Polymer International Polym Int 49:623±626 (2000)

Novel FTIR method for determining thecrystallinity of poly(eeeeee-caprolactone)Yong He and Yoshio Inoue*Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8501, Japan

(Rec

*Co

Yoko

E-ma

Cont

# 2

Abstract: By employing Fourier transform infrared spectroscopy (FTIR) and curve±®tting techniques,

the degree of crystallinity of poly(eeeeee-caprolactone) (PCL) aged at room temperature for 1 month was

estimated to be 49�2%. The degree of crystallinity determined by FTIR in this work is comparable

with those found by other conventional techniques. It is suggested that the FTIR procedure established

here for the crystallinity determination of PCL should also be suitable for the quantitative analysis of

solid-state morphology of polymers containing carbonyl or other functional groups.

# 2000 Society of Chemical Industry

Keywords: poly(e-caprolactone); crystallinity; Fourier transform infrared spectroscopy; FTIR

INTRODUCTION

As is well known, crystalline polymers are not fully

ordered, but are partially disordered; typically the

crystallinity is less than about 80%. Generally all the

physical and mechanical properties are strongly

affected by the crystallinity and therefore the study of

the crystalline state in polymers is of paramount

importance.1 Various techniques have been employed

to assess the relative and absolute degree of crystal-

linity, including X-ray diffraction, density methods,

thermal analysis, nuclear magnetic resonance (NMR)

and infrared (IR) spectroscopy.1±3 Each technique has

its advantages and limitations in terms of crystallinity

determination. Wide-angle X-ray scattering (WAXS)

affords direct determination of three-dimensional

order, but even for this rigorous method, there are

still problems in constructing the baseline delineating

the crystalline and amorphous scattering. Accuracy in

the determination of the crystallinity from the density

method is limited by the reliability of the density values

for completely amorphous and crystalline polymers.

Thermal analysis may be the most routine and simple

method for evaluating crystallinity, but this method

also has a disadvantage in that the melting enthalpy of

the full crystalline polymer DH° must be accurately

known. Crystallinity determination by NMR is mainly

based on associating either the relaxation times or

lineshape, found in a ®t to the experimental data, with

the crystalline and amorphous phases in the polymer.

IR spectra are sensitive to the conformation and

packing of molecular chains, and this sensitivity has

been widely exploited to characterize semicrystalline

polymers in terms of crystallinity.4 Two main methods

were employed to estimate the crystallinity through IR

eived 25 October 1999; accepted 7 February 2000)

rrespondence to: Yoshio Inoue, Department of Biomolecular Engin

hama 226-8501, Japan

il: yinoue@bio.titech.ac.jp

ract/grant sponsor: Ministry of Education, Science, Sports and Culture

000 Society of Chemical Industry. Polym Int 0959±8103/2000/$1

spectra. The ®rst is based on the evaluation of the

crystallinity using the intensity ratio of the `crystalline'

bands and the `amorphous' bands in the IR spectrum

of a given polymer.5 It should be noted that this

approach faces two problems; one is that the true

`crystalline' peaks or `amorphous' peaks are rarely

available; the other is that the intensity ratio represents

only the crystallinity index and not the absolute

crystallinity. A more recent method considers a

characteristic band of the semicrystalline polymer as

the superposition of the crystalline and the amorphous

spectral components, and assesses the crystallinity as

the fraction of the crystalline part.4,6,7 Obviously, this

method is stricter than the ®rst one. However, this

method also suffers from a disadvantage: the absorp-

tion coef®cient ratio of the crystalline and the

amorphous parts must be known to calculate the

crystalline fraction. It is dif®cult to determine the

absorption coef®cient ratio directly because no fully

crystalline polymer sample is actually available. In

many works, the absorption coef®cient ratio has been

simply assumed to be unity; thus the calculated

crystalline fraction can only regarded as the relative

crystallinity.

In this work, the absorption coef®cient ratio of the

crystalline part and the amorphous part in the carbonyl

vibration bands of PCL spectra and the absolute

crystallinity of PCL were accurately determined using

FTIR spectroscopy as an independent technique.

