Substituent-induced delocalization effects on hydrogen-bonding interaction in poly(N-phenyl methacrylamide) derivatives

Article abstract of DOI:10.1016/j.polymer.2011.04.016

Within small molecules, the hydrogen-bonding behaviors affected by delocalization have been studied thoroughly, but rare publication in macromolecules. Therefore, three poly(N-phenyl methacrylamide)s, poly(N-phenyl methacrylamide) (PNPAA), poly(N-4-methox

Full text of DOI:10.1016/j.polymer.2011.04.016

Polymer 52 (2011) 2600e2608
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Polymer
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Substituent-induced delocalization effects on hydrogen-bonding interaction
in poly(N-phenyl methacrylamide) derivatives
,
b
b
b
b
Shiao-Wei Kuo a , Chien-Ting Lin , Jem-Kun Chen , Fu-Hsiang Ko , Feng-Chih Chang ,
*
Kwang-Un Jeong c
a Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC
b Institute of Applied Chemistry, National Chiao Tung University, Hsin Chu 300, Taiwan, ROC
c Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Within small molecules, the hydrogen-bonding behaviors affected by delocalization have been studied
thoroughly, but rare publication in macromolecules. Therefore, three poly(N-phenyl methacrylamide)s,
poly(N-phenyl methacrylamide) (PNPAA), poly(N-4-methoxyphenyl methacrylamide) (PMPMA) and
poly(N-4-bromophenyl methacrylamide) (PBPMA), with different inductive substitution at para position
of benzene ring are prepared to investigate the substituent-induced delocalization effects on the
hydrogen-bonding interaction behaviors. In this study, the variable-temperature FTIR spectrum is used as
tool to study the self- and inter-association hydrogen-bonding interaction. FTIR analyses could provide
evidences that there is relatively stronger inter-associative hydrogen bonding in poly(N-4-bromophenyl
methacrylamide)/P4VP blends. High resolution 13C CP/MAS solid-state NMR analyses indicate that the
spin-lattice relaxation time (T1rH) in all PBPMA blends are homogeneous on the scale at which spin-
Received 10 December 2010
Received in revised form
25 March 2011
Accepted 10 April 2011
Available online 15 April 2011
Keywords:
Polymer blend
Hydrogen bonding
Miscibility
diffusion occurs within the time
delocalization.
T
1rH
,
also due to the enhancement by substituent inductive
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
bonding, to decrease the van Laar type energy of interaction char-
acterized by a FloryeHuggins interaction parameter.
Specific interactions between various polymers concerning with
different structures have been intensively reported, especially for
those in hydrogen-bonding blend systems such as homopolymer/
homopolymer [1e7], homopolymer/copolymers [7e19], and
ternary blends [20e23]. Most studies have concentrated on the
understanding of structural effects on self- or inter-associative
hydrogen bonding in polymer blends. The accessibility of func-
tional groups in hydrogen-bonded polymer blends has significantly
affected the intramolecular screening effects which are a direct
consequence of the chain connectivity [24e26]. In addition, the
acidity or basicity of hydrogen-bond donor, steric hinderance, and
temperature would also affect the inter-association hydrogen
bonding in polymer blend system [2,27]. For blends of nonpolar
polymers [28], the mixing process is thermodynamic unfavorable
because of decreasing the combinational entropy of mixing as
blending two or more polymers. However, the miscibility can be
enhanced by introducing specific interaction, especially hydrogen
There are some factors which could affect the strength of
hydrogen bonding, including the nature of hydrogen donors and
acceptors, temperature, solvent, and so on [29,30]. Particularly, in
the nature of hydrogen-bond donors and acceptors, the resonance
of electrons between donors and acceptors plays an important role
in affecting the strength of hydrogen bonding. This resonance effect
can redistribute the electron density on the hydrogen donors and
acceptors by the driving force that having the conformation of
structure to reach a most stable state. In addition, it can affect the
basicity of the acceptors, further influence the strength of hydrogen
bonding. Therefore, the strength of hydrogen bonding between the
chemical structure which containing the resonance effect, or not,
may exist some difference. In the small molecules field, the reso-
nance effect on the intra-associative hydrogen bonding could be
analyzed by the atoms in molecules (AIM) and density functional
theory (DFT) [31,32]. The donor-acceptor distance of cis-enol
tautomer of malonaldehyde was reduced by the
p-electron delo-
calization between the donor and acceptor atoms to enhance the
strength of malonaldehyde’s intra-association, because the
p
-electron delocalization would change the electron-cloud distri-
* Corresponding author. Tel./fax: þ886 7 5254099.
E-mail address: kuosw@faculty.nsysu.edu.tw (S.-W. Kuo).
bution both on donors and acceptors, inducing the charge
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.04.016

S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
2601
formation and thus enhancing the strength of self-association.
These theories may be difficult to apply in polymers because of
chain connectivity that can complicate polymer hydrogen-bonding
interaction, but the resonance effects cannot be ignored.
The resonance effect on hydrogen-bonding behaviors in a binary
polymer blend has been disclosed in our previous publication [33].
