PU-PHEMA

The third group of polymer-polymer hybrid networks, studied with the application of CRS and other techniques, were polyurethane-poly(2-hydroxyethyl methacrylate) (PU-PHEMA) ones [19,169-171]. Among different applications, PU materials are extensively used for biomedical aims, for instance, in blood contacting apparatus and for organ reconstruction [172-174]. Although PUs are relatively biocompatible materials, they are also known to be prone to biodegradation [175], stress... 

In Sect. 3.3.3, a peculiar dynamics in the hybrid PU-PHEMA semi-IPNs was discussed. In the work described in [239,240], a series of PU-PHEMA-ND nanocomposites with different matrix compositions and ND contents of 0.25, 1, or 3 wt% were studied. Their nanostructure, glass transition dynamics, and elastic properties were investigated in the combined CRS/AFM/DSC experiments. We revealed a possibility of large and specific impact of low content of 3D nanofiller on polymer matrix, without performing a special functionalization of its surface. For preparing nanocomposites, the NDs obtained by the shock-wave method, with the particle sizes of 2-lOOnm and specific surface area of 220m g were used. NDs were introduced into the reaction mixture at the stage of PU synthesis. 

Using AFM analysis, three kinds of ND dispersion/distribution in the PU-PHEMA matrices were detected (1) individual nanoparticles 50-100 nm in size (2) their agglomerates with the size of 0.2-0.5 Lim and (3) their larger aggregates from 0.6 pm up to several micrometers in size (Fig. 59). Generally, all these... 

Such differences in the dispersion states of NDs in the PU-PHEMA matrices had to provide the respective differences in the interfacial area values. It could be expected that the maximal interfacial area and maximal impact of nanofiller on properties were to be attained at 3 wt% NDs in the 63PU-37PHEMA matrix but the largest impact of NDs on the 83PU-17PHEMA matrix might be at 0.25 wt% NDs. 

Really, according to the calculations [220], polymer matrix is considered to be entirely nanoscopically confined by 3D nanofiller only in the case when the average inter-particle distance, L, is close to or less than the dimensions of unperturbed macromolecular random coil the latter is estimated by radius of gyration Rg. At the same time, in our case 8nm for PHEMA and L > 300-500 nm when 0.25% of 3D particles with 50-100 nm size are introduced into the matrix. It means that the PU-PHEMA-ND nanocomposites exhibit an unexpectedly low percolation threshold. 

Fig. 61 A scheme of peculiarly cross-linked structure of PU-PHEMA-ND nanocomposite. ND particles are incorporated via covalent bonding into a network [239,240]...

The hybrid PU-PHEMA semi-IPNs were prepared as described previously [169]. To prepare the nanocomposites, fumed silica nanoparticles were introduced into the polymer system at the stage of PU synthesis. The silica particles were used (1) without special functionalization of their surface with initial surface silanol groups ( -OH cover ), (2) after their functionalization by... 

Contrary to the majority of papers on polymer-silica nanocomposites, the very low content of 3D nanosilica particles in the polymer matrix in this work resulted in average inter-particle distance L larger by an order of magnitude than the radius of gyration Rq of PHEMA. In spite of that, a considerable impact of small 3D silica additives on matrix dynamics was found due to double PU/PHEMA and silica/matrix hybridization. 

Figure 4.12 Effect of grafting PHEMA and PPEGMA on PU surfaces. (Top) Reduced microfouling—that is, bacterial cell viability on surface grafted with PHEMA and PPEGMA compared to pristine PU and PU with covalently bound macroinitiator (PU-Br). (Bottom) Reduced macrofoulmg—that is, reduction m the amount of settled, live barnacle cyprids. Source Adapted from Pranantyo et al. [24], with permission from RSC Publishing.

These semi-IPNs were synthesized [169] by the sequential method the PU network was synthesized (Sect. 2.3) and then swollen with 2-hydroxyethyl methacrylate (HEMA) monomer followed by its photopolymerization. The PHEMA content in these semi-IPNs varied from 10 to 57 wt%. 

IR spectra [19], this constrained dynamics effect was caused by chemical bonding of some of PHEMA hydroxyls with unreacted, residual isocyanate groups of PU network, i.e., by some hybridization of the semi-IPN constituents. 

In principle, similar results were obtained in all subsequent investigations done for various systems [189-191]. As a rule, the temperature transition for the elastomeric component shifts to higher temperatures, whereas the temperature transition for the more rigid component shifts to lower temperatures. Now we can say that these shifts are determined by the formation of two phases with different compositions (initially these shifts were ascribed to the changing compatibility of two components). Eor semi-IPNs based on PU and PHEMA [191], the segregation degree varies in the limits 0.22-0.35,... 

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