Rp(

Ha

b

a

ARRAA

KRTNTsPL

1

anptmaiapitprmc

t

T

0h

Vibrational Spectroscopy 70 (2014) 53– 57

Contents lists available at ScienceDirect

Vibrational Spectroscopy

journa l h om epage: www.elsev ier .com/ locate /v ibspec

heo-optical near-infrared (NIR) spectroscopy study of low-densityolyethylene (LDPE) in conjunction with projection two-dimensional2D) correlation analysis�

ideyuki Shinzawaa,∗, Wataru Kanematsua, Isao Nodab

National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, JapanDepartment of Materials Science and Engineering, University of Delaware, Newark DE 19716, USA

r t i c l e i n f o

rticle history:eceived 29 October 2013eceived in revised form 5 November 2013ccepted 5 November 2013vailable online 21 November 2013

eywords:

a b s t r a c t

A rheo-optical characterization technique based on near-infrared (NIR) spectroscopy is developed specif-ically to probe the submolecular-level deformation caused during a mechanical test. An illustrativeexample of the mechanical deformation of low-density polyethylene (LDPE) is provided to show howit can be utilized. A set of NIR spectra of the polymer sample were collected by using an acousto-optictunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device.While the substantial level of variation of spectral intensity was readily captured during the mechanical

heo-opticsensile testeat-infrared (NIR)wo-dimensional (2D) correlationpectroscopyrojection 2D correlation

deformation of the LDPE, main feature of the NIR spectra was overwhelmed by the contribution fromthe baseline change. Projection 2D correlation analysis was then applied to selectively extract the signalcontribution from the baseline fluctuation. The 2D correlation spectra revealed the predominant exten-sion of amorphous tie chains followed by the rotation of crystalline lamellae, which induce elastic andplastic deformation of the LDPE, respectively.

© 2013 Elsevier B.V. All rights reserved.

ow-density polyethylene (LDPE)

. Introduction

Mechanical properties of polymeric materials are of consider-ble importance to their engineering applications. The increasedeed for a better understanding of the mechanisms involved inolymer deformation has led to the search for new experimen-al techniques to characterize transient structural changes during

echanical processes [1]. Simultaneous vibrational spectroscopicnd mechanical measurements have been developed as a verynformative probe to study the submolecular-level deformationnd relaxation phenomena in polymer films. For example, Sieslerioneered the rheo-optical studies of polymer samples under var-

ous deformation modes by using an infrared probe to establishhe correlation between macroscopic and molecular dynamics ofolymers [2–5]. Such rheo-optical study of solid polymeric mate-

ials has been mainly challenged by the strong absorption in theid IR region [4–6]. The use of IR light often faces a serious obsta-

le in analysis where substantially thick samples are required for

� Selected paper presented at 7th International Conference on Advanced Vibra-ional Spectroscopy, Kobe, Japan, August 25–30, 2013∗ Corresponding author at: National Institute of Advanced Industrial Science and

echnology (AIST), Nagoya 463-8560, Japan. Tel.: +81527367563.E-mail address: h-shinzawa@aist.go.jp (H. Shinzawa).

924-2031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.vibspec.2013.11.005

mechanical integrity during the deformation. Thus, there is a strongincentive to expand the rheo-optical measurement to near-infrared(NIR) region with much less intense absorption.

Polyethylene polymer consists of seemingly very simple chem-ical structure but is one of the most basic and importantpolymers. The crystalline and amorphous phases of polyethyl-ene readily undergo substantial variation to generate differentsupermolecular structures when the external forces (e.g., temper-ature, mechanical force, etc.,) are applied [7–9]. The developmentof different structures, in turn, provides different physical prop-erty of the polyethylene. For example, Watanebe et al. havestudied temperature-dependent behavior of polyethylene by NIRspectroscopy. They revealed that the different density of thepolyethylene (i.e., branching) provides different variation of NIRspectral features when the samples undergo temperature-inducedalternation [7,10]. In this study, a rheo-optical characterizationtechnique based on the combination of NIR spectroscopy andmechanical analysis was carried out to measure strain-dependentNIR spectra as well as tensile stress of a low-density polyeth-ylene (LDPE) sample. A set of NIR spectra were collected byusing an acousto-optic tunable filter (AOTF) NIR spectrometer

coupled with a tensile testing machine as an excitation device.The utilization of the AOTF, which is a wavelength tunable fil-ter based on high-frequency transducer, offers a clear advantageover the conventional grating-monochromator or interferometer

