Synthesis, Characterization, and Biodegradability of Novel Fully

Subscriber access provided by University of Cincinnati Libraries

Article

Synthesis, Characterization, and Biodegradability of Novel Fully Biobased poly(decamethylene-co-isosorbide 2,5furandicarboxylate) Copolyesters with Enhanced Mechanical Properties Yosra Chebbi, Nejib Kasmi, Mustapha Majdoub, Pierfrancesco Cerruti, Gennaro Scarinzi, Mario Malinconico, Giovanni Dal Poggetto, George Z. Papageorgiou, and Dimitrios N. Bikiaris ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Synthesis, Characterization, and Biodegradability of Novel Fully Biobased poly(decamethylene-co-isosorbide 2,5-furandicarboxylate) Copolyesters with Enhanced Mechanical Properties

Yosra Chebbi1,2, Nejib Kasmi2, Mustapha Majdoub1, Pierfrancesco Cerruti3, Gennaro Scarinzi3, Mario Malinconico3, Giovanni Dal Poggetto3, George Z. Papageorgiou4, Dimitrios N. Bikiaris2* 1Laboratoire

des Interfaces et Matériaux Avancés, Université de Monastir, 5000 Monastir,

Tunisia 2Laboratory

of Polymer Chemistry and Technology, Department of Chemistry, Aristotle

University of Thessaloniki, GR-541 24, Greece 3Institute

for Polymers, Composites and Biomaterials (IPCB-CNR), via Campi Flegrei 34,

Pozzuoli, Na 80078, Italy 4Chemistry

Department, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece

* Corresponding authors. Tel.: +302310997812, E-mail: [email protected]

Abstract This study spotlighted a successful synthesis of a novel series of biobased poly(decamethyleneco-isosorbide

2,5-furandicarboxylate)s

dicarboxylate

(DMFD),

isosorbide

(PDIsFs) (Is)

and

copolyesters

from

1,10-decanediol

dimethylfuran-2,5-

(1,10-DD)

by

melt

polycondensation, using Titanium (IV) isopropoxide (TTIP). The chemical structure and composition of prepared polymers were confirmed in detail by

1H

NMR and FTIR

spectroscopies. Satisfactory weight-average molecular weights (Mw) in the 55300-84500 g/mol range and random microstructure were obtained for PDIsFs. It was shown that Is unit incorporation into the copolyesters molecular chains was dramatically effective in increasing the glass transition temperatures (Tg) and in delaying the onset decomposition temperatures of PDIsFs. Hence, an excellent improvement of the thermal stability exceeding 405 °C for all copolymers was obtained. In addition, the degradation behavior in soil as well as the mechanical properties of PDIsFs were duly investigated in detail. The biodegradation rate of the copolyesters 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

depended on comonomer ratio. Rotational rheometry characterization of polymer melts revealed prevailing viscous properties for all formulations, whereas the presence of isosorbide favored a Newtonian behavior. Oxygen Induction Time (OIT) measurements by chemiluminescence (CL) demonstrated that isosorbide incorporation also dramatically increases polymer thermo-oxidative stability. Taking advantage of their features, PDIsFs have the potential to serve as promising and innovative biobased polymers for practical applications such as eco-friendly and sustainable plastic packaging. Keywords: 2,5-furandicarboxylic acid; isosorbide; 1,10-decanediol; melt polycondensation; Mechanical property, biobased polymers; copolyesters. Introduction Over the last decades, sparkling researches have been written about biobased polymers derived from renewable resources.1–3 This is understandable owing to the rapid increase in the atmospheric CO2 concentration, the widespread use of fossil oil reserves and their price increase. In fact, the development of eco-friendly polymeric materials, instead of the traditional fossil oilbased polymers produced from petrochemical sources, have witnessed a spiraling growth of interest.4–7 Poly(3-hydroxyalkanoate)s (PHAs), poly(L-lactic acid) (PLA) and poly(butylene succinate) (PBS) are such outstanding examples of biobased polyesters, which have become widely available and then have been successfully commercialized.8–10 In spite of that, the latter materials still exhibit some shortcomings concerning their poor properties.11–15 In this context, 2,5-furandicarboxylic acid (FDCA), derived from 5-hydroxymethylfurfural (HMF), stands out as a highly promising biobased monomer, readily available today. It has awakened a giant research activity in both scientific and industrial communities.16–19 As result of its outstanding features such as superior performance, emphasis is now being placed on FDCA as an environmentallyfriendly monomer to synthesize biobased homopolyesters

20–28

as well as copolyesters.

