Polymers linear thermal expansion

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). 

The Rheometric Scientific RDA II dynamic analy2er is designed for characteri2ation of polymer melts and soHds in the form of rectangular bars. It makes computer-controUed measurements of dynamic shear viscosity, elastic modulus, loss modulus, tan 5, and linear thermal expansion coefficient over a temperature range of ambient to 600°C (—150°C optional) at frequencies 10 -500 rad/s. It is particularly useful for the characteri2ation of materials that experience considerable changes in properties because of thermal transitions or chemical reactions. 

The uncertainty of the expansion data was evaluated to be less than 5%. The relative thermal expansion AL/L versus T is shown in Fig. 13.3. Curves for other materials are also reported for the sake of comparison. Torlon shows a linear thermal expansion lower than most polymers. As Stycast 2850FT, Torlon thermal contraction closely matches that of some common metals (i.e. aluminium [53] and brass [54]). Smoothed values for the thermal expansion of Torlon relative to 4.2K are reported in Table 13.1. The data... 

These expressions show that a deformed polymer network is an extremely anisotropic body and possesses a negative thermal expansivity along the orientation axis of the order of the thermal expansivity of gases, about two orders higher than that of macromolecules incorporated in a crystalline lattice (see 2.2.3). In spite of the large anisotropy of the linear thermal expansivity, the volume coefficient of thermal expansion of a deformed network is the same as of the undeformed one. As one can see from Eqs. (50) and (51) Pn + 2(iL = a. Equation (50) shows also that the thermoelastic inversion of P must occur at Xim (sinv) 1 + (1/3) cxT. It coincides with F for isoenergetic chains [see Eq. (46)]. 

Fig. 15. Hypothetical dependence of the linear thermal expansion coefficient 3 on the degree of orientation 7). 1 — crystallizable stretched rubber, 2, 3 — crystalline drawn polymers, 4 — p n...

There is a very large anisotropy in the linear thermal expansivities of these oriented polymers. Early studies showed that in polyethylene the expansivity parallel to the orientation direction a, was negative and apparently very close to the value (—12 X 10 K ) for the c-axis expansion of the crystalline regions obtained from X-ray measurements. This result was attributed to the high degree of crystal continuity, and did not appear to be controversial. More recent work however, has... 

Fig. 4. Linear thermal expansion coefficients as a junction of temperature ( ) isotropically cured at C (o) and ( a ) oriented polymer filament (200 pm diameter) measured in the axial and lateral direction, resp.

The TMA technique can be used for Tg-value determinations, resin cure studies, penetration experiments or orientation effect determinations. The most important application is thought to be the linear thermal expansion coefficient (l.e.c.) determination of engineering polymers. An example of this application is given in chapter 3.1.2. The results of a polymer shrinkage experiment monitored by TMA are described in chapter 3.1.3. 

D 2766 (1995) Test method for specific heats of liquids and solids D 3286 (1991) Test method for gross calorific value of coal and coke by the isoperibol bomb calorimeter D 3350 (1999) Polyethylene Pipes and Fitting Materials D 3386 (1994) Test method for coefficient of linear thermal expansion of electrical insulating materials D 3417 (1999) Test method for heats of fusion and crystallization of polymers by thermal analysis D 3418 (1999) Test method for transition temperatures of polymers by thermal analysis... 

In general, the coefficient of thermal expansion of latex-modified mortar and concrete is directly influenced by that of the aggregates used, as in ordinary cement mortar and concrete. Latex-modified mortar and concrete usually have coefficients of thermal expansion equal to or slightly larger than that of ordinary mortar and concrete. Table 4.6l gives the coefficient of linear thermal expansion of SBR- and PVDC-modified mortars with variation of polymer-cement ratio. 

Type of Mortar Polymer-Cement Ratio (%) Coefficient of Linear Thermal Expansion (10-6/°C, -18to38°C) ... 

Preferably, thermomechanical analysis (TMA) is used to determine the linear thermal expansion coefficient of polymers according to ISO 11359. TMA uses a constant applied load (0.1 g to 5 g) and cylindrical or rectangular specimens with plane-parallel surfaces. The test is conducted with a low heating rate. An average or a differential coefficient of thermal expansion can be obtained, according to Eq. 3.5 and Eq. 3.6. 

Linear thermal expansion testing helps to determine if failure by thermal stress may occur in products and materials. Precise knowledge of the CTE can be utilized to estimate the thermal stresses. This aspect makes CTE to an important property of the used fiber for composite materials. A rule of mixtures is sufficient for calculating the CTE of polymers filled with powder or short fibers. In case of long libers, the rule of mixtures is valid perpendicular to the reinforcing fibers. Molecular orientation affects the thermal expansion of polymers. Processing also affects CTE, for semicrystalline polymers this fact is very important. For that reason, CTE measurements are often used to predict shrinkage in injection moulded parts. 

They observed that in injection-molded copolyesters containing 40-90 mol% of p-hydroxybenzoic acid (PHB), the linear thermal expansions were highly anisotropic. They also observed that the linear thermal expansion is zero along the flow but not across the flow. The anisotropy in linear thermal expansion is due to the orientation of polymer chains during molding. 

For the following composite materials, consisting of aligned, continuous fibres in a polymer matrix, estimate the axial and transverse coefficients of linear thermal expansion (percentages given are fibre volume fractions)... 

TMA is used to determine the linear thermal expansion coefficient (a) of polymers, defined as... 

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