Effect of the biobased linear long-chain monomer on crystallization and biodegradation behaviors of poly(butylene carbonate)-based copolycarbonates

Abstract

To improve the crystallization ability of poly(butylene carbonate) (PBC), a monomer with a linear long chain as a biobased derivative of castor oil was randomly introduced into the PBC main chain. A series of aliphatic copolycarbonates poly(butylene-co-decamethylene carbonate)s (PBDCs), with weight-average molecular weights of 125 000 to 202 000 g mol⁻¹, were synthesized from dimethyl carbonate, 1,4-butanediol, and 1,10-decanediol via a two-step polycondensation process, using sodium acetylacetonate as the catalyst. The PBDCs, being statistically random copolymers, showed a single T_g over the entire composition range. The DSC results testified that the introduction of a decamethylene carbonate (DC) unit can significantly enhance the crystallization rate of PBC. The PBDC copolycarbonates had a minimum melting point in the plot of melting point versus composition. Wide-angle X-ray diffraction patterns showed that the copolycarbonates with up to 20 mol% DC units formed PBC type crystals, while those with higher DC unit content crystallized in poly(decamethylene carbonate) (PDC) type crystals. This indicates that the PBDC copolycarbonates show isodimorphic cocrystallization. The thermal stability, crystalline morphology, and enzymatic degradation of the PBDC copolycarbonates were also studied.

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Introduction

Aliphatic polycarbonates (APCs) are a greatly important class of biodegradable polymers for their potential as biocompatible, biodegradable, and nontoxic materials.^1–4 They have received remarkable interest due to the absence of acidic degradation products compared with conventional biodegradable polyesters such as poly(lactic acid).^5–8 High-molecular-weight (weight-average molecular weight (M_w) ≥ 166 000 g mol⁻¹) APCs in which the carbonate linkages are connected by more than three carbon atoms can be prepared by condensation polymerization of dimethyl carbonate (DMC) and aliphatic diols.^9–12

Among the resulting APCs, poly(butylene carbonate) (PBC) has attracted the most interest because of its favorable comprehensive properties and competitive cost. It can be economically prepared by the condensation polymerization of DMC and 1,4-butanediol (BD) which are both inexpensive commercial chemicals on a large scale. Especially, DMC is an eco-sustainable reagent which can be produced by a green process using carbon dioxide as a building block.^13,14 PBC with high number-average molecular weight up to 100 000 has been successfully obtained very recently.^9,10 PBC has been chosen as a toughening blend partner for poly(lactic acid) and poly(butylene succinate) due to its high flexibility.^15,16 Moreover, PBC can also be used as the flexible segment to toughen PBS according to block copolymerization.¹⁷ The obtained fully biodegradable homopolymer, blends, and copolymers based on PBC show potential application fields such as flexible packaging, agricultural film, and compost bags. However, PBC is a relatively slowly crystallizing polymer which can be crystallized adequately after annealing for more than 1.5 h at 30 °C, resulting in a low processing efficiency.¹⁸

Random copolymerization is a convenient method to modulate the properties of homopolymer through varying the composition of copolymer.²⁰ In most of copolymers where both components are crystallizable, the degree of crystallinity decreases drastically as the minor component increases because of the incompatibility in crystal lattices of the two components. In contrast, if the two comonomers are compatible in each crystal lattice, the cocrystallization of these comonomer units may be expected.²¹ Polyesters with linear long-chain aliphatic repeat units are receiving increasing interest since the long-chain segments endow high crystallinity, enhanced melting and crystallization points comparable to those of linear polyethylene.^22–26 Apart from the excellent property, the monomer with long methylene sequence can be obtained from animal or vegetable species and to be used as renewable materials.²⁴ Among them, poly(ω-pentadecalactone) is a highly crystalline polyester (T_m = 97 °C, T_c = 78 °C), due to its long methylene sequence.²⁷ It has been used to enhance the thermal properties of PBC, poly(trimethylene carbonate), and poly(ε-caprolactone) via random copolymerization.^28–31 However, The M_w values of the obtained poly(ω-pentadecalactone-co-butylene carbonate) were lower than 23 000 Da which is not sufficiently high to meet mechanical performance requirements for polymers.²⁸

