Glass transition after cure

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Preparation and thermal crosslinking reactions of oc, -vinylbenzyl terminated polysulfone-b-polydimethylsiloxane, ABA type block copolymers have been discussed 282,313) However, relatively little characterization was reported. Molecular weights of polysulfone and PDMS segments in the copolymers were varied between 800-8,000 and 500-11,000 g/mole, respectively. After thermal curing, the networks obtained showed two phase morphologies as indicated by the detection of two glass transition temperatures (—123 °C and +200 °C) corresponding to PDMS and polysulfone phases, respectively. No mechanical characterization data were provided. 

Enns and Gillham [55] report values of Ex/Em, Fx/Fm, Tgo, Tgoo, and c/.g for various systems. After gelation, expressions for the glass transition temperature as a function of extent of cure and molecular weight can be found in Wisanrakkit and Gillham [59]. 

T9 prepolymer = Glass transition temperoture of prepolymer T, cured polymer = after 8th postcure at 370 °C... 

SiLK resin is a solution of low molecular weight, aromatic, thermosetting polymer. The polymer s molecular weight and solution concentration were tuned to enable precise and convenient deposition by spin coating, a technique universally used by the industry for the deposition of photoresist materials. After deposition on a wafer, the polymer is thermally cured to an insoluble film that has a high glass transition temperature. The polymer has good mechanical properties at process temperatures, which is required for the application, and it is also resistant to process chemicals. 

As was mentioned above (Sect. 4), for all considered polymers prepared at Tciire < T , their experimental glass transition temperature T p is close to their Tcure. The thermosetting reaction becomes quenched by vitrification, and for a new reinitiation of the cure process the polymer is to be softened by an increase of Tcure. Experimentally, in all cases, two consecutive processes take place after a sudden increase of T r (1) the softening of the polymer followed by (2) the next step of cure up to a new txdir (Fig, 25). 

After isothermal cure, temperature scans are conducted in order to measure the Tg after cure and Tg . However, due to thermal degradation, postcures can lead to lower glass transition temperatures than those obtained after cure. Thus, the determination of Tg , for high T, systems is a difficult problem. One approach is to establish a relationship between Tg and theoretical crosslink density for systems of lower Tgoc and similar chemical structure, and extrapolate to the system with higher crosslink density, thereby obtaining an estimate of Tg, ... 

Figure 17. Temperature scan of the quinoxaline resin after exposing to nitrogen and air at 380 C. After post curing in nitrogen, the glass transition temperature remained unchanged. However, after curing in air, there was no observable Tg.

The softening peak in E" at 3.5 Hz for the 5 hour cure occurs at about 91°C while the same peaks for the 11.5 hour cure occur at approximately 129°C. This indicates the resin is significantly below its glass transition temperature at times after the first DSA peak in the isothermal 124°C scan, whereas the resin is in the... 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

Table 2 compares the glass transition temperature of both cured and uncured Navy P3-2300-PE resin before and after exposure to boiling water, using DSC for the uncured samples and TMA for the cured samples. The absorption of water can produce both a reversible decrease in Tg due to plasticization, as well as irreversible changes in Tg due to chemical degradation. If the Tg can be... 

Figure 3.3. Some DSC traces from the exothermic reaction of an epoxy resin after various times of cure, showing the progressive reduction in reaction enthalpy, A//. The expanded region shows the change to the baseline due to the enthalpy change at the glass transition. Note the progressive increase in Jg after partial cure.

Figure 1. Bi-material strip subject to bending by internal stresses after cooling from its cure temperature to below the glass transition temperature of the polymer layer 1. ...

Upon cooling, after a 2 hour cure, both materials exhibit linear stress-temperature profiles. This indicates that the glass transition temperature is at or above the cure temperature, and that measurements have been made in the glassy elastic regime. The glassy-state Ea can be calculated from the slopes of these curves. For the polyimide it is 0.13 MPa/°C and for the BCB it is 0.16 MPa/°C. Note that the polyimide bears a higher cumulative stress at room temperature because of the stress induced by solvent evaporation, in spite of its lower Ea. 

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