Synthesis and application of styrene/4-hydroxystyrene gradient copolymers made by controlled radical polymerization: Compatibilization of immiscible polymer blends via hydrogen-bonding effects

doi:10.1016/j.polymer.2006.06.030

Polymer

Volume 47, Issue 16, 26 July 2006, Pages 5799-5809

https://doi.org/10.1016/j.polymer.2006.06.030 Get rights and content

Abstract

Styrene (S)/4-hydroxystyrene (HS) copolymers are synthesized by hydrolysis of S/4-acetoxystyrene copolymer precursors; two gradient copolymer precursors are made by semi-batch, nitroxide-mediated controlled radical polymerization, and a random copolymer precursor is prepared by conventional free radical polymerization. Conventional heat curves from differential scanning calorimetry indicate two glass transition temperatures (T_gs) and a broad T_g in well-annealed 59/41 mol% and 25/75 mol% S/HS gradient copolymers, respectively, both of which contain short S end-blocks. In contrast, a narrow T_g is observed in a 57/43 mol% random copolymer. Each S/HS copolymer is added at 5 wt% by solution mixing to an 80/20 wt% polystyrene (PS)/polycaprolactone (PCL) blend and tested for its ability to compatibilize the blend during melt processing; the hydroxyl groups on the HS units can form hydrogen bonds with the PCL ester groups. The S/HS random copolymer fails as a compatibilizer while both gradient copolymers are good compatibilizers. Relative to the blend without copolymer, the blend with 59/41 mol% S/HS gradient copolymer also exhibits a major reduction in initial dispersed-phase domain size and irregularly shaped domains, which are indicators of a sharply reduced interfacial tension. In contrast, the blend with 25/75 mol% S/HS gradient copolymer has an average PCL domain size comparable to the blend without copolymer and a broad domain size distribution. The presence of S/HS copolymers in the blend leads to reduced PCL crystallization and melting temperatures as well as reduced enthalpies of crystallization and melting, consistent with some solubilization of copolymer in the PCL domain interiors.

Introduction

Blending two or more immiscible polymers has the potential to lead to synergistic material properties. A common requirement for this potential to be realized is that the interparticle distance between dispersed-phase domains is maintained below a value called the critical ligament thickness [1], [2]. In turn, this usually means that the average dispersed-phase diameter must be maintained to be less than a few microns [1], [2], [3], [4]. Hence, stabilization of the dispersed-phase domain size against melt-state coarsening, taken as the criterion for compatibilization [5], is important in processing immiscible polymer blends.

Accordingly, many compatibilization strategies have been studied. In particular, addition of various types of copolymer (e.g., block [3], [5], [6], [7], [8], [9], [10], [11], [12], [13], tapered block [14], [15] and graft [9], [16], [17]) during melt processing has been heavily examined and shown to be successful in small-scale (although not large-scale [3]) studies. Addition of block copolymer to immiscible blends during solid-state shear pulverization (SSSP) has recently been shown to achieve compatibilization [18]. Reactive blending involving in situ formation of block or graft copolymers during melt processing [13], [19], [20], [21] or SSSP [22], [23] has also yielded compatibilization. Regardless of the presence or absence of specific attractive interactions between the added copolymer and any of the blend components, it is well appreciated that interfacial block and graft copolymers can reduce interfacial tension and provide steric hindrance to dispersed-phase coalescence [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29].

The advent and development of controlled radical polymerization (CRP) have offered the possibility of synthesizing new classes of polymers with well-defined molecular structure [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. Among others, CRP can produce statistical or random copolymers with narrower molecular weight distribution [30], [31] than conventional free radical polymerization (ConvFRP) and block copolymers that cannot be made by anionic polymerization (e.g., copolymers with 4-acetoxystyrene blocks [39], [40]). In contrast to anionic polymerization, CRP can yield gradient copolymers with a gradual change in composition along the chain. The nature of CRP, providing for both long propagating radical lifetimes and a broad array of co-monomers that can cross-propagate, allows for the formation of gradient copolymers by modifying as a function of time the co-monomer composition in a copolymerization reaction mixture [32], [33], [34], [35], [36], [37], [38]. (Anionic methods do not yield gradient copolymers with batch or semi-batch polymerization due to the very large differences of reactivity ratios associated with anionic copolymerization.) Gradient copolymers are expected to be highly effective compatibilizers when added to immiscible blends during melt processing due to a much higher critical micelle concentration and better interfacial activity than block copolymers of the same composition [41]. A study demonstrating enhanced interfacial activity of gradient copolymers, as compared with random or block copolymers, supports these expectations [42]. Using a polystyrene/poly(methyl methacrylate) blend and styrene/methyl methacrylate gradient copolymers, we recently provided the first demonstration that blend compatibilization can be achieved by gradient copolymer addition during melt processing [43].

