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Effect of radiation-induced oxidative degradation on the non-isothermal crystallization of ethylene-butene copolymer
Polymer Degradation and Stability 170 (2019) 109001
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Effect of radiation-induced oxidative degradation on the nonisothermal crystallization of ethylene-butene copolymer rez c, * V.A. Alvarez a, M.D. Failla b, C.J. Pe sticos (CoMP) Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Facultad de Ingeniería, Grupo Materiales Compuestos Termopla n 10850, 7600, Mar Del Plata, Universidad Nacional de Mar Del Plata (UNMdP) and Consejo Nacional de Ciencia y Tecnología de Materiales (CONICET), Colo Buenos Aires, Argentina b PLAPIQUI (CONICET) - Departamento de Ingeniería (UNS) Camino “La Carrindanga” Km 7, 8000, Bahía Blanca, Buenos Aires, Argentina c Grupo Ciencia e Ingeniería de Polímeros, Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Facultad de Ingeniería, Universidad n 10850, 7600, Mar Del Plata, Buenos Nacional de Mar Del Plata (UNMdP) and Consejo Nacional de Ciencia y Tecnología de Materiales (CONICET), Colo Aires, Argentina a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 8 October 2019 Accepted 15 October 2019 Available online 15 October 2019
A random ethyleneebutene copolymer was irradiated with high ionizing energy in environments with different oxygen concentration. The non-isothermal crystallization process of the materials was studied by differential scanning calorimetry. When the polymer was exposed to radiation under free oxygen ambient, the temperature and the crystallinity degree decreased almost linearly with dose because chain-linking reaction prevails. On the contrary, those thermal parameters increased in the material obtained by irradiating the copolymer in environments with oxygen availability where chain scission reactions dominate. It was also found that, at equivalent irradiation dose, the crystallization rates decreased with the dose at a given cooling rate and with the reduction in oxygen content. The parameters obtained from different applied models also confirm the tendencies observed for the experimental variables. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Non-isothermal crystallization Ethylene-butene copolymer Gamma radiation oxidative degradation
1. Introduction The use of gamma irradiation provides an alternative mechanism for initiating oxidations of polyethylene's at temperatures close to the ambient one. The oxidative process is initiated by gamma irradiating the polymer in the presence of oxygen that causes chemical changes including chain scission, chain linking and the incorporation of polar oxygen-containing groups, such as acid, alcohol and ketone among others [1e5]. The oxidation worsens some important properties of the polymer, for instance the mechanical ones, but it can favorably affect others. For example, the introduction of functional groups increases the adhesion of the polyethylene to other materials with polar characteristics, making it easier to create more homogeneous blends containing polar additives or polymers, or to enhance adherence to other material [6]. Furthermore, the oxidized polymer might be used as additive for enhance the photo degradability of polyethylene's or changes its
* Corresponding author. rez). E-mail address: cjperez@fi.mdp.edu.ar (C.J. Pe https://doi.org/10.1016/j.polymdegradstab.2019.109001 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
hydrophobic nature making it less resistant to microorganism's absorption and so increasing its biodegradation rate [7,8]. The improvement in the mentioned properties has led to foresee oxidation by gamma radiation as alternative method for recycling polyolefins [9]. The crystallization of polyethylene's from the molten state has been extensively studied and the main factors controlling the dynamics of the process are reasonably known [10e12]. However, literature data relative to the crystallization process of oxidized polyethylene's are very scarce, even though this has an inherent importance in the recycling of the degraded materials [13e15]. The crystallization of oxidized polyethylene from the melt is probable to depend on the average molecular weight and on the chemical molecular structure. These two factors are altered respect to the untreated polymer, thus some differences in the crystallization of the materials are expected to exist. The objective of this work was to study the non-isothermal crystallization process of a set of materials obtained by the oxidative degradation of a random ethylene-butene copolymer induced by gamma ray irradiation under environments having different oxygen concentrations. This article follows a previous one dealing
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with the effect of the presence of oxygen in the irradiation medium on the resulting molecular structure of ethylene-butene copolymers [16]. The copolymer was irradiated using 60Co g-ray with doses in the range from 29 to 138 kGy under different atmospheres, which initially had a concentration of oxygen between 0 and 100 % v/v. All materials were characterized by combining the techniques of Size Exclusion Chromatography (SEC), Fourier Transform Infrared Spectroscopy (FTIR) and soluble extraction to obtain information on average molecular weights, oxidation level, and gel content, respectively. The non-isothermal crystallization behavior of the materials was access by differential calorimetry. The analysis of the crystallization behavior carried out as a function of the molecular characteristics of the copolymers allows increasing the information on the effect that the oxidation process has on the crystallization of ethylene-butene copolymers. 2. Experimental 2.1. Materials and samples preparation The ethylene copolymer used was a hydrogenated polybutadiene obtained by anionic polymerization of butadiene as described in previous studies [16,17]. The molecular structure of the copolymer is chemically similar to an ethylene-ran-1-butene copolymer with a composition of about 2.2 wt% of butene. The copolymer has a mass and number-average molecular mass of 72000 g/mol (Mw) and 65500 g/mol (Mn), respectively, which were determined by SEC. 2.2. Irradiation procedure Films of the copolymer were prepared by compression molding at 150 C using a hydraulic press with thermostatically controlled platens. The samples were molded between steel plates separated by a brass frame of 0.5 mm thick. Strips cut off from the films were included inside a glass vials in quantity so that the vials had the same mass of material. A vials set was placed inside a gloves box and kept under a media formed by a mixture of nitrogen/oxygen
where the concentration of oxygen was either 0, 21, 50, 75 and 100% v/v at 1 atm. Each set of vials was kept inside the box for two days, then the vials were tightly capped and exposed to g-rays generated by a60Co source at room temperature. The dose applied was in the range from 29 to 138 kGy at a rate of 8 kGy/h, determined by dosimetry using a radiochromic thin-film dosimeter. The error in dose can be estimated in 5%. 2.3. Sample analysis Gel fraction measurement was performed using xylene solvent and baskets made of cellulose paper, and the soluble fraction was extracted for different periods of 4 h at the boiling temperature (around 125 C). Between 0.4 and 0.8 g of sample were used by extracting of soluble portion. In order to prevent oxidation during extraction, a small amount of Irganox 1010 antioxidant was added to the xylene. The extraction was considered complete when, after two consecutive periods of extraction, there was no detectable change of weight in the dried gel. The percentage of gel reported is obtained by averaging four data results, the gel standard deviation is about 3e4%. The average molecular mass and molecular mass distributions of soluble samples were determined by Size Exclusion Chromatography (SEC). The SEC measurements were performed with a Waters 150-C ALP/GPC chromatograph equipped with set of PLGel columns from Polymer Labs and operating at 135 C. The solvent used was 1, 2, 4 trichlorobenzene. The number and mass average molecular mass, Mn and Mw, are included in Table 1, which were estimated following standard procedures. The oxidation was monitored by determining the presence and evolution of the 1710 cm1 IR absorption band attributed to carbonyl stretching. Fourier Transform Infrared (FTIR) spectrum of materials was recorded using a Nicolet 520 spectrometer operating in transmission mode and using a resolution of 4 cm1. The oxidation level of the samples was qualitatively compared by means of a carbonyl index, which was defined as the absorbance of the 1710 cm1 band associated to carbonyl stretching mode divided by those of a reference band centered at 2020 cm1 [18].
