Toward understanding the
cross-linking from molecular chains to aggregates by engineering
terminals of supramolecular hyperbranched polysiloxane
Yuanbo Zhang, Junshan Yuan, Jingzhi Hu, Zhixuan Tian, Weixu Feng,
Hongxia Yan*
Y. Zhang, J. Yuan, J Hu, Z. Tian, Prof. W. Feng, Prof. H. Yan
Shaanxi Key Laboratory of Macromolecular Science and Technology, Xi’an
Key Laboratory of Hybrid Lumines-cent Materials and Photonic Device,
School of Chemistry and Chemical engineering, Northwestern Polytechnical
University, Xi’an 710129, China
E-mail: hongxiayan@nwpu.edu.cn
Keywords:
hyperbranched
polymers, aggregates,
crosslinking
Abstract:Crosslinking
thermosets with hyperbranched polymers confers them superior
comprehensive performance. However, it still remains a further
understanding of polymer crosslinking from the molecular chains to the
role of aggregates. In this study, three hyperbranched polysiloxane
structures (HBPSi-R) are synthesized as model macromolecules, each
featuring distinct terminal groups (R denotes amino, epoxy, and vinyl
groups) while similar molecular backbone (Si-O-C). These structures were
subsequently copolymerized with epoxy monomers to construct
interpenetrating HBPSi-R/epoxy/anhydride co-polymer systems. The spatial
molecular configuration and flexible Si-O-C branches of HBPSi-R endow
them with remarkable reinforcement and toughening effects. Notably, an
optimum impact strength of 28.9 kJ mol-1 is achieved
with a mere 3% loading of HBPSi-V, nearly three times that of the
native epoxy (12.9 kJ mol-1). By contrasting the
terminal effects, the aggregation states and crosslinking modes were
proposed, thus clarifying the supramolecular-dominant aggregation
mechanism and covalent-dominant dispersion mechanism, which influences
the resulting material properties. This work underscores the
significance of aggregate science in comprehending polymer crosslinking
and provides theoretical insights for tailoring material properties at a
refined molecular level in the field of polymer science.
1.
Introduction
Aggregation and
dispersion have long been regarded
as mutually exclusive concepts, but
this dichotomy is in fact a
fundamental feature of the natural world. From the gravitational
coalescence of dispersed celestial bodies in the
universe, to the islands in the
ocean, and even to the intricate network-formation of polymer
cross-linking, this trend seems like ubiquitous across all scales of the
natural world. This universal significance underscores the importance of
understanding the mechanisms that govern aggregation and dispersion
phenomena in diverse fields of
science.[1- 5]
Conventionally, the
cross-linking of polymer is known as
a network-formation process from linear monomer to three-dimensional
structures, namely the crosslinked
network.[6- 8] But if substitute the linear
monomer with a spatial topological macromolecule, such as
dendrimer,[9]star
polymer,[10]hyperbranched
polymer.[11] These topological macromonomers will
occupy all or part of the cross-linking site to form a
topologically-extended network with respect to their three-dimensional
one, we coined it topological crosslinking networks (TCNs) that link
through all or part of topological units. In such system, it could be
recognized that the macromonomers should undergo both aggregation and
dispersion behaviors during polymer crosslinking, which relies on their
interface nature (for example,
functional groups, surface energy, system
temperature).[12- 14] Therefore, the
aggregate state and nano-interface
control over the cross-linked structure, and attributes the final
properties of the resulting polymer.[15- 19]
Aggregate science contributes a
fresh enlightenment to incline the view of the polymer crosslinking into
a more refined and precise
direction.[15- 19] People customarily
consider the crosslinking of chemical bonds at the molecular level,
however, in case of a multi-components system,[23]although many efforts have been made toward concepting
supramolecular interpenetration or
other related ideas among hydrogels
and
elastomers,[24- 28] The crosslinking modes
should be included but not limited in covalent-bonds and weakly
supramolecular interactions. Especially, in case of a thermoset polymer
system, over-agglomeration of additives has always been regarded as an
undesired matter that dampens material
properties,[29, 30] but the aggregates at
nano- or micro- scales could be the positive one reasoned from their
nano-phase separation and forming a strong interface between
double-phase
polymer.[31]
As a representative crosslinked example, epoxy resins (EPs) feature
excellent thermal and mechanical properties due to their
highly-crosslinked network-forming
architecture,[32- 34] which are widely
involved in human society for aerospace, navigation, wind power. To meet
the ever-growing demands for advanced application, the integrals on
realizing high-performance and multi-function purposes still require an
incisive understanding from
structure-to-property relations toward in-principle building and
tailoring polymers. The
state-of-the-art work focuses their insights overlapping supramolecular
chemistry,[35- 38]dynamic
covalent and/or no-covalent
chemistry[39- 42] to engineer the polymer
network and to regulate the chemical crosslinks and physical
entanglement, which inspires us insightful viewpoints to further
understand the cross-linking, and the real formation of
polymer.[43]
In this study, three kinds of hyperbranched polysiloxane (HBPSi-R) were
designed and synthesized, respectively, each featuring with different
terminals but similar molecular backbone (Si-O-C), and they were
copolymerized with epoxy resin/anhydride system to construct
supramolecular HBPSi-R/epoxy interpenetrating polymer networks. The
thermal performance, curing behaviors and mechanical properties of
resulting materials were studied in a fresh viewpoint of aggregation and
dispersion to co-polymer crosslinking. To contrast the terminal effects,
the aggregation states and nano-interface were revealed, as the
double-crosslinking modes and their aggregate mechanism were proposed in
combining their mechanical properties and different terminal groups.
Highlighting the importance for understanding the polymer crosslinking
from the concept of aggregate science, this work provides theoretical
guidance toward in-principle tailoring material properties from a more
refined molecular structure in polymeric science.