Toughening of amorphous poly(propylene carbonate) by rubbery CO₂-based polyurethane: transition from brittle to ductile

Abstract

Amorphous poly(propylene carbonate) (PPC) is brittle at room temperature, but the studies related to the toughening of PPC is rare. Herein, two types of polyurethane (PCO₂PU) synthesized from a CO₂-based diol and toluene diisocyanate were used as rubbery particles to toughen PPC. The notched impact strength of PPC increased from 20.8 J m⁻¹ to 54.2 J m⁻¹ at a PCO₂PU loading of 20 wt%, comparable with that of neat nylon 6, and reached 228.3 J m⁻¹ at a PCO₂PU loading of 30 wt%, 10.9 fold that of neat PPC and even higher than bisphenol A polycarbonate. Matrix yielding as well as cavitation was observed during the impact process, which was responsible for the increase of impact strength. Moreover, the toughening efficiency was related with the carbonate content of PCO₂PU, and the transition of fracture behavior from brittle to ductile occurred when the PCO₂PU with a weight average diameter of 0.20 μm was uniformly dispersed in PPC substrate.

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1. Introduction

Poly(propylene carbonate) (PPC) is an alternative copolymer of CO₂ and propylene oxide that has received considerable attention in the past decades because it is biodegradable and has effective CO₂ utilization.¹ Now it has found applications not only in high value-added areas like tissue scaffolds² or polymer electrolytes,³ but also as a low-cost biodegradable packaging material.⁴ However, PPC is amorphous and brittle with elongation at break below 10% and low notched impact strength of about 20 J m⁻¹ at 20 °C. These values are comparable to polystyrene, which has a notched impact strength of 15.8 J m⁻¹. Generally, the notched impact strengths of high density polyethylene (HDPE) and high impact strength polystyrene (HIPS) are 50–70 J m^−1 5 and 72–148 J m⁻¹,⁶ respectively. Therefore, the notched impact strength of PPC is considerably low, which has severely limited its application.

Considerable effort has been made to overcome the brittleness of PPC. Our previous efforts indicate that PPC can be plasticized by diallyl phthalate⁷ or low molecular weight urethane compounds,⁸ or partly plasticized and enhanced via hydrogen bonding interaction by introducing a hydro-branched poly(ester amide).⁹ The elongation at break has been improved to over 700% while a completely miscible blend has been obtained.⁸ However, currently no report has been related to the notched impact test even though it represents the ability to absorb fracture energy under high loading in a notched state.

The notched impact strengths of general polymers like polyamide¹⁰ or polystyrene¹¹ have been investigated for a long time. The introduction of elastomers (with appropriate domain size or interparticle distance) into the matrix can change the stress state around the particles and form microstructures that dissipate impact energy. These microstructures include intensified stable crazing in HIPS^11b,11c and large area shear yielding^10b,10c that accompany cavitation in polyamides.^10a,10d,10e Both the notched impact strength and fracture morphology are significantly related to the rubber particle diameter and volume fraction.¹² Increasing the content of small rubber particles favors local plane stress with the consequence of a transition from crazing to shear deformation. For a given polymer blend, maximum toughness can be obtained in a limited range of rubber particle sizes^12b where microvoids are formed, thus avoiding premature fracture due to the extension of craze from the notch tip before significant energy has been dissipated, which facilitates the occurrence of shear yielding. For example, in a polyamide/polyolefin elastomer blend,^12a energy dissipated by shear yielding was optimal at particle sizes between 0.1 μm and 0.3 μm.

The rubbery CO₂-based polyurethane (PCO₂PU) (Scheme 1) was synthesized from the chain extending reaction of CO₂-based diols with toluene diisocyanate (TDI). CO₂-based diols with different molecular weights and carbonate content were prepared in our previous study by the copolymerization of propylene oxide and CO₂ ¹³ to obtain low molecular weight (number average molecular weight of ca. 1000–1500 g mol⁻¹) polymers with perfect OH functionalities. It is noteworthy that such a CO₂-based diol has a 20–30% lower cost than conventional polypropylene glycol (PPG). It may be a competitive raw material for polyurethane formation. Most importantly, the presence of a carbonate unit in both PCO₂PU and PPC may enhance the miscibility of the two components. Moreover, the formation of intermolecular hydrogen bonding between N–H in the main chain of PCO₂PU and carbonyl group in PPC may prove to be beneficial. Therefore, two types of rubbery PCO₂PU with different carbonate content were used to toughen PPC. A sudden increase in notched impact strength compared to the value of neat polyamide (PA6) was observed accompanying the fracture behaviour transition from brittle to ductile, and matrix shear yielding appeared for the domain size of PCO₂PU at 0.20 μm.

