Polypropylene ionic thermoplastic elastomers: Synthesis and properties

Polymer Degradation and Stability 95 (2010) 363e368

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Polypropylene ionic thermoplastic elastomers: Synthesis and properties Dimitri D.J. Rousseaux a, b, Xavier Drooghaag a, Michel Sclavons b, Pierre Godard b, Veronique Carlier b, Jacqueline Marchand-Brynaert a, * a b

Unité de chimie organique et médicinale, Université catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium Unité de chimie et physique des hauts polymères, Université catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 27 November 2009

Polypropylene ionic thermoplastic elastomers have been prepared by melt radical grafting of maleic anhydride onto polypropylene in the presence of N-bromosuccinimide followed by neutralization of the resulting elastomeric grafted polypropylene using sodium salts. Sodium hydroxide and sodium acetate were compared in aqueous solution, as anhydrous or hydrated powders. The neutralization reaction was followed by Fourier transform infrared spectroscopy, allowing the development of a method to determine the effective neutralization degree. Important physical changes were recorded upon neutralization. Especially thermal stability, shear storage modulus and complex viscosity in the flow region were largely increased as a function of the neutralization degree. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Polypropylene Ionomer Thermoplastic elastomer Maleic anhydride

1. Introduction Ionomers are macromolecules composed of a small but significant proportion of constitutional units (less than 10%) containing an ionic and/or ionizable group [1]. The ionic moieties are usually obtained by full or partial neutralization of the acid groups by metal salts. The acid groups are mainly carboxylic acids but also sulfonic and phosphonic acids have been used. The ionic groups cause a micro-phase separation of the ionic moieties (ionic aggregation) within the non-ionic matrix and act as physical crosslinks [2e7]. The ionic aggregation induces strong modification in the physical, mechanical and rheological properties of the material. Nevertheless, ionomers can be melt-processed due to the reversibility of the crosslinking based on the increased mobility of the ionic moieties at high temperature. Ionomers of low glass-transition non-crystalline polymers can be used as ionic thermoplastic elastomers (TPEs) if the ionic aggregates are sufficiently weakened at the processing temperatures [8,9]. TPEs are melt-processable polymers composed of a continuous elastomeric phase reinforced by a dispersed hard phase acting as thermo-reversible junction points [10]. Different types of carboxylated ionic TPEs have been studied in the past, for example: Kutsumizu et al. investigated non-crystalline poly(ethylene-co-methacrylic acid) ionomers [11,12]; more recently, the group of Goossens-van Duin studied extensively

* Corresponding author. Tel.: þ3210472740; fax: þ3210474168. E-mail addresses: [email protected] (D.D.J. Rousseaux), jacqueline. [email protected] (J. Marchand-Brynaert). 0141-3910/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.11.031

ionomers based on maleated ethylene/propylene copolymers [9,13e16]. In this paper, carboxylated ionic TPEs of polypropylene (PP) will be introduced. PP elastomeric ionomers are based on the elastomeric polypropylene-graft-maleic anhydride (elPP-g-MA) described by Henry et al. [17]. Both the precursor elPP-g-MA and the ionomers were synthesized by melt-processing. The neutralization reaction was monitored by infrared spectroscopy and a method is proposed to determine the effective neutralization degree (ND). Various forms of sodium hydroxide and sodium acetate were used to compare their efficiency of elPP-g-MA neutralization. The influence of the base nature and amount on the ND was studied. Thermal and rheological properties of the ionomers were examined and are discussed as a function of the ND.

2. Experimental 2.1. Materials Isotactic homopolymer PP powder (Moplen HF500N) was purchased from Basell. The free radical initiator was Luperox 101XLS50 from Arkema (2,5-bis(tert-butyl-peroxy)-2,5dimethylhexane (DHBP), 50 wt% blend with silica). Maleic anhydride (MA, Acros, 99% purity), N-bromosuccinimide (NBS, Acros, 99% purity), sodium acetate (NaAc) trihydrate (Aldrich, 99% purity) and anhydrous (Aldrich, 99% purity), sodium hydroxide (NaOH) in pellets (Aldrich, 99þ% purity) and in powder (Aldrich, 97% purity) and toluene (Fisher, analytical grade) were used as received.