The electron-cloud distribution of (O])CeNeH group in poly(-
N-phenyl methacrylamide) (PNPAA) is almost a planar, and the
aromatic ring in PNPAA can make the carbonyl group electron-
deficient through delocalization via the lone-paired electrons on
nitrogen atom, resembling an electron-withdrawing group. Further-
more, when a pyridine group in P4VP forms a hydrogen bond with the
NeH group in PNPAA, a planar delocalization, constructed by the
aromatic-
p
system of pyridine in P4VP, the aromatic-p system and
the carbonyl-
p
system of amide group in PNPAA via the lone-paired
electrons on nitrogen atom of PNPAA, is formed. This planar delo-
calization is a driving force to replace the self-association of PNPAA
with inter-association to P4VP, and thus the inter-associative equi-
librium constants (KB) in PNPAA/P4VP at different temperatures are
higher than those inpoly(N-methyl methacrylamide)/P4VP (PNMAA/
P4VP), even though the phenyl group is more steric hindered to
prevent the formation of any kinds of hydrogen bonding. This planar
delocalization is impossible in poly(N-cyclohexyl methacrylamide)/
P4VP (PNCHAA/P4VP) blends, so the steric bulky cyclohexyl group
hinders the formation of inter-association and self-association to
obtain smaller KA/KB at different temperatures.
Moreover, based on the definition of the Bronsted-Lewis acid and
base, the acidity of NeH and the basicity of the lone-paired electrons
on nitrogen atom in aniline derivatives depend upon on the
substitutions of aromatic ring. Introducing different electronegative
substitution at para-position of aniline, the basicity of the lone-
paired electrons on nitrogen atom and the acidity of NeH group
Scheme 1. The chemical structures of poly(N-phenyl methacrylamide), poly(N-4-
methoxyphenyl methacrylamide) and poly(N-4-bromophenyl methacrylamide).
solution. Methacryloyl chloride, 4-bromoaniline and 4-methox-
yaniline were used as received without further purification.
can be affected through delocalization over the
p
system of aromatic
2.2. Synthesis
ring. Comparing to aniline, if the para-substitution is an electron-
donating group, such as methoxy group, the electron-donating
group makes eNH2 more electron-rich and electron-shielding,
enhancing the basicity of the lone-paired electrons on nitrogen
atom but reducing the acidity of the NeH bond with electron. In
contrast, if the para-substitution is an electron-withdrawing group,
such as bromo group, the electron-withdrawing group at para-
position makes eNH2 more electron-deficient and electron-
deshielding, reducing the basicity of the lone-paired electrons on
nitrogen atom in 4-methoxy but enhancing the acidity of the NeH
bond. The basicity of lone-paired electrons on nitrogen atom and the
acidity of NH2 in aniline can be affected by para-substituent through
the delocalization of the benzene ring, to speak nothing of the NeH
bonding in their amide derivatives. In this work, a series of poly(N-
phenyl methacrylamide) derivatives (Scheme 1) were designed to
investigate the substituent-induced delocalization effects on self-
associative hydrogen bonding and inter-associative hydrogen
bonding by blending with poly(4-vinylpyridine) (P4VP). FTIR, DSC
and solid-state NMR spectroscopy were used as tools to investigate
the specific interaction.
2.2.1. Synthesis of N-4-methoxyphenyl methacrylamide and N-4-
bromophenyl methacrylamide
Two N-4-methoxyphenyl methacrylamide and N-4-bromo-
phenyl methacrylamide were synthesized by adding different types
of 4-methoxyaniline and 4-bromoaniline with methacroloyl chlo-
ride by the same reaction procedure. Methacryloyl chloride
(0.05 mol, 5.82 mL) in 20 mL dried tetrahydrofuran (THF) in
dropping funnel was slowly added to a stirred cold (0e5 ꢀC) solu-
tion of 4-bromoaniline (0.5 mol) and triethylamine (0.06 mol,
8.31 mL; as catalyst) in 80 mL dried tetrahydrofuran in round
bottom flask. The mixture was magnetically stirred for 24 h at room
temperature. After removing the formed solid quaternary ammo-
nium salt, the solution was transferred to a separating funnel,
washed thoroughly with 5 wt% sodium hydroxide solution, dilute
hydrochloric acid, and water, and then dried over anhydrous
magnesium sulfate. On evaporation of the solvent, N-4-bromo-
phenylmethacrylamide was obtained. 1H NMR (CDCl3, ppm) of N-4-
bromophenyl methacrylamide: 2.0 (3H, methyl), 5.43 and 5.76 (2H,
C]CH2), 7.38w7.45 (4H, benzene), 7.88 (1H, NH). N-4-methox-
yphenyl methacrylamide: 2.01 (3H, methyl), 3.76 (3H, OCH3), 5.38
and 5.75 (2H, C]CH2), 6.82w7.44 (4H, benzene), 7.74 (1H, NH).
