Polypropylene Blends

Article pubs.acs.org/IECR

Reactive Compatibilization of Polylactide/Polypropylene Blends Yuewen Xu,† Jesse Loi,‡ Paula Delgado,† Vasily Topolkaraev,§ Ryan J. McEneany,§ Christopher W. Macosko,*,‡ and Marc A. Hillmyer*,† †

Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States § Kimberly-Clark Corporation, Corporate Research and Engineering, Neenah, Wisconsin 54956, United States S Supporting Information *

ABSTRACT: Polylactide (PLA) was melt blended with either polypropylene (PP) or a polypropylene based elastomer (PBE, Vistamaxx) in an effort to improve its mechanical properties. An ethylene−glycidyl methacrylate−methyl acrylate terpolymer (PEGMMA, Lotader) was utilized as compatibilizer through coupling to the end groups of PLA. Graft copolymers formed enhanced the adhesion between PLA and polyolefin phases and lowered the interfacial tension. The morphological, mechanical, and rheological properties of the PLA/polyolefin compatibilized blends were investigated, and the blends exhibited substantial improvement in elongation at break and tensile toughness as compared to the corresponding binary blends. The remarkable efficacy of PEGMMA as a reactive compatibilizing agent allows the bridging of two immiscible but important classes of thermoplastics, polylactide and polypropylene, and the production of ductile PLA/PP blend materials.

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INTRODUCTION Polylactide (PLA) has been largely applied as an environmentally friendly but relatively brittle thermoplastic. General methodologies for toughening polylactide materials include the introduction of a rubbery domain in block copolymers,1,2 plasticization with a miscible component,3−5 or blending with an immiscible rubbery material.6−11 In blends containing an immiscible rubbery component, compatibilization is usually required to achieve a fine dispersed morphology, resistance to phase coalescence, interfacial adhesion, and enhanced mechanical properties. Typical strategies for blend compatibilization include the use of premade graft or block copolymers,12−14 utilization of nonbonding interactions,15,16 and reactive compatibilization.17−19 Various attempts have been made to compatibilize PLA and another immiscible rubbery phase, such as introducing PLA−polyolefin copolymers in melt blending as premade compatibilizers.20−22 The addition of copolymers in blends will aid the interfacial adhesion between the PLA and the rubbery component and consequent energy dissipation during deformation. For the premade compatibilizer, extra synthetic procedures will be inevitable, while an approach using reactive compatibilization can be favored for large scale production of PLA blend materials. In this case, a reactive component could be introduced during the blending that is capable of reacting with the matrix and/or dispersed phase in situ, reducing the interfacial tension and enhancing the adhesion between phases. For example, maleic anhydride grafted polyolefins and glycidyl methacrylate containing copolymers have been applied to compatibilize PLA and polyolefin blends.23−26 Polylactide/polypropylene blends are clearly of great technical interest, but rarely has work been done to compatibilize these two important classes of thermoplastics and produce a PLA/PP blend with enhanced mechanical properties. Herein, we present a PLA/polypropylene blend © 2015 American Chemical Society

compatibilized by an ethylene−glycidyl methacrylate−methyl acrylate terpolymer (PEGMMA, Lotader). Thermoplastic polypropylene and a polypropylene based elastomer, PBE (Vistamaxx), were selected as the dispersed phase (10 wt %) due to their semicrystalline, tough, and flexible nature, as well as their commercial significance.27 A small amount of reactive component (0.5−5 wt %), PEGMMA, was utilized. The epoxide group in the backbone can react with the end groups of PLA under melt processing to form graft copolymers that are interfacially active. The morphology, mechanical, and rheological properties of the compatibilized blends were evaluated.

