Synthesis of copolyesters with bio-based lauric diacid: structure and physico-mechanical studies

Lauric diacid (LCDA), also known as 1,12-dodecanedioic acid, is used to develop a series of copolyesters along with 1,4-cyclohexanedicarboxylic acid (CHDA) and 1,4-butanediol (BDO). The resulting poly(butylene lauric dicarboxylate-co -butylene 1,4-cyclohexanedicarboxylate) (PBLC) is proved to be a random copolyester with three triad sequences. When the LCDA content increases from 20 to 60 mol%, T m of the copolyester decreases from 133 to 57 (cid:1) C. At the same time, the tensile modulus and strength decrease from 94 and 14 MPa to 40 and 5 MPa, respectively. Moreover, the elongation at break also drops from 640 to only 50%. However, further increasing the LCDA content to 80 mol%, the copolyester becomes amorphous with no T m , and its tensile modulus, strength, and the elongation at break all improve signi ﬁ cantly to 68 and 7 MPa, and over 1400%, respectively. More importantly, for the homo-polymer poly(butylene lauric dicarboxylate) (PBL), it has a relatively high T m of 73 (cid:1) C compared to that of polycaprolactone (PCL), but lower tensile modulus and strength, and signi ﬁ cantly higher ductility, compared to those of PCL, linear low density polyethylene, and polybutylene succinate.


Introduction
Polyesters with linear aliphatic chains have attracted more and more attention from people over the last decades, because they can meet the growing demand in the development of biodegradable and renewable alternatives to non-biodegradable and fossil-based polyesters such as poly(ethylene terephthalate) (PET), and poly(butylene terephthalate) (PBT).Among them, poly(lactic acid) (PLA), 1 poly(butylene succinate) (PBS), 2 and poly(hydroxyalkanoates) (PHAs) 3 have been studied extensively.They have been successfully commercialized and used in many elds such as food packaging, medical devices, 3D printing, and agriculture.These polyesters with linear aliphatic chains typically have ten carbons or less in the backbone, and therefore have low melting points but good biodegradability. 2On the other hand, other types of polyester with linear aliphatic chains usually have 12 carbons or more.They have structures and properties similar to those of polyethylene because of the low density of the ester functional group. 4For example, they have higher melting temperatures and hydrophobicities, and better crystallization abilities, which results in decreased biodegradabilities. 4,5ikewise, monomers used for the synthesis of long-chain aliphatic polyesters are mostly derived from renewable resources such as fatty acids, via biotransformation or chemical synthetic routes. 4Monomers with carbon numbers up to 194 have been developed. 6And polyesters with up to 44 -CH 2units between two ester groups have been reported. 7[10][11][12][13][14] Of particular interest, long-chain dicarboxylic acids with 12 to 20 carbons have been produced in large scale and are commercially available. 4Polyesters and polyamides have been synthesized from these monomers. 4Interestingly, the longchain dicarboxylic acids with 14 to 20 carbons are building blocks of naturally occurring polyesters such as suberin in cork. 15However, the one with 12 carbons, namely lauric diacid (LCDA), also known as 1,12-dodecanedioic acid, does not exist in nature.It is therefore a brand new compound created by human beings.Both chemical [16][17][18] and biological 19,20 methods have been developed for the preparation of LCDA.7][18] On the other hand, the biological way involves fermentation process using wild type or engineered yeast strains, such as Candida sorbophila. 19Fatty acid or alkane can be used as substrates for this process. 19,20Nowadays, LCDA has become one of the most widely used monomers for the synthesis of long-chain aliphatic polyesters [21][22][23][24][25][26][27] and polyamides. 28,29It is worth noting that the commercially available LCDA is mainly produced via biotransformation from alkane (i.e.dodecane) by companies such as Cognis in US, and Cathay Industrial Biothech in China. 4olyesters synthesized from LCDA and different diols with an even number of methylene units have melting temperatures ranging from 67 to 90 C. [30][31][32][33] Among them, polyester derived from LCDA and 1,4-butanediol (BDO), namely poly(butylene lauric dicarboxylate) (PBL), has melting point of 74 C, 24 which is higher than that of poly(butylene adipate) (PBA) (58 C) and poly(caprolactone) (PCL) (60 C), 4 but lower than that of PBS (113 C). 2 Moreover, Celli et al. found that PBL has high thermal stability and good crystallization ability, similar to that of polyethylene, 26 but with fairly good biodegradability. 24Modication on PBL has been performed via introduction of aromatic 24 as well as aliphatic ring structure. 34It was found that incorporation of 70 mol% aromatic ring dramatically hinders the biodegradability of the copolyester, 24 while the same amount of aliphatic ring greatly increases the degradation rate of the copolyester. 34Since the later one, namely poly(butylene lauric dicarboxylate-co-butylene 1,4-cyclohexanedicarboxylate) (PBLC), retains similar biodegradation ability compared to PBL, but has better and tunable properties in a relatively wide range, it is of great importance to reveal the structure-property relationship in this copolyester with different compositions.Therefore, in the current work, we will focus on the sequence distribution and mechanical properties of PBLC, which have not been investigated before.Comparison between PBLC copolyesters and other polyesters with shorter chain as well as polyethylene will be made, in order to provide valuable information to nd potential application for the copolyesters.