2. Experimental
2.1. Materials
2.2.2. Synthesis of poly(N-phenyl methacrylamide) (PNPAA),
poly(N-4-methoxyphenyl methacrylamide) (PMPMA) and poly(N-4-
bromophenyl methacrylamide) (PBPMA)
The degassed solutions of N-phenyl methacrylamide, N-4-
methoxyphenyl methacrylamide and N-4-bromophenyl meth-
acrylamide were carried out in dioxane at 70 ꢀC under an argon
atmosphere in a glass reaction flask equipped with a condenser.
AIBN (1 wt% based on monomers) was employed as the initiator.
The mixture was stirred for ca. 36 h before being poured into excess
N-phenyl methacrylamide was purchased from TCI Chemical
Company that was purified by column chromatography and
recrystallization before its polymerization. Poly(4-vinyl pyridine)
(P4VP) was purchased from Aldrich Chemical. The radical initiator
azobisisobutyronitrile (AIBN) was recrystallized from ethyl alcohol
prior to use. Dioxane was distilled under vacuum and then used as
the solvent for the polymerization experiments performed in

2602
S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
hexane vigorous agitation to precipitate the product. The crude
copolymer product was purified by redissolving it in dioxane and
then adding this solution dropwisely into a large excess of hexane.
This procedure was repeated several times and then the residual
solvent of the final product was removed under vacuum at 70 ꢀC for
1 day to yield pure white poly(N-phenyl methacrylamide), poly(N-
4-methoxyphenyl methacrylamide) and poly(N-4-bromophenyl
methacrylamide). The molecular weight of PNPAA, PMPMA, and
PBPMA is 1600, 2000, 1800 g/mol, respectively.
TCE is able to dissolve neither PMPMA nor PBPMA, and thus, DMF is
used as the common solvent in this study.
Fig. 1 shows all the second run DSC thermalgrams of PNPAA/
P4VP, PMPMA/P4VP and PBPMA/P4VP blends in various composi-
tion of weight ratio, and the Tgs are summarized in Table 1. In
Fig. 1(a), PNPAA/P4VP blends are immiscible based on DSC analyses
since most compositions of these blends show two glass transition
temperatures, indicating less effective inter-associative hydrogen
bonding between the amide units of PNPAA and the pyridine units
of P4VP. The lower Tg in the range of 130e156 ꢀC can be identified
with the P4VP phase with PNPAA segments, and the higher Tg can
be identified with the PNPAA phase with P4VP segments. More
interestingly, the value of the lower-Tg phase in PNPAA/P4VP
composition of 20/80 is even lower than that of pure P4VP. Such
phenomenon had been discovered and demonstrated. Lu et al.
reported that the values of Tg in miscible poly(styrene-co-vinyl-
phenol dimethylsilano)/poly(N-vinyl-2-pyrrolidone) (PVP) blends
containing 10 and 20 wt% of PVP were lower than that of the low-Tg
2.3. Polymer blends
Blends of various binary PNPAA/P4VP, PMPMA/P4VP and
PBPMA/P4VP blends were prepared by solution casting. N,N0-
Dimethylformamide (DMF) solution containing 5 wt% polymer
mixture was stirred for 2e3 days and then it was casting in a Teflon
dish. The solution was left to evaporate at 100 ꢀC in oven for 1 day,
and the blend films were then dried at 120 ꢀC in vacuum oven for 3
days to ensure no residual solvent.
2.4. Characterizations
a PNPAA/P4VP
P4VP
The glass transition temperature of the copolymer was deter-
mined using a Du-Pont DSC-9000 DSC system. The sample was kept
at 200 ꢀC for 1 min and then cooled quickly to 30 ꢀC from the melt of
the first scan. The value of Tg was obtained as the inflection point of
the jump heat capacity at a scan rate of 20 ꢀC/min within the
temperature range of 5e280 ꢀC. Infrared spectra of the copolymer
films were determined by using the conventional NaCl disk method.
The DMFsolution containing the blendwas castontoa NaCl disk. The
film used in this study was thin enough to obey the BeereLambert
law. FTIR measurements were performed on a Nicolet Avatar 320
FTIR spectrophotometer; 32 scans were collected at a spectral
resolution of 1 cmꢁ1 1H NMR spectra of these homopolymers
were recorded on a Brucker ARX300 spectrometer using CDCl3
as the solvent. High-resolution solid-state 13C NMR spectroscopy
experiments were performed at 25 ꢀC using a Bruker DSX-400
spectrometer operating at a resonance frequency of 100.47 MHz.
