SPECIALTY GRAND CHALLENGE ARTICLE
published: 04 December 2013
doi: 10.3389/fchem.2013.00031
Grand challenges in polymer chemistry: energy,
environment, health
Pellegrino Musto *
Institute of Chemistry and Technology of Polymers, National Research Council of Italy, Pozzuoli, Italy
*Correspondence: pellegrino.musto@ictp.cnr.it
Reviewed by:
Gaetano Guerra, University of Salerno, Italy
Keywords: polymer chemistry, nanocomposites, nanomedicine, polyelectrolytes, computational polymer science
Polymer science is a so pervasive and relevant discipline in the contemporary scenario that it is unnecessary to spend much
words to emphasize its role. As a matter
of fact, it has been proposed to designate
our time as the polymer age, to mark its
distinction from previous mankind eras
dominated by a series of diverse materials (the stone, the bronze, the iron ages)
and to remark that our lifestyle would be
hardly conceivable without polymers. The
advent and the global scale establishment
of the polymer technology has shaped
the world around us and has profoundly
changed its perspectives, as it occurs for
any revolutionary technology. Despite the
astonishing achievements we have witnessed along the years, many exciting challenges remain to be faced; these are well
worth to tackle because of their impact
on our everyday life: examples include
green polymer chemistry, environmental
pollution issues, polymers for energy storage and delivery, polymers for the human
health.
In the following, the research subjects
considered most challenging and richer
of promise will be touched upon; these
will form the main focus of the Specialty
Section Frontiers in Polymer Chemistry.
Traditionally, physical chemistry of
polymeric systems is a field of great interest
and activity, witnessed by the awarding of
two important Nobel prizes (Flory, 1974;
de Gennes, 1991). The extensive knowledge accumulated so far makes it a mature
subject, but areas in rapid evolution
still exist. Thus, although classical thermodynamic approaches are considered
to be well established, major refinements are being proposed on the theme
of H-bonding systems. Technologically
relevant fields such as membranes for
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gas mixtures separation, drug delivery
materials, barrier structures for food
packaging, durability, and aging issues
in polymers and polymer-composites, are
all impacted by the ubiquitous occurrence of H-bonding interactions between
the substrate and low molecular weight
penetrants, especially water. Theories
describing the transport properties of
these systems by extending available
mean-field models to include self- and
cross-interactions are being actively investigated (Panayiotou and Sanchez, 1991;
Panayiotou et al., 2007). The comparison of the theoretical predictions with
recent experimental data on the molecular
aggregates’ populations, made available
by spectroscopic techniques such as timeresolved FTIR and solid-state NMR has
contributed to check and refine the theoretical approaches. Improvements are
expected in the near future along the
directions of classical lattice fluid theories
[notably, with the introduction of NonRandom lattice fluid Hydrogen Bonding
models (NRHB)] and on the side of the
available schemes to treat non-equilibrium
glassy polymers (Scherillo et al., 2013).
A further physical-chemistry area in
turbulent evolution is computational
polymer science. The fast progress in terms
of computing resources, coupled with the
advent of powerful, less computationally intensive methodologies (notably, the
Density Functional Theory, DFT, and the
associated hybrid functionals like B3LYP,
which alleviated many of the deficiencies
of previous formulations) provided the
theoreticians (and an increasing number of experimentalists) with a tool to
tackle large chemical systems, well exceeding 100 atoms. This has opened up
new opportunities, especially in regard
to the interpretation of spectroscopic
observables (IR, Raman, UV-VIS, NMR)
in terms of electronic structure methods
(Castiglioni, 2007; Zerbi, 2007). This is a
point of utmost interest: we are getting
closer and closer to a complete understanding of the molecular spectra, which,
in turn, will disclose us the wealth of
molecular level information contained
therein. Clearly, the well established atomistic approaches continue to dominate the
scene: molecular mechanics and molecular
dynamics methods, owing to their robustness and computational efficiency, still
represent the work-horses to face the more
demanding problems related to biopolymers, synthetic polymers both organic and
inorganic, and for coarse-grained, multiscale modeling (Zang et al., 2008; Tadmor
and Miller, 2011). The relevance of this
research field has been recently recognized
with the awarding of the 2013 Nobel Prize
in Chemistry to three major players in this
arena, Martin Karplus, Michael Levitt and
Arieh Warshel “for the development of
multiscale models for complex chemical
systems” as stated by the Prize motivation. Major advancements can be foreseen
in areas such as molecular mechanics
and quantum mechanical calculations of
intramolecular and intermolecular energies, refinement of ab-initio simulation
of spectroscopic observables (frequency
and, especially, intensity) chain-packing
in the crystalline and liquid-crystalline
states, rotational isomeric state theory,
and in the simulation of the solid state,
both crystalline and amorphous.
