Molecular Mobility of an Amorphous Chain in the CrystallizationProcess of Poly(ε-caprolactone)

Shigeo Hara,† Katsuhiro Yamamoto,† Shigeru Okamoto,†Shigetaka Shimada,*,† and Masato Sakaguchi‡

Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan, andNagoya Keizai University, 61 Uchikubo, Inuyama 484-8503, Japan

Received March 30, 2004; Revised Manuscript Received May 12, 2004

ABSTRACT: The chain end of poly(ε-caprolactone) was spin-labeled to study the molecular mobility ofthe polymer chain in an amorphous region under the crystallization. Time-resolved ESR measurementswere carried out in the crystallization process. The three stages of the time evolution of the molecularmobility at the crystallization temperatures were found; the correlation time, τc, of the polymer chainincreased gradually at the early stage and remarkably at the late stage. At the final stage, the τc increasedgradually again. The early stage is the induction period where no change of the scattering intensity isobserved by the time-resolved SAXS and WAXS measurements. The gradual decrease of the molecularmobility is caused by the precursory aggregation of the polymer chain before the crystallization. At thelate stage, the crystalline grows and the molecular mobility in the amorphous region becomes remarkablyrestricted by the rigid chains in the crystalline regions. At the final stage, the gradual decrease of themolecular mobility by the late growth of the crystalline observed again, where no structural change wasdetected by the measurements of X-ray scatterings.

1. Introduction

Numerous studies on polymer crystallization havebeen reported. In recent years, many authors developedvarious devices that carried out in situ and time-resolved measurements in the crystallization process.1-5

For example, the long- and short-range orders weredetected simultaneously by the SAXS and WAXS time-resolved measurements. In the process, the mobility ofthe polymer chain in the amorphous region as well asin the crystalline region may change with an increasein crystallinity. However, a few authors studied themolecular mobility under the structural change in thecrystallization. Fukao and Miyamoto performed in situstudies of the isothermal crystallization of poly(ethyleneterephthalate) (PET) by using simultaneous time-resolved measurements of dielectric relaxation andX-ray scattering.7,8 They concluded that the polymerchain in the amorphous phase make a dynamicaltransition from the so-called R process to anotherrelaxation process, called the R′ process, prior to thecrystallization. Moreover, Matsuba et al. found that inthe induction period the absorbance of trans conforma-tion bonds of syndiotactic polystyrene (sPS) began toincrease by using time-resolved Fourier transforminfrared spectroscopy.5 Strobl reviewed various re-searches of the crystallization process and proposed forthe model of the growth of the lamellar crystallites inan entangled polymer melt.6 In the present study, weobserved the relaxation of the polymer chain during thecrystallization of poly(ε-caprolactone) (PCL) by using theelectron spin resonance (ESR) technique, which is veryeffective for detection of a molecular mobility with ahigh sensitivity. And it is possible to evaluate themobility of the polymer chain in a particular region byusing the spin-label method. The structural change wasalso evaluated by the simultaneous and time-resolved

measurements of small- and wide-angle X-ray scatter-ings. The purpose of this study is to clarify the molecularmobility with the structural change in the crystalliza-tion process by using the time-resolved measurements.We took a typical crystalline polymer, PCL, as a modelsystem because the crystalline growth rate of PCL issufficiently slow to observe the isothermal crystalliza-tion process easily and the melting temperature is low(ca. 60 °C).

2. Experimental SectionMaterials. PCL used in this study was synthesized by

anionic polymerization of ε-caprolactone (Tokyo Chemical Co.,Ltd.) in toluene using diphenylethylene lithium as an initiator,and the polymerization was terminated by CO2 and CH3COOHto introduce a functional group of COOH. Mn and Mw/Mn ofthe synthesized PCL were ca. 40 000 g/mol and 1.2, respec-tively. The spin-label reagent used in this study was 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpoperidinyl-1-oxy) (Ald-rich). The spin-label reagent was bonded to the chain end ofthe synthesized PCL by transesterification with N,N′-dicyclo-hexylcarbodiimide as a condensing agent. The chemical struc-ture is shown in Figure 1. The glass transition temperature(Tg) and melting temperature (Tm) of the PCL are ca. -70 °Cand ca. 60 °C, respectively.

Measurements. Time-resolved SAXS and WAXS measure-ments were performed at beamline BL-9C in Photon Factory(PF) of High Energy Accelerator Research Organization inTsukuba, Japan (KEK). SAXS and WAXS detectors were one-dimensional position-sensitive proportional counter (PSPC)with an effective length of 10 cm. The wavelength λ of X-rayswas 0.15 nm. The detector (PSPC) at the measurements of theSAXS and WAXS was located at the distance of 100 and 70

† Nagoya Institute of Technology.‡ Nagoya Keizai University.

Figure 1. Chemical structure of spin-labeled PCL.