Poly(decamethylene carbonate) (PDC) with long methylene sequences is a relatively new biodegradable polymer possessing a relatively fast crystallization rate.^18,19 Moreover, 1,10-decanediol (DD) is a biobased aliphatic diol which has been reported as a derivative of castor oil.³² The aim of this study is to synthesize high-molecular-weight poly(butylene-co-decamethylene carbonate) copolycarbonates with satisfactory crystallization rate and processing efficiency. A full series of PBDCs were synthesized for the first time, via transesterification and polycondensation route using DMC, DD, and BD. Apart from the synthesis and characterization of high-molecular-weight PBDCs, the cocrystallization behavior, crystalline morphology, thermal and enzymatic degradation analyses were also presented.

Experimental section

Materials

DMC was purchased from Shandong Shida Shenghua Chemical Co. (China). BD was purchased from Beijing Chemical Reagents Corp. (China). DD, lithium acetylacetonate (LiAcac), sodium acetylacetonate (NaAcac), and potassium acetylacetonate (KAcac) were purchased from Beijing InnoChem Science & Technology Co. (China). The lipase from Pseudomonas sp. was purchased from Sigma-Aldrich (USA). All the chemicals were used as received.

Synthesis of PBDCs

The copolycarbonates were prepared by a two-step melt polycondensation method, transesterification and polycondensation, as shown in Scheme 1. In this work, alkali metal acetylacetonates were selected as a new catalytic system for copolycarbonates synthesis. The copolymers are named as PBDC_ϕDC where ϕDC mol% is the molar percentage of DD monomer in feed, i.e., ϕDC mol% = DD/(DD + BD) × 100%. A representative procedure for PBDC50 is described below.

Scheme 1 Synthetic route to PBDC via two-step melt polycondensation.

In the first step of transesterification, BD (18 g, 0.2 mol), DD (34.8 g, 0.2 mol), DMC (72 g, 0.8 mol), and sodium acetylacetonate (0.02 wt% of the final polymer) were added into a 250 mL four-necked round-bottom flask equipped with a mechanical stirrer, nitrogen inlet, fractionating column, thermometer, and feeding funnel under nitrogen atmosphere. The reaction mixture was heated to 98 °C and stirred continually for about 2 h. The temperature was then increased to 180 °C slowly over a period of ca. 2 h and maintained for 1 h to ensure unreacted DMC and methanol formed as transesterification byproducts removed completely. The temperature of the distillation head was kept between 60 and 66 °C to prevent too much volatilization of DMC.

In the second step of polycondensation, the fractionating column was replaced by a short path distillation, and a vacuum system was connected to the reflux condenser through a cold trap immerged in liquid nitrogen. A high vacuum (ca. 10 Pa) was applied slowly over a period of ca. 20 min to avoid excessive foaming and to minimize oligomer sublimation. The polycondensation reaction was carried out for 6–8 hours for all prepared samples. After completion, the copolymers were cooled to room temperature naturally and were used without any purification. PBC and PDC homopolymer were synthesized using the same procedure.

Characterization

The intrinsic viscosity of all the samples in the mixture of phenol–1,1,2,2-tetrachloroethane (1/1, w/w) was measured using Ubbelohde viscometer at 25 °C. ¹H NMR spectroscopy obtained with a Bruker spectrometer operating at a frequently of 400 MHz was used for determining the composition and microstructure of the copolymers. Chloroform (CDCl₃) was used as solvent and the concentration of each sample solution was of about 5% w/v. Molecular weight determinations were estimated by GPC performing on a Waters 1515 HPLC pump and a refractive index detector (Waters 2414) at 35 °C. Chloroform was used as the solvent. Polystyrene standards were used for calibration.