5 nal Sp

bwsdtfte

mfotnithottonapstbdtldprsLm

2

2

wss�

˚

�

tN

2

tstv

R

wm

4 H. Shinzawa et al. / Vibratio

ased approach in terms of high speed data acquisition, especiallyhen the variation of the system occurs in a relatively short time

cale [11]. In fact, such short time scale is true to the mechanicaleformation of the ductile LDPE polymer caused during the tensileest. Consequently, side-by-side comparison between the spectraleature and the corresponding stress or strain behavior provideshe key information on the evolution of supermolecular structuressentially governing the macroscopic deformation of LDPE.

While the rheo-optical NIR measurement readily captures theolecular-level deformation caused by the applied strain, the main

eature of the spectra can also be influenced by physical changesf the sample dimension and morphology. For example, tensileest of the sample induces the substantial decrease in the thick-ess and subsequent change in the NIR light scattering, which both

nevitably provide unwanted fluctuations of the spectral intensi-ies. In addition, the limited resolution and assignment of NIR bandsave also been the obstacles for extracting meaningful informationn the deformation phenomenon. Two-dimensional (2D) correla-ion analysis in conjunction with projection treatment was usedo selectively filter out such unwanted portion of the informationf spectral data [12,13]. A notable feature of the projection tech-ique lies in the fact that selective rejection or augmentation of

certain correlational feature becomes possible by utilizing therojecting vector from various sources, including a part of originalpectral data or even the perturbation variables themselves. Impor-antly, such selective attenuation is also available as an effectiveaseline correction method by utilizing a part of original spectralata, predominantly reflecting spectral intensity change caused byhe change in the sample thickness and light scattering. Substantialevel of detailed changes in the spectral feature was readily eluci-ated when the strain-dependent NIR spectra were subjected to therojection 2D correlation analysis [14]. The 2D correlation spectraevealed the seemingly complicated variation of the spectral inten-ity associated with crystalline and amorphous structures of theDPE, which in turn, provide useful insight into the deformationechanism of the LDPE.

. Theory

.1. 2D correlation spectra

Assume the spectral data matrix A consists m rows of spectraith n columns of spectral variables. We define a set of dynamic

pectra by subtracting a reference spectrum, typically the averagepectrum. The synchronous and asynchronous correlation spectra,

and �, are obtained as

= 1m − 1

AT A (1)

= 1m − 1

AT NA (2)

he superscript T denotes the transpose operation of the matrix and is the so-called Hilbert–Noda transformation matrix [15,16].

.2. Vector projection

Projection 2D correlation operation [12] is a useful data pre-reatment technique to sort out dynamic spectra into two separateets: one which is fully aligned with a chosen projecting vector andhe other which is orthogonal to the same vector. For the givenector y, the projection matrix Ry is defined as

y = y(yT y)−1

yT (4)

here the superscript −1 stands for the inverse operation of theatrix. The m-by-m matrix Ry acts as a projector for the space

ectroscopy 70 (2014) 53– 57

spanned by y. The projected data matrix AP is obtained by themultiplication of Ry with A,

AP = RyA (5)

the projected data Ap represents the projection of A onto theabstract space spanned by y. The portion of dynamic spectra pro-jected onto the space spanned by such a projecting vector y willhave the same trend of y. Thus, all signals contained in the projecteddataset are fully synchronized.

The corresponding null-space projection is carried out as

AN = (I − Ry)A = A − AP (6)

where I means m-by-m identity matrix. The null-space projecteddata matrix AN represents the projection of A onto the spacespanned by the vectors orthogonal to y. AN is the residual after theremoval of AP from A by using the information contained withiny. The null-space projection selectively eliminates the portion ofdynamic spectra which is synchronized with the projecting vector.Finally 2D correlation spectra generated from the projected or null-space projected data are given by substituting AP or AN into Eqs. (1)and (2).

