Hermán2013 Article Thermodegradativestudyofhdpeha

Polym. Bull. (2013) 70:81–96 DOI 10.1007/s00289-012-0781-3 ORIGINAL PAPER

Thermodegradative study of HDPE–HA nanocomposites: IKP and E2 function V. Herma´n • C. Albano • A. Karam • G. Gonza´lez M. Covis

•

Received: 13 November 2011 / Revised: 9 April 2012 / Accepted: 28 May 2012 / Published online: 7 June 2012 Ó Springer-Verlag 2012

Abstract IKP and E2 function methods were used to study thermal stability of HDPE–HA nanocomposites synthesized by in situ ethylene polymerization at different volumes of solvent and temperatures. Thermal analysis was carried out at five different heating rates, b = 3, 5, 7, 10, and 13 °C/min, under N2 atmosphere. Kinetics parameters calculated by IKP method presented a slight increase on activation energy when HA was incorporated in HDPE. A similar tendency was observed in the results obtained from the E2 function method, where the activation energy of the nanocomposites increased 100 kJ/mol with respect to unfilled polymer (420–460 kJ/mol). These results implied higher stability of HDPE due to HA incorporation. HDPE and HDPE–HA degradation mechanisms are represented by a set of functions, those with the highest probability were: nucleation and nucleus growth (S3) 23 %, reaction order (S5) 16 %, reaction in the interface (S6. S7, S8) 11–14 %, and potential law (S14, S17) 3 %. Keywords Nanocomposites  Thermal stability  Degradation mechanisms  IKP method  E2 function method

V. Herma´n (&)  A. Karam Centro de Quı´mica, Laboratorio de Polı´meros, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Carretera Panamericana Km11, Caracas 1020, Venezuela e-mail: [email protected] C. Albano  M. Covis Universidad Central de Venezuela, Facultad de Ingenierı´a, Escuela de Ingenierı´a Quı´mica, Caracas 1020, Venezuela e-mail: [email protected] G. Gonza´lez Laboratorio de Materiales, Centro de Ingenierı´a de Materiales y Nanotecnologı´a. Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Carretera Panamericana Km11, Caracas 1020, Venezuela

123

82

Polym. Bull. (2013) 70:81–96

Introduction In recent years hydroxyapatite (HA) nanoparticles have been used as reinforce filler for different polymeric materials [1–5]. In this type of materials high density polyethylene–hydroxyapatite composites (HDPE–HA) have been used widely in biomedicine since 1980s and have been available commercially as HAPEXTM from 1990, mainly due to the bioinert characteristics of HDPE and to the bioactive properties of HA [6–9]. These composites have an interfacial interaction between the filler and the polymeric matrix, which is translated to an improvement in mechanical properties of the polyolefin [10, 11]. The study of the thermal properties of these materials and possible mechanisms of degradation are very important in order to understand their behavior under thermal heating [12]. To study the thermal stability, the composites are set under a progressive heating, and the data obtained is evaluated using mathematical models to calculate the kinetic parameters (Ea, A, and f(a)), involved in the degradation process [13]. All the mathematical models that have been used are based on the Arrhenius law, and the analysis can be made using this principal equation [14]: da da b ¼ kf ðaÞ dt dT

ð1Þ

where a is the degree of conversion and k ¼ AeðEa=RT Þ according to the Arrhenius Law. The Coast–Redfern is an integral method applied to study thermogravimetric data in which the reaction order is assumed to be equal or different to 1, justified by most of the decomposition reactions in solid state, following the next equations: For n = 1 !   1  ð1  aÞ1n AR 2 RT E 1  ð2Þ log  ¼ log BE E 2:3 RT T 2 ð1  nÞ For n = 1      logð1  aÞ AR 2 RT E log 1  ¼ log T2 BE E 2:3 RT

ð3Þ

On the other hand, the invariant kinetics parameters (IKP) method is a versatile approach that allows us to calculate the kinetics parameters independent of the heating rate, and to determine the possible degradation mechanism. This method is based on the studied of the compensation effect, suggested by Lesnikovich and Levchik [15–17], were the kinetic parameters are evaluated using a single experimental curve: a = a(T) with different analytical forms, which are correlated through the compensation effect: ln A ¼ a þ b Ea

