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Keywords Poly(lactic acid) · Poly(butylene adipate-co-terephthalate) · Co-continuous phase · Biodegradable · Blends Extended author information available on the last page of the article Vol:.(1234567890) 1 3 Journal of Polymers and the Environment (2018) 26:3802–3816 3803 hydrophilic properties . The glass transition temperature Introduction (T ) of PBAT is about − 30 °C, which indicates that it is very ductile at room temperature. Figure 2 is the molecular struc- Recently there has been a huge growth in the development of ture of PBAT. It contains two different co-monomers: one is bio-degradable plastics to reduce the environmental impact butylene terephthalate, which is derived from terephthalic acid of the disposal of conventional oil-based plastics [1–3]. and 1,4-butanediol, and the other co-monomer is butylene adi- Those bio-degradable polymers that are also bio-based, i.e. pate, which is ductile and is produced from adipic acid and derived from renewable feedstocks, have the added advan- 1,4-butanediol . PBAT can be blended with other polymers tage of a reduced carbon footprint. In the growing bio-based to enhance their performance . It is regarded as a good can- economy, poly(lactic acid) (PLA) is one of the most promi- didate for toughening PLA because of its low elastic modulus nent thermoplastics . It is used in packaging and other and high elongation-to-break (> 500%), which are similar to short-life disposable applications, as well as in biomedical the performance of a thermoplastic elastomer, and because of products because of its biocompatibility. its complementarity with PLA . The molecular structure of PLA is shown in Fig. 1. Lactic There are a number of interesting studies in the literature acid is obtained by fermentation of starch that comes from reporting on PLA/PBAT blends. An important factor that maize or corn. The polymer is produced via ring-opening determines the success of melt blending of two polymers is polymerisation of the lactide, which is a cyclic dimer com- their mutual miscibility. In the case of melt blending of two prising two lactic acid molecules . The extent to which bio-degradable polyesters, there would be expected to be PLA will crystallise is highly dependent on the amount of attractions between polar groups leading to stronger interac- l -and d -lactic acid in the polymer. Higher chain symme- tions and hence some miscibility [7, 11]. Liu et al.  have try, and therefore higher crystallinity, occurs in the more reported that the solubility parameters, δ, of poly(l -lactide) optically pure forms. Compared with other biodegradable 0.5 1.5 (PLLA) and PBAT are 19.70 and 19.83 J /cm respectively. polymers, PLA is relatively cheap because of its lower cost The closeness of these values suggests that these two polymers polymerisation method. However PLA has a glass transition are potentially miscible. In their investigation of miscibility of about (57 °C) and so it is rigid and quite brittle at room by dynamic mechanical analysis (DMA), thermal mechanical temperature . This disadvantage significantly limits the analysis (TMA), differential scanning calorimetry (DSC) and application of PLA in ductile products. atomic force microscopy (AFM), these authors concluded that To toughen PLA, a number of methods such as melt the blend with 25% PBAT exhibited the highest miscibility. blending, plasticization, copolymerization and incorporation Yeh et al.  reported that PBAT molecules are miscible with of impact modifiers have been applied [6 , 7]. Melt blending PLA molecules up to 2.5 wt% addition of PBAT but above with ductile polymers is an effective and low cost way to this amount phase separated droplets can be distinguished. enhance the toughness of PLA . It is a process of physi- Also phase separated ‘sea-island’ morphologies are reported cal blending in the melt to mix different polymers without in other studies [14, 15] indicating that miscibility between chemical reactions taking place. Obviously, to maintain bio- PLA and PBAT is limited. degradability, the blending component used to toughen PLA Several papers have reported on mechanical properties, must be not only ductile but also biodegradable. morphology and crystallinity of PLA/PBAT blends. Jiang Poly(butylene adipate-co-terephthalate) (PBAT) is a ductile et al.  found that elongation-to-break of PBAT/PLA blends and biodegradable polymer which has good processability and increased to 100% at an addition level of 5 wt% PBAT and to 200% at an addition level of 20 wt% PBAT. They suggested that PBAT was evenly dispersed in the form of domains with a size of around 300 nm within the PLA matrix. They attrib- uted the large improvement in ductility to rubber-toughening and a debonding-initiated shear yield mechanism. Chiu et al.  found a two-phase sea-island morphology in PBAT/PLA