Photopolymerization of Polyfunctional Acrylates and Methacrylate Mixtures: Characterization of Polymeric Networks by a Combination of Fluorescence Spectroscopy and Solid State Nuclear Magnetic Resonance 1 Wolter F. Jager, Adrian Lungu, D. Y. Chen, and Douglas C. Neckers* Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received October 2, 1996; Revised Manuscript Received December 17, 1996 X ABSTRACT: Fluorescence spectroscopy and solid state NMR methods were used to elucidate the detailed molecular structure of poly(meth)acrylate networks. Both techniques can be employed on identical samples and give complementary information. In this study a series of diacrylates, a series of dimethacrylates, and 1:1 mixtures of the triacrylate TMPTA with diacrylates (DA’s) or dimethacrylates (DMA’s), respectively, was investigated. Fluorescence spectroscopy was performed, using the shift in the fluorescence maximum of 4-(dimethylamino)-4′-nitrostilbene (1), a charge transfer probe. This probe monitors both the rigidity and the polarity of the medium in which it is incorporated. CPMAS 13 C NMR spectroscopy was employed monitoring the relaxation times, T1, of carbon atoms from the main chains and pendant groups, determining cross-link densities, and measuring T1F( 1 H) values. With these techniques information about the mobility of individual atoms and the homogeneity of the polymeric networks is obtained. Introduction Polyacrylate Networks. Multifunctional acrylates or acrylated oligomers are widely used as photopoly- merizable resins employed in information storage sys- tems, in rapid cure coatings, as restorative materials, 2 and in stereolithography. 3 Though their radical poly- merizations are quenched by oxygen, acrylates form hard glassy polymeric networks, and for practical ap- plications complex mixtures of multifunctional mono- mers, photoinitiators, and other additives are em- ployed. 4 Though the use of multifunctional acrylates for the formation of thin films by photopolymerization is rather old and numerous studies concerning these systems have appeared in the literature, 5 many ques- tions concerning the detailed molecular structure of the networks obtained and the exact mechanism by which they are formed remain. The complex structures of acrylate networks are, in part, a consequence of the radical chain reactions by which they are formed. 6,7 One particularly complicating factor, not specific for networks formed by a chain reaction, is that topological factors and vitrification limit final double-bond conversion to well below 100%. Fully cured networks only form with oligomers that have relatively long and flexible spacers between acrylate functionalities. 8 As a result, pendant double-bonds or even unreacted monomers 9 are present in most poly- meric networks. Such unreacted groups are unevenly distributed throughout the network. For diacrylates it has been shown that the first stages of photopolymer- ization form a polymeric network that is inhomogeneous in nature and best described by a random walk percola- tion model. 9 On the basis of this observation, the final polymer is also expected to be inhomogeneous on a molecular scale. Another factor that determines the structure of acrylate networks is that there seems to be no regularity as to which main chains are connected by which side chains. Main chains in poly(meth)- acrylates formed by free radical polymerizations are atactic, a factor that further diminishes order in the system. If mixtures of multifunctional monomers are used, the situation becomes even more complicated. The reactiv- ity of the monomeric units, which apart from the chemical nature of these units 10 is influenced by steric factors that govern accessibility of the reactive sites and diffusion rates of free monomers, plays an important role. Another important factor determining network structure is solubility of the monomers and oligomers in one another. For binary mixtures of multifunctional monomers different scenarios can be anticipated. In the simplest, most straightforward case, both resins are completely miscible at all stages of the polymerization process and the reactivities of the reac- tive groups in both are equal throughout the polymer- ization process. In this case a homogeneous network (at the NMR scale, <20 nm) is formed in which both monomers are incorporated in a statistical manner. If the corresponding linear polymers are not compatible, either due to differences in reactivity or to a limited solubility of one species in the other, a homogeneous network will not form. Discrete segments of networks of different compositions will form. Finally, it should be noted that the structure of photoformed polymers depends on the photopolymeri- zation conditions. 11 Therefore, to obtain identical poly- meric networks, photopolymerizations need to be con- ducted using exactly the same conditions in terms of photoinitiator composition and concentration, light in- tensity, duration of irradiation, and so on. Also, analy- sis needs to be carried out promptly because a thermal aftercure 12 and physical aging alter the structure of the network. CPMAS NMR. Cross-polarization magic angle spin- ning (CPMAS) 13 C NMR provides information about the detailed molecular structure of polymeric networks and X Abstract published in Advance ACS Abstracts, February 1, 1997. 780 Macromolecules 1997, 30, 780-791 S0024-9297(96)01468-4 CCC: $14.00 © 1997 American Chemical Society