Elsevier

European Polymer Journal

Volume 65, April 2015, Pages 202-208
European Polymer Journal

Macromolecular Nanotechnology
Generation of large and locally aligned wormlike micelles in block copolymer/epoxy blends

https://doi.org/10.1016/j.eurpolymj.2014.11.001Get rights and content

Highlights

  • Locally aligned wormlike micelles were generated in a PS-b-PEO/epoxy network.

  • The strategy was to increase the molar mass of a PS-b-PEO generating HPC domains.

  • The large molar mass frustrated the generation of the HPC phase.

  • Instead, locally aligned cylindrical micelles were generated.

Abstract

A dispersion of wormlike micelles, empirically found in some block copolymer (BCP)/epoxy blends, has been reported to produce a significant toughening of epoxy networks. In this study, a rationale procedure to generate and trap large and locally aligned wormlike micelles in an epoxy matrix is reported. A BCP/epoxy/hardener blend was selected that was homogeneous at the polymerization temperature but became nanostructured in the course of polymerization leading to hexagonally packed cylinders (HPC) domains. When a similar BCP with a molar mass about three times larger than the first one and with the same ratio between blocks was used, the nanostructuration into HPC domains was frustrated by diffusional limitations of the large cylindrical micelles generated. A morphology consisting of a dispersion of large and locally aligned wormlike micelles was trapped in the cross-linked epoxy. The selected BCP was polystyrene (PS)-b-poly(ethylene oxide) (PEO), with molar masses M = 43 kDa or 136 kDa and a mass fraction of PEO close to 25 wt%. The network precursors were based on diglycidylether of bisphenol A (DGEBA) and 4,4′-methylenebis(2,6-diethylaniline) (MDEA). Low- and high-molar-mass BCP generated, respectively, HPC domains and wormlike micelles, as supported by TEM images and SAXS spectra.

Introduction

Hillmyer et al. [1], [2] first reported the use of block copolymers (BCP) to generate nanostructures in epoxy networks. The strategy was to self-assemble a BCP in the precursors of the epoxy network, by selecting one immiscible block and one miscible block from the beginning of reaction up to high conversions. In this way, the initially self-assembled nanostructure could be fixed by the cross-linking reaction. Keeping miscibility of one of the blocks up to high conversions enabled to avoid macrophase separation of the whole BCP. However, at high conversions the initially miscible block can also undergo phase separation leading to the occurrence of ordered-ordered phase transitions at particular concentrations [2]. Zheng and colleagues shown that a similar nanostructuration could result when both blocks are initially miscible in the thermoset precursors but one of the blocks becomes phase separated in the course of polymerization while the other block keeps its miscibility up to high conversions [3], [4]. This process was called reaction-induced microphase separation (RIMPS). While the nanostructuration via the self-assembly approach is based on equilibrium thermodynamics of the initial system, for RIMPS the resulting nanostructures depend on the competitive kinetics between polymerization and phase separation. This provides RIMPS with a high versatility to convey the nanostructuration process [4], [5], [6], [7], [8], [9], [10], [11], [12].

Regarding the applications of BCP/epoxy blends, focus has been placed on the role of the BCP as a processing aid [13], [14], as a template for the self-assembly of different type of nanoparticles [15], [16], [17], and as toughening agents [18], [19], [20], [21], [22], [23], [24], [25]. At low BCP concentrations (<10 wt%), spherical micelles, wormlike micelles or vesicles are generated in the epoxy matrix depending on the ratio of epoxy-philic to epoxy-phobic blocks [18], [23], [26]. In water solutions the evolution from spherical micelles  wormlike micelles  octopi  jellyfish  vesicles, was observed when increasing the length of the hydrophobic block of the BCP with respect to the hydrophilic one [27]. Wormlike micelles are convenient morphologies for toughening purposes [20], [21], [22], [23] (the lack of verification of plane-strain conditions might invalidate some of these results [18]).

Usually, small concentrations of BCP (<5 wt%) are required for a significant toughening of the cured epoxy. In the diluted concentration limit, the composition of the BCP that generates wormlike micelles is difficult to predict [26], but may be empirically found [23]. A paper by Hermel-Davidock et al. [28], illustrates the difficulty in predicting conditions to generate wormlike micelles. They investigated BCP/epoxy formulations where nanostructuration was produced before polymerization. It was shown that the nature of a solvent used to homogenize the initial blend and that was later vaporized, determined the generation of either spherical or wormlike micelles that were trapped in the cured material. For one of the blends, wormlike micelles were generated during polymerization from the evolution of spherical micelles present in the initial material. Several papers reported the presence of wormlike micelles in cured BCP/epoxy blends [20], [21], [22], [23], [26], [29], [30], [31], [32]. It is difficult, however, to extract from these studies general rules enabling to obtain wormlike micelles in the final material.

