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Spatially resolved degradation in heterophasic polymers by ESR imaging and FTIR: the case of propylene-ethylene copolymers.

Heterophasic propylene-ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25% and 10% ethylene (E), respectively, as ethylene/propylene rubber (EPR). Electron spin resonance (ESR) spectra of nitroxide radicals in HPEC were studied in the temperature range 100-433 K; the nitroxides were derived from the HAS and are termed HAS-NO. The results were compared with ESR spectra of the same radicals obtained first by oxidation of Tinuvin 770 and then were doped in HPEC and related homopolymers, polyethylene (PE) and polypropylene (PP); these nitroxides are termed "spin probes." ESR spectra indicated that HAS-NO and the spin probes reside in a range of amorphous sites differing in their dynamic properties. The relative population of the sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains. Spatial and temporal effects of the aging process were studied by ESR and ESR imaging (ESRI) of HAS-derived nitroxide radicals, and by FTIR of films prepared by compression molding. 1D ESRI enabled the visualization of an outer region of thickness [approximately equal to]100 [micro]m that contained a lower amount of nitroxides, and is believed to result from the loss of the stabilizer by diffusion ("blooming") and possibly also in chemical reactions during aging. Two-dimensional spectral-spatial ESRI indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E, as compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.

Keywords: Hindered amine stabilizer, FTIR, ATR, non-destructive testing, thermal analysis, ESR and ESR imaging, stablization, thermal properties, thermoplastic olefins

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Heterophasic propylene-ethylene copolymers (HPEC) systems, known commercially as impact polypropylene copolymers (IPC), are an important class of polymers, due to their attractive mechanical properties and low cost. (1,2) The polymers consist of crystalline polypropylene (PP) modified by an elastomeric component, typically ethylene-propylene rubber (EPR), and are prepared by the polymerization of propylene in the presence of catalysts, and the sequential polymerization of a propylene-ethylene mixture with the same catalysts. (3) The resulting polymeric materials are heterophasic, but the specific morphology depends on the preparation method and monomer ratio. Many studies have demonstrated the presence of four phases in HPEC: crystalline PP, amorphous PP, crystalline EPR (mostly polyethylene, PE), and amorphous EPR. (4-7) The morphology of HPEC is of considerable interest because processing them at high temperatures can lead to morphological changes. Therefore, an understanding of the morphology and of the temperature dependence of domain size has enormous practical importance. For samples prepared by injection molding of PP/EPR blends, depth-profiling studies have revealed a skin consisting of several layers ("stratification"), whose composition is different in comparison to the bulk phase. (8,9) The effect was explained by the post-processing temperature variations in the sample upon cooling. Similar effects had been detected earlier in PP samples and were assigned to flow, shear, and temperature variations during and processing. (10)

This article consists of two parts. First, we describe details about HPEC morphology obtained from a study of ESR spectra of the HAS-derived nitroxides formed in thermally treated HPEC, and a comparison with the spectra of nitroxides as probes doped in HPEC and in PP and PE. (11) The results were interpreted in terms of the "rigid amorphous phase," whose extent and dynamics can be quantified by analysis of the ESR spectra. Additional support for this interpretation was obtained by DSC and FTIR measurements.

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Second, we present thermal aging in HPEC using 1D and 2D spectral-spatial ESRI and FTIR. These studies made it possible to assess the effect of ethylene content (as EPR) on the extent of degradation and to evaluate the degree of stabilization by HAS. (12)

EXPERIMENTAL SECTION

Materials

Two HPEC samples differing in their ethylene (E) content were from Dow Chemical Company: HPEC1 (IPC, C 708, [M.sub.n] = 60,700, [M.sub.w] = 227,000), and HPEC2 (IPC, C104-01, [M.sub.n] = 90,400, [M.sub.w] = 428,000). The E content in the HPEC samples was 25 wt% in HPEC1 and 10 wt% in HPEC2, within [+ or -]2%.