THEORETICAL APPROACH

For a given crystalline polymer, one or more charac-

teristic IR bands are often observed to be sensitive to

eering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku,

, Japan; contract/grant number: 11217204(1999)

7.50 623

Y He, Y Inoue

the changes in crystallinity. In practice, the integrated

intensity A of a characteristic band can be regarded as

the superposition of the crystalline part Ac and the

amorphous part Aa. According to the Beer±Lambert

law, Aa and Ac can be expressed as follows:4

Aa � bca

Z �10

ea���d� �1�

Ac � bcc

Z �10

ec���d� �2�

Here e is the absorption coef®cient, b is the thickness,

c is the concentration, n is the wavenumber, and the

subscripts a and c denote the crystalline phase and the

amorphous phase, respectively. The crystallinity Xc

can be calculated from cc and ca using the relation

Xc � cc

�cc � ca� �3�

On the basis of equations ((1)±(3)) and substituting

g �R�1

0ec���d�R�1

0ea���d�

�4�

it is possible to express Xc in terms of Ac and Aa as

follows:

Xc � Ac

�Ac � gAa� �5�

It is not dif®cult to obtain Ac and Aa using a curve-

®tting technique, so the key problem associated with

calculating Xc by eqn (5) is how to determine the

absorption coef®cient ratio g. In this work, g was

accurately measured through monitoring the changes

of Ac and Aa during isothermal crystallization from the

melt state with time-resolved FTIR spectroscopy.

From eqns (1) and (2) together with c =ca�cc, eqn

(6) can be easily derived:

Ac � bc

Z �10

ec���d� ÿ gAa �6�

All the FTIR measurements in this work were carried

out on polymer ®lms cast on silicon wafers During the

isothermal crystallization process, there was no move-

ment of the ®lms. Therefore, the number of molecules

in the IR beam remained constant, that is, bc is a

constant quantity for a given sample. Obviously, the

®rst term on the right hand side of eqn (6) is a constant

for a given sample at a given temperature. Thus, there

should be a linear relationship between Ac and Aa

during isothermal crystallization and then the ratio gcan be determined from the slope of the line.

Figure 1. FTIR spectra in the carbonyl vibration region of PCL as a function

of isothermal crystallization time. From left to right, the crystallization times

are 0, 18, 27, 36, 45, 54, 63, 80, 110, 180min (see text for details).

EXPERIMENTAL

Poly(e-caprolactone) (PCL) (Mn=5.35� 104g molÿ1,

MW/Mn=1.47; Celgreen1-PH4) was supplied by

courtesy of Daicel Chemical Co, Japan. PCL ®lms

for FTIR measurements were prepared by directly

dropping the polymer solution (about 1.5wt%) in

624

chloroform onto the surface of a silicon wafer. The

silicon wafer is transparent for the IR incident beam

and was used as the substrate. The cast ®lm was

controlled to be thin enough to ensure that the studied

absorption was within the linearity range of the

detector.

IR measurements were carried out on a Perkin

Elmer Spectra 2000 single-beam IR spectrometer

under N2 purging. All the spectra were recorded at a

resolution of 4cmÿ1 and with an accumulation of 8±32

scans. Differential scanning calorimetry (DSC) analy-

sis was performed on a Seiko DSC 220 system. The

samples were heated from ÿ100 to 150°C at a heating

rate of 20°Cminÿ1. The value of melting enthalpy

(DH) was calculated as the integral of the endothermal

peak in the DSC curve.

A curve-®tting program was used to resolve the PCL

carbonyl vibration bands into amorphous and crystal-

line fractions. This program is based on the least-

squares parameter adjustment criterion using the

Gauss±Newton iteration procedure. This ®t adjusts

the peak position, the lineshape (a Gaussian fraction

whereby zero represents a pure Lorentzian and unity

represents a pure Gaussian), and the peak width and

height in such a way that a best ®t is obtained.

RESULTS AND DISCUSSION

The FTIR spectra of the carbonyl vibration region for

PCL sample crystallizing at 48°C are shown as a

function of crystallization time in Fig 1. The PCL

sample was ®rst melted at 110°C for 10min and then

cooled in air (under N2 purging) to the crystallization

temperature of 48°C. The crystallization time zero is

the time at which the temperature reaches 48°C. With

an increase of crystallization time, three changes can

clearly be seen: at ®rst the band shifted to low

wavenumber; then the shape of the band became

asymmetric; ®nally the integrated intensity of this

band increased. These changes indicate that this band

is sensitive to the crystallinity of the polymer.