There are several options to choose the source of the projector. Inmost cases, a single column is selected as a projector vector y fromthe data matrix. For example, it was demonstrated by the projec-tion 2D IR correlation analysis of a poly(methyl methacrylate) filmunder a small amplitude deformation by using the intensity con-tribution of the side group ester as a source of the projector [13].Importantly, it is also possible to utilize the baseline fluctuation asa source of the projector. For example, Shinzawa et al. [14] reportedthat the unwanted baseline change can be removed by utilizing thespectral intensity change at a certain wave number, representingbaseline fluctuation, as a projection vector.

3. Experiment

3.1. Sample preparation

The sample film was prepared from LDPE provided by AlfaAesar. Pellets of LDPE were melted and pressed at 120 ◦C to form a5 × 5 cm2 piece of approximately 800 �m thick sheet.

3.2. Rheo-optical NIR measurement

Fig. 1 illustrates a schematic illustration and images of an AOTF-NIR spectrometer (Systems Engineering Inc., Tokyo) equipped witha tensile testing machine MX2-2500N (Imada Co. Ltd., Aichi).The polymer sample was gradually deformed by the mechani-cal stretcher while being probed with NIR beam. The sample wasstretched at the speed of 0.1 mm/min. A set of NIR spectra wascollected every 4 s by co-adding 64 scans over the 1200–1900 nmregion, and the corresponding stress and strain were also recordedsimultaneously.

4. Results and discussion

4.1. Tensile test

Fig. 2 shows a stress–strain curve of the LDPE sample. At theonset of the tensile test, the stress gradually increases, reflecting theelastic deformation of the sample. Further stretching of the sam-ple causes the plastic deformation, which results in the irreversible

variation of the structure. The deformation finally ends up withthe breaking of the sample. Semicrystalline polyethylene samples,prepared from the melt, often show a complex supermolecularstructure consisting of folded-chain crystal lamellae embedded in

H. Shinzawa et al. / Vibrational Spectroscopy 70 (2014) 53– 57 55

Fig. 1. (A) Schematic illustration of a rheo-optical NIR spectrometer and (B) its actualimage.

al[uspagbmaNi

4

ito

Fig. 2. Stress–strain curve of LDPE sample.

liquid-like amorphous matrix. It is believed that adjacent lamel-ae are linked with the so-called amorphous tie-chain molecules17–19]. Because of differences in their mechanical properties, evennder a common macroscopic deformation, each phase is oftenubjected to a different level of local mechanical stimuli. For exam-le, crystalline structure generally makes a material strong, but itlso makes it brittle. The amorphous structure, on the other hand,ives the polymer it’s toughness , i.e., the ability to bend withoutreaking. The generation of such different deformation behaviorsay be closely associated with the co-existence of the crystalline

nd amorphous structures. Thus, the analysis of the correspondingIR spectra becomes important to derive the in-depth understand-

ng of deformation at the submolecular level.

.2. Projection 2D correlation analysis

The strain-dependent NIR spectra of the LDPE sample are shownn Fig. 3(A). This region of the NIR spectrum of LDPE is dominated byhe overtone and combination modes associated with CH2 groupsf LDPE. The corresponding second derivative spectra are also

Fig. 3. (A) Strain-dependent NIR spectra of LDPE sample and (B) correspondingsecond derivative spectra.

provided as reference in Fig. 3(B). A peak observed around 1730 nmis assigned to the first overtone of antisymmetric stretching modeof CH2 group having the electric dipole-transition moment in thedirection perpendicular to that of the polymer backbone [10,20].Although it is not readily identified, the amorphous componentalso provides spectral intensity change at around 1710 nm [10,20].It is important to point out that the spectra show gradual decreasein the intensity over the region. The baseline changes from onespectrum to the other, and the fluctuations caused by the macro-scopic changes of the sample are much larger than those caused bythe molecular level structural changes caused by the mechanicaldeformation, making the identification of pertinent feature diffi-cult. Such baseline changes are mostly arising from the decreasein the thickness of the LDPE sample induced by stretching. Impor-tantly, such predominant baseline change causes some difficulties,especially in the practice of 2D correlation analysis. Fig. 4 shows(A) synchronous and (B) asynchronous correlation spectra directlycalculated from the raw NIR spectra shown in Fig. 3. The plot of thereference spectrum is placed at the top and side of the contour map.Negative correlation intensity areas of the contour map are repre-sented by the shading. The problem caused by the baseline changeis especially acute for the asynchronous correlation spectrum. Base-line fluctuations tend to have strong asynchronous elements withrespect to intensity changes arising from molecular level responses