Table 1 Gel content (wt.%), average molecular weights (g/mol) from MALLS-SEC, Carbonyl index and Density (g/cm3). Sample
Gel (wt.%)
Mn (g/mol)
Mw (g/mol)
Carbonyl Index
Density (g/cm3)
Original PE Irradiated Samples Dose Atmosphere Nitrogen 29 84 138 21% Oxygen 29 84 138 50% Oxygena 34 69 136 75% Oxygena 34 69 136 100% Oxygena 34 69 136
e
65500
72000
e
0.9166
0 68 88
84000 e 29300b
126100 e 53000b
e e e
0.9168
0 ~0.4 39
28100 40400b 33000b
46100 97000b 93000b
0.9 3.7 3.8
0.9196 0.9211 0.9207
0 0 0
35900 22500 19000
51700 37700 52200
1.1 3.1 5.7
0.9172 0.9189 0.9255
0 0 ~1c
32000 21500 9600
47000 36000 31000
1.0 2.5 7.1
0.9164 0.9193 0.9277
0 0 0
36000 22000 18000
52000 37000 51000
1.0 2.3 5.5
0.9163 0.9199 0.9252
a b c
Initial concentration of oxygen (% v/v). Average molecular weights of the soluble fraction. The gel fraction was too low to be determined accurately.
0.9162
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The density of the material was also determined by means of the density gradient column method. Specimens of the materials were placed into a glass column filled with a diethylene glycolisopropanol mixture to give a gradient density range from 0.90 to 0.94 g/cm3. Standard glass floats were used to calibrate the column that was maintained at 23 C. 2.4. Non-isothermal crystallization process Differential scanning calorimetry (DSC) measurements were conducted with a PerkinElmer Pyris 2 DSC system using N2 as purge gas. The instrument was calibrated with indium standard for temperature and heat change. For non-isothermal crystallization, samples, of approximately 10 mg, was first heated up to 160 C and held at molten state for 2 min in order to remove thermal history. These heating and cooling steps were repeated to cover several cooling rates (5, 10, 20 and 30 C/min) with the same sample. Whole (not extracted) samples were used throughout for all DSC characterization. 3. Results and discussion 3.1. Molecular structure analysis In previous studies it was shown that molecular cross-linking reactions are dominant when ethylene-butene copolymers are exposed to high energy radiation in oxygen free atmosphere [16,17]. However, if the polymers are irradiated under oxygen containing ambient, scission reactions might also take place that can prevail under certain conditions. The competition between chain links and scission reactions determines the evolution of the molecular structure. The average molecular weight and the gel fraction of irradiated samples are included in Table 1, where the carbonyl index and density measured for the samples are also included. It can be observed in Table 1 that the reactions that produce chain linking prevail in the case of the materials obtained by irradiating the copolymer under nitrogen. The irradiation to a total dose of 29 kGy causes the average molecular weight to increase with respect to the original polymer while a macroscopy network is produced at higher dosages, which increases in proportion with dose. The Mn value of the soluble part of the material irradiated with the highest dose is much lower than that of the original copolymer indicating that scission is likely to occur. These results are consistent with those find in other studies dealing with the irradiation under inert atmosphere of ethylene-butene copolymers having similar molecular structure to the one used here [17,19]. The irradiation of the copolymer under media containing oxygen produces chain scission and chain linking due to the participation of the dissolved oxygen in reactions conducting to oxidation [16,20e23]. Thus, chain scission prevails when the copolymer is irradiated with the lowest doses under media having oxygen indicated by the reduction of the average molecular mass value of the copolymer respect to the initial one, while the irradiation of the copolymer to higher doses crosslinks becomes more important. The formation of macromolecular network is observed in the material obtained by irradiating the copolymer under media having initially 21% v/v of oxygen. When the irradiation process was performed under media having larger oxygen concentration, chain scission prevails under the whole dose range studied that is evidenced by a progressive decrease in the values of Mn with dose. Nevertheless, the contribution of chain linking process can be noted at the highest doses that is indicates by the upturn in the Mw values and concurrent widening of the molecular mass distribution. Moreover, a very small fraction of gel was detected in the case of samples
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irradiated in media with 75%v/v of oxygen. Early works on gamma irradiation of polyethylene's, showed that film thickness and dosage rate are factors controlling the level of oxidation, chain scission and crosslinks, which depend on the availability of oxygen in the bulk [20,21]. Thus, taking into account that the copolymer was irradiated in an atmosphere with limited oxygen concentration, it is possible that at the highest doses the amount of oxygen available to participate in reactions with macroradicals is low. In this way it is likely that molecular linking reaction may be a little more efficient. The shortage of dissolved oxygen can be due to a restricted diffusion of the gas from the surrounding medium to the depth of the material together with a reduction of its concentration in the media with increasing dosage. In parallel to chain linking and scission reactions, the exposure of polyethylene to radiation in oxygen-containing environments leads to the incorporation of various chemical groups having oxygen atoms, for example among other carbonyls from acids, ketones, esters, etc. [22e24]. The oxidation process of the polymer can be verified by analyzing the IR spectra of the samples in the region between 1600 and 1800 cm1 where carbonyl groups show characteristic absorption bands. The graphs in Fig. 1 contain IR spectra region for some of the samples that were chosen as illustrative examples of the progressive oxidation experience by the polymer. It can be seen in the spectra of the irradiated samples a set of overlapping bands centered at 1710 cm1, which can be associated to many oxidation products bearing carbonyl groups formed. The band due to the carbonyl groups in the irradiated samples, which is absent from the spectrum of the original polymer, grows with dosage reflecting the progressive oxidation of the material upon its irradiation under atmosphere with oxygen. In Table 1 the carbonyl index is presented for all the materials. It can be seen in the Table, that oxidation was not detected in the samples irradiated under nitrogen atmosphere. While the increase on the carbonyl index with dose indicates the progressive incorporation of carbonyl groups in the molecular structure after the irradiation of the copolymer under atmospheres with oxygen. A comparison of the carbonyl index values between samples receiving equivalent dose indicates that there are not significant differences in the oxidation level among the materials. Only in the case of the sample irradiated in 21% v/v oxygen with the highest doses seems to have suffered a somewhat lower level of oxidation than the others materials. To further analyze the effect of oxidation on the structure of the copolymer the density of the materials was measured, which it is another parameter that may be used to show the incorporation of oxygen atoms into the molecular structure of the copolymer [25]. The density is included in Table 1. The density measured for the pristine copolymer corresponds well with values already published for random copolymers with equivalent co-monomer concentration [26]. As it can be seen in Table 1, the density of the copolymer is not affected noticeably after being irradiated under nitrogen. However, this parameter shows an increasing trend with the dose in the oxidized samples. Within the limited amount of experimental data available, the density seems to reach a plateau in the case of irradiated samples with an initial oxygen concentration of 21%v/v, which corresponds with the marginal carbonyl index increment observed in these materials. On the other hand, in the case of materials obtained by irradiating the copolymer under atmospheres with larger oxygen concentration, the density increases progressively with the dosage. This increment in density can be partially associated with the incorporation of oxygen into the molecular structure of the copolymer, which is supported by the corresponding augment in the carbonyl index. The material irradiated to 136 kGy under media having initially 75% v/v of oxygen, displays the largest density and carbonyl index as well. It is necessary to
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Fig. 1. Region of IR spectra of the sample obtained by irradiating the copolymer in atmosphere of: 75% og oxygen (a) and 100% of Oxygen (b).
