phase transitions in polymer systems fried 4.1-4.3

1: Phase Transitions CHEE 490 3.1 Phase Transitions in Polymer Systems Fried 4.1-4.3 Most products use polymers in their bulk (solid, condensed) state. For these applications, the physical properties detailed in lecture 2 are strongly dependent on phase morphology and, as a result, on temperature. To clarify key concepts, we will handle a few different polymers: poly(methylmethacrylate) high density poly(ethylene) low density poly(ethylene-co-hexene) poly(tetrafluoroethylene) poly(isoprene), cis and trans. By the end of this lecture topic, you should be able to identify amorphous and crystalline states, relate these to mechanical properties and predict how each material will behave with respect to temperature changes.

2: Phase Transitions CHEE 490 3.2

3: Phase Transitions CHEE 490 3.3 Crystallinity in Nylon-6,6

4: Phase Transitions CHEE 490 3.4 Identifying the Crystalline Melting Temperature A transition in which the first derivatives of the molar Gibbs energy are discontinuous is defined as a first-order phase transition. The chemical potential of the material changes abruptly at the transition point, Tt. Heat Capacity

5: Phase Transitions CHEE 490 3.5 Identifying the Crystalline Melting Temperature Dilatometry studies involve confining the polymer by a well-characterized, inert liquid and recording the change in volume as the temperature is varied.

6: Phase Transitions CHEE 490 3.6 Identifying the Crystalline Melting Temperature A Differential Scanning Calorimeter (DSC) controls the energy input to a sample and reference so they remain at the same T throughout a programmed temperature rise. A DSC trace is a plot of energy (DH=Hsample-Href) as a function of T.

7: Phase Transitions CHEE 490 3.7 Factors Influencing Crystallinity Chain architecture and composition distribution determines whether a polymer exists in a semi-crystalline or completely amorphous state. 1. Chain symmetry: symmetrical structures that permit close packing of chains favour crystallinity. atactic poly(propylene) versus isotactic (polypropylene) poly(tetrafluoroethylene)? 2. Intermolecular forces: hydrogen bonding and attractive van der Waals forces promote crystallization atactic-poly(vinyl alcohol) 3. Branching and molecular mass: packing efficiency deteriorates with increasing branching and the relative number of free chain ends. isotactic(polypropylene)

8: Phase Transitions CHEE 490 3.8 Factors Influencing Tm The fundamental equation of thermodynamics for a closed system states: DGm = DHm - T DSm where DHm and DSm represent the enthalpy and entropy of fusion per repeat unit, respectively. At the equilibrium temperature, Tm, DGm= 0, therefore: Polymers in which DHm is relatively large (strong intermolecular attraction) and DSm relatively small (minimal ordering from melt to crystalline state), the temperature of melting is high.

9: Phase Transitions CHEE 490 3.9 Amorphous Bulk State An amorphous state is one of relative disorder, where chain orientation is not present on a large scale. Physical properties derived from an amorphous phase are strongly dependent on temperature. Consider, Plexiglass - poly(methyl methacrylate) Natural rubber - cis-poly(isoprene) Both exist in an amorphous phase under conditions of common use, but exhibit very different mechanical properties. If Plexiglass is heated above 105�C, it becomes rubbery. Cool natural rubber below -73 �C and it becomes a brittle, rigid material. The transition from a glassy to a rubbery state in amorphous materials is called the glass transition temperature, Tg. Below Tg, there is insufficient thermal energy to allow significant chain mobility or even chain segmental motion. Only cooperative motion of a few atoms of the main chain or side-groups is present, as well as atomic vibrations.

10: Phase Transitions CHEE 490 3.10 Identifying the Glass Transition Temperature A transition in which the first derivatives of the molar Gibbs energy are continuous, but the second derivatives are discontinuous is, by definition, a second-order phase transition. Molar Volume

11: Phase Transitions CHEE 490 3.11 Identifying the Glass Transition Temperature Transition from a glass amorphous state to a rubbery amorphous state can be detected by a number of methods. Dynamic mechanical testing Specific volume determinations Differential Scanning Calorimetry Shown here is the specific volume vs. T plot for poly(vinyl acetate). Note that the thermal expansion coefficient changes at Tg, and a discontinuity is observed at the glass transition point.

12: Phase Transitions CHEE 490 3.12 Identifying the Glass Transition Temperature DSC trace of poly(ethylene terephthalate-co-p-oxbenzoate), quenched, reheated, cooled at 0.5?K/min through the glass transition, and reheated for measurement at I0?K/min. Tg is taken at the temperature at which half the increase in heat capacity has occurred. The width of the transition is indicated by DT.

13: Phase Transitions CHEE 490 3.13 Factors Influencing Tg Polymers whose structures are flexible, do not provide for strong intermolecular attraction, and do not �pack� well are those with relatively Tg�s. Four factors are generally accepted to affect Tg: 1. Free volume - volume of the material that is not occupied by polymer molecules 2. Attractive forces - hydrogen bonding, dipole association 3. Internal chain mobility - rotational freedom along the chain as influenced by side chains. 4. Chain length - shorter chains have greater relative free volume.