High-resolution solid-state 13C NMR spectrawereacquiredusing the
cross-polarization (CP)/magic-angle spinning (MAS)/high-power
20/80
30/70
40/60
50/50
60/40
70/30
80/20
PNPAA
bPMPMA/P4VP
P4VP
20/80
30/70
40/60
50/50
60/40
70/30
dipolar decoupling (DD) technique, with a 90ꢀ pulse width of 3.9
ms,
a pulse delay time of 3 s, an acquisition time of 30 ms, and 2048
scans. A magic-angle sample-spinning rate of 5.4 kHz was used to
avoid absorption overlapping. The proton spin-lattice relaxation
time in the rotating frame (T1rH) was determined indirectly via
80/20
PMPMA
carbon observation using a 90 ꢀC
s-spin lock pulse sequence prior to
CP. The data acquisition was performed at delay times (
from 0.1 to 10 ms with a contact time of 1.0 ms.
s) ranging
c PBPMA/P4VP
P4VP
20/80
3. Results and discussion
30/70
40/60
50/50
3.1. Thermal analyses
60/40
Differential scanning calorimetry (DSC) is one of the convenient
methods to observe thermal characteristic arising from different
interactions in polymer blends. In addition, it is also useful to
determine the miscibility in polymer blends. However, there exist
many factors such as the nature of the solvent, composition,
molecular weights, etc., affecting the formation of interpolymer
interactions to change blends’ miscibility. Besides these factors, we
are particularly interested in the substituent-induced delocaliza-
tion effects on the hydrogen-bonding behaviors. For this purpose,
DMF is used as casting solvent, even though PNPAA/P4VP blends
are miscible over the entire composition range casted from tetra-
chloroethane (TCE) in our previous publication [26]. Unfortunately,
70/30
80/20
PBPMA
80
100
120
140
160
180
200
220
o
Temperature ( C)
Fig. 1. DSC thermograms of (a) PNPAA/P4VP, (b) PMPMA/P4VP and (c) PBPMA/P4VP
blend with different compositions.

S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
2603
Table 1
210
Tg behaviors of PNPAA/P4VP, PMPMA/P4VP and PBPMA/P4VP blend with different
Experimental datas
compositions.
Kwei prediction k = 1, q = 90
Fox prediction
Composition
Tg (ꢀC)
200
190
180
170
160
150
140
130
(amide/pyridine)
PNPAA/P4VP
PMPMA/P4VP
PBPMA/P4VP
0/100
20/80
30/70
40/60
50/50
60/40
70/30
80/20
100/0
138
146
147
132
134
138
155
169
178
185
195
198
202
194
149
150
155
155
193
193
194
195
141
149
156
193
199
201
196
201
198
196
component [34]. Huang et al. also certificated this phenomenon in
their studies that in the poly(2,2,3,3,3-pentafluoropropyl methac-
rylate-co-4-vinylpyridine) (PFX, where X was denoted the mol% of
4-vinylpyridine unit in the copolymer)/poly(4-vinylphenol) blends,
and the values of Tg were even lower than that of pure PF28 [35].
They suggested that the hydrogen-bonding interaction was not
extensive and thus the chain segments were poorly packed, which
give rise to a large increase in free volume and decreased Tg values.
This explanation is reasonable for interpreting the abatement of Tg
for the PNPAA/P4VP compositions of 20/80 and 30/70 comparing
with the Tg of homo-P4VP in this study.
Fig. 1(b) shows the DSC thermograms of PMPMA/P4VP blends
with different compositions. There are two Tgs in most composi-
tions, indicating that there is not effective inter-associative
hydrogen bonding between PMPMA and P4VP. The lower Tg in
the range of 145e155 ꢀC is assigned as P4VP domains and the
higher Tg is assigned as PMPMA domains. The single Tg of the
composition of 80/20 indicates that the few amount of P4VP
contaminates PMPMA matrix; on the contrary, the single Tg of the
compositions of 30/70 and 20/80 reveals that the few amount of
PMPMA contaminates P4VP matrix, with higher value of Tg upon
increasing PMPMA content. In addition, all the Tg of P4VP domains
are higher than that of pure P4VP because of the hydrogen-bonding
interaction is extensive and free-volume effect does not occur.
The DSC thermograms of PBPMA/P4VP blends are shown in
Fig. 1(c). PBPMA is miscible with P4VP over the entire composition
range with single glass transition, meaning that there is more
effective inter-association between PBPMA and P4VP than that of
PNPAA/P4VP and PMPMA/P4VP blends. Furthermore, the Tgs of
miscible PBPMA/P4VP blends increases upon increasing the
content of PBPMA. For miscible blends, Tg values can be described
by several empirical equations [36e39] and the Kwei equation is
usually employed for systems with specific interactions,
0.0
0.2
0.4
0.6
0.8
1.0
PBPMA Weight Fraction
Fig. 2. Plot of Tg versus the composition of PBPMA/P4VP blends prepared from DMF.
The curve is calculated using the Kwei equation with k ¼ 1 and q ¼ 90.
