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Near-surface crystallization of PET
Polymer 53 (2012) 5554e5559
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Polymer journal homepage: www.elsevier.com/locate/polymer
Near-surface crystallization of PET Kei Shinotsuka a,1, Valery N. Bliznyuk b, Hazel E. Assender a, * a b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK College of Engineering and Applied Sciences, Western Michigan University, Kalamazoo, MI 49008, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2012 Received in revised form 18 September 2012 Accepted 24 September 2012 Available online 27 September 2012
The near-surface crystallization behaviour of poly(ethylene terephthalate) films of various thickness, spin-cast from solution onto silicon substrates, has been studied. In the as-cast state, the films are amorphous, with featureless morphology. A relationship is mapped between the film thickness (in the range 3 nme700 nm), the isothermal annealing temperature (ambient to 150 C), and the crystalline morphology. Two distinct morphologies are observed which are associated with surface-specific crystallization and bulk crystallization (producing much rougher film surfaces). The surface crystallization process is associated with a top-surface region (measured to be of about 13 nm in depth) of lower glass transition temperature, where, at temperatures between the surface and bulk crystallization temperatures, only the material in the more-mobile surface region is able to reorganize to form crystals. This phenomenon is observed for films of all the thicknesses studied and it is therefore concluded that this is not specifically a thin-film phenomenon, rather a surface-specific phenomenon possible in material of any thickness. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Crystallization Polyester Surface glass transition
1. Introduction Polymer thin films and the near-surface properties of polymers are of substantial interest both in terms of the science of constrained systems, and a range of technologies involving coating, patterning, self-assembly etc. There have been extensive recent reports of crystallization processes in polymer thin films, generally reporting distinct morphologies termed ‘dendritic’, ‘seaweed’ or ‘2D spherulitic’ associated with the crystal nucleation and/or growth constrained by the two-dimensional nature of the films [1,2]. Previous research has reported crystalline morphologies in thin films of a range of semicrystalline polymers including polyethylene [3,4], polypropylene [5,6], polystyrene [7e10], poly(ethylene oxide) [11e15], poly(ethylene terephthalate) [16e19], polyamide [20,21], polylactides [22,23], poly(ε-caprolactone) [24,25], polyhydroxybutyrate [26], and poly(di-n-alkylsilanes) [27]. In considering the various morphologies during crystallization in polymer thin films, Wang et al. [28] proposed a three-layer model in which a mobile region at the polymereair interface and a constrained region at the polymeresubstrate interface will
* Corresponding author. Tel.: þ44 1865 273781; fax: þ44 1865 273789. E-mail address: [email protected] (H.E. Assender). 1 Now at Oji Paper Co. Japan. 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.048
modify the nucleation and growth behaviour and hence morphology of polymer crystals. In this study we will consider crystallization in the more-mobile surface layer region in of polymer films of thickness right up to micron dimensions and also set the phenomenon of surface crystallisation in these films in the context of measurements of the surface glass transition temperature. A surface-specific crystallization effect can be induced at the surface of a bulk polymer at certain temperatures by the constraint in chain mobility due to the difference in glass transition temperatures between the surface and the bulk. A study of the cold-crystallization of polymers in the nearsurface region is closely linked with an understanding of the glass transition (Tg) behaviour of thin films because coldcrystallization from glassy amorphous material begins once the molecules are sufficiently mobile to order into crystals, i.e. at some temperature above the local Tg. The Tg of thin films has been the focus of many recent studies using a range of techniques such as ellipsometry [29e31], local thermal analysis [29], Brillouin scattering [32e34], muons [35], and SFM measurements [36e38]. A lower Tg in PET thin films has previously been reported by Zhang [39]. The overall Tg behaviour in polymer thin films is determined by a number of factors, such as polarity of the substrate and polymer chemical structure. However, at the free polymereair interface, the Tg is thought to be depressed over some thin surface layer. In this study, we present the surface crystallization behaviour of PET spin-coated thin films as a function of the film thickness and
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
Snap-off Displacement ( m)
1.5 surface Tg
bulk T g
1.0
0.5
30
40 50 60 70 Temperature (ºC)
80
Fig. 1. Snap-off displacement measured from the SFM force-distance curve of a 66.1 nm PET film as a function of temperature. The gradient increases at TgS (around 48 C), and again at TgB (around 71 C).