29–45

The

resulting materials can be a real alternative to fossil-derived polymers, e.g. poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET), which are considered as the most important materials used in packaging, textile fibers and films, medical applications.46,47 Among several eco-friendly monomers, isosorbide (Is) is a promising renewable diol currently produced at industrial scale by dehydration of sorbitol, the latter being obtained by hydrogenation of D-glucose derived from corn starch. Is has emerged as an attractive building 2 ACS Paragon Plus Environment

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


block in the field of polymer chemistry owing to a plenty of advantages, which indeed gave this monomer a great deal of attention in recent decades, such as interesting rigidity and chirality properties, non-toxicity, bioavailability, and good thermal stability.48 Thanks to its abovementioned features, isosorbide has been extensively employed to produce several aliphaticaromatic polyesters. This is due to the fact that the insertion of this diol into polymer main chain is very helpful to thermal stability enhancement, as well as leading to a spectacular increase in the glass transition temperature (Tg).49–51 In this context, to name of few, a contribution by Noordover et al. 52 has been reported few years ago describing the synthesis of oligomers derived from isosorbide and succinic acid with several diols for renewable coating applications. Additionally, amorphous isosorbide-based homopolymers and copolymers prepared from terephthalic and succinic acids have been recently reported.53 These materials revealed appealing thermal and optical properties. However, although its numerous advantages, the insertion of isosorbide units into polymers backbone often leads to a destruction of the chain’s regularity and thus to a very poor crystallinity and in many cases to totally amorphous materials, which drastically hinders their applications at industrial level. As a relevant example, a fully renewable and wholly amorphous homopolyester (PIsF) derived from isosorbide and FDCA has been described by Gomes.54 Furanic-long chain aliphatic polyesters are of great interest and they have been broadly studied thanks to their reported features.20, 31, 32, 55, 56 This is due to the availability of a wide variety of aliphatic diols having long linear chains and therefore leading to different end-properties of the resulting polyesters. The presence of such hydrocarbon chains into furan-based polyesters structure enhances the crystallization ability, as well as boosts the hydrophobicity and thermal properties of the prepared materials, wherein it can adjust their melting temperatures to ones suitable for thermoplastic processing. In this sense, one among numerous studies published by our group on furan-based polyesters has dealt with the synthesis, as well as the structural, thermal and mechanical characterization of an aliphatic polyester, poly(decamethylene 2,5furandicarboxylate) (PDF).28,31 The latter, showing a significant degree of crystallinity, was prepared from a renewable aliphatic diol, 1,10-decanediol (1,10-DD). This building block can be prepared from the conversion of sebacic acid, originating from castor oil, via esterification and hydrogenation processes,57 as well as by applying a green synthetic route through supercritical dimethyl sebacate hydrogenation, as disclosed in a recent patent.58 3 ACS Paragon Plus Environment


Page 4 of 39

Copolymerization still the most well-established technique, which affords a facile approach to resolve the tunability issue of properties for the prepared materials exhibiting weak points such as low degree of crystallinity, poor thermal properties, among others.