3.2. Strength of specific interaction, as determined by FTIR
spectroscopy
3.2.1. Self-associative hydrogen bonding in PNPAA, PMPMA and
PBPMA
FTIR spectroscopy has been proven to be a very powerful tech-
nique to detect the presence of self-association within a given
molecule or inter-association between two chemically dissimilar
molecules. Fig. 3 shows FTIR spectra recorded at room temperature
for the three neat poly(N-phenyl methacrylamide) derivatives and
scale-expanded FTIR spectra in the region of the carbonyl stretch-
ing vibration. In Fig. 3, PMPMA are observed for (1) the free
carbonyl groups units at 1680 cmꢁ1, (2) the self-associated,
hydrogen-bonded carbonyl groups at 1651 cmꢁ1. In PBPMA, the
peak positions of free and hydrogen-bonded carbonyl groups are
located at 1685 and 1655 cmꢁ1, respectively [40]. The peak posi-
tions of free and hydrogen-bonded carbonyl groups in PNPAA,
which were obtained in our previous publication, are located at
1684 and 1659 cmꢁ1, respectively [26]. Here, we ignore the self-
association hydrogen bonding with relatively weak aromatic—H-
N bonding [33,41]. Zhang et al. pointed out that the difference in
wavenumber (ΔnH) between the absorption bands of hydrogen-
bonded and free functional groups can be used as a measure of
the relative strength of hydrogen bonding [42], so the difference in
wavenumber (ΔnH) between the absorption peaks of hydrogen-
bonded and free carbonyl group in PNPAA is smaller than
PMPMA and PBPMA, indicating that PNPAA, PMPMA and PBPMA
possess different self-association hydrogen bonding strengths.
Moreover, according to the PaintereColeman association model
[40], the thermodynamics of polymer blends with hydrogen
bonding can be written using the following scheme:
W1Tg1 þ kW2Tg2
Tg
¼
þ qW1W2
(1)
ðW1 þ kW2Þ
where Tg is the glass transition temperature of the blend; Tg1 and Tg2
are those of the pure component; W1 and W2 are the weight frac-
tions of the two components; k and q are adjustable parameters.
The application of Eq. (1) on the Tg-composition data of the PBPMA/
P4VP blends is best fitted by a curve (as shown in Fig. 2) with k ¼ 1
and q ¼ 90, where q is a parameter which is corresponding to the
strength of hydrogen bonding in the blend and reflecting a balance
between the breaking of the self-association and the forming of the
inter-association hydrogen bonding. The positive q of 90 means
a strong intermolecular interaction between PBPMA and P4VP. The
reason for the immiscible behavior of PNPAA and PMPMA or
miscible behavior of PBPMA with P4VP could be characterized by
FTIR spectroscopy analyses.

2604
S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
C¼O
1659
self-association of amide groups is related to the fF
through
values
(2)
1684
1 ꢁ fFC¼O
fFC¼O
KB
¼
h
i
2
PNPAA
1680
1651
1655
and the KB at different temperatures could be calculated. Obviously,
the equilibrium constants, and hence the stoichiometry of
hydrogen boning, vary with temperature. This is simply described
through the usual dependence on the enthalpy of hydrogen-bond
formation:
PMPMA
1685
ꢀ
ꢁ
ꢂꢃ
D
R
hB
1
T
1
T+
KB ¼ KBexp ꢁ
ꢁ
(3)
where KB is the value of the equilibrium constant determined at To
K (usually 298 K), R is the universal gas constant, and ΔhB is the
molar enthalpy of the formation of individual hydrogen bonds.
Using Eq. (2) and Eq. (3), we can calculate the enthalpy of self-
associative hydrogen-bond formation in PNPAA, PBPMA, except
PMPMA. The lone-paired electrons on oxygen atom in ether group
of PMPMA would be a hydrogen-bond acceptor, competing with
the lone-paired electrons on oxygen atom in carbonyl group.
However, the hydrogen-acceptor ability of the lone-paired elec-
trons on ether oxygen is much smaller than that of carbonyl group
in amide [42]. Thus, the small fraction of hydrogen-bonded
methoxy group can be ignored, and this methodology still can be
employed to calculate the enthalpy of self-associative hydrogen-
bond formation in PMPMA.
PBPMA
4000
3500
3000
2500
2000
1500
1000
500 1700 1675 1650 1625
Wavenumber (cm-1)
Fig. 3. FTIR spectra for three neat polymethacrylamides: PNPAA, PMPMA and PBPMA.
K2
B1 + B1
Bh + B1
Bh + A1
B2
Self-association
Inter-association
Kh+1
KA
Bh+1
BhA1
The curve-fitting limits were set to 1560e1740 cmꢁ1, and the
spectra were curve resolved into two bands using a lease-squares
fitting procedure [40]. For PBPMA, Lorezian band shapes were
fixed, the
n of the two bands were restricted to ranges of
1685 ꢂ 1 cmꢁ1 and 1655 ꢂ 1 cmꢁ1, and w1/2 was restricted to range
31.5 ꢂ 1 cmꢁ1 and 32.0 ꢂ 1 cmꢁ1, respectively, for all the least-
squares fits. Values of aHB/aF ¼ 1.2 is employed to calculate the
fraction of free and hydrogen-bonded carbonyl groups [39]. The
calculations of KB for the different temperatures are listed in the final
column of Table 2. A van’t Hoff plot of ln KB versus the reciprocal of
temperatureis shownin Fig. 4, and wecan obtain avalue of 1.41 kcal/
mol for the enthalpy ΔhB of hydrogen-bond formation from the
slope of least-squares linear fit of the data in Table 2. The same
methodology was employed to calculate the enthalpy of hydrogen-
bond formation of PMPMA, with the value of 1.39 kcal/mol, and the
ΔhB of PNPAA was obtained with value of 1.29 kcal/mol [33].
