Linear thermoviscoelasticity and characterization of slightly crystalline EPDM networks

Three EPDM network families with various degrees of chemical crosslinking and slightly crystalline below about 350 K have been characterized by thermoviscoelastic and equilibrium swelling measurements. Between 300 and 450 K the thermoviscoelasticity of these elastomers, whether cured or not, is complex. Important differences from previous results on noncrystalline EPDM networks are their dependence on waiting time and temperature history below 350 K, and an equilibrium modulus for the uncured elastomers. These effects increase with rising ethylene content of the EPDM and are related to the crystallinity in the networks. A previously proposed modified time-temperature (t-T) superposition scheme is also applicable to these data, and the resulting frequency shift functions a_T are of the usual WLF type. The vertical shift functions for the relaxational components b_T are different from those for the equilibrium moduli. Experimental data demonstrate that the absolute values of the relaxational components, i.e., G″ (ω) and H(τ), are not changed by small variations in crystallinity. In addition, larger variations in crystallinity do not affect their frequency dependence, only their absolute values. On the contrary, the equilibrium modulus, and therefore all characteristics containing this as a component, is extremely sensitive to small variations in crystallinity, and hence in temperature. The linear viscoelastic characteristics of these slightly crystalline networks and their deviations from thermoviscoelastically simple behavior are discussed as functions of the ethylene content, temperature, and degree of crosslinking of the elastomer. The data provide evidence for three contributing factors, namely chemical crosslinks, crystallinity, and a more disordered type of crystallinity termed microparacrystallinity.     

Estimation of the equilibrium modulus of rubber networks by nonlinear regression analysis

A nonlinear regression analysis is presented to estimate the equilibrium modulus of relaxing rubber networks from G'(ω) or G(t) data. Various semiempirical curve-fitting equations are tested with two-function minimization algorithms.     

Molecular architecture and physicochemical properties of some vinyl alcohol-vinyl acetate copolymers

Two series of vinyl alcohol-vinyl acetate copolymers were prepared by homogeneous and heterogeneous acetylation of the same precursor poly(vinyl alcohol). Their intramolecular monomer distributions were analyzed by IR spectrometry, calorimetry, and differential thermal analysis. The results show a more blocky distribution for the heterogeneously prepared copolymers. The properties of these (co)polymers in dilute aqueous solution were determined by means of viscometry. Whereas the copolymer-solvent interaction parameter of the homogeneously acetylated, random copolymers hardly varied with acetate content, a definite minimum was found for the blocky copolymers at about 7 mole% vinyl acetate. These findings were attributed to the incompatibility of dissimilar sequences, which sharply decreases with decreasing vinyl acetate sequence length. Up to about 17 mole% vinyl acetate content, the solvent quality for the copolymers is at least as good as for poly(vinyl alcohol). In addition, the dilute solution properties of the samples were established in water saturated with 1-butanol. For copolymers with up to about 17 mole% vinyl acetate, at 25°C this mixture is a better solvent than water. The highest increase in solvent quality was found for the homopolymer, whereas the increase diminished with acetate content, irrespective of the intramolecular vinyl acetate distribution. These findings are explained in terms of preferential adsorption of 1-butanol onto the (co)polymer backbone due to hydrophobic interactions and prevention of this process by the bulky acetate groups.     

Development and validation of real-time PCR screening methods for detection of cry1A.105 and cry2Ab2 genes in genetically modified organisms

Primers and probes were developed for the element-specific detection of cry1A.105 and cry2Ab2 genes, based on their DNA sequence as present in GM maize MON89034. Cry genes are present in many genetically modified (GM) plants and they are important targets for developing GMO element-specific detection methods. Element-specific methods can be of use to screen for the presence of GMOs in food and feed supply chains. Moreover, a combination of GMO elements may indicate the potential presence of unapproved GMOs (UGMs). Primer-probe combinations were evaluated in terms of specificity, efficiency and limit of detection. Except for specificity, the complete experiment was performed in 9 PCR runs, on 9 different days and by testing 8 DNA concentrations. The results showed a high specificity and efficiency for cry1A.105 and cry2Ab2 detection. The limit of detection was between 0.05 and 0.01 ng DNA per PCR reaction for both assays. These data confirm the applicability of these new primer-probe combinations for element detection that can contribute to the screening for GM and UGM crops in food and feed samples.