Phase behaviour and morphology of composite ...

Composite Interfaces, 2014 Vol. 21, No. 9, 797–805, http://dx.doi.org/10.1080/15685543.2014.960321

Phase behaviour and morphology of composite comprising of poly (ethylene oxide), polyacrylate and lithium perchlorate Amirah Hashifudina, Lai Har Simb*, Chin Han Chana and Hairunnisa Ramlib

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Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia; bCentre of Foundation Studies, Universiti Teknologi MARA, Kampus Puncak Alam, 42300 Bandar Puncak Alam, Selangor, Malaysia (Received 6 June 2014; accepted 6 August 2014) Thin films of poly(ethylene oxide) (PEO) and polyacrylate (PAc) blend and the composite PEO/PAc/lithium perchlorate (LiClO4) are prepared via solution casting method. Thermal analysis using differential scanning calorimetry depicts that both the PEO/PAc blend and the PEO/PAc/LiClO4 composite form single phase systems, as evident from the presence of single composition-dependant glass transition temperature (Tg). Polarized optical microscopy (POM) was applied to study the spherulite growth rate and morphology of PEO in both the PEO/PAc blend and PEO/PAc/LiClO4 composite. Homogeneity in the phase behaviour of the PEO/PAc blend and the PEO/PAc/LiClO4 composite is reaffirmed by the continuous suppression of the growth rate and the severe deformation and coarseness of the PEO spherulite with ascending PAc content in the blend and increasing LiClO4 concentration in the composite. Additionally, the amount of salt that causes macro-phase separation in the neat polymers and the PEO/PAc blends is governed by the coordination ability of the polymer, especially PEO, to the Li+ ion. Results from both thermal analysis and POM reflect the higher preference of LiClO4 for PEO than PAc. Therefore, PEO/PAc 85/15 blend is observed to exhibit the highest capability to interact with the Li+ ion. Keywords: poly(ethylene oxide); polyacrylate; homogeneity; morphology; spherulite growth rate

1. Introduction Polymer blending is a cost-effective and convenient way of producing new materials by physically mixing two or more homopolymers or copolymers. Since the production of the first polymer blend by Parkes [1], blending has been extensively studied over the past century, aiming at producing cheaper materials with synergistic properties to meet great demands from the construction, automobile and electrochemical industries.[2–4] The performance of polymer blends is governed by the properties of the polymeric components, phase behaviour and the blend morphology.[5–7] From the viewpoint of phase behaviour and applications, polymer blends are differentiated as miscible and immiscible on the molecular scale. Miscibility is an important parameter in blending because of its effect on the glass transition temperature (Tg) which plays an important role in determining the overall physical properties and eventually, the performance of polymer blends.[5] Generally, most of the multi-component polymer systems *Corresponding author. Email: [email protected] © 2014 Taylor & Francis


