and Biodegradable Thermoplastic Elastomers - ACS Publications

17 May 2017 - ranging from 0 to 9 mol %. The degradation .... Polymer films containing 0.1 wt .... retention time while maintaining monomodal peaks (F...
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Photo- and Biodegradable Thermoplastic Elastomers: Combining Ketone-Containing Polybutadiene with Polylactide Using RingOpening Polymerization and Ring-Opening Metathesis Polymerization Kyle J. Arrington, John B. Waugh, Scott C. Radzinski, and John B. Matson* Department of Chemistry and Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: We report the synthesis of a new bio- and photodegradable ABA triblock copolymer that combines the high modulus of biodegradable poly(lactic acid) (PLA) with a novel photodegradable polybutadiene (PB). First, the central block was made by ring-opening metathesis polymerization (ROMP) of a mixture of 1,5-cyclooctadiene and an unsaturated cyclic ketone in the presence of cis-1,4-butenediol. The resulting hydroxyl-functionalized telechelic PB macroinitiator was then chain-extended by DBU-catalyzed ring-opening polymerization (ROP) of lactide. Using this combination of ROMP and ROP, the molecular weights of the A and B blocks were varied, and the resulting copolymers formed robust films with rubbery plateau moduli ranging from 180 to 1600 MPa as measured by dynamic mechanical analysis (DMA). Thermal analysis revealed that the polymers were thermally stable up to 273 °C and exhibited glass transition temperatures at −111 and 55 °C. Accelerated UV weathering was performed on thin polymer films with concentrations of the photodegradable monomer in the PB backbone ranging from 0 to 9 mol %. The degradation occurred through Norrish photocleavage and was tracked by IR spectroscopy, size exclusion chromatography, and DMA until complete mechanical loss.

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lengths of the hard and soft blocks.12 The most common topology for TPEs is symmetric ABA triblock polymers, where two hard segments are attached to the ends of a soft middle segment. This topology leads to microphase separation in the bulk, where the hard segments serve to physically cross-link the soft polymer matrix. The Hillmyer group and others have demonstrated that the toughness of PLA can be improved by synthesizing TPEs containing a polybutadiene (PB) soft segment and two PLA hard segments.13−16 These polymers represent an important step forward in improving the properties of PLA, but they suffer from a lack of degradability from the PB middle block. We envisioned that a photodegradable soft block could allow for complete degradation of a PLA-based ABA triblock copolymer. One class of photodegradable polymers is aliphatic polyketones, which degrade under UV light through Norrish reactions.17,18 Norrish cleavage reactions result in continuous chain scission of the polyketone until the polymer degrades enough to completely lose its mechanical properties. Weathering studies have been conducted to follow the photodegradation of polyketones, which are typically synthesized by

INTRODUCTION Plastics that are both renewable and biodegradable have been posed as alternatives to legacy petroleum-based commodity polymers. Poly(lactic acid) (PLA) is an attractive renewable bioplastic due to its biocompatibility1 and biodegradation.2,3 The monomer is made by the cyclization of lactic acid, which is commonly obtained from renewable sources such as glucose, sucrose, or lactose.4 PLA is used commercially in a variety of products, but its widespread use is limited by its brittleness.5 To solve this problem, researchers have investigated several methods to improve the properties of PLA, including copolymerization,6−8 plasticization,9 and blending.10,11 Of these methods, copolymerization has great potential for achieving mechanically robust materials while maintaining biodegradability. Copolymerization is widely used to design materials that maintain desirable properties of two homopolymers but minimize their deficiencies. For example, block copolymerization of styrene and butadiene is used to make SBS rubber, a type of thermoplastic elastomer (TPE). TPEs are block(y) copolymers that contain hard polymer (thermoplastic) segments that flank either side of the soft polymer (elastomer) segment(s). These polymers gain their mechanical properties as a sum of both the soft and hard segments, making them versatile materials with properties depending on the relative © XXXX American Chemical Society

