Two Step Extrusion Process: From Thermal Recycling of PHB to

Apr 17, 2015 - A green and industrially viable two-step process was demonstrated for toughening polylactide (PLA) without compromising the biobased an...
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Two Step Extrusion Process: From Thermal Recycling of PHB to Plasticized PLA by Reactive Extrusion Grafting of PHB Degradation Products onto PLA Chains Xi Yang, Jocelyn Clénet, Huan Xu, Karin Odelius, and Minna Hakkarainen* Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden ABSTRACT: A green and industrially viable two-step process was demonstrated for toughening polylactide (PLA) without compromising the biobased and biodegradable nature. First, poly(3-hydroxybutyrate) (PHB) biopolymer was thermally degraded in an extruder to create PHB oligomers (dPHB) with functional end-groups suitable for further reactions. Second, a reactive extrusion process was developed to covalently anchor dPHB onto the main chain of PLA. PLA with 20% (w/w) grafted dPHB demonstrated an impressive elongation at break of 538%, 66 times higher than that of pure PLA and significantly higher than the elongation at break of the corresponding physical blend. At the same time WAXD measurements illustrated that grafting significantly increased the crystallization ability of PLA. We present a viable recycling route for PHB and a highly promising approach for fully biobased toughened PLA with covalently anchored PHB plasticizers.

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1).12−14 Recently, the rapid high yield production of crotonic acid and 3-methoxybutanoic acid by microwave-assisted degradation in alkaline methanol was demonstrated.15 An intriguing alternative would be to utilize the crotonate end groups of the PHB thermal degradation products to covalently couple these oligomers onto the PLA chain. This could further improve the miscibility between PLA and PHB phases as well as prevent the premature migration of oligomeric PHB plasticizers from the PLA matrix. Covalently bound plasticizers in more general terms offer an attractive and novel approach to migration resistant materials that do not release harmful compounds to the environment. In the case of biodegradable material they could permit more controlled service-life before material deterioration. This approach has so far been very sparingly investigated, but it has been suggested as one solution for preventing migration of harmful phthalate plasticizers from PVC.16 And the migration rate of the covalently bound plasticizers was shown to be very low. The decrease in Tg was, however, much more moderate as compared to the material plasticized with free phthalate plasticizers. This could indicate that the mechanical property enhancement also was moderate, although this was not investigated. An attractive and at the same time industrially viable method to covalently attach unsaturated products to the main chain of aliphatic polyesters is reactive extrusion, which has been shown to drastically improve the mechanical properties of inherently brittle materials like polylactide.17,18 Free radical initiated

INTRODUCTION Sustainable closed-loop society using renewable, recyclable and/ or biodegradable materials is still, to a large extent, at a visionary stage. Some positive material exceptions include the commercially available polyesters, poly(lactide) (PLA) and poly(3hydroxybutyrate) (PHB). Yet, there are still challenges associated with the wider use of these polymers, such as their limited impact strength and ductility. A compelling approach to increase the flexibility and broaden the application areas while simultaneously retaining a low total environmental impact of biopolymers and bioplastics is to incorporate plasticizers derived from renewable resources1,2 or from recycled plastic and biomass waste.3,4 Previous progress has shown that the linear and cyclic lactic acid oligomers5,6 are excellent plasticizers for PLA. One of the main problems is, however, the poor migration resistance of these compounds in contact with aqueous solutions.7 PHB oligomers, due to their more hydrophobic nature, could prove to be an appealing alternative. The literature concerning miscibility of PLA/PHB blends is not fully unanimous. Some studies indicate partial miscibility with two slightly affected glass transition temperatures,8,9 while in other cases full miscibility was claimed.10,11 However, low molecular weight for the PHB oligomers and a relatively low weight fraction should improve the miscibility as compared to mixing PLA with high molecular weight PHB. PHB and PLA can be biologically recycled, but material recycling would render desirable access to versatile functional chemicals and intermediates leading to more environmentally friendly and economical solutions. PHB can be thermally degraded to crotonic acid (CA) and well-defined oligomers with one unsaturated and one carboxylic acid chain end (Scheme © XXXX American Chemical Society

