Hydrogen-Bonding Interaction and Miscibility betweenPoly(«-caprolactone) and Enzymatically Polymerized NovelPolyphenols

YONG HE,1 JIANCHUN LI,1 HIROSHI UYAMA,2 SHIRO KOBAYASHI,2 YOSHIO INOUE1

1Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku,Yokohama 226-8501, Japan

2Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan

Received 6 July 2001; revised 20 August 2001; accepted 31 August 2001Published online 00 Month 2001; DOI 10.1002/polb.0000

ABSTRACT: The intermolecular hydrogen-bonding interaction and miscibility betweenenzymatically prepared novel polyphenols [poly(bisphenol A) and poly(p-tert-butyl phe-nol)] and poly(«-caprolactone) (PCL) were investigated as a function of composition byFourier transform infrared spectroscopy (FTIR) and DSC. The blend films of PCL andpolyphenols were prepared by casting polymer solution. The FTIR spectra clearlyindicated that PCL and polyphenols interact through strong intermolecular hydrogenbonds formed between the PCL carbonyls and the polyphenol hydroxyl groups. Themelting point and degree of crystallinity of the PCL component decreased with anincreased polyphenol content. A single glass-transition temperature was observed forthe blend, and its value increased with the content of polyphenol, indicating that PCLand polyphenols are miscible in the amorphous state. © 2001 John Wiley & Sons, Inc. JPolym Sci Part B: Polym Phys 39: 2898–2905, 2001Keywords: hydrogen bond; miscibility; poly(«-caprolactone); polyphenol; blends; glasstransition

INTRODUCTION

Recently, many works have focused on the blendsof polyphenols, such as poly(4-vinylphenol)(PVPh), because the phenolic hydroxyl group ofpolyphenol possesses the potential ability to formstrong intermolecular hydrogen bonds with car-bonyl, ether, or other functional groups of thesecond polymer.1–4 A variety of binary blends con-taining polyphenols have been studied, and al-most all of these blends have been reported to bemiscible or at least partially miscible in the amor-phous state over wide ranges of blend composi-tion.4–7 Some works have also focused on the ter-

nary blends including polyphenol as the thirdcomponent. Some polyphenols were found to im-prove the miscibility between the first and secondimmiscible component polymers that had func-tional groups capable of forming hydrogen bondswith phenols.8–10 Knowing that most of the poly-mer blends are generally immiscible as a result ofthe small value of the mixing entropy, it is sur-prising and interesting that PVPh can form mis-cible blends with so many kinds of structurallydissimilar polymers.

On the other hand, the blends of low molecularweight diphenols with polyesters have also beenextensively examined.11–13 A variety of diphenolcompounds interact with many kinds of polyes-ters through strong intermolecular hydrogenbonds, and a hydrogen-bonded network is formedin the blends. As a result, the thermal and dy-

Correspondence to: Y. Inoue (E-mail: yinoue@bio.titech.ac.jp)Journal of Polymer Science: Part B: Polymer Physics, Vol. 39, 2898–2905 (2001)© 2001 John Wiley & Sons, Inc.

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namic mechanical properties of polyesters aregreatly modified through blending with diphenolcompounds.

In this article, we investigate the intermolecu-lar hydrogen-bonding interaction and miscibilitybetween poly(bisphenol A) (PA) and poly(«-capro-lactone) (PCL) as well as between poly(p-tert-bu-tyl phenol) (PB) and PCL. Polyphenols PA and PBare prepared by enzymatical polymerization ofcorresponding phenols.14,15 They are novel poly-phenols having a lot of ‘free’ hydroxyl groups. PCLis selected as a representative of the carbonyl-containing polymers. It is a crystalline aliphaticpolyester, and the weak self-association of thispolymer makes it a good component used to ex-amine the interaction between the polyester car-bonyl group and the hydroxyl group of polyphe-nol. In this work, first, the specific interactionbetween polyphenol and PCL is studied by Fou-rier transform infrared spectroscopy (FTIR).Then, the miscibility and thermal properties ofpolyphenols/PCL blends are investigated by DSC.The effect of the chemical structure of polyphe-nols on the specific interaction and thermal prop-erties is also discussed.