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EXPERIMENTAL SECTION

Materials and Molecular Characterization. Commercial grade PLA was obtained from Natureworks (PLLA, 6201 with D-content of 1.4%). Commercial grades of polypropylene (PP 3155) and polypropylene based elastomer (PBE, Vistamaxx 2120) were purchased from ExxonMobil. PEGMMA (Lotader AX8900, ∼2 mol % glycidyl methacrylate content) was obtained from Arkema. Differential scanning calorimetry was performed on a TA Instruments Discovery Series DSC. Polymer molar mass distributions (PLA, PEGMMA, PBE) were measured using an Agilent 1100 Series size exclusion chromatograph (SEC) equipped with HP1047A refractive index (RI) detector using THF as eluent. Polymer samples (1 mg mL−1) were passed through the Varian PLgel Mixed C columns at 35 °C under a constant flow rate (1.0 mL min−1). Polymer molar mass distribution of polypropylene was measured using a high-temperature SEC system PL-GPC 220 (Agilent Systems) with 1,2,4-trichlorobenzene as an eluent (1.0 Received: Revised: Accepted: Published: 6108

March 9, 2015 May 12, 2015 May 15, 2015 June 3, 2015 DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114

Article

Industrial & Engineering Chemistry Research Table 1. Molecular and Thermal Characteristics of Polymers Used in This Study melt index tempa (°C) PLA PP PBE PEGMMA

230 230 230 190

melt flow rate (g per 10 min)

Mn (kg mol−1) e

15−30 36 80 6

72 52b 55e 45e

Đ

Tmb (°C)

Tgc (°C)

XEd (%)

1.95 3.30 2.00 5.01

168 160 160 65f

59 2 −29 −32

34 48

Temperature melt flow rate was measured at; load was 2.16 kg for all polymers. bMeasured using an RI detector on an SEC instrument with 1,2,4trichlorobenzene at 160 °C. cHeating and cooling were both at a rate of 10 °C min−1; glass transition temperature was determined from the second heating. dDegree of crystallinity. XE = (ΔHm/ΔHm0)·100%, where ΔHm0 is 209 J g−1 for PP and 93 J g−1 for PLA.28,29 eMeasured using an RI detector on an SEC instrument with chloroform at 35 °C. fBroad. a

mL min−1 at 160 °C). Polystyrene standards (Polymer Laboratories) were used for calibration of molar mass. See Table 1 for a summary of the molecular and thermal properties of the polymers used in this study. Preparation of PLA/Polyolefin Blends. Polymer blends for mechanical, morphological, and rheological studies were first dry blended at the desired ratios and then melt blended using a corotating twin-screw extruder, ZSK-30, with a diameter of 30 mm and L/D ratio of 44 manufactured by Werner and Pfleiderer Corp. The extruder had 14 barrel zones. The first barrel zone received the resins at a specified ratio via gravimetric feeder at a total throughput of 15 lb/h. The extruder screw speed was 200 revolutions per minute (rpm). The compounding temperature along the extruder zones was in the range 200−215 °C and the melt temperature was 220 °C (2 min residence time). The extruded blend was passed through a three hole strand die and quenched on a fan-cooled conveyor belt. Strands were cut into pellets using a Conair pelletizer. For the epoxide conversion study, polymer blends were prepared using a recirculating, conical twin screw mixer. All components in the blends were premixed manually, introduced into the instrument simultaneously, and melt mixed at the desired temperature and time (200 rpm). The blend was then extruded and allowed to cool to room temperature. Scanning Electron Microscopy (SEM). Polymer blends were subjected to cryo-microtoming at −140 °C and sputter coated with platinum (100 Å thickness) prior to SEM analysis. Images of samples were obtained using a JEOL 6700 scanning electron microscope with an accelerating voltage of 10 kV. Mechanical and Rheological Testing. Tensile properties were investigated at ambient conditions using a Shimadzu AGSX tensile tester with an extension rate of 5 mm min−1 and under the ASTM D1708 standard. All samples were prepared as dog-bone shapes by compression molding (22 mm gauge length, 5 mm gauge width, 0.5 mm gauge thickness) and were aged at 25 °C for 18 h prior to test. The Young’s modulus (E, calculated from the slope of the initial linear portion of the stress−strain curve), yield strength (σYS), tensile strength (σTS), and elongation at break (εb) were calculated using Trapezium software. Five samples were tested for reported averages and standard deviations. Adhesion strength was assessed by T-peel tests performed at ambient conditions with the same Shimadzu AGS-X tensile tester at a peel rate of 5 mm min−1. Dynamic frequency sweeps were performed with a 25 mm parallel plate geometry using an ARES rheometer (TA Instruments, ω = 0.01−100 rad s−1 with autostrain) at 180 °C in the linear viscoelastic regime. Spectroscopic Analysis. 1H NMR spectra were recorded on a Varian Inova VI-500 spectrometer at room temperature using CDCl3 as solvent. Proton chemical shifts were referenced

to TMS (0 ppm). Fourier transform infrared (FT-IR) spectra were obtained at 4 cm−1 resolution with a Bruker Alpha-P Platinum FT-IR spectrometer equipped with a platinum attenuated total reflectance (ATR) sampling module hosting a diamond crystal (single bounce). The software used for data collection and analysis was OPUS 7.0.

