- 學位論文
Crystallization and Melting Behavior of Enantiomeric Polylactide and the Racemic Blend
左旋聚乳酸及左/右旋聚乳酸等比例摻混之結晶及熔融行為
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摘要
The approach of combined small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC), as previously proposed by Su et al. (Macromolecules 2009, 42, 4200) was adopted to determine the equilibrium melting temperature (Tm°) and the fold surface energy (σe) for stereocomplex βc crystals of poly(L-lactide)/poly(D-lactide) (PLLA/PDLA). With gradually increased temperature, the crystalline lamellae approached solid-melt equilibration for reliable construction of the Gibbs-Thomson (GT) melting line. We successfully extracted from the SAXS data the thickness of crystalline lamellae via both Kratky-Porod approximation and model-fitting based on a combination of disk form factor and 1D-lattice structure factor. With crystallinity- corrected heat of fusion ΔHf° ≈ 165 J/g obtained from a combination of DSC and WAXS results, we have estimated from the intercept and the slope (= 2σeTm°/ρΔHf°) of the melting line that equilibrium melting temperature Tm° ≈ 260 °C and fold surface energy σe ≈ 190 mJ m–2. Melt-crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 C/min from 270 C) was studied via WAXS, DSC, and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (32/31) helical pairs below 220 C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 C; intensity of this mesophase peak started to decrease at 150 C, with concomitant emergence of WAXS- or DSC-discernible formation of βc crystals. Isothermal measurements at 200 vs. 170 C revealed the presence of low vs. high populations of helical pairs; βc crystals were observed to develop only at 170 C but not at 200 C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of βc nuclei. The clear change in the melt structure well before the formation of incipient βc crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of βc crystals. Melt-crystallization of PLLA upon slow cooling (1 C/min from 200 C) was studied via SAXS/WAXS, FTIR, and DSC. Results indicated emergence of the form factor of precursory nanograins in the SAXS profiles before presence of α crystals of identifiable WAXS reflections and 107 helices from FTIR, implying that the nucleation of α crystals involves clusters of non-helical conformers (NHC). Results of model-fitting of SAXS profiles to dispersed ellipsoids indicated constant cluster size before saturated population of nanograins as precursors toward crystallization. Synchronous and asynchronous 2D-correlation maps of the amorphous halo in WAXS profiles showed that NHC in the precursory clusters are arranged in primitive order related to the (110) reflection of α crystals. Synchronous and asynchronous 2D-correlation maps between WAXS and FTIR results also indicated strongly correlation between the (110) reflection and 107 helical structure during subsequent crystallization process, indicating adjustment toward helical regularity within the precursory nanograins during transformation to crystalline α form. Structural evolution during cold-crystallization of quenched poly(L-lactide) (PLLA) glass upon slow (1 °C/min) heating was examined via a combination of FTIR, SAXS/WAXS, and DSC. The as-quenched glass showed broad absorption in the range of 890 to 940 cm−1 (instead of sharp absorption at 922 cm−1 for 107 helices in α crystals), suggesting distributed helical conformers (DHC) around 32 conformation. Upon heating above the glass transition temperature (Tg ≈ 60 °C), a moderate decrease in the population of DHC and partial loss of interhelical contacts were observed, indicating partial disintegration of clusters of distributed helices; this was then followed by specific emergence of 107 helices at 78 °C, lagging slightly (by 2 °C) behind the start of crystallization endotherm in the DSC trace and emergence of crystalline reflections in the parallel WAXS profiles. In contrast, synchronous 2D correlation map derived from FTIR spectra in the range of 1480−1420 cm−1 revealed splitting of the 1457 cm−1 methyl deformation band in the amorphous matrix to give an additional peak at 1448 cm−1 for interchain contacts upon heating to the temperature range of 60−75 °C, which was replaced by sharp peaks at 1445 and 1466 cm−1 upon subsequent formation of α′ crystals up to 140 °C. This picture of clustered NHC as precursory nuclei was further supported by increases in nucleation density upon cold crystallization at 70 °C for glassy specimens prepared at decreasing cooling rates (ranging from direct liquid-nitrogen quench to 50 °C/min), for which the population of NHC as mesophase showed similar increases. Combined WAXS and DSC results indicated occurrence of α′-to-α transformation in the range of 140 to 160 °C; model-fitting of SAXS profiles with dispersed ellipsoids indicates that this solid-solid transformation proceeds with concurrent nanograin coalescence in view of the simultaneous increases in the oblate major axis 2A, the minor axis 2B, and the center-to-center distance d. For stretch-orientated (100% elongation at 60 ºC) PLLA specimens, 2D WAXS patterns revealed amorphous arcs at ambient temperature, amorphous ring upon heating above Tg (with concomitant shrinkage along the drawing direction), followed by emergence of crystalline arcs retaining the original orientation due