of the system. In fact, all the correlation features associated withthe amorphous and crystalline peaks are obviously overshadowedby the intense variation caused by the baseline change, generatingmeaningless artifacts in the 2D correlation spectrum.

56 H. Shinzawa et al. / Vibrational Spectroscopy 70 (2014) 53– 57

Fs

rwcFLtosldi1smtcwosvnalm

f

chronous correlation spectra is covered with positive correlation

ig. 4. (A) Synchronous and (B) asynchronous correlation spectra derived fromtrain-dependent NIR spectra shown in Fig. 3.

One reasonable solution to improve the quality of the 2D cor-elation spectra is to selectively remove the portion of the spectrahich is synchronized with the baseline change. A simple offset

orrection may be considered as one of such possible approaches.or example, Fig. 5(A) represents the offset-corrected spectra of theDPE sample. Note that the offset treatment was carried out by sub-racting the spectral intensities at 1650 nm. Despite the eliminationf the offset deviation, the spectra still show gradual downwardhift. The fluctuation becomes especially acute in the longer wave-ength region, revealing the contribution from the baseline changeue to the multiplicative scatter factor. Fig. 5(B) illustrates the

ntensity variations at 1710 (crystalline), 1730 (amorphous), and800 nm along strain direction derived from the offset-correctedpectra shown in Fig. 5(A). The intensity variation at 1800 nmostly reflects the baseline increase arising from the multiplica-

ive scatter factor. It should be noted that the entire features of therystalline and amorphous bands result in a very similar patternith that at 1800 nm, suggesting that the baseline change provides

verwhelming contribution to the intensity variations in the wholepectral region. It is important to point out here that the intensityariation at 1800 nm in the offset spectra purely reflects the sig-al contribution from the multiplicative scatter factor. Thus, thettenuation of the portion, which is synchronized with this trend,eads to the highly selective removal of the contribution from the

ultiplicative scatter factor in the whole spectral region.Fig. 5(C) represents projection-corrected spectra calculated

rom the offset spectra by the projection onto the space spanned

Fig. 5. (A) Offset-corrected spectra, (B) spectral intensity variations at 1710, 1730,and 1800 nm, and (C) Projection-corrected spectra derived from offset-correctedspectra onto null-space of their signals at 1800 nm.

orthogonal to the intensity variation at 1800 nm. The projection-corrected spectra contain contributions from all other constituentsexcept for the baseline fluctuation, while the spectra are nowfree from the intensity contribution from the intensity varia-tion related to the multiplicative scatter factor. Such selectiveattenuation becomes useful, especially for the subsequent 2Dcorrelation analysis. For example, Fig. 6 shows (A) synchronousand (B) asynchronous correlation spectra calculated from theprojection-corrected NIR spectra. The entire plane of the syn-

peaks of LDPE, reflecting the same deformation trend in thevariation within this NIR region. On the other hand, in the asyn-chronous correlation spectrum, the influence of overwhelming

H. Shinzawa et al. / Vibrational Spectroscopy 70 (2014) 53– 57 57

Fp

ptctviotp

attatmtmsriid

[[

[[[[

[[

ig. 6. (A) Synchronous and (B) asynchronous correlation spectra derived fromrojection-corrected spectra shown in Fig. 5(C).

redominance of the baseline change is removed, and small fea-ures overshadowed by the intense baseline change now becomeslearly visible. The cross peak at the coordinate (1710, 1730) revealshe different behavior of the crystalline and amorphous bands in theariation of the spectral intensity, which is not readily identifiedn the original set of one dimensional spectra. The developmentf the positive correlation peak suggests that, at the onset of theensile testing, the deformation of the amorphous structure occursredominantly before that of the crystalline structure.