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2.2. Synthesis

Table 1 Characteristics of PP and elPP-g-MA.

2.2.1. elPP-g-MA synthesis elPP-g-MA was synthesized by reactive extrusion as described by Henry [17]. Briefly, isotactic polypropylene was treated with 3.26 wt% of supported peroxide, 5 wt% of maleic anhydride and 0.8 wt% of N-bromosuccinimide. elPP-g-MA was produced in a Brabender single screw extruder equipped with a feeding screw of L/D ¼ 20. The barrel was heated at three zones at 220, 240 and 260  C, respectively, and the screw speed was set to 30 rpm.

Grafting level (wt%MA) Crystallinity (%) Mn Mw

PP

elPP-g-MA

/ 46.2 65 k 245 k

3.5 1.6 22 k 60 k

2.3.4. Fourier transform infrared (FTIR) spectroscopy FTIR spectra of compression moulded films were recorded on a Nicolet Nexus 670 FTIR apparatus, over a spectral range from 4000 to 400 cm1 at a resolution of 4 cm1. All the FTIR spectra were normalized by setting the peak height of the 973 cm1 absorption band to an absorbance of 1. This band, which is assigned to isotactic helices, is usually considered as an internal reference for the comparison of films of different thickness [19]. The FTIR spectra were mathematically analyzed by an iterative curve-fitting method developed in the Igor software provided by Wavemetrics, Inc. The absorption bands area in the carbonyl frequency range of the anhydride (1750e1830 cm1) and of the carboxylic acid (1650e1750 cm1) were approximated by a Voigt function.

2.2.2. Ionomer synthesis All the ionomers were synthesized by reactive blending in a Brabender Plasticorder equipped with an electrically heated W50EH mixing device of 40 g capacity. The mixing speed was set to 100 rpm and the temperature at 220  C. The alkaline base was added to the melt elPP-g-MA either in fine powder, or in 2.5 molar aqueous solution, and the reaction was stopped after 5 min by quenching the melt ionomer at room temperature.

2.3. Characterization

2.3.5. Thermogravimetric analysis (TGA) Thermal analyses were performed on a Mettler Toledo TGA/ SDTA 851e using a heating rate of 10  C/min under air atmosphere (100 ml/min) from 25 to 600  C.

2.3.1. Quantification of the grafted species elPP-g-MA samples were first washed as described in a previous paper [17], then, the anhydride content was determined by acidbase titration according to a standard procedure [18].

2.3.6. Rheological properties Rheological properties were measured using a Rubber Process Analyser RPA 2000 from Alpha Technologies. Frequency sweep tests were performed at 120  C and at a strain rate of 10% over an angular frequency ranging from 300 to 1 rad/s.

2.3.2. Differential scanning calorimetry (DSC) Crystallinity and glass-transition temperature (Tg) of elPP-g-MA and ionomers were evaluated from a second heating ramp from 50 to 220  C at 10  C/min using a Mettler Toledo DSC 821e module. The device was calibrated with indium and zinc. 2.3.3. Size exclusion chromatography (SEC) Number-and weight-average molar mass (Mn and Mw, respectively) of the elPP-g-MA were measured on a Waters Alliance GPCV 2000 equipped with two Styragel HT 6E and one Styragel HT 2 columns. A differential refractometer and a viscometer were used as detectors. The mobile phase was 1,2,4-trichlorobenzene (TCB). The concentration of the sample was 2 mg/mL in TCB and dissolution was achieved by shaking at 160  C for 1 h. The injection volume was 215 mL. Temperature was held constant at 145  C throughout the analysis. The system was calibrated prior to the analysis with PS standards according to ISO 16.0142 specification.

3. Results and discussion 3.1. elPP-g-MA characterization Characteristics of PP and elPP-g-MA are summarized in Table 1. elPP-g-MA reaches a grafting level of 3.5 wt% and its molar mass is clearly reduced in comparison with the starting PP. The degradation of the molar mass is caused by a side reaction, b-scission, occurring during the functionalisation. The crystallinity of elPP-g-MA is strongly reduced with respect to the starting PP, due to the epimerization reaction occurring in the presence of peroxide and NBS.

O O O

H 2O

1 base equiv.