Thermal analysis

The thermal property of the samples was record with a Pyris Diamond DSC apparatus (Perkin Elmer) equipped with a CryoFill liquid nitrogen cooling system under a nitrogen atmosphere to minimize the oxidative degradation. Relatively small sample sizes (5 ± 0.3 mg) were used to minimize the effect of thermal conductivity of polymers. The samples were isothermally crystallized from the melt at about the same supercooling (ΔT = T_m – T_c ≈ 40 °C) firstly. Then the samples were heated to 100 °C at a rate of 10 °C min⁻¹ to measure the melting temperature and enthalpy. Then the samples were cooled to −80 °C with the highest achievable rate in order to obtain amorphous materials and reheated to 100 °C at a rate of 10 °C min⁻¹. The glass transition temperature was obtained during the second heating step. For isothermal crystallization the same melting procedure as mentioned above was followed, and then the samples were cooled to the designated crystallization temperature. The curves of heat flow as a function of time were recorded. TGA was carried out using a TA Instruments TGA2950 thermogravimetric analyzer from room temperature to 500 °C under a nitrogen atmosphere, with a heating rate of 10 °C min. Sample weight was 2–3 mg.

WAXD analysis

The diffraction pattern was obtained with a Ragakumodel D/max-2B diffractometer using Ni-filtered Cu-Kα radiation (λ = 0.1542 nm, 40 kV, 200 mA) in the range from 5 to 60° at a scanning rate of 5° min⁻¹. The samples were compression-molded into the films using a hot press at 20 °C higher than their melting temperature.

Polarized optical microscope

Polarized optical microscope observation was carried out on an Olympus BX51 polarizing microscope equipped with a heating-cooling stage and a temperature controller (Linkam THMS600). A small mount of sample, sandwiched between a microscope slide and a cover glass, was first melted at 100 °C for 3 min on the heating-cooling stage to completely melt the crystallites. Then the sample was rapidly quenched to a desired crystallization temperature and allowed to crystallize isothermally. The crystallization morphology was recorded in a computer with a CCD camera (Leica DMLP).

Enzymatic degradation

Polycarbonate films sized 1 cm × 1 cm × 50 μm were prepared in duplicate. The samples were placed in vial tubes containing enzyme and potassium phosphate buffer solution (pH = 6.86) at 37 ± 0.1 °C. The enzyme used was lipase from Pseudomonas sp. (activity of 35 units per milligram). The initial concentration of enzyme in the buffer solution was 10 units per mL. The enzymatic concentration for the samples was 5 units per mg. The films were removed from the enzymatic solution after 24 h, washed with distilled water several times, and dried under vacuum at room temperature to constant weight. The degree of biodegradation was estimated by the sample weight loss.

Results and discussion

Synthesis and characterization

It has been reported that metal acetylacetonates are used as effective initiators/catalysts for the synthesis of polycarbonate.^33–35 The catalytic activity of the metal compounds is affected by the basic of acetylacetonate anion and the chelating capacity of the metal species. Alkali metal acetylacetonates show high catalytic activity on carbonylation reaction due to their strong coordination with carbonyl oxygen.³⁵ Thus we chose LiAcac, NaAcac, and KAcac as the catalyst for the polycondensation of copolycarbonates, and the results are summarized in Table 1. It shows that copolycarbonate with unsatisfactory low M_w was obtained in the presence of LiAcac, while higher M_w values of 155 000 g mol⁻¹ and 177 000 g mol⁻¹ were obtained in the presence of NaAcac and KAcac, respectively. We speculate that for the alkali metal acetylacetonates, sodium ion and potassium ion have stronger coordination with carbonyl oxygen compared to lithium ion. However, KAcac with the highest reactivity may lead to more side reactions and resulted in a low yield (76%). These results indicate NaAcac is more efficient and selective than the other two catalysts. Thus all the random copolycarbonates were prepared by using NaAcas as the catalyst. The detailed synthetic route is presented in the Experimental section.

