AbstractPoly(n‐butyl acrylate) (PnBA) chemically crosslinked with tetraethylene glycol dimethacrylate (TEGDM) and physically crosslinked PnBAs produced by neutralization of poly(n‐butyl acrylate‐stat‐acrylic acid) with NaOH or Ca(OH)2were prepared as a polymer I network. Each polymer I was swollen with styrene and curedin situinto semi‐IPN‐TEGDM, semi‐IPN‐Na, or semi‐IPN‐Ca, respectively. Both physically crosslinked polymers maintained their shapes during the swelling procedure. Dynamic mechanical spectroscopy indicated that good mixing of the two polymers took place in the semi‐IPN‐Ca as well as in semi‐IPN‐TEGDM, but a distinct phase separation occurred in the semi‐IPN‐Na. These results were supported by their transparent or optical opaque appearances, respectively. Annealing at 180°C developed further phase separation in the semi‐IPN‐Na, but very little in the semi‐IPN‐Ca. Analyses by the incompatibility number (based on the modulus–temperature curve) and the calculation of individual phase compositions (from the glass transition temperature shifts) were used in
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