Received: February 3, 2015 Revised: April 2, 2015

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DOI: 10.1021/acs.macromol.5b00235 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Schematic Presentation of the Thermal Degradation of PHB to Oligomers with Crotonate End Groups

formed pellets were subsequently thermally degraded by extrusion at preset temperatures (200−240 °C) and for different degradation times (5−60 min) with a screw speed of 90 rpm. The formed thermal degradation products were then collected for the second step, the reactive extrusion. To facilitate the feeding procedure, PLA and PHB degradation product, dPHB, were premixed to a film by solvent casting. 0.5% (w/w) of the radical initiator was added on the surface of the dry films and the films were fed and extruded at 185 °C, the reaction time was 2.5 min, and the screw speed was set to 90 rpm. Film Formation. The formed reactive extrusion products were compression-molded to films using a Hot Press (Fontujne Presses) at 180 °C with a two cycle method. First a holding time of 10 s and a pressing force of 200 kN were used followed by a holding time of 2 min and a pressing force of 200 kN. Finally the pressed films were cooled down to 25 °C by a water cooling system during 10 min. For each film, 3 g of extruded material was placed in a 15 * 15 cm2 square-shaped metal frame and covered by two Teflon sheets on each side. Films with a thickness of 0.225 ± 0.01 mm were formed. Hydrolytic Degradation and Migration test. Migration test of compression-molded films of PLA, PLA20B and PLA20G were performed in water at 60 °C for 1−4 weeks. Migration was conducted in 20 mL vials containing 10 mL of LC-MS grade water. About 20 mg of film was put in each vial and the vials were closed with butyl/ polytetrafluoroethylene septa and aluminum lids. After the predetermined migration times the remaining solid was taken out and dried under vacuum. The remaining solutions were mixed with methanol with volume ratio 1:3 (water: methanol) for further analysis. Nuclear Magnetic Resonance (NMR). The chemical structure and molecular weight of the thermal degradation products formed were determined using 1H NMR. The NMR spectra were obtained by a Bruker Avance DPX-400 NMR instrument operating at 400 MHz. Analysis was performed at room temperature using chloroform-d (CDCl3) as s solvent. Differential Scanning Calorimetry (DSC). Thermal properties of the thermal degradation products and PLA films were evaluated using a Mettler Toledo DSC 820 Module. A temperature program was set in which the samples were first heated from −50 to 185 °C, then cooled to −50 °C followed by a second heating scan to 200 °C at a heating speed of 20 °C/min under a nitrogen atmosphere. The melting temperature (Tm) and glass transition temperature (Tg) were determined from both first and second heating scan. Tm was noted as the maximum of the melting peak and Tg as the midinflection point of the glass transition. Size Exclusion Chromatography (SEC). The molecular weight and dispersity index (Đ) of thermally degraded PHB (dPHB) was evaluated with a Verotech PL-GPC 50 plus size exclusion chromatography system. The mobile phase was chloroform, and polystyrene standards were used for calibration. This chromatography system is equipped with a PL-RI detector and two PLgel 5 μm MIXEDD (300 × 7.5 mm) columns from Varian. Thermal Gravimetric Analysis (TGA). The thermal stability of the films was determined by Mettler Toledo TGA/DSC 851e module instrument. 3−4 mg sample was loaded into a ceramic cup and heated from 25 to 500 °C at a rate of 5 °C/min under a nitrogen atmosphere. Tensile Testing. Tensile tests conducted on the materials were performed using an INSTRON 5944 module equipped with pneumatic grips. The measurements were performed using a load cell with a maximum load of 500 N at a crosshead speed of 20 mm/min. Films were cut into strips with a thickness of 0.23 ± 0.01 mm, width of 5 mm, and a gauge length of 20 mm was used. The samples were preconditioned prior to being tested following the standard method described in ASTM D618−96 (40h at 50% ± 5% relative humidity and 23 °C ± 1 °C). At least three specimens from each material were tested.

grafting of maleic anhydride is one of the early examples of reactive extrusion modification of polylactide (MAG-PLA), which was performed to improve the interfacial adhesion between PLA and starch.19 A partial reaction between maleic anhydride functionalized PLA and poly(ethylene glycol) (PEG) was achieved during subsequent melt-blending process leading to MAG-PLA/PEG blends where a small fraction of PEG chains were covalently attached to MAG-PLA.20 This approach gave rise to significantly lower glass transition temperature as compared to the corresponding physical blend of PLA/PEG. Probably it also improved the migration resistance even though this was not investigated. However, there was no significant effect on viscoelastic and viscoplastic mechanical behavior. In another study the in situ chemical grafting of PLA was achieved during reactive blending with acrylated-poly(ethylene glycol).21 The Tg temperatures gradually decreased from 59.1 to 35.4 °C when going from PLA to 40% (w/w) PEG grafted PLA. Meanwhile, the elongation at break increased from 4.7% to 17.9%, which is still a very moderate value. This could be related to the relatively high Tg, as 35 °C has been identified as critical temperature where the transition from brittle to ductile takes place for polylactide.22 Successful chemical grafting of tributyl citrate plasticizer onto MAG-PLA has also been shown.23 In this case slightly increased Tg values were recorded for the grafted materials and the effect on mechanical properties was very moderate in comparison to the corresponding physical blend. Covalent bond between endfunctionalized polylactide and conjugated soybean oil was achieved in a twin-screw extruder leading to improved tensile toughness.24 Here we unveiled a two-step process for achieving novel flexible fully biobased and sustainable materials with low plasticizer migration rate. The first step included effective material recycling of PHB to functional bioplasticizers by a simple and industrially viable process where PHB was thermally degraded in an extruder to oligomeric PHB with crotonate end groups (dPHB). The second step consisted of covalent anchoring of the obtained oligomeric plasticizer candidate onto the PLA chains by simple reactive extrusion step. In addition physical blends of PLA/dPHB were prepared through the same process but in the absence of a radical initiator.

