Studies on poly(ε-caprolactone)/thiodiphenol blends: The specific interaction and the thermal and dynamic mechanical properties

Studies on Poly(«-caprolactone)/Thiodiphenol Blends:The Specific Interaction and the Thermal andDynamic Mechanical Properties

YONG HE, NAOKI ASAKAWA, YOSHIO INOUE

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

Received 28 November 1999; revised 16 February 2000; accepted 28 April 2000

ABSTRACT: The effects of several low molecular weight compounds with hydroxylgroups on the physical properties of poly(«-caprolactone) (PCL) were investigated byFourier transform infrared (FTIR) spectroscopy and high-resolution solid-state 13CNMR. PCL and 4,49-thiodiphenol (TDP) interact through strong intermolecular hydro-gen bonds and form hydrogen-bonded networks in the blends at an appropriate TDPcontent. The thermal and dynamic mechanical properties of PCL/TDP blends wereinvestigated by differential scanning calorimetry (DSC) and dynamic mechanical ther-mal analysis, respectively. The melting point of PCL decreased, whereas both theglass-transition temperature and the loss tangent tan d of the blend increased with anincrease in TDP content. The addition of 40 wt % TDP changed PCL from a semicrys-talline polymer in the pure state to a fully amorphous elastomer. The molecules of TDPlost their crystallizability in the blends with TDP contents not greater than 40 wt %. Inaddition to TDP, three other PCL blend systems with low molecular weight additivescontaining two hydroxyl groups, 1,4-dihydroxybenzene, 1,4-di-(2-hydroxyethoxy) ben-zene, and 1,6-hexanediol, were also investigated with FTIR and DSC, and the effects ofthe chemical structure of the additives on the morphology and thermal properties arediscussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1848–1859, 2000Keywords: poly(«-caprolactone); 4,49-thiodiphenol; hydrogen bond; polymer blend

INTRODUCTION

In the last few years, there have been many stud-ies on poly(4-vinylphenol) (PVPh) blends becausethe hydroxyl group of PVPh possesses the poten-tial ability to form strong intermolecular hydro-gen bonds with carbonyl, ether, or other func-tional groups of the second polymer.1–3 A varietyof binary blends containing PVPh have been stud-ied, and almost all of these blends have beenreported to be miscible or at least partially mis-

cible over a wide composition range in the amor-phous state.4–24 Some works also have focused onthe ternary blends of PVPh, and it has been foundthat PVPh improves the miscibility between im-miscible blends.25–27

Because most polymer blends are immiscibledue to the small value of the mixing entropy, it issurprising and interesting that PVPh can formmiscible blends with so many structurally dissim-ilar polymers. This brings us to the followingquestions:

1. Are intermolecular hydrogen bonds formedbetween dihydric phenol (as the dimermodel of PVPh in terms of the chemical

Correspondence to: Y. Inoue (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 1848–1859 (2000)© 2000 John Wiley & Sons, Inc.

1848

structure) and a carbonyl, ether, or otherfunctional-group-containing polymer?

2. Is it possible that the two hydroxy groupsin a molecule of dihydric phenol form twohydrogen bonds at the same time with twocarbonyl groups that come from two differ-ent polymer chains?

3. If the answer to the second question is yes,can dihydric phenol be used as a compati-bilizer to improve the miscibility betweenpolymers with hydrogen-acceptable func-tional groups?

However, a variety of low molecular weightadditives, such as fillers, plasticizers, and antioxi-dants, nowadays are included in polymer materi-als in industry to modify their proprieties andthen greatly widen the application field of poly-mer materials. For example, carbon black, cal-cium carbonate, and silicon dioxide are added asfillers to rubber or plastic to improve the mechan-ical properties; dialkyl phthalates are incorpo-rated as plasticizers in poly(vinyl chloride) mate-rials to improve their processability and flexibil-ity. It is well-known that the type and strength ofthe interactions between low molecular weightcompounds and polymers are crucial to the uses ofadditives. Nonetheless, to our knowledge it is stillnot clear whether or how a low molecular weightadditive will influence the properties of polymermaterials if a small molecule forms two hydrogenbonds with two polymer chains at the same time.

In this investigation, an effort was made toelucidate some of the aforementioned questions.We selected poly(«-caprolactone) (PCL) as a rep-resentative of the carbonyl-containing polymersand used 4,49-thiodiphenol (TDP) as the dihydricphenol. PCL is a highly crystalline polyester, andthe weak self-association of this polymer makes ita good component to study the interaction be-tween the carbonyl group and the hydroxyl groupof dihydric phenol. First, the specific interactionbetween TDP and PCL was studied by Fouriertransform infrared (FTIR) spectroscopy and high-resolution solid-state 13C NMR. Then the thermalbehaviors and dynamic mechanical properties ofPCL/TDP blends were monitored by differentialscanning calorimetry (DSC) and dynamic me-chanical thermal analysis (DMTA), respectively.Furthermore, three other low molecular weightsubstances containing two hydroxyl groups andTDP were also added to PCL, and the correspond-ing blends were investigated by FTIR and DSC.

