Sustainable Thermoplastic Elastomers Derived from Fatty Acids

Article pubs.acs.org/Macromolecules

Sustainable Thermoplastic Elastomers Derived from Fatty Acids Shu Wang,† Sameer Vajjala Kesava,‡ Enrique D. Gomez,‡ and Megan L. Robertson*,† †

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-4004, United States Department of Chemical Engineering and the Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States

‡

S Supporting Information *

ABSTRACT: Vegetable oils are an attractive source for polymers due to their low cost, abundance, annual renewability, and ease of functionalization. Stearyl and lauryl acrylate, derived from vegetable oils such as soybean, coconut, and palm kernel oil, have been polymerized through reversible addition−fragmentation chain transfer polymerization, resulting in poly(styrene-b(lauryl acrylate-co-stearyl acrylate)-b-styrene) (SAS) triblock copolymers. Varying the length of the side chain on the polyacrylate midblock (C18 and C12 in stearyl and lauryl acrylate repeat units, respectively) is a convenient tool for tuning the physical properties of the triblock copolymers. The SAS triblock copolymers exhibit properties appropriate for thermoplastic elastomer (TPE) applications. Small-angle X-ray scattering and transmission electron microscopy experiments have elucidated the microphase-separated morphology of the SAS triblock copolymers, consistent with a spherical morphology lacking long-range order. The physical properties of the polymers can be readily tuned by varying the acrylate midblock composition, including the melting temperature, viscosity, and triblock copolymer tensile properties. Tensile testing reveals elastomeric behavior with high elongation at break. Surprisingly, the order−disorder transition temperature of the triblock copolymer is not dependent on the acrylate composition in the midblock. This indicates that the acrylate composition can be used as a tool to manipulate the physical properties of the triblock copolymers without affecting the order−disorder transition temperature, or processing temperature, of the TPEs.

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functionalization.8,25−30 The carbon−carbon double bonds on the triglyceride structure are amenable to a variety of functionalization chemistries that can lead to subsequent polymerization. Vegetable oils are often epoxidized through a reaction with performic acid,31 or alternatively through a lipasecatalyzed process,32 resulting in the synthesis of epoxy resins.33,34 Epoxidized vegetable oils have also been converted to polyols for polyurethanes.35,36 Epoxidized oils can be acrylated through a ring-opening reaction with acrylic acid, leading to polyacrylates.37−40 Vegetable oils can be functionalized with cyclic structures and then polymerized through ringopening metathesis polymerization (ROMP).41−44 The carboxylic acid end group of a fatty acid can be converted to a hydroxyl end group45,46 and subsequently converted to an acrylate or methacrylate group,47 appropriate for radical polymerization techniques. The application of triblock copolymers as TPEs requires careful control over their nanoscale structure. Given that the phase behavior of triblock copolymers has been wellcharacterized,48−50 desired mechanical properties can be achieved by tuning the molecular composition and architecture. When the glassy outer blocks self-assemble into cylindrical or

INTRODUCTION Thermoplastic elastomers (TPEs), which combine the processing advantages of thermoplastics with the flexibility and extensibility of elastomeric materials, have found versatile applications in industry, including electronics, clothing, adhesives, and automotive components.1 Styrene-based linear ABA triblock copolymers, such as poly(styrene-b-butadiene-bstyrene) (SBS) or poly(styrene-b-isoprene-b-styrene) (SIS), are among the most important and widely used TPEs. Nevertheless, those TPEs are derived from fossil fuels. The finite availability of fossil fuels and environmental impact of petroleum manufacturing has led to an increased interest in the development of alternative polymeric materials from sustainable sources.2 Additionally, there are opportunities in developing TPEs with high thermal stability beyond SBS and SIS triblock copolymers, which have limited oxidative stability and UV resistance owing to the unsaturated carbon double bonds of the midblock in the polymer.3−6 A variety of sustainable sources have been explored for polymers, including vegetable oils,7,8 plant sugars,9 terpenes,10 polysaccharides,11 rosins,12 and lignin.13 Relatively few studies have focused on renewable resources for the derivation of TPEs.14−24 We have chosen to use vegetable oils as a raw material source for TPEs. Vegetable oils and their fatty acids are a particularly attractive source due to their low toxicity, biodegradability, availability, relatively low price, and ease of © 2013 American Chemical Society