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RESULTS AND DISCUSSION Reaction of Epoxide in Melt Blending. The reaction between PLA acid and/or hydroxyl end groups and epoxide usually proceeds within the time scale of melt processing effectively, as has been reported.23,30,31 The epoxide ring on the PEGMMA will undergo acid-catalyzed ring-opening reaction in the molten state with either a hydroxyl or carboxylic acid PLA end group. The carboxylic acid group likely reacts much faster. Due to the low concentration of epoxide groups along the PEGMMA backbones, no gelation was observed and the samples were completely soluble in chloroform. The efficiency of glycidyl methacrylate containing PEGMMA as a reactive compatibilizing agent was first assessed by FT-IR spectroscopy. An absorption peak at 910 cm−1 corresponded to the C−O stretching deformation of the oxirane ring and was used to monitor the reaction between the PLA and PEGMMA (Figure 1, Supporting Information, Figure S1). This band nearly

Figure 1. Proposed graft copolymer structures formed. PEGMMA composition: x:y:z ≈ 44:5:1 with about 25 glycidyl groups per PEGMMA chain on average.

disappeared completely in the PLA/PP/PEGMMA (90/10/5) and PLA/PBE/PEGMMA (90/10/5) blends, which suggested fast reaction kinetics (([−OH]0 + [−COOH]0)/[epoxide]0 = 4.5:1). Due to the relatively low glycidyl methacrylate concentration in PEGMMA (∼2 mol %) and low loading ratio of PEGMMA in the blends (0.5−5 wt %), quantitative determination of the epoxide conversion by IR spectroscopy during melt blending proved challenging. Nevertheless, 1H NMR spectroscopy was successfully utilized to estimate the conversion of epoxide groups at different compounding conditions. For example, at a concentration of 2 wt % PEGMMA in a PLA/PEGMMA blend, 6109

DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114

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Industrial & Engineering Chemistry Research we were able to achieve a reasonable signal-to-noise ratio to effectively calculate the extent of grafting. However, at 80b

no. of grafts Mwc formed per chaina (kg mol−1) ∼3 ∼5 ∼10 ∼20

143 147 152 123

Đ 1.99 2.04 2.10 2.19

a

Calculated using 1H NMR spectroscopy. bPLA was degraded under these compounding conditions. cMeasured using an RI detector on an SEC instrument with chloroform as the mobile phase at 35 °C.

melt blending time, the conversion increased with temperature to up to about 10 grafts per PEGMMA chain (at 240 °C). At extended reaction time of 5 min at 240 °C, although over 80% conversion was achieved, the PLA chain started degrading as evidenced by the molar mass drop in SEC (Mw = 123 kg mol−1 after blending) and color change (to yellowish). The molar mass (Mw) of blended PLA (2 min, 220 °C) showed a slight increase as compared to the neat PLA, further supporting the chain extension of PLA upon addition of PEGMMA. Blend Morphology. To investigate the influence of PEGMMA as a compatibilizing agent on blend morphology in immiscible PLA/Polyolefin blends, cryo-microtomed blend samples were analyzed by scanning electron microscopy (SEM). Figure 2 presents the effect of polyolefin droplet size changes before and after the inclusion of PEGMMA, both with PP and PBE as the dispersed phase. The binary blends of PLA/ PP (90/10) and PLA/PBE (90/10) displayed microdomain droplets with number-average diameters (dn) over 6 μm, indicating a high interfacial tension between phases. It is known that PP and ethylene−propylene random copolymer (EPR) form a homogeneous single-phase mixture with various ethylene contents in the EPR.32 Therefore, the polypropylene and polypropylene based elastomer used in this study should also be miscible and show similar compatibility with the PLA matrix material. This is corroborated by similar dispersed phase droplet sizes in PLA/PP (90/10) and PLA/PBE (90/10) blends (Figure 2a and Figure 2d). Upon the addition of PEGMMA into the PLA/PP and PLA/ PBE binary blends, the morphology displays a dramatic reduction in the droplet size of polyolefin minor phase, typically an indication of interfacial tension reduction.33,34 Particle analysis further reveals that the droplet size of minor phase decreases with the increasing loading of PEGMMA content in the blends. At 0.5 wt % PEGMMA, the particle size drops by about 3-fold over the corresponding binary blends to 2.2 ± 0.5 and 2 ± 0.3 μm for PP and PBE as the dispersed phase, respectively. A higher PEGMMA loading concentration of 5 wt % does not reduce the droplet size as significantly, indicating a near saturation of graft copolymer at the interface

