The state of ion aggregation in ionomers based on copolymers of styrene and acrylic acid

Eur. Polym. J. Vol. 34, No. I, pp. 127-132, 1998 0 1997 Elsevier Science Ltd. All rights reserved

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THE STATE OF ION AGGREGATION IN IONOMERS BASED ON COPOLYMERS OF STYRENE AND ACRYLIC ACID KAZIMIERA Faculty

of Chemistry,

N. Copernicus

SUCHOCKA-GALAS University,

Gagarina

7, 87-100 Toruti,

Poland

(Received 25 March 1996; accepted in final form 9 January 1997) Abstract-Styrene-based ionomers containing alkali metal salts of acrylic acid have been investigated by far IR spectroscopy and optical microscopy. A study has been made of the influence of the content and nature of alkali metal acrylate in the styrene-based ionomers on the state of ion aggregation, It seems that for alkali metal acrylates at low acrylate content the ions aggregate into multiplets, but as the concentration of acrylate increases clusters are formed, which may further form interpenetrating phases. We may suppose that sizes and shapes of aggregates depend on the nature of the introduced alkali metal. 0 1997 Elsevier Science Ltd

formation of ionic aggregates as a function of acrylate and nature of alkali metal.

INTRODUCTION

the past two decades, extensive studies have been performed aimed at improving our understanding of the microstructure of ionomers [l-8]. Due to ionic interactions, these materials possess unique physical properties that make them interesting from both academic and industrial points of view. It has been suggested [9] that ionic groups aggregate to form multiplets and that, at least in some systems, they may be clustered at sufficiently high ion content. Several models concerning ionomer morphologies have been proposed, including the core-shell model of MacKnight et al. [lo] and the hard-sphere liquid-like interference model of Yarusso and Cooper [ 111, none of which satisfactorily accounts for all the observed experimental phenomena. Recently, Eisenberg, Hird and Moore (EHM) presented a new model [ 121 which unified the interpretations of mechanical properties and morphologies of random ionomers. They suggested that the ionic aggregates, or multiplets, restrict the mobility of the adjacent polymer chains. With increasing ion content, the restricted mobility regions begin to overlap and may form fairly large domains (clusters) which exhibit phase-separated behaviour and possess their own T,. As the ion content increases, the addition of ions causes the size of the clustered region to grow until continuity of the clustered phase is established throughout [13]. In our earlier papers [14-173 we indicated that the state of ion aggregation in ionomers based on styrene-acrylic acid copolymers depends on the amount and type of introduced acrylate. This paper reports our results of far IR spectroscopy and optical microscopy studies of styrene-based ionomers containing alkali metal acrylates. Specifically, we have followed the

of content

Over

EXPERIMENTAL The copolymers of styrene (S) and acrylic acid (AA) were obtained by copolymerization in bulk [18, 191. Suitable ionomers were then obtained by titrating (under Nz) a 3-S% solution in benzene (for AA content ca. 5 mol%, a 9: 1 v/v mixture of benzene and methanol was employed) by standard solution of alkali metal hydroxide in methanol

U81. Films for investigations were obtained from the neutralized and unneutralized solutions by evaporation of solvent at room temperature. All polymers were dried to constant weight at 50°C in vacuum. The far IR spectra in the region 8&450 cm-’ of films of the copolymers and ionomers were recorded with a Perkin-Elmer Model 180 at ambient temperature. The films were examined with an optical microscope Ampival Carl Zeiss Jena equipped with a camera at ambient temperature. The materials are labelled by a number in parentheses, giving the concentration of AA or appropriate salt comonomer in mol%. For example, S-ANa (2.45) is a copolymer of styrene (S) and sodium acrylate (ANa) in which 2.45 mol% of the comonomer units correspond to ANa. RESULTS AND DISCUSSION Far IR spectroscopy

Figure 1 shows the spectra of ionomers containing increasing concentrations of sodium acrylate (ANa). When the concentration of ANa increases, a new band appears on the side of the main Na+ motion band at ca. 155 cm-‘. The band at about 240 cm-’ is the primary cation motion band; it is present at low ionic concentration. This band is assigned to cation motion in the anionic field and the hydrocarbon backbone. It is assigned to the vibratrion of an aggregate involving a few ions (low order multiplets). 127

K. Suchocka-Gala6

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Fig. 1. Far IR spectra

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of a series of styrene-based

Figure 1 also shows that the main band, assigned to a motion of cations in the anionic field of the copolymer, shifts to lower frequencies with increasing content of sodium acrylate introduced into the ionomer. The band at ca. 155 cm-’ may be assigned to the vibration of aggregates involving many cations and anionic sites together. This may correspond to the formation of higher aggregates or clusters. In such ionic aggregates the attraction becomes increasingly screened and so the vibrational frequency becomes lower than that of the simple cation site or multiplets. This is consistent with the observation that in solution the ion motion frequencies for simple ion pairs are higher than those for higher aggregates [20]. These spectra indicate that for the investigated ionomers the band at ca. 155 cm-’ appears even for

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ionomers

with various

mol%

of sodium

acrylate.

concentrations of sodium acrylate low ion (3.85 mol%). Figures 2 and 3 show the spectra of ionomers containing potassium and a caesium acrylate. We see that in these two cases on the side of the main band there appears a new band at ca. 155 cm-’ for ionomers containing potassium acrylate and at ca. 95 cm-’ for ionomers containing caesium acrylate. By analogy with ionomers containing Na+ ions, these bands may be assigned to the vibrations of aggregates involving many cations and anionic which may correspond to the sites together, formation of higher aggregates or clusters. It follows that the bands assigned to the formation of the higher aggregates or clusters in the case of the investigated ionomers also depend on the mass of the cation.

