Measuring Diffusion Coefficients of Nitroxide Radicals in Heterophasic

Dec 30, 2010 - †Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols Road,. Detroit, Michigan 48221, United St...
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Macromolecules 2011, 44, 325–333

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DOI: 10.1021/ma101745p

Measuring Diffusion Coefficients of Nitroxide Radicals in Heterophasic Propylene-Ethylene Copolymers by Electron Spin Resonance Imaging Krzysztof Kruczala†,‡ and Shulamith Schlick*,† †

Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols Road, Detroit, Michigan 48221, United States, and ‡Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Received July 31, 2010; Revised Manuscript Received December 17, 2010

ABSTRACT: Heterophasic propylene-ethylene copolymers (HPECs), also known as impact-modified polypropylene copolymers (IPC), are an important class of materials because of their attractive mechanical properties and low cost. Their chemical composition and phase heterogeneity have important effects on the physical and mechanical properties of these systems. Commercially used HPECs usually contain stabilizers such as hindered amines, which can be lost by diffusion (blooming). We present the measurement of the diffusion coefficient of nitroxide radicals by electron spin resonance imaging (ESRI); the radicals were obtained by oxidation of the commercial Tinuvin 770 hindered amine stabilizer (HAS) with m-chloroperoxybenzoic acid and consist of a mixture of mono- and biradicals. The diffusion rates were measured in two polymers, HPEC1 and HPEC2, which differed in the ethylene (E) to propylene (P) ratio, 25:75 and 10:90 wt %, respectively. One dimensional (1D) ESRI allowed the visualization of the radical distribution (“profiles”) within the sample as a function of storing time at 393 K. Simulation of this distribution led to the determination of the macroscopic diffusion coefficients, D, of the paramagnetic tracer in HPEC1 and HPEC2. The results indicate that the main factor determining the diffusion rates in these polymers is the amount of the crystalline phase, mostly composed of isotactic polypropylene (iPP). The D value was the same within experimental error in both systems, 1.42  10-8 cm2 s-1. Comparison with literature data of the temperature variation of D for the same diffusant in polyethylene (PE) and polypropylene (PP) suggest that in both HPEC1 and HPEC2 the diffusion takes place in the amorphous phase restrained by the proximity of the crystalline domains. The results also indicate that ≈20% of the diffusant is lost by evaporation (“blooming”). Results of spectral-spatial 2D ESRI show that the ESR spectra of the nitroxides do not change significantly along the sample length during diffusion, suggesting that the loss of stabilizer is due to diffusion and not to chemical reactions.

*Corresponding author. Telephone: 1-313-993-1012. Fax: 1-313-9931144, E-mail: [email protected].

and developing protective additives.5-9 Hindered amine stabilizers (HAS) are often used for stabilization of polymeric materials. Nitroxides and hydroxylamines are major products of reactions involving HAS. The HAS-derived nitroxides (HAS-NO) are thermally stable, but can scavenge free radicals to yield diamagnetic species; the hydroxylamines can regenerate the original amine, thus resulting in an efficient protective effect. Some of these events are shown in Scheme 1, where >NH denotes the amine, >NO• the nitroxide, and R•, ROO•, and ROOH the reactive intermediates derived from polymer chains exposed to oxygen and irradiation or heat. The spatial distribution of polymer properties during degradation is critical for an understanding of the underlying mechanism, and methods for its determination have been developed: density profiling measures the change in density, which is expected to increase in aged samples, along the irradiation depth;10 and modulus profiling measures the tensile modulus, which decreases during degradation.10,11 The spatial variation of these properties is an excellent indicator of degradation, especially at advanced stages in the degradation process. Both profiling methods are destructive: The sample is cut into sections and each section is studied separately. However, cutting a crystalline polymer sample can lead to morphological changes; for this reason nondestructive methods are preferable. Electron spin resonance imaging (ESRI) is capable of providing spatial details on the evolution of the degradation process within sample depth in a nondestructive way.12 Important papers

