1. Vibrational Spectroscopy 60 (2012) 163–167
Contents lists available at SciVerse ScienceDirect
Vibrational Spectroscopy
journal homepage: www.elsevier.com/locate/vibspec
Projection 2D correlation analysis of spin-coated film of biodegradable
P(HB-co-HHx)/PEG blend
Min Kyung Kima,1
, Soo Ryeon Ryua,1
, Isao Nodab
, Young Mee Junga,∗
a
Department of Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chunchon 200-701, Republic of Korea
b
The Procter & Gamble Company, West Chester, OH 45069, USA
a r t i c l e i n f o
Article history:
Received 28 August 2011
Received in revised form 13 February 2012
Accepted 13 February 2012
Available online 21 February 2012
Keywords:
P(HB-co-HHx)/PEG blend
Thermal behavior
Infrared-reflection absorption (IRRAS)
spectroscopy
2D correlation spectroscopy
Projection 2D correlation analysis
Null-space projection
a b s t r a c t
We investigated thermal behavior of spin-coated film of P(HB-co-HHx)/PEG blend by using infrared-
reflection absorption (IRRAS) spectroscopy, 2D correlation spectroscopy, and projection 2D correlation
analysis. From the analysis of 2D IRRAS correlation spectra, we could determine the sequence of spectral
intensity changes with increasing temperature that PEG band changes first, and then a band for crystalline
component of P(HB-co-HHx) changes before a band for amorphous component. Null-space 2D projection
correlation spectra clearly showed the contribution of PEG in spin-coated film of P(HB-co-HHx)/PEG
blend during heating process, which is barely observable in conventional 2D IRRAS correlation spectra.
Projection and null-space projection 2D correlation analysis seem to selectively accentuate the subtle
contribution of thermal behavior of PEG in spin-coated film of P(HB-co-HHx)/PEG blend during heating
process.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Biodegradable poly(hydroxyalkanoate) (PHA) polymers, includ-
ing poly(3-hydroxybutyrate) (PHB) and PHB-based copolymers,
such as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-
co-HHx)) have been studied extensively [1–4]. Phase separation
and thermal behavior of blends of PHB with poly(l-lactic acid)
(PLLA), PHB with poly(4-vinylphenol) (PVPh), and P(HB-co-HHx)
with PLLA have also been studied [5–7]. We have recently
investigated the transition temperature and thermal behavior of
spin-coated films of P(HB-co-HHx) (HHx = 3.8, 7.2 and 10.0 mol%)
copolymers by using 2D correlation analysis [8–10]. Generalized 2D
correlation spectroscopy is a well-established analytical technique
that provides considerable utility and benefit in various spectro-
scopic studies of polymers [11–13].
P(HB-co-HHx) copolymers are fully miscible with biodegrad-
able polyethylene glycol (PEG). Blends of PEG and several aliphatic
polyesters have been studied [14–16]. However, thermal behavior,
like crystallization of P(HB-co-HHx) in the presence of PEG has not
yet been explored much. We have recently reported on thermal
behavior of thin films of P(HB-co-HHx)/PEG blend [17].
∗ Corresponding author. Tel.: +82 33 250 8495; fax: +82 33 253 7582.
E-mail address: ymjung@kangwon.ac.kr (Y.M. Jung).
1
These authors contributed equally to this work.
In this study, thermal behavior of spin-coated film of P(HB-
co-HHx)/PEG blend was investigated by 2D infrared-reflection
absorption spectroscopy (IRRAS) correlation spectroscopy. To
better understand the effect of PEG in spin-coated film of P(HB-co-
HHx)/PEG blend, we performed projection 2D correlation analysis.
Projection 2D correlation analysis can simplify and stream-
line the information contained in 2D correlation spectra [18]. This
method is based on the use of mathematical matrix projection to
selectively filter out the unwanted portion of the information of
spectral data. The combination of the projection and null-space pro-
jection operations makes it possible to attenuate or augment select
features within congested 2D correlation spectra for easier inter-
pretation. The details of this technique are described previously
[18].
2. Experimental
Biodegradable P(HB-co-HHx) (HHx = 10.0 mol%) copolymer was
obtained from the Procter & Gamble Company, Cincinnati, OH.
It was dissolved in hot chloroform, and then precipitated in
hexane. The same process was repeated again, re-precipitated
in methanol, and vacuum-dried at 60 ◦C. PEG (number-average
molecular weight, Mn is 1500) was purchased from Sigma–Aldrich
Co., Ltd., at highest purity available and used as received without
further purification.
