Thickness Dependence of the Melting Temperature of

Thin Polymer Films

Jae Hyun Kim,1 Jyongsik Jang,* 1 Wang-Cheol Zin2

1 School of Chemical Engineering and Hyperstructured Organic Materials Research Center, Seoul National University,Shinlimdong 56-1, Seoul 151-742, KoreaFax: +82-2-888-1604; E-mail: jsjang@plaza.snu.ac.kr

2 Department of Material Science & Engineering and Center for Advanced Functional Polymers, Pohang University ofScience & Technology, Pohang, Kyungbuk 790-784, Korea

Introduction

The glass transition temperature (Tg) of thin amorphous

polymer films is important in science and in technology

and many studies have been done on this property.[1 – 11]

When the interaction between the polymer and the sub-

strate is not large enough to affect the glass transition, Tg

usually decreases with a decreasing film thickness.[1, 9 – 11]

An increase of Tg in thin polymer films was also found for

the cases where there are specific interactions between the

polymer and the surface of the substrate.[2, 4 – 6] According

to these results, Tg is dependent on film thickness and such

dependence increases with decreasing film thickness.

For semicrystalline polymers, the crystallinity and the

melting temperature (Tm) are perhaps the most important

parameters determining their final properties. Schultz

estimated the effect of specimen thickness on the crystal-

lization rate by modifying the Avrami equation and found

that the film thickness could affect the rate of crystalliza-

tion.[12] Despotopoulou et al. revealed that the crystalliza-

tion of thin films is substantially hindered in thinner films

(i.e., both the crystallinity and the crystallization rate

decrease dramatically as the film thickness decreases).[13]

Because the melting behavior is closely correlated with

crystallization, such differences in the crystallization phe-

nomena in thin films could affect the melting temperature

itself.

Until now, there has been no work dealing with the

effect of thickness on the melting temperature of thin

polymer films. In this study, the melting temperatures of

the semicrystalline polymer films were measured by

ellipsometry as a function of film thickness. For the semi-

crystalline polymer, poly[ethylene-co-(vinyl acetate)] (25

wt.-% vinyl acetate) was used. The factors that affect the

melting temperature in thin films will be discussed.

Communication: The melting temperature (Tm) of thinpoly[ethylene-co-(vinyl acetate)] films coated on a siliconwafer was investigated. Ellipsometry was used to measurethe Tm which was found to decrease dramatically whenthe thickness of the film is less than 300 A. The relation-ship between the lamellar thickness and the Tm wasthought to be responsible this thickness dependence of theTm in thin polymer films.

Macromol. Rapid Commun. 2001, 22, No. 6 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1336/2001/0603–0386$17.50+.50/0

The melting temperature (Tm) or crystallization temperature(Tc) vs. the thickness of the films for EVA25. The closedsquares indicates Tm and the open squares and triangles indi-cates Tc for the EVA25 thin films.

386 Macromol. Rapid Commun. 2001, 22, 386–389

Thickness Dependence of the Melting Temperature of Thin Polymer Films 387

Experimental Part

The investigated sample consists of a thin polymer filmcoated onto the clean native-oxide surface of a silicon wafer(100). Poly[ethylene-co-(vinyl acetate)] (25 wt.-% vinylacetate) denoted EVA25 (M

—w = 150 000 g N mol – 1) was pur-

chased from Aldrich. Thin films of different thickness wereprepared by spin coating toluene solutions of various concen-trations at a speed of 2 000 rpm for about 40 s on a siliconwafer. After coating, the film was kept in a desiccator atroom temperature for about 2 d to induce slow solvent eva-poration. The heating stage, devised in our laboratory, wasused in conjunction with an ellipsometer (Rudolf research;AutoEL II). The ellipsometric angles (w and d) were continu-ously monitored while the sample was heated or cooledrepeatedly at a constant rate of 18C N min – 1 between 358Cand 1158C. At 115 8C. About 3 min separated the heatingand cooling scans. Differential scanning calorimetry (DSC)was used to measure the bulk Tm for the samples and thescanning rate (18C N min1) was the same as that used withthe ellipsometric experiments.

Results and Discussion

The melting temperatures in thin films are measured

directly by ellipsometry as a function of film thickness.

