Shear Induced Crystallization Kinetics And Morphologies For Semi-crystalline Polymers

Polymers are inevitably subjected to shear flow field during industrial processing,which dramatically alters the crystallization kinetics and morphologies and consequently the final product properties.The developing mechanisms for shear induced crystallization have contributed much to polymer processing,for which the extension to the study on the shear induced crystallization behavior for polymers with application value is essential.In this thesis,the chosen polymeric systems are not limited to the neat and/or linear polymers.Rheology and optical microscopy coupled with a shear stage are primarily used to assess the effect of long chain branching,second immiscible component,pre-existing particles on the crystallization kinetics and morphologies under shear flow field.The main contents are listed below:1.The effects of long chain branching degree on the shear-induced isothermal crystallization kinetics of a series of long chain branched polylactides(LCB PLAs)have been investigated by using rotational rheometer,polarized optical microscopy(POM)and scanning electronic microscopy(SEM).Dynamic viscoelastic properties obtained by small-amplitude oscillatory shear(SAOS)tests indicate that LCB PLAs show more broadened relaxation time spectra with increasing branching degree,especially in the long relaxation time region.Upon an application of pre-shear at the shear rate of 1 s-1 LCB PLAs show much faster crystallization rate than linear PLA and the crystallization rate is enhanced with increasing long chain branching degree.By modeling the system as a suspension the quantitative evaluation of nucleation density can be derived from rheometry experiments.The nucleation density is greatly enhanced with increasing long chain branching degree and a saturation in shear time on the nucleation density enhancement is observed.Crystalline morphologies observed by POM and SEM for LCB PLAs also demonstrate the enhancement of nucleation density with increasing branching degree and a transformation from spherulitic to orientated crystalline morphologies.The observation can be ascribed to the longer relaxation time of the longest macromolecular chains and broadened,complex relaxation behaviors due to the introduction of LCB into PLA,which is essential in stabilizing the orientated crystal nuclei after pre-shear.2.The morphological development of sheared isotactic polypropylene(iPP)containing preformed spherulites with micrometer scale has been investigated by means of a Linkam shearing cell coupled with an optical microscope.The preformed spherulites were produced by intentionally quenching the iPP melt to 130? for short duration and then the iPP melt was heated to higher temperatures below and near the nominal melting point of iPP spherulite(164?),which was then subjected to pre-shear.Afterward the sheared melt was quenched to 135 ? for isothermal crystallization.Both phase contrast optical micrographs(PCOM)and polarized optical micrographs(POM)allowed to gain in situ observations of the crystalline morphological evolutions under various pre-shear conditions.Two distinct cylindrite morphologies were induced by pre-shear when preformed spherulites were present,while only spherulites were formed in the absence of preformed spherulites.The shear rate and pre-shear temperature dependences of the morphological evolutions were all systematically investigated,which demonstrated that the formations of the cylindrite morphologies were closely related to the applied pre-shear temperatures.The distinct morphologies are attributed to the different movement between the melt-substrate interface and the preformed spherulites during shear at temperature either below or near the melting point of iPP spherulite.Atomtic force microscopy(AFM)revealed the nature of the core structures for these distince cylindrical morphologies.3.The crystallization morphologies of polylactide/poly(?-caprolactone)(PLA/PCL)blends under shear flow field are investigated primarily by optical microscopy.Viscoelastic properties suggest the immiscible nature and different relaxation behavior of PLA and PCL phases.The isothermal crystallization behavior is unaltered for PLA with addition of PCL.However by applying shear,oriented cylindrical crystalline morphologies are induced while the crystalline morphology of neat PLA remains spherulitic.The shear rate,shear temperature and PCL content all show dramatic influence on PLA crystalline morphologies.The formation of oriented bead-like morphology is mainly attributed to the competition between the relaxation of phase interface after shear and the nucleation of PLA at the phase interface.Although the oriented morphology is not shish-kebab,it provide guidance for processing of this biodegradable polymer blends.4.The shear induced crystallization behavior for poly(L-lactic acid)/poly(D-lactic acid)/poly(ethylene-glycidyl methacrylate)(PLLA/PDLA/EGMA)blend prepared with different screw rotation speed were studied.Differential scanning calorimetry results indicate the enhancement of PLLA cold crystallization assisted by EGMA phase and stereocmplex(SC)crystallites as the screw rotation speed increases.The rheological results indicate the much slower relaxation behavior due to the increased amount of SC crystallites and enhanced dispersibility of EGMA phase with the increase in screw rotation speed.The distinct morphologies for PLLA/PDLA/EGMA blends under shear flow conditions were attributed to the joint effect of SC crystallites,enhanced dispersibility of EGMA phase and the strong interfical effect between EGMA and the matrix.The main innovations involved in this thesis:1)Explor the effect of branching degree of long chain branched polylactide on the crystallization behaviors under shear flow field and establish the relationship between the enhancement of crystallization rate and morphological transition.Provide the theoretical basis for processing of this biodegradable material and extent its application.2)Emphasize the role of pre-existing crystallites and melt-substrate interface in the crystalline morphologies under shear flow field.3)Systematic study on the effect of shear flow on the formation of unique oriented crystalline morphologies for immiscible PLA blends,which is different from the conventional shish-kebab morphology.