Robert W. Seymour

Effect of segment size and polydispersity on the properties of polyurethane block polymers

Abstract The dynamic-mechanical and thermal scanning behaviour of a series of specially synthesized linear segmented polyurethanes is presented. These materials possess a well defined segment molecular weight and molecular weight distribution and no possibility of intermolecular hydrogen bonding. The dynamic-mechanical transitions observed are assigned to specific molecular motions and the effect of segment size and molecular weight distribution is described. The results are explained in terms of hard segment domain perfection. Differential scanning calorimetry (d.s.c.) curves of annealed samples indicate that domain morphology may be affected by thermal treatment. A comparison of dynamic mechanical and d.s.c. results to hydrogen-bonded polyurethanes is made.

Segmental orientation studies of block copolymers: 2. Non-hydrogen bonded polyurethanes

Abstract Uniaxial orientation of a series of polyurethane block copolymers of well characterized segmental molecular weight distribution has been studied by differential infra-red dichroism. These materials contain hard blocks incorporating piperazine units which have no possibility for hydrogen bonding. The soft segments orient into the direction of stretch at all strains but relax towards a random configuration when the load is removed. Hard segment orientation is transverse to the stretch direction at low strains, turning into the direction of stretch at higher strains. At the higher strains the hard segment orientation relaxes very little when the load is removed. Compared to hydrogen bonding polyurethanes the polyurethanes of this study showed higher hard segment orientability and strain hysteresis. This may be partly attributed to the reduced strength of the intersegment bonding and more crystalline nature of the hard segment in the piperazine extended polymers. Segment polydispersity appears to have only a secondary influence on the sub-failure properties studied in this investigation.

Infrared Dichroism of Segmented Polyurethane Elastomers

The mechanical and optical properties of block polymers in uniaxial extension have been well-documented (l). Characteristic of these materials are high moduli, high extensibility, and high resilience relative to random copolymers of equivalent composition. Also common to block polymers is the phenomenon of stress-softening (2, 3), whereby the stress and modulus of the material are substantially lowered by the cyclic application of strain. It is generally agreed that these properties are the product of a heterophase micro-structure, which arises from the segregation of unlike blocks into separate domains (l). Higher modulus domains serve both as filler particles and as physical, multifunctional cross-link points to reinforce the elastomer. Stress-softening is postulated to result from the deformation, orientation, or partial destruction of the reinforcing domains.

Orientation Studies of Polyurethane Block Polymers

Recent results from orientation studies on polyurethane block polymers are presented. Birefringence measurements are of limited value in the study of multiphase systems because of the importance of non-orientational sources of birefringence. However, both conventional and differential infrared dichroism are very useful methods. The application of these techniques in the study of orientation mechanisms, orientation hysteresis, and time dependent orientation processes are discussed and illustrated with pertinent results.