Table 1 Results of melt polycondensation of PBDC50 using various catalysts

Catalyst	Mw^a (g mol^−1)	PDI^a	[η]^b (dL g^−1)	Yield (%)
a Weight-average (Mw) molecular weights and polydispersity index (PDI) determined by GPC.b Intrinsic viscosity ([η]) measured in the mixture of phenol–1,1,2,2-tetrachloroethane (1 : 1) at 25 °C.
LiAcac	74 000	1.72	0.78	89
NaAcac	155 000	1.81	1.03	87
KAcac	177 000	1.83	1.07	76

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The composition of the copolycarbonates was estimated from the ¹H NMR spectrum using the relative intensities of the proton peaks arising from butylene carbonate (BC) and decamethylene carbonate (DC) repeating units. Fig. 1 shows the ¹H NMR spectra of the two homopolymers and the PBDC50 copolycarbonate. The ¹H NMR spectrum of PBC showed a triple peak at 4.14–4.17 ppm attributed to a proton, and a multiple peak 1.72–1.80 ppm attributed to b protons. The PDC ¹H NMR spectrum showed the multiple peaks at 4.09–4.13 ppm attributed to c proton g and peaks at 1.61–1.70 ppm attributed to d proton. The peak positions in the copolymer spectrum are in accordance with those for the homopolymers. Moreover, there was no signal at 3.4–3.5 ppm within experimental limits for all ¹H NMR spectrum of copolymers. This result indicates that there is no ether linkage (–CH₂–O–CH₂–) in the molecular chain which is not hydrolysable and decreases the mechanical properties of the polymers.

Fig. 1 Chemical structure, ¹H NMR spectra, and peak assignments of PBC, PBDC50, and PDC.

The molar composition of the PBDC copolymers was calculated as the integral area ratio of peaks for b protons of BC unit and d protons of DC unit according to the following equation:

where X_BC and X_DC are the molar fraction of BC and DC units, respectively. I_b and I_d are the integral intensities of the corresponding peaks. The results are summarized in Table 2. It is worth noticing that the BC unit content in copolymer is always slightly lower than that in feed. This might be attributed to the sublimation of methyl carbonate terminated oligo(butylene carbonate) (BMBC) and thermal degradation of BC unit during the polycondensation stage. During the PBC homopolymer polycondensation, that oligomer sublimation and thermal degradation of main chain happen, giving BMBC, cyclic tetramethylene carbonate dimmer (TeMC₂), tetrahydrofuran (THF), and a small quantity of cyclic tetramethylene carbonate monomer (TeMC).¹²

Table 2 Molecular characteristics of PBC, PBDC copolycarbonates, and PDC

Sample	[DC]/[BC] in feed	[DC]/[BC] in polymer	LnBC^a	LnDC^a	R^a	Mw (g mol^−1)	PDI	[η] dL g^−1
a Number-average sequence length of BC unit (LnBC), number-average sequence length of DC units (LnDC), and degree of randomness (R) were measured by ^13C NMR analysis.
PBC	—	—	—	—	—	139 000	1.76	1.01
PBDC10	10/90	11/89	5.75	1.35	0.91	202 000	2.07	1.24
PBDC20	20/80	24/76	3.16	1.66	0.92	128 000	1.75	0.94
PBDC30	30/70	33/67	2.46	1.87	0.91	130 000	1.78	0.99
PBDC50	50/50	55/45	2.08	1.96	0.99	155 000	1.81	1.03
PBDC70	70/30	75/25	1.69	3.25	0.90	151 000	1.90	0.96
PBDC90	90/10	94/6	1.21	11.9	0.91	125 000	1.73	0.97
PDC	—	—	—	—	—	167 000	1.68	1.04