Scheme 1 Synthesis of PCO₂PU with CO₂-based diol.

2. Experiment

2.1 Materials

Toluene diisocyanate (TDI) and dibutyltin dilaurate were purchased from Tianjin Guangfu Chemical Research Institute (China) and used as received. CO₂-based diols with carbonate unit content of 50.4% and 62.3% were prepared in our laboratory according to the literature.¹³ The number-average molecular weight (M_n) of the CO₂-based diol with 50.4% carbonate unit content was 1377 g mol⁻¹. The number-average molecular weight (M_n) of the CO₂-based diol with 62.3% carbonate unit content was 1245 g mol⁻¹.

PPC was supplied by Zhejiang Bangfeng Plastic Co. (China), whose technique was licensed under our laboratory. To remove residue rare earth metal ternary catalyst, the copolymer was purified twice by a repeated dissolution/precipitation procedure with dimethyl carbonate as solvent and ethanol as a precipitant. The number-average molecular weight (M_n) and the polydispersity index (PDI) of the purified PPC were determined by gel permeation chromatography (GPC) to be 17.3 × 10⁴ g mol⁻¹ and 3.65, respectively. The carbonate unit content of the purified PPC was 92%, as estimated from its ¹H NMR spectrum according to the literature.¹⁴

2.2 Synthesis of PCO₂PUs

PCO₂PU was synthesized by the polyaddition of CO₂-based diols and TDI under N₂ protection. Briefly, CO₂-based diols (0.035 mol) were dried at 80 °C under vacuum for 40 min to complete dehydration. TDI (0.035 mol) and dibutyltin dilaurate (3.0 μL) were injected into the reaction vessel and stirred at 110 °C for 30 min. The resultant PCO₂PUs from CO₂-based diol with 50.3% and 62.4% carbonate unit were denoted as PCO₂PU1 and PCO₂PU2, respectively. The ¹H NMR (d₆-CHCl₃, TMS, 300 MHz) data were listed as follows.

PCO₂PU1. δ (ppm) = 7.61, 7.21, 7.07 (–ArH), 5.17–4.76 (–CH₂CH(CH₃)OCOO–), 3.97–4.34 (–CH₂CH(CH₃)OCOO–), 3.25–3.81 (–CH₂CH(CH₃)O–), 2.30 (–CH₂COO–), 2.20 (–ArCH₃), 1.29 (–CH₂CH(CH₃)OCOO–, –CH₂CH₂CH₂CH₂COO–), 1.14 (–CH₂CH(CH₃)O–).

PCO₂PU2. δ (ppm) = 7.67, 7.20, 7.06 (–ArH), 5.20–4.78 (–CH₂CH(CH₃)OCOO–), 3.93–4.36 (–CH₂CH(CH₃)OCOO–), 3.30–3.88 (–CH₂CH(CH₃)O–), 2.30 (–CH₂COO–), 2.20 (–ArCH₃), 1.28 (–CH₂CH(CH₃)OCOO–, –CH₂CH₂CH₂CH₂COO–), 1.16 (–CH₂CH(CH₃)O–).

2.3 Melt-blending procedure

Prior to the blending, PPC and PCO₂PUs were dried at 45 °C in vacuum for 24 h. Then, PPC and PCO₂PU were mixed in calculated weight ratio (PCO₂PU/PPC = 2.5/97.5, 5/95, 10/90, 15/85, 20/80, 30/70). The mixing was operated on a Haake batch-intensive mixer (Haake Rheomix 600) at a speed of 60 rpm for 5 min at 140 °C. All the blends were placed in a desiccator before use.

2.4 Characterization

Tensile performance was evaluated using a dumbbell-shaped sample punched out from the molded sheet in a screw-driven universal testing machine (Z010, Zwick Co., Germany) equipped with a 10 kN electronic load cell and mechanical grips. The test was conducted at 20 °C using a cross-head rate of 20 mm min⁻¹ according to the ASTM standard, and the data reported were the mean of the parallel values in five determinations.

The Izod notched impact strength of the specimens was measured on a JJ-20 instrumented impact machine at 20 °C according to the ASTM D256-04. At least five specimens were tested for each sample to obtain an average value.