mention that another factor contributing to the density growth is the increment of crystallinity that might occur due to scissions of macromolecules in the non-crystalline regions. Molecular cleavage can favor the release of physical entanglements allowing sequences of molecular chains to crystallize during the oxidation process. This phenomenon has been observed in many studies dealings with the oxidative degradation of polyethylene's by radiation or thermal aging [18,24,27,28]. 3.2. Non-isothermal crystallization analysis The non-isothermal crystallization from the molten state of the materials was studied by differential scanning calorimetry at four cooling rates of 5, 10, 20, and 30 C/min. The curves in Fig. 2 includes crystallization exotherms recorded at different cooling rate for the original copolymer and some of the irradiated materials that were chosen to illustrate the effect induced by the radiation process on the crystallization. The vertical lines in the figures were drawn for helping to a visual comparison of the exotherms, and signals the crystallization temperature peak corresponding to the copolymer crystalized under a cooling rate of 5 C/min. The original copolymer shows a crystallization process that covers a wide temperature range, which is characterized by a main crystallization exotherm located at about 94 C and a weaker exotherm at ~65 C. The analysis of the crystallization exotherms provides with the crystallization temperature (Tp) and the crystallinity degree (Xc) that are included in Fig. 3. Tp was determined from the position of the minimum of the exotherm and Xc from the heat of crystallization (area of the exotherm) divided by 288 J/g which is the heat of fusion considered for a perfect crystalline polyethylene [12]. The crystallization temperature and the degree of crystallinity measured for the copolymer included in Fig. 3 are consistent with values reported in the literature for random ethylene copolymers with equivalent concentration of comonomers [29,30,31,32,33]. It can be seen in Fig. 2b that the crystallization process of the
materials irradiated under N2 shifts towards lower temperatures respect to that of the original copolymer (Fig. 2a). The changes in the features of the crystallization exotherms were similar for all cooling rates. As it can be seen in Fig. 3, the crystallization temperature and crystallinity slightly decrease with dosage from the value corresponding to the original copolymer. The material irradiated with the highest dose shows a Tp and Xc which is 4 C and 4% respectively, lower than that of the original material. Several literature reports have presented changes in the crystallization behavior of crosslinked polyethylene's qualitatively similar to the one found here [29,34e36]. The changes observed in the crystallization of the polymers irradiated under nitrogen reflect the molecular structure changes experimented by the copolymer as presented before. The incorporation of cross-linking into the molecular structure induce topological restrictions and reduces chain mobility that, in turn, limit the development and growth of crystals, which is reflected by a decrease in crystallinity and crystallization temperature with dosage [15,19,36]. The comparison between DSC traces in Fig. 2a and c shows that those materials obtained by irradiating the copolymer to low doses in an oxygen medium present a crystallization process that is slightly displaced towards higher temperature respect to that of the original copolymer. However, at the highest dose there is a reversion of the crystallization process towards lower temperatures. The material irradiated with a total dose of 136 kGy in 21% v/v of oxygen, shows a crystallization process that occurs in a temperature range almost equal to that of the original copolymer. It can be observed in Fig. 3 that the crystallization temperature and the degree of crystallinity initially increased with the dose and then decreased with the subsequent increase thereof. This behavior could be explained by a competition between molecular chain cleavage, which predominates in the low dose range, and crosslinking and incorporation of oxygenated groups that occurs as the dose increases, which may reduce molecular mobility and length of crystallizable sequences. In order to further analyze the crystallization process, the
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5
time por original copolymer was include for comparison. From the curves of relative crystallinity as a function of time some relevant data to describe non-isothermal crystallization behavior of the studied materials can be extracted. Fig. 5 show reciprocal of time at 10% of relative crystallinity as a function of cooling rate for (a) samples irradiated with different gamma doses in nitrogen and (b) samples irradiated at 136e138 kGy in 5 atm (0, 21, 50, 75 and 100% oxygen). It is possible to observe that, as expected, the crystallization rate increased with the cooling rate for all the materials. For a given cooling rate the rate of crystallization decreases with the radiation dose in nitrogen atmosphere. The same behavior is observed in other atmospheres (see Table 2). On other hand, Fig. 5b show irradiated samples to total doses of 136e138 kGy under different atmosphere. If a comparison is made at a certain cooling rate, the crystallization rate of the samples irradiated under a nitrogen atmosphere and at a low oxygen content (21% v/v) was lower than that of the original copolymer. While those materials irradiated in atmosphere with higher oxygen content presented a higher crystallization rate than the non-irradiated material. The results of Fig. 3b indicate that the greater the initial oxygen concentration in the irradiation medium, the greater the crystallization rate of the material. This behavior could be explained, as previously mentioned, by a competition between molecular chain cleavage, which predominates in the low dose range, and cross-linking and incorporation of oxygenated groups that occurs as the dose increases. In order to further analyzed the non-isothermal crystallization process, the Avrami's and Mo's methods were used to characterize the process. The commonly named Avrami's equation used for analyzing the crystallization kinetics is
X ¼ 1 expð kt n Þ:
Fig. 2. Non-isothermal crystallization exotherms obtained at different cooling rate of samples a) Non irradiated, b) Irradiated in nitrogen atmosphere at 84 kGy and c) Irradiated in 100% oxygen atmosphere at 69 kGy.
crystallization kinetics of the material was related by comparing the evolution of the relative crystallinity with time. The relative crystallinity was calculated as a function of temperature as the ratio of the exothermic peak areas according to
ðT dH dt T0 dT X ¼ ðT ∞ dH dt dT T0
(1)
where To and T∞ are the onset and end crystallization temperature, respectively, T is an arbitrary crystallization temperature. To was defined as the temperature where the tangent of the high temperature side of the exotherms cross the baseline. The temperature scale was transformed to a time scale by means of:
t¼
To T ∅
(2)
where T is the temperature at time t, and 4 is the cooling rate. Fig. 4 shows the relative degree of crystallinity as a function of time obtained using a cooling rate of 10 C/min for the samples irradiated under nitrogen (Fig. 4a) and under 100% v/v oxygen (Fig. 4b). In the figure the evolution of the relative crystallinity with
(3)
where n and k are adjustable parameters to fit the experimental results helping in the analysis of the crystallization process. The parameters do not have the physical meaning as in the original Avrami model applied to study isothermal crystallization process. As an example, the development of relative degree of crystallinity X(t) as a function of time for the original copolymer at various cooling rates is shown in Fig. 6. The parameters of the Avrami equation were obtained by fitting the relative crystallinity as function of time data using a non-linear multivariable regression fit. The Avrami approach was applied only to the early stages of crystallization (X from 2 to 30%) in order to avoid complications arising from the effects of secondary crystallization process. The internal graphic in Fig. 6 show an amplified zone of the early stages of crystallization in which the red curves results from the fit to the experimental data. Fig. 7 shows the n and k parameters, obtained from the best fit of the experimental data as a function of the cooling rate. Samples irradiated at different doses in nitrogen atmosphere were evaluated. On the other hand, in order to study the effect of the presence of oxygen, the higher dose was chosen because the differences are more significant than at lower doses. From Fig. 7a can be observed that n takes values from 1.9 to 2.3 (at 5 C/min) and decrease as the cooling rate increases in almost all cases in the irradiated samples with nitrogen [29]. These values of n were consistent with reported data in the literature for modified polyethylene [29,37,38]. Changes were associated to differences in the morphology of the gel fractions, changed from heterogeneously nucleated disks to fibrils or rods. It was also observed that Avrami coefficients changed as a function of the crosslink density which could be related to the large amounts of rod-like units contained in
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Fig. 3. Crystallinity degree (Xc,%) and crystallization temperature (Tc) measured from exotherms obtained at a cooling rate of 10 C/min.