59,60

This approach was

applied herein to overcome the drawback of the total absence of crystallinity in PIsF. To the best of our knowledge, the study of the relationship between the structure and properties for the long chain aliphatic PDF polyester with incorporated isosorbide has been examined for the first time in the present work. In this context, we aimed to synthesize through melt polycondensation procedure a new fully renewable resources-based furanoate copolyesters series containing FDCA and varied rigid and flexible segments molar ratio, respectively Is and 1,10DD diols. The effect of Is comonomer insertion on the properties of resulting materials, including the thermal and mechanical properties as well as their sensitivity to the biodegradation in soil, has been assessed in detail. Experimental Materials Dimethyl-2,5-furan dicarboxylate (DMFD, purum 99%) was obtained as described in our previous reports using 2,5-furan dicarboxylic acid (FDCA) (Aldrich, 97%).28 1,10-decanediol (1,10-DD, purum 98%), titanium (IV) isopropoxide (TTIP, purum 97%) catalyst and isosorbide (Is, purum 99%) reagents were purchased from Aldrich Co. All other materials and solvents used were of analytical grade. Copolyester synthesis As shown in Scheme 1, three-step melt polycondensation method (transesterification, polyesterification and then polycondensation) was used to synthesize PDIsFs copolyesters from DMFD, Is and 1,10-DD as follows: in the first stage, the transesterification took place in bulk under N2 atmosphere to prepare the bis(hydroxyisosorbide)-2,5-furan dicarboxylate (BHIsF) and bis(hydroxy-decamethylene)-2,5-furan dicarboxylate (BHDF): Predetermined amount of DMFD and 1,10-DD or Is with the molar ratio of diester:diol of 1:2 with 400 ppm of Titanium (IV) isopropoxide (TTIP) catalyst were charged in a round bottom reactor equipped with a mechanical stirrer and the mixture was first heated at 160°C for 2h. The reaction temperature was gradually raised to 170°C for 1h, at 180°C for an additional 1h and finally it was left to proceed for 0.5h at 190°C. The reaction mixture was cooled down and was discharged. The first 4 ACS Paragon Plus Environment

Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesis step totally lasted for 4.5 h. The second step of PDIsFs synthesis, which is the polyesterification, was accomplished by reacting mixtures of BHDF/BHIsF with the selected molar compositions ranged from 95/5 to 60/40 and an equimolar amount of DMFD relative to the total molar ratio of monomers (BHIsF and BHDF). The mixture was placed into the glass reactor provided with a N2 flow and heated at 160°C for 2h. The reactor temperature was then set at 170°C for 1h and was increased to 180°C for an additional 1h and finally it was maintained at 190°C for 0.5h. Then, the polycondensation reaction was performed and the system pressure was gradually reduced to 5.0 Pa over a time of about 30 min to minimize oligomer sublimation and to avoid excessive foaming, which is a potential problem during melt polycondensation. The reaction temperature was raised to the temperature range of 220-250°C (0.5h at 220°C, 0.5h at 230°C, 0.5h at 240°C and for additional 0.5h at 250°C). After cooling at room temperature, the target copolymers were obtained by dissolution in a mixture of trifluoroacetic acid and chloroform (25/75 v/v) followed by a precipitation in cold methanol. Copolyesters characterization All characterization methods are summarized in the Supporting Information. Results and Discussion Copolyester Synthesis In this study, a novel series of biobased copolyesters derived from DMFD, Is and 1,10-DD was successfully synthesized via an esterification-polyesterification-polycondensation procedure as depicted in Scheme 1.

5 ACS Paragon Plus Environment


Page 6 of 39

Transesterification

H3CO C O

O DMFD

C OCH3

+

HO R

OH

Is/1,10-DD

O

160 °C - 190 °C N2, TTIP - 4.5 h

HO R

O C

C O R

O

OH + CH3OH

O O BHDF (R= 1,10-DD) BHIsF (R= Is)

HO O With HO R

Is ,

OH =

HO

(CH2)10

OH 1,10-DD

O OH Polyesterification and Polycondensation

x BHIsF + y BHDF + (x+y) DMFD

05% 10% 15% 20% 30% 40%

95% 90% 85% 80% 70% 60%

1) Polyesterification 160-190 °C, Nitrogen, 4.5 h 2) Polycondensation Under vacuum, 2 h, 220-250 °C

O O O

O

O

O O

x

O (CH2)10

O O

O PDIsFs copolyesters

y

O n

Scheme 1. Synthesis route of copolyesters PDIsFs from Is, 1,10-DD and DMFD. First, the transesterification bishydroxy-products were prepared from DMFD and 1,10-DD or Is using TTIP catalyst, and then the corresponding bishydroxyalkylene-2,5-furan carboxylate monomers (BHIsF and BHDF) were subjected to the polyesterification in presence of DMFD with the molar ratio of 1:1. As a result of the bulk structure and high boiling point of isosorbide and 1,10-DD that block their removal from reactor during melt polycondensation stage even with high vacuum application, DMFD was added in the polyesterification step instead of carrying out the melt polycondensation step. This innovative trick used herein and reported for the first time by our group in our previous study,31 guarantees that all added amount of 1,10-DD and Is are totally reacted. In the last step of preparation, the polycondensation stage was carried out at 220250 °C and the reaction was left to proceed for 2 h under vacuum condition of 5.0 Pa applied slowly over a time of 30 min to minimize oligomer sublimation and avoid excessive foaming, which is a potential problem during the melt polycondensation. The weight-average molecular weight (Mw) of the purified PDIsFs copolyesters was measured by GPC and determined to be in the range of 55300-84500 g/mol (see Table 1). The number-average molecular weight (Mn) showed a significant decrease by increasing the Is content in the copolymer. The outcome can be 6 ACS Paragon Plus Environment

