Long-Chain Hyperbranched Comb Block Copolymers: Synthesis, Microstructure, Rheology, and Thermal Behavior Carlos R. Ló pez-Barró n,* Patrick Brant, Maksim Shivokhin, Jiemin Lu, Shuhui Kang, Joseph A. Throckmorton, Trent Mouton, Truyen Pham, and Rebecca C. Savage ExxonMobil Chemical Company, Baytown, Texas 77520, United States * S Supporting Information ABSTRACT: A series of poly(ethylene-co-acrylic acid)-cb-atactic poly- propylene (EAA-cb-aPP) comb block copolymers were synthesized by grafting aPP-OH macromonomers onto a commercial EAA copolymer made by the high-pressure free radical process. The starting EAA copolymer contains 11 wt % of EAA units and has a significant amount of long chain branches. Therefore, the EAA-cb-aPP copolymers can be classified as hyper- branched. Room temperature atomic force microscopy and X-ray scattering measurements reveal strong, finely textured, phase segregation of the amor- phous aPP and semicrystalline EAA domains, which persists in the melt state. The amorphous aPP side chains have an unexpected nucleating effect that facilitates crystallization of the EAA backbone, as evidenced by an increase in crystallization temperature. Moreover, phase segregation has a strong effect on both the linear and nonlinear viscoelastic response of the copoly- mers. Increases in both the branching density and branch chain length result in an improvement of melt strength as well as an increase in the extensional strain hardening (SH). We postulate that the SH enhancement may arise from the interfacial anchoring of the aPP side chains in the aPP homopolymer domains. This would produce additional resistance for the EAA backbone to stretch under uniaxial load due to an energetically unfavorable process of pulling the aPP arms into the EAA phase where they would face strong repulsions. ■ INTRODUCTION Despite the vast amount of studies devoted to the different aspects of polymers with block structure, their viscoelastic response has received very little attention. A typical test to characterize the order−disorder or order−order transitions (ODT or OOT) in block copolymers (BCP) is via dynamic temperature sweeps, where ODT and OOT are identified as sharp changes is the complex shear modulus. 1−7 However, only a few attempts to establish relationships between microstructure and viscoelastic response of BCPs have been reported. 8,9 Kossuth and co-workers 8 proposed a “universal viscoelastic behavior” of block copolymers with cubic microstructure, consisting of a plateau in the elastic modulus, G′(ω) (denoted G cubic 0 ). In entangled systems, this plateau occurs at considerably lower values of frequency and modulus than those characteristic of the rubbery plateau, G N 0 . In contrast, the low-frequency complex modulus of BCPs with lamellar and hexagonal phases have been observed to exhibit power law dependences G*(ω) ∝ (iω) α , with exponent α ≃ 0.5 for lamellar phases 10−12 and α ≃ 0.3 for hexagonal phases. 13 Note that most of the previous studies are on linear BCPs, whereas the rheological properties of block copolymers with different architectures (such as graft copolymers) are barely explored. Of particular interest for us are the poly(A-cb-B) comb blocks (also known as PA-g-PB graft copolymers). Recent studies have shown effective use of comb block (CB) copolymers as compatibilizers of immiscible blends of A and B homopol- ymers 14−19 and as rheology enhancers in blends with A homo- polymers and in A/B immiscible blends. 19,20 However, only a few studies on the viscoelastic response of CB copolymers have been reported to date. 21−26 Stadler et al. reported creep and dynamic mechanical measurements of CB copolymers having polyethylene (PE) backbone and ethylene−propylene (EP) side chains (poly(PE-cb-EP)). 23 They observed that the molecular weight−viscosity relation in these CB copolymers deviates significantly from the corresponding relations for linear and conventional long-chain branched (LCB) PEs. They explain this deviation based only on the branching architecture. 23 However, they did not consider the possible phase segregation of the PE backbone from the EP side chains as a source of the unusual rheological response, although it is well-known that blends of PEs with different degrees of short branches do phase separate. 27−30 Lin et al. studied the relations between melt microstructure and linear rheology of a series of poly(styrene-co- 4-(vinylphenyl)-1-butene)-g-polyethylene (PSVS-g-PE) copoly- mers with varying branching density. 24 The found that low branching density resulted in microphase-separated structure and a rheological response typical of a network-like structures; Received: January 11, 2018 Revised: July 1, 2018 Published: July 23, 2018 Article Cite This: Macromolecules 2018, 51, 5720-5731 © 2018 American Chemical Society 5720 DOI: 10.1021/acs.macromol.8b00068 Macromolecules 2018, 51, 5720−5731 Downloaded via UNIV OF HOUSTON MAIN on November 12, 2020 at 20:59:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.