Endotherm just above glass transition in uniaxially drawn poly(lactic acid)s films with various d-isomer contents

Lee, Sang Cheol; Han, Jae Il; Heo, Jin Wook

doi:10.1016/j.polymer.2013.04.063

Cited by 14 publications

(9 citation statements)

References 38 publications

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“…Isothermal (ISO) crystallization has been carried out to improve the properties of PLLA but is often done at high annealing temperature and/or duration unfavourable for industrial production . Deformation of PLLA in its amorphous form at temperatures above the glass transition temperature ( T g ) and below the cold crystallization temperature ( T cc ) has been widely investigated in relation to its crystallinity and orientation of the molecular chains within the amorphous regions . Deformation stimulates the otherwise slow crystallization kinetics for PLLA at lower temperatures, observed during cold crystallization or from the melt .…”

Section: Introductionmentioning

confidence: 99%

Characterization of biaxial strain of poly(l‐lactide) tubes

Løvdal

Andreasen

Mikkelsen

et al. 2015

Polymer International

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Poly(L-lactide) (PLLA) in its L-form has promising mechanical properties. Being a semi-crystalline polymer, it can be subjected to strain-induced crystallization at temperatures above T g and can thereby become oriented. Following a simultaneous (SIM) biaxial strain process or a sequential (SEQ) biaxial strain process, the mechanical properties of biaxial strained tubes can be further improved. This study investigated these properties in relation to their morphology and crystal orientation. Both processes yield the same mechanical strength and modulus, yet exhibit different crystal orientation. Through further WAXS analysis it was found that the SEQ biaxial strain yields larger interplanar spacing and distorted crystals and looser packing of chains. However, this does not influence the mechanical properties negatively. A loss of orientation in SEQ biaxial strained samples at high degrees of strain was detected, but not seen for SIM biaxial strain, and did not correlate with mechanical performance in either case. However, post-annealing reduced the orientation to the same level in both cases, and the modulus and strength is decreased for both SIM and SEQ biaxial. It is therefore concluded that mechanical properties after biaxial strain are related to strain-induced amorphous orientation and the packing of crystals, rather than strain-induced crystallinity.

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Section: Introductionmentioning

confidence: 99%

Characterization of biaxial strain of poly(l‐lactide) tubes

Løvdal

Andreasen

Mikkelsen

et al. 2015

Polymer International

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“…Considering the PLA chains' low flexibility and entanglement density, when the molecule chains are stretched, the entanglement part would limit neighbor folding of segments, the high entanglement region and low entanglement region would appear alternately. The flow‐induced phase separation plays a dominant role on creating a series of various transient structures, leading to the “shish‐kebab like structure.”…”

Section: Resultsmentioning

confidence: 99%

“…The flow-induced phase separation plays a dominant role on creating a series of various transient structures, leading to the "shish-kebab like structure." [38] 3.2 | Crystallization behavior of pre-oriented PLA films Figure 2 gives the DSC curves of PLA films prepared at different MDRs. All curves exhibit an enthalpy relaxation peak, a broad coldcrystallization peak, and two melting peaks.…”

Section: Surface Morphology Of Pre-oriented Filmsmentioning

confidence: 99%

The structure transformation of pre‐oriented polylactic acid film during uniaxial stretching at room temperature

Tian

Xie

et al. 2019

Polymer Crystallization

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In this article, the pre‐oriented polylactide (PLA) films were prepared by extrusion casting under melt‐stretching flow and cast roll temperature field. Shish‐kebab like structure was induced and the elongation at break under room temperature of PLA film prepared with melt‐draw ratio of 40 was high to 270%. To clarify the origin of this ductile behavior, the pre‐oriented PLA structure evolution during stretching at room temperature was followed. It was found that melt‐stretching makes the chains fully extended to form amount of rigid amorphous fraction (RAF). The low chain entanglement induces weak connection strength between RAF and mobile amorphous fraction. The craze is easily formed at the interface between these two phases. The initial ductile deformation during stretching of pre‐oriented PLA films comes from the rigid extended chain slip and free amorphous chain network deformation. When the strain is higher than 40%, the RAF region appears periodic ordered structure. With the strain increasing, the RAF gradually transforms to mesophase. The molecular chains are stretched to extend in the crazes. The strain‐hardening behavior comes from the crystal destroy and chain extension. The room temperature stretching of pre‐oriented PLA film can only increase chain's orientation and package tightness, but could not induce the formation of highly perfect crystals.

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“…The exotherm depicted in the insets of Figure 2A-D, tends to shift closer to Tg as the draw temperature increases. This small exotherm is identified as the Tcc, and its shift towards Tg is due to the nucleating effect of the oriented PLLA chains and small crystallites formed during drawing [50,52,54,56]. The orientation of the molecular chains is associated with a decrease in conformational entropy, which act as nucleation points for cold crystallization to occur at lower temperatures.…”

Section: Thermal Analysismentioning

confidence: 99%

“…This is also true for all samples regardless of increasing draw temperature (Figures 2B-D). However, the inset forFigure 2A, shows two thermal events: one being a small endotherm around Tg (which tends to mask this transition), the other a small exotherm between 75 -85 o C. The endotherm around Tg has been well documented for drawn and electrospun PLLA and attributed to the relaxation process of oriented molecular chains termed strain induced enthalpy relaxation[50][51][52][53][54][55]. Therefore, the oriented PLLA chains post-draw, which are not yet part of a crystallite, have a tendency to relax upon heating.…”

mentioning

confidence: 99%

An investigation into the crystalline morphology transitions in poly-L-lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regime

Billimoria

Heeley

Parsons

et al. 2018

European Polymer Journal

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A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription.For more information, please contact the WRAP AbstractThe mechanical behaviour, crystalline and macromorphology structure development during uniaxial deformation and annealing of poly-L-lactic acid (PLLA), with varying strain rate and draw temperatures (Td) above Tg, have been investigated using small-and wide-angle Xray scattering (SAXS/WAXS), microscopy, thermal and mechanical techniques. The mechanical behaviour of PLLA, was strongly dependent on Td where embrittlement and eventual failure were observed as Td was increased, during uniaxial drawing of the amorphous polymer. This was mirrored in the bulk surface morphology where crazing, microvoiding and cavitation occurred with increasing Td. SAXS/WAXS data showed that strain-induced crystallization occurs on drawing, but crystallite orientation decreased with increasing Td, due to chain relaxation at temperatures ≥30 o C above Tg. However, no longrange oriented lamellar macromorphology was observed post-draw directly and only developed in the samples that were step annealed at temperatures above Td. Also, the disordered α' crystal form was observed post-draw at Td between 60 -80 o C, whereas Td ≥ 90 o C, resulted in the ordered α crystal form directly. However, on annealing at temperatures of ≥110 o C, the α'-α crystal transition ensued and in all samples, an oriented lamellar macromorphology developed. Therefore, Td and post-draw annealing, have a significant influence on the mechanical properties, crystallinity and crystalline phase transformation in PLLA, which in-turn, affects the polymers medical and industrial applications.

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Endotherm just above glass transition in uniaxially drawn poly(lactic acid)s films with various d-isomer contents

Cited by 14 publications

References 38 publications

Characterization of biaxial strain of poly(l‐lactide) tubes

Characterization of biaxial strain of poly(l‐lactide) tubes

The structure transformation of pre‐oriented polylactic acid film during uniaxial stretching at room temperature

An investigation into the crystalline morphology transitions in poly-L-lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regime

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