EXPERIMENTAL

Materials

Both PA and PB are the products of the enzymaticpolymerization of bisphenol A and tert-butyl phe-nol, respectively. They were prepared accordingto the literature procedure.14,15 Both PA and PBare copolymers of the phenylene and oxyphe-nylene units. On average, there are about threefree hydroxyl groups per two bisphenol A units inPA and about one free hydroxyl group per twotert-butyl phenol units in PB as determined by 1HNMR.14,15 The number-average molecular weight(Mn) and molecular weight distribution (Mw/Mn)are 1300 and 2.2 for PA and 1130 and 1.3 for PB.The PCL sample [Mn 5 5.35 3 104, Mw/Mn 5 1.47;Celgreen(-PH4)], supplied by the courtesy of Dai-cel Chemical Industries, Ltd., Japan, was used asreceived.

Preparation of Blend Samples

Polyphenol and PCL in appropriate weight ratioswere first dissolved in tetrahydrofuran (THF)(polymer concentration was ca. 5 wt %). Then, thesolution was cast onto a glass Petri dish, and the

solvent was evaporated at room temperatureovernight. The resultant cast films were placed inan oven at 60 °C under vacuum for 2 days toremove the residual solvent. After that, the filmswere further aged at room temperature for morethan 4 weeks before the DSC measurement.

FTIR Spectra

The blend films with a thickness suitable forFTIR measurements were prepared by directlydropping the polymer solution in THF (polymerconcentration was ca. 1.5 wt %) to the surface of asilicon wafer. The silicon wafer was transparentfor IR incident beams and used as the substrate.The thickness of the cast film was carefully con-trolled to be thin enough to ensure that the stud-ied IR absorption was within the linearity rangeof the detector.

IR measurements were carried out on a single-beam IR spectrometer of PerkinElmer Spectra2000. All FTIR spectra were recorded under agiven temperature at a resolution of 4 cm21 withan accumulation of 16 scans.

DSC

DSC thermograms of the samples (ca. 5 mg),which were presealed in aluminum pans, wererecorded on a Seiko DSC 220 system connectedwith a workstation SSC5300. The samples werefirst heated from 2100 °C to an end temperatureranging from 150 to 200 °C (different end temper-atures were chosen because of the differences inthe thermal properties of component polymers),at a heating rate of 20 °C/min (the first heatingscan). After that, samples were rapidly quenchedto 2120 °C with liquid nitrogen, and then theywere again heated to the end temperature at arate of 20 °C/min (the second heating scan). Themelting temperature (Tm) was determined fromthe endothermic peak of the DSC curve observedby the first heating scan. The glass-transitiontemperature (Tg) was taken as the peak top of theDDSC (the differentiation of DSC) curve obtainedfrom the second heating scan. The value of melt-ing enthalpy was calculated as the integral of theendothermal peak in the DSC curve.

Line-Shape Analysis of FTIR

A curve-fitting program was used to resolve thePCL carbonyl vibration bands into three compo-nents, that is, the amorphous, crystalline, and

HYDROGEN-BONDING INTERACTION 2899

hydrogen-bonded components. This program isbased on the least-squares parameter adjustmentcriterion using the Gauss–Newtonian iterationprocedure.16 In this fitting process, the peak po-sition, line shape, peak width, and height of theresolved bands were adjusted in such a way thatthe best fit between the calculated and experi-mental bands is obtained.

RESULTS AND DISCUSSION

Hydrogen Bond between PCL and Polyphenols

FTIR spectroscopy is a particularly suitable tech-nique for investigating specific intermolecular in-teractions. For the PCL/polyphenol blends stud-ied here, one of the components (PCL) containsthe carbonyl group yielding a VCAO stretchingmode at about 1727 cm21, whereas polyphenolshows no absorption in the carbonyl vibrationregion ranging from 1650 to 1800 cm21. There-fore, any change of the FTIR spectrum in thisregion should be directly attributed to the changein the chemical environment of the carbonylgroup, such as the formation of hydrogen bonds.

The FTIR spectra of PCL/PA blends in the car-bonyl vibration region are shown in Figure 1 as a

function of blend composition. For pure PCL, thecarbonyl vibration band centers at 1727 cm21.Blending with PA, a second band appears at alower wave number (1706 cm21) beside the bandcentered at about 1727 cm21. The second bandshould be attributed to the hydrogen-bonded car-bonyl vibration,11–13 indicating the formation ofintermolecular hydrogen bonds between PCL andPA. With the increase of the PA content in theblends, the relative absorbance of the hydrogen-bonded carbonyl vibration increases, whereasthat of the “free” carbonyl band decreases, indi-cating the reduction of the percentage of “free”carbonyl groups relative to the hydrogen-bondedones.