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cm-’ Fig. 2. Far IR spectra

of styrene-based

ionomers

with various

mol%

of potassium

acrylate.

State of ion aggregation

in S-AA-based

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ionomers

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Fig. 3. Far IR spectra

of styrene-based

ionomers

with various

mol%

of caesium

acrylate.

Fig. 4. Optical micrographs: (a) copolymer S-AA (11.67); (b) copolymer S-ANa (2.45); (c) copolymer S-ANa (5.16); (d) copolymer S-ANa (6.41); (e) copolymer S-ANa (11.67). (Scale bar = 100 pm.)

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Optical microscop,p?~ Figure 4 shows the optical micrographs of copolymer SAA (11.67) and styrene ionomers containing increasing content of sodium acrylate. It can be seen [Fig. 4(b)] that introduction of small amounts of sodium ions into the copolymer of S and AA causes formation of small aggregates, probably multiple& dispersed in the PS phase. This may be in agreement with the EHM model [12] in which multiplets are surrounded by polymers with restricted mobility. The relative volume of the regions of restricted mobility grows with increasing ion content and the matrix phase becomes more heterogeneous. Further increase of ANa content to 556 mol% [Fig. 4(c)] causes formation of high-order aggregateclusters, which are of different sizes and shapes and are dispersed in the PS phase containing a small amount of ion pairs, multiplets and small aggregates. Figure 4(d) shows that at the content ca. 6.41 mol% of ANa, the cluster phase becomes dominant and even continuous [13]. It seems reasonable to suggest the existence of two continuous phases, i.e. cluster and matrix. We see that as the ion content increases, the volume fraction of the cluster phase becomes bigger, while that of the matrix phase becomes smaller [Fig. 4(d)]. We can also see that at relatively high ion contents [Fig. 4(e)] the interpenetrating phases are formed. Figure 5 shows the optical micrographs of the studied ionomers containing increasing amounts of caesium acrylate. We see [Fig. S(a)] that similarly to ionomers containing sodium ions. at low concentrations of

Fig. 5. Optical

micrographs:

caesium ions multiplets dispersed in the PS phase are formed. When the content of caesium acrylate increases [Fig. 5(b)]. bigger aggregate clusters are formed, which are dispersed in the PS phase. Further increase of ACs content causes the volume fraction of the cluster phase to become bigger and the matrix phase to become smaller. Therefore. at high ion contents, the cluster phase becomes a continuous phase [Figs 5(c) and 5(d)]. We also observed that in the case of ionomers containing lithium and potassium ions. likewise as in the case of ionomers containing sodium and caesium ions, clusters. and at higher contents of acrylate interpenetrating phases, are formed. Figure 6 shows the optical micrographs of copolymers SSAA (5.16) containing different alkali metals. We see that in all cases in the PS phase containing a small amount of ion pairs and multiplets are formed bigger aggregate clusters, with sizes and shapes dependent on the nature of the introduced alkali metal. In the case of ionomer containing Li’ ions, the sufficiently regular and more coherent clusters are dispersed in the PS phase. It has been suggested [16, 21, 221 that the forces between the lithium cations and the carboxylate groups are covalent in nature and thus these small, highly polar groups interact more strongly and tend to be more firmly held than larger groups. Thus, in these multiplets strong electrostatic interactions must be operative in the multiplets. When a sufficient number of multiplets is close enough together to form a continuous region of

(a) copolymer SSACs (3.85); (b) copolymer SACS (5.16); (c) copolymer copolymer S-ACs (1 I .67). (Scale bar = 100 pm.)

SACS

(6.41); (d)

State of ion aggregation

Fig. 6. Optical

micrographs

in SAA-based

of copolymer S-AA (5.16) containing different (c) K+; (d) Cs+. (Scale bar = 100 pm.)

restricted mobility, a sufficiently large region constitutes a cluster. Figure 6(b) shows that sodium ionomers also form sufficiently regular clusters dispersed in the PS phase, containing ion pairs and other small aggregates. In the case of potassium ionomers [Fig. 6(c)] we can see that the PS phase contains clusters with more irregular shapes. We may also observe that in these cases the clusters are looser than in the case of ionomers containing lithium and sodium ions, because the cationic radius of K+ is considerably larger than that of lithium and sodium. Figure 6(d) shows that in the case of caesium ionomer in the PS phase with ion pairs and other small aggregates are formed bigger, more irregularly shaped and very loose aggregate clusters.

CONCLUSIONS

The experimental methods employed here (far IR spectroscopy and optical microscopy) indicate that clusters are formed at higher concentrations of ions. The far IR studies indicate that the bands which are assigned to the vibrations of higher aggregates or clusters were observed for 3.85 and 5.16 mol% acrylate. However, optical microscopy studies indicate that clusters are formed at ca. 5.16 mol% alkali metal acrylate. This effect may be due to the fact that a wide distribution of cluster sizes may exist, and that not all of them will affect all properties equally. The studies indicate that the state of ion aggregation depends on the content of alkali metal

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ionomers

alkali metal: (a) Li+; (b) Na+;

acrylate in the S-AA copolymers as well as on the nature of the alkali metal. At low and medium contents of alkali metal acrylate the ions are aggregated into the multiplets, but at higher contents clusters are formed and interpenetrating phases are observed. The shapes and sizes of formed aggregates are dependent on the nature of the alkali metal introduced into the copolymer. Further studies of these ionomers are in progress.

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