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


explained on the basis of the lower reactivity of secondary hydroxyl groups of isosorbide compared to primary -OH moieties of decanediol. In addition, the presence of the endo secondary hydroxyl functionality in the comonomer produces a further reactivity decrease due to the remarkable steric effects. Carboxyl end group analysis (-COOH) was carried out for the resulting PDIsFs copolyesters samples and the result was listed in Table 1. The existence of the carboxyl end-groups can be explained by the decomposition reactions such as thermal degradation that are occurring during melt polycondensation stage. As can be seen, the concentration of –COOH content decreases when Mw value increases. These findings are in good agreement with what reported in our previous work.61 Table 1. Molar composition obtained by 1H NMR, Molecular weight, as well as carboxyl end groups of the purified PDIsFs samples. Copolyesters

PDIsFs feed

PDIsFs

Samples

molar ratios

PDIsF 95/5

95/5

93/7

21600

84500

30.52

PDIsF 90/10

90/10

88/12

21500

72700

46.85

PDIsF 85/15

85/15

84.3/15.7

25400

80800

46.37

PDIsF 80/20

80/20

81/19

15900

55300

55.81

PDIsF 70/30

70/30

72/28

11500

61700

67.56

PDIsF 60/40

60/40

61/39

17200

71300

60.13

1H

Mn (g.mol-1) Mw (g.mol-1)

NMR

-COOH (eq/106g)

Structural Characterization and composition of PDIsFs The chemical structures of PIsF, PDF and the prepared PDIsFs copolyesters samples were ascertained by 1H NMR spectroscopy as elucidated in Figure 1. The obtained spectra were in good agreement with the expected structures, where all peaks are correctly assigned to the different protons in the copolyester’s chain as shown in Scheme 2. The relative peak areas of the Is proton j to that of peak a of the furan ring bonded to 1,10-DD was used to determine the molar composition of PDIsFs copolymers. The obtained results presented in Table 1, were in good agreement with the corresponding feed molar ratio. This is an indication that the composition of PDIsFs copolymers is readily controlled by varying the feed molar ratio of 1,10-DD to DMFD. 7 ACS Paragon Plus Environment


Page 8 of 39

This confirms the success of the trick used herein which states the addition of DMFD during the polyesterification stage to prevent the removal by distillation of the diols Is and 1,10-DD from the reactor during melt polycondensation procedure. In the spectrum of homopolymer PIsF, the two protons of the furan ring (a1 and a2) revealed different chemical shifts at 7.51 and 7.61 ppm, as they are nonequivalent. This arises from the non-planar Is structure and the different spatial orientations of the hydroxyl groups: the one at C-2 having the exo position with respect of the Vshaped molecule structure, while the other is in endo position.45 As a result of the exo and endo stereochemistry of Is and through the formation of the ester linkage, the two furan protons a1 and a2 become nonequivalent, and therefore appear as two different doublets (7.51-7.61 ppm). Similar behavior was recently reported for PBS copolyesters based on isosorbide,62 in which for dyads Is-Is, the ester bond is formed between succinic acid-carbonyl groups with endo and exohydroxyls of Is, thereby two different carbons signals have been revealed in the