r 2010 American Chemical Society

Published on Web 12/30/2010

Introduction Degradation of polymers by the action of chemical agents, heat, mechanical stresses, or ultraviolet radiation in the presence of oxygen is the polymer equivalent to corrosion of metals. The term “degradation of polymers” includes all changes in the chemical structure and physical properties of the polymers as a result of external chemical or physical agents. Oxidative degradation of polymers is due to the formation of reactive free radicals such as R• and ROO•, and of hydroperoxides ROOH.1-4 The chemical structure can be modified due to chain scission, crosslinking, formation of double bonds, and increase or decrease of the molecular mass. The degradation chemistry is complicated because even small amounts of chromophores, free radicals, and metallic species from polymerization reactions can introduce additional reaction pathways that enhance the rate of degradation. The deleterious effects are often not detected immediately but may develop over periods of months or even years. While the time scale of structure changes may vary, the final results are degradation of the structure and collapse of the mechanical properties. Research on the effects of radiation and thermal treatment of polymeric materials is focused on several major goals: understanding the degradation mechanism, predicting polymer lifetimes,

pubs.acs.org/Macromolecules

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Macromolecules, Vol. 44, No. 2, 2011 Scheme 1. Stabilization by Hindered Amines

on the degradation of polypropylene (PP) by one-dimensional (1D) ESRI have been published by Lucarini and co-workers13-15 and on PP and polystyrene by Marek et al.16 We have developed 1D and 2D spectra-spatial ESRI methods for the study of heterophasic systems such as poly(acrylonitrilebutadiene-styrene) (ABS)17-23 and heterophasic propylene-ethylene copolymers (HPEC)24-27 containing Tinuvin 770 as the HAS, when exposed to thermal treatment or UV irradiation. The major objectives were to examine polymer degradation under different conditions; to assess the effect of rubber phase (polybutadiene in ABS and ethylene-propylene rubber in HPEC) on the extent of degradation; and to evaluate the extent of stabilization by HAS. The combination of the ESRI study with the temperature variation of the ESR spectra of HAS-derived nitroxides (HAS-NO) has made possible the deduction of important morphological and structural details on the thermal- and photodegradation of these systems. The thermal degradation of HPEC systems containing Tinuvin 770 as the HAS has been studied at 393 and 433 K.25 Two types of HPECs were examined: HPEC1 containing 25 wt % ethylene (E), and HPEC2 containing 10 wt % E (as ethylene-propylene rubber). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR), 1D and 2D spectral spatial ESRI of HAS-NO, and FTIR of films prepared by compression molding. The spatial distribution of HAS-NO was obtained by 1D ESRI. The 2D ESRI experiments allowed the nondestructive acquisition of ESR spectra as a function of sample depth for different treatment times or other aging conditions. Much of the interpretation was based on the detection of two components in the ESR spectra of the nitroxides that differ in their dynamical properties, fast (F) and slow (S).24-27 These two spectral components are shown in Figure 1-S in the Supporting Information for HPEC1 thermally aged at 433 K for 10 d.27 The line shape at 340 K is typical for rotation of the nitroxide along their long axis.17 Similar spectra were obtained for HAS-NO in HPEC2; at 300 K, however, the intensity of the F component is lower (compared to HPEC1), a result that was attributed to the lower E content. On the basis of ESRI data and on ESR spectra of HAS-NO in the HPEC systems and in the related homopolymers (polypropylene and polyethylene), we have suggested that the F spectral component observed at 300 K represents unrestrained ethylene-propylene copolymer chains, and the S component (at the same temperature) represents amorphous polypropylene and the copolymer chains in the rigid amorphous phase (RAP).24 RAP is the elastomer phase that is restrained by proximal crystalline domains. These studies have demonstrated the exceptional sensitivity of the ESR spectra of nitroxide radicals to polymer morphology and degree of crystallinity, and of the ESRI data to the degradation in the different polymer morphological domains. The 1D ESRI experiments in the thermally treated HPEC systems enabled the visualization of an outer region of thickness ≈100 μm that contained a lower amount of nitroxides.25 We have proposed that NO-depleted regions may arise from loss of stabilizer by diffusion (“blooming”), or/and in chemical reactions during aging. The mobility of the stabilizer is a very important parameter, since it has great impact on stabilizer performance: An effective stabilizer should be very soluble in the polymer,