Received: March 5, 2017 Revised: April 30, 2017

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Macromolecules Scheme 1. Synthesis of PLA-b-P(B-co-COK)-b-PLA by ROMP Followed by ROP

the copolymerization of carbon monoxide and ethylene.17−19 As little as 0.5% incorporation of carbon monoxide (CO) allowed for degradation of mechanical properties after 20 weeks, while increasing the incorporation to 1.5% led to complete degradation of mechanical properties after just 4 weeks.18 These materials were commercialized in the 1970s to make Hi-Cone photodegradable six-pack rings that are still used today. Although not widely used outside of ethylene/CO copolymers, we hypothesized that small amounts of aliphatic ketones could potentially provide photodegradability to a wide variety of polymers without dramatically affecting mechanical or thermal properties. This concept could enable the preparation of new polymers that degrade when exposed to direct sunlight but have an indefinite shelf life in the absence of UV radiation. In this contribution, we focus on synthesizing and evaluating photodegradable PLA-based TPEs, with the goal of preparing a new family of ABA triblock copolymers. We envisioned that these polymers could be synthesized by incorporating a ketonecontaining comonomer into the ring-opening metathesis polymerization (ROMP) of cyclooctadiene (COD) in the presence of a chain transfer agent that would allow for the ringopening polymerization (ROP) of lactide.

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Dawn Heleos 2 light scattering detector and a Wyatt Optilab Rex refractive index detector. No calibration standards were used, and dn/ dc values were obtained by assuming 100% mass elution from the columns. Thermogravimetric analysis (TGA) of the polymers was carried out with a TA-Q50 under a dry nitrogen purge (40 mL/min for the balance and 60 mL/min for the sample) from room temperature to 600 °C at a heating rate of 10 °C/min. Differential scanning calorimetry studies (DSC) were carried out on a Q-2000 DSC in aluminum pans operated with a dry nitrogen purge from −150 to 70 °C with a heating and cooling rate of 10 °C/min. Dynamic mechanical analysis (DMA) experiments were carried out with a TA Instruments DMA Q800 in oscillatory tension mode at 1 Hz and 3 °C/min. Atomic force microscopy (AFM) was conducted using a Veeco BioScope II AFM in tapping mode in air at room temperature using Nano World Point Probe silicon SPM sensor tips (spring constant = 42 N m−1, resonance frequency = 320 kHz). UVC Accelerated Weathering. Accelerated weathering studies were conducted in a cylindrical chamber containing a nitrogen gas inlet with an 18 W fluorescent UVC bulb with a peak emission wavelength at 280 nm. Samples were prepared by cutting polymer films into 1 cm by 3 cm rectangles and taping them at an equal height to the sides of the chamber, about 6 cm away from the outside of the bulb. A control sample of each polymer was placed in a separate amber vial that was included in the weathering chamber during all experiments. During irradiation, the chamber was placed under a constant nitrogen flow in order to remove oxygen and to maintain the temperature at 30 °C. At predetermined time points, one or more polymer films were removed and tested by IR, DMA, and SEC. Images of the UVC weathering chamber are in the Supporting Information (Figure S25). Artificial Sunlight Accelerated Weathering. Artificial sunlight weathering studies were conducted in a box with a UVB 200 26 W fluorescent bulb with an irradiance at 90 μW/(cm2 nm) at 310 nm at 20 cm from the light source as specified from the manufacturer, Exo Terra. Samples were prepared by cutting polymer films into 6 cm by 6 cm squares and placing them at the bottom of the box approximately 20 cm from the light source. A thermometer was placed in the box to monitor the temperature in the weathering chamber. The temperature was maintained at 25 °C with a small fan that was placed in the box. Twice each day, the sample films were tested for brittleness by creasing the films. After the films were tested, they were rearranged in the box to make sure each film received a similar amount of light. This procedure was continued until the films were no longer creaseable and broke to the touch. Images of the artificial sunlight weathering