13C

NMR

spectrum. The same finding was also reported by Noordover et al.52 for copolyesters derived from Is, succinic acid, and 1,3-propanediol, where the chemical shifts of succinic acid were detected between 2.6 and 2.8 ppm. The Is unit peaks split into four multiplets at δ 4.43, δ 5.11, δ 5.51 and δ 5.83 ppm stand for the protons g+h, j, i and e+f, respectively. In the 1H NMR spectrum of PDF 31, the two protons of the furanoate ring labeled as (a3) are the most deshielded, and they appear at 7.43 ppm due to their highest π-deprotection. The chemical shifts of outer methylene groups (b), middle CH2 (c) and inner CH2 (d) in 1,10-DD unit appear at 4.38 ppm, 1.75 ppm and 1.31 ppm, respectively. For PDIsFs copolyesters, all the proton peaks from PIsF and PDF homopolymers are retained, the chemical shifts at 1.47 ppm and 1.94 ppm are attributed to the (d) and (c) protons of the 1,10-DD moiety. The resonances appeared at the 4.13-5.81 ppm range were assigned to isosorbide protons. The peak attributed to (h) and (g) protons of Is overlapped those of (b) of 1,10-DD moiety at around 4.42-4.49 ppm (4H of 1,10-DD and 4H of Is).


Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

j O

g,h f

e

O i O g,h f

c

d

c

O C O

a1 a2

O Is.Is

g,h

e

C O O

O f

g,h

IsFIs

i O

j O

O i

b

g,h

O j

e

g,h

O C O

a4 a5

O Is.D a3

a3

b

O H2C H2C (CH2)6 H2C H2C O C O

b

c

d

c

b

C O CH2 CH2 (CH2)6 CH2 CH2 O O

b

c

d

c

b

C O CH2 CH2 (CH2)6 CH2 CH2 O O

O D.D

Scheme 2. Chemical structures of IsFIs, IsFD and DFD units in PDIsFs copolyesters.


IsFD

DFD


Figure 1. A: The whole 1H NMR spectra of PIsF, PDF and PDIsFs, and B: Magnification of protons chemical shifts of PIsF, PDIsFs and PDF. The resonances related to the furanic ring protons were divided into three different signals, this is due to the three possible environments for furan ring: the homo moieties, D.D and Is.Is, and the hetero moiety, Is.D, in which D and Is represent the 1,10-DD and isosorbide units, respectively. A new central signal, which is not spotted in the 1H NMR data of the corresponding homopolymers, was divided into a multiplet. This peak, detected at 7.43-7.48 ppm for all copolyesters, was assigned to the CH of furan fragment (a4+a5) owned by the Is.D unit, where its intensity describes the amount of the heterolinkage. The presence of this hetero-unit confirms that the two bishydroxyalkylene-2,5-furan carboxylate monomers (BHIsF and BHDF) were successfully subjected to the transesterification reaction in the synthesis of the PDIsFs copolyesters. 1H NMR spectra of the aromatic protons in furan ring (Figure 1B) were used to determine the IsFIs, DFD and IsFD sequence distribution in PDIsFs. The chemical shift of the latter is affected by its sensitivity to the different connected ester linkage. The aromatic protons of furan unit of PDIsFs copolymers were split into four signals in the chemical shift range of 7.43-7.60 ppm, which are attributed to five different types of aromatic protons (a1+a2, a3 and a4+a5). The intensity of their peaks depends on the Is/1,10-DD molar ratio in PDIsFs. The indexation of the different repeating units in copolymer chains is elucidated in Scheme 2.


Page 10 of 39

Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The degree of randomness (R) of the PDIsFs copolyesters was calculated using the following equations (1-6): R= PD.Is + PIs.D

(1)

PDIs= [(∫D.Is + ∫Is.D)/2] / [(∫D.Is + ∫Is.D)/2 + ∫D.D] = 1/LnD

(2)

PIsD= [(∫Is.D + ∫D.Is)/2] / [(∫Is.D+∫D.Is)/2 + ∫Is.Is] = 1/LnIs

(3)

∫D.D= Ia3 / (Ia1 + Ia2 + Ia3 + Ia4 + Ia5)

(4)

∫D.Is= (Ia4 + Ia5) / (Ia1 + Ia2 + Ia3 + Ia4 + Ia5)

(5)

∫Is.Is= (Ia1 + Ia2) / (Ia1 + Ia2 + Ia3 + Ia4 + Ia5)