Received: June 9, 2013 Revised: July 17, 2013 Published: September 3, 2013 7202

dx.doi.org/10.1021/ma4011846 | Macromolecules 2013, 46, 7202−7212

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Scheme 1. Triblock Copolymer Synthesisa

a

Poly(LAc-co-SAc) is a random copolymer. For LAc, x = 9, and for SAc, x = 15. Synthetic Procedures. Synthesis of the RAFT Agent (BTBTMB). 1,4-Bis(thiobenzoylthiomethyl)benzene (BTBTMB) was synthesized according to ref 73. Tetrahydrofuran (THF, J.T. Baker, low water, HPLC grade) was purified using a SG Waters solvent purification system. In a round-bottom flask, bromobenzene (99%, 5.94 g) and magnesium turnings (∼1/8 in., 99.95% trace metals basis, 0.965 g) were added to 75 mL of dry THF and heated to 40 °C for 30 min. Carbon disulfide (≥99%, 2.88 g) was added dropwise over 15 min. A solution of α,α′-dibromoxylene (97%, 2.5 g) in 25 mL of dry THF was then added dropwise over 15 min to the resulting dark brown solution. The reaction temperature was maintained at 40 °C for 1.5 h. Ice water (100 mL) was added, and the organic products were extracted with diethyl ether (ACS reagent). The combined organic phase was washed with water and brine and dried over anhydrous magnesium sulfate (≥99.5%). Removal of the solvent yielded a red solid. The solid was recrystallized from ethanol (Koptec, 100%) to yield the product as a bright red solid. Synthesis of Poly(lauryl acrylate-co-stearyl acrylate) [Poly(LAcco-SAc)] Copolymers. Poly(LAc-co-SAc) copolymers (which we will also refer to as polyacrylate or PA) were prepared with reversible addition−fragmentation chain transfer (RAFT) polymerization. Lauryl acrylate (LAc, 90%) was eluted through a silica gel (Fluka, 70−230 mesh) column to remove the monomethyl ether hydroquinone inhibitor. Stearyl acrylate (SAc, 97%) and 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%) were recrystallized in ethanol (Koptec, 100%). LAc/SAc, AIBN (0.05 equiv relative to BTBTMB), BTBTMB, and 1,4-dioxane (99.8%) were added at the predetermined molar ratio to a pressure vessel. After purging with argon for 20 min, the pressure vessel was sealed, immersed into a preheated oil bath at 70 °C and vigorously stirred for 2 days. The polymerization was quenched by ice water. PA copolymers were purified by precipitation in excess cold isopropyl alcohol (Macron, ACS reagent) twice and dried under vacuum overnight at 40 °C. Synthesis of Poly(styrene-b-(LAc-co-SAc)-b-styrene) (SAS) Triblock Copolymers. Styrene (99%) was purified with a basic aluminum oxide column to remove the 4-tert-butylcatechol inhibitor. Toluene (J.T. Baker, OmniSolv, HPLC grade) was purified using a SG Waters solvent purification system. The SAS triblock copolymers were synthesized by a two-step procedure (Scheme 1). PA was polymerized according to the procedure described above and subsequently was used as a macro-chain-transfer agent and mixed with AIBN (0.05 equiv relative to BTBTMB) and styrene in toluene in a pressure vessel. After

spherical domains, the rubbery midblocks form bridges between the glassy domains; the physical behavior of the polymer is that of an elastomeric material.1 When heated above the order−disorder transition temperature, the triblock copolymer becomes disordered and the elastomeric behavior disappears. Though the effect of polydispersity has been well documented,51−53 the most facile approach to achieving a desired microstructure in block copolymers is the implementation of controlled or living polymerization techniques to produce polymers with relatively narrow molecular weight distributions. Relatively few studies have focused on controlled polymerization techniques for functionalized fatty acids.54−68 Reversible addition−fragmentation chain transfer (RAFT) polymerization is a highly attractive candidate for polymerizing fatty acrylates, based on ease of implementation, tolerance to functional groups, mild conditions, and production of polymers with well-controlled molecular weight distributions and architectures.69 In this paper, we discuss the synthesis and physical properties of poly(styrene-b-(lauryl acrylate-co-stearyl acrylate)-b-styrene) triblock copolymers, which act as TPEs and contain midblocks derived from vegetable oils such as soybean, coconut, and palm kernel oil. We have chosen to focus on a copolymer of lauryl acrylate (LAc) and stearyl acrylate (SAc) as the midblock of the triblock copolymers as these monomers offer a number of advantages, including low glass transition temperatures55,70 and long alkyl side chains which are highly hydrophobic and provide a greater resistance to degradation.71,72 Additionally, we hypothesize that varying the length of the side chains on the polyacrylate midblock (through varying the LAc to SAc ratio) will allow us to tune the physical properties of the triblock copolymer. The synthesis, morphology, and thermal and mechanical properties of these triblock copolymers are investigated.

