayed extraction, time lag), were optimized for each sample to achieve the higher mass resolution, expressed as the molar mass of a given ion divided by the full-width at half-maximum (FWHM). The laser irradiance was maintained slightly above threshold. About 2-mg samples were dissolved in 1 mL of hexafluoroisopropanol (HFIP), and 2,5-dihydroxybenzoic acid (DHB), indoleacrylic acid, or 2-(4-hydroxyphenylazo)-benzoic acid (HABA) (0.5 M in HFIP as solvent) were used as matrix. Appropriate volumes of polymer solution and matrix solution were mixed to obtain 1:1 and 1:3 (v/v) ratio. One microliter of each sample/matrix mixture was spotted on the MALDI sample holder and slowly dried to allow matrix crystallization. The best mass spectra were obtained by using DHB and HABA as matrix for PET and PEN samples, respectively. The best MALDI spectra reported here present a mass resolution of 1000-1200 FWHM and were recorded using a (matrix solution)/(polymer solution) ratio of 1:1. The mass peaks corresponding to the macromolecular species were measured with an accuracy of ~0.4 Da. All spectra were calibrated using external calibration file made by MALDI analysis of a Ny6 sample terminated with only amino groups and having a [M.sub.n] of about 6000 Da, appropriately synthesized.

TG and DTG Measurements

A Mettler TA 3000 TG analyser, coupled with a Mettler TC 10A processor, was used for thermal degradations. The temperature calibration of thermobalance was made according to the procedure reported in the user's manual of the equipment (331, based on the change of the magnetic properties of three metal samples (Isatherm, Nickel and Trafoperm) at their Curie points (142.5, 357.0, and 749.0[degrees]C, respectively). The temperature calibration was repeated every month. Degradations were performed in dynamic heating conditions, from 35 up to 600[degrees]C, in both flowing nitrogen (0.02 L*[min.sup.-1]) and static air atmosphere, at heating rates of 5, 10, and 15[degrees]C[min.sup.-1]. As the experimental conditions, and the scanning rate in particular, largely influence the shape of TG curve, the values of each studied compound at various heating rates were different with each other, but the trend was the same independently on the used scanning rate. For the sake of simplicity, only the TG scans at 10[degrees]C[min.sup.-1]were considered and reported. Additional TG measurements were performed at fixed temperatures comparable with those which are generally reached within injection-blowing machines, namely 280-300[degrees]C, in a nitrogen flow for a maximum time of 60 min. In both cases, samples of 10 mg, held in alumina open crucibles, were used and their weights were measured as a function of temperature and time. In both dynamic and isothermal experiments, data averaged from those of three runs for each sample, the maximum difference between the average and the experimental values being within -[+ or -]1[degrees]C, were used to plot the percentage of undegraded polymer (1 - D)% as a function of temperature and time, where D = ([[W.sub.0] - W)/[W.sub.0], and [W.sub.0] and W were the weights of sample at the starting point and during scanning.

DSC Measurements

A Mettler DSC 20 differential scanning calorimeter, coupled with the same Mettler TC 10A processor used for TG experiments, was employed for the determination of glass transition temperature ([T.sub.g]). Both heat flow and temperature of calorimeter were calibrated following the procedure suggested by Mettler and reported in the operating instructions of equipment (33). Samples of about 10 mg, held in sealed aluminum crucibles, at a heating rate of 10[degrees]C*[min.sup.-1], and a static air atmosphere were used for measurements.