EXPERIMENTAL SECTION

Materials and Methods. All reagents were obtained from SigmaAldrich and used as received unless otherwise stated. The ketonecontaining monomer, cyclooct-4-en-1-one (COK, cyclooctene ketone), was synthesized according to a previously published procedure.20 The Hoveyda−Grubbs second-generation catalyst was obtained from Ark Pharm. Dry solvents were purified by passage through a solvent purification system (MBraun). NMR spectra were measured on an Agilent 400 MHz spectrometer. 1H NMR chemical shifts are reported in ppm relative to internal solvent resonances. Yields refer to chromatographically and spectroscopically pure compounds unless otherwise stated. Polymer films containing 0.1 wt % butylated hydroxytoluene (BHT) were prepared by solution casting from a 10 wt % solution in chloroform on a leveled table and were placed under vacuum overnight. Typical film thickness from this casting method was 0.1 mm. IR spectra were obtained on a Varian 670-IR spectrometer by placing the folded film sample directly onto the IR crystal. SEC was carried out in THF at 30 °C on two Agilent PLgel 10 μm MIXED-B columns connected in series with a Wyatt B

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Macromolecules chamber and the lamp spectrum are in the Supporting Information (Figure S26).

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RESULTS AND DISCUSSION Synthesis and Thermomechanical Characterization. Our general synthetic route for preparing photodegradable PLA-based TPEs is shown in Scheme 1. Photodegradable PB midblocks were made through copolymerization of cyclooctadiene (COD) and cyclooct-4-en-1-one (cyclooctene ketone, COK) in varying ratios, which affords 1,4-PB with incorporated ketone units. COD was distilled prior to use to remove vinyl cyclohexene (Figures S1 and S2), and COK was prepared according to a previously published route.20 For all midblocks, ROMP was conducted using (H2IMes)(Cl)2Ru CH-o-OiPrPh (Hoveyda−Grubbs second-generation catalyst, HG2) in the presence of cis-butenediol, which acts as a chain transfer agent (CTA), to prepare hydroxyl ditelechelic PB-coCOK. These polymers are labeled HO-XX-OH, where XX is the Mn of the PB block and the OH indicates the hydroxyl end groups. Molecular weights were controlled by tuning the [monomer]/[CTA] ratio, and a high degree of end-group functionalization was achieved by using a [CTA]/[HG2] ratio of 200. Dispersity values (Đ) for midblocks ranged from 1.6 to 1.8, as is typical of ROMP with CTAs, a nonliving process dominated by chain transfer (Table 1). The terminal diols on

Figure 1. SEC traces of P(B-co-COK) macroinitiator and PLA-b-P(Bco-COK)-b-PLA triblock copolymers: (a) HO-10-OH (maroon line); (b) 10-10-10 (blue line); (c) 15-10-15 (red line).

lactide. Two ABA triblock copolymers were grown from each midblock through addition of either 10 or 15 kDa PLA outer blocks (Table 2). Polymers are named according to the Table 2. Characterization of Different Mn ABA Triblock Copolymers Containing 3 mol % COK sample ID

Mna (kDa)

Đ

wPLAb

storage modulusc (MPa)

15-10-15 10-10-10 15-25-15 10-25-10 15-70-15 10-70-10

46.1 33.3 58.5 47.3 100 87.8

1.3 1.3 1.6 1.6 1.5 1.5

0.76 0.67 0.55 0.45 0.32 0.22

1600 1170 774 499 218 182

Table 1. Ditelechelic P(B-co-COK) Characterization sample ID HO-10-OH HO-25-OH HO-70-OH

Mna

(kDa)

11.0 26.1 68.5

Mnb

(kDa)

10.8 25.2 69.9

Đ

% COK

1.8 1.6 1.6

3 3 3

c

cis/trans

d

3:1 3:1 3:1

Tgd (°C) −110, −110, −113, −109, −111, −108,

53 54 55 55 52 55

Absolute Mn determined by SEC in THF at 30 °C. bDetermined by H NMR end-group analysis in CDCl3. cDetermined by 1H NMR in CDCl3 by comparing the COD to COK ratio. dDetermined by 1H NMR spectroscopy by comparing integration of the trans olefin peak at 5.45−5.40 and the cis olefin peak at 5.40−5.35.

