Isotactic Polystyrene Reactor Blends with Tailored Bimodal Molar

Oct 9, 2013 - Reactor blend formation of soluble highly isotactic polystyrene (iPS) enables tailoring of bimodal iPS molar mass distributions containi...
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Isotactic Polystyrene Reactor Blends with Tailored Bimodal Molar Mass Distribution Maximilian Vielhauer,†,‡ Simon Bodendorfer,†,‡ Pierre J. Lutz,§ Christian Friedrich,† and Rolf Mülhaupt*,†,‡ †

Freiburg Materials Research Center (FMF), Albert-Ludwigs-Universität Freiburg i.Br., Stefan-Meier-Straße 21, D-79104 Freiburg, Germany ‡ Institute for Macromolecular Chemistry, Albert-Ludwigs-Universität Freiburg i.Br., Stefan-Meier-Straße 31, D-79104 Freiburg, Germany § Institute Charles Sadron, University of Strasbourg, CNRS UPR 22, 23, rue du Loess, 67034 Strasbourg, France ABSTRACT: Reactor blend formation of soluble highly isotactic polystyrene (iPS) enables tailoring of bimodal iPS molar mass distributions containing variable amounts of ultrahigh molar mass iPS (UHMWiPS). A key feature is the facile iPS molar mass control, achieved by homogeneous catalytic styrene polymerization on a MAO-activated titanium bisphenolate catalyst, using 1,9-decadiene as chain transfer agent. Whereas UHMWiPS (Mw of 947 000 g mol−1) is formed in the absence of the diene, the molar mass Mw increases from 191 000 to 482 000 g mol−1 with decreasing diene/styrene molar ratio. In a cascade of two parallel reactors, polymerizing styrene in the presence and the absence of diene, the mixing ratio of the resulting two iPS solutions governs the UHMWiPS content of the reactor blends (RB-2). Hence, the contents of iPS and UHMWiPS are varied without affecting the average molar mass of both blend components. In reactor blends (RB-1), produced in a single reactor with delayed diene injection, molar mass and polydispersity of iPS/UHMWiPS as well as molar mass of the iPS fraction and UHMWiPS depend on the diene/styrene molar ratio and the delay time of the diene injection. In this study, we investigate the influence of both iPS molar mass and iPS molar mass distributions on crystallization behavior and viscoelastic properties. The correlation of zero shear viscosity with the iPS molar mass exhibits scaling of 3.4, typical for linear polymer chains. Below 10 wt % UHMWiPS content, bimodal iPS molar mass distribution enhances processability by shear thinning.

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INTRODUCTION Whereas atactic polystyrene (aPS), produced by free-radical polymerization, represents a prominent commodity, thermoplastic, semicrystalline isotactic (iPS) and syndiotactic (sPS) polystyrene, obtained by stereospecific catalytic polyinsertion, are specialty engineering plastics.1−4 As compared to aPS, they exhibit improved stiffness and higher dimensional stability.5 In the early days of stereospecific styrene polymerization, heterogeneous Ziegler catalysts3 and anionic styrene polymerization6−8 in the presence of additives produced iPS with rather low stereoregularity and broad molar mass distribution. During the past decade, remarkable progress has been made in stereospecific styrene polymerization on single site catalysts. Today, there exist three different types of single site catalyst systems, producing highly isotactic polystyrene (mmmm > 0.9) with narrow distributions.5 For example, Arai et al.9 described the use of racemic isopropylidene(benzindenyl)−zirconium complex activated by MAO, while Carpentier et al.10 used ansabis(indenyl)−rare earth complexes for isospecific styrene polymerization. As highly versatile catalyst system, Okuda and co-workers employed titanium complexes, containing 1,4dithiabutane-bridged bisphenolate ligands (Ti-OSSO catalyst).11−13 In isospecific styrene polymerization, iPS remains soluble. Subsequently, during melt compounding or by © 2013 American Chemical Society