bends. They obtained the best tensile and impact strength in the blend containing a PBAT content of 70 wt%. Farselti et al. Fig. 1 Molecular structure of PLA  found that elongation-to-break increased from 3% (pure Fig. 2 Molecular structure of PBAT 1 3 3804 Journal of Polymers and the Environment (2018) 26:3802–3816 PLA) to 45% at a PLA/PBAT blend ratio of 80/20, which F() = 1 + 2.25 log + 1.81(log ) (3) they attributed to a rubber toughening effect because of the A different model for the prediction of co-continuous small spherical inclusions of PBAT in the PLA matrix. They phase morphologies in polymer blends was developed by also found an increase in the critical strain energy release rate Willemse et al. . This model was centred on the geo- (G ) with increasing PBAT content, which they ascribed to a IC metric requirements for co-continuous structures. It results debonding effect between the phases. Xiao et al.  investi- in an equation [Eq. (4)] describing the critical volume gated the crystallization behaviour of PLA/PBAT blends and fraction of the minor phase for complete co-continuity found that the crystallinity of PLA was markedly increased in (φ ) as a function of the matrix viscosity (η ), interfacial the presence of PBAT but that the crystallization mechanism cc m tension (σ), shear rate ( ) and phase dimensions (R being remained unchanged. Li et al.  observed three distinct the radius of a spherical particle as it deforms into a long morphologies for PBAT/PLA blends in their SEM images: cylinder). spherical droplets (PBAT < 20 wt%), elongated fibrous struc- tures (20 wt% < PBAT < 50 wt%) and a co-continuous struc- 4.2 ture (50 wt% < PBAT < 70 wt%). When the PBAT content m = 1.38 + 0.0213 R (4) 𝜑 𝜎 reached > 70 wt%, the morphology reverted to droplets again cc but with PLA now dispersed in a matrix of PBAT. This model predicts a range of compositions within The properties of immiscible melt blended polymers will which fully co-continuous structures can exist. The criti- be dependent on the morphology produced, and this obvi- cal volume fraction is not dependent on the viscosity of the ously depends on the concentration ratio of the two polymers dispersed phase, and so these authors concluded that the as well as the processing history, which will determine the relationship between the volume fraction at phase inver- melt Rheology of the blend. It is most often the case that sion and the viscosity ratio of the blend components is not the major component will form the continuous phase with generally valid. the minor component dispersed in it as spherical droplets, Although PLA is often blended with other biodegrad- although elongated fibrils may occur depending on the able polymers, there have been very few investigations into flow conditions. Existence of two continuous phases as an predicting a co-continuous phase structure for these sys- interpenetrating network structure will occur near the phase tems. Wu et al.  studied the phase behaviour of poly- inversion point and this region of dual phase continuity of lactide/poly(caprolactone) (PLA/PCL) blends and the vis- two polymers gives a unique combination of their properties. coelastic response of these materials. For this system they It is generally accepted that as well as the relative vol- proposed that the elasticity ratio had an important effect ume fractions of the two polymers, the ratio of their melt on the phase inversion behaviour, in addition to viscosity, viscosities is important in predicting co-continuous phase because PLA/PCL blends have such a high viscosity ratio. behaviour [19, 20]. Paul and Barlow  and Jordhamo In a recent paper, Deng and Thomas  investigated syn- et al.  proposed a semi-empirical equation for predicting ergistic effects of blending PLA with poly(butylene suc- dual phase continuity in polymer blends and simultaneous cinate) (PBS). In this system there was found to be a dra- interpenetrating networks, which is given by Eq. 1. matic improvement in ductility with as little as 10 wt% of 1 1 PBS added. This was shown to be due to a co-continuous (1) 2 2 phase morphology, which could be explained by the rela- tive viscosities of the components. In this equation, φ and φ are the volume fractions of 1 2 As discussed above, a number of researchers have polymer blend components 1 and 2, and η and η are their 1 2 blended PBAT with PLA to achieve better performance, respective shear viscosities at the relevant processing tem- particularly with respect to mechanical properties. How- perature and shear rate. This equation predicts that if η / ever, there has been no investigation into producing a co- η > φ /φ , then component 2 will be the continuous phase 2 1 2 continuous phase structure in a blend of PLA and PBAT. with component 1 forming the dispersed phase. However, The aim of this paper is to verify whether PBAT/PLA when η /η = φ /φ , then components 1 and 2 will form a 1 2 1 2 blends can form a co-continuous phase as predicted by co-continuous phase. the viscosity ratio model of Eq. (1) and to exam the effect A further model based on the ratio of melt viscosities of the co-continuous phase on mechanical properties. was developed by Metelkin and Blekht [23, 24], which is represented in Eqs. (2) and (3), where λ = η /η and φ2 is 1 2 the inversion point of component 2. 