Research Article Received: 7 January 2012 Revised: 8 March 2012 Accepted: 10 March 2012 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/pi.4227 Particle-size-dependent properties of sulfonated polystyrene nanoparticles Mousumi Hazarika, Kuruma Malkappa and Tushar Jana ∗ Abstract The inﬂuence of sulfonation reaction time, temperature and the parent polystyrene (PS) particle size on the degree of sulfonation (DS), ion exchange capacity (IEC), morphology and glass transition temperature (T g ) of sulfonated polystyrene (SPS) particles was investigated. A longer reaction time (ca 2 h) at 40 ◦ C and a smaller particle size resulted in SPS particles with a high DS. It was found that a larger PS particle size did not readily yield SPS particles with a high DS even with a longer reaction time. Contrary to the popular belief in the literature that a higher DS ensures a high IEC, we observed that the proportionality of IEC to DS is primarily controlled by the SPS particle size. Larger IEC values were obtained for larger particles rather than smaller ones despite their similar DS, owing to the presence of strong interactions between −SO 3 H groups within the particles in the latter case which restricts the availability of free H + for ion exchange. The SPS particles displayed a core-shell morphology in which the outer shell appeared because of sulfonation on the PS. The DS value and the SPS particle size signiﬁcantly inﬂuenced the shell thickness and thereby the morphology of the SPS particles. c  2012 Society of Chemical Industry Supporting information may be found in the online version of this article. Keywords: sulfonated polystyrene; heterogeneous sulfonation; degree of sulfonation; ion exchange capacity; core-shell morphology INTRODUCTION The chemical modiﬁcation of preformed polymers can be done by functionalizing the polymer backbone. This is a well studied avenue to obtain materials with desirable chemical and physical properties. More appropriately, this approach can be easily used to tune the polymer properties, e.g. hydrophobic or hydrophilic character, solubility, miscibility, changing the soft or rigid nature, polymer solution or melt viscosity, crystallinity, glass transition temperature etc. 1 One such approach which is well recognized in the literature is the sulfonation of rigid hydrophobic polystyrene (PS) to prepare soft hydrophilic sulfonated polystyrene (SPS). In recent years, a large number of studies on SPS have been undertaken because of its wide range of applications in numerous areas, e.g. drug delivery applications, 2 catalysis, 3 humidity sensors, 4 as membranes for reverse osmosis 5 and chromatography techniques 6,7 etc. Hydrated SPS exhibits high proton conductivity and has been used as a polymer electrolyte in proton exchange membrane fuel cells. 8,9 SPS is also used as a template for synthesis of composite materials with hollow nano structures and core–shell structures. 10–12 The sulfonate groups of SPS readily interact with other functional groups of a variety of polymers, resulting in a large number of miscible blends of SPS with other polymers. 13–15 He et al. have demonstrated that SPS latex particles can be used very efﬁciently to make varieties of structure by self-assembly of the SPS latex at the emulsion – droplet interface. 16,17 Sulfonation reactions of PS are often carried out by a sulfona- tion process either homogeneously or heterogeneously. 18–22 In homogeneous conditions, reagents like sulfuric acid, sulfur triox- ide and its complexes, e.g. acyl sulfates and chlorosulfonic acid, are used to sulfonate PS. 18 Turbak 19 prepared sulfonated poly(vinyl toluene) and PS in a homogeneous phase by using trialkyl phos- phate complex of sulfur trioxide as sulfonating agent. Lightly sulfonated PS was prepared by the reaction of a dichloroethane solution of PS with acetyl sulfate at ambient temperature for 1 h. 20,22 The preparation of SPS in a heterogeneous phase as in the copolymerization of styrene and sodium styrene sulfonate has ad- vantages for industrial application over the homogeneous process because it avoids problems with solvents and the separation of the sulfonated product from the reaction mixture. 21–25 Another ad- vantage is that sulfonation proceeds without polymer degradation and crosslinking. 26,27 In this article we have prepared SPS using concentrated sulfuric acid as sulfonating agent. 28–30 We used PS particles of various sizes obtained by emulsion polymerization and immersed them in concentrated sulfuric acid for different times and temperatures. The advantage of this sulfonating agent is that we can readily vary the degree of sulfonation (DS) by varying the reaction time and temperature. Despite the large amount of literature related to SPS preparation, no study has been focused on the effect of PS particle size on the DS and other properties of SPS. Hence, a detailed study is required to establish the method for the preparation of SPS with desirable DS for different particle sizes of PS. In the present work we focused on controlling the DS by varying parameters such as temperature and time of the reaction, and made an effort to understand the effect of PS particle size on the DS. We have developed a series of recipes to obtain SPS with various DSs of different PS particle sizes. Finally, we have characterized the resulting SPS by using ∗ Correspondence to: Tushar Jana, School of Chemistry, University of Hyderabad, Hyderabad, India. E-mail: tjsc@uohyd.ernet.in; tjscuoh@gmail.com School of Chemistry, University of Hyderabad, Hyderabad, India Polym Int (2012) www.soci.org c  2012 Society of Chemical Industry