The effect of the chemical structure of the smallmolecules on the specific interaction and thermalproperties is discussed.

EXPERIMENTAL

Materials

The PCL sample (number-average molecularweight 5 5.35 3 104; polydispersity index 5 1.47;Celgreent PH4) was supplied courtesy of DaicelCo. (Japan). TDP, 1,4-dihydroxybenzene (DHB),1,4-di-(2-hydroxyethoxy) benzene (DHEB), and1,6-hexanediol (HDO) were purchased from To-kyo Kasei Kogyo Co. (Japan). The polymer sampleand all the reagents were used as received.

Preparation of the Blend Samples

PCL and low molecular weight substances in ap-propriate weight ratios were first dissolved in 1,4-dioxane (total polymer concentration 5 5 wt %).Then the solution was cast onto a glass petri dish,and the solvent was evaporated at room temper-ature. All the cast films were placed in an oven at60 °C under vacuum for 2 days, except for thePCL/HDO blend, because of the low boiling pointof HDO. All film samples were aged at room tem-perature for more than 4 weeks before the DSCand NMR measurements.

For the DMTA measurement, the cast films ofPCL/TDP blends were subsequently compression-molded between two Teflon sheets with an appro-priate spacer at 160 °C and under a pressure of 5MPa for 2 min with a Toyo Seiki laboratory press(Mini Test Press 10). The molded films werequickly cooled to room temperature between twoiron plates, and afterward they were aged at roomtemperature for more than 4 weeks before theDMTA experiment.

FTIR

Films of the blends with a thickness suitable forFTIR measurements were prepared by the poly-mer solution in 1,4-dioxane being directeddropped to the surface of a silicon wafer. Thesilicon wafer was transparent for an IR incidentbeam and was used as a substrate. The maximumabsorption of the resulting films was lower than 1absorbance unit, which ensured that all absorp-

POLY(e-CAPROLACTONE)/THIODIPHENOL BLENDS 1849

tions were within the linearity range of the detec-tor.

IR measurements were carried out on a single-beam IR spectrometer (PerkinElmer Spectra2000) at room temperature under N2 purging. Allthe spectra were recorded at a resolution of 4cm21 and with an accumulation of 16 scans.

High-Resolution Solid-State 13C NMR

The 13C FT-NMR spectra were measured at roomtemperature on a Varian UNITY 400 NMR spec-trometer (100 MHz for 13C nucleus). The DD/MAS(dipolar decoupling/magic angle spinning) spectraof pure PCL and the blends were measured underhigh-power proton dipolar decoupling (ca. 59.5kHz). The pulse repetition time was set at 5 s, andthe magic angle spinning rate was optimized at4500 6 5 Hz. The CP (cross-polarization)/MASspectrum of pure TDP was measured with a 1H–13C cross-polarization time of 2.5 ms and a pulserepetition time of 90 s. To simplify the CP/MASspectrum of TDP, the TOSS (TOtal Sideband Sup-pression) technique was used to eliminate theside band.28 For each spectrum, at least 512 tran-sients were accumulated, and the 13C NMR chem-ical shifts were referenced to the CH3 resonanceof hexamethylbenzene (HMB) as the externalstandard [17.4 ppm from tetramethylsilane (TMS)].

DSC

DSC thermograms of the samples (ca. 5 mg),which were presealed in aluminum pans, wererecorded on a Seiko DSC 220 system connected toa SSC5300 workstation. The samples were firstheated from 2100 °C to an end temperature rang-ing from 150 to 200 °C (different end tempera-tures were chosen because of the differences inthe melting point of low molecular weight addi-tives) at a heating rate of 20 °C/min (the firstheating scan). After that, samples were rapidlyquenched to 2120 °C with liquid nitrogen, andthen they were again heated to the end tempera-ture at a rate of 20 °C/min (the second heatingscan). The melting temperature (Tm) was deter-mined from the endothermic peak of the DSCcurve from the first heating scan. The glass-tran-sition temperature (Tg) was taken as the peak topof the DDSC (differentiation of DSC) curve ob-tained from the second heating scan. The value ofthe melting enthalpy was calculated as the inte-gral of the endothermal peak in the DSC curve.

DMTA

Dynamic mechanical spectra were recorded on aSeiko DMS210 instrument under the tensilemode. Each test sample was 30 mm long, 8 mmwide, and ca. 200 mm thick. The experiment wascarried out at 1 and 5 Hz at a thermal scanningrate of 2 °C/min.

RESULTS AND DISCUSSION

Specific Interaction between PCL and TDP

FTIR spectroscopy is a particularly suitable tech-nique for investigating the specific intermolecularinteraction. For the PCL/TDP blends studiedhere, one of the components (PCL) contains acarbonyl group, yielding a nCAO stretching modeat 1727 cm21, whereas TDP shows no absorptionin the carbonyl vibration region ranging from1650 to 1800 cm21. Therefore, any change in theFTIR spectrum in this region should be directlyattributed to a change in the carbonyl group en-vironment, such as the formation of hydrogenbonds.