Fig. 4. Relative crystallinity degree as a function of time at 10 C/min, for the samples irradiated at different doses in: a) nitrogen and b) 100% oxygen atmosphere.
the gel fractions. In the present work, the samples were tested without separating the sol and gel fractions, but the presence of both is useful to justify the fluctuating values obtained when comparing with the copolymer.
In addition, for a fixed radiation dose (Fig. 7c), n increased slightly as a function of oxygen content, because the material exhibited lower (or nothing) gel fraction and crosslinking degree. The k parameter (Fig. 7b and d) increases with the radiation
V.A. Alvarez et al. / Polymer Degradation and Stability 170 (2019) 109001
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Fig. 5. Overall crystallization rate as a function of cooling rate at 10% of relative crystallinity for: a) irradiated under nitrogen atmosphere and b) irradiated to total doses of 136e138 kGy under different atmosphere.
Table 2 Reciprocal of time at 10% of relative crystallinity as a function of cooling rate. Material/Cooling rate
5
10
20
30
Original copolymer
0.013
0.021
0.032
0.050
29-nitrogen 29-21% oxygen 34e50% oxygen 34e75% oxygen 34e100% oxygen
0.010 0.012 0.017 0.018 0.020
0.016 0.017 0.026 0.026 0.028
0.027 0.028 0.037 0.038 0.041
0.039 0.043 0.049 0.050 0.055
84-nitrogen 84-21% oxygen 69-50% oxygen 69e75% oxygen 69e100% oxygen
0.011 0.013 0.015 0.016 0.022
0.014 0.024 0.029 0.024 0.030
0.024 0.038 0.047 0.036 0.045
0.035 0.045 0.058 0.061 0.065
138-nitrogen 138-21% oxygen 136-50% oxygen 136-75% oxygen 136-100% oxygen
0.008 0.016 0.023 0.032 0.046
0.014 0.018 0.036 0.055 0.057
0.022 0.029 0.063 0.091 0.091
0.031 0.043 0.102 0.128 0.147
dose and decrease with the oxygen content. Similar behavior was observed in previous works [39]. This could be explained by the enhanced mobility of the polymer chains due to scission of the copolymer. The method proposed by Mo was applied as alternative approach to analyze the non-isothermal crystallization process of the materials [33]. The method combines the Avrami and Ozawa formalism, where K(T) and m are parameters. At a particular relative crystallinity, the equations are related as follows:
ln k þ nlnt ¼ ln KðTÞ m ln f
(4)
Fig. 6. Relative crystallinity as a function of time for crystallization of non irradiated material.
by reorganizing equation (3) the final form of the Mo is
ln f ¼ ln FðTÞ a ln t
(5)
where F(T) ¼ [K(T)/k]1/m represents the cooling rate needed to reach a defined degree of crystallinity at unit crystallization time, and a ¼ n/m is the ratio between the Avrami's to the Ozawa's exponent. Table 3 show the parameters obtained by analyzing the experimental data with the Mo's model. The parameters (a and F(t)) were obtained using equation (5), plotting ln q versus ln t. F(t) is obtained
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Fig. 7. Parameters of the Avrami's Model (n, k) as a function of the cooling rate for a-b) Irradiated under nitrogen atmosphere and c-d) Irradiated to total doses of 136e138 kGy under different atmosphere.
Table 3 Mo's Parameters of the Mo's model (a, F) of irradiated and original HPB at different crystallinity. Radiation/Relative crystallinity
Non irradiated 29 kgy - Nitrogen 84 kgy - Nitrogen 138 kGy - Nitrogen 138 kGy - 21% Oxygen 138 kGy-50% Oxygen 138 kGy 75%Oxygen 138 kGy-100% Oxygen
10%
20%
30%
40%
a
F(T)
a
F(T)
a
F(T)
a
F(T)
0.87 1.18 1.34 1.41 1.37 1.11 1.07 0.97
5.83 7.78 8.17 8.36 7.98 6.39 5.56 5.00
0.91 1.21 1.27 1.32 1.28 1.08 1.07 0.89
6.10 7.82 8.39 8.17 7.77 6.52 5.75 4.98
0.88 1.19 1.24 1.30 1.20 1.08 1.01 0.81
6.09 7.90 8.41 8.23 7.60 6.67 5.69 4.86
0.78 1.20 1.21 1.31 1.23 1.08 0.98 0.76
5.92 7.65 8.03 7.87 7.10 6.78 5.75 4.87
from slope and m from intercept between axes. The fit of the experimental data was done for the relative crystallinity range between 1 and 40%. It was observed, in all cases, that the values of F(T) showed small variations with the relative degree of crystallinity. On the other hand, for a given degree of crystallinity, F(T) increased with the radiation dose, indicating the higher difficulty of polymer for crystallize [39]. In addition, by considering the same radiation dose (136e138 kGy) and at a given degree of crystallinity, the values of F(T) decreased with increasing oxygen content. a Displayed a similar behavior.
3.3. CCT diagrams The crystallization behavior of the materials was further studied by analyzing continuous cooling transformation (CCT) diagrams that help comparing the extent of transformation as a function of time for a continuously decreasing temperature. Fig. 8 shows the CCT, where for a given crystallization extension (5% or 10% of relative crystallinity) the time and temperature are related at a constant cooling rate. Theoretically, each point on these curves has been obtained by integration of the full model (nucleation and growth) at a constant cooling rate. So, when the degree of
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Fig. 8. CCT diagrams at 5 and 10% of relative crystallinity for: a-b) irradiated under nitrogen atmosphere and c-d) irradiated to total doses of 136e138 kGy under different atmosphere.
crystallization curve is intercepted by a constant cooling rate one, the obtained point represents the time necessary to reach a specific relative degree of crystallinity under specific thermal conditions. Fig. 8a and b reveals that crystallization process of copolymer decrease with the radiation dose, crystallizing at lower temperature and at longer time when irradiated under nitrogen atmosphere showing the retarding effect observed previously. In the case of Fig. 8 c and d, that correspond to data results for samples irradiated at 136e138 kGy, the crystallization process occurs at higher temperature and shorter time as the oxygen content increases.
the decrease on oxygen concentration, for a given radiation dose, produced an increment on the same parameters, which in turns indicates that the crystallization process is restricted by the crosslinking degree. In addition, the parameters obtained from several studied models also confirm the observed tendencies. CCT diagrams, which allow determining the evolution of crystallinity degree under different processing conditions, were successfully constructed from experimental data. The obtained diagrams are powerful tools for the design and optimization of processing steps of ethylene-butene copolymers. The tendencies observed in such diagrams were in agreement with the changes in the as determined experimental parameters.
4. Conclusions The crystallization behavior of copolymers, which were obtained by oxidative degradation of a random ethylene-butene copolymer induced by gamma irradiation in an oxygen atmosphere, was studied by means of differential scanning calorimetry and analyzed by applying several kinetic models. We have found that several experimental parameters: the crystallization rate, the crystallization temperature and the crystallinity degree decreased as a function of the radiation dose in nitrogen atmosphere or the absence of oxygen. On the other hand,
Declaration of competing interest None declared. Acknowledgements The authors would like to acknowledge the financial support of the National Research Council of Argentina (CONICET) and the National Agency Promotion Scientific and Technological (ANPCyT).