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commercially utilized today are two-phased blends which have poor interfacial adhesion between the respective phases governing the ultimate mechanical properties attainable with the blends. In view of this drawback, immiscible blends, unlike miscible blends, require extra processes like grafting, the use of copolymers, etc. to enhance its compatibility between the two immiscible polymers.[8–10] Homogeneity in the phase behaviour of a binary blend is marked by some or all of the following characteristics, such as the presence of a single, composition-dependant Tg, suppression in crystallinity, isothermal crystallization kinetics, spherulite growth rates and melting temperature, as well as the distortion of the morphology of the crystalline polymer in the presence of the amorphous polymer.[11–14] Blends of poly(ethylene oxide) (PEO), a semicrystalline polymer, and polyacrylate (PAc), a non-commercial amorphous random copolymer, forms a one-phase system as reported in the previous paper.[15] PEO is well documented to be good polymer host for solid polymer electrolytes due to its high solvation ability of inorganic salts, low Tg and good mechanical property, except for its low conductivity resulting from its high crystallinity of ~70–80%.[2–4] On the other hand, PAc is transparent and has good adhesiveness which may enhance the electrode–electrolyte adhesion in a solid polymer electrolyte. The present article discusses the homogeneity of the PEO/PAc blend, based on firstly, the effect of the PAc; and secondly, the effect of LiClO4 on the radial growth rate and the morphology of the PEO spherulite using polarized optical microscopy (POM). 2. Experimental 2.1. Materials PEO (Mν = 3 × 105 g/mol) was purchased from Sigma-Aldrich Chemical Company and used after purification. PAc (Mw = 1.7 × 105 g/mol, estimated by gel permeation chromatography) was supplied by the Chemistry Department, Faculty of Science, University of Malaya.[16,17] Anhydrous lithium perchlorate (LiClO4) was purchased from Acrōs Organics and applied after being vacuum dried at 120 °C for 24 h. The common solvent used to prepare the PEO/PAc blend and PEO/PAc/LiClO4 composite is methanol (Fisher Scientific, Leicestershire, UK). 2.2. Sample preparation Polymer solution of 1% w/v concentration was prepared by dissolving 0.04 g of the neat polymer or the blend in 4 mL of the solvent in an oven heated at 50 °C for 2–3 h or until the polymer completely dissolved. The polymer solution was cast drop-by-drop onto a glass cover slip. The cast film was allowed to dry in the oven for 24 h at 50 °C and further dried in a vacuum oven for 24 h at 50 °C. The salt concentration used in the discussion is defined in Equation (1). Ys ¼

Mass of salt Mass of polymer or polymer blend

(1)

2.3. Characterization Glass transition temperature of the PEO/PAc blend and PEO/PAc/LiClO4 composite was investigated using a differential scanning calorimetry (DSC) analyser. Tg value was

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extracted as the half extrapolation tangent at the reheating cycle of the DSC thermogram. Radial spherulite growth rate (G) and morphology of PEO/PAc blend and PEO/PAc/LiClO4 composite were determined using a Nikon Eclipse ME600D (Yokohama, Japan) POM attached with Linkam hot stage.


3. Results and discussion 3.1. Glass transition temperature The Tg of both neat PEO and PAc extracted from the reheating cycles of DSC thermograms are −54 and 16 °C, respectively. The presence of a single compositiondependant Tg which obeys the Fox equation for all blend compositions of PEO/PAc as shown in Figure 1, indicates that the amorphous phase of the two components of the blend form a single phase. The Fox equation is defined in Equation (2), where WPEO, WPAc and Tg refer to the weight fractions of PEO, PAc and the Tg of the blend. TgPEO and TgPAc denote the Tgs of the neat PEO and neat PAc, respectively. 1 WPEO WPAc ¼ þ Tg TgPEO TgPAc

(2)

Figure 2 shows the variation of the Tg values of neat PEO, neat PAc and its blends as a function of mass ratio of lithium salt to polymer (Ys). Addition of low concentrations of LiClO4 (Ys ≤ 0.02) causes a sharp and linear increase in the Tgs of both neat PEO, neat PAc and their blends, indicating that the salt is very soluble in both the polymer components and their blends. However, the Tg of PAc remains relatively constant while that of PEO and the blends ascend gradually with salt concentration 0.02 < Ys ≤ 0.12. The plateau observed for PAc at Ys ≥ 0.05 implies that no further coordination between Li+ ions and PAc occurs, whereas coordination of Li+ ion to the ether oxygen atom of

Figure 1. The plot of Tg values of the PEO/PAc blend vs. weight fraction of PEO. (▲) denotes the experimental Tg value and the dashed line represents values calculated using the Fox equation.


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Figure 2. Glass transition temperatures of PEO/PAc (○) 0/100, (●) 25/75, (Δ) 50/50, (□) 85/15 and (■) 100/0 at different salt concentrations.