Scanning electron microscopy (SEM) experiments were performed using a XL30 ESEM FEG (FEI Co.) instrument with an acceleration voltage of 8 kV. The fracture surfaces of Izod notched impact test were coated with gold to increase the contrast.

Differential scanning calorimetry (DSC) analysis was performed on a Perkin-Elmer DSC-7 instrument under N₂ atmosphere. The sample was first heated from −50 °C to 50 °C at 10 °C min⁻¹ and then rapidly quenched to −50 °C, followed by a second heating process to obtain the glass transition temperature (T_g).

Rheological measurements were performed at 140 °C using an AR 2000 (TA, USA) rheometer with a parallel plate geometry, and the diameters of plates was 25 mm. Dynamic amplitude sweeps from 0.1% to 100% strain at 1 rad s⁻¹ frequency were executed to determine the linear viscoelasticity range. Then, dynamic frequency sweeps were performed from 0.01 to 100 rad s⁻¹ at a small strain of 1% within the linear viscoelastic zone.

The TEM images were recorded on a transmission electron microscope (Tecnai G2 F20 S-TWIN) with an acceleration voltage of 200 kV. The samples for TEM analysis were prepared by microtoming films with 50–100 nm thickness from the blends with an ultramicrotome (LEICA ULTR CUTR ME1-057) and stained with 1% aqueous phosphotungstic acid. The rubber particle size was calculated by TEM using Nanomeasure software. The number and weight average rubber particle sizes were calculated using the following equations.

3. Results and discussion

3.1 Tensile properties and impact strength

The stress–strain curves of PPC/PCO₂PU blends are plotted in Fig. 1, and the corresponding parameters are listed in Table 1. Neat PPC displayed brittle fracture with elongation at break of 9.83%. This value increased to 28.74% when the loading of PCO₂PU1 reached 5 wt%. It was noteworthy that both the tensile strength and the modulus of the blend with 5 wt% PCO₂PU1 increased by 10.21 MPa and 89.48 MPa, respectively compared with those of neat PPC. Further increasing the PCO₂PU1 loading to 20 wt% increased the elongation at break to 49.83%, whereas the tensile strength remained at 47.66 MPa. When the loading of PCO₂PU1 was increased to 30 wt%, the elongation at break reached 320.76% with the tensile strength decreasing to 27.36 MPa. Similar toughening effect can be observed in PPC/PCO₂PU2 blends. When the PCO₂PU2 loading was 20 wt%, the elongation at break increased to 33.06% while the tensile strength was still 4.32 MPa higher than that of the neat PPC. Therefore, a simultaneous increase of tensile strength and elongation at break was realized in this polyblending system, especially at relatively low PCO₂PU loading. This may be attributed to the intermolecular hydrogen bonding between the C=O of PPC and the NH of PCO₂PU, as reported for the PLA/hyperbranched polyamide blend¹⁵ and PPC/low-molecular weight urethane blend.⁸ It is common that the addition of rubbery particles toughens the matrix while decreasing the tensile strength. However, the existence of hydrogen bonds can enhance interfacial adhesion and improve tensile strength.⁹ Therefore, the tensile strength of the blend increased with PCO₂PU1 loading below 15 wt%. When the PCO₂PU1 loading was above 15 wt%, the steric hindrance and dilution effect became dominant, suppressing the reinforcing effect of PCO₂PU. Compared with PCO₂PU1, PCO₂PU2, with a higher carbonate content, showed a more significant reinforcing effect, which may be related to the increased content of hydrogen bonded C=O groups in PPC. Moreover, it appears that PCO₂PU1 is more effective than PCO₂PU2 in toughening PPC. Therefore, an impact test was conducted to confirm the toughening effect.

Fig. 1 Stress–strain curves of PPC/PCO₂PU1 (a) and PPC/PCO₂PU2 (b).

