Article pubs.acs.org/Macromolecules
Role of Nanoparticles and Relaxation on Strain-Induced Crystallization Behavior in Uniaxially Stretched Polyethylene Naphthalate Films: A Mechano-Optical Study K. Kanuga and M. Cakmak* Polymer Engineering Department, The University of Akron, Akron, Ohio 44325-0301, United States ABSTRACT: The effects of nanoparticle concentration and processing conditions on the relaxation behavior of PEN nanocomposites are investigated using mechano optical techniques where birefringence, true stress and true strain are measured while subjecting the polymer films to uniaxial deformation followed by relaxation. For this purpose, two different stretching temperatures were employed to stretch the films containing 0.5 and 2 wt % nanoparticles: one above and the other below the Tll (liquid−liquid transition). Increasing the temperature as well as the addition of nanoparticles suppresses the spontaneous deformation processes leading to sharp increase in true strain. An instantaneous stress drop is observed during relaxation below the Tll corresponding to the initial glassy component observed in the stress-optical behavior during stretching. This stress is attributed to the presence of segmental rigid correlations that were not broken during the stretching stage. This behavior was found to be absent above the liquid−liquid transition as they are already melted. Nanoparticles were found to act as suppressors of crystallinity during stretching. Their presence reduces likelihood of strain crystallization as they reduce the relaxation of oriented chains into favorable registry with each other to crystallize. During relaxation, the presence of nanoparticles was found to increase the crystallinity. Their presence increases the population of oriented amorphous chains during deformation that relax into favorable registry with each other leading to increase in crystallinity.
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INTRODUCTION During the deformation of polymers at sufficiently high temperatures orientation and relaxation mechanisms often take place concurrently. As it was demonstrated earlier by Martins et al.1 and Mulligan et al.,2 stretching polymer films with slow crystallization character such as poly(lactic acid) (PLA) and polyethylene naphthalate (PEN) from amorphous precursors at high stretching rates and/or at low temperatures between Tg and Tcc (cold crystallization) lead to formation of nematic-like order. If sufficient chain relaxation is allowed through the use of low rates, longer time at high temperatures, then this nematic-like order was found to convert to oriented crystalline state. Hence, the relaxation during stretching of polymer films allows the development of three-dimensional crystalline order in the material. In certain film processing operations, the relaxation is induced in the film as a separate processing step where the stretching process is stopped and the film is allowed to relax as it is held between the clamps while being transported along the process line. This increases the stability of the film minimizing subsequent shrinkage during its use. During the relaxation process, the magnitude of stress relaxation, structure evolution in the polymer, and development of polymer orientation are all affected by the processing conditions applied during the deformation process. In other words, the processing history has a direct link between the structure of the material during the stretching process and at © 2013 American Chemical Society
the end of the relaxation process and therefore it is essential to delineate the details of these complex relationships between the thermo-mechanical history applied to the films during the stretching and the relaxation and consequent structural organization processes. There have been limited studies on the relaxation behavior of the preoriented films. For example, the relaxation behavior in PET has been investigated by Ryu et al.,3 Ito et al.,4 Pearce et al.,5 Oultache et al.,6 and Matthew et al.7 Effect of stretch ratio, temperature, and stretching rate have been studied using birefringence and infrared analysis. Martins et al.8 has characterized the influence of three stress-optical regimes observed during uniaxial stretching of PEN on subsequent relaxation process. They have found that the material’s behavior during relaxation is very much dependent on the regime at which the material has been deformed to. If the relaxation takes place within regime I, the stress relaxation is accompanied by a decrease in birefringence following the linear stress-optical behavior and the material remains amorphous. Relaxation within regime II can either show a decrease or increase in the birefringence while the stress is relaxing. In regime III, relaxation occurs with the decrease of stresses while maintaining the same birefringence level attained during the Received: March 15, 2013 Revised: July 11, 2013 Published: July 25, 2013 6300
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negligible during most part of the experiment and maximum error− though quite low- occurs at the end of large deformation. In addition, assuming any changes in volume would affect all dimensions equally and the width being measured real-time reduces the committed error. We have verified this assumption with off-line thickness measurements taken for different deformation levels as well as different stretching conditions and compared them with online thickness values obtained from width measurements. (not shown) Additional details of the system can be found elsewhere.10,11 The birefringence measurements taken in this study are reported for the wavelength of 546 nm. Sample Preparation. Dumbbell-shaped specimens were cut from the original melt cast and amorphous films with the following dimensions: 75 mm long, 40 mm wide, and 30 mm wide in the narrowest region. The samples were clamped and fixed on the arms of the uniaxial stretching system inside the environmental oven. The distance between the clamps was taken as 30 mm. Thermal Characterization. The thermal properties of stretched and relaxed samples were measured using a universal 2920 MDSC V2.6A TA Instruments DSC. The samples of approximately 6−10 mg were crimped in aluminum pans and were scanned at a heating rate of 20 °C/min under a dry nitrogen blanket. The degree of crystallinity of the nanocomposites was determined using the equation below.