Figure 2 illustrates the FTIR spectra ofPCL/PA blends in the hydroxyl vibration region.The vibration band in the spectrum of pure PAcenters at 3381 cm21. This band should be as-signed to the hydrogen-bonded hydroxyl groups inpure PA because the wave number is low. Forpure PCL, only a very weak band centered at3447 cm21 is observed in this region, whichshould be attributed to the vibration of the hy-droxyl group in the chain terminal of PCL. Be-cause of its low intensity as compared with thebands of PA in this vibration region, the contri-bution of this band to the spectra of blends shouldbe negligible when the PA content is not too low.

Figure 1. Carbonyl vibration bands in the FTIR spec-tra of pure PCL, PA, and their blends with a PA contentof 15 wt % (85/15), 30 wt % (70/30), and 45 wt % (55/45)(A: PCL, B: 85/15, C: 70/30, D: 55/45, and E: PA).

Figure 2. Hydroxyl vibration bands in the FTIR spec-tra of pure PCL, PA, and their blends (A: PCL, B: 85/15,C: 70/30, D: 55/45, and E: PA).

2900 HE ET AL.

In the PCL/PA blends, the wave number of thepeak top of the hydroxyl band decreases, whereasthe relative absorbance of this band increaseswith the PA content. As compared with the bandof pure PA, the band of blend shifts to a higherwave number, indicating that the intermolecularhydrogen bonds between the PCL carbonyl andthe PA hydroxyl in the blend are weaker than theself-associated ones in pure PA.

The case is similar for PCL/PB blends. Asshown in Figure 3, a new band appears in the lowwave-number side of the carbonyl vibration re-gion in the spectra of PCL/PB blends indicatingthe formation of hydrogen bonds between PCLand PB.

Application of the Beer–Lambert law permits aquantitative analysis of the fraction of the PCLcarbonyl groups involved in the intermolecularhydrogen bond by using FTIR spectroscopy.1,17–21

The spectra of the PCL/PA blends in the carbonylregion exhibit three distinct components. Thecomponents at approximately 1736 and 1724cm21 are attributed to PCL in the amorphous andcrystalline phases, respectively.22,23 The contri-bution observed at 1706 cm21 can be attributed tothe hydrogen-bonded carbonyl vibration as previ-ously mentioned. According to the Beer–Lambertlaw, the integrated intensities of the amorphousAa, the crystalline Ac, and the hydrogen-bondedpart Ab can be expressed as

Ai 5 bci E0

1`

«i~v! dv (1)

where the subscript i is a, b, or c denoting theamorphous, hydrogen-bonded, and crystallinecarbonyl groups part, respectively; «i(v) is the ab-sorption coefficient; b is the thickness; ci is theconcentration of i; and v is the wave number.

On the other hand, the fraction of hydrogen-bonded carbonyl groups Fb can be determinedfrom cb and the total concentration c (c 5 ca 1 cc1 cb) as

Fb 5 cb/~cb 1 ca 1 cc! (2)

On the basis of eqs 1 and 2 and appointing

gi/j 5 E0

1`

«i~v! dv/E0

1`

«j~v! dv (3)

subscript j is a, b, or c, and then Fb can be ex-pressed by Ai and gi/j as

Fb 5 Ab/~Ab 1 Aa z gb/a 1 Ac z gb/c! (4)

Figure 4. Experimental and curve-fitting FTIR spec-tra in the carbonyl vibration region of the PCL/PA55/45 blend (open circle: experimental points, brokenline: fitting results, A: amorphous part, B: crystallinepart, and C: hydrogen-bonded part).

Figure 3. Carbonyl vibration bands in the FTIR spec-tra of pure PCL, PB, and their blends with a PB contentof 15 wt % (85/15), 30 wt % (70/30), and 45 wt % (55/45).

HYDROGEN-BONDING INTERACTION 2901

From eq 4, the problem associated with calculat-ing Fb is how to determine Ai and gi/j. For thecarbonyl stretching band of PCL, gc/a and gb/chave been estimated to be 1.46 and 1.34, respec-tively, in previous research.11,12 By using a curve-fitting program,16 the integrated intensities of thethree components were also obtained.