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EXPERIMENTAL SECTION

Materials. Poly(3-hydroxybutyrate) powder, PHB, with Mn = 311 600 g/mol, Mw = 561 400 g/mol, and Đ = 1.80, was supplied by Tianan Biologic Materials Co., Ltd., and the poly(lactide), PLA, was supplied by NatureWorks PLA (5200D) with Mn = 189 100 g/mol, Mw = 289 600 g/mol, Đ = 1.53, and L-lactic acid/D-lactic acid: 96/4. The reactive extrusion initiator Luperox 101, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane organic peroxide (SigmaAldrich), and chloroform (FisherScientific) were used as received. Isothermal Degradation and “Grafting to” by Reactive Extrusion. For the thermal degradation and subsequent reactive extrusion, a DSM micro 5 compounder (Co/counter option) with corotating intermeshing conical-twin-screw and three temperature zones was used. Prior to the thermal degradation, the PHB powder was extruded at 185 °C (all zones). Retention time was set to 0 min and screw speed was 25 rpm to form PHB pellets in order to make the feeding of neat PHB easier during the thermal degradation process. The B

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Macromolecules Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). To identify the structural features for the crystalline entities developed in the compression-molded samples, 2D-WAXD measurements were carried out using a homemade laboratory instrument (Bruker NanoStar, Cu Kα-radiation) in the Crystallography Lab, Department of Molecular Biology and Biotechnology, University of Sheffield. The X-ray beam with a wavelength of 0.154 nm was focused to an area of 4 × 4 μm2, and the distance from sample to detector was held at 350 mm for WAXD measurements. An X-ray CCD detector (Model Mar 345, a resolution of 2300 × 2300 pixels, Rayonix Co. Ltd., USA) was employed to collect the 2D-WAXD images. Electrospray Ionization−Mass Spectrometry (ESI−MS). The solutions from the migration study, containing the migrated watersoluble compounds, were analyzed by ESI−MS. In addition ESI−MS was utilized to identify the methanol soluble compounds in the materials before the migration study by placing 5 mg sample into 1 mL methanol for 10 min. A Finnigan LCQ ion trap mass spectrometer (Finnigan, San Jose, CA) in positive mode was used for analysis. Ion source was operating at 4.5 kV. The capillary heater was set to 200 °C. The flow rate of the syringe pump was 5 μL/min. Nitrogen was used as nebulizing gas.

trans-crotonic acid monomer started. This is illustrated by the appearance of shoulders beside the peaks at 5.8 ppm (γ′), 6.9 ppm (β′) and 1.8 ppm (α′) corresponding to the crotonate end groups of PHB oligomers. At the same time, a small amount of cis-crotonic acid was also formed. The peak at 4.1 ppm (β″) corresponds to the proton which is next to the hydroxyl end group (OH−CH(CH3)−CH2−) and the integral of this peak remained very small. Molecular Weight of Degradation Products. The number-average molecular weight (Mn) of the degraded PHB samples, calculated from 1H NMR, is shown in Figure 3. Mn was calculated according to the following equation (β, β′, β″ and γ′ correspond peaks are shown in Figure 2b: Mn =