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Effect of radiation-induced oxidative degradation on the nonisothermal crystallization of ethylene-butene copolymer rez c, * V.A. Alvarez a, M.D. Failla b, C.J. Pe sticos (CoMP) Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Facultad de Ingeniería, Grupo Materiales Compuestos Termopla n 10850, 7600, Mar Del Plata, Universidad Nacional de Mar Del Plata (UNMdP) and Consejo Nacional de Ciencia y Tecnología de Materiales (CONICET), Colo Buenos Aires, Argentina b PLAPIQUI (CONICET) - Departamento de Ingeniería (UNS) Camino “La Carrindanga” Km 7, 8000, Bahía Blanca, Buenos Aires, Argentina c Grupo Ciencia e Ingeniería de Polímeros, Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Facultad de Ingeniería, Universidad n 10850, 7600, Mar Del Plata, Buenos Nacional de Mar Del Plata (UNMdP) and Consejo Nacional de Ciencia y Tecnología de Materiales (CONICET), Colo Aires, Argentina a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 8 October 2019 Accepted 15 October 2019 Available online 15 October 2019
A random ethyleneebutene copolymer was irradiated with high ionizing energy in environments with different oxygen concentration. The non-isothermal crystallization process of the materials was studied by differential scanning calorimetry. When the polymer was exposed to radiation under free oxygen ambient, the temperature and the crystallinity degree decreased almost linearly with dose because chain-linking reaction prevails. On the contrary, those thermal parameters increased in the material obtained by irradiating the copolymer in environments with oxygen availability where chain scission reactions dominate. It was also found that, at equivalent irradiation dose, the crystallization rates decreased with the dose at a given cooling rate and with the reduction in oxygen content. The parameters obtained from different applied models also confirm the tendencies observed for the experimental variables. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Non-isothermal crystallization Ethylene-butene copolymer Gamma radiation oxidative degradation
1. Introduction The use of gamma irradiation provides an alternative mechanism for initiating oxidations of polyethylene's at temperatures close to the ambient one. The oxidative process is initiated by gamma irradiating the polymer in the presence of oxygen that causes chemical changes including chain scission, chain linking and the incorporation of polar oxygen-containing groups, such as acid, alcohol and ketone among others [1e5]. The oxidation worsens some important properties of the polymer, for instance the mechanical ones, but it can favorably affect others. For example, the introduction of functional groups increases the adhesion of the polyethylene to other materials with polar characteristics, making it easier to create more homogeneous blends containing polar additives or polymers, or to enhance adherence to other material [6]. Furthermore, the oxidized polymer might be used as additive for enhance the photo degradability of polyethylene's or changes its
* Corresponding author. rez). E-mail address: cjperez@fi.mdp.edu.ar (C.J. Pe https://doi.org/10.1016/j.polymdegradstab.2019.109001 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
hydrophobic nature making it less resistant to microorganism's absorption and so increasing its biodegradation rate [7,8]. The improvement in the mentioned properties has led to foresee oxidation by gamma radiation as alternative method for recycling polyolefins [9]. The crystallization of polyethylene's from the molten state has been extensively studied and the main factors controlling the dynamics of the process are reasonably known [10e12]. However, literature data relative to the crystallization process of oxidized polyethylene's are very scarce, even though this has an inherent importance in the recycling of the degraded materials [13e15]. The crystallization of oxidized polyethylene from the melt is probable to depend on the average molecular weight and on the chemical molecular structure. These two factors are altered respect to the untreated polymer, thus some differences in the crystallization of the materials are expected to exist. The objective of this work was to study the non-isothermal crystallization process of a set of materials obtained by the oxidative degradation of a random ethylene-butene copolymer induced by gamma ray irradiation under environments having different oxygen concentrations. This article follows a previous one dealing
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with the effect of the presence of oxygen in the irradiation medium on the resulting molecular structure of ethylene-butene copolymers [16]. The copolymer was irradiated using 60Co g-ray with doses in the range from 29 to 138 kGy under different atmospheres, which initially had a concentration of oxygen between 0 and 100 % v/v. All materials were characterized by combining the techniques of Size Exclusion Chromatography (SEC), Fourier Transform Infrared Spectroscopy (FTIR) and soluble extraction to obtain information on average molecular weights, oxidation level, and gel content, respectively. The non-isothermal crystallization behavior of the materials was access by differential calorimetry. The analysis of the crystallization behavior carried out as a function of the molecular characteristics of the copolymers allows increasing the information on the effect that the oxidation process has on the crystallization of ethylene-butene copolymers. 2. Experimental 2.1. Materials and samples preparation The ethylene copolymer used was a hydrogenated polybutadiene obtained by anionic polymerization of butadiene as described in previous studies [16,17]. The molecular structure of the copolymer is chemically similar to an ethylene-ran-1-butene copolymer with a composition of about 2.2 wt% of butene. The copolymer has a mass and number-average molecular mass of 72000 g/mol (Mw) and 65500 g/mol (Mn), respectively, which were determined by SEC. 2.2. Irradiation procedure Films of the copolymer were prepared by compression molding at 150 C using a hydraulic press with thermostatically controlled platens. The samples were molded between steel plates separated by a brass frame of 0.5 mm thick. Strips cut off from the films were included inside a glass vials in quantity so that the vials had the same mass of material. A vials set was placed inside a gloves box and kept under a media formed by a mixture of nitrogen/oxygen
where the concentration of oxygen was either 0, 21, 50, 75 and 100% v/v at 1 atm. Each set of vials was kept inside the box for two days, then the vials were tightly capped and exposed to g-rays generated by a60Co source at room temperature. The dose applied was in the range from 29 to 138 kGy at a rate of 8 kGy/h, determined by dosimetry using a radiochromic thin-film dosimeter. The error in dose can be estimated in 5%. 2.3. Sample analysis Gel fraction measurement was performed using xylene solvent and baskets made of cellulose paper, and the soluble fraction was extracted for different periods of 4 h at the boiling temperature (around 125 C). Between 0.4 and 0.8 g of sample were used by extracting of soluble portion. In order to prevent oxidation during extraction, a small amount of Irganox 1010 antioxidant was added to the xylene. The extraction was considered complete when, after two consecutive periods of extraction, there was no detectable change of weight in the dried gel. The percentage of gel reported is obtained by averaging four data results, the gel standard deviation is about 3e4%. The average molecular mass and molecular mass distributions of soluble samples were determined by Size Exclusion Chromatography (SEC). The SEC measurements were performed with a Waters 150-C ALP/GPC chromatograph equipped with set of PLGel columns from Polymer Labs and operating at 135 C. The solvent used was 1, 2, 4 trichlorobenzene. The number and mass average molecular mass, Mn and Mw, are included in Table 1, which were estimated following standard procedures. The oxidation was monitored by determining the presence and evolution of the 1710 cm1 IR absorption band attributed to carbonyl stretching. Fourier Transform Infrared (FTIR) spectrum of materials was recorded using a Nicolet 520 spectrometer operating in transmission mode and using a resolution of 4 cm1. The oxidation level of the samples was qualitatively compared by means of a carbonyl index, which was defined as the absorbance of the 1710 cm1 band associated to carbonyl stretching mode divided by those of a reference band centered at 2020 cm1 [18].