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The random, alternate, or block nature is a vital factor for a copolymer, significantly influencing its final properties. High-resolution ¹³C NMR has often been used to determine the microstructure of backbone, such as the degree of randomness, sequence distribution, and number-average sequence length.^36,37 Fig. 2 shows the chemical structure and ¹³C NMR spectrum of PBDC50. The signals around 155 and 67 ppm were split into three and four peaks, which are assigned to carbonyl and –OCH₂ carbon resonances, respectively. According to the carbon resonances of the two homopolymers, the three split peaks located at 155.49, 155.36, and 155.23 ppm are further attributed to the presence of dyad sequence distribution denoted as DC/DC, DC/BC (or BC/DC), and BC/BC respectively, and the four split peaks located at 68.17, 68.03, 67.33, and 67.19 ppm are assigned to the DC/DC, DC/BC, BC/DC, and BC/BC respectively. In this work, the carbonyl carbon is chosen for the microstructure analysis of the PBDC copolymers. The degree of randomness (R) is defined as:

where f_BC/BC, f_BC/DC, and f_DC/DC represent the dyads fraction, calculated from the integral intensities of the resonance signals of BC/BC, BC/DC, and DC/DC, respectively. L_nBC and L_nDC stand for the number-average sequence length of the BC and DC units, respectively. In the case of a random copolymer R takes a value equal to 1, while for an alternate copolymer equals to 2 and for a block copolymer close to zero.

Fig. 2 Chemical structure and ¹³C NMR spectrum of PBDC50.

As summarized in Table 2, the degree of randomness was very close to 1, indicating random structure of the copolymers. The L_nBC decreases and L_nDC increases with the increasing of DC unit content. The M_w and PDI were detected by GPC. The weight-average molecular weight of the obtained polymers ranged from 125 000 to 202 000 g mol⁻¹, with dispersities of around 1.68–2.07. The measured intrinsic viscosity of the samples was in the range of 0.94–1.24 dL g⁻¹, as shown in Table 2. These results reveal that high-molecular-weight aliphatic random copolycarbonates are successfully synthesized.

Calorimetric properties

Thermal analysis carried out by DSC for PBC, PBDCs, and PDC is shown in Fig. 3 and 4. The results of thermal transition data are summarized in Table 3. In amorphous random copolymers, glass transition temperature is usually a monotonic function of composition. Fig. 3 shows the DSC heating traces for the copolymers quenched from melt. It shows that all the copolymers have a single T_g in agreement with the random sequence structure validated by ¹³C NMR. As presented, the glass transitions of PBC to PBDC30 were obvious, while those of PBDC50 to PDC were very weak. Compared with PBC to PBDC30, the quenched PBDC50 to PDC samples showed sharp melting peaks without cold crystallization. This indicates that the quenched PBDC50 to PDC samples were highly crystallized during the fast quenching process, resulting in a weak glass transition during the heating scan. As can be seen in Table 3, the T_g values decreased with DC unit from PBC to PBDC30, while those of PBDC50 to PDC were a little higher than that of PBDC30. It can be ascribed to the highly crystallinity of PBDC50 to PDC. The crystalline fraction play a role in amorphous regions of the polymeric network and thus suppress the mobility of amorphous regions, resulting in higher T_g.^38,39

Fig. 3 DSC heating scans for the quenched samples: PBC, PBDC copolycarbonates, and PDC.

Fig. 4 DSC heating (A) and cooling (B) curves of PBC, PBDC copolycarbonates, and PDC.

