J. Phys. Chem. 1993,97, 12061-12066
12061
Infrared Depth Profiling Studies of Recrystallization in Laser-Amorphized Poly(ethy1ene terephthalate) Philip D. Richards and Eric Weitz’ Department of Chemistry, Northwestern University, Evanston, Illinois 60208
A. J. Ouderkirk’ and D. S. Dunn 3M Corp., CRPTL,St Paul, Minnesota 55144 Received: July 16, 1993’
Infrared depth profiling and time-resolved infrared spectroscopy have been used to study recrystallization in laser-amorphized films of poly(ethy1ene terephthalate) (PET). Laser amorphization of the sample film produces a situation in which both primary and secondary crystallization processes can be independently observed in the subsequent recrystallization of the film. The data are consistent with a model in which recrystallization is nucleated by the interface of laser-amorphized and untreated PET followed by an oriented primary growth process which propagates from this interface toward the surface of the film. Random secondary crystallization processes increase the degree of order in portions of the film which have undergone primary crystallization. Spectral features associated with various functional groups of PET show differing responses to primary and secondary crystallization processes and may provide insight into the nature of these processes.
I. Introduction Crystallization in polymers is a complex transformation which can involve several different processes. This transformation is often depicted as beginning with a nucleation step. Crystal nucleation has been the subject of considerable study and is fairly well understood.l.2 In contrast to this process, the further growth of crystallites following nucleation has been the subject of considerable speculation regarding the different processes which can be involved in crystal growth. Multiple growth processes have often been characterized as primary and secondary crystallization processes, where primary crystallization refers to the formation of crystallites and secondary crystallization refers to processes which increase thedegree of crystallinitywithin existing crystallites.3 In some cases, the term secondary crystallization has also been used to describe increases in crystallinity which do not fit the standard kinetic models often used to analyze experimental data.3~~ Although these concepts have been used to discuss the crystallization process, only recently have there been a few direct observations of multiple crystal growth processe~.~.~ However, there is still little insight into what types of processes are involved in secondary crystallization at the molecular level. Previous studies have shown that pulsed ultraviolet excimer lasers can amorphize the surface of a semicrystalline polymer film.8 When a short pulse of UV light is incident on a polymer its surface can be heated above its melting point, resulting in a film of amorphous polymer on a base of unmodified semicrystalline polymer. As has been seen previously, and also in the work presented here, this laser-amorphized layer provides a medium which is conducive to the study of crystallization processes in a p~lymer.~J In order to characterize these crystallization processes, it is necessary to have information regarding the initiation of recrystallization and subsequent crystal growth. We previously reported results on the kinetics of recrystallization of laser-amorphized poly(ethy1ene terephthalate) (PET).’ In that study, the degree of crystallinity in a sample undergoing recrystallization was followed in real time using time-resolved infrared spectroscopy. The data were found to fit a simple kinetic model which explicitly included primary and secondary crystal~~
Abstract published in Aduance ACS Abstracts, October 15, 1993.
0022-3654/93/2097-12061$04.00/0
lization processes. Direct observation of the primary and secondary crystallization processes could be made in these experiments due to the unique set of conditions which were generated through laser amorphization of the polymer. There are two likely mechanisms for the initiation of recrystallization in suc8 a film. In the first mechanism, recrystallization is initiated by nucleation sites available at the interface of the amorphous and semicrystalline polymer. Crystallization then proceeds from this interface toward the surface of the film. The second mechanism involves homogeneous nucleation taking place throughout the amorphous region with crystal growth originating at randomly distributed’nuclei. These two possible mechanisms will clearly affect the spatial growth of a recrystallized region in the sample in very different manners. To differentiate between these mechanisms, it is desirable to have a technique which can investigate the morphology of the sample as a function of the depth into the film. An infrared depth profiling technique, developed previously, can be used to study thin films of polymers? The technique uses infrared reflection absorption spectroscopy with oxygen sputter etching to characterize the chemical and physical nature of thin films. Infrared spectra are taken between steps of oxygen plasma etching. By subtracting spectra taken before and after a particular etching sequence, the spectra of the etched material can be obtained. In this manner, the spectrum of a thin film can be studied as a function of its position in the film. The technique can be useful in studying both multilayered and modified films. It has been seen previously that changes in morphology in samples of PET produce changes in the vibrational spectrum of PET.IoJ1 Thus, infrared spectroscopy is also a powerful tool for studying polymer systems: not only does it provide information regarding the chemical nature of a material but also information regarding its physical structure. We report here the application of this infrared depth profiling technique to study recrystallization in laser-amorphized films of poly(ethy1ene terephthalate). The samples were laser treated and allowed to recrystallize near the temperature at which the crystallization rate reaches its maximum. Samples were allowed to recrystallize for a defined period of time and then quenched to “freeze” the sample’s structure in the state that it existed in at that time. Then, with the depth profiling technique, the full spatial nature of the sample could be investigated as it stood at 0 1993 American Chemical Society
Richards et al.