stretching process. No significant changes were observed structurally. Within the range investigated, no appreciable influence of molecular weight was found on the relaxation behavior of the material. This was found to depend mainly on the structure formed prior to relaxation. In an earlier publication,9 we have observed that PEN− nanocomposites stretched above Tg but below 1.07Tg (K) is in a fixed liquid state and possesses local structure due to the segmental correlations existing at this temperature regime. These correlations exhibit themselves as an initial glassy component in the stress-optical behavior that disappears when stretched above the liquid−liquid transition temperature as the rigid segmental correlations disappear. The present studies aim to investigate the effect of this liquid−liquid transition on the relaxation behavior of PEN−nanocomposites and specifically the role played by these segmental correlations on the relaxation behavior. Effect of these local interactions is expected to play significant role on the long-range connected network formation composed of crystalline regions connected with amorphous chains. The effect of nanoparticles on the mechano-optical behavior was also investigated earlier.9 The present work aims to investigate the influence of mostly exfoliated nanoparticles on the relaxation behavior of PEN and resulting structure evolution. Since the addition of nanoparticles suppresses the development of crystallinity but at the same time stiffens the material by decreasing the average degree of freedom the polymer chains experiences in the vicinity of rigid particle surfaces, it is important to investigate which effect plays a dominant role in the relaxation behavior of nanocomposites. Furthermore, the influence of stretching rate on the orientation/relaxation processes and consequent structural hierarchy development is also explored.
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crystallinity (%) = (ΔHexp/ΔH o) × 100 Where ΔH exp = ΔH melting − ΔH cold crystallization and ΔHo is the heat of fusion for 100% crystalline PEN 103.4 J/g19. WAXD Measurements. Bruker AXS Generator equipped with a copper target tube and a two-dimensional detector was used to obtain the one-quadrant of the WAXD patterns stretched samples. The generator was operated at 40 kV and 40 mA with a beam monochromatized to Cu Kα radiation. A typical exposure time of 20 min was used.