Table 1 Gel content (wt.%), average molecular weights (g/mol) from MALLS-SEC, Carbonyl index and Density (g/cm3). Sample
Gel (wt.%)
Mn (g/mol)
Mw (g/mol)
Carbonyl Index
Density (g/cm3)
Original PE Irradiated Samples Dose Atmosphere Nitrogen 29 84 138 21% Oxygen 29 84 138 50% Oxygena 34 69 136 75% Oxygena 34 69 136 100% Oxygena 34 69 136
e
65500
72000
e
0.9166
0 68 88
84000 e 29300b
126100 e 53000b
e e e
0.9168
0 ~0.4 39
28100 40400b 33000b
46100 97000b 93000b
0.9 3.7 3.8
0.9196 0.9211 0.9207
0 0 0
35900 22500 19000
51700 37700 52200
1.1 3.1 5.7
0.9172 0.9189 0.9255
0 0 ~1c
32000 21500 9600
47000 36000 31000
1.0 2.5 7.1
0.9164 0.9193 0.9277
0 0 0
36000 22000 18000
52000 37000 51000
1.0 2.3 5.5
0.9163 0.9199 0.9252
a b c
Initial concentration of oxygen (% v/v). Average molecular weights of the soluble fraction. The gel fraction was too low to be determined accurately.
0.9162
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The density of the material was also determined by means of the density gradient column method. Specimens of the materials were placed into a glass column filled with a diethylene glycolisopropanol mixture to give a gradient density range from 0.90 to 0.94 g/cm3. Standard glass floats were used to calibrate the column that was maintained at 23 C. 2.4. Non-isothermal crystallization process Differential scanning calorimetry (DSC) measurements were conducted with a PerkinElmer Pyris 2 DSC system using N2 as purge gas. The instrument was calibrated with indium standard for temperature and heat change. For non-isothermal crystallization, samples, of approximately 10 mg, was first heated up to 160 C and held at molten state for 2 min in order to remove thermal history. These heating and cooling steps were repeated to cover several cooling rates (5, 10, 20 and 30 C/min) with the same sample. Whole (not extracted) samples were used throughout for all DSC characterization. 3. Results and discussion 3.1. Molecular structure analysis In previous studies it was shown that molecular cross-linking reactions are dominant when ethylene-butene copolymers are exposed to high energy radiation in oxygen free atmosphere [16,17]. However, if the polymers are irradiated under oxygen containing ambient, scission reactions might also take place that can prevail under certain conditions. The competition between chain links and scission reactions determines the evolution of the molecular structure. The average molecular weight and the gel fraction of irradiated samples are included in Table 1, where the carbonyl index and density measured for the samples are also included. It can be observed in Table 1 that the reactions that produce chain linking prevail in the case of the materials obtained by irradiating the copolymer under nitrogen. The irradiation to a total dose of 29 kGy causes the average molecular weight to increase with respect to the original polymer while a macroscopy network is produced at higher dosages, which increases in proportion with dose. The Mn value of the soluble part of the material irradiated with the highest dose is much lower than that of the original copolymer indicating that scission is likely to occur. These results are consistent with those find in other studies dealing with the irradiation under inert atmosphere of ethylene-butene copolymers having similar molecular structure to the one used here [17,19]. The irradiation of the copolymer under media containing oxygen produces chain scission and chain linking due to the participation of the dissolved oxygen in reactions conducting to oxidation [16,20e23]. Thus, chain scission prevails when the copolymer is irradiated with the lowest doses under media having oxygen indicated by the reduction of the average molecular mass value of the copolymer respect to the initial one, while the irradiation of the copolymer to higher doses crosslinks becomes more important. The formation of macromolecular network is observed in the material obtained by irradiating the copolymer under media having initially 21% v/v of oxygen. When the irradiation process was performed under media having larger oxygen concentration, chain scission prevails under the whole dose range studied that is evidenced by a progressive decrease in the values of Mn with dose. Nevertheless, the contribution of chain linking process can be noted at the highest doses that is indicates by the upturn in the Mw values and concurrent widening of the molecular mass distribution. Moreover, a very small fraction of gel was detected in the case of samples
3
irradiated in media with 75%v/v of oxygen. Early works on gamma irradiation of polyethylene's, showed that film thickness and dosage rate are factors controlling the level of oxidation, chain scission and crosslinks, which depend on the availability of oxygen in the bulk [20,21]. Thus, taking into account that the copolymer was irradiated in an atmosphere with limited oxygen concentration, it is possible that at the highest doses the amount of oxygen available to participate in reactions with macroradicals is low. In this way it is likely that molecular linking reaction may be a little more efficient. The shortage of dissolved oxygen can be due to a restricted diffusion of the gas from the surrounding medium to the depth of the material together with a reduction of its concentration in the media with increasing dosage. In parallel to chain linking and scission reactions, the exposure of polyethylene to radiation in oxygen-containing environments leads to the incorporation of various chemical groups having oxygen atoms, for example among other carbonyls from acids, ketones, esters, etc. [22e24]. The oxidation process of the polymer can be verified by analyzing the IR spectra of the samples in the region between 1600 and 1800 cm1 where carbonyl groups show characteristic absorption bands. The graphs in Fig. 1 contain IR spectra region for some of the samples that were chosen as illustrative examples of the progressive oxidation experience by the polymer. It can be seen in the spectra of the irradiated samples a set of overlapping bands centered at 1710 cm1, which can be associated to many oxidation products bearing carbonyl groups formed. The band due to the carbonyl groups in the irradiated samples, which is absent from the spectrum of the original polymer, grows with dosage reflecting the progressive oxidation of the material upon its irradiation under atmosphere with oxygen. In Table 1 the carbonyl index is presented for all the materials. It can be seen in the Table, that oxidation was not detected in the samples irradiated under nitrogen atmosphere. While the increase on the carbonyl index with dose indicates the progressive incorporation of carbonyl groups in the molecular structure after the irradiation of the copolymer under atmospheres with oxygen. A comparison of the carbonyl index values between samples receiving equivalent dose indicates that there are not significant differences in the oxidation level among the materials. Only in the case of the sample irradiated in 21% v/v oxygen with the highest doses seems to have suffered a somewhat lower level of oxidation than the others materials. To further analyze the effect of oxidation on the structure of the copolymer the density of the materials was measured, which it is another parameter that may be used to show the incorporation of oxygen atoms into the molecular structure of the copolymer [25]. The density is included in Table 1. The density measured for the pristine copolymer corresponds well with values already published for random copolymers with equivalent co-monomer concentration [26]. As it can be seen in Table 1, the density of the copolymer is not affected noticeably after being irradiated under nitrogen. However, this parameter shows an increasing trend with the dose in the oxidized samples. Within the limited amount of experimental data available, the density seems to reach a plateau in the case of irradiated samples with an initial oxygen concentration of 21%v/v, which corresponds with the marginal carbonyl index increment observed in these materials. On the other hand, in the case of materials obtained by irradiating the copolymer under atmospheres with larger oxygen concentration, the density increases progressively with the dosage. This increment in density can be partially associated with the incorporation of oxygen into the molecular structure of the copolymer, which is supported by the corresponding augment in the carbonyl index. The material irradiated to 136 kGy under media having initially 75% v/v of oxygen, displays the largest density and carbonyl index as well. It is necessary to
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Fig. 1. Region of IR spectra of the sample obtained by irradiating the copolymer in atmosphere of: 75% og oxygen (a) and 100% of Oxygen (b).
