Polymer chain stiffening

Polymer chain stiffening A polymer strengthening mechanism significandy different from linear, branched or cross-linked. It has a monomer that is physically large and unsymmetrical. The ability of a chain to flex is impaired. A typical example is with polystyrene plastic. 

As might be expected from a consideration of the factors discussed in Section 4.2, the imidisation process will stiffen the polymer chain and hence enhance Tg and thus softening points. Hence Vicat softening points (by Procedure B) may be as high as 175°C. The modulus of elasticity is also about 50% greater than that of PMMa at 4300 MPa, whilst with carbon fibre reinforcement this rises to 25 000 MPa. The polymer is clear (90% transparent) and colourless. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Very active catalysts for the preparation of strictly alternating butadiene-propylene copolymers (BPR) consist of V0(0R)2C1/i-Bu Al (R = neopentyl). The CH3 side groups in BPR stiffen the polymer chain and were expected to promote the formation of strain-induced structures. The fact that we could not detect strain-induced crystallization is probably due to an atactic configuration of the propylene units. 

A low melting entropy is experienced if the intramolecular motions of the polymer are hindered, for example, the free rotation of chain segments. This is preferentially be done by stiffening of the polymer chain. Table 2.12 shows a selection of building blocks for temperature-resistant polymers. 

The Tm of condensation polymers such as polyesters and polyamides is decreased as the number of methylene (CHj) groups in the reactants is increased. The presence of stiffening groups, such as phenylene groups in a polymer chain, increases the Tm. 

Table 7.1 Chain-Stiffening Effects of Groups Present in Polymer Backbones...

PPO has a high Tm because of the presence of the chain-stiffening 1,4-phen-ylene moiety, which makes up the major portion of the backbone of this polymer (the oxygen atom constitutes the remainder). Because of its very high T this polymer cannot be processed by traditional extrusion or molding techniques. 

The simple theory presented accounts for the fall-off of the modulus of polymers with increased extension. The stiffening of the polymer at very large distortion results from interactions between the extended polymer chains, which is called strain-induced crystallization. 

In contrast, presence of certain groups in the polymer backbone renders the polymer chain less flexible. Examples of stiffening groups are ... 

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