Preparation and characterisation of blends of poly(ethylene ... - umexpert

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Int. J. Materials Engineering Innovation, Vol. 4, Nos. 3/4, 2013

Preparation and characterisation of blends of poly(ethylene oxide) and functionalised epoxidised natural rubber Wan Nurhidayah A. Karim* and Jin Guan Ng Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia E-mail: [email protected] E-mail: [email protected] *Corresponding author

Chin Han Chan Faculty of Applied Science, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia E-mail: [email protected]

Seng Neon Gan Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia E-mail: [email protected] Abstract: Blending of polymers has been a popular technique for making new materials with improved properties or processability. The synergistic properties in the blends are related to the miscibility of the components when there are significant interactions between the constituents. This paper reports the preparation of reactive blends of poly(ethylene oxide) (PEO) and acetic acid-modified epoxidised natural rubber (ENR50, which contains 50% of the isoprene units converted to epoxide groups) by solution casting. The carboxylic acid modification of ENR50 was carried out by reacting the ENR50 dissolved in toluene with excess acetic acid at 100°C. Ring-opening of epoxide group by acetic acid has led to an increase in the Tg. The effects of blend ratio of the acetic acid-modified ENR50 and PEO on the thermal properties were studied. The initial ENR50 has a Tg of −29°C and the acetic acid-modified ENR50 has led to a new Tg of 10°C. FTIR results showed that there was no chemical reaction between the acetic acid-modified ENR50 and PEO in the blends. This is in agreement with the DSC results where two distinct Tgs were observed in the blends. Keywords: polymer blending; poly(ethylene oxide); PEO; acetic acid-modified ENR50; modification; acetic acid; differential scanning calorimetry; DSC; Tg; epoxidised natural rubber; ring-opening. Copyright © 2013 Inderscience Enterprises Ltd.

Preparation and characterisation of blends of poly(ethylene oxide)

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Reference to this paper should be made as follows: Karim, W.N.A., Ng, J.G., Chan, C.H. and Gan, S.N. (2013) ‘Preparation and characterisation of blends of poly(ethylene oxide) and functionalised epoxidised natural rubber’, Int. J. Materials Engineering Innovation, Vol. 4, Nos. 3/4, pp.314–324. Biographical notes: Wan Nurhidayah A. Karim is an MSc student in Polymer Chemistry. He is carrying out research in polymer blends and modification on epoxidised natural rubber. Jin Guan Ng is an MSc student in Polymer Chemistry. He is carrying out research in polymer blends and modification on epoxidised natural rubber. Chin Han Chan received her PhD in 1999–2002, (Polymer Blends) in School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia. She is a Lecturer in Universiti Teknologi Mara. Seng Neon Gan received his PhD in 1976 from the University of Malaya, and spent one year of post-doctorate training at the Centre des Recherches sur les Macromolecules, Strasbourg, France. He is currently a Professor at the Department of Chemistry, University of Malaya. His research interest includes natural rubber, biodegradable polymers, emulsions, palm oil alkyds, polyurethanes, and Ziegler-Natta catalysts. He has served as consultant to a number of local and foreign companies. In response to the encouragements from university to researchers to participate in exhibitions and file patents he has filed 18 patents, and won 17 medals in various exhibitions during 2004–2011. This paper is a revised and expanded version of a paper entitled ‘Preparation and characterization of blends of poly(ethylene oxide) and functionalized epoxidized natural rubber’ presented at 2nd Malaysia Polymer International Conference 2011 (MPIC2011) Universiti Kebangsaan Malaysia (UKM), 18–20 October 2011.

1

Introduction

Studies on polymer blends are being pursued with keen interest with the target of creating new materials having improved properties and processability (Inoue, 1984). Desirable properties can be tailored via polymer blending which are not obtainable from a single polymer (Kammer et al., 1993). Miscibility and morphology of semicrystalline/semicrystalline (Chan et al., 2004, Tan et al., 2006) and semicrystalline/amorphous (Chee et al., 2005) polymer blends have been extensively studied in recent years. Polymers that are immiscible with poly(ethylene oxide) (PEO) form heterogeneous blends (Chan and Kammer, 2008). Epoxidised natural rubber with 50% epoxidation level (ENR50) is a chemically modified natural rubber prepared by converting nominally half of the carbon-carbon double bonds of the polyisoprene structure into epoxide groups. Under suitable conditions, the epoxide group can react with electrophiles such as carboxylic acids (Teik, 1988; Gan and Burfield, 1989). Carboxylic acid concentration, temperature and the reaction time appear to be the main factors that determine the extent of reaction as reported from the previous research work of the ring-opening reaction of the epoxide

