Chapter 12 Polymer Structure and Physical Properties

1. The crystalline Melting Point T_m

During the melting process  ΔG = ΔH_m – T_mΔS_m = 0

So Tm = ΔH_m / ΔS_m

Effect of chain flexibility and other steric factors

Chain flexibility.

The flexibility of chain molecules arises from rotation round the saturated chain bond. The potential energy barriers hindering this rotation range from 0.2 to 1.2 kJ/mol. It is not surprising therefore that the flexibility of polymer chains is an important factor in determining their melting points. Thus polytetrafluoroethylene (T_m=327 ^oC), which is much higher than polyethylene because of it low entropy of fusion, which is results from the high stiffness of the polymer chains. The high melting point of isotactic polypropylene ((T_m=165 ^oC) is attributed to low entropy of fusion arising from stiffening to the chain in the melt because of the higher energy barrier for rotation about C-C bonds than in polyethylene. In neither case can a high heat of fusion account for the high value of Tm, since for either case, its ΔH_m is well below that of polyethylene.

The involvement of inflexible group like the p-phenylene group would marked raise the melting point of the polymer as shown in table 12-1

Table 12-1 Effect of a-phenylene group on the melting point of condensation polymers

Other chain stiffening groups are p,p’-diphenyl, 1,5-or 2,6-naphthyl, diketopiperazine, triazole, etc.

Side chain Substitution

In most cases, the substitution of nonpolar groups for hydrogens of a polymer chain leads to a reduction in Tm or possibly complete loss of crystallinity.

1. If the substitution is random, as in branched polyethylene, the primary effect is a decrease in the degree of crystallinity. The Tm is lowered 20-25 ^oC on going from the linear to the branched material.
2. Substitution of an amide hydrogen with an alkyl group have significant effect (In general, lower by 100 ^oC), since the hydrogen bond is destroyed.
3. When alkyl groups are regularly grafted onto the methylene chain, two effects can compete in setting Tm.  A) an increase of the side chain results in a looser crystal structure with an lower Tm.  B) an increase in the bulkness of the side chain increases Tm, since the entropy decrease

Table 12-2 Effect of side-chain structure on the crystalline melting point of Isotactic poly(a-olefins)

4. The melting point of copolymers is determined by A) 1/T_m – 1/T_m⁰ = -Rlnn/ΔH_m where T_m ⁰ is the melting point of homopolymer and Hm is its heat of fusion. A typical case is the copolymer of hexamethylene terephalamide and hexamethylene sebacamide.  B) if the comonomers are capable of replacing each other in the crystals (isomorphous), the melting point could vary smoothly over the composition range. One example is copolymer of hexamethylene terelphthalamide and hexamethylene adipamide.

5. Block and graft copolymers may exhibit two crystalline melting points, on for each type of chain segment.

B. The glass transition

With few exceptions, polymer structure affects the glass transition Tg and crystalline melting point Tm similarly. Tg (K) is approximately ½ to 2/3 of Tm (K).

1. Tg is more dependent on molecular weight than Tm.

T_g = Tg ^∞ – k / Mn

Tg ^∞ is the glass transition temperature at infinite molecular weight. k is about 2x10⁵ for polystyrene and PMMA and 3.5x10⁵ for atactic poly(α-methyl styrene).

2. Bulky group on the side chain could increase the Tg of the polymer of the same series. For example: polybutadiene (-85 ^oC), polystyrene (100 ^oC), poly(α -methyl styrene) (150 ^oC), polyacenaphthalene (285 ^oC).
3. The Tg of random copolymers are usually between those of the corresponding homopolymers.   a₁w₁(T_g-T_g1) + a₂w₂(T_g-T_g2) = 0

Where a1 and a₂ dependent on the polymer type. w1 and w₂ are the weight fraction of monomers 1 and 2 in the copolymer, T_g1 and T_g2 refer to the homopolymers.

4. Branching polymers have lower Tg, and crosslinking polymers have higher Tg.

Homework:

2. Predict and explain the difference in crystalline melting point between the polymers with the repeat units –(CH2)6NHCO(CH2)4CONH- and –(CH2)6N(CH3)CO(CH2)4CON(CH3)-.