Measured by SEC in THF at 30 °C using absolute MW determined by light scattering. bMass fraction of PLA determined by SEC using the formula (Mn,triblock − Mn,PB)/Mn,triblock. cMeasured by DMA at 25 °C. dMeasured by DSC with a 10 °C heating and cooling ramp; data reported are from the second heat cycle.

the PB midblocks were then used to initiate the DBU-catalyzed ROP of DL-lactide to afford the desired ABA triblock copolymers. Because DBU-catalyzed ROP of DL-lactide is a living process, Đ values decreased to 1.3−1.6 for the ABA triblock copolymers. We first sought to determine how the weight fraction of PLA (wPLA) affects the thermomechanical properties of the final TPE. Three hydroxyl ditelechelic P(B-co-COK) midblocks containing 3 mol % of COK relative to COD were first synthesized using catalyst HG2 in toluene at 50 °C. Polymers were prepared with near-quantitative conversions with Mn values ranging from 11.0 to 68.5 kDa, as measured by size exclusion chromatography (SEC) (Table 1 and Figure 1). 1H NMR spectroscopy showed the presence of the expected methylene protons neighboring the hydroxyl end groups at ∼4.1 ppm (Figures S3−S5), and Mn values calculated by endgroup analysis were in good agreement with SEC, suggesting complete incorporation of the CTA. COK incorporation was confirmed by 1H NMR integrations, with incorporation of 3% as expected based on the feed ratios of the monomers. The ratio of cis to trans olefins for each polymer was determined by 1 H NMR spectroscopy, with a typical ratio of ∼3:1 or 75% cis (Table 1). PLA-b-P(B-co-COK)-b-PLA triblock copolymers were then prepared by addition of PLA using DBU-catalyzed ROP of DL-

rounded Mn value of each of the three blocks; for example, polymer 15-10-15 contains a soft P(B-co-COK) block of 10 kDa flanked by two hard PLA blocks of 15 kDa each. SEC chromatograms of the 10 kDa midblock and the 10-10-10 and 15-10-15 ABA triblock copolymers show a distinct shift in retention time while maintaining monomodal peaks (Figure 1). The final polymers were stored with 0.1 wt % BHT added in order to prevent cross-linking of PB. The morphology of the final ABA triblock copolymers was assessed by atomic force microscopy (AFM). AFM was performed on films casted from chloroform for the polymers described in Table 2. The images revealed microphase separation for all samples (Figure 2 and Figures S10−S14). Next, we evaluated the thermal properties of the polymers by thermogravimetric analysis (TGA). The onset of thermal degradation (5% weight loss) occurred near 280 °C for all six polymer samples (Figure 3A and Figures S20−S24). This initial loss is attributed to the thermal degradation of PLA, which is similar to literature reports.21 The second feature in the TGA trace, which occurs near 400 °C for all samples, is attributed to the degradation of the PB block, which is similar to reported degradation temperatures.22 Each polymer was also evaluated by differential scanning calorimetry (DSC), which revealed two distinct glass transition temperatures (Tg’s) at −110 and 55 °C (Figure 3B) for all samples. The lower Tg is consistent with

a

a

1

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Dynamic mechanical analysis (DMA) was then utilized to measure the thermomechanical response of the polymers (Figure 4). All six polymer samples showed similar behavior.

Figure 2. AFM height image of polymer 15-70-15 film showing phase separation between the soft PB-rich domains and the hard PLA-rich domains.

Figure 4. Thermomechanical analysis of ABA triblock copolymers. (a) DMA traces of each polymer show that storage modulus in the rubbery plateau region decreases with decreasing wPLA. Analysis was carried out in tension mode at a frequency of 1 Hz and a heating rate of 3 °C/min. (b) Tan δ curve of polymer 15-10-15 with peak maxima at −97 and 55 °C, corresponding to the onset of the rubbery plateau and polymer flow, respectively.

From −120 to −97 °C, the polymers are hard and glassy. At −97 °C, the polymers start to transition to the rubbery plateau, with a tan δ maximum at this temperature indicating the onset of long-range segmental motion of the P(B-co-COK) block (Figure 4b). The rubbery plateau then holds until 55 °C, where another tan δ maximum indicates flow of the PLA blocks. For the six different ABA triblock copolymers, the storage modulus in the rubbery plateau region (measured at 25 °C) decreases as wPLA decreases, with a maximum rubbery plateau modulus of 1600 MPa for polymer 15-10-15 and a minimum rubbery plateau modulus of 180 MPa for polymer 10-70-10. Taken together, the thermomechanical properties of these polymers are typical of TPEs based on ABA triblock copolymers that form clear and creaseable films. All polymer samples showed two thermal transitions, one for each type of block, in both the DSC and DMA traces. Thermal degradation as measured by TGA was consistent with PLA and PB homopolymers. Finally, DMA revealed a rubbery plateau for each polymer sample, and storage modulus at the rubbery plateau increased with increasing weight fraction of the hard (PLA) segment. Next, we aimed to evaluate how the