The toughening of high-Tg (glass transition temperature) epoxies with BCP is much more complex. In this case, there is a smaller shear yielding zone ahead of the crack tip determining the need to increase the BCP concentration to allow yielding to take place [25]. However, incorporating large amounts of BCP (>10 wt%) to the epoxy precursors leads to ordered nanostructures such as body-centered cubic (BCC), hexagonally-packed cylinders (HPC), gyroid (G) or lamella (L) [2], [33], instead of the desired micelles.

In this article, a rationale procedure to generate and trap wormlike micelles using a 20 wt% BCP in an epoxy matrix is reported, employing the versatility of the RIMPS process. The starting point is a BCP selected in such a way that both blocks are initially miscible in the epoxy/hardener solvent and that generates hexagonally packed cylinder (HPC) domains in the cured material via RIMPS. The way in which HPC domains are generated in the course of polymerization was recently analyzed [12]. Phase separation starts by the formation of spherical micelles that evolve through a BCC structure to micellar columns, then to cylindrical rods that finally form the HPC phase [12]. The driving force of these transformations is the decrease in the reactive solvent affinity for the epoxy-philic block with the increase in conversion, mainly due to the decrease in the entropic contribution to the free energy of mixing [34]. The aim of this study was placed on trapping the morphology at the stage of cylindrical rods (wormlike micelles), avoiding their packing into HPC domains. This should be accomplished by increasing the molar mass of the BCP keeping the same ratio between blocks. Diffusional restrictions for the packing of large cylindrical micelles should avoid the generation of the HPC domains and arrest the nanostructuration at the level of cylindrical micelles.

The selected BCP to investigate this hypothesis was polystyrene (PS)-b-poly(ethylene oxide) (PEO), with two different molar masses M = 43 kDa or 136 kDa and a mass fraction of PEO close to 25 wt%. The network precursors were based on diglycidylether of bisphenol A (DGEBA) and 4,4′-methylenebis(2,6-diethylaniline) (MDEA). In this system, PEO is the epoxy-philic block and PS is the epoxy-phobic block that is initially miscible at the polymerization temperature (135 °C) but phase separates during polymerization [12]. The solubility of PEO in the reactive solvent decreases with conversion leading to an evolution of the nanostructures. It is also known that the solubility of PEO arises from H-bonds formed among ether groups of PEO and OH groups generated in the epoxy-amine reaction [4], [35], [36]. A temperature increase should decrease its solubility by a decrease in the fraction of H-bonded ether groups (LCST, lower-critical-solution temperature behavior). Cured samples were subjected to a prolonged thermal treatment at 190 °C to investigate a possible effect of PEO insolubility on the nanostructures generated at 135 °C.

Section snippets

Materials

The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA), with an epoxy equivalent of 179 g/mol, determined by chemical titration. The hardener was 4,4′-methylenebis(2,6-diethylaniline) (MDEA, Aldrich), used in an almost stoichiometric proportion (stoichiometric epoxy/amine ratio equal to 0.974 in equivalents of both monomers). Two commercial polystyrene (PS)-b-poly(ethylene oxide) (PEO) BCP (Polymer Source) were selected. The low-molar-mass BCP, L-BCP, had a number average molar

Thermal characterization of the BCP

Both BCP were characterized by DSC to analyze the variation of the melting peak of PEO with its molar mass, a necessary information for the discussion of results shown in next section. Fig. 1 shows DSC thermograms for the BCP of low (L-BCP) and high (H-BCP) molar masses.

The melting point of PEO in L-BCP (Mn PEO = 11,000 Da) is 51.5 °C while in H-BCP (Mn PEO = 34,000 Da) it is 65 °C. The glass transition temperature of the PS blocks is also present in both thermograms (inset in Fig. 1).

Influence of temperature on the solubility of PEO in the cured epoxy network

While the amount

Conclusions

A procedure to generate a large concentration of locally aligned wormlike micelles in BCP/epoxy blends, was presented. It is based on selecting a BCP with a ratio of blocks that generates HPC domains but with a high molar mass that frustrates the self-assembly of the large cylindrical micelles generated during polymerization due to diffusional restrictions. In this way, a large concentration of locally aligned wormlike micelles is generated in the cured blend. The procedure was illustrated

Acknowledgments

We acknowledge the financial support of the University of Mar del Plata (Argentina), the National Research Council (CONICET, Argentina) and the National Agency for the Promotion of Science and Technology (ANPCyT, Argentina). The grant SAXS1-13459 from the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil) is gratefully acknowledged.