2 (2) 1 + F() 1 3 ̇𝛾 ̇𝛾 Journal of Polymers and the Environment (2018) 26:3802–3816 3805 Ram velocities for PLA were varied between 1000 and Experimental −1 200 mm s and those of PBAT between 2000 and 800 mm −1 s . Higher velocities were required for PBAT because of its Materials low melt viscosity. Poly(lactic acid) (PLA) (Ingeo™ 4032D) was procured from Natureworks LLC (Minnetonka, MN, USA). This Differential Scanning Calorimetry (DSC) grade of PLA has an l -lactide content of 98.6 w% and is a crystallisable grade of PLA with a melting point in the DSC was used to investigate the melting and crystalliza- −3 range 160–180 °C. It has a density of 1.24 g cm . Its weight tion behaviour of PBAT/PLA blends. Measurements were average molecular weight (M ) was determined as 94 × 10 g w conducted using a DSC Q200 (TA Instruments, USA) fitted −3 mol from gel permeation chromatography (GPC) meas- with an auto-sampler and mechanical cooler. Samples of urements. Poly(butylene adipate-co-terephthalate) (PBAT, approximately 10–15 mg were cut from the polymer sheet biosafe 2003) with a glass transition temperature of − 34 °C and sealed in aluminium pans before being loaded into the and a melting point around 109 °C was obtained from Xinfu chamber. Specimens were heated in a nitrogen atmosphere −1 Pharmaceutical Co., Ltd, China. from 20 to 200 °C at a heating rate of 10 °C min . For every composition, at least three specimens were tested to calcu- late the average value and standard deviation. Sample Preparation The amount of overall crystallinity, X , was calculated using Eq. 5. Melt blending of PBAT with PLA was carried out at a range ΔH −ΔH m c of composition ratios (PBAT/PLA by weight: 0/100, 10/90, X = × 100% (5) ΔH × Wp 20/80, 40/60, 50/50, 60/40, 80/20 and 100/0). Absorbed 100 moisture was first removed by drying the two polymers in a where ΔH is the enthalpy of melting; ΔH is the enthalpy m c vacuum oven for 24 h at a temperature of 65 °C. After that of cold crystallization; ΔH is the enthalpy of fusion they were melt blended in a Haake Rheomix OS counter- for 100% crystalline polymer; and W is the weight frac- rotating mixer, which promotes dispersive and distributive tion of polymer. For PLA, ΔH = 93 J∕g and for PBAT, mixing. The total sample weight in the mixing chamber was ΔH = 114 J∕g [8, 17], although, being a random co-pol- 58 g and the mixing process was carried out at 175 °C for ymer, PBAT does not crystallize to a great extent. 10 min at a rotor speed of 60 rpm. Neat PBAT and PLA were also melt processed in the Haake at the same condi- Optical Microscopy tion as the blends so that all samples had the same thermo- mechanical history. The polymer samples from the mixer Optical microscopy was used to observe the morphology were then compression moulded into sheets. This was done of the blends and hence to study the dispersion of the two by preheating the polymer for 15 min and compressing it components. The microscope used was a Leica® DMRX into a sheet for 3 min under a pressure of 15 tons (creating (Leica Microsystems Ltd, Germany) binocular transmitted a pressure on the sheet of 11.3 MPa) at a temperature of light microscope. Specimens of roughly 10 µm thickness 180 °C, followed by cooling to room temperature over a were cut from the compression moulded sheets with a glass period of 3 min under a pressure of 5 tons (creating a pres- knife using a cryosectioning technique. The specimens were sure on the sheet of 3.8 MPa). The sheets were of thickness placed on a glass slide and covered with a glass slip. They 1 ± 0.10 mm and from these tensile bars were cut. were observed in bright field illumination at a magnification of ×400. Characterization and Testing Scanning Electron Microscopy (SEM) Capillary Rheometry The morphology of the fracture surfaces of the PBAT/PLA A flowmaster (ROSAND) capillary rheometer was used to blends was examined using a scanning electron microscope determine the shear viscosities of both PLA and PBAT at equipped with a field emission gun (FEGSEM, LEO 1530 175 °C. This test was carried out on pristine samples of VP). The samples were gold coated before examination. The the two polymers. The twin-bore barrel contained a die of FEGSEM was operated at a voltage of 5 kV at various mag- length/diameter ratio of 16 and a ‘zero length’ die to gener- nification levels. ate a Bagley correction and hence eliminate pressure end- In addition, PBAT/PLA of compositions 20/80, 40/60, effects. Pressure was measured at various ram velocities. 