where B1 represents amide-NH group, A1 represents pyridine
group, B2 is the hydrogen-bonded dimmer formed between two
amide groups, Bh is the hydrogen-bonded multimer, and BhA1 is
the inter-associative hydrogen bonding. The change in the rela-
tive intensities of the free and hydrogen-bonded carbonyl bands
vary systematically in favor of the former as temperature
increasing, reflecting the decreasing fraction of hydrogen-bonded
carbonyl groups present as a function of temperature. This is
a measure of the enthalpy of hydrogen-bond formation. The
methodology employed to determine the enthalpy of hydrogen-
bond formation from infrared spectra has been discussed in detail
on previous publication [40,41]. The equilibrium constant KB of
Table 2
Curve fitting of the fraction of free and hydrogen-bonded carbonyl group of PNPAA, PMPMA, and PBPMA.
Temp.
(ꢀC)
PNPAA
PMPMA
PBPMA
The fraction of free and
KB
The fraction of free and
KB
The fraction of free and
hydrogen-bonded C]O
KB
hydrogen-bonded C]Oa
hydrogen-bonded C]Ob
fF (%)
fb (%)
fF (%)
fb (%)
fF (%)
fb (%)
50
70
90
110
130
150
170
190
210
46.4
48.4
50.2
51.4
52.9
54.2
55.7
56.7
57.8
53.6
51.6
49.8
48.6
47.1
45.8
44.3
43.3
42.2
2.49
2.20
1.97
1.84
1.68
1.56
1.42
1.34
1.26
67.7
69.9
71.4
72.9
74.9
76.3
77.2
78.1
79.2
32.3
30.1
28.6
27.1
25.1
23.7
22.8
21.9
20.8
0.71
0.62
0.56
0.51
0.45
0.41
0.38
0.36
0.33
59.9
62.0
63.4
65.1
66.5
67.5
68.9
69.9
71.4
40.1
38.0
36.6
34.9
33.5
32.5
31.1
30.1
28.6
1.12
0.99
0.90
0.82
0.76
0.71
0.66
0.62
0.56
a
The
The
n
n
of the two bands were restricted to ranges of 1684 ꢂ 1 cmꢁ1 and 1659 ꢂ 1 cmꢁ1, and the w1/2 was restricted to range 27.0 ꢂ 1 cmꢁ1 and 34.0 ꢂ 1 cmꢁ1, respectively.
b
of the two bands were restricted to ranges of 1680 ꢂ 1 cmꢁ1 and 1651 ꢂ 1 cmꢁ1, and the w1/2 was restricted to range 31.5 ꢂ 1 cmꢁ1 and 27.0 ꢂ 1 cmꢁ1, respectively.

S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
2605
hydrogen-bonded carbonyl groups blue-shift. However, the
strength of self-association between carbonyl and NeH of amide
group in three poly(N-phenyl methacrylamide) derivatives are
comparable to each others. As shown in Fig. 5, through delocal-
1.4
1.2
PNPAA
PBPMA
PMPMA
1.0
0.8
ization of benzene-
p system, electron-donating group makes the
0.6
= 656
0.4
amide group electron-rich, enhancing the hydrogen-bond
acceptor ability of carbonyl group, but the proton on nitrogen in
amide (NeH) is electron-shielded, with reduced hydrogen-bond
donor ability. In contrast, electron-withdrawing group makes the
amide group electron-deficient, reducing the hydrogen-bond
acceptor ability of carbonyl group; but the proton on nitrogen in
amide is electron-deshielded, with enhanced hydrogen-bond
donor ability. Two factors are offset by each other, so that the
values of enthalpy of self-association in poly(N-phenyl meth-
acrylamide) with different inductive substitution at para position
of benzene ring are not affected by the substituent-induced
delocalization.
0.2
0.0
= 708
= 701
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
2.0
2.2
2.4
2.6
2.8
3.0
3.2
1000/T
Fig. 4. The van’t Hoff plots of ln KB versus 1/T for PNPAA (-), PBPMA (C) and PMPMA
(:).
a
PNPAA/P4VP
From the calculation of the enthalpy of self-associative forma-
tion in PNPAA, PMPMA and PBPMA, similar values of enthalpy are
also obtained as the difference in wavenumber (ΔnH) between the
absorption bands of hydrogen-bonded and free carbonyl groups;
even though the equilibrium constants of PBPMA at each temper-
ature is smaller than those of PNPAA. Bromine atom is a more steric
bulky group than a hydrogen atom, the self-association between
carbonyl and NeH group in PBPMA is much more difficult than the
self-association in PNPAA, and thus equilibrium constants in
PBPMA at different temperatures are smaller than those in PNPAA.