RESULTS AND DISCUSSION

Reactive blending of PET/PEN blends (80/20 w/w) were carried out at 300[degrees]C for 30, 60, and 120 min, in the presence of Ti[(isopropoxide).sub.4] as catalyst, under nitrogen ([N.sub.2]) flow, using the procedure reported in the EXPERIMENTAL section. Purified PET sample, by solvent extraction of the cyclic oligomers, and a PET with the end hydroxyl groups capped with benzoate groups (referred as Bz-PET) were used to investigate the role of the hydroxyl end groups of PET sample in the exchange reactions occurring between PET and PEN sample at the processing temperature (300[degrees]C). All PET and PEN samples used in the present study were analyzed by NMR ([.sup.1]H and [.sup.13]C) and, in particular, by MALDI-TOF MS to characterize their end groups. In fact, it is known that the MALDI-TOF MS is able to detect individual and intact polymer molecules even in a complex mixture, including species present in smaller amounts in a polymer sample (31), (32). It has permitted the identification of repeat units, chain ends, cyclic oligomers, and also of species present in a smaller amount in a lot of class of polymer samples. Positive-ion MALDI-TOF mass spectrum of PEN sample shows three family of peaks corresponding to cyclic oligomers and to linear oligomers terminated with methyl esters ([CH.sub.3]0/[0CH.sub.3]) at both ends and the other with methyl ester at one end and hydroxyl groups at the other end ([CH.sub.3]0/0H). Peaks owing to cyclic oligomers are present in the mass range of m/z = 1000-3500, whereas the peaks corresponding to oligomers terminated with ([CH.sub.3]0/[0CH.sub.3]) groups are the most intense in the mass range higher than ni/z = 3500. The intensity of peaks owing to the oligomers terminated with CH30/0H groups is very lower (<10%) with respect to those corresponding to the oligomers ([CH.sub.3]0/[0CH.sub.3]) ended. This result suggests that PEN chains are mostly ended with methyl ester moiety. The MALDI-TOF analysis of PEN heated at 300[degrees]C for different minutes (ranging from 5 up to 120 min) gives mass spectra close to that of unheated initial polymer, suggesting that thermal degradation processes does not occur in the used experimental conditions. Heated and unheated unmodified commercial PET samples give the same MALDI-TOF mass spectra using DHB as a matrix, confirming that this polymer is stable at 300[degrees]C under inert atmosphere ([N.sub.2] flow) [1]. Five important series of peaks are present in the MALDI spectrum of all unmodified PET samples, which correspond to the protonated ions of linear and cyclic oligomers. Peaks owing to the cycles dominate in the lower mass range m/z = 2500, whereas peaks corresponding to linear oligomers terminated with methyl ester groups at both ends (species A in Fig. 1) dominate in the higher mass range m/z = 2500. Low-intense peaks owing to oligomers terminated with [CH.sub.3]0/H groups, with HO/H species, and HO[([CH.sub.2]).sub.2]O/H moieties are also present. Each family of peaks is also accompanied by low-intense peaks at higher mass of 44 Da, corresponding to the same family of oligomers containing one diethylene glycol (DEG) unit along the chains. The presence of this unit was also confirmed by [.sup.1]H-NMR analysis, as shown in Fig. 2. On the basis of these data, the PET chains, as well as the PEN, are essentially terminated with methyl ester groups. Peaks owing to the cycles disappear in the mass spectrum of the PET sample treated with [CH.sub.2][Cl.sub.2] to extract the cyclic oligomers. Figure 1 shows the MALDI-TOF mass spectrum of the benzoylcapped PET sample. All peaks correspond to the protonated and sodiated ions of PET species highlighted in Fig. 1. As expected, the most intense peaks correspond to the oligomers terminated with methyl esters at both ends (species A); peaks owing to the chains terminated with benzoate groups are also observed (species B, C, and D). Each family of oligomers also shows the mass peaks owing to the corresponding macromolecular chains containing one DEG unit, which are labeled with the symbol * in Fig. 1. This mass spectrum confirms that the Bz-PET sample does not have reactive OH end groups. MALDI analyses of the Bz-PET heated at 300[degrees]C for different minutes within 5 and 60 min gave mass spectra close to that of the unheated sample shown in Fig. 1, confirming the thermal stability of this sample in the melt-mixing conditions used for the reactive blending of the PET/PEN systems. Direct evidence of the successful end-capping of the hydroxyl end groups can be also supported by focusing on the resonance of the alkyl protons adjacent to the hydroxyl end group and noting the shift on esterification with benzoyl chloride as shown in Fig. 2, which displays the resonance peaks of methylene protons ([CH.sub.2]) along the PET chains. The signals around 4.69 ppm are from the ethylene glycol residues in the main chains (protons H"); however, for the present purposes, interest is confined to the signals at 3.95 and 4.54 ppm corresponding, respectively, to [H.sup.d] and [H.sup.e] of the ethylene unit linked to the hydroxyl end group. The triplet signals at 3.85 and 4.45 ppm are owing to the [CH.sub.2] belonging to DEG units (protons [H.sup.f]] and [H.sup.g], respectively) along the PET chains. On esterification with BZC, the environment experienced by these protons changes, the resonance of the protons shifts to a higher frequency and overlaps with the 4.6-4.8 ppm region. In contrast, the resonance of the [H.sup.b] protons shifts to a lesser extent, appearing as a shoulder on the peak at 4.65 ppm. A decrease in the intensity of the signals from the protons in the alkyl end groups, relative to the DEG protons at 4.13 and 4.64 ppm, consistent with the capping reaction, is clearly evident in the proton spectra. This is demonstrated by comparing Fig. 2a with Fig. 2b. After 24 h, the signals at 3.95 and 4.54 ppm have disappeared (Fig. 2b), indicating completion of the esterification reaction. The extent of this conversion is so sufficiently high that no residual--OH ends were identifiable within the accuracy of the NMR spectrometer used. In addition, the achievement of complete functionalization can be supported on the basis of the appearance of three additional peaks at 8.50 (doublet), 7.62 (double doublet), and 7.50 (double doublet) ppm owing to aromatic protons [H.sup.[alpha]], [H.sup.[beta]], [H.sup.[gamma]], respectively; the aromatic protons of the terephthalate groups belonging to the PET structural units give a large singlet at 8.095 ppm. The capped polymer was maintained in the reactor in a nitrogen atmosphere at 300[degrees]C for 120 min to assess the presence of the benzoic groups for times similar to transesterification reaction ones. As shown in Fig. 2c, the peaks assigned to the benzoic groups are still present, confirming the MALDI-TOF MS data discussed above (Fig. 1).