mention that another factor contributing to the density growth is the increment of crystallinity that might occur due to scissions of macromolecules in the non-crystalline regions. Molecular cleavage can favor the release of physical entanglements allowing sequences of molecular chains to crystallize during the oxidation process. This phenomenon has been observed in many studies dealings with the oxidative degradation of polyethylene's by radiation or thermal aging [18,24,27,28]. 3.2. Non-isothermal crystallization analysis The non-isothermal crystallization from the molten state of the materials was studied by differential scanning calorimetry at four cooling rates of 5, 10, 20, and 30 C/min. The curves in Fig. 2 includes crystallization exotherms recorded at different cooling rate for the original copolymer and some of the irradiated materials that were chosen to illustrate the effect induced by the radiation process on the crystallization. The vertical lines in the figures were drawn for helping to a visual comparison of the exotherms, and signals the crystallization temperature peak corresponding to the copolymer crystalized under a cooling rate of 5 C/min. The original copolymer shows a crystallization process that covers a wide temperature range, which is characterized by a main crystallization exotherm located at about 94 C and a weaker exotherm at ~65 C. The analysis of the crystallization exotherms provides with the crystallization temperature (Tp) and the crystallinity degree (Xc) that are included in Fig. 3. Tp was determined from the position of the minimum of the exotherm and Xc from the heat of crystallization (area of the exotherm) divided by 288 J/g which is the heat of fusion considered for a perfect crystalline polyethylene [12]. The crystallization temperature and the degree of crystallinity measured for the copolymer included in Fig. 3 are consistent with values reported in the literature for random ethylene copolymers with equivalent concentration of comonomers [29,30,31,32,33]. It can be seen in Fig. 2b that the crystallization process of the
materials irradiated under N2 shifts towards lower temperatures respect to that of the original copolymer (Fig. 2a). The changes in the features of the crystallization exotherms were similar for all cooling rates. As it can be seen in Fig. 3, the crystallization temperature and crystallinity slightly decrease with dosage from the value corresponding to the original copolymer. The material irradiated with the highest dose shows a Tp and Xc which is 4 C and 4% respectively, lower than that of the original material. Several literature reports have presented changes in the crystallization behavior of crosslinked polyethylene's qualitatively similar to the one found here [29,34e36]. The changes observed in the crystallization of the polymers irradiated under nitrogen reflect the molecular structure changes experimented by the copolymer as presented before. The incorporation of cross-linking into the molecular structure induce topological restrictions and reduces chain mobility that, in turn, limit the development and growth of crystals, which is reflected by a decrease in crystallinity and crystallization temperature with dosage [15,19,36]. The comparison between DSC traces in Fig. 2a and c shows that those materials obtained by irradiating the copolymer to low doses in an oxygen medium present a crystallization process that is slightly displaced towards higher temperature respect to that of the original copolymer. However, at the highest dose there is a reversion of the crystallization process towards lower temperatures. The material irradiated with a total dose of 136 kGy in 21% v/v of oxygen, shows a crystallization process that occurs in a temperature range almost equal to that of the original copolymer. It can be observed in Fig. 3 that the crystallization temperature and the degree of crystallinity initially increased with the dose and then decreased with the subsequent increase thereof. This behavior could be explained by a competition between molecular chain cleavage, which predominates in the low dose range, and crosslinking and incorporation of oxygenated groups that occurs as the dose increases, which may reduce molecular mobility and length of crystallizable sequences. In order to further analyze the crystallization process, the
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time por original copolymer was include for comparison. From the curves of relative crystallinity as a function of time some relevant data to describe non-isothermal crystallization behavior of the studied materials can be extracted. Fig. 5 show reciprocal of time at 10% of relative crystallinity as a function of cooling rate for (a) samples irradiated with different gamma doses in nitrogen and (b) samples irradiated at 136e138 kGy in 5 atm (0, 21, 50, 75 and 100% oxygen). It is possible to observe that, as expected, the crystallization rate increased with the cooling rate for all the materials. For a given cooling rate the rate of crystallization decreases with the radiation dose in nitrogen atmosphere. The same behavior is observed in other atmospheres (see Table 2). On other hand, Fig. 5b show irradiated samples to total doses of 136e138 kGy under different atmosphere. If a comparison is made at a certain cooling rate, the crystallization rate of the samples irradiated under a nitrogen atmosphere and at a low oxygen content (21% v/v) was lower than that of the original copolymer. While those materials irradiated in atmosphere with higher oxygen content presented a higher crystallization rate than the non-irradiated material. The results of Fig. 3b indicate that the greater the initial oxygen concentration in the irradiation medium, the greater the crystallization rate of the material. This behavior could be explained, as previously mentioned, by a competition between molecular chain cleavage, which predominates in the low dose range, and cross-linking and incorporation of oxygenated groups that occurs as the dose increases. In order to further analyzed the non-isothermal crystallization process, the Avrami's and Mo's methods were used to characterize the process. The commonly named Avrami's equation used for analyzing the crystallization kinetics is
X ¼ 1 expð kt n Þ:
Fig. 2. Non-isothermal crystallization exotherms obtained at different cooling rate of samples a) Non irradiated, b) Irradiated in nitrogen atmosphere at 84 kGy and c) Irradiated in 100% oxygen atmosphere at 69 kGy.
crystallization kinetics of the material was related by comparing the evolution of the relative crystallinity with time. The relative crystallinity was calculated as a function of temperature as the ratio of the exothermic peak areas according to
ðT dH dt T0 dT X ¼ ðT ∞ dH dt dT T0
(1)
where To and T∞ are the onset and end crystallization temperature, respectively, T is an arbitrary crystallization temperature. To was defined as the temperature where the tangent of the high temperature side of the exotherms cross the baseline. The temperature scale was transformed to a time scale by means of:
t¼
To T ∅
(2)
where T is the temperature at time t, and 4 is the cooling rate. Fig. 4 shows the relative degree of crystallinity as a function of time obtained using a cooling rate of 10 C/min for the samples irradiated under nitrogen (Fig. 4a) and under 100% v/v oxygen (Fig. 4b). In the figure the evolution of the relative crystallinity with
(3)
where n and k are adjustable parameters to fit the experimental results helping in the analysis of the crystallization process. The parameters do not have the physical meaning as in the original Avrami model applied to study isothermal crystallization process. As an example, the development of relative degree of crystallinity X(t) as a function of time for the original copolymer at various cooling rates is shown in Fig. 6. The parameters of the Avrami equation were obtained by fitting the relative crystallinity as function of time data using a non-linear multivariable regression fit. The Avrami approach was applied only to the early stages of crystallization (X from 2 to 30%) in order to avoid complications arising from the effects of secondary crystallization process. The internal graphic in Fig. 6 show an amplified zone of the early stages of crystallization in which the red curves results from the fit to the experimental data. Fig. 7 shows the n and k parameters, obtained from the best fit of the experimental data as a function of the cooling rate. Samples irradiated at different doses in nitrogen atmosphere were evaluated. On the other hand, in order to study the effect of the presence of oxygen, the higher dose was chosen because the differences are more significant than at lower doses. From Fig. 7a can be observed that n takes values from 1.9 to 2.3 (at 5 C/min) and decrease as the cooling rate increases in almost all cases in the irradiated samples with nitrogen [29]. These values of n were consistent with reported data in the literature for modified polyethylene [29,37,38]. Changes were associated to differences in the morphology of the gel fractions, changed from heterogeneously nucleated disks to fibrils or rods. It was also observed that Avrami coefficients changed as a function of the crosslink density which could be related to the large amounts of rod-like units contained in
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Fig. 3. Crystallinity degree (Xc,%) and crystallization temperature (Tc) measured from exotherms obtained at a cooling rate of 10 C/min.