The FTIR spectra of PCL/TDP blends in thecarbonyl vibration region are shown in Figure 1as a function of TDP composition. For pure PCL,

Figure 1. FTIR spectra in the carbonyl vibration re-gion of pure PCL, TDP, and their blends with TDPcontents of 10 wt % (PCLTDP10), 20 wt %(PCLTDP20), 30 wt % (PCLTDP30), and 40 wt %(PCLTDP40).

1850 HE, ASAKAWA, AND INOUE

the carbonyl vibration band centered at 1727cm21. Blending with TDP, a second band ap-peared at a lower wave number (1706 cm21) inaddition to the band centered at about 1727 cm21.The second band should be attributed to the hy-drogen-bonded carbonyl vibration, indicating theformation of intermolecular hydrogen bonds be-tween PCL and TDP. With an increase in the TDPcontent in the blends, the relative absorbance ofthe hydrogen-bonded carbonyl vibration in-creased, whereas that of the free carbonyl banddecreased, indicating a reduction of the percent-age of free carbonyl groups relative to the hydro-gen-bonded ones.

In Figure 2 are shown the FTIR spectra of thePCL/TDP blends in the hydroxy vibration region.Three vibration bands appeared in the spectrumof pure TDP centered at 3318, 3265, and 3183cm21. These bands should be assigned to the hy-drogen-bonded hydroxyl groups in pure TDP, astheir wave numbers are low. For pure PCL, only avery weak band centered at 3447 cm21 was ob-served in this region, and it should be attributedto the vibration of the hydroxy group in the chainterminal of PCL. Because of its low intensity com-pared with the bands of TDP in this vibrationregion, the contribution of this band to the spectraof the blends should have been negligible when

the content of TDP was not too low. In the PCL/TDP blends, a new band appeared at 3383 cm21,and its relative absorbance increased with theTDP content. This band should have come fromthe vibration of the hydroxyl group of TDP, whichformed intermolecular hydrogen bonds with thecarbonyl group of PCL chain. Compared with thethree bands of pure TDP, this band shifted to ahigher wave number, which indicated that thehydrogen bond formed between PCL and TDPwas weaker than those formed between TDP mol-ecules.

High-resolution solid-state 13C NMR is also apowerful tool for studying the intermolecular hy-drogen-bonding interaction. In general, the 13C

Figure 3. High-resolution solid-state 13C NMR spec-tra of pure PCL, TDP, and their blends with TDPcontents of 20 wt % (PCLTDP20) and 40 wt %(PCLTDP40): the DD/MAS NMR spectra of pure PCLand the PCLTDP20 and PCLTDP40 blends and theCP/MAS NMR spectrum of pure TDP (the side bands inthe CP/MAS spectrum of TDP were eliminated with theTOSS technique28).

Figure 2. FTIR spectra in the hydroxyl vibration re-gion of pure PCL, TDP, and their blends with TDPcontents of 10 wt % (PCLTDP10), 20 wt %(PCLTDP20), 30 wt % (PCLTDP30), and 40 wt %(PCLTDP40).


nuclei exhibit more or less downfield shifts whenthey are involved in hydrogen bonds.29–31 Forpoly(vinyl alcohol)/poly(vinyl pyrrolidone) blends,the formation of a hydrogen bond between thecarbonyl group in poly(vinyl pyrrolidone) and thehydroxyl group in poly(vinyl alcohol) results in adownfield shift of about 2 ppm in the carbonylcarbon resonance.31

As expected, a significant downfield shift of thecarbonyl carbon resonance was observed in thehigh-resolution solid-state 13C NMR spectra ofPCL/TDP blends (Fig. 3 and Table I). Moreover,the chemical shift of the CH2 («) also changedfrom 64.4 ppm in pure PCL to 65.2 ppm in aPCLTDP40 blend (40 means that the TDP con-tent was 40 wt % in this PCL/TDP blend), asshown in Figure 3 and Table I. Thus, the NMRspectra further confirmed the formation of inter-molecular hydrogen bonds between the hydroxylgroups in TDP and the carbonyl groups in PCLchain.

Quantitative Analysis of the Fractions ofHydrogen-Bonded Carbonyl and Hydroxyl Groups

As mentioned earlier, FTIR and high-resolutionsolid-state 13C NMR spectra confirmed qualita-tively that intermolecular hydrogen bonds areformed between the hydroxyl groups in TDP andthe carbonyl groups in PCL chain. Moreover, em-ploying the Beer–Lambert law permits a quanti-

tative analysis of the fraction of the carbonylgroups and the hydroxyl groups involved in theintermolecular hydrogen bond between PCL andTDP via FTIR spectroscopy. In fact, quantitativeevaluations of similar systems have been dis-cussed many times in the literature.1,32–36

The spectra of the PCL/TDP blend in the car-bonyl region exhibit three distinct components.37

