Journal of Molecular Structure 974 (2010) 165–172

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Journal of Molecular Structure

journal homepage: www.elsevier .com/ locate /molst ruc

Structural understanding of polyethylene crystal by retardance mapping technique

Shin Watanabe a,*, Isao Noda b, Yukihiro Ozaki c

a Procter & Gamble Innovation Godo Kaisya, Research & Development Division, Analytical GCO, Asia, 1-17 Naka, Koyo-cho, Higashi-Nada, Kobe 658-0032, Japanb The Procter & Gamble Company, 8611 Beckett Road, West Chester, OH 45069, USAc Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan

a r t i c l e i n f o

Article history:Received 29 December 2009Received in revised form 6 March 2010Accepted 8 March 2010Available online 15 March 2010

Keywords:Two-dimensional correlation spectroscopyPolyethyleneRetardance mapping

0022-2860/$ - see front matter Published by Elsevierdoi:10.1016/j.molstruc.2010.03.025

* Corresponding author. Tel.: +81 78 845 7240; faxE-mail address: watanabe.s@pg.com (S. Watanabe

a b s t r a c t

The higher-order structure of polyethylene (PE) crystal has been studied by means of retardance mappingtechnique. This technique allows one to visually and quantitatively examine crystal structures in polymersamples from macroscopic point of view. The retardance map obtained in the present study consists of640 � 480 individual retardance data. The distribution of retardance in the view field or change in theretardance under an external perturbation can therefore be evaluated by mathematically treating thehistograms. We obtained the second derivatives of the histograms of retardance maps that have beencollected at ambient condition and detected multiple peaks in the histograms of PE specimens. Thepresence of shoulder peaks indicates the presence of locally developed higher-order crystalline structurein the view field. A series of retardance maps were collected during the heating of PE specimens. Whentemperature is increased, PE crystal starts to melt and this process can be studied based on retardancemapping technique. The difference in the melting of higher-order structure among PE samples was stud-ied and clear difference was depicted. We performed two-dimensional (2D) correlation analysis againsthistogram of the retardance map to elucidate structural changes that happen before and during the melt-ing process. As a result, we were able to conclude the disordering of higher-order structure in PE startsfrom temperature far below Tm.

Published by Elsevier B.V.

1. Introduction

The crystal structures of semicrystalline polymers, such as poly-ethylene, undergo characteristic transition processes when externalphysical variables, like temperature and pressure, change [1]. Thesetransitions include the partial melting, annealing, and lamella rear-rangement [2]. To probe changes in the crystalline structure of PE,analytical techniques, such as IR [3–6], NIR [7–9], Raman [10–13]and X-ray diffraction [4,10,14,15] have been used. In particular, wereported the usefulness of NIR spectroscopy for the study of crystal-line structure in PE [16–18]. In our study, we explored the use of NIRspectroscopy for the study of structural and conformational state ofPE crystals. We determined band assignments in NIR spectral regionand established technical foundations for the fundamental polymerresearch by using NIR. NIR spectroscopy has significant advantagesover conventional IR spectroscopy for the study of polymers, as itcan handle thick and bulk specimens by utilizing its superior trans-missibility. The spectral band assignments in IR spectral region of PEwere also re-examined by using 2D correlation spectroscopy tech-nique. Based on the difference in the temperature dependency ofconformational bands in IR region, new spectral assignments as wellas sequence in the formation of conformational defects against tem-

B.V.

: +81 78 845 6976.).

perature were proposed. In linear low density PE (LLDPE) with shortside chains, the occurrence of lamella thickening has been expected.In our recent paper [19], we demonstrated the occurrence of suchrearrangement process by studying the incorporation of conforma-tional defect sequences into the crystalline phase and the thickeningof crystalline lamella measured by small angle X-ray diffraction(SAXS) technique. Furthermore, IR and wide-angle X-ray diffraction(WAXS) were used to provide comprehensive understanding on thestructural change in PE crystals during heating.