Figure 3. Thermal analysis of ABA triblock copolymers. (a) TGA trace of polymer 10-10-10 from rt to 600 °C with a heating rate of 10 °C/ min. The onset of degradation occurs at 273 °C. (b) DSC second heating trace (exo down) of polymer 10-10-10 at 10 °C/min after cooling from 70 °C at 10 °C/min. Two glass transitions are observed at −110 and 55 °C. TGA and DSC data for the other polymers are included in the Supporting Information.

highly cis-1,4-PB, while the higher Tg is in line with literature values for PLA. These two glass transitions provide a large range in which the P(B-co-COK) block is rubbery while the PLA blocks are glassy. D

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modulus of each weathered sample was measured and compared to its respective unweathered film. Sample thin films were removed every 30 min and tested until complete loss of mechanical integrity, where the films would crumble before they could be loaded onto the DMA instrument. As expected, polymer 10-150%-10 maintained its storage modulus over the duration of exposure. In contrast, the ketone-containing samples showed a steady decrease in modulus over time. The magnitude of the modulus decrease for polymers 15-103%-15, 15-106%-15, and 15-109%-15 depended on the amount of ketones in the backbone. Polymer 15-109%-15 lost 23% of its modulus in 30 min, while 15-106%-15 and 15-103%-15 only lost 11% and 7%, respectively. While the initial loss of mechanical properties depended on the amount of ketone incorporation in the midblock, by 2 h all of the ketone-containing polymers had lost about 40% of their modulus. Polymer 15-109%-15 was the first film with a complete loss of mechanical integrity, which occurred at 2 h. Polymer 15-106%-15 lost mechanical integrity after 2.5 h, while polymer 15-103%-15 lost mechanical integrity at 3 h. We speculate that as the mol % ketone in the backbone of the PB block increases, the likelihood of chain scission increases. Initial chain scission along the P(B-co-COK) backbone leads to a loss of mechanical integrity by causing the ABA triblock copolymers to form AB diblock copolymers. Diblock copolymer elastomeric systems lack the mechanical properties seen in ABA triblock copolymer elastomers, which causes the modulus of the film to decrease.24 As more chains in the film continue to form AB diblock copolymers, the mechanical properties continue to decrease until the film becomes brittle to the touch. To establish whether Norrish-type fragmentation was leading to chain scission, weathered films were examined by attenuated total reflectance IR spectroscopy. A growing broad peak ranging from 3550 to 3200 nm was observed in the spectrum as UV irradiation time increased (Figure 6). This peak is

mechanical properties of these polymers changed upon degradation of the midblocks through photolysis. Weathering Studies. In ethylene/CO copolymers, UV irradiation triggers Norrish cleavage events that degrade the polymer backbone, leading to loss of mechanical properties. Higher CO incorporation leads to faster loss of mechanical integrity.17−19 To study the effect of weathering through exposure to UV irradiation on the PLA-b-P(B-co-COK)-b-PLA ABA triblock copolymers synthesized here, we prepared three new P(B-co-COK) midblock polymers with varied COK mol %. We chose to keep the Mn of the midblock consistent at 10 kDa for these studies to maintain a high modulus, allowing for a wide range of moduli to be observed before complete loss of mechanical properties. In addition to the PLA-b-P(B-co-COK)b-PLA ABA triblock copolymer containing 3 mol % COK already discussed (Table 2, sample ID 15-10-15, referred to here as 15-103%-15), midblocks were also prepared with 6 mol % COK, 9 mol % COK, and 0 mol % COK as a control. The COK concentrations for the 6% and 9% were confirmed by 1H NMR (Figures S6 and S7). ROP of DL-lactide from these midblocks, initiated by the terminal alcohols, afforded the final ABA triblock copolymers: 15-100%-15, 15-103%-15, 15-106%-15, and 15-109%-15. Accelerated weathering was performed on the four polymers containing different mol % COK in a home-built weathering chamber (Figure S25) equipped with a UVC florescence bulb with a peak emission at 280 nm. The weathering chamber provides more intense light than ambient sunlight, which enables fast degradation and rapid comparison between samples. Studies were performed under a constant nitrogen flow, which allowed for a steady temperature to be maintained at 30 °C. Nitrogen flow also prevented oxygen from triggering possible unwanted chain scission and/or cross-linking of PB.23 Samples were prepared by cutting out 1 cm by 3 cm rectangular solution-cast films and taping them along the cylindrical wall of the weathering chamber at a known height in order to make sure each film was receiving the same amount of irradiation. Sample films were then irradiated for a total of 4 h, removing samples for analysis every 30 min. Degradation of the films during the weathering studies was first tested by DMA (Figure 5 and Tables S1−S4). The storage