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RESULTS AND DISCUSSION Mechanical Behavior. In our earlier publication,9 we mapped the dynamic phase diagram for the PEN nanocomposites wherein we quantified the Tll as influenced by the nanoparticle concentration as well as deformation rates. In order to assess the influence of the rigid “segmental crystals” on the mechano optical as well as structural evolution that exists between Tg and Tll, we selected two temperatures to investigate for this work. One is at 130 °C between Tg and Tll and the other is150 °C between Tll and Tcc. The engineering stress−strain behavior for the nanocomposites during uniaxial stretching at 130 and 150 °C is plotted in Figures 1 and 2. The data indicate that 0.5 wt %
EXPERIMENTAL PROCEDURE
Materials. Melt cast amorphous PEN 100 μm thick sheets containing a series of organically modified clay concentrations ranging from 0.5 to 2 wt % nanoparticle concentrations was provided by Teijin Chemical Co. All samples were transparent. Nanoparticles were found to be well exfoliated as reported in earlier publication9 Online Birefringence and True Mechanical Measurements. A custom built uniaxial stretching machine10,11 was used to simultaneously measure the mechanical and optical properties of polymer films during deformation and following relaxation process. The design of this machine includes three parts: the uniaxial stretching system with two moving cross heads allows the horizontal mid symmetry plane of the sample remain stationary during stretching. The sample is surrounded by an air circulating environmental chamber with external heater system. With the designed-in optical flats on front and back walls of the chamber, the spectral birefringence and sample width (with a laser micrometer) are continuously monitored during and after the stretching. Real time measurements of optical retardation, sample width, force and elongation are recorded simultaneously. Assuming (1) simple extension and (2) uniaxial symmetry, time variation of the local thickness is calculated and thus birefringence, local true stress and local true strain values are determined. The equation used is:
λMDX(λND)2 = 1 Change in volume due to development of crystallinity in the material would be expected to occur. But as will be shown in this paper the degree of crystallinity developed in the samples ranges anywhere from 0% to 30%. The volume change between these states is approximately 2.5% (ρc = 1.407 g/cm3, ρa = 1.325 g/cm3). Since the development of crystallinity occurs toward the end of high strain stretching, the error committed by this incompressibility assumption is
Figure 1. Engineering stress−strain curve for 0.5 wt % nanocomposite at 130 and 150 °C. 6301
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neck formation in the material. In the latter conditions, the steep stress decline under relatively constant strain is followed by a large increase in true strain that takes place even though the crossheads of the stretching machine do not move in this holding stage (Figure 3a for 0.5% nano). Stretching during the relaxation stage under the action of already developed stresses during deformation is still observable even beyond the strain hardening point. The magnitude of these additional strains decreases with increasing strain applied during initial deformation. Similar effect has been observed by Martins et al.,1 where it has been shown that a spontaneous deformation process occurs that corresponded to neck formation for PEN causing the sharp true strain increase. Another important observation in the relaxation behavior is the occurrence of an initial instantaneous stress drop during relaxation at 130 °C (below Tll) that is absent at 150 °C (above Tll). This initial stress drop corresponds well with the early glassy stress observed in the material when stretching in the Tg−Tll temperature range where the material is still in a fixed liquid state. Moreover, it also represents the elastic nature of these segmental correlations that we have shown to exist below Tll9 that manifested as glassy component in the mechanooptical behavior. With further stretching, a majority of these segmental correlations “break” as higher stresses are experienced in the material. During relaxation when the material is held between the clamps, the remaining segmental correlations forming a connected network initially relax. However, as majority of these segmental correlations break with increased stretching, the initial stress drop observed during relaxation becomes much smaller at higher strains. Increasing the temperature from 130 to 150 °C and increasing the nanoparticle concentration from 0.5 wt % to 2 wt % suppresses
Figure 2. Engineering stress−strain curve for 2 wt % nanocomposite at 130 and 150 °C.