Figure 2. Scanning electron microscopic (SEM) images of different blends explored in this study. Scale bar represents 2 μm.

(Figure 3).35 However, the droplet size of PP and PBE reaches sub-1 μm at 5 wt % PEGMMA concentration (0.9 ± 0.2 and 0.6 ± 0.1 μm for PP and PBE, respectively).

Figure 3. Droplet sizes for PP in PLA/PP/PEGMMA and PBE in PLA/PBE/PEGMMA blends as a function of PEGMMA weight concentration. PLA/PP and PLA/PBE were kept at 90/10 ratio. Number-average particle diameter dn = ∑nidi/∑ni; approximately 150 particles were circled and analyzed by ImageJ software. Error bars correspond to the standard deviation.

We also noticed that PEGMMA undergoes phase separation with PLA, as demonstrated by the 100−200 nm scale fine microdomains identified by TEM (Supporting Information, Figure S3). These results suggest that only a fraction of PEGMMA is localized at the PLA/polyolefin interface and reflects a very high efficacy of PEGMMA as a reactive 6110

DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114

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Industrial & Engineering Chemistry Research Table 3. Mechanical Characteristics of Homopolymer and Polymer Blends Useda material PLA PP PBE PLA/PP (90/10) PLA/PP/PEGMMA (90/10/1) PLA/PP/PEGMMA (90/10/5) PLA/PBE (90/10) PLA/PBE/PEGMMA (90/10/1) PLA/PBE/PEGMMA (90/10/5)

εb (%)

E (GPa) 2.2 1.0 0.014 2.0 1.8 1.6 1.6 1.8 1.7

± ± ± ± ± ± ± ± ±

0.2 0.03 0.002 0.1 0.2 0.1 0.1 0.1 0.1

7.6 28 780 7.8 17 23 18 41 62

± ± ± ± ± ± ± ± ±

1 5 80 1 3 2 2 6 5

σYS (MPa) 78 41 2.8 54 54 50 50 53 50

± ± ± ± ± ± ± ± ±

6 5 0.3 2 4 1 3 3 4

σTS (MPa) 65 30 5 45 41 35 39 37 36

± ± ± ± ± ± ± ± ±

5 4 0.2 5 2 5 2 4 6

Homopolymer PLA and PLA blends were aged for 18 h at 25 °C (see Experimental Section). Weight ratios of polymer components are listed in parentheses.