to development of crystallites upon further heating. These observations are explained in terms of relaxation of stretched/oriented NHC to facilitate interchain interaction/aggregation of helical conformers (which remained oriented), followed by development of oriented nuclei and subsequent crystal growth. Cold-crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow heating (1 C/min from 30 C) was studied via WAXS, DSC, and FTIR. Results indicated that the βc phase of racemic polylactide crystallized first, followed by the formation of α′ crystals. Crystallization mechanisms for α′ and βc crystals are completely different in the racemic blend: intensity increase of the 107 helices of PLLA and 103 helices of PDLA in 922 cm−1 of FTIR are consistent with increase tendency of α′ crystals in terms of the (110)/(200) reflection of WAXS, but intensity of (110) reflection in βc crystal sequentially increased after extensive development of racemic (32/31) helical pairs, as similarly observed in the melt-crystallization case. Incubation at 60 °C resulted in clearly increased α′ crystallinity and decreased βc content in the racemic PLLA/PDLA blend during the subsequent cold-crystallization process, indicating clustering of NHC as precursors of α′ nuclei. Isothermal FTIR measurements between 70 to 85 °C revealed increasing population of 107 or 103 helices (vs. decreasing population of paired 32/31 helices) with increasing incubation temperature Ti above 76 °C, suggesting control of the α′ vs. βc phase ratio via proper incubation. To summarize, the process of polymer crystallization is not a one-step process as depicted in the classic theory of polymer crystallization: it is a multi-step process, involving intermediate stages of mesomorphic order. Even though there are nanograin existence before crystals appear for melt crystallization and cold crystallization of PLLA, however, they are different inside conformers, like non-helical arrangement aggregating for melt crystallization but distributed helical cluster for cold crystallization. Nevertheless, detailed structural features within the mesomorphic nanodomains can be different, i.e., non-helical conformers for PLLA but racemic helical pairs in the case of PLLA/PDLA.
關鍵字
左旋聚乳酸並列摘要
The approach of combined small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC), as previously proposed by Su et al. (Macromolecules 2009, 42, 4200) was adopted to determine the equilibrium melting temperature (Tm°) and the fold surface energy (σe) for stereocomplex βc crystals of poly(L-lactide)/poly(D-lactide) (PLLA/PDLA). With gradually increased temperature, the crystalline lamellae approached solid-melt equilibration for reliable construction of the Gibbs-Thomson (GT) melting line. We successfully extracted from the SAXS data the thickness of crystalline lamellae via both Kratky-Porod approximation and model-fitting based on a combination of disk form factor and 1D-lattice structure factor. With crystallinity- corrected heat of fusion ΔHf° ≈ 165 J/g obtained from a combination of DSC and WAXS results, we have estimated from the intercept and the slope (= 2σeTm°/ρΔHf°) of the melting line that equilibrium melting temperature Tm° ≈ 260 °C and fold surface energy σe ≈ 190 mJ m–2. Melt-crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 C/min from 270 C) was studied via WAXS, DSC, and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (32/31) helical pairs below 220 C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 C; intensity of this mesophase peak started to decrease at 150 C, with concomitant emergence of WAXS- or DSC-discernible formation of βc crystals. Isothermal measurements at 200 vs. 170 C revealed the presence of low vs. high populations of helical pairs; βc crystals were observed to develop only at 170 C but not at 200 C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of βc nuclei. The clear change in the melt structure well before the formation of incipient βc crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of βc crystals. Melt-crystallization of PLLA upon slow cooling (1 C/min from 200 C) was studied via SAXS/WAXS, FTIR, and DSC. Results indicated emergence of the form factor of precursory nanograins in the SAXS profiles before presence of α crystals of identifiable WAXS reflections and 107 helices from FTIR, implying that the nucleation of α crystals involves clusters of non-helical conformers (NHC). Results of model-fitting of SAXS profiles to dispersed ellipsoids indicated constant cluster size before saturated population of nanograins as precursors toward crystallization. Synchronous and asynchronous 2D-correlation maps of the amorphous halo in WAXS profiles showed that NHC in the precursory clusters are arranged in primitive order related to the (110) reflection of α crystals. Synchronous and asynchronous 2D-correlation maps between WAXS and FTIR results also indicated strongly correlation between the (110) reflection and 107 helical structure during subsequent crystallization process, indicating adjustment toward helical regularity within the precursory nanograins during transformation to crystalline α form. Structural evolution during cold-crystallization of quenched poly(L-lactide) (PLLA) glass upon slow (1 °C/min) heating was examined via a combination of FTIR, SAXS/WAXS, and DSC. The as-quenched glass showed broad