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EXPERIMENTAL METHODS

Materials. All chemicals were purchased from Sigma-Aldrich unless otherwise noted below. 7203


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Figure 1. 1H NMR data obtained from (a) poly(LAc-co-SAc) random copolymer and (b) SAS triblock copolymer; (c) SAS chemical structure. purging with argon for 20 min, the pressure vessel was sealed, immersed into a preheated oil bath at 70 °C, and vigorously stirred for 2 days. The polymer was precipitated into cold methanol (BDH, ACS grade, 99.8%) twice and dried under vacuum overnight at 60 °C. One series of triblock copolymers (SAS4) were synthesized by a slightly modified procedure: polystyrene was first synthesized using dibenzyl trithiocarbonate as the RAFT agent, followed by chain extension with the acrylate monomers to form SAS triblock copolymers (Scheme S1 in the Supporting Information). Reaction Kinetics. To investigate the reaction kinetics of PLAc and PSAc homopolymers, a stock solution containing LAc or SAc, BTBTMB, AIBN (0.05 equiv relative to BTBTMB), and 1,4-dioxane (99.8%) was prepared and separated into 5−8 pressure vessels in the glovebox. Once sealed, the vessels were taken out of the glovebox and heated at 70 °C. The reactions were quenched at a desired reaction time. The reaction conversion and molecular weight distribution were characterized using proton nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC), respectively. To verify the composition of the statistical copolymer, poly(LAc-co-SAc), the same procedure was followed with a 50/50 mixture of LAc/SAc. The polymer was characterized with 1H NMR for the reaction conversion and LAc/SAc ratio in the copolymer and GPC for the molecular weight distribution. The nomenclature SASX-Y-Z refers to SAS triblock copolymers with the following characteristics: X specifies a series (1−4), in which each series contains triblock copolymers with a fixed molecular weight, Y specifies the wt % of LAc in the midblock, and Z specifies the vol % of styrene in the triblock copolymer. Characterization Procedures. Molecular Characteristics. Proton nuclear magnetic resonance (1H NMR) experiments were performed on a JEOL ECA-500 instrument using deuterated chloroform (99.96 atom % D) as the solvent. Molecular weight and molecular weight distribution (including the polydispersity index, PDI) were measured by a Viscotek gel permeation chromatography (GPC) instrument using THF (OmniSolv, HPLC grade) as the mobile phase at 30 °C. The flow rate was 1 mL/min, and the injection volume was 100 μL. A triple detection system, including light

scattering, a viscometer, and refractometer, was employed to characterize the absolute molecular weight. Density. The densities of selected polyacrylates (those that are amorphous at room temperature) were measured at room temperature (around 21 °C) through the following protocol. A small portion of the polymer was placed on a glass slide and cooled with liquid nitrogen. The cold sample was fractured with a blade and quickly transferred to a vial containing a methanol/water mixture of known composition. The methanol/water content was adjusted to determine the composition range over which the polymer transitioned from being suspended in the solvent to sinking in the solvent. This range was narrowed until the density could be determined to 2 significant digits after the decimal point. The exact solvent mixture density was determined through weighing a known volume of the mixture (dispensed through a glass syringe) on an analytical balance (Mettler Toledo MS204S). The resulting densities were as follows: poly(lauryl acrylate) homopolymer, 0.94 g/mL; poly(LAc-co-SAc) copolymer with 76 wt % LAc, 0.94 g/mL; poly(LAc-co-SAc) copolymer with 61 wt % LAc, 0.92 g/mL. Thermal Properties. Melting temperatures (Tm) were characterized by a TA Instruments Q2000 differential scanning calorimeter, calibrated with an indium standard, with a nitrogen flow rate of 50 mL/min. The sample was placed in the calorimeter (using a Tzero aluminum pan) and was heated from −50 to 100 °C, cooled back to −50 °C, and subsequently heated to 100 °C, at a rate of 10 °C/min. The value of Tm was determined as the peak position of the second heating. The thermal degradation properties were probed by thermogravimetric analysis (TGA), using a TA Instruments Q500 analyzer. The sample was heated from 40 to 600 °C at a rate of 10 °C/ min in an argon environment (the balance argon purge flow was 40 mL/min and the sample purge flow was 60 mL/min). Mechanical and Rheological Properties. The viscosity of PA random copolymers was characterized by a TA Instruments DHR-2 rheometer. Two experiments were conducted with a cone and plate geometry (40 mm diameter, cone angle 2.319°). Flow curves were obtained through measuring the viscosity at shear rates ranging from 7204


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Table 1. Characteristics of SAS Triblock Copolymers midblock [poly(LAc-co-SAc)]

SAS triblock copolymers

series

namea

Mn (kg/mol)b

PDIb

wt % LAcc

Mn (kg/mol)b

PDIb

vol % Std

1

SAS1-100-23 SAS1-76-23 SAS1-61-24 SAS2-100-23 SAS2-76-24 SAS2-61-23 SAS3-100-17 SAS3-76-18 SAS3-61-18 SAS4-100-18 SAS4-73-18 SAS4-62-18