Kruczala and Schlick Chart 1. Structures of Tinuvin 770 (1), the Corresponding Nitroxide Monoradical (2), and the Nitroxide Biradical (3)

and its translational mobility (diffusion and volatility) should be low.28,29 We present the measurement by ESRI of the diffusion coefficient of Tinuvin 770-derived nitroxide radicals (Chart1) in HPEC1 and HPEC2 at 393 K, under conditions of negligible degradation (diffusion time shorter than that necessary for the detection of carbonyls by FTIR24). As will be clearly seen below, the mobility of the radicals is significant, and leads to loss of stabilizer even when the extent of degradation is negligible. Experimental Section Materials. Two HPEC samples differing in their ethylene (E) content were a gift from the Dow Chemical Company: HPEC1 (IPC, C 708, Mn = 60,700, Mw = 227 000), and HPEC2 (IPC, C104-01, Mn = 90 400, Mw = 428 000). The polymers were prepared by polymerization of propylene in the presence of the catalyst, and sequential polymerization of the product with a propylene-ethylene mixture with the same catalyst.30 The E content (as the ethylene-propylene copolymer) in the HPEC samples, determined as described in ref 24, was 25 wt % in HPEC1 and 10 wt % in HPEC2, within (2%. The Tinuvin 770-derived radicals were prepared by oxidation of Tinuvin 770 from Ciba Specialty Chemicals, Chart 1, with m-chloroperoxybenzoic acid (mCPBA), and were a mixture of monoradicals (≈25%) and biradicals (≈75%).31 These radicals are referred to as “nitroxides” or “nitroxide radicals”, whereas radicals generated from HAS during polymer degradation processes are denoted as HAS-NO. Preparation of Samples for ESR Measurements. Polymers doped with nitroxide spin probes are usually prepared by the dissolution method, which involves solubilization of the polymer and probe in the solvent, followed by solvent evaporation. This was the method used in the case of ABS polymers doped with spin probes.32 Dissolution of HPEC1 and HPEC2 can be achieved in xylene at ≈400 K, but the process is slow and sometimes incomplete. For this reason, in this study the polymers were doped with the nitroxides by the melting method: Weighed amounts of the radical and polymer were mixed slightly above the melting point of the polymer.24 The typical temperature for mixing of HPEC samples and nitroxide was ≈450 K. The mixtures were held at the high temperature for ≈4 min or less in most cases. Preparation of Samples for ESRI Measurements. HPEC plaques (dimension 10 cmx10 cmx3 mm) were prepared by injection molding. The nitroxide was dissolved in toluene (10 wt %) and a layer of solution was applied to one side of the plaques, followed by drying for 10 min at 433 K. These samples were kept at 393 K, the temperature of the diffusion experiments. At selected time

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For this reason all 1D ESRI measurements were performed at 340 ( 1 K; at this temperature the F and S spectral components are not distinguishable and the spatial dependence of the ESR signal is avoided. The 1D concentration profiles were determined based on the concept of virtual components and on the genetic algorithm (GA) that enables optimization of the results.33 In this approach the concentration profile P(r) is expressed as a superposition of several Gaussian functions and two Boltzmann functions, which allow a better representation of the changes in concentration at the edges of the sample, as in eq 2. The Boltzmann (B) and Gaussian (G) functions are given in eqs 3 and 4. PðrÞ ¼ Bðr1 , a1 , h1 Þ þ

nX -1

Gðrk , ak , hk Þ þ Bðrn , an , hn Þ

ð2Þ

k¼2

Figure 1. Configuration of the samples used in the ESRI diffusion experiments. The blue arrow points to the layer of radicals and the red arrow shows the diffusion direction (“from-top-to-bottom”). The magnetic field gradient, 200 G/cm, is along the diffusion direction. The sample height is also indicated.