annealing temperature. Characteristic surface crystallites are reported for a specific range of temperature at all film thicknesses, which appears to be associated with the depression in the surface glass transition temperature (TgS) 2. Experimental methods PET pellets (density ¼ 1.375 g/cm3, melting point 250e255 C) were purchased from Sigma Aldrich. Their viscosity average molecular weight was measured to be 26,000 g/mol. To prepare
a
thin films, the PET was dissolved in a mixed solvent of 70 wt% 2chlorophenol (Fisher Scientific) and 30 wt% 1,1,1,3,3,3-hexafluoro2-propanol (Sigma Aldrich). Solutions were filtrated with PTFE filters of 0.2 mm pore size. Amorphous polymer films were formed on single crystal silicon substrates (orientation (100) plane) by spin coating at 3000 rpm. By varying solution concentration, film thicknesses were controlled in the range 3 nme700 nm, measured by means of an ellipsometer (Rudolph Research/Auto EL) and AFM. SFM force-distance curve measurements of PET were carried out on an in-situ heating stage mounted on an Autoprobe CP microscope (Park Scientific Instruments) with V-shaped cantilevers (Thermo microscopes ‘Ultralever’, effective nominal tip radius of 10 nm, spring constant ranging from 1.1 to 1.6 N/m) with a contact force of 0.24 nN and contact frequency of 0.7 Hz. One point in graph used to determine the Tg is the average of 30 data taken from different parts of the film surface. The morphology and RMS observations of the surface were carried out by AFM operated in contact mode with a scan rate of 0.5e0.7 Hz. The annealing was performed with stepwise increases in temperature with samples kept at each increasingly elevated temperature for 2 h before the morphology was recorded. The bulk Tg of PET as-received pellets was measured by differential scanning calorimetry (Perkin Elmer DSC-7) at a heating rate of 5 C/min in an argon gas atmosphere. Infrared spectra of PET films on a Si wafer were measured on a PerkineElmer Spectrum 2000 Explorer Spectrometer using a multipurpose attenuated total reflection accessory (The SeagullÔ) purchased from Harrick Scientific Corporation. The measurements were made at 65 angle of incidence with p-polarised light.
b
75
d
70 65
Thickness of Surface Layer (nm)
c
TgTotal by Ellipsometry Tg (d) by eq.1
60 55 50
c
d-c Thermal Expansion
Tg of the total thickness
Bulk Tg = 71.1(ºC)
45
5555
αr
αi αg
Surface Tg = 48.1(ºC) 0
100 200 300 400 500 600 700 Film Thickness (nm)
50
TgS
TgTotal TgB
Temperature
40 30
Average thickness of surface layer = 13.6 nm
20 10 0
0
200 400 600 Film Thickness (nm)
Fig. 2. (a) Tg of the total thickness of the PET films obtained by ellipsometry with an in-situ heater. The TgB, as measured by DSC, TgS, as measured by SFM, and a curve fit to Equation (1) are indicated. (b) Schematic illustration of how the thickness of the surface layer, c, can be calculated from the ellipsometry data, using Equation (2), once TgS and TgB are known. (c) Thickness of the surface layer of PET films calculated by Equation (2). The average thickness of surface layer is determined to be a constant value of 13.6 nm.
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a d Tg ðdÞ ¼ 1 TgB d
(1)
where Tg(d) is Tg of film thickness d, a and d are the characteristic length and exponent, which were found to be 0.89 nm and 1.0 respectively for these samples. The thermal expansion coefficient of the material at temperatures between the surface and bulk Tg (ai), as well as that for the bulk glass (ag) and the bulk rubber (ar) can be extracted from the ellipsometry data because we know both the TgB and TgS values from independent measurements. The equation:
dai ¼ ðd cÞag þ car Fig. 3. AFM images of 680 nm thick PET films. (a) As-cast, with RMS roughness 1.8 nm, (b) after annealing for 2 h at 95 C, with RMS roughness 29.9 nm.
3. Results and discussion TgS of PET films of various thicknesses was determined by measurement of the SFM force-distance interaction [38]. This allowed measurements to be carried out even in films which had started to crystallize, as the measurement could be highly localized in the regions between crystallites. In this study, the increase in the snap-off displacement of the tip (the difference in tip height between the point at which contact is first made during approach of the tip and the point at which contact is lost on tip retraction) was used to determine the TgS. Fig. 1 is a typical example of a plot of the snap-off displacement as a function of temperature. TgS is the first temperature at which there is a distinct change in gradient in linear fits of the slope. In the thicker films, a second change in gradient is seen at the bulk glass transition temperature (TgB), which corresponds well with that observed by DSC. Due to the limited volume of rubbery material inside the surface layer, the change in slope at TgS is relatively small, whereas there is a greater change in slope at TgB at which all the material becomes rubbery. Taking the average of 30 measurements for each film thickness, PET showed a constant value of TgS, (48.1 0.5) C, independent of the film thickness over all the films measured (covering the range 10 nme680 nm). DSC measurements of the same material gave a value of TgB of 72.6 C. Measurements of the Tg through the whole film thickness were made by monitoring the change in thermal expansion of the film with ellipsometry. Fig. 2(a) shows the Tg (determined as the intersection of linear fits to the high and low temperature thermal expansion) as a function of film thickness, as measured. Tg of the total thickness varies between TgB and TgS depending on the changing ratio of the surface and bulk layer as the film thickness varies, and the data give a good fit to the equation [31]:
(2)