Table 1 Main mechanical properties of PPC/PCO₂PU with various PCO₂PU loading

Sample	PCO2PU ratio (wt%)	Young's modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
PPC/PCO2PU1	0	1392.91 ± 60.84	53.47 ± 2.48	9.83 ± 5.28
PPC/PCO2PU1	2.5	1407.07 ± 51.74	59.83 ± 3.13	24.60 ± 9.32
PPC/PCO2PU1	5	1482.39 ± 67.85	63.68 ± 3.31	28.74 ± 5.07
PPC/PCO2PU1	10	1420.6 ± 134.29	60.69 ± 4.85	28.71 ± 4.17
PPC/PCO2PU1	15	1144.66 ± 79.13	49.01 ± 3.04	21.02 ± 5.28
PPC/PCO2PU1	20	1086.76 ± 126.49	47.66 ± 1.90	49.48 ± 7.53
PPC/PCO2PU1	30	680.05 ± 109.49	27.36 ± 1.40	320.76 ± 51.44
PPC/PCO2PU2	2.5	1424.79 ± 72.52	61.99 ± 3.39	21.97 ± 4.96
PPC/PCO2PU2	5	1449.79 ± 67.56	63.28 ± 2.89	22.37 ± 4.96
PPC/PCO2PU2	10	1474.11 ± 25.50	66.23 ± 1.79	20.22 ± 4.59
PPC/PCO2PU2	15	1323.54 ± 48.24	63.78 ± 2.72	33.74 ± 6.31
PPC/PCO2PU2	20	1164.90 ± 40.76	57.79 ± 2.96	33.06 ± 5.15
PPC/PCO2PU2	30	730.59 ± 109.01	28.03 ± 1.44	233.01 ± 49.70

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The notched impact strengths of PPC/PCO₂PU blends are displayed in Table 2 and Fig. 2. When the PCO₂PU1 loading was below 20 wt%, the notched impact strength increased very little. It increased suddenly by 33.4 J m⁻¹ from 20.8 J m⁻¹ to 54.2 J m⁻¹ when PCO₂PU1 loading was increased to 20 wt%. Further increasing the PCO₂PU1 loading to 30 wt% increased the notched impact strength to 228.3 J m⁻¹. PCO₂PU2 showed nearly no toughening effect in the range of compositions studied. PCO₂PU1 was more effective in toughening PPC, which was more obvious in the notched impact test.

Table 2 Notched impact strengths of PPC/PCO₂PUs with various PCO₂PU loadings

Sample	0	2.5 wt%	5 wt%	10 wt%	15 wt%	20 wt%	30 wt%
PPC/PCO2PU1 (J m^−1)	20.8 ± 1.4	24.7 ± 3.2	24.2 ± 2.9	22.2 ± 3.2	20.5 ± 1.3	54.2 ± 5.1	228.3 ± 46.3
PPC/PCO2PU2 (J m^−1)	20.8 ± 1.4	21.3 ± 1.6	22.7 ± 2.1	24.7 ± 3.1	20.8 ± 3.1	22.8 ± 2.1	25.5 ± 8.6

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Fig. 2 Notched impact strengths of PPC/PCO₂PUs with various PCO₂PU loadings.

It is reasonable that the elongation at break of PPC/PCO₂PU2 blends increased significantly, while the notched impact strength was improved little because notched impact strength was more accurate. As far as the PPC/PCO₂PU1 blend was concerned, PCO₂PU1 effectively toughened PPC at 20 wt% loading and the notched impact strength was comparable with that of neat PA6.¹⁶ When loading was increased to 30 wt%, although the tensile strength decreased to 27.36 MPa, the notched impact strength increased to 10.9 fold that of neat PPC, which was even higher than the bisphenol A polycarbonate.¹⁷

The improved notched impact strength may indicate the occurrence of a brittle–ductile transition. This transition may be accompanied by the formation of microstructures such as shear yielding, crazes and microvoids. To confirm the microstructure formed during the impact test and understand the toughening mechanism, the fracture surfaces of neat PPC and PPC/PCO₂PU blends under 20 wt% and 30 wt% loading were studied.

3.2 Toughening mechanism

SEM images obtained on the fracture surface of PPC and the polyblends are shown in Fig. 3. In Fig. 3a, the fracture surface of neat PPC had a typical brittle appearance; it was relatively smooth with elliptical marks, indicating that the secondary-crack-front velocity was bigger than that of main-crack-front.¹⁸ When PPC was blended with 20 wt% PCO₂PU1, the corresponding fracture surface showed no elliptical mark and became coarser while matrix shear yielding appeared (Fig. 3c). Moreover, a few cavitations could be found in the magnified image (Fig. 3d). At 30 wt% PCO₂PU1 loading, a large area of shear yielding with cavitation was observed and the fracture surface showed a typical ductile appearance (Fig. 3e). Compared with PCO₂PU1, the addition of 20 wt% and 30 wt% PCO₂PU2 did not considerably change the fracture surface of PPC. No cavitation and massive shear yielding can be observed, corresponding to the limited increase in the impact strength of PPC/PCO₂PU2 (Fig. 3g and i).