60/40 and 80/20 were immersed in acetone for 8 h with 1 3 3806 Journal of Polymers and the Environment (2018) 26:3802–3816 magnetic stirring to dissolve the PLA phase because acetone fluids. To describe the relationship between shear viscosity is a good solvent for PLA . Then the residual blends and shear strain rate, the power law, shown in Eq. (6), was were taken out of the solvent and examined using SEM. This applied. process was carried out to etch the PLA from the surface (n−1) 𝜂 = 𝜂 (6) and reveal the phase structure, as suggested by the work of In this equation, η is shear viscosity; η is the consistency Galloway and Macosko . 0 index; is shear strain rate and n is the power law index. Taking logarithms of both sides of Eq. (6) gives the rela- Tensile Testing tionship between shear viscosity and shear strain rate shown in Eq. (7). This is plotted in Fig. 3 to obtain the values of A universal testing machine (LLOYD Instruments) was n and η . used to determine the tensile properties of the samples. The 0 compression moulded sheets were cut into dumbbell shapes log (𝜂 )=(n − 1)log ( )+ log (𝜂 ) (7) 10 10 10 0 with thickness of 1 mm, a gauge length of 25 mm and width The slope and intercept of PLA in Fig. 3 are − 0.524 and −1 of 4 mm. The crosshead speed used was 10 mm min . In 4.1455, so for PLA; n = 0.476, η = 13,980, η = 13,980 0 PLA order to determine the critical concentrations at which a −0.524 . For PBAT, the slope from Fig. 3 is − 0.282 and co-continuous phase of PBAT/PLA was formed and subse- the intercept is 3.0806 and so the relevant parameters are quently disappeared, a range of compositions were tested. −0.282 n = 0.718, η = 1204 and η = 1204 . 0 PBAT These included 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 40, 50, To transform the rotor speed of the Haake mixer to shear 60, 80 and 100 wt% of PBAT. For each composition ratio, strain rate, the Newtonian equivalent expression , shown at least eight specimens were tested. in Eq. (8), was applied. 2𝜋 N Υ= 16𝜋 N ≈ (8) 2 2 Results and Discussion (1 + 𝛽 )(𝛽 − 1) ln (𝛽 ) In Eq. (8), Y is the shear strain rate, N represents the rotor Melt Rheology speed, β is the value of the wall radius ( R ) divided by the rotor radius ( R ). The rotor speed of the Haake mixer was The shear viscosities of PLA and PBAT were measured by 60 rpm, which means 60 revolutions per minute. The wall using capillary rheometry at 175 °C. The variation of the radius and rotor radius are 20 and 17.5 mm. So the shear shear viscosities of PLA and PBAT with increasing shear strain rate of the mixing process in the Haake mixer was strain rate is shown in Fig. 3. It is obvious from Fig. 3 −1 calculated to be 47 s . By substituting a shear strain rate that the melts of both PLA and PBAT are shear thinning Fig. 3 Shear strain rate depend- ence of the shear viscosities of PBAT and PLA 1 3 ̇𝛾 ̇𝛾 ̇𝛾 ̇𝛾 ̇𝛾 Journal of Polymers and the Environment (2018) 26:3802–3816 3807 −1 of 47 s into the appropriate viscosity equations, the shear model gives a better fit to the viscosity data, the value of viscosities of PLA and PBAT were calculated to be 1859 and 19 wt% is taken as the more accurate prediction of the co- 406 Pa.s respectively. continuous phase. These results show that the viscosity of PBAT is much Lu et al.  also reported that viscosity ratio helps to lower than that of PLA at the processing conditions. The vis- determine the morphology of PBAT/PLA blends. They cosity ratio of PBAT to PLA is 0.218. According to Eq. (1), investigated PBAT/PLA blends at 30/70 w/w containing the ratio of the volume fractions of two polymers at which a various amounts of dicumyl peroxide. They did not find co-continuous phase structure begins to form is determined a co-continuous phase structure, only a typical sea-island by the ratio of their melt viscosities during processing. So morphology. Another paper that investigated rheological when the value of ∕ reaches 0.218, a co-continu- properties of compatibilized PBAT/PLA blends (at 20/80 PBAT PLA ous phase should begin to form if Eq. (1) is valid in this case. w/w) is that of Al-Itry et al. . Reactive compatibilization −3 Because the density of PBAT (1.25 g cm ) and that of PLA was found to give improvement in mechanical properties −3 (1.25–1.27 g cm ) are almost same, the weight fraction of through chain extension and copolymer formation but not a each component was regarded to be the same as its volume co-continuous phase morphology. fraction. Hence, the critical value of to form a co- PBAT continuous phase is calculated to be 17.9 wt%. Thus when Crystallinity and Thermal Properties the content of PBAT reaches 18 wt%, a co-continuous phase structure should start to form and significant improvement DSC traces of PBAT/PLA blends are shown in Fig. 5. On in ductility would be expected. heating from 