Article pubs.acs.org/Macromolecules
Melt and Solid-State Structures of Polydisperse Polyolefin Multiblock Copolymers Sheng Li and Richard A. Register* Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States
Jeffrey D. Weinhold and Brian G. Landes Core R&D, The Dow Chemical Company, Midland, Michigan 48674, United States, and Freeport, Texas 77541, United States S Supporting Information *
ABSTRACT: Crystallization in polydisperse ethylene−octene multiblock copolymers, polymerized via chain shuttling chemistry, is examined using two-dimensional synchrotron small- and wide-angle X-ray scattering on flow-aligned specimens. The multiblocks are composed of alternating crystalline (hard) blocks of low 1-octene content and amorphous (soft) blocks of high 1-octene content; the block lengths and the number of blocks per chain are characterized by most-probable distributions. These polymers self-assemble into lamellar domain morphologies in the melt, and the melt morphology is retained in the solid state. Despite extensive mixing between hard and soft blocks, the high crystallinity (>50%) of the hard blocks leads to an alignment of the crystallites within the domain structure, with the orthorhombic polyethylene c-axis generally perpendicular to the lamellar domain normal. The interlamellar domain spacings exhibited by the multiblocks, which exceed 100 nm, are estimated to be 5 times larger than those in near-monodisperse block copolymers having a similar chemical composition and a number-average molecular weight equivalent to the multiblock’s “constituent diblock” repeating unit. This swelling factor exceeds the value of 3 previously reported for analogous polydisperse olefin diblock copolymers, due to the lower segregation strength and enhanced phase mixing of the multiblocks studied here.
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INTRODUCTION Recent advances in polymerization catalysis have allowed the production of olefin block copolymers (OBCs) with alternating crystallizable and noncrystallizable blocks.1 These OBCs can be made in a continuous flow process, employing a chain shuttling polymerization technology developed by The Dow Chemical Company.2 In this process, the block copolymers can be prepared in a single reactor, containing both ethylene and 1octene monomers, along with a mixture of catalysts with distinctively different monomer selectivities: a Zr-based catalyst which is a poor incorporator of 1-octene to catalyze the growth of semicrystalline (hard) blocks, and a Hf-based catalyst which is a good incorporator of 1-octene to catalyze the growth of amorphous (soft) blocks. A chain shuttling agent (CSA), typically a metal alkyl such as diethylzinc, is also added to pass the growing polymer chains between the two different types of catalyst sites, resulting in the production of linear multiblock copolymers with alternating hard and soft blocks. With this chain shuttling chemistry in a continuous-flow stirred tank reactor, the multiblocks should exhibit most-probable distributions of both the block lengths and the number of blocks per chain. © 2012 American Chemical Society
Several characterization studies on OBC multiblocks have been reported.3−8 In the earlier studies, the copolymers had a relatively small differential in octene content between the hard and soft blocks, and thus formed homogeneous melts.3,4 Their solid-state structure was then dictated by crystallization, resulting in alternating crystalline and amorphous lamellae with a spherulitic superstructure. More recently, OBCs with higher octene differentials have been examined; based on their melt rheology5 and crystallization behavior,6,7 it was concluded that at sufficiently high octene differential, these OBC multiblocks self-assemble into domain structures in the melt, giving rise to more complex solid-state structures. However, while the total molecular weight and distribution of an OBC multiblock can readily be determined, no information has been available on the average lengths or distributions of the constituent hard and soft blocksunlike the better-known case of a “living” polymerization in batch, where the reactor can be sampled at different times during the polymerization to provide such information. This lack of information on the block Received: May 4, 2012 Revised: June 19, 2012 Published: June 29, 2012 5773
dx.doi.org/10.1021/ma300910m | Macromolecules 2012, 45, 5773−5781
Macromolecules
Article
Table 1. OBC Multiblock Characterization Data sample code
C8 in hard block (wt %)
C8 in soft block (wt %)
wt % hard block ( f hard)
Mtot n (kg/mol)
PDItot
MDi n (kg/mol)
Tm,peak (°C)
Tc,onset (°C)
wc
χN at 167 °C
dDS (nm)
OBC-4767 29 OBC-4884 23 OBC-4881 22 OBC-3688 22
5.4 5.4 5.4 5.0
58.9 60.3 61.5 56.7
47 48 48 36
67 84 81 88
2.7 2.1 2.1 2.2
29 23 22 22
119 116 116 118
104 103 102 101
0.56 0.58 0.53 0.57
3.7 3.2 3.3 2.5
123 102 107 136
multiblocks, we are able to estimate the average block lengths in these materials. This then allows the structural characteristics of the OBC multiblocks to be compared with those of the OBC diblocks, as well as near-monodisperse block copolymers, on a comparable N basis.