intervals, cylindrical samples with diameter ≈7 mm were cut through the plaque thickness, trimmed to fit the ESRI sample tube of 5 mm od, and placed in the ESR resonator with the symmetry axis along the long (vertical) axis of the resonator and parallel to the direction of the magnetic field gradient, as shown in Figure 1. ESR Measurements. Spectra were recorded on Bruker X-band EMX spectrometers operating at 9.7 GHz with 100 kHz magnetic field modulation and the ER 4101 STE standard resonator, and equipped with the Acquisit 32 Bit WINEPR data system version 3.01 for acquisition and manipulation, and the ER 4111 VT variable temperature units. The microwave frequency was measured with a Hewlett-Packard 5350B microwave frequency counter. The sensitivity profile of the cavity for the sample of height ≈3 mm is essentially constant. Most spectra were collected 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 points. The modulation amplitude was varied in the range 0.5-1.2 G, depending on the line width. The temperature was controlled within (1 K. All samples were allowed to equilibrate for at least 10 min after reaching the desired temperature. Additional experimental details have been described.24 ESR Imaging and Data Acquisition. The ESRI system is based on the EMX spectrometer equipped with two figure-eightshaped Lewis coils connected to two regulated power supplies. Measurements were performed using vertical (perpendicular to the external magnetic field) gradients of 200 G/cm for 1D ESRI, and 250 G/cm for 2D ESRI measurements. The instrumentation has been described in detail.17,22,25-27 The 1D concentration profiles were determined by ESRI from two spectra: the regular ESR spectrum and the 1D image, which is the ESR spectrum measured in the presence of the magnetic field gradient. For samples less than 4 mm in height the sensitivity response of the resonator can be treated as a constant. For these conditions, the 1D image, I(B), is a convolution of the ESR spectrum S(B) measured in the absence of the gradient with the function P(r), which describes the spatial distribution of the paramagnetic centers (the profile) along the gradient direction r, as shown in eq 1: Z þ¥ IðBÞ ¼ ðSðB - gradr BÞPðrÞÞ dr ð1Þ -¥

The convolution is legitimate if the gradient-off ESR spectrum, S(B), has no spatial dependence. In our previous ESRI study of HPEC systems we determined that the relative intensities of the F and S components vary with sample length.25-27

Bðri , ai , hi Þ ¼ ai =½1 þ expðr - ri Þ=hi 

ð3Þ

Gðri , ai , hi Þ ¼ ai exp½ - ðr - ri Þ2 =2hi 2 

ð4Þ

In eqs 2-4, r is the spatial coordinate, ai is part of a virtual stored profile, and hi is the half-width of a virtual component. While the Fourier transform followed by the Monte Carlo optimization method leads to noisy concentration profiles, the approach based on virtual components and the genetic algorithm (VC-GA) allows the automatic determination of concentration profiles without noise and without prior knowledge about their shapes. The noiseless profiles make possible a more confident interpretation of results, as seen clearly in Figure 3, below.26,33 The 2D spectral-spatial ESR images were reconstructed from a complete set of projections, typically 256, obtained as a function of the magnetic field gradient, using a convoluted backprojection algorithm, as described elsewhere.18,25 In the first reconstruction stage, the projections at the missing angles, typically 93 out of 256, 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) with 2-10 iterations.34 The 2D ESRI experiments were performed at 300 ( 1 K, and the twodimensional images were saved as 256  256 matrices. Determination of the Diffusion Coefficients. The translational diffusion coefficient D was determined using the program SCENARIOS,35,36 in a Matlab environment. The program allows an automatic determination of the diffusion coefficient for different experimental settings. Figure 1 describes the “fromtop-to-bottom” configuration; in this case the symmetrization procedure35 implemented in the software was applied in order to correct for possible imperfections in the diffusant distribution perpendicular to the diffusion direction. If we assume that the diffusion is Fickian (eq 5), and the initial radical distribution can be described by a delta function, the concentration profile can be described by the Green function, eq 6.37 Since the initial distribution of nitroxides cannot be easily predicted, a series of radical concentration profiles at different diffusion times were determined and the early profiles were taken as starting profiles for the later ones. If the diffusion follows Fick’s law, combinations of several pairs of concentration profiles are expected to lead to the same diffusion coefficient, and the plot of DΔt as a function of Δt for the pair (Δt = t2 - t1) is expected to be a straight line crossing the origin, and the slope of the line is the diffusion coefficient. DCðx, tÞ D2 Cðx, tÞ ¼ Dx Dt Dx2 2