20 to 180 °C, PLA goes through the glass A polynomial model can also be applied to predict the transition (55 ± 1.5 °C), cold cr ystallization (100 ± 3 °C) viscosity of PLA and PBAT, shown in Eq. (9). (Coefficients and melting (169 ± 2 °C). The glass transition temperature A A and A are three material parameters.) (Fig. 4). of PBAT is − 30 °C. PBAT is a random co-polymer and 0, 1 2 therefore does not have a sufficiently symmetrical structure ̇ ̇ log 𝜂 = A + A log(Υ) + A log (Υ) (9) 0 1 2 to give high levels of crystallinity. However, there is a very Comparing value of R in the Power law and Polynomial broad and shallow endotherm around 100–120 °C indicating models, it can be seen that the Polynomial model gives a bet- some crystallisation of PBAT. −1 ter fit. Substituting a shear strain rate of 47 s into Eq. (9) DSC results show that the glass transition tempera- gives the value of shear viscosity of 1760 and 413 Pa.s for ture (T ) of the blends barely changes regardless of the PLA and PBS respectively, which gives a viscosity ratio of concentration of PBAT, which indicates that PBAT is not 0.235. The critical value of to form a co-continuous PBAT miscible with PLA. According to the rule of mixing, if phase is calculated to be 19.0 wt%. Since the Polynomial the amorphous regions of the PBAT are miscible with the Fig. 4 Shear strain rate depend- ence of the shear viscosities of PBAT and PLA; regression analysis with Polynomial model 1 3 3808 Journal of Polymers and the Environment (2018) 26:3802–3816 Fig. 5 DSC traces of PBAT/PLA blends Fig. 6 Effect of PBAT content on PLA crystallinity amorphous regions of the PLA, then there should be a shift in the glass transition temperature of the blend according Crystallinity was also measured from the DSC traces. to the Fox equation : PBAT is not highly crystalline but from the very broad and shallow endotherm it was calculated that the degree of crys- (PBAT) (PLA) = + (10) tallinity of 100% PBAT was 8.8 ± 0.7%. T T (PBAT) T (PLA) g g g Results for the cold crystallisation temperature (T ) and In Eq. (10), T is the glass transition temperature of the enthalpy (ΔH ), melting temperature (T ) and enthalpy g c m blend in K; T (PBAT) is the glass transition temperature of (ΔH ), and degree of crystallinity (%X ) of PLA in the m c PBAT, which is 239 K; T (PLA) is the glass transition tem- blends are summarised in Table 2. It is seen that the cold perature of PLA, which is 332K; (PBAT) is the weight crystallisation temperature is slightly reduced, particularly fraction of amorphous PBAT in the total amorphous for blends 20/80 and 80/20, implying that crystallisation of region; (PLA) is the weight fraction of amorphous PLA PLA is facilitated. Also, PLA is found to show a greater in the total amorphous region. When PBAT is 20 wt%, degree of crystallisation as the amount of PBAT in the for- (PBAT) is 21.46% and (PLA) is 78.54%. The measured mulation increased. This somewhat surprising result is plot- and theoretical values of T are compared in Table 1. From ted in Fig. 6. It implies that PBAT can act as a nucleating these data it appears that PLA is not miscible with PBAT agent for crystallisation of PLA. at any point above 20 wt% of PBAT. There are a number of studies that have reported on the crystallisation behaviour of PLA/PBAT blends. There are Table 1 Glass transition PBAT (wt%) 0 20 40 60 80 temperature of PBAT/PLA blends Measured T (°C) 55.8 ± 1.8 54.4 ± 0.2 54.8 ± 0.3 56.3 ± 0.3 56.6 ± 0.2 Theoretical T (°C) N/A 33.4 11.5 − 7.2 − 22.8 Table 2 Cold crystallisation, PBAT/PLA T (ºC) T (ºC) ΔH (J/g) ΔH (J/g) %X c m c m c melting and crystallinity of PLA blends in PBAT/PLA blends 0/100 102.2 ± 0.4 170.8 ± 0.4 33.6 ± 1.0 35.1 ± 1.0 2.1 ± 0.1 20/80 99.5 ± 0.5 169.0 ± 0.3 19.2 ± 0.5 25.6 ± 0.1 8.5 ± 0.8 40/60 100.9 ± 0.5 168.7 ± 0.5 13.2 ± 0.1 19.5 ± 0.6 11.2 ± 1.1 60/40 102.5 ± 0.9 168.7 ± 0.7 7.4 ± 0.6 13.3 ± 0.9 15.6 ± 1.1 80/20 96.7 ± 0.5 167.7 ± 0.2 1.8 ± 0.03 6.2 ± 0.3 23.5 ± 1.5 1 3 Journal of Polymers and the Environment (2018) 26:3802–3816 3809 mixed results on whether PBAT increases or decreases the The other three compositions in Fig. 7 all show sea-island crystallinity of PLA. However, there does appear to be a morphologies. The structure of the 10/90 blend has small consensus that PBAT increases the crystallisation rate of spheres of PBAT, around 1 μm in size, in a matrix of PLA. PLA. Yeh et al.  found that the percentage crystallinity The PBAT spherical domains are small because the melt of PLA in melt-compounded blends reduced gradually as viscosity of PBAT is much less than that of PLA and so the the PBAT content increased. Chiu et al.  investigated PBAT is easily broken down in the melt into small drop- heat treatment effects and obtained high levels of crystal- lets. At the 60/40 composition ratio PBAT has become the linity but again reported a decrease in crystallinity