Figure 6. IR spectra of films of polymer 15-109%-15 after weathering from 0 to 2.5 h with 0.5 h increments. The arrow indicates the appearance of the enol O−H stretch.

attributed to the O−H stretch of the enol that forms during the Norrish type II photoprocess (Scheme 2). This result was expected since previous reports established that ketonecontaining polymers in the rubbery state typically undergo Norrish type II cleavage events.25,26 Furthermore, because the ketones have a neighboring δ−ε unsaturation, Norrish type II cleavage is likely thermodynamically favored over Norrish type I cleavage through formation of a vinyl radical.26 Finally, to measure the effect of irradiation on molecular weight, irradiated polymer films removed from the weathering chamber at various time points were dissolved in THF and

Figure 5. Retention of storage modulus vs irradiation time for polymers 15-100%-15 (black), 15-103%-15 (yellow), 15-106%-15 (blue), and 15-109%-15 (red). Storage modulus was measured by DMA in tension mode at a frequency of 1 Hz held at 25 °C. Triangles indicate the last film measured before complete loss of mechanical integrity. Modulus values are included in the Supporting Information (Table S1−S4). E

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Table 3. Final Mn of UVC Weathered Triblock Copolymers

Scheme 2. Norrish Type II Elimination along the PB Backbone

sample ID

Mn of PBa (kDa)

Mn of triblocka (kDa)

theor of final Mnb (kDa)

exptl Mna (kDa)

15-103%-15 15-106%-15 15-109%-15

10.8 10.7 11.2

46.1 46.7 46.0

20.4 19.5 18.5

22.3 19.4 19.2

Measured by SEC in THF at 30 °C using absolute MW determined by light scattering. bDetermined using the formula (Mn,triblock − Mn,PB)/2 + (Mn,PB/X + 1) where X is the theoretical number of ketones per chain. a

evaluated by SEC. As expected, polymer 15-100%-15 showed no change in retention time, peak shape, or Mn as a result of UVC exposure (Figure 7a). For the ketone-containing polymers, SEC chromatograms of polymer films removed from the chamber after 30 and 60 min showed the expected increase in retention time indicative of chain scission; however, some high molecular weight species were also observed for these polymers (Figure 7b−d). We attribute these low retention time peaks to polymer−polymer oligomerization/cross-linking reactions that occur through addition of radicals formed during the Norrish II mechanism to the backbone olefins in the P(B-co-COK) midblock. However, by 2 h only low-Mn peaks were observed. This indicates that enough chain scission had occurred along the polymer backbone to degrade the oligomerized/crosslinked polymer chains to the final lower Mn polymers. For all three ketone-containing samples, the final Mn of the degraded polymers (Figure 7, red traces) corresponded to their final theoretical Mn (Table 3). With random ketone placement along the backbone of the PB segment, fully degraded polymer

samples should contain only PLA blocks with short PB fragments attached. The measured Mn values are consistent with this conclusion. Weathering the ABA triblock copolymers under intense UVC light allowed for a rapid understanding of polymer degradation; however, this method does not give a clear understanding of the time it takes for the polymers to degrade under ambient sunlight. We expected that accelerated weathering under artificial sunlight would allow for a direct interpretation of the time needed to degrade the PB segments in the ABA triblock copolymers. Sample films of polymers 15100%-15, 15-103%-15, 15-106%-15, and 15-109%-15 were prepared and weathered under irradiation from a UVB200 terrarium lamp. This lamp mimics the relative intensities of UV and visible light from the sun. The UV irradiance for the lamp is 90 μW/(cm2 nm) at 310 nm, corresponding to ∼4.5 times higher intensity than ambient sunlight at 310 nm on the surface of the

