Inverse Gas Chromatography on Graft and Random Copolymers of Styrene and 2-Hydroxyethyl Methacrylate

Polymer Journal, Vol. 15, No. 8, pp 577-583 (1983) Inverse Gas Chromatography on Graft and Random Copolymers of Styrene and 2-Hydroxyethyl Methacrylate Koichi ITO, Yutaka MASUDA, Takuji SHINTANI, Toshiaki KITANO, and Yuya YAMASHITA* School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan *Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received January 31, 1983) ABSTRACT: Inverse gas chromatography (IGC) was applied to amphiphilic graft copolymers of poly(2-hydroxyethyl methacrylate) (PHEMA) with polystyrene (PSt) branches using various probes at 160°C. Retention oftetradecane or amylbenzene, which selectively interacts only with PSt phase, suggested that a microphase inversion occurs around 20-30wt% PSt, below which PSt segments constitute a discontinuous phase (islands). Rather nonselective probes such as dimethylformamide and 2-(2-methoxyethoxy)ethanol showed a retention which exceeds that expected from a simple additive relation, suggesting a considerable contribution of their interaction with the interface of the microphase-separated domains. In contrast, random copolymers showed a retention behavior as expected for a statistical distribution of the monomer units. General discussion is given on IGC as a means of characterizing binary polymer systems. KEY WORDS Inverse Gas Chromatography I Amphiphilic Graft Copolymer I 2-Hydroxyethyl Methacrylate I Styrene I Microphase Separation I Morphology I Selective and Nonselective Probes I Inverse gas chromatography (IGC) has been developed by Guillet and others1.2 as a simple and convenient method for evaluating various properties of polymers such as melting point (Tm), glasstransition point (Tg), crystallinity, interaction parameter, and solubility parameter. In principle, it relies on the interaction of an appropriate volatile compound as a probe with a target polymer as a stationary phase in conventional gas chromatography. Since any change in polymer phase would be reflected in the retention behavior of a probe, IGC also appears to be useful as a method for examining a multi-phase structure in block or graft copolymers and in polymer blends, in addition to being a simple method of evaluating their interaction with the probe. Galin and Rupprecht3 investigated the retention of decane on polystyrene (PSt)-polydimethylsiloxane (PDMS) block copolymers to estimate the domain size of PSt. Ward et a/. 4 similarly evaluated the morphology of PDMSbisphenol A polycarbonate block copolymers and blends. Decane as a probe was assumed in these cases to interact independently with the PDMS matrix and surface (interface) of PSt- or polycarbonate domains at a temperature below their Tg. Suzuki et a/. 5 studied the surface morphology of PSt-poly(ethylene oxide) blend by following the retention behavior of octane in comparison with scanning electron-microscope observation. We also examined a retention of dodecane on PSt-polytetrahydrofuran (PTHF) block copolymers6 and on poly(methyl methacrylate) (PMMA}poly(stearyl methacrylate) (PSMA) graft copolymers7 to show that PTHF or PSMA segments, which have a lower Tm and a lower surface-energy, tend to make a continuous phase as compared to PSt or PMMA segments, respectively. In general, the IGC method may be expected to give more clear-cut and interesting information on the morphology of these multi-phase polymer systems by analyzing the retention behavior of several probes which may interact selectively with a par577 K. ITO eta!. ticular component. To this end, the present paper describes the application of various probes to IGC on amphiphilic graft and random copolymers of styrene (St) and 2-hydroxyethyl methacrylate (HEMA). The graft copolymers were found by 1 H NMR and contact-angle measurements to have a distinct tendency to form micellar domains in contrast with a single-phase structure of random copolymers. 8 The morphology of the same graft or block copolymer systems were examined by means of transmission e!ecron-microscopy (TEM) by Yamashita et a/. 9 and Okano et a/. 10 EXPERIMENTAL Materials Graft copolymers, random copolymers, and homopolymers were prepared, purified, and characterized as reported before. 8 Table I shows the polymers used and their column data. Acid- and silane-treated diatomaceous earth, Uniport HP of Gasukuro Kogyo Co., Ltd., 60-80 mesh with a nominal specific surface area I m2 g-1, was used as an inert support for polymers. Methane and probes for IGC were used as supplied commercially. J!:o= VR 0 =f·tR 273 Po-Pw (p)p 0 ) 2 -1 9 W W T,. Po (1) (pJpY-1 where w is a polymer loading weight,/, the flow rate of the carrier gas (helium) measured by a soapbubble flow meter at an ambient temperature T,; Pi and Po are the column inlet and outlet pressures, respectively, which were measured by a mercury manometer, and Pw is a water-vapor pressure at T,. The flow rate f was kept constant at 15 mljmin. The column temperature T was changed in the ranges 150-170°C and 50-70°C, which are well above and below the T 8 's of the homopolymers, around 100°C for PSt 11 and !20°C for PHEMA.U A linear regression line was obtained between In V8° and 1/T, as given in Figure 1 as an example. V8° at 60 and 160°C were read on this line within a precision of 1 ml g -I, and the interaction enthalpy 11H between the probe and the polymer was calculated from their slope, which should be effectively equal to (11Hv -11H)j R/· 2 •13 •14 where 11Hv is a heat of vaporization of the probe, which was in turn obtained from a literature source 15 •16 or estimated from the vapor-pressure data. The 95% confidence limit of 11H was in a range of ± 1 to ± 2 kcal mol- 1 at 160°C. Methods Polymers were dissolved in tetrahydrofuran (or methanol for PHEMA homopolymer) and coated on the inert support, Uniport HP, by slowly evaporating the solvent with gentle stirring, and finally dried under vacuum. The polymer loading was determined by the calcination method. The polymer-coated supports were packed into a copper column (4mm i.d., 80-100cm length) and conditioned at 60°C. The column data were given in Table I. Retention data were collected on a Yanaco gas chromatograph G-180 equipped with a flame ionization detector. A probe in less than 0.1 .ul was injected simultaneously with methane as a noninteracting marker, and the net retention time tR was determined from the peak-to-peak distance from the probe to methane. tR thus determined was independent of the probe size injected. Retention volume VR0 and specific retention volume V8°, corrected to 0°C, were calculated from tR as follows. 1 •2 578 THEORETICAL CONSIDERATION In general, the gas chromatographic retention of a probe by a polymer is due to two mechanisms, absorption into the polymer bulk phase and adsorption onto the polymer surface, so that1 •2 •13 Column temperature, °C 5.0,...----'-l"c-0 4.5 ] 0 0'1 > c 4.0 3.5 3.0L_ (...truncated)