of PLA sole continuous phase with droplets of PLA of diverse sizes with increasing PBAT content. Liu et al.  investigated (20–40 µm) within the PBAT matrix. The reason for the the non-isothermal crystallisation kinetics of poly(l -lactide) large droplets of PLA in the PBAT matrix is because of PLLA/PBAT blends. They also reported that crystallinity the difference in melt viscosities of the two polymers. Due of the PLLA-rich phase decreased with increasing PBAT to their high melt viscosity, PLA droplets will not become content. However, they found that the crystallisation rate easily broken down and dispersed in the PBAT matrix. How- coefficients of the blend membranes were higher than those ever, for the 80/20 sample, there is a finer structure because of the original PLLA, suggesting that amorphous domains of at the much higher PBAT/PLA concentration ratio, the PLA PBAT serve as effective nucleation sites for PLLA. In their droplets break up much more readily. study, Xiao et al.  observed that the degree of crystal- linity of PLA in PLA/PBAT blends was markedly increased and there was found to be an increase in crystallisation rate Mechanical Properties with increase in PBAT content. Jiang et al.  investigated recrystallization of both neat PLA and a PLA-5% PBAT Tensile Test Results blend. They found that the blend started to crystallise at a lower temperature than the neat PLA, suggesting the pres- Tensile testing was used to determine the mechanical prop- ence of a new crystalline structure induced by PBAT. erties of PBAT/PLA blends and to look for evidence of Our results verify that the degree of crystallization of co-continuous phase formation. There was expected to be PLA increases with increasing content of PBAT, implying enhanced ductility of the blends in a region of dual phase that PBAT serves as nucleation sites for PLA crystallization. continuity. The results of Young’s modulus and tensile strength of Optical Microscopy the various blends are plotted as a function of PBAT content in Figs. 8 and 9 respectively. Both Young’s modulus and Optical microscopy was used to study the phase structure tensile strength decrease with increasing PBAT content. This of the blends. The images for bright field illumination are result was not unexpected on adding increasing levels of a shown in Fig. 7. soft, flexible material to a hard, rigid one. From the bright field micrographs in Fig. 7, it is seen Two models that are often used to predict the behaviour that there is phase separation in all six of the PBAT/PLA of composites or blends are the Parallel and Series models, blends shown. The two phases will either be continuously written for modulus in Eqs. (11) and (12). interdispersed in the form of a 3D network or will be present Parallel Model E = E + E (11) b 1 1 2 2 as discrete spherical domains embedded in a surrounding matrix, which is often described as a sea-island structure. E E 1 2 Series Model E = (12) The sample at the composition of 20/80 PBAT/PLA has ( E + E ) 1 2 2 1 a fine structure with the PBAT well dispersed in the PLA. In these equations E and E are the moduli of compo- From the viscosity ratio calculation, it was predicted that a 1 2 nents 1 and 2 respectively, while E is the modulus of the co-continuous phase structure should be formed at a PBAT blend. and are the volume fractions of components concentration of 19 wt%. The optical micrograph of the 1 2 1 and 2. These two models represent the upper and lower 20/80 composition is typical of that of two interpenetrating predicted boundaries of behaviour. The Parallel model phases , thereby giving credence to the co-continuous assumes that the continuous phase consists of the higher phase prediction. The morphologies of the 30/70 and 40/60 modulus polymer and therefore represents the upper bound- blends are also typical of co-continuous phase structures. ary, whereas the lower boundary is represented by the Series At the 40/60 composition ratio, the structure has started to model, which assumes that the lower modulus component is coarsen with distinct droplets being visible, although the co- the continuous phase. In this case the higher modulus poly- continuous phase structure is still evident inside the droplets. mer is PLA (1672 MPa) and the lower modulus polymer is This coarsening of the structure implies that the 40/60 blend PBAT (50 MPa). is near the upper limit of the co-continuous range. 