nanocomposite necks at both of these deformation temperatures while the 2 wt % nanocomposite necks at 130 °C but does not exhibit necking at 150 °C as shown by the absence of stress drop in Figure 2. The true mechanical behavior of PEN-Nanocomposites during stretching followed by 25 min of relaxation is shown in Figure 3a−d. The samples stretched to low deformations exhibit a rapid stress decrease at early stages of relaxation and this is followed by small increase in strain indicating that the films could undergo deformation during relaxation stage as the stress developed during stretching decays. This is particularly pronounced in films stretched to intermediate true strains of 0.8 to 1.5. This deformation zone corresponds to the regions of
Figure 3. True stress−strain curves for (a) 0.5 wt % nanocomposite at 130 °C, (b) 0.5 wt % nanocomposite at 150 °C, (c) 2 wt % nanocomposite at 130 °C, and (d) 2 wt % nanocomposite at 150 °C at 0.01 s−1 engineering rate. The data represent stretching and subsequent relaxation stages (stretch ratios applied during deformation are indicated on each curve). 6302
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Below the Tll, beyond a deformation level of 0.5 (true strain) (Figure 4) the birefringence remains constant in the first stage of stress recovery, but beyond a certain time it begins to rise. These initial constant stress recovery regions shorten as the initial deformation levels increase. X-ray pattern taken at the end of relaxation indicates that material underwent crystallization during relaxation. Above the T ll , (Figure 5)
necking leading to suppressed spontaneous deformation process and hence smaller change in true strain levels during relaxation. Stress-Optical Behavior. PEN−Nanocomposite−0.5 wt %. The stress optical behavior of PEN filled with 0.5% nanoclay exhibit initial glassy (photoelastic) behavior at low stresses and this gives way to rapid rise in birefringence at higher stresses and ultimately reaching a plateau at higher deformation levels (Figure 4) . The data obtained during 25 min relaxation stage
Figure 6. Stress−optical behavior for 2 wt % nanocomposite stretched at 130 °C and 0.01 s−1 engineering rate. Figure 4. Stress−optical behavior for 0.5 wt % nanocomposite stretched at 130 °C and 0.01 s−1 engineering rate. Crystallinities of each sample indicated next to the corresponding WAXS pattern.
after stretching to a series of deformation levels are also presented in this figure. As it can be observed in Figure 4, below the Tll (at 130 °C), the relaxation following small deformation (true strain below 0.5) starts with a stress recovery stage with no change in the birefringence. After a certain level of stress relaxation, birefringence decrease is observed following nearly the same slope as observed during stretching. At this stage, the structure of the films remains amorphous indicated by the X-ray patterns taken before and after the relaxation steps. Above the Tll (at 150 °C) (Figure 5), birefringence decrease is observed from the start of the relaxation and the initial stress recovery stage is absent.
birefringence and true stress decrease during relaxation but no longer follow back the trend of the birefringence change observed during the stretching stage. The material at the beginning of the relaxation is amorphous finally developing a nematic-like structure at the end of 25 min relaxation. Above the true strain of 0.8 the material undergoes strain induced crystallization in regime II, and it is crystallized or has a nematic-like order before the start of the relaxation. Below the Tll (130 °C) relaxation shows an initial stress drop followed by a steep birefringence increase and further leveling off at the longest relaxation times. However, following stretching above the Tll (150 °C) (Figure 7), relaxation stage exhibits a steeper birefringence rise. Finally, stretching the material above the true strain of 1.5, the stress-optical behavior enters regime III. At these deformation levels, the film developed significant crystallinity.
Figure 5. Stress−optical behavior for 0.5 wt % nanocomposite stretched at 150 °C and 001 s−1 engineering rate.
Figure 7. Stress−optical behavior for 2 wt % nanocomposite stretched at 150 °C and 0.01 s−1 engineering rate (crystallinities obtained by DSC are shown next to X-ray patterns). 6303
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Ensuing relaxation of the material in this zone leads to gradual increase in birefringence as the stress decreases when stretched below Tll (Figure 4). Relaxation of the material when stretched above Tll still shows a steep birefringence increase with stress relaxation (Figure 5). PEN−Nanocomposite−2 wt %. Following stretching below the Tll (Figure 6), relaxation stage shows an initial stress drop before the start of birefringence decline during relaxation. When the film stretched above the Tll (Figure 7) relaxation from such small deformation levels show a monotonous drop in birefringence as well as stresses however the material at the end of the relaxation stage has a faint nematic order observable in WAXS patterns. This appears when the deviation from the linear stress optical behavior is observed. This behavior is not present in the 0.5 wt % nanocomposite material where comparably stretched and relaxed samples show amorphous structure in X-ray patterns. When the material is stretched above regime I (true strain >0.5), no significant differences are observed in the relaxation behavior above and below Tll except the initial stress drop observed below Tll similar to that observed in the 0.5 wt % material. Birefringence as well as true stresses decrease initially followed by leveling-off of the birefringence and finally a steep birefringence rise is observed without any further drop in true stress. This is different from the relaxation behavior at 0.5 wt % which does not show any rise in birefringence after leveling off, in this stage of the stress optical behavior for a relaxation time of 25 min. Strain Optical Behavior. Birefringence as a function of true strain is plotted in Figures 8-− for 0.5 and 2 wt %
Figure 9. Strain−optical behavior for 0.5 wt % nanocomposite stretched at 130 °C and 0.01 s−1 engineering rate (% crystallinities obtained from DSC are indicated on each WAXS pattern).