Fig. 3 SEM images of the fracture surface of PPC/PCO₂PU impact sample: (a and b) PPC, (c and d) PPC/PCO₂PU1 (80/20), (e and f) PPC/PCO₂PU1 (80/30), (g and h) PPC/PCO₂PU2 (80/20), (i and j) PPC/PCO₂PU2 (80/30).

It has been reported that for polyblends showing massive shear yielding in the matrix upon impacting, most energy will be dissipated from matrix yielding.¹⁹ Though the formation of microvoids is the secondary factor contributing to toughening, it is necessary in the formation of massive shear yielding. The cavitation reduces the critical stress at the onset of shear yielding to avoid obtaining the critical strain energy release rate first.^12b In the PPC/PCO₂PU1 blends with 20 wt% PCO₂PU1, cavitation as well as shear yielding was observed, indicating that a transition from brittle to ductile occurred and the impact strength was increased. However, the absence of massive cavitation determined that shear yielding was not as massive as that observed in PLA blend²⁰ or in PP blend.²¹ At 30 wt% PCO₂PU1 loading, the cavitation as well as the shear yielding was expanded to the whole range and the impact strength was improved largely. Therefore, the cavitation and shear yielding resulted in the increase of notched impact strength. In addition, the occurrence of cavitation and shear yielding in the PPC/PCO₂PU1 blends at the loading of 20 wt% may be related to the morphology of the blend. Therefore, the miscibility and morphology were studied.

3.3 DSC analysis of various PPC/PCO₂PU blends

To study the miscibility between PPC and two types of PCO₂PU, the DSC curves of PPC/PCO₂PU1 and PPC/PCO₂PU2 with various compositions were recorded in Fig. 4. There was only a single glass transition temperature at the low loading of PCO₂PU for all the blends, suggesting good miscibility between PPC and PCO₂PU at the loading below 5 wt%. However, another glass transition temperature appeared when PCO₂PU loading was above 5 wt%, indicating the occurrence of obvious phase separation. The glass transition temperature of the miscible blend was between that of neat PPC and PU, which was a little higher than the value calculated by the Fox equation. In immiscible blends composed of PPC and PCO₂PU1, the two glass transition temperatures, displayed in all the blends, were also between those of the two components due to the diffusion of a mutually soluble T_g-reducing component from one phase to another. But interestingly, the lower T_gs in the PPC/PCO₂PU2 blends, corresponding to rubbery particle rich phase, were lower than that of the PCO₂PU2. As reported in the PS/PE blends, the phenomenon may be due to the interactions between the two components.²²

Fig. 4 DSC curves of PPC/PCO₂PUs with various PCO₂PU loadings: (a) PPC/PCO₂PU1 and (b) PPC/PCO₂PU2.

3.4 Rheology analysis of various PPC/PCO₂PU blends

To confirm the miscibility of PPC and PCO₂PUs, rheology analysis has been conducted. Fig. S1 and S2† show the dynamic frequency sweeps of PPC and PPC/PCO₂PU blends with various compositions. All the blends exhibited a decrease in viscosity with an increase in frequency, indicating an obvious shear thinning behavior and pseudoplastic characteristic. Moreover, the addition of PCO₂PU decreased the viscosity of PPC, which may favor processability.

Due to the sensitivity of dynamic rheology to changes in morphology, it is generally used to study the miscibility of blends. For homopolymer or miscible blends, when the frequency is small enough, the slope of log G′ vs. log ω is 2 and the slope of log G′′ vs. log ω is 1.²³ When phase separation occurs, the slope becomes smaller due to the contribution of the elasticity of the interface and the time–temperature superposition fails. The plots of log G′ vs. log ω for the PPC/PCO₂PU blends with various compositions are displayed in Fig. 5. As the content of PCO₂PU increased, the slope of log G′ vs. log ω decreased gradually. When the loadings of two types of PCO₂PUs were 5 wt%, the slope was very close to 2, indicating the miscibility of the blends. With the loading increased to 10 wt%, the slope decreased to 1.78 and 1.68 for PPC/PCO₂PU1 and PPC/PCO₂PU2, respectively. The slope further decreased when PCO₂PU loading was 30 wt%. Therefore, the blends became immiscible with PCO₂PUs loading above 10 wt%, which was in accordance with the DSC analysis. The different toughing effects of PPC/PCO₂PU blends may be related with phase morphology.