Polym Int 49:623±626 (2000)

Figure 2. Experimental and curve-®tting FTIR spectra in the carbonyl

vibration region of PCL, isothermally crystallized at 48°C for 45min.

Abbreviations: expt., amor., crys., base. and calc. denote experimental

result, amorphous part, crystalline part, baseline and calculated curve,

respectively.

Figure 3. Relationship between the integrated intensity of the amorphous

(Aa) and crystalline (Ac) bands for three PCL samples that were

isothermally crystallized at 45 (*), 48 (&) and 50°C (*). The regression

lines and values of the coef®cient R2 were: 45°C, Y =45.784ÿ1.441X,

R2=0.999; 48°C, Y =28.432ÿ1.508X, R2=0.999; 50°C,Y =44.512ÿ1.430X, R2=0.998. (a.u. is the abbreviation for arbitrary unit.).

Poly (e-caprolactone) crystallinity

A curve-®tting program was used to resolve the

carbonyl vibration region into two bands: amorphous

and crystalline. During the curve ®tting, the peak

position of the amorphous band was ®xed at

1736cmÿ1, which was determined from the spectrum

of the fully amorphous sample and was the same as

that reported in the literature,8,9 but left the peak

widths, heights and shapes of the two bands, and the

peak position of the crystalline bands, as adjustable

parameters. As an example, Fig 2 illustrates the

experimental and ®tted spectra in the carbonyl

vibration region of a PCL sample crystallized at

48°C for 45min. The excellent agreement between

the experimental and ®tted spectra indicates the

reliability of the curve-®tting technique. In this way,

quantitative data regarding the integrated intensity of

the amorphous and crystalline bands were obtained.

Figure 3 depicts the relationship between the inte-

grated intensity of the amorphous (Aa) and crystalline

(Ac) bands for three PCL samples crystallized at 45, 48

and 50°C. By a linear least-squares ®t, three lines with

slopes of ÿ1.44, ÿ1.51 and ÿ1.43 were obtained. The

values of the regression coef®cient R2 were greater

than 0.998, suggesting good linear relation between Aa

and Ac. From eqn (6) and the slopes of the three lines

in Fig 3, the ratio g was determined to be 1.46�0.03.

Then based on eqn (5), the crystallinity of the cast

®lms of PCL, after ageing at room temperature for 1

month, was measured by FTIR spectroscopy to be

49�2%, as an average of three samples.

Assuming the melting enthalpy of 100% crystalline

PCL to be either 136Jgÿ1 (ref 10) or 166Jgÿ1,11 the

crystallinity of the PCL sample used for the FTIR

study was determined by DSC to be, respectively,

either 61�2% or 50�2%. The crystallinity of PCL

has also been reported by others to be 42% by small-

angle X-ray scattering (SAXS)11 and NMR,12 and

60% by the density method.13 This suggests that the

crystallinity of PCL determined in this study by FTIR

Polym Int 49:623±626 (2000)

is comparable with values determined by other

conventional techniques.

Finally, it is worthwhile pointing out that the

procedure exploited here for crystallinity determina-

tion can be further used to investigate the crystal-

lization kinetics of PCL, and that the procedure can

also be employed for the quantitative analysis of

polymers with carbonyl or other functional groups.

CONCLUSIONS

A procedure has been established here for the

quantitative crystallinity analysis of PCL using FTIR

spectroscopy and curve-®tting techniques. Through

this procedure, the crystallinity of PCL aged at room

temperature for 1 month was estimated to be 49�2%;

this value is comparable with those estimated by other

conventional techniques. It is suggested that the

procedure established here should also be suitable

for the quantitative analysis of polymers containing

carbonyl or other functional groups.

ACKNOWLEDGEMENTS

This work was partly supported by a grant-in-aid for

scienti®c research on priority area Sustainable Bio-

degradable Plastics, no. 11217204(1999) from the

Ministry of Education, Science, Sports and Culture

(Japan).

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Polym Int 49:623±626 (2000)