Figure 10. Strain−optical behavior for 2 wt % nanocomposite stretched at 150 °C and 0.01 s−1 engineering rate.
Figure 8. Strain−optical behavior for 0.5 wt % nanocomposite stretched at 150 °C and 0.01 s−1 engineering rate (% crystallinities obtained from DSC are indicated on each WAXS pattern).
nanocomposite films. The samples stretched below Tll (Figures 9 and 11) exhibit nearly linear behavior beyond initial slight nonlinear stage. For the 0.5 wt % nanocomposite taken above the Tll (150 °C) (Figure 8), the film stretched to the true strain of 0.5 shows a small increase in strain during relaxation. X-ray pattern taken after relaxation shows that the material has little crystallinity. When the material is stretched above the true strain levels of 0.5 spontaneous deformation takes place during the relaxation due to necking as discussed earlier leading to a rise in true strain. Birefringence during relaxation in this zone decreases initially and then starts showing a rise toward the end of the relaxation process. X-ray patterns show that the material at the end of the relaxation process has some oriented
Figure 11. Strain−optical behavior for 2 wt % nanocomposite stretched at 130 °C and 0.01 s−1 engineering rate.
crystalline structure developed. At low deformation levels, the structure of the material at the end of the relaxation process remain largely amorphous and the development of true strain and birefringence follow completely opposite trends as there is little long-range connectivity in the polymer network leading to 6304
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Figure 12. (a) % crystallinity change with true strain for 0.5 wt % nanocomposite at 130 °C; (b) % crystallinity change with true strain for 0.5 wt % nanocomposite at 150 °C.
Figure 13. (a) % crystallinity change with true strain for 2 wt % nanocomposite at 130 °C; (b) % crystallinity change with true strain for 2 wt % nanocomposite at 150 °C.
relaxation process dominates leading to a decrease in birefringence monotonously following the same trend as the material stretched at 150 °C. When strained to a value of 0.8 followed by relaxation, the film does not show any decrease in birefringence initially and toward the end of relaxation process the birefringence starts to increase. For true strain of 0.8 and below, X-ray patterns taken at the beginning of relaxation show significant orientation but crystallization is still absent. Comparing this to the relaxation behavior at 150 °C at same strain levels, the birefringence initially decreases significantly unlike at 130 °C and then start increasing toward the end. This shows that below the Tll the chain relaxation process is substantially suppressed. Finally at high deformation levels (true strain of 1.5 and higher) birefringence starts increasing from the beginning of the relaxation process and the slope of the birefringence increase becomes steeper similar to the relaxation behavior below Tll. For films containing 2 wt % nanoclay (Figures 10 and 11) stretched up to 0.5 true strain, exhibit an increase in birefringence toward the end of the relaxation stage. This
domination of polymer chain relaxation process. Finally stretching the material above true strain of 0.8 the birefringence shows a rapid rise immediately from the beginning of the relaxation process. X-ray patterns show that the material has a well-defined crystal structure at the beginning of the relaxation and has 20% crystallinity, establishing the long-range connected network that allows the orientation process to dominate chain relaxation mechanism. Similar behavior is also observed in Figure 10 for 2 wt % nanocomposite film stretched at 150 °C. At true strain of 0.78, birefringence initially drops during relaxation and the XRD patterns shows that the material is predominantly amorphous. However at true strain of 1.15, birefringence shows a rapid rise from the beginning of relaxation and the XRD patterns indicate a clear crystal structure and crystallization has increased to about 23% at the end of the stretching process. This suggests that a critical amount of crystallinity needed during stretching for the longrange connected network to be established in PEN. Stretching the 0.5 wt % nanocomposite at 130 °C (below Tll) (Figure 9) below the true strain levels of 0.5 the chain 6305
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Figure 14. (a) Orientation factor change vvith true strain for 0.5 and 2 wt % nanocomposite at 130 °C; (b) orientation factor with true strain for 0.5 and 2 wt % nanocomposite at 150 °C.