can be used to calculate the thickness of the surface layer, c, as illustrated in Fig. 2(b). Fig. 2(c) shows the measured values for c as a function of film thickness. This clearly indicates that a surface layer of lower Tg, of thickness about 13 nm, occurs on the polymer whatever its thickness, even for ‘thick’ films of over half a micron. The crystallization behaviour of the thin films was first examined upon heating at temperatures substantially above TgB, at a temperature above which crystallization can occur in the bulk of the film. AFM images indicate that the as-deposited spin-cast films are very flat and featureless (Fig. 3(a)). After annealing, the surface becomes very much rougher with traces of an underlying spherulitic morphology (Fig. 3(b)). The amorphous and crystalline natures of the respective films were confirmed with ATR-FTIR spectroscopy by following the change in the intensity of the 1340 cm1 trans and 1370 cm1 gauche bands in the spectrum of the polymer [40e42]. The spectrum of the unannealed PET film is typical for amorphous PET: the weakly absorbing band at 1370 cm1 (gauche) is well defined, whereas the trans absorption at 1340 cm1 is low in intensity and appears more like a shoulder of the strong CeO vibration next to it. After annealing, the 1370 cm1 band appears much weaker, in contrast to the now fully developed trans conformer absorption peak. The IR measurement covers a depth of around one micron, i.e. the whole depth of the film. After annealing at slightly lower temperatures, the morphology observed in the samples is characteristic thin-film crystals and similar to that previously observed for PET [16], which has previously been confirmed as crystallites by electron diffraction [17] and GIXRD [18]. Fig. 4 shows an example of the progression in morphology of the sample as the annealing temperature is increased, in this case for films of thickness 28 nm. After annealing at 75 C the as-cast smooth, featureless morphology is interrupted by the nucleation of dendritic structures fanning out from a number of points (Fig. 4(a)). This morphology is characteristic of that observed on thin-film constrained systems, and we postulate that they are associated with crystallization of the near-surface region of the polymer which has a lower Tg, while the underlying bulk of
Fig. 4. AFM micrographs of annealed 28 nm thick films. (a) Annealed at 60 C, RMS roughness 2.2 nm, (b) annealed at 75 C, RMS roughness 4.7 nm, and (c) annealed at 150 C, RMS roughness 12.7 nm.
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
material is, at these temperatures, not yet able to crystallize. The sample is of course not completely pure, monodisperse, PET and so there is a danger that these effects are contributed to by surface segregation of low molecular weight material and/or impurities. The consistency of observed lower surface Tg observed previously on other monodisperse polymers [31e38], and the crystalline morphologies associated with surface crystalline systems in PET and other polymers, gives us confidence that the effects observed are surface-driven. After the film has been subsequently annealed at 85 C (Fig. 5(b)) the dendritic features have grown to impinge upon one another and to fill the whole surface. Annealing at a still higher temperature results in the break-down of this morphology, and an associated increase in roughness of the films to produce a surface topography more reminiscent of the bulk crystallized films, for example shown in Fig. 3(b). It cannot be determined from these images whether the surface crystals contribute to the nucleation of the bulk crystallization, or whether the bulk material crystallizes completely independently of the surface morphology. A similar break-down in the lamellar morphology after annealing at higher temperatures was observed in PEO thin films that had been annealed just below the melting temperature [11]. At these, higher, temperatures, the material throughout the thickness of the PET film has become sufficiently mobile to crystallize, and a bulk crystallization morphology results. A similar progress of morphology is seen for all film thicknesses. The surface roughness as measured by AFM correlates with the different morphologies for all film thicknesses (Fig. 5). In all cases the initial amorphous surface has a roughness of less than 3 nm. After surface crystallization starts, the RMS roughness lies in the range 5e7 nm. The onset of bulk crystallization increases the surface roughness in all the thicknesses; the thicker the film, the greater the roughness. A characteristic surface crystalline morphology, after annealing at an appropriate temperature, is seen for all film thicknesses, although the width of the crystalline lamellae increases with film thickness (Fig. 6). Interestingly, the reverse trend was seen with lamellar thickness in isotactic polystyrene crystals in films of 17 nm and thinner [9]. The dimensionality of the crystallization was considered with Avrami analysis of the nucleation and growth of crystals. Growth rates were measured at various film thicknesses at a temperature of 80 C by repeated AFM imaging of the crystals in the same area during in-situ heating of the sample over times up to 400 s. The
Thickness (nm) bulk crystalline 680 212 97 30 66 39 27 20 17 9 surface crystalline 10 amorphous
Fig. 6. AFM images of surface crystallites in (a) 10 nm, (b) 39 nm, and (c) 212 nm thick films annealed at 80 C show how the thickness of the dendritic branches of surface crystallites increases with film thickness. (d) Both direct measurement of AFM line profiles (width of lightedark repeat distances) and Fourier transform analysis of the images suggest a linear increase in arm thickness with film thickness.
focus was on the thinner film thicknesses as in the thicker films the thicker dendritic width meant that the surface crystallite edges were more difficult to define. The analysis is based on the Avrami equation [43], which describes the relationship between crystallinity and crystallization time:
Xt ¼ 1 expðkt n Þ XN
4.0
50 70 90 110 130 150
Annealing Temperature (ºC) Fig. 5. The development of RMS roughness, measured over a 5 mm 5 mm area on three micrographs, under stepwise annealing. The boxes indicated the different morphologies, as observed by AFM (see Fig. 8).
Crystallite morphology 3.5 Avrami Exponent, n
RMS Roughness (nm)
10 30
(3)
where Xt and XN are the mass fraction of crystallized material at time t and at the end of the crystallization respectively, k is a rate constant, and n the Avrami exponent which can be related directly to the nucleation (sporadic or predetermined) and growth dimensions of the crystal. In this case, the ratio Xt/XN is assumed to
40
0
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3.0 2.5 2.0
10 nm 17 nm 39 nm
1.5 1.0 0
90 180 270 Angle of growth front (º)
360
Fig. 7. Avrami exponent, calculated from AFM analysis of growth rate, at range of film thicknesses plotted to illustrate how the dimensionality (n) of the crystals increases as the range of the direction of growth increases.
