Mechanisms of Microscopic Lamellar Assembly in Polycrystalline Spherulites of Polyesters

Abstract

Banded spherulite in polymer has aroused considerable interest over a century; not only because of their beautiful outward appearances and systematic arrangements but also because of their potential application values Much effort has been put in explaining the crystallization mechanism of banded spherulite; yet the discussion is still going on The two most widely accepted theories rhythmic deposition and especially radial lamellar twisting are still poorly understood with many unexplainable defects Hence in this study novel approaches and schematic models have been taken to provide insights in viewing lamellar assembly in ring-banded spherulites of two model polyesters poly(ethylene adipate) (PEA) and poly(trimethylene terephthalate) (PTT) via correlations between outer-surface and three-dimensional interior morphology PEA an aliphatic polyesters in thick bulk forms exhibits a ring-banded top surface with bowl-like and dome-like height profile centering on its nucleus when crystallized at 28 oC The 3-D periodical assembly leads to not only noticeable ring bands on exterior surfaces but also corrugated-board like multi-shell structures in the interior of the spherulites The concentration density of crystalline polymer chains in the growth front (x1) has been found to change gradually with respect to the radius forming a corrugated-board structure which firstly grew in tangential direction then turned to radial direction The radial plates taper to form thinner cilia-like lamellae due to the polymer chain concentration gradient being periodically precipitated during the growth Alternating sequences of plate-like lamellae in two perpendicular orientations (bending from tangential to radial) in PEA ring-banded spherulites during growth in radial direction account for the spherulites confined in thin films to display two contrast circumferential rings with alternating interference colors The SEM dissection graphs clearly reveal interior corrugated layer thickness in bulk forms which matches well with the inter-ring spacing in thin-films Moreover faster-crystallizing poly(1 4-butylene adipate) (PBA) and much slower-crystallizing poly(ethylene adipate) (PEA) each with the ability to form ring-banded morphology at same Tc were simultaneously crystallized from mixtures of various compositions Investigations on morphology phase and thermal behavior were conducted in order to reveal lamellar packing and spherulitic structures in this binary system At intermediate compositions PBA/PEA (10/90 ~ 40/60) PEA has the ability to simultaneously crystallize with PBA in forming a bird-nest-like spherulites with porous structure as a result of interpenetration and re-orientations of PBA and PEA lamellae The corrugated-board structure of PBA is to be followed by another corrugated-board structure of PEA leading to interwoven lamellae in 3-D forms observed from bulk fractured interiors The interaction between PBA and PEA along with the composition ratio in the blend are supposed to be the driving force of such porous structure formation Etching agents with methylamine being one among several others have been used in many studies for exposing the crystal assembly in contrast to amorphous domains in polymer samples for better morphology contrast Apparently the methylamine does not simply just induce physical etching of peeling off a covered layer from polymer samples but also causes severe chemical changes in not only aliphatic but also aromatic polymers The chemical reactions between methylamine and polyesters ultimately lead to different chemical structures alteration in spherulites and lamellae and sometimes completely different lamellar structures with a different melting peak with –C(=O)O– in polymers being replaced with –C(=O)NH– bonding Cautions should be taken when using methylamine as etching agent or else misinterpretation may be a consequence PTT a highly birefringent aromatic polyester exhibits spherulites with three main types of banded structure (i e neat concentric single spiral and double spiral) co-existing in a same sample film melt-crystallized at 165 oC regardless of sample thickness (1 ?m ~ 100 ?m) The nature of high birefringence in PTT ring-banded spherulites is utilized for probing the origins of diversification of lamellar assemblies into spherulites of multiple optical patterns Three controlling factors: core diameter band spacing and height difference (Δz) around the core are the key parameters of different banding patterns in PTT spherulites Formation mechanism for three types of ring-banded spherulites has been interpreted and found to be highly correlated with the initial nuclei geometry shapes

Date of Award	2015 Aug 19
Original language	English
Supervisor	Eamor Woo (Supervisor)