annealing iPS crystallizes. This is in contrast to sPS, which precipitates immediately as soon as it is formed. In fact, sPS is insoluble in most common solvents at room temperature. Owing to the formation of ultrahigh molar mass iPS with narrow molar mass distribution, melt processing of iPS, produced on Ti-OSSO catalysts, is difficult. In a recent advance, 1-olefins and nonconjugated dienes were identified as highly effective chain transfer agents controlling molar mass by varying the 1-olefin/styrene or diene/styrene molar ratios, respectively.14 In addition to the efficient molar mass control, 1,9-decadiene afforded vinyl-terminated iPS. These vinylterminated iPS were used as intermediates in coupling reactions, thus enabling the preparation of iPS di- and triblock copolymers, containing both iPS and polydimethylsiloxane segments.15 Moreover, similar silane coupling reactions were employed, producing novel molecular silica nanocomposite materials such as linear and star-shaped iPS/POSS hybrids.16 The living styrene polymerization on a modified Ti-OSSO catalyst, as first disclosed by Okuda et al.,12,17 enabled the preparation of iPS-block-cis-1,4-polybutadiene diblock copolyReceived: August 23, 2013 Revised: September 18, 2013 Published: October 9, 2013 8129

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Scheme 1. Single Reactor iPS/UHMWiPS Blend Formation by Delayed Injection of 1,9-Decadiene as Chain Transfer Agent during Styrene Polymerization

mers.18 In recent years, the progress made in iPS polymerization catalysis has been accompanied by a better insight into the crystallization behavior and viscoelastic properties of iPS. For instance, Liu et al. reported on the melting behavior of iPS, investigated by DSC and TEM.19 Cebe et al. described the dual reversible crystal melting and relaxation of the rigid amorphous fraction investigated by heat capacity studies20 and dielectric spectroscopy.21 The effect of the tacticity on the viscoelastic properties of polystyrene was elucidated by Wang et al.22 Moreover, binary stereoisomer blends of iPS with aPS or sPS were prepared.23−25 For achieving a better understanding of iPS processing and properties, it is highly desirable to simultaneously control both iPS molar mass and molar mass distribution. While extensive research has been published on polyolefins with bimodal molar mass distributions,26 very little is known with respect to the influence of iPS molar mass and molar mass distribution on iPS crystallization, processing, and properties. Herein, we exploit the facile iPS molar mass control in the presence of 1,9decadiene as chain transfer agent in order to tailor iPS with bimodal molar mass distribution by means of reactor blend technology.

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under a nitrogen atmosphere. The glass transition temperatures were determined from the second heating scan from 25 to 260 °C to ensure pure amorphous samples. To measure the melting temperatures, the polymer samples were annealed at 170 °C for 10 h after a first heating scan from 25 to 260 °C. Then the melting temperature was measured using two heating scans from 25 to 260 °C. For calibration, an indium and a tin standard were employed. The viscoelastic properties of the different blends were investigated using an Anton Paar Physica MCR301 rheometer with a parallel-plate geometry having a disk diameter of 8 mm under nitrogen atmosphere. After melting the samples at 250 °C in the rheometer, a time sweep with a constant frequency of 10 rad s−1 and 1% deformation at 250 °C for 15 h was carried out to ensure thermal stability. The rheological properties were measured by isothermal frequency sweeps in the temperature ranges between 230−270 °C and 130−150 °C due to the limitation of crystallization. The isothermal frequency sweeps covered the range of 0.1−100 rad s−1 with a deformation of 1% for the high temperature and 0.1% for the low temperature range due to torque limitations. To ensure no degradation during the measurements, repeated experiments at 260 °C were performed in the end of every measurement cycle. Furthermore, the samples were double checked in the end with HT-SEC. To obtain master curves, the different isotherms were shifted with IRIS Rheo-Hub 2008 data analysis using horizontal shift factors to the selected reference temperature of 230 °C. Synthesis of Monodisperse Vinyl-Terminated iPS (iPS-Deca). The low molar mass vinyl-terminated iPS (iPS-Deca) was produced by polymerizing styrene on the MAO-activated Ti-OSSA catalyst, using 1,9-decadiene as chain transfer agent. A typical polymerization is as follows: To a 2 L Schlenk flask, equipped with a magnetic stirrer under a dry argon atmosphere, toluene (427 mL), 1,9-decadiene (1.1 mL, 0.006 mol, 0.01 mol L−1), and then MAO (37.5 mL, Al:Ti = 1500) were added. After stirring the mixture for 5 min at room temperature, styrene (115 mL, 1 mol, 1.67 mol L−1) was added, and the reaction mixture was heated to 40 °C. The bisphenolate titanium catalyst (20.1 mg, 37.5 μmol, 62.5 μmol L−1) was dissolved in toluene (20 mL) for 5 min at room temperature and then injected into the reaction mixture in order to start the polymerization. After 1 h, the resulting iPS solution was precipitated in acidified methanol (2.5 L), filtered off, washed with methanol, and dried at 65 °C at reduced pressure to constant weight. Synthesis of Monodisperse UHMWiPS. Monodisperse ultrahigh molar mass iPS was prepared by styrene polymerization on the MAOactivated bisphenolate titanium catalyst. A typical polymerization is as follows: To a 2 L Schlenk flask equipped with a magnetic stirrer under a dry argon atmosphere, toluene (448 mL) and MAO (37.5 mL, Al:Ti = 1500) were added. After stirring the mixture for 5 min at room temperature, styrene (115 mL, 1 mol, 1.67 mol L−1) was added, and the reaction mixture heated to 40 °C. The bisphenolate titanium catalyst (20.1 mg, 37.5 μmol, 62.5 μmol L−1) was dissolved in toluene