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160 150 140 130 120 110
Temperature (ºC)
100 90 80 70 60 50 40 30 20 10 0 0
100
200
300
400
500
600
700
Film Thickness (nm) Fig. 8. Surface morphology of PET thin films that have been heated up stepwise and kept at each elevated temperature for 2 h. The surface features are categorized as amorphous (circles), surface crystalline (diamonds), and bulk crystalline (squares, with triangles illustrating points where there is some residual surface crystalline morphology still evident after the onset of bulk crystallization).
be the areal fraction of the observed surface with the crystalline morphology. The Avrami exponent is found to vary between 1.5 and 2.5 increasing with time during the growth. When this is matched with the morphology observed, it is clear that following the initial ‘nearly closed fans’ of crystal lamellae growing from the nucleus along a small range of angles, the fans ‘open out’ over time, creating
a larger angle of growth front (examples of these different stages may be found in Figs. 4a, b and 6). Hence the dimensionality of the crystals moves from near-one dimensional to two-dimensional depending upon this angle of growth front. This is found to be the case for all the three film thicknesses measured, with the thicknesses following the same curve on a plot of exponent against angle of growth (Fig. 7). Therefore we can make the generalisation that the growth starts as near-one dimensional from the nucleus and becomes two-dimensional, constrained to mobile near-surface region at these temperatures. Fig. 8 summarises the range of temperatures over which the different morphologies are seen for films of various thickness, and Fig. 9 illustrates schematically the proposed crystallization processes. Starting from the as-cast amorphous layers (Fig. 9(a)), as the temperature is raised, the near-surface region becomes mobile above TgS (48 C) and by 70 C the surface crystalline morphology can be seen (Fig. 9(b)). The first bulk crystalline morphology is observed at 85 C for the thicker films once the bulk of the film becomes sufficiently mobile (Fig. 9(c)). The difference between the surface and the bulk crystallization temperature, 15 C, observed here reflects the difference in temperature between TgS and TgB (24 C) measured previously. The slightly smaller difference between the crystallization temperatures perhaps reflects the more constrained nucleation and growth required in the surface layer, slightly increasing the gap between the measured Tg and the first observed crystallization in this case. The onset of surface crystallization happens to occur close to (but just beneath) the bulk Tg. This is likely coincidental, although it would be interesting to observe whether such a correlation is observed in other systems.
Fig. 9. Schematic representation of the cross section of the polymer film on the substrate illustrating the surface and bulk crystallization processes. (a) The as-cast film is amorphous, but can be thought of as three layers where the top and bottom layers are influenced by the proximity of the free surface and the substrate respectively. (b) At temperatures in which the surface region contains mobile molecules, but the bulk region does not, a confined region near the surface is able to crystallize with a dendritic morphology. (c) Above a temperature at which bulk crystallization is possible, this subsumes the surface crystalline morphology. (d) If the films are sufficiently thin that no bulk region exists (e.g. 3 nm) only dendritic morphologies are observed at any temperature.
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
For very thin films (in this case of thickness less than about 70 nm), the replacement of the surface crystalline morphology with that of the bulk crystals is delayed to higher temperatures as the volume of bulk material becomes small. At extremely low thickness (3 nm) even the onset of surface crystallization appears to be delayed to higher temperatures, presumably due the influence of the underlying substrate (Fig. 9(d)). Above 70 nm of film thickness the range of temperature over which the surface crystalline morphology is seen remains constant even to films of thickness in excess of half a micron (illustrated by a ‘green corridor’ of diamonds in Fig. 8). This range reflects the difference in Tg between the surface and bulk, and suggests that this surface crystallization phenomenon is a general one and not a purely thinfilm phenomenon. 4. Conclusions All thicknesses of the PET films have a surface region of lower Tg, of thickness approximately 13 nm. Consequently there is a nearsurface region that has a lower crystallization temperature than the bulk, so if the sample is annealed at temperatures between the surface and bulk crystallization temperatures, a surface-specific crystallization can occur with a morphology characteristic of crystallization in a constrained 2-dimensional layer at the surface of films of any thickness. The 2-dimensional nature of the crystals was confirmed with Avrami analysis. At temperatures above which bulk crystallization is possible, the typical bulk spherulitic morphology is observed leading to a rougher surface and hence obscuring, or recrystallizing any previously induced surface crystals. As the overall film thickness decreases to become comparable with the thickness of the surface region (plus any second modified region close to the substrate that may exist), the bulk region of the film becomes small and eventually is extinguished. This reduction in thickness of the ‘bulk’ layer is manifested first in an increase in the temperature at which the bulk crystallization is observed to overcome the surface crystal morphology, until for very thin films no such bulk crystallization is observed. This observation with PET is consistent with the thin-film crystallization observations that have been made previously on a range of polymers, which have not previously been discussed in the context of a low Tg region of the polymer in films of all thickness. Acknowledgements The authors acknowledge funding from the Toppan Printing Company, Japan, and the Oji Paper Company, Japan in support of this work.
