Hierarchical reinforcement of polyurethane-based composites with inorganic
micro- and nanoplatelets
Rafael Libanori, Frédéric H.L. Münch, Davi M. Montenegro, André R. Studart
⇑
Complex Materials, Department of Materials, ETH Zurich, Zurich 8093, Switzerland
article info
Article history:
Received 24 June 2011
Received in revised form 27 September 2011
Accepted 3 December 2011
Available online 13 December 2011
Keywords:
A. Particle-reinforced composites
B. Mechanical properties
C. Anisotropy
D. Rheology
E. Casting
abstract
Hierarchically reinforced structures are widespread in nature but less common among man-made mate-
rials. In this paper, we show that polyurethane-based thermoplastic polymers can be hierarchically rein-
forced with laponite nanoplatelets and alumina microplatelets to reach strength and elastic modulus that
are, respectively, 7- and 29-fold higher than that of the pure polymer matrix (91.7 MPa and 6.97 GPa,
respectively). We find that the selective reinforcement of the polyurethane hard domains with laponite
nanoplatelets is key to keep the polymer matrix sufficiently ductile for the incorporation of high concen-
trations of alumina microplatelets. Effective reinforcement of the polymer with microplatelets of differ-
ent surface chemistries was only possible after annealing the composite at 130 °C to promote strong
bonding at the oxide/polymer interface. Large-area composite films and bulk parts exhibiting good align-
ment of alumina microplatelets were obtained through conventional tape-casting. The concept of hierar-
chical reinforcement demonstrated here can be explored to obtain composite materials covering a wide
range of mechanical properties using only a few reinforcing building blocks within the same polymer
matrix.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Polyurethanes find widespread use in biomedical, structural
and automotive applications [1,2]. The mechanical behavior of this
copolymer can be tuned from soft and rubbery to hard and stiff by
simply changing the ratio of hard to soft segments in the macro-
molecule’s backbone [1]. The addition of reinforcing filler particles
into polyurethanes greatly increases their versatility by extending
the range of mechanical properties that can be achieved and possi-
bly incorporating further functionalities [3–6].
Materials exhibiting mechanical behavior that can be tuned
over a wide range by changing the fraction of one or more of their
constituents are important in many technological and natural sys-
tems. Polymeric substrates potentially used in flexible electronics
for example should have locally tuned mechanical response in or-
der to reduce the mechanical mismatch between the flexible sub-
strate and the hard metallic circuitry [7,8]. Keeping one of the
constituents as continuous phase while varying the concentration
of other building blocks to control the mechanical properties is
an interesting approach because it potentially eliminates interfaces
that work as stress concentrators [7].
Because of their rather limited choices with regards to chemical
compositions, living organisms are able to build biological materials
with very different mechanical properties by just controlling the dis-
tribution and arrangement of a few types of building blocks within
the same continuous matrix. This is the case for example of the long
threads used by mussels to anchor themselves on rocks, where high
strength, elasticity and surface wear resistance are combined by lo-
cally changing the cross-linking density of the organic matrix
throughout the material [9]. Likewise, materials like seashells, fish
scales, bone and teeth have their degree of mineralization and the
orientation of inorganic building blocks locally adjusted to vary
the material’s stiffness by nearly an order of magnitude [10–16].
Remarkably, the concentration of inorganic phase dispersed within
a polymeric matrix can be as high as 95 vol% in such mineralized bio-
logical materials. Although the underlying design principles are still
being investigated [17], the organization of building blocks of differ-
ent sizes into hierarchical structures is a reoccurring approach used
by living organisms to tailor the properties of natural materials.
In contrast to hard biological materials, artificial polymers rein-
forced with inorganic particles become remarkably brittle above a
critical volume fraction of the inorganic phase, which is character-
ized by a substantial decrease in the work of fracture [18–20].
Since the critical particle concentration leading to such reduction
in the work of fracture is typically lower than 30–50 vol%, the
range of mechanical properties that can be covered using a given
polymer matrix reinforced with inorganic particles is rather lim-
ited in artificial composites.
0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compscitech.2011.12.005
⇑
Corresponding author.
E-mail address: andre.studart@mat.ethz.ch (A.R. Studart).
Composites Science and Technology 72 (2012) 435–445
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology
journal homepage: www.elsevier.com/locate/compscitech