Preparation of Samples for ESR and ESRI

HPEC samples containing 1 wt% Tinuvin 770 (Scheme 1) as plaques prepared by injection molding were aged in a convection oven at 393 or 433 K.

For the ESR experiments, cylindrical samples with diameters of [approximately equal to]7 mm were cut from the plaques at selected time intervals, trimmed to fit the 5-mm diameter of the ESR sample tube, and placed in the ESR resonator with the symmetry axis along the long (vertical) axis of the resonator, parallel to the direction of the magnetic field gradient. The ESR spectra of HAS-NO were measured in the temperature range 100-433 K as a function of treatment time at a given temperature. Some ESR spectra of layers obtained by microtoming the cylinders were also measured. Slices 50 [micro]m in thickness were obtained using a Spencer-820 rotatory microtome equipped with a stainless steel knife.

ESR Measurements

Spectra were collected with Bruker X-band EMX spectrometers operating at 9.7 GHz with a 100 kHz magnetic field modulation, and equipped with the Acquisit 32-bit WINEPR data system version 3.01 for acquisition and manipulation, and ER 4111 VT variable temperature units. The microwave frequency was measured with a Hewlett Packard 5350B microwave frequency counter. Most spectra were measured with the following parameters: sweep width 120 G, microwave power 2 mW, time constant 40.96 ms, conversion time 81.92 ms, 4-10 scans, and 1024-2048 points. The modulation amplitude was varied in the range 0.5-1.2 G, depending on the line width. The temperature was controlled within [+ or -] 1 K. All the samples were allowed to equilibrate for at least 10 min after reaching the desired temperature. Additional experimental details, including the determination of HAS-NO concentration in whole samples and of the relative intensity of the F and S components, have been described. (11,13,14)

ESR Imaging and Data Acquisition

One of the ESR spectrometers was equipped with two Lewis coils and two regulated DC power supplies for the imaging experiments. The intensity profile of radicals was deduced from 1D ESRI experiments. Two spectra were needed: the regular ESR spectrum and the spectrum measured in the presence of the magnetic field gradient ("1D image"). The 1D image is a convolution of the ESR spectrum in the absence of the gradient with the distribution of the paramagnetic centers along the gradient direction ("the profile"). The convolution is correct if the ESR line shape has no spatial dependence. The two spectra needed were measured at 240 K in order to "freeze" the fast component, or at 340 K in order to reach the motional narrowing regime of both spectral components; in this way, the spatial dependence of the ESR signal was avoided. (13a) The 1D images were obtained with a field gradient of 206 G/cm, unless otherwise indicated.

[FIGURE 1 OMITTED]

In our initial ESRI studies, the concentration profiles of the radicals were deduced by Fourier transform followed by optimization with the Monte-Carlo (MC) procedure. (13a,c,15) The disadvantage of this method is the high frequency noise present in the optimized profiles. In our most recent publications, the intensity profile was fitted by analytical functions and convoluted with the ESR spectrum that was measured in the absence of the field gradient in order to simulate the 1D image. (13d,e) The best fit was obtained by variation of the analytical function (Gauss or Boltzmann, for example) and by the numerical parameters of the chosen function in order to obtain good agreement with the 1D image; the best fit was selected by visual inspection. Recently, the genetic algorithm (16) was used for minimizing the difference between simulated and experimental 1D images; this procedure allows for the best fit to be chosen automatically. A typical genetic algorithm (GA) consists of the following basic operations: creation of the initial population, calculation of the fit to experimental data, selection of the couples, reproduction, and mutation. (16) The terminology is adopted from biology. The ability of the GA to obtain a global minimum energy has been demonstrated and discussed. (17)

The 2D spectral-spatial ESR images were reconstructed from a complete set of projections, typically 128, collected as a function of the magnetic field gradient, using a convoluted back-projection algorithm. (13b-e) In the first reconstruction stage, the projections at the missing angles were assumed to be identical to the projection measured at the largest available angle. In the second stage, the projections at the missing angles were obtained by the projection slice algorithm (PSA) (18,19) with 2-10 iterations. The 2D image was saved as a 256 X 256 matrix.