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Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Near-surface crystallization of PET Kei Shinotsuka a,1, Valery N. Bliznyuk b, Hazel E. Assender a, * a b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK College of Engineering and Applied Sciences, Western Michigan University, Kalamazoo, MI 49008, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2012 Received in revised form 18 September 2012 Accepted 24 September 2012 Available online 27 September 2012
The near-surface crystallization behaviour of poly(ethylene terephthalate) films of various thickness, spin-cast from solution onto silicon substrates, has been studied. In the as-cast state, the films are amorphous, with featureless morphology. A relationship is mapped between the film thickness (in the range 3 nme700 nm), the isothermal annealing temperature (ambient to 150 C), and the crystalline morphology. Two distinct morphologies are observed which are associated with surface-specific crystallization and bulk crystallization (producing much rougher film surfaces). The surface crystallization process is associated with a top-surface region (measured to be of about 13 nm in depth) of lower glass transition temperature, where, at temperatures between the surface and bulk crystallization temperatures, only the material in the more-mobile surface region is able to reorganize to form crystals. This phenomenon is observed for films of all the thicknesses studied and it is therefore concluded that this is not specifically a thin-film phenomenon, rather a surface-specific phenomenon possible in material of any thickness. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Crystallization Polyester Surface glass transition
1. Introduction Polymer thin films and the near-surface properties of polymers are of substantial interest both in terms of the science of constrained systems, and a range of technologies involving coating, patterning, self-assembly etc. There have been extensive recent reports of crystallization processes in polymer thin films, generally reporting distinct morphologies termed ‘dendritic’, ‘seaweed’ or ‘2D spherulitic’ associated with the crystal nucleation and/or growth constrained by the two-dimensional nature of the films [1,2]. Previous research has reported crystalline morphologies in thin films of a range of semicrystalline polymers including polyethylene [3,4], polypropylene [5,6], polystyrene [7e10], poly(ethylene oxide) [11e15], poly(ethylene terephthalate) [16e19], polyamide [20,21], polylactides [22,23], poly(ε-caprolactone) [24,25], polyhydroxybutyrate [26], and poly(di-n-alkylsilanes) [27]. In considering the various morphologies during crystallization in polymer thin films, Wang et al. [28] proposed a three-layer model in which a mobile region at the polymereair interface and a constrained region at the polymeresubstrate interface will
* Corresponding author. Tel.: þ44 1865 273781; fax: þ44 1865 273789. E-mail address: [email protected] (H.E. Assender). 1 Now at Oji Paper Co. Japan. 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.048
modify the nucleation and growth behaviour and hence morphology of polymer crystals. In this study we will consider crystallization in the more-mobile surface layer region in of polymer films of thickness right up to micron dimensions and also set the phenomenon of surface crystallisation in these films in the context of measurements of the surface glass transition temperature. A surface-specific crystallization effect can be induced at the surface of a bulk polymer at certain temperatures by the constraint in chain mobility due to the difference in glass transition temperatures between the surface and the bulk. A study of the cold-crystallization of polymers in the nearsurface region is closely linked with an understanding of the glass transition (Tg) behaviour of thin films because coldcrystallization from glassy amorphous material begins once the molecules are sufficiently mobile to order into crystals, i.e. at some temperature above the local Tg. The Tg of thin films has been the focus of many recent studies using a range of techniques such as ellipsometry [29e31], local thermal analysis [29], Brillouin scattering [32e34], muons [35], and SFM measurements [36e38]. A lower Tg in PET thin films has previously been reported by Zhang [39]. The overall Tg behaviour in polymer thin films is determined by a number of factors, such as polarity of the substrate and polymer chemical structure. However, at the free polymereair interface, the Tg is thought to be depressed over some thin surface layer. In this study, we present the surface crystallization behaviour of PET spin-coated thin films as a function of the film thickness and
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
Snap-off Displacement ( m)
1.5 surface Tg
bulk T g
1.0
0.5
30
40 50 60 70 Temperature (ºC)
80
Fig. 1. Snap-off displacement measured from the SFM force-distance curve of a 66.1 nm PET film as a function of temperature. The gradient increases at TgS (around 48 C), and again at TgB (around 71 C).