absorption in the range of 890 to 940 cm−1 (instead of sharp absorption at 922 cm−1 for 107 helices in α crystals), suggesting distributed helical conformers (DHC) around 32 conformation. Upon heating above the glass transition temperature (Tg ≈ 60 °C), a moderate decrease in the population of DHC and partial loss of interhelical contacts were observed, indicating partial disintegration of clusters of distributed helices; this was then followed by specific emergence of 107 helices at 78 °C, lagging slightly (by 2 °C) behind the start of crystallization endotherm in the DSC trace and emergence of crystalline reflections in the parallel WAXS profiles. In contrast, synchronous 2D correlation map derived from FTIR spectra in the range of 1480−1420 cm−1 revealed splitting of the 1457 cm−1 methyl deformation band in the amorphous matrix to give an additional peak at 1448 cm−1 for interchain contacts upon heating to the temperature range of 60−75 °C, which was replaced by sharp peaks at 1445 and 1466 cm−1 upon subsequent formation of α′ crystals up to 140 °C. This picture of clustered NHC as precursory nuclei was further supported by increases in nucleation density upon cold crystallization at 70 °C for glassy specimens prepared at decreasing cooling rates (ranging from direct liquid-nitrogen quench to 50 °C/min), for which the population of NHC as mesophase showed similar increases. Combined WAXS and DSC results indicated occurrence of α′-to-α transformation in the range of 140 to 160 °C; model-fitting of SAXS profiles with dispersed ellipsoids indicates that this solid-solid transformation proceeds with concurrent nanograin coalescence in view of the simultaneous increases in the oblate major axis 2A, the minor axis 2B, and the center-to-center distance d. For stretch-orientated (100% elongation at 60 ºC) PLLA specimens, 2D WAXS patterns revealed amorphous arcs at ambient temperature, amorphous ring upon heating above Tg (with concomitant shrinkage along the drawing direction), followed by emergence of crystalline arcs retaining the original orientation due to development of crystallites upon further heating. These observations are explained in terms of relaxation of stretched/oriented NHC to facilitate interchain interaction/aggregation of helical conformers (which remained oriented), followed by development of oriented nuclei and subsequent crystal growth. Cold-crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow heating (1 C/min from 30 C) was studied via WAXS, DSC, and FTIR. Results indicated that the βc phase of racemic polylactide crystallized first, followed by the formation of α′ crystals. Crystallization mechanisms for α′ and βc crystals are completely different in the racemic blend: intensity increase of the 107 helices of PLLA and 103 helices of PDLA in 922 cm−1 of FTIR are consistent with increase tendency of α′ crystals in terms of the (110)/(200) reflection of WAXS, but intensity of (110) reflection in βc crystal sequentially increased after extensive development of racemic (32/31) helical pairs, as similarly observed in the melt-crystallization case. Incubation at 60 °C resulted in clearly increased α′ crystallinity and decreased βc content in the racemic PLLA/PDLA blend during the subsequent cold-crystallization process, indicating clustering of NHC as precursors of α′ nuclei. Isothermal FTIR measurements between 70 to 85 °C revealed increasing population of 107 or 103 helices (vs. decreasing population of paired 32/31 helices) with increasing incubation temperature Ti above 76 °C, suggesting control of the α′ vs. βc phase ratio via proper incubation. To summarize, the process of polymer crystallization is not a one-step process as depicted in the classic theory of polymer crystallization: it is a multi-step process, involving intermediate stages of mesomorphic order. Even though there are nanograin existence before crystals appear for melt crystallization and cold crystallization of PLLA, however, they are different inside conformers, like non-helical arrangement aggregating for melt crystallization but distributed helical cluster for cold crystallization. Nevertheless, detailed structural features within the mesomorphic nanodomains can be different, i.e., non-helical conformers for PLLA but racemic helical pairs in the case of PLLA/PDLA.
參考文獻
20. Yang, C. F.; Su, A. C.; Su, C. H.; Jeng, U. Unpublished results.
24. Chen, S. H.; Wu, Y. H.; Su, C. H.; Jeng, U.; Hsieh, C. C.; Su, A. C.; S. A. Chen Macromolecules 2007, 40, 5353–5359.
25. Su, C. H.; Jeng, U.; Chen, S. H.; Lin, S. J.; Ou, Y. T.; Chuang, W. T.; Su, A. C. Macromolecules 2008, 41, 7630–7636.
26. Su, C. H.; Jeng, U.; Chen, S. H.; Lin, S. J.; Wu, W. R.; Chuang, W. T.; Tsai, J. C.; Su, A. C. Macromolecules 2009, 42, 6656–6664.
8. Chen, S. H.; Wu, Y. H.; Su, C. H.; Jeng, U.; Hsieh, C. C.; Su, A. C.; Chen, S. A. Macromolecules 2007, 40, 5353–5359.
延伸閱讀
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- 馮冠倫（2020）。左旋聚乳酸－聚乙二醇與左旋聚乳酸－乙二醇團聯共聚合物對聚乳酸結晶行為以及結晶型態的影響〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU202004367
- 吳承軒（2019）。四臂聚乙二醇-左旋聚乳酸共聚物對左旋聚乳酸結晶行為之影響〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU201904218
- 陳雅鉉（2014）。不同溶劑處理效應對左旋聚乳酸結晶行為的影響〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU.2014.10813
- 黃祖浩（2013）。左右旋聚乳酸摻合物對左旋聚乳酸結晶行為的影響〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU.2013.02153