(6)

where PD.Is and PIs.D are the probability of finding unit of hetero-linkage: a decanediol (D) unit next to an isosorbide (Is) unit and the probability of finding an Is unit next to a D unit respectively, while LnD and LnIs are the number average sequence length of the repeating units, also called block length, calculated according to Yamadera and Muand by the equations (2) and (3). Furthermore, ∫D.Is, ∫D.D and ∫Is.Is represent the triads fractions of DFD, IsFD and IsFIs and were calculated from the integral intensities of the resonance signals using the equations (4)-(5), in which the integration of shift I was tagged as Ii. The corresponding results were summarized in Table S1. According to the Bernoullian statistics, for the prepared PDIsFs having Is molar content ranging from 15 to 40%, the R values were nearly close to 1, suggesting that random copolyesters were the preferred structures. PDIsFs with low Is fraction (5 and 10 mol%) respectively revealed R values of 0.6 and 0.64, indicating that a complete randomization has not been attained. Bernoullian statistics state that for random copolyesters, R is ⩽1; while for alternating copolymer, R is equal to 2; and for block copolymers, R is close to 0.63 As can be clearly seen, R values of the PDIsFs copolymers gradually increased from 0.60 to 0.90 with increasing of Is feed content from 5% to 40%. This finding pointed out that neat polyester (PIsF) during polyesterification step is much easier to be subjected to the transesterification than the neat (PDF). The chemical structure of the copolymers was further confirmed by FT-IR spectroscopy as depicted in Figure S1. A characteristic absorption band at 1720-1730 cm-1 attributed to the ester bond (v C=O) is observed for all PDIsFs samples. Another characteristic band at 1280 cm-1 is 11 ACS Paragon Plus Environment


assigned to C-O-C stretching mode (v C-O-C) of the ester group. The appearance of two absorption bands at 2845 and 2930 cm−1 is due to the C-H symmetrical and asymmetrical stretching modes of (CH2) groups of 1,10-DD and isosorbide fragment. These groups also produce a characteristic band at 1450 cm−1 in the fingerprint region. Two weak bands were detected at 3120 and 3180 cm-1 arising from =C-H stretching vibration of furan moieties (v =CH). Additionally, the presence of a broad detectable band between 3400 and 3500 cm-1, particularly for copolyesters with high Is molar content, originated from stretching vibration of hydroxyl group (-OH) confirms the reasonable amount of hydroxyl end-groups. This hence leads to obtaining of copolymers with a low molecular weight that was proved by the obtained Mw values illustrated in Table 1. Thermal Properties of PDIsFs Copolyesters The thermal properties of the PDIsFs copolyesters were analyzed by DSC. The related thermal transition data collected in this analysis, including the melting temperature (Tm), normalized melting enthalpy (Hm), crystallization temperature (Tc), normalized crystallization enthalpy (Hc), glass transition temperature (Tg) and the cold crystallization temperature (Tcc), were gathered in Table 2. The DSC thermograms of the as received copolymers samples were presented in Figure 2. It can be concluded from the latter that the resulting copolyesters are semicrystalline materials. The DSC traces of PDF homopolyester showed a glass transition temperature close to 1°C and a melting point of 116 °C. This sample crystallizes from its melt (Tc) at 76.5 °C. It was found that with the increasing molar amount of isosorbide units from 5% to 40% in the copolyesters’ backbone, the crystallibility of these samples gradually decreased, showing a decrease of both melting point and melting enthalpy respectively from 110.9°C to 64.3°C and from 37.9 to 30.1 J/g. This is a consequence of the strong repressing effect that isosorbide bicyclic structure exerts on the destruction of the chain’s regularity, leading therefore to a poor crystallizability. In contrast, as clearly shown in the DSC traces of the quenched samples (Figure 2.c) where all obtained polymers except PDIsF 95/5 are amorphous, the glass transition temperature (Tg) follows the opposite trend of Tm and Hm discussed above, that is, increasing from -1.2 to 20.5°C as the content of the polyester in Is increases from 5 to 40 mol%. This is associated with the replacement of the highly flexible decamethylene fragment by the


Page 12 of 39

Page 13 of 39

much more rigid isosorbide structure which, by reducing chain mobility, leads to an increase in Tg. Several equations have been proposed to describe the Tg-composition dependence of miscible polymer blends. In the equation of Fox 64 (7):

w w 1  1  2 Tg Tg 1 Tg 2

(7)

w1 and w2 are the weight fractions of the comonomers and Tg1 and Tg2 the glass transition temperatures of the respective homopolymers. As can be seen in Figure 2.d, the Fox equation results in somehow higher values Tg values compared to the experimental ones.