Cðx, tÞ ¼ ð1=4πDtÞe - x

=4Dt

ð5Þ

ð6Þ

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Figure 2. ESR spectra of the nitroxides at the indicated temperatures in HPEC1 (A) and HPEC2 (B). The extreme separation (ES, in G), at 300 K for the F component and at 120 K for the S component, are given in part A. Two spectral components are clearly visible in both parts A and B at 300 K. At this temperature % F is 36 in part A and 29 in part B.

Figure 3. Concentration profiles of the nitroxide radicals in HPEC1 (A) and HPEC2 (B) obtained by Fourier transform followed by Monte Carlo optimization (black), and optimized profile calculated by the VC-GA method with 5 Gaussian and 2 Boltzmann functions. In both cases at least 100,000 iterations were necessary for profile determination. The 1D ESRI images were measured at 340 K with a magnetic field gradient of 200 G/cm. Upward arrows in parts A and B point to the maximum of the initial radical profile.

Results and Discussion In this section we will discuss the determination of the diffusion coefficients D measured in the two HPEC systems, deduce in which phase diffusion takes place by comparing with literature D values in the corresponding homopolymers PP and PE, and comment on the implications of the present results for the degradation and stabilization of the HPECs. Determination of the Diffusion Coefficients in HPEC1 and HPEC2. Representative ESR spectra of the nitroxide radicals in HPEC1 and HPEC2 are presented in Figure 2, parts A and B, respectively. Spectra typical of two interacting >NO groups (a five-line spectrum) in a long molecule such as the nitroxide biradical can be expected only in fluid media and have not been detected in the present systems. Therefore, use of the biradical nitroxide will not interfere with the interpretation of the ESR spectra of the monoradical. In support of this assumption, we recall that the ESR spectrum of HAS-NO at 300 K in polybutadiene, which has a lower glass transition temperature, Tg, compared to copolymers based on P and E units, was simulated by assuming a rotation about the long axis of the HAS, indicating a rigid probe conformation.17 The most

important feature is the presence of two spectral components in the temperature range 300-320 K, with different ES values; % F is 36 in HPEC1, and 29 in HPEC2, a result that is assigned to the higher ethylene content in HPEC1. The % F is slightly lower than that detected for HAS-NO in HPEC (41 and 35% respectively) because the nitroxides are detected in the original polymers, while HAS-NO appears in the plaques prepared by injection molding and the % F is higher. Therefore, the ESR spectra shown in Figure 2 indicate that when measuring diffusion coefficients the behavior of nitroxide radicals will reflect that of the HAS-NO in the thermally degraded HPEC systems. The concentration profiles used for calculation of the diffusion coefficient were determined by the virtual components and genetic algorithm (VC-GA) method that was described in detail elsewhere.33 Representative profiles obtained by this approach and also by Fourier transform followed by Monte Carlo optimization are presented in Figure 3. The profiles were plotted only up to 2 mm (even though the sample height was ≈3 mm) because no radicals were detected beyond this region of the sample. We note the noiseless profiles calculated by the VC-GA method.

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Figure 4. Concentration profiles of the nitroxide radicals in HPEC1 and HPEC2 systems calculated from the 1D ESRI experiments after diffusion at 393 K for the indicated time. For both HPEC1 and HPEC2 the profiles are normalized to the same total area, and the maximum of the profile at t = 0 (see arrows in Figure 3) was taken as 1, for comparison with the other profiles.

Figure 5. 2D spectral-spatial ESRI perspective image and corresponding spectral slices measured at 300 K for the nitroxide radicals in HPEC2 after 48 h of diffusion at 393 K.

