Transition, first-order polystyrene

While Tm is a first order transition, Tg is a second order transition and this precludes the possibility of a simple relation between them. There is, however, a crude relation between Tm and Tg. Boyer [25] and Beamen [265] inspected data for a large number of semicrystalline polymers, some of which are shown in Table 2.4. They found that the ratio TgjTm ranged from 0.5 to 0.75 when the temperatures are expressed in degrees Kelvin. The ratio is closer to 0.5 for symmetrical polymers such as polyethylene and polybutadiene, but closer to 0.75 for unsymmterical polymers, such as polystyrene and polyisoprene. The difference in these values may be related to the fact that in unsymmterical chains with repeat units of the type -(CH2-CHX-)- an additional restriction to rotation is imposed by steric effects causing Tg to increase, and conversely, an increase in symmtery lowers Tg. 

Both first- and second-order transitions are observed in polymers. Melting and allotropic transformations are accompanied by latent-heat effects and are known as first-order transitions. During second-order transitions, changes in properties occur without any latent-heat effects. Below the second-order-transition temperature (glass transition temperature) a rubberlike material acts like a true solid (see Chapter 1). Above this temperature the fixed molecular structure is broken down partially by a combination of thermal expansion and thermal agitation. The glass transition temperature of polystyrene is 100°C below 100°C polystyrene is hard and brittle, and above 100°C it is rubberhke and becomes easily deformed. 

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

A novel variation of this technique (62) involves depression of the first-order, nematic-isotropic melting transition of A(-(p-ethoxybenzylidene)-p-ra-butylaniline. Polystyrene and poly(ethylene oxide) are soluble in both phases, and Mn values of up to 10 have been studied. 

Techniques.—A novel method for the determination of the number average molar mass (M ) is reported by Kronberg and Patterson, based on the observation that polystyrene and poly(ethylene oxide) are soluble in the nematic and isotropic phases of the liquid crystalline iV-(/>-ethoxybe zylidene)/ n-butylanaline. Presence of a polymer depresses the first-order nematic-Tsotropic melting transition, by decreasing the nematic order, and as liquid crystals tend to exhibit large values of the cryoscopic constant, molar masses of up to 10 may be studied with some accuracy. 

First, we measured the flow rate dependent relative retention [(Co — C)/Co] of hyperbranched polystyrene fractions by gradually increasing the flow rate. Figure 5.9 shows that (Co — C)/Co decreases as the flow rate increases but not as sharp as the first-order coil-to-stretch transition of linear chains observed before [38, 43]. Presumably, this is because even for a given overall molar... 

The first study utilizing this method was reported by Schuller in 1966 [65]. Schuller used polystyrene latex beads that were spread on a salt-containing aqueous subphase in order to keep the particles at the interface. tt-A plots of the floating particles were determined, which showed several phase regions with reproducible transition points. The author determined the particle diameters from the A-value, at which a steep rise in the isotherm occurred. Moreover, Schuller also spread millimeter-sized Styropor particles and found isotherms of similar shape [66]. By taking pictures at different surface pressure, he was able to correlate the shape with different states of order in the monolayer. Shortly after that. 

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

Polyparaphenylenevinylene based macro-initiator 2 was used for NMRP of various monomers (styrene, methyl aciylate, butyl acrylate). From this compound various well defined rod-coil blocks copolymers with polystyrene and polyaciylate based coil blocks have been obtained. Furthermore, in each case, it is possible to random copolymeiize a second monomer for instance chloromethylstiyrene. The first monomer determines mechanical properties and phase transitions of the coil block, for example, bytulacrylate based coils have low Tg and can provide easy processabihty towards thin films. The second monomer (between 5% and 10% in molar ratio) provides the introduction of functional moieties which are necessary for a further modification in order to tune the electronic properties of the copolymer. NMRP from DEH-PPV macroinitiator 2 is schematically presented in Figure 2. 

In order to understand the pecuHarities in the deformation of hypercrosslinked polystyrenes, let us first consider the deformation of conventional network styrene copolymers (Fig. 7.38). The copolymer incorporating 3% DVB exhibits two physical states, glassy and rubbery, with a narrow transition zone... 

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