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cm, respectively. Collagen and tripalmitin were used as astandard specimen to calibrate SAXS and WAXS, respectively.The scattering intensities were corrected for the backgroundscattering and sample absorption. The sample was containedin a quartz tube, which was evacuated to a pressure of 10-4

Torr and sealed before ESR measurements. ESR spectra wereobserved at 0.2 mT with 100 kHz field modulation (RE1XGX-band). The signal of 1,1-diphenyl-2-picryhydrazyl (DPPH)was used as a g tensor standard. The magnetic field wascalibrated with the well-known splitting constant of Mn2+. Thesample was first annealed at ca. 75 °C for 10 min to erase theprevious thermal history. Then the sample was quenched tothe crystallization temperature, Tc, and the time-resolvedmeasurements were carried out at the temperature to observethe crystallization process.

3. Results and Discussion

Crystallization Process by the SimultaneousMeasurements of SAXS and WAXS. Figure 2 showsthe time evolutions of the SAXS and WAXS profiles inthe crystallization process. The SAXS intensity, I, iscorrected with a Lorentz factor, and the values ofIq2 are plotted for the scattering vector, q () (4π/λ)sin(θ/2), where θ is the scattering angle). The SAXSpeak is caused by the periodicity in an alternativestructure of crystalline and amorphous layers that isusually observed in crystalline polymers. At the earlystage of the crystallization, the intensity Iq2 increasesslightly with increasing the storage time. Then, the peakintensity begins to increase remarkably at ca. 1500 s.The early and late processes can be attributed to theinduction period of crystallization and the growth ofspherulite structure, respectively. The peaks of theWAXS profile are assigned to be the crystallographicreflections of the plane indexed (110), (111), and (200)in the crystalline structure of PCL.

To evaluate the structural change in the crystalliza-tion process, the dependence of the intensity of SAXSand WAXS at the crystallization temperature (Tc) of 47and 50 °C should be discussed. Figure 3 shows the timeevolution of the normalized intensity to that at the longcrystallization time. The intensity of SAXS is obtainedby integrating the value of Iq2 in the range of 0.15 nm-1

< q < 0.8 nm-1, whereas the peak area of the reflection(110) is taken as the intensity of WAXS. In the presentstudy, we can see that the intensities of SAXS andWAXS increased in the same manner. The time depen-dence of the intensities of SAXS and WAXS coincidewith each other. These results suggested that crystallinestructure and the higher-order structure such as alter-native lamella one were formed simultaneously. In thisstudy, no peak at low q in SAXS due to spinodaldecomposition9,10 observed in the induction period.

Molecular Mobility by ESR Measurement. Figure4 shows the time dependence of ESR spectra of the spin-labels bonded to the PCL chain end, during the crystal-lization process at 47 °C. The main triplet spectrum dueto hyperfine coupling caused by the nitrogen nucleusnarrows with an increase in mobility of the radicalsbecause of motional averaging of the anisotropic inter-action between an electron and a nucleus. A slightchange of the spectrum with the storage time is de-tected. For instance, the relative intensity ratios of threelines change with the time. It is possible to evaluatethe relaxation rate of the polymer chain by the analysesof the anisotropic spectra. The rotational correlationtime τc of nitroxide spin-label which undergoes a fastmotion (10-11 < τc < 10-9) was estimated by theprocedures of Freed et al.,11,12 in which the anisotropicrotational motion is taken into account. The line widthof ESR spectra, ∆Hmsl, is expressed as follows

Figure 2. Simultaneous SAXS and WAXS profiles duringisothermal crystallization at 50 °C.

Figure 3. Normalized SAXS (4, O) and WAXS (2, b)intensities as a function of the logarithm of crystallization timeat 47 °C (4, 2) and 50 °C (O, b). The scattering intensity at along time is taken to be 1.0.

Figure 4. Time dependence of ESR spectra of spin-labelbonded to the PCL chain end during the isothermal crystal-lization at 47 °C.

∆Hmsl ) A + B‚M + C‚M2 (M ) -1, 0, 1) (1)

5324 Hara et al. Macromolecules, Vol. 37, No. 14, 2004

where M is the projection quantum number of 14N spin.A, B, and C are given by

where H0 is the resonance magnetic field, g0 is the gvalue at H0, â is the Bohr magneton, and h is the Planckconstant. The Ai tensor and gi tensor (i ) x, y, z) thatrefer to the coordinate system fixed to axially symmetricrotor are the hyperfine splitting and the g value,respectively. The value of A, B, and C are obtained fromeq 6, derived from eq 1:

Here IM (M ) -1, 0, 1) are the peak height as indicatedin Figure 4. τ0 and τ2 are expressed as the followingequations:

D| and D⊥ are rotational diffusion coefficients about themajor and minor axes of axially symmetric rotor,respectively. The major axis was taken to be parallel tothe X-axis of the coordinate fixed to the nitroxide radicalwhose definition was given elsewhere.13 τc is the averagerotational correlation time of the anisotropic rotationalmotion. It is possible to evaluate the mobility of polymerby calculating τc in the crystallization process.