Although ellipsometry is widely used for measuring Tg in

thin polymer films by detecting the volumetric change in

temperature dependence between glassy and rubbery

states,[1 – 3, 11] this is the first time Tm of thin polymer films

has been measured using this method. One of the ways to

observe the melting point is to observe changes in the

specific volume with temperature.[14] Since the melting

constitutes a first-order phase transition, a dramatic

change in volume is expected over a range of several

degrees. Figure 1 shows a typical scan of our usual ellip-

sometric experiment. The sample temperature was raised

to 1158C at a rate of 18C N min – 1 and kept at that tem-

perature (1158C) for about 3 min. The ellipsometric

angles (w) were continuously monitored while the sample

was being cooled at a constant rate of 18C N min – 1 to

358C and this experiment was repeated to confirm the

reproducibility with cooling and heating the samples. The

ellipsometric angle w is closely correlated with the physi-

cal properties of the sample such as thickness and refrac-

tive index, thus the drastic change around 708C would be

caused by crystallization for a cooling scan or melting for

a heating scan. According to the DSC of the bulk sample,

these transition temperatures around 708C correspond to

the crystallization or the melting temperature of the

EVA25. There is no difference between the first and sec-

ond cooling scans but a difference exists between the

heating and the cooling scan, as can be confirmed from

Figure 1. The abrupt decrease of w in the cooling scan is

mainly correlated with the crystallization of the sample

and the abrupt increase of w in the heating scan is closely

connected with the melting. Therefore, the point whose

rate of change of w with temperature is a maximum can

be defined as the crystallization temperature (Tc) in the

cooling scan and the melting temperature (Tm) in the heat-

ing scan. The difference between the heating scan and the

cooling scan is considered as the hysteric difference

between the melting and the crystallization behavior with

temperature, which is usually observed in bulk polymer

samples.[15] Figure 2 shows the usual ellipsometric heat-

ing scans and the differential curves for those scans. The

maximum peak point in the differential curve is regarded

as the melting temperature for the sample. The peak tem-

perature of the differential curve for the 215 A film is

depressed from the bulk value (~768C). For films below

about 200 A, it was difficult to measure the transition

temperatures since the total change in w related with the

transition is too small for each transition temperature to

be defined. Therefore, the data for these very thin films

could not be obtained. From this procedure, the thick-

ness-dependent melting temperatures and crystallization

temperatures for each sample are determined and

expressed in Figure 3. Neither Tc nor Tm change with

repetition and both show reduced values when the thick-

ness of the film is less than 300 A. Therefore, it can be

confirmed that the melting temperature of the EVA25

film is dependent on the film thickness, when the thick-

ness of the film is less than 300 A. The crystallization

temperature of the EVA25 seems to be slightly less

dependent on thickness than the melting temperature.

More experiments are needed to verify such a difference

in the degree of thickness dependence between Tc and Tm.

Figure 1. Typical ellipsometric cooling and heating scans forthe EVA25 thin film. The thickness of this film is 495 A. Theangle of incidence of radiation is 70 8 and its wavelength is6 328 A. These data points are obtained every 1 min while thesample is being cooled or heated at a rate of 1 8C N min – 1.

388 J. H. Kim, J. Jang, W.-C. Zin

Until now, there has been no general theory to predict

the thickness dependence of the melting temperatures in

thin polymer films. However, it is well known that there

is a strong dependence of the observed melting tempera-

ture of a polymer crystal, Tm, upon the crystal thickness,

l, as is described in Equation (1).[16]

Tm ¼ T0m ÿ

2ceT0m

lDHv

(1)

The equilibrium melting temperature (T0m) is the tem-

perature at which a crystal without any surfaces would

melt, ce is the fold surface energy and DHv is the enthalpy

of fusion per unit volume.[16] Equation (1) implies that Tm

will always be less than T 0m for crystals of finite size and

that Tm will decrease as l decreases for given values of

T 0m, ce and DHv. The typical lamellar thickness (l) for the

semicrystalline polymers is of the order of 100–200 A.[15]

From the viewpoint of chain packing, there is a possi-

bility of hindrance resulting from the geometric dimen-

sion as the film thickness decreases. The thinner the film,

the more difficult it is for a crystal to form, and this

would affect the lamellar thickness. As can be estimated

from Equation (1), such a decrement in the thickness of

the lamellae will induce a depression of the Tm in thin

polymer films. Frank et al. found that the crystallization

rate decreases drastically as the film thickness decreases

and crystallization is strongly hindered, particularly for

thicknesses less than 300 A.[17] Astonishingly, the crystal-

lization does not occur until the thickness approaches

150 A. This finding implies that the crystallization is

greatly influenced by the film thickness. The effect from

the thickness starts to operate on the crystallization

behavior of thin films when the thickness of the film is

below 300 A.[17] As is evident in Figure 3, the melting

temperatures also decrease drastically when the thickness

of the film is reduced below 300 A. Therefore, this thick-

ness of the order of ~300 A seems to be a critical thick-

ness for semicrystalline polymers, which causes deviation

of the crystallization and melting behaviors of thin poly-

mer films from those of the bulk. In this experiment,

regardless of sample thickness, all the crystallization con-

ditions are exactly the same. Judging from these results,

the crystallization may be hindered by the film thickness

and this hindrance could affect the lamellar thickness for

a given crystallization condition. This reduction in lamel-

lar thickness might be an origin for the thickness depen-

dence of the melting temperature in semicrystalline thin

polymer films. This study demonstrates that ellipsometry

Figure 2. Typical ellipsometric heating scan and the differen-tial curve for that scan. (a) Film thickness 330 A, (b) film thick-ness 215 A. The open square is the change in w with tempera-ture and the solid circle is the differential change of the w valuewith temperature. The vertical arrow indicates the maximumpeak temperature of the differential curve and this temperatureis regarded as Tm of the sample.

Figure 3. The melting temperature (Tm) or crystallization tem-perature (Tc) vs. the thickness of the films for EVA25. Theclosed squares indicates Tm and the open squares and trianglesindicates Tc for the EVA25 thin films.

Thickness Dependence of the Melting Temperature of Thin Polymer Films 389

can be used as a method to determine the Tm and Tc in

thin polymer films, and that the thickness of the film can

affect the Tm of the samples.

Acknowledgement: This research has been supported by theKorea Science and Engineering Foundation through the Hyper-structured Organic Materials Research Center and the authorsgratefully acknowledge the support received from this organiza-tion.

Received: June 7, 2000Revised: October 4, 2000

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