12062 The Journal of Physical Chemistry, Vol. 97,No. 46, 1993
a particular time during the recrystallization process. From these data, it is possible to directly observe the initiation of recrystallization and subsequent crystal growth. Also, by observing the behavior of the infrared absorption belonging to different functional groups within the polymer chain it is possible to draw conclusions regarding the fundamental nature of primary and secondary crystallization processes involved in crystal growth.
SCHEME I kl
A-B-C
t < t*
-
dAldt = kj dBldt = - kj - kB dCldt = kB
k2
t>
t,
A=O dBldt = - k2B dCldt= k2B
XI. Experimental Section Sample films of poly(ethy1ene terephthalate) (PET) were prepared by spin coating a solution of the polymer onto goldcoated silicon wafers. The starting resin had a number-average molecular weight of 27 000. An o-chlorophenolsolution of PET was cast onto the wafers and dried in a vacuum oven at 75 OC for 2 h in order to produce an amorphous film. Semicrystalline films were generated by thermal crystallization of amorphous films. The amorphous films were heated in a vacuum oven for 2 h at 200 OC in order to produce semicrystalline films. Samples were irradiated with the output of a Lambda Physik 201 MSC excimer laser operating on KrF. Pulse energies at 248 nm were typically 8-12 mJ/cm2. Sampleswere heated toa desired temperature, laser treated, allowed to recrystallize for a predetermined interval of time, and rapidly quenched to terminate and "freeze out" crystallization at the time of interest. The sample was held in a vertical position, against a hot plate, by a mechanical arm controlled by a stepping motor while being purged from above by nitrogen. The temperature of the sample was monitored by a thermocouple on the hot plate. After sufficient time had elapsed for the temperature of the sample to equilibrate with the hot plate, the sample was exposed to a single pulse from the laser. An output pulse from the laser was passed into a delay generator in order to control the length of time for recrysttllization. After the desired recrystallization time had elapsed, a pulse from the delay generator was sent to the stepping motor to release the samplefromcontact with thehot plate. Thesamplesubsequently fell into a bath of liquid fluorocarbon which quenched the crystallization process. In this work, when discussing recrystallization times, we report the delay time before releasing the sample. The sample is actually at an elevated temperature longer than this time due to the finite time required for the sample to reach the quenching bath. This time for the sample to reach the bath is on the order of 0.1 s. However, the exact time the sample is heated is not as important as the incremental difference in time between various samples. Thus, the delay time gives a repeatable measurement of the recrystallization time. Infrared measurements were made using an infrared reflection absorption spectroscopy (IRRAS) configuration. The details of the IRRAS experiment have been given elsewhere.12 A brief description is provided here for convenience. There is an enhancement of the electric field of infrared radiation which is incident on a highly reflective surface at a glancing angle.13 In the configuration used here, the IR radiation is incident on the sample at an angle of 7 8 O relative to the surface normal. Spectra were collected using a Nicolet 60SX spectrometer equipped with a liquid nitrogen cooled HgCdTe detector. Spectra taken for the depth profiling experiments were collected at a resolution of 4 cm-1 and averaged over 400 scans. Time-dependent measurements were made using the rapid scan feature of the spectrometer. Using this feature, interferograms are collected on each pass of the interferometer mirror and saved in memory for additional processing. The details of the time-resolved experiment are given in ref 7. Briefly, semicrystalline films of PET on gold-coated silicon wafers were placed within a nitrogenpurged compartment of the spectrometer in an IRRAS configuration. The rapid scan sequence was initiated, and a pulse from the spectrometer triggered the eximer laser resulting in a single UV laser pulse which amorphized a sample held within the spectrometer. Spectra with a resolution of 10 cm-1 were then collected at 29-ms intervals as the sample recrystallized.