PET/PEN melt-mixed blends were analyzed by [.sup.1]H-NMR to characterize their composition, and then the copolymers were eventually formed by the exchange, which occurred during the process in the molten state (300[degrees]C). The sequence structure of PET/PEN copolyesters can be analyzed from the resonance signals that represent three types of ethylene units: TET (ethylene unit between two terephthalate groups), NEN (ethylene unit between two naphthalate groups), and TEN (ethylene unit between terephthalate and naphthalate groups). The [.sup.1]H-NMR chemical shifts appear at 4.85, 4.90, and 4.80 ppm for the TET, TEN, and NEN triads, respectively. Initially, the copolymers have large block lengths, which become increasingly random as the reactions proceed. The extent of reaction between PET and PEN is given by a factor called the degree of randomness (RD) which is calculated from proton NMR measurements. RD can be calculated from the three peak areas ([I.sub.NEN], [I.sub.TET], and [I.sub.TEN]) corre-sponding to the triad sequences: NEN, TET, and TEN, using the Eqs. 1 and 2 elsewhere described (19).

[X.sub.NEN] = [I.sub.NEN]/[I.sub.Tot]; [X.sub.TET] = [I.sub.TET]/[I.sub.Tot]; [X.sub.TEN] = [I.sub.TEN]/[I.sub.Tot];

[I.sub.Tot] = [I.sub.NEN] + [I.sub.TET] + [I.sub.TEN]; (1)

RD = [X.sub.NEN]/[[2X.sub.pEN](1 - [X.sub.pEN])] (2)

where [X.sub.pEN] corresponds to the molar fraction of the PEN in the PET/PEN blends.

Based on this definition of the RD, value of 0 indicates the presence of a mixture of homopolymers, a value included between 0 and 1 indicates that the copolymer has a block character, an RD value of I indicates the presence of a random copolymer, whereas a value of 2 shows the presence of an alternating copolymer. Figure 3 shows the [.sub.1]H-NMR spectra of the PET/PEN and Bz-PET/PEN blends reacted at 300[degrees]C for different reaction times. The resuits indicate that 30-min melt reacting leads to an RD of 0.39 in the case of the unmodified blend and 0.29 in the case of the modified one. For reaction times of 60 min, similar reductions in the RD values were obtained with a value of 0.58 for the unmodified system and of 0.39 in the case of the modified system. After 120 min of reaction, the unmodified system is fully randomized (RD 1), whereas the modified system has still an RD of 0.75. This is strong and direct evidence that the end-capping of the hydroxyethyl chain ends of the PET influences the mechanism and the kinetic of transesterification. This result suggests that using polyester having end groups capped with thermally and hydrolytic stable moieties, only an exchange reaction involving the inner ester groups occurs during the melt mixing of PET/PEN blends, in the presence of a transesterification catalyst such as Ti[(isopropoxide).sub.4].

