DDS systems

The highly polar nature of the TGMDA—DDS system results in high moisture absorption. The plasticization of epoxy matrices by absorbed water and its effect on composite properties have been well documented. As can be seen from Table 4, the TGMDA system can absorb as much as 6.5% (by weight) water (4). This absorbed water results in a dramatic drop in both the glass transition temperature and hot—wet flexural modulus (4—6). 

Table 4. Effect of Absorbed Water on the Mechanical Properties of the TGMDA—DDS System ...

The curves of Fig. 6, relative to the TGDDM-DDS systems cured with 50 PHR of hardener, have been drawn using the same procedures, although, according to the discussion of the previous section, a low activity term associated with the adsorption on the hydrophilic sites, has also been considered. 

The simple 3,4-dihydro-l,2-diazete 1,2-dioxide (1,2-diazetine 1,2-dioxide, DD) system 53 is unknown. 3,3,4,4-Tetramethyl derivative 54 was the first compound of the class to be synthesised [50]. It has an unusually low triplet energy and is a useful triple quencer [51, 52]. Many other derivatives of 53 have recently been prepared in view of their NO-donor properties. 

Figure 4.9 shows the lit of experimental results obtained for the epoxidized novolac DDS system, using Eq. (4.12) with X = 0.15 (Oyanguren et al., 1993b). As Acpmax could not be determined with enough precision, X was used as an adjustable coefficient. 

Figure 4.9 Tg as a function of conversion for an epoxidized novolac-DDS system (xmax = 0.80). The dashed line is the curve predicted using Eq. (4.12) with X =0.15 (Oyanguren and Williams, 1993b - Copyright 2001 - Reprinted by permission of John Wiley Sons, Inc.)...

Figure 8.7 SEM photograph of a fully cured rubber-modified epoxy network. The rubber CTBN (26 wt% AN) is first pre-reacted with a large excess of diglycidyl ether of bisphenol A (DGEBA) to obtain an epoxy-terminated rubber. Then an equivalent of 15 wt% initial CTBN is introduced in DGEBA-4,4 -diamino diphenyl sulfone, DDS, system precured at 135°C (time > tgei) and then postcured at 230°C. Rubber-rich particles are spherical, D 2.8 0.5 gm, and well dispersed. (From LMM Library.)...

Another approach for improving adhesion is to introduce a small amount of a third component that may simultaneously be dissolved in the TP and form chemical bonds with the thermoset. One example is the addition of CTBN in a PSF DGEBA DDS system, where CTBN is preferentially dissolved in the PSF phase but can react with the epoxy groups, promoting adhesion between both phases (Pascault and Williams, 2000). 

Woo et al. (1994) studied a DGEBA/DDS system with both polysul-fone and CTBN. The thermoplastic/rubber-modified epoxy showed a complex phase-in-phase morphology, with a continuous epoxy phase surrounding a discrete thermoplastic/epoxy phase domain. These discrete domains exhibited a phase-inverted morphology, consisting of a continuous thermoplastic and dispersed epoxy particles. The reactive rubber seemed to enhance the interfacial adhesive bonding between the thermoplastic and thermosetting domains. With 5 phr CTBN in addition to 20 phr polysul-fone, Glc of the ternary system showed a 300% improvement (700 Jm-2 compared with 230 J m 2 for the neat matrix). 

Figure 3 FT-IR monitoring of isothermal curing of the MY720-DDS system curing temperature 177 C ...

The main reactions in the TGDDM-DDS system are shown in Scheme 1. 

Figure 5. First order kinetic analysis of conversion rate data on the MY720-DDS system cured Isothermally at 177 C and 153 C.

In conclusion, we have obtained a cure reactivity model for the TGDDM-DDS system within the temperature range 153-177 C. Extension of the model to higher and lower temperatures and its refinement via direct measurement of rates of specific reaction is in progress. 

The kinetic activation energy, E (, is determined to be 16.1 kcal/mole for the BFj MEA accelerated system versus - 17.5 kcal/mole for the non-accelerated TGDDM/DDS system. 

Thus E32- can be computed In the system H2S-S3-H2P knowing J [s], which can be done by titration of the dissolved species. In the H2S-S8 H20-NaCl(0.7 DD system the relations obtained between pH and E32- are in good agreement with the relations computed from equation 2 as can be seen in Figure 3. At pH > 9, slight deviations were observed owing to polysulfide interaction with the Ag/Ag2S electrode. 

Based on Donnan s [18, 19] and Onsager s [41, 42] fundamental works, the theories for Donnan dialysis systems were developed [20-26, 32-36]. The BAHLM system could be considered as two DD systems, operating in consecutive order, continuously in one module (see Fig. 6.2) the first is composed of feed/LM and the second is composed of LM/strip compartments, separated by ion-exchange membranes. Therefore, the Kedem-Katchalsky equations [43, 44] can be applied to our case ... 

When these results are applied to the TGDDM/DDS system the likely chain extension, crosslinking and competing reactions (e.g. cyclization from the intramolecular reaction) are those shown in Scheme 1.20 (St John, 1993). 

The results give a good fit to an accelerated (by BF3-MEA) TGDDM/DDS system however, they do not predict results for the unaccelerated system well at longer times. The introduction of the order of reaction greatly improved fits. 

Table VI. Interlaminar Fracture Energies of Epoxy-Graphite Composites TGDDM-DGEBA-DDS System Toughened with MEAc Polymer...

The material used was a diglycidyl ether of bisphenol A (DGEBA) based epoxy resin (Ciba-Geigy, GY250) cured using stoichiometric amounts of 4,4 -diamin-odiphenyl sulfone (DDS). The rubber used for the modifications was Hycar car-boxy-teminated butadiene-acrylonitrile (CTBN) rubber (1300 x 13). The curing schedule for all the rubber-modified epoxy-DDS systems was as follows first the rubber and then DDS were mixed with the epoxy resin and stirred at 135 °C until the DDS was dissolved the systems were cured for 24 h at 120 °C and then postured for 4 h at 180 °C. The control epoxies were cured according to the same schedule. 

Dynamic mechanical and differential scanning calorimetry experiments were used to analyze the BF catalyzed TGDDM-Novalac-DDS system. Dynamic mechanical experiments have been utilized in the past to characterize this epoxy resin (1,2,5,6,7). They have provided information concerning the viscoelastic response of the polymer as well as important morphological information. On the other hand. Differential Scanning Calorimetry (DSC) experiments provide a means for measuring extent of reaction, reaction kinetics and mechanisms of the epoxy system curing process (2). 

The purpose of this work is to combine the results from dynamical mechanical and DSC experiments to understand the effect of BF catalysis on the TGDDM-Novalac-DDS epoxy system. The dynamic mechanical and DSC results for solution processed epoxy films are compared to melt processed results to determine the effect of processing conditions on the properties of the catalyzed TGDDM-Novalac-DDS system. 

DDS systems may be ascribed to more than one type, e.g. nanoparticles or microspheres coated with multilayer films. Cationic polymers are extensively used in nucleic acid delivety and the respective delivery systems may also be considered as DDSs. However, since non-viral nucleic acid delivery with a great potential in gene therapy has become such an intensively studied area, it is out of the scope of the present chapter. 

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