∫β ∫β′ + ∫γ′ 2

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× 86

+ ∫ β″

Mn decreased as a function of time and temperature. After 30 min of extrusion at 200 °C the molecular weight of the sample had decreased to approximately 15 000 g/mol. The same extrusion time at 220 °C decreased the molecular weight to approximately 3000 g/mol. The differences became less obvious at longer degradation times, but the trends were similar in all cases. 240 °C was too high to be used as extrusion temperature as the materials became too extensively degraded resulting in sticky product with pungent odor. Overall, as could be expected, at higher degradation temperature Mn decreased faster. 1/Mn increased linearly as a function of degradation time until Mn reached around 6000 g/mol, indicating a constant degradation rate. The constant degradation rate further implies that degradation proceeded through random β-elimination scission, which has been proposed as the main degradation mechanism for PHB producing oligomers with crotonate end group.25 It was noticed that after 45 min at 210 °C and after 20 min at 220 °C the slopes of the 1/Mn curves changed and the 1/Mn started increasing faster. This faster increase of 1/Mn is explained by accelerated degradation.26 Similar to the results of Nguyen et al., as chains shortened to Mn around 6000, autoacceleration of thermal degradation became significant.13 It has been proposed that the increasing scission rate is induced by the conjugation of the crotonate end groups and not by the higher concentration of carboxylic acid end groups.13,27 During degradation at 200 °C, 1/ Mn remained linear as the critical molecular weight around 6000 was not reached. Thermal Properties of Degradation Products. The thermal behavior of the extruded degradation products as a function of time and temperature shown in Figure 4 confirmed the 1H NMR results. Before the thermal degradation by extrusion the melting temperature, Tm (recorded from the second heating), was 179 °C and the glass transition temperature, Tg (recorded from the second heating), was 6.8 °C. A continuous decrease in Tm and Tg values was then observed upon prolonged degradation at 220 °C ending at Tm = 146 °C and Tg = −3.1 °C after 45 min, indicating a significant reduction in molecular weight. The observation of double melting peaks in degraded PHB, instead of a single melting peak as presented in undegraded PHB, demonstrates the coexistence of different crystallite size. This has been reported in other studies as well.28,29 For further reaction by reactive extrusion the product formed after extrusion at 220 °C for 45 min was chosen. Molecular weight characteristics for the selected oligomer as determined by 1 H NMR and SEC is provided in Table 1. The correlation

RESULTS AND DISCUSSION To create plasticized degradable bioplastics through a green and industrially viable approach, a two-step process was developed. First a process to thermally degrade PHB in an extruder was examined and optimized to create oligomeric PHB plasticizers with appropriate molecular weight and with functional endgroups suitable for further reaction. Subsequently, as a second step, a reactive extrusion method was developed to covalently bind the plasticizers to the main chain of PLA. Isothermal Degradation of PHB by Extrusion. Thermal degradation of PHB is known to result in crotonic acid and oligomeric degradation products with crotonate end groups (Scheme 1). To develop a fast, straightforward and up-scalable approach for creating such functional oligomeric degradation products, thermal degradation by extrusion was investigated. High molecular weight PHB was extruded at temperatures ranging from 200 to 240 °C, i.e., from approximately 20 to 60 °C above the melting temperature of PHB. After degradation, products showed increasingly dark brown color, Figure 1. At

Figure 1. Images of the PHB samples (a) before extrusion, (b) after pellet formation, (c) after 60 min degradation in extruder at 200 °C, and (d) after 45 min degradation in extruder at 220 °C.

regular time intervals, extruded materials were characterized with respect to molecular weight, type of products formed and thermal behavior. Degradation rate at different temperatures was evaluated as well (Figure 2−4). To determine the type and amount of degradation products formed during the extrusion process, the materials were evaluated by 1H NMR as exemplified in Figure 2. Already after 5 min degradation at 220 °C, the trans crotonate end groups of PHB oligomers were clearly seen in the 1H NMR spectrum (shifts at 5.8 ppm, (γ′), (CH3−CHCH−), at 6.9 ppm, (β′), (CH3−CHCH−) and at 1.8 ppm, (α′), (CH3−CHCH−)). The relative amount of crotonate end groups increased as a function of time indicating increasing extent of degradation. When PHB was degraded for more than 30 min, formation of C

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Figure 2. 1H NMR spectra showing the (a) formation of thermal degradation products as a function of time during extrusion at 220 °C and (b) the component of extrusion products after 45 min degradation at 220 °C.

Figure 3. (a) Decreasing number-average molecular weight and the (b) inverse of number-average molecular weight as a function of degradation time (5−60 min) at specific temperatures (200−220 °C) as determined by 1H NMR.