The components approximately at 1736 and 1724cm21 are attributed to PCL in the amorphous andcrystalline conformations, respectively. The con-tribution observed at 1706 cm21 can be attributedto the hydrogen-bonded carbonyl vibration as pre-viously mentioned. With a curve-fitting pro-gram,38 quantitative data regarding the inte-grated intensity of the three separated bandswere obtained (Table II). The fraction of the car-bonyl groups involved in the intermolecular hy-drogen bond (F(B,CO)) can be calculated fromeq 1:23,33

F(B,CO) 5 A(B,CO)/~A(B,CO) 1 A(A,CO) z gB/A

1 A(C,CO) z gB/C) (1)

where A(B,CO), A(A,CO), and A(C,CO) are the inte-grated intensities corresponding to the hydrogen-bonded, amorphous, and crystalline carbonylbands, respectively. gB/A and gB/C are absorptionratios that take into account the differences be-tween the absorptivities of the hydrogen-bondedand amorphous carbonyl groups and betweenthose of the hydrogen-bonded and crystalline car-bonyl groups, respectively:

gi/j 5 *01`«i(n)dn/*0

1`«j~n!dn (2)

where «i (n) is the absorption coefficient, n is thewave number, and i, j is A, B or C. The values ofgB/A and gB/C were determined to be 1.95 and 1.34,

Table I. The 13C Chemical Shifts (d/ppm) of theCarbonyl and CH2 («) Groups of PCL

Sample CAO CH2 («)

PCL 173.1 64.4PCLTDP20 174.7 65.0PCLTDP40 175.1 65.2

Table II. The Fractions of Hydrogen-Bonded Carbonyl and Hydroxyl Groups

BlendsTDP Content

(wt %) A(A,CO) A(C,CO) A(B,CO) F(B,CO) F(B,OH) FDHBTM

PCL 0 39.6 60.4 0 0PCLTDP10 10.0 36.0 47.5 16.5 0.11 0.95 ^0.90PCLTDP20 20.0 26.1 39.0 34.9 0.25 0.96 ^0.92PCLTDP30 30.0 36.5 6.0 57.5 0.42 0.94 ^0.88PCLTDP40 40.0 33.8 0 66.2 0.50 0.72 ^0.44


respectively,38,39 comparable to published datafor similar systems.1,32,33,36

In principle, the number of the hydroxyl group(N(B,OH)) involved in the intermolecular hydrogenbond formed between PCL and TDP in a blendshould be equal to that of the hydrogen-bondedcarbonyl group (N(B,CO)):

N(B,OH) 5 N(B,CO) (3)

Based on eq 3, the fraction of the hydroxyl groupinvolved in the intermolecular hydrogen bondformed between PCL and TDP (F(B,OH)) can beexpressed as follows:

F~B,OH! 5 F~B,CO!$~1 2 w!/~2w!% z ~MTDP/MPCL) (4)

where w is the weight content of TDP in theblend, MPCL and MTDP are the molecular weightsof the PCL monomer unit and TDP, respectively,and 2 denotes that there are two hydroxyl groupsin a TDP molecule.

In this study, the TDP molecule that simulta-neously formed two intermolecular hydrogenbonds with two carbonyl groups of PCL throughits two hydroxyl groups was defined as the doublehydrogen-bonded TDP molecule (DHBTM). WithF(B,OH), the fraction of DHBTMs (FDHBTM) can beexpressed as follows:

FDHBTM ^ 2F~B,OH! 2 1 (5)

From eqs 1 and 4, the values of F(B,CO) and F(B,OH)for the PCLTDP40 blend were calculated to be0.50 and 0.72, respectively. From eq 5, FDHBTMwas found to be not less than 0.44 (Table II). Thevalue of F(B,CO) (0.50) seems relatively high,whereas it is lower than that (0.71) of the PCL/PVPh blend with a PVPh content of 40%.37

On average, there were 487 carbonyl groups ineach of the PCL chains studied here (estimatedfrom a number-average molecular weight of 5.353 104). For the PCLTDP40 blend, F(B,CO) with avalue of 0.50 indicated that in a PCL chain therewere 243 carbonyl groups involved in the inter-molecular hydrogen bond. However, in thePCLTDP40 blend, at least 44% of the TDP mole-cules formed two hydrogen bonds at the sametime with PCL chains (FDHBTM ^ 0.44). Thesefacts strongly suggested that there was a hydro-gen bond network in the PCLTDP40 blend. Thecase was similar for the PCLTDP10, PCLTDP20,

and PCLTDP30 blends. The hydrogen bond net-work should act as a physical crosslink in theblend and then exert a great influence on theproperties of the blend.

It is worthwhile to discuss here the followingtwo points. First, there were two cases for theformation of two intermolecular hydrogen bondsbetween the two hydroxyl groups of a TDP mole-cule and two carbonyl groups of PCL. One wasthat the two carbonyl groups belonged to thesame PCL chain, and the other was that each ofthem came from two different PCL chains. Thefirst case should make no contribution to the hy-drogen bond network. From a statistical point ofview, the second case should be the main case.Thus, the presence of the first case might notaffect the presence of a hydrogen bond network.