(a)

Normalized Heat Flow (w/g) Endo Up


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PDF PDIsF 95/5 PDIsF 90/10 PDIsF 85/15 PDIsF 80/20 PDIsF 70/30 PDIsF 60/40 PIsF -20

0

20

40

60

80

(b)

100 120 140 160 180

PDF PDIsF 95/5 PDIsF 90/10 PDIsF 85/15 PDIsF 80/20 PDIsF 70/30 PDIsF 60/40 PIsF 20

Temperature (°C)

40

60

80

100

Temperature (°C)


120

140


Page 14 of 39

Glass transition temperature (oC)


1 2 3 4 5 40 (c) (d) 6 Fox 7 Experimental 30 PDF 8 9 PDIsF 95/5 10 20 PDIsF 90/10 11 12 PDIsF 85/15 13 10 14 PDIsF 80/20 15 PDIsF 70/30 0 16 PDIsF 60/40 17 PIsF 18 -10 19 -20 0 20 40 60 80 100 120 140 160 180 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 20 Weight fraction of IsF Temperature (°C) 21 22 23 Figure 2. DSC thermograms of copolyesters (PDIsFs) with different compositions: a) First 24 heating scans of the as received samples, b) cooling scans (by 10 °C.min-1), c) heating scan after 25 26 melt quenching (by 20 °C.min-1), and d) variation of the glass transition temperature with the 27 28 isosorbide content in the copolymers. 29 30 Only three quenched samples, PDIsF 90/10, PDIsF 85/15 and PDIsF 80/20, exhibited cold 31 32 crystallization during the heating scan, respectively at Tcc values of 33.6, 49.3 and 73.1°C. These 33 34 copolyesters could crystallize upon heating thanks to their fast crystallization rate. Furthermore, 35 a noticeable exothermic crystallization peak (Tc) at 57.4°C, in addition to another extremely 36 37 weak broad Tc at 39°C were spotted during the first DSC cooling scan (Figure 2.b) respectively 38 39 for PDIsF 95/5 and PDIsF 90/10, indicating thus that these samples were easily able to 40 crystallize upon the cooling process from the melt. The first DSC heating scans for PDIsFs 41 42 copolymers samples moreover revealed a further endothermic peak at temperatures above the Tg. 43 44 This behaviour could be ascribed to the melting of less perfect crystals of the PDF phase 36,49,65. 45 46 Such phenomenon were also studied in detail by Zhao in the semi crystalline poly(ethylene 47 terephthalate) 66 and by Kwiatkowska in biobased furanoate copolyesters 35. 48 49 50 Table 2. Thermal properties of the as received copolyesters PDIsFs with different compositions. 51 52 Xc** PDIsFs Td,5% Td,10% Td,max R 500°C Tg Tcc Tc* ∆Hc* ∆Hm* Tm* OIT (s) 53 (%) samples (%) (°C) (°C) (°C) (J/g) (J/g) (°C) 54 55 49.3 PDF 406.1 416.1 436.6 8.4 1 76.5 42.1 64 116 56 57 58 14 59 ACS Paragon Plus Environment 60

