nanoparticles atom transfer radical polymerization was used to incorporate...
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NanoparticlesAtom transfer radical polymerization was used to incorporate alkoxyamine-functionalized
monomers into a poly(methyl methacrylate)-based polymer.3 The polymer was heated to
homolyze the alkoxyamine C–O bonds, enabling radical exchange and crosslinking.3
Intramolecular crosslinking should cause the polymers to fold into SCNPs.
Crosslinking at 1.0 mg/mL broadened the size distribution, indicating intramolecular and
intermolecular crosslinking. The 0.5 mg/mL reaction shows an increase in retention time,
which seems to indicate that intramolecular crosslinking and SCNP formation is favored.4
15 17 19 21 23 25 27 29 31 33 35
RI
Retention Time (min)
AlkoxyaminesHydroxyl-substituted alkoxyamines were synthesized by a published method (1) and by a method provided by Dr. Hideyuki Otsuka (2).1 Methacrylate monomers were synthesized by
reacting the hydroxyl-substituted alkoxyamines with 2-isocyanatoethyl methacrylate.2 Alkoxyamines were activated for functionalization of materials with 1,1’-carbonyldiimidazole (CDI).
N
HO
ON
HO
OBrCuBr Cu0PMDETA
+anisole
/
1
N
O
O
+ N
O
ONaHCO3 OHMeOH
FeSO4
HO
OH+
2
40°C
40°C
Scheme 1. Synthesis of hydroxyl-substituted alkoxyamines.
O
ONCO
O
ONCO
N
HO
OO
O HN O
O NO
N
O
O
O
O HN O
OO
N
O
+
+
dibutyltindilaurate
dibutyltindilaurate
DMF
DMF
OH
rt
rt
O N
O
NNN
O
N NN
O
O+
THFrt
OH ON
O
N
O
O
N
O
NNN
O
N NN
HO
O
+THFrt
Radically-Exchangeable Alkoxyamines as Heat-Responsive Crosslinkers for Polymeric Nanostructures and Nanocomposites
Odin Achorn, Danming Chao, Erik Berda,* and Johan Foster*[email protected]; Parsons Hall, 23 Academic Way, Durham NH 03824
IntroductionWe report progress toward the synthesis of single-chain polymer nanoparticles (SCNPs) and cellulose-reinforced nanocomposites with the use of radically-exchangeable alkoxyamine
crosslinkers. Crosslinking was achieved by complementary interactions between two different alkoxyamines in the materials. Heat was used to cleave the two alkoxyamines homolytically,
leaving nitroxide radicals bonded to some points of the material and benzylic radicals to other points. Bonding between these complementary radicals caused the materials to crosslink.
Crosslinked nanoparticles were analyzed by size exclusion chromatography (SEC), and crosslinked nanocomposites were analyzed by dynamic mechanical thermal analysis (DMTA).
References1. Nicolay, R.; Matyjaszewski, K. Macromolecules. 2011, 44, 240-247.
2. Amamoto, Y.; Higaki, Y.; Matsuda, Y.; Otsuka, H.; Takahara, A. J. Am. Chem. Soc. 2007, 129, 13298-13304.
3. Su, J.; Amamoto, Y.; Nishihara, M.; Takahara, A.; Otsuka, H. Polym. Chem., 2011, 2, 2021-2026.
4. Tuten, B. T.; Chao, D.; Lyon, C. K.; Berda, E. B. Polym. Chem., 2012, 3, 3068-3071.
5. Biyani, M.V.; Foster, E.J.; Weder, C. ACS Macro Lett. 2013, 2, 236−240.
1.0 mg/mL Reaction MN (g/mol) MW (g/mol) PDI Rh(n) (nm)
Before Crosslinking 2.42×104 2.65×104 1.10 3.2
After Crosslinking 4.76×104 7.05×104 1.48 5.5
Scheme 4. Copolymerization of the two alkoxyamine monomers with methyl methacrylate.
