Acrylic networks

IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

As discussed in Chapter 7, radical polymerization can induce marked inhomogeneities, easily observable by DMTA. For example, in photocured polyurethane-acrylate networks, two a peaks corresponding to distinct phases, separated by about 40-100 K, depending on composition and cure conditions, can be observed in the DMTA dissipation spectrum at 10 Hz (Barbeau et al., 1999). 

The last case concerns inhomogeneous networks often produced by chain polymerization (acrylate networks, unsaturated polyesters), where a gradient of crosslink densities is the result of the reaction mechanism and, in some cases, thermodynamic effects (Chapter 7). 

A pendulum may be used (Charpy, Izod, tensile impact) to determine the work of fracture (Brown, 1999). Instrumented devices provided with piezoelectric transducers are also available load-time or load-displacement curves can be recorded (Merle et al., 1985), giving as much information as static tests. Servohydraulic or pneumatic setups and falling weight devices are also used. The drop ball test from the US Food and Drug Administration, is especially useful for optical lenses (acrylate networks). 

The impact properties of acrylate networks used in optical lenses have been widely investigated, mainly from an industrial point of view. Due to internal stresses which develop during the reaction, the cure schedule and the heating and cooling rates have to be carefully adjusted. The structural complexity of these networks does not allow us to obtain correlations of the observed impact strength values (Matsuda et al., 1998). 

An IPN of two different polymers is shown in Equation 12, where the poly (ethyl acrylate) network is formed first, then swollen with styrene and crosslinker, and polymerized in situ (16). 

Similar reversible contraction/dilation experiments under constant load were recently performed by Eisenbach on stretched poly(ethyl acrylate) networks, cross-linked however with 4,4 -dimethacryloylaminoazobenzene (0.02 mol-%). 

Fig. 14. Schematic representation of the photomechanical effect induced in poly(ethyl acrylate) network with azoaromatic cross-links upon irradiation [35]...

Eisenbach, C.D. (1980) Isomerization of aromatic azo chromophores in poly(ethyl acrylate) networks and photomechanical effect. Polymer, 21, 1175—1179. 

In addition, the solvent generates pores and, in conjunction with other parameters such as temperature, influences the morphology of the material (size, shape and size distribution of the cavities). For chromatographic applications, macroporous networks are preferable. As the pores are more accessible, recognition is enhanced and retention times reduced. For instance, the use of acetonitrile as solvent in acrylate networks leads to a more macro-porous structure than chloroform [130]. 

FTIR experiments show a loss of ether functionality at 990 cm at 150 C which is further evidence that condensation of alkoxy functionality is occurring. A loss of acrylic functionality at 1640 cm is observed for the AH as t ell as for the epoxy acrylate during thermal cure. This may be the result of thermal generation of radicals or of radical trapping in the photoploymerized acrylic network which has been reported. 

Experimental determination of the contributions above those predicted by the reference phantom network model has been controversial. Experiments of Oppermann and Rennar (1987) on endlinked poly(dimethylsiloxane) networks, represented by the dotted points in Figure 4.4, indicate that contributions from trapped entanglements are significant for low degrees of end-linking but are not important when the network chains are shorter. Experimental results of Erman and Wagner (1980) on randomly crosslinked poly(ethyl acrylate) networks fall on the solid line and indicate that the observed high deformation limit moduli are within the predictions of the constrained-junction model. 

Eisenbach (1980) investigated the photomechanical effect of poly(ethyl acrylate) networks (3) cross-linked with azobenzene moieties and observed that the polymer network contracted upon exposure to UV light (caused by the trans-cis isomerization of the azobenzene cross-links) and expanded upon irradiation of visible light (caused by cis-trans back-isomerization Fig. 3.5). This photomechanical effect is mainly due to the conformational change of the azobenzene crosslinks by the trans-cis isomerization of the azobenzene chromophore. However, the degree of deformation was small (0.2%). 

The transfer of the geometrical change caused by photoisomerization on the molecular level to macroscopic shape changes was demonstrated in different concepts. While azo-dye loaded nylon filament fabrics showed shrinkage of approximately 0.1% after irradiation under load [162], the incorporation of azobenzene-containing crosslinkers in poly(ethyl acrylate) network films enhanced this photomechanical effect to 0.25% [163]. This is a significant difference to... 

DuPont Cyro American Polymers AtoHaas Continental Acrylics Cyro DuPont ICI Acrylics LG Chemical Network Polymers Plaskolite RTP Network Polymers NOVA Chemicals AtoHaas Continental Acrylics Cyro DuPont ICI Acrylics Network Polymers RTP AtoHaas Cyro ICI Acrylics Network Polymers Plaskolite RTP Cyro BP Chemicals BP Chemicals BP Chemicals PPG... 

Figure 1.4 Schematic structure of grafted silica nanoparticles and radiation-cured grafted silica/acrylate networks (adapted from reference [41]).

C. F. Ryan and R. J. Crochowski, Acrylic Modifiers which Impart Impact Resistance and Transparency to Vinyl Chloride Polymers, U.S. Pat. 3,426,101 (1969). Three-layered latex IPN Poly(butyl acrylate), network I. Polystyrene, network II. Poly(methyl methacrylate), linear polymer III. Latex dispersed in poly(vinyl chloride) or copolymers. 

Neuss, S., Blomenkamp, I., Stainforth, R., Boltersdorf, D., Jansen, M., Butz, N., Perez-Bouza, A., Knuchel, R., 2009. The use of a shape-memory poly(is an element of-caprolactone)dimeth-acrylate network as a tissue engineering scaffold. Biomaterials 30,1697-1705. 

Lightly crossUnked acrylic networks Bulk rheological properties, elastic modulus, resistance to interfacial crack propagation Dynamic mechanical spectroscopy 129... 

Figure 9 shows that increase in the excited state quencher concentration gives rise to an increased width of the reflection band. The fact that intensity plays a very important role indicates the competition between the phase separation process which causes the increase as opposed to the polymerisation speed which tends to freeze in the structure and the pitch present within the system. Such a pitch gradient has been observed in polymerized acrylate networks (9). In the case of studies of other gels, the band broadening occurred as the system phase separated into two regions containing liquid crystal and the liquid crystal swollen network. 

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