EXPERIMENTAL SECTION

Materials. Dichloro[1,4-dithiabutandiyl-2,20-bis(6-tert-butyl-4methylphenoxy)]titanium was obtained from MCAT GmbH. Methylaluminoxane (MAO, 10 wt % in toluene) was purchased from Aldrich and used without purification. Styrene (99%, Aldrich) and 1,9-decadiene (98%, ABCR GmbH) were dried over CaH2 and distilled. Toluene (Aldrich) was dried using the Vacuum Atmospheres Co. solvent purification system. All polymerizations were performed under dry argon atmosphere using standard Schlenk flasks and glovebox techniques. Characterization. NMR spectra were measured in CDCl3 at 25 °C on a Bruker ARX300 spectrometer with 32 scans for 1H and at 75 MHz with 3000 scans for 13C. The solvent peak (CDCl3) was used as standard and calibrated at 7.26 ppm for 1H and 77.36 ppm for 13C. Weight-average molar mass (Mw) and polydispersity index were determined by high-temperature size exclusion chromatography (HTSEC) at 150 °C using a PL-220 chromatograph (Polymer Laboratories) equipped with three PLGel Olexis columns, using a refraction index (RI) detector and a differential viscometer 210 R (Viscotek). The SEC measurements were carried out in 1,2,4trichlorobenzene (Merck) stabilized with 0.2 wt % 2,6-di-tert-butyl(4-methylphenol) (Aldrich) against an aPS standard. The thermal behavior was measured on a differential scanning calorimeter (DSC 7 PerkinElmer) with a heating rate of 10 K min−1 8130