References (41)

  • E. Serrano et al.

    Eur Polym J

    (2009)
  • D. Hu et al.

    Eur Polym J

    (2009)
  • B. Nandan et al.

    Eur Polym J

    (2011)
  • J. Gutierrez et al.

    Polymer

    (2011)
  • A. Tercjak et al.

    Eur Polym J

    (2012)
  • L. Ruiz-Pérez et al.

    Polymer

    (2008)
  • E.M. Redline et al.

    Polymer

    (2014)
  • L. Cano et al.

    Polymer

    (2014)
  • Q. Guo et al.

    Polymer

    (2001)
  • M. Rico et al.

    Eur Polym J

    (2012)
  • M.A. Hillmyer et al.

    J Am Chem Soc

    (1997)
  • P.M. Lipic et al.

    J Am Chem Soc

    (1998)
  • F. Meng et al.

    Macromolecules

    (2006)
  • F. Meng et al.

    Macromolecules

    (2006)
  • E. Serrano et al.

    Macromol Rapid Commun

    (2005)
  • E. Serrano et al.

    Marcromolecules

    (2006)
  • E. Serrano et al.

    Macromol Chem Phys

    (2007)
  • W. Fang et al.

    Polymer

    (2008)
  • R. Yu et al.

    Macromolecules

    (2011)
  • H.E. Romeo et al.

    Macromolecules

    (2013)
  • Cited by (20)

    • Nanostructure transformation in epoxy/block copolymer composites with good mechanical properties

      2022, Reactive and Functional Polymers
      Citation Excerpt :

      The peaks located in 1, 30.5 and 90.5 Q values were observed for epoxy containing 22.2 wt% diblock copolymer. It revealed that block copolymer self-organized into cylinders which were hexagonally packed [29,30]. With further increasing the content of PCL1.4W-PS2.8W diblock copolymer, lamellar nanostructures with a layer thickness of ∼74 nm were obtained (Fig. 3d).

    • Blends of tri-block copolymers and addition curing resins: Influence of block copolymer-resin compatibility on toughness and matrix properties on toughenability

      2019, Reactive and Functional Polymers
      Citation Excerpt :

      It seems difficult to find generalizations since in BCP-chemistry there is a multitude of variables that can be modified. BCP can be A-B [29–46], A-B-A [47–54], A-B-C [55–58] or even tetra-block-copolymers [59,60], and in any of these types, the chemical structure of the building blocks as well as their respective molecular weights (MW) can be varied. Though there is a good qualitative understanding of the relationship between the BCP chemical structure and the resulting morphology in the BCP modified thermoset [42–44,53,56,60–63] it is less well understood how these complex morphologies control the fracture toughness.

    • Effect of miscible PMMA chain length on disordered morphologies in epoxy/PMMA-b-PnBA-b-PMMA blends by in situ simultaneous SAXS/DSC

      2016, Polymer
      Citation Excerpt :

      Results indicated that the resultant microphase separated structures are very sensitive to the molecular structure and fraction of the BCPs. For example, the chain length of the philic block is important for the symmetry of cylindrical micelles [3] and for micro/macro-phase separation [5,7], indicating that relaxation of the philic chains in a cross-linked epoxy network is an important factor for the stabilization of nano-sized micelles. The morphology of microphase separation based on self-assembly of BCPs has been investigated in blends using PEO-b-PEE [1], PEO-b-PEP [1,12–15], P(MMA-co-GMA)-b-PEHMA [16], PEO-b-PBO [17], PH-b-PDMS [18], PS-b-PB-b-PMMA [19], and ABCD type BCPs [20,21].

    • Remote activation by green-light irradiation of shape memory epoxies containing gold nanoparticles

      2015, European Polymer Journal
      Citation Excerpt :

      In order to avoid phase separation, the stabilizing ligands must be extremely soluble during the whole conversion range. Poly(ethylene oxide) (PEO) exhibits this behavior, particularly with typical epoxies based on diglycidylether of bisphenol A (DGEBA) [36–39]. Therefore, Au NPs stabilized with PEO chains were synthesized and dispersed in the epoxy monomers.

    View all citing articles on Scopus
    View full text