| Poly(lactic acid)(PLA)exhibits excellent performance inbiocompatibility,biodegradability and mechanicalproperties,thus allowing for wide potential applications inbiomedicaland packaging areas.Unfortunately,owing to its intrinsically slow crystallization kinetics,PLAproducts are usually amorphous,especially under actualprocessing conditions,which leads to someundesirable properties such as poor barrier property andthermal resistance.For fiber/polymer composites,fiber reinforcement is an effective method to improve the crystallization of PLA.The PLA film was prepared by the solution method and then a single silk fiber was sandwiched between two pieces of the PLA film.The influences of isothermal temperature,fiber-pulling speed and fiber-pulling time on the morphology of cylindrites were investigated by POM and FTIR.Finally,we investigated a small content of(5 wt%)Poly(ethylene oxide)(PEO)on the crystallization behavior of PLA.This paper consists of several aspects as following:(1)The silk fiber has no nucleating ability toward PLA in the quiescent melt,when the isothermal temperature was around 100-130 °C.At the lower temperature(<120 °C),the nuclei rate of PLA matrix was so fast that a large amount of smaller spherulites formed rapidly.And it was difficult to observe the crystallinemorphologyof PLA on the SF surface.Our results showed that 120-130°C was an ideal range to investigatecrystalline behavior of PLA inducing by SF.(2)The effect of fiber-pulling speed on the crystalline morphology of PLA was investigated by means of POM.The results showed that different fiber-pulling speed lead to different crystalline morphology.For lower fiber-pulling speed,it could be observed the “string shape spherulite” on the SF surface.Increasing fiber-pulling speed,the “string shape spherulite” would turn to cylindrites.The SF would be broken,when the pulling rate was too high.The nucleation rate and the saturated nucleation density increased with increasing speed of fiber pulling,but the induction time shown the opposite effect.When the crystallization temperature was 120 °C,the speed of formation “string shape spherulite”,cylindrites,and the brokenpulling rate of SF was 5,40,and 150 μm/s,respectlvely.However,when the crystallization temperature was 130 °C,the three critical value was 10,50,250 μm/s,respectively.Given the fiber-pulling speed,the cylindrites formed at 120 °C had the earlier induction period,higher nuclei density and larger growth rate than those at 130 °C.(3)SF was pulled at a speed of 40 μm/s,and the effect of fiber-pulling time on the crystallization morphology was similar result to the case of fiber-pulling speed.Given fiber-pulling speed,the nuclei density wasnotably increasing versus fiber-pulling time.A shorter pulling time caused “string shape spherulite”,which could turn to cylindrites structure when the pulling time was long enough.Compared to 130 °C,the case at 120 °C,the cylindrites were more compact,the growth rate of cylindrites was much higher.(4)The structure of cylindrites induced by pulling SF was investigated FTIR.The results showed that the cylindrites structure were α form.The degree of crystallinity and orientation of the cylindrites crystallized at 120°C,increased with fiber-pulling speed.However,the degree of crystallinity and orientation of the cylindrites crystallized at 130°Creached a plateau,when the pulling rate was above the saturation critical value(100μm/s).(5)With a small content of PEO(5wt%),the critical fiber-pulling speed inducingthe formation of “string shape spherulite” and cylindrites became larger than pure PLA.And the crystal growth rate of PEO/PLA was higher than pure PLA. |
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