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(20 mL), stirred for 5 min at room temperature, and then injected into the reaction mixture in order to start the polymerization. After 1 h, the resulting polymer solution was precipitated in acidified methanol (2.5 L), filtered off, washed with methanol, and dried at 65 °C at reduced pressure to constant weight. Synthesis of Bimodal iPS by Single Reactor Blend Technology (RB-1). The bimodal iPS reactor blends were produced by postmetallocene polymerization of styrene on Ti-OSSO in the part time presence of 1,9-decadiene as chain transfer agent. A typical polymerization is as follows: A 0.25 L Schlenk flask equipped with a magnetic stirrer under a dry argon atmosphere was consecutively filled with toluene (73.5 mL) and MAO (7.5 mL, Al:Ti = 1500). After stirring the mixture for 5 min at room temperature, styrene (22.9 mL, 0.2 mol, 1.67 mol L−1) was added, and the reaction mixture was heated to 40 °C. The Ti-OSSO (4.02 mg, 7.5 μmol, 62.5 μmol L−1) was dissolved in toluene (5 mL) for 5 min at room temperature and injected to the reaction mixture to start the polymerization. After 0.5 h, the chain transfer agent 1,9-decadiene (9.0 mL, 0.049 mol, 0.41 mol L−1) was added, and the reaction mixture was stirred for further 0.5 h at 40 °C. To stop the polymerization, the resulting polymer solution was precipitated in methanol (0.4 L), acidified with HCl (6 mol L−1), filtered off, washed with methanol, and dried at 65 °C at reduced pressure to constant weight. Synthesis of Bimodal iPS by Cascade Reactor Blend Technology (RB-2). The different monodisperse iPS fractions were synthesized on Ti-OSSO/MAO as described in the presence and absence of the diene in two parallel reactors. Then, the resulting iPS toluene solutions were blended together in a third reactor. In a 0.1 L round-bottom flask high molar mass iPS (0.6 g) and the low molar mass iPS-Deca (2.4 g) were dissolved at 125 °C in toluene (0.04 L). The solvent was removed under reduced pressure, and the resulting polymer was dried at 65 °C at reduced pressure to constant weight. Polymer Stabilization. All polymers and polymer blends were stabilized with Irganox 1010/Irgafos 168 (4:1, 0.5 wt %).

Scheme 2. Strategies for Producing iPS/UHMWiPS Reactor Blends with Bimodal Molar Mass Distribution by Means of Single (RB-1) and Cascade (RB-2) Reactor Blend Formation

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RESULTS AND DISCUSSION As is apparent from Scheme 1, styrene was polymerized in toluene using the homogeneous MAO-activated dichloro[1,4dithiabutandiyl-2,20-bis(6-tert-butyl-4-methylphenoxy)]titanium catalyst (Ti-OSSO/MAO). Either at the beginning or in the course of catalytic styrene polymerization, the 1,9decadiene was added as chain transfer agent. This chain transfer reaction, involving insertion of the diene vinyl group followed by β-H-elimination, accounts for the formation of vinyl end groups.14 Independent of the iPS molar mass, the polymerization system remains homogeneous. Both single and cascade reactor blend strategies, displayed in Scheme 2, enable tailoring of iPS reactor blends containing variable ultrahigh molar mass iPS (UMWiPS) content. In the iPS cascade reactors (RB-2), styrene is polymerized in the absence and in the presence of 1,9-decadiene in two different parallelized reactors. The diene is injected at the beginning of the polymerization (tI = 0, see Scheme 2). In a sequenced third reactor, the resulting iPS solutions are blended together. Whereas the molar mass of the low molar mass iPS is controlled by the diene/styrene molar ratio,14 the iPS/UHMWiPS ratio is governed by the mixing ratio of the two solutions. In biomodal iPS produced by RB-2, the average molar mass of both iPS and UHMWiPS are independent of the mixing ratio. In the single reactor blends (RB-1), the diene is injected during the isospecific styrene polymerization, typically at around half of the total polymerization time (tI = tp/2, see Scheme 2). Table 1 lists the reaction conditions of isospecific styrene polymerization on Ti-OSSO/MAO together with the molar masses, polydispersity, and the thermal properties of RB-1. In the absence of 1,9-decadiene, styrene polymerization yielded

UHMWiPS with weight-average molar mass of 947 000 g mol−1 and polydispersity of 1.5. The delayed injection of 1,9decadiene during polymerization markedly reduced the average molar mass and drastically broadened the molar mass distribution with increasing diene/styrene molar ratio. Hence, on increasing the diene/styrene molar ratio from 0.004 to 0.24, the average iPS/UHMWiPS (RB-1) molar mass decreased from 484 000 to 400 000 g mol−1, whereas the polydispersity markedly increased from 3.6 to 47.3. In contrast to iPS, as evidenced by SEC analysis (see Figure 1), both the average molar mass and content of UHMWiPS were not affected by the 1,9-decadiene addition and the diene/styrene molar ratio. As a rule, the molar mass of the iPS fraction decreased with increasing delay time of the diene injection, owing to the increased diene/styrene molar ratio at higher conversion when injecting the same amount of diene at different times. The thermal behavior of the RB-1 was examined by means of differential scanning calorimetry. The glass transition temperatures are listed in Table 1. Figure 2 displays the DSC traces of RB-1. The glass transition temperature increased from 84 to 97 °C with increasing molar mass of the low molar mass iPS fraction. This is in accord with the free volume theory, taking into account the higher concentration of end groups typical for low molar mass iPS.27 8131