Fig. 5 G′ of PPC/PCO₂PUs with various compositions at 1% strains. (a) 5 wt%, (b) 10 wt%, (c) 20 wt%, (d) 30 wt%.

3.5 Phase morphology

Phase morphology is of vital importance for the toughening effect. The size,²⁴ distribution, and even the shape of the distribution phase²⁵ affect the microstructure formed during the impact process and thus influence the value of impact strength. For cavitation observed initially at 20 wt% PCO₂PU1 loading, the phase morphologies of PPC/PCO₂PU blends under 20 wt% PCO₂PU loadings were studied and the diameters of the dispersed phase are displayed in Table S1† and Fig. 6. As shown in Fig. 6, PCO₂PU was dispersed uniformly as spherical particles in the PPC matrix in all the blends. When the content of PCO₂PU1 increased from 15 wt% to 20 wt%, the weight average diameter increased from 0.09 μm to 0.20 μm. For the blend with PCO₂PU2 content of 20 wt%, the weight average diameter of the dispersed phase was 0.11 μm, which indicated that PCO₂PU2 had better miscibility with PPC because of larger carbonate content. However, it appeared that the impact strength could be increased when the diameter of the dispersed phase was as large as 0.20 μm. When the dispersed phase had a diameter around 0.10 μm, there was nearly no contribution to the impact strength. A similar phenomenon was observed in PLA blends, where optimum particle size for increasing toughness was between 0.5 and 0.9 μm.²⁶ In the PLA/P(CL-co-LA) blend, the observed particle size was 0.70 μm, and the impact strength increased to 2 times of neat PLA.^25a According to the theory developed by Paul,^12b the shear yielding of the matrix occurs when the stress reaches the critical value, which is influenced by the cavitation of the dispersed phase. When the diameter of the rubber phase is too small to form the microvoids, the critical stress at the onset of shear yielding is so large that the strain energy release rate is sufficient to initiate crazing and crack growth in the plane strain region. As a result, the development of shear yielding is limited and fracture occurs first. It is noteworthy that the resistance of cavitation is related to the diameter of rubber phase, i.e., the critical volume strain at cavitation increases as the diameter decreases. Therefore, there is a critical diameter beyond which the cavitation occurs before shear yielding. The cavitation favors the shear yielding by reducing the critical value at which shear yielding occurs. As a result, energy dissipation is promoted and the impact strength increases.

Fig. 6 Phase morphologies of PPC/PCO₂PUs with various compositions: (A) PPC/PCO₂PU1 (85/15), (B) PPC/PCO₂PU1 (80/20), (C) PPC/PCO₂PU2 (80/20).

In the PPC/PCO₂PU blends, the diameter of the dispersed phase is believed to be a determining factor. The cavitation was found in the PPC/PCO₂PU1 (80/20) blend, in which the diameter of the dispersed phase reached 0.20 μm, and shear yielding occurred with energy dissipation during the process of impact. However, in other blends, the small diameter of the dispersed phase led to the absence of microvoids, and unstable crazes propagated before the shear yielding occurred. As a result, the notched impact strength increased little. The smaller toughening efficiency of PCO₂PU2 can be efficiently explained by the smaller diameter of dispersed phase, which results from better miscibility of PCO₂PU2 with PPC; this is unfavorable for the occurrence of cavitation.

4. Conclusion

Polyurethane containing carbonate units was synthesized and used to toughen PPC taking advantage of their potential miscibility. Both the elongation at break and the tensile strength were improved and simultaneous reinforcement and toughening were realized. The notched impact strength of PPC was improved to 228.3 J m⁻¹ for the first time, which was comparable to traditional bisphenol A polycarbonate. It was found that the increase in impact strength was related to the diameter of dispersed phase. When the diameter of dispersed phase reached 0.20 μm, cavitation and matrix yielding occurred; this led to the dissipation of energy and ductile fracture. Our investigations were helpful to understand the relationship between morphology and toughness in addition to determining the range of efficient diameters of the rubber phase for toughening PPC.

Acknowledgements

The work was financially supported by the National Natural Science Foundation of China (Grant no. 51321062 and no. 21134002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07142a

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