with 2 wt % nanocomposite film is less compared to 0.5 wt % nanocomposites. Below Tll (130 °C), crystalline orientation increases during relaxation and finally at high deformation levels the change in orientation decreases. This certainly fits the molecular description of chains reaching their finite extensibility limit and cannot orient further. Above Tll, for the 0.5 wt % nanocomposite, crystalline orientation increases during relaxation but at high deformation levels it decreases below the orientation levels observed at the end of deformation. This is manifested significantly more for the 2 wt % nanocomposite film indicating that above Tll, absence of an amorphous network causes significant chain relaxation during the relaxation process leading to the drop in crystalline orientation observed in Figure 14b. Relaxation Rate. The stress relaxation behavior of the nanocomposites was fitted to an exponential function:
behavior is not observed in the 0.5 wt % nanocomposite wherein the increase in birefringence in the relaxation starts only for the samples stretched above the true strain of 0.5. This suggests that increasing the nanoparticle content suppresses the relaxation process for polymer chains in its sphere of influence allowing crystallization to be initiated early compared to 0.5 wt % nanocomposite. Crystallinity and Orientation Development during Relaxation. Development of crystallinity and crystalline orientation before and after relaxation is plotted as a function of initial true strain for different deformation levels to evaluate the structural changes taking place during relaxation in Figures 12−14. For the 0.5 wt % nanocomposite, below the Tll material stretched to small deformation levels (∼0.5) exhibit small crystallinity change during relaxation. Beyond the true strain of 0.8, crystallization of 10 to 15% occurs during relaxation. Finally stretching beyond the true strain of 1.5 the material is highly strain crystallized at the end of stretching leaving no room for crystallization during relaxation at this stage and changes in the range of 2−3% are observed. The behavior above the Tll (150 °C) is similar (Figure 12b). For 2 wt % nanocomposite, below the Tll (130 °C) no crystallization is observed during relaxation at small deformation levels. Beyond the true strain of 0.8 development of crystallinity in the range of 8 −10% is observed at the end of relaxation and with higher deformation levels the crystallinity development is suppressed but still significant changes in crystallinity during relaxation in the range of 5% are observed. Above the Tll (150 °C) with increasing deformation the crystallinity developed during relaxation consistently increases and reaches its highest levels above the true strain of 1.5. This is substantially different behavior as compared to other three cases described above. Figure 14 shows the crystalline orientation factor at the beginning and at the end of relaxation. Crystalline orientation achieved for 0.5 wt % nanocomposite film is higher than 2 wt % nanocomposite films indicating that polymer chains are confined by the nanoparticles which hinder their orientation. This is also supported by the fact that the true strain achieved
σ = σ∞ + (σO − σ∞)e(−t / t *)
Here σ is the true stress at time t, σo is the initial true stress at t = 0, σ∞ is the stress at infinite time, and 1/t* is the rate parameter where t* is the characteristic relaxation time of the polymer chain. Figure 15 shows a plot of the stress relaxation as a function of time along with the curve fit of the equation. A reasonably good fit is obtained. Figures 16 and 17 show the relaxation rates as a function of initial true strain induced in the material for 0.5 and 2 wt % nanocomposites stretched at 130 and 150 °C. These figures show that the relaxation rate first increases at early stages of deformation and having shown a peak value begin to decrease as the crystallinity development become substantial. This increase may be attributed to temporary increase in relaxation rate due to detachment of polymer chains from the nanoparticle surfaces at intermediate deformation levels and this trend reverses as the crystallinity is increased and oriented chains begin to press against the rigid surfaces of nanoparticles while the nanoparticles themselves also continue to orient in the direction of stretching. The sample with higher clay concentration obviously has more chains adjacent to the particles and the effect, as a result, is more pronounced. As expected, the relaxation rate parameters exhibit higher 6306