Synthesis and chemical modification of polymers is another area of
exceptional progress. Fueled by the
development of the controlled-synthesis
routes [Radical Addition Fragmentation
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Musto
Transfer polymerization (RAFT, BarnerKowollik, 2008); Atom Transfer Radical
Polymerization (ATRP, Siegwart et al.,
2012),
click-chemistry
(Lutz
and
Sumerlin, 2009) and supramolecular
chemistry routes (Bertrand et al., 2012)]
this field has experienced an unprecedented period of growth after years of
neglect. The ability to manipulate and
tune the matter at an atomic scale achieved
by contemporary polymer chemists is
truly remarkable: some scientists (the
most enthusiastic) even suggest that, at
the current state of the art, any architectural design can be achieved within very
narrow limits of molecular weight and
end-group fidelity. In fact, with the synthetic techniques available nowadays it is
possible to develop structures (micelles)
that may reversibly self-assemble according to a specific stimulus (Zhang et al.,
2012), shape-memory structures (Hu
et al., 2012), supramolecular assemblies
mimicking the biological macromolecules
and their functions (Harada and Kataoka,
2006). The current focus is no longer on
structural but rather on functional polymers, with an ever increasing range of
capabilities and applications. These developments are strictly related to human
health issues: many of the new synthetic
polymers find application as drug delivery
systems (Bajpai et al., 2008; Oh et al.,
2008) as molecularly imprinted polymers for clinical diagnostics (Kim et al.,
2006, 2007), as intelligent targeting carriers in modern therapeutic strategies (Park
et al., 2008; Wei et al., 2013), to mention only a few of the most remarkable
achievements. The outcomes of these forefront researches have generated an entirely
new field of activity, Nanomedicine (Park
et al., 2008; Mignani et al., 2013), dealing with highly specific, molecular-scale
medical intervention for treating diseases
or repairing damaged tissues. Polymerbased nanomedicine, including the use
of polymer–DNA complexes (polyplexes)
(O’Rorke et al., 2010) polymer–drug conjugates, and polymer micelles bearing
hydrophobic drugs, has emerged as one
of the most attractive approaches for
improving the efficacy of cancer therapeutics (Park et al., 2008). Among the
most actively investigated and promising areas mention should be made of
(i) stimuli-sensitive systems and their
Frontiers in Chemistry | Polymer Chemistry
Grand chjallenges in polymer chemistry
synthesis, especially in relation to polymer hydrogels; (ii) the preparation and
characterization of gradient polymer surfaces for biomedical applications, that is,
surfaces with a gradually varying chemical
composition along one dimension; (iii)
the development of increasingly efficient
polymer systems for gene delivery, a field
whose great promises have not been fulfilled yet, because of the many challenging
issues which remain to be settled (Wong
et al., 2007).
Two issues are driving worldwide
research toward polymers from natural and renewable sources: the growing
concern about environmental deterioration caused by the increasing production
and disposal of conventional synthetic
polymers, and the will to reduce the
dependence of western economies from
petrochemical products. Vegetable oils are
one of the most readily available alternatives: they are cheap, renewable resources
and the functional groups present therein
can be suitably activated for polymerization (Yu et al., 2006; Raquez et al., 2010).
Vegetable oils are obtained from naturally occurring plants, such as sunflower,
cotton, linseed and consist, predominantly, of triglycerides. The structure
of these monomers can be modified
according to different strategies which,
in turn led to various synthetic routes,
including condensation, radical, cationic,
and methathesis polymerization (Güner
et al., 2006; Sharma and Kundu, 2008).
Recently, the focus is being shifted toward
thermosetting materials, i.e., highly crosslinked networks cured by heat, heat and
pressure, and/or light irradiation. Very
interesting opportunities are emerging in
this area in connection with some commonly used networks such as phenolics,
epoxy, polyester, and polyurethane resins
(Raquez et al., 2010).
Polymers from renewable sources suffer
two major drawbacks: the lesser mechanical properties and the sensible price compared to their conventional counterparts.
To alleviate both these disadvantages, various blends and composites have been
developed along the years: in this field
improvements are required in connection to the synthesis of compatibilizers,
which ensure optimum dispersion and
better interfacial adhesion between the
components. Furthermore, the technology
of reactive extrusion, a complex and
challenging area of polymer rheology, is
actively investigated as a means to optimize the current products (Yu et al., 2006).