Fig. 4. Relative crystallinity degree as a function of time at 10 C/min, for the samples irradiated at different doses in: a) nitrogen and b) 100% oxygen atmosphere.
the gel fractions. In the present work, the samples were tested without separating the sol and gel fractions, but the presence of both is useful to justify the fluctuating values obtained when comparing with the copolymer.
In addition, for a fixed radiation dose (Fig. 7c), n increased slightly as a function of oxygen content, because the material exhibited lower (or nothing) gel fraction and crosslinking degree. The k parameter (Fig. 7b and d) increases with the radiation
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Fig. 5. Overall crystallization rate as a function of cooling rate at 10% of relative crystallinity for: a) irradiated under nitrogen atmosphere and b) irradiated to total doses of 136e138 kGy under different atmosphere.
Table 2 Reciprocal of time at 10% of relative crystallinity as a function of cooling rate. Material/Cooling rate
5
10
20
30
Original copolymer
0.013
0.021
0.032
0.050
29-nitrogen 29-21% oxygen 34e50% oxygen 34e75% oxygen 34e100% oxygen
0.010 0.012 0.017 0.018 0.020
0.016 0.017 0.026 0.026 0.028
0.027 0.028 0.037 0.038 0.041
0.039 0.043 0.049 0.050 0.055
84-nitrogen 84-21% oxygen 69-50% oxygen 69e75% oxygen 69e100% oxygen
0.011 0.013 0.015 0.016 0.022
0.014 0.024 0.029 0.024 0.030
0.024 0.038 0.047 0.036 0.045
0.035 0.045 0.058 0.061 0.065
138-nitrogen 138-21% oxygen 136-50% oxygen 136-75% oxygen 136-100% oxygen
0.008 0.016 0.023 0.032 0.046
0.014 0.018 0.036 0.055 0.057
0.022 0.029 0.063 0.091 0.091
0.031 0.043 0.102 0.128 0.147
dose and decrease with the oxygen content. Similar behavior was observed in previous works [39]. This could be explained by the enhanced mobility of the polymer chains due to scission of the copolymer. The method proposed by Mo was applied as alternative approach to analyze the non-isothermal crystallization process of the materials [33]. The method combines the Avrami and Ozawa formalism, where K(T) and m are parameters. At a particular relative crystallinity, the equations are related as follows:
ln k þ nlnt ¼ ln KðTÞ m ln f
(4)
Fig. 6. Relative crystallinity as a function of time for crystallization of non irradiated material.
by reorganizing equation (3) the final form of the Mo is
ln f ¼ ln FðTÞ a ln t
(5)
where F(T) ¼ [K(T)/k]1/m represents the cooling rate needed to reach a defined degree of crystallinity at unit crystallization time, and a ¼ n/m is the ratio between the Avrami's to the Ozawa's exponent. Table 3 show the parameters obtained by analyzing the experimental data with the Mo's model. The parameters (a and F(t)) were obtained using equation (5), plotting ln q versus ln t. F(t) is obtained
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Fig. 7. Parameters of the Avrami's Model (n, k) as a function of the cooling rate for a-b) Irradiated under nitrogen atmosphere and c-d) Irradiated to total doses of 136e138 kGy under different atmosphere.
Table 3 Mo's Parameters of the Mo's model (a, F) of irradiated and original HPB at different crystallinity. Radiation/Relative crystallinity
Non irradiated 29 kgy - Nitrogen 84 kgy - Nitrogen 138 kGy - Nitrogen 138 kGy - 21% Oxygen 138 kGy-50% Oxygen 138 kGy 75%Oxygen 138 kGy-100% Oxygen
10%
20%
30%
40%
a
F(T)
a
F(T)
a
F(T)
a
F(T)
0.87 1.18 1.34 1.41 1.37 1.11 1.07 0.97
5.83 7.78 8.17 8.36 7.98 6.39 5.56 5.00
0.91 1.21 1.27 1.32 1.28 1.08 1.07 0.89
6.10 7.82 8.39 8.17 7.77 6.52 5.75 4.98
0.88 1.19 1.24 1.30 1.20 1.08 1.01 0.81
6.09 7.90 8.41 8.23 7.60 6.67 5.69 4.86
0.78 1.20 1.21 1.31 1.23 1.08 0.98 0.76
5.92 7.65 8.03 7.87 7.10 6.78 5.75 4.87
from slope and m from intercept between axes. The fit of the experimental data was done for the relative crystallinity range between 1 and 40%. It was observed, in all cases, that the values of F(T) showed small variations with the relative degree of crystallinity. On the other hand, for a given degree of crystallinity, F(T) increased with the radiation dose, indicating the higher difficulty of polymer for crystallize [39]. In addition, by considering the same radiation dose (136e138 kGy) and at a given degree of crystallinity, the values of F(T) decreased with increasing oxygen content. a Displayed a similar behavior.
3.3. CCT diagrams The crystallization behavior of the materials was further studied by analyzing continuous cooling transformation (CCT) diagrams that help comparing the extent of transformation as a function of time for a continuously decreasing temperature. Fig. 8 shows the CCT, where for a given crystallization extension (5% or 10% of relative crystallinity) the time and temperature are related at a constant cooling rate. Theoretically, each point on these curves has been obtained by integration of the full model (nucleation and growth) at a constant cooling rate. So, when the degree of
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Fig. 8. CCT diagrams at 5 and 10% of relative crystallinity for: a-b) irradiated under nitrogen atmosphere and c-d) irradiated to total doses of 136e138 kGy under different atmosphere.
crystallization curve is intercepted by a constant cooling rate one, the obtained point represents the time necessary to reach a specific relative degree of crystallinity under specific thermal conditions. Fig. 8a and b reveals that crystallization process of copolymer decrease with the radiation dose, crystallizing at lower temperature and at longer time when irradiated under nitrogen atmosphere showing the retarding effect observed previously. In the case of Fig. 8 c and d, that correspond to data results for samples irradiated at 136e138 kGy, the crystallization process occurs at higher temperature and shorter time as the oxygen content increases.
the decrease on oxygen concentration, for a given radiation dose, produced an increment on the same parameters, which in turns indicates that the crystallization process is restricted by the crosslinking degree. In addition, the parameters obtained from several studied models also confirm the observed tendencies. CCT diagrams, which allow determining the evolution of crystallinity degree under different processing conditions, were successfully constructed from experimental data. The obtained diagrams are powerful tools for the design and optimization of processing steps of ethylene-butene copolymers. The tendencies observed in such diagrams were in agreement with the changes in the as determined experimental parameters.
4. Conclusions The crystallization behavior of copolymers, which were obtained by oxidative degradation of a random ethylene-butene copolymer induced by gamma irradiation in an oxygen atmosphere, was studied by means of differential scanning calorimetry and analyzed by applying several kinetic models. We have found that several experimental parameters: the crystallization rate, the crystallization temperature and the crystallinity degree decreased as a function of the radiation dose in nitrogen atmosphere or the absence of oxygen. On the other hand,
Declaration of competing interest None declared. Acknowledgements The authors would like to acknowledge the financial support of the National Research Council of Argentina (CONICET) and the National Agency Promotion Scientific and Technological (ANPCyT).
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