Reactive Extrusion of the dPHB onto the PLA Chain. To graft the produced dPHB oligomers onto the PLA chains, a reactive extrusion process initiated by a peroxide working at elevated temperatures was developed. Reactive extrusion can be counted as a green process, because it is fast and hence a low energy consuming technique that does not involve the use of solvents and limits the use of auxiliaries in general. It is presumed that the oligomers with unsaturated crotonate end groups formed during the isothermal degradation in the extruder in the first step could react with the methanetriyl carbon in the main chain of PLA, Scheme 2. To evaluate the influence of the dPHB grafting reaction on the PLA properties, corresponding physical blends were prepared by extrusion without the addition of peroxide but otherwise under

Figure 4. DSC traces of the thermal degradation products formed during extrusion at 220 °C as a function of time.

Scheme 2. Schematic Presentation of the Most Likely Grafting of the Thermally Degraded PHB with Unsaturated End Groups to the Main Chain of PLA

Table 1. Molecular Weight and Molecular Weight Distribution of dPHB after 45 min Degradation at 220 °C Mn(NMR)

Mn(SEC)

Mw(SEC)

Đ(SEC)

1600 g/mol

2200 g/mol

4600 g/mol

2.1

between the number-average molecular weight from NMR and SEC is relatively good, although the SEC value is somewhat higher due to the calibration with PS standards, which usually overestimates the molecular weight of PLA. A relatively low molecular weight oligomer was selected to ensure a high enough concentration of crotonate end groups. In addition a lower molecular weight oligomer is expected to have better plasticization effect in the final product. D

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Macromolecules Table 2. Thermal Properties of the Neat, Blended, and Grafted PLA Films from the First DSC Heating Scans PLA10B Tg (°C) Tm (°C)

PLA

PLA101

62.6 ± 0.3 146.1 ± 0.6

61.0 ± 0.7 148.2 ± 1.1

PLA20B PLA10G

45.9 ± 0.4 145.1 ± 0.2

158.9 ± 0.2

47.4 ± 0.2 140.0 ± 0.3

PLA20G 33.3 ± 0.4 132.7 ± 0.01

146.8 ± 0.3

38.1 ± 0.2 137.0 ± 0.3

it was dPHB that reduced the intermolecular forces between the PLA chains. Furthermore, dPHB is expected to increase the number of end groups and free volume. The chain mobility increases with decreasing glass transition temperature. When the thermal history was erased, PLA blends and grafted PLA showed very different thermal behaviors although the glass transition temperatures remained lower than for neat PLA. PLA10G and PLA20G both had distinct melting peaks, which were observed at 142.0 ± 0.1 and 140.0 ± 0.3 °C, respectively, while the PLA blends, PLA10B and PLA20B, had inconspicuous melting peaks. The grafted internal dPHB plasticizers led to cold crystallization during the second heating scan, while the external dPHB plasticizer did not have this effect. Several earlier studies have reported similar behavior for branched polymers. A study on crystallization of long-chain branched PLA showed that branching points acted as nucleation points leading to faster crystallization for branched polylactide as compared to linear polylactide.30,31 Thermal stability of the films, as determined by the onset temperature of degradation, decreased with increasing amount of dPHB. Similar to DSC results, PLA and PLA101 showed similar behavior, Figure 6a. The incorporation of low molecular weight dPHB in the PLA matrix could lead to a decrease in the overall thermal stability of the material and this was also observed. This might be due to early evaporation of the low molecular weight compounds or as suggested by others the PHB scission rate could be induced by the conjugation on the crotonate end groups with ester segments.13,27 The grafted materials, had a slightly higher thermal stability compared to the blended materials. PLA blends with 10% (w/w) and 20% (w/w) dPHB had onset temperatures of 259 and 256 °C, respectively, while the onset temperatures were 271 °C for PLA10G and 265 °C for PLA20G. The plasticizer content in the films was confirmed by an additional step in the TGA weight loss curves corresponding to the degradation of the plasticizer, Figure 6b. The weight loss during this step matched relatively well with the feed composition. TGA indicated that dPHB content in PLA was approximately 13% (w/ w) when the feed ratio was 10% (w/w) and 23% (w/w) when the feed ratio was 20% (w/w).

the same condition as the grafting reaction. In both cases, for grafted and blended PLA films, two compositions with 10 or 20% (w/w) of the plasticizer, dPHB, were prepared. The materials were denoted based on the type of reaction performed and their composition. For example PLA10G is a material with 10% (w/w) of dPHB grafted to a PLA main chain and PLA10B is a blend of PLA and 10% (w/w) dPHB. The reference PLA materials were extruded with (PLA101) and without peroxide (PLA) to evaluate the effect of peroxide on the PLA properties and low molecular weight product content. Thermal Behavior of PLA Films. Thermal properties of neat, blended, and grafted PLA films of varying compositions were determined both from the first and second heating scan by DSC, Table 2 and Figure 5. PLA and PLA101 had approximately