[FIGURE 2 OMITTED]

FTIR Measurements

Films of HPEC containing 0 (HPECn-0H) and 1% HAS (HPECn-1H) were obtained, respectively, from neat polymer pellets and from plaques prepared by injection molding. The films were suspended inside a convection oven and thermally treated at 433 K. FTIR spectra were acquired at selected time intervals. Table 1 lists the treatment time for the films prepared for FTIR measurements.

FTIR spectra of the films were acquired in transmission mode using the Perkin Elmer Spectrum 2000 FTIR spectrometer equipped with a Mid IR (MIR) globar source and a triglycine sulfate (TGS) detector. During acquisition, the film was held in place with a magnetic film holder. Spectra were measured in the range 4000-400 [cm.sup.-1] with 4 [cm.sup.-1] resolution and eight scans; each scan was automatically corrected for open-beam background.

RESULTS

ESR Spectra of HAS-NO in HPEC1 and HPEC2

In Figure 1A we present selected ESR spectra in the temperature range 120-360 K, for HAS-NO in HPEC1 thermally treated at 393 K for 107 days. The rigid limit spectrum at 120 K changed as the temperature increased, and at 300 K the dynamically fast (F) component emerged. Similar spectra were observed for HPEC2. The spectrum at 300 K of HAS-NO in HPEC2 that was similarly treated is shown in Figure 1B. Inspection of the spectra at 300 K indicated that the relative intensity of the F component was lower in HPEC2 (%F=35) as compared to HPEC1 (%F = 41). Typical melting points for the HPEC components are 395 K for crystalline PE and [approximately equal to]441 K for crystalline PP, as determined by DSC (Figure 2). A probe molecule located in the crystalline domains is expected to be represented, below the polymer melting point, by a dynamically slow (S) spectral component. In the spectra shown in Figure 1, however, the slow component was not visible at and above 320 K, indicating that HAS-NO is not located in the crystalline domains. Therefore, the nitroxide radicals reflect the dynamics in the amorphous domains.

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[FIGURE 4 OMITTED]

While performing the ESR and ESRI experiments, we observed that cylindrical samples cut with a 5-mm diameter cutting tool had a lower %F in their ESR spectra at 300 K, as compared to samples cut with a tool of larger diameter, 7 mm, and trimmed to fit the 5-mm diameter ESR sample tube. A series of controlled experiments were then performed in order to quantify the effect, and the results are shown in Figure 3. The upper spectrum was measured at 300 K for HAS-NO in HPEC1 aged at 393 K for 107 days; the cylindrical sample was obtained with a 7-mm diameter cutting tool, then trimmed by a sharp blade to fit the ESR tube; in this sample %F = 41 [+ or -] 2. The same sample was subsequently microtomed into 50-[micro]m slices and all slices were transfered to the ESR sample tube. In the ESR spectrum of the slices (bottom spectrum in Figure 2) %F = 29 [+ or -] 2.

[FIGURE 5 OMITTED]

As increased ordering and further crystallization in the PP domains was expected upon microtoming ("stress"), the lowering of %F was assigned to the restraining effect of these domains on nitroxide probes located in their vicinal amorphous phases, as illustrated in Figure 4.

ESR spectra in the temperature range 100-433 K for HAS-NO in HPEC1 and HPEC2 thermally treated at 433 K for 10 days are presented in Figure 5. The rigid limit spectrum at 100 K changed as the temperature increased, and at 300 K both F and S components appeared. The line shapes at 340 K are typical for rotation of the probe along the long axis of HAS-NO. The relative intensities of the two spectral components at 300 K in the HPEC samples was deduced by deconvolution, as described (13); at 300 K, %F = 38 in HPEC1 and 40 in HPEC2. The vertically expanded portions of the spectra measured at 433 K (downward arrows in Figure 5) indicate signals from the biradical, 2NO-HAS, in both samples. These signals were more intense in HPEC1, even though the nitroxide concentration was slightly lower in comparison to HPEC2. This effect was assigned to the larger E content (as EPR) in HPEC1, which leads to lower microviscosity.