0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.vibspec.2012.02.008

2. 164 M.K. Kim et al. / Vibrational Spectroscopy 60 (2012) 163–167
0.18
0.12
0.18
ce
(a) 30 oC
150 oC
0.06
Absorban
90012001500180027002900
0.00
(b)
0.16
orbance
(b)
30 oC
65 oC
0 00
0.08
Abs
2900 2700 1800 1500 1200 900
0.00
Wavenumber(cm
-1
)
Fig. 1. IRRAS spectra of spin-coated film of P(HB-co-HHx)/PEG blend (a) and PEG (b) obtained during the heating process from 30 to 150 ◦
C.
Blend of P(HB-co-HHx) and PEG was prepared by dissolving each
component together in hot chloroform. The blending ratio of P(HB-
co-HHx)/PEG blend was 98/2 by weight.
Pt-coated silicon wafer from Siltron Inc. (Korea) was used as
the substrates for spin coating. To prepare spin-coated film, about
2 wt% P(HB-co-HHx)/PEG blend solution dissolved in chloroform
was spun onto a Pt-coated silicon wafer at 2000 rpm for 60 s. The
thickness of spin-coated film of P(HB-co-HHx)/PEG blend was about
400 nm.
The infrared-reflection absorption (IRRAS) spectra were mea-
sured at a spectral resolution of 4 cm−1 with a Thermo NICOLET
6700 FT-IR spectrometer equipped with a liquid nitrogen-cooled
MCT detector. A seagull accessory of Harrick Scientific Product
Inc., which includes a heating block attachment, was used for
the IRRAS measurement, and an infrared ray was used at an
angle of incidence of 79 ◦C. To ensure a high signal-to-noise ratio,
64 interferograms were co-added for each measurement. The
temperature-dependent IRRAS spectra of spin-coated film of P(HB-
co-HHx)/PEG blend were measured at an increment of 5 ◦C in the
range of 30–150 ◦C.
Synchronous and asynchronous 2D correlation spectra were
obtained by using the same software as at described previously
[19]. Projection and null-space projection 2D correlation spectra
were obtained using an algorithm developed by Noda [18].
3. Results and discussion
Fig. 1(a) shows the temperature-dependent IRRAS spectra of
spin-coated film of P(HB-co-HHx)/PEG blend, obtained during
the heating process from 30 to 150 ◦C. As shown in Fig. 1(a),
there are two distinct bands in the C O stretching region for
P(HB-co-HHx)/PEG blend, a crystalline band at 1724 cm−1 and
an amorphous band at 1755 cm−1. The intensity of a crystalline
band decreases with increasing temperature, while that of an
amorphous band increases. Similar trends of the spectral changes
of C H deformation, the C O C stretching mode, and the C O
stretching mode in 1000–1500 cm−1 region and C H stretching
mode in 2800–3050 cm−1 region are also observed for the heat-
ing process. However, in Fig. 1(a), it is very difficult to detect the
spectral changes of PEG component due to small content of PEG in

3. M.K. Kim et al. / Vibrational Spectroscopy 60 (2012) 163–167 165
1294
1282
2931
2985
900
982
1198
1134
2820
2840
3008
2968
2
(a) (c)
avenumber(cm
-1
)
900
1000
1100
1200
avenumber(cm
-1
)
2860
2880
2900
2920
2940
ν2
,wa
wavenumber (cm
-1
)
1300
1400
1500
900100011001200130014001500
ν2
,wa
wavenumber (cm
-1
)
2960
2980
3000
3020
2850290029503000
1
)
900
1000
1
)
2820
2840
2860
2880
ν1
, wavenumber (cm ) ν1
, wavenumber (cm )
(b) (d)
ν2
,wavenumber(cm
-
1100
1200
1300
ν2
,wavenumber(cm
-
2900
2920
2940
2960
2980
ν1
, wavenumber (cm
-1
)
1400
1500
900100011001200130014001500
ν1
, wavenumber (cm
-1
)
3000
3020
2850290029503000
Fig. 2. Synchronous and asynchronous 2D correlation spectra in the region of C H deformation, C O C stretching, and C O stretching modes in 1000–1500 cm−1
(a, b)
and C H stretching mode in 2800–3050 cm−1
(c, d) for spin-coated film of P(HB-co-HHx)/PEG blend, respectively. The red and blue lines represent positive and negative
cross peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
P(HB-co-HHx)/PEG blend. The temperature-dependent IRRAS spec-
tra of spin-coated film of PEG itself are shown in Fig. 1(b) for
comparison. As shown in Fig. 1(b), spectral intensities of bands
at 1344, 1122, and 966 cm−1 changed greatly during heating pro-
cess, which are not readily detected in IRRAS spectra of spin-coated
film of P(HB-co-HHx)/PEG blend. In this study, we only focused on
regions in 1000–1500 cm−1 and 2800–3050 cm−1 in which spectral
changes of PEG component are observed.
