Published: May 11, 2011
r 2011 American Chemical Society 4470 dx.doi.org/10.1021/ma200979h |Macromolecules 2011, 44, 4470–4478
ARTICLE
pubs.acs.org/Macromolecules
Structural Investigation on Thermoresponsive PVA/
Poly(methacrylate-co-N-isopropylacrylamide) Microgels
across the Volume Phase Transition
Shivkumar V. Ghugare,† Ester Chiessi,† Rainer Fink,‡ Yuri Gerelli,^ Andrea Scotti,§ Antonio Deriu,§
Geraldine Carrot,|| and Gaio Paradossi†,*
†Dipartimento di Scienze e Tecnologie Chimiche, Universit
a di Roma Tor Vergata, 000133 Roma, Italy
‡Physikalische Chemie II and ICMM, Friedrich-Alexander Universit€at Erlangen-N€urnberg, Egerlandstrasse 3,
D-91058 Erlangen, Germany
§Dipartimento di Fisica, Universit
a di Parma, Parma, Italy
^Institute Laue Langevin, 6 rue Jules Horowitz, 38000 Grenoble, France
)Laboratoire Leon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
b
S Supporting Information
’ INTRODUCTION
Switchable hydrogels were described first in the inspiring works
of Tanaka in the eighties.1
8 It is now clear that “soft”, polymer
based, switchable systems can be a powerful answer to the several
requirements raised in modern drug delivery,9 transducer
technologies,10 and biotechnologies.11 Changes in the physical state
of monolithic hydrogels were already studied and rationalized by
Flory within the framework of the Flory
Rehner theory,12
14 an
extension of the well-known theory on polymer mixing process.14
Further insight was provided by considering gels as fractal systems.15
Issues such as responsivity to physiological changes of pH and
temperature, localized drug delivery, biodegradability, bioadhesion,
hydration can bematched, in principle, with biocompatible colloidal
dispersions of polymeric micro- or nanoparticles. In the past decade
attention of the researchers has moved to microgels, i.e., systems
able to combine the features of macroscopic hydrogels such as
connectivity and response to external stimuli with the colloidal
properties of nano/micro sized systems. Because of their large
specific surface, these systems enable a variety of surface modifica-
tions. Moreover, moving from macro- to microscopic gels allows a
systemic administration of the drug loaded microgel, a decrease in
time response to an external trigger as pH or temperature changes
and the exploitation of this feature for a fast and targeted delivery of
the drug. In switchable microgels designed for the controlled drug
release, the combination of the colloidal properties with injectability
for a systemic administration of a large drug loading capacity and
biocompatibility is a requirement for potential application.
Thermoresponsivity is one of the most studied features for a
shrinking-assisted drug delivery. The great majority of the studies
addressing the design of thermoresponsive systems finalized to the
release of a drug were dealing with N-isopropylacrylamide,
NiPAAm, based polymers,16 copolymers,17 hydrogels,18 and nano-
gels/microgels.19
22 Hydrophilic polymer systems incorporating
Received: March 22, 2011
ABSTRACT: Characterization of switchable microgels is a major task in drug
delivery science. The study of soft polymeric devices requires a combined use of
spectroscopy, microscopy, and scattering approaches enabling the characteriza-
tion of nanostructured features across a volume phase transition. In this work the
structural changes of poly(vinyl alcohol) based thermoreversible microgel
particles which incorporate p(NiPAAm-co-methacrylate) chains across the
transition temperature occurring at 33 C have been addressed by utilizing
reciprocal and direct space approaches such as small angle neutron scattering,
SANS, dynamic light scattering, DLS, soft transmittance X-ray microscopy,
STXM, and confocal laser scanning microscopy, CLSM, respectively. The
comparison between the results obtained from those approaches allows an evaluation of the driving forces acting in the transition
and reveals the changes in themicrogel structure at nanoscale level. The structure of the poly(vinyl alcohol) basedmicrogel particles,
incorporating p(NiPAAm) sequences, consist of a hydrogel core and of a crown of polymer chains projected, at room temperature,
in aqueous medium. An increase of the temperature above 33 C causes a volume phase transition of the system characterized by the
collapse of the particle core and of the chains grafted at the particle surface. This transition is accompanied by a massive release of
water and an increase of the interface with the dispersing aqueous medium, causing the passage from a permeable to a
semipermeable structure.

4471 dx.doi.org/10.1021/ma200979h |Macromolecules 2011, 44, 4470–4478
Macromolecules ARTICLE
residues with lower critical solution temperature (LCST) close to
physiological temperature can be considered as thermoresponsive
systems. Recently, we reported on stable thermoresponsive
microgels based on soluble poly(vinyl alcohol) grafted with
methacryloyl side chains with a degree of substitution (DS) of
5%, PM5.23,24 Water-in-water microemulsion technique25can be
applied to PM5 aqueous solutions using aqueous dextran T40 as
dispersion phase. The immiscibility of the two aqueous phases
offers the possibility to photopolymerize the emulsion with a
separation of microgel particles.25,26 Addition of NiPAAm and
photopolymerization of the emulsion allows the incorporation of
this residue in a PVA/poly(methacrylate-co-NiPAAm), PMN-II,
network (see Scheme 1).27
Furthermore, we addressed the dynamic behavior of water
associated with the network probing the relaxation processes
occurring in the microgels by QENS.27 Here we focus on how the
microgel
water interface changes on the micro/nanoscopic level
upon a temperature-driven volume phase transition, using direct
and reciprocal space investigation methods having negligible
impact on the polymer soft matter devices. Confocal laser
scanning microscopy (CLSM) coupled with a permeation study
provided a pore size evaluation across the transition temperature.
