Increased glass transition temperature

The polymers can either be amorphous or semicrystalline, witli increased glass transition temperatures by increasing the phenyl units in the backbone. 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

PHAs containing bromine can be prepared by chemical modification of PHAs containing unsaturated repeating units. For example, bromination of PHA-10UND= or PHA-10UND proceeded to completion rapidly to yield polymers with increased glass transition temperatures [76]. 

So far we have not considered the influence of the constitution of the polymer main chain on the formation of the nematic phase. If the same mesogenic group is linked to different backbones, the nematic phase can be preserved, as shown for one example in Table 3. Owing to the different flexibilities of the backbones, the nematic state is shifted with respect to the temperature. With falling flexibility of the main chain, as indicated by the increasing glass transition temperature, the phase transformation temperatures nematic to isotropic are shifted towards higher temperatures. This clearly indicates that the restriction of motions, due to the polymer-fixation, directly reflects on the phase transformation temperature. If this restriction... 

Oxynitride glasses are silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. As nitrogen increases, glass transition temperature, elastic modulus, viscosity and hardness increase while thermal expansion coefficient decreases. 

The oligomers 23 and 24a-c show enhanced solubility and increased glass transition temperatures when compared to their linear counterparts 25 and 26 (Scheme 3.10). No melting transition is observed for 23 whereas the linear molecule 25 (Scheme 3.10) melts at 254 °C. The electronic absorption spectra of the cruciforms 23 and 24a-c exhibit structureless, broad absorption peaks. Assuming a nonplanar ground state, the also featureless emission spectra indicate no significant increase of planarity upon photoexcitation. In addition, the cruciform molecules 23 and 24a-c show lower photoluminescence quantum yields (41-70%)... 

It is interesting to contrast these films with the other films being considered for flexible electronics especially for the higher performance flexible display market. The main candidates are shown in Fig. 7.2 which lists the substrates on the basis of increasing glass transition temperature (Tg) [5, 6],... 

Couchman and Karasz (11) recently have made some calculations indicating that spherical microphases should exhibit increased glass-transition temperatures because of an increased pressure inside such microphases attributed to the surface tension between microphases. Since there is some doubt about the existence of a surface of tension in the Gibbs sense (12) between chemically linked microphases, we shall simply note that these calculations are the only ones in existence that indicate a possible reason for an increase in the Tg of a glassy microphase and, in addition, that these calculations also postulate differences in Tg with differences in morphology. For example, this surface-tension-dependent effect would not be expected in samples with lamellar morphology, no matter how small the width of each lamella. 

The other independent transformation that can take place during network formation is vitrification. This transition occurs at the particular conversion where the increasing glass-transition temperature (Tg) of the reacting system equals the instantaneous value of the cure temperature. At this time, the macroscopic behavior of the system changes from a liquid or rubber to a glass. This means an increase of several decades in the value of the storage modulus. 

Also similar to styrene and methyacrylates is the use of divinylbenzene as a comonomer. Acrylated, epoxidized soybean oil was polymerized with divinylbenzene or modified with phthalic anhydride before polymerization to give materials with increased glass transition temperatures [43]. Modification of the soybean oil with cinnamate esters and subsequent... 

The dependence of the glass-transition temperature on the copolymer composition varies with the chemical structure of the comonomer unit. Copolymerization with methyl succinic acid, dimethyl succinic acid, adipic acid, and propanediol results in a decreased glass-transition temperature, whereas copolymerization with benzyl succinic acid and terephthalic acid brings about an increased glass-transition temperature, as shown in Fig. 11. 

Bis(4-phenoxybenzoyl)diphenyl and NJ -bis (4-phenoxybenzoyl)-p-phenylenediamine yield polymers that are semi-crystalline and show remarkably increased glass transition temperatures in comparison to conventional PEEK [12]. It is believed that this occurs due to the incorporation of the diphenyl moiety and amide linkages in the main chains. 

Increased glass transition temperatures, hardness and chemical durability are claimed for these nitrided glasses in which some of the O atoms have been replaced by N or NH. The N atoms are capable of three connections and may therefore participate in extra cross-linking (12.65b) or tighter bonding to neighbouring P atoms (12.65c). 

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