Figure 5. DSC traces for the neat, blended, and grafted PLA films from the second heating scan.

the same Tg, slightly above 60 °C, and their melt temperature was 146.1 and 148.2 °C. However, both PLA blends and grafted PLAs showed lower glass transition temperature than the neat PLAs in both heating scans. The more dPHB the film contained, the lower glass transition temperature it had, indicating miscible blends. The decrease in glass transition temperature was about 15 °C when 10% (w/w) dPHB was added and 25 °C when 20% (w/w) dPHB was added. Altogether this indicates that the peroxide did not change the thermal properties of neat PLA, but

Figure 6. Thermal stability of the PLA films. E

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Macromolecules Crystalline Structure of PLA Induced by Grafted dPHB. Two-dimensional wide-angle X-ray diffraction was utilized to identify the structural features for the crystalline entities developed in PLA, PLA101, PLA10/20B, and PLA10/20G, as shown in Figure 7. Interestingly, blending and grafting of dPHB

peaks were observed for PLA10G and PLA20G. Typical peaks located at 2θ = 14.8°, 16.7°, 19.0°, and 22.3° are assigned to the lattice planes (010), (200)/(110), (203), and (015) of α-crystal form of PLLA, respectively.32 No diffraction peaks deduced to PHB could be observed. This evidence that the crystallinity originates from crystallization of PLA chains and the PHB grafts will be located in the amorphous regions. According to 1DWAXD intensity profiles, PLA10G was characterized by a relatively high crystallinity of 26.5%, which was further enhanced to 41.2% for PLA20G. This indicates that well dispersed dPHB grafts enhanced and accelerated the crystallization of PLA main chains, while blended dPHB oligomers did not have this capacity. Mechanical Properties of PLA films. Figure 8 illustrates the mechanical properties of the prepared PLA films. Blending

Figure 8. Mechanical properties of the PLA-based grafted and blended films. (a) Modulus. (b) Stress at break. (c) Elongation at break.

PLA with 10% (w/w) dPHB decreased the elongation and tensile stress from 8% and 51 MPa to 4% and 43 MPa. PLA10G showed similar, but slightly higher elongation and tensile stress of 11% and 45 MPa, respectively. When 20% (w/w) dPHB was added in the PLA matrix by either blending or grafting, elongation increased significantly. In particular, PLA20G had a remarkably high elongation of 538%, which is 66 times higher than the elongation of PLA. At the same time PLA20G exhibited exceptionally high crystallinity of 41.2%. Elongation of PLA20B was 380%, which is also a large increase from PLA10B, but still significantly inferior to the elongation of the grafted analogue PLA20G. Interestingly, PLA20G demonstrated not only higher elongation, but also higher modulus (1124 MPa) and higher stress at break (30 MPa) than PLA20B, which had a modulus of 866 MPa and stress at break of 21 MPa. Addition of 20% (w/w) dPHB through grafting or blending, thus, significantly improves the elongation at break of PLA and it is evident that the grafted dPHB exhibited even higher plasticization efficiency than the blended dPHB. In comparison with previous studies on grafted PLA and PLA−PHB copolymers, the obtained improvement is exceptional. As an example Choiet al.21 grafted acrylated PEG on PLA by extrusion. 20% (w/w) grafted PEG increased the elongation at break from 4.65% to 9.54% and decreased the tensile modulus from 1293 to 884 MPa. In another study the addition of 20% (w/w) PEG into PLA matrix increased the elongation at break from 3% to 260%, while the modulus was decreased from 2500 MPa to 150 MP. However, stress at break

Figure 7. 2D-WAXD patterns and 1D-WAXD intensity profiles extracted from the corresponding 2D-WAXD patterns for PLA, PLA101, PLA10/20B and PLA10/20G.

both significantly improved the chain mobility and decreased the Tg of PLA, but grafted dPHB also greatly increased the ability of PLA to crystallize. This is explained by increasing crystallization rate due to the grafting points that acted as nucleation points, which is also supported by earlier studies on, e.g., long-chain branched polylactide.30,31 dPHB-grafted PLA, PLA10G, and PLA20G, exhibited strong diffraction rings, attributed to the lattice planes (010), (200)/(110), (203), and (015) of PLLA αcrystal. However, only a weak, amorphous halo was observed in PLA, PLA101, PLA10B, and PLA20B. On the basis of 2DWAXD, 1D-WAXD intensity profiles were extracted. All the materials except PLA10G and PLA20G showed a broad amorphous peak, but interestingly four distinct diffraction F

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Figure 9. ESI−MS mass spectra of low molecular weight compounds extracted from original dPHB and PLA.