The concentration of HAS-NO in HPEC1 and HPEC2 (whole samples) as a function of treatment time, t, is presented in Figures 6A and 6B, for treatment at 393 and 433 K, respectively. For treatment at 393 K, the radical concentration increased with t, and was lower for t < 110 hr and higher for t [greater than or equal to] 110 hr in HPEC2, as compared to HPEC1. This "crossover" in nitroxide concentration can be explained by the higher consumption of the stabilizer in HPEC1, due to the higher content of the more degradable EPR component, vide infra. For treatment at 433 K, the radical concentration reached a maximum in HPEC2 and then decreased; the HPEC1 sample shattered for t >10 days and it was not possible to measure the radical intensity. The crossover in radical concentration for the two polymers was also seen for samples treated at 433 K, but after shorter treatment times, [approximately equal to]5 days (Figure 6B). The corresponding relative intensity of the F component, %F, is shown in Figures 6C and 6D. The F content in samples treated at 393 K was fairly constant: 35-37% in HPEC2 and 41-43% in HPEC1 (Figure 6C). For treatment at 433 K, however, %F reached a maximum and then decreased as the time of treatment increased.

1D and 2D Spectral-Spatial ESRI

Concentration profiles of HAS-NO in HPEC1 and HPEC2 for the same treatment times, 29 and 168 days, at 393 K are presented in Figure 7. All the profiles were corrected for the sensitivity profiles of the resonator. For ease of comparison, the resonator profile measured for the longer cylindrical sample (5.65 mm) in the region of interest is also shown in Figure 7A, together with the uncorrected profile. The 1D profiles show the formation of an outside layer that contained a lower amount of nitroxide radicals. The layer was more pronounced for HPEC1; in this sample the effect was already seen after 29 days of treatment. For the same treatment times the outside layer in HPEC2 was not as visible, but became more pronounced after 168 days of treatment.

The nitroxide-depleted region was not detected in the 1D profiles of HAS-NO in HPEC1 samples treated at 433 K for four and seven days (Figure 8A), nor in HPEC2 samples treated for 7, 10, and 50 days (Figure 8B). After 10 days of treatment at 433 K, the diffusion-limited oxidation (DLO) regime (20) was clearly observed for HPEC1.

Two-dimensional spectral-spatial ESRI perspective plots at 300 K and corresponding spectral slices in the derivative mode are shown for HAS-NO in HPEC1 treated at 393 K for 168 days (Figure 9A), and at 433 K for 10 days (Figure 9B). Comparison of the spectral slices in Figures 9A and 9B indicates a lower %F in Figure 9B throughout the sample depth. This result, combined with the DLO regime evidenced in the perspective plot, indicates more advanced degradation for the sample treated at the higher temperature, even though the treatment time was much shorter, 10 versus 168 days.

In Figure 10 we present spectral profiling: variation of %F at 300 K in the plaques as a function of sample depth, in HPEC1 and HPEC2 treated at 393 and 433 K, respectively. The most significant result is the formation of a narrow skin, [approximately equal to]100 [micro]m thick, that contains a lower %F at both treatment temperatures (Figures 10A-D).

FTIR Measurements

All FTIR spectra were normalized to the [CH.sub.3] symmetric bend (umbrella) peak at 1377 [cm.sup.-1]. Spectra in the carbonyl region for HPEC1-0H and HPEC2-0H for selected treatment times at 433 K are shown in Figure 11. We noticed the appearance of the carbonyl peak after shorter treatment times for HPEC1 (88 hr) as compared to HPEC2 (136 hr).