ð4Þ

where a* and b* are constant compensation effect parameters. Compensation effect is observed on each of these functions fj(a), when log (Aj) is plot versus Ej, a straight line is obtained for each heating rate, define as [18, 19]:

123

Polym. Bull. (2013) 70:81–96

83

log Ajv ¼ bv þ Iv Ejv

ð5Þ

where Ajv is the apparent calculated pre-exponential factor with fj(a) for bv, and Ejv is the apparent calculated activation energy [20]. Values of bv and Iv are calculated from the intercept and the slope of the straight lines obtained from Eq. (3). Lesnikovich and Levchik [15–17] and Levchik et al. [20] discussed the significance of these values and demonstrated the following relationships: bv ¼ logðkv Þ

ð6Þ

1 2:3RTv

ð7Þ

Iv ¼

where kv is the rate constant of the system at the temperature Tv; these two parameters are characteristic of the experimental conditions. The log kv curves are then plotted against 1/Tv, following the next equation: logðkv Þ ¼ logðAinv Þ 

Einv 2:3RTv

ð8Þ

Thus, the values of the invariant activation energies and pre-exponential factors can be calculated from the slopes and intercepts of the curves. On the other hand, the probabilities associated with the eighteen apparent activation energies (Ejv) and pre-exponential factors (Ajv) are calculated using the Coast–Redfern method [21], and then computing modeled. Kinetic functions fj(a) may then be discriminated using the log (Ainv) and Einv values, where n is the ith term of the experimental values of (da/dT)iv, the residual sum of squares for each fj(a) and for each heating rate bv may be computed as [19, 22]: 2  i¼n  X   da  Ainv eðEinv=RTinv Þ fj ðainv Þ ðn  1ÞS2jv ¼ ð9Þ  dT  bv i¼1 The most probable function is then chosen by the average minimum value of Sj defined by the relationship: v¼p 1X SJ ¼ Sjv ð10Þ p v¼1 where p is the number of heating rates used. The probability associated with each fj(a), value can be calculated by defining the ratio: S2 Fj ¼ 2J Smin

ð11Þ

P 2 2 where S2 ¼ 1p v¼p v¼1 Sjv and Smin is the average minimum of residual dispersion. This ratio obeys the F distribution: v

1

Fj2 CðvÞ qðFj Þ ¼ 2 v  v C =2 ð1 þ Fj Þ

ð12Þ

123

84

Polym. Bull. (2013) 70:81–96

where n is equal to the number of degrees of freedom for every dispersion and C is the gamma function (function that extends the concept of factorial to complex numbers). The probabilities of the jth function are computed on the assumption that the experimental data with L kinetic functions is described by a complete and independent system of events: j¼L X

Pj ¼ 1

ð13Þ

j¼1

In this way, Pj can be obtained, using the function fj(a): qðFiÞ Pj ¼ j¼L P qðFiÞ

ð14Þ

j¼1

Another model used is E2 function [23], an integral method that consists of a mathematical model based on the equation of Arrhenius, that allows to calculate the activation energy once the reaction order (n) is known, using Coats–Redfern method [21]. In addition, three conversions (a1 = 0.05, a2 = 0.50, and a3 = 0.95) with their decomposition temperatures are used. The equations in this mathematical model differ according to the reaction order (n) as follows: n=1 ð1  a1 Þ1n  ð1  a2 Þ1n ð1  a2 Þ

1n

n=1 ln ln

 ð1  a3 Þ







¼

1a1 1a2 1a2 1a3

1n

¼

T2 E2 ðE=RT2 Þ  T1 E2 ðE=RT1 Þ T3 E2 ðE=RT3 Þ  T2 E2 ðE=RT2 Þ

T2 E2 ðE=RT2 Þ  T1 E2 ðE=RT1 Þ T3 E2 ðE=RT3 Þ  T2 E2 ðE=RT2 Þ

ð15Þ

ð16Þ

The principal advantage of E2 function model is that only one heating rate is required, also it is reliable and easy to apply. However, more computational efforts are required and n-value has to be found using other methods such as Coast– Redfern. The mathematical models previously explained were used to study the thermal stability and possible decomposition mechanisms of HDPE–HA composite, due to its potential applications in biomedicine.