Page 15 of 39


1 2 3 29.2 408.7 418.3 439.2 8.5 -1.2 57.4 39.3 37.9 110.9 5765±375 4PDIsF 95/5 5 28.4 440.1 6.6 -0.1 33.6 39 9.8 36.9 108.2 4468±985 6PDIsF 90/10 411.7 420.5 7 25.9 PDIsF 85/15 412.8 420.7 440.1 8.1 3.7 49.3 33.6 105.5 6603±1560 8 9PDIsF 80/20 406.3 418.7 26.7 442.6 6.8 6.3 73.1 34.6 99.5 10887±1856 10 11 25.2 PDIsF 70/30 404.9 417.9 439.2 9.71 9.7 32.7 91.5 69240 12 23.2 PDIsF 60/40 408.6 420.6 443.8 12.1 20.6 30.1 64.3 83700 13 14 * These values refer to first calorimetric scan. Controlled cooling rate scan is 10 °C.min-1 15 ** These values were calculated by dividing the ΔΗ values by the ΔΗ o = 129.8 J/g32 m m 16 17 To recapitulate, it is very worthy to note that the thermal behaviour of the materials developed 18 19 herein strongly depends on their isosorbide molar ratio. The aforementioned results proved that 20 21 Is units content can be used as an effective control parameter to tune the properties of end 22 23 products by affecting their crystallization behavior and structural properties, preparing thereby 24 new sustainable PDIsFs copolyesters with enhanced and tailored thermal properties. 25 26 27 Thermal degradation of the copolymers 28 29 As the thermal degradation behavior and stability of polymers have a great importance for 30 determining their potential application, the thermogravimetric analysis (TGA) was performed 31 32 under nitrogen atmosphere to assess the thermal stability of the copolyesters, where their TGA 33 34 and DTG (differential TG) traces are depicted in Figure 3. Table 2 lists the most representative 35 36 decomposition parameters, determined from TGA curves, such as the maximum decomposition 37 temperature (Td, max), the degradation temperatures at 5% and 10% weight loss (Td,5%, Td,10%) and 38 39 the residue at 500°C (R500°C). All copolyesters samples revealed very high thermal stability up to 40 41 405°C with Td,5% oscillating from 405 to 413°C and they underwent a single stage degradation, 42 43 and just 6.6-12.1% of their residual weight remains at 500°C. PDIsFs synthesized herein 44 exhibited slightly enhanced Td,5% values to those of the neat PDF. It is obvious that this finding 45 46 could be mainly related to the incorporation of the Is unit into the copolymers’ backbone. This 47 48 allows concluding that the presence of Is fragment in PDIsFs samples is responsible for the 49 delaying the onset of thermal degradation. To sum up, the excellent thermal stability of the 50 51 prepared copolyesters PDIsFs ensures the safe processing of the resulting materials at 52 53 temperatures higher than their melting points, hence making them appropriate candidates for 54 55 injection moulding applications. The benefit of Is insertion into the backbone’s polymers in 56 57 58 15 59 ACS Paragon Plus Environment 60


enhancing their thermal properties was confirmed in several studies in the literature.67-69 This is in full accordance with was reported in the present work.

100 80

PDF PDIsF 95/5 PDIsF 90/10 PDIsF 85/15 PDIsF 80/20 PDIsF 70/30 PDIsF 60/40

60 PDF PDIsF 95/5 PDIsF 90/10 PDIsF 85/15 PDIsF 80/20 PDIsF 70/30 PDIsF 60/40

40 20 0 250

DTG

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

a)

b) 300

350

400

450

500

550

600

420

o

Temperature ( C)

490

Temperature (°C)

Figure 3. TGA thermograms of the pure copolyesters (PDIsFs), a) remaining mass and b) first derivative of mass loss. Wide angle X-Ray diffraction patterns (WAXD) of PDIsFs Copolymers To assess the crystalline structure of the resulting materials, wide-angle X-ray diffraction analysis (WAXD) was conducted. The WAXD patterns of the PDIsFs copolymers under investigation are shown in Figure 4. As reported in our previous studies, the semicrystalline PDF homopolyester shows four main diffraction peaks at 11.2°, 16.7°, 21.5°, and 23.8° 56 while PIsF homopolymer displayed the amorphous halo in its diffractogram

22.

This is due to the exo and

endo position of the two hydroxyl groups of Is that hamper the spacial molecule arrangement. The PDF WAXD pattern recorded in this work is the same with those shown in previous papers.56,70 The PDIsFs copolymers show similar patterns to that of neat PDF, as can be seen in Figure 4, suggesting that PDIsFs and PDF adopt similar crystal unit cells. As can be seen from WAXD patterns, all PDIsFs samples revealed strikingly similar peak positions and shapes to those of the parent PDF homopolyester, indicating that in their microstructure only PDF crystals are present.


Page 17 of 39

PDF PDIsF 95/5 PDIsF 90/10 PDIsF 85/15

PDIsF 80/20 PDIsF 70/30 PDIsF 60/40 PIsF

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

2theta (degree) Figure 4. WAXD patterns of the PDIsFs copolyesters

Melt rheology The effect of the shear angular frequency on the copolyester complex moduli at a melt temperature of 120 °C are shown in Figure 5. For all compositions, the polymer melt behavior was mainly viscous (G’

Synthesis, Characterization, and Biodegradability of Novel Fully

Recommend Documents