Figure 4 presents the concentration profiles of radicals in HPEC1 and HPEC2, calculated from the 1D ESRI experiments and normalized to the same total area. The first profiles (time = 0 h) for both polymers were obtained immediately after evaporation of the solvent at 433 K. The other profiles were determined after the samples were placed in the oven at 393 K for the indicated times. Changes in the profiles with time clearly indicate the diffusion of the nitroxides at 393 K. The 2D spectral-spatial ESRI perspective plots for the radicals in HPEC2 after 48 h at 393 K, together with the spectral slices recorded nondestructively, are shown in Figure 5. The spectra were recorded at 300 K to allow visualization of the ESR spectra from the radicals, which are located in the different domains of the heterophasic polymers. The spatial profile indicates the spread of the radicals along the sample height due to diffusion, as also seen in the concentration profiles, Figure 4. The determination of the diffusion coefficients by simulation of the distribution profiles for HPEC1 (A) and HPEC2 (B) is illustrated in Figure 6. The variation of D  Δt as a function of Δt together with the linear fit for radical diffusion in HPEC1 (C) and in HPEC2 (D) are also

presented in Figure 6. In both cases the fit is very good, with a correlation factor R2 better than 0.99 and an estimated error for the slope less than 4%. The determined diffusion coefficients are similar for both investigated polymers: HPEC1: Dniroxide ¼ ð1:47 ( 0:04Þ  10 - 8 cm2 s - 1 HPEC2: Dnitroxide ¼ ð1:38 ( 0:02Þ  10 - 8 cm2 s - 1 Deducing in Which Phase Diffusion Takes Place. The D values given above suggest that the diffusion coefficients in the two HPEC systems are essentially the same. To understand this result in polymers containing 25 wt % E (HPEC1) and 10 wt % E (HPEC2), we go back to the description of the morphology of these systems in terms of four phases; crystalline PP, amorphous PP, crystalline part of ethylenepropylene copolymer and amorphous ethylene-propylene rubber.24-26 These studies have provided evidence for the location of the nitroxides in a range of amorphous sites differing in their dynamical properties. As determined

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Figure 6. Pairs of profiles used for the determination of diffusion coefficients for HPEC1 (A) and HPEC2 (B). In parts A and B, the two pairs of profiles were deduced after 4 and 24 h of diffusion, and the diffusion coefficients D deduced by simulation of the profile after 24 h were 1.40  10-8 cm2/s (A) and 1.45  10-8 cm2/s (B). The dependence of the values D  Δt versus the time interval Δt = t2 - t1 for the indicated initial-final concentration profile pairs for HPEC1 and HPEC2 is shown in parts C and D, respectively. Table 1. Diffusion Coefficients, D, of Tinuvin 770-Derived Nitroxide in LDPE and HDPE at the Indicated Temperaturesa system

T/K

D/cm2 s-1

ref

-10

38 296 4.23  10 39 313 2.05  10-9 -9 39 323 6.49  10 -9 38 323 6.75  10 28a 323 7.4  10-9 38 331 1.95  10-8 -8 39 333 1.58  10 -8 28b 333 1.86  10 38 338 3.2  10-8 38 340 3.5  10-8 -9 39 343 4.75  10 -8 38 348 5.65  10 39 353 1.20  10-7 extrapolation 393 1.82  10-6 39 HDPE 333 6.79  10-10 -9 39 343 2.64  10 39 353 9.85  10-9 39 363 1.80  10-8 39 373 4.16  10-8 -7 extrapolation 393 2.72  10 a Values in italic script are averages of different results at a given temperature measured in the original paper. LDPE

elsewhere,24 the amorphous phase is 63% in HPEC1 and 60% in HPEC2. Therefore, the amount of the amorphous phase in both polymers is similar. Since the D values are also very close, we conclude that at 393 K the most important factor that determines the diffusion is the ratio of amorphous to crystalline domains, not the composition (PE and PP) of