Experimental Materials Calcium hydroxide and di-ammonium hydrogen phosphate were supplied by Fisher Chemicals. For the polymerization the following reactants were used: ethylene with

123

Polym. Bull. (2013) 70:81–96

85

a 5.0 grade of purity (Boc Gases), toluene (Riedel de Hae¨n, p.a.), Cp2ZrCl2 (Sigma Aldrich), and methylaluminoxane (MAO) with 12.77 % of aluminum (Akzo Chemicals).

Synthesis and characterization of HA A wet chemical precipitation method was used to synthesize HA [24], with equimolar solution of calcium hydroxide and di-ammonium hydrogen phosphate. The resulting suspension was washed with de-ionized water and centrifuged several times until pH neutral was achieved. Afterwards, HA was dried at 80 °C for 48 h, and then pulverized. Fourier transformed infrared spectroscopy (FTIR, Nicolet iS10) was used to determine characteristic functional groups of HA, using KBr tablets and 64 scans [24]. X-ray diffraction was used to determine the crystalline characteristics of HA (XRD, Siemens D5005 with a Cu source) [25]. Transmission electron microscopy was used to study morphology and dispersion of HA nanoparticles into the polymeric matrix, samples were prepared employing a suspension technique. TEM, Jeol JEM 1220 100 keV was used. In addition, the thermal behavior of the HA was studied by thermogravimetric analysis using a heating rate of 10 °C/min from 25 to 700 °C, under N2 atmosphere (TGA, Mettler Toledo TGA/STA851e) [26, 27].

Synthesis of HDPE–HA composite HDPE–HA nanostructure composites were synthesized by in situ ethylene polymerization. Toluene was used as solvent, previously dried, and Cp2ZrCl2/MAO as the catalytic system. In situ ethylene polymerization was carried out at different volumes of solvent (100–300 ml) and temperatures (10–75 °C), with a constant percentage of HA (15 % % 0.8720 ± 0.0001 g). HA was suspend in 25 ml of toluene and transferred to Bu¨chi reactor using Schlenck techniques [28]. Co-catalyst was dissolved in 10 ml of toluene, and also added to the Bu¨chi reactor. After 10 min of agitation a solution of pre-catalysts was transferred into the reactor. Polymerization was carried out at constant ethylene pressure (1 bar) for 30 min [29]. The reaction was quenched by addition of 10 % HCl solution in ethanol. The polymer obtained was washed three times with ethanol, and dried in vacuum at 60 °C for 12 h.

Thermal studied of HDPE–HA composite Thermal analysis was carried out in a Metler Toledo TGA/STA851e. All samples were analyzed under the following conditions: *9.0–10.0 mg of HDPE–HA nanostructure composites were heated until 700 °C under N2 atmosphere, at five different heating rates, b = 3, 5, 7, 10, and 13 °C/min. The data obtained was evaluated using IKP and E2 function methods.

123

86

Polym. Bull. (2013) 70:81–96

Fig. 1 FTIR of HA synthesized nanocrystals

Fig. 2 DRX of HA synthesized nanocrystals

Results and discussion HA synthesis was extremely effective, resulting in a high yield (99 %). Characterization by FTIR showed the characteristic HA functional groups (Fig. 1). Vibration of the hydroxyl group was observed around 3,700 cm-1, while the bands at 1429, 1089–1035, 962, 603, and 565 cm-1 correspond to the vibration modes of the phosphate group (PO4-3) [24, 30, 31]. X-ray diffraction presented all characteristic reflections of HA, with broad peaks, as a result of its nanometric crystal size (Fig. 2) [24]. HA nanoparticles have needle morphology with an average length of (55 ± 9) nm and average diameter equal to (10 ± 1) nm [25, 32], as can be observed in Fig. 3. HA present high thermal stability, only 5 % of mass was lost at 80 °C (Fig. 4), this mass is associated to the loss of water molecules adsorbed on the nanoparticle surface [24]. HDPE–HA nanocomposites were obtained by ethylene in situ polymerization, synthesized at different toluene volume and temperatures (Table 1). Each of these compounds were analyzed by TGA at five different heating rates, b = 3, 5, 7, 10, and 13 °C/min. IKP and E2 function methods were used to study their thermal stability and to determine the kinetic parameters.