81.9 80.7 81.6 57.3 58.2 56.8 57.3 58.2 56.8 39.2f 39.5f 38.8f

1.28 1.31 1.24 1.26 1.30 1.28 1.26 1.30 1.28 N/Ag N/Ag N/Ag

100 76 61 100 76 61 100 76 61 100 73 62

108.9 107.5 109.7 76.2 78.4 75.7 70.4 72.3 70.7 48.7 49.0 48.3

1.68 1.69 1.65 1.67 1.65 1.61 1.44 1.43 1.42 1.38 1.42 1.44

23 23 24 23 24 23 17 18 18 18 18 18

2

3

4e

a The nomenclature SASX-Y-Z refers to triblock copolymer series X (1−4), with wt % of LAc in the midblock of Y% and PSt vol % of Z%. In each series, the polymer molecular weight is held constant (within experimental error). bMn and PDI are measured by GPC (light scattering, with triple detection). cThe wt % LAc was calculated based on NMR. dThe vol % St was calculated based on NMR data and measured polyacrylate densities (procedure described in Experimental Methods) and the PSt density of 1.04 g/cm3 reported in ref 75. eSeries 4 polymers were synthesized through chain extension from PSt with Mn = 9.5 kg/mol and PDI = 1.13. Synthesis shown in Scheme S1 of the Supporting Information. fCalculated by subtracting Mn of the PSt end blocks from the Mn of the triblock copolymer. gThese values cannot be measured.

100 to 10−4 s−1, using the DHR-2 instrument’s automatic steady state sensing. In select cases transient tests were also conducted in which the stress was held constant until steady state conditions (in both the shear rate and viscosity) were observed. This process was repeated for a variety of shear rates. The viscosity was determined by averaging the viscosity at the plateau. The viscosity of SAS4 triblock polymers was also determined in the disordered state; the samples were placed between 25 mm parallel plates and heated in an electrically heated plates sample environment containing nitrogen gas. The measurements were taken at 150 °C (50 °C above the order−disorder transition temperature as determined by the rheometer, described below). Tensile testing was carried out with an Instron tensile tester with a 100 N load cell at a speed of 10 mm/min to obtain tensile strength and elongation at break of SAS triblock copolymers at room temperature. Dog-bone-shaped testing bars (following ASTM D638, bar type 5, thickness 1.5 mm) were prepared by compression molding on a Carver Hotpress at an applied load of 4000 lbs at 210 °C. Pneumatic grips (maximum 2 kN) were used to affix the sample in the testing frame, at a compressed air pressure of 9 psi. Each measurement was repeated with 4−6 test specimens. Morphology and Order−Disorder Transition. Small-angle X-ray scattering (SAXS) measurements were performed at the Advanced Photon Source (APS) at Argonne National Laboratories at Sector 5ID-D beamline, maintained by the Dow−Northwestern−Dupont Collaborative Access Team (DND-CAT). The X-ray wavelength was 0.7293 Å, and the sample-to-detector distance was 4578 mm. The scattering intensity was monitored by a Mar 165 mm diameter CCD detector with a resolution of 79 μm/pixel. The two-dimensional scattering patterns were azimuthally integrated to a one-dimensional profile of intensity versus scattering vector, q = 4π sin(θ/2)/λ [θ is the scattering angle; λ is the wavelength]. The triblock copolymers were prepared by compression molding on a Carver Hotpress at an applied load of 4000 lbs at 210 °C for 5 min prior to SAXS experiments. Selected samples characterized with SAXS were subsequently characterized with transmission electron microscopy (TEM). Sections of approximately 50 nm thicknesses were prepared for TEM experiments using a Leica EM UC6 microtome operating at 0 °C with a Leica EM FC6 cryo-attachment and a glass knife. Contrast between the polystyrene and polyacrylate domains was enhanced using 0.5 wt % (aq) RuO4 vapor staining (5 min staining time). Imaging was done at the Materials Research Institute of the Pennsylvania State University on a JEOL 2010 LaB6 transmission electron microscope. Bright field images were recorded using a Gatan energy filter such that inelastic scattering is removed. Order−disorder transition temperatures (TODT) of SAS triblock copolymers were also probed using the

TA Instruments DHR-2 rheometer. The linear viscoelastic region was first determined in a strain sweep from 1% to 50% at a frequency of 10 rad/s. A frequency sweep was then completed (using a strain in the linear region) from 1 to 50 rad/s every 10 °C from 270 to 50 °C (200 to 20 °C for series 4). The storage modulus at a frequency of 10 rad/s was plotted versus temperature, and the sharp decrease in storage modulus indicated the TODT.

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RESULTS AND DISCUSSION Synthesis of PA Random Copolymers and SAS Triblock Copolymers. The NMR spectrum of poly(LAc-coSAc) is shown in Figure 1a. The peaks at 4.0, 2.25, 1.87, 1.59, 1.26, and 0.87 ppm indicate the incorporation of the acrylate monomers, and the three peaks from 7.3 to 8.0 ppm (inset of Figure 1a) show the presence of the functional groups on the RAFT agent (BTBTMB) after polymerization, allowing for subsequent chain extension to synthesize triblock copolymers. For pure poly(LAc), the theoretical ratio of peak area b to peak area a (Figure 1a) is 18:3, and for pure poly(SAc), the theoretical ratio is 30:3. In the copolymer, the LAc/SAc ratio is calculated from the ratio of these two peak areas (eq 1). In eqns 1a and 1b, xLAc and xSAc are the mole fractions of LAc and SAc repeat units, respectively, and R is peak area b divided by peak area a in the NMR spectra (Figure 1a). The wt % LAc calculated with this method (eq 1c) was very close to the feed concentration (each series in Table 1 contained polymers with a target LAc concentration of 60, 75, and 100 wt %). x LAc =