Figure 10. ESI−MS mass spectra after 2 weeks of hydrolysis at 60 °C (o-LA, red and purple rectangle; dPHB, green oval).

for the PEG blends was only 4 MPa.33 Aluthgeet al.34 synthesized triblock copolymer, PLLA−PHB−PLLA, with 5% (w/w) to 28% (w/w) elastomeric atactic PHB. Longer block of PHB decreased tensile strength and increased elongation at break of the triblocks. Because of phase separation, elongation at break of the copolymer with 28% PHB only increased moderately to 21%. Hydrolytic Degradation and Migration Testing. The hydrolytic degradation and migration behavior of PLA20B and PLA20G were fingerprinted. PLA and PLA101 were included for comparison. To identify the low molecular weight compounds originally present in PLA and dPHB, the methanol soluble compounds were extracted and analyzed by ESI−MS, Figure 9. Peaks corresponding to Na+ adducts of cyclic lactic acid oligomers, (m/z = 72n + 23) (marked with yellow rectangle) and adducts with Na+ and methanol (m/z = 72n + 23 + 32) (marked with pink rectangle) were detected in PLA. Same cyclic

oligomers were extracted from PLA101. Corresponding cyclic oligomers were also detected in previous study, where similar extraction was performed on solution casted PLA films.35 In the methanol extracts of dPHB the main series of peaks were the Na+ adducts of the low molecular weight linear dPHB oligomers with crotonate end groups (m/z = 69 + 86n + 17 + 23) (marked with green oval). In addition two additional series of oligomers with two (marked with purple oval) or three (marked with gray oval) sodiums were detected. No distinct degradation product peaks were detectable in any of the materials after 1 week of degradation. After 2 weeks of degradation, Figure 10, no degradation products or migrant were detected in the case of PLA, while PLA101 and PLA20G exhibited similar degradation product patterns. The main series of peaks in the mass spectra of PLA101 and PLA20G migrants were adducts of linear lactic acid oligomers [o-LA + 2Na]+ G

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Figure 11. ESI−MS mass spectra after 3 weeks of hydrolysis at 60 °C (o-LA, red, purple, and yellow rectangle; dPHB, green oval; o-PHB, blue oval).

Figure 12. ESI−MS mass spectra after 4 weeks of hydrolysis at 60 °C (o-LA, red, purple, and yellow rectangle; dPHB, green oval; o-PHB, blue oval).

(marked with a red rectangle) and [o-LA + 3Na + H2O]+ (marked with a purple rectangle). No unreacted dPHB or dPHB hydrolysis products were observed among the PLA20G degradation products. The earlier detection of o-LA in the case

of PLA101 and PLA20G in comparison with PLA can be due to some degradation taking place during the extrusion in the presence of free radical initiator. In addition, the hydrolysis temperature was above the Tg of PLA20G, which could enhance H