123

Polym. Bull. (2013) 70:81–96

87

Fig. 3 TEM of HA synthesized nanocrystals

Fig. 4 TGA of HA synthesized nanocrystals

Table 1 HDPE–HA composites codification

Reaction conditions 2,000 rpm, 1 bar, 30 min, [Al]/[Zr] = 500

Sample

Polymerization conditions

HDPE

100 ml of solvent, 25 °C

HDPE–HA_100 ml

100 ml of solvent, 25 °C, 15 % of HA

HDPE–HA_200 ml

200 ml of solvent, 25 °C, 15 % of HA

HDPE–HA_300 ml

300 ml of solvent, 25 °C, 15 % of HA

HDPE–HA_10 °C

100 ml of solvent, 10 °C, 15 % of HA

HDPE–HA_75 °C

100 ml of solvent, 75 °C, 15 % of HA

The thermograms obtained (Fig. 5) showed that the decomposition process of HDPE–HA composites goes on a single step between 430 and 509 °C. As can be seen on Table 2 when the heating rate increased it is observed that the initial (Tid), final (Tfd) decomposition temperatures, and the peak temperatures (Tp) were shifted to higher values, this could be attributed to a reduced heat transfer requiring greater energy for the degradative processes to occur resulting in an increase in the temperatures (Fig. 6) [23, 33].

123

88

Polym. Bull. (2013) 70:81–96

Fig. 5 TGA of: a HDPE, b HDPE–HA_100 ml, c HDPE–HA_200 ml, d HDPE–HA_300 ml, e HDPE– HA_10 °C, and f HDPE–HA_75 °C

Even though a significance change in the Tid, Tfd, and Tp was not observed when the filler was incorporated into the polymer, the slope of the thermograms changed, as can been seen on Fig. 7, thermograms of HDPE–HA composites present a higher slope compared to HDPE. This could indicate an increase in the stability of the materials, although it is not reflected in the decomposition temperatures. The IKP method allowed to determine the invariant kinetic parameters, such as activation energy and Arrhenius pre-exponential factor that are independent of the temperature at which the experiment was carried out (Table 3). Figure 8 showed the

123

Polym. Bull. (2013) 70:81–96 Table 2 HDPE–HA composite decompositions temperatures

89

Sample

Heating velocity (°C)

Tid (°C)

Tfd (°C)

Tp (°C)

HDPE

3

436

483

465

5

437

491

470

7

437

496

474

10

447

503

480

13

460

506

486

3

436

482

464

5

443

490

470

7

454

496

479

10

458

503

484

HDPE–HA_100 ml

HDPE–HA_200 ml

HDPE–HA_300 ml

HDPE–HA_10 °C

HDPE–HA_75 °C Tid initial decomposition temperature, Tfd final decomposition temperature, Tp maximum decomposition temperature

13

453

505

483

3

441

481

466

5

445

488

471

7

455

496

480

10

458

502

483

13

462

508

487

3

436

482

465

5

448

489

473

7

455

496

475

10

459

502

483

13

461

507

486

3

439

480

464

5

447

487

472

7

451

494

477

10

458

502

483

13

461

509

488

3

440

481

466

5

447

489

472

7

452

494

477

10

454

500

481

13

461

507

488

compensation effect for HDPE, (log (Ajv) vs Ej), similar results were obtained for each HDPE–HA composites [18, 19]. These results demonstrate a slight increase in the invariant kinetic parameters when the filler was added into the polymeric matrix. This suggests that HA confers thermal stability to polyethylene, and this can be attributed to a good dispersion of the filler, that delays the thermodegradatives processes, in good agreement with the observed change in slope of the thermograms (Fig. 7) [22]. By adding HA into the polymer matrix the chemical reaction kinetics slowed down, similar results were obtained by Chrissafisa et al. [33, 34], who studied the effect of different inorganic nanoparticles on the thermal stability of polymers, and