10 − R 4

(1a)

xSAc =

R−6 4

(1b)

wt %LAc =

x LAc

x LAc × 240.38 × 100 × 240.38 + xSAc × 324.54

(1c)

A representative NMR spectrum of a SAS triblock copolymer is presented in Figure 1b. The peaks at 6.5−7.2 and 1.44 ppm are attributed to the polystyrene (PSt) repeat unit while the other peaks from the acrylates are maintained. 7205


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Figure 2. Mn and PDI as a function of monomer conversion for the RAFT polymerization of (a) LAc, (b) SAc, and (c) a 50/50 feed mixture of LAc/ SAc. The NMR Mn values are calculated by multiplying the target molecular weight by the conversion determined by NMR. The weight fraction of LAc in the copolymer obtained from the 50/50 feed mixture of LAc/SAc is shown in (d). The dashed line in (d) indicates the feed weight fraction of LAc.

order to measure the polymer composition as a function of monomer conversion. Figure 2c shows the Mn and PDI as a function of monomer conversion for a 50/50 feed mixture of LAc/SAc. The LAc/SAc ratio in the copolymer was determined by 1H NMR, revealing that the LAc/SAc ratio is consistent throughout the reaction, without any detectable composition drift (Figure 2d). Previous studies have also reported a lack of composition drift and reactivity ratios close to 1 for polyacrylate copolymers with varying side-chain lengths.79,80 Thermal Stability of Polyacrylates and SAS Triblock Copolymers. TGA data obtained from PSt and poly(LAc-coSAc) copolymers are shown in Figure S2a. There is little difference between them, and therefore we anticipated that the triblock copolymers would exhibit similar thermal degradation behavior. TGA data obtained from the SAS triblock copolymers are shown in Figure S2b. All triblock copolymers are quite stable up to 320 °C, indicating that the polymers can be processed at elevated temperatures without degradation. Tunable Physical Properties of Polyacrylate Copolymers. The physical properties of the PA copolymers that comprise the midblock of the SAS triblock copolymers were probed by varying the ratio of LAc/SAc in the copolymer. The presence of side-chain crystallinity and the melting and crystallization behavior of poly(alkyl acrylates) with long side chains have been previously reported.40,70,79−83 The melting temperature (Tm) of the PA copolymers is shown in Figure 3. The Tm values decrease linearly as the LAc fraction increases and are consistent with ref 40. Interestingly, a recent report in the literature has utilized the presence of side-chain crystallization in similar triblock copolymers to produce shape

Four series of SAS triblock copolymers were synthesized, in which the midblock composition (SAc/LAc ratio) was varied to study the impact of the midblock composition on the ultimate macroscopic properties of the triblock copolymer (Table 1). In each series the polymer molecular weight is held constant while the midblock composition is varied. Representative GPC data are shown in Figure S1 of the Supporting Information for SAS2-100-23 (and the PA random copolymer that was chain extended with styrene to form this triblock copolymer). There is a distinct shift to the left in the data when comparing the PA random copolymer and triblock copolymer. The GPC data for the triblock copolymers show trimodal distributions. The shoulder to the right of the primary peak may indicate the presence of PA random copolymers that were not chain extended or diblock copolymer. The trimodal nature of the GPC data are also similar to that observed in ref 74, in which the largest size mode (to the left of the primary peak) was attributed to chain branching. Reaction Kinetics of Polyacrylate Polymerization. The kinetics of polymerization of LAc and SAc were monitored by measuring the Mn and PDI of the polymer as a function of reaction conversion, shown in Figure 2. The Mn of PLAc (Figure 2a) and PSAc (Figure 2b) increase linearly with the monomer conversion, indicating the controlled behavior of the RAFT polymerization.76 The PDI is relatively low, around 1.3. The slightly higher PDIs as compared to that obtained for other monomers such as tert-butyl acrylate77 can be attributed to the higher rate of termination in polyacrylates with long side-chain lengths.68,78 Similar results have been observed in the literature for LAc.59 A copolymer of LAc and SAc was also monitored in 7206