These studies have given good understandings on the disorder-ing in the crystal structure of polyethylene that happens below Tm.However, the crystalline structural changes have not been wellstudied from the viewpoint of macroscopic higher-order structure.In the present study, we introduce the use of a light microscope-based retardance mapping system called LC-PolScope [20] for thestudy of structural changes in PE that happen during heating.LC-PolScope is an automated polarized light microscope with a li-quid crystal universal compensator and circular polarizer. Regularlight microscope has been routinely used for the qualitativemorphological study of PE samples to probe superstructure of PEcrystal during the crystallization process [21]. Moreau et al. useda fluorescent microscope for the study of defect structure in PEpower cable [22]. However, no quantitative study based on itsoutput has been carried out so far. As the outputs from LC-PolScopeare provided in the form of retardance value, the quantitative data

166 S. Watanabe et al. / Journal of Molecular Structure 974 (2010) 165–172

analysis can be easily performed. Its usage has so far been limitedto the observation of biological specimens, but its application isnow growing into the field of polymer science as well [19,23].

Generalized 2D correlation spectroscopy proposed by Noda in1993 has become a powerful and versatile tool for elucidating sub-tle spectral changes induced by an external perturbation, such astemperature, concentration, and time [24–26]. 2D correlation anal-ysis has been widely applied to various spectroscopic data. Thetechnique allows one to extract pertinent information from com-plex datasets, such as spectral data collected under an externalperturbation. In the present study, we applied 2D correlation tech-nique to the in-depth analysis on retardance histogram obtainedfrom LC-PolScope. The technique successfully detected changes inthe retardance histogram during the heating process even at a tem-perature well below Tm.

2. Experimental

2.1. Materials

Six PE samples with different structures and crystallinities wereused in this study. Specifically, two HDPE, two LLDPE, and twoLDPE were kindly supplied by Japan Polyethylene Corporation, Ja-pan. Table 1 shows the crystallinity and melting point estimatedfrom DSC traces provided by the courtesy of Japan PolyethyleneCorporation. All samples were supplied in the form of pellets.

2.2. LC-PolScope analysis

The retardance maps were obtained by using LC-PolScope ver.4.724 (Cambridge Research Institute, CRI, Woburn, MA) attachedto Nikon Eclipse E800M light microscope with 60� transmissionlens (CFI Plan APO TIRF, 60� H, N.A. 1.45, W.D. 2.1 mm). In orderto carry out temperature dependent experiment, a heating stagewas used (Mettler FP82HT hot stage controlled by Mettler FP90).Thin slice of LLDPE pellet (ca. 10–20 lm depending of overall retar-dance intensity) was prepared by using a microtome and placed in-between slide and cover glass. The specimen was placed on the hotstage and kept at 150 �C for 10 min. After the complete melting, thespecimen was cooled to room temperature at the rate of 2 �C perminute. The melting behavior observation was carried out by heat-ing the stage at 2 �C per minute from room temperature to their Tm.The retardance map (ca. 140 lm � 100 lm) was recorded at 5 �Cintervals.

This instrument is equipped with a CCD camera (640 � 480 pix-els) for detection. As one image is acquired against ca. 140 lm �105 lm when 60� objective lens is used, one pixel is calculated tocover 220 nm, which is nearly about diffraction limit.

2.3. Second derivative calculation

The second derivative of a spectrum was calculated by Savitsky-Golay method (second-order polynomial with 15 data points).

Table 1Density, crystallinity and melting point of six PE samples.

Density (g/cm3) Crystallinity (%) Melting point (�C)

HDPEHDPE-L 0.953 68.3 128.4HDPE-H 0.964 74.7 132.4LLDPELLDPE-L 0.921 31.2 123.2LLDPE-H 0.937 52.2 127.5LDPELDPE-L 0.918 26.1 106.1LDPE-H 0.923 33.1 111.0

2.4. 2D correlation analysis

To obtain 2D spectra, 2Dshige ver. 1.3 (developed by ShigeakiMorita, Kwansei-Gakuin University) was used. In the 2D correla-tion maps, unshaded regions indicate positive correlation intensi-ties, while shaded regions indicate negative correlation intensities.

The 2D correlation maps were constructed on retardance histo-grams collected during heating at following temperatures. LDPE-H:30–97 �C, HDPE-L: 30–100 �C, LLDPE-H: 30–100 �C, LLDPE-L: 30–70 �C.