123

90

Polym. Bull. (2013) 70:81–96

Fig. 6 First derivate of: a HDPE, b HDPE–HA_100 ml, c HDPE–HA_200 ml, d HDPE–HA_300 ml, e HDPE–HA_10 °C, and f HDPE–HA_75 °C

attributed the thermal stability improvement to different reasons, such as, shielding effect, the gas impermeability of inorganic nanoparticles, which inhibit the formation and escape of volatile by products during degradation, reducing the chain mobility and consequently delaying degradation in polymer–ceramics composites. The kinetic analysis demonstrates that the process of degradation requires higher activation energy for the composite than for the polymer. The highest energy barrier indicates that the presence of HA decreases molecular mobility and consequently, increasing chemical and physical stability. During

123

Polym. Bull. (2013) 70:81–96

91

Fig. 7 Thermogram zooms (in the range 420–500 °C) of HDPE–HA composites at different heating rates: a 3 °C/min and b 10 °C/min

Table 3 Invariant kinetic parameters obtained by IKP method

Sample

Eainv (kJ/mol)

Ainv (l/min)

HDPE

303

3.149E ? 20

HDPE–HA_100 ml

320

5.561E ? 21

HDPE–HA_200 ml

325

1.281E ? 19

HDPE–HA_300 ml

323

7.335E ? 21

HDPE–HA_10 °C

327

3.260E ? 18

HDPE–HA_75 °C

318

3.855E ? 21

Fig. 8 The straight line log (Aj) vs. Ej for the thermal degradation of HDPE, at all used heating rates (corroborating compensation effect)

degradation, an increase in the activation energy also means an increase in stability due to the retarding effect. On the other hand, the distributions of probabilities associated with the 18 degradation functions proposed in the literature are presented in Fig. 9. Degradation of HDPE and HDPE–HA composites are a complex phenomenon and are represented by a set of functions: ‘‘nucleation and nucleus growth kinetic model’’

123

92

Fig. 9

Fig. 10

Polym. Bull. (2013) 70:81–96

Probabilities evaluated by IKP

Possible routes of radical formation involved in HDPE degradation

with approximately 23 % of probability (S3) and ‘‘reaction order’’ with a 16 % of probability (S5). Also, mechanisms with lower probabilities occurred, like reaction at the interface 11–14 % (S6, S7, and S8), and potential law, with very low probability, around 3 % (S14 and S17). The probability of the latter mechanism decreases when HA is incorporated into the polymeric matrix; this might indicate that HA avoided fast reactions between molecules with low molecular weight. From these results it can be inferred that thermal degradation of HDPE and HDPE–HA composites initiate on the polymer melt. Therefore, at this stage the reactions that might be involved are ‘‘nucleation and nucleus growth kinetic’’ mechanism (S1, S2, S3, and S4), forming free radicals by two possible routes (Fig. 10) [35], these isolated radicals begin to form very close radical groups (called clusters). When part of this polymer is carbonized forming a solid, the mechanisms of degradation might be changed, due to the diffusion of low molecular weight

123

Polym. Bull. (2013) 70:81–96

93

Fig. 11 Activation energy obtained by E2 function

chains into the interface, allowing the degradation processes to continue. However, as can be observed in Fig. 9, diffusion processes do not occurred. Also, degradation mechanisms of ‘‘reaction at the interface’’ are present, corresponding to S6, S7, and S8; this is attributed to crosslinking reactions between the melt polymer, degraded material, and decomposition gases. Finally, the molecules of low molecular weight formed in the first stage of decomposition lead to the formation of a new phase that corresponds mainly to a reaction order mechanism (S5) and to potential law (S14, S17), similar results were obtained by Albano et al. [36]. These results indicated that incorporation of HA into the polymeric matrix do not modified significantly the degradation mechanisms of HDPE. The kinetic parameters calculated by the E2 function method (Fig. 11), presented a similar tendency to the results obtained by IKP. The activation energy exhibits an increase when HA is incorporated into the polyethylene matrix, this energy is between 420 and 460 kJ/mol, which confirms that HA produces an increase in the stability of the polymer. It is worth to mention that the differences between Ea calculated by both methods are based on the approximations of the mathematical models employed by each method. The Ea obtained by the two mathematical methods is high compared to reports in the literature for polyethylene of low molecular weight (Mw = 23.000) [35]. However, the Mw of our polymer is Mw = 425.000 (measured by viscosimetry method). This result suggests a direct relationship between molecular weight and activation energy, when Mw increases a decrease in the chain mobility is obtained, so a greater energy is required to degrade the polymer. A good dispersion of HA was observed by TEM, with the characteristic needle morphology and nanometric size. The images obtained indicate that the best HA dispersion was achieved when polymerization was carried out at 10 °C and 200 ml (Fig. 12), whereas under the other conditions employed formation of agglomerates was observed. However, these agglomerates have small sizes that did not produce significant change in the kinetic parameters, calculated by the IKP and E2 function methods.