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(Figure S3). The difficulty in reaching steady-state conditions in these polymers may indicate the presence of structural rearrangement or alignment of the polymer chains under nonsteady-state conditions. Additionally, the upturn in viscosity at low shear rates was observed in the PA polymer containing 61% LAc at 25 °C even when the transient test was conducted (Figure S3c). This behavior may be the result of the proximity to the crystallization temperature in this polymer (determined to be 19 °C using DSC analysis). Morphology of SAS Triblock Copolymers. The morphology of the SAS triblock copolymers was investigated by both SAXS and TEM. The integrated 1D SAXS profiles are shown in Figure 5 for series 1 and 2 and in Figure S4 for all other polymers. The six polymers shown in Figure 5 exhibit similar features in the SAXS profiles. The 2D SAXS data for these polymers (Figure S5) show a small degree of anisotropy, likely due to the presence of shear forces during compression molding. Broad scattering peaks are observed in Figure 5, with the presence of higher order peaks. The higher order peaks at √3, √4, √7, √12, and √13 (labeled by the arrows) are consistent with a cylindrical microstructure. (The presence of the √3 and √4 peaks observed in the cylindrical morphology is very sensitive to the volume fraction of the phases.85,86) The additional peaks associated with a spherical morphology (such as the √2 and √3 peaks), however, could be obscured by the breadth of the peaks. For this reason, we also obtained TEM micrographs on one of the samples, SAS1-100-23. A representative micrograph for SAS1-100-23 is shown in Figure 6a where the dark regions correspond to PSt domains because of RuO4 staining. The micrograph in Figure 6a suggests a poorly ordered spherical morphology. The lack of long-range order is consistent with the broad peaks that are observed in the SAXS data. The micrograph in Figure 6a is similar to that in a publication by Mahanthappa and co-workers,87 where the microstructure was surmised to be influenced by crystallization of the triblock copolymer matrix. In our study, however, we prepared sections at 0 °C, in the vicinity of the Tm of the matrix of SAS1-100-23 (2 °C), and imaged the sections at 25 °C (above the Tm). Thus, we expect that crystallization of the matrix to have a minimal effect on the microstructure. A slightly different scenario as compared to the other samples is observed in the SAXS data obtained for SAS2-61-23, as there is a small shoulder to the left of the primary peak and there also exists a √3 higher order peak. The 2D SAXS data (Figure S5) indicate the highest degree of anisotropy as compared to the other samples. TEM data were also obtained on SAS2-61-23, shown in Figure 6b. A poorly ordered microphase-separated morphology is also observed in Figure 6b, although in this case there appears to be connectivity between some of the PSt spherical domains. To lend further evidence to the presence of a disordered spherical structure in which some of the spheres are connected, SAS1-100-23 and SAS2-61-23 were viewed in an optical microscope through crossed polarizers; birefringence was not observed (note that the grain size and refractive indices of the polymers will influence the efficacy of this method for detecting anisotropic structures). Cylindrical or spherical microstructures are highly desirable for TPE applications, as they provide physical cross-linking and elastomeric behavior resulting from bridged PA chains connecting distinct glassy PSt domains.1 It is important to note that the data shown in Figures 5 and 6 are not necessarily representative of an equilibrium structure (the

Figure 3. Melting temperatures (Tm) of poly(LAc-co-SAc) random copolymers (characteristics given in Table S1) (▲), the midblock in the SAS1 series of triblock copolymers (red ×), and the midblock in the SAS2 series of triblock copolymers (blue ○).

memory materials.84 In TPE applications, which are the focus of this paper, the Tm of the soft segment should be less than the usage temperature (usually room temperature);1 thus, a LAc fraction higher than 60% is needed. The viscosity of the PA random copolymers at 30 °C is plotted as a function of copolymer composition in Figure 4 (the complete viscosity as a function of shear rate data are included in Figure S3). The increase in viscosity as the LAc content increases likely results from differences in the glass transition temperature (Tg) of the random copolymers.55,70 Additionally, increasing the side-chain length (which also reduces the number of monomer repeat units as the midblock molecular weight is held constant within a series) is expected to reduce the hydrodynamic volume of the polymer, also reducing the viscosity. The shear rate dependence of the viscosity in the PA copolymers (Figure S3) shows unusual behavior at low shear rates. In the flow curves (obtained by sweeping through various shear rates, allowing the instrument to sense the steady-state conditions), an increase in viscosity is observed at low shear rates. However, when a transient test was conducted (in which the stress was held constant for a long period of time to manually ensure that the steady-state shear rate and viscosity were attained at each stress value) this upturn disappeared

Figure 4. Viscosity of poly(LAc-co-SAc). The data symbols indicate: midblock polyacrylates in SAS series 1 at 30 °C (▲), midblock polyacrylates in SAS series 2 at 30 °C (□), and SAS4 triblock copolymers measured at 150 °C (50 °C above the TODT) (●). Viscosity as a function of shear rate is shown in Figure S3. 7207


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Figure 5. 1D SAXS profiles for SAS triblock copolymers at room temperature.

