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Semi-interpenetrating polymer networks and mixtures based on functionalised polyurethane and polymethacrylate prepolymers: motional heterogeneity and microphase separation

Semi-interpenetrating polymer networks (SIPN) and polymer mixtures based on segmented polyurethane and methacrylate copolymers are extensively used because of their interesting physical properties and high versatility in chemical structures. Most of the properties are directly related to the molecular-level dispersion of polymer segments and phase separation. These properties are enhanced by the polymer-polymer noncovalent interactions induced by additional functional groups incorporated in the backbone of the constituent polymers. In the present work SIPNs and polymer mixtures (1 : 1 mass ratio) based on segmented polyester polyurethane (PU) with additional carboxylic groups and methacrylic copolymer (PM) with tertiary amino groups were studied using electron spin resonance (ESR) - spin label method. The concentration of functional groups varied from 0 to 0.45 mmol g-1 in both prepolymers. Segmental motional heterogeneity, motional transitions and phase separations were extensively explored from the temperature-dependent ESR spectra of spin-labelled polymer components. More specifically the degree of mixing or interpenetration in the SIPNs is observed in a nano-range. The parallel analysis of the composite ESR spectra of the SIPNs and polymer mixtures, either with a labelled PU or PM component, make it possible to relate the segmental dynamics of the one polymer component in the presence of the other component to the complex polymer-polymer interactions and the extent of the miscibility. The ESR spectra of the chain-labelled PM component reveal that a part of the PM chains in the mixtures and SIPNs are plasticized by PU soft segments. The ratio of the fast and slow motion derived from the composed spectra indicates a critical level of functional groups' concentration, above which additional hydrogen bonding interactions lead to a change in the network structure. The better interpenetration and interaction of the two polymers is confirmed in the SIPNs. An increased miscibility and disorganisation of the ordered domains is seen from the loss of spherulitic morphology and crystallinity at higher functional groups concentration, as shown by the wide-angle X-ray diffraction (WAXD) measurements and optical microscopy. The motional behaviour of the PU labelled prepolymer (end labelled hard segments) also confirms a higher degree of phase separation of the PU segments with additional functional groups. The appearance of the critical concentration of functional groups (0.35 mmol g-1) with the largest fraction of restricted motion is discussed in terms of chain structure due to the noncovalent interactions that at a critical level of interactions leads to the formation of more open hard segment structure accessible to interactions with the soft segments. The analysis of the ESR spectra of spin-labelled hard segments indicates better miscibility of the prepolymers with additional functional groups in both SIPNs and mixtures. However, in the SIPNs motional restriction increases as a consequence of crosslinking, grafting and hydrogen bonding, thus enhancing molecular level dispersion. Motional transitions are compared with the glass-transition temperatures measured by DSC and the differences between the two methods are discussed in terms of domain size and motional heterogeneity distribution.

Semi-interpenetrating polymer networks; polymer mixtures; ESR

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