In Fig. 5, the scale-expanded spectra in the region of the
carbonyl stretching vibration reveals that the peak positions of
free and hydrogen-bonded carbonyl group shift because of para-
substitutions. Electron-withdrawing group (eBr) makes the peaks
of free and hydrogen-bonded carbonyl groups red-shift, and
electron-donating group (eOCH3) makes the peaks of free and
3360
3368
3438
20/80
40/60
60/40
80/20
PNPAA
3600
3400
3200
3000
3000
3000
2800
2800
2800
b
PMPMA/P4VP
3362
3438
3368
20/80
40/60
60/40
80/20
PMPMA
3600
3400
3200
c
PBPMA/P4VP
3438
3275
3368
20/80
40/60
60/40
80/20
PBPMA
3600
3400
3200
Wavenumber cm-1
Fig. 5. The electron-flow pathway induced by para-substitution in poly(N-phenyl
Fig. 6. FTIR spectra at 120 ꢀC for (a) PNPAA/P4VP blends, (b) PMPMA/P4VP blends, and
methacrylamide).
(c) PBPMA/P4VP blends.

2606
S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
3.2.2. Inter-associative hydrogen bonding in PNPAA/P4VP, PMPMA/
P4VP and PBPMA/P4VP blends
group in poly(N-phenyl methacrylamide)s via the lone-paired
electrons on nitrogen atom [26]. The para-substitution can affect
the inter-associative hydrogen bonding between amide and
pyridine through this planar delocalization. Electron-donating
group in PMPMA makes the amide group electron-rich, so the
proton on nitrogen in amide is electron-shielded, with lower
hydrogen-bond donor ability than that of PNPAA. In contrast,
electron-withdrawing group in PBPMA makes the amide group
electron-deficient, so the proton on nitrogen in amide is electron-
deshielded, with higher hydrogen-bond donor ability than that of
PNPAA.
Fig. 6 gives FTIR spectra of the NeH stretching region for PNPAA/
P4VP, PMPMA/P4VP and PBPMA/P4VP blends at 120 ꢀC, because
P4VP and polymethacrylamides absorb moisture from air very
easily. Clearly, pure PNPAA, PMPMA, and PBPMA all show the two
absorption band corresponding to free amide NeH group at ca.
3438 cmꢁ1 and hydrogen-bonding amide group at ca. 3368 cmꢁ1
.
The intensity of free amide NeH group was decreased and the
hydrogen-bonding amide group from shifts to a lower wavenumber
with the increase of P4VP contents in all three binary blend
systems. This result reflects a new distribution of hydrogen-
bonding formation resulting from the competition between ami-
deeamide and amideepyridine interactions. Zhang et al. pointed
out that the difference in wavenumber (ΔnH) between the absorp-
tion bands of hydrogen-bonded and free phenolic groups could be
used as a measure of the relative strength of hydrogen bonding
[42]. The wavenumber of amideepyridine hydrogen bond was
located at 3360 cmꢁ1, 3362 cmꢁ1, and 3275 cmꢁ1 as shown in Fig. 6
for PNPAA/P4VP, PMPMA/P4VP, PBPMA/P4VP blends, respectively,
indicating that the fraction of hydrogen bonding in PBPMA/P4VP
blends is higher than those in PNPAA/P4VP and PMPMA/P4VP
blends. From the DSC thermograms, PBPMA/P4VP blends are
homogeneous over entire composition range, while most compo-
sitions of PNPAA/P4VP and PMPMA/P4VP blends are heteroge-
neous, indicating that the possession of higher inter-associative
strength than self-associative strength does not guarantee to obtain
a miscible polymer blend. Actually, much larger KA than KB does
ensure a miscible blend [1,43,44]. As shown in Fig. 6, the intensity
of inter-associative hydrogen-bonded NeH stretching with P4VP in
PBPMA/P4VP blends is obviously higher than that in PNPAA/P4VP
and PMPMA/P4VP blends, meaning a larger KA/KB ratio in PBPMA/
P4VP blends.
The relative strength of self-associative hydrogen-bond
formation in PNPAA, PMPMA and PBPMA are comparable to each
other, because the substituent-induced delocalization effects on
NeH and C]O is offset by each other as the description above.
Nevertheless, the relative strength of inter-associative hydrogen-
bond formation and the further miscibility in PNPAA/P4VP,
PMPMA/P4VP and PBPMA/P4VP blends are different from each
others, indicating that the hydrogen-bond donor ability of NeH in
amide group is the factor affected by substituent-induced delo-
calization. When a pyridine group in P4VP forms a hydrogen bond
with the NeH group in amide (see in Fig. 7), a planar delocal-
3.3. 13C CP/MAS solid-state NMR spectroscopic analyses
In addition to FTIR, evidences on interactions in the blends can
also be obtained from solid-state NMR spectroscopy as demon-
strated by changes in chemical shift and/or line shape. Fig. 8 shows
the selected 13C CP/MAS spectra of PNPAA/P4VP, PMPMA/P4VP and
PBPMA/P4VP at the composition of 20/80, comparing with that of
pure PNPAA and pure P4VP. Five peaks are also observed for pure
P4VP where the peak at
d
¼ 123.9 ppm and corresponds to the
pyridine carbon atom. Clearly, the pyridine carbons shift in PBPMA/
P4VP-20/80 downfields about 3.8 ppm significantly also indicating
that the hydrogen-bonding interaction between the amide group of
PBPMA and the pyridine group of P4VP. The pyridine carbons shift
in PNPAA/P4VP-20/80 and PMPMA/P4VP-20/80 downfield about
3.2 and 3.0 ppm, respectively, and this finding is consistent with the
results of our FTIR spectroscopic analysis.