1 3 3810 Journal of Polymers and the Environment (2018) 26:3802–3816 Fig. 7 Optical micrographs of PBAT/PLA blends—bright field The Young’s moduli of all the blends fall into the range Between 50 and 100 wt% of PBAT, Young’s modulus data between the Parallel and Series models, suggesting that are tracking the Series model. This indicates that from PBAT and PLA are compatible even though they are not 50 wt% and above, PBAT is the continuous phase with miscible. Up to 40 wt% of PBAT, the blend modulus tracks PLA dispersed within it. The dramatic drop in modulus the Parallel model, which indicates that PLA is acting as a from 1000 MPa at 40 wt% PBAT to about 400 MPa at continuous (or co-continuous phase). However it is inter- 50 wt% PBAT indicates that the upper limit of the co- esting to see that when the PBAT content increases above continuous range falls between 40 and 50 wt% of PBAT. 40 wt%, there is a dramatic decrease in Young’s modulus. 1 3 Journal of Polymers and the Environment (2018) 26:3802–3816 3811 Fig. 10 Effect of PBAT content on elongation-at-break Fig. 8 The effect of PBAT content on Young’s modulus Fig. 11 Elongation at break of PBAT/PLA blends when PBAT con- tent ranges from 0 to 20 wt% Fig. 9 The effect of PBAT content on tensile strength In Fig. 9 it is seen that tensile strength drops below the between 40 and 50 wt% of PBAT marks the upper limit of lower boundary when the PBAT content increases above the co-continuous phase structure i.e. the co-continuous 30 wt%. There is a very steep drop in tensile strength phase is replaced by another structure, which is that of large between 40 and 50 wt%. This confirms a very significant PLA particles dispersed in a PBAT matrix, as shown in change in morphology in this region with PBAT becoming the optical micrographs (Fig. 6) and discussed below for the continuous phase and with poor interaction between the the SEM images of the fracture surfaces. However, when phases. the PBAT content is increased above 60 wt%, the ductility The results of elongation-at-break are plotted as a func- increases further. This is because pure PBAT is very ductile tion of PBAT content in Fig. 10, which shows a very signifi- and when the PBAT content reaches 80 wt%, the droplets cant increase (from around 10% up to 300%) in the composi- of PLA have become very fine and well dispersed, so the tion range between 10 and 20 wt% PBAT. This improvement negative effect on elongation-at-break is compensated for is evidence that a co-continuous phase structure has been by the higher PBAT concentration. formed in this composition range. Elongation-at-break The establishment of a co-continuous phase at a remains above 300% in the composition range from 20 to low concentration of PBAT was further investigated 40 wt% PBAT but then drops back down to around 100% at by studying blends with compositions at 2 wt% incre- the composition of 50 wt%. This drop in elongation-at-break ments of PBAT between 0 and 20 wt%. The results of 1 3 3812 Journal of Polymers and the Environment (2018) 26:3802–3816 elongation-to-break tests are plotted in Fig. 11. It is clearly they have a well-developed co-continuous phase structure, seen that below 14 wt%, ductility is very poor, implying whereas others are more brittle because the co-continuous that PBAT particles are dispersed in a PLA matrix phase. phase structure is incomplete. This result strongly con- However, between 16 and 19 wt%, elongation-at-break firms the predicted value from the empirical viscosity starts to increase significantly and there is a large standard model discussed in see "Melt Rheology" section, i.e. that deviation in the data. This indicates that at around 18 wt% the critical value of PBAT to form a co-continuous phase of PBAT, a co-continuous phase structure starts to form. structure is calculated to be 19 wt% and when the content Hence some specimens have a very high ductility because of PBAT reaches this value a significant improvement in ductility is expected. Fig. 12 Scanning electron micrographs of fracture surfaces of PBAT/PLA blends 1 3 Journal of Polymers and the Environment (2018) 26:3802–3816 3813 there are droplets of PLA in a PBAT matrix and their num- Scanning Electron Microscopy of Fracture Surfaces ber and size reduces as the PLA content reduces. To analyse the morphology of the PBAT/PLA blends fur- ther, Scanning Electron Microscopy was used. The SEM Melt Viscosity and Co‑continuous Phase Structure images of PBAT/PLA blend fracture surfaces from the ten- sile tests are shown in Fig. 12. The fracture surface for pure The viscosity ratio of the two polymers during melt blending is a key factor in determining the morphology of the blends. PLA shows a flat, featureless structure that is typical of a brittle fracture surface. For 20 and 40 wt% of PBAT, the In this study the results of capillary rheometry experiments (see "Melt Rheology" section) show that at the temperature fracture surfaces show that fibrils have been drawn from the −1 surface, which is a common feature of ductile failure. These and shear strain rate (i.e. 175 °C and 47 s ) at which the PBAT and PLA were processed in the Haake mixer, their micrographs indicate that PBAT and PLA have a co-contin- uous phase structure at compositions of PBAT between 20 melt viscosities were 413 and 1760 Pa.s respectively. From Eq. (1) the composition