Table 3 Thermal properties of PBC, PBDC copolycarbonates, and PDC

Sample	Tg^a (°C)	ΔHm^a (J g^−1)	Tm^a (°C)	Tc^a (°C)	Td,max^b (°C)	Td,5%^b (°C)
a Glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (ΔHm), and crystallization temperature (Tc) measured using DSC.b Decomposition temperatures of the maximum rate (Td,max) and of 5% weight loss (Td,5%) estimated from the TGA thermogram.
PBC	−32	25.4	51.9	—	297	256
PBDC10	−38	18.0	42.2	—	(315, 348)	283
PBDC20	−43	11.0	38.1	—	(318, 350)	286
PBDC30	−46	17.4	40.4	12.0	(318, 361)	286
PBDC50	−45	32.4	45.7	18.6	(324, 362)	292
PBDC70	−45	42.3	52.7	26.9	(334, 370)	310
PBDC90	−44	57.8	60.0	36.5	382	332
PDC	−43	65.6	63.6	40.3	385	337

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In order to give a fair representation of the melting temperature (T_m) and enthalpies (ΔH_m), all polymers were crystallized adequately at roughly the same supercooling (ΔT = T_m − T_c ≈ 40 °C), then were heated to 100 °C at a rate of 10 °C min⁻¹. The heating scan in Fig. 4(A) indicates that the homopolymers and all the copolycarbonates are semicrystalline. PBDC10 and PBDC20 showed broad melting peaks with less melting enthalpy. Furthermore, the thermal transition behavior of PBDC significantly depends on composition and a depression of melting temperature and enthalpy with increasing DC unit content were observed for PBDCs. The lowest melting temperature and enthalpy were observed for PBDC20. Then the T_m and ΔH_m increased with increasing DC unit content when DC unit content was higher than 20 mol%.

For PBDCs with those of reference PBC and PDC homopolymers, the T_m and ΔH_m are also plotted in Fig. 5 to better understand their composition dependence. As previously reported by the studies on the cocrystallization behavior of aliphatic copolyesters,^40–42 the change of the melting temperature and enthalpy with the DC unit composition exhibits a typical isodimorphic cocrystallization behavior. A similar variation tendency of melting temperature dependent on copolymer composition was also discovered in poly(ω-pentadecalactone-co-butylene-co-succinate)⁴³ and poly(butylene succinate-co-butylene furandicarboxylate)⁴⁴ random copolyesters, indicating isodimorphic cocrystallization in nature. In addition, as can be seen in the cooling thermograms (Fig. 4(B)), the crystallization temperatures became higher and the crystallization peaks became narrower as the DC unit content increased.

Fig. 5 Composition dependences of T_m and ΔH_m.

Fig. 6 shows the crystallization rate for PBC, DC-rich copolymer, and PDC as a function of temperature. Crystallization rate is represented by the reciprocal of the halftime of complete crystallization (t_1/2). For all samples, the crystallization rates decreased with temperature increase. As shown, the crystallization rates of DC-rich copolymers at higher temperatures were much faster than that of PBC at lower temperatures, suggesting that the introduction of DC unit considerably improves the crystallization rate of PBC. On the basis of these results, it can be concluded that randomly copolymerizing with certain amount of DC units with linear long-chain can enhance the melting temperature and crystallization rate of PBC.

Fig. 6 Crystallization rate (reciprocal of half-time of complete crystallization measured by DSC) of PBC, PBDC copolycarbonate, and PDC.

WAXD patterns and crystalline morphology

Two different cases of cocrystallization behavior (i.e., isomorphism and isodimorphism) in random copolymers have been reported.^45–47 In isomorphism, only one crystalline phase containing both comonomer units is observed at all compositions, and the melting temperature of copolymers increases linearly with increasing comonomer content.⁴⁸ In isodimorphism, two crystalline phases are observed depending on the composition, and a minimum melting temperature is found.^49–51

Fig. 7 shows the WAXD patterns of PBC, PDC, and PBDC copolycarbonates in order to investigate the nature of crystalline phase. The main reflections of PDC appeared at 20.97° and 23.88°, and those of PBC appeared at 21.04° and 21.50°. The WAXD patterns of all the copolymers suggest that the crystal packing changes from that of PBC to PDC with increasing DC unit content. The copolymers containing 10 and 20 mol% of DC units developed PBC type crystal. The reflections for the DC-rich (≥30 mol%) copolymers are those associated with the PDC crystal form. It is valid that the crystal type of PBDCs changes from PBC type to PDC type crystals at the composition of 20–30 mol% DC unit content which is equivalent to the composition for the lowest melting temperature.