1 base equiv. O

O HO

-O M+

O

O HO

pKa1 = 4.2

HO

O

pKa2 = 5.6 Fig. 1. PP-g-MA neutralization steps.

O M+

-O M+

O

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3.2. Ionomer ND determination The neutralization reaction can be assessed by FITR by monitoring the decrease of the carbonyl symmetric stretching vibration of the succinic anhydride band at 1790 cm1. The asymmetric stretching of the succinic anhydride band is visible at 1864 cm1 but is weaker than the symmetric one. The grafted succinic anhydride moieties can be neutralized twice (Fig. 1) whereas the first neutralization step is easier because the first acid dissociation constant (pKa1 ¼ 4.2) is smaller than the second one (pKa2 ¼ 5.6). After the first neutralization step, the anhydride ring is opened into a carboxylate group and a carboxylic acid. Fig. 2 shows an enlargement of the carbonyl region of the FTIR spectra of the starting elPP-g-MA and the ionomer neutralized with 0.5 equivalents (equiv.) of 2.5 M NaOH solution. The elPP-g-MA spectrum shows only two absorption bands, characteristic for cyclic anhydrides at 1863 cm1(weak, asymmetrical stretching) and at 1790 cm1 (strong, symmetrical stretching). As shown in Fig. 2, the intensity of the anhydride bands decreases upon neutralization while two new bands appear at 1715 and 1570 cm1. These new bands correspond to the carbonyl stretching of carboxylic acid and metal-carboxylate, respectively, [9,16]. If the ND were calculated on the basis of the decrease of the anhydride band at 1790 cm1 from elPP-g-MA (AanhelPPgMA) to ionomer (Aanhionomer), using Equation (1), it would be overestimated due to the carboxylic acid functions contribution.

Aanhionomer ND ¼ $100 AanhelPPgMA

Anhydride bands Carboxylic acid band

1800

1850

1800

1750

1700

1650

1600

1550

1500

-1

Wavenumber (cm ) Fig. 3. Enlargement of the carbonyl region of the FTIR spectra for dry and wet elPPg-MA.

the acid band that would have been measured if all anhydride functions were hydrolyzed into acid and this value corresponds to the Y intercept (Y0) of Equation (3). The corrected acid band area can be calculated by measuring the anhydride and acid bands areas of a sample using Equation (4). A corrected neutralization degree (cND) based on the decrease of the corrected acid band area from elPP-g-MA ðAacid correlPPgMA Þ to ionomer ðAacid corrionomer Þ can be calculated using Equation (5).

Aacid ¼ ðAanh $SÞ þ Y0

(2)

1750

1700

Carboxylate band

1650

1600

1550

1500

Aacid

corr

¼ Y0

(3)

Aacid

corr

¼ Aacid  ðAanh ,SÞ

(4)

Aacid corrionomer ,100 Aacid correlPPgMA

(5)

cND ¼

3.3. Influence of the nature of the base on the ND The influence of the nature of the base on the neutralization degree has been studied. Two different bases were used: sodium

Normalized absorbance (A.U.)

Normalized absorbance (A.U.)

elPP-g-MA 0.5 equiv. NaOH 2.5M

1850

1900

(1)

To solve this problem, elPP-g-MA was hydrolyzed in a water atmosphere in order to open the anhydride functions into diacids properly. Fig. 3 shows an enlargement of the carbonyl region of the FTIR spectra for dry elPP-g-MA (starting material) and wet elPP-gMA. Wet elPP-g-MA was then melt-pressed into films at 200  C. By melt-pressing the wet sample for increasing times, the diacid functions progressively closed to form anhydrides (Fig. 4). The evolution of the acid band area as a function of the anhydride band area for several melt-pressed samples is presented in Fig. 5. A linear regression, using Equation (2), can be used to convert the anhydride band area (Aanh) into acid (Aacid) in order to calculate a corrected acid band area (Aacid_corr). The corrected acid band area is the area of

1900

dry elPP-g-MA wet elPP-g-MA

Normalized absorbance (A.U.)

The additive allows the grafting level to be improved while limiting excessive degradation of the molar mass by b-scission [17].