Second, there were also hydrogen bonds be-tween TDP molecules (self-association) in blendsin addition to the intermolecular hydrogen bondsbetween the molecules of TDP and PCL (interas-sociation), and there should be an equilibriumbetween these two kinds of hydrogen bonds. Theincrease in the TDP content facilitated the forma-tion of the self-associated hydrogen bonds, whichshould lead the reduction of F(B,OH) and FDHBTMvalues with the increase in the TDP content (theresults in Table II confirmed this prediction).

Thermal and Dynamic Mechanical Properties

DSC analysis was performed to study the thermalbehavior of the blends. Figure 4 shows the DSCtraces of PCL/TDP blends with various TDP con-tents recorded during the first heating scan. Themelting point of the PCL component in the blendsdecreased with the TDP content: it was 64 °C forpure PCL and only 46 °C for the PCLTDP30blend. In that there were strong intermolecularinteractions between PCL and TDP, this resultwas quite reasonable. What was interesting wasthat no melting peak corresponding to the TDPcomponent was observed for the blends studiedhere. This should suggest that the TDP compo-nent existed in an amorphous state and that thecrystallization of the TDP component was com-pletely suppressed in the blends. Furthermore,the melting of the PCL component and the TDPcomponent was undetectable for the PCLTDP40blend, as shown in the DSC curve. This indicatedthat a semicrystalline PCL became a fully amor-phous elastomer by blending with 40% TDP.


Figure 5 depicts the relationship between thecrystallinity of the PCL component and the TDPcontent in the blends. The crystallinity was cal-culated from the melting enthalpy of the PCLcomponent (the area of the DSC melting peak inthe first heating scan), with the melting enthalpy

of 100% crystalline PCL assumed to be 166 J/g.40

Clearly, the PCL component in the blends pos-sessed a lower crystallization degree than in thepure state and its crystallinity decreased with theincrease in the TDP content. Similar behaviorwas also observed for the PCL/PVPh blend andwas attributed to the increasing Tg.41 For PCL/TDP blends, this behavior should mainly comefrom the hydrogen bond network in addition tothe increasing Tg, as the Tg of the PCL/TDP blendwas much lower than that of the PCL/PVPh blendat the same PCL content.41

In Figure 6 are shown the DSC traces of PCL/TDP blends recorded in the second heating scan.Obviously, the Tg’s of the blends were higher thanthe Tg of pure PCL. The relationship between theTg and TDP content is depicted in Figure 7. TheTg of a PCL/TDP blend increased with the contentof TDP in the blend except for the Tg ofPCLTDP10 (244 °C), which was higher than thatof PCLTDP20 (247 °C). Because of the intermo-lecular hydrogen bond, the molecules of TDP

Figure 6. DSC traces of pure PCL, TDP, and theirblends with various TDP contents recorded during thesecond heating scan.

Figure 4. DSC traces during the first heating scan ofpure PCL, TDP, and their blends with various TDPcontents.

Figure 5. Relationship between the crystallinity ofthe PCL component and the TDP content in the blends.


might act as a physical bulk side group of the PCLchain in the blend. Furthermore, because of theformation of a hydrogen-bond network, TDPshould also act as a physical crosslinking agent inthe blend. Both the physical bulk side group andcrosslink network should lower the flexibility ofthe PCL chain and then heighten the Tg of theblend.

It seemed abnormal that the PCLTDP10 blendpossessed a higher Tg than the PCLTDP20 blend.As shown in Figure 6, there was no crystallizationpeak of the PCL component after the glass tran-sition in the DSC curve of PCLTDP10; that is, thecrystallization was completed during quenchingjust as for pure PCL. In contrast, a crystallizationpeak appeared after the glass transition in theDSC curve of the PCLTDP20 blend, and the areaof this peak was almost the same as that of themelting peak. This indicated that the PCLTDP20blend was in an amorphous state before the glasstransition was observed during the second heat-ing scan. It is well-known that the crystallizationof a polymer causes an increase in its Tg as thecrystallization lowers the mobility of the polymerchains. Thus, it should be acceptable to attributethe anomaly to the difference in the phase struc-tures of the PCLTDP10 and PCLTDP20 blends;that is, PCLTDP10 was in a semicrystalline state,whereas the PCLTDP20 blend was in an amor-phous state before the glass transition occurred.

Figures 8 and 9 depict the dynamic mechanicalspectra of pure PCL and PCL/TDP blends withTDP contents ranging from 10 to 30%. It was

impossible to measure the relaxation spectra ofthe PCLTDP40 blend because it was too soft toprepare a test film.

From Figure 8, it could be seen for each systemthat, with increasing temperature, the logarithmof the storage modulus, log(E9), varied very littlein the first section of the curve (the first plateauzone). Then a sharp decrease was observed andwas attributed to the change in the segmentalmobility related to the glass transition. After a

Figure 7. Relationship between the Tg of the blendsand the TDP content in the blends from the measure-ments of DSC (F) and DMTA (E).