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group with carboxylic acid. Changes in physical properties, such as modulus, hardness and resilience and the increased Tg of the ENR50 after treatment with dicarboxylic acid are interpretation as due to crosslinking reaction that has led to an increase in Tg (Teik, 1988). This was disputed by the results of increase in Tg when ENR50 was reacted with small amount of benzoic acid that was monofunctional that could not incur any crosslinking (Gan and Burfield, 1989). The ring opening of epoxide group by a carboxylic acid would lead to structural change when the epoxide group was replaced by new –OH and –OCOR groups, contributing to greater amount of polar interactions and reduction of chain mobility. The ring-opening of ENR50 by acetic acid has produced hydroxyl and acetate functional groups in the polymer chains. During the preparation of acetic acid modifiedENR50, excess acetic acid was use to ensure complete ring-opening of all the epoxy groups. After the reaction, the excess unreacted acid could be easily separated and removed by precipitating the modified rubber in methanol followed by repeated washing with methanol. The new functional groups give rise to the higher polarity to the modified rubber. The acetic acid-modified ENR50 was used for blending with PEO. Compatibility of the polymer blends can be studied by differential scanning calorimetry (DSC), which measures the glass transition temperature (Tg) of the polymeric materials. Miscible blends will show only single Tg, locate between those of the blend components. On the other hand, immiscible blends will exhibit the two Tgs of the components.

2

Experimental

2.1 Materials ENR50 was a kind gift from Malaysia Rubber Board (MRB) (Sungai Buloh, Malaysia) and used as supplied. PEO (Mη = 1×105 g mol−1) was purchased from Sigma-Aldrich Co. All other chemicals were used as commercially supplied: toluene, chloroform and glacial acetic acid were from Merck (Darmstadt, Germany) and methanol from BumiPharmaSdn. Bhd.

2.2 Preparation of acetic acid-modified ENR50 90 g of ENR50 was mechanically masticated on a laboratory two-rolls mill at room temperature for 40 passes, cut into small pieces with a pair scissors, and placed into a one litre round bottom reaction flask, equipped with mechanical stirrer, water condenser and a dropping funnel were attached. 450 ml of toluene was added into the reaction flask and stirred at 150 rpm until the ENR50 has dissolved. The heating mantle was turned on and the solution was heated slowly until the temperature reached 100±5°C, 120 ml of glacial acetic acid was added through a dropping funnel. The stirrer was maintained around 200 rpm. Reaction was carried out for 18 hours and the content in the flask was mixed with five times excess of methanol to precipitate the modified ENR50, which was then isolated by filtration. The filtrate containing excess unreacted acetic acid was discarded,


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and the isolated rubber was washed with fresh methanol before being dried in vacuum oven at 50°C for 24 hours. Samples were stored in desiccators.

2.3 Preparation of the blends Acetic acid-modified ENR50/PEO polymer blends were prepared by solution casting method. The modified ENR50 and PEO were separately dissolved in chloroform to form 5% (w/w) solutions. The solutions were mixed and continuously stirred for 24 hours. Thin films of the blends were prepared by casting the homogenised solution mixture in Teflon dishes. Chloroform was allowed to evaporate off at room temperature overnight in the fume hood. Thin films of polymers were further dried in vacuum oven at 50°C for another 24 hours to ensure they were free of solvent. The thin films obtained were stored in desiccators before further analyses. Compositions of the blends were varied with content of modified ENR50from 25%, 50% to 75% by weight.

2.4 Characterisation of acetic acid-modified ENR50, PEO and acetic acid-modified ENR50/PEO blends 2.4.1 FTIR spectroscopy A thin layer of the sample dissolved in minimum amount of toluene was coated directly onto the sodium chloride cell and the solvent was removed under reduced pressure in a vacuum oven at 50°C to deposit a thin polymer film on the sodium chloride cell. FTIR spectra of the polymer films were recorded on a Perkin Elmer Spectrum 400 Spectrum (Waltham, Massachusetts, USA) FTIR instrument. The spectra were recorded in the absorbance mode over the range of 450−4,000 cm−1 at a resolution of 4 cm−1.