Synthesis of PBLC copolyesters
PBLC copolyesters were prepared via a two-stage melt polycondensation process involving esterication reaction at 230 C under N 2 , followed by transesterication reaction at 260 C under vacuum (Scheme 1).A typical process for the synthesis of PBLC (with 20 mol% of PBL) is described as following.trans-CHDA (13.76 g, 0.08 mol, 4 eq.),LCDA (4.60 g, 0.02 mol, 1 eq.), and BDO (14.40 g, 0.16 mol, 8 eq.) were charged to a 250 mL round bottom ask equipped with an overhead mechanical stirrer and distillation device.Ti cat.(1.84 mg, 0.1 wt% of the amount of total diacid) was added to the mixture.The ask was then sealed and applied vacuum ($10 Pa), followed by purging with dry N 2 .This cycle was repeated for three times to remove air and moisture.Subsequently, the ask was heated to 230 C under N 2 for 3-5 h to nish the esterication reaction.The system was then evacuated to about 1 kPa and simultaneously increased the reaction temperature to 260 C, and kept for about half hour to start the second stage, namely the trans-esterication reaction.It was then placed under vacuum below 20 Pa for about 6 h to remove the extra amount of BDO in order to get high molecular weight products.
Four PBLC copolyesters were synthesized and denoted as PBLC-x, where x stands for the molar ratio of PBL, which is in the range of 20 to 80 mol%.For example, PBLC-20 means that the polymer contains 20 mol% of PBL.Besides, PBL homopolymer was also synthesized for comparison.All resulted polymers were used without further treatment.