Figure 7. SEC traces of weathered samples over a 2 h period showing a decrease in Mn: (a) 15-10-15; (b) 15-103%-15; (c) 15-106%-15; (d) 15-109%15. F

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Macromolecules Earth.27,28 Sample films were placed under the lamp and periodically tested for degradation by creasing the films. Once films broke upon creasing, they were considered degraded. Polymer 15-100%-15 showed no loss in mechanical properties and remained a creaseable film over the course of 40 days of constant light exposure. As expected based on the accelerated weathering studies discussed earlier, polymer 15-109%-15 was the first to shatter upon creasing, maintaining its mechanical integrity for 14 consecutive days under the lamp. Polymer 15106%-15 broke after 19 days while polymer 15-103%-15 was the last film to break, maintaining its mechanical integrity for 29 days. On the basis of the intensity of the terrarium bulb and the fact that sunlight is not present at night, we estimate that the films would last for approximately 4−8 months outside.

(3) Shi, B.; Palfery, D. Temperature-dependent polylactic acid (PLA) anaerobic biodegradability. Int. J. Environ. Waste Manage. 2012, 10, 297−306. (4) Nampoothiri, K. M.; Nair, N. R.; John, R. P. An overview of the recent developments in polylactide (PLA) research. Bioresour. Technol. 2010, 101, 8493−8501. (5) Rasal, R. M.; Hirt, D. E. Toughness decrease of PLA-PHBHHx blend films upon surface-confined photopolymerization. J. Biomed. Mater. Res., Part A 2009, 88, 1079−1086. (6) Fukuzaki, H.; Yoshida, M.; Asano, M.; Kumakura, M.; Mashimo, T.; Yuasa, H.; Imai, K.; Hidetoshi, Y. Synthesis of low-molecularweight copoly (l-lactic acid/ε-caprolactone) by direct copolycondensation in the absence of catalysts, and enzymatic degradation of the polymers. Polymer 1990, 31, 2006−2014. (7) Fukuzaki, H.; Aiba, Y.; Yoshida, M.; Asano, M.; Kumakura, M. Synthesis of biodegradable poly (L-lactic acid-co-D, L-mandelic acid) with relatively low molecular weight. Makromol. Chem. 1989, 190, 2407−2415. (8) Kylmä, J.; Seppälä, J. V. Synthesis and characterization of a biodegradable thermoplastic poly (ester-urethane) elastomer. Macromolecules 1997, 30, 2876−2882. (9) Martin, O.; Averous, L. Poly (lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer 2001, 42, 6209−6219. (10) Broz, M.; VanderHart, D. L.; Washburn, N. Structure and mechanical properties of poly (D, L-lactic acid)/poly (ε-caprolactone) blends. Biomaterials 2003, 24, 4181−4190. (11) Iannace, S.; Ambrosio, L.; Huang, S.; Nicolais, L. Poly (3hydroxybutyrate)-co-(3-hydroxyvalerate)/poly-L-lactide blends: Thermal and mechanical properties. J. Appl. Polym. Sci. 1994, 54, 1525− 1535. (12) Odian, G. Principles of Polymerization; Wiley-Interscience: New York, 2004; Vol. 3. (13) Pitet, L. M.; Hillmyer, M. A. Combining Ring-Opening Metathesis Polymerization and Cyclic Ester Ring-Opening Polymerization To Form ABA Triblock Copolymers from 1, 5-Cyclooctadiene and d, l-Lactide. Macromolecules 2009, 42, 3674−3680. (14) Kim, N.-Y.; Yun, Y. S.; Lee, J.-Y.; Choochottiros, C.; Pyo, H.-r.; Chin, I.-J.; Jin, H.-J. Enhanced impact properties of polylactide by poly (lactide-b-butadiene-b-lactide) triblock copolymer. Macromol. Res. 2011, 19, 943−947. (15) Jiao, M.; Yang, K.; Pan, W.; Zhang, W.; Xie, S. Preparation and characterization of poly (L-lactic acid)/polybutadiene block copolymers. J. Macromol. Sci., Part B: Phys. 2010, 49, 886−896. (16) Jiao, M.; Yang, K.; Zhang, W.; Pan, W.; Zhang, R.; Xie, S. Designing and characterization of biodegradable multiblock poly (Llactic acid)/polybutadiene elastomers. J. Macromol. Sci., Part B: Phys. 2011, 50, 2103−2115. (17) Hartley, G. H.; Guillet, J. Photochemistry of ketone polymers. I. Studies of ethylene-carbon monoxide copolymers. Macromolecules 1968, 1, 165−170. (18) Bremer, W. P. Photodegradable Polyethylene. Polym.-Plast. Technol. Eng. 1982, 18, 137−148. (19) Drent, E.; Budzelaar, P. H. Palladium-catalyzed alternating copolymerization of alkenes and carbon monoxide. Chem. Rev. 1996, 96, 663−682. (20) Arrington, K. J.; Murray, C. B.; Smith, E. C.; Marand, H.; Matson, J. B. Precision Polyketones by Ring-Opening Metathesis Polymerization: Effects of Regular and Irregular Ketone Spacing. Macromolecules 2016, 49, 3655. (21) Babanalbandi, A.; Hill, D. J. T.; Hunter, D. S.; Kettle, L. Thermal stability of poly (lactic acid) before and after γ-radiolysis. Polym. Int. 1999, 48, 980−984. (22) Brazier, D. W.; Schwartz, N. V. The effect of heating rate on the thermal degradation of polybutadiene. J. Appl. Polym. Sci. 1978, 22, 113−124. (23) Scott, G.; Wiles, D. M. Programmed-life plastics from polyolefins: a new look at sustainability. Biomacromolecules 2001, 2, 615−622.