Figure 8. Storage modulus E9 of pure PCL and PCL/TDP blends with TDP contents of 0–30% (E 5 PCL; h

5 PCLTDP10; 1 5 PCLTDP20; ƒ 5 PCLTDP30) mea-sured at 5 Hz and a heating rate of 2 °C/min.

Figure 9. Loss tangent tan d of pure PCL and PCL/TDP blends with TDP contents of 0–30% (E 5 PCL; h

5 PCLTDP10; 1 5 PCLTDP20; ƒ 5 PCLTDP30) mea-sured at 5 Hz and a heating rate of 2 °C/min.


second plateau zone, the value of log(E9) de-creased drastically again because of the meltingof the crystalline phase.

In a comparison of the storage modulus of purePCL with the moduli of the PCL/TDP blends, itwas clear that in the first plateau zone, the stor-age modulus of the blend was higher than that ofpure PCL; however, in the second plateau zone, itwas lower than that of pure PCL. Table III liststhe values of the storage modulus, E9, for purePCL and the PCL/TDP blends at 2100 and 0 °Cas the representative values of the first and sec-ond plateau zones, respectively. It was obviousthat at 2100 °C the storage modulus of the blendfirst increased and then decreased with the in-crease in the PCL content, whereas at 0 °C, itdecreased rapidly with the increase in the PCLcontent. This result may be related to the changein the phase structure and density of the hydro-gen-bond network with the change in the TDPcontent.

Figure 9 displays the loss tangent (tan d) ofpure PCL and the PCL/TDP blends. Three differ-ences can be noted in the relaxation behavioramong the samples. First, the a-transition peakbecame sharper with the increase in the TDPcontent. It may be reasonably assumed that thereis an interfacial region in addition to the crystal-line and amorphous regions in pure PCL, and thisregion decreased after the addition of TDP be-cause of the decreasing of the crystalline region. Ifso, this should be one of the possible reasons forthe peak sharpening. Second, the value of tan d onthe peak top increased with the increase in theTDP content, which may be related to the phasestructure and density of the hydrogen-bond net-work increasing with the TDP content. Third, theTg, determined as the temperature correspondingto the maximum of tan d in the a transition,

increased with the TDP content (see also Fig. 7).This was basically in accordance with the DSCresults.

Effect of the Chemical Structure of the LowMolecular Weight Molecules on the SpecificInteraction

In this study, the interactions between PCL andDHB, DHEB, and HDO were also investigated byFTIR. A content of 30 wt % was selected for theselow molecular weight additives in the blend. Sim-ilar to TDP, all these small molecules contain twohydroxyl groups at the molecular termini. Figure10 summarizes the FTIR spectra in the carbonylvibration region for pure PCL and the PCL blendswith the small molecular substances. All thesmall molecules showed no infrared absorptivityin this region.

In a comparison with the spectrum of purePCL, little difference was observed for the spectraof the PCLHDO30 and PCLDHEB30 blends, in-dicating that after the blending with HDO andDHEB, there was little change in the environ-ment of the PCL carbonyl group. Hence, therewas little interaction between the carbonylgroups of PCL and the hydroxyl groups of HDOand DHEB. The peak of the carbonyl group in-volved in the hydrogen bond, the shoulder peak at1707 cm21, appeared in the spectrum of thePCLDHB30 blend, indicating the formation of hy-

Figure 10. FTIR spectra in the carbonyl vibrationregion of pure PCL and its blends with 30 wt % lowmolecular weight additives.

Table III. The Dynamic Mechanical Properties ofPCL and PCL/TDP Blends

Sample

Storage Modulus E9 (Pa) Tan d atthe Peakof the a

Transition2100 °C 0 °C

PCL 2.97 3 109 6.80 3 108 0.089PCLTDP10 4.89 3 109 5.25 3 108 0.207PCLTDP20 4.66 3 109 3.11 3 108 0.323PCLTDP30 3.77 3 109 9.90 3 107 0.526


drogen bonds between the carbonyl group of PCLand the hydroxyl group of DHB. However, thispeak was very weak compared with that of thePCLTDP30 blend. Obviously, the fraction of thePCL carbonyl groups involved in the intermo-lecular hydrogen bond (F(B,CO)) in the blendsincreased in the following order: PCLHDO306 PCLDHEB30 ! PCLDHB30 , PCLTDP30.

In these blends, F(B,CO) may mainly depend ontwo factors, the molar ratio of the hydroxyl tocarbonyl groups (ROH/CO) in the blend and theintensity of the interaction between the twogroups. With other conditions unchanged, thehigher the ROH/CO is, the higher the F(B,CO) shouldbecome when the content of the two-hydroxyl-containing substances is not too high. Similarly,the stronger the interaction is, the higher F(B,CO)should become. In the studied blends, ROH/CO was0.83 for the PCLHDO30 blend, 0.49 for the PCLD-HEB30 blend, 0.89 for the PCLDHB30 blend, and0.45 for the PCLTDP30 blend. Because thePCLTDP30 blend possessed the lowest ROH/CObut had the highest F(B,CO), it could be concludedthat the interaction between the hydroxyl andcarbonyl groups in the PCLTDP30 blend was thestrongest among these blends.