Characterization
Gel permeation chromatography (GPC).Number average molecular weights (M n ) and molecular weight distributions (Đ M ) of PBL, PBC, and PBLC copolyesters were measured on PL-GPC220 gel permeation chromatography (GPC) equipped with a PLgel 5 mm MIXED-D column with a dimension of 300 Â 7.5 mm and a refractive index detector.HPLC grade chloroform was used as elution solvent at 40 C with a ow rate of 1.0 mL min À1 , and molecular weight was calibrated with polystyrene standard (3070-258 000 g mol À1 ).The concentration of copolyesters was about 6.7 mg mL À1 .
Nuclear magnetic resonance (NMR).The composition and the amount of trans-CHDA in the nal molecular chains of PBL, PBC, and PBLC copolyesters were determined by 1 H and 13 C NMR in CDCl 3 solvent using a Bruker AVIII400 NMR spectrometer at room temperature.
X-ray diffraction (XRD).XRD patterns of PBL, PBC, and PBLC copolyesters were recorded with a Bruker AXS D8 Advance with an X-ray wavelength of 0.1541 nm covering a 2q range from 5 to 50 within 9 min.
Differential scanning calorimetry (DSC).Thermal properties of PBL, PBC, and PBLC copolyesters were characterized using differential scanning calorimeter (METTLER-TOLEDO DSC I).Temperature calibration was carried out using an indium standard.Measurements were performed under a nitrogen atmosphere at a ow rate of 50 mL min À1 .About 7 mg of sample was placed in an alumina sample pan and the measurement was carried out according to the following process: the sample was heated up to 200 C at 10 C min À1 and held at this temperature for 2 min to erase the heat history.It was then cooled down to À30 C at 10 C min À1 .Subsequently, a second heating scan was performed at 10 C min À1 to 200 C.The melting point (T m ) was obtained from the second heating scan, the crystallization temperature (T c ) was obtained from the cooling scan.The degree of crystallinity was calculated according to the following equation: where c c is the degree of crystallinity, f is the weight fraction, DH m is the experimental melting heat of fusion, and DH q m is the heat of fusion of 100% crystalline (170 J g À1 for PBL and 141 J g À1 for trans-PBC calculated according to the group contribution theory). 35ermal stability.The thermal stability of PBL, PBC, and PBLC copolyesters were measured using a Mettler-Toledo TGA/ DSC thermogravimetric analysis (TGA).6-10 mg sample was placed in a ceramic furnace and the TGA curve was recorded in a temperature range from 50 to 800 C with a heating rate of 10 C min À1 under dry N 2 atmosphere with a ow rate of 50 mL min À1 .The temperature at which the weight loss of 5% (T 5% ) was taken as an index to evaluate the thermal stability of the copolyesters.
Tensile property.Tensile testing was performed in an Instron-5567 tensile testing machine with a 500 N load cell.The stretching rate was 100 mm min À1 and the test temperature was 25 C. Dumb-bell-shaped sample bars with dimensions of 35.0 mm (length), 2.0 mm (neck width) and 0.5 mm (thickness) were prepared by press-molding at temperature 20 C higher than T m or T f of the sample and subsequently cooled down to temperature below its T c or simply to room temperature, without releasing the pressure.All data were obtained by averaging the data from ve parallel measurements.