Figure 12 presents the carbonyl region for HPEC2-0H and HPEC2-1H, for the indicated treatment times at 433 K. The carbonyl peak appeared in the neat polymer for t = 136 hr (Figure 12A), and at shorter t, 113 hr, in the HAS-stabilized polymer (Figure 12B). The disappearance of the carbonyl peak from HAS after t [greater than or equal to] 5 hr was clearly seen for HPEC1-1H (Figure 12B). Samples in which the HAS disappeared also showed a negligible ESR signal from HAS-NO.

DISCUSSION

Effect of Stress

The results presented in Figures 1-4 are compatible with the concept of a rigid amorphous phase. (21,22) The restraining effect of the crystalline domains is seen from the broad lines in the ESR spectra of the nitroxide radicals, and from the lowering of the F/S intensity ratio as a result of stress. In addition to the existence of the interphase between amorphous and crystalline domains, the present study suggested a "gradient" in dynamics of the interphase, which can be visualized directly from the spectra. The amorphous phase was expected to be more restricted in the region between two crystalline domains, as shown in Figure 4. In other words, we expected that the restricted amorphous phase exists not only due to the gradual loss of order from polymeric crystals to the disordered phase, as suggested in the original paper by Flory, (23) but also because of restrictions arising from more than one proximal crystalline domain. This idea is supported by the results in HPEC1, where the presence of even a small amount of crystalline PE (2-3 wt% of the entire sample, corresponding to [approximately equal to]10 wt% of the total E amount) (11) leads to a larger extreme separation (ES) and, therefore, slower dynamics in the ESR spectra of HAS-NO. The schematic representation of the restricted phase in Figure 4 also implies that higher crystallinity leads to a larger amount of the restricted amorphous phase, and to a more dynamically restricted amorphous phase.

[FIGURE 6 OMITTED]

Degradation of HPEC as a Function of EPR Content

Figure 6A indicates that for treatment at 393 K the concentration of HAS-NO in whole samples is higher in HPEC1 for t [less than or equal to] 110 days, and lower for t >110 hr, as compared to HPEC2. The higher nitroxide content in HPEC1 in the early stages of thermal treatment can be explained by the expected higher diffusion of oxygen, HAS, and HAS-NO in samples containing a larger amount of E (as EPR). The diffusion coefficient, D, of Tinuvin 770 in PP is 2.7 X [10.sup.-10] [cm.sup.2]/sec in the temperature range 298-348 K, and much higher in PE (74 X [10.sup.-10] [cm.sup.2]/sec) in a similar temperature range (328-353 K).(24-26) The lower nitroxide content in HPEC1 as the treatment time increased cannot be explained if the only difference between the two HPEC samples is the rate of diffusion of oxygen and the mobility of other reactants. The explanation we offer is a higher consumption of nitroxides due to a higher degradation rate in HPEC1, as compared to HPEC2. This idea is reinforced by data in Figure 6D, which show that the disappearance of the fast component for treatment at 433 K was much faster for HPEC1 than for HPEC2. We recall that the F component represents radicals located in unrestrained (by crystallites) EPR domains.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The concentration profiles shown in Figures 7 and 8 provide additional evidence for this interpretation. The DLO regime was clearly reached for HPEC1 after 10 days of treatment at 433 K (bottom profile in Figure 8A), indicating the advanced stage of oxidation; for HPEC2, the 1D profiles were almost flat even after 50 days of treatment at the same temperature (Figure 7B).