The sputter etcher used here is similar to one used in prior experiment^.^ The etching apparatus was assembled within a 2-ft-diameteraluminum cylindrical chamber. The lower electrode was coupled to a radio frequency generator operating at 13.56 MHz, and the upper electrode was grounded. The electrodes are 3 in. in diameter and water cooled. Ground shields were installed around the electrodes to confine the plasma to the area between the faces of the electrodes. The IR beam passed through the chamber via NaCl windows installed on opposite sides of the chamber and was incident on the sample which was held on the lower electrode. External connections for pumping and oxygen were made through the bottom of the chamber. Pumping of the chamber was carried out using a Roots type blower backed by a mechanical pump. Oxygen flow was regulated at 7.5 sccm using a mass flow controller. Total pressure in the chamber was held at 30 mTorr as measured by a capacitance manometer. Under these conditions, an etching time of 15 s removed approximately a 9-nm layer from the film. 111. Model
In order to interpret crystallization data, it is necessary to have a model to describe relevant crystallization processes. A kinetic model that was developed to describe time-dependent recrystallization data in ref 7 is presented in Scheme I. This model is based on crystallization being nucleated by the interface of laseramorphized PET and untreated polymer with crystal growth proceeding from this interface toward the surface of the film. This basic mechanism for the initiation of crystallization is consistent with thedepth profiling data presented here. Themodel explicitly includes nucleation from preexisting sites at the interface followed by oriented primary crystalline growth and a random secondary Crystallization process taking place in material which has undergone primary crystallization. Recrystallization in this system is described as an A B C process, where A describes the population of PET in the amorphous state, B is the population of the primary crystal, and Cis the population of the secondary crystal. The transition from A to B is described by an idealized planar crystallization front moving at constant velocity from the initial interface to the surface of the film. At times before the recrystallizationfront has reached the surface of the film, t < t,, the recrystallization front moves through the amorphous material at a constant linear velocity, resulting in a zero-order decay of the population of the amorphous material. The primary crystal is formed by this constant rate process and depleted by the secondary crystallization process which is first order in the population of the primary crystal B. Finally, this second process, which is first order in the population of B, produces the secondary crystal C. At times after the recrystallization front has reached the surface, t > t,, the population of A is 0 since the primary crystallization front has passed completely through the formerly amorphous layer. Since at this point B is nolonger being produced by the primary process, the populations of B and C are now controlled solely by the transformation of B to C. This gives an exponential decay of B and rise of C with a time origin of t,.
--
Recrystallization in Laser-Amorphized PET
"'1
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12063
Amorphous PET
A
................. Semicrystalline PET
8c
4w
< I
4 1175
1150
1125
1100
1075
Wavenum ber (cm-l) 1800
2000
1600
1400
1200
1000
800
600
Wavenumbers (cm" )
Figure 1. IRRAS spectra of semicrystallineand amorphous thin films of PET. 'A" indicates absorption bands characteristic of amorphous PET, and 'C" indicatesabsorptionbands characteristicof semicrystalline PET.