A curve-fitting program was used to resolve thecarbonyl vibration region of the spectra of thePCL/polyphenol blend into three bands: amor-phous, crystalline, and associated carbonyl groupvibration bands. During the curve fitting, thepeak positions of the amorphous and crystallinebands are fixed at 1736 and 1724 cm21, respec-tively, which are the same as those reported inthe literature,22,23 but left the peak widths andheights of the three bands and the peak positionof the hydrogen-bonded carbonyl bands as theadjustable parameters. Figure 4 illustrates theexperimental and fitting spectra in the carbonylvibration region of PCL/PA 55/45 blends. The ex-cellent agreement between the experimental andfitting spectra indicates the reliability of thiscurve-fitting technique. In this way, quantitativedata regarding the relative integrated intensity ofthe amorphous, crystalline, and associated bandsare obtained, as listed in Tables I and II.

The values of fraction Fb of intermolecular hy-drogen-bonded carbonyl groups in PCL/PA andPCL/PB blends are summarized in Tables I andII, respectively. Fb increased as the polyphenolcontent increased. Furthermore, the value of Fbfor the PCL/PA blend is much higher than that forthe PCL/PB blend at the same polyphenol con-tent. This results from the following two aspects:(1) the molar ratio of the hydroxyl group to thecarbonyl group for the PCL/PA blend is higherthan that for the PCL/PB blend at the same poly-phenol content; and (2) as compared with PA, thestructure of PB is unfavorable to the formation ofinterassociated hydrogen bonds. Because of thesimilar reasons, the value of Fb for the PCL/poly-phenol blends investigated here are much lowerthan those found for the PCL/4,49-thiodiphenol11

and PCL/poly(4-vinyl phenol) blends.22

Thermal Properties of PCL/Polyphenol Blends

Figure 5 depicts the DSC thermograms ofPCL/PA blends recorded in the first heating scan.The Tm of the PCL component decreases afterblending with PA. For the PCL component, Tm is71 °C in the pure state and 50 °C in the PCL/PA55/45 blend, that is, Tm decreases about 21 °C

Table I. Fitting Results of the Carbonyl Vibration Bands in the FTIR Spectra of PCL/PA Blendsa

PCL/PA

Amorphous Band Crystalline Band Hydrogen-Bonded Band

Fbv/cm21 W1/2/cm21 Aa/% v/cm21 W1/2/cm21 Ac/% v/cm21 W1/2/cm21 Ab/%

100/0 1736 16.0 41.1 1724 15.2 58.985/15 1736 15.2 36.7 1724 11.4 51.6 1706 30.8 11.6 0.07670/30 1736 15.8 38.0 1724 11.6 46.3 1706 29.8 15.7 0.10355/45 1736 15.0 30.9 1724 11.4 42.2 1706 27.1 26.9 0.187

a v, W1/ 2, and Ai (i 5 a, b, c) are peak position, peak width at the half-height, and relative peak intensities, respectively. Fb isthe fraction of hydrogen-bonded carbonyl groups of PCL.

Table II. Fitting Results of the Carbonyl Vibration Bands in the FTIR Spectra of PCL/PB Blendsa

PCL/PB

Amorphous Band Crystalline Band Hydrogen-Bonded Band

Fbv/cm21 W1/2/cm21 Aa/% v/cm21 W1/2/cm21 Ac/% v/cm21 W1/2/cm21 Ab/%

100/0 1736 16.0 41.1 1724 15.2 58.985/15 1736 16.2 42.8 1724 14.4 52.8 1706 29.6 4.4 0.02870/30 1736 15.4 39.5 1724 14.3 52.1 1706 28.8 8.4 0.05455/45 1736 15.2 39.3 1724 14.1 45.1 1706 27.1 15.6 0.102

a v, W1/ 2, and Ai (i 5 a, b, c) are peak position, peak width at the half-height, and relative peak intensities, respectively. Fb isthe fraction of hydrogen-bonded carbonyl groups of PCL.

2902 HE ET AL.

(Fig. 5 and Table III). This result indicates thatthe crystallization of PCL is greatly affected byPA.

The DSC thermograms in the first heating scanof the PCL/PB blends are displayed in Figure 6.Similar to the PCL/PA blend, Tm of the PCL com-ponent also decreases but little after blendingwith PB (Fig. 6 and Table IV).