Further evidence was seen in the carbonyl region of FTIR spectra for HPEC1 and HPEC2 without the HAS treated at 433 K (Figure 11). The formation of carbonyl groups in HPEC1 was detected after a shorter treatment time (88 hr), as compared to HPEC2 (136 hr). It is clear that the EPR component is an important factor in degradation. The initial point of attack may be the tertiary carbon in propylene. (27,28) The data presented here indicate, however, that the rate of aging processes in HPEC is determined by the increased rate of oxygen diffusion and reactants mobility in polymers with higher EPR content. The higher proportion of the amorphous domains in HPEC1 in comparison to HPEC2, 6% versus 60%, and the corresponding higher amount of the EPR represented by the F component, 26 vs 21, (11) may also enhance the local reactivity. The presence of stronger signals for 2HAS-NO in HPEC1 in the ESR spectra measured at 433 K (Figure 5), are evidence for the higher mobility in this system, as compared to HPEC2.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Behavior of HAS During Thermal Treatment

The 1D profiles shown in Figure 7 for HPEC samples treated at 393 K indicate less nitroxides in the outer regions of the plaques; the effect was seen even after only 29 days of treatment. At 393 K the degradation process was very slow, as clearly seen by the absence of carbonyl peaks in the FTIR data (data not shown), and in Figures 7A and C, which show increasing HAS-NO and steady %F for both polymers with time of treatment. Therefore, no serious complications due to consumption of HAS as stabilizer were expected. We suggest that the outer layer depleted in nitroxide radicals was due to the loss of HAS; the loss was initially more pronounced in HPEC1 because of the higher E content, which increased the mobility of the additive. In HPEC2 treated at 393 K, the effect became pronounced after longer treatment times (bottom profile in Figure 5B, t = 168 days), most likely because the diffusion of the additive was slower in this system, which contained less E. The higher nitroxide content in the plaque center is also seen in the 2D spectral-spatial ESRI perspective plot shown in Figure 9A, for HPEC1 treated at 393 K.

For treatment at 433 K, the loss of additive and the formation and consumption of nitroxides were enhanced; the balance of these processes is the most reasonable cause for the essentially flat profiles seen in Figure 8, with the exception of HPEC1 after 10 days of treatment (lower profile in Figure 8A). The DLO regime reached in HPEC1, also seen in 2D ESRI experiments (Figure 9B), clearly shows the formation of more HAS-NO at the sample extremities, due to access to oxygen.

In the spectral profiles deduced from 2D ESRI (Figure 10) we noticed an outer slice of thickness ~~100 [micro]m depleted in the F component. The effect was detected for both HPEC samples, for treatment at 393 and 433 K. The lower %F might be due to the loss of the stabilizer in the outer region, as was also seen in the loss of HAS-NO in the 1D profiles, and also to consumption of nitroxides in specific morphological domains (see discussion below). The loss of the additive during thermal treatment ("blooming") is directly seen in the FTIR spectra of HPEC2-1H films (Figure 12B); the carbonyl peak from the HAS disappeared after five hours of treatment at 433 K.

The effect of HAS on the degradation process can be assessed by comparing FTIR spectra in the carbonyl region for HPEC2-0H and HPEC2-1H films (Figures 12A and B). The spectra suggest more extensive degradation in HPEC2-1H, judging by the time necessary to detect the carbonyl peak: 136 hr in HPEC2-0H, and only 113 hr in HPEC2-1H. In parallel, HAS-NO was not detected in the ESR spectra after five hours of treatment. We tentatively suggest that fragments from HAS were reactive and led to degradation. Evidence for reaction products of HAS with HPEC have been presented (27); some of these products might promote degradation.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Effect of Morphology on Thermal Degradation

The spectral slices deduced from 2D ESRI experiments (Figure 9), as well as spectral profiling (Figure 10), indicated that %F is lower in the center of the plaques; the effect is small but consistent in all samples, even for the plaque that exhibits the DLO regime (Figure 9B). We note that the lower %F was seen in plaque regions that clearly showed a higher total nitroxide content in the 1D profiles (Figures 7 and 8). This effect can be rationalized by the assumption that the F component, which reflects nitroxides located in the dynamically unrestricted amorphous domains, was consumed preferentially during the aging process. Support for this idea is provided by Figure 6D, which shows the significant decrease in %F for HPEC1 with treatment time; the decrease in %F is also seen in the same figure for HPEC2, but less dramatic in comparison to HPEC1 for the reasons outlined in the previous section. This is a significant conclusion, because it emphasizes the ability of ESRI experiments to detect variations in the degree of aging in different morphological domains. It became possible to distinguish "preferential" aging in EPR domains represented by the F component of HAS-NO.