365

1900

1850

1800

1750

1700

1650

1600

1550

1500

-1

Wavenumber (cm ) Fig. 2. Enlargement of the carbonyl region of the FTIR spectra for elPP-g-MA and 0.5 equiv. NaOH 2.5 M ionomer.

Fig. 4. Enlargement of the carbonyl region of the FTIR spectra for melt-pressed wet elPP-g-MAs for increasing times.

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350 340

80

-1

Aacid (Abs.cm )

330 320

60

310 40

300

elPP-g-MA NaAc.3H2O sol. NaAc 2.5M NaAc anh. sol. NaOH 2.5M

290

20

280 0 0

10

20

30

40

50

60

70

80

270

90

0

10

20

30

40

50

60

70

cND (%) Fig. 5. Acid band area (Aacid) evolution as a function of the anhydride band area (Aanh) for the melt-pressed elPP-g-MAs.

hydroxide (NaOH) and sodium acetate (NaAc). These bases were added in various forms to the molten elPP-g-MA. One base equiv. refers to the amount of base required to neutralize all the anhydride functions twice. NaOH was added in 2.5 M aqueous solution (NaOH 2.5 M) at 0.1, 0.2, 0.3, 0.5 and 0.75 equiv. or as powdered solid (NaOH sol.) at 0.75 equiv. NaAc was added as hydrated solid (NaAc.3 H2O sol.) and as 2.5 molar aqueous solution(NaAc 2.5 M) at 0.1, 0.2, 0.5, 0.75 and 1 equiv. or as anhydrous solid (NaAc anh. sol.) at 1 equiv. Fig. 6 shows the evolution of the cND of ionomers, as a function of the added amount of base. Up to 0.2 equiv., the cND is the same for all ionomers, and proportional to the base amount. Differences are observed above 0.2 equiv. The cND of the ionomers synthesized with NaOH 2.5 M continues to rise proportionally to the base amount, indicating that the reaction yield is maintained. The cND of the ionomer synthesized with 0.75 equiv. of NaOH sol. is about the half of the one synthesized with 0.75 equiv. of NaOH 2.5 M. Over 0.2 equiv., the cND of the ionomers synthesized with NaAc 3 H2O sol. continues to rise but less importantly than with NaOH 2.5 M while the cND of the ionomers synthesized with NaAc 2.5 M reaches a plateau. The cND of the ionomer prepared with 1 equiv. of NaAc anh. sol. is 2.7 times lower than the one with 1 equiv. of NaAc 3 H2O sol. and 1.6 times lower than the one with 1 equiv. of NaAc 2.5 M. The neutralization reaction requires water to ionize the base. Although hygroscopic, NaOH sol. powder contains much less water

Fig. 7. Ionomers onset temperature (To) versus cND.

than NaOH in solution. This explains the observed difference in cND between NaOH 2.5 M and NaOH sol. The difference in cND observed between the ionomers neutralized by NaOH and NaAc results from their pKa which are 15.7 for NaOH (H2O/OH) and 4.76 for NaAc (CH3CO2H/CH3CO 2 ). NaOH is a stronger base than NaAc and is therefore more efficient in neutralizing elPP-g-MA under the same conditions. Considering the pKa difference between the grafted succinic moieties and the bases, complete reaction is theoretically expected with NaOH and an equilibrated reaction with NaAc. However, the neutralization reaction is displaced towards product formation since the reaction residue (H2O for NaOH, CH3CO2H for NaAc) is evaporated during the ionomer synthesis. The difference in cND observed between the ionomers neutralized by NaAc 3H2O sol. and NaAc 2.5 M cannot be rationalized. As for NaOH, the presence of water, either from salt hydration for NaAc 3H2O sol., or from solution for NaAc 2.5 M, explains the higher cND obtained when neutralizing elPP-g-MA with a water-containing NaAc salt rather than with NaAc anh. sol. The neutralization efficiency order for all bases is the following: NaOH 2.5 M > NaOH sol. z NaAc 3 H2O sol. > NaAc 2.5 M > NaAc. anh. sol. The ionomer prepared with 0.75 equiv. of NaOH 2.5 M reaches a critical neutralization degree. When synthesizing this ionomer, the torque increased until the blender-knives broke the ionic network and expelled it. The crosslinking density reaches a plateau and a higher neutralization degree cannot be obtained. For that reason, the characterization of this sample is omitted.