I
IV. Results and Discussion A. Spectra of Crystalline and Amorphous PET. The infrared spectrum of PET has been well studied.lOJ1 It has previously been seen to be highly dependent on the morphology of the sample. This can be seen by comparing the spectra of semicrystalline and amorphous PET shown in Figure 1. Absorbances which show an increase in intensity in a crystalline environment are indicated with a C, while absorptions characteristic of an amorphous environment are marked with an A. It is obvious that significant differences exist between the spectra of semicrystalline and amorphous PET. In this work, we will concentrate on a few specific regions of interest in the spectrum. In a previous work, information about the crystallization kinetics of PET was obtained from observing the behavior of the crystalline-sensitive band at 1342 cm-I. This band has been associated with a CH2 wagging mode of the ethylene glycol linkage in PET.'OJ1 Differences in the overall intensity of various spectra collected in these experiments were corrected for by normalization to the peak at 1411 cm-I. As seen in Figure 1, this peak is insensitive to the morphology of the sample and so can be used as a direct measure of the thickness of the sample. The other region of the spectrum which we have examined is between 1050 and 1200 cm-I. The absorptions in this region of the spectrum have been associated with deformations of the ring moiety in PET. It is clear from a cursory inspection of the spectra that there are a t least two different absorptions in this region. Using a spectral deconvolution program, we have fit this region with three bands centered at 1134, 1122, and 1105 cm-1. Fits of spectra of the amorphous and semicrystalline samples with these three bands are shown in Figure 2a and Figure 2b, respectively. The band centered a t 1134 cm-I shows a significant increase in intensity with increased crystallinity, while the bands at 1122 and 1105 cm-1 are less sensitive to morphology and show a slight increase in intensity in an amorphous environment. B. Mechanism of Recrystallization. Spectra of sample films were collected following each incremental etching step. As indicated in section 11,subtracting thespectrum obtained following a particular etching step from that taken prior to the etching step allows thespectrumof each 9-nm-thicketchedlayer to beobtained. In this manner, the morphology of the film can be investigated as a function of depth. Figure 3 shows the intensity of the crystalline-sensitive absorption at 1342 cm-1 as a function of depth for laser-treated samples held at 180 OC and allowed to recrystallize for the indicated time. The data have been normalized by dividing the intensity of the 1342-cm-1 peak by the intensity of the morphologyinsensitive peak at 1411 cm-' to account for possible variations in the thickness of etched layers. The ordinate of the plot is thus
-
1175
1150
1125
1100
1 75
Wavenumber (cm-') Figure 2. Fits of the 1200-1050-~m-~ portion of the PET spectrum. 'A" shows the fit to the spectrum of a sample of amorphous PET with three bands centered at 1134, 1122, and 1105 cm-I. 'B" shows the fit to the spectrum of semicrystallinePET using three peaks centered at the same positions.
+0 s
--e-- .35 s
delay
1.90
delay
...A,....5 s delay
I
1.50
1.10
0.70
0.30 0.00
crv' 0
50
100
150
Depth (d Figure 3. Depth profiles for the 1342-cm-I absorption band. The normalized absorbance of the crystalline-sensitiveband is plotted as a function of depth in the film for samples with the indicated delay times: (+) 0-s delay; (0)0.3% delay, (A) 0.5-s delay. a measure of the degree of crystallinity of the sample. From the data for the sample which was quenched after no delay, it can be seen that the sample is amorphous to a depth of 35 nm. Below
12064 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
this depth, the intensity of the 1342-cm-1 absorption grows as the degree of crystallinity increases in etched layers deeper into the sample. The majority of this increase occurs within a transition region between 50 and 90 nm from the surface of the film. Below this region, the absorption measurement approaches a constant value as material is reached which was untreated by the laser amorphization process. The data in Figure 3, for a sample with a recrystallization delay time of 0.35 s, show a higher degree of crystallinity at the surface than is seen in the layers immediately below this surface layer. Below this surface layer, two data points show a small increase in crystallinity in comparison to the corresponding layers in the film which were quenched with no delay. There is a transition at 30-40 nm followed by a relatively constant degree of crystallinity corresponding to material which has undergone recrystallization. Finally, there is another transition between 70 and 100 nm and then a leveling off of the absorbance as the untreated film is reached. Thus, in this sample there is a slightly recrystallized surface, a laser-amorphized layer, a transition to material which was recrystalllized following laser treatment, and a second transition (at the laser treatment depth) to the untreated material. In the sample with a recrystallization delay time of 0.5 s, there is again a higher degree of crystallinity a t the surface than in the layers immediately below. Below this surface layer, there is a relatively high degree of crystallinity as opposed to the amorphous material seen just below the surface (or at the surface for the zero delay sample) in the other samples. For this time delay, recrystallization has taken place throughout the laser-treated material. This recrystallized material, however, has not reached the same degree of crystallinity that is seen in the untreated material. Thus, there is still a transition region a t 70-100 nm between the laser-treated material and the untreated material. In summary, in this sample there is a higher degree of crystallinity at the surface, recrystallization throughout the laser-treated region, and a transition to the unmodified material. The data presented above are consistent with a model in which recrystallization is nucleated by the interface between the lasertreated amorphous PET and untreated semicrystalline PET with recrystallization preferentially oriented perpendicularly to this interface. The idealized recrystallization front would proceed uniformly from the initial interface toward the upper surface of the film. Given this model, if a sample were laser treated and allowed to recrystallize for a time such that the recrystallization front had passed through a significant fraction of the material, yet was quenched before reaching the surface, one would expect to see an amorphous region and a transition to recrystallized material above the initial laser treatment depth. This prediction qualitatively fits the data quite well for the sample in Figure 3 which had a delay time of 0.35 s before quenching. In these data, the recrystallization front has passed through approximately twothirds of the laser-amorphized material. With a longer delay of 0.5 s, there is no longer an amorphous region as the recrystallization front has passed through the entire film to the upper surface before quenching has occurred. The other mode of recrystallization that was discussed involves random nucleation of crystal growth throughout the laseramorphized region. Random nucleation would result in recrystallization proceeding in all directions throughout the amorphous region. Nucleation is a time-dependent process with new nucleation sites constantly being formed. Thus, there is not a single starting time for this recrystallization process such as that which exists in the model described by Scheme I, where nucleation sites (a nucleation layer) are created by laser amorphization. In the case of random nucleation, since there is no preferred direction for crystal growth and no unique starting time for the recrystallization process, this process would be expected to result in a homogeneous change in the degree of crystallinity within the
Richards et al.