123

94

Fig. 12

Polym. Bull. (2013) 70:81–96

TEM of: a HDPE–HA_10 °C and b HDPE–HA_200 ml

Conclusions IKP and E2 function methods allowed determining the kinetic parameters of the HDPE–HA composites obtained by in situ ethylene polymerization. The activation energy, Ea, calculated using IKP and E2 function presented the same tendency; both results indicated an increase in the activation energy of HDPE when HA was included. This increase in the activation energy indicates an increase in the thermal stability of the polymer. Also, not significant differences were observed when the synthesis was carried out at different conditions. HDPE and HDPE–HA degradation mechanisms are represented by a set of functions, those with higher probability are: nucleation and nucleus growth (S3) 23 %, reaction order (S5) 16 %, reaction at the interface (S6, S7, S8) 11–14 %, and potential law (S14, S17) 3 %.

References 1. Dalby MJ, Di Silvio L, Harper EJ, Bonfield W (2002) Increasing hydroxyapatite incorporation into poly(methylmethacrylate) cement increases osteoblast adhesion and response. Biomaterials 23(2): 569–576 2. Hing K, Best A, Bonfield W (1999) Characterization of porous hydroxyapatite. J Mater Sci Mater Med 10(3):135–145 3. Yumei X, Dongxiao L, Hongsong F, Xudong L, Zhongwei G, Xingdong Z (2007) Preparation of nano HA–PLA composites by modified PLA for controlling the grow of HA crystals. Mater Lett 61:59–62 4. Wang M, Bonfield W (2001) Chemically coupled hydroxyapatite-polyethylene composites: structure and properties. Biomaterials 22(11):1311–1320 5. Jia L, Wei Z, Li L, Yufeng Z, Lou X (2009) Thermal degradation kinetics of g-HA/PLA composites. Thermochim Acta 493:90–95