3. Results and discussion

3.1. Retardance map

Fig. 1A–C shows temperature-dependent retardance map ofHDPE-H collected at 30 �C, 100 �C and 128 �C. The map is con-structed from 256 gray scale, higher the retardance, higher inbrightness. As temperature increases, the high retardance area de-creases and average retardance decreases. It is well known that thecrystalline structure in PE starts to disorder along with the temper-ature increase [27]. The high retardance area is assumed to corre-spond to the area where PE crystals densely exist. i.e. local higher-order structure. The decrease in high retardance area due to tem-perature increase is therefore reasonably explained.

Figs. 2A–C, 3A–C and 4A–C show temperature-dependent retar-dance map of LDPE-L, LLDPE-H and LLDPE-L collected at tempera-ture indicated in the figures. The average retardance on the mapobtained for LDPE-L is quite low comparing to the other specimensreflecting this specimen’s low crystallinity.

3.2. Retardance histogram

Fig. 5A–D shows temperature-dependent retardance histogramof HDPE-H, LDPD-H, LLDPE-H and LLDPE-L collected during heat-ing, starting from 30 �C to Tm. Each histogram consists from ca.300,000 pixels that correspond to total pixels of the CCD detector(640 � 480). The x-axis shows retardance while y-axis shows cor-responding number of pixels. The absolute value of retardance,however, does not hold critical information as the thickness ofthe specimen controls the absolute value of retardance. The thick-ness of the specimen was not precisely controlled.

As shown in the figures, the peak position of the retardancecurves exist at a relatively lower retardance region for the speci-men with lower crystallinity, such as LDPE-H and LLDPE-L. In con-trast, the peak of the retardance curve is located nearly at thecenter of the distribution curve for specimens with relatively highcrystallinity, namely, HDPE-H and LLDPE-H. The data obtained inthis experiment is based on spectral information obtained throughlight microscope. Its special resolution is estimated around220 nm. The crystalline structure detected in this experiment istherefore considered due to higher-order structure. The differencein the population distribution pattern in retardance suggests dif-ference in the higher-order crystalline structure between the twospecimen groups. The results reflect the fact that the specimenswith high crystallinity hold well-developed higher-order structurewhile those with low crystallinity do not. In order to further eluci-date the difference and similarity in the higher-order crystallinestructure of the specimen, the second derivative was calculatedagainst retardance histogram obtained at 30 �C. Fig. 6A–D shows,respectively, second derivative of retardance histogram of HDPE-H (A), LDPD-H (B), LLDPE-H (C) and LLDPE-L (D). In the secondderivatives, peaks in addition to the main peak were detected(indicated with arrowheads). The presence of multiple peaks inretardance histogram very likely reflects the presence of locally

Fig. 1. Retardance maps of HDPE-H at 30 �C (A), 100 �C (B) and 128 �C (C).

Fig. 2. Retardance maps of LDPE-L at 30 �C (A), 80 �C (B) and 100 �C (C).

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developed higher-order structure such as aggregation or orienta-tion. In the specimens with higher crystallinity, the presence ofmultiple peaks in retardance is more prominent than that in spec-imens with low crystallinity such as LDPE or LLDPE-L. This differ-ence is considered due to difference in the development degreeof such higher-order structure.

3.3. Temperature dependency in the disordering of higher-orderstructure

Fig. 7 shows the temperature dependence of average retardanceduring heating. The average retardance was obtained withretardance histogram collected from one view field captured by

Fig. 3. Retardance maps of LLDPE-H at 30 �C (A), 100 �C (B) and 120 �C (C).

Fig. 4. Retardance maps of LLDPE-L at 30 �C (A), 100 �C (B) and 118 �C (C).

168 S. Watanabe et al. / Journal of Molecular Structure 974 (2010) 165–172

LC-PolScope. As shown in the figure, significant difference in thetemperature dependency is detected among specimens. Specifi-cally, the higher-order crystalline structure in PE with high crystal-linity such as HDPE does not start to melt until the temperaturebecomes close to its Tm. On the other hand, the structure in PEspecimens with lower crystallinity such as LDPE starts to melt from

low temperature, which is far below its Tm. In our study of explor-ing melting behavior of PE specimens by using mid-infrared (MIR)and near-infrared (NIR) spectroscopy, we plotted integrated inten-sity of spectral bands due to crystalline phase during heating [17].Comparing the result of the study with that of present study, it isnoted that there is considerable difference in the start of melting