B
e P
2
I
2000
1650
1300
950
00
Wavenumber (cm-') Figure 4. Spectra at various depths in a sample of PET that was laser treated at 180 OC and allowed to recrystallizefor 0.35 s: A, the spectrum for a layer 9-18 nm from the surface; B, the spectrum for a layer 44-53 nm from the surface; C, the spectrum for a layer of untreated material
below the laser treatment depth.
treated region of the film with the degree of crystallinity increasing with time. The data in Figure 3 clearly show a depth dependence to the degree of crystallinity and are not consistent with a dominant random nucleation mechanism. The final observation from the data in Figure 3 is the higher degree of crystallinity at the surface of the film in samples with longer delay times prior to quenching. This higher degree of crystallinity is assigned to heterogeneous nucleation of crystallization from defects or impurities at the surface of the film. This surface crystallization process has a significantly slower rate than the linear growth process nucleated by the interface of laser amorphized and untreated PET and is thus only a relatively minor perturbation to the data and does not need to be considered in further analysis of the experimental data. A slower rate for this surface-nucleated process is not surprising since there are not preexisting nuclei at the surface, such as are available at the interface of semicrystalline and amorphous PET, and thus nucleation must occur before significant recrystallization can occur. C. Primary and Secondary Crystallization. Figure 4 shows the spectra of various layers of a film which was allowed to recrystallize for 0.35 s prior to quenching. Figure 4a shows the spectrum of the layer from 9 to 18 nm below the surface of the film. This spectrum is nearly identical to that of amorphous PET shown in Figure 1. Figure 4c shows the spectrum of a layer of the sample below the laser amorphization depth. In this case, as expected, the spectrum corresponds well to the spectrum of semicrystalline PET, shown in Figure 1. Figure 4b shows the spectrum of the layer 44-53 nm from the surface. Figure 3 indicated that the recrystallization front has passed through this region of the sample. Though there is a significant absorption at 1342 cm-1 corresponding to that seen in crystalline PET, examination of this spectrum in the region from 1200 to 1050 cm-1 shows that this portion of the spectrum closely resembles amorphous PET. It is apparent then that the recrystallization process that has taken place in Figure 3 does not affect all functional groups, and thus all portions of the polymer chain, in the same manner. Figure 5 shows depth profiles similar to those in Figure 3 for the crystalline-sensitive peak a t 1134 cm-1. Again the intensity of the band a t 1134 cm-1 has been normalized for variations in etching times (and thus depths) by dividing its intensity by the intensityof the morphology-insensitive band a t 1411 cm-1. Since
Recrystallization in Laser-Amorphized PET --t
0
--e-- 3 5
g
delay
g
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12065
....I....5
delay
delay
3.50,
I
3.00
B I .Y
m
2.00
E i2
1
1.50
2
3
4
5
6
1
8
9
Time (seconds) Figure 6. Time-dependent growth of 1342- and 1134-cm-1 absorption bands. Fits of the data to eq 1 are shown as solid lines.