ratio at which a co-continuous phase and 40 wt%. It is expected that these fibrils are due to the PBAT continuous phase because PBAT has a much lower morphology is formed can be calculated, and so for this sys- tem the composition ratio is 0.235 [Eq. (13)]. yield stress than PLA and will undergo plastic deformation at lower stress. PBAT PBAT = = = 0.235 When the PBAT content has reached 60 wt%, it is clear (13) PLA PLA that now the PLA continuous phase has disappeared. It is seen from the 60/40 sample that PLA is present as large This corresponds to 19.0 wt% of PBAT in the formulation. particles dispersed within the PBAT continuous phase. As shown in Fig. 11 and discussed above, this com- These large PLA particles become debonded from the PBAT position corresponds to where there is a very significant matrix and so cracks and flaws will be induced at the inter - improvement in ductility as shown by the increase in elon- face, resulting in relatively poor mechanical properties. This gation-at-break. This is advantageous because it predicts that accounts for the dramatic decreases in both modulus (Fig. 8) relatively low additions of the low viscosity polymer will and tensile strength (Fig. 9) between 40 and 60 wt% PBAT. give large benefits in terms of enhanced ductility. It also accounts for the unexpected drop in ductility observed Figure 14 is a schematic diagram showing the phase mor- in the same composition range (Fig. 10). A dramatic drop phologies of the PBAT/PLA blends over the full range of in elongation-at-break with increasing PBAT content has compositions, as deduced from the melt viscosities, optical also been found by other authors  although there was no micrographs, tensile properties and SEM fracture surfaces. explanation of the cause. When the PBAT content reaches The lower viscosity component, PBAT, is observed to form a 80 wt%, it is seen from Fig. 10 that the samples show very continuous phase over a larger composition range than PLA. ductile behaviour in the tensile test. The SEM image shows that PLA is still dispersed in the PBAT in the form of par- ticles. However, the size of the particles has become much Conclusions finer and the dispersion has become much more uniform. Therefore, at this point, the influence of PLA on the mechan- The synergistic effects of melt blending two biodegradable ical properties is very much diminished, and the blend per- polymers, poly(lactic acid) and poly(butylene adipate-co- forms in a similar way to PBAT. terephthalate), have been investigated. A range of melt To further investigate the co-continuous morphology blended compounds were prepared at various PBAT/PLA in the blends, the acetone etched PBAT/PLA blends were weight ratios of 0/100, 20/80, 40/60, 60/40, 80/20 and 100/0. investigated using SEM (Fig. 13). When the PBAT content Melt viscosities, thermal properties, crystallinity, mechani- is only 20 wt%, a continuous network of PBAT remains after cal properties and phase morphology were studied. the PLA has been dissolved. The structure consists of a frag- In particular it was the aim of this study to investigate ile PBAT skeleton with interconnected voids from where the whether PBAT/PLA blends can form a co-continuous phase PLA has been removed. structure and to predict the conditions under which this At 40 wt% of PBAT, there is still a co-continuous phase would occur. Capillary rheometry experiments were carried structure with a PBAT network interconnected with less out at 175 °C to measure the melt viscosities of PLA and voids. When the PBAT content reaches 50 wt%, it is seen PBAT. Data at six different volumetric flow rates were fitted that PBAT is the only continuous phase and droplets of PLA to a Power Law model. It was calculated that the shear rate at −1 have been dissolved. Hence the upper limit of the co-con- which the blends were mixed in the Haake mixer was 47 s tinuous structure lies in the range between 40 and 50 wt% and at this shear rate the viscosities of PBAT and PLA were of PBAT in agreement with what was predicted from the found to be 413 and 1760 Pa.s respectively. The viscosity mechanical properties. Also for 60 and 80 wt% of PBAT, ratio of the polymers is reported to be equal to the ratio of 1 3 3814 Journal of Polymers and the Environment (2018) 26:3802–3816 Fig. 13 Scanning electron micrographs of PBAT/PLA blends after etching with acetone 1 3 Journal of Polymers and the Environment (2018) 26:3802–3816 3815 Fig. 14 Schematic diagram of phase structure as a function of composition 2. Vroman I, Tighzert L (2009) Biodegradable polymers. Materials the volume fractions at which a co-continuous phase can be 2:307–344 formed. Using this relationship, it was calculated that the 3. 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Journal of Polymers and the Environment – Springer Journals
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