The samples have the same thermal history.However, they have different melting points andshow the melting point depression. The observedmelting point depression seems to suggest thatthe crystallization behavior of PCL is affected bythe addition of polyphenol, especially PA. Fur-thermore, this opinion is supported by two facts inthe second heating scan of the DSC thermogramsof the PCL/PA and PCL/PB blends in Figures 7and 8. First, both 55/45 blends with PA and PB

have no melting peaks, explaining why PCL isamorphous when quenched and cannot crystallizeeven if heated. Second, the 70/30 blend with PBhas an exothermic peak, the area of which issimilar to that of the melting peak. It explains the70/30 blend with PB is also almost amorphouswhen quenched, whereas 70/30 with PA has noexothermic peak. On the other hand, the Tm de-crease alone is not absolute evidence of miscibil-ity. For instance, isotactic polystyrene and poly-(methyl methacrylate) (PMMA) are not miscible,but isotactic polystyrene single crystals do show aTm depression when embedded in PMMA.24 Con-versely, the miscible blend of poly(ethylene oxide)(PEO) and PMMA does not show a clear decreasein PEO melting peak temperature.25

In the blends, the crystallinity (Xc) of PCL canbe calculated by the following equation:

Figure 5. DSC thermograms of pure PCL, PA, andtheir blends with various PA contents recorded duringthe first heating scan.

Table III. Thermal Properties of PCL/PA Blends

Sample Tm of PCL/°C DH/J z g21 Xc/% Tg/°C

PCL 71 81 60 25985/15 70 70 61 24570/30 69 56 59 23155/45 50 24 32 28PA — — — 141

Figure 6. DSC thermograms of pure PCL, PB, andtheir blends with various PB contents recorded duringthe first heating scan.

Table IV. Thermal Properties of PCL/PB Blends

Sample Tm of PCL/°C DH/J z g21 Xc/% Tg/°C

PCL 71 81 60 25985/15 71 69 60 25270/30 70 53 56 24655/45 67 41 55 230PB — — — 126

HYDROGEN-BONDING INTERACTION 2903

Xc 5 DH/~DH0 3 W! (5)

where DH0 is the thermodynamic melting en-thalpy per gram of pure-state PCL (136 J/g26), DH(J/g) is the apparent melting enthalpy corre-sponding to the PCL component, and W is theweight content of this component in the blend.The calculated crystallinity is listed in Tables IIIand IV. As a general trend, the crystallinity ofPCL in the blends decreased with an increasedpolyphenol content in the blends.

As shown in Figures 7 and 8, only one glasstransition is observed for all PCL/PA blends, andthe glass-transition temperature (Tg) increasesfrom 259 to 28 °C with the increase of PA contentfrom 0 to 45% (Fig. 7 and Table III). This resultsuggests that PCL is miscible with PA in theamorphous state over the studied compositionrange. Similarly, a single Tg is observed for thePCL/PB blend (Fig. 8 and Table IV), indicatingthat PCL and PB are also miscible in the amor-phous state.

CONCLUSIONS

As detected by FTIR, there were strong interas-sociated hydrogen bonds between PCL and poly- phenols (PA and PB). The quantitative analysis of

the FTIR spectra revealed that the fractions ofhydrogen-bonded carbonyl groups in PCL/poly-phenol blends increased with an increased poly-phenol content. Thermal analysis revealed thatboth the melting point and crystallinity of thePCL component in the blend decreased with anincreased polyphenol content. A single Tg wasobserved for all investigated PCL/polyphenolblends, indicating that PCL and polyphenols (PAand PB) are miscible in the amorphous state.

This work was partly supported by a grant-in-aid forScientific Research on Priority Area, “Sustainable Bio-degradable Plastics,” 11217204(2000) from the Minis-try of Education, Culture, Sports, Science and Technol-ogy (Japan). The authors are grateful to Daicel Chem-ical Industries, Ltd., Japan for kindly supplying thePCL sample.

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Figure 7. DSC thermograms of pure PCL, PA, andtheir blends with various PA contents recorded duringthe second heating scan (Tg’s are indicated by the ar-rows).

Figure 8. DSC thermograms of pure PCL, PB, andtheir blends with various PB contents recorded duringthe second heating scan (Tg’s are indicated by the ar-rows).

2904 HE ET AL.

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