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Table 1. Compositions, Molar Mass, and Thermal Properties of Bimodal Single Reactor iPS Blends samplea

[1,9-decadiene]/[styrene]

Mnb [g/mol]

Mwb [g/mol]

Mw/Mnb

Tgc [°C]

Tm1d [°C]

Tm2d [°C]

ΔHmd [J/g]

RB-1-0.24 RB-1-0.16 RB-1-0.065 RB-1-0.008 RB-1-0.004 UHMWiPS

0.24 0.16 0.065 0.008 0.004

8 000 13 000 25 000 67 000 134 000 612 000

400 000 407 000 419 000 436 000 482 000 947 000

47.3 32.1 16.5 6.5 3.6 1.5

84 85 92 96 97 98

202 201 202 204 205 205

218 218 219 220 221 219

30 33 31 26 28 27

a Reaction conditions: [titanium bisphenolate] = 65.5 μmol L−1, Al:Ti = 1500, [styrene] 1,67 mol L−1, T = 40 °C, t = 1 h. bDetermined by high temperature SEC in 1,2,4-trichlorobenzene at 150 °C, using refraction index detector and a aPS standard for calibration. cDetermined from the second heating scan. dAfter isothermal crystallization at 170 °C for 10 h.

Figure 3. DSC traces of RB-1 at a heating rate of 10 K min−1 after isothermal crystallization at 170 °C ((a) RB-1-0.004, (b) RB-1-0.008, (c) RB-1-0.065, (d) RB-1-0.16, (e) RB-1-0.24).

Figure 1. HT-SEC traces of bimodal iPS/UHMWiPS (RB-1) reactor blends with delayed injection of the diene chain transfer agent as a function of the diene/styrene molar ratio. For comparison, UHMWiPS produced in the absence of the diene is displayed as reference.

crystalline RB-1 show three endothermic peaks corresponding to an annealing peak (Ta), and two melting temperatures (Tm1 and Tm2). All RB-1 samples melt at temperatures around 219 °C, only marginally affected by variations of iPS molar mass. For comparison, a series of isotactic PS cascade reactor blends (RB-2) were prepared by solvent blending iPS solutions in toluene, prepared by catalytic polymerization in the presence and absence of 1,9-decadiene. Through the selection of the 1,9decadiene/styrene molar ratio of 0.006, the weight-average molar mass of the low molar mass iPS (iPS-Deca) was adjusted to Mw = 168 000 g mol−1 with a polydispersity of 2. Then iPSDeca toluene solutions were blended with solutions of UHMWiPS (Mw = 947 000 g mol−1 with a polydispersity of 1.5). Thus, the UHMWiPS content varied between 0 and 20 wt % without affecting the average molar mass of iPS and UHMWiPS. This is verified by the SEC traces displayed in Figure 4. The properties of RB-2 are summarized in Table 2, listing compositions, molar mass, and thermal properties of iPS/UHMWiPS (RB-2). With increasing amount of the ultrahigh molar mass iPS, the total weight-average molar mass increased from 168 000 g mol−1 for the pure iPS-Deca up to 335 000 g mol−1 for RB-2 containing 20 wt % UHMWiPS. The broadening of the molar mass distribution is reflected by the changes of the polydispersity, which increases from 2 to 4.4 for RB-2 containing 20 wt % UHMWiPS. The thermal behavior of the bimodal RB-2 was investigated by means of differential scanning calorimetry and compared with that of RB-1. Clearly, according to Table 2, both glass

Figure 2. DSC traces of amorphous iPS-RB prior to annealing (heating rate of 10 K min−1, (a) RB-1-0.004, (b) RB-1-0.008, (c) RB1-0.065, (d) RB-1-0.16, (e) RB-1-0.24).

