Study on the effect of structure of polydimethylsiloxane grafted with polyethyleneoxide on surface activities

https://doi.org/10.1016/j.colsurfa.2008.11.020Get rights and content

Abstract

Twelve kinds of polydimethylsiloxane grafted with polyethyleneoxide (PDMS-g-PEO) were synthesized by two-step reactions: the preparation of silicone backbone containing methyl-hydrogen siloxane units and the following hydrosilylation reactions with PEO containing allyl groups. When the effects of structure of PDMS-g-PEO on the surface properties were investigated by comparing the each effect caused by EO content, EO chain length or the size of hydrophobe, the size of hydrophobe was found to be a critical factor for deciding surface tension rather than the content of EO. Significant decrease in the contact angle of the aqueous solution was observed when EO content of PDMS-g-PEO or the surfactant concentration increased. On the other hand, the viscosity of the surfactant solution at constant concentration slightly increased as EO content increased, but the difference was not significant. Dynamic surface tension measurement showed a decrease in the surface tension of the aqueous surfactant solution with an increase in the surfactant concentration. In addition, much longer times were required to reach an equilibrium compared with conventional nonionic or ionic surfactants due to the lower mobility of PDMS-g-PEO with high molecular weight.

Introduction

Silicone surfactants have been widely used in many industrial fields such as foam stabilizers, emulsifying agents, detergents and antifoaming agents because of their thermal stability and low surface tension [1], [2], [3]. Silicones modified with hydrophilic polyethyleneoxide (PEO) have a strong tendency to orient at the interface between two incompatible phases caused by their surface active properties, and provide maximum benefit at low concentrations. The molecular structure of PEO-modified silicone copolymers is typically classified into branch type and tri-block or multi-block types and the differences in their physical properties are well summarized in the earlier literature [4]. Among them, graft copolymer (PDMS-g-PEO) composed of polydimethylsiloxane (PDMS) backbone and branched PEO has been most extensively studied. Since the structure–property relation of PDMS-g-PEO has been investigated in many earlier studies [5], [6], surface activity of PDMS-g-PEO was found to be sensitive to weight fraction of EO (EO content), the length of the EO chain and the size of the hydrophobe as seen in conventional hydrocarbon surfactants. However, most studies have been focused on the effect caused by individual factor; on the other hand, the direct comparison of the each effect caused by these three factors were not sufficiently investigated.

In general, PDMS-g-PEO is prepared by two-step reactions as shown in Fig. 1. The first step is the synthesis of silicone fluid intermediate (SH copolymer) by the equilibrium polymerization and the second is the hydrosilylation reactions of silicone fluid intermediate with PEO containing allyl groups in the presence of platinum-based catalysts. The first step, which affords silicone backbone containing methyl-hydrogen siloxane units, is the equilibrium polymerization of octamethylcyclotetrasiloxane (D4), poly(methylhydrosiloxane) (PMHS) using hexamethyldisiloxane (HMDS) as the end-capping reagent. In most studies, the chemical composition denoted with D and D′ in Fig. 1 and molecular weight of SH copolymer were calculated from the feeding ratios without any proper analysis. Therefore, the structures of PDMS-g-PEO did not represent discrete, pure, and individual components. However, this insufficient analysis of functional silicones has been improved by the recent development in 29Si NMR studies [7], [8], [9].

In this study, PDMS-g-PEO was synthesized according to the standard techniques as shown in Fig. 1. The characterization of SH copolymer prepared by the equilibrium polymerization was performed by 29Si NMR and 1H NMR and the compositions were compared with the expected composition calculated from the equilibrium distribution of feeding materials. Using SH copolymer, twelve kinds of PDMS-g-PEO were synthesized by hydrosilylation reaction with unsaturated PEO (UPEO). The effects of structure of PDMS-g-PEO on the surface properties such as contact angle, viscosity and surface tension were investigated by comparing each effect caused by EO content, EO chain length or the size of hydrophobe.

Section snippets

Materials

Octamethylcyclotetrasiloxane (D4), poly(methylhydrosiloxane) (PMHS, the range of molecular weight is 1700–3200 g/mol), hexamethyldisiloxane (HMDS), and hydrogen hexachloroplatinate (IV) hydrate (CPA) were purchased from Aldrich and used without further purification. Four kinds of UPEOs, whose average molecular weights are 340, 440, 800 and 1540 g/mol, were kindly supplied by NOF Cooperation, Japan. The commercial trade names of UPEOs are UNIOX PKA-5006, 5008, 5118 and 5010.

Speier's catalyst was

Synthesis of SH copolymer

SH copolymers were synthesized by the equilibrium polymerization of D4 and PMHS using HMDS as the end-capping reagent to control molecular weight [11]. Incorporation of HMDS in feed led to the copolymer containing methylhydrosiloxy units with randomness, and the ratios of methylhydrosiloxy units to dimethylsiloxy units in the copolymer can be regulated by the equivalent ratio of HMDS/D4 in feed.

The structure of SH-2004 copolymer was analyzed by 1H NMR and 29Si NMR. The 1H NMR spectrum of SH

Conclusions

PDMS-g-POEs with various compositions were synthesized by the hydrosilylation reactions of SH copolymer with UPOE in the presence of various platinum-based catalysts. When the surface activities in aqueous solution were compared, the following results are drawn:

  • (1)

    For series I surfactants, where the same SH copolymer was coupled with EO having a different chain length, the surface tension increases as an increase in chain length of EO as well as in EO content.

  • (2)

    The result obtained from Series II

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