1.00
0
60
120
180
240
Depth (W Figure 5. Depth profiles for the 1134-cm-' absorption band. The normalized absorbance of the crystalline-sensitiveband is plotted as a function of depth in the film for samples with the indicated delay times. Symbols are as in Figure 3.
the band at 1134 cm-l is crystalline sensitive, an increase in the Yvalue in the plot again corresponds to an increase in the degree of crystallinity. In the sample that was quenched with 0-s delay, the observed data are seen to mirror the corresponding data obtained from the 1342-cm-l peak. There is an amorphous region at thesurfacegenerated by thelaser treatment, a transition region, and a semicrystalline region below the penetration depth of the laser. In the sample with a delay time of 0.35 s there is no obvious change in the data versus the 0-s time delay sample. With a delay of 0.5 s, there is a higher degree of crystallinity at the surface and possibly a somewhat higher degree of crystallinity throughout the amorphized material. In these data for the 1134-cm-1 absorption, there is however no clear sign of a crystallization front as was observed in the data for the 1342-cm-1 peak. This lack of an obvious effect of a crystallization front in the 1132-cm-1 data can be readily explained due to the effects of primary and secondary crystallization processes on an absorption. The primary crystallization process can be interpreted as leading to long-range order in the material, in effect producing a crystalline phase in a configuration which has not yet reached a minimum energy. Secondary crystallization processes can be pictured as much more localized processes which serve to further order material within the existing framework produced by the primary crystallization process. Since each functional group of the chain has a different local environment and therefore experiences a different potential, secondary crystallization would be expected to proceed at different rates for each group based on the differing environments they are in. Additionally, it can be seen that, although the primary process must affect all portions of the chain at the same rate since it is a long-range ordering process, it does not have to affect each functional group to the same degree. The spectra of some functional groups may be sensitive to the crystalline environment produced by the long-range ordering involved in the primary process, whereas the spectra of other groups may be much more sensitive to local ordering and may not exhibit a spectrum indicative of a crystalline environment until these groups have undergone secondary processes. The differences that have been seen in thedepth profiling data for the two peaks we have examined can be attributed to these differences
in the effect of the crystallization process. The band associated with the ethylene glycol linkage a t 1342 cm-l shows a strong sensitivity to the primary crystallization process, as can be seen in the data in Figure 3. On the other hand, the band a t 1134 cm-l, associated with deformations of the ring moiety, is not as sensitive to this long-range ordering process and hence does not exhibit an absorption characteristic of a recrystallized film. The recrystallization times used in these experiments were not long enough to observe secondary crystallization processes in these samples. Since the secondary process occurs randomly within the material which has undergone primary crystallization, it is not as spatially well defined as the primary process, and hence it is not as well suited to probing by depth profiling as is the primary process. However, secondary crystallization is readily seen in time-resolved measurements of recrystallization such as reported in ref 7. The time-dependent changes in the absorptions at 1342 and 1134 cm-1 during recrystallization a t 180 OC are shown in Figure 6. The plot shows the real time recovery of the bands whose intensity initially decreased due to amorphization and then recovered as crystallization proceeded. The intensity of the 1342-cm-1 peak rises immediately, whereas the 1134-cm-l peak shows an induction time and a slower rise. Thus, at times less than 1 s there is considerable growth in the 1342-cm-l peak but much less change in the intensity of the 1134-cm-1 peak. This behavior is consistent with the observations discussed for depth profiling experiments where on a similar time scale there is significant growth in the 1342-cm-1 peak and little change in the 1134-cm-1 peak. The time-dependent change in absorptions presented in Figure 6 can be fit to the kinetic model in Scheme I according to eq 1.
X J t ) = (abs B)B(t)
+ (abs C)C(t)
(1) In this equation, Xc(t)is a measure of the degree of crystallinity in the sample as given by the intensity of a crystalline-sensitive absorption. The terms B ( t ) and C(t) are the time-dependent concentrations of B and C as given by Scheme I, while (abs B) and (abs C) are the relative absorption coefficients of B and C . The value of (abs C ) is defined to be the asymptotic level reached for the peak absorbance in the plots of absorbance as a function of time. The three parameters kl, k2, and (abs B) were obtained from a nonlinear least-squares fit of the data to eq 1. Fits were obtained for multiple values of the time, t,, in Scheme I until a value of tswas reached which provided a minimum in the error of the fits to the data. The value oft, which gives the best fit of the data to eq 1 then corresponds to the time at which the primary crystallization front reaches the surface of the film. Given
Richards et al.