123

Polym. Bull. (2013) 70:81–96

95

6. Bonfield W, Grynpas MD, Tully AE, Bowman J, Abram J (1981) Hydroxyapatite reinforced polyethylene—a mechanically compatible implant material for bone replacement. Biomaterials 2(3):185–186 7. Bonfield W (1988) Hydroxyapatite-reinforced polyethylene as an analogous material for bone replacement. J Am Chem Soc 523:173–177 8. Dalby MJ, Di Silvio L, Davies GW, Bonfield W (2000) Surface topography and HA filler volume effect on primary human osteoblasts in vitro. J Mater Sci Mater Med 11(12):805–810 9. Rea SM, Best SM, Bonfield W (2004) Bioactivity of ceramic-polymer composites with varied composition and surface topography. J Mater Sci Mater Med 15(9):997–1005 10. Wang M, Joseph R, Bonfield W (1998) Hydroxyapatite-polyethylene composites for bone substitution: effects of ceramic particle size and morphology. Biomaterials 19(24):2357–2366 11. Shahbazi R, Javadpour J, Khavandi AR (2006) Effect of nanosized reinforcement particles on mechanical properties of high density polyethylene-hydroxyapatite composites. Adv App Ceram 105(5):253–258 12. Albano C, Catan˜o L, Perera R, Karam A, Gonza´lez G (2010) Thermodegradative and morphological behavior of composites of HDPE with surface treated hydroxyapatite. Polym Bull 64(1):67–79 13. Budrugeac P (2005) Some methodological problems concerning the kinetic analysis of non-isothermal data for thermal and thermo-oxidative degradation of polymers and polymeric materials. Polym Degrad Stab 89:265–273 14. Bojan J, Borivoj A (2007) The use of the IKP method for evaluating the kinetic parameters and the conversion function of the thermal decomposition of NaHCO3 from nonisothermal thermogravimetric data. Int J Chem Kinet 39(8):462–471 15. Lesnikovich AJ, Levchik SV (1983) A method of finding invariant values of kinetic parameters. J Therm Anal 27:89–93 16. Levchik SV, Levchik GF, Lesnikovich AJ (1985) Analysis and development of effective invariant kinetic parameters finding method based on the non-isothermal data. Thermochim Acta 92:157–160 17. Lesnikovich AJ, Levchik SV (1985) Isoparametric kinetic relations for chemical transformations in condensed substance. J Therm Anal 30:677–702 18. Francois D, Serge B, Rene D, Le Bras Michel (2000) Kinetic medelling of the thermal degradation of polyamide-6 nanocomposite. Eur Polym J 36:273–284 19. Bourbigot S, Flambard X, Duquesne S (2001) Thermal degradation of poly(p-phenylenezobisoxazole) and poly(p-phenylenediamine terephthalamide) fibres. Polym Int 50:157–164 20. Levchick SV, Levchick GF, Guslev VG (1984) Thermolysis of potassium tetraperoxochromate (V). II Linear heating. Thermochim Acta 77(1–3):357–365 21. Coats AW, Redfern JP (1964) Kinetic parameters from thermogravimetric data. Nature 201:68 22. Catan˜o L, Albano C, Karam A, Perera R, Silva P (2007) Thermal stability evaluation of PA6/LLDPE/ SEBS-g-DEM blends. Macromol Symp 257:147–157 23. Chen H, Lai K, Lin Y (2004) Methods for determining the kinetic parameters from nonisothermal thermogravimetry: a comparison of reliability. J Chem Eng Jpn 37(9):1172–1178 24. Koutsopoulos S (2002) Synthesis and a characterization of hydroxyapatite crystals: a review study in analytical methods. J Biomed Mater Res 62(4):600–612 25. Keller L (1995) X-ray powder diffraction pattern of calcium phosphate analyzed by Rietveld method. J Biomed Mater Res 29(11):1403–1413 26. Albano C, Karam A, Dominguez N, Sanchez Y, Perera R, Gonzalez G (2006) Optimal conditioning for preparation of HDPE-HA composite in an internal mixer. Mol Cryst Liq Cryst 448:251–259 27. Minkova L, Magagnini PL (1993) Crystallization behaviour and thermal stability of HDPE filled during polymerization. Polym Degrad Stab 42:107–115 28. Shiver DF, Drezdozn MA (1986) The manipulation of air-sensitive compounds. The manipulation of air-sensitive compounds. Wiley, Chichester 29. Xiaochen D, Li W, Libo D, Jianhua L, Jia H (2006) Preparation of nanopolyethylene wire with carbon nanotubes supported Cp2ZrCl2 catalyst. J Appl Polym Sci 101(3):1291–1294 30. Arman E, Mats J, Richardson CF, Nancollas GH (1993) The characterization of hydroxyapatite preparations. J Colloid Interface Sci 159(1):158–163 31. Fowler BO (1974) Infrared studies of apatites. I. Vibrational assignments for calcium, strontium, barium hydroxyapatites utilizing isotopic substitution. Inorg Chem 13:194–207 32. Bale WF, Bonner JF, Hodge HC, Adeler H, Wreath AR, Bell R (1945) Optical and X-ray diffraction studies of certain calcium phosphates. Ind Eng Chem 17(8):491–495

123

96

Polym. Bull. (2013) 70:81–96

33. Chrissafisa K, Bikiaris D (2011) Can nanoparticles really enhance thermal stability of polymers? Part I: an overview on thermal decomposition of addition polymers. Thermochi Acta 523:1–24 34. Bikiaris D (2011) Can nanoparticles really enhance thermal stability of polymers? Part II: an overview on thermal decomposition of polycondensation polymers. Thermo Acta 523:25–45 35. Jeffery D, Vyazokin PS, Wight CA (2001) Kinetics of the thermal and thermo-oxidative degradation of polystyrene, polyethylene and poly(propylene). Macromol Chem Phys 202:775–784 36. Albano C, Catan˜o L, Figuera L, Perera R, Karam A, Gonza´lez G (2009) Evaluation of a composite based on high-density polyethylene filled with surface-treated hydroxyapatite. Polym Bull 64:45–55

123