OO
HN
OO
ON
O
OO
O
HN
O
HN
O O
OO O
Br
ON
O
O
NO
0.76x/0.12x/0.12xOO
HN
OO
NO
O
O
BrO
O
OO
PMDETACuBr
Anisole+ + +
50°C
Future WorkOnce conditions are optimized for making SCNPs, the alkoxyamine crosslinks will be used
to polymerize styrene.1 This may lead to phase-separated nanoparticles with immiscible
polystyrene blocks on the surface of the poly(methyl methacrylate) core.
AcknowledgementsWe thank the UNH Chemistry Department, the UNH Hamel Center for Undergraduate
Research, and the Adolphe Merkle Institute for funding this project. Additional thanks go
to Dr. Hideyuki Otsuka for his advice on synthesizing alkoxyamines and to the members
of the Berda group and the AMI Polymer Chemistry & Materials group for their help.
NanocompositesNanocomposites are materials with two phases finely dispersed at the nanometer scale.
In this project, rigid cellulose nanocrystals (CNCs) were dispersed in a soft poly(vinyl
acetate) (PVAc) matrix. The CNCs set up a scaffold to impart their rigidity to the
material.5 The two materials were functionalized with complementary alkoxyamine
crosslinkers.
Heat-induced crosslinking between the scaffold and matrix increased the interaction
between the two phases, increasing the temperature at which the material transitions
from a strong, glassy state to a weaker, rubbery state (glass transition temperature, Tg).
0 10 20 30 40 50 60 70 80 90 1001
10
100
1000
10000
Temperature (°C)
Stor
age
Mod
ulus
(MPa
)
O OH O
OO
NO
O0.6x/0.35x/0.05x
OOO
O
HO
OHO
OH
HOOH
OO
ON
O
O OH O
OOO
OOO
O
HO
OHO
OH
HOOH
OO
NO100°C
m/n
m/n
24 hrs.
OO
OH
OHHO
p
+ DMAP
50°CDMF
O N
O
NO
N
O
O OHO
0.6x/0.4x
DMAPDMF
N
OO
NO
N
+
50°C0.6x/0.35x/0.05x
Figure 5. Dynamic Mechanical Thermal Analysis of nanocomposite film before and after crosslinking.
ConclusionsAlkoxyamines have been shown to be effective at making crosslinks in polymeric
nanoparticles and nanocomposites. Heating polymers with pendant complementary
alkoxyamines resulted in different sized particles due to intramolecular and
intermolecular crosslinking. A reaction concentration of 0.5 mg/mL seems to favor the
formation of smaller particles. Heating a nanocomposite film of alkoxyamine-
functionalized CNCs in an alkoxyamine-functionalized PVAc matrix increased its Tg.
Scheme 2. Synthesis of alkoxyamine monomers. Scheme 3. Synthesis of CDI-activated alkoxyamines.
15 20 25 30 35
LS
Retention Time (min)
Table 1. Polymer size data
Figure 4. Plain PVAc on the left and nanocomposite on the right.
Figure 3. TEM of CNCs.5 The CNCs set up a scaffold in the PVAc matrix.
Scheme 5. Crosslinking between PVAc and CNCs.
Figure 3. Phase-separated nanoparticle formation by polymerization of an immiscible block through SCNP crosslinks.
Figure 1. Crosslinking reaction.
Figure 2. Light Scatter (LS) and Refractive Index (RI) SEC traces of polymers before and after crosslinking at 1.0 and 0.5 mg/mL in toluene.
0.5 mg/mL Reaction MN (g/mol) MW (g/mol) PDI Rh(n) (nm)
Before Crosslinking 1.29×105 1.50×105 1.16 8.4
After Crosslinking 2.04×103 3.20×103 1.57 1.2
18 20 22 24 26 28 30 32 34
LS
Retention Time (min)18 23 28 33
RI
Retention Time (min)
1.0 mg/mL Reaction 0.5 mg/mL Reaction