Macromolecules 2011, 44, 471–482
471
DOI: 10.1021/ma102134y
Synthesis, Characterization, and Electrospinning of Architecturally-Discrete Isotactic-Atactic-Isotactic Triblock Stereoblock Polypropene Elastomers Carl Giller,† Giriprasath Gururajan,† Jia Wei,‡ Wei Zhang,‡ Wonseok Hwang,‡ D. Bruce Chase,*,† John F. Rabolt,*,† and Lawrence R. Sita*,‡ †
Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716, United States, and ‡Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, 20742, United States
Received September 15, 2010; Revised Manuscript Received December 7, 2010
ABSTRACT: Stereochemically homogeneous and architecturally discrete isotactic-atactic-isotactic triblock stereoblock PP (sbPP) thermoplastic elastomers in which the block lengths for each domain type can be varied in programmed fashion while keeping total molecular weight and molecular weight polydispersity the same has been achieved for the first time. Five sbPP materials were prepared: sbPP-1 (6iso-88a-6iso), total isotactic content, 12%; sbPP-2 (12iso-76a-12iso), 24%; sbPP-3 (18iso-64a-18iso), 36%; sbPP-4 (24iso-50a26iso), 50%; and sbPP-5 (20iso-64a-33iso), 53%. All five sbPP materials were successfully processed by solution-based electrospinning to provide fiberous mats with feature sizes on the nanometer to micrometer length scale. Extensive characterization by analytical (SEM, AFM, tensile testing, DSC,), spectroscopic (FT-IR, FT-Raman), and synchrotron X-ray diffraction techniques of bulk samples, electrospun fibers and solvent cast films of the sbPP samples revealed well-defined trends in elastic properties, morphologies and crystallinity that are associated with a higher degree of crystallinity that emerges with higher isotactic contents. The results of these investigations serve to provide an important foundation that can be used to potentially identify the best combination of stereoerror level incorporation within the isotactic domains and total isotactic content for these architecturally discrete sb-PP materials for maximizing desirable elastomeric traits and solution-based (electrospinning) processing methodology with the goal of achieving the best possible structural forms for potential product applications.
Introduction Thermoplastic polypropene (PP) elastomers have been the subject of intense academic and industrial interest ever since Guido Natta1 first described such a material that was isolated through fractionation of bulk polypropene prepared with the stereoselective heterogeneous Ziegler-Natta catalyst, TiCl3/ AlEt3.2,3 The idealized solid-state structural model suggested by Natta to account for this elastomeric behavior is that of an isotactic-atactic stereoblock polypropene architecture in which long runs of several consecutive stereoregular enchainments of propene give rise to crystalline (hard) block domains possessing an isotactic microstructural configuration that are chemically linked through amorphous (soft) block domains associated with an atactic configuration generated from stereorandom sequences of propene insertions. As proposed for thermoplastic elastomers in general, the crystalline domains that are dispersed throughout the amorphous matrix serve as interchain physical cross-links that provide for elastic recovery after strain-induced deformation.4 In addition to isotactic-atactic stereoblock PP, other stereochemical microstructural forms for PP that are claimed to impart elasticity have been reported, such as isotactichemiisotactic and syndiotactic-atactic stereoblocks.5,6 From a commercial perspective, global annual production of PP-based materials stood at ∼44 million metric tons in 2009,7 and accordingly, the potential of expanding the technological range and *Authors to whom correspondence should be addressed. r 2011 American Chemical Society
applications of PP with a variety of different grades of stereoblock thermoplastic elastomers through “simple” manipulation of stereochemical microstructure during transition-metal-mediated coordinative polymerization of commodity-volume propene monomer is exceedingly attractive. Although a stereoblock microstructural form for PP is easy to conceptualize, in practice, it has been exceedingly difficult to prepare PP materials that meet the criteria for having discrete stereoblock architectures, and Busico2 has recently presented a well-balanced and in-depth critical evaluation of reported claims that have appeared in the literature related to the production of stereoblock PP. In general, the specific challenge that renders stereoblock PP an extremely difficult target is the requirement that a mechanistic scheme must exist that incorporates a credible dynamic and reversible process that can “switch” the nature and degree of stereoselective transition-metal-centered chain-growth propagation from one well-defined state (e.g., highly stereoselective) to another state (e.g., aspecific). Unfortunately, observation of elastic behavior by itself cannot be used as definitive proof of a stereoblock microstructure,8 and many of the reports of homogeneous “two-state” coordination polymerizations of propene actually produce elastomeric PP that is best described as having rather poorly defined stereochemical microstructures in which short runs of monomer insertions occur with a low degree of stereoselectivity within a generally much more stereoregular polymer backbone.2,5,9,10 If irreversible chain-termination and chain-transfer is also occurring at rates that are competitive with propagation, then the final PP microstructure will also be, by Published on Web 01/10/2011
pubs.acs.org/Macromolecules
