Microphase separation kinetics in segmented polyurethanes: effects of soft segment length and structure

Abstract

Synchrotron small-angle X-ray scattering (SAXS) was used to investigate the microphase structure and microphase separation kinetics of two segmented polyurethanes with 4,4'-diphenylmethyl diisocyanate (MDI) and l,4'-butanediol (BD) as the hard segment and poly(tetramethylene oxide) (PTMO) and poly (propylene oxide) end-capped with poly (ethylene oxide) (PPO-PEO) (M" ~2000) as the soft segments. A more complete phase separation was observed in the PTMO based sample although PTMO and PPO-PEO have almost identical solubility parameters. This phase separation behavior could be explained as due partially to a kinetic factor. The microphase separation kinetics from quenching a sample in the melt state to lower annealing temperatures could be described by a relaxation process. A single-relaxation time process was observed for the PTMO based sample. By variation of the soft segment molecular weight from 1000 to 2000, the relaxation time was reduced from ~103 to 64 s. This behavior strongly supports our argument that in a segmented polyurethane, hard segment mobility, system viscosity, and hard segment interactions are the three controlling factors. In the PPO-PEO-based sample a double-relaxation time process was observed. One of the relaxation times was 54 s while the other secondary process was 1.48 X 103 s.

Specific interactions, structure and properties in segmented polyurethane elastomers

“…The results showed that structural units of various forms and sizes develop during polymerization [13][14][15]. Besides a crystalline or at least highly ordered phase, soft and hard segments form corresponding phases which are partially soluble in each other [16][17][18][19][20][21]. Solubility depends very much on the interaction of the components, the urethane groups are capable of forming specific interactions, H-bonds with each other, but also with the soft segments [22,23].…”

Segmented shape memory polyurethane and its water vapor transport properties

“…Strong interaction of the ester functions of oligoester chains with the urethane groups of the rigid segments than with the ether functions of oligoether chain, microphase separation takes place to greater extent in polyetherurethanes than in polyesterurethanes [6]. The degree of microphase separation increased with increasing the block length of the polyol, with the shortest block length in the resulting SPU being almost single-phase [11,32]. The PU properties improve with more complete phase separation and greater ordering of the segments in the hard domain phase.…”

Comparative dielectric studies of segmental mobility in novel polyurethanes

“…Polyurethanes (PUs) are characterized by a two-phase microstructure, consisting of hard-segment (HS) microdomains dispersed in a soft-segment (SS) phase [1][2][3]. It is this microstructure that is responsible for most of the extraordinary properties of PUs utilized in many applications.…”

“…According to Koberstein and co-workers [5], a critical HS length is required for microphase separation, whereas DMS generally improves with an increase in the HS content. Chu et al [2] pointed to the significance of the kinetic factor, besides the thermodynamic one, for the phase structure formation.…”

“…PUs are often prepared by a two-step process: formation of prepolymer (PP) with reactive NCO groups by the reaction of a low-molecular-weight polyether or polyester with an aromatic or aliphatic diisocyanate (DIC) at the first stage, which is extended with a low-molecular-weight diol or diamine (chain extender, CE) at the second stage [2,3,8,13]. The ratio PP:CE is very often chosen close to the stoichiometric 1:1, as it has been anticipated that no linear PUs are formed when the mole fraction of CE is significantly reduced [33].…”