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process. In the result of the study by using the vibrational spectros-copy techniques, the melting process was observed linearly pro-ceeds against temperature. This is particularly obvious in PE withlow crystallinity such as LDPE. However, the result of present studyshown in Fig. 7, there is a clear offset in the start of melting. Thepresent study is concerned with higher-order crystalline structurewhile that of the study by vibrational spectroscopy technique re-flects the environment that molecular chains in the structure ofPE. The observed difference could be explained in this way. Whentemperature is increased, a PE chains in crystal with relativelyloosely packing structure starts to melt. As present study only con-cerns with the melting in the higher-order crystalline structurewith high crystallinity, the melting phenomenon that happens inloosely packed structure may not be captured. The difference inthe results of two studies is most likely due to the difference inwhat is being detected by the two different techniques.

3.4. 2D correlation analysis on histograms

To study changes of retardance during heating, 2D correlationmaps were constructed on retardance histogram. Fig. 8 shows syn-chronous 2D correlation map (m1, m2) (A) and asynchronous corre-lation map W(m1, m2) (B) on retardance histogram obtained forLDPE-H that collected during heating up to 97 �C. In the synchro-nous map shown in Fig. 8A, two autopeaks appear, and the signs

of the corresponding cross peaks are negative. This result indicatesthat the intensity of the two peaks changes in the opposite wayduring heating. In the asynchronous map shown in Fig. 8B, crosspeaks lay across the retardance region. This ‘‘butterfly” pattern ofthe peak cluster appearing on the asynchronous 2D map suggestsgradual shift of high-retardance peak (high crystallinity) to lowerretardance (low crystallinity) along the temperature hike [28]. Thisis a common phenomenon in all the examined specimen.

Fig. 9 shows synchronous 2D correlation map (m1, m2) on retar-dance histogram of HDPE-L (A), LLDPE-H (B) and LLDPE-L (C) dur-ing the heating up to 100 �C (70 �C for LLDPE-L). In temperatureranges well below Tm, the histograms show only slight changesthe shape and intensity for all samples (Fig. 5). Interestingly, how-ever, the synchronous spectra for above three specimens indicatethe development of correlation peaks between high and low retar-dance region. This unexpected result demonstrates the occurrenceof disordering in crystalline structure even at a temperature farlower than Tm. Considering the fact that the structural changes de-tected in the 2D correlation maps are based on retardance mea-sured by using light microscope-based technique, it is likelyassociated with the disordering in the higher-order structure ofcrystalline phase of PE, such as orientation.

It is known that the structural changes happen in PE even attemperature far below Tm. In our series of studies, we demon-strated the occurrence of variety of structural changes in PE below

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Fig. 7. Temperature dependence of averaged retardance of six PE specimens in thetemperature range of 35 �C to its Tm.

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Tm [16–19,29]. Above result suggests that structural changes belowTm associated with the pre-melting process also happen at thehigher-order supermolecular structure of PE, not just at individualmolecules.

4. Conclusion

We have investigated crystal structure of PE by using retar-dance mapping technique in order to provide new insights intothe pre-melting process from the macroscopic view point. Specifi-cally, the histograms obtained on the retardance maps were ana-lyzed by spectral analysis techniques, including 2D correlationanalysis. We found that:

(1) Several shoulder peaks can be detected in the second deriv-atives of retardance histograms obtained at ambient condi-tion for HDPE-H, LLDPE-H and LLDPE-L. This finding maysuggest that the presence of uneven distribution of higher-order crystalline structure in PE.

(2) The higher-order crystalline structure in PE with higher crys-tallinity mainly starts to melt when heated close to its Tm.

(3) 2D correlation analysis on the retardance histogram con-firms that structural changes happen in HDPE-L, LLDPE-Hand LLDPE-L even below 100 �C. The changes are due to

Fig. 8. The contour map representation of the 2D synchronous (A) and asynchronous (B) constructed from the retardance histogram obtained for LDPE-H.

Fig. 9. The contour map representation of the 2D synchronous spectra of the retardance histogram obtained for HDPE-L (A), LLDPE-H (B) and LLDPE-L (C).

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the evolution of macroscopic supermolecular structure, asthe retardance maps are constructed based on spectral datacollected through light microscopy.

Acknowledgements

The authors are grateful to Japan Polyethylene Corporation forsupplying the specimens used in this study.

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