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default, stereochemically heterogeneous, and, just as in the case of Natta’s original sample, these materials almost invariably can be fractionated into components that possess either more, or less, stereoregularity.11 Perhaps for this latter reason, the current body of literature regarding stereoblock PP is essentially silent on the topic of the postsynthesis processing of these materials into technologically interesting structural forms, such as films, fibers, blends, or composites. Thus, given the present state of the art, it is fair to say that the structural, morphological, thermal, and mechanical properties of well-defined and architecturally discrete stereoblock PP materials still remain enigmatic 60 years after having first been proposed by Natta. Within the past decade, several groups have unequivocally demonstrated that certain classes of molecular transition-metal complexes can serve as initiators for the “living” coordination polymerization of ethene, propene, longer-chain R-olefins, and R,ω-nonconjugated dienes. 12 Our contribution to this effort was the introduction in 2000 of the class of group 4 metal dimethyl, monocyclopentadienyl, monoacetamidinate complexes of general structure, (η5-C5R5)M(Me)2[N(R1)C(R2)N(R3)] (I), which in the case where M = Zr, R = R2 = Me, R1 = t-Bu, and R3 = Et (1), could serve, after stoichiometric “activation” by the borate cocatalyst, [PhNHMe2][B(C6F5)4] (2), as a highly active initiator for the isospecific living coordination polymerization of 1-hexene.13 On the basis of the degenerative group transfer polymerization mechanism involving rapid methyl group exchange between configurationally stable, cationic active propagating centers and configurationally unstable, neutral dormant sites that is depicted in Scheme 1,14 we more recently reported the successful development of a highly versatile, living polymerization protocol by which temporal control over polyolefin stereochemical microstructure can be exerted simultaneously over all propagating centers in such a manner as to provide the first unequivocal examples of architecturally discrete isotactic-atactic stereoblock and stereogradient PP.15-17 Given the strict living character of coordination polymerization that is achieved with 1, this stereomodulation strategy can be employed to produce an endless variety of programmed linear isotactic-atactic stereoblock sequences in which the identity, length, and relative ordering of each stereoblock (atactic or isotactic) is known with certainty, and where the final PP materials are monodisperse with narrow molecular weight polydispersities and with polymer microstructures that are stereochemically homogeneous. Our preliminary investigations of architecturally discrete stereoblock PP were limited to the synthesis and characterization of several isomeric isotactic-atactic multiblock materials in which the total isotactic content and the number-average molecular weight index, Mn, and molecular weight polydispersity, D (=Mw/Mn), were held approximately constant at 40%, 170 kDa, and 1.19, respectively, while the stereoblock architecture was varied according to an atactic-isotactic diblock, an atacticisotactic-atactic triblock and an atactic-isotactic-atacticisotactic tetrablock.15 Gratifyingly, these results revealed a strong dependence of tensile strength and elasticity of melt-pressed samples on the material’s specific stereoblock architecture (i.e., di- vs tri- vs tetrablock). However, not included in these initial studies was the synthesis of discrete stereoblock PP triblock materials possessing an isotactic-atactic-isotactic block sequence that might potentially exhibit the most favorable elastomeric properties due to the end-capping of a middle amorphous atactic domain with crystalline isotactic segments.18 This omission was not due to oversight, but instead, it was deemed necessary at the time due to synthetic difficulties originally encountered with a limited solubility of the active polymeryl species when attempting to carry out the stereomodulated living coordination polymerization of propene at low temperature (∼10 °C) in chlorobenzene in which the first block is both
Giller et al. Scheme 1
isotactic and of a significant degree of polymerization. Fortunately, further optimization of synthetic procedures have now circumvented this restriction, and herein, we report, for the first time, a family of the desired symmetric isotactic-atactic-isotactic stereoblock PP triblocks in which Mn and D are held constant while the total isotactic content is allowed to vary. Characterization and comparison of the mechanical properties of these new stereoblock PP materials with those reported previously15 serve to expand the database of structure-property relationships for this intriguing new class of polyolefin elastomer. Regarding postsynthesis processing, electrospinning is an electrostatically driven polymer processing technique that is useful for the production of polymeric fibers with diameters ranging from tens of nanometers to tens of micrometers.19,20 In addition, the interconnected web-like porous morphology and the large surface area to volume ratio typically observed for electrospun mats make this fiber production process desirable for applications in biomedical engineering, filtration, and textile technologies. Although electrospinning has seen revitalization since 1995, most of the polymers studied to date have been processed at room temperature from common solvents and those polymers that cannot be processed under these conditions still present a special challenge. In this respect, Rabolt and co-workers21-25 have begun to address this challenge and this group has now reported the electrospinning of a number of highly crystalline polyolefins, including