Results and discussions
3.1 Molecular characterization PBL, PBC, and four PBLC copolyesters with different compositions were successfully obtained.The number average molecular weight (M n ) and molecular weight distribution (Đ M ) were determined by GPC analysis.Fig. 1 shows the GPC curves of PBL, PBC, and PBLC copolyesters.Only a single peak was observed centered at elution time of about 14 min, indicating that there is no oligomer in the product.All samples have high M n around 30 000 g mol À1 and Đ M ranging from 1.9 to 2.5.
The molecular structure was analyzed and conrmed by 1 H NMR spectra.As shown in Fig. 2, all peaks with different chemical shis are carefully attributed to correspondent protons in PBL, PBC, and a typical copolyester PBLC-20. 34,36The Fig. 1 GPC profiles of PBL, PBC, and PBLC copolyesters.compositions of the PBLC copolyesters were determined according to the integration (I e , I g-trans , and I f-cis ) of three peaks located at d ¼ 1.294 ppm (e), 1.466 ppm (g-trans), and 2.478 ppm (f-cis) (eqn (1)).The molar ratio of trans-CHDA isomer in the nal products was calculated according to eqn (2), due to the fact that the protons g and f have different chemical shis for different CHDA isomers.The correspondent results are listed in Table 1.All samples have PBL molar percentage close to their feed ones.Moreover, the trans-CHDA amount of all samples are controlled at around 90 mol%, indicating small amount of isomerization.
trans-CHDA mol% ¼ I g-trans Although the compositions of PBLC copolyesters can be determined easily from 1 H NMR, the sequence distribution, number-average length of units, and randomness of the copolyesters were unable to obtain according to the 1 H NMR analysis.Previous studies on a copolyester containing 2,5furandicarboxylic acid have suggested that the protons of -OCH 2unit in different triads have different chemical shis resulting from the conjugation effect because of aromatic furan ring. 37When the aliphatic CHDA is used instead, there is no conjugation effect.Thus the protons of -OCH 2unit in different sequences have the same chemical shi around 4.10 ppm as show in Fig. 2. Similar observation has been reported in the literature before for PBLC as well as poly(butylene adipate-cobutylene 1,4-cyclohexanedicarboxylate) (PBAC). 34,36However, in the case of PBAC, it was possible to determine the sequence distribution, number-average length of units, as well as randomness from 13 C NMR analysis in a sample with 50 mol% of PBC. 36Therefore, we performed detailed analysis on the 13 C NMR of PBLC copolyesters in order to nd out the sequence distribution of these copolyesters, which has failed to be revealed in a previous study. 34e performed 13 C NMR for the four PBLC copolyesters and found that for both PBLC-40 and PBLC-60, there are distinguishable peaks for the chemical shis in the range of 63.50-63.90ppm, which could be assigned to different triads sequences in PBLC copolyesters (Fig. 3).The signal in this range is due to the carbons in the butylene unit connected to the ester bond. 38From Fig. 3, it can be seen clearly that the three different triads sequences present in the PBLC copolyesters (LBL, LBC, and CBC, see Fig. 3) would result in four different peaks, namely a1, a2, a3, and a4.Their chemical shis are located at 63.642, 63.669, 63.765, and 63.790 ppm, respectively.The assignment of the peaks to different carbons was based on the fact that the ester bond connected to the aliphatic ring structure has stronger inductive effect than the one connecting to the long aliphatic chain.Therefore, carbon a4 has the highest chemical shi at 63.790 ppm, while carbon a1 has the lowest one at 63.642 ppm. 36,38By performing a peak tting on the original curve, four independent peaks can be resolved as displayed in Fig. 4. As a result, the number-average length of BL and BC units (L BL and L BC ), and randomness (R) can be calculated from the integration of the four resolved peaks according to eqn (3)-( 5), where I a1 , I a2 , I a3 , and I a4 are integration of the four resolved peaks as showed in Fig. 4. The calculated results are summarized in Table 1.Both PBLC-40 and PBLC-60 have randomness of 1.04, which is very close to 1, indicating that both of them are random copolyesters.However, since they have different compositions, their unit length are different.PBLC-40 has shorter BL unit length but longer BC unit length than those of PBLC-60.This result is reasonable since PBLC-40 has less amount of BL unit and more amount of BC unit in its molecular chain, compared with that of PBLC-60.For the rest two PBLC copolyesters, namely PBLC-20 and PBLC-80, we were unable to nd four distinguishable peaks in the same region.Probably because the signal from the dominate component makes that of the minority one become negligible.Therefore, no further information can be found on the sequence distribution for these two samples.Nevertheless, the results from PBLC-40 and PBLC-60 copolyesters can be used to conclude that PBLC samples are random copolyesters. 34