those amorphous domains. We note that we have assumed that the nitroxides reside only in the amorphous domains. Since we are unable to find literature data for diffusion of Tinuvin 770-derived nitroxides in HPECs, we have compared our findings with the D values of these radicals in LDPE, HDPE and PP. The literature D values for Tinuvin 770-derived nitroxides (HAS-NO) for LDPE and HDPE are given in Table 1, and for PP in Table 2. Table 2 also contains the D values deduced in this study. In some cases two or three different literature values for D at the same temperature are given in the cited references but determined after different diffusion times; for simplicity, these multiple values were averaged and the average numbers are given in the Tables in italic script. As mentioned above, the D values deduced for the nitroxide radicals in HPEC1 and HPEC2 at 393 K in this study are very similar and can be represented by average number (1.42 ( 0.05)  10-8 cm2/s. This value is lower than the D values in both LDPE (by a factor of 125) and HDPE (by a factor of 20) at the same temperature, as calculated by extrapolation (Arrhenius relationship) of the literature data presented in Table 1. The diffusion coefficients obtained in this work for the HPEC polymers are in the range expected for the diffusion of Tinuvin-derived radicals in PP. These D values are slightly lower than the interpolated value at 393 K. We note however that the literature data exhibit fluctuations not only between two studies,38,39 but even for the same authors.38 As seen in Figure 7, the value in this study, 1.42  10-8 cm2 s-1, can be placed in the range of predicted results for D determined on

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the basis of literature data collected in Table 2. The upper and lower confidence limits in Figure 7 show the region of all possible fitted lines for a confidence level of 95%: we have 95% confidence that the best-fit line lies within the confidence limits.40 Table 2. Diffusion Coefficients, D, of Tinuvin 770-Derived Nitroxide in PP and HPEC at the Indicated Temperaturesa system

T/K

D/cm2 s-1

ref

-10

323 2.7  10 28a 38 330 4.50  10-10 -10 28b 333 7.36  10 -9 38 336 1.10  10 38 341 1.65  10-9 38 343 1.40  10-9 -9 39 343 4.75  10 -9 38 346 3.05  10 39 353 1.49  10-9 15 355 2.55  10-9 38 356 6.10  10-9 -9 39 363 4.25  10 39 373 7.38  10-9 39 378 2.31  10-8 39 387 2.24  10-8 -8 39 394 5.43  10 interpolation 393 3.36  10-8 this work HPEC1 393 1.47  10-8 this work HPEC2 393 1.38  10-8 a Values in italic script are averages of different results at a given temperature measured in the cited reference. PP

Figure 7. Arrhenius plot for the diffusion coefficients of nitroxide radicals in PP (black squares) and in HPEC (red circle). The green triangle represents the interpolated value for D in PP at 393 K.

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Taking into account that essentially the same D values were determined for both HPEC polymers, and the fact that this value is characteristic for diffusion of HAS-NO radical in PP, we suggest that at 393 K the rate of diffusion for the nitroxide radicals might be related to the amount of crystalline phase, which is predominantly iPP: DSC measurements have indicated no crystalline PE in HPEC2, and a very small amount of crystalline PE in HPEC1.24 The composition of amorphous phase (ratio PE/PP) at this temperature has no important effects. It could be argued that the D value measured here represents diffusion in an E-rich region, and that the lower value compared to values given in Table 1 may be due to the presence of obstacles. We note however that, due to the method of synthesis by sequential polymerization of PP with an ethylene-propylene rubber, we do not expect PE chains in the HPEC systems. For this reason the picture of diffusion in amorphous PP-rich regions is preferable. Implications for Polymer Stabilization. The importance of the stabilizer solubility (S) and diffusion coefficient (D) have been discussed in numerous papers, and the empirical coefficient S2/D has been proposed as a criterion for an effective stabilizer; good agreement has been reported for HASstabilized LDPE films during accelerated aging and for three HAS types in LDPE and PP.28,29,38,39 Measurement of the loss of stabilizer due to diffusion from the outer sections of a polymer sample is an important additional information on the effectivity of a stabilizer; this information for the polymer systems presented here can be visualized from Figure 8 below. Figure 8 presents the concentration of the radicals, determined as the ESR signal intensity in the sample depth as a function of diffusion time. We note that a pronounced decrease of the total signal intensity with diffusion time is observed only for the outermost layer (9, 0 mm). For the next layer (red dot, 0.4 mm) we clearly see a slight increase of radical intensity that we assign to diffusion from the outermost layer, followed by a plateau; for the next layers we observe only slight increases in radical intensity. Also visible is a large decrease in the ESR intensity for the external layer between diffusion times t = 0 and t = 0.5 h; in the same time interval, however, the intensity increase in the next layer is much smaller. This result suggests a loss of nitroxide by evaporation of the radicals from the surface of polymer sample. The detection of a similar behavior for HPEC1 and HPEC2 suggests that the presence of the PE component does not alter the properties of the stabilizer. Quantitative determination of the loss of nitroxide due to evaporation was performed by measuring the total radical intensity (in the entire sample). The results suggest that the loss of the diffusant is on the order of 20%.

