Polymer double glass transition

The blends of polysulfone with the a-methyl styrene polymers are immiscible, as evidenced by the double glass-transition temperatures in Table II. To improve the miscibility characteristics, polysulfone was modified in two ways. First, 25% of the bisphenol A was replaced by monomer I which contains a pendant ester group and, when no improvement resulted, the whole 50% of the bisphenol A was replaced. Again, the blends remain immiscible as evidenced from Figures 4 and 5 and from Table II. Further, the presence of the pendant ester group in polymer C does not improve the miscibility picture even though one would expect a favorable contribution from the carbonyl group on account of the miscibility of polycarbonate with the a-methyl styrene polymers. 

Immiscibility dominates polymer blends. It reveals itself as opacity, delamination, double glass transition, or combination of these properties. Most immiscible polymer blends require compatibilization and toughening. 

Zhou, Z., Chudnovsky, A. and Bodnyak C.P (1995) Cold-drawing (necking) behavior of polycarbonate as a double glass-transition. Polym. Eng. Sci., 35, 304. 

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

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. 

In principle, the reaction can take place at temperatures between the glass transition and the melting temperature of the polymer. However, sufficient mobility of the end groups is required to ensure reaction. It has been shown that the reaction doesn t begin until temperatures of 150°C [36] although it doesn t become industrially significant until temperatures above about 200 °C. As a rule of thumb, the reaction rate doubles every 12-13 °C. This is based on the data shown in Figures 4.5 and 4.6 and has been confirmed by others [37],... 

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

In this contribution we present results obtained with tetra-ethyleneglycol diacrylate (TEGDA). This compound was chosen since its polymer shows an easily discernible maximum in the mechanical losses as represented by tan 5 or loss modulus E" versus temperature when it is prepared as a thin film on a metallic substrate. When photopolymerized at room temperature it forms a densely crosslinked, glassy polymer, just as required in several applications. Isothermal vitrification implies that the ultimate conversion of the reactive double bonds is restricted by the diffusion-limited character of the polymerization in the final stage of the reaction. Therefore, the ultimate conversion depends strongly on the temperature of the reaction and so does the glass transition. 

The relationship between Tg and molecular weight, M, is often described by the Fox and Flory equation 266 Tg = T/XJ - K/M, where T/XJ is the glass transition temperature of the polymer with infinite molecular weight and K is a constant characteristic of the polymer. For the malto-oligomer series, a double reciprocal plot of... 

Ethylene-norbornene copolymers, which have thermoplastic properties when heated above their glass transition temperatures of ca. 200-250°C, have been commercialized by Ticona GmbH under the trade name TOPAS (Tliermoplas-tic Olefin Polymer of Amorphous Structure). Their properties - exceptional transparency, low double refraction, high stiffnes and hardness, low permeability for moisture and excellent biocompatibility - make these ethylene-norbornene copolymers particularly valuable as engineering polymers, for optical applications and as materials for food and medical packaging. 

A real example of the effect of temperature on the viscoelastic functions at T > Tg is shown in Figure 8.2. Here double logarithmic plots of the compliance function J t) versus time are shown at several temperatures for a solution of polystyrene My — 860,000) in tri-ra-tolyl phosphate (1) in which the weight fraction of polymer is 0.70. Because the glass transition temperature of the solution is 15°C, the isotherms were registered at... 

Before use, all the polymer pellets were dried at Ta +20 K for at least 48 h. Then, sheets of controlled thickness B were firstly compression molded under vacuum at Ta + 50 K and then cooled down slowly through the glass transition region. Next, samples of dimensions suitable for the SENB tests were cut from the sheets with a diamond saw, machined to produce the notch, and annealed once more at Ta +20 K for 72 h. The purpose of this thermal treatment is double first, it allows the elimination of the residual stresses induced by sample pressing and machining and secondly, it permits removal of eventual moisture which is known to affect dramatically the SAPA mechanical properties [12]. The pre-crack was produced in the samples just before testing. 

---