molecular weights has, to some extent, hindered the development of structure−property relationships in these novel OBC materials. Crystallization in block copolymers has attracted interest because in addition to block incompatibility, block crystallization can also lead to domain formation in the solid state.9−11 Block incompatibility can be gauged by the quantity χN, where χ is the Flory−Huggins interaction parameter and N is the degree of polymerization; for diblock copolymers, N is usually taken as the total diblock degree of polymerization, while for (AB)n multiblocks such as the OBCs, N is more usefully related to the degree of polymerization of the AB repeat. Unfortunately, it is precisely this quantity which is not currently measurable on the OBCs. In diblocks, when χN is small, crystallization proceeds from either a disordered12,13 or weakly segregated melt,14,15 giving rise to a solid-state morphology that is typically alternating crystalline/amorphous lamellae, regardless of the copolymer compositionvery similar to the case of OBC multiblocks with low octene differential.3,4 But when the value of χN is large, crystallization may be confined within the existing microdomains, such that the melt morphology is preserved in the solid state.16−19 This type of confined crystallization can also affect the crystal orientation.20−23 For example, when crystallization is confined within lamellar microdomains, the crystals orient their stems perpendicular to the lamellar normal to alleviate the incommensurability between the preferred crystal thickness and the thickness of the pre-existing lamellar domains.20,21 At intermediate values of χN, the solid-state morphology exhibited by the block copolymer depends strongly on crystallization history.24,25 While fast cooling may confine crystallization within the existing domains, slow cooling can lead to substantial reorganization of the copolymer’s structure. In a previous publication, we examined the melt and solidstate morphologies of OBC diblocks, and found that despite both blocks being polydisperse, they self-assembled into wellordered lamellae in the melt, which were preserved upon cooling into the solid state.26 The interlamellar domain spacings exhibited by these OBC diblocks were in excess of 100 nm, nearly 3-fold larger than those in near-monodisperse polyethylene block copolymers of similar number-average molecular weights. Interestingly, although the lamellar melt morphology was preserved in the solid state, the diblocks lacked crystal orientation, which was attributed to extensive interblock mixing that allowed growing crystallites to “pass through” the soft domain without disturbing the overall domain morphology. In the present study, we use simultaneous smallangle (SAXS) and wide-angle X-ray scattering (WAXS) on flow-aligned polymer specimens to examine the morphologies of OBC multiblocks. These multiblocks have a much lower hard block octene content and thus a higher hard block crystallinity than the previously studied diblocks, which leads to crystal orientation relative to the lamellar domains. Furthermore, by extracting unattached soft segments from the OBC
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EXPERIMENTAL SECTION
OBC Multiblocks. Four OBC multiblocks were examined in this study, with their relevant characteristics given in Table 1. Each multiblock copolymer is identified by the code “OBC-f hard xy ”, where f hard indicates the polymer’s weight percent hard block, and x and y indicate the polymer’s number-average total molecular weight (Mtot n ) and constituent diblock molecular weight (MDi n , defined below) in kg/ mol, respectively. The details of polymer synthesis have been described elsewhere,2 and the melt rheology and some solid-state 5 structural data for OBC-4884 23 have been reported previously. The octene contents in the hard and soft blocks, and f hard, were measured by 13C NMR on the whole polymer, in a 50/50 mixture of tetrachloroethane-d2/orthodichlorobenzene at 130 °C.27 The overall molecular weights and polydispersity indices (PDItot) of the multiblocks were measured by high-temperature gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene (TCB) at 140 °C, relative to linear polyethylenes of the same elution time.27 These apparent tot ) were then “polyethylene-equivalent” molecular weights (MPEeqv converted to the true values (Mtot) according to28,29
M tot =
tot MPEeqv tot 1 + woct (roct − 1)
(1)
wtot oct
is the total weight fraction of octene in the multiblock, and where roct is the hydrodynamic equivalence ratio for polyoctene, relative to polyethylene in TCB, estimated as 0.37.26 OBC Extracts. Unattached soft blocks were extracted from the OBC multiblocks by stirring the as-received polymer pellets (diameter ∼3 mm) in n-hexane at room temperature for 10 days. The liquid phase was then slowly transferred into a heated polytetrafluoroethylene dish to evaporate the solvent, leaving behind the extracted fraction, which was subsequently characterized by GPC and NMR. OBC-3688 22 pellets recovered at the end of the first 10-day extraction were combined with fresh n-hexane and stirred at room temperature for another 10 days, resulting in