To investigate thermal behavior of P(HB-co-HHx)/PEG blend, 2D
correlation analysis was applied to temperature-dependent IRRAS
spectra of spin-coated film of P(HB-co-HHx)/PEG blend. Fig. 2(a)
and (b) shows, respectively, synchronous and asynchronous 2D
correlation spectra in the region of C H deformation, the C O C
stretching mode, and the C O stretching mode in 1000–1500 cm−1
of spin-coated film of P(HB-co-HHx)/PEG blend. Autopower spec-
trum extracted along the diagonal line in the synchronous 2D
correlation spectrum is given on top of each synchronous 2D
correlation spectrum. In the synchronous 2D correlation spec-
trum, we clearly observed two bands for the crystalline band at
1294 cm−1 and the amorphous band at 1282 cm−1, which is not
readily detectable in the original 1D spectra. The intensities of these
two bands are changed greatly during heating process. We also

4. 166 M.K. Kim et al. / Vibrational Spectroscopy 60 (2012) 163–167
1294
1282
2985
982
1198
1134
3006
2968
2933
1
)
900
1000
1
)
2820
2840
2860
2880
(a) (b)
wavenumber(cm
-1
1000
1100
1200
wavenumber(cm
-1
2880
2900
2920
2940
ν2
,w
1300
1400
1500
ν2
,w
2960
2980
3000
3020
ν1
, wavenumber (cm
-1
)
1500
900100011001200130014001500
ν1
, wavenumber (cm
-1
)
2850290029503000
Fig. 3. Synchronous projection 2D correlation spectra in the region of C H deformation, C O C stretching, and C O stretching modes in 1000–1500 cm−1
(a) and C H
stretching mode in 2800–3050 cm−1
(b) for spin-coated film of P(HB-co-HHx)/PEG blend, respectively. The red and blue lines represent positive and negative cross peaks,
respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
observed the intensity change of a band at 1198 cm−1 assigned to
PEG. From the analysis of 2D correlation spectra, the sequence of
intensity changes in 1000–1500 cm−1 region with increasing tem-
perature is such that a band of PEG is changing first, and then
crystalline component of P(HB-co-HHx) changes before a band for
amorphous component.
The corresponding 2D correlation spectra in the region of C H
stretching mode in 2800–3050 cm−1 are shown in Fig. 2(c) and (d).
In synchronous 2D correlation spectrum, the intensity changes of
the bands at 2931 and 2985 cm−1 are significant during the heating
process, which can be assigned to P(HB-co-HHx). It is quite difficult
to detect the spectral changes of PEG component during the heating
process even in 2D correlation spectra. Detailed band assignments
for IRRAS spectra and 2D correlation spectra of spin-coated film of
P(HB-co-HHx)/PEG blend are summarized in Table 1.
To better understand the contribution of PEG in spin-coated film
of P(HB-co-HHx)/PEG blend during heating process, we performed
Table 1
Band assignments and wavenumber (cm−1
) for IRRAS spectra of spin-coated film of
P(HB-co-HHx)/PEG blend.
Wavenumber (cm−1
) Assignments
1D 2D Projection 2D
982 CH2 rocking (PHA C)
1105 C O C asymmetric stretching (PEG)
1198 1198 C O C asymmetric stretching (PEG)
1277 C O C asymmetric stretching (PEG)
1296
1282 C O C asymmetric stretching (PHA A)
1294 C O C asymmetric stretching (PHA C)
1313 C O C asymmetric stretching (PEG)
2875 CH2 asymmetric stretching (PEG)
2931 2931 CH2 asymmetric stretching (PHA C)
2968 2968 CH3 asymmetric stretching (PHA C)
2972 CH3 asymmetric stretching (PEG)
2985 2985 CH3 asymmetric stretching (PHA A)
3008 3008 Hydrogen bonding
C: crystalline, A: amorphous, PHA: P(HB-co-HHx).