Figure 6. TEM micrographs obtained from (a) SAS1-100-23 and (b) SAS2-61-23. Fourier transforms of each image are shown in Figure S6. Figure 7. Storage modulus as a function of temperature of SAS triblock copolymers. Series 1 data sets are blue, series 2 data sets are red, series 3 data sets are black, and series 4 data sets are green. The symbols indicate polyacrylate compositions. In series 1−3: 100 wt % LAc (×), 76 wt % LAc (△), 61 wt % LAc (■). In series 4: 100 wt % LAc (×), 73 wt % LAc (△), 62 wt % LAc (■). The TODT is determined from the temperature at which the storage modulus drastically decreases.

processing temperature of the polymers was in the vicinity of the TODT) yet are very relevant to understanding of the mechanical testing data as the same processing conditions were used for morphological and mechanical testing studies. Order−Disorder Transition of SAS Triblock Copolymers. The order−disorder transition temperatures (TODT) of SAS triblock copolymers are summarized in Figure 7. Within each series, when the molecular weight of the triblock copolymer and styrene content are kept constant, but the ratio of LAc/SAc in the midblock is varied, there is negligible effect on the TODT (i.e., compare each series in Figure 7 in which the color of the data points is held constant, and the symbols are varying). Note that the polymers in series 4 have the smallest deviations in molecular weight and St fraction (Table 1) due to the alternative synthetic scheme employed (Scheme S1). Importantly, in SAS triblock copolymers we are potentially able to tune the physical properties of the triblock copolymer without varying the TODT. The TODT is important for TPE applications as it affects the temperature at which the materials must be processed in the melt state. In series 4, the TODT is in the vicinity of the Tg of PSt end blocks. As the molecular weight of the triblock copolymer is increased, two distinct transitions emerge, and the TODT and Tg can be independently characterized. Comparing series 2 and 3, the midblock molecular weights are similar to one another, but the two series contain polymers with different PSt fractions. A

higher PSt fraction results in a higher TODT, as observed in Figure 7. For series 1, the TODT was too high to be determined (an upper temperature limit of 270 °C was chosen to avoid degradation of the polymer). It is quite unexpected that the TODT would be independent of the midblock composition. The order−disorder transition of the triblock copolymer is defined by the thermodynamic parameter χN, where χ is the Flory−Huggins interaction parameter and N is the number of repeat units in the triblock copolymer (both χ and N are based on a reference volume, vref). The value of χN at the order−disorder transition will depend on the triblock copolymer composition (vol % St). In each triblock copolymer series in Figure 7, the triblock copolymer composition (vol % St) and molecular weight are fixed at a constant value. As the only parameter that is varying within a series is the midblock composition (% LAc repeat units in the midblock), the consistency of the TODT within a series implies that χN(T) is independent of the midblock composition. The number of monomer units (Nmon) for each polyacrylate 7208


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Figure 8. (a) A representative stress−strain curve of the triblock copolymers (obtained from SAS1-61-24). (b) Tensile stress and elongation at break of the SAS triblock copolymers: series 1 tensile strength (▲) and elongation (●) and series 2 tensile strength (△) and elongation (○). Additional stress−strain curves are shown in Figure S7.

mechanical properties of TPEs are highly dependent on the molecular weight between entanglements (Me) of the midblock.1 In order to obtain high tensile strength, the soft matrix needs to be entangled, requiring Mn of the midblock to be 2−3 times larger than Me (the exact relationship between the critical molecular weight, Mc, and Me depends on the polymer structure93). In the commercial SBS and SIS triblock copolymers, the Me for poly(butadiene) and poly(isoprene) are 1.7 and 6.1 kg/mol, respectively.94,95 However, polyacrylates have higher Me’s, and as the alkyl chain length increases, Me increases substantially, resulting in lower tensile strengths (refer to Table S3). The Me for poly(lauryl methacrylate) has been reported to be 225 kg/mol,16 and we anticipate the Me for poly(LAc) to be similarly large. We have synthesized three high molecular weight poly(LAc) polymers in order to characterize the Me (up to Mn = 170 kg/mol). In all cases, the rheology data are consistent with an unentangled polymer (Figure S8). Therefore, we can conclude that the SAS triblock copolymers summarized in Table 1 will contain PA midblocks that are unentangled. The unentangled matrices in the SAS triblock copolymers result in lower tensile strengths as compared to SBS or SIS TPEs. Though this may be a disadvantage for some TPE applications, many recent articles in the literature have focused on the use of polyacrylates for TPEs due to their hydrophobicity and soft matrix.16,96 A soft matrix may be an advantage in pressure sensitive adhesives or applications employing oil-extended TPEs. In some cases, the midblock composition can be used to manipulate the SAS triblock copolymer mechanical properties. In series 1, as the LAc fraction increases, the ultimate tensile stress decreases whereas the elongation at break increases (Figure 8). Thus, in this higher molecular weight series, it is possible to tune the mechanical properties of the triblock copolymers by changing the side chain length of the midblock. In contrast, in series 2 the SAS triblock copolymer mechanical properties did not appear to be affected by the midblock composition.