The overall evidence presented in the literature indicated that the pyrolysis of pure PET proceeds through the primary formation (at about 300[degrees]C) of cyclic oligomers, and then the linear PET chains decompose further (at about 400[degrees]C) via [beta]-hydrogen transfer, leading to the formation of low-molar-mass PET chains with vinyl ester and carboxylic end groups (1), (2). In this process, new carboxyl end groups will produce [CO.sub.2] as a result of decarboxylation, whereas the vinyl end groups may decompose to produce acetaldehyde in considerable amounts. Ethylene glycol chain ends produce acetaldehyde either by their own decomposition, leading also to the formation of new carboxyl end groups, or by reacting with the formed vinyl end groups (14). They also react with the anhydride linkages formed from additional carboxyl chain ends (14). Especially in oxygen-free atmosphere processing of PET, the sensitivity of chain ends to thermal and thermo-oxidative degradation seems to be greater than the internal unit especially in the case of low-molecular-weight polymers (13). The end-capping of the hydroxyl chain ends of PET samples can inhibit thermal degradation processes occurring at the processing temperature (270-300[degrees]C) which involve these end groups, thus inhibiting the acetaldehyde formation from the ethylene hydroxyl ends. Moreover, as the polyester carboxyl end groups have a promoting effect on thermal degradation (15), the capping reaction could partially inhibit the formation of carboxyl groups from the ethylene glycol terminal degradation reactions. At the same time, the end-capping of the DEG end units, which. has been referred as the main impurity in PET (protons [H.sup.f, g] in Fig. 2), could inhibit the spontaneous cyclization/1,4-diox-ane and 2-methyl-1,3-dioxolane, a significant source of acetaldehyde at high melt temperatures (13), and could also inhibit the reaction with the formed vinyl end groups which yields additional acetaldehyde (14). As compounds showing the most high initial decomposition temperature value must be considered most thermally stable (34), (35), better improvement in thermal stability was observed for the func-tionalized PET than the original one (Figs. 4 and 5). In fact, considering the initial decomposition temperatures (TO, graphically obtained by the TG curves as the intersection between the starting mass line and the maximum gradient tangent to the TG curve, in flowing nitrogen, the functionalized PET showed Ti (421[degrees]C) clearly higher than PET (410[degrees]C) and obviously lower than PEN (436[degrees]C). Analogous trend was observed in static air atmosphere where [T.sub.i] was 399, 404, and 416[degrees]C, respectively, for PET, functionalized PET (Bz-PET), and PEN.

The DTG curves in both atmospheres showed for all studied compound a main degradation stage immediately followed, only in oxidative atmosphere, by a more little another one at higher temperature. On considering that the most part of mass loss is associated with the main degradation stage, we hypothesize that the degradation mechanism is not affected by the environment used.

Isothermal degradation experiments of functionalized PET in static air atmosphere were performed to verify mass loss near the service temperature. In Fig. 6, we report TG isothermal scans at 180, 190, and 200[degrees]C for 1 h of Bz-PET from which no appreciable mass loss is noted. Moreover, DSC runs showed that characteristic temperatures ([T.sub.g], [T.sub.c], and [T.sub.m]) of the PET sample do not change by end-capping functionalization. In fact, the Bz-PET sample showed very similar values ([T.sub.g] 80[degrees]C; [T.sub.c] = 125[degrees]C; [T.sub.m] = 244[degrees]C) compared to that of the initial neat PET ([T.sub.g] = 78[degrees]C; [T.sub.c] = 127[degrees]C; [T.sub.m] = 251[degrees]C).

The information presented above could be very useful in designing production as well as processing protocols for PET homopolymer and for PET/PEN block copolymers, especially in situations where low level of acetaldehyde generation and low RD are highly desirable.

CONCLUSIONS

The data reported in the present study indicate that the end-capping of the hydroxyethyl chain ends of the PET influences the mechanism and the kinetic of transesterification reactions occurring during the melt mixing of PET/PEN blends, favoring the formation of a block copolymers also a longer reaction time. This chemical modification also gives most thermally stable PET, and thus limiting the formation of acetaldehyde at high melting temperatures (270-300[degrees]C).

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Ignazio Blanco, (1) Gianluca Cicala, (1) Carmelo Luca Restuccia, (2) Alberta Latteri, (1) Salvatore Battiato, (3) Andrea Scamporrino, (3) Filippo Samperi (3)

(1.) Department of Industrial Engineering, University of Catania, 95125 Catania, Italy

(2.) Cytec Engineered Materials Limited, LL13 9UZ Wrexham, UK

(3.) Institute of Chemistry and Technology of Polymers (ICTP)-Sez, Catania, CNR, 95126 Catania, Italy

Correspondence to: Ignazio Blanco; e-mail: iblanco@dmfci.unict.it

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

DOI 10.1002/pen.23206
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Date:Dec 1, 2012
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