PEG blend

Vibrational Spectroscopy 60 (2012) 163–167

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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 Kim a,1 , Soo Ryeon Ryu a,1 , Isao Noda b , Young Mee Jung a,∗ a b

Department of Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chunchon 200-701, Republic of Korea 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 infraredreflection 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, including poly(3-hydroxybutyrate) (PHB) and PHB-based copolymers, such as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HBco-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 spectroscopic studies of polymers [11–13]. P(HB-co-HHx) copolymers are fully miscible with biodegradable 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: [email protected] (Y.M. Jung). 1 These authors contributed equally to this work. 0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2012.02.008

In this study, thermal behavior of spin-coated film of P(HBco-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-coHHx)/PEG blend, we performed projection 2D correlation analysis. Projection 2D correlation analysis can simplify and streamline 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 projection operations makes it possible to attenuate or augment select features within congested 2D correlation spectra for easier interpretation. 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.

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Wavenumber(cm ) 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(HBco-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 measured 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(HBco-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 heating 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

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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 spectra 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 process, 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 spectrum 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 spectrum, 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

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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 temperature 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 ) 1D

2D

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2972

CH2 rocking (PHA C) C O C asymmetric stretching (PEG) C O C asymmetric stretching (PEG) C O C asymmetric stretching (PEG) C O C asymmetric stretching (PHA A) C O C asymmetric stretching (PHA C) C O C asymmetric stretching (PEG) CH2 asymmetric stretching (PEG) CH2 asymmetric stretching (PHA C) CH3 asymmetric stretching (PHA C) CH3 asymmetric stretching (PEG) CH3 asymmetric stretching (PHA A) 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 projection 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 synchronous 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 projected 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 correlation 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 correlation 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-coHHx) was observed in both conventional and null-space projection 2D correlation spectra. It means that the contribution of amorphous 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

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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.

authors thank the Central Laboratory of Kangwon National University for the measurements of IRRAS spectra.

4. Conclusion

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We demonstrated the thermal behavior of spin-coated films of P(HB-co-HHx)/PEG blends by using 2D IRRAS correlation spectroscopy. 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 projection 2D correlation analysis, to better understand the contribution of PEG in spin-coated film of P(HB-co-HHx)/PEG blend during heating process. Null-space 2D projection correlation spectra clearly showed the subtle contribution of PEG in spin-coated film of P(HBco-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

References