DOI: 10.1021/acs.macromol.5b00235 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the hydrolysis rate compared to PLA and PLA101 that were both aged at approximately Tg temperature. This accelerating effect on PLA20G hydrolysis is on the other hand counteracted by the higher crystallinity of PLA20G material. After 2 weeks of hydrolysis the same linear low molecular weight o-LA were also detected as degradation products from PLA20B. In this case the degradation of PLA matrix could be accelerated, in comparison to PLA, as the hydrolysis temperature was above the Tg of PLA20B. It was particularly interesting that dPHB oligomers appeared among the water-soluble migrants, while they were absent from the product patterns of PLA20G. This shows that dPHB oligomers had started to migrate from the PLA matrix already after 2 weeks in aqueous environment. Together with the low Tg, this could further facilitate the water permeation and accelerate the hydrolysis of PLA. The degree of degradation had significantly increased after 3 weeks (Figure 11). Na+ adducts of o-LA [o-LA + Na]+ (marked with yellow rectangle) were now detected in all the materials. However, interesting differences in the degradation product patterns and relative intensities of the different products were observed. o-LA were the most dominant products in the mass spectra containing the water-soluble degradation products and migrants from PLA, PLA101 and PLA20G, while dPHB oligomers were the most dominant products in the mass spectra of PLA20B, further evidencing the relatively fast migration of dPHB from the physical blends. The ESI−MS spectra after 3 weeks also showed that the dPHB grafts in PLA20G had started to hydrolyze. PHB oligomers with hydroxyl and carboxylic acid end groups, [o-PHB + Na]+, (marked with blue oval) were observed in the mass spectra of PLA20G, but the intensity was relatively low. A small amount of dPHB oligomers with crotonate end groups was now also detected among the degradation products of PLA20G, indicating small amount of unreacted dPHB in the grafted materials. After 4 weeks of hydrolysis (Figure 12), there were no changes in the identified water-soluble products, but the relative intensities of the water-soluble migrants from PLA20B and PLA20G had changed. The migrated dPHB oligomers were even more clearly the dominating products in the mass spectra of water-soluble products from PLA20B. At the same time the relative intensities of the lower molecular weight PHB oligomers had increased over the longer ones indicating some further hydrolysis to lower molecular weight oligomers. After 4 weeks of hydrolysis, o-PHB were the most dominant products in the mass spectra of PLA20G aging solutions. PLA20G had high crystallinity for the PLA part, which protects the PLA chains against hydrolytic degradation. The PHB grafts on the other hand are expected to be located in the amorphous regions, which was also supported by the 2D-WAXD results presented above. This will make the dPHB grafts more susceptible to hydrolysis, in comparison to the highly crystalline PLA matrix, leading to over presentation of o-PHB among the degradation products, if compared to the original composition of the material. The hydrolytic degradation study further confirmed the successful grafting of dPHB onto PLA chains. In addition to superior mechanical properties grafting also improved the migration resistance of the dPHB plasticizers. Weight loss of all the materials was followed during the hydrolytic aging. After 4 weeks of degradation, PLA20B and PLA101 had the highest weight loss followed by PLA20G (Figure 13), while PLA had the slowest degradation rate. The hydrolytic aging temperature, 60 °C, was around PLA’s Tg, while it was much higher than the Tg of the PLA20B and PLA20G,

Figure 13. Weight loss for PLA, PLA101, PLA20B, and PLA20G during hydrolysis at 60 °C.

which probably explains at least partly the differences in degradation rate. PLA started to degrade from 3 weeks which is consistent with the ESI−MS results. Extrusion of PLA101 in the presence of free radical initiators might have caused chain scissions and formation of some shorter oligomers, which could explain the increased weight loss compared to PLA extruded without free radical initiators. PLA20B and PLA20G followed the same trend however they had highly different degradation products patterns according to the ESI−MS results. Both materials were aged above Tg, which is expected to accelerate the degradation rate. PLA20B had fast weight loss due to the migration of dPHB. The additional carboxylic acid groups in the dPHB oligomers might have further accelerated the hydrolysis process of PLA. No dPHB migrated from PLA20G during the first 2 weeks, but the higher concentration of carboxylic acid group and the presence of branching as well as the dPHB grafts being in the amorphous regions might have increased the hydrolysis rate of the amorphous regions leading to migration of PLA and PHB oligomers.

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CONCLUSION A viable recycling route for PHB and a highly promising approach for fully biobased and biodegradable toughened PLA through covalently anchored oligomeric PHB plasticizers were demonstrated. After 45 min at 220 °C, PHB was thermally degraded in an extruder to well-defined dPHB with molecular weight of 1600 g/mol. The dPHB was then successfully grafted onto PLA chains by reactive extrusion. PLA with 20% (w/w) grafted dPHB demonstrated impressive ductility and good toughness. The elongation at break was approximately 66 times higher than that of pure PLA. Grafting significantly improved the crystallization rate of PLA and the crystallinity of the originally amorphous PLA increased to 26.5% and 41.2% for PLA10G and PLA20G, respectively. The PLA blends with the same dPHB contents remained amorphous. Moreover, WAXD of dPHB grafted PLA illustrated α-crystal form of PLLA with typical lattice planes (010), (200)/(110), (203), and (015) giving clear evidence that the crystallinity originated from crystallization of PLA and the dPHB grafts remained amorphous. In the case of the physical blends dPHB started to migrate out already after 2 weeks of hydrolytic aging. At this time point only lactic acid oligomers had migrated from the PLA with grafted dPHB. On the other hand, the dPHB grafts, susceptible to hydrolysis due to location in the amorphous regions, started to hydrolyze after 3 weeks leading to release of 3-hydroxybutanoic acid oligomers. These I

DOI: 10.1021/acs.macromol.5b00235 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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differences in the migrant and degradation product patterns further confirmed the successful grafting of dPHB onto PLA chain.

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AUTHOR INFORMATION

Corresponding Author

*(M.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement No. 311815.

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DOI: 10.1021/acs.macromol.5b00235 Macromolecules XXXX, XXX, XXX−XXX