Figure 5 shows the time evolution of τc. To clarify thecorrelation between the molecular mobility and struc-tural changes in the crystallization process, the timeevolution of the normalized SAXS intensity is alsodepicted in Figure 5. τc increased with an increase inthe crystallization time; that is, the mobility of polymer

chain decreased in the crystallization process. Moreover,the remarkable change of the increase of τc agrees withthe increase of the normalized SAXS intensity. There-fore, these changes of τc in the crystallization processare originated from the structural change of PCL.However, at the early stage of the duration time shorterthan ca. 700 s at 50 °C, no change of the normalizedSAXS intensity from the alternative lamella structureis observed, whereas the correlation time τc increasesgradually with increasing crystallization time. At theearly stage of the crystallization process, the mobilityof the polymer chain decreased even if the crystallinestructure was not definitely formed. When the normal-ized SAXS intensity started to be detected (the crystal-line structure appeared), τc increased remarkably. Andthen, the normalized SAXS intensity increased gradu-ally to an asymptotic value with the increase in τc.Finally, no increase of the SAXS intensity was detected,but τc continued to increase gradually.

The nitroxide radicals of large size, labeled at thepolymer chain end, should be trapped in the amorphousregion. The remarkable decrease in the mobility (theincrease in τc) with the change of the SAXS intensity iscaused by the growth of the crystalline. For instance,the polymer segments in the amorphous layer betweenthe crystalline layers should be suppressed by theimmobile segments in the crystalline. The alternativestructure of the crystalline and amorphous layers startsto generate, and then the amorphous chains are con-siderably restricted and the correlation time, τc, in-creases with an increase in crystallinity. At the finalstage in the crystallization process, each spherulitecollided and the crystallization rate has considerablydecreased, where no structural change was observedclearly with the measurements of X-ray scatterings. On

A (gauss) ) 2π15x3

‚g0âh

‚H02{(Fg

0)2‚τ0 + 2(Fg2)2‚τ2} +

M(M + 1) π5x3

‚g0âh

{(FA0)2‚τ0 + 2(FA

2)2‚τ2} (2)

B (gauss) ) 8π15x3

‚g0âh

‚H0{Fg0FA

0 ‚τ0 + 2Fg2FA

2‚τ2} (3)

C (gauss) ) 2π6x3

‚g0âh

{(FA0)2‚τ0 + 2(FA

2)2‚τ2} (4)

FA0 ) (2/3)1/2{Az - 1/2(Ax - Ay)}

Fg0 ) (2/3)1/2{gz - 1/2(gx - gy)}

FA2 ) 1/2(Ax - Ay)

Fg2 ) 1/2(gx - gy) (5)

(∆Hmsl(M))2IM ) const

B ) 12(xI0

I1- x I0

I-1)∆Hmsl(0)

C ) 12(xI0

I1+ x I0

I-1- 2)∆Hmsl(0) (6)

1τm

) 6D⊥ + m2(D| - D⊥) (m ) 0, 2)

τ| ) 16D|

, τ⊥ ) 16D⊥

, τc ) xτ|τ⊥ (7)

Figure 5. Normalized SAXS intensity (solid line) and thecorrelation time τc (closed circle) plotted against the crystallization time at (a) 47 °C and (b) 50 °C.

Macromolecules, Vol. 37, No. 14, 2004 Crystallization of Poly(ε-caprolactone) 5325

the other hand, the mobility of polymer chain in theamorphous layer is also strongly influenced by the lategrowth of crystalline. Consequently, the τc detected bythe ESR measurements continues to increase with thecrystallization time.

At the early stage in the crystallization process,various structural changes are generated, for example,spinodal decomposition, the conformational change, andthe nucleation. Strobl proposed the sequential growthof the crystallites at the early stage.6 It is consideredthat these structural change influences the mobility ofthe polymer chain. Therefore, the increase in τc at theearly stage in the crystallization process is originatedfrom the structural change such as the self-aggregation,but the detail is open to the further study. One key maybe opened by studying the change of the molecularmobility at higher crystallization temperature where theinduction period is more longer.

Conclusions

In the present study, to clarify the molecular mobilitycorrelated with the structural change in the crystalliza-tion process was carried out by using time-resolvedSAXS, WAXS, and ESR measurements on PCL. It wasfound that the correlation time τc increases with thegrowth of crystalline. When the intensity of X-rayscattering began to increase with the generation of awell definite structure, the correlation time τc increasedsimultaneously and remarkably. Furthermore, at theinduction period in the crystallization process, the τcincreased gradually. This result indicates the existenceof the precursory aggregation of the polymer chain inthe crystallization process.

Acknowledgment. We thank Prof. S. Ueno(Hiroshima University Japan), Prof. H. Takahashi(Gunma University Japan), and Prof. S. Sakurai (KyotoInstitute of Technology) for their help in measurementsat PF BL-9C (KEK).This study was performed underthe approval of the Photon Factory Program AdvisoryCommittee (Proposal No. 2001G275 & 2003G275). Thefinancial support of a part of this study is by a grantfrom the NITECH 21st Century COE Program “WorldCeramics Center for Environmental Harmony”.

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