It is believed that the elastic properties of polyethylene is closelyssociated with the interlamellar bridges, providing high elasticityo the semicrystalline polymer [18,19]. Thus, the sequential order ofhe event revealed by the 2D correlation analysis, in turn, providesn even clearer picture to the mechanical deformation induced byhe tensile test. Fig. 7 schematically summarizes a possible defor-

ation mechanism proposed by the rheo-optical analysis. Initiallyhe LDPE sample consists of the crystalline lamellae oriented in

any directions. The deformation is mainly achieved by the exten-ion of the amorphous tie chains. Such extension of the tie chains

esults in the elastic deformation. Further stretching of the tie chainnduces the rotation and eventual breakup of the lamellae, lead-ng to the irreversible plastic deformation. Consequently, it wasemonstrated that the rheo-optical NIR spectroscopy coupled with

[[

[

Fig. 7. A possible deformation mechanism of LDPE under tensile test.

projection 2D correlation analysis can probe fine details of the sub-molecule deformation of the polymer sample.

5. Conclusion

A rheo-optical characterization technique based on the com-bination of NIR spectroscopy and mechanical analysis wasdemonstrated in this study. A set of strain-dependent NIR spec-tra as well as tensile stress of a LDPE sample were collected usingan AOTF NIR spectrometer coupled with a tensile testing machineas an excitation device. While the substantial level of variationof spectral intensity was readily captured during the mechanicaldeformation of the LDPE, the main feature of the NIR spectra wasoverwhelmed by the contribution from the baseline change. Projec-tion 2D correlation analysis was applied to selectively separate thesignal contribution of interest from the baseline fluctuation. The 2Dcorrelation analysis of the spectra revealed that submolecular-leveldeformation mechanism of the LDPE, namely the extension of thetie chains and subsequent rotation of the lamellae, which induceselastic and plastic deformation of the LDPE, respectively.

References

[1] R.S. Stein, Polym. J. 17 (1985) 289.[2] H.W. Siesler, Polym. Bull. 9 (1983) 382.[3] H.W. Siesler, Adv. Polym. Sci. 65 (1984) 1.[4] M. Unger, H.W. Siesler, Appl. Spectrosc. 36 (2009) 1351.[5] C. Vogel, G.G. Hoffmann, H.W. Siesler, Vib. Spectrosc. 49 (2009) 284.[6] I. Noda, A.E. Dowrey, C. Marcott, J. Polym. Sci. Polym. Lett. Ed. 21 (1983) 99.[7] S. Watanebe, I. Noda, Y. Ozaki, Polymer 49 (2008) 774.[8] K. Tashiro, M. Kobayashi, Polymer 37 (1996) 1775.[9] K. Tashiro, K. Ishino, T. Ohta, Polymer 40 (1999) 3469.10] S. Watanebe, I. Noda, Y. Ozaki, J. Mol. Str. 73 (2008) 883-884.11] A.P. Goutzoulis, D.R. Pape, Design and Fabrication of Acousto-Optic Devices,

Marcel-Dekker Inc., New York, 1994.12] I. Noda, J. Mol. Struct. 974 (2010) 116.13] I. Noda, Vib. Spectrosc. 60 (2012) 146.14] H. Shinzawa, K. Awa, I. Noda, Y. Ozaki, Vib. Spectrosc. 65 (2013) 28.15] I. Noda, Y. Ozaki, Two-dimensional Correlation Spectroscopy, Wiley, Chich-

ester, West Sussex, 2004.16] I. Noda, Appl. Spectrosc. 54 (2000) 994.17] F. Nilsson, X. Lan, T. Gkourmpis, M.S. Hedenqvist, U.W. Gedde, Polymer 53

(2012) 3594.18] Z. Spitalsky, T. Bleha, Polymer 44 (2003) 1603.19] S. Humbert, O. Lame, J.-M. Chenal, C. Rochas, G. Vigier, J. Polym. Sci. 48 (2010)

1535.20] S. Watanebe, J. Dybal, K. Tashiro, Y. Ozaki, Polymer 47 (2006) 2010.