2.4.2 Differential scanning calorimetry Glass transition temperature (Tg), melting temperature (Tm), and melting enthalpy (ΔHm) were determined using TA DSC Q200 (New Castle, Delaware, USA) equipped with cooling system (RCS 90, New Castle, Delaware, USA). The DSC was calibrated with indium standard under nitrogen atmosphere. About 6−10 mg of sample was encapsulated in an aluminium sample pan. For isothermal crystallisation determination, the samples were held at 80°C for five minutes followed by cooling to 49°C at a cooling rate of 20°C min−1. Afterwards, samples were heated to 80°C at a heating rate of 10°C min−1. For Tg analysis, the same procedure as above except cooling the samples to −70°C and held for one minute.

2.4.3 Thermal gravimetric analysis (TGA) Perkin Elmer TGA 6 (Norwalk, Connecticut, USA) was used to investigate the thermal stability by heating 10 mg of the sample from 50°C to 900°C at a heating rate of 10°C min−1 under nitrogen atmosphere. Onset temperature, Td, is the onset of the weight loss.

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Table 1

Characteristics of the thermal gravimetric analysis (TGA) curves obtained in nitrogen atmosphere with a heating rate of 10°C/min

Acetic acid-modified ENR50 content in blend %

Temperature First Td (°C)

Second Td (°C)

0

-

381.4

25

259.9

380.6

50

259.6

379.5

75

260.2

375.1

100

259.9

357.6

3

Results and discussion

3.1 Characterisation of acetic acid-modifiedENR50 Ring-opening reaction of the epoxide group by carboxylic acid has been reported by a number of earlier papers (Hayashi et al., 1981; Gan and Burfield, 1989; Lee et al., 2011). A plausible reaction of the epoxide group with acetic acid is illustrated in Figure 1. Figure 1

A plausible reaction of the epoxide group with acetic acid

Figure 2 shows the overlay FTIR spectra for initial ENR50 and the acetic acid-modified ENR50. For the initial ENR50 before modification, the characteristic bands of saturated aliphatic C-H bonds are observed at 2,963, 2,926 and 2,860 cm−1 for C-H stretching, while 1,451, 1,326, and 1,250 cm−1correspond to CH2 scissoring, CH2 wagging and CH2 twisting respectively. C = C stretching band is at 1664 cm−1. The peak of epoxide group is seen at 873 cm−1 as reported earlier (Gan and Hamid, 1997). For the acetic acid-modified ENR, the broad peak at 3469 cm−1 is due to –OH stretching. The characteristic bands of saturated aliphatic C-H stretching are observed at 2,859, 2,933 and 2,963 cm−1. The band for –CH2 scissoring is located at 1,449 cm−1 while C = C stretching band at 1,664 cm−1. The band for –C-CH3 is located at 1,374 cm−1 while C-O-C stretching at 1,069 cm−1. The acetate structure gives rise to peaks at 1,026, 1,241 and 1,735 cm−1.The peak of epoxide group at 873 cm−1 is hardly visible.

Preparation and characterisation of blends of poly(ethylene oxide) Figure 2

Absorbance FTIR spectra of (a) ENR50 and (b) acetic acid-modified ENR50

Figure 3

TGA thermogram for acetic acid-modified ENR50

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Thermogravimetric analysis in nitrogen atmosphere has been performed. There was no noticeable weight loss below 200°C. The onset temperature of weight losses (Td) were obtained from the thermogravimetric curves by bi-tangent method provided by the software of the instrument. For the initial ENR50 sample, the thermal degradation just proceeded in one-step decomposition from 320°C−550°C, whereas for acetic acid-modified ENR50, two-steps thermal decomposition was observed. This is being illustrated in Figure 3, the first Td was observed at 259°C and the second Td observed at 358°C. Presumably the modified ENR has undergone some complicated molecular changes during the first stage of thermodegradation, leading to a more stable structure which further degradated at higher temperature. More detail studies would be needed to investigate the nature of these changes. Table 2, Tg of ENR50 was −29°C. Complete chemical modification by acetic acid has led to higher Tg, of 10°C. This presumably is related to the formation of new chemical structure. The replacement of epoxide by hydroxyl and acetate groups would introduce higher polar interaction and possibly some hydrogen bonding. The corresponding increase in Tg from the modification of ENR50 by benzoic acid has been noted previously (Gan and Burfield, 1989). Table 2

First and second Tg values for acetic acid-modified ENR50/PEO blend samples


First Tg (°C)

Second Tg (°C)