Solid-state NMR spectroscopy is often used to better understand
the phase behavior and interactions within polymer blends
[45e48]. As discussion above, the DSC analyses reveal that phase
separation occurred in PNPAA/P4VP and PMPMA/P4VP blends in
certain compositions. However, it is must be kept in mind that the
compositions with single transition temperature may be hetero-
geneous instead of homogeneous, because the dimension of phase
separation may be too small to be detected by DSC analysis. One of
the methodology employed to analyze the dimension of phase
separation smaller than 20 nm is a measurement of the spin-lattice
relaxation time in the rotating frame (T1rH) [34]. The T1rH value of
the blends was measured through the delayed-contact 13C/MAS
experiments. The T1rH values are calculated from Eq. (4):
ization is constructed by the aromatic-
p
system of pyridine in
system of amide
P4VP, the aromatic- system and the carbonyl-
p
p
PNPAA
PNPAA/P4VP-20/80
PMPMA/P4VP-20/80
PBPMA/P4VP-20/80
P4VP
200 180 160 140 120 100 80 60 40 20
0
Fig. 7. The planar delocalization over carbonyl, phenyl aromatic and pyridine-aromatic
systems via the lone-paired electrons on nitrogen atom constructed by inter-
associative hydrogen bond between poly(N-phenyl methacrylamide) derivatives and
Chemical Shift (ppm)
p
Fig. 8. 13C CP/MAS spectra of PNPAA/P4VP, PMPMA/P4VP and PBPMA/P4VP at the
P4VP units.
composition of 20/80.

S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
2607
0.0
a PNPAA/P4VP = 80/20
b PNPAA/P4VP = 20/80
0.0
-0.5
-1.0
-1.5
-0.5
-1.0
-1.5
-2.0
-2.0
1.0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
12
12
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
c PMPMP/P4VP = 80/20
d PMPMP/P4VP = 20/80
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
1.0
0
2
4
6
8
10
0
2
4
6
8
10
1.0
0.5
e PBPMA/P4VP = 80/20
f PBPMA/P4VP = 20/80
0.5
0.0
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-4.0
12 -2
-2
0
2
4
6
8
10
0
2
4
6
8
10
Delay Time (ms)
Fig. 9. Logarithmic plots of magnetization intensities of 176 ppm versus delay time for PPMA/P4VP blends (a) 80/20 and (b) 20/80; for PMPMA/P4VP blends (c) 80/20 and (d) 20/80;
for PBPMA/P4VP blends (e) 80/20 and (f) 20/80.
lnðMs=MoÞ ¼ ꢁ
s
=T1rH
(4)
20 and 20/80 appearing single Tg in DSC analyses. This result is
consistent with the assumption that the phase separation may not
be measured in DSC. In contrast, single-exponential decays in T1rH
for PBPMA/P4VP blends are obtained in Fig. 9, meaning that both
where is the delay time used in the experiment, M0 and Ms are
the intensities of the peaks initially and at time , respectively.
s
s
s
Fig. 9 shows the plots of ln (Ms/M0) against . We estimated the
homogeneities of these polymer blends through quantitative
analyses based on the main-chain-methylene carbon atom’s
resonance in poly(methacrylamide) at 46 ppm. Two exponential
decays in T1rH for PNPAA/P4VP ¼ 80/20, 20/80 and PMPMA/
P4VP ¼ 80/20, 20/80 can be observed in Fig. 9 and Table 3, indi-
cating that two domains possessing distinct mobility are present
in PNPAA/P4VP and PMPMA/P4VP blends for entire compositions,
except PNPAA blends and PMPMA blends with compositions of 80/
Table 3
Value of relaxation time (T1rH) for PNPAA/P4VP, PMPMA/P4VP and PBPMA/P4VP
blends.
Composition amide/pyridine
T
1rH(ms)
PNPAA/P4VP
PMPMA/P4VP
PBPMA/P4VP
100/0
80/20
20/80
4.14
3.33/9.09
3.03/7.69
4.71
3.25/8.33
2.75/4.76
5.36
5.16
3.12

2608
S.-W. Kuo et al. / Polymer 52 (2011) 2600e2608
PBPMA blends are homogeneous on the scale at which spin-
diffusion occurs within the time Thus the interaction
1rH
Acknowledgment
T
.
between PBPMA and P4VP is the largest among these three binary
blends. These conclusions are also consistent with the results of
infrared and DSC analyses.
This work was supported financially by the National Science
Council of the R.O.C. under Contract No. NSC 97-2221-E-110-013-
MY3 and NSC 98-2221-E-110-006.
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