Preparation of poly(tert-butyl acrylate-g-styrene) as precursors of amphiphilic graft copolymers: 2. Relaxation processes and mechanical behavior

doi:10.1016/S0032-3861(02)00063-0

Polymer

Volume 43, Issue 9, April 2002, Pages 2803-2810

https://doi.org/10.1016/S0032-3861(02)00063-0 Get rights and content

Abstract

Viscoelastic relaxations of four poly(tert-butyl acrylate-g-styrene) copolymers are studied over a wide range of temperatures. The temperature location and apparent activation energy of the distinct relaxations observed under tension mode are discussed. As grafted polystyrene content increases in the copolymer, microdomains of both components occur and, consequently, two relaxations associated with cooperative motions of either the acrylate backbone or the graft are observed. A bending deformation mode is also analyzed in the region of the glass transition of the two components to study the phase separation. Moreover, the glass transition temperature of the acrylic backbone is estimated by microhardness measurements. The relationship between microhardness and storage modulus is established either below or above the glass transition of the tert-butyl acrylate backbone.

Introduction

One of the purposes of researches during the years has been to create polymers where the final behavior can be fine-tuned based on the applications. The control of macroscopic properties, such as glass transition temperature, melting temperature, solubility, particle size, and viscoelasticity, can be attained by regulating the degree of branching. Graft and block copolymers have found numerous practical applications because of the interaction between segments belonging to the same or different polymer chains. They represent a class of macromolecular architecture with unique features, which include as structural variables for instance, the composition, backbone length, branch length, and branch spacing. Accordingly, such graft copolymers possess a great potential to provide new properties and, consequently, spread out their application in fields which include, at the present time, polymer alloys, surface modification, membranes, and coating [1]. On the other hand, graft copolymers present distinct physical properties to those exhibited by their linear counterparts in either solution or in solid state. The rheological and mechanical behavior is strongly influenced by the existence of microdomains. These structures are in turn determined by the segment composition and architecture, temperature (or segregation power), and concentration and solvent quality (for the cases of copolymer solution).

Dynamic mechanical thermal analysis (DMTA) allows to elucidate the different motions taking place in the macromolecular chains and, in addition, to learn about the mechanical behavior because of the relationship of the storage modulus, E′, with the stiffness. The study of viscoelastic relaxations of poly(tert-butyl acrylate-g-styrene), PtBAS, copolymers can be performed by taking into account the different relaxation processes exhibited in both ‘parent’ homopolymers [2], poly(tert-butyl acrylate), PtBA, and polystyrene (PS). The comparison of viscoelastic spectra in these copolymers with those displayed by homopolymers seems to be an adequate route to assess the influence of composition on the viscoelastic behavior and mechanical properties of such copolymers. Moreover, DMTA provides valuable information about, on the one hand, the existence of the mentioned microdomains by the observation of the glass transition temperature, T_g, of both the pure components or, on the other hand, the assumption of a homogeneous graft copolymer if a unique T_g is empirically seen.

The Vickers microindentation hardness test measures the resistance of a given material to a plastic deformation produced by the impact of an indenter. Therefore, amorphous and semicrystalline polymers provide a different response related to the rigidity of the system. Differences in microhardness can even exist within amorphous polymeric materials if they are composed by microdomains of distinct stiffness (for instance, block or graft copolymers). Microhardness (MH) tests allow a rapid evaluation of variations in mechanical properties that are affected by changes in chemical or processing conditions [3], [4], [5]. In addition, MH results can be related to other mechanical parameters such as the elastic modulus and yielding stress [6], [7].

In a previous article [8] the preparation, kinetic study, and thermal properties of poly(tert-butyl acrylate-graft-polystyrene) copolymers have been reported. The simple desprotection of tert-butyl acrylate (tBA) group by hydrolysis allowed to obtain the amphiphilic graft copolymers. The aim of the current work is to study the relaxation processes of these graft copolymers by DMTA, either under a tensile deformation mode or a bending one, and to determine whether they exhibit or not the phase separation at the microscopic level. In addition, the T_g of the tert-butyl acrylate backbone is measured by microhardness measurements. Finally, the relationship between storage modulus and microhardness is established for the distinct graft copolymers at temperatures either below or above the glass transition of the backbone.

Section snippets

Materials

Macromonomer SR-4500 (ARCO Chemical Company), a poly(styrene) carrying a methacryloyloxy group at the chain end with a number-average molecular weight of 13 000 g mol⁻¹ and M_w/M_n=1.05, was used as received. 2,2′-Azobis(isobutyronitrile), AIBN, (Fluka) was purified by successive crystallizations from methanol. Benzene (Merck) and tert-butyl acrylate, tBA, (Merck) were purified by conventional methods [9].

Graft copolymer

The copolymer reactions were conducted in benzene solution using 8.0×10⁻³ mol l⁻¹ of AIBN as

Viscoelastic behavior

The viscoelastic response of graft copolymers is primarily determined by the mutual solubility of the two homopolymers [11]. Fig. 1 shows the storage, E′, and loss, E″, moduli and $tan δ$ for one of the graft copolymers, PtBAS4, at different experimental frequencies used under tensile deformation. Temperature location and apparent activation energy of the distinct relaxation processes are listed in Table 2. E′ values do not practically change with frequency up to approximately 50 °C since below

Acknowledgements

This research has been supported by the Comisión Interministerial de Ciencia y Tecnologı́a (CICYT), (MAT2000-1008). M. Fernández-Garcı́a and M.L. Cerrada are grateful to the Comunidad Autónoma de Madrid and Ministerio de Educación y Cultura, respectively, for their financial support.

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Preparation of poly(tert-butyl acrylate-g-styrene) as precursors of amphiphilic graft copolymers: 2. Relaxation processes and mechanical behavior

Abstract

Introduction

Section snippets

Materials

Graft copolymer

Viscoelastic behavior

Acknowledgements

Polym Testing

Polymer

Polymer

Adv Polym Sci

Anelastic and dielectric effects in solid polymers

Current Trends Polym Sci

Angew Macromol Chem