Fig. 7 WAXD diffractograms of PBC, PBDC copolycarbonates, and PDC.

There are some continuous shifts in all the reflection peaks with increasing the DC unit, inferring cocrystallization of the two comonomers. The d-spacing value of the crystalline planes of PBDCs was calculated with the Bragg equation based on these WAXD patterns. Fig. 8 presents the plots of d-spacing versus the DC unit content (mol%). As shown in Fig. 8, the d-spacing of both planes of the PBC crystal increased with increasing the DC unit content from 0 to 20 mol%. This indicates that the inclusion of the DC unit into the crystalline lattice of the PBC crystal results in the slight change of the unit size and the content of the DC unit included in the host crystalline lattice increase with increasing the DC unit content in the bulk polymer. In the case of the PDC type crystal, the d-spacing of the (2θ = 20.97°) plane decreased and that of the (2θ = 23.88°) plane increased linearly with increasing BC unit content. These indicate that the BC and DC units are compatible in each crystal lattice throughout the whole compositions. Thus, in accordance with the DSC results, these WAXD results further support the isodimorphism of PBDC.

Fig. 8 d-spacing of characteristic planes as a function of the DC unit content in the PBDC copolycarbonates.

The spherulitic morphology of PBC, PBDC50, and PDC were observed under the isothermal crystallization condition at the temperatures of 0, 40, and 55 °C respectively. The neat PBC sample yielded typical spherulites from the melt and it showed mixed spherulites as seen in the polarized micrographs in Fig. 9, due to the absence of clear Maltese Cross. The size of PDC is significantly smaller even crystallized at a higher temperature indicating high nucleation density. The size of PBDC50 spherulite was between neat PBC and PDC. The enhancement of primary nucleation rate leads us to speculate that the energy barrier of primary nucleation is lowered down with the introduction of DC unit. This also proves that randomly copolymerizing with DC unit can enhance the crystallization ability of PBC, in accordance with the DSC results.

Fig. 9 Spherulitic morphology for (a) PBC at 0 °C, (b) PBDC50 at 40 °C, (c) PDC at 55 °C.

Thermal stability

The thermal stability of polymers is a critical parameter for their potential applications. Thermogravimetric analysis provides important information about thermal stability of polymeric materials. Therefore, the thermal stability of the resulted copolycarbonates was characterized under a nitrogen atmosphere, compared with PBC and PDC homopolymers.

Fig. 10 presents TGA (a) and DTG (b) curves of PBDC copolycarbonates, PBC, and PDC homopolymers. The decomposition temperature at 5% weight loss, T_d,5% and decomposition temperature at the maximum rate, T_d,max are shown in Table 3. Before 220 °C, all the copolymers did not have any weight loss. Fig. 10(a) shows that the main weight loss of all copolymers lay between those of the two homopolymers and that the T_d,5% increased continuously from 256 °C (for PBC homopolymer) to 337 °C (for PDC homopolymer). Thus, copolymerization with DC unit is demonstrated to be capable of improving the thermal stability of PBC.

Fig. 10 TGA (a) and DTG (b) curves of PBC, PBDC copolycarbonates, and PDC.

As shown in Fig. 10 curves, both PBC and PDC homopolymers degrade in a single step. Generally, in random copolymer where the corresponding homopolymers degrade in one-step, the copolymer should degrade in one-step too.^52,53 It is worth to note that, PBDC10–70 showed a peculiar two-step degradation behavior with two peaks in the DTG curves as observed in Fig. 10(b). The first degradation step starts well above the thermal degradation range of PBC homopolymer, and its location and steepness depend on the specific copolymer analyzed. The second step occurs at a higher temperature comparable with that of PDC. Similar two-step degradation behaviors were also reported in aliphatic poly(carbonate-ester)s, such as poly(butylene carbonate-co-butylene succinate)⁵⁴ and poly(ω-pentadecalactone-co-trimethylene carbonate) copolymers.⁵⁵ However, the origin of the degradation behavior was not clarified. We speculate that this two-step degradation behavior is attributed to the special thermal degradation mechanism of PBC homopolymer. Based on our previous research,¹⁰ at low temperature, the intramolecular transesterification of BC units takes place during the thermal degradation, producing TeMC monomer and dimmer and not changing the main chain structure (Scheme 2). At high temperature, the thermal degradation of PDC takes place via simple random scission of the polymer chain. Thus, two-step degradation behavior happened.