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birefringence drop following the same trend as in stretching. With increasing deformation to true strain levels of 0.8 and above, spontaneous deformation is dominant during relaxation as the deformation levels are in the necking zone leading to steep rise in the true strain levels during relaxation causing significant strain induced crystallization. This induced crystallization causes the material to hold itself well due to the longrange connectivity allowing the birefringence to rise from this point onward. Finally, increasing the deformation levels to 1.5 and above the material has significant crystallinity developed to hold itself well establishing long-range connected network whose nodes are crystalline regions and entanglements allowing the orientation process to dominate preventing the birefringence to decrease during relaxation. However, material at this point has a significant long-range connected network already developed and does not undergo significant crystallinity increase during relaxation resulting in a small birefringence change. Increasing the nanoparticle concentration suppresses necking and the resulting spontaneous deformation process. The presence of nanoparticles also increases the effective “network nodes” by their presence as they contribute as additional entanglement sites. Hence the true strain does not show a sharp increase and correspondingly the birefringence change does not show such a sharp rise at the intermediate deformation levels as in the 0.5 wt % nanocomposite. Above the Tll and at small deformation levels, the birefringence decreases steadily following the same trend as observed during stretching as discussed above. With the addition of 2 wt % nanoparticles the birefringence decreases but does not follow the same trend as the one shown during stretching and the material at the end of 25 min relaxation shows a faint nematic-like structure. This indicates that the polymer chains present in the intergallery spacing of the nanoparticles or in their proximity lead to suppressed relaxation and even though the bulk of the material relaxes back, these chains close to the nanoparticles undergo deformation and becoming ordered appearing as nematic-like order in the X-ray patterns. Above the true strain of 0.8, birefringence shows a steep rise from the beginning of relaxation for the 0.5 wt % nanocomposite as influenced by the spontaneous deformation process occurring in this zone as well as the significant crystallinity rise during relaxation. However with the 2 wt % nanocomposite birefringence remains flat during the early stages of relaxation and then shows a sharp rise toward the end. This is explained by following the structural evolution process at different stages of relaxation. The 2 wt % nanocomposite stretched to true strain above 1 and relaxed for different times showed that during the initial flat portion of the relaxation no significant crystallinity development is observed and since there is no spontaneous deformation taking place in this condition birefringence remains nearly constant. With higher relaxation times significant crystallization takes place that is followed by a corresponding steep birefringence rise toward the end. Further stretching above the true strain of 1.5, the material has already significant crystallinity developed before relaxation and the birefringence shows a significant rise as governed by the crystallinity development during relaxation which shows a 4− 5% increase. The orientation of crystalline chains was observed to be higher than as stretched films. This is influenced by two factors: One is the further deformation takes place during relaxation process. Even though the ends of the sample being held at both ends remain stationary, stresses already developed at the end of deformation stage cause further stretching of the
Figure 15. True stress relaxation along with a plot of the curve-fitting equation.
Figure 16. Relaxation rate parameter {1/t*) for 0.5 and 2 wt % nanocomposites along with corresponding crystallinity values as a function of initial true strain at 130 °C.
