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If the hydroxyl group reacted further along the chain then higher oligomers would be produced. The presence of potassium hydroxide within the polydimethylsiloxane mixture has a powerful destabilizing effect and it is clear that a different mechanism is responsible for oligomer formation. Since the cyclic oligomers are formed in a step­ wise reaction, each step must be catalysed by potassium hydroxide, or a derivative i.e. a silonate.

When volatile degradation products are continuously removed, 169

Grassie found that the depolymerisation reaction proceeded 30 times

faster than in a closed system. The removal of the low molecular weight oligomers, prevents the polymerisation reaction taking place. This seems to be an application of the Le Chatelier Principle, and thus if hexamethylcyclotrisiloxane was selectively removed, the equilibrium would shift so as to produce more of the trimer. The results obtained from the small scale (lOg) experiments are given in Table 3.1.

However the reactions that we have considered so far, do not involve gas evolution. But on opening one of the small glass tubes,

168

very short time. This is most likely to have been methane resulting

from the cleavage of a silicon-carbon bond. Another possibility could be hydrogen gas, but this would have exhibited different behaviour on burning. However such gas evolution would explain the explosions which occurred with the large scale reactions.

Although a route was availabe to enrich hexamethylcyclotrisiloxane within the mixture of oligomers, it was clear that this method was unsuitable for use on the desired scale. Although the desired starting material was a relatively expensive material, it was purchased for this study rather than synthesised.

3.3.3 Copolymerisation using Hexamethylcyclotrisiloxane

The physical appearance of the copolymer product was dependant on the molecular weight of the components and the composition of each product. A white powder was indicative of a high styrene content, whilst a gum-like material was very characteristic of a high molecular weight silicone.

The characteristic absorption bands resulting from the phenyl ring and siloxane linkage have been used to confirm the presence of both components (Fig.3.5). To give a first indication as to the composition of the copolymers, the Si-CH^ absorption at 1265 cm \ and styrene at 560 cm ^ peaks were compared. Correlation data is given in Table 3.2.

Nuclear magnetic resonance spectra were recorded (Fig.3.6, Table 3.3) and the composition of the block copolymer calculated, from the

integrated peak areas.

In the GPC analysis, two detectors are used; infrared (poly­ dimethylsiloxane specific) detector and refractive index (polystyrene specific) detector. The infrared trace simply shows a single

peak indicative of polymer chains having polydimethylsiloxane components, any homopolystyrene that may be present, not being detected. However the refractive index trace gives.a positive response for homo­

polystyrene, and negative for polystyrene chains containing poly­ dimethylsiloxane. The sample Pss 23 appears to contain a small amount of high molecular weight homopolystyrene, amongst the polystyrene- polydimethylsiloxane block copolymer. It is possible that a few high molecular weight, living polystyryl anions terminated, prior or during to the addition of the hexamethycyclotrisiloxane, but with the majority of the styryl anions acting as an initiator for the cyclic monomer, thereby giving rise to a block copolymer. The retention time at which there is the highest number of copolymer chains, equates to a molar mass of 42,300g mole \ based upon polystyrene calibration standards.

The thermal characteristics of the copolymers were investigated, as described in section 2.5.1. The polydimethylsiloxane glass trans­ ition temperature (Tg) was found to be the same for both copolymers and identical to that obtained for the polydimethylsiloxane homopolymer in the blends. The polystyrene exhibited a glass transition in the

region 98°C - 104°C. It might be expected that a copolymer would exhibit a single glass transition, the position of which would be

dependent on the composition of such a copolymer. However, it is found 169 that two Tg sire obtained as with polymer blends. Noshay and McGrath have commented that, the thermal properties of block copolymers resemble those of physical blends, and have been found to display glass trans­ ition temperatures and crystalline melt temperature characteristic of each of the components. This is because block copolymers often display two-phase morphology and in the case of the polystyrene-polydimethyl- siloxane block copolymers, there is evidence that the system separates

B Blocks B Blocks A Blocks

■<?

This polymer system is prevented from forming separate macrophases, 170

due to the chemical bonds linking the blocks together. Even so,

these microphases are able to exhibit their characteristic thermal 171

properties. A number of theories have been developed to predict

the lengths of the blocks required for such phase separation. In a 172

polystyrene - polybutadiene AB block copolymer, phase separation

would occur with the polybutadiene molecular weight 50,000 and

polystyrene 5000-10,000. It is evident that a relatively short

polystyrene chain is required for phase separation. Although no figures are available for our particular system, it is anticipated that the polystyrene block length would ensure that phase separation took place.

Transition broadening (ATg = 4.7°C) has been observed with

polysiloxanes when blended with polystyrene. This observation has been explained as due to the influence of the polystyrene component. The polystyrene in these copolymers appears to have a much greater influence, with A T g = 7°C. This is in good agreement with the work of Krause11^ on polystyrene-polydimethylsiloxane (AB) block copolymers where the average range over which the transition takes place was found to be 7°C, with little or no shift in the polysiloxane transition temperature. Thus the thermal characteristics of the siloxane component does point to some interaction existing between the two phases. Whereas the glass transition of the polystyrene component and the temperature over which the transition takes place is the same as that in homopolystyrene. 3.3.4 Preparation of Hydroxyl-terminated Polystyrene

Infrared analysis of the product (Fig.3.8) from a thin film, showed absorption bands at 1250 cm 1 (Si- CH^) and 810 cm-1 (Si-CHg) and 810cm-1

(Si-C) indicative of the presence of the silane group. The absence of any peaks in the region 460-470 (Si-Cl), and the presence of the broad

band at 3600-3200 cm 1 confirm that the chlorosilane terminating group

c H =c H2

+ BuLi

Bu- C H 2— C H

Li.+

x

Bu

- K H 2— C H'

Li + Cl— Si— Cl

Jx

Bu--CHo— C H — Si— Cl

+ Li Cl

CH

Bu-{-CH2— CH

Si—Cl + HoOr3

► Bu--CH2— C H

CHq

Si—OH

+ HCl

Equation 3.8 The preparation of hydroxy terminated polystyrene.