polyethene,21 syndiotactic PP,22 isotactic poly(1-butene),23 and isotactic poly(4-methyl-1-pentene),24 as well as the structural and thermal characterization of the corresponding electrospun fibers. The successful electrospinning of these crystalline polyolefins at room temperature is significant, in part, due to the exhibited limited solubility of these materials in commercially available solvents. The now ready availability of a variety of monodisperse architecturally discrete isotactic-atactic stereoblock PP elastomers that are obtained through a living polymerization, coupled with successful demonstration of the electrospinning processing technique for polyolefins provides strong impetus for investigating the ability to prepare electrospun fibers of these novel stereoblock materials that might find use in applications such as medical stents, joint replacements and tissue engineering. Toward this end, further results are reported herein that now provide suitable conditions for the successful electrospinning of the targeted isotactic-atactic-isotactic stereoblock PP materials. Structural characterization of the morphology of the electrospun fibers as a function of isotactic content serve to enhance our understanding of the structure-processing-property relationships of these novel stereoblock PP elastomers. Experimental Section (a). General Considerations. All synthetic manipulations were performed under an inert atmosphere of dinitrogen using standard glovebox techniques. Dry, oxygen-free solvents were employed throughout with chlorobenzene (PhCl) being distilled from calcium hydride under an atmosphere of dinitrogen. Polymer grade propene was purchased from Matheson Trigas and further purified by passing through sequential packed columns of activated Q5 and 4 A˚ molecular sieves. The precatalyst,
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Macromolecules, Vol. 44, No. 3, 2011
Table 1. Block Compositions and Molecular Weight Indices for the Stereoblock PP Materials sample
block composition
Mn (kDa)
Mw/Mn
total % isotactic
sbPP-1 sbPP-2 sbPP-3 sbPP-4 sbPP-5
6iso-88a-6iso 12iso-76a-12iso 18iso-64a-18iso 24iso-50a-26iso 20iso-47a-33iso
200 206 195 220 385
1.28 1.22 1.28 1.33 1.24
12 24 36 50 53
(η5-C5Me5)ZrMe2[N(Et)C(Me)N(t-Bu)] (1), and the methyl donator reagent for stereomodulation, (η5-C5Me5)ZrMe2[N(t-Bu)C(Me)N(CH2C(CH3)3)] (3), were prepared according to previously reported procedures,13,15 while [PhNHMe2][B(C6F5)4] (2) was purchased from Boulder Scientific and used without further purification. Gel permeation chromatography (GPC) was performed using a Viscotek GPC system equipped with a column oven and differential refractometer, both maintained at 45 °C, and four columns also maintained at 45 °C. Tetrahydrofuran (THF) was used as the eluant at a flow rate of 1.0 mL/min. Mn, Mw and Mw/ Mn values were obtained using the Viscotek OmniSEC software (conventional calibration) and 10 polystyrene standards (from Polymer Laboratories). 13C{1H} NMR spectra were recorded at 150 MHz using 1,1,2,2-tetrachloroethane-d2 as the solvent at 90 °C. (b). Polymer Synthesis of Isotactic-Atactic-Isotactic Stereoblock PP (General Procedure). To a solution of 39.8 mg (100 μmol) of 1 in 1.5 mL of PhCl, cooled to -10 °C in a 10 mL vial was added a solution of 84.1 mg (105 μmol) of 2 in 1.5 mL of PhCl all at once via pipet. The resulting yellow solution was then rapidly transferred by pipet to a 500 mL Schlenk flask that was previously charged with 200 mL of PhCl, cooled to -10 °C, and pressurized to 5 psi with propene. After the transfer of this catalyst mixture was complete, the Schlenk flask was pressurized once again to 5 psi and this pressure maintained for 1.5 h with continuous magnetic bar stirring of the polymerization mixture maintained at -10 °C. At this time, a solution of 24.5 mg (55 μmol) of 3 in 3 mL of PhCl was added all at once by pipet to the polymerization mixture by very briefly removing the septum. After repressurizing to 5 psi, the polymerization mixture was stirred at -10 °C for an additional 38 h, whereupon a 1 mL aliquot of the diblock solution was withdrawn and rapidly quenched with 1 mL of acidic methanol. At nearly the same time, a solution of 48.1 mg (60 μmol) of 2 in 4 mL of PhCl was added via pipet and the mixture stirred at -10 °C for a final 4.5 h, at which time the polymerization was quenched by addition of 1 mL of methanol. After purification through precipitation from a concentrated toluene solution into acidic methanol, 14 g of the stereoblock PP product was isolated as a white elastomeric material after drying in vacuo for 18 h. According to previously published procedures,15 GPC and 13C NMR stereochemical microstructural analyses of the intermediate diblock (Mw 217 kDa, PDI 1.18) and triblock (Mw 251 kDa, PDI 1.22) served to establish the desired stereoblock PP architecture and quantitatively determine the relative block lengths as being: 12% isotactic-76% atactic-12% isotactic, or hereafter more conveniently referred to as follows: 12iso-76a-12iso. In similar synthetic fashion, other isotactic-atactic-isotactic stereoblock PP materials were prepared and Table 1 provides a summary of isotactic-atactic block composition and final molecular weight indices that define the desired stereoblock PP products. (c). Phase-Sensitive Tapping Mode Atomic Force Microscopy (ps-tm-AFM). Each of the stereoblock PP materials were dissolved into toluene to provide 1% (w/w) solutions that were then used to form ultrathin polymer films onto either a cleaned Si(111) substrate [pretreated by soaking in 70:30 conc. H2SO4: 30% H2O2 “pirahna” solution (CAUTION!), followed by rinsing with copious quantities of nanopure (