The differences in the intensity of the interac-tion, or the intermolecular hydrogen bond, be-tween the carbonyl group and the hydroxyl groupshould arise from the differences in the chemicalstructure of the low molecular weight molecules.TDP is a phenol, whereas HDO and DHEB arealcohols. The benzene ring in TDP should stabi-lize the intermolecular hydrogen bond because ofthe hydroxyl group directly linking to the aro-matic ring, whereas there is no such stabilizationfor either HDO or DHEB systems.

Effect of the Chemical Structure of the SmallMolecules on the Thermal Properties of the Blends

Figure 11 shows the DSC curves in the first heat-ing scan for pure PCL and the PCL blends withthe low molecular weight substances. In the DSCcurves of the PCLHDO30 and PCLDHEB30blends, there were two melting peaks correspond-ing to those of PCL and the low molecular weightmolecules, respectively. Still, in the DSC curves ofthe PCLDHB30 and PCLTDP30 blends, the melt-ing peaks corresponding to the small moleculesdisappeared, indicating that both DHB and TDPmolecules existed in an amorphous state in theseblends (according to DSC, the melting points ofpure TDP and DHB are 156 and 183 °C, respec-tively). However, the melting point of the PCLcomponent in the PCLHDO30 and PCLDHEB30blends almost remained 64 °C, the same as that inthe pure state, but it decreased to 56 °C inPCLDHB30 and 46 °C in the PCLTDP30 blend(Table IV).

In addition, the Tg of the PCL component (or ofthe blends) in the PCLHDO30 and PCLDHEB30blends was the same as that of PCL in the purestate (260 °C), whereas it increased to 244 °C in

Figure 11. DSC traces of the PCL/TDP blends withvarious low molecular weight additives recorded duringthe first heating scan.

Table IV. The Thermal Properties of PCL Blendswith Low Molecular Weight Additives

Sample Tm (°C)a Tg (°C)b Tc (°C)b

PCL 64 260 —c

PCLHDO30 64 260 —c

PCLDHEB30 64 260 —c

PCLDHB30 56 244 6PCLTDP30 46 241 13

a Obtained from the first heating scan.b Obtained from the second heating scan.c No crystalline peak was observed in the second heating

scan.


PCLDHB30 and 241 °C in the PCLTDP30 blend(Table IV).

It was clear that the addition of HDO andDHEB did not affect the thermal properties ofPCL, whereas the melting point of PCL decreasedand the Tg of PCL increased with the addition ofDHB and TDP. Obviously, this result was in ac-cord with the fact that there were strong intermo-lecular hydrogen bonds between PCL and TDPand between PCL and DHB, whereas there werevery weak or no hydrogen bonds between PCLand HDO or between PCL and DHEB, as shownpreviously.

CONCLUSIONS

As detected by FTIR and high-resolution solid-state 13C NMR, there were strong intermolecularhydrogen bonds between PCL and TDP. An inter-molecular hydrogen-bonded network was con-cluded to be present in the PCL/TDP blendsthrough a quantitative evaluation of the popula-tion of the free and hydrogen-bonded carbonylgroups of PCL.

The thermal and dynamic mechanical proper-ties of PCL were greatly modified through blend-ing with TDP. The melting point of PCL de-creased, whereas the Tg of the blends increasedwith the increase in the TDP content. The addi-tion of 40 wt % TDP changed PCL from a semi-crystalline polymer in the pure state to a fullyamorphous elastomer. The molecules of TDP losttheir crystallizability in the blends with TDP con-tents not larger than 40 wt %. At the a-transitionpeak of the DMTA spectra, the loss tangent tan dof the blends also drastically increased with theTDP content: the loss tangent of the blend with 30wt % TDP was about five times larger than that ofpure PCL. There are differences between the ef-fects of TDP and the effects of PVPh on the ther-mal properties, although they have many similarpoints.

Intermolecular hydrogen bonds between PCLand HDO (or DHEB) could not be detected byFTIR, whereas the formation of strong intermo-lecular hydrogen bonds was found between PCLand TDP (and DHB). Correspondingly, the ther-mal properties of PCL were not affected by theaddition of HDO or DHEB but were greatly mod-ified by TDP and DHB. These facts suggest thatdihydric phenol, but not dihydric alcohol, canform strong hydrogen bonds with PCL and effec-

tively modify the properties of PCL. The resultsalso suggest that the formation of hydrogen bondswith low molecular weight compounds can modifythe morphology and properties of polyesters suchas PCL.

This work was partly supported by a Grant-in-Aidfor Scientific Research on a Priority Area [SustainableBiodegradable Plastics, No. 11217204 (1999)] from theMinistry of Education, Science, Sports, and Culture(Japan).