12066 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 the model of recrystallization which has been developed, the linear primary recrystallization rate can be obtained simply by dividing the original thickness of the amorphous layer by the time required for the recrystallization front to reach the surface. Fits to the data in Figure 6 are shown as solid lines. Minimum error in the fits of both sets of data was reached a t the same value of t , giving a primary recrystallization rate of 92 nm/s. This agreement in t , for the curve a t two different frequencies is consistent with primary crystallization involving the formation of long-range order which affects all portions of the polymer chain at the same rate. The secondary recrystallization rates were determined to be 0.32 s-* for the 1342-cm-’ data and 0.49 s-1 for the 1134-cm-1 data. As previously stated, rates for secondary recrystallization are not expected to be the same for different functional groups since they can be dependent on the local environment. Finally, the ratio of (abs B) to (abs C) can be used as an indicator of the relative importance of the primary versus the secondary process in the overall development of a crystalline environment around a functional group. For the 1342-cm-1 data this ratio is 0.53, while for the 1134-cm-’ data the value is 0.12. In other words, for the peak at 1342 cm-’ 53% of the increase in the peak intensity which occurs during recrystallization is attributable to the primary recrystallization process, whereas only 12% of the increase seen in the peak at 1134 cm-1 during recrystallization is due to the primary process. The different shapes of the curves in Figure 6 are a result of both the different secondary rates and the relative extent to which each of the two processes increase the absorbance of these crystalline-sensitive bands. Since the 1342-cm-1 data are most sensitive to the primary process, the curve initially approximates a straight line mirroring this linear process. The secondary process produces the exponential shape for the curve a t long time. The 1134-cm-1 data are most sensitive to the secondary process and thus approach an exponential shape with an induction time due to the need to form the primary crystalline material before the secondary process can take place.
V. Conclusions Laser treatment of a PET film produces an amorphous layer on top of untreated semicrystalline PET. Using an infrared depth profiling technique, the existence of a crystallization front passing
through a laser-amorphized film of PET held a t elevated temperatures has been observed. The behavior of this recrystallization front is consistent with a recrystallization mechanism where crystallization is nucleated by the interface between amorphous and semicrystalline PET followed by crystal growth oriented toward the surface of the film. Time-dependent data indicate the existence of a subsequent second crystallization process that affects regions of the film that have been transformed by the primary crystallization process. Two “crystalline-sensitive” infrared absorptions were examined and were found to exhibit very different responses to the primary and secondary recrystallization processes. These studies provide direct evidence for the existence of multiple recrystallization processes and, taken together with prior studiesofthe PETsystem,givesomeindication of the nature of these processes and the kinetics of recrystallization.
Acknowledgment. The work a t Northwestern University was supported by 3M Corp. under their University Cooperation Research Program. References and Notes (1) Van Krevelen, D. W.; Hoftyzer, P. J. Properties of Polymers; Elsevier: Amsterdam, 1976;Chapter 19. (2) Wunderlich, B. Macromolecular Physics. Crystal Nucleation, Growth, Annealing; Academic: New York, 1976;Vol. 2. (3) Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley-Interscience: New York, 1989;p VI/279. (4) Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Kroschwitz, J. I., Eds.; Wiley-Interscience: New York, 1989;p S 231. ( 5 ) Lin, S. B.; Koenig, J. L. J. Polym. Sci.: Polym. Phys. Ed. 1983,21.
2365. ( 6 ) Bulkin, B. J.; Lewin, M.; McKelvy, M. L. Spectrochim. Acta 1985, 41A, 251. (7) Richards, P. D.; Weitz, E.; Ouderkirk, A. J.; Dunn, D. S.Macromolecules 1993,26, 1254. (8) Dunn. D. S.: Ouderkirk. A. J. Macromolecules 1990.23. 770. (9) McClure, D: J.; Ouderkirk, A. J.; Hill, J. B.; Dum, D. S . J.’ Yac.Sci. Technol., A 1990,8 (3), 2295. (10)Stokr, J.;Schneider, B.;Doskocilova,D.;Lovy, J.;Sedlacek,P. Polymer 1982,23, 714. (11) Boerio, F. J.; Bahl. S. K.; McGraw, G. E. J . Polym. Sci.: Polym. Phys. Ed. 1976, 14, 1029. (12) Dunn, D. S.;McClure, D. J. J. Vac. Sci. Technol., A 1987,5 (4),
1327. (13) Rabolt, J. F.;Jurich, M.; Swalen, J. D. AppLSpectrosc. 1985,39(2), 269.