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CONCLUSION In summary, we have described the synthesis of a bio- and photodegradable TPE using a combination of ROMP and ROP. The key component is the aliphatic ketone-containing monomer in the soft block, which allows for photodegradation of this family of polymers. Accelerated weathering studies confirmed photodegradation, and IR spectroscopic data indicated that degradation occurred through Norrish type II photocleavage. DMA of the weathered films showed that polymers with higher concentrations of backbone ketones lost their mechanical properties with less radiation time, while films without ketones did not lose their mechanical properties under the testing conditions. We expect that this strategy of aliphatic ketone incorporation can be used widely to enable photodegradation in other polymer types.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00479. Full characterization data and additional AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.B.M.). ORCID

John B. Matson: 0000-0001-7984-5396 Funding

This work was supported in part by the NSF (DMR-1454754) and in part by the Virginia Tech Department of Chemistry. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Joseph M. Dennis for help with thermomechanical testing and the ICTAS Nanoscale Characterization and Fabrication Laboratory for help with AFM.

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REFERENCES

(1) Conn, R.; Kolstad, J.; Borzelleca, J.; Dixler, D.; Filer, L.; LaDu, B.; Pariza, M. Safety assessment of polylactide (PLA) for use as a foodcontact polymer. Food Chem. Toxicol. 1995, 33, 273−283. (2) Kale, G.; Auras, R.; Singh, S. P.; Narayan, R. Biodegradability of polylactide bottles in real and simulated composting conditions. Polym. Test. 2007, 26, 1049−1061. G

DOI: 10.1021/acs.macromol.7b00479 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (24) Noshay, A.; McGrath, J. E. Block Copolymers: Overview and Critical Survey; Elsevier: 2013. (25) Heskins, M.; Guillet, J. Photochemistry of ketone polymers. III. Energy transfer in ethylene-carbon monoxide polymers. Macromolecules 1970, 3, 224−231. (26) Wagner, P. J. Type II photoelimination and photocyclization of ketones. Acc. Chem. Res. 1971, 4, 168−177. (27) Standard Practice for Fluorescent Ultraviolet; Exposure of Photodegradable Plastics. ASTM International, 2014. (28) Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials. ASTM International, 2012.

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DOI: 10.1021/acs.macromol.7b00479 Macromolecules XXXX, XXX, XXX−XXX