1000000

60

cND (%)

50

elPP-g-MA NaAc.3H2O sol. NaAc 2.5M NaAc anh. sol. NaOH 2.5M NaOH sol.

100000

G' at 12 rad/s (Pa)

70

40 30 20

10000

elPP-g-MA NaAc.3H2O sol. NaAc 2.5M NaAc anh. sol. NaOH 2.5M

1000 10 0 0.0

0.2

0.4

0.6

0.8

Base equivalent Fig. 6. Ionomers cND evolution versus base equivalent.

1.0

100

0

10

20

30

40

50

60

cND (%) 0

Fig. 8. Ionomers shear elastic modulus (G ) at 12 rad/s versus cND.

70

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η* at 12 rad/s

100000

10000

elPP-g-MA NaAc.3H2O sol. NaAc 2.5M NaAc anh. sol. NaOH 2.5M

1000

100 0

10

20

30

40

50

60

70

cND (%)

367

solution, is required to ionize the base in order to allow the acidbase reaction to occur. A method was developed to determine the effective neutralization degree by infrared spectroscopy. The choice of the base has a strong influence on the neutralization degree. Sodium hydroxide allowed the neutralization yield to be kept almost constant even at high neutralization degree. However, as the neutralization yield never reaches 100%, the excess of the added sodium hydroxide may lead to corrosion. Sodium acetate is a weak base but the neutralization reaction is displaced towards product formation since the reaction residue (acetic acid) is evaporated during the ionomer synthesis. Furthermore, no corrosion is expected with sodium acetate. For these reasons, sodium acetate is preferred over sodium hydroxide. The ionomer physical properties were improved strongly. The thermal stability in air atmosphere, shear storage modulus and complex viscosity in the flow region were largely increased as a function of the neutralization degree.

Fig. 9. Ionomers complex viscosity (h*) at 12 rad/s versus cND.

Acknowledgments 3.4. Ionomer characterization 3.4.1. DSC While the DSC trace of elPP-g-MA (not shown here, but similar to the one presented by Henry et al. [17]) exhibits a glass-transition around 4.2  C and a weak broad melting peak, the DSC traces of the ionomers show only a glass-transition. Tg for all the ionomers are similar to the one of elPP-g-MA. 3.4.2. TGA Thermogravimetric analysis was performed on the ionomers. In order to compare all the samples, the temperature at 10% weight loss, which corresponds to the onset of the degradation, is plotted as a function of the cND in Fig. 7. There is a significant increase in the onset temperature (To) with respect to the cND. At high ND, the To is measured about 60  C higher than for elPP-g-MA. This phenomenon can be explained by the ionic crosslinking which holds together the polymer segments resulting from thermal degradation (of the ionomer). As the neutralization degree increases, the crosslinking density increases proportionally. Hence the polymer segments are tied together and the onset temperature is raised. 3.4.3. Rheology Frequency sweep experiments were performed on all the samples. To facilitate the comparison between the different samples, only the values of shear elastic modulus and complex viscosity, arbitrarily taken at 12 rad/s, are presented. Figs. 8 and 9 0 show the evolution of the shear elastic modulus (G ) and the complex viscosity (h*) respectively, as a function of the cND. Both the shear elastic modulus and the complex viscosity increase about 2 decades with the cND. The phenomenon is explained by the matrix reinforcement as a result of the ionic crosslinking. By increasing the neutralization degree, the extent of ionic aggregation rises and enhances the reinforcement effect.

4. Conclusions Elastomeric highly-functionalized poly(propylene-graft-maleic anhydride) was produced and characterized. From this material, new carboxylated ionic thermoplastic elastomers of polypropylene were successfully synthesized. Two sodium bases were used under different physical forms to compare their efficiency to neutralize the elastomeric polypropylene-graft-maleic anhydride. It was shown that water, either from salt hydration or from salt aqueous

The authors thank La Région Wallonne and FRIA (Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture) for financial support. J. Marchand-Brynaert is Senior Research Associate of the FRS-FNRS (Fonds National de la Recherche Scientifique, Belgium). Prof. A. Jonas is acknowledged for help and useful discussions about the FTIR fitting routine and Prof. J. Devaux for his kind support.

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