We report on the mechanical properties of optically curable stereolithographic resins (SLRs) which were reinforced through the addition of small amounts of cellulose nanocrystals (CNCs). The resin/filler mixtures are readily accessible via simple mixing processes. A detailed rheological investigation of such mixtures and the successful processing of these materials on a commercial SLR machine show that at low filler concentrations (below 5%) the processability of the materials is barely impacted. The storage modulus, E', increased steadily with increasing CNC content in the regimes below and above the glass transition. A remarkable modulus enhancement was observed in the rubbery regime, where E' increased by 166, 233, and 587% for CNC/SLR nanocomposites with 0.5, 1.0, and 5.0% w/w CNC, respectively. The modulus increase was less pronounced in the glassy state, where E' increased by 21, 32 and 57%, for the same compositions. The increase in tensile strength was of similar magnitude. In comparison to previously reported CNC and carbon-nanofiller based nanocomposites, the presently investigated nanocomposites display a comparably large increase of stiffness and strength, which appear to originate from the high level of dispersion and the intimate contact of the CNCs with the SLR matrix. Through the fabrication of 3-dimensional parts, it was shown that the CNC-filled resins can be processed with standard equipment in a stereolithographic process that is widely used for rapid prototyping and rapid manufacturing.

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SynopsisThe bifunctional epoxides bisphenol A diglycidyl ether (BADGE) and hexahydrophthalic diglycidyl ester (HHDGE) a s well as the monoepoxides phenyl glycidyl ether (PGE) and cyclohexane carboxylic acid glycidyl ester (CHGE) were cured with hexahydrophthalic anhydride (HHPA) in the presence of benzyldimethylamine (BDMA) or 1-methyhmidazole (1-MI) as catalysts a t 100-140°C. Investigations of the curing kinetics gave sigmoidal-shaped curves with marked induction periods. IR analysis of the cured products revealed that the propagation proceeds not only by the esterification reaction of epoxide with anhydride but also by chain anhydride formation by the reaction of carboxylate with anhydride groups. '"C-NMR investigations of the soluble polymers showed that most of the peaks resulting from double bonds could not be assigned t o structures formed by initiation reactions that had previously been proposed for the anhydride curing of epoxides. In analogy to a postulated mechanism for the decarboxylation condensation of HHPA alone in the presence of tertiary amines, it is proposed that an isomerization product of HHPA is one of the molecules that initiate the curing reaction. INTRODUCTIONThe curing of epoxy resins with cyclic anhydrides is an important process for the fabrication of electrical insulating materials. The mechanism of this reaction has been intensively studied during the last years.'-I8 The noncatalyzed polymerization is now quite well understood.'.2,"* l2,I7 It is started by the reaction of anhydride groups with hydroxyl groups that are present in the reaction mixture (as part of the epoxy resin) to generate a monoester and a carboxylic acid group. The carboxylic acid group then reacts with the epoxide to form a diester with a hydroxyl group, which in turn reacts with another anhydride: Science, Vol. 37, 1753-1776 (1989 The situation is not as clear when tertiary mines are used as catalysts, In the case of technical epoxy resins, the reaction might be initiated by complex formation of an amine with a proton donor and proceed in a similar way as above, the active centers being the carboxylate and hydroxylate anions.3, l3In the absence of proton donors, however (i.e., if pure monomeric epoxides without OH groups and pure anhydrides are used), the amine must react either with the anhydride7 or the epoxide'' to initiate the polymerization. The second possibility, the complex formation of a tertiary amine with an epoxide, was detected by Matejka et al.15 by NMR investigations of model compounds, The following initiation mechanism was postulated'':The authors found that the tertiary amine was irreversibly bound to the epoxide. For the reaction of monoepoxides with cyclic anhydrides, this would mean that no termination or transfer reactions occur and that only the catalyst concentration determines the resulting molecular weight of the polymer. Thus, each polymer chain would contain a quarternary ammonium ion at one end. From investigations of the oligomers formed by the reaction of phenylglycidyl ether with ...

SynopsisThe activity and conformation of lysozyme solubilized in apolar solvents via reverse micelles was investigated. The systems used were sodium di-Zethylhexylsulfosuccinate (AOT)/isooctane/H,O, cetyltrioctylammoniumbromide (CTAB)/CHCl,, isooctane/H,O tetraethyleneglycoldodecylether (E0,C ,,)/isooctane/H,O, and bulk water. CD spectra of lysozyme in reverse micellar solutions were investigated as a function of[AOT]) and were compared to the spectra in aqueous solutions. No marked changes were found in the E04C12 or in the CTAB systems with respect to water, which indicates that no sizeable conformational changes of the enzyme occurred upon solubilization in the reverse micellar systems. In agreement with previous studies [C. Grandi, R. E. Smith, and P. L. Luisi (1981) J. BWZ. Chern. 256, 837-8431 dramatic conformational changes can be inferred in the AOT system on the basis of CD studies. This is taken as an indication that the enzyme denatures in this micellar system. This is particularly striking because the enzyme is fully active in AOT reverse micelles. The apparent paradox is solved by the observation that the native CD spectrum (and by inference, the native conformation) is maintained when lysozyme is bound to NAG or NAG3, and by inference, when the substrate is bound, e.g., during enzyme turnover. However, in the absence of added NAG, NAG3, or substrate, the enzyme in the AOT reverse micellar system rapidly denatures. Together with CD studies, fluorescence and nmr data confirm the hypothesis of an irreversible denaturation of lysozyme in the AOT system, the denaturation being slowed down when the substrate is present. The activity of the enzyme has been studied as a function of pH and w,, using the chromophoric substrate 3,4dinitrophenyl-tetra-N-acetyl-0-D-chitotetraoside (3,4-DNP-NAG4 ). Generally speaking, the kinetic parameters are comparable to those found in bulk water solution. More detailed, in the CTAB system, k,, tends to be smaller than in aqueous solution (with quite similar KM), whereas in the EO,C,, system (at pH 7.0) the turnover number is larger and K M is smaller than in water. In the AOT system, the kinetic parameters a t pH 7.0 are also quite comparable to those found in water.

SynopsisThe anhydride curing of epoxides was studied by performing copolymerizations of epichlorohydrin, phenyl glycidyl ether (PGE), or bisphenol-A-diglycidyl ether (BADGE) with phthalic anhydride (PSA). As initiators, tertiary amines or ammonium salts were used. In the case of epichlorohydrin, linear polyesters were obtained a t 100°C. At higher temperatures (140-16OoC), a side reaction of the CH,CI group took place which caused branching and partial crosslinking of the polymer. The reaction of phenyl glycidyl ether with phthalic anhydride gave linear, strongly alternating copolymers a t temperatures of 120-160°C. Molecular weights (an) were in the range of 4ooO-87,000, depending on the purity of the starting materials and the initiator used. The reaction of the diepoxide BADGE with phthalic anhydride yielded highly crosslinked products.Their crosslink densities (which correlate with the glass transition temperature T ' ) , however, did not show the same dependence on initiator and purity of the starting materials as the molecular weights of the linear polyesters obtained by the "model reaction" of PGE with PSA. Possible reasons for this effect are discussed.