Grafting polystyrene on Cellulose (CNC) by surface initiated Atom Transfer Radical Polymerization (SI ATRP) Zhen Zhang, Gilles Sebe, Xiaosong Wang Abstract
Grafting polymers on the surface of nanoparticles is a very promising way to prepare
hybrid nanomaterials or nanocomposites. if the polymer grafting is in a controlled
way, including grafting density, chain length and dispersity, well designed
nanocomposites can be obtained. In this paper, polystyrene was grafted on the surface
of Cellulose NanoCrystal (CNC) nano-initiator by surface initiated Atom Transfer
Radical Polymerization (SI ATRP), which was confirmed by FTIR, TGA, DTG, DSC
and DLS. To initiate SI ATRP on CNC, it is necessary to introduce initiating sites on
the surface of CNC to prepare CNC nano-initiators first.
1 Introduction
Grafting soft polymer onto the surface of rigid nanoparticles by covalent bonds is a
very promising bottom up approach to prepare nanocomposites[1, 2], which can not
only take fully advantage of the synergistic effect of the soft and rigid parts to obtain
optimal balance between durability, mechanical properties and thermal properties, but
also introduce novel functionalities [3, 4]. The diversity of nanoparticles and
polymers offers a big resource pool of new ideas. Fabrication of nanomaterials with
designed structures is essential for nanotechnology. However, grafting polymers on
the surface of nanoparticles in a controlled way to prepare well designed
nanocomposites is still a challenge, including controlled grafting density, chain length
and dispersity of grafted polymer.
Cellulose nanocrystal (CNC) is extracted from cellulose by acid hydrolysis with
diameter from 5 to 20 nm and length from 50 to 500 nm which depends on different
resources of cellulose and extraction method. Due to the excellent properties of CNC
[5], for example, renewable, biocompatible, excellent mechanical properties with low
density, potential to modification due to abundant hydroxyl group on the surface,
self-assembly to chiral nematic crystal and low thermal expansion coefficient, CNC
have drew a lot of attention in recent years in the applications of nano-metal particles
stabilizer, transparent substrate for electronic, nano-template, reinforcing agent for
nanocomposites, emulsifier for Pickering emulsion, and so on. With the scalable
production in the demonstration plant, the cost of CNC will decrease dramatically in
the near future, and the applications of CNC will be exploited much more in industry
field. However, the hydrophilic nature of CNC surface limits its further application
due to the poor miscibility with hydrophobic materials[6]. Sometimes it is necessary
to modify the surface for specific applications. The modification of the surface also
can introduce some other novel functionality. Grafting polymer on CNC is a very
effective and promising method to by modify the surface and introduce novel
functionalities[7].
Since the development of the Atom Transfer Radical Polymerization (ATRP) in
1995[8], it has become a very important method and widely use “living”
polymerization technique to prepare well-controlled polymeric chains. A variety of
monomer can be polymerized by ATRP with well-controlled molecular weight and
low dispersity. ATRP has also been initiated efficiently and extensively on the surface
of different kinds of substrate (defined as surface initiated (SI) ATRP) to yield hybrid
materials by grafting polymers precisely. SI ATRP is a “grafting from” method, which
can obtain high graft density of polymer on the surface when compared with “grafting
to” method [9].
2 Results and discussion
2.1 Preparation of CNC nano-initiator: CNC-Br
Before grafting polymer on CNC by SI ATRP, initiating sites Br were introduced on
the surface of CNC. BIBB was used to modify the surface of CNC with TEA and
DAMP as the catalyst. In the modification process, the dispersibility of CNC was
crucial for the modification. So it is necessary to disperse CNC in DMF by sonication.
As shown in Figure 1, compared with the FTIR of CNC, the new strong absorption
peak at 1739 cm-1 was attributed to the C=O of the modified moieties, which
indicated the successfully modification of CNC by BIBB.
Figure 1. FTIR of CNC and CNC-Br
The thermal stability of CNC and CNC-Br were also characterized by TGA and DTG,
as shown in Figure 2. As CNC was more hydrophilic than CNC-Br, CNC showed a
little more weight loss than CNC-Br below 100 °C due to absorbed moisture.
Normally the main decomposition temperature range of cellulose is between 320 to
400 °C with the maximum degradation temperature (Tmax) at 390 °C [10]. The sulfuric
ester groups on the surface of CNC catalyzed the thermal degradation at low
temperature [11]. The main decomposition temperature range of CNC was between
250 to 320 °C with the onset decomposition temperature (Tonset) at 280 °C and Tmax
at 297 °C respectively. The obviously decrease of thermal stability of CNC-Br was
due to the introduction of Br, which may eliminate HBr upon heating and the HBr
formed catalyzed decomposition of CNC at lower temperature [10].
