Polyurethane crystalline systems

Crystalline Systems. Lower Tqs, flatter rubbery plateau modulus, higher hard segment crystallinity, all support the view that phase separation is more complete MDI-BDO and MDI-ED0 based polyurethanes as catalyst concentration decreases. 

The physical properties of polyurethanes are derived from their molecular stmcture and deterrnined by the choice of building blocks as weU as the supramolecular stmctures caused by atomic interaction between chains. The abiHty to crystalline, the flexibiHty of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, eg, polyurethane foams, other factors such as density determine the final properties. 

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

The substantial work on polystyrene/polybutadiene and polystyrene/ polyisoprene blends and diblock and triblock copolymer systems has lead to a general understanding of the nature of phase separation in regular block copolymer systems (5,6). The additional complexities of multiblocks with variable block length as well as possible hard- and/or soft-phase crystallinity makes the morphological characterization of polyurethane systems a challenge. 

James DF and Walters K, "A Critical Appraisal of Available Methods for the Measurement of Extensional Properties of Mobile Systems" in Collyer AA (Ed), "Techniques in Rheological Measurement", Chapman Hall, London, 1993. Jin S, "Synthesis and Characterization of Side-Chain Liquid Crystalline Polyesters and Polyurethanes", PhD Thesis, Delft, 1995. 

The successful utilization of Reaction Injection Molding (RIM) to fabricate complex polyurethane shapes In a single step from relatively low viscosity streams has led to a search for other chemical systems which can be fabricated by the RIM process. The rapid polymerization of molten caprolactam by anionic catalysis has been utilized to develop attractive nylon RIM systems. The incorporation of a rubber segment In the polymer chain allows the fabrication of high Impact or even elastomeric nylon parts. The combination of a rubber phase with the high melting (215°C) crystalline nylon phase provides useful properties at low temperatures as well as at elevated temperatures. 

The Mesogenic Index has been applied to a substantial body of data on polyesters and polyamides drawn from from published literature including 22 systems where the critical concentration for onset of liquid crystallinity has been established with reasonable accuracy (Tables 2 and 3). The characteristic value of the MI at the onset of a liquid crystalline phase is generally found to be remarkably close to 10. When applied to the more limited data available on polyurethanes, polyethers, polycarbonates, the rule holds up well. 

A rigid foam is defined as one in which the polymer matrix exists in the crystalline state or, if amorphous, is below its Tg. Following from this, a flexible cellular polymer is a system in which the matrix polymer is above its Tg. According to this classification, most polyolefins, polystyrene, phenolic, polyycarbonate, polyphenylene oxide, and some polyurethane foams are rigid, whereas rubber foams, elastomeric polyurethanes, certain polyolefins, and plasticized PVC are flexible. Intermediate between these two extremes is a class of polymer foams known as semirigid. Their stress-strain behavior is, however, closer to that of flexible systems than to that exhibited by rigid cellular polymers. 

In reaction injection moulding (RIM), two monomers are injected into a mould, where polymerisation and crosslinking occur. It is used mainly with polyurethanes to make large automotive panels. The chemistry of polyurethanes was described in Chapter 4. Other systems used include a block copolymer between a crystalline polyamide (nylon 6) and a rubbery polyether (polypropylene oxide). In principle, any polymerisation reaction that can be substantially completed after about 30 s in the mould is a candidate for RIM. 

Information about surface morphology is also obtained from SEM studies. In general, the intercalated clay layers show an intense peak in the range of 1.5° ° (29 value), whereas exfoliated systems have no distinct peak in that range for their loss of structural integrity shown in the XRD pattern of the nanocomposites. XRD studies indicate that there is no infiu-ence of nanomaterial on poly(e-caprolactone)diol PCL crystallinity in sunflower oil-based hyperbranched polyurethane/silver nanocomposite, but that crystallinity is enhanced in castor oil or Mesua ferrea oil-based hyperbranched polyurethane/MWCNT nanocomposites. ... 

In 2007, Cao and Jana at Akron tethered Cloisite 30B clay particles onto SMP polyurethanes [70]. The polyurethanes were synthesized from aromatic diisocyanates, a crystalline polyester polyol, and poly(e-caprolactone) (PCL) diol. The monomer ratios were cleverly balanced between an excess ratio of isocyanate in the polymer system and the pendant alcohol groups (-CH2CH2OH) on the quaternary ammonium ions fi om the Cloisite 30B. The team observed an increase in the rubbery modulus at 100°C (Tm-I- 50°C) of nearly one order of magnitude, from 4—5 MPa without loading to more than 20 MPa with loading. In the carefully crafted study, the authors determined that at high (5%) clay content, a more rapid relaxation of induced tensile stress reduces the recovery force of SMPs. In addition,... 

Polyurethanes with liquid crystalline behaviour have been produced from bischloroformates of 4,4 -alkylenedioxydiphenols by polycondensation with piperazine (37), tr<2 x-2,5-dimethylpiperazine (38) and 4,4 -bipiperidine (39). These molecules exhibit smectic properties. In general the hydrogen bonding between the neighbouring polymer chains is sufficiently strong to suppress the formation of mesophases however, some other systems have been reported. ... 

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