Elsevier

Polymer

Volume 48, Issue 14, 29 June 2007, Pages 4087-4096
Polymer

Synthesis and rheological properties of an associative star polymer in aqueous solutions

https://doi.org/10.1016/j.polymer.2007.04.069Get rights and content

Abstract

Rheological properties of aqueous solutions and hydrogels formed by an amphiphilic star block copolymer, poly(acrylic acid)-block-polystyrene (PAA54-b-PS6)4, were investigated as a function of the polymer concentration (Cp), temperature, and added salt concentration. The water-soluble polymer synthesised by atom transfer radical polymerization (ATRP) was found to form hydrogels at room temperature at polymer concentrations, Cp, over 22 g/L due to the interpolymer hydrophobic association of the PS blocks. Increasing Cp leads to stronger elastic networks at room temperature that show a gel-to-solution transition with increasing temperature. Increase of ionic strength decreases the moduli compared with the pure hydrogel but did not affect the gel–sol transition temperature significantly. Small-angle X-ray experiments showed two distinct scattering correlation peaks for samples above the gelling Cp, which indicates the aggregates formed due to hydrophobic association. Upon heating the intensity of the scattering correlation peaks was found to decrease indicating the loss of the network structure due to thermal motion.

Introduction

Associative polymers are of great industrial importance owing to their adjustable rheological properties, for example in oil recovery, paints, cosmetics formulations and pharmaceutical as well as medical applications such as drug delivery and tissue engineering [1]. Particularly interesting are the stimuli-responsive systems of associating polymers in which free-standing gels transform to free-flowing liquids or vice versa. To obtain such systems water-soluble polymers modified with a small number of hydrophobic groups are often used [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Enhanced viscosity and reversible gelling behaviour originate from transient intermolecular associations between the hydrophobic groups.

Responsive properties in aqueous solutions are attained by using either neutral (i.e. non-ionizable) water-soluble polymers [4], [5], [6], [7], [21], [30], [31], [32] or polyelectrolytes [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [22], [23], [24], [25], [26], [27], [28], [29]. The self-assembly of block polyelectrolytes gives additional versatility with regards to the responsiveness and polyelectrolytes may create strong elastic gels at lower concentrations than neutral polymers [2]. By changing the polymer concentration [4], [6], [7], [8], [9], [10], [11], [13], [14], [16], [17], [18], [19], [26], [27], temperature [5], [6], [15], [17], [18], [21], [22], [23], [24], [25], [26], ionic strength [10], [27], [28], pH [19], [29] or by adding analytes [12], [23] the solution interactions and the rheological properties may be varied significantly. The characteristics of the polymers may be tuned for example by varying the molar mass of the polymer or the mass ratios of the comonomers, but also by the topology of the polymers, using structures such as diblocks [2], [3], [10], [14], [23], [24], triblocks [2], [3], [6], [15], [16], [17], [18], [19], [25], [29] or stars [20], [21].

In this work, we have synthesised a relatively monodisperse water-soluble associating starlike polyelectrolyte, four-armed poly(acrylic acid)-block-polystyrene, and investigated the stimuli-responsive behaviour of its hydrogels by rheological methods. The starlike topology of the polymer represents a special case of a branched architecture, in which all polymer chains have the same branching point. Compared with linear diblock and triblock polymers the four-armed topology may enable more effective association. Indeed, according to Lin and Cheng [31], four-arm amphiphilic stars based on poly(ethylene oxide) inner blocks and poly(N-isopropyl acryl amide) outer blocks are able to form gels that have higher strength than stars with higher number of arms, due to the low intramolecular aggregation in the former case. Another advantage over linear polymer systems is the existence of the central core of the star polymer. Due to the multifunctionality, and often to the hydrophobic nature of the core such polymers have been proposed as carriers for example for fragrance molecules [33], dyes [34] or catalysts [35], [36]. The polyelectrolyte core in the present case may also be suitable for encapsulation of ionizable compounds and additionally the polyelectrolyte is sensitive to changes in environmental conditions. Thus in addition to polymer concentration, we have examined the effects of ionic strength and temperature on the rheological properties of the aqueous polymer solutions. Furthermore, small-angle X-ray scattering (SAXS) was used to study the structure of the non-saline polymer solutions at different temperatures.

Section snippets

Materials

The synthesis of a tetrafunctional initiator (diTMP–Br) by acylation of di(trimethylolpropane) (diTMP) with 2-bromoisobutyrylbromide is described elsewhere [37]. tert-Butyl acrylate and styrene were distilled from CaH2 prior to use. CuBr, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), trifluoroacetic acid (all from Aldrich) were used without further purification.

Synthesis of starlike poly(tert-butyl acrylate), (P(tBA)4)

tert-Butyl acrylate (15.34 g, 0.12 mol), CuBr (0.26 g, 1.81 mmol) and diTMP–Br (0.32 g, 0.38 mmol) were placed in a dry Schlenk tube.

Synthesis and characterisation of the amphiphilic star polymer

The amphiphilic star block copolymer was synthesised by ATRP of tert-butyl acrylate, tBA, followed by block copolymerisation of styrene and the hydrolysis of poly(tert-butyl acrylate) blocks to poly(acrylic acid). The structure and 13C NMR spectrum of the di(trimethylolpropane)-based initiator diTMP–Br used in the polymerization of tBA is presented in Fig. 1. This particular initiator was chosen instead of the often-used pentaerythritol-based one, due to its lower steric hindrance and better

Conclusions

A water-soluble amphiphilic star polymer, (PAA54-b-PS6)4, based on poly(acrylic acid) and polystyrene was synthesised and its rheological properties were studied in aqueous solutions. In water the polymer formed hydrogels with increasing polymer concentration (Cp). The gelling concentration in water was found to be 22 g/L and elastic gels (storage modulus > 1100 Pa) were observed at the highest studied Cp of 51 g/L. The observed physical network formation at such low concentrations was rationalized

Acknowledgment

S. Strandman wishes to thank the Finnish National Graduate School in Nanoscience (NGS-NANO) for funding.

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