annealing temperature. Characteristic surface crystallites are reported for a specific range of temperature at all film thicknesses, which appears to be associated with the depression in the surface glass transition temperature (TgS) 2. Experimental methods PET pellets (density ¼ 1.375 g/cm3, melting point 250e255 C) were purchased from Sigma Aldrich. Their viscosity average molecular weight was measured to be 26,000 g/mol. To prepare
a
thin films, the PET was dissolved in a mixed solvent of 70 wt% 2chlorophenol (Fisher Scientific) and 30 wt% 1,1,1,3,3,3-hexafluoro2-propanol (Sigma Aldrich). Solutions were filtrated with PTFE filters of 0.2 mm pore size. Amorphous polymer films were formed on single crystal silicon substrates (orientation (100) plane) by spin coating at 3000 rpm. By varying solution concentration, film thicknesses were controlled in the range 3 nme700 nm, measured by means of an ellipsometer (Rudolph Research/Auto EL) and AFM. SFM force-distance curve measurements of PET were carried out on an in-situ heating stage mounted on an Autoprobe CP microscope (Park Scientific Instruments) with V-shaped cantilevers (Thermo microscopes ‘Ultralever’, effective nominal tip radius of 10 nm, spring constant ranging from 1.1 to 1.6 N/m) with a contact force of 0.24 nN and contact frequency of 0.7 Hz. One point in graph used to determine the Tg is the average of 30 data taken from different parts of the film surface. The morphology and RMS observations of the surface were carried out by AFM operated in contact mode with a scan rate of 0.5e0.7 Hz. The annealing was performed with stepwise increases in temperature with samples kept at each increasingly elevated temperature for 2 h before the morphology was recorded. The bulk Tg of PET as-received pellets was measured by differential scanning calorimetry (Perkin Elmer DSC-7) at a heating rate of 5 C/min in an argon gas atmosphere. Infrared spectra of PET films on a Si wafer were measured on a PerkineElmer Spectrum 2000 Explorer Spectrometer using a multipurpose attenuated total reflection accessory (The SeagullÔ) purchased from Harrick Scientific Corporation. The measurements were made at 65 angle of incidence with p-polarised light.
b
75
d
70 65
Thickness of Surface Layer (nm)
c
TgTotal by Ellipsometry Tg (d) by eq.1
60 55 50
c
d-c Thermal Expansion
Tg of the total thickness
Bulk Tg = 71.1(ºC)
45
5555
αr
αi αg
Surface Tg = 48.1(ºC) 0
100 200 300 400 500 600 700 Film Thickness (nm)
50
TgS
TgTotal TgB
Temperature
40 30
Average thickness of surface layer = 13.6 nm
20 10 0
0
200 400 600 Film Thickness (nm)
Fig. 2. (a) Tg of the total thickness of the PET films obtained by ellipsometry with an in-situ heater. The TgB, as measured by DSC, TgS, as measured by SFM, and a curve fit to Equation (1) are indicated. (b) Schematic illustration of how the thickness of the surface layer, c, can be calculated from the ellipsometry data, using Equation (2), once TgS and TgB are known. (c) Thickness of the surface layer of PET films calculated by Equation (2). The average thickness of surface layer is determined to be a constant value of 13.6 nm.
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a d Tg ðdÞ ¼ 1 TgB d
(1)
where Tg(d) is Tg of film thickness d, a and d are the characteristic length and exponent, which were found to be 0.89 nm and 1.0 respectively for these samples. The thermal expansion coefficient of the material at temperatures between the surface and bulk Tg (ai), as well as that for the bulk glass (ag) and the bulk rubber (ar) can be extracted from the ellipsometry data because we know both the TgB and TgS values from independent measurements. The equation:
dai ¼ ðd cÞag þ car Fig. 3. AFM images of 680 nm thick PET films. (a) As-cast, with RMS roughness 1.8 nm, (b) after annealing for 2 h at 95 C, with RMS roughness 29.9 nm.
3. Results and discussion TgS of PET films of various thicknesses was determined by measurement of the SFM force-distance interaction [38]. This allowed measurements to be carried out even in films which had started to crystallize, as the measurement could be highly localized in the regions between crystallites. In this study, the increase in the snap-off displacement of the tip (the difference in tip height between the point at which contact is first made during approach of the tip and the point at which contact is lost on tip retraction) was used to determine the TgS. Fig. 1 is a typical example of a plot of the snap-off displacement as a function of temperature. TgS is the first temperature at which there is a distinct change in gradient in linear fits of the slope. In the thicker films, a second change in gradient is seen at the bulk glass transition temperature (TgB), which corresponds well with that observed by DSC. Due to the limited volume of rubbery material inside the surface layer, the change in slope at TgS is relatively small, whereas there is a greater change in slope at TgB at which all the material becomes rubbery. Taking the average of 30 measurements for each film thickness, PET showed a constant value of TgS, (48.1 0.5) C, independent of the film thickness over all the films measured (covering the range 10 nme680 nm). DSC measurements of the same material gave a value of TgB of 72.6 C. Measurements of the Tg through the whole film thickness were made by monitoring the change in thermal expansion of the film with ellipsometry. Fig. 2(a) shows the Tg (determined as the intersection of linear fits to the high and low temperature thermal expansion) as a function of film thickness, as measured. Tg of the total thickness varies between TgB and TgS depending on the changing ratio of the surface and bulk layer as the film thickness varies, and the data give a good fit to the equation [31]:
(2)
can be used to calculate the thickness of the surface layer, c, as illustrated in Fig. 2(b). Fig. 2(c) shows the measured values for c as a function of film thickness. This clearly indicates that a surface layer of lower Tg, of thickness about 13 nm, occurs on the polymer whatever its thickness, even for ‘thick’ films of over half a micron. The crystallization behaviour of the thin films was first examined upon heating at temperatures substantially above TgB, at a temperature above which crystallization can occur in the bulk of the film. AFM images indicate that the as-deposited spin-cast films are very flat and featureless (Fig. 3(a)). After annealing, the surface becomes very much rougher with traces of an underlying spherulitic morphology (Fig. 3(b)). The amorphous and crystalline natures of the respective films were confirmed with ATR-FTIR spectroscopy by following the change in the intensity of the 1340 cm1 trans and 1370 cm1 gauche bands in the spectrum of the polymer [40e42]. The spectrum of the unannealed PET film is typical for amorphous PET: the weakly absorbing band at 1370 cm1 (gauche) is well defined, whereas the trans absorption at 1340 cm1 is low in intensity and appears more like a shoulder of the strong CeO vibration next to it. After annealing, the 1370 cm1 band appears much weaker, in contrast to the now fully developed trans conformer absorption peak. The IR measurement covers a depth of around one micron, i.e. the whole depth of the film. After annealing at slightly lower temperatures, the morphology observed in the samples is characteristic thin-film crystals and similar to that previously observed for PET [16], which has previously been confirmed as crystallites by electron diffraction [17] and GIXRD [18]. Fig. 4 shows an example of the progression in morphology of the sample as the annealing temperature is increased, in this case for films of thickness 28 nm. After annealing at 75 C the as-cast smooth, featureless morphology is interrupted by the nucleation of dendritic structures fanning out from a number of points (Fig. 4(a)). This morphology is characteristic of that observed on thin-film constrained systems, and we postulate that they are associated with crystallization of the near-surface region of the polymer which has a lower Tg, while the underlying bulk of
Fig. 4. AFM micrographs of annealed 28 nm thick films. (a) Annealed at 60 C, RMS roughness 2.2 nm, (b) annealed at 75 C, RMS roughness 4.7 nm, and (c) annealed at 150 C, RMS roughness 12.7 nm.