Figure 8. Variation of the normalized intensity as a function of diffusion time for the indicated heights: (A) HPEC1 and (B) HPEC2.

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In our previous studies, we have proposed that the loss of the stabilizer is due either to blooming, or to chemical reactions taking place during degradation.25 As degradation is not expected in the present samples because of the limited time the sample spent at 393 K (e48 h), we can conclude that the loss of nitroxides is due to evaporation and not to degradation processes: Degradation was detected at 393 K for both HPEC1 and HPEC2 only after thermal treatment of 29 days or longer. Additional support for this conclusion is in Figure 5, which shows no change in the ESR spectra of the slices for conditions similar to those used in this study for the determination of the diffusion coefficient. Conclusions The ESR imaging (ESRI) technique was used to track the diffusion processes of nitroxide radicals derived from Tinuvin 770 as the hindered amine stabilizer in HPEC polymers and to determine their macroscopic diffusion coefficients. The ethylene (E) content in the two HPEC samples studied were different, 25 wt % E in HPEC1 and 10% wt in HPEC2. The diffusion coefficients determined by simulation of the 1D ESR profiles as a function of diffusion time at 393 K were essentially the same for HPEC1 and HPEC2: 1.42  10-8 cm2 s-1. This value was considered in the context of literature data for D in LDPE, HDPE, and PP. The D value measured in this study is in the range expected for the diffusion of the nitroxides in PP, with a confidence level of 95%. Plots of the normalized intensity of the radicals as a function of diffusion time at 393 K suggested loss of the stabilizer by evaporation from the outer layer of the sample. Moreover, we conclude that during treatment conditions (not longer than 48 h at 393 K) the loss of stabilizer is due to blooming, and not to chemical reaction. The results deduced in this study prove that 1D ESRI is suitable for the measurement of macroscopic diffusion coefficients for paramagnetic species not only in solution or gel systems, but also in polymer plaques. Acknowledgment. This study was supported by the Polymers Program of the National Science Foundation. We are grateful to Antonin Marek for sharing with us the SCENARIOS software for the determination of diffusion coefficients, Gian Franco Pedulli and Marco Lucarini (University of Bologna) for the gift of the radicals prepared by oxidation of Tinuvin 770, and to John L. Gerlock (Ford) for his help with plaque preparation and for numerous insightful discussions. We are grateful to the three reviewers for their careful reading of the manuscript and constructive criticism. Supporting Information Available: Figure 1-S presenting the ESR spectra of HAS-derived onnitroxides in thermally aged HPEC1 registered at the indicated temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Donnell, J. H. Radiation Chemistry of Polymers. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., O'Donnell, J. H., Eds.; American Chemical Society: Washington, DC, 1989; Chapter 1, pp 1-13. (2) Handbook of Polymer Degradation, Hamid, S. H., Amin, M. B., Maadhah, A. G., Eds.; Marcel Dekker: New York, 1992. (3) Irradiation of Polymeric Materials: Processes, Mechanisms, and Applications; Reichmanis, E., Frank, C. W., O'Donnell, J. H., Eds.; American Chemical Society: Washington, D.C., 1993. (4) Polymer Durability: Degradation, Stabilization and Lifetime Prediction; Clough, R. L.; Billingham, N. C.; Gillen, K. T., Eds.; Advances in Chemistry Series 249, American Chemical Society: Washington, DC, 1996.

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