Here we provide experimental results on the application of gradient copolymers made by nitroxide-mediated controlled radical polymerization (NM-CRP) in compatibilizing a polymer blend in which a favorable thermodynamic interaction, hydrogen bonding, exists between the dispersed phase and one of the repeat units of the added gradient copolymer. In other words, our study involves the compatibilization of an A/B polymer blend by an A/C gradient copolymer where the B and C repeat units can participate in hydrogen bonding [24], [25], [26]. We add low levels (5 wt% relative to total blend weight) of styrene/4-hydroxystyrene gradient copolymer to immiscible 80/20 wt% polystyrene/polycaprolactone blends. (The 4-hydroxystyrene repeat units, also called 4-vinyl phenol repeat units, are known to form hydrogen bonds with the oxygen atoms in the ester groups in the polycaprolactone [44], [45], [46], [47], [48], [49], [50].) The effects of gradient copolymer addition on initial dispersed-phase domain size and coarsening during high-temperature static annealing are studied as a function of gradient copolymer composition and are compared to results obtained with addition of S/HS random copolymer. We find that added S/HS gradient copolymers are effective in the suppression of PCL domain coarsening during static, high-temperature annealing, i.e., in achieving compatibilization, while added S/HS random copolymers do not compatibilize PS/PCL blends.

We also present the non-isothermal crystallization and melting behavior of PCL domain in each PS/PCL blend. The added S/HS copolymers exhibit strikingly different degrees of impact on the crystallization and melting behavior of the PCL dispersed phase as a function of the detailed copolymer structure. We also note that irregular, non-spherical dispersed-phase domains are formed in the 80/20 wt% PS/PCL blends compatibilized with low levels of S/HS gradient copolymers and that the irregular interfaces remain stable during high-temperature annealing. Finally, given the multi-step nature of the production of S/HS gradient copolymers via hydrolysis of S/4-acetoxystyrene gradient copolymers, we also provide descriptions of the synthesis and characterization of the gradient copolymers.

Section snippets

Materials and methods

Styrene (Aldrich, 99%) and 4-acetoxystyrene (AS, Aldrich, 96%) were deinhibited using tert-butylcatechol inhibitor remover and dried over CaH₂ before use. The unimolecular initiator A-T (N-(α-methylbenzyloxy)-di-tert-butylamine) was synthesized as previously reported [37], [38], [51], and AIBN (Aldrich) was used as received. Polystyrene (Pressure Chemical; nominal M_w = 30,000 g/mol) and polycaprolactone (Aldrich; nominal M_w = 80,000 g/mol) were used as received for the major and the minor phases of

Gradient and random copolymer characterization

Table 1 provides the characterization of apparent molecular weight (MW) and overall styrene mole fraction for the S/4-acetoxystyrene and S/HS gradient and random copolymers synthesized in this study. One gradient copolymer and the random copolymer have nearly identical overall styrene mole fractions (F_S = 0.59 for SgradHS1 and F_S = 0.57 for SranHS); a second gradient copolymer has a much lower styrene mole fraction (F_S = 0.25 for SgradHS2). The apparent M_n and M_w values are roughly similar in the

Summary

Two S/AS gradient copolymers were synthesized by NM-CRP, and one S/AS random copolymer was made by ConvFRP; these copolymers were hydrolyzed to obtain S/HS copolymers. The S/HS random copolymer and the S/HS gradient copolymer of nearly identical overall composition, 57 mol% S and 59 mol% S, respectively, yield one T_g and two T_gs, respectively, after high-temperature annealing. Thus, the gradient copolymer is ordered (microphase separated) while the random copolymer is disordered. A S/HS gradient

Acknowledgments

The support of the NSF-MRSEC program (grants DMR-0076097 and DMR-0520513), Northwestern University and a 3M Fellowship (to JK) are gratefully acknowledged. We also acknowledge the use of the scanning electron microscope in a shared user facility of the Northwestern University Materials Research Center.

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