Figure 17. Relaxation rate parameter (1/t*) for 0.5 and 2 wt % nanocomposite along with corresponding crystallinity values as a function of initial true strain at 150 °C.
magnitude at their maxima at higher temperature (Figure 17). At both temperatures, the use of higher concentration of nanoparticles was found to suppress the relaxation much more effectively particularly when the crystallinity begins to develop. Structural Interpretation. The mechano optical measurements indicate that, the films when stretched below Tll contain rigid segmental inter or intra chain correlations. Majority of these relatively stiff segments break with deformation. The remaining segmental correlations in the material recover elastically exhibiting the instant stress recovery observed initially during relaxation. Material at this stage is essentially amorphous even after relaxation allowing the orientation relaxation processes to dominate resulting in a steep 6307
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(4) Ito, H.; Suzuki, K.; Kikutani, T.; Nakayama, K. PPS 18 Conference Proceedings, Guimarães, Portugal, Polymer Processing Society: Lawrence, KS, 2002. (5) Pearce, R.; Cole, K.; Ajji, A.; Dumoulin, M. Polym. Eng. Sci. 1997, 37 (11), 1795−1800. (6) Oultache, A. K.; Kong, X.; Pellerin, C.; Brisson, J.; Pézolet, M.; Prud’homme, R. E. Polymer 2001, 42, 9051−9058. (7) Matthews, R. G.; Ajji, A.; Dumoulin, M.; Prud’homme, R. E. Polymer 2000, 41, 7139−7145. (8) Martins, C. I.; Cakmak, M. Macromolecules 2006, 39 (14), 4824− 4833. (9) Kanuga, K.; Cakmak, M. Polymer 2007, 48, 7176−7192. (10) Serhatkulu, T.; Cakmak, M. ANTEC Conf. Proc. 1999, 57, 1645−1649. (11) Koike, Y.; Cakmak, M. Polymer 2003, 44 (15), 4249−4260.
sample in the midsection while they relax. Furthermore, the relaxation of highly oriented amorphous chains that have not had a chance to register and become parallel with adjacent highly oriented chains begin to relax and become parallel and crystallize with their chain axes oriented in the stretching direction and these regions also increasingly contribute to the orientation increase in crystalline domains as the relaxation proceeds. Hence both the crystallinity and crystalline orientation increase particularly at higher deformation levels. The role of presence of clay particles is to suppress the relaxation behavior of the chains in their immediate vicinity and add as additional “nodes” to the long-range connected network of entanglements and crystalline domains to increase the efficiency of local chain orientation. Too much of relaxation suppression in the presence of nanoparticles is not conducive to increases in the overall orientation induced crystallization. The chains may be substantially oriented but oriented in the wrong directions relative to each other. Unless they become parallel to their neighboring chains they will not crystallize. Local relaxation of these highly oriented but “misregistered” chains lead to their adjacent parallel alignment and consequent oriented crystallization.
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CONCLUSIONS The relaxation behavior of PEN nanocomposites following deformation was investigated as influenced by temperature and nanoparticle concentration. Relaxation behavior is significantly influenced by the spontaneous deformation process associated with necking in PEN. Structural evolution during relaxation was found to be controlled by the interplay of chain relaxation and orientation processes as influenced by the presence of stiff nanoparticles. Following small deformations, the material exhibit amorphous structure and ensuing relaxation is dominated by orientational relaxation manifested in rapid birefringence decline. Following intermediate level of stretching, the material start to form oriented ordered regions that nucleate the formation of long-range connected network. Relaxation from such state typically occurs with stress recovery with little change in birefringence followed by rise in birefringence at longer times caused by strain crystallization. Finally, at high deformation levels, significant crystalline structure has already been developed before relaxation allowing the chain orientation process to dominate right from the start of the relaxation process resulting in birefringence rise from the beginning of the relaxation. The presence of nanoparticles were found to suppress the orientation relaxation during stretching and this results in increased crystallinity development during relaxation as the increased population of amorphous yet oriented chains relax and register with one another and crystallize.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (M.C.)
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Martins, C. I.; Cakmak, M. Macromolecules 2005, 38 (10), 4260− 4273. (2) Mulligan, J.; Cakmak, M. Macromolecules 2004, 38, 2333−2344. (3) Ryu, D. S.; Inoue, T.; Osaki, K. Polymer 1998, 39 (12), 2515− 2520. 6308
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