Thermal properties and crystal structure
The thermal properties including melting temperature (T m ), crystallization temperature (T c ), and thermal stability of PBL, PBC, and PBLC copolyesters were investigated by using DSC and TGA analysis.
Fig. 5 shows the DSC curves of these samples in the second heating, and cooling scans.Table 2 collects the relevant thermal data.In the rst place, from Fig. 5 we can see that both PBL and PBC homo-polymers are able to crystallize, and show strong and sharp peaks in the second heating and cooling scans.PBL has T m of 73 C, T c of 54 C, melting enthalpy (DH m ) of 7 J g À1 and degree of crystallinity (c c ) of 4.0%.All these values are lower than those of PBC.Especially, the low c c of PBL indicates that it has weaker crystallization ability compared to that of PBC.Comparing with other linear aliphatic polyesters derived from BDO and dicarboxylic acids with different carbon numbers, PBL has higher T m than those with shorter chain length in the dicarboxylic acids, except for succinic acid (PBS, T m 113 C) and ethanedioic acid (PBE, T m 105 C), and lower T m than those with longer chain length in the dicarboxylic acids. 4In the second place, for the rest four PBLC copolyesters, both T m and T c decrease steadily with the increasing of BL content.In addition, both DH m and c c decrease to fairly small value, indicating poor crystallization ability.As we can see from Fig. 5   a Melting temperature T m , melting enthalpy DH m and degree of crystallinity c c are determined from second heating scan of DSC curves and the crystallization temperature T c is determined from the cooling scans of DSC curves, the temperature at 5% weight loss T 5% is determined by TGA in N 2 atmosphere.peaks become broad and weak.When BL content is 80 mol%, namely PBLC-80, the copolyester shows neither melting nor crystallization peak in the DSC curves.Therefore, from the DSC analysis, PBLC-80 is an amorphous copolyester.For PBLC-60 on the other hand, it shows a very small peak in the second heating scan but no peak in the cooling scan, indicating fairly poor crystallization ability, compared with other copolyesters and the homo-polymers.Fig. 6 shows the TGA curves of PBL and PBLC copolyesters, the related data are collected in Table 2. PBL has slightly lower T 5% but higher T d,max than those of PBC.Moreover, for the four PBLC copolyesters, both T 5% and T d,max increase gradually with the increasing of BL content, indicating increased thermal stability.Nonetheless, all samples have high thermal resistance with T 5% and T d,max over 370 and 405 C, respectively.As a result, these materials can be processed safely without thermal degradation at temperatures 30-50 C higher than their corresponding T m .

that melting
The crystalline structure of PBL, PBC, and PBLC copolyesters was studied by using X-ray diffraction (XRD).The XRD patterns displayed in Fig. 7 clearly indicate that PBL and PBC homopolymers have totally different crystalline structure.The crystal lattice of PBC is of triclinic type, 36 with diffraction peaks at 2q ¼ 15.16, 16.34, 18.24, 19.64, 20.59, and 22.53 .On the other hand, PBL is of the orthorhombic type similar to that of polyethylene, 4,26 with diffraction peaks at 2q ¼ 21.39, and 23.72 .For the other four PBLC copolyesters, their crystalline structure can be divided into two groups.For PBLC-20, 40, and 60, they are the triclinic type similar to that of PBC.For PBLC-80, it is the orthorhombic type similar to that of PBL. 39or the rst group, with increasing amount of BL unit, the intensity of the diffraction peaks becomes weak, and the peak at 2q ¼ 19.64 disappears.This observation is similar to what has been reported in a PBAC copolymer series. 36Therefore, this result suggests that the crystalline phase in this group of PBLC copolyesters is the PBC crystalline phase. 39Incorporation of BL unit hinders the crystallization of BC unit, and only small amount of BC unit is able to organize into crystalline phase.This explains the fact that this group of PBLC copolyesters have very low heat of fusion and degree of crystallinity, as showed in Table 2.Moreover, the BL unit is in the amorphous region.
Based on these results, it is reasonable to imply that the decreasing of T m from 133 to 57 C for the rst group of PBLC copolyesters is due to the depression effect of the noncrystalline BL unit on the BC unit.This depression effect can be quantitatively described by the Flory equation. 40The Flory equation deals with the relationship between the T m of the PBLC copolyester (T m,PBLC ) and the equilibrium T m of the PBC homo-polymer À T m;PBC Á , as showed in eqn ( 6): where R is the universal gas constant, p is the molar fraction of the crystallizable BC unit, and DH u is the enthalpy of fusion per crystallizable repeating BC unit.According to our previous nding, the crystallizable BC unit is formed merely from the trans-CHDA isomer, therefore, p is the molar fraction of trans-BC unit. 36By using the Flory equation, we nd a perfect linear relationship between ln p and 1/T m,PBLC , for the rst group of PBLC copolyesters, as showed in Fig. 8.The correlation coefficient (R 2 ) for the linear tting is 0.999.Therefore, it can be concluded that when the BL content is lower than 60 mol%, the random PBLC copolyesters have decreased T m due to the depression effect of the non-crystalline BL unit on the crystallizable trans-BC unit.