PEO continues, leading to a gradual increase in its Tg. This observation points towards the fact that LiClO4 has higher preference for PEO than PAc. Additionally, it is noted that blends with increasing PAc content exhibit levelling off of Tg values at progressively lower salt content except for PEO/PAc 85/15, where the Tg values continue to increase at ascending salt concentration. The levelling off of Tg values at different salt concentrations for the neat polymers as well as the blends is the result of macro-phase separation in which two phases occur, with one rich in PEO–Li+ and/or PAc–Li+ complexes and another rich in LiClO4 ion pairs, which means that the salt has reached its maximum dissolution limit in the polymer matrix. The Tg results are in accordance with the conductivity behaviour reported in the previous study.[15] On the whole, no phase separation between the polymer components is observed for all compositions of the composite PEO/PAc/LiClO4 at salt concentration 0.02 ≤ Ys ≤ 0.12. 3.2. Radial growth rate To determine radial spherulite growth rate, the cast film of each sample was heated from room temperature to 80 °C at a rate of 10 °C min−1, annealed at 80 °C for 5 min, then cooled down at the same rate to the crystallization temperature (Tc) of 49 and 45 °C for the PEO/PAc blend and the PEO/PAc/LiClO4 composite, respectively, to allow the PEO spherulite to crystallize isothermally. Micrographs of the growing PEO spherulites were captured in appropriate time intervals and the diameters of spherulites were estimated using Q-win software. An average of four diameters of each growing spherulite is applied for the characterization of the growth rate of the spherulite. Figure 3 depicts that the radius (R) of the PEO spherulite in all the compositions of the blend and the composite samples investigated increase linearly with time: the value of G for each sample is extracted from the slope of the linear curve. The growth rate of the PEO spherulite is observed, in Figure 4, to decrease exponentially with increasing PAc content, indicating that PEO/PAc is completely



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Figure 3. Variations in the radius of growing PEO spherulites (R) as functions of time for the PEO/PAc 85/15 blend at Tc = 49 °C.

Figure 4. The radial growth rate of PEO spherulites (G) as functions of weight fraction of PEO (WPEO) for PEO/PAc blends at Tc = 49 °C.

homogenous down to a macro-structural level, forming a single phase blend. Special interaction between PAc and PEO in the amorphous phase of the homogeneous PEO/ PAc blend restricts the migration of the crystallisable material to the growth front of the PEO crystal, thus, reducing the growth rate of the PEO spherulite. No crystals are observed when PAc content exceeds 30 wt%.

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Figure 5 depicts that the spherulite growth rates of neat PEO and PEO/PAc 85/15 blend decrease linearly with accending salt concentration. Deviation from the linear decreasing trend of values of G is observed with the concentration of LiClO4, Ys ≥ 0.12, due to macro-phase separation in which the dissolution of salt in the PEO matrix has reached its maximum limit at Ys between 0.10 and 0.12. Additionally, a significant suppression of the spherulite growth rate is observed in the PEO/PAc 85/15 blend with the addition of the same salt concentration, implying that the homogeneity in the phase behaviour of the PEO/PAc blend persists in the presence of LiClO4.

3.3. Morphology The as-prepared sample of neat PEO, as shown in Figure 6(a), is volume-filled with large spherulites, demonstrating a clear birefringence Maltese cross. With the addition of 15 wt% of PAc, the Maltese cross of each spherulite disappears and the spherulites become smaller in size and coarser in texture, as shown in Figure 6(b). The morphology of the as-prepared PEO/PAc 85/15 blend added with salt of concentration Ys = 0.02 (the composite), as shown in Figure 6(d), displays a co-continuous strip of the amorphous domain meandering around and within each distorted PEO spherulite, enabling the transportation of Li+ ions through the PEO matrix, leading to its highest conductivity among all the blend compositions studied.[18] The extent of deformation and coarseness of spherulite is more severe for the PEO/PAc 85/15 blend as compared to the neat PEO shown in Figure 6(c), upon addition of salt as small as Ys = 0.02. Figure 7 shows that the micrographs of PEO spherulites crystallize isothermally at Tc = 49 and 45 °C, respectively, from the melt of PEO/PAc blends and the composites PEO/PAc/LiClO4 added with LiClO4 concentrations Ys = 0.02 and 0.10, respectively.