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
material is, at these temperatures, not yet able to crystallize. The sample is of course not completely pure, monodisperse, PET and so there is a danger that these effects are contributed to by surface segregation of low molecular weight material and/or impurities. The consistency of observed lower surface Tg observed previously on other monodisperse polymers [31e38], and the crystalline morphologies associated with surface crystalline systems in PET and other polymers, gives us confidence that the effects observed are surface-driven. After the film has been subsequently annealed at 85 C (Fig. 5(b)) the dendritic features have grown to impinge upon one another and to fill the whole surface. Annealing at a still higher temperature results in the break-down of this morphology, and an associated increase in roughness of the films to produce a surface topography more reminiscent of the bulk crystallized films, for example shown in Fig. 3(b). It cannot be determined from these images whether the surface crystals contribute to the nucleation of the bulk crystallization, or whether the bulk material crystallizes completely independently of the surface morphology. A similar break-down in the lamellar morphology after annealing at higher temperatures was observed in PEO thin films that had been annealed just below the melting temperature [11]. At these, higher, temperatures, the material throughout the thickness of the PET film has become sufficiently mobile to crystallize, and a bulk crystallization morphology results. A similar progress of morphology is seen for all film thicknesses. The surface roughness as measured by AFM correlates with the different morphologies for all film thicknesses (Fig. 5). In all cases the initial amorphous surface has a roughness of less than 3 nm. After surface crystallization starts, the RMS roughness lies in the range 5e7 nm. The onset of bulk crystallization increases the surface roughness in all the thicknesses; the thicker the film, the greater the roughness. A characteristic surface crystalline morphology, after annealing at an appropriate temperature, is seen for all film thicknesses, although the width of the crystalline lamellae increases with film thickness (Fig. 6). Interestingly, the reverse trend was seen with lamellar thickness in isotactic polystyrene crystals in films of 17 nm and thinner [9]. The dimensionality of the crystallization was considered with Avrami analysis of the nucleation and growth of crystals. Growth rates were measured at various film thicknesses at a temperature of 80 C by repeated AFM imaging of the crystals in the same area during in-situ heating of the sample over times up to 400 s. The
Thickness (nm) bulk crystalline 680 212 97 30 66 39 27 20 17 9 surface crystalline 10 amorphous
Fig. 6. AFM images of surface crystallites in (a) 10 nm, (b) 39 nm, and (c) 212 nm thick films annealed at 80 C show how the thickness of the dendritic branches of surface crystallites increases with film thickness. (d) Both direct measurement of AFM line profiles (width of lightedark repeat distances) and Fourier transform analysis of the images suggest a linear increase in arm thickness with film thickness.
focus was on the thinner film thicknesses as in the thicker films the thicker dendritic width meant that the surface crystallite edges were more difficult to define. The analysis is based on the Avrami equation [43], which describes the relationship between crystallinity and crystallization time:
Xt ¼ 1 expðkt n Þ XN
4.0
50 70 90 110 130 150
Annealing Temperature (ºC) Fig. 5. The development of RMS roughness, measured over a 5 mm 5 mm area on three micrographs, under stepwise annealing. The boxes indicated the different morphologies, as observed by AFM (see Fig. 8).
Crystallite morphology 3.5 Avrami Exponent, n
RMS Roughness (nm)
10 30
(3)
where Xt and XN are the mass fraction of crystallized material at time t and at the end of the crystallization respectively, k is a rate constant, and n the Avrami exponent which can be related directly to the nucleation (sporadic or predetermined) and growth dimensions of the crystal. In this case, the ratio Xt/XN is assumed to
40
0
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3.0 2.5 2.0
10 nm 17 nm 39 nm
1.5 1.0 0
90 180 270 Angle of growth front (º)
360
Fig. 7. Avrami exponent, calculated from AFM analysis of growth rate, at range of film thicknesses plotted to illustrate how the dimensionality (n) of the crystals increases as the range of the direction of growth increases.
