Polystyrene Thermal expansion coefficient

EXAMPLE 10.4 Hamaker Constant of Liquid Polystyrene. Estimate the Hamaker constant of liquid polystyrene at 298 K. The thermal expansion coefficient a for polystyrene at 298 K is approximately 5.7 10 4 K 1. Compare your result with the experimentally obtained value of A = 7.8 10 2°J reported by Croucher( 1981, Fig. 1). 

Figure 5. Thermal expansion coefficients for TR-41-2443. Arrow indicates polystyrene T0.

Figure 6. Thermal expansion coefficients for polybutadiene (PBD), polystyrene (PS), and PS-PBD blend.

The specific volume and expansion coefficient of the solution-blended material are shown in Figure 6, along with data for pure polybutadiene and pure polystyrene. None of the three polymers has any distinguishing features below the polystyrene Tg> illustrating that the observed transition and minimum are the results of the unique structural morphology of the block copolymers. It should be noted that the substantial difference in the thermal expansion coefficients of polybutadiene and polystyrene can be expected to be an important factor affecting the structure and properties of block copolymer samples prepared under various conditions. 

Another explanation for an abnormal increase in Tgl in polymer blends has been proposed by Manabe, Murakami, and Takayanagi 125). They used a three-layered shell model, which accounts for interaction between the dispsersed and continuous phases of the blend. Abnormal increases in the glass transition of polystyrene in blends with various rubbers were explained by thermal stresses which arise from the difference in thermal expansion coefficients of the component polymers. However shifts in the glass transition temperatures of the SIN s do not appear to arise from differences in the expansion coefficients of the components because samples with the same overall composition and almost identical microstructures have significantly different glass transition temperatures. 

According to more recent theories, the toughness of high impact polystyrene is caused by flow and energy dissipation processes in the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and in the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution in impact polystyrene. Stress maxima create zones of lower density, called crazes (3), in which the polystyrene molecules are extended parallel to the direction of stress. Macroscopi-cally craze formation appears as whitening the flow processes result in irreversible deformation (cold flow). 

As was already stated (see Figure 6), the temperature dependence of the shift factor aT is a function of the elastomer phase content. The strong effect of the rubber content on the temperature dependence of the shift factor aT could be explained by an increase in free volume of the SAN resin induced by the elastomer phase, as was suggested by Prest and Porter (13) for polystyrene-poly (phenylene oxide) blends. In order to verify this hypothesis, log aT experimental data for SAN and relative blends were used to calculate the WLF parameters and, in turn, the free volumes (f0) at the reference temperature (T0) and the thermal expansion coefficients (a) by the equation ... 

At (ft—> 0 equation [7.2.37] gives a linear dependence of the relative vapor pressure, P°v(/Pvo on the solvent volume concentration with the angle coefficient exp(l+x). At 1 solution obeys the Raul s law. Note that the value of < )i in [7.2.37] is temperature-dependent due to difference in thermal expansion coefficients of components. The % value for a given solvent depends on the concentration and molar mass of a polymer as well as on temperature. However, to a first approximation, these features may be ignored. Usually % varies within the range 0.2 - 0.5. For example, for solutions of polyethylene, natural rubber, and polystyrene in toluene % = 0.28,0.393 and 0.456, correspondingly. 

The first application we will discuss in this section is the influence of the pure component properties (basically the volumetric properties) on the phase behavior in a mixture of two homopolymers. A mixture for which all necessary information is available(7,10) is the mixture of Polystyrene(PS) with the Poly(vinyl methyl ether)(PVME). Scaling constants for PS are obtained from Reference 7 while scaling constants for PVME and binary parameters are obtained from Reference 10. In Figure 1 is shown the influence of the thermal expansion coefficient a of PS on the spinodal curve of the mixture of PS and PVME considered both monodisperse with molecular weight 51000. As shown in the Figure a small increase in the thermal expansion coefficient of PS of the order of 1 % enhances the compatibility of PS with PVME by raising the spinodal curve and the critical temperature by about 30 degrees. 

The relative insulation characteristics of polyurethane foam and polystyrene foam as compared to brick and wood is given in Fig. 3.3. Thermal conductivity coefficients, thermal expansion coefficients and dielectric constants for various polymers and other materials are given in Table 3.3. 

Another phenomenon associated with T. G Fox was the glass transition. A classic paper with Rory appeared in the Journal of Applied Physics in 1950 on the behavior of fractions of polystyrene as a function of temperature near the glass transition [28]. At this point, Fox and Flory referred to the observed phenomenon as a second-order transition, a designation they would live to regret. Specific volumes were measured as a function of temperature on each fraction and the temperature where the thermal expansion coefficient showed a break in slope was taken as the glass transition temperature, Tg. An empirical correlation was constructed for this data. 

According to Hocker et at., above 443 K the thermal expansion coefficient in polystyrene increases as... 

Figure 13 shows the relaxation map of oriented polystyrene. Hie elementary processes isolated between 9VC and 102°C are characterized by relaxation times following a compensation law, eq. (8) they have the same relaxation time (t. = 0.11 s) at the compensation temperature = 145°C. This mode is the dielectric manifestation of the glass transition. The elementary peak isolated at 13S°C is well-described by a Fulcher-Vogel equation, eq. (7), with a critical temperature Too = 50"C, a thermal expansion coefficient of the free volume of 1.7 x lO" and a pre-exponential factor of 0.74 s. 

Coefficient of Linear Thermal Expansion. The coefficients of linear thermal expansion of polymers are higher than those for most rigid materials at ambient temperatures because of the supercooled-liquid nature of the polymeric state, and this applies to the cellular state as well. Variation of this property with density and temperature has been reported for polystyrene foams (202) and for foams in general (22). When cellular polymers are used as components of large stmctures, the coefficient of thermal expansion must be considered carefully because of its magnitude compared with those of most nonpolymeric stmctural materials (203). 

CNT can markedly reinforce polystyrene rod and epoxy thin film by forming CNT/polystyrene (PS) and CNT/epoxy composites (Wong et al., 2003). Molecular mechanics simulations and elasticity calculations clearly showed that, in the absence of chemical bonding between CNT and the matrix, the non-covalent bond interactions including electrostatic and van der Waals forces result in CNT-polymer interfacial shear stress (at OK) of about 138 and 186MPa, respectively, for CNT/ epoxy and CNT/PS, which are about an order of magnitude higher than microfiber-reinforced composites, the reason should attribute to intimate contact between the two solid phases at the molecular scale. Local non-uniformity of CNTs and mismatch of the coefficients of thermal expansions between CNT and polymer matrix may also promote the stress transfer between CNTs and polymer matrix. 

Polystyrene is an amorphous polymer and shrinkage and coefficient of thermal expansion are rather low depending on the possible rubber content. The absorption and alteration by moisture exposure are low. 

The only effects on the thermal properties seen from the incorporation of a fire retardant additive occurs in the case of high-impact polystyrene (HIPS) where, as shown in Table 8.4, the incorporation of a fire retardant leads to a decease in expansion coefficient and, in the case of the polyesters, where the incorporation of a fire retardant produces a small improvement in heat distortion temperature. 

SAN copolymers are linear, amorphous materials with improved heat resistance over pure polystyrene. The polymer is transparent but may have a yellow color as the acrylonitrile content increases. The addition of a polar monomer, acrylonitrile, to tbe backbone gives these polymers better resistance to oils, greases, and hydrocarbons when compared to polystyrene. Glass-reinforced grades of SAN are available for applications requiring higher modulus combined with lower mold shrinkage and lower coefficient of thermal expansion. ... 

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