In the case of samples prepared by injection molding of PP/EPR blends, depth-profiling studies have revealed the formation of a skin consisting of several layers ("stratification"), whose composition is different compared to the bulk phase (8,9,28); a transcrystalline layer (~~10 [micro]m thick) followed by an elastomer region (~~20 [micro]m thick) were detected in the outer regions of these samples. The resolution in the ESRI experiment is ~~100 [micro]m; therefore, we cannot visualize these narrow regions. It is quite possible, however, that the outer slice depleted in the F component seen in Figure 10 was due to the presence of a crystalline PP layer; if that is the case, the effect was seen in the ESRI experiments as part of a larger region, and therefore not as pronounced as it really is.

CONCLUSIONS

The ESR spectra provided unambiguous evidence for location of the nitroxide radicals in a range of amorphous sites differing in their dynamical properties. The distribution of sites was explained by assuming that the crystalline domains exerted a restraining effect on chains located in amorphous domains. Additional support for this assumption was provided by FTIR spectra and DSC of HPEC.

Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E, as compared to 10% E. The spatial distribution of the HAS-derived nitroxide radicals obtained by 1D ESRI enabled the visualization of an outer region of thickness ~~100 [micro]m that contained a lower amount of nitroxides, and was believed to result from the loss of stabilizer during aging, by diffusion ("blooming") and by chemical reactions.

Nondestructive ("virtual") slicing of the 2D spectral-spatial ESR images resulted in a series of ESR spectra, which indicated the presence of nitroxide radicals in two amorphous sites, fast (F) and slow (S); the corresponding relative intensity varied with sample depth. 1D and 2D ESRI allowed for the detection of faster degradation in the amorphous EPR phase represented by the fast spectral component. This study emphasized the ability of ESRI experiments to track aging in different morphological domains.

The effect of HAS is antiprotective. The presence of a greater Tinuvin 770 content in the polymers led to less efficient stabilization in the thermally treated samples. In addition, FTIR spectra indicated increased regularity of polypropylene segments in HPEC during aging at 433 K; and ESR spectra showed the formation of nitroxide biradicals as a result of thermal treatment at 433 K.
Table 1 -- Treatment Times of Films Measured by FTIR

 Treatment Time (hr) at 433 K
Sample 0 39 63 88 113

HPEC1-0H [check] [check] [check] [check] (a) --
HPEC1-1H [check] [check] (a) -- -- --
HPEC2-0H [check] [check] [check] [check] [check]
HPEC2-1H [check] [check] [check] [check] [check] (a)

 Treatment Time (hr) at 433 K
Sample 136
HPEC1-0H --
HPEC1-1H --
HPEC2-0H [check]
HPEC2-1H --

(a) At this stage the films disintegrated and treatment could not be
continued.


ACKNOWLEDGMENTS

This study was supported by the Polymers Program of the National Science Foundation. We are grateful to Rose A. Ryntz for illuminating discussions on polymer properties.

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(27) Delprat, P., Duteurtre, X., and Gardette, J.-L., Polym. Degrad. Stab., 50, 1 (1995).

(28) Gensler, R., Plummer, C.J.G., Kausch, H.-H., Kramer, E., Pauquet, J.-R., and Zweifel, H., Polym. Degrad. Stab., 67, 195 (2000).

Shulamith Schlick** -- University of Detroit Mercy*

Krzysztof Kruczala -- Jagiellonian University([dagger])

* Department of Chemistry and Biochemistry, Detroit, MI 48219-0900.

[dagger] Faculty of Chemistry, 30-060 Krakow, Poland.

** Author to whom correspondence should be addressed. Email: schlicks@udmercy.edu.
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Author:Kruczala, Krzysztof
Publication:JCT Research
Date:Jan 1, 2005
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