Scanning transmission X-ray microscopy (STXM) was used for
the evaluation of the overall size of individual particles in wet
condition at high spatial resolution. The momentum transfer
values probed by small angle neutron scattering (SANS) allowed
to exploring the surface behavior and the pore size of the
microgels across the transition. The picture emerging from these
approaches is compared with the dynamic light scattering results
disclosing the role of interfacial water across the VPTT.
’EXPERIMENTAL SECTION
Materials. N-Isopropylacrylamide (NiPAAm) purchased from Al-
drich, was recrystallized in n-hexane prior to use. All other chemicals
were used as received. Poly(vinyl alcohol), PVA, with number and
weight-average molecular weights of 30 000 ( 5 000 and 70 000 (
10 000 g/mol, respectively, dextranT40 with average molecular weight
of 35 000
40 000 g/mol, and D2Owere purchased from Sigma-Aldrich.
4-(N,N-Dimethylamino)pyridine (DMAP), glycidyl methacrylate
(GMA) and fluorescein isothiocynate isomer 1 (FITC) were Fluka
products. Photoinitiator 2-hydroxy-1-[4-(hydroxyethoxy)phenol]-2-
methyl 1-propanone (Irgacure 2959) was purchased from Ciba. Di-
methyl sulfoxide (DMSO), inorganic acids, and bases were RPE grade
products supplied by Carlo Erba (Italy).
Water was Milli-Q purity grade (18.2 MΩ
3
cm) produced with a
deionization apparatus (PureLab) from USF, Elga. Dialysis membranes
(cutoff 12 000 g/mol) were purchased from Medicell International Ltd.
and prepared according to standard procedure.
Methods. Preparation of PVA-Based Thermoresponsive Microgels.
According to the procedure described elsewhere.25,28
30 PVA backbone
was grafted with glycidyl methacrylate with a degree of substitution of 5%.
Microgels based on 5% methacrylated PVA (PM5) were prepared,
adapting a procedure originally introduced by Franssen and Hennink,25
which employs a water-in-water emulsion technique based on poly-
mer
polymer immiscibility in aqueous solution. In a typical experiment,
an aqueous dispersion containing dextran T40 at a concentration of 16%
(w/v), PM5 at 2% (w/v), 1.3% NiPAAm (w/v), and the UV photo-
initiator Irgacure 2959 at 0.3% (w/v) was vigorously stirred by an
UltraTurrax emulsifier at 16 000 rpm. After emulsification, PM5 in the
dispersed aqueous phase was cross-linked by photopolymerization using a
365 nm light source at an intensity of 7 mW/cm2 for 5 min. The cross-
linked PVA/poly(methacrylate-co-N-isopropylacrylamide) microgels
were purified by repeated steps of centrifugation and resuspended in
Milli-Q water. The effective amount of NiPPAm incorporated in the
network (NiPPAm/methacrylate molar ration: 2.4 and NiPPAm weight
fraction: 22% (w/w)) were determined by elemental analysis.24Here after
the obtained microgels were labeled as PMN-II.
Scanning Electron Microscopy (SEM). The morphology of the
microgel particles was probed by scanning electron microscopy (Zeiss
Supra VP55) high-resolution field emission scanning electron micro-
scope at the Paul Scherrer Institute (PSI, Villigen, Switzerland). Diluted
dispersions of microgel particles were dropped onto mica substrates and
dried at room temperature in air prior to observation.
Dynamic Light Scattering (DLS). Evaluation of the average hydro-
dynamic diameter and size distribution of the microgels was carried out
by dynamic light scattering (DLS) experiments using a BI-200SM
goniometer (Brookhaven Instruments Co.) equipped with a solid state
laser source emitting at 532 nm with an external water bath thermostat.
The temperature was controlled from 20 to 43 C with an accuracy of
(0.2 deg. The autocorrelation functions were analyzed with a standard
software package using cumulant analysis to obtain the hydrodynamic
diameter of themicrogels and the size distribution from the linear,Γ, and
quadratic, μ2, terms of the power series expansion, respectively:
ln½gð1ÞðτÞ ¼
Γτ þ
μ2
2!
τ
2


μ3
3!
τ
3
þ
μ4
4!
τ
4
þ ::: ð1Þ
Differential Scanning Calorimetry (DSC). Volume phase transition
temperature of the PMN-II microgels was examined using a TA Q2000
differential scanning calorimeter. Known amounts of dry microgels were
equilibrated inMilli-Qwater at room temperature to reach themaximum
swelling and centrifuged to eliminate the excess of water. An exactly
weighted amount of slurry, about 20
30 mg, was placed inside an
aluminum pan and then hermetically sealed with an aluminum lid. The
scans were performed from 25 to 55 C on the swollen microgels at a
heating rate of 3 C/min under a flux of 50 mL/min of dry nitrogen.
Confocal Laser Scanning Microscopy (CLSM). Freeze-dried micro-
gels were first immersed in Milli-Q water at room temperature to reach
their swollen state. The swollen microgels were labeled with FITC by
coupling the dye to the hydroxyl group of microgels.31 Fluorescent dye
at a concentration of 10 μM was added in the dispersion. The unreacted
Scheme 1. Structure of the PVA/Poly(methacrylate-co-
NiPAAm), PMN-II, Network (m = 0.05; n = 0.12)