a

nearly constant in the cold-drawn stage, with values of 37 and 40 MPa for PLA/PP/PEGMMA (90/10/5) and PLA/PBE/ PEGMMA (90/10/5) blends, respectively. Both necking and cold-drawing could be attributed to a shear yielding mechanism, and were not typically the case for the PLA homopolymer.38 The elongation at break of compatibilized blends shows a dependence on the inclusion of different dispersed phases, with the rubbery PBE leading to a significant improvement in the elongation at break over that with polypropylene as a dispersed phase. This result correlates with the rubbery nature of PBE (εb = 780%), and is a good indication of efficient stress transfer across the matrix-dispersed phase interface. The enhanced miscibility between PBE and olefin segments of PEGMMA compatibilizer may also play a role. The rubbery content in composite material should be minimized in order to achieve a toughening effect without compromising other mechanical responses dramatically, i.e., elastic modulus and yield strength.39 Therefore, the mass content of polyolefin phase was kept at 10% in the binary blends. The Young’s modulus (E) of compatibilized blends reached 1.8 GPa for both PLA/PP/PEGMMA (90/10/1) and PLA/PBE/PEGMMA (90/10/1) blends and was lowered only by 18% as compared to the PLA homopolymer (2.2 GPa). This result further confirmed the role of the reactive component, PEGMMA, in forming graft copolymers at the interface in melt blending and the resulting stress transfer across the glassy− rubbery phases. Rheology of Blends. The dynamic elastic modulus further illustrates the interfacial activity of these blends. At a high frequency, the G′ of both uncompatibilized and compatibilized blends displayed similar values, with either PP or PBE as a dispersed phase (Figure 5a). However, at low frequency, the G′ curves of these blends deviate from terminal behavior (G′ ∝ ω2). In particular, the slopes of compatibilized blends show a slight leveling off and higher values as compared to the uncompatibilized blends, reflecting an enhanced interaction between PLA and polyolefin phases. The slopes of G′ in the terminal region of PLA/PP (90/10) and PLA/PBE (90/10) blends are 1.00 and 0.98, respectively, whereas, for the compatibilized blends, the slopes of PLA/PP/PEGMMA (90/ 10/5) and PLA/PBE/PEGMMA (90/10/5) blends are 0.41 and 0.48, respectively. The graft copolymers formed likely localize at the interface and link two immiscible phases leading to an increased elasticity of the blend and longer relaxation times.23,40 PLA homopolymer has a higher complex viscosity (η*) than PP and PBE, and the viscosities of compatibilized blends with

component in compatibilizing PLA and PP; furthermore, the morphology did not exhibit a large structural dependence on the dispersed phase used, as the droplet sizes of both polypropylene and polypropylene based elastomer (PBE) were reduced substantially upon inclusion of PEGMMA. Mechanical Properties. The mechanical responses of homopolymer and blend materials were obtained by uniaxial tensile testing of compression molded dog-bone samples. The Young’s modulus (E), yield strength (σYS), tensile strength (σTS), and elongation at break (εb) of blend materials were tested and compared with those of PLA homopolymer (see Table 3). The representative engineering stress−strain curves for homopolymers and blends are shown in Figure 4. The PLA

Figure 4. Representative stress−strain curves for homopolymers and polymer blends used in this study. All samples were aged for 18 h at 25 °C prior to data acquisition.

homopolymer underwent a brittle failure without necking, as the deformation of PLA homopolymer is dominated by a crazing mechanism.36,37 The elongation at break (εb) of PLA homopolymer reaches only 7.6%, comparable with the reported value for PLA.20 The binary blends also displayed a brittle failure, although a slightly higher elongation of 18% was observed when elastomeric PBE was used as a dispersed phase. Upon the addition of PEGMMA as a compatibilizing agent, the elongation at break displays significant improvement as compared to the binary blends. Notably, the PLA/PBE/ PEGMMA (90/10/5) elongates to 62%, suggesting a very effective toughening behavior of this blend considering the low mass fraction of polyolefin phase used. Moreover, the compatibilized blends display necking and cold-drawing in the plastic deformation regime. The stress at break remained 6111

DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114

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cooling to room temperature, and then laminating the PLA on top of the PEGMMA side at the same temperature before cooling to room temperature (1 min lamination time, cooling rate 40 °C min−1) to form a PLA−PEGMMA−polypropylene sandwich sample. The thickness of each layer was kept at 1, 0.05, and 0.35 mm for PLA, PEGMMA, and polypropylene, respectively. The resulting sandwich rectangular bar (60 × 7.9 × 1.4 mm, L × W × H) gave a peel strength of 2300 ± 350 N/ m. The peel force vs displacement curves from the peel tests are shown in Supporting Information (Figure S4). Applying the same methodology, the peel strength between PLA and PBE laminated with PEGMMA was measured to be 700 ± 250 N/ m. The high adhesion strengths indicate an efficient interfacial reaction between layers. However, the quantitative determination of epoxide conversion by spectroscopy (e.g., by NMR or IR spectroscopy) is difficult in this case due to the very low concentration of epoxide groups at the surface between layers. The large difference in adhesion when polypropylene and PBE were used is likely related to the bulk polymer deformation energy. In other words, the high elastic modulus of polypropylene may also contribute to the enhanced adhesion value under the peel test. Nevertheless, the adhesion values for both systems were relatively high, where virtually no adhesion was observed between PLA and polypropylene/PBE in the absence of PEGMMA, as the two films readily delaminated. Varying the lamination time from 5 to 60 min did not seem to alter the peel strength between PLA and polypropylene/PBE with PEGMMA as an adhesion promoter. These results suggest a fast reaction kinetics between epoxide and PLA and an effective interfacial layer that is responsible for the relatively strong adhesion observed.41 The apparent fast reaction kinetics was consistent with the epoxy conversion studies mentioned above.