Mechanical properties
The mechanical properties of PBL and PBLC copolyesters were characterized by tensile testing.The representative tensile strain-stress curves are showed in Fig. 9, and the mechanical properties data are listed in Table 3.In the rst place, for the two homo-polymers, PBL has very similar tensile modulus, but higher tensile strength and elongation at break compared to Fig. 6 TGA curves for PBC, PBL, and PBLC copolyesters.those of PBC. 41However, replacing BDO with 1,5-pentanediol, the resulted polyester PPeDo has tensile modulus of 344 MPa, which is signicantly higher than that of PBL. 42Moreover, compared with some other polymers such as linear low density polyethylene (LLDPE), PBS, and PCL, PBL also has smaller modulus but higher elongation at break, 42,43 probably because PBL has relatively low degree of crystallinity than those of the above mentioned polymers. 42In the second place, as showed in Fig. 10 and 11, when 20 mol% of a second unit is introduced to the homo-polymer, both PBLC-20 and PBLC-80 have largely decreased tensile modulus and strength, but slightly increased elongation at break, compared with those of PBC and PBL, respectively.This phenomenon is due to the fact that the introduction of the second units (i.e.BL or BC) breaks the crystalline of the homo-polymer.If we consider the two homopolymers as rigid and tough materials, these two copolyesters can be considered as so but tough materials.However, further increasing the amount of the second unit to 40 mol%, both PBLC-40 and PBLC-60 have further decreased tensile modulus and strength, and signicantly decreased elongation at break.Moreover, both copolyesters break before they yield, therefore, no yield strength or strain can be observed.As a result, these two copolyesters are so and brittle.In another word, increasing the amount of one unit from 0 to 50 mol%, namely decreasing the content of the other one from 100 to 50 mol%, the PBLC copolyester experiences a ductile-to-brittle transition.This transition is very different from other copolyesters series, in which the elongation at break of the copolyester with composition near 50/50 is usually the highest. 43,44A possible explanation to this phenomenon is due to the low M n and possible high entanglement molecular weight for PBLC-40 and PBLC-60, in which the molecular chain can not form effective entanglements that are necessary for good ductility. 45

Conclusion
In conclusion, we have developed a series of copolyesters from a bio-based long chain dicarboxylic acid and an aliphatic ring structure.The copolyesters with different compositions have very different structure and properties compared with their parenting homo-polymers.It was found that the copolyester is a random copolymer with three triad sequences according to the 13 C NMR analysis.The copolyester with increasing amount of BL unit to 60 mol% tends to have decreased T m as well as T c , compared with those of the PBC homo-polymer.This is because the BL unit would form defect in the PBC crystalline region as proved by applying the Flory equation.Moreover, these copolyesters have the same crystalline lattice type to PBC.However, further increasing the BL unit to 80 mol% in the copolyester, it becomes amorphous as showed in DSC scans, and its XRD pattern is similar to that of PBL.For the rst time the mechanical properties of the copolyesters are revealed.It was found that when the composition is getting close to 50/50, the tensile modulus and strength decrease gradually, but surprisingly, the elongation at break also drops sharply, compared with the two homo-polymers.In addition, for the homo-polymer PBL, although the tensile modulus is relatively low, it has better ductility compared to polymers like LLDPE, PBS, and PCL.

Fig. 10 Fig. 11
Fig. 10 Trends of tensile modulus and strength with different LCDA content.

Fig. 8
Fig. 8 Plot of Flory equation (black squares) and linear fitting (red line) for the first group of PBLC copolyesters.

Table 1
Molecular structure of PBL, PBC, and PBLC copolyesters

Table 2
Thermal properties of PBL, PBC, and PBLC copolyesters a

Table 3
Mechanical properties of PBL and PBLC copolyesters a