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160 150 140 130 120 110
Temperature (ºC)
100 90 80 70 60 50 40 30 20 10 0 0
100
200
300
400
500
600
700
Film Thickness (nm) Fig. 8. Surface morphology of PET thin films that have been heated up stepwise and kept at each elevated temperature for 2 h. The surface features are categorized as amorphous (circles), surface crystalline (diamonds), and bulk crystalline (squares, with triangles illustrating points where there is some residual surface crystalline morphology still evident after the onset of bulk crystallization).
be the areal fraction of the observed surface with the crystalline morphology. The Avrami exponent is found to vary between 1.5 and 2.5 increasing with time during the growth. When this is matched with the morphology observed, it is clear that following the initial ‘nearly closed fans’ of crystal lamellae growing from the nucleus along a small range of angles, the fans ‘open out’ over time, creating
a larger angle of growth front (examples of these different stages may be found in Figs. 4a, b and 6). Hence the dimensionality of the crystals moves from near-one dimensional to two-dimensional depending upon this angle of growth front. This is found to be the case for all the three film thicknesses measured, with the thicknesses following the same curve on a plot of exponent against angle of growth (Fig. 7). Therefore we can make the generalisation that the growth starts as near-one dimensional from the nucleus and becomes two-dimensional, constrained to mobile near-surface region at these temperatures. Fig. 8 summarises the range of temperatures over which the different morphologies are seen for films of various thickness, and Fig. 9 illustrates schematically the proposed crystallization processes. Starting from the as-cast amorphous layers (Fig. 9(a)), as the temperature is raised, the near-surface region becomes mobile above TgS (48 C) and by 70 C the surface crystalline morphology can be seen (Fig. 9(b)). The first bulk crystalline morphology is observed at 85 C for the thicker films once the bulk of the film becomes sufficiently mobile (Fig. 9(c)). The difference between the surface and the bulk crystallization temperature, 15 C, observed here reflects the difference in temperature between TgS and TgB (24 C) measured previously. The slightly smaller difference between the crystallization temperatures perhaps reflects the more constrained nucleation and growth required in the surface layer, slightly increasing the gap between the measured Tg and the first observed crystallization in this case. The onset of surface crystallization happens to occur close to (but just beneath) the bulk Tg. This is likely coincidental, although it would be interesting to observe whether such a correlation is observed in other systems.
Fig. 9. Schematic representation of the cross section of the polymer film on the substrate illustrating the surface and bulk crystallization processes. (a) The as-cast film is amorphous, but can be thought of as three layers where the top and bottom layers are influenced by the proximity of the free surface and the substrate respectively. (b) At temperatures in which the surface region contains mobile molecules, but the bulk region does not, a confined region near the surface is able to crystallize with a dendritic morphology. (c) Above a temperature at which bulk crystallization is possible, this subsumes the surface crystalline morphology. (d) If the films are sufficiently thin that no bulk region exists (e.g. 3 nm) only dendritic morphologies are observed at any temperature.
K. Shinotsuka et al. / Polymer 53 (2012) 5554e5559
For very thin films (in this case of thickness less than about 70 nm), the replacement of the surface crystalline morphology with that of the bulk crystals is delayed to higher temperatures as the volume of bulk material becomes small. At extremely low thickness (3 nm) even the onset of surface crystallization appears to be delayed to higher temperatures, presumably due the influence of the underlying substrate (Fig. 9(d)). Above 70 nm of film thickness the range of temperature over which the surface crystalline morphology is seen remains constant even to films of thickness in excess of half a micron (illustrated by a ‘green corridor’ of diamonds in Fig. 8). This range reflects the difference in Tg between the surface and bulk, and suggests that this surface crystallization phenomenon is a general one and not a purely thinfilm phenomenon. 4. Conclusions All thicknesses of the PET films have a surface region of lower Tg, of thickness approximately 13 nm. Consequently there is a nearsurface region that has a lower crystallization temperature than the bulk, so if the sample is annealed at temperatures between the surface and bulk crystallization temperatures, a surface-specific crystallization can occur with a morphology characteristic of crystallization in a constrained 2-dimensional layer at the surface of films of any thickness. The 2-dimensional nature of the crystals was confirmed with Avrami analysis. At temperatures above which bulk crystallization is possible, the typical bulk spherulitic morphology is observed leading to a rougher surface and hence obscuring, or recrystallizing any previously induced surface crystals. As the overall film thickness decreases to become comparable with the thickness of the surface region (plus any second modified region close to the substrate that may exist), the bulk region of the film becomes small and eventually is extinguished. This reduction in thickness of the ‘bulk’ layer is manifested first in an increase in the temperature at which the bulk crystallization is observed to overcome the surface crystal morphology, until for very thin films no such bulk crystallization is observed. This observation with PET is consistent with the thin-film crystallization observations that have been made previously on a range of polymers, which have not previously been discussed in the context of a low Tg region of the polymer in films of all thickness. Acknowledgements The authors acknowledge funding from the Toppan Printing Company, Japan, and the Oji Paper Company, Japan in support of this work.
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