Figure 2.
(A) TGA of CNC and CNC-Br; (B) DTG of CNC and CNC-Br
2.2 Grafting PS on CNC nano-initiators by SI ATRP
As the initiator sites on the surface of CNC are negligible to some extent when
compared with sacrificial initiator, it is easy to tailor the length of grafted polymer by
changing the initial molar ratio of monomer to sacrificial initiator and the conversion.
Moreover, the addition of sacrificial initiators will increase the concentration of Cu2+
in the system, which limits the termination to obtain a better controlled ATRP.
Although the molecular weight of free polymer initiated by sacrificial initiator was
not exactly the same with the grafted polymer, the free polymer was still a tool to
study the grafted polymer. Now sacrificial initiator has been extensively used in SI
ATRP to have a better controlled ATRP and provide some information about the
grafted polymer. In this paper, EBiB was used as the sacrificial initiator in SI ATRP
system.
From the comparison of FTIR spectra of CNC-Br, CNC-g-PS 2h (reaction time was 2
h) and the free PS, as shown in Figure 3, it was easy to conclude that PS was
successfully grafted on CNC-Br by SI ATRP. After grafting PS, the peaks in
CNC-g-PS FTIR spectra, for example, 3025 cm-1 (C-H stretching of PS), 1494 cm-1
(C=C stretching of PS), and 700 cm-1 (C-H bending of PS), were the characteristic
peaks of PS.
Figure 3. The FTIR spectra of CNC-Br, CNC-g-PS 2 h and free PS
The kinetic research of the free PS was studied first. Figure 4A showed the monomer
conversion and ln (M0/Mt) vs reaction time curve, in which the monomer conversion
and ln (M0/Mt) was calculated by 1H NMR. The liner increase of ln (M0/Mt) vs time
indicated a first-order kinetic with a constant concentration of the radical. Figure 4B
showed the Mn and dispersity of free PS vs monomer conversion curve, and the Mn
and dispersity was determined by SEC. The Mn increased linearly with the monomer
conversion, and all the dispersity values (1.06-1.13) are below 1.15. The kinetic
research results demonstrated the well-controlled living polymerization of free PS by
ATRP.
0.4
B
monomer conversion
ln ([M0]/[Mt])
0.25
Fit line for ln ([M0]/[Mt])
0.3
15000
0.2
0.15
0.10
0.1
Mn
0.20
Mn
Dispersity
Linear fit for Mn
1.4
10000
1.2
Dispersity
0.30
ln([M0]/[Mt])
Monomer conversion
A
5000
0.05
0.00
0.0
0
1
2
3
4
Time (h)
5
6
7
8
0
0.00
0.05
0.10
0.15
0.20
0.25
1.0
0.30
Monomer conversion
Figure 4. The kinetic research of free PS by ATRP (A) Monomer conversion and
ln(M0/Mt) vs reaction time; (B) Mn and dispersity of free PS vs monomer conversion
A
1.0
0.8
CNC-Br
free PS 2H
free PS 4H
free PS 6H
free PS 7.3H
2.5
2.0
Deriv. Weight
0.6
Weight
B
Free PS 2h
Free PS 4h
Free PS 6h
Free PS 7.3h
PS Mn=250000
0.4
1.5
1.0
0.2
0.5
0.0
100
200
300
400
0.0
500
100
200
Temperature (oC)
300
400
500
600
Temperature (oC)
Figure 5. The TGA (A) and DTG (B) vs reaction time of free PS
2.4
0.8
2.0
Deriv. Weight
Weight
CNC-Br
CNC-g-PS 2H by ATRP
free PS 7.3H by ATRP
2.8
CNC-Br
CNC-g-PS 2H by ATRP
free PS 7.3H by ATRP
1.0
0.6
0.4
1.6
1.2
0.8
0.2
0.4
0.0
0.0
100
200
300
400
Temperature (oC)
500
100
600
200
300
400
500
600
Temperature (h)
Figure 6. The TGA and DTG of CNC-g-PS
The thermal stability of the free PS and CNC-g-PS was characterized by TGA and
DTG. As shown in Figure 5A, the Tonset of the free PS increased a little bit with the
increase of the Mn of the PS. Then Mn of PS used here as a comparison was 250, 000
g/mol. The main decomposition temperature range of PS was from 350 to 450 °C
(Figure 5B). Figure 6 showed the TGA and DTG of CNC-g-PS. After grafted with PS,
the thermal stability of CNC-g-PS increased a lot.
3 Conclusion
BIBB was employed to modify CNC to prepare CNC-Br nano-initiator for SI ATRP.
And then PS was grafted on CNC-Br by SI ATRP with presence of sacrificial initiator.
The successful grafting was confirmed by FTIR, TGA, DTG and so on.
4 References
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