REFERENCES AND NOTES

1. Coleman, M. M.; Painter, P. C. Prog Polym Sci1995, 20, 1.

2. Pehlert, G. J.; Painter, P. C.; Coleman, M. M. Mac-romolecules 1998, 31, 8423.

3. Li, L.; Chan, C. M.; Weng, L. T.; Xiang, M. L.;Jiang, M. Macromolecules 1998, 31, 7248.

4. Pedrosa, P.; Pomposo, J. A.; Calahorra, E.; Cor-tazar, M. Macromolecules 1994, 27, 102.

5. Zhang, X.; Takegoshi, K.; Hikichi, K. Macromole-cules 1993, 26, 2198.

6. Akiba, I.; Akiyama, S. Polym Networks Blends1997, 7, 147.

7. Pehlert, G. J.; Painter, P. C.; Veytsman, B.;Coleman, M. M. Macromolecules 1997, 30, 3671.

8. Zhou, X.; Goh, S. H.; Lee, S.; Tan, K. L. Appl SurfSci 1997, 119, 60.

9. Moskala, E. J.; Varnell, D. F.; Coleman, M. M.Polymer 1985, 25, 228.

10. Hong, J.; Goh, S. H.; Lee, S. Y.; Siow, K. S. Polymer1995, 36, 143.

11. Landry, C. J. T.; Coltrain, B. K.; Teegarden, D. M.;Ferrar, W. T. Macromolecules 1993, 26, 5543.

12. Pomposo, J. A.; Etxeberria, A.; Cortazar, M. Mac-romolecules 1992, 25, 6909.

13. Li, D.; Brisson, J. Macromolecules 1997, 30, 8425.14. Luo, X. F.; Goh, S. H.; Lee, S. Y. Macromolecules

1997, 30, 4934.15. Dong, J.; Ozaki, Y. Macromolecules 1997, 30, 286.16. Li, D.; Brisson, J. Macromolecules 1996, 29, 868.17. Bhagwagar, D. E.; Painter, P. C.; Coleman, M. M.

Macromolecules 1994, 27, 7139.18. Landry, C. J. T.; Ferrar, W. T.; Teegarden, D. M.;

Coltrain, B. K. Macromolecules 1993, 26, 35.19. Chen, H. L.; Wang, S. F.; Lin, T. L. Macromolecules

1998, 31, 8924.20. Zhang, L. L.; Goh, S. H.; Lee, S. Y. J Appl Polym Sci

1998, 70, 811.21. Zhang, L. L.; Goh, S. H.; Lee, S. Y. Polymer 1998,

39, 4841.22. Xing, P. X.; Dong, L. S.; An, Y. X.; Feng, Z. L.;

Avella, M.; Martuscelli, E. Macromolecules 1997,30, 2726.


23. Iriondo, P.; Iruin, J. J.; Fernandez-Berridi, M. J.Macromolecules 1996, 29, 5605.

24. Belfiore, L. A.; Qin, C.; Ueda, E.; Pires, A. T. N. JPolym Sci Part B: Polym Phys 1993, 31, 409.

25. Hosokawa, M.; Akiyama, S. Polym J 1999, 31, 13.26. Pomposo, J. A.; Cortazar, M.; Calahorra, E. Macro-

molecules 1994, 27, 252.27. Pomposo, J. A.; Calahorra, E.; Eguiazabal, I.; Cor-

tazar, M. Macromolecules 1993, 26, 2104.28. Dixon, W. T. J Magn Reson 1985, 64, 32.29. Maciel, G. E.; James, R. V. J Am Chem Soc 1964,

86, 3893.30. VanderHart, D. L.; Earl, W. L.; Garroway, A. N. J

Magn Reson 1981, 44, 361.31. Zhang, X.; Takegoshi, K.; Hikichi, K. Polymer

1992, 33, 712.32. Li, D.; Brisson, J. Polymer 1998, 39, 801.

33. Li, D.; Brisson, J. Polymer 1998, 39, 793.34. Pehlert, G. J.; Yang, X. M.; Painter, P. C.; Coleman,

M. M. Polymer 1996, 37, 4763.35. Coleman, M. M.; Hu, Y.; Sobkowiak, M.; Painter,

P. C. J Polym Sci Part B: Polym Phys 1998, 36, 1579.36. Coleman, M. M.; Graf, J. F.; Painter, P. C. Specific

Interactions and the Miscibility of Polymer Blends;Technomic: Lancaster, PA, 1991.

37. Sanchis, A.; Prolongo, M. G.; Salom, C.; Masegossa,R. M. J Polym Sci Part B: Polym Phys 1998, 36, 95.

38. He, Y.; Inoue, Y. Polym Int, in press.39. He, Y.; Asakawa, N.; Inoue, Y.; submitted for pub-

lication.40. Chen, H. L.; Li, L. J.; Lin, T. L. Macromolecules

1998, 31, 2255.41. Lezcano, E. G.; Salom Coll, C.; Prolongo, M. G.

Polymer 1996, 37, 3603.


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