Figure 5. The effect of salt concentration on the radial growth rate of PEO spherulite isothermally crystallized at Tc = 45 °C in blends of PEO/PAc (■) 100/0 and (□) 85/15.



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Figure 6. Micrographs of as prepared sample of PEO/PAc blends (a) 100/0, (b) 85/15 and PEO/PAc/LiClO4 composite (c) 100/0/0.02, (d) 85/15/0.02. Micrograph taken at 5× magnification; bar corresponds to 100 μm.

The micrograph was taken at 5× magnification. The spherulite of neat PEO displays a fine fibrillar texture with a well-defined border and a clear birefringence Maltese cross, as shown in Figure 7(a). Upon addition of 15 wt% of PAc, the PEO spherulite becomes slightly irregular in shape and smaller in size, as demonstrated in Figure 7(b). The PEO spherulite in PEO/PAc 70/30, displayed in Figure 7(c), adopts a severely distorted morphology, with dark regions covering almost half of the spherulite, irregular outlines, much smaller sizes and the disappearance of the Maltese cross pattern. The dark region is the extended amorphous phase caused by the interaction between the PAc and the PEO leading to the restricted crystallization of PEO. This result correlates with the decrease in the growth rate of PEO with increasing PAc content and together with the Tg result, conclude the formation of the single phase PEO/PAc blend. Figure 7(d) and (e) show that the incorporation of a small amount of LiClO4 (Ys = 0.02) to neat PEO results in the coarsening of the crystalline lamellae and slightly irregular spherulite; however, this amount of salt causes severe deformation and coarsening of the spherulite for the PEO/PAc 85/15 blend. In addition, a number of dark spots are observed within the spherulite, which is the amorphous domain resulting from the formation of Li+–polymer complexes. The extremely small and distorted spherulite observed in the PEO/PAc/LiClO4 100/0/0.10 composite reflects the growth rate shown in Figure 5, in which the PEO spherulite experiences a sharp reduction in growth rate due to higher coordination between the Li+ ions and the ether oxygen atoms of PEO, leading to the formation of an extended amorphous phase in the PEO spherulite.


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Figure 7. Micrographs of PEO spherulites crystallized isothermally at Tc = 49 °C from the melt of PEO/PAc (a) 100/0, (b) 85/15 and (c) 70/30 blends, and at Tc = 45 °C from the melt of composite PEO/PAc/LiClO4 (d) 100/0/0.02, (e) 85/15/0.02 and (f) 100/0/0.10. Micrograph taken at 5× magnification, bar corresponds to 100 μm.

4. Conclusions The formation of a one-phase PEO/PAc blend is confirmed by ramification from thermal analysis in which a single composition-dependant Tg that accords with the Fox equation is observed. Furthermore, the suppression of growth rate of PEO spherulite and the formation of distorted morphology of PEO in the presence of PAc obtained using POM reaffirms the homogeneity of the blend. The phase behaviour of all the compositions of the PEO/PAc/LiClO4 composite on all salt concentrations from Ys = 0.02–0.12 remain the same as that of the one-phase PEO/PAc blend. The amount of salt that causes macro-phase separation in the neat polymers and the blends is


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governed by the coordination ability of the polymer to the Li+ ion. PEO/PAc 85/15 blend exhibits the highest capability to interact with the Li+ ion and thus explains the preference of the LiClO4 towards PEO rather than PAc. Funding This work is supported by the Ministry of Higher Education (MOHE), Malaysia [grant number 600 RMI/RAGS 5/3 (14/2012)].


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