projection 2D correlation analysis. We used the transformation
data by the vector projection with the spectral intensity variation
signal at 1724 cm−1, which is dominated by the contribution from
the crystalline C O stretching vibration of P(HB-co-HHx), for pro-
jection 2D correlation spectra. Fig. 3(a) and (b) shows, respectively,
synchronous projection 2D correlation spectra constructed from
the spectra obtained by positively projected dynamic spectrum at
1724 cm−1 in regions of C H deformation, the C O C stretching
mode, and the C O stretching mode in 1000–1500 cm−1 and C H
stretching mode in 2800–3050 cm−1. They are the same as the syn-
chronous conventional 2D correlation spectra shown in Fig. 2(a)
and (c). It means that all spectral changes observed in conventional
2D correlation spectra indicate the contribution of not PEG but
P(HB-co-HHx) in P(HB-co-HHx)/PEG blend during heating process.
To selectively filter out the contribution of P(HB-co-HHx), we
performed null-space projection 2D correlation analysis. Fig. 4(a)
and (b) shows, respectively, synchronous null-space projection
2D correlation spectrum constructed from the null-space pro-
jected data with the crystalline signals of P(HB-co-HHx) removed in
regions of C H deformation, the C O C stretching mode, and the
C O stretching mode in 1000–1500 cm−1 and C H stretching mode
in 2800–3050 cm−1. The synchronous null-space projection 2D cor-
relation spectrum in the region of C H deformation, the C O C
stretching mode, and the C O stretching mode in 1000–1500 cm−1
shown in Fig. 4(a) is completely different with conventional 2D cor-
relation spectrum. New bands at 1313, 1105, and 1065 cm−1 are
observed in Fig. 4(a), which are barely observable in conventional
2D correlation spectrum. These new bands can thus be assigned
to PEG. A band at 1277 cm−1 assigned to amorphous of P(HB-co-
HHx) was observed in both conventional and null-space projection
2D correlation spectra. It means that the contribution of amor-
phous of P(HB-co-HHx) was not completely filtered out due to
the projection by the band of crystalline C O stretching vibration
of P(HB-co-HHx). In Fig. 4(b), bands at 2972 and 2875 cm−1 are
observed, which are not detected in conventional 2D correlation
spectrum. These two bands can also be assigned to PEG. It suggests

5. M.K. Kim et al. / Vibrational Spectroscopy 60 (2012) 163–167 167
277
2972
1
1065
1313
1105
2875
2925
2
3010
900
1000
2820
2840
2860
2880
(a) (b)
avenumber(cm
-1
)
1000
1100
1200
avenumber(cm
-1
)
2880
2900
2920
2940
ν2
,wa
1300
1400
ν2
,wa
2960
2980
3000
3020
ν1
, wavenumber (cm
-1
)
1500
900100011001200130014001500
ν1
, wavenumber (cm
-1
)
2850290029503000
Fig. 4. Synchronous null-space projection 2D correlation spectra in the region of C H deformation, C O C stretching, and C O stretching modes in 1000–1500 cm−1
(a)
and C H stretching mode in 2800–3050 cm−1
(b) for spin-coated film of P(HB-co-HHx)/PEG blend, respectively. The red and blue lines represent positive and negative cross
peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
that the spectral changes of PEG in C H stretching region during
the heating process are much more significant than those in C H
deformation, the C O C stretching mode, and the C O stretching
regions.
4. Conclusion
We demonstrated the thermal behavior of spin-coated films
of P(HB-co-HHx)/PEG blends by using 2D IRRAS correlation spec-
troscopy. From the analysis of conventional 2D correlation spectra,
we can determine the following sequence of spectral intensity
changes that PEG band changes first and then a band for crystalline
component of P(HB-co-HHx) changes before a band for amorphous
component.
Furthermore, we performed projection and null-space projec-
tion 2D correlation analysis, to better understand the contribution
of PEG in spin-coated film of P(HB-co-HHx)/PEG blend during heat-
ing process. Null-space 2D projection correlation spectra clearly
showed the subtle contribution of PEG in spin-coated film of P(HB-
co-HHx)/PEG blend during heating process. The combination of the
projection and null-space projection operations might be a very
useful technique to attenuate or augment select features within
congested 2D correlation spectra for easier interpretation.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grants funded by the Korea government (MEST) (no.
2009-0065428 and no. 2009-0087013) and the BK 21 program from
the Ministry of Education, Science and Technology of Korea. The
authors thank the Central Laboratory of Kangwon National Univer-
sity for the measurements of IRRAS spectra.
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