Dynamic Mechanical Analysis, crosslinking

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

A difunctional bisphenol-A-based benzoxazine has been synthesized and characterized by GPC and 1II NMR (Fig. 7.39). A small of amount of dimers and oligomers also formed. Thermal crosslinking of bisphenol-A benzoxazine containing dimers and oligomers resulted in networks with relatively high Tgs. Dynamic mechanical analysis of the network showed a peak of tan 8 at approximately 185°C. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Electron irradiation causes chain scission and crosslinking in polymers. Both of these phenomena directly affect the glass transition temperature (Tg) of the materials. Thermomechanical (TMA) and dynamic-mechanical analysis (DMA) provide information about the Tg region and its changes due to radiation damage. Therefore, DMA and TMA were performed on all irradiated materials. 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

In more recent studies from Gonzalez and co-workers [88-90] it was concluded from dynamic mechanical analysis of peroxide-cured NR that a non-uniform crosslinked network results if a large amount of peroxide is used. This result seems to be in line with the optical spectroscopy studies discussed. 

Tg values can be determined via either calorimetric, dynamic scanning calorimetry (DSC) or mechanical dynamic mechanical analysis (DMA) measurements. However, since three dimensional highly crosslinked systems have relatively small amounts of molecular motion, the DSC method is not particularly sensitive for Tg determination [131,132]. Fry and Lind have reported that DSC is misleading, as reactive groups are often sufficiently entrapped in the vitrified structure to give spurious results [133]. 

In conclusion, it has been shown that the predicted order of miscibility in composite latex particle systems is not necessarily bourne out when the extent of miscibility is guaged by dynamic mechanical analysis, and, very recently, by the same authors using solid-state NMR spectroscopy. Control over particle morphology, and, hence, over damping behaviour can be exercised by the differences in hydrophilicity between the polymer pair in question, by the degree of crosslinking in the first network and by whether or not the first-formed polymer is above or below its Tg when the second monomer is polymerised. 

Demonstration of Dual Cure. To demonstrate thermal cure after UV cure three sets of films were prepared for Dynamic Mechanical Analysis (DMA). In the first set, four formulations were based on AM 1, AH 2, the epoxy acrylate, amd the urethane acrylate. Formulations used the test resin at 65% in TPGDA, and included 3% Darocur 1173 as a source of free radicals. DMA plots for the epoxy acrylate and urethane acrylate are shown in Figure 9. The elastic or storage modulus (E ) in the rubbery region is often used as a measure of crosslink density. The maximum of the tan delta plot, not shown, is... 

K. Tamareselvy, F.A. Rueggeberg, Dynamic mechanical analysis of two crosslinked systems, Dent. Mater. 10 (1994) 290-297. 

Dynamic mechanical analysis (DMA) is a sensitive method for glass transition temperature measurement, for detection of side-chain and main-chain motions, and for local mode relaxation measurements. Loeal mode relaxation can not be measured by DSC. DMA can give information about the crosslinking process of modified phenolic prepolymer [218] and about the erosslinked material [132]. During DMA measurements, sinusoidally varying stress of frequency is applied to the sample. Frequency and the stress are connected by equation 57, where is the maximum stress amplitude and is the phase angle at which the stress proceeds the strain. 

Crosslinked NR nanocomposites were prepared with montmorillonite. Morphology was characterized using transmission electron microscopy (TEM), wide-angle X-ray scattering (WAXS), and dynamic mechanical analysis (DMA). X-ray scattering patterns revealed clay intercalation and TEM showed dispersion with partial delamination. The loss modulus peak broadened with clay content, while Tg remain constant. Montmorillonite reinforced the rubber. The DMA exhibited non-linear behaviour typified as a Payne effect (see Section 20.11) that increased with clay content and was more pronounced for this type of nanocomposite. Viscoelastic behaviour was observed under large strains via recovery and stress relaxation. ... 

As could be expected, the glass transition temperature of different samples increased with curing periods. However, the values were close to each other for both curing methods. Dynamical mechanical analysis (DMA) showed that crosslinking density of the samples increased with an increase in cure time but the changes were more noticeable under microwave irradiation compared to conventional thermal curing [55]. 

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