ABSTRACT:The shape memory behavior of a series of polycaprolactone/methane diisocyanate/ butanediol ( PCL /MDI /BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600 -7000, and the hard segment content varied from 7.8 to 27% by weight. Film specimens for shape memory measurements were prepared by drawing at temperatures above the melting temperature of the soft segment crystals and subsequent quick cooling to room temperature under constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temperature, ability, and speed were used to study the influence of structure and processing conditions on the shape memory behavior of the sample. It was found that the high crystallinity of the soft segment regions at room temperature and the formation of stable hard segment domains acting as physical crosslinks in the temperature range above the melting temperature of the soft segment crystals are the two necessary conditions for a segmented copolymer with shape memory behavior. The response temperature of shape memory is dependent on the melting temperature of the soft segment crystals. The final recovery rate and the recovery speed are mainly related to the stability of the hard segment domains under stretching and are dependent on the hard segment content of the copolymers.
SYNOPSISA series of polycaprolactone/4,4'-diphenylmethane diisocyanate/l,4-butanediol (PCL/MDI/ BDO) segmented polyurethanes of different compositions was synthesized by solution polymerization. The molecular weight of PCL diols used was in the range of 1600-7000. The crystallinity and morphology of these polymers were studied by using DSC, dynamic mechanical analysis, WAXD, and polarizing microscopy methods. It was found that the crystallinity of PCL prepolymers was depressed in segmented polyurethanes. A lower limit of PCL molecular weight was found, below which the PCL segments were not able to crystallize a t the usual processing conditions. This limit of molecular weight is in the range of 2000-3000 and exhibits a slight increase with increasing hard-segment content of polyurethanes.The glass transition temperature related to the PCL segment regions in polyurethane specimens deviated from that of pure amorphous PCL prepolymer to a higher temperature. The deviation resulted from the crystallization of PCL segments and also the influence of hard segments. The formation of hard-segment domains becomes very difficult for polyurethanes having low hard-segment content and short hard-segment length. There is a lower limit of hard-segment content and segment length. Only above that limit do the polyurethanes have enough hard-segment domains acting as physical crosslinks at temperatures above the melting point of the PCL crystals. The structural characteristics of segmented polyurethanes which may exhibit a shape memory effect are also discussed.
SYNOPSISIn this paper, the crystallization characteristics of polypropylene and low ethylene content polypropylene copolymers with and without nucleating agents were studied by differential scanning calorimetry (DSC) and polarized light microscopy ( PLM) . In order to determine the nonisothermal crystallization rate of these materials, a new estimation method was introduced. Comparing with the crystallization rate coefficient (CRC) , which was proposed by Khanna, we found the new approach is more reasonable. From the analysis of results of DSC and PLM, it can be concIuded that nucleating agent is more efficient than it is in PP homopolymer.
A series of ethylene oxide-ethylene terephthalate segmented copolymers (EOET) with different long soft segments and different hard-segment contents were synthesized. The thermally stimulated shape-memory behavior of EOET segmented copolymer is characterized by the maximum recovery (R f ), the recovery temperature (T r ), and the recovery speed (V r ) or the temperature ( T M ) of the maximum deformation recovery rate against temperature and the maximum recovery speed (V M ). These parameters show that the features of thermally stimulated shape-memory effects of EOET segmented copolymers are following: The crystallinity of soft segments determines the temperature T r and T M , and the parameters R f , V r , and V M depend on the stability of the physical crosslinks formed by the hard segments, which at the same time are influenced by the length of soft segment. R f , V r , and V M decrease with prolongation of the keeping time (t k ) and increasing stretching ratio.
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