Upon annealing at 170 °C for the duration of 10 h, all RB-1 samples crystallized. In accord with previous reports on the melting behavior of monodisperse iPS,19,20,28 as evidenced by DSC traces of the second heating cycle (see Figure 3), all 8132

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Figure 5. Reduced viscosity η′ vs reduced angular frequency aTω shifted to the reference temperature of 230 °C for the iPS/UHMWiPS reactor blends and high and low molar mass iPS.

Figure 4. HT-SEC traces of bimodal iPS/UHMWiPS (RB-2), containing iPS together with 1, 5, 10, and 20 wt % of UHMWiPS (Mw = 947 000 g mol−1).

Table 3. Zero Shear Viscosity of RB-1 and RB-2 As Compared to the Corresponding Monodisperse High and Low Molar Mass iPS

transition temperature and melting temperature of iPS-SB are not affected by either the molar mass or polydispersity. This is in agreement with the absence of low molar mass iPS, which is present in RB-1 prepared at higher diene/styrene molar ratios. To determine the viscoelastic properties of the iPS reactor blends, rheological experiments were carried out. Thermal stability was confirmed by time sweeps at 250 °C for 15 h. In addition, isotherms at 260 °C were measured before and after every measurement, whereby the congruent curves again confirmed good thermal stability. The isotherms were shifted with the shift factors aT (c1 = 4.8, c2 = 213.1 °C) based on the Williams−Landel−Ferry (WLF) equation29 to the reference temperature at 230 °C. Thereby all samples show simple rheological behavior according of the WLF equation. Figure 5 displays the reduced viscosity (η′) versus the reduced angular frequency (aTω). At high frequency, all samples showed similar viscosity, owing to molar mass independent dynamics of segmental movements, whereas that of high molar mass iPS was slightly lower. All iPS samples exhibit shear thinning behavior. With increasing content of UHMWiPS (compare Table 3), as expected, the plateau value of the reduced viscosity (η′) increased. Zero shear viscosity (η0) was determined by extrapolation of the η′ to zero frequencies (see Figure 5). A strong molar mass dependence on the zero shear viscosity was observed and varied between 320 000 Pa·s for the monodisperse UHMWiPS and 1139 Pa·s for the monodisperse iPS-Deca. The η0 values of the different reactor blends depended on their molar mass and

sample

content UHMWiPS [%]

Mwa [g/mol]

Mw/Mna

η0 [Pa·s]

100 50b 50b 50b 50b 50b 20 10 5 1

947 000 482 000 436 000 419 000 407 000 400 000 335 000 256 000 222 000 191 000 168 000

1.5 3.6 6.5 16.5 32.1 47.3 4.4 3.8 3.2 2.3 2.0

320 000 35 875 34 113 13 838 18 072 26 113 7 917 3 276 2 202 1 489 1 139

UHMWiPS RB-1-0.004 RB-1-0.008 RB-1-0.065 RB-1-0.16 RB-1-0.24 RB-2-20% RB-2-10% RB-2-5% RB-2-1% iPS-Deca a

Determined by high temperature size exclusion chromatography (HT-SEC) in 1,2,4-trichlorobenzene at 150 °C using a refraction index detector and an aPS standard. bCalculated from HT-SEC.

were in the range expected from the two monodisperse iPS reference samples (see Table 3). The different η0 as a function of the Mw are displayed in Figure 6. The measured η0 are proportional to Mw3.39 with an error of 0.15. This is in good agreement with the theoretical value of 3.4 for linear polymers.29 The scattering of η0 of some reactor blends are caused by the their much higher polydispersities.

Table 2. Compositions, Molar Mass, and Thermal Properties of Bimodal iPS/UHMWiPS (RB-2) samplea

content UHMWiPS [wt %]

Mnb [g/mol]

Mwb [g/mol]

Mw/Mnb

Tgc [°C]

Tm1d [°C]

Tm2d [°C]

ΔHmd [J/g]

iPS-Deca RB-2-1% RB-2-5% RB-2-10% RB-2-20%

1 5 10 20

85 000 84 000 69 000 68 000 77 000

168 000 191 000 222 000 256 000 335 000

2,0 2.3 3.2 3.8 4.4

95 95 95 94 95

204 204 204 204 204

220 220 220 220 220

32 29 31 30 29

a iPS cascade reactor blends. bDetermined by high temperature size exclusion chromatography of solutions in 1,2,4-trichlorobenzene at 150 °C using a refraction index detector and calibrating against an aPS standard. cDetermined from the second heating scan. dAfter isothermal crystallization at 170 °C for 10 h.