Energy conversion, storage and delivery
is a further sector where polymer chemistry is playing a major role (Rabek, 1988;
Page et al., 2012). Analysis of the literature shows that research efforts in this area
date back from the early 1990s, but the
most significant achievement have been
reported in the last 10 years. It is therefore, a dynamic and up-to-date subject,
strictly related to economic and social
issues and capable of attracting conspicuous funding. The state-of-the-art polyelectrolytes developed so far for fuel-cell
and battery applications still need important improvements, which can only be
achieved through a deeper understanding
of the molecular mechanisms at the background of their mesoscale behavior. The
relevance of chain microstructure, chain
dynamics, and nanoscale morphology on
the performances of these materials cannot
be overestimated. It is a highly interdisciplinary field in which the synthetic polymer chemist has to work face-to-face with
the physicist and the chemical engineer,
which is not always easy; this same reason, however, makes the subject so exciting
and challenging. An emerging approach
to overcome the current drawbacks consists in adding suitable ionic-liquids to the
polyelectrolytes. These are molten salts at
room temperature and possess peculiar
properties such as high chemical, thermal and electrochemical stability, high
conductivity and no measurable vapor
pressure. Organic, semiconductor-based
photovoltaic devices offer the promise of a
low cost technology based on existing rollto-roll printing techniques. However, these
devices are currently limited to solar power
conversion efficiencies of 3–5%. This is
because of poor overlap between the
absorption spectrum of the organic chromophores and the solar spectrum, nonideal band alignment between the donor
and acceptor species, and low chargecarrier mobility (Krebs, 2009). An active
research area is the development of specialized polymer architectures capable of
a better overlap with the sun’s spectrum
which can be fabricated by cheap, high
throughput and high purity technologies (solution printing). Examples of such
December 2013 | Volume 1 | Article 31 | 2
Musto
developments are ZnO nanocomposites
and dendritic structures. Finally, microporous and nanoporous polymers are showing promise for energy gas storage, with
the added advantage of versatility in the
synthetic routes and ease of chemical modification of the resulting polymer structures (Bhoje Gowd et al., 2009; Dawson
et al., 2012).
Enormous research efforts have been
devoted during the last decade toward the
development of nanocomposites. These
were introduced in the 1990s as an extension of the classical polymer-composite
concept (Pavlidou and Papaspyrides,
2008; Kango et al., 2013). The rationale
behind the new approach arises from the
opportunity of obtaining enhancements of
properties far exceeding those achievable
with macro-composites, taking advantage of the exceptional surface area of the
nanofillers. However, the initial enthusiasm for the new materials was intended to
be, at least partially, frustrated. After years
of intense and costly research, the technology of nanocomposites has not been
exploited to its full potential yet. Many
opportunities still remain for improving the techniques to achieve optimum
dispersion, avoiding unwanted coalescence of nanoparticles in larger aggregates.
Functionalization of the filler with suitable
reactants that promote dispersion and
improve the interfacial adhesion is an area
of intense activity (Kango et al., 2013).
With respect to the preparative technologies involving the in-situ formation of the
inorganic phase, modified sol-gel routes
whose chemistry can be tailored so as
to be compatible with the presence of
the polymeric matrix are being actively
explored (Kickelbick, 2003). Carbonbased nanocomposites represent the new
research frontier on hybrid materials.
Interest on these systems has flourished
with the advent of well characterized
and molecularly controlled carbonbased nanostructures such as fullerenes,
graphene and nanotubes (Sahoo et al.,
2010; Roy et al., 2012; Naffakh et al.,
2013). It has been demonstrated that
the incorporation of these motifs into
diverse polymer matrices produces a
relevant enhancement of the material’s
performances, most notably in terms of
mechanical, thermal, electrical, and magnetic properties. Here again, the critical
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Grand chjallenges in polymer chemistry
issues reside in the efficient dispersion of
the nanofillers as well as, for the case of
carbon nanotubes, their controlled alignment within the matrix. Several methods
are being currently explored, among
which solution-mixing, melt-mixing,
electrospinning. Efficient methods for
the chemical functionalization of the
carbon-based nanostructures have been
recognized as a key factor toward the
development of high-performance materials based on this class of nanocomposites
(Roy et al., 2012).
From the considerations enumerated so
far emerges a discipline rich of opportunities and challenges, whose development
is likely to produce significant advancements in our society. The Specialty Section
Frontiers in Polymer Chemistry was born
to witness, disseminate, and foster these
advancements.
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Received: 14 October 2013; accepted: 18 November
2013; published online: 04 December 2013.
Citation: Musto P (2013) Grand challenges in polymer
chemistry: energy, environment, health. Front. Chem.
1:31. doi: 10.3389/fchem.2013.00031
This article was submitted to Polymer Chemistry, a
section of the journal Frontiers in Chemistry.
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