Figure 5. Plots of (a) storage modulus (G′) and (b) complex viscosity (η*) as a function of frequency for various blends studied.

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CONCLUSIONS An ethylene−glycidyl methacrylate−methyl acrylate terpolymer (PEGMMA) was utilized as a reactive compatibilizer for polylactide (PLA) blended with polypropylene (PP) or a polypropylene based elastomer (PBE). The facile reaction between epoxide and PLA end groups at the interface during melt processing allowed the rapid and effective formation of graft copolymers. Approximately five PLA grafts per PEGMMA chain could be achieved when processed for 2 min at 220 °C. The conversion of epoxide groups on the PEGMMA backbone clearly displayed a temperature and time dependence. The compatibilized PLA/polyolefin blends showed substantial reduction in the droplet sizes of the minor phase and transition from brittle failure to ductile shear yielding and necking resulting in a significant increase in the elongation at break (up to 62%) upon the addition of 0.5−5 wt % PEGMMA as compared to the corresponding binary blends. Both PP and PBE were explored as dispersed phases in the blends. PBE reduced the droplet size more effectively and yielded a more ductile PLA composite material in the presence of PEGMMA as compared to PP as a dispersed phase, likely due to the slightly enhanced miscibility between PBE and the olefin segment of PEGMMA, as well as the more rubbery nature of PBE over PP. Rheological studies also provided evidence of strong interfacial activity as demonstrated by the elevated viscosity and storage modulus of compatibilized blends over the corresponding binary blends at low frequency. Adhesion tests gave a quantitative analysis of improved adhesion values between PLA and polypropylene based polymers laminated

PEGMMA incorporated surpass those of the PLA and polyolefin homopolymers at the low frequency regime (Figure 5b, Table 4). In addition, the zero shear viscosities (η0) of Table 4. Zero-Shear Viscosity from Cross Model Fita material

η0 (Pa s)

PLA PP PLA/PP/PEGMMA (90/10/5) PLA/PP (90/10) PBE PLA/PBE/PEGMMA (90/10/5) PLA/PBE (90/10)

1500 900 3000 1300 290 2100 1400

a

For a description of the zero-shear viscosity determinations of the homopolymer and blends, please refer to Supporting Information (Table S2).

PLA/PP/PEGMMA (90/10/5) and PLA/PBE/PEGMMA (90/10/5) blends increase by 130 and 50% as compared to the corresponding binary blends, respectively (Table 4). This high viscosity of compatibilized blends at low frequencies may be attributed to the improved interfacial interaction, as well as the high molar mass component (graft copolymers) formed in melt. Adhesion Test. To probe adhesion between PLA and polyolefins in the presence of PEGMMA, T-peel tests were performed. Samples were constructed by first laminating the PEGMMA to the polypropylene at 180 °C for 2 min and 6112

DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114

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with PEGMMA as a tie layer. The methodology developed herein certainly opens an avenue in bridging the two commercial polymers of high interest, polylactide and polypropylene, and we are currently exploring the utility of these compatibilized blend materials with enhanced properties.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed NMR, IR spectra, TEM, and adhesion tests; determination of zero-shear viscosities of polymers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00882.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Kimberly-Clark for funding support and providing polymer samples. We also acknowledge the Center for Sustainable Polymers at the University of Minnesota, a National Science Foundation supported Center for Chemical Innovation (CHE-1413862). Parts of this work were carried out in the University of Minnesota College of Science and Engineering Characterization Facility, which receives partial support from NSF through the NNIN program.

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DOI: 10.1021/acs.iecr.5b00882 Ind. Eng. Chem. Res. 2015, 54, 6108−6114