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state compliance increased by more than 500%. Clearly, iPS reactor blend technology enables adjusting both melt viscosity and compliance.

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CONCLUDING REMARKS Adding 1,9-decadiene as chain transfer agent to homogeneous isospecific styrene polymerization enables tailoring iPS reactor blends with bimodal molar mass distribution and variable content of UHMWiPS. In contrast to sPS, regardless of its molar mass, iPS remains soluble in toluene during polymerization but crystallizes either during melt compounding or by postpolymerization annealing. In a cascade reactor system, UHMWiPS and iPS solutions are produced in two parallelized reactors by catalytic polymerization in the absence or presence of 1,9-decadiene, respectively. By blending together both solutions, iPS/UHMWiPS reactor blends with bimodal molar mass distribution are obtained. The solution mixing ratio governs iPS/UHMWiPS reactor blend composition without affecting the average molar mass of both reactor blend components. In a single reactor system, the delay time and the diene/styrene molar ratio control UHMWiPS content, molar mass, and molar mass distribution. With increasing diene/styrene molar ratio the molar mass distribution drastically broadens, thus producing tailored iPS with ultrabroad molar mass distribution. In principle, these strategies can be applied to tailor tri- and multimodal iPS molar mass distributions. When the molar mass of the low molar mass iPS fraction exceeds 50 000 g mol−1, the glass transition temperature of around 97 °C and the iPS melting temperature of 219 °C are independent of both iPS average molar mass and iPS polydispersity. In contrast to polyolefins, the content of low molecular weight iPS does not appear to significantly increase crystallization rate. The correlation of zero shear viscosity and molar mass is in accord with the behavior of linear polymers. Below UHMWiPS content of 10 wt %, the UHMWiPS incorporation improves processability, as reflected by enhanced shear thinning.

Figure 6. Zero shear viscosity η0 vs the weight-average molar mass Mw (η0 was shifted at 230 °C) for the iPS reactor blends and UHMWiPS.

Processability of polymer melts in shear and/or elongation is related to their viscous and elastic properties expressed by zero shear viscosity η0 and steady state compliance J0e . For instance, the characteristic shear rate γ̇0, for which the deviation from zero shear viscosity plateau sets in and the shear rate dependent viscosity becomes prominent (shear thinning regime), is inverse proportional to the product of both parameters.30 Moreover, the stretchability of melt filaments and their stability during this process are higher the higher the steady state compliance is. Therefore, optimizing the processability of polymers by tuning the molecular weight distribution via the addition of small amounts of high molecular weight component is the method of choice. The corresponding data are displayed in Figure 7 for the reactor blends containing 1, 5, 10, and 20 wt % UHMWiPS. While zero shear viscosity increased monotonically from the low toward the high molecular weight component, the steady state compliance shows the expected maximum for low UHMWiPS content. For instance, the addition of only 5 wt % UHMWiPS increased the viscosity by 100% with respect to the neat low molar mass iPS, while, at the same time, the steady

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG), IRTG 1642 Soft Matter Science. The authors thank Julia Eckerle for her technical assistance, Alfred Hasenhindl for measuring NMR spectroscopy, and Marina Hagios for measuring HT-GPC.

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REFERENCES

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Figure 7. Steady-state compliance J0e and zero shear viscosity η0 vs the UHMWiPS content for the reactor blends containing 1, 5, 10, and 20 wt % UHMWiPS. For comparsion, the high and low molar mass references iPS are included. The compliance is linear fitted to aTω = 10−2 rad s−1. 8134

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, this paper was published on the Web on October 9, 2013, with errors in Scheme 2. The corrected version was reposted on October 11, 2013.

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dx.doi.org/10.1021/ma401770m | Macromolecules 2013, 46, 8129−8135