0

−58

-

25

−58

9

50

−58

10

75

−59

9

100

-

10

3.2 Compatibility of acetic acid-modified ENR50/PEO blends FTIR is used to study the chemical interactions of acetic acid-modified ENR50 and PEO in the blends. Spectra for neat acetic acid-modified ENR50, neat PEO, and blends containing acetic acid-modified ENR50 at 25%, 50% and 75% by weight were shown in Figure 4. There is no shift of the band position of –OH group. Peak at 2,879 cm−1 is due to C-H stretching of PEO. The peak at 1,735 cm−1 corresponds to the carbonyl in the ester group of the modified ENR50. This is present in the blend containing 25% acetic acid-modified ENR50 and its intensity increases as the amount of modified ENR50 is increased to 50% and 75%. The characteristic triplet bands of C–O–C stretching at 1,060, 1,094 and 1,144 cm−1 was observed in all samples containing PEO (Bailey and Koleske, 1976). The presence of the triplet peaks of PEO was clearly observed in the acetic acid-modified ENR50/PEO blends. No band shifting was detected. These results have shown that there was no significant interaction between the two blend components. These results clearly shows that the modified ENR and PEO are immiscible.

Preparation and characterisation of blends of poly(ethylene oxide) Figure 4

FTIR absorbance spectra for acetic acid-modified ENR50/PEO blend samples of (a) 100% (b) 75% (c) 50% (d) 25% weight of acetic acid-modified ENR50 and (e) PEO 100%

Figure 5

First Td, onset [Δ] and second Td, onset [●] as a function of weight fraction of acetic acid-modified ENR50 in acetic acid-modified ENR50/PEO blend system

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TGA analysis is carried out on acetic acid-modified ENR50/PEO blend system. Samples were heated from 50°C to 600°C under nitrogen atmosphere with a constant heating rate at 10° C min−1 (Table 1). For pure PEO, there is only one Td observed at 381.4°C. While for the blend samples, there were two Td, as observed from the thermal degradation of the neat modified ENR. The first Td was observed at around 260°C for all blends and the value was the approximately the same as for the acetic acid-modified ENR50. While the second Td showed a decreasing trend as the amount of PEO was decreased in the blend as shown in Figure 5. As the amount of acetic acid-modified ENR50 was increased in the blend samples, it has caused the second Td to decrease and approaches that of the neat acetic acid-modified ENR50. DSC is employed to study the acetic acid-modified ENR50/PEO blend system. Tg of PEO is −58°C whereas the Tg of acetic acid-modified ENR50 is 10°C. All the three blends have two Tg values shown in Table 2 corresponds to the Tg of the acetic acidmodified ENR50 and Tg of PEO. This is in agreement with the results from FTIR studies that the two components in the blends are immiscible. Tm and ΔHm of PEO for acetic acid-modified ENR50/PEO blends are summarised in Table 3. Acetic acid-modified ENR50 constituent does not exert any strong interfere with the PEO spherulites in the blends. Tm values of the three blends are at 60±2°C. The enthalpy of melting ΔHm is due to the PEO component in the blends. As the % modified ENR is increased, the %PEO is decreased leading to smaller ΔHm. Table 3

Tm, ΔHm of PEO and χPEO, could be calculated from the equation


Tm (°C)

ΔHm (J g–1)

χPEO

0

67

139

0.74

25

67

94

0.67

50

62

49

0.52

75

58

8

0.17

-

-

-

Acetic acid-modified ENR50

Degree of crystallinity for PEO, X PEO = ΔH m / (WPEO • ΔH m ref )

(

ΔH m = Melting enthalpy for PEO J g –1

)

WPEO = Weight fraction of PEO in the blend ΔH m ref = Melting enthalpy of 100% crystalline PEO, 188.3 J g –1 is adopted from Cimmino et al. (1990 ) .


4

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Conclusions

The modification of ENR50 was carried out by reacting the ENR50 dissolved in toluene with excess acetic acid. FTIR is used to characterise the chemical structure of acetic acid-modified ENR50 formed. Formation of acetate and hydroxyl functional groups in the acetic acid-modified ENR50 causes Tg to increase from −29°C to 10°C. The blends of PEO and acetic acid-modified ENR50 has been prepared by solution casting technique. The FTIR results show that there is only physical mixing of all constituents in the blends. DSC further confirms the immiscibility between PEO and the acetic acid-modified ENR50 by recording two glass transition temperatures.

Acknowledgements This project has been funded by Postgraduate Research Fund PS337/2010A, University of Malaya.

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