Scheme 2 Intramolecular transesterification of BC Units during PBDC thermal degradation.

Enzymatic degradation

Aliphatic polycarbonates are more susceptible to degradation in a wide variety of ecosystems compare to aromatic ones, and thus they are currently proposed commercially as biodegradable materials.^56,57 It is known that many factors such as the degree of crystallinity, molecular weight, copolymer ratios, and molecular mobility of the amorphous phase affect the enzymatic degradation rate of polymers.^58–60 In order to examine the biodegradability of the synthesized polycarbonates in a short time scale, enzymatic hydrolysis degradation was carried out. Fig. 11 shows the weight loss of neat polycarbonates, in comparison with their copolycarbonates during enzymatic hydrolysis for several days. As can be seen, PDC showed a slow biodegradation rate with a weight loss of 6.1% after 7 days. On the other hand, PBC has a higher biodegradation rate with a weight loss close to 13 wt% for the same time. It is expected that PBC is less crystalline than PDC, and that additionally, the carbonate bond density of PBC chain is higher than that of PDC. This is in accordance with a recent study in amorphous polyester nanoparticles showing that polymer structure and mainly the ester bond density have a direct influence on enzymatic hydrolysis.⁶¹ Consequently, the biodegradation rates of PBDC50 and PBDC70 were between PBC and PDC homopolymer. Moreover, the biodegradation rate of PBDC50 is a little higher than PBDC70 because of the lower crystallinity and higher carbonate bond density. It is worth noting that the copolycabonate containing 20 mol% DC shows the highest degradation rate and the weight loss reached to 40 wt%. One possible explanation for such higher rate could be that the PBDC20 copolycarbonate has a much lower degree of crystallinity and melting point than PBC as can be seen in DSC and WAXD results. It has proven that the enzymatic hydrolysis rates of copolymers are significantly influenced by the crystallinity and mainly melting point.⁶²

Fig. 11 Weight loss against time of enzymatic degradation of the PBDC copolycarbonates.

Conclusions

Aliphatic poly(butylene-co-decamethylene carbonate) copolycarbonates with M_w of 125 000 to 202 000 g mol⁻¹ were synthesized via a transesterification and polycondensation process, using NaAcac as an effective catalyst. Good agreement between theoretical and experimental BC/DC molar ratios was attested by ¹H NMR. Information about L_nBC, L_nDC, and R of the PBDCs was obtained by ¹³C NMR. All the copolycarbonates present random microstructure with R independent of the copolycarbonate composition. As expected, the introduction of DC unit can significantly enhance the crystallization ability of PBC. The PBDC copolycarbonates had a minimum melting point and enthalpy in the plot of melting point versus composition. WAXD patterns showed that the copolycarbonates with up to 20 mol% DC units formed PBC type crystal, while those with higher DC unit content crystallized in PDC type crystal. These evidenced an isodimorphic cocrystallization behavior of PBDC copolymers. The biodegradation rate of PBDCs can be adjusted by the DC unit content which determines the crystallinity, melting point, and carbonate bond density of the copolycarbonate.

Acknowledgements

Financial support from National Natural Science Foundation of China (Grant no. 21304100 and no. 51373186) and Cultivation Project of Institute of Chemistry Chinese Academy of Sciences (Grant no. CMS-PY-201330) is gratefully acknowledged.

Notes and references

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