midblock will certainly change as the % LAc is varied (and the total midblock molecular weight is held constant). However, in order to define χ and N based on vref, we will use the following expression: N=

vmonNmon M M 1 M = mon = ρNA M mon vref ρNAvref vref

(2)

where Mmon is the molecular weight of a monomer repeat unit, vmon is the volume of a monomer repeat unit, M is polymer molecular weight (taken to be Mn), ρ is the density, and NA is Avogadro’s number. Therefore, within a triblock copolymer series, N will vary only if ρ is dependent on the % LAc of the polyacrylate. In the Experimental Methods, a simple procedure is described to measure ρ for polyacrylates with varying % LAc. The densities were very similar, between 0.92 and 0.94 g/mL. Therefore, the N values for the polymers within each triblock copolymer series in Figure 7 will not differ to a great extent. The conclusion that we can draw from this analysis is that if the polyacrylate composition does not affect either the TODT or the value of N, then χ must be independent of the polyacrylate composition of the midblock. This conclusion departs from the literature precedent, as past determinations of χ values between polystyrene and poly(n-alkyl methacrylates) have indicated that the value of χ decreases as the alkyl chain length increases (for example, at 120 °C, χ is 0.023 for polystyrene and poly(methyl methacrylate), 0.017 for polystyrene and poly(nbutyl methacrylate), and 0.015 for polystyrene and poly(npentyl methacrylate; all χ values were calculated based on a reference volume of 100 Å3 employing the equations given in ref 88, which uses the χ values reported in refs 89−91). However, the side-chain lengths of the polyacrylates used in this study are significantly larger than in refs 89−91. Corroborating evidence for our results are found in a study by Siepmann and co-workers, who predicted the solubility parameters of poly(n-alkyl acrylates) and poly(n-alkyl methacrylates).92 The solubility parameters became independent of the length of the side chain when the side chain approached 10 carbon atoms. Mechanical Properties of SAS Triblock Copolymers. Tensile testing data obtained from SAS triblock copolymers (series 1 and 2) are shown in Figure 8 and Figure S7. The tensile strength and elongation at break are much lower than commercial SBS triblock copolymers.1 It is known that the

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CONCLUSIONS

Four series of poly(styrene-b-(lauryl acrylate-co-stearyl acrylate)-b-styrene) triblock copolymers were synthesized by RAFT polymerization with a symmetric chain transfer agent. The polymerizations of the fatty acid-derived acrylate monomers 7209


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06CH11357. S.W. and M.L.R. acknowledge financial support from University of Houston start-up funds and the University of Houston Provost’s Undergraduate Research Scholarship Program. S.V.K. and E.D.G. acknowledge financial support from NSF under Award DMR-1056199.

were well-controlled with a linear increase of molecular weight as a function of monomer conversion and relatively low values of the PDI (around 1.3). The SAS triblock copolymers exhibited microphase separation, likely a spherical morphology without strong long-range order, as indicated by SAXS and TEM. The physical properties of the polymers can be readily tuned by varying the acrylate midblock composition, including the melting temperature, viscosity, and triblock copolymer tensile properties. Mechanical testing reveals the thermoplastic elastomeric behavior of the triblock copolymers. The acrylate midblock composition does not affect the order−disorder transition temperature of the triblock copolymer. This indicates that the acrylate composition can be used as a tool to manipulate the physical properties of the triblock copolymers without affecting the order−disorder transition temperature, or processing temperature, of the TPEs.

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ASSOCIATED CONTENT

S Supporting Information *

A synthesis scheme for the SAS triblock copolymers using the dibenzyl trithiocarbonate RAFT agent (Scheme S1); representative GPC data (Figure S1); TGA data for poly(styrene), poly(LAc-co-SAc), and SAS triblock copolymers (Figure S2); characteristics of polymers used to determine polyacrylate melting temperatures (Table S1); viscosity as a function of shear rate for poly(LAc-co-SAc) and SAS triblock copolymers (Figure S3); 1D SAXS pattern for all SAS triblock copolymers at room temperature (Figure S4); characteristics of SAS triblock copolymers shown in Figure S4 (Table S2); 2D SAXS data for selected triblock copolymers (Figure S5); Fourier transforms of the TEM images (Figure S6); tensile stress−strain curves for six SAS triblock copolymer series (Figure S7); mechanical properties of poly(n-alkyl acrylate)based TPEs (Table S3); and characterization of molecular weight between entanglements for poly(LAc) (Figure S8). This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 713-743-2748 (M.L.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Avantika Singh, Brian Rohde, and Guozhen Yang for helpful discussions and Kim Mai Le for assistance with experiments. The authors appreciate the assistance of Dr. Charles Anderson for access and training in the University of Houston Department of Chemistry Nuclear Magnetic Resonance Facility, Dr. Ramanan Krishnamoorti and Daehak Kim for access and training on the TGA instrument, and Dr. Rigoberto Advincula and Dr. Kevin Cavicchi for advice on the RAFT process. SAXS measurements were performed at the DuPont−Northwestern−Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., the Dow Chemical Company, and Northwestern University. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC027210


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