Page 502 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Testing and Spectrometric Characterization of Polymers 465

in a cyclic fashion as a function of temperature. The polymer molecules store some of the imparted
energy and dissipate a portion in the form of heat. Since the amount of energy stored and converted
to heat is related to molecular motion, changes in the ratios of energy stored to energy converted
to heat is used to measure T . Sperling compared literature reports of T values for some common
g g
polymers and found differences of several decades of degrees in the reported T values.
g
13.6.4 THERMAL CONDUCTIVITY

As energy in the form of heat, magnetic, or electric is applied to one side of a material, the energy
is transmitted to other areas of the sample. Heat energy is largely transmitted through the increased
amplitude of molecular vibrations. The heat fl ow Q from any point in a solid is related to the tem-
perature gradient dt/dl with the thermal conductivity λ as follows:

Q = − λ (dt/dl) (13.4)

Table 13.2 contains a listing of the thermal conductivities of selected materials. Notice the typi-
cal much smaller values for polymers compared with metals.
–1
Most polymers have thermal conductivity values in the general range between 10 and 1 W/m-K.
For polymers, transmission of thermal energy, heat, is favored by the presence of ordered crystal-
line lattices and covalently bonded atoms. Thus graphite, quartz, and diamond are relatively good
thermal conductors. Crystalline polymers such as HDPE and i-PP exhibit somewhat higher thermal
conductivities than amorphous polymers such as LDPE and a-PS. In general, thermal conductivity
increases with increasing density and crystallinity for the same polymer. For amorphous polymers,
where energy is transmitted through the polymer backbone, thermal conductivity increases as the
chain length increases. Addition of small molecules, such as plasticizers, generally decreases ther-
mal conductivity.
As long as a polymer does not undergo a phase change, thermal conductivity is not greatly
affected by temperature changes. Aligning of polymers generally increases their thermal conduc-
tivities along the axis of elongation. For instance, the conductivity of HDPE increases 10-fold along
the axis of elongation at 10% strain.
Foamed cellular materials have much lower thermal conductivities because the gas employed to
create the foam is a poor conductor. Thus, foams are employed as commercial insulators in build-
ings, thermal jugs, and drinking mugs.

TABLE 13.2
Thermal Conductivities of Selected Materials
Approximate Thermal Approximate Thermal
Material Conductivity (W/m-K) Material Conductivity (W/m-K)
Copper 7,200 a-PS 0.16
Graphite 150 PS(foam) 0.04
Iron 90 PVC 0.16
Diamond 30 PVC(foam) 0.03
Quartz 10 Nylon 66 0.25
Glass 1 PET 0.14
PMMA 0.19 NR 0.18
HDPE 0.44 PU 0.31
LDPE 0.35 PU(foam) 0.03
i-PP 0.24 PTFE 0.27
PVC (35% plasticizer) 0.15

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